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The FHI poster session for 2024 has been scheduled for the week of November 25th to 28th, with the aim of fostering interpersonal connections and engaging in scientific discussions. To facilitate this, we will host four poster sessions featuring contributions from the departments and emeritus groups.
| Date | Time | Department | Location |
|---|---|---|---|
| 25.11.2024 | 14:00 - 16:00 | TH dept. | |
| 26.11.2024 | 14:00 - 16:00 | MP dept. | |
| 27.11.2024 | 14:00 - 16:00 | PC dept. | |
| 28.11.2024 | 14:00 - 16:00 | AC & ISC dept. |
If you have any questions or comments, please contact me!
Abstract view at TH poster session in building A (2nd floor) on Nov. 25th, 2024, from 2:00 pm.
Abstract view at TH poster session in building A (2nd floor) on Nov. 25th, 2024, from 2:00 pm.
Abstract view at TH poster session in building A (2nd floor) on Nov. 25th, 2024, from 2:00 pm.
Abstract view at TH poster session in building A (2nd floor) on Nov. 25th, 2024, from 2:00 pm.
Abstract view at TH poster session in building A (2nd floor) on Nov. 25th, 2024, from 2:00 pm.
Abstract view at TH poster session in building A (2nd floor) on Nov. 25th, 2024, from 2:00 pm.
Abstract view at TH poster session in building A (2nd floor) on Nov. 25th, 2024, from 2:00 pm.
Abstract view at TH poster session in building A (2nd floor) on Nov. 25th, 2024, from 2:00 pm.
Abstract view at TH poster session in building A (2nd floor) on Nov. 25th, 2024, from 2:00 pm.
Abstract view at TH poster session in building A (2nd floor) on Nov. 25th, 2024, from 2:00 pm.
Abstract view at TH poster session in building A (2nd floor) on Nov. 25th, 2024, from 2:00 pm.
Abstract view at TH poster session in building A (2nd floor) on Nov. 25th, 2024, from 2:00 pm.
Abstract view at TH poster session in building A (2nd floor) on Nov. 25th, 2024, from 2:00 pm.
Abstract view at TH poster session in building A (2nd floor) on Nov. 25th, 2024, from 2:00 pm.
Abstract view at TH poster session in building A (2nd floor) on Nov. 25th, 2024, from 2:00 pm.
Abstract view at TH poster session in building A (2nd floor) on Nov. 25th, 2024, from 2:00 pm.
Abstract view at TH poster session in building A (2nd floor) on Nov. 25th, 2024, from 2:00 pm.
Abstract view at TH poster session in building A (2nd floor) on Nov. 25th, 2024, from 2:00 pm.
Abstract view at TH poster session in building A (2nd floor) on Nov. 25th, 2024, from 2:00 pm.
Abstract view at TH poster session in building A (2nd floor) on Nov. 25th, 2024, from 2:00 pm.
We present on the realization of the first Bose-Einstein-Condensate of Dysprosium atoms. We present details about the experimental techniques, i.e. laser cooling and evaporative cooling, that allow us to increase the phase-space density of our gas until the onset of Bose-Einstein Condensation, signaled by the emergence of a bimodal distribution in time-of-flight.
The goal of our experiment is to study how optical cavities can modify ultracold reactions. To do so, we need to upgrade our ultracold dysprosium experiment by including two new elements in the science chamber. 1) A high-finesse optical cavity for controlling light-matter coupling. 2) An ion optics system for detecting reaction products. This requires the development of a new atomic source, i.e. a 2D-MOT, in place of our Zeeman slower. We present initial investigation of how a 2D-MOT can be integrated in our experiment. We also discuss how we plan to incorporate machine-learning for optimizing the duty cycle of our experiment.
Since 2013 the infrared FEL at the Fritz Haber Institute (FHI FEL) has been providing intense, pulsed mid-infrared (MIR) radiation, continuously tunable from <3 μm to >50 μm for in-house users. This has resulted in more than 100 peer-reviewed publications so far. In 2023 an additional short-Rayleigh-range far-infrared (FIR) FEL has been commissioned lasing from <5 μm to >170 μm. In addition, a 500 MHz kicker cavity has been installed downstream of the electron accelerator. It deflects the electron bunches alternatingly left and right by an angle of ±2° thereby splitting the high-repetition-rate (1 GHz) electron bunch train into two bunch trains of half the repetition rate each; one is steered to the MIR FEL and the other one to the FIR FEL. The wavelengths in both FEL’s can be tuned independently over wide ranges of up to a factor of four by undulator gap variation. In addition, 2-color operation is also available at reduced repetition rates (e.g. 55.5 MHz of both MIR and FIR pulses). Furthermore, two additional small dipole magnets upfront and behind the kicker cavity permit conventional single-color operation of either the MIR or the FIR FEL when the 500 MHz kicker field is off. Regular user operation in 2-color mode is scheduled to start within a few months.
In 1896, Edward Charles Pickering (1846-1919), Director of the Harvard College
Observatory (HCO), reported in a trio of publications the observation of “peculiar
spectra” of the southern star ζ Puppis, which he attributed to an “element not yet
found in other stars or on earth.” Supported by laboratory spectra obtained by Alfred
Fowler (1868-1940), Niels Bohr (1885-1962) showed in 1913 that this “element” was in
fact ionized helium, He+. Its spectrum has become known as the Pickering Series, even
though Pickering credited Williamina Fleming (1857-1911) for the discovery. Fleming
was one of HCO’s “computers” and the future Curator of the Astronomical Photographic
Glass Plate Collection. The series of spectral lines associated with Pickering’s name
played a unique role on the path to quantum mechanics by serving as a proving ground
for Bohr’s model of the atom. Our examination of the discovery of the Pickering series
relied on the records held at the Center for Astrophysics | Harvard & Smithsonian
(the successor institution to HCO), especially the Notebooks and Diaries of Williamina
Fleming and others as well as on the Center’s Glass Plate Collection. Glimpses of the
“peculiar sociology” of a research institution, half of whose staff were women employed
on grossly unequal terms with men, are given in the course of the narrative.
We present a combined experimental and theoretical investigation of the radiationless decay spectrum of an O 1s double core hole in liquid water. Our experiments were carried out using liquid-jet electron spectroscopy from cylindrical microjets of normal and deuterated water. The signal of the double-core-hole spectral fingerprints (hypersatellites) of liquid water is clearly identified, with an intensity ratio to Auger decay of singly charged O 1s of 0.0014(5). We observe a significant isotope effect between liquid H$_2$O and D$_2$O. For theoretical modeling, the Auger electron spectrum of the central water molecule in a water pentamer was calculated using an electronic-structure toolkit combined with molecular-dynamics simulations to capture the influence of molecular rearrangement within the ultrashort lifetime of the double core hole. We obtained the static and dynamic Auger spectra for H$_2$O, (H$_2$O)$_5$, D$_2$O, and (D$_2$O)$_5$, instantaneous Auger spectra at selected times after core-level ionization, and the symmetrized oxygen-hydrogen distance as a function of time after double core ionization for all four prototypical systems. We consider this observation of liquid-water double core holes as a new tool to study ultrafast nuclear dynamics.
Photoelectron circular dichroism (PECD) has emerged as an extremely sensitive probe of the molecular and electronic structure of chiral molecules, but its suitability for application to aqueous solutions had not yet been proven. Here, we provide an update on our recent PECD measurements of aqueous-phase alanine, the simplest chiral amino acid. We demonstrate that the PECD response of alanine in water is different for each of alanine’s carbon atoms, and is sensitive to molecular structure changes (protonation states) related to the solution pH. PECD of dilute aqueous-phase chiral molecules currently requires challenging and time-consuming measurements, but new enabling technologies including velocity map imaging of aqueous phase targets promises to dramatically expedite such experiments in the near future. We anticipate that liquid-phase PECD will become a powerful tool for the study of aqueous-phase chiral molecules of biological relevance in the coming years.
Intermolecular Coulombic decay (ICD) is a non-local autoionization process that has the potential to selectively probe the first hydration shell of solvated molecules. Here, we demonstrate the applicability of ICD spectroscopy to biomolecules in a complex environment. Firstly, we access site-specific information on the interaction of adenosine triphosphate in the aqueous phase (ATP(aq)) with magnesium ions (Mg2+(aq)). Because ICD is sensitive to the immediate environment of the target, information about such interactions is conveyed by the emerging ICD electrons following the ionization of core-level Mg 1s orbitals. A recently observed variant of ICD that follows resonant X-ray excitation of the target has an increased sensitivity to the chemical composition of the solvation shell and enables the determination of the absolute binding energies of the solvating molecules, for the first time. In a second example, we extend the resonant ICD to investigate the intermolecular interactions of aqueous-phase proline with Ca2+ ions. We provide evidence of ion pairing between the Ca2+ ions and the carboxylic acid moiety of proline replacing the water molecules in the ions's coordination shell. Therefore, we establish ICD spectroscopy as a new and powerful tool for biomolecular structure investigations.
Tens-of-µm-sized liquid jets have revolutionized experimental research on the physical chemistry of aqueous solutions. Our work on improved liquid jet techniques unlocks further interesting opportunities in this field.
We show first results of a novel set-up that enables velocity map imaging (VMI) of electrons emitted from a liquid jet, thus allowing the measurement of the full angular and energy distribution in one shot.
Flatjets - planar liquid sheets in vacuum - have interesting properties for experiments on liquids. We give an overview of various designs we have produced, and opportunities associated with them. In particular, using the FHI Free Electron Laser we have gained new insights into the diffusion dynamics in liquids on a µs scale.
Liquid microjets (liquid jets, LJ), introduced in the 80s–90s by M. Faubel, S. Schlemmer and J.P. Toennies, enabled the investigation of volatile liquids under high-vacuum conditions and opened a breadth of new fields [1]. One of such fields is the combination of LJs with photoelectron spectroscopy, LJ–PES, capable of revealing subtle energy shifts in molecular-orbital binding energies (BEs) of both solvent and solute [2]. Here we present our most recent LJ-PES study applied to proline solutions [3].
Proline is one of the proteinogenic amino acids, being the only one with a heterocyclic amine group, which imparts higher rigidity and reduces its conformational space. As happens with many biomolecules, proline is found as a zwitterion in the neutral aqueous phase. PES studies of such molecules in the gas phase can only bring limited insight about its electronic structure when in aqueous solution, which is its biologically relevant environment. We can also easily switch to proline’s cationic and anionic forms by changing the solution’s pH to low and high values, respectively. These assisted us to interpret our XPS (X-rays Photoelectron Spectroscopy) results.
Even though theoretical and indirect experimental evidence on the evaporation dynamics of water using LJs have been gathered in the last decades, no direct study on that matter has been reported. We are setting up a novel experiment: crossing a supersonic beam of metastable carbon monoxide (CO($a^3$Π) or CO$^*$) with a LJ. By detecting the projection of the beam or imaging its fluorescence, one can determine the density profile of the vapor phase around a LJ.
[1] M. Faubel, S. Schlemmer, and J. P. Toennies, A molecular beam study of the evaporation of water from a liquid jet, Z. Phys. D: At. Mol. Clusters 10, 269 (1988).
[2] B. Winter and M. Faubel, Photoemission from liquid aqueous solutions, Chem. Rev. 106, 1176 (2006).
[3] B. Credidio, S. Thürmer, D. Stemer, M. Pugini, F. Trinter, J. Vokrouhlický, P. Slavíček, and B. Winter, From gas to solution: The changing neutral structure of proline upon solvation, J. Phys. Chem. A (2024).
The ytterbium monofluoride (YbF) molecule has gained attention for being a system well-suited for measuring the electric dipole moment of the electron (eEDM). In this work we present REMPI measurements of Rydberg states, i.e. states with a single (highly) excited electron, of YbF. Assignment of these measured Rydberg states to series that converge to various rotational and vibrational levels of the YbF+ cation leads to a very accurate value for the ionization energy (IE) of YbF, along with values for the rotational constant B and the vibrational transition energy ΔG1/2 of YbF+. In addition, we have performed photo induced Rydberg ionization (PIRI) measurements via a previously identified Rydberg state of YbF. These measurements, that had already been successfully performed on the dysprosium oxide (DyO) molecule within our group,leading to a first reliable gas-phase reference value for ΔG1/2 of DyO+, confirm our ΔG1/2 value for YbF+ obtained from Rydberg state analysis.
Photoelectron Circular Dichroism (PECD) is a chiral optical effect that manifests in the angle-dependent photoemission of an electron upon irradiation of a chiral molecule by circularly polarized light. PECD can aid in our fundamental understanding of electron dynamics as this effect is acutely sensitive to the molecular state and electron emission conditions. The magnitude and sign of PECD can vary greatly depending on the emission channels linking the initial and final states of the molecule, the conformation of the molecule and its vibrational states, and the kinetic energy of the departing electron. This sensitivity is a double edge sword: PECD studies can offer a wealth of information, if one can disentangle the individual contributions to the overall measured PECD. The use of anions in PECD studies allows for pre-photodetachment mass selectivity and eliminates the need for X-ray based ionization sources or multiphoton ionization schemes. Furthermore, photodetachment from anions is a photoemission regime that has historically been understudied in conjunction with PECD studies. We provide an energy-resolved PECD signal for mass-selected deprotonated chiral anion of (R)-1-phenylethanol. The photoelectron spectrum of deprotonated 1-phenylethanol is clearly assigned to a single electronic transition of a single conformer, which allows for the straightforward investigation of the PECD dependence on the electron kinetic energy, as well as vibrational state of the molecule. The PECD of 1-phenylethanol was measured at different photon energies from 2.38 eV to 3.59 eV, showing a change of the resulting PECD value for a given transition with differing electron kinetic energies. If electron kinetic energy is held constant, the PECD effect at low electron kinetic energies is found to be dominated by a single vibrational mode.
The stereochemistry and conformational flexibility of chiral molecules have a strong impact on their biological, biochemical, and pharmacological properties. A central analytical challenge is the generally applicable differentiation of enantiomers, as well as the fast and accurate determination of the enantiomeric excess of a chiral sample.
Gas phase vibrational action spectroscopy is a highly sensitive, selective, and fast tool for this purpose. Chiral ionic analytes are transferred into the gas phase, where they interact with volatile chiral selector molecules in a gas-filled ion guide under the formation of diastereomeric complexes. These are then mass-selected, cryogenically cooled, messenger-tagged and an infrared photodissociation (IRPD) spectrum is measured. The spectra of the vibrationally cold diastereomers exhibit sufficiently different IR fingerprints, such that they can be spectrally distinguished and quantified.
Different intermolecular non-covalent interactions can be present in diastereomers, among them H-bonds, π-π interactions and steric hindrance. We study a set of different chiral selector molecules and chiral amino acid analytes with different structural motifs to identify the decisive interactions in the present complexes. We aim at maximizing the differences in the vibrational action spectra of the diastereomers.
Studies on metal oxide clusters in the gas phase are aimed at gaining a better atomistic understanding of single-site catalysts. Here, we study the structure and reactivity of cationic model systems using a combination of mass spectrometry, infrared photodissociation (IRPD) spectroscopy, ion mobility and electronic structure calculations ranging from density functional theory to multi-reference electron correlation methods.
The reactivity of the aluminium oxide cations [Al3O4]+ is studied upon the substitution of Al atoms by the transition metal ones, Fe or Co. The electronically closed-shell [Al3O4]+ has a cone like structure and is unreactive towards methane activation,[1] while [FeAl2O4]+[2] and [CoAl2O4]+ show a planar bicyclic structure with a terminal oxygen radical (AlO•-I) that can abstract H from CH4. However, the doubly transition metal substituted species, [Fe2AlO4]+ and [Co2AlO4]+ are unreactive towards methane, and the structure assignment could not be done solely comparing IRPD spectra to harmonic vibrational spectra for different isomers. Therefore, we used ion mobility spectrometry and ab initio molecular dynamics simulations to unambiguously determine the structure of these systems. They exhibit a “key-like” structure with a planar four-membered ring connected to a nearly linear terminal O–TM+III–O-II unit, that display large amplitude motions that can only be captured by anharmonic simulations. Ion mobility data for the cobalt system, [Co2AlO4]+, were also obtained and confirm the assignment made.
We present our recent progress on laser cooling AlF molecules using deep lasers. AlF is distinctively different from the molecular species that have been laser-cooled so far: it is a stable molecule that can be produced in large quantities and it has a strong $A^1\Pi\leftarrow X^1\Sigma^+$ transition near 227.5 nm that can be used for rapid slowing and cooling in a magneto-optical trap (MOT) with a large capture velocity. Similar to alkaline-earth atoms, AlF has narrow, spin-forbidden $a^3\Pi\leftarrow X^1\Sigma^+$ transitions near 367 nm for precision spectroscopy and narrow-line cooling.
Using two deep ultraviolet lasers, we demonstrate laser slowing of AlF molecules produced in a cryogenic buffer gas molecular beam. Molecules are decelerated from 150 to 70 m/s using the chirped-frequency laser slowing method. We experimentally measured the loss probability from the laser cooling scheme to the second vibrationally excited level of the $X^1\Sigma^+$ state, finding $1.8(5) \times 10^{-4}$. This implies a photon scattering limit of about 5,000 with only a single vibrational repump laser.
In addition, we present new experiments using our continuous AlF molecular beam oven. We compare the output flux of the oven to that a of supersonic beam of AlF, finding that its continuous output exceeds the peak flux of the supersonic source at around $J=7$. The continuous source is then used to measure high $(v,J)$ levels of the $c^3\Sigma^+$ state, providing new information about electronic states near the first dissociation limit of the molecule. We observe that the molecular beam, produced at a temperature near 1,000K, thermalises with the vacuum walls of our experiment and produces a transient room temperature vapour. This presents a possibility to laser cool and trap molecules from a greatly simplified and inexpensive source, in a multitude of rotational states.
Aluminum monofluoride (AlF) is a promising candidate for laser cooling and trapping experiments. To support research in this area, we have developed the AlF Spectroscopy Database (alf.mp.fhi-berlin.mpg.de), which features the latest spectroscopic constants for the AlF molecule across multiple electronic states.
The database is hosted on an interactive website that enables users to compute Franck-Condon factors and transition wavelengths between vibrational states of different electronic states. More importantly, it provides estimates of rovibrational energy levels and their transitions for selected electronic states, specified vibrational and rotational levels. Furthermore, the database includes quantum chemistry predictions of transition dipole moments for various excited states. These features provide useful information about laser frequencies during experimental setups.
We have developed a Python-based toolkit designed to investigate the dynamic properties of molecular systems using molecular dynamics (MD) simulation techniques. The toolkit supports simulations under various ensembles, including the microcanonical (NVE), canonical (NVT), isoenthalpic–isobaric (NPH), and isothermal–isobaric (NPT) ensembles, enabling the study of diverse dynamic processes and chemical reactions under different conditions. Users can also define the initial state of species to simulate scattering processes.
The toolkit incorporates enhanced sampling methods such as replica exchange molecular dynamics (REMD) and integrated temperature sampling (ITS). It is further equipped to utilize machine-learning potentials and integrates with electronic structure theory calculations, e.g. coupled-cluster theory. These capabilities are made possible through compatibility with popular quantum chemistry tools like Molpro and ASE-supported calculators. In addition, the toolkit provides several analysis tools, for example, clustering and vibrational spectrum calculations.
Beyond chiral analysis, Enantiomer-Specific State Transfer (ESST) enables the control and manipulation of chiral molecules at the quantum level. Using tailored microwave fields, a chosen rotational state can be enriched for a selected enantiomer. Although ESST can theoretically achieve 100% transfer efficiency, early ESST studies reported only modest state-specific enantiomeric enrichment, limited to a few percent [1,2]. This limitation was primarily due to the thermal population of rotational states [1,2] and the spatial degeneracy of these states [3].
To mitigate the effect of thermal population, we developed a new experimental scheme combining ultraviolet radiation with microwave spectroscopy. This approach allows for depleting one of the rotational states before the ESST process [4,5], significantly enhancing transfer efficiency. This advancement has enabled quantitative studies of ESST under various conditions [4,5].
[1] S. Eibenberger, J. Doyle, and D. Patterson, Phys. Rev. Lett. 118, 123002 (2017)
[2] C. Pérez, A. L. Steber, S. R. Domingos, A. Krin, and M. Schnell, Angew. Chem. Int. Ed. 56, 12512 (2017)
[3] K. K. Lehmann, J. Chem. Phys. 149, 094201 (2018)
[4] J. H. Lee, J. Bischoff, A. O. Hernandez-Castillo, B. Sartakov, G. Meijer, and S. Eibenberger-Arias, Phys. Rev. Lett. 128, 173001 (2022)
[5] J. H. Lee, J. Bischoff, A. O. Hernandez-Castillo, E. Abdiha, B. Sartakov, G. Meijer, and S. Eibenberger-Arias, New J. Phys. 26, 033015 (2024)
In this poster, we present near-complete chiral selection in rotational quantum states [1]. In our study we combine UV laser and microwave radiation to realize near-ideal initial conditions for Enantiomer-Specific State Transfer (ESST). With this we overcome previous limitations of ESST due to initial thermal population in all three states in a triad of rotational states connected to the absolute ground state. Our results show that 96% state-specific enantiomeric purity can be obtained from a racemic mixture, in an approach that is universally applicable to all chiral molecules of C1 symmetry. With its capability to create enantiopure quantum states starting from a racemic mixture, this approach has the potential to significantly advance the experimental methods to measure parity-violation effects in chiral molecules [2].
We will also present information on ongoing efforts to address triads of rotational states with higher orientational degeneracy which are intrinsically more difficult to control. For this, we will incorporate theoretically tailored pulse schemes particularly designed to overcome this challenge [3].
[1] JuHyeon Lee, Elahe Abdiha, Boris Sartakov, Gerard Meijer, Sandra Eibenberger-Arias,
Nature Communications 15, 7441 (2024)
[2] I. Erez, E. R. Wallach, and Y. Shagam. Phys. Rev. X, 13, 041025, (2023)
[3] M. Leibscher, E. Pozzoli, C. Pérez, M. Schnell, M. Sigalotti, U. Boscain, C. P. Koch, Commun Phys 5, 110 (2022).
We present high-resolution UV spectra of three different chiral molecules — 1-Indanol, Styrene oxide and 1-Phenylethanol. All of these molecules are interesting candidates for performing experiments on enantiomer-specific quantum state control.
We show vibrationally resolved REMPI spectra together with theoretical predictions to facilitate assignment of the transitions. We also experimentally determined the S1 excited state lifetime using two-color REMPI and via Lamb-dip measurements [1]. In addition, we present rotationally resolved laser-induced fluorescence spectra of the molecules. The fits to the spectra provide the rotational energy level structure in the first electronically excited state and provide important information for future experiments on enantiomer-specific state transfer [2,3].
[1] A. O. Hernandez-Castillo, J. Bischoff, J. H. Lee, J. Langenhan, M. Karra, G. Meijer, and S. Eibenberger-Arias, Phys. Chem. Chem. Phys. 23, 7048 (2021)
[2] J. H. Lee, J. Bischoff, A. O. Hernandez-Castillo, B. Sartakov, G. Meijer, and S. Eibenberger-Arias, Phys. Rev. Lett. 128, 173001 (2022)
[3] JuHyeon Lee, Elahe Abdiha, Boris Sartakov, Gerard Meijer, Sandra Eibenberger-Arias, Nature Communications 15, 7441 (2024)
Gas-phase vibrational spectroscopy has been proven to provide a nearly ideal method for the investigation of ions, unperturbed by solvent effects. In addition, ions can be placed in helium nanodroplets as a matrix, where the interaction with the dopant ion is very weak. As the helium droplets are at very low temperatures (0.37 K), thermal broadening and therefore spectral congestion is significantly reduced. In the experiment, the ions that are embedded in a helium droplet are irradiated with a tuneable infrared free electron laser. Resonant absorption of photons can lead to an evaporation of helium, as the energy of the excitation gets dissipated through the molecule and the droplet, eventually leading to evaporation of helium. Detection of bare ions as a function of infrared wavelength then gives an infrared spectrum. This technique can be used to study a wide variety of molecular ions and ionic clusters, ranging from small carbocations through proton-bound complexes to large organic molecules and biomolecules.
Carbocations are playing an important role in many areas of chemistry. They can be found in the interstellar medium, as the availability of carbon and the long lifetime of gaseous ionic species in astrophysical environments causes a high abundance of ionised carbonaceous molecules, such as polyaromatic molecules and fullerenes.Furthermore, carbocations are intermediates in a variety of organic chemical reactions. Some of those can undergo facile chemical conversions, such as hydride shifts, which can occur due to the overlap of the empty p-orbital and a vicinal CH $\sigma$-orbital. Others can form three-center two-electron bonds and are therefore of a more fundamental interest. These so-called non-classical carbocations involve the delocalization of a chemical bond, which results in C-C and C-H bonds of fractional bond order.
Here we show a selection of infrared spectra of carbocations of interest in the spectral range of 800-1600~cm$^{-1}$ to investigate diverse processes such as hydride shifts or the formation of non-classical carbocations.
Ultracold helium nanodroplets provide an ideal matrix for gas-phase vibrational spectroscopy, reducing thermal broadening and spectral congestion while unperturbed significantly by solvent effects and interactions with dopant ions. In the experiment, ions are embedded in a helium droplet and irradiated with a burst-mode infrared free electron laser (FEL). Resonant absorption of photons from the FEL pulse train by the ion dissipates energy to the surrounding helium, causing evaporation and detection of the bare ion as as a function of wavelength and generating low noise spectra. While the majority of previous FEL ultracold helium experiments have utilised a single FEL beam (one-colour), there exist may opportunities to use one-colour split-pump and two-colour beams for novel molecular detection and probing experiments. Here we explore the experimental design and implementation for controlling split pump and simultaneous two-colour FEL experiments with adjustable spatial and temporal pulse overlap using second-harmonic generation at near to far IR.
Cryogenic Infrared action spectroscopy has been proven to be effective for the experimental characterization of the vibrational modes of a large variety of molecular ions and ionic clusters in the gas phase[i]. Here, the ion of interest is isolated by a quadrupole mass filter and accumulated in a hexapole ion trap, where is traversed by a beam of helium nanodroplets that pick up the selected ions and when left the ion trap are probed by infrared light of a free electron laser (FHI-FEL). When the frequency of the laser is resonant with a vibrational transition of the ion the absorption of a photon occurs, with a subsequent intramolecular vibrational redistribution (IVR) among all vibrational modes in the picosecond (ps) timescale. This energy is quickly transfer to the helium droplet, which by evaporation of helium atoms (evaporative cooling) returns to its equilibrium temperature.. This has an outstanding consequence; as the absorption-dissipation process occurs at the ps timescale, every micro-pulse of the FEL (separated by 1 ns) encounters the ion at the equilibrium temperature of the helium droplet (0.37 K), always probing the ion at its vibrational ground state. The spectrum, then, is obtained by the detection of the bare ions in a time-of-flight analyzer as a function of the FEL wavenumber
Using two FEL pulse trains with a controlled pulse delay, we can perform one- and two-color IR-IR pump-probe spectroscopy on molecular ions in helium droplets. With the one-color pump-probe methodology, vibrational lifetimes can be measured by integrating the signal intensity as a function of the delay time between the two pulse trains. By employing a probe of a different wavelength (two-color), the flow of the energy from one mode to another can be followed, and the vibrational dynamics can be described.
Exposure of human body to ultraviolet light can induce formation of dimeric crosslinks at bipyrimidine sites within deoxyribonucleic acid. Nucleotide excision repair enzymes normally recognize the crosslinks and remove them. In-born genetic mutations of the enzymes result in severe photosensitivity and high risk of skin cancer. Completely avoiding sun or consuming medication for pain-free light exposure are the only treatments available. Two main types of crosslinks are known to occur: cyclobutane pyrimidine dimer (CPD) and 6-4 pyrimidine-pyrimidone (64PP) adduct. Several diastereoisomers of CPD and a Dewar isomer of 64PP-adduct are known to form. Isomeric and isobaric nature of the system complicates unambiguous assignment of structures with traditional mass spectrometry based methods. Other traditional spectroscopy techniques, such as nuclear magnetic resonance spectroscopy, require large amount of purified samples to elucidate the structures. Here, we use cryogenic gas-phase infrared action spectroscopy technique coupled with nano-electrospray ionization instead. A custom-built instrument is used for spectroscopic analysis of mass-selected ions trapped in helium nanodroplets cooled down to 0.37 K.
Crosslinked nucleotides produced by exposing a solution of mono- or di-nucleotides to 254 nm ultraviolet radiation was used to test the feasibility of the technique for analyzing such molecules. Mass-selected ions of the crosslinked nucleotides trapped in helium nanodroplets are irradiated with infrared laser to vibrationally excite the ions. The ions released post evaporation of the helium nanodroplets due to the absorption of energy from the photons are detected to plot a highly resolved spectrum. The experimental spectrum is compared with theoretical spectra, of different possible isomers, calculated using density-functional theory. Clearly cis isomers of CPD and non-Dewar isomer of 64PP-adduct are the main products. The presence of other isomers of CPD cannot be ruled out. Cryogenic infrared action spectroscopy of mobility-separated and mass-selected ions will likely provide a conclusive answer.
Conventional condensed-phase bioanalytical approaches often require large amounts of high-purity samples and are, therefore, not universally applicable. Mass spectrometry (MS), on the other hand, requires only minute sample amounts and its purity is often not a critical factor. Although, the extent of structural information obtained directly by MS is limited,it can be combined with complementary techniques, such as ion-mobility mass spectrometry (IM-MS) and gas-phase infrared (IR) spectroscopy to yield both conformational- (IM-MS) and vibrational information (IR) on the molecules of interest.
Here, we report on the combination of IM-MS and cryogenic IR spectroscopy. Biomolecules are transferred to the gas phase under soft conditions by nano-electrospray ionisation and are separated based on their size, mass, and charge in a linear drift tube by collisions with an inert buffer gas followed by a quadrupolar mass analyser. Based on the arrival time of the molecules travelling through the drift tube, different conformations of molecules can be isolated and selected for the transfer to the cryogenic ion trap. In the trap, ions are cooled to ~40 K and tagged with weakly-bound neutral N2 molecules. When such tagged ions absorb one (or more) IR photons from a benchtop OPO/OPA laser or the FHI free electron laser, the tag can dissociate from the ion, which leads to a mass shift of the analyte in the time-of-flight mass analyser. Finally, the IR spectrum of mass- and conformer-selected ions is recorded by monitoring the signal of the tagged (or untagged) ion in the time-of-flight mass spectrum as a function of excitation wavelength. To demonstrate the wide-range applicability of the instrument, vibrational data have been collected from a wide range of biomolecular species, such as peptides and carbohydrates.
FHI-aims (Fritz Haber Institute ab initio materials simulations) [1-3] is a versatile electronic-structure software package developed for computational studies in molecular and materials science. Widely used by a global network of developers, researchers at the Fritz Haber Institute, academic institutions, and industry, FHI-aims leverages numeric atom-centered basis sets to deliver computational precision on par with leading benchmark codes for density functional theory (DFT) and many-body methods. Notably, it achieves this high level of precision [4] while retaining computational efficiency similar to plane-wave pseudopotential methods. The code demonstrates remarkable applicability, routinely handling systems comprising thousands of atoms with semi-local and hybrid density functionals – recently demonstrated up to 30,000 atoms for hybrid density functionals [5]. Additionally, it exhibits excellent scalability on modern high-performance computing platforms. FHI-aims boasts advanced electronic-structure capabilities for both molecules and solids and seamlessly integrates into complex simulation environments. This integration includes the ability to serve as a parallel library accessible through Python or via internet sockets or through its graphical user interface GIMS [6], making it a powerful tool for a wide range of scientific investigations. Recent updates include integration with high-throughput simulation frameworks like atomate2 (based on pymatgen) [7] and Taskblaster (based on ASE) [8], enhancing its capability to explore material properties efficiently. The FHI-aims community actively develops new features and interfaces with new external frameworks, such as dispersion correction models (XDM [9] and D3 [10]), band unfolding, the Kubo-Greenwood formula [11], crystal orbital overlap population analysis [12], and improvements to periodic GW calculations and to DFPT functionality, making FHI-aims a continuously evolving tool. As an outlook, we present a framework for active learning with SISSO [13], which is being tightly integrated with FHI-aims with the goal of broadening the usability of AI materials discovery.
References
[1] V. Blum et al., Comp. Phys. Commun. 180, 2105 (2009)
[2] V. Havu, et al., J. Comp. Phys. 228, 8367 (2009)
[3] V. Gavini, et al., Model. Simul. Mater. Sci. Eng. 31.6, 063301 (2023)
[4] K. Lejaeghere, et al. Science 351.6280, aad3000 (2016)
[5] S. Kokott, et al., J. Comp. Phys. 161, 024112 (2024)
[6] Visit: https://gims.ms1p.org
[7] Visit: https://materialsproject.github.io/atomate2/
[8] Visit: https://taskblaster.readthedocs.io/en/latest/
[9] A. J. A. Price, A. Otero-de-la-Roza, and E. R. Johnson, Chem. Sci. 14, 1252 (2023)
[10] S. Grimme, S. Ehrlich, and L. Goerigk, J. Comp. Chem., 32, 1456 (2011)
[11] J. Quan, C. Carbogno, and M. Scheffler, arXiv:2408.12908 (2024)
[12] I.Takahara et al, Modelling Simul. Mater. Sci. Eng. 32, 055028 (2024)
[13] T. A. R. Purcell, M. Scheffler, and L. M. Ghiringhelli, J. Chem. Phys. 159, 114110 (2023)
A highly intricate interplay of underlying processes governs certain materials properties and functions. This prevents a realistic description by physical models or atomistic simulations. AI can identify nonlinear correlations between materials’ parameters and the measured performance. Thus, AI might better capture the materials’ behavior compared to the theory of the past. However, the data is often inconsistent and the flexibility of AI usually comes together with a lack of interpretability. To address these issues, we combine systematic experiments and simulations with interpretable, data-efficient AI to identify key physical parameters that describe complex materials properties, the “materials genes”.[1] We discuss the concept and recent applications in heterogeneous catalysis.[2,3]
References
[1] L. Foppa, L.M. Ghiringhelli, F. Girgsdies, M. Hashagen, P. Kube, M. Hävecker, S. Carey, A. Tarasov, P. Kraus, F. Rosowski, R. Schlögl, A. Trunschke, and M. Scheffler, Materials genes of heterogeneous catalysis from clean experiments and artificial intelligence, MRS Bulletin 46, 2021; https://doi.org/10.1557/s43577-021-00165-6
[2] G. Bellini, G. Koch, F. Girgsdies, J. Dong, S. J. Carey, O. Timpe, G. Auffermann, M. Scheffler, R. Schlögl, L. Foppa, A. Trunschke. CO Oxidation Catalyzed by Perovskites: The Role of Crystallographic Distortions Highlighted by Systematic Experiments and Artificial Intelligence. Angew. Chem. Int. Ed. 2024 https://doi.org/10.1002/anie.202417812
[3] R. Miyazaki, K. S. Belthle, H. Tüysüz, L. Foppa, M. Scheffler, Materials Genes of CO2 Hydrogenation on Supported Cobalt Catalysts: An Artificial Intelligence Approach Integrating Theoretical and Experimental Data. J. Am. Chem. Soc. 146, 5433, 2024; https://doi.org/10.1021/jacs.3c12984
Sequential active learning (SAL)-driven workflows can efficiently guide experiments and simulations towards the discovery of materials with desired properties [1]. However, AI and machine-learning approaches commonly used in these workflows rely on the knowledge of key physical parameters describing the materials property of interest. These low-dimensional representations are typically unknown. Here, we address this challenge by developing a SAL workflow based on the sure-independence screening and sparsifying operator (SISSO) approach [2,3]. SISSO identifies, even based on moderate amounts of data, models for materials properties as analytical expressions depending on key parameters, out of many offered ones. Crucially, we train ensembles of SISSO models in order to obtain not only mean predictions but also to quantify the uncertainty of the predictions, which are used to navigate the previously unexplored regions of materials space [4]. We demonstrate the SISSO-guided workflow by identifying acid-stable oxides for the water-splitting reaction by using high-quality DFT-HSE06 calculations [5].
[1] J. H. Montoya, K. T. Winther, R. A. Flores, T. Bligaard, J. S. Hummelshøj, M. Aykol, Autonomous intelligent agents for accelerated materials discovery, Chem. Sci. 11, 2022; https://doi.org/10.1039/D0SC01101K
[2] R. Ouyang, S. Curtarolo, E. Ahmetcik, M. Scheffler, L. M. Ghiringhelli, SISSO: A compressed-sensing method for identifying the best low-dimensional descriptor in an immensity of offered candidates, Phys. Rev. Mater. 2, 083802, 2018; https://doi.org/10.1103/PhysRevMaterials.2.083802
[3] T. A. R. Purcell, M. Scheffler, C. Carbogno, L. M. Ghiringhelli, SISSO++: A C++ Implementation of the Sure-Independence Screening and Sparsifying Operator Approach, J. Open Source Softw. 7, 3960, 2022; https://doi.org/10.21105/joss.03960
[4] J. Behler, G. Csanyi, L. Foppa , K. Kang, M. F. Langer, J. T. Margraf, A. S. Nair, T. A. R. Purcell, P. Rinke, M. Scheffler, A. Tkatchenko, M.Todorovic, O. T. Unke, Y. Yao, Workflows for Artificial Intelligence, ChemRxiv, 2024; https://doi.org/10.26434/chemrxiv-2024-vw06p
[5] A. S. Nair, AI-guided Workflow for the Discovery of Acid-Stable Oxides, https://gitlab.com/akhilsnair/sl-sisso
Widely used machine-learning (ML) approaches in materials science and catalysis are designed to accurately describe, in average, a wide range of materials. Nonetheless, only a handful of compounds might show the desired properties to be suitable for a given application. Thus, global ML models may overlook these statistically exceptional materials of interest. Here, we discuss how the subgroup-discovery (SGD) approach can be utilized to identify descriptions focused on exceptional materials [1]. We present a systematic analysis of the Pareto front of optimal SGD solutions with respect to generality and exceptionality, two conflicting objectives in SGD [2]. The concepts are illustrated by the identification of “rules” describing single-atom alloys (SAAs) capable of providing a strong activation of adsorbed CO$_2$, the first step towards the catalytic conversion of the molecule towards chemicals and fuels [3].
[1] B. R. Goldsmith, M. Boley, J. Vreeken, M. Scheffler, and L. M. Ghiringhelli, Uncovering structure-property relationships of materials by subgroup discovery, New. J. Phys. 19, 013031, 2017; https://doi.org/10.1088/1367-2630/aa57c2
[2] L. Foppa and M. Scheffler, Coherent Collections of Rules Describing Exceptional Materials Identified with a Multi-Objective Optimization of Subgroups, arXiv:2403.18437, 2024; https://doi.org/10.48550/arXiv.2403.18437
[3] H. I. Rivera-Arrieta and L. Foppa, Rules Describing CO$_2$ Activation on Single-Atom Alloys from DFT-meta-GGA Calculations and Artificial Intelligence, ChemRxiv, 2024; https://doi.org/10.26434/chemrxiv-2024-1dr10
An accurate first-principles description of the electronic band structure at finite temperatures is the prerequisite to quantitatively predict the electronic and optical properties of real materials. Theoretically, this requires proper consideration of the self-energy contributions from both electron-electron (e-e) and electron-vibration (e-vib) interactions. For the latter, the widely used electron-phonon coupling (EPC) model [1] fails for strongly anharmonic materials. Furthermore, while the self-energy contributions from the two types of interactions are considered separately within the EPC model, they can be treated collectively in a statistical manner [2] by combining the $GW$ method with ab initio molecular dynamics. To realize this approach, a robust and efficient $GW$ implementation for periodic systems is of key importance.
In this poster, we present our recent efforts to implement this approach to study electron self-energy using FHI-aims, an all-electron full-potential framework with compact numeric atom-centered orbitals (NAOs).[3] The one-shot periodic $G_0W_0$ method in FHI-aims is implemented based on the localized resolution-of-identity (RI) technique. [4,5] We first benchmark the performance of our $G_0W_0$ implementation in terms of accuracy and efficiency. We then show that a proper treatment of the dielectric response at long wave-length limit can significantly accelerate the convergence of quasiparticle band gap with respect to k-grids and auxiliary basis sets. Finally, using the band unfolding technique implemented in the NAO framework, we present temperature-dependent electronic band structure of silicon accounting for e-e and e-vib interactions, as a proof of concept.
References
[1] F. Giustino, Electron-phonon interactions from first principles, Rev. Mod. Phys. 89, 015003, 2017; https://doi.org/10.1103/RevModPhys.89.015003
[2]M. Zacharias, M. Scheffler, and C. Carbogno, Fully anharmonic nonperturbative theory of vibronically renormalized electronic band structures, Phys. Rev. B 102, 045126, 2020; https://doi.org/10.1103/PhysRevB.102.045126
[3]V. Blum, R. Gehrke, F. Hanke, P. Havu, V. Havu, X. Ren, K. Reuter, and M. Scheffler, Ab initio molecular simulations with numeric atom-centered orbitals, Comput. Phys. Commun. 180, 2175–2196, 2009; https://doi.org/10.1016/j.cpc.2009.06.022
[4]A. C. Ihrig, J. Wieferink, I. Y. Zhang, M. Ropo, X. Ren, P. Rinke, M. Scheffler, and V. Blum, Accurate localized resolution of identity approach for linear-scaling hybrid density functionals and for many-body perturbation theory, New J. Phys. 17, 093020, 2015; https://doi.org/10.1088/1367-2630/17/9/093020
[5] X. Ren, F. Merz, H. Jiang, Y. Yao, M. Rampp, H. Lederer, V. Blum, and M. Scheffler, All-electron periodic G0W0 implementation with numerical atomic orbital basis functions: Algorithm and benchmarks, Phys. Rev. Materials 5, 013807, 2021; https://doi.org/10.1103/PhysRevMaterials.5.013807
*This project was supported by the NOMAD Center of Excellence (European Union's Horizon 2020 research and innovation program, Grant Agreement No. 951786) and the ERC Advanced Grant TEC1p (European Research Council, Grant Agreement No. 740233)
Molecular dynamics (MD) has been popularly utilized to understand the dynamical properties of materials such as thermal, electrical, and ionic conductivities. Ab initio MD provides universal, high-quality predictions for energy, forces, and stress of any material, but its usage is limited due to high computational costs. Recent machine-learned interatomic potentials (MLIPs), with their excellent size scalability and remarkable calculation efficiency, can address this issue. However, the reliability of MLIPs could not be guaranteed for out-of-domain configurations. Particularly, rare events, such as defect creation or phase transition precursors, are often missed in training data or have regularized away due to insufficient data during MLIP training. But, owing to their significant impact on dynamical properties [1], their behavior should be reproduced by MLIP. This study systematically investigates how an active learning (AL) scheme can deal with rare event training and accelerate the whole training process [2]. First, the configurational space is examined by MD using explorative MLIPs, such as NequIP [3] and SO3KRATES [4]. Second, all generated MD snapshots are evaluated based on the MLIP prediction uncertainty, which enables qualitative identification of unfamiliar data. Finally, an iterative loop is formed by incorporating unfamiliar data into training data to retrain MLIP models for the next round. Applying AL to 122 materials [1, 5] identifies two representative corrections for erroneous MLIP predictions: a loss of real rare events and a prediction of false events. In addition, under-(over-)estimation of phonon lifetimes in AgGaSe2(CuI) shows the impact of erroneous MLIP predictions in dynamical properties, while AL rectifies them. In the end, we make NequIP enable the direct evaluation of unfolded heat flux prediction via automatic differentiation [6], resulting in efficient heat conductivity evaluation using MLIPs. Finally, a whole AL process leads to a systematic MLIP approach for thermal conductivity predictions of thermal insulators.
[1] F. Knoop, T. A. R. Purcell, M. Scheffler, and C. Carbogno, Anharmonicity in thermal insulators: An analysis from first principles. Physical Review Letters, 130(23), 236301, 2023; https://doi.org/10.1103/PhysRevLett.130.236301
[2] K. Kang, T. A. R. Purcell, C. Carbogno, and M. Scheffler, Accelerating the Training and Improving the Reliability of Machine-Learned Interatomic Potentials for Strongly Anharmonic Materials through Active Learning. arXiv preprint arXiv:2409.11808, 2024; https://doi.org/10.48550/arXiv.2409.11808
[3] S. Batzner, A. Musaelian, L. Sun, M. Geiger, J. P. Mailoa, M. Kornbluth, N. Molinari, T. E. Smidt, and B. Kozinsky. E (3)-equivariant graph neural networks for data-efficient and accurate interatomic potentials. Nature communications, 13(1), 2453, 2022; https://doi.org/10.1038/s41467-022-29939-5
[4] J. T. Frank, O. T. Unke, K.-R. Müller, and S. Chmiela, A Euclidean transformer for fast and stable machine learned force fields. Nature Communications, 15(1), 6539, 2024; https://doi.org/10.1038/s41467-024-50620-6
[5] NOMAD repository for aiMD data, DOI: 10.17172/NOMAD/2021.11.11-1
[6] M. F. Langer, F. Knoop, C. Carbogno, M. Scheffler, and M. Rupp, Heat flux for semilocal machine-learning potentials. Physical Review B, 108(10), L100302, 2023; https://doi.org/10.1103/PhysRevB.108.L100302
This project was supported by the NOMAD Center of Excellence (European Union's Horizon 2020 research and innovation program, Grant Agreement No. 951786) and the ERC Advanced Grant TEC1p (European Research Council, Grant Agreement No. 740233).
First-principles approaches for phonon-limited electronic transport are typically based on many-body perturbation theory [1] and thus rely on the validity of a quasi-particle picture for phonons and electrons. However, both these pictures can become questionable in strongly anharmonic systems [2,3]. We overcome this hurdle by combining ab initio molecular dynamics (aiMD) calculations with the Kubo-Greenwood (KG) formalism [4]. This non-perturbative, stochastic method allows us to account for all orders of anharmonic and vibronic couplings in the calculation of carrier mobilities. We discuss the implementation of this formalism in the ab initio material simulation package FHI-aims [5] and the strategy to extrapolate to the direct current limit [6]. Furthermore, to analyze the impact of strong electron-nuclei interactions on the electronic structure, we also developed an efficient band unfolding method for linear combination of atomic-centered orbital (LCAO) basis sets [7], enabling us to define a non-perturbative, temperature-dependent electronic spectral function. Finally, we demonstrate the capabilities of these methods by calculating and analyzing the temperature-dependent electron mobility of the strongly anharmonic oxide perovskites SrTiO3 and BaTiO3 across a wide range of temperatures [8].
References
[1] S. Poncé, W. Li, S. Reichardt, and F. Giustino, First-principles calculations of charge carrier mobility and conductivity in bulk semiconductors and two-dimensional materials, Rep. Prog. Phys. 83, 036501, 2024; https://doi.org/10.1088/1361-6633/ab6a43
[2] F. Knoop, T.A.R. Purcell, M. Scheffler, and C. Carbogno, Anharmonicity in Thermal Insulators: An Analysis from First Principles, Phys. Rev. Lett. 130, 236301 2023; https://doi.org/10.1103/PhysRevLett.130.236301
[3] M. Zacharias, M. Scheffler, and C. Carbogno, Fully anharmonic nonperturbative theory of vibronically renormalized electronic band structures, Phys. Rev. B 102, 045126, 2020; https://doi.org/10.1103/PhysRevB.102.045126
[4] B. Holst, M. French, and R. Redmer, Electronic transport coefficients from ab initio simulations and application to dense liquid hydrogen, Phys. Rev. B 83, 235120, 2011; https://doi.org/10.1103/PhysRevB.83.235120
[5] V. Blum, R. Gehrke, F. Hanke, P. Havu, V. Havu, X. Ren, K. Reuter, and M. Scheffler, Ab initio molecular simulations with numeric atom-centered orbitals, Comp. Phys. Comm. 180, 2175-2196, 2009; https://doi.org/10.1016/j.cpc.2009.06.022
[6] M. French, G. Röpke, M. Schörner, M. Bethkenhagen, M. P. Desjarlais, and R. Redmer, Electronic transport coefficients from density functional theory across the plasma plane, Phys. Rev. E 105, 065204, 2022; https://doi.org/10.1103/PhysRevE.105.065204
[7] J. Quan, N. Rybin, M. Scheffler, and C. Carbogno, in preperation.
[8] J. Quan, C. Carbogno, and M. Scheffler, Carrier Mobility of Strongly Anharmonic Materials from First Principles, arXiv 2408.12908, 2024; https://doi.org/10.48550/arXiv.2408.12908
The physical response of condensed matter is determined by the microscopic interactions between internal degrees of freedom, such as charge, spin, or lattice. Ultrafast methods provide insights into these interactions by studying a materials ultrafast nonequilibrium response, which helps us to understand how macroscopic material properties arise from oftentimes complex microscopic interactions. However, established ultrafast spectroscopy approaches spatially average over large sample areas, although materials are usually inhomogeneous on nanometer or even Angstrom length scales. Gaining comprehensive insight into the local ultrafast response of surfaces, molecules, and quantum materials, and correlating this with the spatially averaged global response, therefore requires new experimental tools that combine ultrafast temporal with ultrahigh spatial resolution.
Ultrafast scanning tunneling microscopy (USTM) has become a powerful and versatile tool for imaging ultrafast dynamics at surfaces with atomic-scale spatial resolution. One of the most promising approaches that has emerged in the past decade is THz-lightwave-driven STM (THz-STM). THz-STM utilizes broadband single-cycle THz pulses as a quasi-static bias voltage in STM. It combines the free-space illumination of optical radiation with the previous ideas of nonlinear rectification in tunnel junctions driven by AC electric fields. Here we show the conceptual principle of THz-STM and explain the technical aspects of the THz-STM setup we build in the last years at FHI. We highlight the system's flexibility and capabilities for future ultrafast microscopy and spectroscopy at atomic scales.
Charged aqueous interfaces are the subject of extensive investigation due to their prevalence in both natural and industrial processes, with importance ranging across the biological, environmental, and chemical sciences. At such phase boundaries, the excess surface charge generates an electric field that penetrates through the electrolyte, perturbing the ion distributions and electric potential, as well as generating a rotational torque on the water dipoles and thus altering the distribution of molecular orientations. These alterations can persist up to several 10s of nanometers (i.e., 100s of molecular layers) and have important consequences for the role of the electrolyte in interfacial dynamics and reactivity. An important model that is extensively used to describe such charged interfaces is the Gouy-Chapman (GC) model, along with its Stern adaptation (Gouy-Chapman-Stern, GCS model), which describes either a single dielectric regime (GC), or two distinct layers (GCS): the compact layer in close proximity to the charged interface, and the diffuse layer beneath. While this model has successfully been employed to describe the field-driven anisotropy of the electric potential and interfacial ion excess, it nevertheless treats the constituent water molecules as a continuous medium. Clearly, therefore, many questions about the participation of the most abundant component of the electrolyte remain unanswered.
In this work, we directly probe these open questions using our recently developed depth-resolved vibrational spectroscopy technique that combines the phase-resolved sum- and difference-frequency generation (SFG and DFG) responses from an interface. By studying the depth-dependency of the water response, we elucidate the connection between the anisotropy in electric potential and any induced molecular reorientation. This firstly allows us to experimentally demonstrate that the induced water response features at least two distinct layers rather than the single-layer description of the GC model. Beyond this, using the two-layer GCS description, we then extract the spectra of the compact and diffuse layers, showing that they report distinctly different molecular structures and environments. Finally, through concentration-dependent studies, we extract the decay length of the induced orientational anisotropy of the water molecules within the diffuse layer and compare this to the theoretically predicted Debye screening length, finding significant discrepancies and thus highlighting the insufficiencies of the GCS model in describing the response of molecular water.
Liquid-vapor interfaces play a crucial role in the atmosphere. Their composition can alter compared to the bulk. We investigate complex acid-base equilibria, including tautomers and short-lived species, at the liquid-vapor interface with photoelectron spectroscopy.
The angular momentum of lattice vibrations — phonon angular momentum — is a largely unexplored degree of freedom in solid-state systems, playing a key role in the understanding of ultrafast demagnetization processes and offering new pathways for phonon-driven ultrafast material control, particularly relevant to spintronics and valleytronics. While this research area is rapidly growing, phonon angular momentum states have been previously inferred from secondary effects, such as transient magnetic fields or photoluminescence.
In this work [1], we experimentally demonstrate the possibility to directly prepare and monitor coherent states of phonon angular momentum. For this, we utilize intense, helicity-tailored, single-cycle THz pulses [2] to coherently drive a doubly-degenerate Raman-active phonon mode. We consecutively track its vectorial trajectory via the THz-induced Kerr effect. We find that the circularly polarized THz pulse drives a circular Eg phonon state with a precisely controlled helicity via a nonlinear sum-frequency excitation [3]. The identified excitation process corresponds to coherent phonon-phonon angular momentum transfer, where an IR-active circular Eu phonon anharmonically drives a Raman-active Eg phonon of opposite helicity. We confirm the conservation of pseudo phonon angular momentum by the observed helicity reversal between the Eu and Eg helicity states, which is based on the discrete rotational symmetry of the lattice. This fundamental finding provides a new handle for investigating and controlling angular momentum channels in solid-state systems.
References
[1] O. Minakova et al., In preparation (2024).
[2] M. Frenzel et al., Optica 11, 3, p. 362-370 (2024).
[3] D. M. Juraschek and S. F. Maehrlein, PRB 97, 17, p. 174302 (2018)
The geometry of atoms in molecules and materials governs their properties. The controlled manipulation of their arrangement on the atomic scale is however limited with current technology.
This work explores non-contact atomic force microscopy (NC-AFM) as one possible tool to achieve this goal. Individual molecules of nonahelicene ([9]H) and coronene (Cor) are studied on a Ag(110)-surface and compared with each other. It is shown that the observed response of both molecules can be reproduced by empirical models, although an elastic contribution is required to rationalize the difference of the spring-like [9]H to the flat and stiff Cor. Systematic measurements of the [9]H response resulted in dense 3D datasets. Their analysis yields maps which not only reveal the elastic behaviour of [9]H with submolecular resolution but also the relevant contributing factors originating from the tip-molecule interaction.
In summary, this work paves the way towards the controlled systematic manipulation of single-molecule properties with submolecular accuracy.
Cavity electrodynamics offers a unique avenue for tailoring ground-state material properties, excited-state engineering, and versatile control of quantum matter. Merging these concepts with high-field physics in the terahertz (THz) spectral range opens the door to explore low-energy, field-driven cavity electrodynamics, emerging from fundamental resonances or order parameters. Despite this demand, leveraging the full potential of field-driven material control in cavities is hindered by the lack of direct access to the intra-cavity fields. Here, we demonstrate a new concept of active cavities, consisting of electro-optic Fabry-Pérot resonators, which measure their intra-cavity electric fields on sub-cycle timescales. We thereby demonstrate quantitative retrieval of the cavity modes in amplitude and phase, over a broad THz frequency range. To enable simultaneous intra-cavity sampling alongside excited-state material control, we design a tunable multi-layer cavity, enabling deterministic design of hybrid cavities for polaritonic systems. Our theoretical models reveal the origin of the avoided crossings embedded in the intricate mode dispersion, and will enable fully-switchable polaritonic effects within arbitrary materials hosted by the hybrid cavity. Electro-optic cavities (EOCs) will therefore serve as integrated probes of light-matter interactions across all coupling regimes, laying the foundation for field-resolved intra-cavity quantum electrodynamics.
Tommaso Pincelli, Lawson T. Lloyd, Alessandro de Vita, Tania Mukherjee, Túlio de Castro, Amine Wahada, Roberto Sant$^a$, Srdjan Stavrič$^b$, Silvia Picozzi$^b$, Nathan P. Wilson$^c$, Zdeněk Sofer$^d$, Martin Wolf, Laurenz Rettig, Ralph Ernstorfer
$^a$Politecnico di Milano, Milano, Italy
$^b$Consiglio Nazionale delle Ricerche, Chieti, Italy
$^c$Walter Schottky Institute, Technische Universität München, Garching, Germany
$^d$Chemistry Department, University of Chemistry and Technology Prague, Prague, Czech Republic
We illustrate our results on CrSBr, Fe$_3$GeTe$_2$ (FGT), and CrI$_3$, exploring how spin order influences electronic band structure, exciton dynamics, and ultrafast magnetic processes in presence of strong vertical confinement of the electronic stats. We approach this problem by combining spectroscopic techniques, time-resolved experiments, and theoretical modeling: this provides a uniquely direct and momentum-resolved view of electronic properties and quasiparticle dynamics. We uncover a wealth of material-specific behaviors that underscore the potential of 2D spin-ordered systems for spintronic and optoelectronic applications.
In CrSBr, an air-stable, direct-gap semiconductor, exhibits in-plane FM and interlayer AFM ordering (T$_N$~132 K), which drive anisotropic electronic and optical properties. We identify strongly bound excitons (~700 meV) and track their dynamic with a fluence and wavelength-dependent experimental protocol. We are able to build a rate-equation model that quantitatively describes the entire dataset, pointing towards mechanisms of exciton-exciton annihilation rather than Mott transitions for the decay of the excitonic signal. We identify markers of spin order with promising potential for experiments linking magnetic and excitonic dynamics with wide ranging interest for optoelectronic and spintronic devices.
In FGT, a metallic van der Waals ferromagnet, comparison between trARPES experiments and DFT calculations unveil Stoner-like magnetic excitations and coherent oscillations at 3.69 THz, which we attribute to an A$_{1g}$ optical phonon at the Γ point. These findings reveal the complex spectrum of magnetic excitations in FGT as well as a strong coupling between magnetic and lattice dynamics, linking phonon modes to ultrafast demagnetization.
CrI$_3$, a van der Waals Ising ferromagnet, exhibits significant renormalization of the electronic band structure in the FM state. Resonant photoemission spectroscopy (ResPES) and X-ray absorption spectroscopy (XAS/XMCD) reveal a deviation from ionic character resulting from a hybridization between Cr 3d e$_g$ and I 5p orbitals, with DFT predictions corroborating experimental data. A picture of ligand-mediated FM ordering emerges that can be extended to the entire class of 2D Chromium chalcogenides, a key step in understanding the complex forms of magnetism emerging in reduced dimensionality systems.
Surface phonon polaritons (SPhPs) are quasiparticles resulting from the hybridisation of IR photons with transverse optical phonons. The extreme light confinement, the low losses due to the long phonon lifetimes and the high directionality that can result from low crystal symmetries are among the factors that have made phonon polaritons so attractive to the nanophotonics community in the last decade. In this contribution we propose two methods to characterise SPhPs. First, we show that spectroscopic ellipsometry can be combined with prism coupling to study the effect of SPhP excitation on the polarisation states of the reflected light. In addition, we report spectro-imaging in momentum space, which provides a fast and accurate dispersion mapping of SPhPs. As an outlook, we propose a combination of these two approaches - ellipsometry and momentum-space imaging - for an in-depth study of polariton polarisation states in low-symmetry van der Waals materials.
Delving the rich dynamics of many-body quantum systems represents a profound challenge in both fundamental and applied science. This challenge arises due to the intricate correlation between carriers and nuclei, which entails complex dynamic processes occurring across timescales ranging from attoseconds to picoseconds. These processes include electronic and structural phase transitions, Mott physics, and exciton dynamics. While conventional time-stationary spectroscopic methods offer valuable insights, they often lack the necessary temporal resolution to fully understand these rapid processes, emphasising the need for alternative techniques capable of capturing ultrafast dynamics.
In recent strides, the efficacy of time-resolved pump-probe inquiries employing coherent attosecond soft X-ray core-level spectroscopy has been brought to light. Soft X-ray core-level spectroscopy allows for an element-specific exploration of electronic structures in condensed matter physics. Moreover, it boasts exceptional experimental resolution, achieved by seamlessly integrating attosecond coherent ultra-broadband X-ray probe pulses with optically synchronised, carrier-envelope-phase (CEP) stable pump pulses.
The attosecond soft X-ray approach overcomes the typical time-energy uncertainty of the pump-probe intensity measurement. This way, the spectral resolution is entirely determined by the energy resolution of the spectrograph, and the temporal resolution is determined by the convolution between the attosecond soft X-ray probe pulses and the field amplitude of the CEP-stable pump pulses. This method enables unique time-energy resolutions over an ultra-broadband X-ray continuum. However, the low photon flux of the current soft X-ray sources limits the energy resolution and significantly elongates the measuring times for pump-probe time-resolved studies.
This project introduces a novel design for ultrafast shortwave infrared lasers, developing a high-energy CEP-stable Optical Parametric Chirped Pulse Amplification (OPCPA) system, offering an alternative solution to the typical Ti: Sa pumped TOPAS systems. For this purpose, the Dira 200-1 model from TRUMPF Inc. with thin-disc technology delivering 200 mJ pulses at 1 kHz is employed. The front-end we present here is comprised in a footprint of 1.2 m x 0.5 m, includes pulse compression from 650 fs to 9 fs at a 1 µm wavelength, 2.1 µm CEP-stable pulse generation via IP-DFG, and pulse amplification to microjoule energy levels in an OPCPA stage. With this configuration, our goal is to augment the pulse energy of conventional ultrafast shortwave infrared sources by more than an order of magnitude at 1 kHz while preserving the compactness and stability inherent to typical OPCPA systems. Such enhancement is pivotal in generating coherent soft-X-ray radiation boasting unparalleled photon flux within a tabletop setup and consequently reducing pump-probe acquisition times from day-long endeavours to mere minutes. Moreover, with our soft X-ray beamline design we expect to access the iron L3 edge at 710 eV for the first time employing a coherent tabletop X-ray source.
Aqueous solution-vapor interfaces play a major role in atmospheric processes, for example in the interaction of the oceans or of aqueous aerosols with trace gases [1]. The largest contiguous aqueous-vapor interface is that of the oceans with air, covering more than 70% of the Earth’s surface [2]. Studies have shown that the ocean-air interface is covered by a thin film of amphiphilic compounds, including surfactants [3]. This so-called sea surface microlayer significantly influences many processes with importance to the global ecosystem, such as the exchange of trace gases (e.g., CO2) and heat transfer. Additionally, it contributes to the production of sea spray aerosols (SSAs), which are among the largest natural sources of aerosols globally [4,5].
Our previous studies on the interaction of ions and surfactants at liquid-vapor interfaces [6] demonstrated that surfactants can influence the behavior of dissolved ions like Mg²⁺and SO₄². These effects arise mainly from electrostatic interactions between ions and the charged functional groups of surfactants (e.g., -CNH₃⁺, -COO⁻). The goal of our current research is to achieve a molecular-scale, quantitative understanding of how organic surfactants interact with inorganic ions in ocean water. By investigating the directional photoemission propensities, termed photoelectron angular distributions (PADs), of these interfacial components using liquid-jet X-ray photoelectron spectroscopy (XPS), we aim to gain deeper insights into their depth distribution and infer structure properties. Specifically, measuring PADs provides valuable information with Ångstrom resolution on the distance between the charged functional groups of surfactants and the dissolved ions [7].
In this study, we examine the relative distance of divalent (SO₄²⁻) ions to the interface in the presence of surfactants. As prototypes for the latter, we use sub monolayer coverages of octyl ammonium (-CNH₃⁺, positively charged) and octanoate (-COO⁻, negatively charged). Our findings indicate that the presence of positively and negatively charged surfactants can modify the relative depth of species at the interface, which is crucial for understanding molecular-level heterogeneous chemical reactions in the atmosphere.
1T-TaS$_2$ is a strongly correlated material characterized by a rich phase diagram
hosting a commensurate charge density wave (C-CDW) phase at low temperatures
accompanied by the opening of an insulating gap. These properties arise from the
interaction between electronic and lattice degrees of freedom, making 1T-TaS$_2$ a
prototype material for studying the complex quantum systems.
Time-resolved techniques, particularly pump-probe experiments, have revealed
intricate dynamics within 1T-TaS$_2$. However, the spatial-averaging nature of these
techniques limits the exploration of atomic-scale inhomogenities. For instance, it is
known that a photoinduced hidden state exists in 1T-TaS$_2$ and is characterized by
nanometer-sized domains each hosting a C-CDW. Therefore, an experimental
approach combining angstrom-spatial and femtosecond-temporal resolution is highly
desirable.
Terahertz-induced scanning tunneling microscopy (THz-STM) achieves this by
coupling broadband single-cycle THz-pulses to an STM junction and measuring the
resulting THz-induced rectified current (I$_{\rm{THz}}$). In this work, we demonstrate that THz-
STM can locally probe the amplitude mode (AM) phonon in 1T-TaS$_2$. The AM appears
as a global 3% periodic modulation of I$_{\rm{THz}}$ at a frequency of 2.45 THz. We demonstrate
that this modulation is a consequence of a symmetric gap oscillation induced by the
phonon. Our results mark an important step towards the application of THz-STM to
study the non-equilibrium dynamics of quantum materials at the atomic scale.
Antiferromagnetic (AF) spintronics is a promising route towards more efficient and stable devices, because antiferromagnets are less susceptible to external fields and foster a broad range of magnetic interactions with the potential for higher speeds and energy efficient manipulation. However, their self-cancelling magnetic moment makes the interaction with magnetic order challenging. One way to achieve this is to utilize the magnetic anisotropy (MA) to manipulate the spin arrangement which we demonstrated recently using ultrafast optical excitation [1]. External magnetic fields, as regularly used in ferromagnetic materials, can also have a strong influence on MA, providing an additional control knob on the AF magnetic order. Therefore, understanding the interaction of laser excitation induced transient MA with magnetic fields is of strong interest. To this end, we perform femtosecond time-resolved resonant soft X-ray diffraction (RSXRD) in the prototypical A-type antiferromagnet GdRh2Si2. Consistent with our previous study, we observe an ultrafast rotation of the AF arrangement of Gd 4f spins followed by coherent oscillations of the AF order induced by light-induced changes in the MA potential.
Surprisingly, while the AF order undergoes a spin-flop transition upon increasing magnetic field, the oscillations persist and their frequency increases while the amplitude of reorientation upon photoexcitation changes its direction. To understand our observations, a phenomenological model is built based on the MA potential and Zeeman energy as two competing mechanisms, which reproduces the key features of the observed ultrafast dynamics. Our results demonstrate magnetic field control of the MA potential and pave the way towards deterministic control of spin order using combined electromagnetic and magnetic fields.
Tommaso Pincelli, Lawson T. Lloyd, Amine Wahada, Zoè de Granrut, Alexander Enders, Tania Mukhejee, Túlio de Castro, Alessandro de Vita, Samuel Beaulieu, Maciej Dendzik, Shuo Dong, Holger Oertel, Martin Wolf, Laurenz Rettig, Ralph Ernstorfer.
We present our progress in developing the new paradigm of multidimensional photoemission spectroscopy (MPES) to probe quasiparticle wavefunctions. By combining novel experimental approaches and theoretical modeling, we are able to access excitonic states, track their dynamics and map their spatial extent. Using new forms of photoemission dichroism we can access band topology and unravel subtle aspects of the electronic and excitonic wavefunctions.
We demonstrated the observation of excitonic states in WSe$_2$, a layered semiconductor with strong excitonic binding energy. Time- and angle-resolved photoemission spectroscopy (trARPES) captures the distinct behaviors of excitons and free carriers by using varying excitation energies. Moreover, owing to the momentum resolution of trARPES we can track the scattering of excitons into momentum-indirect dark states, as well as the excitonic distribution in real space. These findings demonstrate the capability of photoemission to probe excitonic states and their transitions in real-time.
Advancements in mapping band topology enable direct investigation of quasiparticle eigenfunctions rather than merely eigenvalues and populations. By exploiting photoemission matrix element effects, we can gain insight into the entanglement of spin, orbital, and valley degrees of freedom in 2D materials with strong spin-orbit coupling and broken inversion symmetry. Employing recipes such as rotating the crystal symmetry axes or the light polarization direction, we can e.g. access the orbital pseudospin in WSe2, or even a complete reconstruction of the wavefunction. Moreover, modulation of the polarization of the pump in a trARPES experiments allows tracking valley polarization in the ensuing ultrafast scattering processes.
To achieve such multidimensional experimental parameter control, the trARPES laboratory has been upgraded to significantly enhance its experimental capabilities. A new polarization control scheme allows continuous control of the angle of linear polarization, while a new circular polarization scheme is planned. The instrument is also being upgraded in order to operate with both short (<40 fs) and long (<190 fs) pulse, for high temporal or energy resolution operation respectively.
Ultrathin ZnO on Ag(111) has emerged as an interesting material platform for the atomic-scale investigation of light-matter interaction in plasmonic tunnel junctions. As ultrathin layers, ZnO forms a two-dimensional hexagonal lattice with a layer-thickness dependent electronic structure. In addition, an interface state (IS) is formed at the ZnO/Ag(111) interface, whose coupling to the ZnO-CBE plays an important role for the light-matter interaction in this system. Here, we use scanning tunneling microscopy (STM) combined with optical laser excitation and local light detection to study light-matter interaction of ZnO/Ag(111) inside a plasmonic nanocavity.
Part I: We use STM-induced luminescence (STML) to study the influence of ultrathin ZnO/Ag(111) on plasmonic light emission from a plasmonic nanocavity. At positive sample bias, the plasmonic luminescence - resulting from the radiative decay of localized surface plasmons (LSP) excited by inelastic tunneling - is spectrally modified by the ZnO layers. In particular, we observe a low-pass filtering effect which is absent at negative bias and depends on the local electronic structure of the ZnO, as confirmed by spatial STML mapping. Our findings demonstrate that the conduction band of ZnO serves as initial state for plasmonic luminescence driven by inelastic electron transport across the ZnO/Ag(111) interface.
Part II: While tip-enhanced Raman spectroscopy (TERS) has been implemented in STM with even sub-molecular resolution, time domain vibrational spectroscopy approaches are still under development. We recently showed that optical pump-probe STM on ultrathin ZnO/Ag(111) allows to realize coherent phonon (CP) spectroscopy with few nanometer spatial resolution. Beyond this proof-of-concept study, the mechanisms for CP excitation and detection and the role of the IS-CBE optical resonance are not fully understood. To gain further insights, we implement two-color pump-probe STM for controlled on- and off-resonant ultrafast optical STM and to selectively excite the IS-CBE resonance on 2ML and 3ML. With a new laser setup, we could realize resonant CP spectroscopy on 2ML ZnO, and show first results which indicate that the CP excitation and detection is also possible under non-resonant conditions.
S. Liu, Sci. Adv. 8, 42, eabq5682 (2022)
Liu et al., Nano Lett. 19, 8, 5725 (2019)
Wiedenhaupt et al., in preparation
Hydrogen-bonded (H-bonded) molecular networks are ubiquitous in nature, appearing in systems such as DNA, proteins, and ice, to name a few. Achieving a comprehensive understanding of chemical reactions within these networks requires spatial resolution at the molecular level, which has been particularly challenging for photochemical studies. In this poster, we present our recent investigation into plasmon-induced localized reactions in two-dimensional H-bonded networks on surfaces using a laser-coupled scanning tunneling microscope (STM). We studied triphenylene-2,6,10-tricarboxylic acid (TTC) molecules, which form honeycomb network structures on Ag(111) at room temperature through hydrogen bonds of carboxylic groups. Our observations revealed that photo-induced deformation of the molecular network was highly localized to a few molecules directly beneath an Ag STM tip under visible laser illumination, with minimal impact on neighboring molecules. In contrast, a PtIr tip which lacks plasmonic activity in the visible range, failed to drive the reaction. This indicates that localized surface plasmon resonances at the STM junction play a critical role in the process. By varying the incident laser wavelength, we further examined the reaction mechanism, identifying it as being mediated by non-thermal electrons (hot electrons) generated by plasmons. This conclusion was corroborated by demonstrating the same reaction could be initiated by non-thermal electrons supplied by the STM tip without laser incidence. Our findings highlight the potential of plasmonic STM tips for precise control over the structure and reactivity of H-bonded assemblies.
Inhomogeneous molecular assemblies at interfaces play a critical role in both natural and industrial systems, with examples ranging from lipid rafts in biological membranes to lab-on-a-chip technologies. Investigating these assemblies at the molecular level, particularly their composition, arrangement, and packing structure, is a subject of great scientific interest. However, achieving such detailed characterization requires a technique that combines molecular recognition, orientational sensitivity, and high spatial resolution, ideally capable of probing monolayer-thick structures. Meeting all these requirements has remained a considerable challenge.
A technique that does satisfy the above requirements is vibrational sum-frequency generation (vSFG) microscopy as it gains access to molecular compositions through their characteristic vibrational resonances, is orientationally sensitive owing to its second-order selection rules, and can achieve sub-micron imaging resolution owing to its frequency upconversion. However, achieving such structural characterization with vSFG microscopy has so far been infeasible owing to the technical challenges of spatially mapping such weak signals, limiting its application to much thicker films or those on metal substrates where any in-plane signals are lost. In this contribution, we introduce a novel vSFG microscope design and imaging system which overcomes these limitations, yielding highly improved signal-to-noise ratios. We then demonstrate its ability to characterize heterogeneous structures by spatially mapping the molecular orientations and analyzing the packing structures within phase-separated monolayers of mixed chiral lipids.
The air-water interface is one of the most prevalent interfaces on Earth and is central to a vast range of natural and industrial processes. The sheer presence of the interface induces significant changes in its properties such as density and dielectric function, as well as the distribution of molecular orientations and interconnectivity of the H-bond network. The widespread importance of these perturbations has led the air-water interface to be the subject of intensive research efforts spanning several decades. Nevertheless, many of its fundamental aspects, such as the thickness of its structural anisotropy, remain contentious, with no direct experimental measurements being available. A key technique in these interfacial investigations is sum-frequency generation (SFG) spectroscopy which gains insight into molecular structure and H-bonding through the characteristic vibrational line-shapes. Nevertheless, SFG only yields spatially integrated responses, meaning no depth information is accessible, and the interpretation of any resulting spectra is highly challenging. For this reason, experimental results are often compared to predictions from molecular dynamics (MD) simulations which yield a depth-dependent view specific structural motifs at the interface. Such comparisons, however, necessitate the agreement between the observed and simulated spectra. While this is generally the case for the O-H stretching vibrations, substantial differences are observed for the H-O-H bending mode. This disagreement raises critical questions over much of our current understanding about the structure of aqueous interfaces.
In this work, we directly address these questions using a novel depth-resolved vibrational spectroscopy which involves the simultaneous measurement of phase-resolved sum- and difference frequency generation (SFG and DFG) signals, enabling precise depth-profiling at the sub-nanometre scale. Firstly, by probing the O-H stretching region, we obtain a direct measure of the decay length of the structural anisotropy, comparing the results to predictions from MD simulations. Secondly, by probing the H-O-H bending region, we show that the observed discrepancy between experiment and simulation arises from a dominant bulk quadrupolar contribution that is not included in simulations. Through further measurements at charged interfaces, the experimental spectrum is broken down into its different components and the interfacial dipolar signal predicted by theory is retrieved, thus consolidating many years of simulations on aqueous interfaces. Analysis of the purely dipolar contribution then gives new insight into the dominant factors governing the molecular structure at the interface.
Metal-halide perovskites (MHPs) emerged as exciting novel semiconductors with outstanding optoelectronic properties for applications in photovoltaics and light emission. More recently, these semiconductors also attract interest as promising candidates for spintronics. In the absence of inversion symmetry, spin-orbit coupling (SOC) leads to the Rashba-Dresselhaus effect, offering outlook for spin current control. Therefore, inversion symmetry breaking in MHPs with strong SOC has crucial implications. However, in structurally complex, low-dimensional hybrid organic-inorganic perovskites, the presence and exact mechanisms of inversion symmetry breaking remain elusive.
Here, employing intense, close to single-cycle, THz fields, we coherently drive and identify lattice dynamics carrying optical signatures of inversion symmetry breaking in Ruddlesden-Popper (PEA)2(MA)n-1PbnI3n+1 perovskites, despite their global centrosymmetric structure. We demonstrate coherent control by THz pulses over distinct phonons, which we tentatively assign to inorganic cage and coupled inorganic-PEA+ vibrations. By developing a general polarization analysis for THz-driven phonons, we pinpoint linear and nonlinear driving mechanisms. From this, we identify simultaneous IR- and Raman-activity of inorganic cage modes below 1.5 THz, suggesting the presence of inversion symmetry breaking. We therefore demonstrate a general experimental method to dial into the coherent dynamics of modes bearing broken inversion symmetry fingerprints, paving the way for simultaneous ultrafast control of optoelectronic and spintronic properties of 2D HOIPs.
We report on tip-enhanced Raman spectroscopy (TERS) of H$_2$ and D$_2$ molecules physisorbed on Ag(111) at cryogenic temperatures (around 10 K). The intense Raman peaks resulting from the rotational and vibrational transitions are observed at sub-nanometer gap distances of the junction formed by a Ag tip and the Ag surface. Our results suggest that TERS based on low-temperature scanning tunneling microscopy (STM) can be applied to weakly bound physisorbed molecules and is capable of studying subtle interactions within plasmonic picocavities.
Phonon polaritons are hybrid light-matter particles in solid-state materials that enable waveguiding of light on length scales much smaller than the photon wavelength. Here, we introduce HfSe$_2$ as a new van der Waals material that supports phonon polaritons in the terahertz (THz) spectral range. We image the propagation of these polaritons with a near-field optical microscope that is attached to a free-electron laser. The phonon polaritons of thin material films enable an extreme confinement of light from 61 μm free-space wavelength to 245 nm. Through a combination of experiments and simulations, we show that the origin of this record-high confinement is an exceptionally large light-matter coupling of HfSe$_2$ and its optical anisotropy.
The Kagome lattice offers a plethora of interesting physics ranging from van-Hove singularities to Dirac points and flat bands. In particular, the Kagome metals family AV3Sb5 (A=K, Rb, Cs) features an unconventional superconducting phase, coexisting with a parent charge density wave (CDW) phase. The origin of the CDW is still under debate. Moreover, this family exhibit a flat band below the Fermi level, which offers a unique platform for a highly correlated electronic subsystem. Carrying the hallmark of strong electron-electron interaction, this can give rise to many-body states as well as exotic topological effects. While many of such interactions are difficult to access in thermal equilibrium, a dynamical study after perturbing the system promises additional insights into the nature of such states, and allows disentangling the different interactions at play. Interestingly, studies of flat band dynamics are scarce, and little is known on their response to ultrafast excitation.
In order to provide insight to these questions, we studied the ultrafast dynamics in CsV3Sb3 using time- and angle-resolved photoemission spectroscopy (trARPES). Starting in its CDW phase, we drive the system out of equilibrium and subsequently monitor the dynamics of the CDW gap around the Fermi level as well as the dynamics of the flat band. We discuss the ultrafast melting of CDW order, concomitant with a shift and a broadening of the flat band, all being modulated by various coherent phonon modes. While one prominent mode around 1.3 THz is clearly resolved in all bands, the flat band shows additional faster oscillations in the range of 4THz.
Femtosecond electron diffraction (FED) allows direct observation of a crystal lattice’s response to laser excitation. It is ideally suited to study the ultrafast energy flow from electrons to phonons as well as other photo-induced changes of the lattice, such as structural phase transitions, coherent phonons, and lattice distortions.
We have employed FED to probe lattice dynamics in 2d transition metal dichalcogenides (TMDCs), a family of layered van der Waals bonded materials. We have focused primarily on semiconducting monolayer systems with strong excitonic effects, (e.g. WSe2 and ReS2 presented herein), as well as heterostructures of the such monolayers (namely of WSe2 and MoSe2). We have also begun studying WTe2, a polar metal.
The results presented include collective structural variations on sub picosecond timescales in monolayer ReS2 and in WTe2, as well as element resolved atomic vibrations in monolayer WSe2. The latter is the first reported example of such a real-space picture on ultrafast timescales, and allows identifying stages within the relaxation of the phonon system. We also present a “stamping” setup for synthesizing monolayer-monolayer heterostructure samples, with the first examples being WSe2-MoSe2 heterostructures. “stamping” one monolayer onto another is done controllably such that the angle between the two can be defined. A particular aspect of the setup is the need to center a third layer for FED measurements: a Platinum pinhole is placed with micron precision, which ensures that the femtosecond electron pulses only probe the desired sample area.
We present an ambient pressure X-ray photoelectron spectroscopy investigation of the adsorption of ammonia on ice over the temperature range of -23 °C to -50 °C. Previous flow tube studies showed significant uptake of ammonia at these temperatures to ice, which was linked to the incorporation of ammonium into the ice crystal lattice. Our present investigation shows a significant uptake of ammonia to the ice interface, with ammonia concentrations exceeding those measured in past studies for the case of bulk snow and ice. We also have indication that some of the ammonia is protonated at the ice surface and thus adsorbed there as ammonium ions. The impact of high ammonia concentrations at the air-ice interface on the surface chemistry of ice clouds is discussed. The present study lays the groundwork for investigating the reaction of adsorbed ammonia with other trace gases in the atmosphere, which is demonstrated on the example of a proof-of-principle experiment of its interaction with acetic acid.
As a promising chemical storage medium for hydrogen, the thermal decomposition of ammonia using non-precious metal catalysts, such as Co, Ni, and Fe, has recently gained significant attention. In our work, we are exploring new avenues for the synthesis of such catalysts. We prepare nanostructured oxide precursors that contain both the active element and potential promoters in their crystal structure and generate metal nanoparticles that exhibit exceptional metal-support/promoter interactions by exsolution during reductive pretreatment or under operation conditions. We have chosen spray-flame synthesis (SFS) and continuous-flow hydrothermal synthesis (CFHS) as preparation methods for the oxide precursors, as nanoparticles can be obtained continuously. With regard to the crystal structures of the precursors, we focus on rock salt structures,1 spinels2 and perovskites. In order to be able to compare the properties of the catalysts with those of the materials prepared and tested by the collaborating partners in the project, cross-laboratory standard operating procedures were developed, the tests were automated and integrated into the data infrastructure of our institute, and the compatibility of the data structure with overarching databases was adapted.3
An important result of the project is that both the precursor structure and the synthesis method have a significant influence on the catalytic activity. In particular, catalysts derived from perovskites and spinels with general composition LaₓSr₁₋ₓCoO₃ and La2O3-CoₓNi₁₋ₓAl₂O₄ show exceptionally high activities at 500°C of 0.0423 mmol·gcat⁻¹·s⁻¹ and 0.0356 mmol·gcat⁻¹·s⁻¹, respectively, which exceed those of benchmark catalysts. Complementary operando and in situ spectroscopic and analytical methods such as X-ray diffraction, vibrational spectroscopy, NAP-XPS and electron microscopy were applied to elucidate particle size effects and the influence of promoters on catalytic properties. The function of basic promoters will be discussed.
1. Alkan, B. et al., From “Single Sites” to Stable Nanoparticles Derived from Spray-Flame Synthesized Solid Solutions of Cobalt in MgO for Ammonia Decomposition, to be submitted (2024).
2. Ngo, A. B. et al., Continuous Flow Hydrothermal Synthesis (CFHS) of Mg-doped Co Spinels under Supercritical Conditions, to be submitted (2024).
3. Moshantaf, A. et al., Advancing Catalysis Research through FAIR Data Principles Implemented in a Local Data Infrastructure – A Case Study of an Automated Test Reactor. DOI: 10.1039/D4CY00693C (2024).
In the pursuit of carbon neutrality, the catalytic hydrogenation of CO₂ using renewable H₂ is considered a promising route for the production of fuels and chemicals, while its integration with carbon capture and utilization processes (ICCU) provides an alternative and innovative approach for the synthesis of value-added products.¹ Even though the formation of C₁ molecules by CO/CO₂ hydrogenation is well established, the synthesis of C₂₊ products might be more attractive due to several advantages, such as a higher volumetric energy density.² In this work, structure-function relationships for the selective formation of C₂₊ products by CO₂ hydrogenation in direct and tandem reactions as well as the implementation of ICCU processes are investigated. Firstly, the chemical composition of spinel precursors AB₂₋ₓRhₓO₄ ((A,B) = Mn, Cu, Fe, Zn) is systematically varied, leading to differences in the activity and the products distribution (C₁ and C₂₊ oxygenates and hydrocarbons), which can be related to Rh⁰ exsolution and partial reduction of further spinel components as analyzed by NAP-XPS. Another approach is the development of nanostructured tandem core-shell catalysts for the stepwise conversion of CO₂-to-methanol and methanol-to-ethylene within the same operational window. Such rational design is guided by detailed operando investigations to elucidate the reaction pathway and reaction intermediates. In addition, CeO₂-supported metal particles are prepared by (co)precipitation as potential systems for ICCU. Attractive adsorption capacities under various CO₂ feeds (pure CO₂ and CO₂ from air), even at elevated temperatures (up to 200 ᵒC) are observed, as well as interesting catalytic performance in CO₂ hydrogenation reaction. The aforementioned findings suggest that the rational design of catalysts is essential in order to develop new hydrogenation routes for the utilization of CO₂.
1. Li, J., He,X., & Hu, R. Integrated Carbon Dioxide Capture and Utilization for the Production of CH₄, Syngas and Olefins over Dual-Function Materials. ChemCatChem 16, (2024).
2. Prieto, G. Carbon Dioxide Hydrogenation into Higher Hydrocarbons and Oxygenates: Thermodynamic and Kinetic Bounds and Progress with Heterogeneous and Homogeneous Catalysis. ChemSusChem. 10, 1056-1070 (2017).
CO2 hydrogenation is a kinetically limited reaction, occurring exclusively at the catalyst surface. To achieve satisfactory catalytic conversions, it is essential to form a reactive interface that facilitates the adsorption of CO2 and its subsequent conversion to the desired products, i.e., methanol. The industrially established system for methanol production via CO2 hydrogenation is Cu/ZnO, however, the nature of the active surface is not fully discovered.1,2 Better understanding of the active phase and its deactivation mechanism might be achieved with the Catlab Laterally Condensed Catalyst approach. As CO2 hydrogenation catalysts, two types of model systems were utilized – Laterally Condensed Catalyst (LCC)3 with composition of 3 nm ZnO – 20 nm Cu – Si wafer (100), and Cu2O/ZnO nanocubes with varying Cu/Zn ratios4. In-situ microscopy and spectroscopy measurements such as in situTEM, ESEM, XPS, and XAS were performed to study the microscopic changes and the electronic structure as well as correlate these with the catalytic performance. During the in-situ performed XPS/XAS measurements, the ZnO overlayer acted as a protective layer for Cu against oxidation in the 3 nm ZnO-20 nm Cu LCC structure where Cu remained metallic throughout the reaction (CO2+H2 mixture), performed up to 220 °C. It was observed, comparing TEM images for both types of studied systems, that powder Cu2O/ZnO nanocubes represent an optimal reference for the LCC catalyst, due to their well-defined structure and morphology. In methanol synthesis conditions, ZnO initially segregates and forms a shell around the Cu2O nanocubes4. This shell delays the onset of reduction which helps preserve the cubic morphology.
CO2 reduction should minimize hydrogen use while co-producing base chemicals. Plasma pyrolysis generates black carbon along with ethylene and acetylene1. Due to operational hazards, concentrated acetylene must be selectively hydrogenated into valuable ethylene. Pd-based catalysts are commonly used for this reaction. However, the surface and subsurface dynamics of active catalysts during acetylene hydrogenation are not yet fully understood. A rational design approach involves using 2-D Laterally Condensed Catalysts (LCCs), which features a nonreactive functional interface between a thin metal layer (3 nm Pd, Pd-Au), a buffer layer (20 nm SiO2), and a reactive interface exposed to the feed gas. These interfaces can be examined using operando spectro-microscopy. Pd-Au LCCs were employed to investigate catalyst multidimensional development during acetylene hydrogenation. DFT calculations show that higher selectivity towards semi-hydrogenation can be achieved by introducing C or Au to the Pd LCC. The heteroelement modifies the adsorption energies of acetylene and ethylene, favoring ethylene desorption before full hydrogenation occurs. Au not only mimics atomic carbon in Pd LCC, affecting Pd local electronic properties but also temporarily introduces mesoscopic geometric effects by influencing the distribution of Pd atoms at the surface as well as the concentration of Pd:C and Pd:H species in the subsurface. In the absence of Au, more carbon diffuses into the Pd (XPS), corroborated by the presence of a carbon interlayer beneath the spent Pd LCC (TEM). Prolonged operation (>20 h) paradoxically showed that the superior initial selectivity of Pd-Au LCC catalysts diminishes over time due to significant Pd segregation to the surface (TEM). On the macroscopic scale, less surface carbon blockage was observed (Raman). Additionally, Au in Pd LCC mitigated catalyst agglomeration. Single-phase bulk catalysts with varying Pd-to-Au atomic ratios were also studied as reference catalysts.
Reference
1 Gladisch, H. Acetylen‐herstellung im elektrischen lichtbogen. Chem. Ing. Tech. 41, 204-208, (1969).
Ammonia is industrially produced by the Haber-Bosch process over a fused, multi-promoted iron-based catalyst [1]. Current knowledge about this reaction has been derived from model systems with less structural complexity impeding a clear-cut structure-activity correlation [1,2]. Here, we explore the real structure and its structural evolution of complex, technical, multi-promoted ammonia synthesis catalysts using scale-bridging electron microscopy, complemented by X-ray diffraction, spectroscopy and near-ambient pressure X-ray photoelectron spectroscopy, respectively [3].
Our detailed analysis shows the presence of additional phases and disentangles different members of the ammonia iron family. Operando experiments unravel their interconnectedness suggesting that each component has to fulfill a distinct role in ensuring highly active and stable ammonia synthesis catalysts. These experiments further highlight that activation is the critical step in which the active catalyst structure is formed and decode the pivotal role of the promoters. The synergism between the different promoters contributes simultaneously to the structural stability, hierarchical architecture, catalytic activity, and poisoning resistance. We discuss that the confluence of these aspects is the key for the superior performance of technical catalyst formulations. The study demonstrates that catalysis science can only proceed if we openly explore the full complexity of catalytic systems [3].
Solid oxide cells (SOCs) are highly efficient electrochemical devices that convert electrical energy into chemical fuels, offering a versatile solution for energy storage to mitigate the intermittency of renewable energy sources in modern power systems. Operating at elevated temperatures, typically above 600 °C, SOCs benefit from favorable thermodynamic and kinetic efficiencies. However, these high-temperature conditions often induce structural transformations, which can either enhance$^1$ or impair$^2$ the catalytic performance. In this work, we investigate the thermal stability and atomic-scale structure of a model system consisting of lanthanum-strontium manganite (LSM) thin films epitaxially grown on yttria-stabilized zirconia (YSZ) substrates, synthesized via pulsed laser deposition.
Utilizing a comprehensive multi-modal approach that combines advanced electron microscopy, X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and first-principles calculations, we observe that the LSM/YSZ interface, known for facilitating rapid oxide ion diffusion, is characterized by significant distortions in the oxygen and cation lattices. Additionally, the interface exhibits a localized accumulation of oxygen vacancies, which are closely associated with a reduced Mn²⁺ oxidation state to maintain charge balance.
Furthermore, we conducted operando X-ray spectroscopy analysis on a symmetrical SOC cell (LSM/YSZ/LSM) under high-temperature electrochemical conditions in an oxygen atmosphere. The O K-edge XAS spectra reveal distinct changes in O 2p—Mn 3d hybridization under varying oxidizing and reducing potentials. Simultaneously, in-depth operando XPS measurements of La, Sr, and Mn core levels located at the LSM surface, bulk, and interface with YSZ provide insights into elemental redistribution and segregation processes occurring within the film and at the interface.
These results offer an enhanced atomistic understanding of the structure-function relationships in SOCs, providing a foundation for the rational design of optimized interfaces to improve the efficiency and longevity of these systems.
In recent years, nickel-oxyhydroxide has come into focus as a low-cost and efficient material in the Oxygen Evolution Reaction (OER) via electrochemical water splitting. It is a prototypical active and stable catalyst for alkaline OER, however, its precise reaction mechanism is not yet fully understood. We coupled pulse voltammetry, operando X-ray absorption spectroscopy (XAS) and density functional theory (DFT) simulations to characterize the nature of the active surface and elucidate the OER mechanism on nominally Fe-free NiOOH. Electrochemical and XAS data show the Ni(OH)2 transformation to NiOOH in the known cyclic voltammetry peak before the OER onset and the material stays NiOOH during OER. Nevertheless, oxidative charge accumulates with more and more anodic bias and this charge correlates with the logarithm of OER current. As opposed to the case of IrOx, surface oxyl formation on NiOOH is thermodynamically prohibitive. Instead, surface water deprotonates to surface hydroxyl in a broad potential window relevant to OER, and its coverage increases with increasing anodic bias. Although hydroperoxo and especially superoxo are thermodynamically favoured, their formation is kinetically inhibited, in line with XAS data indicating their coverages being below detection limit. This suggests that the rate determining step is likely O-O bond formation. We find that O-O bond formation proceeds through the involvement of 3 hydroxyl sites; one site participates in O-O bond formation and the other two sites are hydrogen acceptors. Moreover, as the hydroxyl coverage increases, the barrier of this O-O bond formation step decreases linearly via a Brønsted relationship. A simple kinetic model based on this relationship provides reaction rate predictions that are in good agreement with the electrochemical results.
Hence, in strong analogy to Ir-based materials under acidic conditions, we found the potential dependent activity of NiOOH under alkaline conditions is driven by oxidative charge accumulation, rather than direct action of the electrochemical bias on the reaction coordinate. In the case of NiOOH, however, adsorbed hydroxyl is active in water nucleophilic attack.
Cobalt-based oxides are excellent catalysts for oxidation reactions, both in thermal catalysis1 and in electrocatalysis.2 Here, we present an overview of combined operando X-ray spectroscopic and operando transmission electron microscopic studies of cobalt spinel oxides (Co3O4) and perovksites (LaCoxFe1-xO3) in low-temperature oxidation reactions, focusing on the evolution of the electronic and geometric surface structures under reaction conditions and the occurrence of common motifs between all systems.
ILIAS (identical location imaging and spectroscopy) in combination with a quasi in situ approach allows the effects of temperature and gas phase on the catalyst surface to be disentangled from those of the electron beam. As a proof of concept, the structural evolution of a Co3O4 spinel catalyst was investigated, tracking the oxidation state changes across the surface during different heat treatments, which can be correlated with the catalytic activity by CO titration.
For Co3O4, a complex network of solid-state diffusional processes controlling the activity of the selective oxidation of 2-propanol could be unraveled. These include exsolution of reduced CoOx, vacancy formation and collapse, as well as recrystallization and give rise to a unique splitting of the activity into a low- and a high-temperature activity regime,3 possibly with a frustrated state at the transition between them.4 On LaCoxFe1-xO3, distinct differences between dry and wet feeds were observed: the presence of water not only inhibits the reduction of cobalt to Co2+ and carbonaceous deposits, but also increases the number of exsolved nanoparticles at the surface and leads to higher surface hydroxide coverages.
Furthermore, the evolution of Co3O4 during the calcination process between 300 and 600 °C was analyzed, giving rise to short-range atomic migration, micropore formation, and local structural instability at lower temperatures, as well as larger, more defined pores, smoother surfaces and more stable structures at higher temperatures.
In the oxygen evolution reaction on sputter-deposited cobalt thin films, the formation of electrophilic oxygen species was investigated, which were found to correlate with the oxidation waves in the voltammogram – their identity was confirmed using DFT.
Many of these systems are characterized by similar structural features, such as exsolution of nanoparticles, surface roughening or void formation, suggesting the existence of similar processes despite the different chemical potentials.
1. Najafishirtari, S. et al. A Perspective on Heterogeneous Catalysts for the Selective Oxidation of Alcohols. Chem. Eur. J. 27, 16809 (2021).
2. Song, F. et al. Transition Metal Oxides as Electrocatalysts for the Oxygen Evolution Reaction in Alkaline Solutions: An Application-Inspired Renaissance. J. Am. Chem. Soc. 140, 7748 (2018).
3. Götsch, T. et al. Local Solid-State Processes Adjust the Selectivity in Catalytic Oxidation Reactions. Nat Catal. in Revision.
4. Schlögl, R. Heterogeneous Catalysis. Angew. Chem. Int. Ed. 54, 3465 (2015).
CO₂ electrocatalytic reduction reaction allows to convert environmentally harmful CO₂ into useful chemicals. To this aim, Cu stands out as the only catalyst capable of producing valuable hydrocarbons and alcohols, such as ethylene and ethanol.1-2
The catalyst surface structure plays a key role in determining the selectivity towards certain carbon products; in particular, vicinal surfaces were studied to find connections between active sites and selectivity. Earlier works on electropolished surface suggest that Cu(310), i.e. Cu(S)–[3(100) × (110)], produces the highest amount of ethanol.3-4
Following recent works on UHV-prepared Cu(111) and (100) surface, we proved in this work that metallic Cu(310) single crystal does not reduce CO₂, but rather H₂O to H₂; indeed, we identified that only for oxidized surface CO₂ is converted to carbon products. We measured the Faradaic Efficiencies (FEs) of the UHV-prepared Cu(310) single crystal. It initially produces C products but switches gradually to only hydrogen in ≈1h. After the transition, different stimuli were applied and only an oxidative electrochemical stimulus lead to the formation of carbon products, with a total FE for gaseous products of ≈30%. X-Ray Photoelectron Spectroscopy (XPS), Low Energy Electron Diffraction (LEED), and Scanning Electron Microscopy (SEM) allowed us to investigate the Cu oxidation states, surface long-range order, and surface morphology, respectively.
The electrocatalytic reduction of nitrate (NO3RR) presents a promising approach for decentralized clean ammonia (NH3) production, while simultaneously mitigating environmental pollution caused by toxic and carcinogenic nitrate-laden wastewater.[1] Copper-based materials are attractive NO3RR catalysts due to their affordability and high NH3 selectivity.[2] However, the mismatch in adsorption between N-intermediates and active hydrogen (H) makes pure Cu catalysts achieve high selectivity only at higher overpotentials.[3] In this study, we show that doping Cu with Pd significantly enhances NH3 selectivity at lower overpotentials. Combining different in situ/operando characterization techniques and electrochemical analytical methods, we revealed that Pd doping increases electrochemical active surface area, accelerates the reduction of oxidized Cu to metallic Cu, and provides more accessible H. Furthermore, through the strategy of pulsed electrolysis, ammonia was obtained as the main product over a wide range of applied potential, starting at just 0.25 V vs. RHE (reversible hydrogen electrode). Notably, 10 consecutive catalyst recycling experiments exhibited negligible decay in Faradaic efficiency (>90%) and yield rate of ammonia. Such active and stable performance is attributed to the reversible transition between Cu(II) and Cu(0) during pulsed electrolysis, which facilitates the continuous regeneration of active species. This work not only achieves efficient and stable NH3 synthesis at low overpotentials, but also provides insights into the dynamic features and underlying mechanisms responsible for the improved performance.
Electrocatalytic water splitting is one of the most promising technologies for producing green hydrogen from renewable resources. However, the anodic oxygen evolution reaction (OER) remains the bottleneck of this process due to its sluggish kinetics. In alkaline water electrolysis, nanocrystalline spinel-type Co3O4 is a highly attractive anode material due to its low cost, high activity, and stability under OER conditions. It is well-established that Co-based electrocatalysts form an X-ray amorphous CoOx(OH)y surface layer under OER-relevant potentials, which can be directly linked to catalytic activity [1,2]. Despite this, the electronic and structural effects of metal dopants on the formation of the active state and the structure-activity relationships remain largely unknown.
In this work, we investigate the role of dopants in the near-surface structural and electronic transformations of metal-doped Co3-xMxO4 (M = Mn, V) nanoparticles during active state formation. Using time-resolved operando hard X-ray spectroscopies (XRD, XAS), we track the chemical and structural changes that occur during active state formation under potentiodynamic (pulsed) reaction conditions. Complementary, the oxygen evolving state is probed by soft X-ray in situ photoemission (XPS) and electron-yield X-ray absorption spectroscopy (EY-XAS).
Our findings reveal distinct structural processes arising from pseudocapacitive redox chemistry in the pre-OER region, which influence active state formation and, ultimately, catalytic turnover and stability. By integrating time-resolved structural and chemical insights, we elucidate the kinetics of processes that lead to the formation of an oxygen-evolving catalyst under operational conditions. These insights advance our understanding of key structure-activity relationships in oxygen-evolving catalysis on Co-based electrocatalysts.
Literature:
[1] Bergmann, A., Martinez-Moreno, E., Teschner, D. et al. Reversible amorphization and the catalytically active state of crystalline Co3O4 during oxygen evolution. Nat Commun 6, 8625 (2015).
[2] Tim Wiegmann, Olaf M. Magnussen et al. Operando Identification of the Reversible Skin Layer on Co3O4 as a Three-Dimensional Reaction Zone for Oxygen Evolution. ACS Catalysis 12 (6), 3256-3268 (2022).
This study explores the epitaxial growth of cobalt oxide (Co₃O₄) thin films on platinum (Pt(100)) single crystals, focusing on the interfacial phenomena that drive the Oxygen Evolution Reaction (OER) in electrochemical water-splitting.1 By systematically varying film thickness, we uncover the delicate interplay between interfacial charge transfer and surface polarization, both critical for catalytic performance.2 The growth process reliably produces structurally stable films across a range of thicknesses, confirmed through Scanning Tunneling Microscopy (STM) and Low Energy Electron Diffraction (LEED).
A Schottky barrier forms at the Pt/Co₃O₄ interface, enabling directional charge transfer facilitated by band bending, as evidenced by X-ray and Ultraviolet Photoelectron Spectroscopy (XPS/UPS). Thinner films demonstrate enhanced charge transfer kinetics and dynamic surface polarization, which are essential for modulating surface states that interact with oxygen intermediates during the OER.3 These reactive surface states, optimally distributed in thinner films, reduce the energy barrier for oxygen adsorption and activation, thereby improving catalytic efficiency. Conversely, thicker films exhibit higher capacitance due to bulk volumetric contributions, highlighting the trade-off between surface reactivity and bulk effects on performance.
Electrochemical analyses, including Mott-Schottky analysis and capacitance-voltage spectroscopy (CVS), reveal dynamic charge behavior at the interface, with distinct regions of accumulation, depletion, and inversion. Key capacitance peaks at 0.5 V (Co²⁺/Co³⁺ transitions) and 0.65 V (subsequent redox transitions), resolved for the first time as distinct events, are associated to surface states from the fuzzy nature of the skin layer formation. These states enhance oxygen adsorption, lower reaction barriers, and improve catalytic efficiency, suggesting a critical film thickness where charge neutralization optimizes electronic structure and reaction kinetics under operational conditions.4 This study establishes a relationship between controlled film thickness, interfacial charge dynamics, and catalytic performance, bridging fundamental surface science principles with practical electrocatalytic applications. The findings provide a pathway for designing high-performance Co₃O₄-based electrodes to enhance water-splitting efficiency and advance sustainable energy technologies.
Various electrocatalytic reactions are influenced by cationic species in the electrolyte, although their exact role is often debated.1 Resolving the electrochemical interface is thus essential for comprehending the interaction of electrolyte species with electrode surfaces and designing improved catalytic systems. Surface X-ray Diffraction (SXRD) has been shown to be a powerful technique for probing the local ordered atomic scale structure of electrochemical interfaces.2 Here, we show how new SXRD methods can be used to investigate how pivotal variables as pH, potential, and cation identity, affect cation-surface interactions. With a new glass-free, gas and liquid tight SXRD cell design, we maintain a more stable sample and background during specular crystal truncation rod measurements (CTR). Through time-resolved potential step measurements we show how the electrolyte composition influences the cation dynamics at the surface and the diffusion layer. Finally, with novel potentiodynamic specular scans in different electrolytes, we obtain very accurate insights on the structure of the cation layer across a wider potential range than what has been measured so far.
The hydrogen evolution reaction (HER) is the most prominent electrocatalytic reaction. It is needed to generate green hydrogen from water and simultaneously serves as the test reaction for general electrocatalyst function in aqueous media. Despite this, it remains poorly understood. Attempts have been made to describe HER kinetics according to Brønsted-Evans-Polanyi relationships for which the hydrogen binding energy with the surface is the primary activity descriptor (1). However, this approach has been increasingly challenged (2,3). It has been pointed out that catalyst surfaces on the descending volcano branch (such as for W, Mo, Ti) are covered with amorphous oxides under operation, preventing reliable calculations3. Furthermore, even for the ascending branch, barriers of ion (de)solvation are neglected.
Here, we present our extensive study on the temperature dependent HER kinetics across a large set of nanoparticle catalysts in acidic and alkaline membrane electrolyzer environments4. When we extract the bias dependent activation entropy and enthalpy, we discover distinct kinetic fingerprints between the iron triad (Fe, Ni, Co), platinum-group metals (PGMs; Pt, Rh, Ir, Pd) and coinage metals (Au, Co, Ag). Strikingly, except for the PGMs, none of the other groups displays simple Butler-Volmer type behavior - not even in acid. Furthermore, we highlight the importance of bias dependent activation entropy in the pre-exponential factor. Consistent with our recent results (5,6), we separate the entropic changes into changes in the interfacial solvent that can differ between H+ and OH solvation, and surface configurational entropy changes that show how a bias dependent H coverage impacts HER kinetics at higher current densities.
Our results are key to update electrocatalyst research and education and for developing novel electrocatalyst design strategies. Importantly, besides developing faster HER catalysts in alkaline conditions from earth-abundant metals, understanding the slow HER kinetics of the coinage metals, is a first step toward understanding the competing CO2 reduction to C2+ products on the same catalyst surfaces.
References
The oxygen reduction reaction (ORR) is a key reaction in fuel cells and Li-air batteries. Despite this, the fundamental reaction mechanism is still not understood. Frequently, kinetics are described with Butler-Volmer type theory, assuming that the electric bias is completely translated in reducing the activation enthalpy (activation energy). However, at solid-electrolyte interfaces, the bias can also change the activation entropy in the pre-exponential factor, either due to entropic changes on the surface or in the solvent.
Separating surface coverage and interfacial electrolyte effects has been a long-standing challenge in electrocatalysis. For example, Markovic and co-workers showed that the pre-exponential factor can be bias dependent, which was related to bias dependent coverage of competing OHad for the ORR in alkaline conditions (1). On the other hand, Feliu and co-workers reported on the influence of interfacial water on the stabilization of reaction intermediates (2), such as the proposed *OOH- in acid (3).
Here, we present our results on the impact of the bias and pressure on the transition state of the ORR on platin group metal nanoparticles in Nafion-based membrane electrode assemblies (4). We demonstrate that, both, surface coverage and electrolyte effects are key for ORR kinetics. At low bias, we observe a compensation effect between an increasing activation energy and pre-exponential factor with bias, which we interpret as fingerprints of the interfacial solvation kinetics that are impacted by the formation of a charged intermediate at the surface and entropic changes in the interfacial water, consistent with our recent results (5). At higher bias, we observe Butler-Volmer type behavior, although the coverage of reaction intermediates impacts the configurational entropy, and, thus, preexponential factor, too. Finally, by varying the O2 pressure, we probe the activation volume and how it is changing depending on the bias and rate. Taken together, our results substantially advance our understanding on the ORR and, more broadly, the impact of the bias in electrocatalysis. In particular, they highlight the pressing need to go beyond current simplified kinetic models to capture the true nature of complex transition states in electrocatalysis.
Electrocatalysts exist within a complex liquid reaction environment and experience (electro)chemical driving force that can change their structural and composition from the as-synthesized state. It is, therefore, crucial that we have insight into the working state of the catalyst if we are to reliably relate the catalyst morphologies with this associated catalytic performance. Even though this need to provide insight to inform the design of catalysts for various green energy applications has driven the widespread development and adoption of operando techniques, there remains a crucial gap because the current methods are either broad beam spectroscopy techniques that are mostly sensitive to the ensemble chemical signatures of the catalytic system or microscopic techniques that can provide high spatial resolution imaging of the catalyst morphology but limited information of the catalyst’s chemical state.
In this presentation, we will show how a synergistic combination of transmission electron microscopy (TEM) and transmission X-ray microscopy (TXM) using the same operando microscopy platforms allow us to interrogate both restructuring and chemical composition of identical catalysts under reaction conditions. The first example involves looking at the phase stability of Cu2O cubes under nitrate reduction (NO3RR) conditions. Our TEM results reveal that unlike the rapid restructuring previously reported for carbon dioxide electro-reduction1, the Cu2O cubes were able to maintain their structural integrity during NO3RR at moderately reductive potentials together with a slow formation of redeposited particles. TXM measurements performed with the same electrochemical cell holder at the U41-TXM beamline indicate that the cubes also do not experience rapid reduction, and the emergence of the metallic phase arises from the re-deposition and not reduction. This phase co-existence is further used to rationalize the active phase for ammonia formation2. In the next example, we applied the same approach to the study of oxidation state changes in the Co(OH)2 nanosheets under oxygen evolution (OER) conditions in the Fe impurity containing electrolyte. It is known that the presence of Fe drastically improves the OER activity of Co(OH)2 pre-catalysts but the exact mechanism remains under debate. Here, the TEM results show that the presence of Fe suppresses the degradation of the nanosheets during reaction while scanning TXM measurements at MYSTIIC beamline is used to reveal the oxidation states of the transformed pre-catalysts and the secondary particles formed3.
References:
1. Grosse, P. et al. Dynamic Transformation of Cubic Copper Catalysts during CO2 Electroreduction and its Impact on Catalytic Selectivity. Nat. Commun. 12, 6736 (2021).
2. Yoon, A. et al. Revealing Catalyst Restructuring and Composition During Nitrate Electroreduction through Correlated Operando Microscopy and Spectroscopy. Nat. Mater. Accepted (2024).
3. Yang, F. et al. Fe-Induced Stabilization of Co(OH)₂ Nanosheets: Unveiling Structural and Oxidation State Dynamics in Oxygen Evolution Reaction via Operando Microscopy and Spectroscopy. In Preparation (2024).
One of the most important aspects of rational catalyst design is controlling the environment of active sites. In electrocatalysis, the use of organic ligands has emerged as a promising strategy to tune the activity and selectivity of the catalyst. In particular, Arduengo-type N-heterocyclic carbenes (NHCs) and the closely related N-heterocyclic olefins (NHO) have emerged as promising ligands able to strongly bind to metal atoms, in many cases with higher stability than other ligands, such as thiols. Furthermore, NHCs have been successfully used in CO2RR to increase the selectivity of the catalyst towards desired products [1].
Understanding how these organic ligands interact with the catalyst is of paramount importance to further improve catalytic functionalities and stability. To shed light on this question, we combined different surface sensitive techniques to study the adsorption configuration, molecular arrangement and charge transfer of model NHCs and NHOs on Cu(111) [2,3]. Our results highlight the importance of the anchor and N-substituent groups to create tailor-made organic layers and to tune the surface properties. In addition, with the activation of CO2 molecules for CO2RR in mind [4], novel NHC-based assemblies containing nitrile groups were investigated. We found that the thermal stability of nitrile-containing NHCs on Cu(111) is higher than on Ag(111), which can be explained by the formation of molecular networks on Cu(111).
Carbon capture and reutilization is a necessity for transitioning towards a sustainable society. Electrocatalytic carbon dioxide reduction reactions (CO2RR) to hydrocarbons at copper cathodes is one promising route, where the selectivity and activity are dictated by a complex interplay of copper oxidation state, structure, and oxygen content at the (sub)surface.[1] Stabilization of these catalytically active species and tuning of the surface microenvironment is critical for catalyst longevity, selectivity, and to improve mechanistic insights. For instance, copper nanoparticles modified with silica thin films have shown increased ethylene production attributed to suppression of hydrogen evolution and stabilization of key CO2RR intermediates.[2] This work demonstrates the use of a crystalline two-dimensional silica network grown under ultra-high vacuum (UHV) conditions to encapsulate pre-oxidized Cu(111) single crystal surfaces, a proposed catalytically active surface motif for CO2RR, and alter CO2RR reactivity.[3] The silica network stabilizes the surface mesoscopic structure and chemical composition for long-term and repeated CO2RR operations relative to unmodified Cu(111) single crystals based on ex situ analyses.[4] In turn, the passivation and formation of nanoconfined reactive sites by the silica network alters the product distribution relative to pristine Cu(111). Our findings demonstrate the ability to modify operation of a model CO2RR electrocatalyst by a two-dimensional overlayer, where further in situ and in operando measurements will help elucidate the dynamics of the surface during CO2RR.
1. Yu, J., Wang, J., Fan, Z. et al. Recent Progresses in Electrochemical Carbon Dioxide Reduction on Copper-Based Catalysts toward Multicarbon Products. Adv. Fun. Mater. 31, 2102151 (2021).
2. Li, J., Sargent, E.H., Sinton, D. et al. Silica-copper catalyst interfaces enable carbon-carbon coupling towards ethylene electrosynthesis Nat. Commun. 12, 2808 (2021).
3. Navarro, J.J., Tosoni, S., Roldan Cuenya, B. et al. Structure of a Silica Thin Film on Oxidized Cu(111): Conservation of the Honeycomb Lattice and Role of the Interlayer. J. Phys. Chem. C 124, 20942-20949 (2020).
4. Nguyen, K.C., Bruce, J.P., Roldan Cuenya, B., et al. The Influence of Mesoscopic Surface Structure on the Electrocatalytic Selectivity of CO2 Reduction with UHV-Prepared Cu(111) Single Crystals. ACS Energy Lett. 9, 644-652 (2024).
Transition metal-nitrogen-doped carbons (TM-N-C) are promising catalysts for several important electrochemical processes, including CO$_{2}$ electrocatalytic reduction (CO$_{2}$RR) [1]. In these catalysts, nitrogen is incorporated into a carbon matrix, creating binding sites for metal species. The latter are believed to be the active sites for CO$_{2}$RR. Among different TM-N-Cs, Ni-N-C is most studied for the CO$_{2}$RR reduction to CO, exhibiting high selectivity (Faradaic efficiency FE$_{CO}$ >90%) and significant current density. Several studies suggest that Co-N-C catalysts can have comparable performance for CO$_{2}$RR. However, while there is a consensus regarding the Ni-N-C's performance, the selectivity of Co-N-C catalysts varies widely in literature, with FE$_{CO}$ values ranging from 20% to 100% [2]. This discrepancy suggests that dynamic processes in Co-N-C catalysts is more complex than in Ni-N-C, with more than one Co species coexisting during CO$_{2}$RR conditions.
In this study, operando time-resolved XANES data were used to unveil the local structure around Co sites in their as prepared states, but also under realistic working conditions during CO$_{2}$RR. A multi-step approach has been used for the interpretation of the collected XANES data. First, we identified the number of different coexisting Co species, their corresponding kinetic profiles and XANES spectra using unsupervised machine learning methodologies, such as the principal component analysis combined with a multivariate curve resolution technique [3]. In the second step, we deduced the atomistic structures for each of the identified species through a XANES fitting procedure facilitated by a supervised machine learning approach [4]. Afterwards, we validated the predicted structures by comparing the simulated pre-edge regions with experimental results, gaining additional information on the types of ligands appearing and evolving during reaction conditions.
Our results confirm that, similarly to the Ni-N-C system [5], the single Co sites are the active species for the CO$_{2}$RR, but also reveal the dynamic, heterogeneous nature and adaptation of the system to the reaction conditions. In particular, our data suggest that the local environment around Co is affected by the interactions between the Co site and CO adsorbates but also highlight the formation of of partially reduced Co clusters [6]. The presence of these species, which we believe are not active for CO$_{2}$RR but facilitate parasitic HER, could explain the large discrepancies between the CO$_{2}$RR activities for Co-N-C materials reported in the literature.
Overall, these results demonstrate the potential of XAS spectroscopy in combination with advanced data modelling to access an unprecedented level of understanding of a complex multi-component catalytic system, yielding novel insights into the formation of Co-active sites for the CO$_{2}$RR reaction.
References
[1] J. Timoshenko et al. Chem. Review. 2021, 121 (2)
[2] Q. Fan et al. Adv. En. Mat. 2020, 10 (5)
[3] A. Martini et al. Crystals 2020, 10 (8)
[4] A. Martini et al. Comput. Phys. Commun. 2020, 250.
[5] A. Martini et al J. Am. Chem. Soc. 2023, 145 (31)
[6] A. Martini et al. J. Synchr. Rad. 2024, 31(4).
Catalysts evolve dynamically under operating conditions. From atomic-scale restructuring to severe coke formation, the dynamic evolution of the catalyst structure is closely linked to changes in catalytic performance. Operando X-ray absorption spectroscopy (XAS) is a well-suited technique to probe the dynamic structural changes under realistic reaction conditions and to correlate rich information gained on the geometric and electronic structure of catalysts to the observed catalytic properties. Synchrotron radiation (SR) facilities have been widely used for operando XAS experiments. However, access to beamtime at SR facilities is limited by a highly competitive application process, which means that not all groups are guaranteed to secure beamtimes to answer their scientific questions promptly. More importantly, such limited access hinders development of new experimental setups (involving e.g., multiple experimental techniques, and dedicated infrastructure for monitoring catalysts’ performance), and limits possibilities for catalyst screening, considering the risk of wasting precious beamtime without success. Simultaneously, long-term monitoring of catalyst (de-)activation processes is crucial for their practical applications, and for bridging the gap between the lab-based studies and industrial conditions. However, long-term XAS studies are hardly compatible with the conventional SR beamtime access models.
This calls for the development of new methodologies for operando XAS experiments in a more accessible lab-based setting. Thanks to the recent development in X-ray optics and detectors, commercial lab-based XAS spectrometers become now increasingly accessible. However, these devices still must be adapted for operando studies. In this contribution we discuss our recent progress in enabling lab-based operando XAS investigations, on the example of catalysts both for electrochemical and thermally driven processes.
The implementation of single atom catalysts (SACs) critically depends on the stability of single atoms towards sintering. We have recently shown that the catalyst pre-treatment with oxygen plasma improves stability and reactivity of Pt/CeO2 SACs in CO oxidation.1 Here we focused on employing “cold” plasma to SACs for reactions under the reducing (H2 containing) atmosphere. In this work, we investigated the reaction of propane dehydrogenation to propene (PDH) over Pt/Al2O3 SACs, in particular to examine, whether the plasma pre-treatment can improve stability of SACs and maintain high Pt dispersion under reductive conditions or show any other beneficial effect on the catalytic performance. We found that the catalyst, pre-treated with the hydrogen plasma before the conventional calcination/reduction steps, showed considerably increased propane conversion without loss of selectivity, whereas argon plasma caused no such effect. The structural and chemical evolution of the catalysts studied by STEM, XAS, XPS, and DRIFTS showed that the H2 plasma: (i) partially reduces singly dispersed Pt2+ species and thus promotes Pt clustering; (ii) decreases the amount of Cl remaining after use of the hexachloroplatinic acid precursor; and (iii) enhances hydroxylation of the alumina support. Upon subsequent oxidation/reduction at elevated temperatures, the Pt single atoms sinter into nanoparticles of almost the same size (~ 1.5 nm) and surface structure, irrespective of the plasma pre-treatment. We propose that the promotional effect of the H2 plasma lies in specific modification of the alumina surface that in turn alters the metal/support interaction and formation of a more active Pt/Al2O3 interface during the oxidation/reduction steps.
The hydrogenation of CO$_2$ to methanol occurs with high efficiency on Cu/ZnO-based catalysts. However, the nature of the Cu−Zn interaction and especially the role of Zn in Cu/ZnO catalysts are still not fully understood. In the industrial Cu/ZnO/Al$_2$O$_3$ catalyst, Zn was found to migrate onto the Cu surface during the reaction, thus forming a Cu−ZnO interface that is crucial for a high catalytic activity.[1] However, whether a Cu−Zn alloy or a Cu−ZnO structure is formed, the transformation of this interface under working conditions and how this influences the catalytic performance needs further investigation.
Cu$_2$O nanocubes (NCs) have been shown to be a good model system to investigate the interaction of Cu and ZnO.[2-4] In this work, we prepared cubic Cu$_2$O NCs modified with a ZnO shell of various thicknesses, supported on SiO$_2$ or ZrO$_2$. In this way, an intimate contact between Cu and ZnO is created even before the start of the reaction. The evolution of the catalyst’s structure and composition during the CO$_2$ hydrogenation reaction were investigated by means of spectroscopy (XPS, XAS), diffraction (XRD), and microscopy methods (SEM, STEM). [5]
We found that the initial Zn loading affects the structure and oxidation state of Zn, which, in turn, influences the catalytic performance. High Zn loadings result in a stable ZnO shell and lead to an increased methanol production when compared to a Zn-free catalyst. However, a too high loading caused the initially amorphous ZnO layer to crystallize, which inhibits the catalyst’s activity. Low Zn loadings, in contrast, lead to the presence of metallic Zn species (and potential Cu-Zn alloy formation) and show no significant improvement over the bare (Zn-free) Cu NCs regarding their catalytic performance. It therefore appears, that there needs to be a minimum content of Zn to promote the Cu catalyst and that there is an optimum ZnO shell thickness that leads to a disordered ZnO layer on top of Cu, resulting in the highest activity.
References
T. Lunkenbein et al., Formation of a ZnO Overlayer in Industrial Cu/ZnO/Al$_2$O$_3$ Catalysts Induced by Strong Metal–Support Interactions. Angewandte Chemie 127.15 (2015): 4627-4631.
D. Kordus et al., Enhanced Methanol Synthesis from CO$_2$ Hydrogenation Achieved by Tuning the Cu–ZnO Interaction in ZnO/Cu$_2$O Nanocube Catalysts Supported on ZrO$_2$ and SiO$_2$. Journal of the American Chemical Society 146.12 (2024): 8677-8687.
M. Rüscher et al., Tracking heterogeneous structural motifs and the redox behaviour of copper–zinc nanocatalysts for the electrocatalytic CO$_2$ reduction using operando time resolved spectroscopy and machine learning. Catalysis Science & Technology 12.9 (2022): 3028-3043.
A. Herzog et al., Time-resolved operando insights into the tunable selectivity of Cu–Zn nanocubes during pulsed CO$_2$ electroreduction. Energy & Environmental Science 17 (2024): 7081-7096.
D. Kordus et al., Shape-dependent CO$_2$ hydrogenation to methanol over Cu$_2$O nanocubes supported on ZnO. Journal of the American Chemical Society 145.5 (2023): 3016-3030.
Indium oxide (In2O3) has recently received considerable attention in the catalysis community due to its unexpectedly high selectivity in the hydrogenation of CO2 to methanol.1 Metal deposition onto In2O3 substantially promotes the activity, while the selectivity remains close to that of bare In2O3 independent of the metal used.2,3 To get insight into the metal/In2O3 interaction and the role of the metal/oxide interface in the CO2 hydrogenation reaction, we performed Near Ambient Pressure X-ray Photoelectron Spectroscopy (NAP-XPS) study of “inverse” model catalysts prepared by In oxide deposition onto a Ru(0001) substrate. The In2O3(111) film grows on Ru(0001) via the Volmer-Weber growth mode, forming bulk-like In2O3(111) islands from the onset. NAP-XPS measurements of the In2O3(111) films of various film thickness revealed the formation of metallic In species at 200 - 280°C in pure H2 and CO2 + H2 atmosphere if In2O3 partially covers the Ru surface, and not on the dense films. The In(0) formation is assisted by facile H2 dissociation on the Ru surface, and H ad-atoms react at the interface to In2O3. XPS results obtained for the reference system prepared by direct In deposition onto the partially covered films show that metallic In formed in the H2 atmosphere remains at the In2O3/Ru interface and do not migrate onto Ru to form a surface alloy.
Copper (Cu) is a leading catalyst for CO$_2$ electroreduction (CO2RR) to multi-carbon products, though its structure sensitivity and stability remain debated. This study reveals that CO2RR does not occur on perfect Cu(111) and Cu(100) surfaces but rather on defect sites, such as steps and kinks [1,2]. Under reaction conditions, these planar surfaces restructure into more active stepped surfaces [2]. Atomic-scale simulations and experiments demonstrate that CO coverage on pristine surfaces is too low to support efficient CO2RR. In contrast, steps and kink sites enhance CO$_2$ activation and CO coverage, significantly boosting catalytic activity. However, the square motifs near step edges, rather than the undercoordinated atomic sites present at step edges and kink sites themselves, emerge as the primary active sites for C-C coupling [3].
Electrochemical experiments combined with scanning tunneling microscopy measurements show that pristine Cu surfaces mainly produce hydrogen, while stepped surfaces yield hydrocarbons. Surface restructuring under CO2RR reaction conditions, driven by CO adsorption, increases step density and promotes the formation of stepped surfaces with enhanced catalytic performance. These findings emphasize the crucial role of surface defects in CO2RR and the self-activation of Cu surfaces under reaction conditions.
The influence of step density and step orientation on the product distribution in electrochemical CO2RR for UHV- prepared Cu(111) and Cu(100) will be presented. Whereas the defects and step-rich motifs enable CO$_2$ conversion into hydrocarbons, the pre-catalyst’s facet determines the exact product distribution. Hence, this work offers new insights into catalyst design. Future work should investigate these active site structures and develop methods to steer the operando surface restructuring for improved CO2RR efficiency.
CuNi nanoparticles have been successfully employed as catalysts in many chemical reactions. Depending on reaction conditions changes in their surface composition are observed, due to the adsorption of molecules. Here, we studied the Ni/Cu(100) single crystal surface as a model system for CO2 hydrogenation to explore the segregation trends under different reaction atmospheres. Exposure to an oxidizing atmosphere prior to hydrogenation results in the formation of NiO islands. LEEM, LEED, µNEXAFS, XPEEM and depth profile XPS results indicate that the NiO islands are encapsulated by a thin Cu shell (ca. 1.5-4 nm thick). Two reaction environments are explored, namely: CO2+H2 and CO2+CO+H2. In both cases the NiO islands progressively reduce to Ni. However, while in the first case Ni remains encapsulated, a thinning of the Cu shell is detected when CO was added. This suggests a clear effect on the stabilization energy when different intermediates are formed. DFT simulations found two families of intermediaries showing distinctive segregation trends: i) C-bound species that stabilize Ni on the surface and ii) O-bound species that stabilize Cu on the surface, thus identifying the nature of the metal-adsorbate bond as the driving force for segregation. A quite different behaviour is observed with the Ni/Cu(100) system, where no encapsulation is observed.
The dynamicity of the reduction process of the NiO islands could be corroborated by measurements with in situ conditions in a newly installed and state-of-the-art NAP XPEEM/LEEM instrument acquired in the framework of the CatLab project. The microscope, currently installed in the experimental hall of BESSY-II in Adlershof, offers a unique opportunity to observe surface phenomena in pressures up to 0.1 mbar and temperatures up to 800°C. A time dependence analysis of the reduction of NiO islands under hydrogenation conditions allows us to identify potential mechanisms for the phase redistribution.
Methane dry reforming (MDR, CH₄ + CO₂ → 2CO + 2H₂) is a promising pathway to produce syngas while reducing the net emission of two of the most harmful greenhouse gases. The industrially relevant catalyst, Ni, suffers from major drawbacks, such as carbon deposition and sintering at high reaction temperatures1.
One alternative to address these issues is the use of bimetallic catalysts2. The incorporation of copper into nickel catalysts can significantly enhance their performance3. While metallic Ni is chiefly responsible for dissociating methane—a rate-determining step—copper aids in CO₂ activation, reduces coking, and augments the reducibility of NiO, leading to more active sites. To elucidate the role of copper in this catalytic reaction, our study employed Ni nanoparticles (NPs) and NiCu NPs supported on SiO₂@Si(100). These NPs underwent a three-step process at near-ambient pressures: O₂ annealing for cleaning, H₂ annealing for reduction, and three MDR reactions at increasing temperatures (550°C, 650°C, 750°C). We used X-ray photoemission electron microscopy (XPEEM), low energy electron microscopy (LEEM), X-ray photoelectron spectroscopy (XPS), and near edge X-ray absorption fine structure spectroscopy (NEXAFS) to investigate our samples. The quasi-in-situ measurements unraveled the chemical composition, oxidation state (NEXAFS, XPEEM, XPS), and morphology (LEEM) of the NPs. Findings from our H₂ annealing experiments indicated that the activation energy for NiO NPs reduction is 1.2 ± 0.1 eV, while copper addition lowers the onset temperature for this reaction. Moreover, the presence of copper modified the oxidation state of nickel under MDR conditions. These insights pave the way for a deeper understanding of the role of copper in this bimetallic catalyst system.
The structure of interfacial hydration layers plays a crucial role in energy and chemical conversion processes, impacting the kinetics of electrocatalytic reactions such as CO2 electroreduction (CO2RR) and hydrogen evolution (HER). We reveal the intricate interplay between carbon and proton sources within the microenvironment of bicarbonate electrolytes and highlight the inherent impact of the local interfacial water structure on the competing mechanisms of CO2RR and HER on gold electrocatalysts, by combining in situ attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) with online differential electrochemical mass spectrometry (DEMS). We discover carbonate anion radicals (CO3•–) during CO2RR, acting as a carbon source for the formation of aldehyde species in addition to CO. At high cathodic potentials, HER is accelerated by rapid proton delivery from ordered interfacial hydration networks induced by carbonate molecules. Our molecular-level findings indicate water as the primary proton donor for CO2RR and the Volmer step of HER. These findings hold significance for rationally optimizing electrified solid-liquid interfaces and electrocatalysts. To interrogate interfacial hydration layer ordering effects, the interfacial viscosity of a KClO4 aqueous electrolyte can be probed by in situ friction force measurements, employing atomic force microscopy (AFM). We observe a decrease in the friction coefficient with increasing electrolyte concentration over the concentration range up to 25 mM, while adhesion force variations are found to be negligible. Qualitatively, this lubrication effect can be rationalized by accounting for the mildly chaotropic effect of K+ ions. For imaging of spatial variations in electrocatalytic activity across solid-electrolyte interfaces, an advanced scanning electrochemical microscope is being developed, involving electrically conductive AFM microcantilevers with an insulating and passivating top coating made by plasma vapour deposition of inorganic materials.