Ultrahigh Vacuum Surface Research Articles

Thoughts on the past, present and future of UHV surface chemistry and the birth of Single-Atom Alloys

Throughout its relatively short lifetime, ultra-high vacuum (UHV) surface chemistry has progressed quickly. In the 1960′s, pioneers like Ertl and Somorjai started the field using single crystals and gained significant insight into catalytic processes by relating surface structure to reactivity. The more recent proliferation of scanning probes has significantly increased the power of the single crystal approach by enabling the atomic-scale structure of active sites to be correlated with their reactivity. In this perspective we briefly discuss how the field developed, identify some challenges, and highlight Single-Atom Alloys (SAAs), a new class of heterogeneous catalyst that was developed from a fundamental surface science approach. However, despite recent successes, funding for fundamental surface science has declined. Academic hires in the discipline are also declining in part due to the start-up costs. We make the case that fundamental UHV surface chemistry is still too young a field to be in recession.

Switching the on-surface orientation of oxygen-functionalized helicene.

Helicenes represent an important class of chiral organic material with promising optoelectronic properties. Hence, functionalization of surfaces with helicenes is a key step toward new organic materials devices. The deposition of a heterohelicene containing two furano groups and two hydroxyl groups onto copper(111) surface in ultrahigh vacuum leads to different adsorbate modifications. At low coverage and low temperature, the molecules tend to lie on the surface in order to maximize van der Waals contact with the substrate. Thermal treatment leads to deprotonation of the hydroxyl groups and in part into a reorientation from lying into a standing adsorbate mode.

Structural and electronic properties of MoS2 and MoSe2 monolayers grown by chemical vapor deposition on Au(111).

The exceptional electronic and photonic properties of the monolayers of transition metal dichalcogenides including the spin-orbit splitting of the valence and conduction bands at the K points of the Brillouin zone make them promising for novel applications in electronics, photonics and optoelectronics. Scalable growth of these materials and understanding of their interaction with the substrate is crucial for these applications. Here we report the growth of MoS2 and MoSe2 monolayers on Au(111) by chemical vapor deposition at ambient pressure as well as the analysis of their structural and electronic properties down to the atomic scale. To this aim, we apply ultrahigh vacuum surface sensitive techniques including scanning tunneling microscopy and spectroscopy, low-energy electron diffraction, X-ray and angle-resolved ultraviolet photoelectron spectroscopy in combination with Raman spectroscopy at ambient conditions. We demonstrate the growth of high-quality epitaxial single crystalline MoS2 and MoSe2 monolayers on Au(111) and show the impact of annealing on the monolayer/substrate interaction. Thus, as-grown and moderately annealed (<100 °C) MoSe2 monolayers are decoupled from the substrate by excess Se atoms, whereas annealing at higher temperatures (>250 °C) results in their strong coupling with the substrate caused by desorption of the excess Se. The MoS2 monolayers are strongly coupled to the substrate and the interaction remains almost unchanged even after annealing up to 450 °C.

(Invited) Coupling of Electrochemistry with Surface Science and Computational Modeling to Understand Nanoparticle Electrocatalysts

We investigated nanoparticle electrocatalysts in oxygen evolution reaction (OER) and CO2 reduction reaction (CO2RR) via an approach coupling electrochemistry with ultra-high vacuum (UHV) surface science and computational modeling. Well-defined nanoparticle electrocatalysts such as Fe2O3, NiFeOx, and Ag, were prepared in the UHV chamber via physical vapor deposition (PVD) and characterized with surface science techniques including X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM), followed by electrochemistry measurements to establish the structure-property relationships in OER and CO2RR. Calculations based on density functional theory (DFT) and microkinetic modeling (MKM) were then implemented to gain fundamental insight into these systems. We observed the edge-enhanced OER activity of Au-supported, ultra-thin Fe2O3 electrocatalysts and revealed incorporation of Ni at the edge sites (NiFeOx) further boosting their OER activity. We also resolved the size-dependent transition between CO2RR and H2 evolution reaction (HER) selectivity in sub-5 nm Ag electrocatalysts and identified an effective minimum size limit of ∼4 nm for active and selective Ag catalysts: particle diameters below this range experienced increased H2 evolutions due to a high population of Ag edge sites; and larger diameter particles favored CO2RR as the population of Ag(100) surface sites grew but will begin to lose catalyst utilization based on total metal masses.

Bridging Electrochemistry and Ultrahigh Vacuum: "Unburying" the Electrode-Electrolyte Interface.

ConspectusElectrochemistry has a central role in addressing the societal issues of our time, including the United Nations' Sustainable Development Goals (SDGs) and beyond. At a more basic level, however, elucidating the nature of electrode-electrolyte interfaces is an ongoing challenge due to many reasons, but one obvious reason is the fact that the electrode-electrolyte interface is buried by a thick liquid electrolyte layer. This fact would seem to preclude, by default, the use of many traditional characterization techniques in ultrahigh vacuum surface science due to their incompatibility with liquids. However, combined UHV-EC (ultrahigh vacuum-electrochemistry) approaches are an active area of research and provide a means of bridging the liquid environment of electrochemistry to UHV-based techniques. In short, UHV-EC approaches are able to remove the bulk electrolyte layer by performing electrochemistry in the liquid environment of electrochemistry followed by sample removal (referred to as emersion), evacuation, and then transfer into vacuum for analysis.Through this Account, we highlight our group's activities using UHV-EC to bridge electrochemistry with UHV-based X-ray and ultraviolet photoelectron spectroscopy (XPS/UPS) and scanning tunneling microscopy (STM). We provide a background and overview of the UHV-EC setup, and through illustrative examples, we convey what sorts of insights and information can be obtained. One notable advance is the use of ferrocene-terminated self-assembled monolayers as a spectroscopic molecular probe, allowing the electrochemical response to be correlated with the potential-dependent electronic and chemical state of the electrode-monolayer-electrolyte interfacial region. With XPS/UPS, we have been able to probe changes in the oxidation state, valence structure, and also the so-called potential drop across the interfacial region. In related work, we have also spectroscopically probed changes in the surface composition and screening of the surface charge of oxygen-terminated boron-doped diamond electrodes emersed from high-pH solutions. Finally, we will give readers a glimpse into our recent progress regarding real-space visualizations of electrodes following electrochemistry and emersion using UHV-based STM. We begin by demonstrating the ability to visualize large-scale morphology changes, including electrochemically induced graphite exfoliation and the surface reconstruction of Au surfaces. Taking this further, we show that in certain instances atomically resolved specifically adsorbed anions on metal electrodes can be imaged. In all, we anticipate that this Account will stimulate readers to advance UHV-EC approaches further, as there is a need to improve our understanding concerning the guidelines that determine applicable electrochemical systems and how to exploit promising extensions to other UHV methods.

Driving a Third Generation Molecular Motor with Electrons Across a Surface.

Excitation of single molecules with electrons tunneling between a sharp metallic tip of a scanning tunneling microscope and a metal surface is one way to study and control dynamics of molecules on surfaces. Electron tunneling induced dynamics may lead to hopping, rotation, molecular switching, or chemical reactions. Molecular motors that convert rotation of subgroups into lateral movement on a surface can in principle also be driven by tunneling electrons. For such surface-bound motor molecules the efficiency of motor action with respect to electron dose is still not known. Here, the response of a molecular motor containing two rotor units in the form of overcrowded alkene groups to inelastic electron tunneling has been examined on a Cu(111) surface in ultrahigh vacuum at 5 K. Upon vibrational excitation, switching between different molecular conformations is observed, including conversion of enantiomeric states of chiral conformations. Tunneling at energies in the range of electronic excitations causes activation of motor action and movement across the surface. The expected unidirectional rotation of the two rotor units causes forward movements but with a low degree of translational directionality.

Conglomerate Aggregation of 7,12,17‐Trioxa[11]helicene into Homochiral Two‐Dimensional Crystals on the Cu(100) Surface

AbstractDespite the vast development of stereochemistry it remains a great challenge to predict the fate of homochiral or heterochiral recognition in crystallization from racemic solution or melt. Using the well‐defined model system of racemic trioxaundecahelicene on a Cu(100) surface in ultrahigh vacuum, the chiral recognition of this heterohelicene has been investigated by means of state‐of‐the‐art scanning probe microscopy. The combination of scanning tunneling microscopy and tuning fork‐based non‐contact atomic force microscopy allowed assignment of the absolute configuration of single 7,12,17‐trioxa[11]helicene molecules in their two‐dimensional assemblies. At very low coverages, homochiral van der Waals‐bonded tetramers are observed. These also constitute the building blocks of the completely filled monolayer, which is a 2D conglomerate of homochiral domains. The interpretation of the adsorption geometries was supported by density‐functional theory calculations and molecular dynamics simulations, predicting a stronger interaction energy for homochiral structures over the heterochiral assembly. The striking similarity to heptahelicene on the same surface suggests that chiral recognition is dominated by the footprint interaction of the molecules with the metallic substrate.

Examples, Perspectives, and Impact of in-Operando Electrochemistry & Electrodeposition

In order to leave our children not with a huge problem in the future, we quickly have to realize the energy transition to renewable and CO2 neutral sources and circuits, which implies the introduction of the complete H2 economy. In addition, the ever ongoing demand and progress in electronics requires new and smart steps from the semiconducting industry pushing the border for device fabrication and thus material science and nanotechnology.Both extremely important topics crucially depend on the complete understanding of the fundamental atomic processes for device or catalyst manufacturing by electrodeposition as well as their stability during operation under (electrochemical) application.The general problem, however, has been formulated nicely already by Wolfgang Pauli:„God made the bulk; surfaces were invented by the devil.“It took around two decades, before most of the basic concepts and atomic effects have been observed and understood in standard, ultra-high vacuum (UHV) surface science and nanotechnology. Now, it is surely time to open a similar book and start from scratch, as atoms and surfaces in electrochemical environment clearly behave differently than in UHV. Moreover, this time it is not enough to measure after the deposition, after the growth, after the nucleation, or after a reaction!Both electrochemical data clearly show that many surfaces change during application and the complex understanding during electrodepostion, which involves a huge parameter space of tunable variables, can surely only be addressed adequately by in operando measurements.In this talk, I will show examples from in operando measurements ranging from STM over TEM to SXRD and X-ray photoelectron spectroscopy that all demonstrate the power and perspective for gaining new insight. Figure 1

Structure and Reactivity of α-Al2O3(0001) Surfaces: How Do Al–I and Gibbsite-like Terminations Interconvert?

The α-Al2O3(0001) surface has been extensively studied because of its significance in both fundamental research and application. Prior work suggests that in ultra-high-vacuum (UHV), in the absence of water, the so-called Al–I termination is thermodynamically favored, while in ambient, in contact with liquid water, a Gibbsite-like layer is created. While the view of the α-Al2O3(0001)/H2O(l) interface appears relatively clear in theory, experimental characterization of this system has resulted in estimates of surface acidity, i.e., isoelectric points, that differ by 4 pH units and surface structure that in some reports has non-hydrogen-bonded surface aluminol (Al–OH) groups and in others does not. In this study, we employed vibrational sum frequency spectroscopy (VSFS) and density functional theory (DFT) simulation to study the surface phonon modes of the differently terminated α-Al2O3(0001) surfaces in both UHV and ambient. We find that, on either water dosing of the Al–I in UHV or heat-induced dehydroxylation of the Gibbsite-like in ambient, the surfaces do not interconvert. This observation offers a new explanation for disagreements in prior work on the α-Al2O3(0001)/liquid water interface─different preparation methods may create surfaces that do not interconvert─and shows that the surface phonon spectral response offers a novel probe of interfacial hydrogen bonding structure.

Observation of robust superlubricity of MoS2 on Au(111) in ultrahigh vacuum

The structural and superlubric properties of single layer MoS2 on Au(111) forming moiré superlattice structures have been investigated by means of ultrahigh vacuum atomic force microscope with bimodal and contact modes. We synthesize epitaxial monolayer MoS2 flakes on the Au(111) surface in ultrahigh vacuum. Using friction force microscopy, atomic friction measurements indicate a superlubric regime between the tip apex and the moiré corrugated MoS2 surface in which the friction force remains at an ultralow value and is independent from normal load. Superlubricity conditions are observed for different loads and velocities which indicates the absence of out-of-plane deformations. We find that the MoS2 layer including the moiré superlattice modulation originating from the natural misfit between MoS2 and the Au(111) substrate is relatively rigid. We also demonstrate a low friction coefficient of the MoS2 surface crossing a single Au(111) step. Our results open up a new avenue for minimizing friction in nanoscale electronic devices and other dry rigid contacts used in aerospace lubrication.

Anisotropy of Shear-Induced Mechanochemical Reaction Rates of Surface Adsorbates; Implications for Theoretical Models

Mechanochemical reactions occur by an applied force modifying and ideally accelerating the rate of reaction of a mechanically active species, a mechanophore. Thermal reactions are described by the steepest descent pathway (SDP) of the potential energy surface (PES) from the transition state to the reactant state, which are stationary points on the SDP. The activation energy is calculated from the energy difference between these two points. The PES is modified by an imposed force to yield a reaction pathway given by the force-displaced stationary points (FDSPs), which depend on the magnitude and direction of the force and shape of the PES. However, the SDP has zero force in a direction perpendicular to it so that the PES can be visualized as a “harmonic valley” that forms a potential energy trough about the SDP. If the walls of the potential trough are sufficiently steep, a mechanochemical reaction should be constrained to occur along the SDP, and the influence of an applied force should depend only on the component of the force along it. If this is the case, it should be possible to use just the shape of the PES around the initial and transition states to calculate the effect of an imposed force or stress on mechanochemical reaction rates. The postulate is tested for the mechanically induced decomposition of an adsorbed methyl thiolate species on a Cu(100) single-crystal surface by measuring the azimuthal angular dependence of the mechanochemical methyl thiolate decomposition rate by varying the sliding direction of a sharp atomic force microscope tip over the surface in ultrahigh vacuum. The concept is also illustrated using a model 4-fold potential using a Remoissenet–Peyrard function to mimic the potential of a Cu(100) surface. This yields an angular dependence that agrees well with the prediction from the above postulate. This simplification will facilitate the analysis of mechanochemical rates of both surface and bulk reactions.

Macroscopic Single-Phase Monolayer Borophene on Arbitrary Substrates.

A major challenge in the investigation of all 2D materials is the development of synthesis protocols and tools which would enable their large-scale production and effective manipulation. The same holds for borophene, where experiments are still largely limited to in situ characterizations of small-area samples. In contrast, our work is based on millimeter-sized borophene sheets, synthesized on an Ir(111) surface in ultrahigh vacuum. Besides high-quality macroscopic synthesis, as confirmed by low-energy electron diffraction (LEED) and atomic force microscopy (AFM), we also demonstrate a successful transfer of borophene from Ir to a Si wafer via electrochemical delamination process. Comparative Raman spectroscopy, in combination with the density functional theory (DFT) calculations, proved that borophene's crystal structure has been preserved in the transfer. Our results demonstrate successful growth and manipulation of large-scale, single-layer borophene sheets with minor defects and ambient stability, thus expediting borophene implementation into more complex systems and devices.

The vibrational and thermodynamic properties of hydrogen adsorbed on the Pt(100) surface: a theoretical study

The adsorption of hydrogen on the Pt(100) surface is investigated theoretically within the conventional ultrahigh vacuum (UHV) surface modeling. Both density functional theory (DFT) and Monte Carlo (MC) simulation are implemented to determine the stability, the vibrational properties of the adsorbed hydrogen atom, and the H-H pair interactions. From the converged data, it is found that the H adsorbed on the bridge sites (B sites) have the strongest binding energy. Meanwhile, the H adsorbed on the top sites (T sites) have the highest vertical stretching frequency. In the increment of H coverage (), up to 1 ML, only the H on the B sites are counted. The shape of the g-value curve obtained by MC simulation shows a good agreement with the result of the cyclic voltammetry (CV) measurement. Reducing the pair interaction parameters by about 30%, the g-value is also decreased. From 0.3 to 0.6 with respect to the g-value is extremely fitted to the experimental data. This indicates that the interactions of H-H on the Pt(100) surface within the range of are mainly attributed to those hydrogen atoms adsorbed on the B sites.

Charged Particle-Induced Surface Reactions of Organometallic Complexes as a Guide to Precursor Design for Electron- and Ion-Induced Deposition of Nanostructures.

Focused electron beam-induced deposition (FEBID) and focused ion beam-induced deposition (FIBID) are direct-write fabrication techniques that use focused beams of charged particles (electrons or ions) to create 3D metal-containing nanostructures by decomposing organometallic precursors onto substrates in a low-pressure environment. For many applications, it is important to minimize contamination of these nanostructures by impurities from incomplete ligand dissociation and desorption. This spotlight on applications describes the use of ultra high vacuum surface science studies to obtain mechanistic information on electron- and ion-induced processes in organometallic precursor candidates. The results are used for the mechanism-based design of custom precursors for FEBID and FIBID.

Hydroxylation of the Zn terminated ZnO(0 0 0 1) surface under vacuum conditions

Under vacuum conditions, the polar surface ZnO(0 0 0 1) evolves in time forming a layer with H and additional O which changes the top layer stoichiometry and other properties such as its work function. In this work we present a study of the evolution of the ZnO(0 0 0 1) surface in ultra-high vacuum performed with time of flight direct recoil spectroscopy to detect the adsorbed H and O, plus other standard techniques including photoelectron spectroscopy and low energy electron diffraction. The experimental results are complemented with density functional theory (DFT) calculations for surfaces perfectly terminated and with triangular defects plus O adatoms. We show that the ZnO surface reaches a stable condition at an OH coverage of around 0.25 ML. We also studied the effect of increasing the partial pressure of H2 and of H2O on the layer growth. We observe that during the formation of the OH layer the work function decreases. Analysis of this variation of the work function with DFT reveals the role of the O adatoms in the adsorption process. Finally we show that the OH coverage decreases continuously with the increase of the sample temperature, without showing a well-defined desorption peak, which is in agreement with earlier predictions of the thermodynamics evolution. We believe that the present results contribute to clarify the behavior of the ZnO(0 0 0 1) surface under vacuum and will be useful in future works, particularly where H cannot be detected by standard techniques.

Atomic-Scale Investigation of Oxidation at the Black Phosphorus Surface

Black phosphorus (BP) exhibits extraordinary electronic properties that are desirable for a wide variety of electronic and optoelectronic applications. However, applications of BP are hindered by its rapid degradation in ambient conditions. Despite significant advances that have been made in understanding the degradation mechanism, no consensus has yet been reached on how BP oxidation occurs at the atomic scale as experimental studies have been mostly restricted to averaged effects of degradation over a micron- to millimeter-sized region. Here, BP oxidation is investigated using scanning tunneling microscopy/spectroscopy (STM/S). Introducing O2 gas to the BP surface in ultrahigh vacuum at a pressure of 10–5 mbar for 1 min creates two new types of defects on the surface. We identify these defects as dangling atomic oxygen and phosphorus multivacancies using density functional theory simulations. In addition to the structural changes to the surface, the electronic structure is also drastically altered by the introduction of oxygen. The 300 meV band gap of BP is lifted due to dosing. This change in the electronic structure is reversible through STM tip manipulation. These are the first experimental results showing the atomic-scale oxidation of BP, an important step toward understanding the degradation process.

Adsorption and reaction kinetics of SO2 on graphene: An ultrahigh vacuum surface science study

The adsorption of sulfur dioxide, SO2, on graphene, epitaxially grown on Ru(0001) (short Gr/Ru), was studied at ultrahigh vacuum conditions, by thermal desorption spectroscopy, TDS, and Auger electron spectroscopy, AES. TDS detected the parent mass, i.e., a molecular adsorption/desorption pathway was present. However, deviations in the gas-phase fragmentation pattern and multimass TDS were observed. In addition, AES revealed adsorbed sulfur after TDS experiments. Thus, SO2 partially decomposed on Gr/Ru.

Ru(0001) surface electrochemistry in the presence of specifically adsorbing anions

Disentangling the potential dependent surface(electro)chemical processes can be an arduous task, in particular for ill-defined electrode surfaces or when several reactions either coincide or occur without electron transfer. Employing ultrahigh vacuum surface preparation, electrochemical measurements and in situ spectroscopic / structural characterisation we have investigated the potential dependent surface processes proceeding on a Ru(0001) single crystal electrode in the absence and presence of specifically adsorbing anions in acid electrolyte. Online differential electrochemical mass spectrometry (DEMS) measurements clearly identify anodic H2 formation in both cases, a recently reported phenomenon (Scott et al. Catal. Sci. Technol.10 (2020) 6870), suggesting that this can be induced not only by OH adsorption, but also by other strongly adsorbing anions such as bisulfate. Furthermore, operando surface X-ray diffraction (SXRD) measurements demonstrate that the composition of the adlayer depends significantly on the potential history of the electrode, where strongly adsorbed hydroxyl species lead to pronounced hysteresis effects. Such complex processes are proposed to be characteristic for strongly interacting electrodes in general, not only for Ru.

Часопролітна мас-спектрометрична установка

Потреба техніки і технологій привели до різкого прискорення дослідно-конструкторських і науково-дослідних робіт для вивчення в надвисокому вакуумному середовищі як поверхневих, так і об'ємних властивостей твердих тіл. Як відомо, на поверхневі властивості твердих тіл в значній мірі впливають адсорбційні процеси, обумовлені навколишнім середовищем, тобто елементним складом газу, що знаходиться у вакуумній системі. Атоми і молекули газу відрізняються своєю масою, отже їх можна ідентифікувати цієї фізичної величиною. Тому мас-спектрометрія, як метод аналізу складу складних газових сумішей та їх ідентифікації за масами, є одним з найбільш ефективним для визначення молекулярної маси газу, його складу і структури. У даній роботі запропонована часопролітна мас-спектрометрична установка з підвищеною швидкодією, точністю і надійністю вимірювання мас газу у вакуумній системі, а це даэ можливість проводити біль якісний аналіз адсорбційних процесів на поверхні твердого тіла.

Microscopic Visualization of Porous Nanographenes Synthesized through a Combination of Solution and On-Surface Chemistry

On-surface synthesis has recently been regarded as a promising approach for the generation of new molecular structures. It has been particularly successful in the synthesis of graphene nanoribbons, nanographenes and intrinsically reactive and instable, yet attractive species. It is based on the combination of solution chemistry aimed at preparation of appropriate molecular precursors for further ultra-high vacuum surface assisted transformations. This approach also owes its success to an incredible development of characterization techniques, such as scanning tunneling/atomic force microscopy and related methods, which allow detailed, local characterization at atomic scale. While the surface-assisted synthesis can provide molecular nanostructures with outstanding precision, down to single atoms, it suffers from basing on metallic surfaces and often limited yield. Therefore, the extension of the approach away from metals and the struggle to increase productivity seem to be significant challenges toward wider applications. Herein, we demonstrate the on-surface synthesis approach for generation of non-planar nanographenes, which are synthesized through a combination of solution chemistry and sequential surface-assisted processes, together with the detailed characterization by scanning probe microscopy methods.