Covalent Adsorption of N-Heterocyclic Carbenes on a Copper Oxide Surface.

Tuning the properties of oxide surfaces through the adsorption of designed ligands is highly desirable for several applications, such as catalysis. N-Heterocyclic carbenes (NHCs) have been successfully employed as ligands for the modification of metallic surfaces. On the other hand, their potential as modifiers of ubiquitous oxide surfaces still needs to be developed. Here we show that a model NHC binds covalently to a copper oxide surface under UHV conditions. In particular, we report the first example of a covalent bond between NHCs and oxygen atoms from the oxide layer. This study demonstrates that NHC can also act as a strong anchor on oxide surfaces.

Variation of bending rigidity with material density: bilayer silica with nanoscale holes.

Two dimensional (2D) materials are a young class of materials that is foreseen to play an important role as building blocks in a range of applications, e.g. flexible electronics. For such applications, mechanical properties such as the bending rigidity κ are important. Only a few published measurements of the bending rigidity are available for 2D materials. Nearly unexplored is the question of how the 2D material density influences the bending rigidity. Here, we present helium atom scattering measurements on a "holey" bilayer silica with a density of 1.4 mg m-2, corresponding to 1.7 monolayers coverage. We find a bending rigidity of 6.6 ± 0.3 meV, which is lower than previously published measurements for a complete 2D film, where a value of 8.8 ± 0.5 meV was obtained. The decrease of bending rigidity with lower density is in agreement with theoretical predictions.

Dynamics in the O(2 × 1) adlayer on Ru(0001): bridging timescales from milliseconds to minutes by scanning tunneling microscopy.

The dynamics within an O(2 × 1) adlayer on Ru(0001) is studied by density functional theory and high-speed scanning tunneling microscopy. Transition state theory proposes dynamic oxygen species in the reduced O(2 × 1) layer at room temperature. Collective diffusion processes can result in structural reorientations of characteristic stripe patterns. Spiral high-speed scanning tunneling microscopy measurements reveal this reorientation as a function of time in real space. Measurements, ranging over several minutes with constantly high frame rates of 20 Hz resolved the gradual reorientation. Moreover, reversible fast flipping events of stripe patterns are observed. These measurements relate the observations of long-term atomic rearrangements and their underlying fast processes captured within several tens of milliseconds.

Structure and registry of the silica bilayer film on Ru(0001) as viewed by LEED and DFT.

Silica bilayers are stable on various metal substrates, including Ru(0001) that is used for the present study. In a systematic attempt to elucidate the detailed structure of the silica bilayer film and its registry to the metal substrate, we performed a low energy electron diffraction (I/V-LEED) study. The experimental work is accompanied by detailed calculations on the stability, orientation and dynamic properties of the bilayer at room temperature. It was determined, that the film shows a certain structural diversity within the unit cell of the metal substrate, which depends on the oxygen content at the metal-bilayer interface. In connection with the experimental I/V-LEED study, it became apparent, that a high-quality structure determination is only possible if several structural motifs are taken into account by superimposing bilayer structures with varying registry to the oxygen covered substrate. This result is conceptually in line with the recently observed statistical registry in layered 2D-compound materials.

Growth of N-Heterocyclic Carbene Assemblies on Cu(100) and Cu(111): From Single Molecules to Magic-Number Islands.

N-Heterocyclic carbenes (NHCs) have superior properties as building blocks of self-assembled monolayers (SAMs). Understanding the influence of the substrate in the molecular arrangement is a fundamental step before employing these ligands in technological applications. Herein, we study the molecular arrangement of a model NHC on Cu(100) and Cu(111). While mostly disordered phases appear on Cu(100), on Cu(111) well-defined structures are formed, evolving from magic-number islands to molecular ribbons with coverage. This work presents the first example of magic-number islands formed by NHC assemblies on flat surfaces. Diffusion and commensurability are key factors explaining the observed arrangements. These results shed light on the molecule-substrate interaction and open the possibility of tuning nanopatterned structures based on NHC assemblies.

Growth and Atomic-Scale Characterization of Ultrathin Silica and Germania Films: The Crucial Role of the Metal Support.

The present review reports on the preparation and atomic-scale characterization of the thinnest possible films of the glass-forming materials silica and germania. To this end state-of-the-art surface science techniques, in particular scanning probe microscopy, and density functional theory calculations have been employed. The investigated films range from monolayer to bilayer coverage where both, the crystalline and the amorphous films, contain characteristic XO4 (X=Si,Ge) building blocks. A side-by-side comparison of silica and germania monolayer, zigzag phase and bilayer films supported on Mo(112), Ru(0001), Pt(111), and Au(111) leads to a more general comprehension of the network structure of glass former materials. This allows us to understand the crucial role of the metal support for the pathway from crystalline to amorphous ultrathin film growth.

Identifying Structure-Selectivity Correlations in the Electrochemical Reduction of CO2 : A Comparison of Well-Ordered Atomically Clean and Chemically Etched Copper Single-Crystal Surfaces.

The identification of the active sites for the electrochemical reduction of CO2 (CO2 RR) to specific chemical products is elusive, owing in part to insufficient data gathered on clean and atomically well-ordered electrode surfaces. Here, ultrahigh vacuum based preparation methods and surface science characterization techniques are used with gas chromatography to demonstrate that subtle changes in the preparation of well-oriented Cu(100) and Cu(111) single-crystal surfaces drastically affect their CO2 RR selectivity. Copper single crystals with clean, flat, and atomically ordered surfaces are predicted to yield hydrocarbons; however, these were found experimentally to favor the production of H2 . Only when roughness and defects are introduced, for example by electrochemical etching or a plasma treatment, are significant amounts of hydrocarbons generated. These results show that structural and morphological effects are the key factors determining the catalytic selectivity of CO2 RR.

A Silica Bilayer Supported on Ru(0001): Following the Crystalline-to Vitreous Transformation in Real Time with Spectro-microscopy.

The crystalline-to-vitreous phase transformation of a SiO2 bilayer supported on Ru(0001) was studied by time-dependent LEED, local XPS, and DFT calculations. The silica bilayer system has parallels to 3D silica glass and can be used to understand the mechanism of the disorder transition. DFT simulations show that the formation of a Stone-Wales-type of defect follows a complex mechanism, where the two layers show decoupled behavior in terms of chemical bond rearrangements. The calculated activation energy of the rate-determining step for the formation of a Stone-Wales-type of defect (4.3 eV) agrees with the experimental value. Charge transfer between SiO2 bilayer and Ru(0001) support lowers the activation energy for breaking the Si-O bond compared to the unsupported film. Pre-exponential factors obtained in UHV and in O2 atmospheres differ significantly, suggesting that the interfacial ORu underneath the SiO2 bilayer plays a role on how the disordering propagates within the film.

From Crystalline to Amorphous Germania Bilayer Films at the Atomic Scale: Preparation and Characterization.

A new two-dimensional (2D) germanium dioxide film has been prepared. The film consists of interconnected germania tetrahedral units forming a bilayer structure, weakly coupled to the supporting Pt(111) metal-substrate. Density functional theory calculations predict a stable structure of 558-membered rings for germania films, while for silica films 6-membered rings are preferred. By varying the preparation conditions the degree of order in the germania films is tuned. Crystalline, intermediate ordered and purely amorphous film structures are resolved by analysing scanning tunnelling microscopy images.

A Two-Dimensional 'Zigzag' Silica Polymorph on a Metal Support.

We present a new polymorph of the two-dimensional (2D) silica film with a characteristic 'zigzag' line structure and a rectangular unit cell which forms on a Ru(0001) metal substrate. This new silica polymorph may allow for important insights into growth modes and transformations of 2D silica films as a model system for the study of glass transitions. Based on scanning tunneling microscopy, low energy electron diffraction, infrared reflection absorption spectroscopy, and X-ray photoelectron spectroscopy measurements on the one hand, and density functional theory calculations on the other, a structural model for the 'zigzag' polymorph is proposed. In comparison to established monolayer and bilayer silica, this 'zigzag' structure system has intermediate characteristics in terms of coupling to the substrate and stoichiometry. The silica 'zigzag' phase is transformed upon reoxidation at higher annealing temperature into a SiO2 silica bilayer film which is chemically decoupled from the substrate.

Charge Control in Model Catalysis: The Decisive Role of the Oxide-Nanoparticle Interface.

In chemistry and physics the electronic charge on a species or material is one important determinant of its properties. In the present Minireview, the essential requirements for a model catalyst system suitable to study charge control are discussed. The ideal model catalyst for this purpose consists of a material system, which comprises a single crystal metal support, covered by an epitaxially grown ultrathin oxide film, and flat, two-dimensional nanoparticles residing on this film. Several examples from the literature are selected and presented, which illustrate various aspects of electron transport from the support to the nanoparticle and vice versa. Key experiments demonstrate charge control within such model catalysts and give direct evidence for a chemical reaction at the perimeter of Au nanoparticles. The concepts derived from these studies are then taken a step further to see how they may be applied for bulk powder oxide supported nanoparticles as they are frequently found in catalytically active materials.

Modelling the atomic arrangement of amorphous 2D silica: a network analysis.

The recent experimental discovery of a semi two-dimensional silica glass has offered a realistic description of the random network theory of a silica glass structure, initially discussed by Zachariasen. To study the structure formation of silica in two dimensions, we introduce a two-body force field, based on a soft core Yukawa potential. The different configurations, sampled via Molecular dynamics simulations, can be directly compared with the experimental structures, which have been provided in the literature. The parameters of the force field are obtained from comparison of the nearest-neighbor distances between experiment and simulation. Further key properties such as angle distributions, distribution of ring sizes and triplets of rings are analyzed and compared with the experiment. Of particular interest is the spatial correlation of ring sizes. In general, we observe a very good agreement between experiment and simulation. Additional insight from the simulations is provided about the temporal and spatial stability of the rings in dependence of their size.

Carbon dioxide activation and reaction induced by electron transfer at an oxide-metal interface.

A model system has been created to shuttle electrons through a metal-insulator-metal (MIM) structure to induce the formation of a CO2 anion radical from adsorbed gas-phase carbon dioxide that subsequently reacts to form an oxalate species. The process is completely reversible, and thus allows the elementary steps involved to be studied at the atomic level. The oxalate species at the MIM interface have been identified locally by scanning tunneling microscopy, chemically by IR spectroscopy, and their formation verified by density functional calculations.

Ultrathin silica films: the atomic structure of two-dimensional crystals and glasses.

For the last 15 years, we have been studying the preparation and characterization of ordered silica films on metal supports. We review the efforts so far, and then discuss the specific case of a silica bilayer, which exists in a crystalline and a vitreous variety, and puts us into a position to investigate, for the first time, the real space structure (AFM/STM) of a two-dimensional glass and its properties. We show that pair correlation functions determined from the images of this two-dimensional glass are similar to those determined by X-ray and neutron scattering from three-dimensional glasses, if the appropriate sensitivity factors are taken into account. We are in a position, to verify, for the first time, a model of the vitreous silica structure proposed by William Zachariasen in 1932. Beyond this, the possibility to prepare the crystalline and the glassy structure on the same support allows us to study the crystal-glass phase transition in real space. We, finally, discuss possibilities to use silica films to start investigating related systems such as zeolites and clay films. We also mention hydroxylation of the silica films in order to adsorb metal atoms modeling heterogenized homogeneous catalysts.

The Influence of Mesoscopic Surface Structure on the Electrocatalytic Selectivity of CO2 Reduction with UHV-Prepared Cu(111) Single Crystals.

The key role of morphological defects (e.g., irregular steps and dislocations) on the selectivity of model Cu catalysts for the electrocatalytic reduction of CO2 (CO2RR) is illustrated here. Cu(111) single-crystal surfaces prepared under ultrahigh vacuum (UHV) conditions and presenting similar chemical and local microscopic surface features were found to display different product selectivity during the CO2RR. In particular, changes in selectivity from hydrogen-dominant to hydrocarbon-dominant product distributions were observed based on the number of CO2RR electrolysis pretreatment cycles performed prior to a subsequent UHV surface regeneration treatment, which lead to surfaces with seemingly identical chemical composition and local crystallographic structure. However, significant mesostructural changes were observed through a micron-scale microscopic analysis, including a higher density of irregular steps on the samples producing hydrocarbons. Thus, our findings highlight that step edges are key for C-C coupling in the CO2RR and that not only atomistic but also mesoscale characterization of electrocatalytic materials is needed in order to comprehend complex selectivity trends.

Leaning-Based Interfaces Improve Ground-Based VR Locomotion in Reach-the-Target, Follow-the-Path, and Racing Tasks.

Using standard handheld interfaces for VR locomotion may not provide a believable self-motion experience and can contribute to unwanted side effects such as motion sickness, disorientation, or increased cognitive load. This paper demonstrates how using a seated leaning-based locomotion interface -HeadJoystick- in VR ground-based navigation affects user experience, usability, and performance. In three within-subject studies, we compared controller (touchpad/thumbstick) with a more embodied interface ("HeadJoystick") where users moved their head and/or leaned in the direction of desired locomotion. In both conditions, users sat on a regular office chair and used it to control virtual rotations. In the first study, 24 participants used HeadJoystick versus Controller in three complementary tasks including reach-the-target, follow-the-path, and racing (dynamic obstacle avoidance). In the second study, 18 participants repeatedly used HeadJoystick versus Controller (8 one-minute trials each) in a reach-the-target task. To evaluate potential benefits of different brake mechanisms, in the third study 18 participants were asked to stop within each target area for one second. All three studies consistently showed advantages of HeadJoystick over Controller: we observed improved performance in all tasks, as well as higher user ratings for enjoyment, spatial presence, immersion, vection intensity, usability, ease of learning, ease of use, and rated potential for daily and long-term use, while reducing motion sickness and task load. Overall, our results suggest that leaning-based interfaces such as HeadJoystick provide an interesting and more embodied alternative to handheld interfaces in driving, reach-the-target, and follow-the-path tasks, and potentially a wider range of scenarios.

Integrating Continuous and Teleporting VR Locomotion into a Seamless 'HyperJump' Paradigm.

Continuous locomotion in VR provides uninterrupted optical flow, which mimics real-world locomotion and supports path integration . However, optical flow limits the maximum speed and acceleration that can be effectively used without inducing cybersickness. In contrast, teleportation provides neither optical flow nor acceleration cues, and users can jump to any length without increasing cybersickness. However, teleportation cannot support continuous spatial updating and can increase disorientation. Thus, we designed 'HyperJump' in an attempt to merge benefits from continuous locomotion and teleportation. HyperJump adds iterative jumps every half a second on top of the continuous movement and was hypothesized to facilitate faster travel without compromising spatial awareness/orientation. In a user study, Participants travelled around a naturalistic virtual city with and without HyperJump (equivalent maximum speed). They followed waypoints to new landmarks, stopped near them and pointed back to all previously visited landmarks in random order. HyperJump was added to two continuous locomotion interfaces (controller- and leaning-based). Participants had better spatial awareness/orientation with leaning-based interfaces compared to controller-based (assessed via rapid pointing). With HyperJump, participants travelled significantly faster, while staying on the desired course without impairing their spatial knowledge. This provides evidence that optical flow can be effectively limited such that it facilitates faster travel without compromising spatial orientation. In future design iterations, we plan to utilize audio-visual effects to support jumping metaphors that help users better anticipate and interpret jumps, and use much larger virtual environments requiring faster speeds, where cybersickness will become increasingly prevalent and thus teleporting will become more important.

Leaning-Based Interfaces Improve Simultaneous Locomotion and Object Interaction in VR Compared to the Handheld Controller.

Physical walking is often considered the gold standard for VR travel whenever feasible. However, limited free-space walking areas in the real-world do not allow exploring larger-scale virtual environments by actual walking. Therefore, users often require handheld controllers for navigation, which can reduce believability, interfere with simultaneous interaction tasks, and exacerbate adverse effects such as motion sickness and disorientation. To investigate alternative locomotion options, we compared handheld Controller (thumbstick-based) and physical walking versus a seated (HeadJoystick) and standing/stepping (NaviBoard) leaning-based locomotion interface, where seated/standing users travel by moving their head toward the target direction. Rotations were always physically performed. To compare these interfaces, we designed a novel simultaneous locomotion and object interaction task, where users needed to keep touching the center of upward moving target balloons with their virtual lightsaber, while simultaneously staying inside a horizontally moving enclosure. Walking resulted in the best locomotion, interaction, and combined performances while the controller performed worst. Leaning-based interfaces improved user experience and performance compared to Controller, especially when standing/stepping using NaviBoard, but did not reach walking performance. That is, leaning-based interfaces HeadJoystick (sitting) and NaviBoard (standing) that provided additional physical self-motion cues compared to controller improved enjoyment, preference, spatial presence, vection intensity, motion sickness, as well as performance for locomotion, object interaction, and combined locomotion and object interaction. Our results also showed that less embodied interfaces (and in particular the controller) caused a more pronounced performance deterioration when increasing locomotion speed. Moreover, observed differences between our interfaces were not affected by repeated interface usage.

Crystalline-vitreous interface in two dimensional silica.

The interface between a crystalline and a vitreous phase of a thin metal supported silica film was studied by low temperature scanning tunneling microscopy. The locally resolved evolution of Si-Si nearest neighbor distances and characteristic angles was evaluated across the border. Furthermore, we investigated the behavior of the ring size distribution close to the crystalline-vitreous transition. The crystalline order was found to decay gradually within about 1.6 nm into the vitreous state.

Thin silica films on Ru(0001): monolayer, bilayer and three-dimensional networks of [SiO4] tetrahedra.

The atomic structure of thin silica films grown over a Ru(0001) substrate was studied by X-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy, low energy electron diffraction, helium ion scattering spectroscopy, CO temperature programmed desorption, and scanning tunneling microscopy in combination with density functional theory calculations. The films were prepared by Si vapor deposition and subsequent oxidation at high temperatures. The silica film first grows as a monolayer of corner-sharing [SiO(4)] tetrahedra strongly bonded to the Ru(0001) surface through the Si-O-Ru linkages. At increasing amounts of Si, the film forms a bilayer of corner-sharing [SiO(4)] tetrahedra which is weakly bonded to Ru(0001). The bilayer film can be grown in either the crystalline or vitreous state, or both coexisting. Further increasing the film thickness leads to the formation of vitreous silica exhibiting a three-dimensional network of [SiO(4)]. The principal structure of the films can be monitored by infrared spectroscopy, as each structure shows a characteristic vibrational band, i.e., ∼1135 cm(-1) for a monolayer film, ∼1300 cm(-1) for the bilayer structures, and ∼1250 cm(-1) for the bulk-like vitreous silica.