Ultrahigh Vacuum Surface Research Articles

Self-Running Ga Droplets on GaAs (111)A and (111)B Surfaces

Thermal decomposition of GaAs (111)A and (111)B surfaces in ultrahigh vacuum results in self-running Ga droplets. Although Ga droplets on the (111)B surface run in one main direction, those on the (111)A surface run in multiple directions, frequently taking sharp turns and swerving around pyramidal etch pits, leaving behind mixed smooth-triangular trails as a result of simultaneous in-plane driving and out-of-plane crystallographic etching. The droplet motion is partially guided by dislocation strain fields. The results hint at the possibilities of using subsurface dislocation network and prepatterned, etched surfaces to control metallic droplet motion on single-crystal semiconductor surfaces.

Spinning and translational motion of Sb nanoislands manipulated on MoS2

Antimony nanoislands grown on a MoS2 surface in ultra-high vacuum have been manipulated by atomic force microscopy (AFM) in ambient conditions. The island profiles have been digitized and provided as an input to a collisional algorithm based on classical mechanics. Assuming that the islands are rigid and static friction is high enough to prevent further motion after the passage of the probing tip, the direction of motion and the angle of rotation of the islands have been reproduced numerically. For a given spacing between the scan lines, the angle of deflection with respect to the fast scan direction and the angular speed of the islands are expected to vary with the friction between islands and substrate. From a comparison between model and experiment a shear strength in the order of 0.2 MPa is estimated.

The Energetics of Supported Metal Nanoparticles: Relationships to Sintering Rates and Catalytic Activity

Transition metal nanoparticles on the surfaces of oxide and carbon support materials form the basis for most solid catalysts and electrocatalysts, and have important industrial applications such as fuel production, fuels, and pollution prevention. In this Account, I review my laboratory group's research toward the basic understanding of the effects of particle size and support material on catalytic properties. I focus on studies of well-defined model metal nanoparticle catalysts supported on single-crystalline oxide surfaces. My group structurally characterized such catalysts using a variety of ultrahigh vacuum surface science techniques. We then measured the energies of metal atoms in these supported nanoparticles, using adsorption calorimetry tools that we developed. These metal adsorption energies increase with increasing size of the nanoparticles, until their diameter exceeds about 6 nm. Below 6 nm, the nature of the oxide support surface reaches also greatly affects the metal adsorption energies. Using both adsorption calorimetry and temperature programmed desorption (TPD), we measured the energy of adsorbed catalytic intermediates on metal nanoparticles supported on single crystal oxide surfaces, as a function of particle size. The studies reveal correlations between a number of characteristics. These include the size- and support-dependent energies of metal surface atoms in supported metal nanoparticles, their rates of sintering, how strongly they bind small adsorbates, and their catalytic activity. The data are consistent with the following model: the more weakly the surface metal atom is attached to the nanomaterial, the more strongly it binds small adsorbates. Its strength of attachment to the nanomaterial is dominated by the number of metal-metal bonds which bind it there, but also by the strength of metal/oxide interfacial bonding. This same combination of bond strengths controls sintering rates as well: the less stable a surface metal atom is in the nanomaterial, the greater is the thermodynamic driving force for it to sinter, and the faster is its sintering rate. These correlations provide key insights into how and why specific structural properties of catalyst nanomaterials dictate their catalytic properties. For example, they explain why supported Au catalysts must contain Au nanoparticles smaller than about 6 nm to have high activity for combustion and selective oxidation reactions. Only below about 6 nm are the Au atoms so weakly attached to the catalyst that they bind oxygen sufficiently strongly to enable the activation of O₂. By characterizing this interplay between industrially important rates (of net catalytic reactions, of elementary steps in the catalytic mechanism, and of sintering) and their thermodynamic driving forces, we can achieve a deeper fundamental understanding of supported metal nanoparticle catalysts. This understanding may facilitate development of better catalytic nanomaterials for clean, sustainable energy technologies.

Quantitative adsorbate structure determination under catalytic reaction conditions

Current methods allow quantitative local structure determination of adsorbate geometries on surfaces in ultrahigh vacuum (UHV) but are incompatible with the higher pressures required for a steady-state catalytic reactions. Here we show that photoelectron diffraction can be used to determine the structure of the methoxy and formate reaction intermediates during the steady-state oxidation of methanol over Cu(110) by taking advantage of recent instrumental developments to allow near-ambient pressure x-ray photoelectron spectroscopy. The local methoxy site differs from that under static UHV conditions, attributed to the increased surface mobility and dynamic nature of the surface under reaction conditions.

Surface‐Assisted Organic Synthesis of Hyperbenzene Nanotroughs

A hexagonal macrocycle consisting of 18 phenylene units (hyperbenzene) was synthesized on a Cu(111) surface in ultrahigh vacuum by Ullmann coupling of six 4,4''-dibromo-m-terphenyl molecules. The large diameter of 21.3 Å and the ability to assemble in arrays makes hyperbenzene an interesting candidate for a nanotrough that could enclose metallic, semiconducting, or molecular quantum dots.

Self-assembly of binary molecular nanostructure arrays on graphite

The controlled positioning and assembly of functional molecules into ordered nanostructures on surfaces depends on the interplay of multiple interactions on different strength and length scales. On metal surfaces, the relatively strong molecule-substrate interactions can constrain the molecules to adsorb in registry with the surface periodicity and lock them into specific adsorption sites. This can significantly reduce the structural tunability of the molecular nanostructure arrays formed. Inert graphite has a smooth potential-energy surface as well as relatively weak interfacial interactions with adsorbed molecules, and is therefore chosen as a supporting substrate for constructing molecular nanostructures with a high degree of controllability and tunability. The aim of this article is to highlight recent progress in the fabrication of self-assembled molecular nanostructures on inert graphite surfaces in ultra-high vacuum, with particular emphasis on the role of intermolecular interactions in the self-assembly process. We describe the formation of tunable two-dimensional (2D) binary molecular networks by directional and selective hydrogen bonding, as well as the templating effect of these 2D molecular networks, demonstrating the rational design and construction of long-range ordered 2D molecular nanostructures with desired functionality.

Anchored metal nanoparticles: Effects of support and size on their energy, sintering resistance and reactivity

Many catalysts consist of metal nanoparticles anchored to the surfaces of oxide supports. These are key elements in technologies for the clean production and use of fuels and chemicals. We show here that the chemical reactivity of the surface metal atoms on these nanoparticles is closely related to their chemical potential: the higher their chemical potential, the more strongly they bond to small adsorbates. Controlling their chemical potential by tuning the structural details of the material can thus be used to tune their reactivity. As their chemical potential increases, this also makes the metal surface less noble, effectively pushing its behavior upwards and to the left in the periodic table. Also, when the metal atoms are in a nanoparticle with higher chemical potential, they experience a larger thermodynamic driving force to sinter. Calorimetric measurements of metal vapor adsorption energies onto clean oxide surfaces in ultrahigh vacuum show that the chemical potential increases with decreasing particle size below 6 nm, and, for a given size, decreases with the adhesion energy between the metal and its support, Eadh. The structural factors that control the metal/oxide adhesion energy are thus also keys for tuning catalytic performance. For a given oxide, Eadh increases with (deltaHsub,M--deltaHf,MOx)/OmegaM2/3 for the metal, where deltaHsub,M is its heat of sublimation, deltaHf,MOx is the standard heat of formation of that metal's most stable oxide (per mole of metal), and OmegaM is the atomic volume of the bulk solid metal. The value deltaHsub,M--deltaHf,MOx equals the heat of formation of that metal's oxide from a gaseous metal atom plus O2(g), so it reflects the strength of the chemical bonds which that metal atom can make to oxygen, and OmegaM2/3 simply normalizes this energy to the area per metal atom, since Eadh is the adhesion energy per unit area. For a given metal, Eadh to different clean oxide surfaces increases as: MgO(100) approximately TiO2(110) < or = alpha-Al2O3(0001) < CeO2-x(111) < or = Fe3O4(111). Oxygen vacancies also increase Eadh, but surface hydroxyl groups appear to decrease Eadh, even though they increase the initial heat of metal adsorption.

X-ray photoelectron spectroscopic study on interface bonding between Pt and Zn- and O-terminated ZnO

Interface bonding between Pt and Zn- and O-terminated ZnO surfaces was investigated by precise analysis of x-ray photoelectron spectra. The interfaces were formed by vapor depositing Pt onto the ZnO surfaces in ultrahigh vacuum. The changes in the Zn 2p3/2, O 1s, Zn LMM Auger, and Pt 4f7/2 spectra upon Pt deposition were observed. The changes in the shape of the Zn LMM spectra and the shifts in the binding energy of Zn 2p3/2 and O 1s revealed that there was a metallic Zn component in the Zn LMM and Zn 2p3/2 spectra for Zn-terminated ZnO and a Pt-O component in the O 1s spectra for both Zn- and O-terminated ZnO. Peaks were fitted with multiple components accordingly. The binding energy shifts of Zn 2p3/2 and O 1s for the ZnO component were almost the same, which confirmed that the fitting was reasonable. From the fitting results, the interface bonding was concluded to be O-terminated, i.e., Zn-O-Pt bond formation occurred at the interface for both Zn- and O-terminated ZnO. This clearly demonstrated that the stable interface bonding occurring between Pt and ZnO is Zn-O-Pt bonding whether the ZnO substrate is initially Zn-terminated or O-terminated.

Dissociative Adsorption of Dimethyl Sulfoxide at the Ge(100)-2 × 1 Surface

Methods for passivating the germanium surface must be developed in order to fully exploit its electrical properties in devices. Passivation by formation of Ge–S bonds has proven most effective to date, spurring interest in adsorption of sulfur-containing molecules on the germanium surface. In this study, we investigate adsorption of dimethyl sulfoxide (DMSO) on the Ge(100)-2 × 1 surface in ultrahigh vacuum. By a combination of density functional theory calculations, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy, we find that DMSO undergoes two dissociative adsorption reactions on the Ge(100)-2 × 1 surface: S–C dissociation and S═O dissociation. The former produces a surface-bound methyl group and CH3SO fragment bound to germanium through the oxygen, while the latter yields adsorbed atomic oxygen and dimethyl sulfide, which desorbs to vacuum. It is shown that the reactivity of DMSO is greatly affected by the dipolar character of the sulfoxide bond, which drives the reaction ...

Selective Alkyne Hydrogenation over Nano-metal Systems: Closing the Gap between Model and Real Catalysts for Industrial Applications

The relationship between catalytic response and properties of the active phase is difficult to establish in classical heterogeneous catalysis due to the number of variables that can affect catalytic performance. Ultrahigh-vacuum surface methods applied to model catalyst surfaces are useful tools to assess fundamental issues related to catalytic processes but they are limited by the significant differences with catalysts in the working state. In an attempt to overcome this issue, (unsupported) nano-metal systems with controlled size and shape have been synthesized and tested in selective alkyne hydrogenation. The results revealed a dependency of nano-particles (NPs) morphology (size and shape) and allowed the identification of the active sites for this type of reaction. The nature of the stabilizer (steric and electrostatic stabilization) used in the NPs preparation has been shown to influence catalytic performance. The tailored active phase was subsequently immobilized on suitable nano- and micro-structured inorganic (e.g. 3D sintered metal fibers) supports with controlled surface properties in order to corroborate if the results obtained on the optimized nano-metal systems could be extrapolated to real catalysts. This article highlights the advantages and limitations of the analysis of selective alkyne hydrogenation over nano-metal systems that close the gap between model and real catalysts where the main challenges that lie ahead are summarized.

Substrate temperature and electron fluence effects on metallic films created by electron beam induced deposition

Using three different precursors [MeCpPtMe3, Pt(PF3)4, and W(CO)6], an ultra-high vacuum surface science approach has been used to identify and rationalize the effects of substrate temperature and electron fluence on the chemical composition and bonding in films created by electron beam induced deposition (EBID). X-ray photoelectron spectroscopy data indicate that the influence of these two processing variables on film properties is determined by the decomposition mechanism of the precursor. For precursors such as MeCpPtMe3 that decompose during EBID without forming a stable intermediate, the film's chemical composition is independent of substrate temperature or electron fluence. In contrast, for Pt(PF3)4 and W(CO)6, the initial electron stimulated deposition event in EBID creates surface bound intermediates Pt(PF3)3 and partially decarbonylated Wx(CO)y species, respectively. These intermediates can react subsequently by either thermal or electron stimulated processes. Consequently, the chemical composition of EBID films created from either Pt(PF3)4 or W(CO)6 is influenced by both the substrate temperature and the electron fluence. Higher substrate temperatures promote the ejection of intact PF3 and CO ligands from Pt(PF3)3 and Wx(CO)y intermediates, respectively, improving the film's metal content. However, reactions of Pt(PF3)3 and Wx(CO)y intermediates with electrons involve ligand decomposition, increasing the irreversibly bound phosphorous content in films created from Pt(PF3)4 and the degree of tungsten oxidation in films created from W(CO)6. Independent of temperature effects on chemical composition, elevated substrate temperatures (&amp;gt;25 °C) increased the degree of metallic character within EBID deposits created from MeCpPtMe3 and Pt(PF3)4.

The Structure–Sensitivity of n-Heptane Dehydrocyclization on Pt/SiO2 Model Catalysts

The structure sensitivity of n-heptane dehydrocyclization has been evaluated on Pt nanoparticles as a function of Pt particle size. Pt particles were vapor-deposited onto a SiO2 surface in ultrahigh vacuum (UHV) and then run under near-atmospheric pressures (495 Torr) by transferring the samples in situ to a batch reactor connected to the UHV system. The results demonstrate that the reaction rate increases as the particle size is decreased from 4 to 1.5 nm, consistent with results on high surface area technical catalysts. However, as particle size is further decreased below 1.5 nm, the reaction rate decreases, thus providing evidence of an optimum particle size for the dehydrocyclization reaction under the experimental conditions. The reactions on the nanoparticles are also compared with results obtained on Pt(100) and Pt(110) single crystals, which were run in the same apparatus under identical conditions. The differences between supported and unsupported Pt demonstrate that nanoparticles deactivate at a slower rate than single crystals, which suggests participation from the underlying silica support.

Spatially Graded Graphitization on 4H-SiC (0001) with Si-Sublimation Gradient for High Quality Epitaxial Graphene Growth

We report a new approach to produce high quality epitaxial graphene based on the concept of controlling Si sublimation rate from SiC surface. By putting a mask substrate to suppress Si sublimation from the SiC surface in ultrahigh vacuum, epitaxial graphene growth at 4H-SiC (0001) was locally controlled. Spatially graded surface graphitization was confirmed in a scanning electron microscopy contrast from the outside unmasked region to the inside masked region. The contrast was discussed with Raman characterization as the increase of graphene thickness and the surface compositional change of SiC. Results indicate two types of growth processes of epitaxial graphene at 4H-SiC (0001) step-terrace structures.

Formation of Co nanoparticles on a pyrolytic graphite surface in ultrahigh vacuum

Isolated cobalt nanoparticles have been obtained by annealing Co films produced by the electron-beam evaporation of a solid target onto a highly oriented pyrolytic graphite substrate in ultrahigh vacuum. Preliminary ion bombardment of the substrate reduces the average size of the particles formed during coalescence, but the shape of large particles is distorted. It is established that the Co nanoparticles retain their shape when exposed to air. Magnetic force images show that the particles of 20–40 nm in height are single-domain.

A Close Look at Proteins: Submolecular Resolution of Two- and Three-Dimensionally Folded Cytochrome c at Surfaces

Imaging of individual protein molecules at the single amino acid level has so far not been possible due to the incompatibility of proteins with the vacuum environment necessary for high-resolution scanning probe microscopy. Here we demonstrate electrospray ion beam deposition of selectively folded and unfolded cytochrome c protein ions on atomically defined solid surfaces in ultrahigh vacuum (10(-10) mbar) and achieve unprecedented resolution with scanning tunneling microscopy. On the surface folded proteins are found to retain their three-dimensional structure. Unfolded proteins are observed as extended polymer strands displaying submolecular features with resolution at the amino acid level. On weakly interacting surfaces, unfolded proteins refold into flat, irregular patches composed of individual molecules. This suggests the possibility of two-dimensionally confined folding of peptides of an appropriate sequence into regular two-dimensional structures as a new approach toward functional molecular surface coatings.

Nitrogen adsorption and desorption at iron pyrite FeS2{100} surfaces

We have investigated the interaction of nitrogen with single-crystal iron pyrite FeS(2){100} surfaces in ultra-high vacuum. N(2) adsorbs molecularly at low temperatures, desorbing at 130 K, but does not adsorb dissociatively even at pressures up to 1 bar. Atomic surface N can, however, be obtained with nitrogen ions and/or excited neutral species, generated by passing N(2) through an ion gun. Substantial nitrogen-induced disorder is seen with both ions and neutrals, and no ordered N overlayers form; a decrease in the S/Fe ratio is seen when exposing to nitrogen ions. Recombinative desorption leads to temperature-programmed desorption peaks at 410 and 520-560 K which we associate with interstitial atomic N and substitutional ionic N, respectively, in the surface regions. Thermal repair of sputter damage necessitates segregation of bulk S to the surface, which, over repeated experiments, leads to gross cumulative damage to the bulk crystal. The desorption temperatures associated with recombinative desorption of atomic N from FeS(2){100} are significantly lower than those measured for Fe surfaces. This is linked to the inability of FeS(2){100} to dissociate N(2), but suggests that N(ads) will be significantly more able to react with other species than it is on Fe surfaces.

Homo-coupling of terminal alkynes on a noble metal surface

The covalent linking of acetylenes presents an important route for the fabrication of novel carbon-based scaffolds and two-dimensional materials distinct from graphene. To date few attempts have been reported to implement this strategy at well-defined interfaces or monolayer templates. Here we demonstrate through real space direct visualization and manipulation in combination with X-ray photoelectron spectroscopy and density functional theory calculations the Ag surface-mediated terminal alkyne C(sp)-H bond activation and concomitant homo-coupling in a process formally reminiscent of the classical Glaser-Hay type reaction. The alkyne homo-coupling takes place on the Ag(111) noble metal surface in ultrahigh vacuum under soft conditions in the absence of conventionally used transition metal catalysts and with volatile H(2) as the only by-product. With the employed multitopic ethynyl species, we demonstrate a hierarchic reaction pathway that affords discrete compounds or polymeric networks featuring a conjugated backbone. This presents a new approach towards on-surface covalent chemistry and the realization of two-dimensional carbon-rich or all-carbon polymers.

Identification of the Active Species in Photochemical Hole Scavenging Reactions of Methanol on TiO2

Molecular and dissociative forms of adsorbed methanol were prepared on the rutile TiO2(110) surface to study their relative photocatalytic activity for hole-mediated oxidation. Molecular methanol is the dominant surface species on the vacuum-annealed TiO2(110) surface in ultrahigh vacuum (UHV). Coadsorption of methanol with oxygen results in ∼20% of the adsorbed methanol decomposing to methoxy and OH. Subsequent heating of the surface to ∼350 K desorbs unreacted methanol and OH (as water), leaving a surface with only adsorbed methoxy groups. Using temperature-programmed desorption (TPD), we show that adsorbed methoxy is at least an order of magnitude more reactive than molecularly adsorbed methanol for hole-mediated photooxidation. Methoxy photodecomposes through cleavage of a C–H bond forming adsorbed formaldehyde and a surface OH group. These results suggest that methoxy, and not molecular methanol, is the effective hole scavenger in photochemical reactions of methanol on TiO2.

Pyrocatechol as a surface capping molecule on rutile TiO2 (110)

A ‘cap and dip’ method of adsorbing ruthenium di-2,2′-bipyridyl-4,4′-dicarboxylic acid diisocyanate (N3 dye) on a rutile TiO2 (110) surface was investigated using pyrocatechol as a capping molecule. This method involves cleaning the rutile surface in ultra-high vacuum (UHV), depositing pyrocatechol onto the surface to ‘cap’ the adsorption sites, removing from vacuum, ‘dipping’ in an N3 dye solution and returning to vacuum. Photoemission measurements following the return of the crystal to vacuum suggest that the pyrocatechol keeps the surface free from contamination on exposure to atmosphere. Photoemission spectra also indicate that the pyrocatechol capping molecules are replaced by the N3 dye in solution and that the N3 dye is adsorbed intact on the rutile TiO2 (110) surface. This technique may allow other large molecules, which are thermally unstable to evaporation in UHV, to be easily deposited onto TiO2 surfaces.

Ethylene Irradiation: A New Route to Grow Graphene on Low Reactivity Metals

A novel technique for growing graphene on relatively inert metals, consisting in the thermal decomposition of low energy ethylene ions irradiated on hot metal surfaces in ultrahigh vacuum, is reported. By this route, we have grown graphene monolayers on Cu(111) and, for the first time, on Au(111) surfaces. For both noble metal substrates, but particularly for Au(111), our scanning tunneling microscopy and spectroscopy measurements provide sound evidence of a very weak graphene-metal interaction.