Under Ultrahigh Vacuum Research Articles

In situ scanning tunneling microscopy study of Ca-modified rutile TiO2(110) in bulk water.

Despite the rising technological interest in the use of calcium-modified TiO2 surfaces in biomedical implants, the Ca/TiO2 interface has not been studied in an aqueous environment. This investigation is the first report on the use of in situ scanning tunneling microscopy (STM) to study calcium-modified rutile TiO2(110) surfaces immersed in high purity water. The TiO2 surface was prepared under ultrahigh vacuum (UHV) with repeated sputtering/annealing cycles. Low energy electron diffraction (LEED) analysis shows a pattern typical for the surface segregation of calcium, which is present as an impurity on the TiO2 bulk. In situ STM images of the surface in bulk water exhibit one-dimensional rows of segregated calcium regularly aligned with the [001] crystal direction. The in situ-characterized morphology and structure of this Ca-modified TiO2 surface are discussed and compared with UHV-STM results from the literature. Prolonged immersion (two days) in the liquid leads to degradation of the overlayer, resulting in a disordered surface. X-ray photoelectron spectroscopy, performed after immersion in water, confirms the presence of calcium.

KCl ultra-thin films with polar and non-polar surfaces grown on Si(111)7 × 7.

The growth of ultra-thin KCl films on the Si(111)7 × 7 reconstructed surface has been investigated as a function of KCl coverage and substrate temperature. The structure and morphology of the films were characterized by means of scanning tunneling microscopy (STM) under ultra-high vacuum (UHV) conditions. Detailed analysis of the atomically resolved STM images of islands grown at room and high temperatures (400 K–430 K) revealed the presence of KCl(001) and KCl(111) islands with the ratio between both structures depending on the growth temperature. At room temperature, the growth of the first layer, which covers the initial Si(111)7 × 7 surface, contains double/triple atomic layers of KCl(001) with a small fraction of KCl(111) islands. The high temperature growth promotes the appearance of large KCl(111) areas, which are built up by three atomic layers. At room and high temperatures, flat and atomically well-defined ultra-thin KCl films can be grown on the Si(111)7 × 7 substrate. The formation of the above mentioned (111) polar films is interpreted as a result of the thermally activated dissociative adsorption of KCl molecules on Si(111)7 × 7, which produces an excess of potassium on the Si surface.

Enhanced photochemistry of ethyl chloride on Ag nanoparticles.

Enhanced photodecomposition of ethyl chloride (EC) adsorbed on SiO2/Si (100) supported silver nanoparticles (Ag NPs) under ultrahigh vacuum (UHV) conditions has been studied in order to assess the potential contribution of plasmonic effects. The cross section for photodecomposition of EC and overall photoyield were found to increase with increasing photon energy regardless of the plasmon resonant wavelength and with Ag coverage without any noticeable particle size effect. The influence of EC-Ag NPs separation distance on the rate of EC decomposition was studied in order to examine potential local electric field influence on the photodissociation process. Long (∼5 nm) photoactivity decay distance has been observed which excludes local surface plasmon dominance in the photodecomposition event. These findings suggest that the alignment of excited electron energy and adsorbate affinity levels is central for efficient photochemical reactions, whereas short-range electric field enhancement by plasmon excitation on top and at the immediate vicinity of silver nanoparticles does not have any measurable effect.

Investigation of the interaction between a novel unnatural chiral ligand and reactant on palladium for asymmetric hydrogenation.

We report about the mechanistic studies of the reaction between a newly synthesized (S)-2-((R)-3H-dinaphtho[2,1-c:1',2'-e]azepin-4(5H)-yl)-2-phenylethanol based on the binaphthyl skeleton and (E)-2-methyl-5-phenylpent-2-enoic acid for the asymmetric hydrogenation of α,β-unsaturated acids with heterogeneous palladium catalysts. The specific interactions between the chiral ligand and reactant were investigated in solution with palladium nanoparticles, as well as under ultrahigh vacuum (UHV) conditions on the palladium metal surface in the absence of hydrogen. The reactions were explored using nuclear magnetic resonance (NMR) spectroscopy, scanning tunneling microscopy (STM), and high-resolution photoemission spectroscopy (HRPES) combined with density functional theory (DFT) calculations. A NMR study identified the interaction between both molecules with palladium nanoparticles in solution. In addition, STM and HRPES studies revealed the spatial distribution and configuration of both compounds on the palladium metal surface under UHV conditions. The theoretical results support the experimental results with respect to the interaction energy value. It was found that the reaction between the ligand and reactant occurs with hydrogen bonding on palladium surface, simultaneously. The present study provides mechanistic details of the asymmetric hydrogenation reaction, which bears a correlation between the ligand, reactant, and catalyst during the reaction.

Tug-of-war between corrugation and binding energy: revealing the formation of multiple moiré patterns on a strongly interacting graphene-metal system.

The formation of multidomain epitaxial graphene on Rh(111) under ultra-high vacuum (UHV) conditions has been characterized by scanning tunnelling microscopy (STM) measurements and density functional theory (DFT) calculations. At variance with the accepted view for strongly interacting graphene-metal systems, we clearly demonstrate the formation of different rotational domains leading to multiple moiré structures with a wide distribution of surface periodicities. Experiments reveal a correlation between the STM apparent corrugation and the lattice parameter of the moiré unit cell, with corrugations of just 30-40 pm for the smallest moirés. DFT calculations for a relevant selection of these moiré patterns show much larger height differences and a non-monotonic behaviour with the moiré size. Simulations based on non-equilibrium Green's function (NEGF) methods reproduce quantitatively the experimental trend and provide a detailed understanding of the interplay between electronic and geometric contributions in the STM contrast of graphene systems. Our study sheds light on the subtle energy balance among strain, corrugation and binding that drives the formation of the moiré patterns in all graphene/metal systems and suggests an explanation for the success of an effective model only based on the lattice mismatch. Although low values of the strain energy are a necessary condition, it is the ability of graphene to corrugate in order to maximize the areas of favourable graphene-metal interactions that finally selects the stable configurations.

Support effects in the adsorption of water on CVD graphene: an ultra-high vacuum adsorption study.

Experimental data for water adsorption on CVD (chemical vapor deposition) graphene/SiO2 and graphene/Cu studied under ultra-high vacuum (UHV) conditions are discussed, focusing on support effects and hydrophobicity. Under UHV, it seems that graphene wettability is inversely related to wetting properties of the support. Graphene is not transparent to water wetting on the supports studied here.

Morphological variations in AuxSiy nanostructures under variable pressure and annealing conditions

Well-ordered, substrate symmetry-driven, AuxSiy structures of average size ~25 nm were formed under ultra-high vacuum (UHV) conditions using molecular beam epitaxy method. Post-annealing was done at 500 °C in three different vacuum conditions: (1) low vacuum (LV) (10−2 mbar), (2) high vacuum (HV) (10−5 mbar) and (3) UHV (10−10 mbar) (MBE chamber). For both HV and LV cases, the AuxSiy nanostructures were found to have their corners rounded unlike in UHV case where the structures have sharp edges. In all the above three cases, samples were exposed to air before annealing. In situ annealing inside UHV chamber without exposing to air resulted in well-aligned rectangles with sharp corners, while sharp but irregular island structures were found for air exposed and UHV annealing system. The role of residual gases present in LV and HV annealing environment and inhibition of lateral surface diffusion due to the presence of surface oxide (through air exposure) would be discussed. Annealing at various conditions yielded variation in the coverage and correspondingly, the average area of nanostructures varied from a ~329 nm2 (as deposited) to ~2,578 nm2 (at high temperature). High-resolution transmission electron microscopy (planar and cross section) has been utilized to study the morphological variations.

Alternative Low-Pressure Surface Chemistry of Titanium Tetraisopropoxide on Oxidized Molybdenum

Titanium tetraisopropoxide (TTIP) is a precursor utilized in atomic layer depositions (ALDs) for the growth of TiO2. The chemistry of TTIP deposition onto a slightly oxidized molybdenum substrate was explored under ultrahigh vacuum (UHV) conditions with X-ray photoelectron spectroscopy. Comparison of the Ti(2p) and C(1s) peak areas has been used to determine the surface chemistry for increasing substrate temperatures. TTIP at a gas-phase temperature of 373 K reacts with a MoOx substrate at 373 K but not when the substrate is at 295 K, consistent with a reaction that proceeds via a Langmuir–Hinshelwood mechanism. Chemical vapor deposition was observed for depositions at 473 K, below the thermal decomposition temperature of TTIP and within the ALD temperature window, suggesting an alternative reaction pathway competitive to ALD. We propose that under conditions of low pressure and moderate substrate temperatures dehydration of the reacted precursor by nascent TiO2 becomes the dominant reaction pathway and leads to the CVD growth of TiO2 rather than a self-limiting ALD reaction. These results highlight the complexity of the chemistry of ALD precursors and demonstrate that changing the pressure can drastically alter the surface chemistry.

Theoretical study of intermolecular interactions in nanoporous networks on boron doped silicon surface

Supramolecular networks on a doped boron silicon surface under ultra high vacuum (UHV) have been recently obtained (Makoudi et al., 2013). The used molecule contains different end-groups, bearing either bromine, iodine or hydrogen atoms denoted 1,3,5-tri(4′-bromophenyl)benzene (TBB), 1,3,5-tri(4-iodophenyl)benzene (TIB) and 1,3,5-triphenyl-benzene (THB). To explain the formation of the nanoporous structures, interactions of the type aryl-X⋯H hydrogen bonds (X being a halogen atom) have been proposed. In order to obtain a complete insight of the stabilizing interaction in these networks adsorbed on the Si(111)√3x√3R30°-boron surface, we present a full density-functional-theory study taking the van der Waals interactions into account. We investigated the energetic and structural properties of three different nanoporous networks constituted by TBB, TIB and THB molecules. The electronic studies allow us to identify hydrogen bond and dipole–dipole intermolecular interactions in the supramolecular halogen networks, whereas only dipole–dipole interactions are present in the 1,3,5-triphenyl-benzene nanoporous network.

In Situ Observation of Epitaxial Li–Si-Nanostructure Formation on Si(111)

The formation kinetics of lithium–silicon phases was studied in a molecular beam epitaxy environment under ultrahigh vacuum (UHV) conditions. A heated Si(111) substrate was exposed to an incoming lithium flux at a gradually decreasing substrate temperature. The onset of the nucleation and the formation of Li–Si phases were simultaneously observed by in situ synchrotron X-ray diffraction (XRD), azimuthal reflection high-energy electron diffraction (ARHEED), and line-of-sight quadrupole mass spectrometry (QMS) of the desorbing Li. First, Li-induced reconstructions formed, followed by the appearance of nanostructures which grow epitaxially with respect to the Si substrate. Both crystal structures, namely Li12Si7 and Li21Si5, which are thermodynamical stable at elevated temperatures were identified. They form with an epitaxial relationship such that [111]Si ∥ [010]Li12Si7 and [112̅]Si ∥ [100]Li12Si7 as well as [111]Si ∥ [111]Li21Si5 and [011̅]Si ∥ [011̅]Li21Si5. Finally, three-dimensional Li structures appeared on the surface. The in situ measurements were supplemented by ex situ atomic force microscopy and scanning electron microscopy showing the real-space information on three-dimensional Li–Si structures. Similarities and differences compared to a Li-ion battery structure containing a Si anode are discussed. A significant mobility of Si and Li to form nm-sized crystal structures is concluded.

Application of Double Resonance Sum Frequency Generation Spectroscopy to Characterize the Interfacial Electronic and Molecular Structure in Electrochemical Environment

Absorbed CO on Pt surface has attracted much interest among scientists in surface science, catalysis, and electrochemistry. Information about oxidation mechanisms of CO on Pt, which depends on geometric and electronic structures of CO/Pt interface, is important to design and develop new catalysts for various reactions including H2-O2 and direct methanol fuel cells. Small amount of CO present in H2 produced from fossil fuel readily adsorbs on Pt and inhibits H2 oxidation. Adsorbed CO is also formed when Pt is in contact with methanol solution and inhibits the methanol oxidation reaction. Compared to extensive study on CO/Pt system under ultrahigh vacuum (UHV), interfacial electronic structure of CO/Pt interface in electrochemical environment is less understood due to the difficulty in determining it in solution. Accordingly, it is very important to investigate interfacial electronic structure of CO/Pt under electrochemical condition. Because of the presence of solution, it is not possible to use techniques with electron probes, which are most powerful in UHV. Double resonance sum frequency generation (DR-SFG) spectroscopy, which is a highly surface-sensitive nonlinear optical technique with the tunable frequencies of both incident fundamental lights, i.e., visible and IR, can provide information of surface molecular and electronic structures, which is not obtained by traditional surface vibrational technique. Here, we employed DR-SFG technique to probe potential-dependent electronic structure of CO/Pt interface in methanol solution. SFG signal due to C-O stretching was observed and was dependent on frequency of visible light as a result of the energetic coupling of electronic transition with the input visible photon and/or output SF photon, i.e., double resonance effect. Furthermore, resonant visible energy linearly depended on electrode potentials. Thus, the electronic resonance could be explained by the transition from Fermi energy, Ef, of Pt to the unoccupied 5anti-bonding state of CO.

Role of thermal processes in dewetting of epitaxial Ag(111) film on Si(111)

Epitaxially grown silver (Ag) film on silicon (Si) is an optimal plasmonic device platform, but its technological utility has been limited by its tendency to dewet rapidly under ambient conditions (standard temperature and pressure). The mechanisms driving this dewetting have not heretofore been determined. In this study, scanning probe microscopy and low-energy electron microscopy are used to compare the morphological evolution of epitaxial Ag(111)/Si(111) under ambient conditions with that of similarly prepared films heated under ultra-high vacuum (UHV) conditions. Dewetting in both cases is seen to be initiated with the formation of pinholes, which might function to relieve strain in the film. We find that in the UHV environment, dewetting is determined by thermal processes, while under ambient conditions, thermal processes are not required. We conclude that dewetting in ambient conditions is triggered by some chemical process, most likely oxidation.

Nanoporous Gold-Supported Ceria for the Water–Gas Shift Reaction: UHV Inspired Design for Applied Catalysis

Inspired by model studies under ultrahigh vacuum (UHV) conditions, inverse monolithic gold/ceria catalysts are prepared using thermal decomposition of a cerium nitrate precursor on a nanoporous gold (npAu) substrate. Cerium oxide deposits throughout the porous gold material (pores and ligaments 30–40 nm) are formed. npAu disks and coatings were prepared with loadings of about 3 to 10 atom % of ceria. The composite material was tested for the water–gas shift (WGS) reaction (H2O + CO → H2 + CO2) in a continuous flow reactor at ambient pressure conditions. Formation of CO2 was observed at temperatures as low as 135 °C with excellent stability and reproducibility up to temperatures of 535 °C. The considerably increased thermal stability of the material can be linked to the presence of metal oxide deposits on the nanosized gold ligaments. The loss of activity after about 15 h of catalytic conversion with heating to 535 °C was only about 10%. Photoemission spectroscopy indicates a defect (Ce3+) concentration of...

Coverage-Dependent Disorder-to-Order Phase Transformation of a Uracil Derivative on Ag(111)

The self-organization of an angular bis(uracil-ethynyl) benzene derivative is investigated on Ag(111) by means of scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) conditions. It is found—starting at low submonolayer coverage—that upon increasing the molecular coverage a disorder-to-order phase transformation occurs. Specifically, at low and intermediate molecular coverage a glassy phase consisting of one-dimensional (1D) chains and 2D aggregates is observed, while close to a first complete molecular layer, a well-ordered 2D close-packed phase is revealed. The main driving forces responsible for the structure formation are (i) the high self-complementarity of the uracil (U) moiety, resulting in U–U homopairs through 2-fold C═O···H–N H-bonds and (ii) the steric hindrance induced in the system by the alkyl chains. The delicate balance between the molecule–molecule and the molecule–substrate interactions leads to a complex phase behavior of the uracil derivative at the solid–vacuum interface. On the basis of this detailed study, we present a qualitative understanding of the peculiar phase behavior of the system.

Auger Electron Spectroscopy in high vacuum: Nanocharacterisation in the Scanning Electron Microscope

It is generally regarded as impossible to carry out Auger Electron Spectroscopy (AES) analysis in a scanning electron microscope (SEM) due to the high ambient gas pressure in an SEM. This is because standard electron energy analysers such as the Concentric Hemispherical Analyser (CHA) or Cylindrical Mirror Analyser (CMA) are devices that acquire data in a serial manner that can last up to few minutes. This is considered too slow for high vacuum (10−6mbar) and a previously cleaned surface would be re-contaminated before a spectrum could be completed. This has led to AES being traditionally carried out under ultrahigh vacuum (UHV) environment. We report on two devices for fast acquisition of AES data characterising nanoscale objects by the use of AES in an SEM.

Electrochemical stability and restructuring and its impact on the electro-oxidation of CO: Pt modified Ru(0001) electrodes

Structural modifications during electrochemical measurements on well defined Pt modified Ru(0001) electrode surfaces, which were prepared and characterized under ultrahigh vacuum (UHV) conditions, and the influence of the restructuring on the CO oxidation reaction have been investigated in a set-up combining surface preparation and scanning tunneling microscopy characterization under UHV conditions (UHV-STM) and electrochemical flow cell measurements. Bare Ru(0001) and Pt monolayer island modified Ru(0001) electrodes with different Pt coverages were investigated, together with a Pt0.3Ru0.7/Ru(0001) monolayer surface alloy for comparison. Comparing bulk CO oxidation measurements performed upon cycling in base electrolyte (0.5M H2SO4) to 0.90VRHE with similar measurements performed after potential cycling to 1.05VRHE, we find pronounced differences in the current–voltage characteristics, with a distinct new peak at low potentials in the positive-going scan in the latter case, which is centered at 0.67VRHE. STM imaging performed before and after the electrocatalytic measurements revealed a distinct restructuring of the Pt monolayer island modified Ru(0001) surfaces upon potential cycling to 1.05VRHE, while cycling to 0.90VRHE maintains the original structure and morphology of the bimetallic surface. In contrast, for the bare Ru(0001) electrode, restructuring of steps is observed already upon potential cycling to 0.9VRHE. Implications of these findings on the electrochemical stability of the electrodes as well as on the mechanistic understanding of the CO oxidation reaction on bimetallic PtRu electrode surfaces and on the activity of different mono- and bimetallic nanostructures are discussed.

Walking peptide on Au(110) surface: Origin and nature of interfacial process

IGF tri-peptide adsorption on Au(110)-(1×2) under Ultra High Vacuum (UHV) conditions has been investigated using surface science techniques such as synchrotron based Angle Resolved X-ray Photoemission Spectroscopy (AR-PES or AR-XPS), Low Energy Electron Diffraction (LEED) and Scanning Tunnelling Microscopy (STM). The behaviour of IGF molecules has been revealed to be coverage dependent; at low coverage, there is formation of islands presenting a chiral self-organised molecular network with a (4 2, −3 2) symmetry as shown by Low Energy Electron Diffraction (LEED) and Scanning Tunnelling Microscopy (STM) on the unaltered Au(110)-(1×2) reconstruction, suggesting significant intermolecular interactions. When the coverage is increased, the islands grow bigger, and one can observe the disappearance of the self-organised network, along with a remarkable destruction of the (1×2) substrate reconstruction, as shown by STM. The effect of IGF on the surface gold atoms has been further confirmed by angle-resolved photoemission measurements which suggest a modification of the electronic states with the (1×2) symmetry. The resulting molecular organisation, and overall the gold surface disorganisation, prove a strong surface-molecule interaction, which may be probably be explained by a covalent bonding.

Kelvin Probe Force Microscopy in Surface Chemistry: Reactivity of Pd Nanoparticles on Highly Oriented Pirolytic Graphite

The reactivity of 10–50 nm large Pd nanoparticles (PdNPs) supported on the highly oriented pirolytic graphite (HOPG) surface has been studied under ultra-high-vacuum (UHV) conditions by monitoring the work function (WF) during Kelvin probe force microscopy. The WF of as-prepared PdNPs can vary by more than 600 meV under similar deposition conditions. Because of a chemical reaction between the PdNPs and the residual gas of the UHV at room temperature, the WF always continuously decreases until an equilibrium value is reached. We suspect carbon contamination resulting from the dissociation of CO and hydrocarbons for both phenomena. Smaller PdNPs exhibit a higher reactivity (contamination) than larger ones, and site effects can be observed. We show that annealing at high temperatures in an oxygen atmosphere is sufficient to obtain clean PdNPs, which exhibit the expected WF difference between HOPG and Pd.

Catalytic oxidation of methanol to formaldehyde by mass-selected vanadium oxide clusters supported on a TiO2(110) surface.

We report the results of a systematic study of the catalytic activity of mass-selected vanadium oxide clusters deposited on rutile TiO2 surfaces under ultrahigh vacuum (UHV) conditions. Our results show that supported V, VO, and VO2 clusters are not catalytically active for the oxidative dehydrogenation of methanol to formaldehyde but can be made catalytically active by postoxidation. In addition, we found that the postoxidized VO/TiO2 produces the most formaldehyde. Scanning tunneling microscopy (STM) imaging of the postoxidized VO/TiO2 reveals isolated clusters with height and width indicative of VO3 bound to the TiO2 surface. Our results are consistent with previous density functional theory (DFT) calculations that predict that VO3 will be produced by postoxidation of VO and that VO3/TiO2 is an active catalyst.

Microscopic surface structures and ORR activities for vacuum-deposited Pt/Ni/Pt(1 1 1) and Pt/Ni/Pt(1 1 0) sandwich structures

Pt0.3–0.6nm/Ni0.3–0.6nm/Pt(111) and Pt0.3–0.6nm/Ni0.3–0.6nm/Pt(110) atomic sandwich structures were prepared through alternating vacuum depositions of Ni followed by Pt onto clean Pt(111) and (110) substrates at room temperature under ultra-high-vacuum (UHV) conditions. After the samples were transferred from UHV to a 1-atmN2 atmosphere, their oxygen reduction reaction (ORR) activities were evaluated in O2-saturated 0.1M HClO4 at 0.9V vs. reversible hydrogen electrode. Pt0.6nm/Ni0.6nm/Pt(111) and Pt0.6nm/Ni0.6nm/Pt(110) were most active among the respective Pt/Ni/Pt(111) and Pt/Ni/Pt(110) sandwich series: the activities of the former and latter sandwich structures were approximately five- and threefold greater than those of the corresponding clean Pt(111) and (110) substrate surfaces. Scanning tunneling microscopy images of the as-prepared Pt0.6nm/Ni0.6nm/Pt(111) and Pt0.6nm/Ni0.6nm/Pt(110) surfaces revealed three-dimensionally grown hexagonal-shaped small domains of Pt(111) (approximately 2nm in size) and parallelogram-shape (110) terrace islands oriented along 〈110〉, respectively. The results indicate that not only the atomic arrangements of the topmost Pt layers but also the nanoscale morphologies of Pt–Ni in the surface vicinities determine the enhancement of the ORR activity of Pt–M alloy catalysts.