A photoelectron diffraction investigation of vanadyl phthalocyanine on Au(1 1 1)

Scanned-energy mode photoelectron diffraction using the O 1s and V 2p emission perpendicular to the surface has been used to investigate the orientation and internal conformation of vanadyl phthalocyanine (VOPc) adsorbed on Au(1 1 1). The results confirm earlier indications from scanning tunnelling microscopy that the VO vanadyl bond points out of, and not into, the surface. The VO bondlength is 1.60 ± 0.04 Å, not significantly different from its value in bulk crystalline VOPc. However, the V atom in the adsorbed molecule is almost coplanar with the surrounding N atoms and is thus pulled down into the approximately planar region defined by the N and C atoms by 0.52 (+0.14/−0.10) Å, relative to its location in crystalline VOPc. This change must be attributed to the bonding interaction between the molecule and the underlying metal surface.     

Oxidation of ultrathin indium layers on W(1 1 0)--a core-level photoemission study

We have studied the reaction of ultrathin In overlayers on W(1 1 0) with molecular oxygen at 300 K. At a coverage of 0.25 monolayers (ML) oxygen first chemisorbs dissociatively at free tungsten sites and oxidation of In occurs with some delay. At an In coverage of 1.2 ML complete oxidation of the closed overlayer is observed. Layers of 3 ML thickness first show rapid transformation from In to an In₂O₃-like species until an oxide monolayer is formed. Further oxidation occurs at much reduced rate. No oxygen-induced restructuring is observed for In at 300 K, in contrast to the response of Ag monolayers deposited on W(1 1 0).     

The adsorption of ethylene on Au/Pd(1 1 1) alloy surfaces

The adsorption of ethylene on gold-palladium alloys formed on a Pd(1 1 1) surface is investigated using a combination of temperature-programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS). Various alloy compositions are obtained by depositing four monolayers of gold on a clean Pd(1 1 1) surface and annealing to various temperatures. For gold coverages greater than ∼0.7, ethylene adsorbs primarily on gold sites, desorbing with an activation energy of less than 55 kJ/mol. At gold coverages between ∼0.5 and ∼0.7, ethylene is detected on palladium sites in a π-bonded configuration (with a σ-π parameter of ∼0.1) desorbing with an activation energy of between ∼57 and 62 kJ/mol. Further reducing the gold coverage leads to an almost linear increase in the desorption activation energy of ethylene with increasing palladium content until it eventually reaches a value of ∼76 kJ/mol found for ethylene on clean Pd(1 1 1). A corresponding increase in the σ-π parameter is also found as the gold coverage decreases reaching a value of ∼0.8, assigned to di-σ-bonded ethylene as found on clean Pd(1 1 1).     

An effective Hamiltonian for sulfur adsorption at Au(1 0 0) surface

Adsorption of sulfur at the (1 0 0) surface of gold is analyzed with the help of the density functional theory (DFT). Potential energy surface for a single S atom at the Au(1 0 0) surface is computed and a simple analytical formula was found to reproduce the ab initio results to a good accuracy. Vibration frequencies of the adsorbed S atom are computed using the harmonic approximation and the contribution of zero-point motion to the adsorption energy is evaluated. The effects of surface Au atoms relaxation in the sulfur adsorption is analyzed. The interactions between S atoms adsorbed at the nearest and the next nearest equivalent adsorption sites are computed and used to define the effective Hamiltonian describing the interactions between the adsorbed sulfur atoms.     

The adsorption of acetic acid on Au/Pd(1 1 1) alloy surfaces

The adsorption of acetic acid is studied as a function of gold content by temperature-programmed desorption and reflection-absorption infrared spectroscopy on Au/Pd(1 1 1) alloys formed by depositing 5 ML of gold onto a Pd(1 1 1) surface and heating to various temperatures. For mole fractions of gold greater than ∼0.5, acetic acid adsorbs molecularly and desorbs intact with an activation energy of ∼52 kJ/mol. This acetic acid is present as catemers, where the nature of the catemer is found to depend on gold concentration. When the relative gold concentration is less than ∼0.33, adsorption of acetic acid at 80 K and heating to ∼207 K forms η¹-acetate species on the surface. On further heating, these can either thermally decompose to eventually evolve hydrogen, water and oxides of carbon, or form η²-acetate species, where the coverage of reactively formed η²-acetate species increases with decreasing gold concentration in the near surface region.     

CO adsorption on Au(3 1 0) and Au(3 2 1): 6-Fold coordinated gold atoms

The adsorption of CO on Au(3 1 0) and Au(3 2 1) was studied using a combination of thermal desorption spectroscopy and high resolution core level photoemission spectroscopy. These vicinal Au surfaces both have 6-fold coordinated atoms at the step edges but have a different terrace structure. The CO adsorption behavior was found to be very similar for both surfaces. Three different desorption peaks due to chemisorbed CO were identified, which desorb around 100 K(α), 120 K(β) and 180 K(γ), respectively. The C1s and O1s spectra of the chemisorbed CO show a complex shake-up structure. Our experimental results indicate that CO only adsorbs on the step atoms. The different desorption peaks are explained by substrate-mediated long-range interactions between the adsorbates. Comparison with literature results shows that the CO adsorption energy is not only dependent on the coordination number of the Au atoms, but that the exact geometrical structure of the surface also plays a role.     

Adsorption of d-alaninol on Cu(1 0 0)

Adsorption of d-alaninol on Cu(1 0 0) at room temperature has been investigated by photoelectron spectroscopy in the soft X-ray and VUV energy range and low energy electron diffraction (LEED). d-Alaninol was found by LEED to self-assemble at full coverage; core and valence photoemission spectra are presented at low and full coverage. Chemisorption occurs at room temperature. The bonding at low coverage takes place at the hydroxylic group; at full coverage there is evidence of bonding for both hydroxylic and amino groups.     

Oxygen adsorption on a Mo2C(0 0 0 1) surface: Angle-resolved photoemission study

Oxygen adsorption on a C-terminated α-Mo₂C(0 0 0 1) surface has been investigated with Auger electron spectroscopy, low-energy electron diffraction, and angle-resolved photoemission spectroscopy utilizing synchrotron radiation. It is found that the oxygen atoms adsorb on the Mo atoms in the second layer forming a (1 × 1) orthorhombic periodicity. The oxygen adsorption induces a peculiar state around the Fermi level, which is observed at 0.4 eV in the normal-emission spectra. ARPES measurements show that the state is a partially occupied metallic state. The photoionization cross section of the state shows a maximum at the photon energy of 56 eV, which is assigned as originating from the resonance of the Mo 4d photoemission involving Mo 4p → 4d photoexcitation.     

The adsorption of methylcyclopentane on Pt(1 1 1)

The adsorption of methylcyclopentane (MCP) on Pt(1 1 1) has been studied using the atom superposition and electron delocalization (ASED-MO) molecular orbital method. Results show a weak interaction with the metallic surface. The adsorption energy is rather independent of the adsorption site coordination number. We find that Pt 6s, 6p_z and 5d_z² orbitals are involved in the bonding with MCP. There is no bonding between the carbon ring and the Pt surface and the interaction comes from the hydrogen atoms to the surface.     

Chemisorption of alkyl thiols and S-alkyl thiosulfates on Pt(1 1 1) and polycrystalline platinum surfaces

The self-assembled monolayers prepared from 1-dodecanethiol (C₁₂SH) or S-dodecylthiosulfate (Bunte salt, C₁₂SSO₃Na) have been characterised on polycrystalline gold and platinum surfaces and on Pt(1 1 1). Contact angle and impedance measurements show that the film quality decreases in the order Au/C₁₂SH > Pt/C₁₂SH ∼ Au/C₁₂SSO₃Na > Pt/C₁₂S SO₃Na. XPS measurements show that the S-SO₃ bond of organic thiosulfates is broken on platinum surfaces and the state of the surface-bound sulfur is indistinguishable from that of thiolate. On platinum three sulfur species are formed upon SAM formation and we suggest that the catalytic activity of platinum is responsible for their existence in pristine monolayers.     

A scanning tunnelling microscopy investigation of gold island formation from an octanethiol self-assembled monolayer on Au(1 1 1)

The release of gold atoms from an octanethiol monolayer on Au(1 1 1) and the subsequent formation of single-layer-high gold islands have been investigated using a scanning tunnelling microscope (STM) in air. When the bias voltage between the STM tip and the sample is above the threshold for water electrolysis, reactive desorption of the thiol molecules takes place leading to the release of gold adatoms. The number of released atoms has been evaluated as a function of exposure to the tip current under both positive and negative bias voltages. Tip-induced ripening of the gold islands, and more interestingly, tip-induced disintegration of small islands are observed.     

Supramolecular assembly of biphenyl dicarboxylic acid on Au(1 1 1)

We investigate the structure of submonolayer film of 4,4′-biphenyl dicarboxylic acid (BDA) molecules on Au(1 1 1)-22 × √3 reconstructed surface with the use of scanning tunneling microscopy (STM). The BDA molecules form ordered structures on Au(1 1 1) surface which are commensurate with the substrate. We have concluded that the molecule-molecule interaction is mainly through hydrogen bonding formed by a straight dimer of BDA molecules. The straight dimer can be expressed as 4s + 2t or its six crystallographic equivalents using the unit vectors of the gold substrate of s and t. The length of hydrogen bonding (O-H-O) is estimated to be 0.31 nm assuming nearest neighbor distance of gold atoms of 0.275 nm. The ordering shows a clear contrast with the case of BDA on Cu(1 0 0) surface [S. Stepanow, N. Lin, F. Vidal, A. Landa, M. Ruben, J.V. Barth, K. Kern, Nanoletters 5 (2005) 901] in which a square type of ordering of molecules is observed by the formation of hydrogen bonding between a carboxylate (COO) and a benzene ring. The clear difference of the ordered structure on Cu(1 0 0) and Au(1 1 1) surface demonstrates that the absence (presence) of deprotonation of carboxyl group of BDA molecule on Au(1 1 1) (Cu(1 0 0)) switches the straight and square type ordering of BDA molecules.     

Structural stability and electronic structure of iron adsorption on the GaN(0 0 0 1) surface

In this work, we have investigated by means of first-principles spin-polarized calculations, the electronic and magnetic properties of iron (Fe) adsorption and diffusion on the GaN(0 0 0 1) surface using density functional theory (DFT) within a plane-wave pseudopotential scheme. In the surface adsorption study, results show that the most stable positions of a Fe adatom on GaN(0 0 0 1) surface are the H3 sites and T4 sites, for low and high Fe coverage respectively. We found that the Fe-H3 2 × 2 surface reconstruction exhibits a half-metallic behavior with a spin band gap and stable ferromagnetism ordering, which is a desirable property for high-efficiency magnetoelectronic devices. In addition, confirming previous experimental results, we found that the iron monolayers present a ferromagnetic order and a large thermal stability. This is interesting from a theoretical point of view and for its technological applications.     

Anchoring sulphur-headgroup organic molecules at Cu(1 0 0): Tailoring the interface electronic states

Sulphur-headgroup organic molecules have been chemisorbed on Cu(1 0 0) as self-assembled monolayers (SAMs) in highly-ordered two-fold symmetry structures, and the electronic states induced at the interface have been measured by photoemission: a close similarity of the main interface states for methane-thiolate and mercaptobenzoxazole on Cu(1 0 0) in the same p(2 × 2)-phase is observed. The bonding states for methane-thiolate/Cu(1 0 0) in the p(2 × 2) and c(2 × 2) structures have been compared to ab-initio calculation of the total density of states (DOS) for the S/Cu(1 0 0) system in the same phases. The major role of the S-Cu bonding to determine the density of state evolution at the interface is brought to light. The observed differences in the two phases depend mainly on the charge distribution associated to the different molecular packing, with a minor role of the radical group.     

Long range one-dimensional ordering of lead phthalocyanine monolayer on InSb(1 0 0) (4 × 2)/c(8 × 2)

Sub-monolayer and monolayer of lead phthalocyanine deposited on InSb(1 0 0) (4 × 2)/c(8 × 2) surface have been investigated by scanning tunneling microscopy and low energy electron diffraction. Molecules first adsorb on the indium rows of the (4 × 2)/c(8 × 2) structure in the [1 1 0] direction and diffuse at the surface in order to form two-dimensional islands. The molecule-substrate interaction stabilizes the PbPc molecules on the In rows. It weakens the interaction between molecules located in adjacent rows resulting in numerous gliding planes between the molecular chains, in the direction parallel to the rows. At monolayer completion, a long-range one-dimensional order is adopted by the molecules in the [1 1 0] direction.     

Theoretical study on adsorption of thiophenethiolate molecule on Au(1 1 1) surface

We have theoretically studied the adsorption of a thiophenethiolate (C₄H₃S-S) molecule on the Au(1 1 1) surface by first-principles calculations. It is found that the bridge site is the most stable adsorption site with the adsorption energy of 1.02 eV. In the optimized adsorption geometry, the bond between the head S atom and the connected C atom in the tail thiophene molecule is tilted by 57.2° from the surface normal. In addition, the adsorption of thiophenethiolate induces large relaxations of the surface Au atoms around it. Furthermore, weak interactions between the S atom in the tail thiophene ring and the Au atoms also contribute to the adsorption on the Au surface.     

Adsorption and structure of methyl mercaptoacetate on Cu(1 1 1) surface by XPS and NEXAFS spectroscopy

Methyl mercaptoacetate (MA) on Cu(1 1 1) surface was investigated using synchrotron radiation-based X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. MA adsorbs on the surface via thiolate formation and weak interaction of the carbonyl group with the surface. Two different adsorption states previously reported for methanethiolate and ethanethiolate were confirmed, besides atomic sulfur. NEXAFS measurements support gauche-type conformation of MA whose skeleton lies on the surface.     

Density-functional study of the CO adsorption on ferromagnetic Co(0 0 0 1) and Co(1 1 1) surfaces

The regular CO overlayers at coverage θ = 1/3 adsorbed on the (0 0 0 1) surface of hcp Co and (1 1 1) surface of fcc Co are studied by first-principles density-functional theory with the exchange-correlation component in the PBE form. Adsorption in atop, bridge, and three-fold hcp or fcc position are considered. The adsorption energies, CO stretching frequencies, geometry, work function, and local magnetic moments are studied, and, when possible, compared with experimental or theoretical data. Particularly, we show that the recently proposed correction to adsorption energy of CO prefers correctly the atop adsorption site, whereas the remaining sites are almost degenerate in energy. The CO molecule lowers magnetization on neighbouring Co atoms, and the effect decreases with the adsorption site coordination. We show, however, that this trend is not the result of the different C-Co separation at different adsorption sites. A very small magnetic moment appears on CO that couples antiferromagnetically to Co. Most results are very similar for the Co(0 0 0 1) and Co(1 1 1) surfaces.     

Theoretical investigation of the Au/Si(1 1 1)-(5 × 2) surface structure

The atomic structure of the Au/Si(1 1 1)-(5 × 2) surface has been studied by density-functional theory calculations. Two structure models, proposed experimentally by Marks et al. and Hasegawa et al., have been examined on an equal ground. In our total-energy calculations, both models are found to be locally stable and energetically comparable. In our electronic-structure analyses, however, both models fail to reproduce the key features of angle-resolved photoemission spectra and scanning-tunneling-microscopy images, indicating that the considered models need to be modified. Suggestions for the modification are given based on the present calculations.