Simulations of iodine adsorbed Ge(0 0 1) surface and its STM images

A layer of iodine at Ge(0 0 1) surface develops an ordered structure of iodine atoms bound to Ge dimers. Here are discussed atomic structures of Ge(0 0 1) surface covered by 0.25 monolayer of iodine. The p(2×4), p(2×2), c(2×4) and p(1×4) surface structures are found in calculations. The structure with two iodine atoms of the dissociated I₂ molecule adsorbed at both ends of the same germanium dimer is found to be energetically favourable over iodine adsorption at neighbouring dimers. Simulated STM images of the obtained surface structures are presented and compared with experimental data.     

Diluting thiol-derivatized oligonucleotide monolayers on Au(1 1 1) by mercaptohexanol replacement reaction

Surface replacement reaction of thiol-derivatized, single-stranded oligonucleotide (HS-ssDNA) by mercaptohexanol (MCH) is investigated in order to reduce surface density of the HS-ssDNA adsorbed to Au(1 1 1) surface. Cyclic voltammograms (CVs) and scanning tunneling microscopy (STM) are employed to assess the composition and state of these mixed monolayers. It is found that each CV of mixed self-assembled monolayers (SAMs) only shows a single reductive desorption peak, which suggests that the resulted, mixed SAMs do not form discernable phase-separated domains. The peak potential gradually shifts to negative direction and the peak area increases step by step over the whole replacement process. By analyzing these peak areas, it is concluded that two MCH molecules will replace one HS-ssDNA molecule and relative coverage can also be estimated as a function of exposing time. The possible mechanism of the replacement reaction is also proposed. The DNA surface density exponentially reduces with the exposing time increasing, in other words, the replacement reaction is very fast in the first several hours and then gradually slows down. Moreover, the morphological change in the process is also followed by STM.     

One-dimensional defects in iodine adlayers on Pt(1 0 0)

One-dimensional defect structures of closed-packed adlayers of iodine on Pt(1 0 0) were studied with scanning tunneling microscopy (STM). On the terraces of the Pt(1 0 0) surface we observed rotational domains with line defects running in [0 1 0] directions, in coexistence with nearly defect-free domains. In addition to these prevailing line defects (A-defects) with a local coverage lower than that of a defect-free surface, we report on much less frequently observed line defects with higher local coverages (B- and C-defects). The strong dependence of the concentration of these defects on the adsorption temperature is governed by the decrease of the overall iodine coverage with increasing temperature. Iodine adsorption at ∼1100 K leads to self-organization of A-defects in quasi-periodic arrangements. The relevance of these defects as important structural elements of commensurate superstructures of iodine on Pt(1 0 0) is stressed.     

Structure and electronic properties of gold adsorbed on Ti(0 0 0 1)

The Au/Ti(0 0 0 1) adsorption system was studied by low energy electron diffraction (LEED) and photoemission spectroscopy with synchrotron radiation after step-wise Au evaporation onto the Ti(0 0 0 1) surface. For adsorption of Au at 300 K, no additional superstructures were observed and the (1 × 1) pattern of the clean surface simply became diffuse. Annealing of gold layers more than 1 ML thick resulted in the formation of an ordered Au-Ti surface alloy. Depending on the temperature and annealing time, three surface reconstructions were observed by LEED: (√3 × √3) R30°, (2 × 2) and a one-dimensional incommensurate (√3 × √3) rectangular pattern. The Au 4f core level and valence band photoemission spectra provided evidence of a strong chemical interaction between gold and titanium. The data indicated formation of an intermetallic interface and associated valence orbital hybridization, together with diffusion of gold into the bulk. Au core-level shifts were found to be dependent on the surface alloy stoichiometry.     

The surface structure of BaO on Pt(1 1 1): (2 × 2)-reconstructed BaO(1 1 1)

The surface structure of BaO(1 1 1) has been determined using STM and computer modelling. The BaO(1 1 1) surface was prepared in thin film form on Pt(1 1 1) and presents a surface with twice the lattice parameter expected for that of the bulk termination, i.e. a (2 × 2) reconstruction. Computer modelling indicates that the bulk termination is unstable, but that the (2 × 2) reconstructed BaO(1 1 1) surface has a low surface energy and is hence a stable surface reconstruction. The (2 × 2) reconstruction consists of small, three-sided pyramids with (1 0 0) oriented sides and either oxygen or barium ions at the apices. Less regular surface reconstructions containing the same pyramids are almost equally stable, indicating that we may also expect less regular regions to appear with a fairly random distribution of these surface species. The simulations further suggest that a regular (4 × 4) reconstruction built up of bigger pyramids is even more energetically favourable, and some evidence is found for such a structure in the STM.     

Oxygen induced facet formation on Rh(2 1 0) surface

Oxygen induced nanometer-scale faceting of the atomically rough Rh(2 1 0) surface has been studied using Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and scanning tunneling microscopy (STM). The Rh(2 1 0) surface completely covered with nanometer-scale facets when annealed at ≥550 K in the presence of oxygen. LEED studies reveal that the pyramidal faceted surface is characterized by three-sided nanoscale pyramids exposing (7 3 1), (7 3 −1) and (1 1 0) faces. A clean faceted surface was prepared through the use of low temperature surface cleaning method using the reaction with H₂ while preserving (“freezing”) the pyramidal facet structure. The resulting clean faceted surface remains stable for T ∼ 600 K and for higher temperatures; the faceted surface irreversibly relaxes to the planar surface. STM measurements confirms the formation of nanopyramids with average pyramid size ranging from 12 to 21 nm depending upon the annealing temperature. The nanopyramidal faceted Rh surface may be used as a potential template for the growth of metallic nanoclusters and for structure sensitive reactions.     

Ultra-thin Si overlayers on the TiO2 (1 1 0)-(1 × 2) surface: Growth mode and electronic properties

The growth of thin subnanometric silicon films on TiO₂ (1 1 0)-(1 × 2) reconstructed surfaces at room temperature (RT) has been studied in situ by X-ray and ultra-violet photoelectron spectroscopies (XPS and UPS), Auger electron and electron-energy-loss spectroscopies (AES and ELS), quantitative low energy electron diffraction (LEED-IV), and scanning tunneling microscopy (STM). For Si coverage up to one monolayer, a heterogeneous layer is formed. Its composition consists of a mixture of different suboxides SiO_x (1 < x ? 2) on top of a further reduced TiO₂ surface. Upon Si coverage, the characteristic (1 × 2) LEED pattern from the substrate is completely attenuated, indicating absence of long-range order. Annealing the SiO_x overlayer results in the formation of suboxides with different stoichiometry. The LEED pattern recovers the characteristic TiO₂ (1 1 0)-(1 × 2) diagram. LEED I-V curves from both, substrate and overlayer, indicate the formation of nanometric sized SiO_x clusters.     

On the vibrational behaviour of cyanide adsorbed at Pt(1 1 1) and Pt(1 0 0) surfaces in alkaline solutions

This communication deals with the vibrational behaviour of cyanide adlayers formed on Pt(1 1 1) and Pt(1 0 0) surfaces in the electrochemical environment. In situ FTIR spectroscopy can be employed to follow the potential dependence of the C-N stretching frequency in acidic electrolytes with quite a low uncertainty. Owing to the stability of the cyanide adlayer in alkaline solutions, experiments performed in NaOH medium are usually perturbed by the significant overlapping of the reference and the sample FTIR spectra. Deconvolution of the spectra was carried out assuming a Lorentz oscillator. The procedure allowed to confirm that two potential regions with different band centre frequency tuning coexist for Pt(1 1 1)-CN in perchloric acid medium. Conversely, in the alkaline electrolyte a single tuning rate for the band position was found for both surfaces studied. The lack of reorientation of the C-N molecular axis together with the occurrence of a certain screening effect of negatively charged hydroxyl anions on the electric field at the interface could be at the origin of the different behaviour displayed in both electrolytic media.     

Step flow observed on top of oxidized CoAl(1 0 0) surface

Clean and oxidized surfaces of CoAl(1 0 0) were investigated by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), high resolution electron energy loss spectroscopy (HREELS), and scanning tunnelling microscopy (STM). The regrowth or step flow of terraces was observed at 1150 K. The correlation between the growth of oxide and the step flow on the CoAl(1 0 0) surface is discussed in this paper.     

Atomic and electronic structure of methanol on Ge(1 0 0)

We have performed density-functional theory (DFT) calculations to investigate the adsorption structures of methanol on a Ge(1 0 0) surface. Among many possible adsorption configurations, the most favorable configurations at room temperature were found to be those in which the OH-dissociated methanol molecule forms O-Ge bonds, with the methoxy group either parallel or perpendicular to the Ge surface. The spatial arrangement of methoxy group relative to the Ge(1 0 0) surface is not critical. The dissociated H is bonded to an adjacent up-Ge atom, passivating the dangling bond. The possibility of H diffusion to other Ge atoms is also investigated. The corresponding simulated images explain well the adsorption features observed experimentally. The reaction pathways explain the feasibility of OH-dissociative structures at room temperature. The two OH-dissociative configurations where methoxy groups are either parallel or perpendicular to Ge surfaces are similar in thermodynamic and kinetic aspects.     

The growth and stability of Fe layers on the Mo(1 1 1) surface

The initial growth and the stability of Fe layers on the Mo(1 1 1) surface was studied with Auger electron spectroscopy, low energy electron diffraction, scanning tunneling microscopy and thermal desorption spectroscopy. At room temperature at least the first two monolayers grow layer-by-layer. The first layer is stable up to about 1200 K. Excess Fe starts to agglomerate at about 400 K and forms with increasing temperature thick flat-top islands which start to sublime at a somewhat below 1200 K. A strong decrease of the adsorption energy with coverage was found in the first monolayer. No {2 1 1} or { 1 1 0} micro-faceting could be seen at any coverage upon annealing.     

Etching reaction of methylchloride molecule on the GaAs (0 0 1)-2 × 4 surface

Adsorption process of methylchloride (CH₃Cl) on the GaAs (0 0 1)-2 × 4 surface was studied by a scanning tunnelling microscopy (STM) measurement. The arsenic rich 2 × 4 surface, which was prepared by molecular beam epitaxy (MBE), was exposed to a supersonic molecular beam of CH₃Cl with a kinetic energy of 0.06 eV. New bright spots appeared on the CH₃Cl exposed surface. They were largely observed at the “B-type” step edge and divided into two types according to their locations. It was suggested that new spots were due to weakly adsorbed CH₃Cl molecules without any dissociation. The adsorption mechanism of CH₃Cl molecule was also studied by an ab initio Hartree-Fock calculation, which explained the experimental results well.     

Ag induced restructuring of the oxygen precovered Cu(1 1 0) surface

The interplay between chemisorbed oxygen and deposited Ag on the Cu(1 1 0) surface has been studied by scanning tunneling microscopy (STM) and photoelectron emission microscopy (PEEM). The Cu-CuO stripe phase formed on the clean Cu(1 1 0) surface upon oxygen chemisorption at 660 K is partly dissolved by Ag deposition at 300 K. Upon annealing, however, a phase separation is observed, where the Cu-O compounds agglomerate into large CuO islands and the Ag is located in between. Also a strong preference for the Ag to attach to step bunches is observed. Especially on the fully (2×1)O reconstructed Cu(1 1 0) surface, all the deposited Ag is found at the step bunches giving rise to a contrast in PEEM.     

Morphology and electronic structure of bcc Co(1 1 0) and fcc/hcp Co(1 1 1) on Fe(1 1 0) investigated by STM and STS

We report on the growth of ultrathin epitaxial Co films on Fe(1 1 0) examined by scanning tunneling microscopy and spectroscopy (STM and STS). At room temperature Co forms pseudomorphic, ideally ordered body-centered cubic (bcc) layers for the first two monolayers as confirmed by atomically resolved STM images. This is in contrast to the related case of Co/Cr(1 1 0) where a superstructure occurs in the second layer. The third monolayer forms a close-packed structure and causes a transformation of the buried second monolayer into a close-packed structure. The Fe(1 1 0) substrate strongly influences the electronic structure of the first Co monolayer as concluded from the dI/dU spectra. This influence is less important for the second monolayer. The measured local density-of-states function for the bcc Co double layer is in agreement with theoretical predictions for bcc Co.     

Dynamics of confined Xe monolayers adsorbed on the Pt(997) vicinal surface

The dynamics of a confined discrete xenon monolayer adsorbed on the vicinal Pt(997) surface is analyzed within a semi-empirical potential approach. We have investigated two monolayer geometries, namely the commensurate phase and the quasi-hexagonal incommensurate phase, which seem to be stable on the (111) face of Pt, depending on the terrace size. We show that the number and the behavior of localized and side modes characteristic of the confinement are completely different in the two phases. Inelastic helium scattering experiments with a resolution better than 0.2 meV should be an efficient tool to detect the localized modes of the monolayer. This could be an additional way to elucidate the structure of Xe/Pt(997), which has not yet been well established.     

A low temperature surface preparation method for STM nano-lithography on Si(1 0 0)

Registration markers are crucial in connecting scanning tunneling microscope (STM) lithographed nano- and atomic-scale devices to the outside world. In this paper we revisit an ultra high vacuum annealing method with a low thermal budget that is fully compatible with etched registration markers and results in clean 2 × 1 reconstructed Si(1 0 0) surfaces required for STM lithography. Surface contamination is prevented by chemically stripping and reforming a protective silicon oxide layer before transferring the sample to the vacuum system. This allows for annealing temperatures of only 900 °C, where normally carbon contaminants result in the formation of SiC clusters on the surface at annealing temperatures below 950 °C. Reactive ion etched marker structures with an etch depth of 60 nm and a typical lateral dimension of only 150 nm survive a 900 °C flash anneal.     

Vibration of small molecules on Pt(1 1 1) surface

The adsorption of methanol and other small molecules onto transition metal surfaces is an important issue in electrochemistry, fuel cells, etc. Despite the overwhelming interest there are still unresolved issues beginning from the geometry of the adsorbed species to the correct assignments of different vibrational modes of the adsorbed molecules on the surface.In order to understand the adsorption processes, we have performed density functional theory (DFT) calculations for small molecules (methanol, formaldehyde, formic acid) on Pt(1 1 1) surfaces. We investigated the nature of the metal-ligand bonding in these adsorption processes using electron density difference and PDOS (partial density of states) methods. Ab initio vibration spectra have been calculated for these systems.     

A semi-empirical study of polyacene molecules adsorbed on a Cu(1 1 0) surface

To describe the adsorption of large organic molecules on metal surfaces, to calculate the corresponding diffusion and rotation barriers, the semi-empirical mono-electronic Hamiltonian of the ASED molecular orbital method have been completed to take into account three body interaction terms. The full re-parametrization of this ASED+ version of ASED was determined on the specific case of benzene adsorbed on Cu(1 1 0) and a full transferability assumed for the member of the polyacene series also adsorbed on Cu(1 1 0). The adsorption energies, geometries, diffusion and rotation barriers are very well described by this new semi-empirical technique of calculation opening the way of optimizing larger conjugated molecule on surface for uni-molecular mechanics or electronics.     

First principles studies of a Xe atom adsorbed on Nb(1 1 0) surface

We study adsorption sites of a single Xe adatom on Nb(1 1 0) surface using a density functional theory approach: the on-top site is the most favorable position for adsorption. We compare the binding features of the present study to earlier studies of a Xe adatom on close-packed (1 1 1) surfaces of face-centered cubic metals. The different features are attributed through a microscopic picture to the less than half filled d-states in Nb.