Structure of monolayer silica on Mo(1 1 2) investigated by rainbow scattering under axial surface channeling

The structure of a monolayer crystalline silica film grown on a Mo(1 1 2) substrate is investigated via grazing scattering of fast atoms. For scattering along low indexed directions in the surface plane (“axial surface channeling”) the corrugation of the interaction potential leads to an azimuthal out-of plane scattering with an intensity enhancement for the maximum deflection angle, the so called “rainbow”. From the comparison of the experimental angular distributions for scattered projectiles with classical trajectory simulations we obtain information on the arrangement of atoms in the topmost surface layer. Our work provides evidence for the structural model of a two-dimensional network for the silica film.     

First principle study of a bimolecular thin film on Ag(1 1 1) surface

The formation of melamine–PTCDI bimolecular networks deposited on Ag(1 1 1) is studied by means of first principle calculations. Emphasis is placed on the interplay of the inter-molecular hydrogen bonds and the molecule–substrate contacts. Our simulations show rather strong distortions of the adsorbed molecules near the contact points due to the influence the hydrogen bonds. Despite this, the charge transfer from the substrate to a PTCDI molecule remains almost the same (0.9 e⁻) as obtained for an isolated PTCDI molecule. A detailed analysis of the topological features of the electronic density reveals that the charge transfer modifies the two types of hydrogen bonds in opposite ways, weakening the central bond and strengthening the two lateral ones, while roughly keeping a constant binding energy. Altogether, the influence of the substrate on the molecular network is proved to be weak.     

Nanostructuring-induced modification of optical properties of p-GaAs (1 0 0)

A pulsed anodic etching method has been utilized for nanostructuring of p-type GaAs (1 0 0) surface, using HCl-based solution as electrolyte. The resulting porous GaAs layer is characterized by atomic force microscopy (AFM), room temperature photoluminescence (PL), Raman spectroscopy and optical reflectance measurements. AFM imaging reveals that the porous GaAs layer is consisted of a pillar-like of few nm in width distributed between more-reduced size nanostructures. In addition to the “infrared” PL band of un-etched GaAs, a strong “green” PL band is observed in the etched sample. The broad visible PL band of a high-energy (3.82 eV) excitation is found to compose of two PL band attributed to excitons confinement in two different sizes distribution of GaAs nanocrystals. The quantum confinement effects in GaAs nanocrystallites is also evidenced from Raman spectroscopy through the pronounced appearance of the transverse optical (TO) phonon line in the spectra of the porous sample. Porosity-induced a significant reduction of the specular reflection, in the spectral range (400–800 nm), is also demonstrated.     

Chemisorption of isocyanate (NCO) on the Pd(1 0 0) surface at different coverages

The chemisorption of isocyanate (NCO) species on Pd(1 0 0) was studied within the density functional formalism (DFT) using a periodic slab model. The NCO was adsorbed on top, bridge and hollow sites of the metal surface at different coverages. At low coverages, the adsorption energies are in the range of ?2.5/?3.0 eV, indicating an important adsorbate–substrate interaction for the three sites studied. The lateral repulsive interaction between neighboring NCO species is almost negligible or weak at lower and intermediate coverages, and very strong at complete monolayer. At low coverages, both bridge and hollow sites are energetically preferred; yet the bridge site becomes the only favoured site at intermediate and complete coverages. Work function and dipole moment calculations can be interpreted by an important charge transfer from the metal surface to NCO. Interestingly, while on hollow site the charge taken by NCO is essentially the same over all the range of coverage, an increasing depolarization is observed on bridge and top sites as the coverage increases. Symmetric and asymmetric NCO stretching modes were also calculated and compared with recent infrared spectroscopy measurements.     

DFT calculations of adenine adsorption on coin metal (1 1 0) surfaces

Density functional theory is used to analyze in detail the adsorption of the adenine molecule on the (1 1 0) surfaces of Cu, Ag, and Au. While the adsorption configurations are similar in all three cases – the molecule bonds via two nitrogen atoms to the substrate – the details like charge transfer or local strain a rather different. The molecule–substrate interaction in case of Cu is stronger than for the more noble metals Ag and Au. Longe-range dispersion forces stabilize the adsorption configuration in dependence on the specific adsorption geometry. In case of Ag and Au, relativistic effects are found to be important.     

Density functional calculations for Ni adsorption on Al(1 1 0)

The adsorption of 0.25, 0.5 and 1 monolayer (ML) of the transition metal Ni on the metal substrate Al(1 1 0) was studied using first-principles calculations at the level of density functional theory. The metal–metal system was analyzed with the generalized gradient approximation. Four stable atomic configurations were considered, and the optimized geometries and adsorption energies of different Ni adsorption sites on the Al(1 1 0) surface at selected levels of coverage were calculated and compared. The four-fold hollow site was determined to be the most stable adsorption site with adsorption energy of 5.101 eV at 0.25 ML, 3.874 eV at 0.5 ML and 3.665 eV at 1 ML. The adsorption energies of the four sites slightly decreased as the Ni coverage increased. Work function analysis showed that when Ni is adsorbed on the Al(1 1 0) surface, the work function decreased as the coverage increased due to depolarization. The Mulliken population and density of states were calculated to determine the charge distribution of the adsorption site, confirming that a chemisorption interaction exists between the adsorbed Ni atom and Al(1 1 0) surface atoms.     

Observation of charge transfer states of F4-TCNQ on the 2-methylpropene chemisorbed Si(1 0 0)(2 × 1) surface

We have investigated the valence electronic states of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄-TCNQ) on the 2-methylpropene chemisorbed Si(1 0 0)(2 × 1) surface using valence photoelectron spectroscopy. Since the electron affinity of condensed F₄-TCNQ is 5.24 eV and the energy from the valence band maximum of the 2-methylpropene saturated Si(1 0 0)(2 × 1) surface to the vacuum level is 4.1 eV, spontaneous charge transfer would be expected in the present system. At sub-monolayer coverage of F₄-TCNQ, characteristic peaks are observed at 1.1 and 2.5 eV below Fermi energy. The former peak is assigned to a singly occupied affinity level, and the latter is ascribed to a relaxed highest occupied molecular orbital of adsorbed F₄-TCNQ. The work function change is increased up to +1.3 eV as a function of F₄-TCNQ coverage. These results support the occurrence of charge transfer into F₄-TCNQ on the 2-methylpropene saturated Si(1 0 0)(2 × 1) surface.     

High resolution XPS studies on hexadecafluoro-copper-phthalocyanine deposited onto Si(1 1 1)7×7 surface

High resolution X-ray photoelectron spectroscopy measurements have been performed onto ultrathin films of hexadecafluoro copper phthalocyanine deposited, at room temperature and in ultrahigh vacuum conditions, onto clean Si(1 1 1)7×7 substrate (silicon, Si). The measurements, performed at various film thicknesses, show a strong interaction between the molecule and the Si substrate. All the core level peaks present strong modifications induced by the substrate interaction. In particular the fluorine (F) spectrum clearly presents the effect of the interaction of some F atoms of the molecule with the substrate, which determines the formation of F–Si bonds while the copper spectrum indicates a charge transfer from the Si substrate. The changes observed in the other core level spectra have been attributed to a different charge distribution in the molecule, after the formation of F–Si bonds. We suggest a planar growth of these molecules on the Si substrate starting from the first layer.     

Entropy-driven adsorption of carbon nanotubes on (0 0 1) and (1 1 1) surfaces of CeO2 islands grown on sapphire substrate

According to the aim to compose combinatorial material by adsorption of carbon nanotubes onto the structured CeO₂ surface the interaction of the armchair (5,5) and zigzag (8,0) nanotubes with the (0 0 1) and (1 1 1) surfaces of CeO₂ islands have been investigated by theoretical methods. The thermodynamics of the adsorption were studied at the low surface coverage region. The interaction energy between the nanotube and the different CeO₂ surfaces shows significant increase when the size of the interface reaches 7–8 unit cells of CeO₂ and it remains unchanged in the larger interface region. However, the entropy term of the adsorption is significantly high when the distances of CeO₂ islands are equal to 27 nm (adsorption of armchair (5,5) nanotube) or 32 nm (adsorption of zigzag (8,0) nanotube). This property supports adsorption of nanotubes onto CeO₂ surfaces which possesses a very specific surface morphology. A long-wave vibration of nanotubes was identified as background of this unexpected phenomenon. This observation could be applicable in the development of such procedures where the nanotube adsorption parallel to the surface is aimed to perform.     

Nucleation, growth and morphology of Co thin films on Ag(1 1 0)

Scanning tunneling microscopy (STM) experiments reveal that Co growth on Ag(1 1 0), at coverages of Co < 1 ML and low substrate temperatures (150 K), involves a concomitant insertion of Co into the top Ag layer and exchange of Ag out onto the surface. At 300 K, coverages of Co > 1 ML gives rise to a 3D nanocluster growth on the surface, with the clusters covered by Ag. Depending slightly on coverage, the clusters have a typical diameter of 3 nm and a height of 0.4 nm. Upon annealing to 500 K, major changes are observed in the morphology of the surface. STM and AES show that there is a reduction of the number of Co islands on the surface, partly due to subsurface Co cluster migration and partly due to sintering into larger clusters.     

The origin of inter-dimer-row correlated adsorption for NH3 on Si(0 0 1)

We discuss the interaction between adsorbing ammonia molecules and pre-adsorbed ammonia fragments on the Si(0 0 1) surface, searching for experimental evidence of a H-bonded precursor state predicted by modelling. While correlations along dimer rows have already been identified, these mix substrate-mediated effects due to dimer buckling with ammonia–adsorbate effects. Correlations between fragments on neighbouring dimer rows are not affected by substrate effects (in this system), allowing an analysis of direct ammonia–adsorbate effects. We present an analysis of cross-row correlations in existing high-coverage STM data which shows significant correlations between NH₂ groups on neighbouring dimer rows over a significant range, providing evidence for the H-bonded precursor state with a range of around 10 Å. We discuss implications for the interpretation of STM images of ammonia on Si(0 0 1).     

Misfit driven azimuthal orientation of NaCl domains on Ag(1 0 0)

We investigated the growth of thin NaCl films on Ag(1 0 0) by spot-profile-analysis low energy electron diffraction (SPA-LEED), varying extensively the growth temperature (200–500 K) and the film thickness (0.5–14 ML). The incommensurate growth of NaCl on Ag(1 0 0) yields (1 0 0)-terminated epitaxial NaCl domains, which are preferentially oriented with their [0 1 0] axis parallel to that of the substrate. At 300 K, the NaCl domains exhibit an azimuthal mosaicity by 14° around this orientation and the NaCl unit cell is laterally contracted in the first layers by 0.9% with respect to the bulk. At higher growth temperatures, the azimuthal mosaic distribution sharpens and additional distinct orientations appear, presumably due to a higher-order commensurability. The evolution of the azimuthal mosaic distribution with increasing temperature can be ascribed to both the NaCl thermal expansion and higher diffusion rates of NaCl on Ag(1 0 0). The best epitaxy, i.e. that with the highest selectivity of a specific azimuthal domain orientation, is achieved by growing NaCl films at low deposition rate (0.1 ML min⁻¹) on the Ag(1 0 0) substrate at constant high temperature (450–500 K). The observations made here can probably be applied more generally to other heterogeneous interfaces and, in particular, be used to improve the quality of thin insulating films.     

Hydrazine (N2H4) adsorption on Ni(1 0 0) – Density functional theory investigation

A theoretical study on the structure and adsorption mechanism of hydrazine (N₂H₄) on Ni(1 0 0) are presented. The hydrazine molecule was found to adsorb on the surface through one of its nitrogen atom in its anti-conformation. The charge transfer from hydrazine lone pair orbitals played a key role in the formation of the bonding. The mechanism involved in the bonding was found to reduce the necessity of hyper-conjugation interaction, that reduces the gauche effect found in hydrazine at the gas-phase. Upon adsorption to the surface, the reduced interaction resulted in the promotion of a more favored conformation through its anti-conformation.     

Density functional study of the adsorption of aspirin on the hydroxylated (0 0 1) -quartz surface

In this study the adsorption geometry of aspirin molecule on a hydroxylated (0 0 1) α-quartz surface has been investigated using DFT calculations. The optimized adsorption geometry indicates that both, adsorbed molecule and substrate are strongly deformed. Strong hydrogen bonding between aspirin and surface hydroxyls, leads to the breaking of the original hydroxyl–hydroxyl hydrogen bonds (Hydrogenbridges) on the surface. In this case new hydrogen bonds on the hydroxylated (0 0 1) α-quartz surface appear which significantly differ from those at the clean surface. The 1.11 eV adsorption energy reveals that the interaction of aspirin with α-quartz is an exothermic chemical interaction.     

Simulations of germanium epitaxial growth on the silicon (1 0 0) surface incorporating intermixing

We present kinetic lattice Monte Carlo simulations of Ge deposition onto a reconstructed Si (1 0 0) surface. We account for the exchange of Ge with Si atoms in the substrate, considering two different exchange mechanisms: a dimer exchange mechanism whereby Ge–Ge dimers on the surface become intermixed with substrate Si atoms, and the exchange of Ge atoms below the surface to relieve misfit strain. We examine how Si–Ge exchange affects the interface between the materials when the growth simulations are done at different temperatures.     

Electronic charge transfer between Au nano-particles and TiO2-terminated SrTiO3(0 0 1) substrate

The interaction between Au nano-particles and oxide supports is recently discussed in terms of the catalytic activities. This paper reports the electronic charge transfer between Au nano-particles and TiO₂-terminated SrTiO₃(0 0 1) substrate, which is compared with that for stoichiometric(S)-, pseudo-stoichiometric(S¹)- and reduced(R)-TiO₂(1 1 0) supports. We observed the photoelectron spectra of Au 4f, O 2s, Ti 3p, and Sr 4p lines and also measured the work functions for Au/oxides supports using synchrotron-radiation light. As the results, all the O 2s, Ti 3p, and Sr 4p lines for Au/SrTiO₃(0 0 1) show lower binding energy shifts in a quite same manner and abrupt increase in the work function is seen in an initial stage. This clearly evidences an electronic charge transfer from the substrate to Au probably due to a much larger work function of Au than SrTiO₃(0 0 1), which leads to an upward band bending (0.3 eV) just like a Schottky contact. Electronic charge transfers also take place at Au/S- and Au/S¹-TiO₂(1 1 0) and Au/R-TiO₂(1 1 0) interfaces, where electrons are transferred from Au to S- and S¹-TiO₂ and from R-TiO₂ to Au, as predicted by ab initio calculations.     

Point defects along metallic atomic wires on vicinal Si surfaces: Si(5 5 7)–Au and Si(5 5 3)–Au

Point defects on the metallic atomic wires induced by Au adsorbates on vicinal Si surfaces were investigated using scanning tunneling microscopy and spectroscopy (STM and STS). High-resolution STM images revealed that there exist several different types of defects on the Si(5 5 7)–Au surface, which are categorized by their apparent bias-dependent images and compared to the previous report on Si(5 5 3)–Au [Phys. Rev. B (2007) 205325]. The chemical characteristics of these defects were investigated by monitoring them upon the variation of the Au coverage and the adsorption of water molecules. The chemical origins and the tentative atomic structures of the defects are suggested as Si adatoms (and dimers) in different registries, the Au deficiency on terraces, and water molecules adsorbed dissociatively on step edges, respectively. STS measurements disclosed the electronic property of the majority kinds of defects on both Si(5 5 7)–Au and Si(5 5 3)–Au surfaces. In particular, the dominating water-induced defects on both surfaces induce a substantial band gap of about 0.5 eV in clear contrast to Si adatom-type defects. The conduction channels along the metallic step-edge chains thus must be very susceptible to the contamination through the electronic termination by the water adsorption.     

First-principles study of the pentacene/Cu(1 1 1) interface: Adsorption states and vacuum level shifts

We have studied the interaction of pentacene with a Cu(1 1 1) surface using density functional theory (DFT) within a generalized gradient approximation (GGA) and the van der Waals density functional [vdW-DF, M. Dion, H. Rydberg, E. Schröder, D.C. Langreth, B.I. Lundqvist, Phys. Rev. Lett. 92 (2004) 246401]. The adsorption energy is accurately predicted by vdW-DF, while the equilibrium distances between pentacene and the metal substrate (Z_C) are overestimated by both GGA and vdW-DF. The work function changes depend significantly on Z_C. The experimental work function change can be successfully reproduced by GGA if the experimentally reported adsorption geometry is used, whereas the magnitude of the work function change is underestimated if calculated adsorption geometries are applied. We examined the IDIS model [H. Vázquez, R. Qszwaldowski, P. Pou, J. Ortega, R. Pérez, F. Flores, A. Kahn, Europhys. Lett. 65 (2004) 802] to compare it with the GGA results. The interface dipoles estimated by the IDIS model fairly agree with the GGA results, provided that the adsorption distance is large. On the other hand, they tend to deviate from the GGA results as the adsorption distance becomes smaller, where back donation from the metal surface to the adsorbate occurs. Our analysis reveals that at experimentally reported metal–organic distance, back donation is significant enough to induce polarization of pentacene molecules perpendicular to the surface, which leads to a reduction of the work function. Thus, at the experimentally reported metal–organic distance, the work function change estimated by a simple IDIS model deviates from that calculated by self-consistent GGA calculations. We also found that at the experimentally reported metal–organic distance, the transferred electrons create weak chemical bonds between pentacene and the Cu(1 1 1) surface, illustrating the reactive nature of pentacene.     

Theoretical studies on the near edge X-ray absorption fine structure spectra of a hexanethiolate monolayer on Ag(1 0 0)

A theoretical study on the S K-edge near edge X-ray absorption fine structure (NEXAFS) of a hexanethiolate monolayer on Ag(1 0 0) has been performed by employing the multiple-scattering cluster (MSC) method. The unoccupied molecular orbitals of the system, which are closely correlated with resonances of the NEXAFS spectra, have been calculated by using the discrete variational (DV)-Xα method. The physical origins of the resonances are elucidated by these theoretical studies. It has been shown that the leading peak at 2470.3 eV is not a π*(S–C), but a resonance corresponding to the transition of 1s electrons into a hybrid orbital of the S(3p) atomic orbital of a hexanethiolate molecule and Ag atomic orbitals. The interaction between the adsorbate and the substrate induces other two weak resonances at 2475.2 and 2478.2 eV in the NEXAFS. The adsorption structure of a hexanethiolate monolayer on Ag(1 0 0) deduced from the theoretical analysis on the NEXAFS is in agreement with that from the SEXAFS of the system.