First-principles calculations of hydrogen molecule adsorption on Ti (0 0 0 1)-(2 × 1) surface

Adsorption of H₂ molecule on the Ti (0 0 0 1)-(2 × 1) surface was studied by density functional theory with generalized gradient approximation (GGA). The parallel and vertical absorption cases were investigated in detail by adsorption energy and electronic structure analysis, we obtained three stable configurations of FCC-FCC (the two H atoms adsorption on the two adjacent fcc sites of Ti (0 0 0 1) surface, respectively), HCP-HCP (the two H atoms adsorption on the two adjacent hcp sites of Ti (0 0 0 1) surface, respectively) and FCC-HCP (the one H atom adsorption on the fcc site and the other adsorption on the near hcp site) based on the six different parallel adsorption sites after the H₂ molecule dissociates. However, all the end configurations of four vertical adsorption sites were unstable, H₂ molecule was very easy to desorb from Ti surface. The H-H bond breaking and Ti-H bond forming result from the H₂ molecule dissociation. H-H bond breaking length ranges from 1.9 Å to 2.3 Å for different adsorption configurations due to the strong Ti-H bond forming. The H₂ dissociative approach and the end stable configurations formation in parallel adsorption processes are attributed to the quantum mechanics steering effects.     

Study on the adsorption of fluorescein on Ag(1 1 0) substrate

The adsorption of fluorescein on the Ag(1 1 0) surface has been investigated by the first-principles pseudopotential method. Various adsorption geometries have been calculated and the energetically most favorable structure of fluorescein/Ag(1 1 0) was identified. The fluorescein molecule, in most favorable structure, is on hollow site, and the adsorption energy is 2.34 eV. Here the adsorption sites refer to the positions at the first layer of the substrate where the middle carbon atom of the fluorescein molecule is located. The bonding strength of the fluorescein molecule to the Ag substrate is site selective, being determined by electron transfer to the oxygen atoms of the molecule and local electrostatic attraction between the oxygen atoms and the silver atoms.     

The adsorption of In on the surface of (0 0 1) CdTe

To understand CdTe doping with In, first-principle calculations are performed to obtain the various kinds of surface-structure for In on CdTe (0 0 1) surface. Of all the structures examined, the structure of CdTe (0 0 1) as caused by In adsorption atoms at the fourfold hollow sites with 0.25 monolayer coverage is the most energetically favorable. In atoms are adsorbed on the Cd-terminated surface, whereas below the Te-terminated surface. For the Cd-terminated surface, cadmium vacancy can form spontaneously and is energetically favorable. In atoms are likely to be adsorbed/incorporated at an interstitial site on Te-terminated CdTe (0 0 1) surfaces for most of the range of the chemical potential.     

Study of arsenic for antimony exchange at the Sb-stabilized GaSb(0 0 1) surface

In this paper we present a first-principle study on the energetics of a single As₂ molecule on GaSb(0 0 1) reconstructed surface. In order to shed light into the mechanisms of anion exchange at the Sb-rich GaSb(0 0 1) surface, we studied firstly As₂ adsorption and then As for Sb exchange. We identify a surface region where both the processes are energetically favored. The results of this twofold analysis can be combined to derive possible reaction paths for the anion exchange process.     

First-principles calculations of ethanethiol adsorption and decomposition on GaN (0 0 0 1) surface

The adsorption and decomposition of ethanethiol on GaN (0 0 0 1) surface have been investigated with first-principles calculations. The DFT calculations reveal that ethanethiol adsorbs dissociatively on the clean GaN (0 0 0 1) surface to form ethanethiolate and hydrogen species. An up limit coverage of 0.33 for ethanethiolate monolayer on GaN (0 0 0 1) surface is obtained and the position of the sulfur atom and the tilt angle of the thiolate chain are found to be very sensitive to the surface coverage. Furthermore, the reactivity of ethanethiol adsorption and further thermal decomposition reactions on GaN (0 0 0 1) surface is discussed by calculating the possible reaction pathways and ethene is found to be the major product.     

Stable reconstruction and adsorbates of InAs(1 1 1)A surface

We investigated the surface properties of InAs(1 1 1)A by low-temperature scanning tunneling microscopy (LT-STM) with atomic resolution and first-principles calculation. Very clear atom image was observed, showing that the surface reconstruction is an In-vacancy structure. We also observed two kinds of adsorbates on the surface. The first-principles calculations indicate that the In-vacancy structure is the most stable surface reconstruction under any experimental conditions, which is consistent with the LT-STM observation. Investigations of adsorption properties of an In atom, an As atom, and an As₂ molecule by the first-principles calculations imply that the observed adsorbates are an In atom and an As₂ molecule.     

Adsorption of Al on the Si(0 0 1) surface

The first-principles calculations have been presented to study the adsorption of aluminum (Al) on the Si(0 0 1)(2×1) surface. We have investigated the optimized geometries and electronic structures of the adatom-substrate system. The adsorption energy of the system has been calculated. The most stable adsorption sites were consequently determined to be HH site and T3+T4. It is shown that the Si-Si dimer is asymmetric on the reconstructed bare surface and become symmetric upon Al adsorption. In addition, the bond length of Si-Si was found to be considerably elongated in the adsorption system. It is found that the work function change obtained in our work is different from other previous results on the adsorption of alkali metals on the Si(0 0 1) surface. In order to investigate the relative stability of phases at different coverages, the surface formation energy of the adsorption system was calculated. To shed light on the nature of the Al-Si bond and the character of silicon surface, the density of states (DOS) and difference charge density of the system were evaluated.     

Charge transfer in the atomic structure of Ge (1 0 5)

The atomic structure and charge transfer on the Ge (1 0 5) surface formed on Si substrates are studied using scanning tunneling microscopy and spectroscopy (STM and STS). The bias-dependent STM images of the whole Ge (1 0 5) facets formed on a Ge “hut” structure on Si (0 0 1) are observed, which are well explained by the recently confirmed structure model. The local surface density of states on the Ge (1 0 5) surface is measured by STS. The localization of the electronic states expected from charge transfer mechanism is observed in the dI/dV spectra. The surface band gap is estimated as 0.8-0.9 eV, which is even wider than the bulk bandgap of Ge, indicating the strong charge transfer effect to make the dangling bonds stable. The shape of normalized tunnel conductance agrees with the theoretical band structure published recently by Hashimoto et al.     

Atomic structure of the carbon induced Si(0 0 1)-c(4 × 4) surface

The atomic and electronic structures of the Si(0 0 1)-c(4 × 4) surface have been studied by scanning tunneling microscopy (STM) and density functional theory (DFT). To explain the experimental bias dependent STM observations, a modified mixed ad-dimer reconstruction model is introduced. The model involves three tilted Si dimers and a carbon atom incorporated into the third subsurface layer per c(4 × 4) unit cell. The calculated STM images show a close resemblance to the experimental ones.     

Barrier height imaging of Si(1 1 1)3 × 1-Ag reconstructed surfaces

We investigated the bias voltage polarity dependence of atomically resolved barrier height (BH) images on Si(1 1 1)3 × 1-Ag surfaces. The BH images were very similar to scanning tunneling microscopy (STM) images in both the empty and filled states. This similarity strongly supports the interpretation that the BH image reflects the vertical decay rate of the surface local density of states (LDOS). Differences in contrast and protrusion shapes between BH and STM images were observed. We attributed these differences to the geometric contribution to the STM image and to the improved spatial resolution of the BH image due to the lock-in technique.     

Hydrogen-induced metallization on Ge(1 1 1) c(2 × 8)

We have studied hydrogen adsorption on the Ge(1 1 1) c(2 × 8) surface using scanning tunneling microscopy (STM) and angle-resolved photoelectron spectroscopy (ARPES). We find that atomic hydrogen preferentially adsorbs on rest atom sites. The neighbouring adatoms appear higher in STM images, which clearly indicates a charge transfer from the rest atom states to the adatom states. The surface states near the Fermi-level have been followed by ARPES as function of H exposure. Initially, there is strong emission from the rest atom states but no emission at the Fermi-level which confirms the semiconducting character of the c(2 × 8) surface. With increasing H exposure a structure develops in the close vicinity of the Fermi-level. The energy position clearly indicates a metallic character of the H-adsorbed surface. Since the only change in the STM images is the increased brightness of the adatoms neighbouring a H-terminated rest atom, we identify the emission at the Fermi-level with these adatom states.     

Quasi-one-dimensional structures on the Si(1 1 1) surface induced by Ba adsorption

Ba-induced quasi-one-dimensional reconstructions of the Si(1 1 1) surface have been investigated by low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). While the 3 × ‘2’ surface shows double-periodicity along the stripes in STM images consistent with half-order streaks observed in LEED patterns, no sign of the double-periodicity along the chain direction was detected for the 5 × 1 surface. The 5× stripes in STM images show internal structures with multiple rows. The two rows comprising the boundaries of a 5× stripe in the filled-state STM image are found to have 3a × √3/2 spacing across the stripe. The observation of the successive 3× and 2× spacings between the boundary rows supports a structural model proposed for the Ba-induced 5 × 1 Si reconstruction composed of honeycomb chains and Seiwatz chains. The highest coverage 2 × 8 surface does not reveal a quasi-1D row structure in STM images.     

Structure of the SiC (0 0 0 1) 3 × 3 reconstruction studied by surface X-ray diffraction

The surface structure of the 3 × 3 reconstruction of the 4H-SiC (0 0 0 1) surface was investigated with surface X-ray diffraction (SXRD).Of the studied models, the twist model proposed by Starke et al. [U. Starke, J. Schardt, J. Bernhardt, M. Franke, K. Reuter, H. Wedler, K. Heinz, J. Furthmuller, P. Kackell, F. Bechstedt, Phys. Rev. Lett. 80 (1998) 758] gave the best fit to the experimental data. The structural parameters were determined accurately.     

Monolayer adsorption of water on NaCl(1 0 0)

Density functional theory calculations have been applied to investigate the adsorption geometry of water overlayers on the NaCl(1 0 0) surface in the monolayer regime. Competition between H-H intermolecular repulsion and the attraction of the polar molecules to the surface ions results in the most stable structure having a 2 × 1 adsorption symmetry with an adsorption energy of 415 meV. Overlayers of 1 × 1 symmetry, as observed in experiment, have slightly lower adsorption energies. The layers are also unstable with respect to rotation of individual molecules. Multiple hydrogens/oxygens interacting with a single substrate ion can pull that ion out of the surface, although the examples considered are energetically very unfavourable. Overlayers of 1 × 1 symmetry with a coverage of one water molecule per NaCl do not have a high enough adsorption energy to wet the surface.     

GaN/LiNbO3 (0 0 0 1) interface formation calculated from first-principles

The stable adsorption sites for both Ga and N ions on the ideal and on the reconstructed LiNbO₃ (0 0 0 1) surface are determined by means of first-principle total energy calculations. A single N layer is found to be more strongly bound to the substrate than a single Ga layer. The adsorption of a GaN monolayer on the polar substrate within different orientations is then modeled. On the basis of our results, we propose a microscopic model for the GaN/LiNbO₃ interface. The GaN and LiNbO₃ (0 0 0 1) planes are parallel, but rotated by 30° each other, with in-plane epitaxial relationship [1 0 0]_GaN‖ [1 1  0]_LiNbO3. In this way the (0 0 0 1) plane lattice mismatch between GaN and LiNbO₃ is minimal and equal to 6.9% of the GaN lattice constant. The adsorbed GaN and the underlying LiNbO₃ substrate have parallel c-axes.     

Comparative study of H2 adsorption on W(1 0 0)-c(2 × 2)Cu and W(1 0 0): Surface alloying effects

The interactions of H and H₂ with W(1 0 0)-c(2 × 2)Cu and W(1 0 0) have been investigated through density functional theory (DFT) calculations to elucidate the effect of Cu atoms on the reactivity of the alloy. Cu atoms do not alter the attraction towards top-W sites felt by H₂ molecules approaching the W(1 0 0) surface but make dissociation more difficult due to the rise of late activation barriers. This is mainly due to the strong decrease in the stability of the atomic adsorbed state on bridge sites, the most favourable ones for H adsorption on W(1 0 0). Still, our results show unambiguously that H₂ dissociative adsorption on perfect terraces of the W(1 0 0)-c(2 × 2)Cu surface is a non-activated process which is consistent with the high sticking probability found in molecular beam experiments at low energies.     

A theoretical study of Fe adsorption along Bi-nanolines on the H/Si(0 0 1) surface

We have investigated the energetic stability and equilibrium geometry of the adsorption of transition metal Fe atoms near the self-organized Bi lines on hydrogen passivated Si(0 0 1) surface. Our total energy results show that there is an attractive interaction between Fe adatoms along the Bi-nanolines. For the energetically most stable configuration, the Fe adatoms are seven-fold coordinated, occupying the subsurface interstitial sites aside the Bi-nanolines. With increased coverage, Fe atoms are predicted to form two parallel lines, symmetrically on both sides of the Bi line. Within our local spin-density functional calculations, we find that for the most stable geometries the Fe adatoms exhibit an antiferromagnetic coupling.     

Oxygen vacancy pairs on CeO2(110): A DFT + U study

Oxygen vacancy pairs have been suggested to play a role in the reduction of NO molecules on ceria and for the oxidation processes of reducible rare-earth oxides. The formation energy of the oxygen vacancy pairs and the changes in the structural and electronic properties of the ceria (110) surface with oxygen vacancy pairs are investigated using density-functional theory (DFT + U) methodology within the generalized gradient approximation. It is found that the excess electrons localize on the Ce ions neighbouring the vacancies, and the most stable structure for the oxygen vacancy pairs on the ceria (110) surface is at next-nearest-neighbour site.     

(4×2) and (2×4) reconstructions of GaAs and InP(001) surfaces

Received: 21 March 1997/Accepted: 12 August 1997     

Density functional theory study of hydrogenation mechanism in Fe-doped Mg(0 0 0 1) surface

Using density functional theory (DFT) in combination with nudged elastic band (NEB) method, the dissociative chemisorptions and diffusion processes of hydrogen on both pure and Fe-doped Mg(0 0 0 1) surfaces are studied. Firstly, the dissociation pathway of H₂ and the relative barrier were investigated. The calculated dissociation barrier (1.08 eV) of hydrogen molecule on a pure Mg(0 0 0 1) surface is in good agreement with comparable experimental and theoretical studies. For the Fe-doped Mg(0 0 0 1) surface, the activated barrier decreases to 0.101 eV due to the strong interaction between the s orbital of H and the d orbital of Fe. Then, the diffusion processes of atomic hydrogen on pure and Fe-doped Mg(0 0 0 1) are presented. The obtained diffusion barrier to the first subsurface is 0.45 eV and 0.98 eV, respectively. Finally, Chou method was used to investigate the hydrogen sorption kinetic mechanism of pure MgH₂ and Mg mixed with 5 at.% Fe atoms composites. The obtained activation energies are 0.87 ± 0.02 and 0.31 ± 0.01 eV for H₂ dissociation on the pure surface and H atom diffusion in Fe-doped Mg surfaces, respectively. It suggests that the rate-controlling step is dissociation of H₂ on the pure Mg surface while it is diffusion of H atom in the Fe-doped Mg surface. And both of fitting data are matching well with our calculation results.