Bonding geometry of Mn-wires on the Si(100)(2 × 1) surface

The bonding geometry of monoatomic Mn-wires, which form on the reconstructed Si(100)(2 × 1) surface at room temperature, was investigated with scanning tunneling microscopy (STM). The Mn-wire structures are always perpendicular to the Si-dimer rows and the images exhibit a strong modulation of their apparent height as a function of bias voltage. The Mn-wire structures appear as depressions in the empty state images for bias voltages around 0.7 V, and as protrusions for all other bias voltages. It is suggested that the wire-images are defined by mixed Mn-Si states, either through a hybridization between the Mn d-states and the Si-p states, or backbonding from Mn-d electrons into the broken Si-dimer bond. The dominant bonding geometry shows that the Mn-wire maxima are positioned in between the Si-dimer rows, and a small percentage of about 20% is in registry with the Si-dimer rows, and might be described as defective wires. The experimental STM images cannot currently be described in a satisfactory manner with theoretical bonding models from the literature.     

Atomic and nanostructures of monolayer c(2 × 2)NiN on Cu(0 0 1)

Structures of monolayer nickel nitride (NiN) on Cu(0 0 1) surface are studied by X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Formations of Ni–N chemical bonds and NiN monolayer at the surface are confirmed by XPS on the N-adsorbed Cu(0 0 1) surfaces after Ni deposition and subsequent annealing to 670 K. A c(2 × 2) structure is always observed in the LEED patterns, which is a quite contrast to the (2 × 2)p4g structure observed usually at the N-adsorbed Ni(0 0 1) surface. Atomic images by STM indicate the mixture of Ni–N and Cu–N structures at the surface. Density of the trenches on the N-saturated surface decreases and the grid pattern on partially N-covered surfaces becomes disordered with increasing the Ni coverage. These results are attributed to the decrease of the surface compressive stress at the N-adsorbed Cu surface by mixing Ni atoms.     

A Cu(1 1 1)(√3×√3)R30°-Sb structure by impact collision ion-scattering spectroscopy

We have investigated the structures of Cu(1 1 1)(√3×√3)R30°-Sb using time of flight-impact collision ion-scattering spectroscopy. The experimental data and computer simulations support a structural model for the Cu(1 1 1)(√3×√3)R30°-Sb structure in which Sb atoms displace up to 1/3 of the first layer of Cu atoms and incorporate them into the first Cu layer with the Sb atoms displaced outward 0.40 Å with respect to the first-layer Cu atoms. The outermost first layer of Sb and Cu atoms shift from fcc- to hcp-hollow sites (only the top layer of Sb and Cu atoms occupies hcp hollow sites).     

Passivation effect of allylamine molecule on the electronic structure of a Si(001) − (2 × 1) surface

The chemisorption of the allylamine molecule, which contains two functional groups (ethenyl and hydroxyl), on a Si(001) ? (2 × 1) surface was studied using density functional theory (ab-initio DFT) based on the pseudopotential approach. In particular, we focused on the determination of the most stable position of the CC double bond in the ethenyl group and observation of the passivation effect of allylamine on the electronic structure of the clean Si(001) ? (2 × 1) phase. For this purpose, all of the possible interaction mechanisms occurring at the interface were considered: (i) dissociative bonding where the CC bond is parallel to the silicon surface, (ii) dissociative bonding where the CC bond is perpendicular to the silicon surface, and (iii) the [2 + 2] CC cycloaddition reaction. From our total energy calculations, it was found that the bifunctional allylamine molecule attached to the Si(001) ? (2 × 1) surface through the amino functional group, by breaking the N–H bond and forming a Si–H bond and Si–NHCH2CHCH2 surface fragments. During this process, the ethenyl functional group remains intact, and so can be potentially used as an extra reactive site for additional chemical interactions. In addition to these findings, the nudged elastic band method (NEB) calculations related with the reaction paths showed that the parallel position of the CC bond with respect to the surface of the substrate is more favorable. In order to see the influence of the chemisorbed allylamine molecule on the surface states of the clean Si(001) – (2 × 1), we also plotted the density of states (DOS), in which it is seen that the clean Si(001) – (2 × 1) surface was passivated by the adsorption of allylamine.     

3C-SiC(001)-3 × 2 reconstructed surface analyzed by high-resolution medium energy ion scattering

The atomic structure of the 3C-SiC(001)-3 × 2 reconstructed surface was analyzed precisely by high-resolution medium energy ion scattering (MEIS). The present MEIS analysis unambiguously shows that the (3 × 2) surface consists of Si adatoms (1/3 ML, 1 ML = 1.05 × 10¹⁵ atoms/cm²) on top and underlying Si adlayer (2/3 ML) on the bulk truncated Si plane. As the result, the most probable structure is focused on the Two Adlayer Asymmetric Dimer Model predicted by ab initio calculations and the modified versions with alternating long and short dimers in the 2nd adlayer proposed by photoelectron diffraction (PED) and by grazing incidence X-ray diffraction (GIXRD) analyses. Observed MEIS spectra are well reproduced by the structure relatively close to that determined by PED rather than GIXRD. Interestingly, the first principle calculations using VASP (Vienna ab initio simulation package) prefer symmetric dimers in the second Si adlayer and non-relaxed interplanar distance between the top Si and 2nd C plane of the bulk-truncated surface, which are, however, unable to reproduce the observed MEIS spectra. The distorted 2nd adlayer (asymmetric dimers) may correlate with the compressed interplanar distance between the underlying Si and C planes.     

Atomic structure of two-dimensional binary surface alloys: Si(111)-√21 × √21 superstructure

The atomic structures of Au and Ag co-adsorption-induced √21 × √21 superstructure on a Si(111) surface, i.e., (Si(111)-√21 × √21-(Au, Ag)), where the Si(111)-5 × 2-Au surface is used as a substrate, have been investigated using reflection high-energy positron diffraction (RHEPD) and photoemission spectroscopy. From core-level spectra, we determined the chemical environments of Ag and Au atoms present in the Si(111)-√21 × √21-(Au, Ag) surface. From the rocking curve and pattern analyses of RHEPD, we found that the atomic coordinates of the Au and Ag atoms were approximately the same as those of the Au and Ag atoms in other Si(111)-√21 × √21 surfaces with different stoichiometries. On the basis of the core-level and RHEPD results, we revealed the atomic structure of the Si(111)-√21 × √21-(Au, Ag) surface.     

Atomic structure of the (3 × 2) Si–GaAs (0 0 1) reconstructed surface: A clue to δ doping mechanism derived from in situ grazing incidence X-ray diffraction data

In view of understanding silicon incorporation in the δ doping process of GaAs (0 0 1), Si atoms have been deposited, under UHV, on a α(2 × 4) arsenic terminated substrate. In the low coverage regime, a transition to a less As rich (3 × 2) reconstructed Si–GaAs (0 0 1) surface was observed whose atomic structure has been investigated by grazing incidence X-ray diffraction performed in situ. Silicon is found to occupy not only a Ga substitutional site, precursor of a donor dopant but also to form nuclei for neutral clusters, on a template made by the (3 × 2) GaAs (0 0 1) reconstructed surface observed by Martrou et al. [Phys. Rev. B 72 (2005) 241307®]. The maximum surface concentration of donor-like silicon is estimated at 1.04 × 10¹⁴ cm^?2 (1/6th monolayer).     

Topmost-surface-sensitive Si-2p photoelectron spectra of clean Si(1 0 0)-2 × 1 measured with photoelectron Auger coincidence spectroscopy

Topmost-surface-sensitive Si-2p photoelectron spectra of a clean Si(1 0 0)-2 × 1 surface have been measured using Si-2p photoelectron Si-L₂₃VV Auger coincidence spectroscopy (Si-2p–Si-L₂₃VV PEACS). The escape depth of the PEACS electrons is estimated to be ～1.2 Å. The results support the assignments of the Si up-atoms, the Si down-atoms, the Si 2nd-layer, and the Si bulk proposed in previous researches. The Si-2p component with a binding energy of ?0.23 eV relative to the bulk Si-2p_3/2 peak, is shown to originate mainly from the topmost surface. Site selectivity of PEACS is indicated to be achieved to some degree by carefully selecting the kinetic energy of the Auger electrons. Since PEACS can be applied to any surface, the present study opens a new approach to identify PES components.     

Adsorption of small molecules on a (2 × 1) PdZn surface alloy on Pd(1 1 1)

The adsorption of methanol, formaldehyde, methoxy, carbon monoxide and water on a (2 × 1) PdZn surface alloy on Pd(1 1 1) has been studied using DFT calculations. The most stable adsorption structures of all species have been investigated with respect to the structure and the electronic properties. It was found that methanol is only weakly bound to the surface. The adsorption energy only increases with higher methanol coverage, where chain structures with hydrogen bonds between the methanol molecules are formed. The highest adsorption energy was found for the formate species followed by the methoxy species. The formaldehyde species shows quite some electronic interaction with the surface, however the stable η₂ formaldehyde has only an adsorption energy of about 0.49 eV. The calculated IR spectra of the different species fit quite well to the experimental values available in the literature.     

Self-assembled line growth of allyl alcohol on the H-terminated Si(100)-(2 × 1) surface

Using first-principles density-functional calculations, we investigate the growth mechanism of allyl alcohol (ALA) line on the H-terminated Si(100)-(2 × 1) surface. Unlike the allyl mercaptan (CH₂ = CH ? CH₂ ? SH) line, which was observed to grow across the Si dimer rows, we find that ALA (CH₂ = CH ? CH₂ ? OH) has the line growth along the Si dimer row. The self-assembled growth of ALA line occurs via the radical chain reaction mechanism, similar to the case of a typical alkene molecule, styrene. Our calculated energy profile along the reaction pathway shows that the different growth direction of ALA line compared with that of allyl mercaptan line is ascribed to the great instability of the oxygen radical intermediate, which prevents the line growth across the dimer rows.     

Self-assembly of C60 fullerenes on quasi-one-dimensional Si(111)4 × 1-In surface

Using scanning tunneling microscopy observations, self-assembly of C₆₀ fullerenes in the course of room-temperature adsorption onto Si(111)4 × 1-In reconstruction and after subsequent annealing at temperatures ranging from 150 to 450 °C has been studied. Adsorbed C₆₀ fullerenes have been found to occupy off-centered positions on In-atom rows forming linear chains with a maximal length of eight C₆₀ molecules. Intermolecular spacing within the regular chains equals three lattice constants of Si(111) surface. Two energetically different adsorption states of C₆₀ have been detected, one of which is occupied preferentially at room temperature, while occupation of the second (more tight) state dominates at temperature above ~ 150 °C. In the first state, C₆₀ fullerene resides plausibly in a continuous rotation, while in the second state a C₆₀ molecule is fixed tightly in a single orientation with a C₆₀ hexagon pointing upward. Transition of C₆₀ fullerenes to the more stable state is accompanied by expelling In atoms from the Si(111)4 × 1-In reconstruction.     

First principles total energy calculations of the adsorption of germane and digermane on Si(0 0 1)-c(2 × 4)

First principles total energy studies are performed to investigate the energetics, and the atomic structure of the adsorption of germane (GeH₄), and digermane (Ge₂H₆) on the Si(0 0 1)-c(2 × 4) surface. It has been observed experimentally that adsorption of Ge₂H₆ is a dissociative process, which first yields GeH₃ and then GeH₂ fragments as products. We first study the adsorption of GeH₂ considering two different models; the intra-row and the on-dimer geometries. Our results show that the on-dimer site is more stable than the intra-row geometry by 0.44 eV. This is not a surprise since in the absence of H atoms, adsorption in the on-dimer site leaves no dangling bonds. In contrast, when the GeH₂ fragment is considered together with two H atoms, the intra-row geometry is favored energetically as compared with the on-dimer site, in good agreement with experiment. Similar results have been previously obtained for the adsorption of SiH₂ on Si(0 0 1). Digermane adsorption is explored according to two different geometries. In the first one, we have considered the adsorption as two GeH₃ fragments, while in the second, we have considered the adsorption as two GeH₂ fragments plus 2 H fragments. In good agreement with experiments, it is found that the latter geometry is energetically more favorable.     

Interaction of Ag with the Si(1 1 1)1 × 1–H surface

Synchrotron radiation based photoemission spectroscopy (SRPES) and low energy electron diffraction (LEED) are used to study the interaction between Ag atoms and the Si(1 1 1)1 × 1–H surface. At an Ag coverage of 0.063 monolayers (ML) on the Si(1 1 1)1 × 1–H surface, the Si 2p component corresponding to Si–H bonds decreases, and an additional Si 2p component appears which shifts to a lower binding energy by 109 meV with respect to the Si bulk peak. The new Si 2p component is also observed for 0.25 ML Ag on the Si(1 1 1)7 × 7 surface. These findings suggest that Ag atoms replace the H atoms of the Si(1 1 1)1 × 1–H surface and form direct Ag–Si bonds. Contrary to the widely accepted view that there is no chemical interaction between Ag particles and the H-passivated Si surface, these results are in good agreement with recent first-principles calculations.     

Atomic and electronic properties of tert-butanol on the Si(001)-(2×1) surface

The atomic and electronic properties of the adsorption of tert-butanol [(CH₃)₃OH] molecule on the Si(001)-(2×1) surface have been studied by using the ab-initio density functional theory (DFT) based on pseudopotential approach. We have found that tert-butanol bonded the Si(001) surface by oxygen atom, cleaving a O–H bond and producing a Si-H bond and tert-butoxy surface species. We have also investigated the influence of chemisorption of tert-butanol on the electronic structure of the clean Si(001)-(2×1) surface. Two occupied surface states situated entirely below the bulk valence band maximum have been identified, which means that the clean Si(001)-(2×1)surface was passivated by the chemisorption of tert-butanol. In order to explain the nature of the surface components we have also plotted the total and partial charge densities at the [`(K)]\bar{K} point of the surface Brillouin zone (SBZ).     

Selective adsorption of coronene on Si(1 1 1)-(7 × 7)

The adsorption of coronene (C₂₄H₁₂) on the Si(1 1 1)-(7 × 7) surface is studied using scanning tunneling microscopy (STM). Upon room temperature submonolayer deposition, we find that the coronene molecules preferentially adsorb on the unfaulted half of the 7 × 7 unit cell. Molecules adsorbed on different sites can be induced to move to the preferential sites by the action of the tip in repeated image scans. Imaging of the molecules is strongly bias dependent, and also critically depends on the adsorption site. We analyze the results in terms of differential bonding strength for the different adsorption sites and we identify those substrate atoms which participate in the bonding with the molecule.     

Atomic and electronic structures of Ag/Si(100)-c(6 × 2) surface: A first-principles study

Using the experimental data obtained mainly with the scanning tunneling microscopy observations, density functional theory calculations have been applied to examine an atomic structure of the Ag/Si(100)-c(6 × 2) reconstruction. A set of structural models has been proposed having a similar Si(100) substrate reconstruction which incorporates rows of top Si atom dimers and troughs in between the rows. Stability of about twenty models with various Ag coverage ranging from 1/6 to 1 ML has been tested, that allows reducing the number of plausible models to four. Two of these four models have been attributed to the “regular” intrinsic Ag/Si(100)-c(6 × 2) reconstruction, while the other two to its defect-induced modification. The latter is observed in the local areas near defects and domain boundaries and exhibits 3 × 2 periodicity. Comparing the results of calculations with the experimental STM images, it has been concluded that while the Si(100) substrate reconstruction is solid, the Ag subsystem is flexible due to the presence of the lightly bonded mobile Ag atoms.     

Structure determination of the Cu(0 0 1)–c(4 × 4)-Sn surface by low-energy electron diffraction

Low-energy electron diffraction (LEED) have been used to determine the Cu(0 0 1)–c(4 × 4)-Sn structure formed at 300 K. It is demonstrated that a structural model suggested by scanning tunneling microscopy observations is correct: The model consists of one substitutional Sn atom and four Sn adatoms in the unit cell. Optimum parameters of the determined c(4 × 4) structure reveal that Sn adatoms laterally are displaced by 0.30 Å away from ideal fourfold-hollow sites along the 〈100〉 directions. It is proposed that such displacements of the Sn adatoms cause the formation of a network of octagonal rings on Cu(0 0 1). The substitutional Sn atom is located at each center of the octagonal rings. The formation conditions of the network are discussed.