Nitric oxide adsorption on the Si(100)(2×1) surface — a vibrational study

High-resolution vibrational electron energy loss spectroscopy has been used to study the adsorbed state of NO on the Si(100)(2×1) surface. At 300 K, NO is adsorbed dissociatively on the Si(100) surface in the disordered structure, and the Si₃N and SiOSi species are formed. By heating at 1200–1300 K, the O adatoms are removed from the surface and the silicon nitride is formed; the (2×1) structure is recovered, which is interpreted to indicate that the nitride is formed mainly in the subsurface region.     

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).     

NH3-induced c(4 × 2) to (2 × 2) transition on the Ge(001) surface by a domino-like dimer flip

Under the conditions of thermodynamic adsorption-desorption equilibrium, the first strongly bound molecular adsorption state of ammonia on Ge(001) saturates at one molecule per Ge reconstruction dimer (1/2 ML). High-resolution electron diffraction studies show that this adsorption is accompanied by a structural transition from c(4 × 2) on the clean surface to a (2 × 2) structure which is already completed for a coverage of about 0.04 ML, far below saturation. We propose a model implying the formation of NH₃ islands locally covered with 1/2 ML and a (2 × 2) periodicity caused by a flip of the dimer tilt direction of every second dimer Beyond the edge of these islands, the dimer flip continues domino-like along the dimer rows over the clean parts of the surface. Elongated (2 × 2) domains about 280–330Å long and 30–65Å wide are formed, depending on coverage.     

HREELS study of C70 molecules adsorbed on a Si(1 1 1)-(7×7) surface

We have measured the coverage dependence of vibrational excitation spectra of C₇₀ molecules adsorbed on a Si(1 1 1)-(7×7) surface using high-resolution electron-energy-loss spectroscopy. At the monolayer coverage, the intensity of the 57 meV peak increases, and those of the 83 and 178 meV peaks decrease. Taking into account the dipole selection rule, the change in intensity of the 57 meV peak indicates that the average angle between the long axes of C₇₀ molecules and surface normal is about 40°. The decreases in intensities of the 83 and 178 meV peaks suggest that the rotational motion of molecules is quenched upon adsorption. We will discuss the Coriolis interaction between the accidentally degenerate A₂″ and E₁′ modes.     

Sub-10-nm multifacet domelike Ge quantum dots grown on clean Si (001) (2×1) surface

Sub-10-nm multifacet domelike Ge quantum dots (QDs) ensembles with uniform size have been achieved on a clean Si (001) (2×1) reconstructed surface at a substrate temperature of 450°C, total Ge coverage of 7 ML, Ge deposition rate of ∼0.0115 ML/s and no post-annealing. Their areal density and diameter are 5.2×10¹¹ cm⁻² and 7.2±2.3 nm, respectively, which is explained by a pit-mediated mass transferring nucleation mechanism suggested by us. According to the phase diagram analysis, their domelike morphology can be attributed to a relatively high growth temperature. Their high density and small size result in a strong non-phonon peak with a large blue shift of 0.19 eV in the low-temperature photoluminescence spectrum.     

Effects of potassium on the adsorption of methanol on β-Mo2C(001) surface

We have studied the effect of K on the adsorption of methanol on the β-Mo₂C(001) surface and compared our experimental data with theoretical calculations. We have also performed high resolution electron energy loss spectroscopy (HREELS) (LK, ELS3000). For calculations we used the density functional theory under the VASP implementation. The most favorable sites for methanol adsorption are on top of a Mo atom in the clean surface and on top of a K atom in the pre-dosed surface. The changes in the work function fit our model as the surface withdraws charge from the adsorbate. The changes in the computed vibrational frequencies also agree with the HREELS results at very low coverage. The C–O bond distance increases while the O–H bond decreases making a C–O bond breakage a possibility on K covered surfaces.     

Ab initio study of surface electronic structure of phosphorus donor impurity on Ge(111)-(2×1) surface

We present the results of numerical modeling of the electronic properties of the Ge(111)-(2 × 1) surface in the vicinity of a P donor impurity atom near the surface. We have shown that, in spite of well-established bulk donor impurity energy level position at the very bottom of the conduction band, the surface donor impurity might produce an energy level below the Fermi energy, depending on impurity atom local environment. It has been demonstrated that the impurity located in subsurface atomic layers is visible in scanning tunneling microscopy experiment. The quasi-one-dimensional character of the impurity image observed in scanning tunneling microscopy experiments is confirmed by our computer simulations.     

Adsorption and diffusion of an Au atom and dimer on a θ-Al2O3 (001) surface

With density-functional calculations we have investigated adsorption and diffusion of an Au atom and an Au₂ dimer on a θ-Al₂O₃(001) surface. The surface structure of θ-Al₂O₃(001) has an armchair-like configuration containing flat and trench areas and the Au_n (n = 1 or 2) cluster prefers to adsorb on the flat area. A single Au atom adsorbs on an O–Al bridge site with adsorption energy 0.35 eV, whereas an Au₂ dimer bonds to the oxide with adsorption energy 0.78 eV, with one Au coordinated singly to a surface O. Formation of Au₂ from Au₁ is favored, with a negligible energy barrier. The calculated energy barriers for diffusion indicate that an Au atom diffuses more rapidly than an Au₂ dimer but both prefer to diffuse anisotropically, along the flat area of the θ-Al₂O₃(001) surface.     

Cesium adsorption on the β2-GaAs(001) surface

The results of first-principles calculations of the cesium adsorption energy on the β2-GaAs(001) surface performed within approaches of the density functional theory are presented for two possible terminations of the surface. It is shown that, among the considered high-symmetry positions, the energy-preferred position for cesium is position T ₃ when the surface layer contains arsenic and position T ₄ for gallium terminated surface. Cesium introduces insignificant perturbations in the positions of surface-layer atoms, and surface dimers do not break even in the case of adsorption at the dimer bridge and top positions. It is shown that cesium bonding to the GaAs (001) substrate can be explained by sp hybridization of arsenic and gallium orbitals as well as by formation of cesium states mixed with delocalized states of a clean surface. At low coverage, more preferable adsorbate sites are those with nearest neighbor arsenic atoms for both surface terminations.     

First-principles study of O2 adsorption on the α-U(001) surface

First-principles calculations based on density functional theory (DFT) have been performed to investigate the adsorption structures and electronic properties for O₂ on the α-U(001) surface. It was found that O₂ tends to dissociate with significant energetic preference compared to molecular adsorption. When approaching the surface perpendicularly along top site, the O₂ adsorbates were found to remain as molecule on the surface. The density of states of the system showed strong hybridization features for O2p, U6d and U5f states in the case of dissociative adsorption which is weaker for molecular adsorption. Further electronic properties analysis demonstrated that the bonding character of U–O bond is related to the symmetry of the adsorption site. Top site configuration showed stronger covalent component for the U–O bond, while the ionic character was found to be more obvious for hollow site adsorption.     

Kinetic Monte Carlo simulations of Au clusters on Si(111)-7 × 7 surface

Self-assembled growth of Au nanoclusters on the Si(111)-7 × 7 surface has been studied using kinetic Monte Carlo simulations. A model considering various atomic processes of deposition, adsorption, diffusion, nucleation, and aggregation is introduced, and the main energetic parameters are optimized based on the experimental results. The evolution of surface morphology during Au growth is simulated in real time, from which the atomic behaviors of Au could be really captured. Most of Au atoms diffuse on the substrate in the very early stage of growth, and Au clusters nucleate and grow with the increasing coverage. The competition among various atomic processes results in the distinct distribution of Au clusters under different coverages. The growth conditions are further optimized, showing that the higher uniformity of Au clusters would be obtained at a low deposition rate and an optimal substrate temperature of about 380 K.     

Combined theoretical and experimental determination of the atomic structure and adsorption site of Na on the Si(100)2 × 1 surface

The Na/Si(100)2 × 1 interface is studied by both ab initio local density functional total energy DMol molecular calculations using very large cluster models and photoemission EXAFS which provides the unique feature of probing both Na adsorbate and Si substrate environments. Theoretical and experimental bond lengths are found to be in very good agreement and enable a definite assignment of the adsorption site: Na is adsorbed on a single site, the cave, with no Na-Na distance consistent with any “double layer” models. The growth and existence of a second Na layer are shown to occur only in presence of very low level impurities.     

Types of surface modulation in a Ge–Si(111) heterosystem

Three types of elastic modulation in atomic layers of epitaxial Ge films on the surface of Si(111) observed in scanning tunnel microscopy are discussed. Two types of modulation are associated with misfit dislocations accumulating at the interface. Modulation of the third type arises on the surfaces of dislocation-free pseudomorphic films and could be due to the presence on a Si substrate surface of two-dimensional silicon islands with a height of three atomic bilayers and lateral dimensions of 15–20 nm. Measured bending values Δh for the third type of modulation (Δh ≤ 0.1 nm) agree with data calculated within the theory of elasticity     

A first-principles study on initial processes of a Ni adatom on the H-terminated Si(0 0 1)-(2×1) surface

We studied the adsorption, surface diffusion, and penetration, i.e. the initial processes of a Ni adatom on the H-terminated Si(001)-(2×1) surface by the first-principles theoretical calculations. As concerns the adsorption, two different types were found. When Ni is deposited onto the Si dimer row, it once captures H from the dimer Si, though it eventually returns H, with no activation energy barrier. Then, Ni moves to the most stable site, which is the off-centered bridge (B) site between the dimer rows, with the activation energy of 0.65 eV. On the other hand, Ni deposited between the dimer rows captures no H and moves to the B site without the energy barrier. Thus an adsorbed Ni atom invariably arrives at the most stable B site at the room temperature. As for the surface diffusion, it needs the activation energies of 0.66 and 1.19 eV for Ni to migrate from the B site in the directions parallel and perpendicular to the dimer row, respectively. Therefore, we concluded that the surface diffusion of Ni is restricted in the valley between the dimer rows at the room temperature. Furthermore, since the penetration of Ni is blocked on this surface, it was also concluded that the surface hydrogenation suppresses silicidation.     

Surface electronic structure of Ge(001)2 × 1: experiment and theory

Detailed studies of the surface electronic structure of Ge(001)2 × 1 have been performed with angle-resolved photoemission. Five surface structures are identified in the spectra and their initial-energy dispersions E(k_∥) are presented along the [010] direction. Employing the local density approximation and Green's functions, the surface band structure along this direction has been calculated self-consistently for an asymmetric dimer model of the 2 × 1-reconstructed surface. Four of the five experimental surface structures are identified with calculated surface states or resonances, i.e. the dangling-bond state and two different back-bond resonances. Excellent agreement is obtained between the experimental and theoretical dispersions for these surface electronic bands.     

Dangling-bond logic gates on a Si(100)-(2 × 1)-H surface

Atomic-scale Boolean logic gates (LGs) with two inputs and one output (i.e. OR, NOR, AND, NAND) were designed on a Si(100)-(2 × 1)-H surface and connected to the macroscopic scale by metallic nano-pads physisorbed on the Si(100)-(2 × 1)-H surface. The logic inputs are provided by saturating and unsaturating two surface Si dangling bonds, which can, for example, be achieved by adding and extracting two hydrogen atoms per input. Quantum circuit design rules together with semi-empirical elastic-scattering quantum chemistry transport calculations were used to determine the output current intensity of the proposed switches and LGs when they are interconnected to the metallic nano-pads by surface atomic-scale wires. Our calculations demonstrate that the proposed devices can reach ON/OFF ratios of up to 2000 for a running current in the 10 μA range.     

Theoretical simulation of STM image of C60 molecules on Si(100)-(2 × 1) surface

Theoretical simulation of STM image of C₆₀ molecules on Si(100)-(2 × 1) surface was performed by the DV-Xα-LCAO method. The results excellently reproduced the internal stripe pattern of the STM image of C₆₀ observed by experiment. We confirm that it is the interaction between C₆₀ and the Si substrate which causes this internal structure.