Theory of adsorption: Ordered monolayers from Na to Cl on Si(001) and Ge(001)

First principles calculations of clean and adsorbate-covered surfaces of Si(001) and Ge(001) are reported. Chemical trends in the adsorption of ordered Na, K, Ge, As, Sb, S, Se and Cl overlayers are discussed. The calculations are based on the local-density approximation and employ non-local, norm-conserving pseudopotentials together with Gaussian orbital basis sets. The semi-infinite geometry of the substrate is properly taken into account by employing our scattering theoretical method. From total-energy minimization calculations we obtain optimal surface reconstructions which show asymmetric dimers for Si(001), Ge(001) and Ge:Si(001). For As:Si(001), Sb:Si(001) and Sb:Ge(001), we find symmetric adatom dimers in the equilibrium geometries. S or Se adlayers are found to be adsorbed in bridge positions forming a (1×1) unit cell with a geometry very close to the configuration of a terminated bulk lattice. Cl atoms adsorb on top of the dangling bonds of symmetric Si dimers residing in the first substrate-surface layer. Our calculations for Na:Si(001) and K:Si(001) confirm valley-bridge site adsorption for half monolayer coverage. For full monolayer alkali-metal coverage, adsorption in pedestal and valley-bridge positions is found to be energetically most favourable. The calculated optimal adsorption configurations are in excellent agreement with a whole body of recent experimental data on surface-structure determination. For these structural models, we obtain electronic surface band structures which agree very good with a wealth of data from angle-resolved photoemission spectroscopy investigations.     

Angle-resolved study of hydrogen abstraction on Si(1 0 0) and Si(1 1 1): Evidence for non-activated pathways

The abstraction of chemisorbed hydrogen on Si(1 0 0) and Si(1 1 1) induced by atomic hydrogen has been investigated by studying with a rotatable mass spectrometer the angle-resolved molecular hydrogen desorption from a Si surface exposed to a chopped beam of atomic hydrogen. The angular distributions of desorbing molecules can be fitted independent of the surface temperature and the surface reconstruction by a cosⁿθ function with n < 1 for Si(1 0 0) and Si(1 1 1). These results are interpreted by non-activated pathways involving site-specific hot-atom abstraction on two adjacent silicon atoms with one having a dangling bond. Possible mechanisms according to the surface reconstructions are discussed.     

Band bending and quasi‐2DEG in the metallized β‐SiC(001) surface

We study the mechanism leading to the metallization of the β‐SiC(001) Si‐rich surface induced by hydrogen adsorption. We analyze the effects of band bending and demonstrate the existence of a quasi‐2D electron gas, which originates from the donation of electrons from adsorbed hydrogen to bulk conduction states. We also provide a simple model that captures the main features of the results of first‐principles calculations, and uncovers the basic physics of the process. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

碱金属钠原子修饰硅原子团簇的结构及储氢性能研究

提出碱金属钠原子修饰笼形Si_6团簇的结构模型,采用密度泛函理论(DFT)研究钠原子修饰笼形Si_6团簇的结构及储氢性能.研究结果表明,氢分子与笼形Si_6团簇表面相互作用很弱,氢分子在其表面容易脱附.采用钠原子修饰笼形Si_6团簇后可有效避免氢分子的脱附,并且钠原子在笼形Si_6团簇的表面不发生团聚,有利于氢分子在其表面吸附和循环利用.研究发现在两个钠原子修饰笼形Si_6团簇的结构中,每个钠原子可以有效吸附六个氢分子.计算得到Na2Si_6团簇结构储氢的质量分数高达10.08 wt%,且氢分子的平均吸附能约为0.837 kcal/mol.可见,实现钠原子修饰笼形Si_6团簇结构在常温常压条件下储氢是有可能的.     

Water adsorption on hydrogenated Si(1 1 1) surfaces

The adsorption of water on the hydrogen terminated Si(1 1 1) surface is studied by means of first-principles calculations as well as contact angle measurements. Possible initial adsorption configurations for single water molecules and the potential energy surface are calculated. Only small adsorption energies of the order of meV are predicted. Calculations for higher coverage show that the water-water interactions are stronger than the water-surface bonding. The contact angle formed between a water droplet on the surface approximated from the total-energy calculations amounts to 88°, while our measured value is 91°.     

Theoretical study on adsorption of Au and hydrated Au cations on clean Si(1 1 1) surface

To model the adsorption of Au⁺ cation in aqueous solution on the semiconductor surface, the interactions of Au⁺ and hydrated Au⁺ cations with clean Si(1 1 1) surface were investigated by using hybrid density functional theory (B3LYP) and Møller-Plesset second-order perturbation (MP2) methods. Si(1 1 1) surface was described with Si₇H₁₁, Si₁₁H₁₇ and Si₂₂H₂₁ clusters. The effect of the basis set superposition error (BSSE) was taken into account by applying the counterpoise correction. The calculated results indicated that the binding energies between hydrated Au⁺ cations and clean Si(1 1 1) surface are large, suggesting a strong interaction between hydrated Au⁺ cations and the semiconductor surface. The bonding nature of the chemical adsorption of Au⁺ to Si surface can be classified as partial covalent as well as ionic bonding. As the number of water molecules increases, the water molecules form hydrogen bond network with one another and only one water molecule binds directly to the Au⁺ cation. The Au⁺ cation in aqueous solution will safely attach to the clean Si(1 1 1) surface.     

Nitric acid-stabilized superparamagnetic iron oxide nanoparticles studied with X-rays

Using density functional theory, we systematically investigate the adsorption geometries and electrical properties of (3,3) carbon nanotube (CNT) integrated on hydrogen-terminated Si(001):1?×?1 surface. Prior to adsorption of the CNT, the surface is patterned in two different ways by desorbing selective hydrogen atoms from the surface. The (3,3) CNT which is metallic in nature becomes semiconducting with a band gap around the fermi level when it is supported on patterned hydrogen-terminated Si(001):1?×?1 surface. However, the band gap is reduced when a transverse electric field is applied, allowing the (3,3) CNT on the patterned hydrogen-terminated Si(001):1?×?1 to become metallic at a critical field strength. The tuning of electrical properties of the (3,3) CNT integrated with Si surface may have potential technological applications.     

Resistless patterning of a chlorine monolayer on a Si(0 0 1) surface with an electron beam

We achieved electron beam (e-beam) patterning without a photoresist on a Cl-terminated Si(0 0 1) surface. Synchrotron radiation photoemission spectroscopy and scanning photoelectron microscopy were employed to investigate the surface chemical state and pattern formation. The Cl-Si bonds were easily broken by the irradiation with an e-beam of 1 keV, leading to a pattern formation through the adsorption of residual molecules of water and hydrocarbon at the exposed Si dangling bond sites. In addition, we demonstrated the selective adsorption of desired molecules on the surface by e-beam irradiation in environments consisting of different gases, such as oxygen, ammonia, and 1-butanethiol.     

Nonlinear optical investigations of the dynamics of hydrogen interaction with silicon surfaces

Optical second-harmonic generation (SHG) from silicon surfaces may be resonantly enhanced by dangling-bond-derived surface states. The resulting high sensitivity to hydrogen adsorption combined with unique features of SHG as an optical probe has been exploited to study various kinetical and dynamical aspects of the adsorption system H₂/Si. Studies of surface diffusion of H/Si(111)7×7 and recombinative desorption of hydrogen from Si(111)7 × 7 and Si(100)2 × 1 revealed that the covalent nature of hydrogen bonding on silicon surfaces leads to high diffusion barriers and to desorption kinetics that strongly depend on the surface structure. Recently, dissociative adsorption of molecular hydrogen on Si(100)2×1 and Si(111)7×7 could be observed for the first time by heating the surfaces to temperatures between 550 K and 1050 K and monitoring the SH response during exposure to a high flux of H₂ or D₂. The measured initial sticking coefficients for a gas temperature of 300K range from 10^–9 to 10^–5 and strongly increase as a function of surface temperature. These results demonstrate that the lattice degrees of freedom may play a decisive role in the reaction dynamics on semiconductor surfaces.     

A density-functional theory investigation on disorption of O2 on Sn(111) and its comparison with initial oxidation on the X(111) (X=Si,Ge, Sn,Pb) surfaces

The first-principles calculations are performed to investigate the adsorption of O₂ molecules on an Sn(111) 2×2 surface. The chemisorbed adsorption precursor states for O₂ are identified to be along the parallel and vertical channels, and the surface reconstructions of Sn(111) induced by oxygen adsorption are studied. Based on this, the adsorption behaviours of O₂ on X(111) (X=Si, Ge, Sn, Pb) surfaces are analysed, and the most stable adsorption channels of O₂ on X(111) (X=Si, Ge, Sn, Pb) are identified. The surface reconstructions and electron distributions along the most stable adsorption channels are discussed and compared. The results show that the O₂ adsorption ability declines gradually and the amount of charge transferred decreases with the enhancement of metallicity.     

Interpretation of initial stage of 3C-SiC growth on Si(1 0 0) using dimethylsilane

The initial stage of cubic silicon carbide (3C-SiC) growth on a Si(0 0 1) surface using dimethylsilane (DMS) as a source gas was observed using scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED). It was found that the dimer vacancies initially existing on the Si(0 0 1)-(2 × 1) surface were repaired by the Si atoms in DMS molecules, during the formation of the c(4 × 4) surface. From the STM measurement, nucleation of SiC was found to start when the Si surface was covered with the c(4 × 4) structure but before the appearance of SiC spots in the RHEED pattern. The growth mechanism of SiC islands was also discussed based on the results of RHEED, STM and temperature-programmed desorption (TPD).     

Theoretical insights into the effects of the diameter and helicity on the adsorption of formic acid on silicon carbide nanotube

The anchoring of small organic molecules onto the semiconductor surface has a great application for developing various molecular devices, such as novel solar cells, fuel cells, hybrid systems, sensors, and so on. In the present work, by carrying out detailed density-functional theory calculations, we have investigated the adsorption of the formic acid (HCOOH) molecule on planar and various curved silicon carbide (SiC) nanotubes. By considering both the molecular and dissociative adsorptions of HCOOH on these SiC nanomaterials, we found that the HCOOH molecule prefers to dissociate into HCOO and H group. Interestingly, different adsorption modes were found for HCOOH on SiC nanotubes, i.e. dissociative monodentate or bidentate adsorption, which depends on the tube diameter and helicity. For (n, 0) SiC nanotube, the monodentate adsorption mode is energetically favorable when n is less than 10. However, HCOOH prefers to be adsorbed on other (n, 0) SiC nanotubes in a bridged bidentate mode, which is similar to those of on (n, n) SiC nanotubes or planar SiC sheet. Moreover, upon HCOOH adsorption, these SiC nanomaterials remain to be of the semiconducting nature and their band gaps are decreased to different degrees. In addition, we also explored the effects of HCOOH coverage on its adsorption on SiC nanotube.     

Study of Si and C adatoms and SiC clusters on the silicon surface by the molecular dynamics method

Individual Si and C adatoms, as well as SiC clusters, on a Si surface are simulated by the molecular dynamics method in the course of investigation of the initial stages of formation of a SiC layer on silicon with the help of molecular beam epitaxy. The potential energy surfaces for Si and C adatoms on the (2 × 1) reconstructed Si(001) surface and on the nonreconstructed Si(111) surface, as well as on the Si(111) surface with a SiC cluster, are calculated and analyzed. The values of migration barriers for adatoms on these surfaces are calculated. The effect of the SiC cluster on deformation of the surface region of Si(111) and on the migration of adatoms is investigated. The deep minima observed on the potential energy surfaces immediately above a cluster and at its boundaries can trap diffusing adatoms. The distributions of stresses and strains in the silicon lattice under a cluster on the surface are studied and described.     

Surface science of diamond: Familiar and amazing

The crystal structure of diamond is identical with that of its more common semiconductor relatives silicon and germanium. Consequently, a number of surface properties in terms of reconstructions, surface states and surface band diagrams are similar as in the case of Si or Ge. But diamond also exhibits a number of unusual and potentially very useful surface properties. Particularly when the surface dangling bonds are saturated by monovalent hydrogen atoms (donor-like), surface states are removed from the gap, the electron affinity changes sign and becomes negative, and the material becomes susceptible to an unusual type of transfer doping where holes are injected by acceptors located at the surface instead of inside the host lattice. These surface acceptors can in the simplest case be adsorbed molecules conveniently chosen by their electron affinity, but they can also be solvated ions within atmospheric water layers or electrolytes in contact with the hydrogenated diamond surface. In this article the surface properties of diamond will be reviewed with special emphasis on this new kind of doping mechanism.     

Elastic and inelastic properties of SiC/Si biomorphic composites and biomorphic SiC based on oak and eucalyptus

This paper reports on the results of a comparative investigation into the elastic and microplastic properties of biomorphic SiC/Si composites and biomorphic SiC prepared by pyrolysis of oak and eucalyptus with subsequent infiltration of molten silicon into a carbon matrix and additional chemical treatment to remove excess silicon. The acoustic studies were performed by the composite oscillator technique using resonant longitudinal vibrations at frequencies of about 100 kHz. It is shown that, in biomorphic SiC (as in biomorphic SiC/Si) at small-amplitude strains ε, adsorption and desorption of the environmental (air) molecules determine to a considerable extent the Young’s modulus E and the internal friction (decrement of acoustic vibrations δ) and that the changes in E and δ at these amplitudes are irreversible. The stress-microplastic strain curves are constructed from the acoustic data for the materials under study at temperatures of 100 and 290 K.     

Hydrogen adsorption on low-index surfaces of the PdFe alloy

The adsorption of hydrogen on the (001) and (110) surfaces of the PdFe alloy has been investigated using the projector-augmented-wave method within the generalized gradient Perdew-Burke-Ernzerhof (PBE) approximation for the exchange-correlation functional. The most preferred sites of hydrogen adsorption on the two surfaces have been determined. It has been shown that the hydrogen adsorption in hollow sites is more preferred on the (001) surface independently of its termination. The hydrogen adsorption in B1 bridge sites is energetically more favorable on the palladium-terminated (110) surface, whereas B2 bridge sites are more preferred on the iron-terminated (110) surface. The inclusion of the magnetism in the calculation leads to a decrease in the adsorption energy of hydrogen on the (001) and (110) surfaces, regardless of their termination, but does not change the tendencies revealed in variations of the binding energy in the nonmagnetic calculation. In general, the magnetism affects the position of hydrogen with respect to the surface layer and decreases the negative relaxation of interlayer distances in the cases of iron terminations of both surfaces, while hydrogen, in turn, decreases the magnetic moment of the nearest neighbor iron atoms. The paths of hydrogen diffusion from the surface deep into the material have been analyzed.     

Organic molecule adsorption on solid surfaces: chemical bonding, mutual polarisation and dispersion interaction

We discuss some of the most relevant bonding scenarios for the adsorption of organic molecules on solid surfaces from the perspective of first-principles calculations. The adsorption of uracil and phenanthrenequinone on Si(001) and the adsorption of adenine on Cu(110) and graphite(0001) surfaces serve as prototypical examples to highlight relevant molecule–substrate interactions and their consequences for the properties of the adsystem. Covalent bonds formed during organic reactions with semiconductor surfaces significantly modify the structural and electronic properties of both the adsorbed molecules and the substrate. Organic molecule adsorption on metals may be driven by mutual polarisation that leads to substantial charge transfer and rehybridisation, despite small adsorption energies. Subtle effects related to the lowering of the kinetic energy of the valence electrons as well as dispersion forces, finally, govern the interaction between the organic molecules and chemically inert substrates such as graphite. PACS 68.35.Md; 68.43.Bc; 68.43.-h; 73.20.-r; 82.39.Pj     

Stability and band offsets between Si and LaAlO<Subscript>3</Subscript>

The replacement of traditional SiO₂ with high-k oxides allows the physical thickness of the gate dielectric to be thinner without the tunneling problem in Si-based metal-oxide-semiconductor field-effect transistors. LaAlO₃ appears to be a promising high-k material for use in future ultra large scale integrated devices. In the present paper, the electronic properties of Si/LaAlO₃ (001) heterojunctions are investigated by first-principles calculations. We studied the initial adsorption of Si atoms on the LaAlO₃ (001) surface, and found that Si atoms preferentially adsorb on top of oxygen atoms at higher coverage. The surface phase diagrams indicate that Si atoms may substitute oxygen atoms at the LaO-terminated surface. The band offsets, electronic density of states, and atomic charges are analyzed for the various Si/LaAlO₃ heterojunctions. Our results suggest that the Si/AlO₂ interface is suitable for the design of metal oxide semiconductor devices because the valence and conduction band offsets are both larger than 1 eV.     

Atomic configurations during Si incorporation on GaAs(001) in As atmosphere evidenced by reflectance difference spectroscopy

The structural ordering of surface atoms during Si deposition on singular and vicinal GaAs(001) surfaces has been studied by reflectance difference (RD) spectroscopy using the difference function between the Si-covered and the bare surface. In dependence on the Si coverage the difference spectra correspond to RD spectra of the bare Si(001)-(1×2) or of the As-terminated Si(001):As(2×1) surface. This finding and the behaviour of RD transients recorded at 3.8 eV photon energy allows to define a (3×2)α phase with Si dimers in the top layer and Ga dimers in the third layer, and a (3×2)β phase with As-dimer rows on top of Si in the second layer.     

Theoretical modelling of surface phonons

We present a mini review of progress made towards theoretical modelling of surface phonons. We outline the essential ingredients of two theoretical methods, viz. an adiabatic bond charge method for semiconductor surfaces and the ab-initio density-functional perturbation method for solid surfaces in general. From the results of theoretical calculations we establish trends and criteria for the existence of localized phonon modes on group-IV(001) and III-V(110) semiconductor surfaces. We further obtain signatures of characteristic vibrational modes which develop during dissociative molecular adsorption on Si(001) surfaces. The results are compared with available experimental measurements. Some remarks are forwarded regarding manipulation of surface phonon modes for scientific advances and technological applications.