α-P/GaAs(100)界面的光电子能谱研究

本文采用X射线光电子能谱、紫外光电子能谱和低能电子衍射对室温下P在GaAs(100)表面上的生长进行了研究。结果表明,在生长初期P是成团吸附的,随着淀积量的增加而生长成α-P薄膜,该薄膜的价带结构与等离子体淀积的α-P:H薄膜的价带结构相似。在界面处有约一单层的P与衬底表面的Ga成键。α-P覆盖层使GaAs表面势垒下降约0.2eV。 关键词：     

Complete wetting of pits and grooves

For one-component volatile fluids governed by dispersion forces an effective interface Hamiltonian, derived from a microscopic density functional theory, is used to study complete wetting of geometrically structured substrates. Also the long range of substrate potentials is explicitly taken into account. Four types of geometrical patterns are considered: i) one-dimensional periodic arrays of rectangular or parabolic grooves and ii) two-dimensional lattices of cylindrical or parabolic pits. We present numerical evidence that at the centers of the cavity regions the thicknesses of the adsorbed films obey precisely the same geometrical covariance relation, which has been recently reported for complete cone and wedge filling. However, this covariance does not hold for the laterally averaged wetting film thicknesses. For sufficiently deep cavities with vertical walls and close to liquid-gas phase coexistence in the bulk, the film thicknesses exhibit an effective planar scaling regime, which as a function of undersaturation is characterized by a power law with the common critical exponent -1/3 as for a flat substrate, but with the amplitude depending on the geometrical features.     

Titanium carbonitride films on cemented carbide cutting tool prepared by pulsed high energy density plasma

Hard films prepared by pulsed high energy density plasma (PHEDP) are characterized by high film/substrate adhesive strength, and high wear resistance. Titanium carbonitride (TiCN) films were deposited onto YG11C (ISO G20) cemented carbide cutting tool substrates by PHEDP at room temperature. XRD, XPS, SEM, AES, etc. were adopted to analyze the phases (elements) composition, microstructure and the interface of the films, respectively. The results show that, the uniform dense films are composed of grains ranging from 70 to 90 nm. According to the AES result, there is a broad transition layer between the film and the substrate, due to the ion implantation effect of the PHEDP. The transition layer is favorable for the film/substrate adhesion.     

Unifying interfacial self-assembly and surface freezing

X-ray investigations reveal that the monolayers formed at the bulk alkanol-sapphire interface are densely packed with the surface-normal molecules hydrogen bound to the sapphire. About 30-35 °C above the bulk, these monolayers both melt reversibly and partially desorb. This system exhibits balanced intermolecular and molecule-substrate interactions which are intermediate between self-assembled and surface-frozen monolayers, each dominated by one interaction. The phase behavior is rationalized within a thermodynamic model comprising interfacial interactions, elasticity, and entropic effects. Separating the substrate from the melt leaves the monolayer structurally intact.     

Melting of thin films of alkanes on magnesium oxide

Recent incoherent neutron scattering investigations of the dynamics of thin alkane films adsorbed on the Magnesium Oxide (100) surface are reported. There are marked differences in the behaviour of these films, as a function of temperature and coverage, compared to similar measurements on graphite. In particular, it has previously been shown that adsorbed multilayer films on graphite exhibit an interfacial solid monolayer that coexists with bulk-like liquid, well above the bulk melting point. In contrast, these studies show that the alkane films on MgO exhibit no such stabilization of the solid layer closest to the substrate as a function of the film thickness, even though the monolayer crystal structures are remarkably similar. These studies are supported by extensive thermodynamic data, a growing body of structural data from neutron diffraction and state of the art computer modelling     

Impact of an irreversibly adsorbed layer on local viscosity of nanoconfined polymer melts

We report the origin of the effect of nanoscale confinement on the local viscosity of entangled polystyrene (PS) films at temperatures far above the glass transition temperature. By using marker x-ray photon correlation spectroscopy with gold nanoparticles embedded in the PS films prepared on solid substrates, we have determined the local viscosity as a function of the distance from the polymer-substrate interface. The results show the impact of a very thin adsorbed layer (~7 nm in thickness) even without specific interactions of the polymer with the substrate, overcoming the effect of a surface mobile layer at the air-polymer interface and thereby resulting in a significant increase in the local viscosity as approaching the substrate interface.     

Influence of the metal nanofilm-semiconductor interface on surface properties of the nanofilm: The CO-Yb-Si(111) system

The influence of the “ytterbium nanofilm-single-crystal silicon substrate” interface on properties of the films has been investigated. It has been shown that, if the film thickness is less than 10 monolayers, the Friedel oscillations (standing waves of electron density) generated by the interface affect the work function of the films and the rate of adsorption of CO molecules on their surface. In turn, the CO molecules modify the electronic structure of ytterbium during adsorption on the surface of nanofilms by transforming ytterbium from the divalent to trivalent state. The completely filled layer of adsorbed CO molecules consists of two phases. The first phase is a two-dimensional gas whose molecules weakly interact with each other, but their lone electron pairs form a donor-acceptor bond with the Yb 5d level; as a result, this level is located below the Fermi level and the metal transforms into the trivalent state. After filling the two-dimensional phase, the second (island) phase, in which the CO molecule are bound by horizontal π-bonds, begins to grow. The formation of these bonds becomes possible due to the filling of 2π states in the molecules upon compaction of the adsorbed layer. The considered the adsorbed two-phase layer is responsible for the complete transition of ytterbium into the trivalent state.     

Controlling wear failure of graphite-like carbon film in aqueous environment: Two feasible approaches

Friction and wear behaviors of graphite-like carbon (GLC) films in aqueous environment were investigated by a reciprocating sliding tribo-meter with ball-on-disc contact. Film structures and wear scars were studied by scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and a non-contact 3D surface profiler. A comprehensive wear model of the GLC film in aqueous environment was established, and two feasible approaches to control critical factor to the corresponding wear failure were discussed. Results showed that wear loss of GLC films in aqueous environment was characterized by micro-plough and local delamination. Due to the significant material loss, local delamination of films was critical to wear failure of GLC film in aqueous environment if the film was not prepared properly. The initiation and propagation of micro-cracks within whole films closely related to the occurrence of the films delamination from the interface between interlayer and substrate. The increase of film density by adjusting the deposition condition would significantly reduce the film delamination from substrate, meanwhile, fabricating a proper interlayer between substrate and GLC films to prevent the penetration of water molecules into the interface between interlayer and substrate could effectively eliminate the delamination.     

Reversible sphere-to-lamellar wetting transition at the interface of a diblock copolymer system

We use ellipsometry to investigate a transition in the morphology of a sphere-forming diblock copolymer thin-film system. At an interface the diblock morphology may differ from the bulk when the interfacial tension favours wetting of the minority domain, thereby inducing a sphere-to-lamella transition. In a small, favourable window in energetics, one may observe this transition simply by adjusting the temperature. Ellipsometry is ideally suited to the study of the transition because the additional interface created by the wetting layer affects the polarisation of light reflected from the sample. Here we study thin films of poly(butadiene-ethylene oxide) (PB-PEO), which order to form PEO minority spheres in a PB matrix. As temperature is varied, the reversible transition from a partially wetting layer of PEO spheres to a full wetting layer at the substrate is investigated.     

Improvement of metal–ferroelectric–silicon structures without buffer layers between Si and ferroelectric films

Si-based metal–ferroelectric–semiconductor (MFS) structures without buffer layers between Si and ferroelectric films have been developed by depositing SrBi₂Ta₂O₉ (SBT) directly on n-type (100)-oriented Si. Some effective processes are adopted to improve the electrical properties of these MFS structures. Contrary to the conventional MFS structures with top electrodes directly on ferroelectrics, our MFS structures have been developed with thin dense SiO₂ films deposited between ferroelectric films and top electrodes. Due to the SiO₂ films, the leakage current densities of MFS structures are reduced to 2×10^-8 A/cm² under the bias of 5 V. The C-V electrical properties of the MFS structures are greatly improved after annealing at 400 °C in N₂ ambient for 1 h. The C-V memory windows are increased to 3 V, which probably results from the decrease of the interface trap density at the Si/SBT interface. Received: 7 September 1999 / Accepted: 24 November 1999 / Published online: 2 August 2000     

Optical and electrophysical properties of nanocomposites based on PEDOT: PSS and gold/silver nanoparticles

The absorption spectra in the visible region and current-voltage characteristics in a wide range of electric fields have been investigated at the macroscopic level (planar structures) and at the microscopic level (using a conductive atomic force microscope) in films based on the electroactive polymer PEDOT: PSS and gold/silver nanoparticles (PEDOT: PSS + Au/AgNP). It has been shown that the behavior of the current-voltage characteristics of the nanocomposite films depends significantly on the electric field strength. It has been found that the introduction of gold nanoparticles into PEDOT: PSS in weak electric fields leads to an increase in the bulk conductance by almost two orders of magnitude (due to donor-acceptor interactions), a 50% decrease in the conduction activation energy, and an increase in the sensitivity to adsorbed oxygen. It has been demonstrated that electrical conduction of PEDOT: PSS + AuNP films is provided by hopping charge transfer both in the system of intrinsic localized states and in the system of impurity states of adsorbed oxygen. In strong electric fields, the current-voltage characteristics exhibit a different behavior in the forward and reverse scanning modes.     

Luminescence of molecules adsorbed on surfaces of wide gap materials

We report on the photoluminescence spectra of thin films of chain-like π-conjugated molecules on well-defined surfaces of wide gap inorganic materials. The aim is to study the energy transfer processes across the organic/substrate interface which limit the luminescence of molecules in close contact to substrate surfaces. We discuss quaterthiophene (4 T) adsorbed on the c(2×2)-ZnSe(0 0 1) surface and tetracene (Tc) on the surface of a thin epitaxial Al₂O₃ film on Ni₃Al(1 1 1). For thin films vapor-deposited at low substrate temperatures, we observe no luminescence signal for both systems, which indicates the presence of fast luminescence quenching processes at the organic/inorganic interface. After annealing, luminescence spectra corresponding to those of the bulk crystals are obtained. This can be explained by the formation of 3D-crystallites, which effectively separate most molecules from the organic/inorganic interface.     

Second harmonic generation in planar optical waveguides

We study optical second harmonic generation (SHG) in planar waveguide structures composed of several layers with different dielectric constants. We develop a general formalism for the calculation of mode generation by a planar antenna embedded in the waveguide. As an application we consider a monolayer of high second-order susceptibility adsorbed at the interface between two layers of the waveguide structure. Periodic modulation of the nonlinear susceptibility allows phase matching leading to dramatically enhanced second harmonic intensities. We investigate the SHG-efficiency of various experimentally realizable geometries.     

Band structure of Au layers on the Ru(0001) and graphene/Ru(0001) surfaces

The electronic structures of Au monolayers on the Ru(0001) and graphene-coated Ru(0001) surfaces have been calculated by DFT method using the supercell (repeated-slab) approach. The local densities of states (LDOS) and band structures of the monolayer and bilayer Au films adsorbed on the graphene/Ru(0001) and those of free hexagonal Au layers are found to be very similar. This result indicates that the monolayer graphene almost completely screens the Au layers from the Ru(0001) substrate surface, so that electronic properties of Au films adsorbed on graphene are determined predominantly by the electronic structure of the Au adlayers, essentially independent on the electronic structure of the substrate surface.     

Electronic structures and coronene on Ru（0001）： vibrational properties of first-principles study

We calculate the configurations,electronic structures,vibrational properties at the coronene/Ru(0001) interface,and adsorption of a single Pt atom on coronene/Ru(0001) based on density functional theory calculations.The geometric structures and electronic structures of the coronene on Ru(0001) are compared with those of the graphene/Ru(0001).The results show that the coronene/Ru(0001) can be a simplified model system used to describe the interaction between graphene and ruthenium.Further calculations of the vibrational properties of coronene molecule adsorbed on Ru(0001) suggest that the phonon properties of differently corrugated regions of graphene on Ru(0001) are different.This model system is also used to investigate the selective adsorption of Pt atoms on graphene/Ru(0001).The configurations of Pt on coronene/Ru(0001) with the lowest binding energy give clues to explain the experimental observation that a Pt cluster selectively adsorbs on the second highest regions of graphene/Ru(0001).This work provides a simple model for understanding the adsorption properties and vibrational properties of graphene on Ru(0001) substrate.     

Electronic structures and vibrational properties of coronene on Ru(0001): first-principles study

We calculate the configurations, electronic structures, vibrational properties at the coronene/Ru(0001) interface, and adsorption of a single Pt atom on coronene/Ru(0001) based on density functional theory calculations. The geometric structures and electronic structures of the coronene on Ru(0001) are compared with those of the graphene/Ru(0001). The results show that the coronene/Ru(0001) can be a simplified model system used to describe the interaction between graphene and ruthenium. Further calculations of the vibrational properties of coronene molecule adsorbed on Ru(0001) suggest that the phonon properties of differently corrugated regions of graphene on Ru(0001) are different. This model system is also used to investigate the selective adsorption of Pt atoms on graphene/Ru(0001). The configurations of Pt on coronene/Ru(0001) with the lowest binding energy give clues to explain the experimental observation that a Pt cluster selectively adsorbs on the second highest regions of graphene/Ru(0001). This work provides a simple model for understanding the adsorption properties and vibrational properties of graphene on Ru(0001) substrate.     

Thermal stress reduction of GaAs epitaxial growth on V-groove patterned Si substrates

We investigate the thermal stresses for GaAs layers grown on V-groove patterned Si substrates by the finite-element method. The results show that the thermal stress distribution near the interface in a patterned substrate is nonuniform,which is far different from that in a planar substrate. Comparing with the planar substrate, the thermal stress is significantly reduced for the Ga As layer on the patterned substrate. The effects of the width of the V-groove, the thickness, and the width of the SiO₂ mask on the thermal stress are studied. It is found that the SiO₂ mask and V-groove play a crucial role in the stress of the Ga As layer on Si substrate. The results indicate that when the width of V-groove is 50 nm, the width and the thickness of the SiO₂ mask are both 100 nm, the Ga As layer is subjected to the minimum stress. Furthermore,Comparing with the planar substrate, the average stress of the Ga As epitaxial layer in the growth window region of the patterned substrate is reduced by 90%. These findings are useful in the optimal designing of growing high-quality Ga As films on patterned Si substrates.     

A simulation study of films of n-hexane and n-perfluorohexane on a solid surface

The behaviour of silica-supported films of liquid n-hexane and n-perfluorohexane has been investigated by means of molecular dynamics simulations of model compounds. Thin films of both neat liquids and equimolar mixtures were studied with different surface interaction strengths at room temperature (300 K). The molecules tend to form layers parallel to the surface of the substrate. In agreement with recent experimental results for hexane, a low density region was found near the surface, provided that the interaction strength was low. There is no corresponding low density region for perfluorohexane. The study of the mixture indicates that perfluorohexane is preferentially adsorbed at both solid-liquid and liquid-vapour interfaces while the molecules of hexane are concentrated inside the film.     

Enhanced charge transfer in a monolayer of the organic charge transfer complex TTF-TNAP on Au(111)

Electronic doping is a key concept for tuning the properties of organic materials. In bulk structures, the charge transfer between donor and acceptor is mainly given by the respective ionization potential and electron affinity. In contrast, monolayers of charge transfer complexes in contact with a metal are affected by an intriguing interplay of hybridization and screening at the metallic interface, determining the resulting charge state. Using scanning tunneling microscopy and spectroscopy, we characterize the electronic properties of the organic acceptor molecule 11,11,12,12-tetracyanonaptho-2,6-quinodimethane (TNAP) adsorbed on a Au(111) surface. The ordered islands remain in a weakly physisorbed state with no charge transfer interaction with the substrate. When the electron donor tetrathiafulvalene (TTF) is added, ordered arrays of alternating TNAP and TTF rows are assembled. In these structures, we find the lowest unoccupied molecular orbital (LUMO) of the free TNAP molecule shifted well below the Fermi level of the substrate. The TNAP is thus charged with more than one electron.