Effects of level broadening and electron overheating in tunneling structures based on metal clusters

The effect of level broadening and electronic subsystem overheating of disk-shaped and spherical gold clusters/islands on the current-voltage characteristic of the three-electrode structure has been studied. A scheme for calculating the electronic level broadening in the one-dimensional case of rectangular barriers has been proposed. In the two-temperature electron-ion model of a metal cluster, taking into account the size dependence of the Debye frequency, the kinetic electron temperature has been estimated as a function of the bias voltage. At low ion temperatures, the broadening and electron overheating effects result in disappearance of steps of quantum and Coulomb staircases, i.e., a strong smoothness of the current-voltage characteristic even in structures on clusters consisting of a denumerable number of atoms, which is actually observed experimentally.     

Silicon dioxide modification by an electron beam

The overheating temperature of a microvolume of silicon dioxide produced by bombardment by a high specific-power electron beam has been estimated. Calculations showed that the maximum temperature to which a microvolume of silicon dioxide is overheated can be as high as 1200°C for an electron beam current of 100 nA. The variation in the cathodoluminescence characteristics of amorphous silica with different contents of hydroxyl groups was studied for various electron beam specific-power levels. The impact of a high specific-power electron beam was shown to create additional lattice defects up to the formation of silicon clusters.     

Anions of xenon clusters bound by long-range electron correlations

In contrast with the single atom, atomic van der Waals clusters can form stable anions where the excess electron is bound due to long-range correlations with the electrons of the cluster. We report on extensive all-electron many-body ab initio studies on Xe clusters. Three-dimensional, planar, and linear structures of the clusters are investigated and compared. In particular, we find that the minimal number of Xe atoms in the cluster required to form a stable anion is 5 independently of the dimensionality of the cluster. We provide electron affinities for clusters made of 5, 6, and 7 atoms in all dimensions and find that the planar clusters form the most stable anions. The Dyson orbitals of the excess electrons are computed and analyzed.     

Effects of single-electron charging in a tunnel structure with a metallic cluster

The effects of single-electron tunnel charging and Coulomb blockade in a cluster structure (molecular transistor) are studied theoretically with allowance for the quantization of the electronic levels in an island electrode. The electronic spectrum is calculated for small spherical and disk-shaped clusters. Under the assumption that the total energy of the system is conserved with inclusion of the contact potential difference, equations are derived for analyzing the current-voltage characteristic. Limitations associated with the Coulomb instability of a cluster and with electron relaxation are introduced into the theory. For single-electron transistors with small gold clusters, the current gap and the asymmetry in its position on the voltage axis are calculated. The current gap is shown to vary nonmonotonically with the cluster size.     

Electrophonon resonance in narrow band semiconductors

The theory of the electric current splashes on a current-voltage characteristic due to the phonon-induced-electron-transitions between Stark ladder levels is developed. Splitting and broadening of the current peaks because of an electric field vector deviation from the [ 111 ] direction is considered. The recent experimental results on current-voltage characteristic of the tellurium cluster superlattice in ceolite are accounted for by the electrophonon resonance studied in the present paper.     

Supersonic cluster beam deposition of nanostructured titania

Nanostructured titanium dioxide films have been deposited by supersonic cluster beam deposition (CBD). Nanoparticles are produced by a pulsed microplasma cluster source (PMCS) and selected by aerodynamic separation effects. The as-deposited film is a complex mixture where amorphous material coexists, at the nanoscale, with anatase and rutile crystal phases. The nanocrystalline fraction of the film is characterized by crystal size ranging from 100 nm to less than 5 nm. We have characterized the film structure by transmission electron microscopy, Raman spectromicroscopy, X-ray diffraction, and UV-visible spectroscopy showing that correlations exist between cluster size and film properties. In particular if very small clusters are deposited, the film shows a predominant rutile phase whereas larger clusters form films with mainly anatase structure. Our observations suggest that phonon confinement effects are responsible for a significant shift and broadening observed for the Raman peaks. In addition, optical gap tuning is provided by mass selection: large clusters assembling generates a film with 3.22 eV optical gap, while smallest clusters 3.52 eV.     

Electronic excitation processes in rare gas clusters studied by electron energy loss spectroscopy

We present the electron energy loss spectra for Ar clusters as a function of incident electron energy and of cluster size. In spectra measured with 100 eV incident electron energy the bulk excitation peak becomes visible for a mean cluster size above 170 atoms per cluster. For 250 eV incident electron energy the bulk excitation peak is clearly observable even for a mean cluster size of 120 atoms per cluster. These experimental results are qualitatively reproduced by a simple calculation that accounts for the mean free path of electrons in Ar clusters; i.e., the penetration depth of incident electrons into the cluster.     

The anomalous thermal radiation from metals produced by ultrashort laser pulses. I

The spectral-time characteristics of the secondary radiation induced in silver and tungsten by picosecond laser pulses of varying duration and fluence have been studied theoretically and experimentally. It is established that the secondary radiation is due to heating, but in silver, nevertheless, it is not usual and does not correspond, for example, to grey-body radiation. This radiation—the anomalous thermal radiation proceeds by the following mechanism: When the electron and phonon subsystems in a metal are heated (including also the nonisothermal process), there appears a glow with a continuous spectral distribution and an intensity exceeding the radiation intensity of the grey body whose temperature is equal to the temperature of the ion or electron subsystem of a metal. This anomaly is either due to overheating of the electron subsystem with respect to the ion subsystem or due to recombination of electrons and thermions in a microlayer above the metal surface.     

Unsupported lead clusters and electron diffraction

A beam of Pb clusters is produced with the inert gas aggregation method and probed by electron diffraction. Analysis of the diffraction patterns indicates that average cluster size can vary between 3 and 7 nm, according to nucleation conditions. The diffraction patterns from beams with larger average cluster size are very similar to patterns calculated from model decahedron clusters, while those for smaller cluster size do not appear to have simple geometrical face-centred cubic, decahedral, or icosahedral structure. Received 30 November 2000     

Finite cluster studies of electron correlation effects

A review of the physical properties of small systems described by various model Hamiltonians is given. Their physical properties are found either exactly or by using Monte-Carlo methods, and are compared with properties of bulk systems. It is shown that (i) clusters differ in an essential way from the bulk, having electronic states strongly dependent on the geometry and cluster size, but, at the same time, (ii) important insights into the nature of electron correlation and low-energy excitations in solids may be gained by cluster studies. In particular, the cluster calculations allow one to identify the tendencies to various kinds of symmetry-broken states in the (extended) Hubbard model and give a simple qualitative understanding of the low temperature properties of the Kondo and the heavy-fermion systems. Furthermore, cluster calculations allow one to study such problems as the development of magnetic moment at chemisorbed atoms for strong correlations and the suppression of frustration by quantum fluctuations. Recently, cluster calculations gave important information about the possible role of charge transfer processes and magnetic interactions in the microscopic mechanism of pairing in high temperature superconducting oxides.     

Observation of electron localization in rough gold nanoclusters on the graphite surface

The results of scanning tunneling spectroscopy of the electronic states of Au nanoclusters on the graphite surface are presented. The tunneling current is found to be different at different points of a rough-surface nanocluster. The measured differential current-voltage curve of the clusters is nonmonotonic near the Fermi energy, and the tunneling conductance decreases by almost a factor of two as the cluster volume changes from 1 to 0.1 nm³. This decrease can be associated with the change in the density of the electronic states near the Fermi energy. The observed features are qualitatively described within the framework of the mechanism of electron localization in disordered systems.     

Low-energy electron bremsstrahlung from metal clusters

Expressions are derived allowing numerical calculation of the matrix element of the dipole moment between states that belong to a continuous spectrum for the case where a charged particle in an arbitrary orbital state is scattered by a spherically symmetric short-range potential. The calculation scheme is based on expanding a diverging matrix element into a sum of converging integrals. The method is used to study static bremsstrahlung of a slow electron scattered by sodium clusters. The bremsstrahlung cross section is shown to be resonant. The resonances arise at certain energies (depending on a cluster) of an electron having emitted a photon and are due to electron capture on quasi-steady-state levels.     

Effects of solvation on dissociative electron attachment to methyl iodide clusters

Using laser photoelectron attachment to methyl iodide clusters in a differentially-pumped seeded supersonic helium beam and mass spectrometric ion detection, we have measured the rate coefficients for formation of (q = 0-2) ions over the electron energy range 0-100 meV with an effective energy width of about 2.5 meV. Whereas a prominent vibrational Feshbach resonance just below the onset for the C-I stretch vibration ( ) is observed for dissociative attachment to monomers (yielding I^- ions), only weak and broad structure, shifted to lower energies, is detected for formation of ions and essentially no structure is left in the attachment spectrum for . These observations are interpreted by model R-matrix calculations which successfully describe the DA cross-section for the monomer and qualitatively recover the trend observed for cluster ion formation. For the clusters, the effects of increased electron-target long-range interaction and of solvation as well as coupling to soft vibrational modes lead to strong broadening and shifting of the vibrational Feshbach resonance and, ultimately, to its disappearance. Received 29 November 1999 and Received in final form 14 January 2000     

仪器展宽对大气压等离子体电子密度测量的影响

实验使用两台不同的单色仪,采用光谱线型法测量了大气压氩气介质阻挡放电中的电子密度.诊断结果表明,由于不同的单色仪其仪器加宽不同,仪器加宽对总的光谱线型有较大影响.通过考虑等离子体中的各种加宽机制,采用卷积和反卷积的方法对氩原子发射谱线线型进行了分析,从整个光谱线型中分离出Stark线型,排除了仪器加宽对最终诊断结果的影响.从而最终测量了大气压氩气介质阻挡放电中的电子密度.测量得到在大气压氩气介质阻挡放电中单个放电丝存在时,电子温度为10000K时,电子密度约为3.05-3.26×1021 m-3.此方法不仅可以应用在大气压介质阻挡放电中,还可以用于测量其它大气压等离子体电子密度.     

Oscillatory magnetization of two-dimensional electron gas

Diamagnetic moment of the two-dimensional electron gas is calculated for finite temperature using a simple energy band model and assuming no collisional broadening of Landau levels. Numerical results are presented for GaAs band parameters. The diamagnetic moment oscillates around zero value as a function of magnetic field strength and tends to zero at low fields. It is concluded that both the diamagnetic and the paramagnetic susceptibilities of the 2D electron gas vanish in the low field limit.     

Nondissociative electron attachment to molecules and clusters

We present results of effective range theory calculations of nondissociative electron attachment to SF₆ molecules and CO₂ clusters. The first process is strongly influenced by the SF^-₆ virtual state, and the second by vibrational Feshbach resonances associated with electron capture by the long-range polarization field of the cluster with simultaneous vibrational excitation of one molecular unit. We also study how both processes depend on the initial vibrational excitation of the target. We obtain a noticeable dependence of the attachment cross-section on the symmetric stretch vibration of SF₆, although it does not lead to a significant temperature dependence at low electron energies.     

Structure of unsupported bismuth nanoparticles

We present new results of electron diffraction experiments on unsupported nanometer-sized bismuth clusters. The high intensity cluster beam, necessary for electron diffraction, is provided by an inert-gas aggregation source. The cluster beam contains particles with average cluster sizes between 4.5 and 10 nm. When using Helium as a carrier gas we are able to observe a transition from crystalline clusters to a new structure, which we identify with that of amorphous or liquid clusters. Received 28 November 2000     

Analysis of thermal effects accompanying the interaction of electron beams with ferroelectric crystals

A spatial nonstationary temperature distribution in a model ferroelectric specimen exposed to electron beams of a scanning electron microscope (SEM) is simulated. A solution to the problem on the effect of focused heat sources having different configurations on a single ferroelectric crystal and crystal with electrodes deposited on its faces is presented. The model is based on the finite-difference method. A program for calculating temperature fields and maximum overheating temperature for preset specimen characteristics and experimental parameters of different scanning electron microscopes is suggested.     

Sensitivity of PbSnTe:In films to submillimeter radiation under conditions of field electron injection

Based on the concept of injection currents in the PbSnTe:In alloy, the mechanism of photosignal formation in PbSnTe:In films in the submillimeter spectral range at liquid-helium temperatures was considered. It was shown that the existence of energy-distributed electron trap levels controlling the features in current-voltage characteristics can cause extrinsic photoconductivity. For such sensitivity to submillimeter radiation, its spectral dependence should be related to the injection level controlling the filling of traps with different energies. The sensitivity initiation mechanism caused by the interaction of submillimeter radiation with the PbSnTe:In phonon subsystem was considered. Such a process can result in an increase in the static permittivity and, hence, an increase in the injection current.