Z. J. Ding

Self-energy in surface electron spectroscopy: I. Plasmons on a free-electron-material surface

On the basis of a quantum approach, the formula for the complex self-energy, and hence also that for the energy-loss cross-section, are derived for fast electrons penetrating through the surface from the interior of a solid. Both the angular and depth dependence are included in the expression. Use of a Drude-Lindhard model bulk dielectric function for describing a bulk plasmon excitation in a free-electron-like material has yielded an analytical expression for the surface dielectric function which satisfies surface sum rules. The calculated imaginary and real parts of the electron self-energy for Si have indicated, in detail, the excitation processes of the surface plasmon and the bulk plasmon for an electron passing through the surface, with the dependency on electron energy and take-off angle. This approach provides quantitatively the electron inelastic scattering cross-section in the surface region for use in surface electron spectroscopy.

Self-energy in surface electron spectroscopy: II. Surface excitation on real metal surfaces

A scheme for calculating the complex self-energy of electrons moving in a real metal surface region is proposed. The approach is based on a quantum formula that uses a Drude-Lindhard model bulk dielectric function for describing free-electron metal. The experimental energy-loss function is fitted to a finite sum of the modelled energy-loss functions and a corresponding expression for the complex self-energy is derived. Calculated differential inelastic scattering cross-section results are given for Mg, Ag and Au to show the surface and bulk excitation modes in these metals. The z-dependent inelastic mean free path and stopping power near a surface region are also obtained. The approach provides a practical scheme to be used in quantitative surface electron spectroscopy.

Electron inelastic scattering and secondary electron emission calculated without the single pole approximation

In this work, aimed primarily at providing more accurate electron inelastic mean free paths (IMFPs) and stopping powers (SPs) at low energies than are provided by the single pole approximation, the “full Penn” algorithm has been employed to derive the electron inelastic scattering energy loss function in solids. IMFPs and SPs have thus been calculated in the energy range from 1 eV to 10 keV and are in good agreement with the experimental data. This treatment of electron inelastic scattering combined with a consistent model for the cascade secondary electron generation has enabled more elaborate Monte Carlo simulations of secondary electron emission from metals. The calculated results of the energy distributions and the secondary electron emission yields for Al and Cu agree reasonably with experimental results.

Spatially Resolved Distribution Function and the Medium-Range Order in Metallic Liquid and Glass

The structural description of disordered systems has been a longstanding challenge in physical science. We propose an atomic cluster alignment method to reveal the development of three-dimensional topological ordering in a metallic liquid as it undercools to form a glass. By analyzing molecular dynamic (MD) simulation trajectories of a Cu64.5Zr35.5 alloy, we show that medium-range order (MRO) develops in the liquid as it approaches the glass transition. Specifically, around Cu sites, we observe “Bergman triacontahedron” packing (icosahedron, dodecahedron and icosahedron) that extends out to the fourth shell, forming an interpenetrating backbone network in the glass. The discovery of Bergman-type MRO from our order-mining technique provides unique insights into the topological ordering near the glass transition and the relationship between metallic glasses and quasicrystals.

Electronic structure and transport of a carbon?chain between graphene nanoribbon leads

The electronic structure and transport property of a carbon chain between two graphene nanoribbon leads are studied using an ab initio tight-binding (TB) model and Landauers formalism combined with a non-equilibrium Greens function. The TB Hamiltonian and overlap matrices are extracted from first-principles density functional calculations through the quasi-atomic minimal basis orbital scheme. The accuracy of the TB model is demonstrated by comparing the electronic structure from the TB model with that from first-principles density functional theory. The results of electronic transport on a carbon atomic chain connected to armchair and zigzag graphene ribbon leads, such as different transport characters near the Fermi level and at most one quantized conductance, reveal the effect of the electronic structure of the leads and the scattering from the atomic chain. In addition, bond length alternation and an interesting transmission resonance are observed in the atomic chain connected to zigzag graphene ribbon leads. Our approach provides a promising route to quantitative investigation of both the electronic structure and transport property of large systems.

Monte Carlo simulation of secondary electron images for real sample structures in scanning electron microscopy

Monte Carlo simulation methods for the study of electron beam interaction with solids have been mostly concerned with specimens of simple geometry. In this article, we propose a simulation algorithm for treating arbitrary complex structures in a real sample. The method is based on a finite element triangular mesh modeling of sample geometry and a space subdivision for accelerating simulation. Simulation of secondary electron image in scanning electron microscopy has been performed for gold particles on a carbon substrate. Comparison of the simulation result with an experiment image confirms that this method is effective to model complex morphology of a real sample.

Photoluminescence and Raman spectra study of para-phenylenevinylene at low temperatures

The photoluminescence and Raman spectra of poly (para-phenylenevinylene) (PPV) have been investigated at low temperatures down to 83 K. With decreasing temperature the photoluminescence spectra peaks are shifted to the lower energy side while the peak intensities are enhanced. The fitted Huang–Rhys parameter for the conjugated length has indicated that the conjugated length is elongated by 2.2 repeat units from 294 to 83 K. Though a distinct shift for the Raman band frequencies was not found, the Raman band intensities are, however, intensified at low temperatures. The Raman band intensity ratio I1548/I1626 is enhanced with decreasing temperature, indicating that the disorder is reduced and the conjugated length is increased at low temperatures.

Validity of the semi-classical approach for calculation of the surface excitation parameter

The problem of surface plasmon excitation by moving charges has been elaborated by several different approaches, mainly based on dielectric response theory within either semi-classical or quantum mechanical frameworks. In this work, a comparison of the surface excitation effect between two different frameworks is made by calculation of the differential inverse inelastic mean free path (DIIMFP) and a Monte Carlo simulation of reflection electron energy loss spectroscopy (REELS) spectra. A semi-classical modeling of the interaction between electrons and a solid surface is based on analyzing the work done by moving electrons; the stopping power and inelastic cross section are derived with the induced potential. On the other hand, a quantum mechanical approach is based on derivation of the complex inhomogeneous self-energy of the electrons. The numerical calculation shows that the semi-classical model presents almost the same values of DIIMFP as by the quantum model except at the glancing condition. The simulation of REELS spectra for Ag and SiO(2) as well as a comparison with experimental spectra also confirms that a good agreement with the spectral shape is found among the two simulation results and the experimental data.

Gamma-irradiation-induced Ag/SiO2 composite films and their optical absorption properties

Abstract Small Ag particles or clusters dispersed mesoporous SiO 2 composite films were prepared by a new method: First the matrix SiO 2 films were prepared by sol–gel process combined with the dip-coating technique, then they were soaked in AgNO 3 solutions followed by irradiation of γ-ray at room temperature and in ambient pressure. The structures of these films were examined by X-ray diffraction (XRD), high-resolution transmission electron microscope (HRTEM), and optical absorption spectroscopy. It has been shown that the Ag particles grown within the porous SiO 2 films are very small, and they are isolated and dispersed from each other with very narrow size distributions. With increasing the soaking concentration and an additional annealing, an opposite peakshift effect of the surface plasmon resonance (SPR) was observed in the optical absorption measurements.

Monte Carlo Simulation of CD‐SEM Images for Linewidth and Critical Dimension Metrology

In semiconductor industry, strict critical dimension control by using a critical dimension scanning electron microscope (CD-SEM) is an extremely urgent task in near-term years. A Monte Carlo simulation model for study of CD-SEM image has been established, which is based on using Motts cross section for electron elastic scattering and the full Penn dielectric function formalism for electron inelastic scattering and the associated secondary electron (SE) production. In this work, a systematic calculation of CD-SEM line-scan profiles and 2D images of trapezoidal Si lines has been performed by taking into account different experimental factors including electron beam condition (primary energy, probe size), line geometry (width, height, foot/corner rounding, sidewall angle, and roughness), material properties, and SE signal detection. The influences of these factors to the critical dimension metrology are investigated, leading to build a future comprehensive model-based library.