Yan-Hom Li

Electron inelastic interactions with overlayer systems

An overlayer system composed of a thin film on the top of a semi-infinite substrate was studied in this work for electron inelastic interactions. Analytical expressions for the depth-dependent inelastic differential and integral inverse mean free paths were derived for both incident and escaping electrons. The interface (film-substrate) effect and the surface (vacuum-film) effect were analyzed by comparing the results of an overlayer system and a semi-infinite system. It was found that the interface effect extended to several angstroms on both sides of the interface for a 500 eV electron incident into or escaping from the vacuum–SiO2–Si and the vacuum–Au–Ni systems. An application of the spatial-varying inelastic differential inverse mean free paths was made by Monte Carlo simulations of the electron elastic backscattering from an overlayer system. Good agreement was found between results calculated presently and data measured experimentally on the elastic reflection coefficient.

Energy losses of charged particles moving parallel to the surface of an overlayer system

An energetic charged particle moving parallel to the surface of an overlayer system was studied. This system was composed of a thin film on the top of a semi-infinite substrate. Based on the dielectric response theory, the induced potential was formulated by solving the Poisson equation and matching the boundary conditions. The stopping force was built-up using the energy-momentum conservation relations and the extended Drude dielectric functions with spatial dispersion. Surface (vacuum–film) and interface (film–substrate) excitations were included in the formulations of the interaction between charged particles and the overlayer system. Results of the wake potential were presented for protons moving parallel to a vacuum–copper–silicon system. Dependences of the induced potential and the stopping force on film thickness, distance of the proton from surface, and proton velocity were investigated.

Manipulations of vibrating micro magnetic particle chains

We investigate the motion of a micro-chain consisting of several magnetic particles. The chain is firstly formed by a uniform directional field, and then manipulated by a vibrating field. We demonstrate where the chain appears to display distinct behaviors, from rigid body vibrations, bending distortions to breaking failures, by increasing either the chain’s length or vibrating amplitude. In addition, the vibrating chain can be successfully driven forward, mimicking a micro-swimmer by connecting particles of different sizes.

Steering of Magnetic Micro-Swimmers

We experimentally investigate the motion of micro-swimmers consisting of several magnetic particles with different sizes. Swimmers are firstly formed by a static unidirectional field, and then manipulated by an additional dynamical perpendicular field. It is known that such magnetic-particle swimmers (or chains) driven by an external field would oscillate with the orientation of the field but lagging behind by a certain phase angle. In this work, we demonstrate if a swimmer subjected to a strong oscillating field which results in an instantaneous phase lag greater than π/2, the swimmer can be steered perpendicularly to its original direction. Detailed swimming mechanism and trajectory are presented. By this innovative steering technology, orientation of a micro-swimmer can be effectively manipulated without a physical reconfiguration of the external field arrangement.

Magnetic microchains and microswimmers in an oscillating magnetic field

Superparamagnetic micro-bead chains and microswimmers under the influence of an oscillating magnetic field are studied experimentally and numerically. The numerical scheme composed of the lattice Boltzmann method, immersed boundary method, and discrete particle method based on the simplified Stokesian dynamics is applied to thoroughly understand the interaction between the micro-bead chain (or swimmer), the oscillating magnetic field, and the hydrodynamics drag. The systematic experiments and simulations demonstrated the behaviors of the microchains and microswimmers as well as the propulsive efficiencies of the swimmers. The effects of key parameters, such as field strengths, frequency, and the lengths of swimmer, are thoroughly analyzed. The numerical results are compared with the experiments and show good qualitative agreements. Our results proposed an efficient method to predict the motions of the reversible magnetic microdevices which may have extremely valuable applications in biotechnology.

Energy spectra of electrons quasi-elastically backscattered from solid surfaces

The energy distribution of electrons quasi-elastically backscattered from solids has been investigated. Monte Carlo (MC) simulations were performed for the study of the recoil energy shift and the broadening of this distribution for backscattered electrons from Si and Au. In these simulations, electron interaction cross sections were obtained from calculations based on the dielectric response theory for inelastic interactions, including volume and surface excitations, and elastic interactions. The depth-dependent electron inelastic mean free path for volume excitations and the probability of surface excitations were calculated using the dielectric functions derived from optical data. The relativistic partial-wave expansion method was applied to calculate the elastic scattering cross section for a potential of the atom in the solid. The Rutherford-type recoil energy was included in the MC simulations by either considering or neglecting the thermal effect of atomic vibrations. Such an effect was applied using the single scattering model. The intensity of electrons quasi-elastically backscattered from Si and Au was simulated for incident electrons of an energy distribution. The adjustment for the spectrometer energy resolution was allowed. An analytic expression for the intensity of backscattered electrons by a single scattering was derived explicitly. A comparison of simulated results with experimental data was made and discussed.

Retardation effect on energy losses of electrons moving parallel to solid surfaces

When a charged particle moves parallel and close to a solid surface, it suffers an energy loss arising from the induced potentials caused by the interactions between the charged particle and the surface. For the fast moving charged particle, the induced potentials could be affected by the electromagnetic retardation effect. In the present work, the retardation effect on the induced potentials was studied using a dielectric function with spatial dispersion for an electron of high energy moving parallel to the solid surface. Appropriate boundary conditions and the Lorentz gauge were employed to calculate the induced potentials by solving Maxwell equations in the Fourier space using the dielectric response theory. Analytical formulas of the differential inverse inelastic mean free path (DIIMFP), inelastic mean free path (IMFP), and stopping power (SP) were derived by considering the retardation effect using relativistic energy and momentum conservation relations and applying the extended Drude dielectric fun...