Controlled lateral and perpendicular motion of atoms on metal surfaces

Ankara : The Department of Physics and the Institute of Engineering and Science of Bilkent University, 1994.Thesis (Master's) -- Bilkent University, 1994.Includes bibliographical references leaves 64-69.Nanoscale modification of matter has been the subject of interest. Recently, several experimental studies have demonstrated that by using a scanning tunneling microscope one can translate atoms on metal surfaces to a desired position. Furthermore, it has been shown that an atom between surface and tip can be transferred reversibly which results in bistable conductance. The controlled dynamics of adsorbed species has opened a new field of research. This thesis work provides a theoretical investigation of the controlled lateral and perpendicular motion of an inert gas atom (Xe) on metal surfaces. The lateral motion of Xe on the Ni(llO) and P t(lll) surfaces is manipulated by a W tip. The interaction energy of the physisorbed atom with the tip and metal surface is described by an empirical potential. Using molecular statics the energy surfaces are calculated and the adsorbtion sites are determined. By using the molecular dynamics calculations, the variation in the coordinates of the adsorbate Xe with the tip moving at a given height are obtained. Three different modes of Xe translation are distinguished depending on the height of the tip. These are i) carriage on the tip, ii) pushing and, iii) pulling modes. The range of the tip height where one of these modes occur is strongly depended on the relaxation of electrodes and the geometry of the tip. Controlled and reversible transfer of atoms between the metal surface and the tip is studied by the transfer of Xe between two flat P t(lll) surfaces. Physisorption of Xe on the P t(lll) surface is studied by an empirical potential including short and long-range interactions and yielding correct account of several experimental data. Effective charge on Xe and the dipole moment constructed therefrom are calculated as a function of the Xe-surface separation. The potential energy curve of Xe between two P t(lll) surfaces and quantum states of Xe therein are calculated as a function of the applied voltage and separation between two P t (lll) surfaces. Within this model, various mechanisms, such as tunneling of Xe, dipole excitation and resonant tunneling, electromigration contributing to the transfer of Xe are examined. The transfer rate of Xe is then calculated for different mechanisms. Its dependence on the bias voltage is explored. The overall behavior of the total transfer rate is not a power law. While at low bias voltages thermal assisted atom tunneling is effective, the dipole excitation and resonant tunneling becomes dominant at high bias voltages.Buldum, AlperM.S

Theory of atomic scale friction

Ankara : Department of Physics and Institute of Engineering and Science, Bilkent Univ., 1998.Thesis (Ph.D.) -- Bilkent University, 1998.Includes bibliographical references leaves 101-108Friction is an old and important but at the same time very complex physical event. This thesis aims to develop an atomic scale theory of friction. VVe investigate various atomic processes and stick-slip motion by using simple models and by using simulation of realistic systems based on the stateof-the art molecular dynamics and ab-initio electronic structure and force calculations. Theoretical studies of dry sliding friction, which has a close l)earing· on the experiments done by using the atomic and friction force microscope were performed. First, a simple model is used to investigate the basic mechanisms of friction and stick-slip motion, whereby the effect of material parameters and local elastic deformation of the substrate were also examined. Then, atomic scale study of contact, indentation, subsequent |)ulling and dry sliding of a sharp and blunt metal tips on a metal surface were studied. In order to understand the atomic-scale aspects of boundary lubrication such as interesting covera.ge and load dependent behavior and structural transformations, molecular dynamics simulations were performed on a model system that has two .\'i(110) surfaces and a. xenon layer confined between these two surfaces. Finally, in view of the atomic processes revealed from computer simulations an energy dissipation mechanism and quantum heat conduction were studied.Buldum, AlperPh.D

Novel Structures and Properties of Gold Nanowires

The structures of free-standing gold nanowires are studied by using molecular-dynamics-based genetic algorithm simulations. Helical and multiwalled cylindrical structures are found for the thinner nanowires, while bulk-like fcc structures eventually form in the thicker nanowires up to 3 nm in diameter. This noncrystalline-crystalline transition starts from the core region of nanowires. The vibrational, electronic, and transport properties of nanowires are investigated based on the optimal structures. Bulklike behaviors are found for the vibrational and electronic properties of the nanowires with fcc crystalline structure. The conductance of nanowires generally increases with wire diameter and depends on the wire structure

Quantum Interference Effects in Electronic Transport through Nanotube Contacts

Quantum interference has dramatic effects on electronic transport through nanotube contacts. In optimal configuration the intertube conductance can approach that of a perfect nanotube ($4e^2/h$). The maximum conductance increases rapidly with the contact length up to 10 nm, beyond which it exhibits long wavelength oscillations. This is attributed to the resonant cavity-like interference phenomena in the contact region. For two concentric nanotubes symmetry breaking reduces the maximum intertube conductance from $4e^2/h$ to $2e^2/h$. The phenomena discussed here can serve as a foundation for building nanotube electronic circuits and high speed nanoscale electromechanical devices

Contact resistance between carbon nanotubes

Quantum transport properties of intermolecular nanotube contacts are investigated. We find that atomic structure in the contact region plays important roles and resistance of contacts varies strongly with geometry and nanotube chirality. Nanotube end-end contacts have low resistance and show negative differential resistance (NDR) behavior. Contact resistance can be dramatically decreased by exerting small pressure/force between the tubes if the contact is commensurate. Significant variation and nonlinearity of contact resistance may lead to new device applications.Comment: 4 pages, 4 figure

Adhesion and Friction Characteristics of Carbon Nanotube Arrays

There has been a great deal of interest in understanding, design and fabrication of bio-mimetic and bio-inspired adhesives in recent years. In this paper we present theoretical investigations on adhesion, friction behaviors and characteristics of fibrillar arrays composed of noninteracting carbon nanotubes for bio-inspired dry adhesives. Contact, compression, subsequent pulling off and dry sliding friction simulations were performed. It is demonstrated that there are two different adhesion forces during pull off. Static friction force values are in between 40 and 60 N cm(-2) at different loads and they are significantly larger than the normal adhesion forces. Dynamic friction force and load are anisotropic and they depend on the direction of the motion. It is also found that friction force values and friction coefficients decrease although contact length and contact area increase when the loads are high. This is due to the arms of the nanotubes which bend significantly and act as stiffer springs at high loads

A Review on the Molecular Modeling of Argyrodite Electrolytes for All-Solid-State Lithium Batteries

Solid-state argyrodite electrolytes are promising candidate materials to produce safe all-solid-state lithium batteries (ASSLBs) due to their high ionic conductivity. These batteries can be used to power electric vehicles and portable consumer electronics which need high power density. Atomic-scale modeling with ab initio calculations became an invaluable tool to better understand the intrinsic properties and stability of these materials. It is also used to create new structures to tailor their properties. This review article presents some of the recent theoretical investigations based on atomic-scale modeling to study argyrodite electrolytes for ASSLBs. A comparison of the effectiveness of argyrodite materials used for ASSLBs and the underlying advantages and disadvantages of the argyrodite materials are also presented in this article