| In the first part of the thesis, at the atomic scale, friction behaviours and modulations of lamellar Mo S2 have been studied by first principles calculations. These results will be helpful to understand the nanofriction phenomenon at the atomic level, and provide some new ideas and methods for modulating the microscopic friction characteristics. In the other part of the thesis, clusters diffusion on some metal surfaces have been investigated by molecular dynamics studies. It is very important to know in detail about the microscopic mechanisms of single atom and small clusters diffusion on a surface for the understanding of these surface phenomena. The main results have been shown as follows:(1) The atomic-scale friction in Mo S2 has been investigated by first principles method. Our results show that the frictional force increases with the normal load, which obeys almost the Amonton’s law. Moreover, due to different configurations during the friction process, the potential energy surface at a high load appears a different contour profile.(2) Our calculations based on the density functional theory have been carried out to investigate atomic-scale friction between two Mo S2 layers applied with in-plane biaxial strain. Our results show that the in-plane compressive Mo S2 exhibits lower friction than the tensile system. Aided by an in-depth understanding of the relationship between atomic-scale friction and interfacial interactions, one can tune friction by controllable structural modifications to the sliding surfaces, which may shed light on friction control and electronic device lubricant design.(3) Sliding properties of two Mo S2 layers imposed on an external electric field, have been investigated by the use of density functional theory(DFT) calculations. Our results show that the friction between two Mo S2 layers can be reduced by imposing an electric field. The reduced friction is attributed to a weaker chemical interaction enabled by the charge depletion between the adjacent S atomic planes. In-depth understanding of the relationship between friction and interfacial interaction showsthat friction can be tuned with an external electric field.(4) Using embedded-atom-method(EAM) potential, diffusion processes of an adatom and a vacancy have been simulated in detail by molecular dynamics on three Fe surfaces, Fe(110), Fe(100), and Fe(111). Our results reveal that adatom adsorption energies and diffusion migration energies on these surfaces have similar monotonic trend, that is adsorption energy,(110)(100)(111)a a a a a aE <E <E, diffusion migration energy,(110)(100)(111)m m m a a aE <E <E. However, for a vacancy, formation and migration energies have a different trend, formation energy,(111)(100)(110)f f f v v vE <E <E, and migration energy,(111)(110)(100)m m m v v vE <E <E. On the Fe(110) surface, simple jumping of an adatom(or a vacancy) is the main diffusion mechanism with relatively low migration energy barrier; nevertheless, exchange with a surface atom plays a dominant role in surface diffusion on the Fe(100) and Fe(111) surfaces.(5) Using embedded-atom-method(EAM) potential of Ag/Cu heterogeneous system, structural stabilities and diffusion behaviors of small Cu clusters on Ag(111) surface have been investigated by molecular dynamics studies. The binding energies, the effective diffusion barriers and the corresponding prefactors for Cu clusters have been calculated. Structural stabilities and the diffusion properties of small Cu clusters show significant magic size effects: the trimer and heptamer clusters with relatively higher effective diffusion barriers are more stable than the other sizes. These results will help to understand the diffusion mechanism of the film growth at subatomic monolayer, in good agreement with previous experimental and theoretical results. |
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