Molecular Dynamics And First Principles Of The Quantum Size Effect In Pb Thin Films Of Silicon Probe Nanoscale Friction

This paper involves two thesises. Firstly, nanofriction of Si tip-surface is studied by molecular dynamic simulation. Secondly, using first-principles calculations quantum size effects of Pb films are investigated.Molecular dynamic simulation studies of atomic-scale transition from stick-slip to continuous sliding friction have been performed for a number of tip-surface contacts consisting of Si atoms. The surface is designed to Si (001)2X1 structure. The cap's planes of the tips are Si (111). The cap of the tips is fixed at Z orientation, and the bottom layers of the substrate are rooted. Transition from stick-slip to continuous sliding friction is show by means of the abrasion of the tip apex atoms. The schemes of this simulation are mainly from the changes of the height of the tip apex away from the surface and the velocity of tip's moving. The height ranges from 2.5A to 3.5A,velocity from 10m/s to 30m/s.The surface's structure is Si (001)2Xl,as the tip's different locations above the surface gives the change of the transition discipline .For Si (111) tip locates above the dimer atoms nearly, there exist 2.7 A and 3.1 A of distances between the tip and the substrate,which keep the stick-slip on the subsrate and for Si (111) tip locates between the dimer atoms of the substrate,there are 3.0 A and 3.1 A. These heights do not relate to the moving's velocity.Quantum size effects(QSE) of Pb films are investigated fully by first-principles calculations. Inner-layer relaxation and electronic density decay lengths in the vacuum are calculated.for the Pb(111),Pb(100)and Pb(110)free-standing films, there exist oscillations of relative percentages of standard deviations of films' inner-layer spacings to bulk spacings with the film thicks. From Pb(111),Pb(100) to Pb(110) free-standing films,those oscillations' intensity strengthens more and more. The electronic density decay lengths in vacuum of Pb films are investigated by first-principles calculations too. Results show that the decay lengths oscillate with film thickness and the oscillations follow a bilayer pattern interrupted by crossovers for Pb(111) free-standing films and Pb(111) films on the Ge(111) substrate, and a triple-layer pattern for Pb(110) free films. We make the comparisons between the electronic density decay lengths and work function of Pb films. For Pb films, the oscillations are almost consistent with the work function.