Size-and Temperature-dependent Mechanical Properties Of Silicon Nanowires By Theoretical Modeling And Molecular Dynamics Simulations

Due to the promising potential applications of Si nanowires(Si NWs)in nano-/micro-electro-mechanical systems,they deserve further investigations to describe their properties quantitatively.Si nanowires present large surface-to-volume ratios and thus possess unique properties when comparing with their bulk counterpart,showing size-dependence of nominal Young's modulus and surface energy.Temperature also plays an important role in the size-dependent Young‘s modulus.In addition,a change in temperature can induce dimension change of a material,characterized by thermal expansion coefficient(TEC)of the material.In this thesis,we extended the surface eigenstress model and derived a unified analytic equation to describe the effects of size on Young‘s modulus and the temperature on both Young‘s modulus and TEC under small elastic deformation of Si NWs,by employing the linear approximation of Varshni equation.Then,molecular dynamics(MD)simulations were conducted on single crystalline [100],[110] and [111] Si nanowires at temperatures ranging from 100 K to 400 K.At last,MD results were fitted by using the derived model.The perfect fitting of the derived formulas to MD results verifies the availability of the current model.The size-dependent ultimate biaxial tensile strength of [100] Si NWs under large deformation and their size-and temperature-dependent plasticity properties were also investigated by using MD simulations.The atomistic calculations show that the ultimate biaxial tensile strength of Si nanowires becomes smaller when its size becomes smaller.For small size NWs,plasticity deformation under extension was observed in MD simulations,indicated by the formation of dislocations.A single run-through dislocation was formed in very small size Si NWs.When the sizes of Si NWs become larger,many dislocations initiate meanwhile at edges and finally form dislocation forests.The critical size of a Si NW forming a single run-through dislocation is also temperature-dependent.We drew the phase diagram quantitatively describing the size-and temperature-dependence of the formation of a single run-through dislocation in Si NWs.