Thermal transport in nanoengineered silicon nanowires via molecular dynamics simulation

Sean Sullivan, Purdue University

Abstract

With the advent of nanoengineering, examining thermal transport in complex nanostructures is becoming important in areas such as nanoelectronics and energy conversion. Ever-increasing power densities of modern nanoelectronic devices make thermal management a critical factor in device design. Tuning electronic and thermal transport through nanoengineering is also prevalent in thermoelectric applications where the figure of merit, ZT, is optimized by minimizing thermal conductivity while simultaneously maximizing electronic conductivity. As device sizes become smaller and computational resources increase, atomistic simulation of device-scale materials is now a feasible prospect. This allows for the unique opportunity to use theory to predict properties of novel nanoengineered devices in order to wisely guide experimental development. We use nonequilibrium molecular dynamics (MD) simulations to characterize the effects of nanoengineered, modulated structures on phonon thermal transport in Si nanowires with characteristic feature sizes on the order of tens of nanometers. It is found that the thermal conductance of the corrugated nanowires is lower than that of smooth wires with cross-sections equal to the constrictions (inner channel) of the engineered wires. This is surprising given that the maximum local cross-sectional area in the corrugated wire may be up to ~5 times that of its inner channel. However, reduction in thermal conductance reaches a point of diminishing returns when the height of the corrugated ridges becomes too high relative to the inner channel. Regardless of corrugation amplitude, 80% of the heat current is carried within the linear channel of the wires, with the remaining 20% being modulated by the ridges. For high amplitudes, temperature gradient inversion and heat current vorticity occur within the ridges, preventing anything further than a ~30% reduction in thermal conductance as compared with the straight, inner channel wires. A detailed analysis of the MD trajectories allows for the characterization of local quantities with unparalleled resolution. These analysis techniques have been additionally applied to asymmetrically corrugated wires, as well as Si/Ge core-shell corrugated nanowires. This enhanced understanding of structural effects on localized thermal transport and the discovery of a point of diminishing returns may help to guide the development of next-generation nanoelectronic and thermoelectric devices.

Degree

M.S.M.S.E.

Advisors

Strachan, Purdue University.

Subject Area

Nanoscience|Materials science

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