Significant Reduction of Thermal Conductivity in Si/Ge Core−Shell Nanowires

Abstract

We report on the effect of germanium (Ge) coatings on the thermal transport properties of silicon (Si) nanowires using nonequilibrium molecular dynamics simulations. Our results show that a simple deposition of a Ge shell of only 1 to 2 unit cells in thickness on a single crystalline Si nanowire can lead to a dramatic 75% decrease in thermal conductivity at room temperature compared to an uncoated Si nanowire. By analyzing the vibrational density states of phonons and the participation ratio of each specific mode, we demonstrate that the reduction in the thermal conductivity of Si/Ge core-shell nanowire stems from the depression and localization of long-wavelength phonon modes at the Si/Ge interface and of high frequency nonpropagating diffusive modes.

Interface bond relaxation on the thermal conductivity of Si/Ge core-shell nanowires

“…3(a), the thermal conductivity of Si core decreases significantly with increasing shell thickness. As expected, this trend is very well consistent with the results reported by Hu et al 21 Also, they indicated that the minimum of the overall thermal conductivity of the entire CSNW will be appearance due to larger contribution of coating layer to the overall heat transfer. Fig.…”

“…In comparison to the individual silicon nanowires, Si/Ge CSNWs have evident advantages because the covering layer offers the possibility of a partial and internal charge separation, an efficient passivation of the surface trap states and so on. 7 Up to date, significant progresses for Si/Ge CSNWs both experimentally [8][9][10][11][12][13][14] and theoretically [15][16][17][18][19][20][21][22][23][24][25][26][27][28] have demonstrated that the thermal transport properties of Si/Ge CSNWs can be modified at room temperature compared to that of the bare Si or Ge nanowires. Classically, the standard macroscopic approach describing heat transport in semiconductors is the well-known Fourier's law, i.e., J = −κ∇T, where J and ∇T are the heat flux density in the material and the temperature gradient, respectively.…”

Impacts of Atomistic Coating on Thermal Conductivity of Germanium Nanowires

“…For instance, by etching the surface of silicon NWs, it has been experimentally demonstrated that thermal conductivity of Si NW can be reduced more than two orders of magnitude compared with bulk Silicon [5,6]. Moreover, remarkable reduction of thermal conductivity in core-shell [7][8][9][10], tubular [11,12], and surface-decorated [13] NWs has also been reported through various kinds of interface and surface engineering.Previous studies [7][8][9] on the reduction of thermal conductivity in core-shell NWs mainly focus on Si/Ge core-shell NWs, which is quite intuitive as Ge is a low thermal conductivity material compared to Si [14]. However, in experimental realizations, the NWs synthesised are Ge/Si core-shell NWs [1][2][3].…”

“…Previous studies [7][8][9] on the reduction of thermal conductivity in core-shell NWs mainly focus on Si/Ge core-shell NWs, which is quite intuitive as Ge is a low thermal conductivity material compared to Si [14]. However, in experimental realizations, the NWs synthesised are Ge/Si core-shell NWs [1][2][3].…”

Rayleigh waves, surface disorder, and phonon localization in nanostructures

“…At higher frequencies, the participation ratios differ little between free and fixed BCs. While it is hard to definitively prove a connection between localization and conductivity, it is widely believed that localized modes contribute less to the conductivity than delocalized modes [35,[94][95][96].…”