Impacts of Atomistic Coating on Thermal Conductivity of Germanium Nanowires

Abstract

By using non-equilibrium molecular dynamics simulations, we demonstrated that thermal conductivity of Germanium nanowires can be reduced more than 25% at room temperature by atomistic coating. There is a critical coating thickness beyond which thermal conductivity of the coated nanowire is larger than that of the host nanowire. The diameter dependent critical coating thickness and minimum thermal conductivity are explored. Moreover, we found that interface roughness can induce further reduction of thermal conductivity in coated nanowires. From the vibrational eigen-mode analysis, it is found that coating induces localization for low frequency phonons, while interface roughness localizes the high frequency phonons. Our results provide an available approach to tune thermal conductivity of nanowires by atomic layer coating. devices [1][2][3]. In addition to the electronic and optical properties, thermal property of NWs has attracted more and more interests due to the potential thermoelectric applications in both power generation and refrigeration [4]. 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]. Very recently, both theoretical and experimental works have shown that thermal conductivity of Ge/Si core-shell NWs can be lower than that of pure GeNWs with the same cross section area [15,16]. The reduction is caused by the localization of the longitudinal phonon modes induced by the coherent resonance effect between the transverse and longitudinal modes [15].Thus the core-shell structure offers the unique opportunity to further reduce thermal conductivity of low thermal conductivity material even by coating with high thermal conductivity material.Despite these recent progresses about the thermal properties of core-shell NWs, many important and fundamental issues remain unsolved. For example, in our recent 4 work [15], thermal conductivity of Ge/Si core-shell NW is compared with that of pure GeNW with the same entire cross section area. However, thermal conductivity of NW is sensitive to the surface-to-volume ratio (SVR) and increases with the cross section area [17,18]. Coating another material outside the host NW increases the cross section area and thus can lead to the increase of thermal conductivity. Therefore, to study the effect of coating on thermal conductivity from the experimental point of view, one should compare thermal cond...

Thermal transport in nanostructures

“…Very recently, the direct connection between coherent resonance and thermal conductivity reduction was reported 91 in Ge/Si core-shell. Figs.…”

Thermoelectric effects and topological insulators

“…The reduction of lattice thermal conductivity, which is the main reason for the enhanced zT , is induced by two typical mechanisms: (i) incoherent mechanisms that reduce thermal conductivity by scatterings, for instance via interface/surface [10,11] or defect/impurity [12,13] , and (ii) coherent mechanisms that modulate the phonon band structure and group velocity, for instance via nanomesh structure [14,15] or core-shell structure [16][17][18] .…”

Tunable mechanical and thermal properties of ZnS/CdS core/shell nanowires

“…In recent years, many experimental investigations have been carried out to synthesize and characterize different kinds of core/shell structures such as ZnO/ZnS [13], ZnO/CdS, [14] GaN/GaP, ZnO/TiO2 [15,16], CdSe/CdS [17,18], PbSe/CdSe [19,20], ZnS/CdS [21][22][23][24][25], CdS/ZnS [21,26,27], Ge/Si [28]. Both the classical molecular dynamics (MD) and density functional theory (DFT) methods have been used extensively to study the electronic [29][30][31][32][33][34][35], optical [29,36] and thermal [37][38][39][40][41][42][43][44][45][46][47][48] properties of the core/shell nanostructures. In contrast, mechanical properties of the core/shell nanostructures have so far not been investigated in details.…”