Atomic-bond-relaxation Theory For End Effect On Si Nanowire Thermal Conductivity

As the world energy crisis and environmental pollution become increasingly serious, people pay close attention to the exploration of new thermoelectric materials, which have become the forefront of research and one of the focus topics in the fields of nanomaterial sciences and condensed matter physics. As a new type of thermoelectric materials, the structure of silicon nanowires(Si NWs) have become the target of the scientists concerned. As comparable to the ideal bulk nanometer, SiNWs possess some fantastic physical and chemical properties. For example, the atoms located at the end part of nanowires have high coordination imperfection and various atomic energy states, which play the crucial roles for their mechanical, thermal, optical and electronic properties. The thermal conductivity of SiNWs is one of the crucial parameters for their thermoelecric and related properties. Thus, many efforts have been made to pursue the physical mechanism of the depression of thermal conductivity in SiNWs. However, there are many issues remain unsolved,such as the definition of surface roughness factor and interface confinement.Therefore, in this paper, based on the continuum mechanics and atomic-bond-relaxation consideration, we develop a quantitativeanalytical method that describes the effects original from surface relaxation and interface misfit to explore the thermal conductivity of SiNWs on the size and surface or interface. Furthermore, we study the end effect on the SiNWs thermal conductivity under different types of edges, such as surface facet/island, cross-section geometry, size, axial orientation and interface condition. The achievements are shown as follows:(1) We define a surface roughness factor. Also, we develop a theoretical model to address the end effect on thermal conductivity of SiNWs with different surface roughness, nanowire size, cross-section geometries with circle, hexagon, tetragon and trigon, and axial orientations, i.e., [111], [110] and [100]. Firstly, our results show that as comparable to the SiNWs with circle, hexagon and tetragon shapes,trigon SiNWs have the minimum thermal conductivity under the same conditions. Secondly, a [100] Si NW with a triangular cross-section has the minimum value of thermal conductivity comparable with [111] and[100] orientations at a fixed size. Moreover, the specific thermal conductivity of SiNWs can be realized through designing end structures such as the volume, shape of surface island or other factors. Our predictions are satisfactory quantitative agreement with experimental results and simulations, suggesting that the possible methods on tunable thermal conductivity of Si NWs for the desired applications.(2) We investigate the effect of interface confinement modulation on the radial thermal conductivity of SiNWs based on the atomic-bond-relaxation consideration and elastic continuum mechanics.Our results show that the strain induced by the interface mismatch and surface relaxation affects the he cohesive energy and thermal conductivity of Si/Ge core-shell nanowires(CSNWs), which differs from that of bare SiNWs. We have also examined the relationship between the size and the thermal conductivity of Si/Ge CSNWs and found that the he thermal properties in CSNWs can be effectively modulated by the interface conditions. Our results suggest that the method provides a fundamental understanding of the thermal conductivity of CSNWs from a view of the interface effect, providing a useful guidance for the practice applications.