Nano-cross-junction effect on phonon transport in silicon nanowire cages

Abstract

Wave effects of phonons can give rise to controllability of heat conduction beyond that by particle scattering at surfaces and interfaces. In this work, we propose a new class of 3D nanostructure: a silicon-nanowire-cage (SiNWC) structure consisting of silicon nanowires (SiNWs) connected by nano-cross-junctions (NCJs). We perform equilibrium molecular dynamics (MD) simulations, and find an ultralow value of thermal conductivity of SiNWC, 0.173 Wm -1 K -1 , which is one order lower than that of SiNWs.By further modal analysis and atomistic Green's function calculations, we identify that the large reduction is due to significant phonon localization induced by the phonon local resonance and hybridization at the junction part in a wide range of phonon modes. This localization effect does not require the cage to be periodic, unlike the phononic crystals, and can be realized in structures that are easier to synthesize, for instance in a form of randomly oriented SiNWs network.KEYWORDS: Nano-cross-junction, silicon-nanowires-cage, thermal conductivity, local phonon resonance, random network of silicon nanowiresOver the past decades, nanostructures have attracted great attentions due to its unique properties, including the low thermal conductivity. Most-commonly exercised approach is to lower thermal conductivity by phonon scattering at boundaries (surfaces and interfaces) that becomes dominant over intrinsic scattering as the length scales of the nanostructures decreases. Taking silicon nanowires (SiNWs) as a representative material, reduction of thermal conductivity has been realized by enhanced phonon scatterings at surfaces or boundaries due to high surface-to-volume ratio. Another line of effort to further reduce thermal conductivity which works on bulk materials is to utilize wave nature of phonons. Periodic phononic crystals can terminate or inhibit phonon propagation by inducing interference of phonons reflected at boundaries [12][13][14][15]. A challenge here is to ensure the occurrence of wave interferences, which requires strict periodicity of the internal structure with a size on the order of the phonon waves, which is about 1 nm at room temperature. [16] In addition, boundaries of the internal structures need to be smooth enough to specularly reflect phonons. These make production of the phononic crystals by bottom-up synthesis and top-down fabrication extremely challenging. [17] Therefore, there is a strong need for a structure that can give rise to wave effects (interference, localization, and resonance) "locally" so that the periodicity is no longer and planar nanowire cross-junction architectures.[26] These works have shown the advantages of two-dimensional cross-junction over "bridge" junction.In this letter, based on the above bottom-up approach and planar nano-cross-junctions (NCJs), [26] we take a step further and propose a silicon-nanowire-cage (SiNWC) ( Fig. 1(c)) structure consisting of SiNWs ( Fig. 1(a)) and 3D-NCJs ( Fig. 1(b)). Thus, the 1DSiNW is turned into a 3D bulk material as show...

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