Phonon heat conduction in corrugated silicon nanowires below the Casimir limit

Abstract

The thermal conductance of straight and corrugated monocrystalline silicon nanowires has been measured between 0.3 K and 5 K. It is demonstrated that the corrugation strongly reduces the thermal transport by reducing the mean free path of the phonons. The experimental averaged mean free path is remarkably smaller than the smaller diameter of the nanowire, evidencing a phonon thermal transport reduced below the Casimir limit. Monte Carlo simulations highlight that this effect can be attributed to significant multiple scattering of ballistic phonons occuring on the corrugated surfaces. This result suggests an original approach to transforming a monocrystalline material into a phonon glass.

Ultra-low thermal conductivity of two-dimensional phononic crystals in the incoherent regime

“…For representative nanostructures, such as SiNWs, introduction of surface roughness is a commonly adopted method to reduce thermal conductivity. 35,[44][45][46][47][48][49][50][51] However, from the viewpoint of coherent models, this method (introduction of surface roughness) is believed to be a "negative" effect to reduce thermal conductivity of 2D PnCs, because it will destroy the coherence of phonons. In contrast, as demonstrated in our present work, phonon coherent transport is not the necessary condition to achieve ultra-low thermal conductivity in 2D PnCs.…”

Nanophononics: state of the art and perspectives

“…Experimental techniques -be it based on the 3ω [45,46] and the time-domain thermoreflectance techniques [47] or the scanning thermal microscopy [48] -have proven to be robust in estimating the thermal conductivity, thermal conductance and interfacial properties. The need for accurate and reliable thermal conductivity measurements has seen sophisticated developments of the 3ω method [49,50], with a sensitivity of less than 10 nW/(m K) (±5 × 10 −3 nW/(m K) at room temperature) and a very high resolution (Δκ/κ = 10 −3 ).…”

Rayleigh waves, surface disorder, and phonon localization in nanostructures

“…Our results underscore the importance of the ongoing work to extend the specularity parameter model to account for mode conversion [112] and Rayleigh waves [11]. However, even with improvements, the specularity parameter concept only makes sense in the limit of weak roughness; all specularity parameter models have a "Casimir limit" when the specularity parameter is zero and all scattering is diffuse [3], yet in many three-dimensional nanostructures thermal conductivities far below that limit have been measured [1,2,113]. It is also not clear that an improved specularity parameter can result in finite thermal conductivity in two dimensional systems in the absence of other scattering mechanisms.…”

“…Current PMC simulations do not allow for mode conversion or Rayleigh waves. Instead, PMC simulations either have phonons scatter specularly at the surface [3,82,113,116] or have a specularity parameter with the reflected phonon of the same mode as the incident phonon [3,117]. Monte Carlo simulations for chaotic ray-splitting billiards are similar to PMC simulations, and the chaotic ray-splitting billiard simulations have been extended to allow for mode conversion between bulk modes [118].…”