Ultralow Thermal Conductivity of Single‐Crystalline Porous Silicon Nanowires

Abstract

Porous materials provide a large surface-to-volume ratio, thereby providing a knob to alter fundamental properties in unprecedented ways. In thermal transport, porous nanomaterials can reduce thermal conductivity by not only enhancing phonon scattering from the boundaries of the pores and therefore decreasing the phonon mean free path, but also by reducing the phonon group velocity. Herein, a structure-property relationship is established by measuring the porosity and thermal conductivity of individual electrolessly etched single-crystalline silicon nanowires using a novel electron-beam heating technique. Such porous silicon nanowires exhibit extremely low diffusive thermal conductivity (as low as 0.33 W m −1 K −1 at 300 K for 43% porosity), even lower than that of amorphous silicon. The origin of such ultralow thermal conductivity is understood as a reduction in the phonon group velocity, experimentally verified by measuring the Young's modulus, as well as the smallest structural size ever reported in crystalline silicon (<5 nm). Molecular dynamics simulations support the observation of a drastic reduction in thermal conductivity of silicon nanowires as a function of porosity. Such porous materials provide an intriguing platform to tune phonon transport, which can be useful in the design of functional materials toward electronics and nanoelectromechanical systems.

Probing the Physical Origin of Anisotropic Thermal Transport in Black Phosphorus Nanoribbons

“…In this work, we follow the BP NR fabrication process reported by Lee et al and study the anisotropic thermal conductivity of BP NRs with different thicknesses by an electron beam technique . The anisotropic thermal transport of BP NRs is well accounted for by the Young's modulus measured along different directions using a three‐point bending method . The anisotropy ratio of thermal conductivity between ZZ and AC directions is similar to that of the Young's modulus along these two directions, implying that the thermal transport anisotropy is mainly due to the anisotropic phonon dispersion, and barely dependent on the phonon scattering relation time, which is contrary to predictions previously made by others that phonon scattering time would play a significant role in the phonon transport anisotropy of BP .…”

“…In this work, we follow the BP NR fabrication process reported by Lee et al and study the anisotropic thermal conductivity of BP NRs with different thicknesses by an electron beam technique . The anisotropic thermal transport of BP NRs is well accounted for by the Young's modulus measured along different directions using a three‐point bending method .…”

“…Considering , where v s is the speed of sound, E the Young's modulus, and ρ the mass density, a measurement of Young's modulus would provide direct information about the origin of phonon transport anisotropy, since the phonon group velocity is nearly equal to the speed of sound in the low frequency limit. This means we can directly link the anisotropic thermal conductivity of BP to its orientation‐dependent Young's modulus, providing insight into the origin of thermal conductivity anisotropy of BP, as the Young's modulus is relatively insensitive to the thickness of BP .…”

“…Once the BP NRs were fabricated, the silicon substrate with BP NRs was transferred to the SEM chamber for thermal device fabrication, as well as fabrication of mechanical devices, which will be discussed in the next section. As shown in Figure a, a single BP NR was transferred onto a micro‐electro‐thermal systems (METS) device and the ends of the nanoribbons were deposited with platinum (Pt) to make good thermal contact . The transfer process is detailed in Figure S3 in the Supporting Information.…”

“…The thermal conductivity of BP NRs was measured by an electron beam technique, for which the measurement principle is discussed in our previous paper . Briefly, a focused electron beam is used as a heating source to heat up the NR and the temperature rises at the two ends of the NR; T L and T R , were recorded by platinum resistance thermometers.…”