Nanoscale Thermal Transport—Beyond the Classical Size Effect

Nanostructures demonstrate intriguing thermal properties that are of fundamental interest and important engineering implications. The most widely recognized nanoscale effect is due to scattering of energy carriers at materials boundaries, which is called the classical size effect. Interestingly, other nanoscale effects can also play important roles, leading to more exotic thermal properties.

One such effect is due to the coupling between mechanical and thermal properties. For example, acoustic softening occurs in thin silicon nanowires with significantly lower Young’s modulus than that of bulk silicon, which renders reduced thermal conductivity [1,2]. On the other hand, elastic stiffening with an enhanced Young’s modulus can happen in silver nanowires, which manifests its effects on both electrical and thermal transport properties [3].

More interestingly, for quasi-one-dimensional (quasi-1D) van der Waals (vdW) crystal nanowires that are composed of covalently bonded atomic chains with weak interchain vdW interactions, dimensionality transition can occur with 1D phonon-dominated transport. For example, the thermal conductivity of quasi-1D NbSe₃ nanowires demonstrates a non-monotonic diameter dependence, first reduces and then increases as the wire diameter becomes smaller [4]. Surprisingly, the thermal conductivity of thin NbSe₃ nanowires becomes divergent with the wire length, following a one-third power law dependence up to a maximum measured length of 42.5 microns. Analyses indicate that these intriguing observations are due to 1D phonon-dominant transport, which is induced by a five-fold enhancement of the Young’s modulus from the bulk value. As such, these results provide the first solid experimental evidence for superdiffusive transport along 1D lattices, a classical anomaly in thermal physics that can be traced back to the Fermi-Pasta-Ulam-Tsingou (FPUT) paradox.

Beyond electrons and phonons, other energy carriers such as polaritons can also make important contributions to thermal transport through nanostructures because of the large surface-area-to-volume ratio. For example, surface phonon polaritons (SPhPs), hybrid quasi-particles resulting from coupling between infrared photons and optically active phonons, have been predicted to contribute to heat conduction along polar thin films/nanowires. However, because of the low number density of polaritons, experimental attempts taking advantage of the ultralong SPhP decay length have only demonstrated a low SPhP thermal conductivity of <0.5 W/m-K. Through introducing Au coatings at the end(s) of SiC nanowires as SPhP launchers, we show that the SPhPs with number densities much higher than the corresponding equilibrium values can be stimulated to propagate along the uncoated nanowire segment, which leads to a room temperature SPhP-mediated thermal conductivity of ~6 W/m×K [5]. Importantly, we show that modifying the surface of the sample could effectively modulate the SPhP-mediated thermal conductivity through altering the SPhP transport along the nanowire surface.

[1] Wingert et al. Nano letters 15, 2605-2611 (2015).
[2] Yang et al. Nanoscale 8, 17895-17901 (2016).
[3] Zhao et al. Nano Letters 20, 7389-7396 (2020).
[4] Yang et al. Nature Nanotechnology 16, 764-768 (2021).
[5] Pan et al. Nature 623, 307-312 (2023).

This presentation is in memory of Dr. Natalio Mingo.