Perfectly periodic structures modify the transport properties of heat carriers by interference effect and hinder heat transport.

In search of efficient thermoelectric nanostructures, many theoretical works predicted that nanopillars, placed on the surface of silicon membranes, nanobeams, or nanowires, can reduce the thermal conductivity of these nanostructures. To verify these predictions, we experimentally investigate heat conduction in suspended silicon nanobeams with periodic arrays of aluminium nanopillars. Our room temperature time-domain thermoreflectance experiments show that the nanobeams with nanopillars have 20% lower thermal conductivity as compared to pristine nanobeams. We discuss possible explanations of these data, including coherent effects, and conclude that the thermal conductivity is reduced mainly by phonon scattering at the pillar/beam interface due to the intermixing of aluminium and silicon atoms, as supported by the transmission electron microscopy. As this intermixing does not only reduce thermal conductivity but may also increase electrical conductivity, these nanostructures are exceptionally promising for thermoelectric applications.

We present experimental and theoretical investigations on the roles of the limiting dimensions, such as the smallest dimension, surface roughness, and density of holes in the reduction of thermal conductivity of one-dimensional phononic nanostructures at temperatures of 4 and 295 K. We discover that the thermal conductivity does not strongly depend on the period of the phononic crystal nanostructures whereas the surface roughness and the smallest dimension of the structure—the neck—play the most important roles in thermal conductivity reduction. Surface roughness is a very important structural parameter in nanostructures with a characteristic length less than 100 nm in silicon. The importance of the roughness increases as the neck size decreases, and the thermal conductivity of the structure can differ by a factor of four, reaching the thermal conductivity of a small nanowire. The experimental data are analyzed using the Callaway–Holland model of Boltzmann equation and Monte Carlo simulation providing deeper insight into the thermal phonon transport in phononic nanostructures.

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We demonstrate the power enhancement of planar-type uni-leg poly-Si thermoelectric generators by nano-patterning. The thermoelectric generators were fabricated on an SOI wafer by a conventional lithography-based process. The size of the phononic nano-patterning (300 nm in period) was designed based on the thermal phonon mean free path spectrum. The thermal conductivity of the Si membrane was reduced by 60% compared with an unpatterned one, which resulted in the doubled thermoelectric figure-of-merit. The nano-patterning increased the temperature gradient in the Si membrane and resulted in a four times higher voltage and ten times larger power density.

We propose a simple, low-cost, and large-area method to increase the thermoelectric figure of merit (ZT) in silicon membranes by the deposition of an ultrathin aluminum layer. Transmission electron microscopy showed that short deposition of aluminum on a silicon substrate covers the surface with an ultrathin amorphous film, which, according to recent theoretical works, efficiently destroys phonon wave packets. As a result, we measured 30−40% lower thermal conductivity in silicon membranes covered with aluminum films while the electrical conductivity was not affected. Thus, we have achieved 40−45% higher ZT values in membranes covered with aluminum films. To demonstrate a practical application, we applied this method to enhance the performance of a silicon membrane-based thermoelectric device and measured 42% higher power generation.

We measure the thermal conductivity of silicon phononic crystals with asymmetric holes at room and liquid helium temperatures and study the effect of thermal rectification, phonon boundary scattering, neck transmission, and hole positioning. Also, we compare the influence of asymmetric holes on thermal conductivity reduction with the one of conventional circular holes. This reduction is almost 40% larger in the case of pacman shaped holes as compared with circular ones for the same parameters of phononic crystals. Our experimental results can be used to significantly improve the efficiency of thermoelectric devices by using pacman-shaped holes in phononic crystals.

Nanostructuring is the dominant approach for effective thermal conduction control in nanomaterials. In the past decade, researchers have been interested in thermal conduction control by the coherent effects in phononic crystal (PnC) systems. Recent theoretical works predicted that nanopillars on the surface of silicon membranes could cause a dramatic thermal conductivity reduction due to the phonon local resonances. However, this remarkable prediction has not been experimentally verified yet with the deep-nanoscale pillar-based PnCs. Here, we fabricate nanopillars on suspended silicon membranes using damageless neutral-beam etching and investigate the impact of nanopillars on the thermal conductivity of the membranes in the 4–300 K range. We found that thermal conductivity reduction caused by the nanopillars does not exceed 16%, which is much weaker than that predicted by the theoretical works. Moreover, this reduction remains temperature independent. These facts make the coherence an unlikely reason for the observed reduction. Indeed, our Monte Carlo simulations can reproduce the experimental results under a purely incoherent approximation. Our study shows that the coherent control of heat conduction by PnC nanostructures is more challenging to observe experimentally in reality than predicted in near-ideal modeling.