Our first paper of 2024 “Tuning the thermal conductivity of silicon phononic crystals via different defect motifs: Implications for thermoelectric devices and photovoltaics” has just been accepted for publication in ACS Applied Nano Materials, where it will be part of a Forum Focused on South American Authors jointly published with ACS Applied Materials & Interfaces. This joint special issue will be a collection of articles published in a single issue of the journal authored by leaders of applied materials in South America.
In this publication we investigate the thermal conductivity of silicon phononic crystals, materials with a periodic arrangement of modifications which can be tailored to control their thermal conductivity. The introduction of intermediate-sized pores in silicon membranes can reduce their lattice thermal conductivity and increase their thermoelectric efficiency, as long as the pores are not too small to interfere with electron transport, nor too large to cause structural instabilities in the material. We consider thin silicon membranes and structures with holes of different sizes and shapes, forming phononic crystals with different defect motifs, and calculate their thermal conductivity along the [110] and [-110] crystalline directions, with the homogeneous non-equilibrium molecular dynamics method. We find that for the hole sizes considered the thermal conductivity is a logarithmically decreasing function of the defect area, independent of defect shape and transport direction, and verify that the thermal conductivity of silicon phononic crystals is ultimately limited by the neck size, which is the smallest distance between two adjacent defects, in agreement with the literature. However, our results also show that the dependence of the conductivity with neck size follows a power law along the [110] direction, but shows an exponential dependence along the [-110] direction. We attribute this difference to the scattering of phonons by surface dimers which originate in the 2×1 reconstruction of the silicon [001] surface, and are oriented along the [-110], acting as resonators which can scatter phonon more efficiently along that direction. Those findings are relevant for the design of thermoelectric devices and thermal barriers in photovoltaic cells.
The results stem from Higo de Araujo Oliveira mater’s thesis, and it is the first graduate thesis from our group since we moved to UFPE in 2020. It is also another great result from our ongoing collaboration with Ari Harju and Zheyong Fan, both currently at Varian Medical Systems in Finland.