Effect of grain size on thermoelectric properties of n-type nanocrystalline bismuth-telluride based thin films

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The effect of grain size on the thermoelectric properties of n-type nanocrystalline bismuth-telluridebased thin films is investigated. We prepare the nanocrystalline thin films with average grain sizesof 10, 27, and 60 nm by a flash-evaporation method followed by a hydrogen annealing process. Thethermoelectric properties, in terms of the thermal conductivity by a differential 3 method, theelectrical conductivity, and the Seebeck coefficient are measured at room temperature and used toevaluate the figure of merit. The minimum thermal conductivity is 0.61 W m−1 K−1 at the averagegrain size of 10 nm. We also estimate the lattice thermal conductivity of the nanocrystalline thinfilms and compare it with a simplified theory of phonon scattering on grain boundaries. Fornanosized grains, the lattice thermal conductivity of nanocrystalline thin films decreases rapidly forsmaller grains, corresponding to the theoretical calculation. The figure of merit is also decreased asthe grain size decreases, which is attributed to the increased number of defects at the grainboundaries.

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