The effect of grain size on the thermoelectric properties of $n$ -type nanocrystalline bismuth-telluride based thin films is investigated. We prepare the nanocrystalline thin films with average grain sizes of 10, 27, and 60 nm by a flash-evaporation method followed by a hydrogen annealing process. The thermoelectric properties, in terms of the thermal conductivity by a differential $3\omega$ method, the electrical conductivity, and the Seebeck coefficient are measured at room temperature and used to evaluate the figure of merit. The minimum thermal conductivity is $0.61\text{W}{\text{m}}^{-1}{\text{K}}^{-1}$ at the average grain size of 10 nm. We also estimate the lattice thermal conductivity of the nanocrystalline thin films and compare it with a simplified theory of phonon scattering on grain boundaries. For nanosized grains, the lattice thermal conductivity of nanocrystalline thin films decreases rapidly for smaller grains, corresponding to the theoretical calculation. The figure of merit is also decreased as the grain size decreases, which is attributed to the increased number of defects at the grain boundaries.

The thermal conductivity of $\mathit{n}$ -type nanocrystalline bismuth-telluride-based thin films $\left({\text{Bi}}_{2.0}{\text{Te}}_{2.7}{\text{Se}}_{0.3}\right)$ is investigated by a differential $3\omega$ method at room temperature. The nanocrystalline thin films are grown on a glass substrate by a flash evaporation method, followed by hydrogen annealing at $250°\text{C}$ . The structure of the thin films is studied by means of atomic force microscopy, x-ray diffraction, and energy-dispersive x-ray spectroscopy. The thin films exhibit an average grain size of 60 nm and a cross-plane thermal conductivity of $0.8\text{W}∕\text{m}\text{K}$ . The in-plane electrical conductivity and in-plane Seebeck coefficient are also investigated. Assuming that the in-plane thermal conductivity of the thin films is identical to that of the cross-plane direction, the in-plane figure of merit of the thin films is estimated to be $ZT=0.7$ . As compared with a sintered bulk sample with average grain size of $30\mu \text{m}$ and nearly the same composition as the thin films, the nanocrystalline thin films show approximately a 50% reduction in the thermal conductivity, but the electrical conductivity also falls 40%. The reduced thermal and electrical conductivities are attributed to increased carrier trapping and scattering in the nanocrystalline film.

Bismuth telluride (Bi₂Te₃) is a promising thermoelectric material for applications near room temperature. To increase the thermoelectric performance of this material, its dimensions and thermal transport should be decreased. Two-dimensional nanoplates with nanopores are an ideal structure because thermal transport is disrupted by nanopores. We prepared Bi₂Te₃ nanoplates with single nanopores by a solvothermal synthesis and investigated their structural and crystallographic properties. The nanoplates synthesized at a lower reaction temperature (190 °C) developed single nanopores (approximately 20 nm in diameter), whereas the nanoplates synthesized at a higher reaction temperature (200 °C) did not have nanopores. A crystal growth mechanism is proposed based on the experimental observations.

Here, we investigate the combined effect of the nanoscale crystal grains and porosity on the lattice thermal conductivity of bismuth-telluride-based bulk alloys using both experimental studies and modeling. The fabricated bulk alloys exhibit average grain sizes of 30 < d < 60 nm and porosities of 12% < Φ < 18%. The total thermal conductivities were measured using a laser flash method at room temperature, and they were in the range 0.24 to 0.74 W/m/K. To gain insight into the phonon transport in the nanocrystalline and nanoporous bulk alloys, we estimate the lattice thermal conductivities and compare them with those obtained from a simplified phonon transport model that accounts for the grain size effect in combination with the Maxwell-Garnett model for the porosity effect. The results of this combined model are consistent with the experimental results, and it shows that the grain size effect in the nanoscale regime accounts for a significant portion of the reduction in lattice thermal conductivity.

The structure and thermoelectric properties of boron doped nanocrystalline ${\mathrm{Si}}_{0.8}{\mathrm{Ge}}_{0.2}$ thin films are investigated for potential application in microthermoelectric devices. Nanocrystalline ${\mathrm{Si}}_{0.8}{\mathrm{Ge}}_{0.2}$ thin films are grown by low-pressure chemical vapor deposition on a sandwich of ${\mathrm{Si}}_3{\mathrm{N}}_4∕\mathrm{Si}{\mathrm{O}}_2∕{\mathrm{Si}}_3{\mathrm{N}}_4$ films deposited on a Si (100) substrate. The ${\mathrm{Si}}_{0.8}{\mathrm{Ge}}_{0.2}$ film is doped with boron by ion implantation. The structure of the thin film is studied by means of atomic force microscopy, x-ray diffraction, and transmission electron microscopy. It is found that the film has column-shaped crystal grains $\sim 100\mathrm{nm}$ in diameter oriented along the thickness of the film. The electrical conductivity and Seebeck coefficient are measured in the temperature range between 80–300 and $130–300\mathrm{K}$ , respectively. The thermal conductivity is measured at room temperature by a $3\omega$ method. As compared with bulk silicon-germanium and microcrystalline film alloys of nearly the same $\mathrm{Si}∕\mathrm{Ge}$ ratio and doping concentrations, the ${\mathrm{Si}}_{0.8}{\mathrm{Ge}}_{0.2}$ nanocrystalline film exhibits a twofold reduction in the thermal conductivitity, an enhancement in the Seebeck coefficient, and a reduction in the electrical conductivity. Enhanced heat carrier scattering due to the nanocrystalline structure of the films and a combined effect of boron segregation and carrier trapping at grain boundaries are believed to be responsible for the measured reductions in the thermal and electrical conductivities, respectively.