Phonon transport and thermoelectric properties of semiconducting Bi2Te2X (X = S, Se, Te) monolayers.Confinement or dimensionality reduction is a novel strategy to reduce the lattice thermal conductivity and, consequently, to improve the thermoelectric conversion performance.  The phonon transport and thermoelectric properties of Bi2Te2X (X = S, Se, Te) monolayers are systematically investigated by employing density functional theory and the Boltzmann transport equation.  The calculated lattice thermal conductivity of these 2D systems ranges from ∼1.3 W m-1 K-1 (Bi2Te2Se) to ∼1.5 W m-1 K-1 (Bi2Te3) for a 10 μm system size at room temperature and considering spin-orbit coupling in harmonic force constants.  This remarkably low lattice thermal conductivity is attributed to small group velocities and enhanced anharmonic phonon scattering rates.  Our results reveal that the thermal transport in these 2D systems is dominated by in-plane transverse acoustic modes.  Additionally, the thermal conductivity can be further reduced by decreasing the sample size due to phonon-boundary scattering.  The thermoelectric properties including the Seebeck coefficient, power factor and electrical conductivity are calculated using the semi-classical Boltzmann transport equation within the rigid band approximation.  The low thermal conductivities coupled with their high carrier mobilities lead to good thermoelectric power factors.  With optimal carrier doping, a figure of merit ∼0.6 can be achieved at room temperature, which increases to ∼0.8 at 700 K, thus making them promising candidates for thermoelectric applications.
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