The crystallization, thermodynamic and thermoelectric properties of vast off-stoichiometric Sn–Se crystals

Abstract

Pristine tin-deficient layered single crystalline tin selenide (SnSe), showing strong lattice vibration anharmonicity, emerged as a very promising thermoelectric material in 2014 due to its ultralow thermal conductivity. Also, novel three-dimensional (3D) charge transport properties and two-dimensional (2D) phonon transport properties were observed in bromine-doped SnSe samples. However, the origin of the ultralow thermal conductivity of SnSe single crystals has not been fully elucidated so far, although much effort has been put into investigating the influence of intrinsic Sn vacancies in tin-deficient and stoichiometric SnSe. Until now, the crystal growth of Sn-rich SnSe samples has not been reported. This study tries to fill the gaps in the existing knowledge about vast off-stoichiometric SnSe with extra Sn. Sn-Rich single-phase crystals with different Sn/Se atomic ratios were prepared using a self-flux method and subsequent heat treatment at 673 K. Higher lattice thermal conductivities than both Sn-deficient and stoichiometric crystals were recorded from Sn-rich vast off-stoichiometric samples. The higher thermal conductivities of Sn-rich samples suggest that covalent bonding enhancement across the Sn–Se slabs along the a direction dominates the phonon scattering process, and that the phonon scattering effects from defects aroused by extra Sn are less effective in Sn-rich SnSe crystals than those from Sn vacancies in Sn-deficient samples. The adjustment of the Sn/Se ratio allows for the optimization of the carrier mobility and band gap, simultaneously, even though the carrier concentration is nearly unchanged in the Sn-rich SnSe crystal, with a Sn/Se atomic ratio between 1.13 and 1.17. This may also relate to changes in the bonding nature between Sn–Se slabs, weakening the 2D transport characteristics of SnSe. Therefore, the quantum confinement effect/scattering effect along the stacking direction is weakened and higher carrier mobility is obtained along the in-plane direction. For comparison, SnSe₂ crystals with extra Se were grown via the same self-flux method using Se. Phase-pure SnSe₂ crystals with a hexagonal structure (space group: Pm1) were achieved, and their phase evolution with temperature was explored. Similar abnormal SnSe crystal thermal properties have been observed in SnSe₂ compounds. These results suggest that the atomic ratio of Sn/Se related defects (defect clusters) and bonding circumstances have critical influences on the transport properties of both Sn-deficient and Sn-rich SnSe crystals, even though the underlying physics are completely different.