ENGINEERING THERMOELECTRIC PROPERTIES OF TIN SELENIDE SINGLE CRYSTAL, POLYCRYSTAL, AND EPITAXIAL THIN FILM

Abstract

Nowadays, the global climate is drastically changed due to the use of fossil fuels for transport and energy generation. This situation leads to the increasing research on alternative sources of energy field. Thermoelectric (TE) modules which convert heat energy directly into electrical energy without any moving parts are good candidate for this purpose. Historically, the efficiency of these devices is still very low due to the limit of material’s thermoelectric figure of merit ZT. Along a hundred years historical of thermoelectricity research, many efforts have been made to improve the thermoelectric device’s efficiency by improving the material’s thermoelectric figure of merit ZT. However, this is not an easy job due to the interrelation between Seebeck coefficient S, electrical conductivity σ, and thermal conductivity κ. To optimize these parameters for maximize ZT, band engineering, complex crystal, nanostructures, nano-wire, nano-tube, superlattices etc., approaches have been made aiming on the “electron - crystal” and “phonon - glass” concepts. Bismuth telluride (Bi2Te3) and lead telluride (PbTe) have been found to be the best TE material for TE device applications at room temperature and above with ZT ~ 1, respectively and still being used in commercial TE devices. These materials are both binary but contain expensive and toxic elements (Bi and Pb). Researchers have focused on finding out alternative materials, which are economically and environmentally friendly, leading to the robust development of thermoelectric materials. In 2014, SnSe has been reported as the lowest thermal conductivity of any bulk materials assigned to its anharmonic bonding, leading to the high ZT value (ZT = 2.6 in un-doped p-type and ZT = 2.2 in Bi-doped n-type SnSe). It is a layered material with a weak Van de Waals bonding between layers along a-axis. This very weak bonding is one of the main reasons for the low lattice thermal conductivity along a-axis. Our group working on 2D layered chalcogenide materials such as: SnSe, GaSe, SnSe2, SnS, GeS, GeSe, GeTe, SnTe, GaTe etc., focusing on their thermoelectric properties. There are two unsolved questions concerning the SnSe system relate to its ultralow thermal conductivity and its low mass density (5.43 g/cm3) compare to that obtained from neutron diffraction (6.18 g/cm3). Many other groups have reported different thermal conductivity and higher mass density for SnSe. There is a debate that the reported thermoelectric properties are intrinsic SnSe or not. Therefore, deeper understandings about this system are required such as effects of defects, stoichiometry, etc. This dissertation focuses on the engineering defects in SnSe single crystal from bulk to thin film form and polycrystalline SnSe and investigate the influence of the intrinsic defects on the thermoelectric properties of SnSe. Apart of this dissertation, I focus on the fabrication of mixed phase SnSe and SnSe2 and its thermoelectric properties.