Thermoelectrical Study Of Low Dimensional Materials

Thermoelectric effect is a phenomonon that the heat can be transferred to electricity directly.It is a promising eco-friendly energy generation method that attracts lots of attention.However,the low energy convention efficiency limits its further application in practice.As theoretically predicted by Hicks and Dresselhaus,better thermoelectric properties are expected in nano structure materials due to the quantum confinement.In this thesis,we study thermoelectric properties by taking two different low-dimensional materials,such as thermoelectric properties of ultrathin Bi₂Te₃ nanowires,thermoelectric properties of SnSe nanoplates,and thermal conductivity of SnSe nanoplates.Bulk Bi₂Te₃ is an excellent thermoelectric material due to its narrow bandgap and heavy elements,and is the only commercialized thermoelectric material so far.However,the energy convention efficiency is still not high.Electrical and thermoelectric properties of individual ultrathin bismuth telluride?Bi₂Te₃?nanowires with diameter less than 10 nm were studied at room temperature.These sub-10 nm nanowires show significant deviation of electrical and thermoelectrical properties from reported nanowires with much larger diameters including nonmonotonic dependence of Seebeck coefficient with gate modulation and non-turn-off behaviors in conductivity.For certain gate voltage region,Seebeck coefficient increases with conduction increase,also on the contrary to typical semiconducting thermoelectric material.These new observations of sub-10 nm Bi₂Te₃ nanowires can be only understood by taking into account of both electrons and holes in nanowires.Two band transport model is utilized to successfully describe both thermoelectric and electric transport behaviors of these ultrathin nanowires.Enhanced Seebeck coefficients are also observed,reaching-278?V/K for n type Bi₂Te₃ nanowires,much higher than all previous values for Bi₂Te₃nanowire studies.Bulk SnSe is an excellent thermoelectrical material with the highest ZT value of2.8,making it promising in application.We studied temperature-dependent electrical and thermoelectrical properties of SnSe nanoplates at low temperature.Conductivity drops and rises again as temperature is lowered.And the Seebeck coefficient is positive at room temperature and becomes negative at low temperature.The change of the sign of the Seebeck coefficient indicates bipolar transport of the SnSe nanoplate.The bipolar transport is a result of that the Fermi energy changes with temperature due to the different thermal excitations of donors and acceptors at different temperature.SnSe bulk has an ultralow thermal conductivity due to the anharmonicity of crystal structure.The nanocrystallization effect in thermal conductivity of this material was measured successfully using suspended devices.The Seebeck coefficient shows a sign change between 108 K and 325 K,indicating a bipolar transport in SnSe nanoplate.The thermal conductivity is 0.8 W/m K at room temperature comparable with the intrisic SnSe in bc plane??0.7 W/m K?.This similarity confirms that the mean free path of phonon in SnSe is very short??0.84 nm?.The thermal conductivity increases with decreasing temperature,consistant with the Debye model that the phonon decreases with decreasing temperature above Debye temperature,indicating the Debye temperature of SnSe is less than 108 K.