| The investigations of novel materials are deemed to be an essential driving factor for the development of modern science and technology.Two-dimensional(2D)layered van der Waals(vdW)materials have been studied extensively due to their exotic physi-cal properties.Especially 2D materials are widely explored in information devices and spintronics,as they are considered to be an alternative for thin film materials.As a stable 2D semiconducting material,?-In2Se3 was widely studied because of its excellent op-toelectric properties.In addition,room-temperature ferroelectric properties have been proven in their few-layered structures with coupled in-plane and out-of-plane ferroelec-tricity,which may lead to a lowing of lattice thermal conductivity,and thus excellent thermoelectric properties can be expected.In this thesis,we investigated thermoelectric properties of monolayed ?-In2Se3 by means of first-principles calculations.Meanwhile,the ferroelectric properties were also studied by experiments.The main results can be summarized as follows:(1)Thermoelectric(TE)properties of monolayed ?-In2Se3 were investigated by first-principles calculations based on the density functional theory.Within the experi-mentally achievable doping levels,monolayed ?-In2Se3 is proved to be a good candidate for high-performance thermoelectric materials.The power factor PF nad the figure of merit ZT can reach 0.02 W/mK2 and 2.18 at room temperature,respectively.We at-tribute such great TE performance to the large electrical conductivity and low lattice thermal conductivity,which originate from its unique band structures and anharmonic scattering.Furthermore,we also confirm that the quantum confinement effect can real-ize up to an order of magnitude enhancement in the PF in this system.(2)a-In2Se3 crystals were grown using chemical vapor transport(CVT)method and were proven to be 2H phase by means of X-ray diffraction(XRD)?Raman spec-trum and Energy dispersive spectrometer(EDS).Few-layered ?-In2Se3 field-effect tran-sistors were fabricated by mechanical exfoliation method and other nano fabrication procedures,in order to explore its electric transport properties at room temperature.By performing electric transport measurement,few-layered ?-In2Se3 was confirmed to be an n-type semiconducting material,which is consistent with elsewhere reported results.(3)A room-temperature ferroelectricity of few-layered ?-In2Se3 was verified based on the piezo force microscopy(PFM)analysis.Stale ferroelectric domains which were irrelevant with the surface topography were observed.By performing the fixed-point measurement in ferroelectric domain regions,the phase and amplitude scenes show ob-vious ferroelectric hysteresis and butterfly-like characteristics,respectively.The posi-tive and negative coercive voltages of few-layered ?-In2Se3 were measured to be-0.2 V and-1.6V,respectively.Besides,the controllable inversion of ferroelectric polarization was proven by writing/erasing ferroelectric domains with an external voltage applied on the PFM tip.Furthermore,a device structure with a source-drain patterned on the?-In2Se3 channel was designed to meet the requirement of PFM characterization and regulate its ferroelectricity by applying gate voltage to few-layered samples simultane-ously,which provide hints for our future studies in this system. |
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