Study On Physical Properties And Electrical Transport Of Novel Two-dimensional Semiconductors

The physical,optical and chemical properties of materials in the two-dimensional(2D)scale are prominently different from the bulk counterparts due to the quantum confinement effect.With sustained improvement of the fabrication technique for nano-device,2D materials show more and more promising application prospects in electron-ic,optoelectronic devices,optical engineering and new energy fields,and so on.In this thesis,we study and tune the physical properties of graphene,main-group metal dichalcogenides,SnSe2 and InSe.We also investigate the application potentials of these materials,such as the thermoelectric performance of InSe.In chapter three,we study the Raman spectrum of graphene under high excitation laser.We found that the peak of 2D Raman mode can be described by using only three sub-modes with 3.81 eV excitation energy,which is different from the traditional four sub-modes at lower Raman excitation energy.The outer P3322 mode is prominently suppressed at high energy,which is consistent well with the recent theory.In chapter four,we investigate the 2D superconductivity transition in SnSe2 with high carrier density.We used the ionic liquid gating to induce high carrier density on the surface of sample,and observed superconducting transition at Tc = 3.9 K.Combin-ing the measurement of anisotropic magnetoresistance,temperature dependent magne-toresistance,and the V-I characteristic,we can confirm that the superconductor is being 2D superconducting state.By further studying,we found many unconventional superconducting phenomena.For example,under the parallel magnetic field,the upper critical filed is 2-3 times higher than the classical Pauli paramagnetic filed;we also found two phase existing in the different magnetic field and temperature regions:at the temperature slightly below Tc,the Cooper pairings show the thermally activated be-havior;at lower temperatures and small magnetic field,the Cooper pairings transport with the model of temperature-independent quantum tunneling of vortices.In chapter five,we study the electrical transport of InSe at low temperatures,and observe a gate-tunable weak antilocalization.By transferring the InSe onto the hexag-onal boron nitride(hBN)to reduce the impurities scattering,we obtained high mobility InSe/hBN devices.At low temperatures with weak thermal fluctuations,the quantum interference effect plays a dominant role for the carrier transport,and the measure-ments of magnetoresistance further reveal the scattering mechanism.We found that the magnetotransport shows a weak antilocalization behavior,enhances with gate voltage increasing.The dephasing length is up to 320 nm,and determined by the electron-electron interaction.InSe shows a promising potential in electronics due to the high mobility,high current on/off ratio and the high ability in ambient environment.In chapter six,we in-vestigate the temperature,carrier density and thickness dependent thermoelectric per-formance of InSe/hBN devices.The carrier density dependence of Seebeck coefficient behavior reveals that the dominant scattering mechanism at temperature between 90 K-300 K is acoustic phonon scattering.The Seebeck coefficient is up to-800 ?V/K in the 5 nm device,showing excellent thermoelectric performance.We found the ther-moelectric properties can be enhanced by reducing the thickness of sample,especially in the ultrathin sample.With the help of ab initio calculations,we found the density of states can be highly enhanced in the thinner sample.We also found that the ther-moelectric performance significantly enhance only at the regime of the thickness lower than the thermal de Broglie wavelength.