| In 2004,the successful preparation of graphene by Andre Geim and Konstantin Novoselov has aroused a research upsurge of two-dimensional(2D)materials.Graphene with high carrier mobility,large thermal conductivity and high electrical conductivity has been widely used in transparent conductive films and supercapacitor.However,the zero-bandgap electron structure has seriously limited its applications in semiconductor fields.In recent years,two-dimensional transition metal dichalcogenides(TMDCs)hae presented versatile applications in optoelectronics due to their moderate bandgap,large nonlinear susceptibility and fast carrier recombination.Moreover,with dangling-bong-free surface and weak van der Waals(vdW)interlayer interactions,the vdW heterstructures can be formed without lattice mismatching by stacking TMDCs,combing superior properties of individual layers to achieve diverse functions due to the enhanced electron orbitals coupling.As an significant TMDCs semiconductor,molybdenum ditelluride(MoTe2)has attracted much attention due to its excellent photoelectric properties,such as high carrier mobility,strong photoelectric conversion efficiency,and a silicon-like bandgap.Additionally,the theoretical calculation showed that there was a small energy difference between 2H-MoTe2 and 1T’-MoTe2.The phase transition between them can be realized by simple methods,which provides potential for new low-power information storage devices.In this dissertation,the optoelectrical and terahertz modulation properties of MoTe2 were studied.The all-optical terahertz modulator based on MoTe2/Si heterostructure with high modulation performance was designed.The carrier dynamics process of photoinduced 2H-1T’phase transition was explored.These main contents are outlined as following:1、Crystal growth and characterization of 2H-MoTe2To avoid introducing impurity,2H-MoTe2 bulk single crystals were grown by self(Te)-flux method.In the process of crystal growth,2H-MoTe2 single crystals were obtained at high temperature(1180℃,HT)and low temperature(850℃,LT).The basic properties of2H-MoTe2 crystals were characterized by XRD,XPS and Raman spectroscopy.High resolution transmission electron microscope(TEM),selected area electron diffraction(SAED)and Laue diffraction indicated that the crystal quality of 2H-MoTe2 grown at high temperature was poor,while the crystal quality of 2H-MoTe2 grown at low temperature was significantly improved.The crystals will go through the phase transition from IT’ to 2H for high temperature growth,which eventually leads to the poor quality.In addition,the shape of the crystal is regular hexagon,which is consistent with its intrinsic hexagonal structure.Additionally,the maximum size of 2H-MoTe2 is 4×4×1 mm3.The XRD results confirmed that the natural growth plane of crystal was {002} group.2、Optoelectrical properties of 2H-MoTe2To further verify the quality of 2H-MoTe2 crystals,field-effect transistor devices were prepared.The electrical and optoelectrical properties were further investigated.The experimental results indicated that the field-effect mobility of 2H-MoTe2 crystals obtained at high temperature(1180℃)is only 3.4 cm2·V·s-1,while that of crystals grown at lower temperature(850℃)can reach 53.4 cm2·V-1·s-1.To the best of our knowledge,it is the maximum field-effect mobility of intrinsic 2H-MoTe2 so far.Furthermore,the photoresponse results showed that the crystal grown at low temperature also had higher switch ratio,responstivity(R)and external quantum efficiency(EQE),in which the R and EQE increased by about 10 times.3、Ultrasensitive and broadband all-optically controlled THz modulator based on MoTe2/Si van der Waals heterostructureConsidering that the band structure of 2H-MoTe2 is similar to that of silicon,an all-optical terahertz modulator based on the MoTe2/Si vdWs heterostructure was proposed.The terahertz modulation performance of the MoTe2/Si modulator was tested by using a self-built terahertz time-domain spectroscopy(THz-TDS).The MoTe2/Si modulators presented ultrasensitive modulation performance with a MD of 99.9%even under low illumination power of 300 mW at 1064 nm.To the best of our knowledge,it is the highest MD for TMDCs-based all-optically controlled THz modulators to date.The ultrasensitive modulation performance gives rise to reduce the power consumption and thermal effect caused by high power laser.In addition,MoTe2/Si modulator exhibited broadband modulation over the frequency range from 0.3 to 2.0 THz.From the energy band theory point of view,the high performance of MoTe2/Si modulator is attributed to the bandgap similar to Si and the small carriers transport energy barrier induced by small band offset.Therefore,the accumulation of carriers with high concentration can be achieved under a low illumination power,improving the conductivity of device.4、Carrier dynamics of semiconductor-metal phase transition in few-layer MoTe2 driven by laser irradiationIn this paper,we used a laser of 808 nm to irradiate MoTe2 nanosheet,and realized the phase transition from 2H to 1T’ by controlling the irradiation time.Firstly,the samples under different illumination time were verified by TEM,Raman spectrum and XPS.The results indicated that 1T’-phase began to appear at about 10 min,and the phase transition was completed at about 25 min.After that,THz-TDS system was used to characterize the conductivity,carrier scattering rate and mobility under different irradiation time.The transient carrier dynamics was investigated by using the ultrafast laser pump-probe spectroscopy.The change of band structure of the sample was revealed by the analysis of differential reflection spectrum from ground state bleaching(GSB)to excited state absorption(ESA).Finally,the structure changes of the samples before and after illumination were studied by the angle resolution pump detection system.The experimental results showed that the symmetry of crystal structure was broen before and after laser irradiation.Additionally,by comparing the carrier decay time of laser-induced 1T’-MoTe2 with that of pristine IT’-MoTe2,the generation of Te vacancy during the phase transition was confirmed. |
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