Synthesis Of Narrow Bandgap Tow-Dimensional Semiconductor Materials And Investigation Of Photoelectronic Application

Followed by the discovery of graphene,scientists have paid much attention to searching for new types of two-dimensional?2D?materials around the world.Interestingly,several categories of novel 2D materials with unique properties,such as transition metal chalcogenides?TMCs?,black phosphorus?BP?and topological insulators?TI?,have been duly found.Compared with zero-bandgap graphene,these 2D materials with narrow bandgap exhibit more distinct advanstages in optoelectronic applications.These materials have graphene-like structure,which are formed by atomic layers bonded with Van der Waals forces,making the monolayer or multilayer structures available by mechanical exfoliation method.Due to the small bandgap,such 2D materials have broadband light absorption ranging from visible to near-infrared wavelengths,which enables them realize various functional applications.Taking the advantages of 2D materials,integration of different 2D materials to form heterojunctions can further extend their future applications.Herein,we demostrate the synthesis and characterizations of the TMCs?PtS₂ and PtSe₂?,BP,TI?Bi₂Te₃?and graphene based heterojunction?graphene-Bi₂Te₃?.The main results are shown as follows:1.Bi₂Te₃ nanosheets and graphene-Bi₂Te₃ heterojunction are synthesized by solution method and chemical vapor deposition,respectively.Firstly,the surface metal plasmon of Bi₂Te₃ nanosheets are charaterized by scattering-type scanning near-field optical microscopy?s-SNOM?,hence to investigate the effects of the incident wavelengths,different thickness and morphology of Bi₂Te₃ crystals on their plasmon.It can be found there exists a strong plasmonic coupling effect in the stuided graphene-Bi₂Te₃ heterostructure,which implies the extinction spectrum of the graphene-Bi₂Te₃ heterostructure diplays three times higher than that of graphene.Moreover,we also demonstrated that the plasmonic resonance peak of the graphene-Bi₂Te₃ heterostructure can be easily tuned by changing the grating period.Utilizing open-aperture Z-scan technique,the optical nonlinearity of Bi₂Te₃nanosheets and graphene-Bi₂Te₃ heterostructure are investigated,revealing that Bi₂Te₃ possessed ultra-broadband nonlinear saturable absorption property and the graphene-Bi₂Te₃ heterostructure has deeper modulation depth than that of pure graphene.By varying the coverage of Bi₂Te₃ nanoplatelets on graphene,the modulation depth can be tuned from 14.7 to 50.1%.Besides,we further fabricated a highly efficient photodetector and a fiber Bragg grating all-optical photodetector based on the graphene-Bi₂Te₃ heterojunction.Combining the unique properties of graphene with Bi₂Te₃,the device not only shows a greatly enhanced responsivity,but also exhibits broadband photodetection performance ranging from visible to near-infrared wavelengths.2.Centimeter-sized,high-quality Se-doped BP bulk was successfully synthesized by the mineralizer-assisted gas-phase transformation method.Homogeneous Se doping in BP crystals with controllable doping concentration can be obtained under different growth conditions.Phototransistors were further fabricated on mechanically exfoliated Se-doped BP flakes.The phototransistor exhibited reliable electrical characteristics with a high on/off current ratios of 10⁵and mobility of 561 cm²V^-1s^-1 at ambient environment.The responsivity of the doped BP?with a doping concenstration of 1.6 wt%?devices is over 20-fold enhancement than that of the pristine BP.An obvious transistion from p-type to n type in pristine BP can be observed after the deposition of the Si_xN_y on the top of BP flake.The electron mobility of this n-type BP phototransistor was then improved to be¹76 cm²V^–1s^–1.After that,a high-performance in-plane BP p-n diode was also successfully demonstrated by this approach.To be noted here that this doped BP can be kept stable in ambient atmosphere.3.The PtS₂,PtSe₂,PtS₂-PtSe₂,PtS₂-PtS₂,PtSe₂-PtSe₂ heterojunction array photodetectors were produced on a 2"SiO₂/Si substrate by a two-step chemical vapor deposition methods and auxiliary photolithography,where the size,density,and geometry of 2D sheets can be adjusted as desired.The optoelectronic properties of these photodiodes were comprehensivly studied in following part.Theoretical calculation results along with the Kelvin probe force microscopy?KPFM?and ultraviolet photoelectron spectroscopy?UPS?measurements,it can be confirmed a built-in electric field formed at the interface of the heterojunctions.Then dynamic photovoltaic switching in the photodiode is observed at zero-volt state under laser illuminations.The self-driven PtS₂-PtSe₂ photodiodes show excellent characteristics including wide photoresponse ranging from 405 to 2200 nm,high photoresponsivity of 361 mAW^-1,high EQE of 84%and a fast response speed.Open aperture Z-scan measurement of the PtSe₂ film is taken at the wavelength of 1064 nm and the results show efficient saturable absorption characteristics with a modulation depth of 26%and saturable intensity about0.346 GWcm^-2.Simultaniously,an all-fiber ring cavity was also built in which the PtSe₂film was sandwiched between two fiber splices as the saturable absorber and ytterbium-doped fiber was used as the optical gain medium.We found a stable dissipative soliton pulses generated in the normal dispersion regime with a 3-dB spectral bandwidth of 2.0 nm and pulse duration of 470 ps centered at 1064.47 nm.