Vapor Phase Synthesis And Optoelectronic Properties Investigation Of Two-dimensional Nonlayered Cadmium-based Chalcogenide

Two-dimensional(2D)nanomaterials have triggered worldwide interests owing to their unique structures and distinct physical properties,and enabled great potential for applications in electronic and optoelectronic devices.However,the study of 2D materials in the real world are still mainly limited to layered compounds up to now.Many materials with superior characteristics having crystal structures of chemical bonding in three dimensions(nonlayered materials)are neglected because they are difficult to be synthesized.Cadmium-based chalcogenide are the typical?-?group semiconductors with direct bandgap,high optical absorption coefficient,good chemical and thermal stability,etc.It is challenge and significative to make the cadmium-based chalcogenide into 2D structure.Here,the research status and existing problems of the 2D cadmium-based chalcogenide in recent years are comprehensively reviewed.Then we designed and conducted the experiments to systematically study the current problems and challenge.The work are listed follows:(1)We presented the initially self-limited epitaxial growth of 2D nonlayered CdS flakes on mica substrate by employing In₂S₃ as the passivation agent,and fabricated high performances photodetector device based on CdS flake.CdS flakes with thickness of?6nm and lateral size of?40?m were synthesized by adjusting the amounts of In₂S₃.The growth mechanism was evidenced via theoretical calculations,which was attributed to the surface distortion effect of In _Cd-S motif and the preference of local environments for In on the CdS(0001)surface.The vertical growth of CdS along[0001]direction was inactivated by surface In defects.The photodetector exhibited excellent performances,such as a high photoswitching ratio(I_light/I_dark,?10³),a high detectivity(D*,?2.71×10⁹ Jones),and fast photoresponse speed(?_R=14 ms,?_D=8 ms).(2)CdS flakes were as the epitaxial layer to grow high-quality 2D CdS/CsPbBr₃ heterostructures,which showed excellent photovoltaic(PV)performances.The clear interface and crystal orientation relationship were identified by aberration-corrected scanning transmission electron microscopy(AR-STEM).Photoluminescence(PL)and time-resolved PL were adopted to verify the strong interlayer coupling and charges transfer at the interface of the type-II heterojunction.The lateral device based on the heterostructure exhibited excellent excitonic PV performances,such as large short-circuit voltage(?0.76 V),high fill fator(?50.4%)and high power conversion efficiency(?17.5%)under 500nm illumination.Meanwhile,the device also displayed high photodetection performances at the zero bias voltage,including a high photoswitch ratio(?10⁵)and a fast response rate(84?s for rise and 23?s for decay time).(3)Nonlayered wurtzite-zinc blend(WZ-ZB)structure CdSe homojunction flakes were obtained by temperature gradient,which exhibited high photodetection performances.The indium as passivation agent was expanded to the CdSe growth.Different polymorphs of CdSe flakes(WZ at the high temperature zone,WZ-ZB homojunctions at the intermediate temperature zone and ZB at the low temperature zone)were obtained on mica substrates.Interestingly,two pieces of vertical WZ-ZB homojunctions epitaxially integrated into WZ/ZB lateral junctions.These WZ/ZB homojunctions showed a divergent second-harmonic generation intensity,strongly correlated to the multiple twinned ZB phase.It was identified by AR-STEM and theoretical calculations.Finally,the photodetector based on the CdSe homojunction showed excellent performances integrating a high photoswitching ratio(?3.4×10⁵)of WZ structure and high photoresponsivity(3.7×10³ AW^-1)of ZB structure.(4)The optical and electrical properities of WZ structure CdSe flakes were tuned though the strain engineering.The band gap of CdSe flakes increased from the compression strain to the tensile strain.In addition,based on the piezoelectric effect,a uniform negative potential was formed on(0001)face of the CdSe flakes under the tensile strain.Then the negative piezopotential usually raised the schottky barrier height,which decreased the current of the device and vice versa.