Study On Vapor Phase Systhesis And Optoelectronic Properties Of Antimony And Cadmium Group Two-Dimensional Semiconductors

In recent years,two-dimensional(2D)materials have attracted extensive research interest because of abundant species,excellent properties and wide applications.Among them,antimony and cadmium group 2D materials have great promising applications in electronics and optoelectronics,due to their unique and rich structure,simple elemental composition,high absorption coefficient,wide bandgap,thickness-dependent bandgap.However,the development of effective synthesis strategies for the controlled preparation of large-area,atomic-layer thickness,high-quality antimony and cadmium group 2D materials is the premise for the realization of their basic research and wide applications.At present,the fabrication methods of antimony and cadmium group 2D materials mainly include liquid-phase and gas-phase method.Although the liquid-phase method can be used to prepare thin-layer samples in batch,the size small,irregular morphology,nouniform thickness,poor crystalline quality,as well as inevitable impurities pollution,seriously restrict the development and application of the materials.In contrast,the gas-phase method can be used to prepare 2D samples with high quality,large size,atomic-layer thickness,controllable morphology and composition.It is promising to realize large-scale production of 2D materials in practical industry.However,due to the special structure of antimony and cadmium group materials,there are still many problems and challenges in the preparation of 2D morphology by gas-phase synthesis.Based on the above research background,this dissertation focuses on antimony and cadmium group materials as the research object.This dissertation firstly summarizes the basic structure,properties and research status of antimony and cadmium group materials;Three kinds of new gas-phase synthesis strategies were then proposed for the preparation problems;The large-area,large-size and high-quality ultrathin 2D samples were successfully prepared,and their basic structure,optical properties and optoelectronic performance were systematically characterized and analyzed.The main research results and conclusions are as follows:(1)Salt-assisted vapor phase synthesis and optoelectronic properties of 2D nonintrinsic layered antimony-group chalcogenides.The 2D Sb₂Se₃ flakes were successfully prepared by salt-assisted vapor phase synthesis strategy via breaking the inherent one-dimensional chain-like structure limitation of Sb₂Se₃.The as-grown Sb₂Se₃flakes exhibit a typical triangular morphology,ultrathin thickness of 1.3 nm and lateral size up to 24?m.Based on the experimental evidences and theoretical calculations,the sodium-assisted 2D morphology of growth mechanism was proposed to interpret the nature of the growth process,where the 2D Sb₂Se₃ grows along the(010)crystal plane,mainly because the(010)crystal plane has a low formation energy after the adsorption of sodium.The photodetector based on Sb₂Se₃ flakes demonstrates excellent performances including wide spectral response varying from ultraviolet to near-infrared region(300-950 nm),high responsivity(4320 m A W^-1),fast response speed(13.16 and 9.61 ms),and strong anisotropy(anisotropy ratio is 2.5).The present salt-mediated vapor phase synthesis strategy may pave a new avenue to synthesize anisotropic 2D materials with intrinsic one-dimensional chain-like crystal structure.(2)The hot plate-assisted vapor phase synthesis and optoelectronic properties of 2D layered cadmium-group halides.The submillimeter size,atomic-layer thickness,and large-area Cd I₂ flakes were successfully prepared by the facile hot plate-assisted vapor synthesis strategy.The as-grown Cd I₂ flakes are high-quality single crystals with the plane(001)as preferential growth orientation,which are proved by systematic characterizations.The thickness-dependent micro-absorption spectra demonstrates that the Cd I₂ flakes have obvious tunable-bandgap with thickness.The photodetector based on Cd I₂ flakes displays excellent photoelectric performance including low dark current(0.53 p A),high current on/off ratio(56.70),high responsivity(5.55 m A/W),fast response speed(<2 ms),and good stability.The hot plate-assisted vapor phase synthesis strategy has also been successfully extended to the synthesis of large-area,large-size,high-quality,and ultrathin Cd Br₂ and Cd Cl₂ single-crystalline flakes,which provides a new path for the preparation of 2D metal halides.(3)The template-assisted vapor phase synthesis and optoelectronic properties of 2D nonlayered cadmium-group chalcogenides.The layered Cd I₂ flakes as precursor templates were firstly synthesized by a simple hot plate-assisted vapor synthesis strategy.Then large-size(up to submillimeter scale),and ultrathin(down to atomic-layer level)nonlayered Cd S flakes were successfully achieved via a simple low-temperature vapor sulfurization process.The size and thickness of Cd S flakes could be controlled well by the precursor templates of Cd I₂ flakes.The growth mechanism is ascribed to the chemical substitution reaction from I to S atoms between Cd I₂ and Cd S,meanwhile realizing structural transformation from layered to nonlayered 2D materials,which has been evidenced by both experiments and theoretical calculations.The photodetector based on transformed Cd S flakes shows excellent photoelectric properties such as ultrahigh current switching ratio(10³),ultralow dark current(10^-13A)and ultrafast response speed(50?s).In addition,this template-assisted vapor phase synthesis strategy can be expanded to the fabrication of other nonlayered 2D materials like Cd Se,thus offering a new route to prepare large-size ultrathin nonlayered 2D materials.