Chemical Vapor Deposition And Photoelectric Properties Of Two-dimensional Tellurium Single Crystals

With the increasing progress of large-scale preparation technology,more and more new two-dimensional(2D)materials can be further explored and studied.Since 2Dmaterials have atomic-scale thickness,they can greatly reduce the size of devices,improve the integration of electronic assemblies,and have many interesting physical properties such as tunable band gaps and good optoelectronic properties.Those merits make 2D materials great potential in electronic application.As a chalcogenide material,2D Te has excellent properties such as environmental stability,oxidation and hydration catalytic activity,tunable bandgap,thermoelectric and nonlinear optical responses compared with other existing 2D materials.It also has high carrier mobility at room temperature.These properties facilitate fundamental research and practical applications such as high-performance photodetectors,field-effect transistors(FETs),and modulators.Furthermore,the 2D Te nanoflakes have a unique helical chain structure,which endows them with high carrier mobility and strong in-plane anisotropy properties.The flexible mechanical properties and low structural symmetry of 2D Te nanosheets provide high in-plane piezoelectric coefficients,which make them a potential material for piezoelectric devices.Furthermore,2D Te nanoflakes currently have the lowest lattice thermal conductivity of the known family of 2D single-element materials,exhibiting extraordinary topological properties.In this thesis,based on chemical vapor deposition method,we use materials In₂Te₃ as the growth source,fluorphlogopite as the substrate,and use the confined-space epitaxial growth technology to prepare large-scale,high-quality 2D tellurium single-crystal nanosheets.Field effect transistors based on 2D tellurium nanosheets were prepared,and the electrical properties and photoelectric conversion properties of the devices were studied.The research methods and results are as follows:1.Large-scale and high-quality 2D tellurium nanosheets were successfully prepared by high-temperature chemical vapor deposition method using high melting-point material In₂Te₃as growth source.The influence of growth temperature,growth time,flow rate of carrier gas,and growth source concentration on the shape of 2Dtellurium nanosheets were systematically explored.Furthermore,the effect of growth temperature on the thickness of 2D tellurium nanosheets was explored.The optical,morphology and crystal structure characterization of the 2Dtellurium nanosheets were carried out,and the results showed that the surface morphology of the 2Dtellurium nanosheets was smooth and had a perfect hexagonal crystal structure.In addition,X-ray photoelectron spectroscopy(XPS)characterization results show that no indium atoms remain in the 2D tellurium nanosheets.The above results provide inspiration for further research on the preparation technology of 2D tellurium nanosheets.2.2D Te nanosheets were transferred from mica substrate to SiO₂/Sisubstrate by wet transfer to fabricate field effect transistors(FETs)based on 2D Te nanosheets.The optoelectronic properties of FETs were tested using light pulses with wavelengths of 405,532,635 and 808 nm.The I_ds-V_g results show that the 2D Te nanosheet-based FETs exhibit distinct p-type electrical behavior,with hole mobility as high as 850 cm~2/(V·s),when a certain degree of positive gate is applied.FETs exhibit slight n-type electrical behavior under certain gate bias.The photodetector exhibits good stable and repeatable photoresponse performance,and exhibits excellent photoelectric performance under the incident light of 405nm.The photoresponsivity is 1.04×10~4A/W,and the photocurrent rise time and decay time are 4.3 s and 17.6 s,respectively,the detection rate is 1.4×10¹² Jones,the external quantum efficiency is 3.19×10~6%,and the sensitivity is 64 cm~2/W.These results indicate that 2D tellurium nanosheets have great potential in the fields of electronics and optoelectronics.3.The polarized light Raman test was performed on the 2D tellurium nanosheets.The Raman measurement showed that the 2D tellurium nanosheets had strong in-plane anisotropy with peak/valley ratio of 14.96.The angle-resolved electrical test of the FETs is carried out.and found that the 2D tellurium shows obvious electrical anisotropy with peak/valley ratio of1.73.In addition,the anisotropic photoresponse characteristics of the device were preliminarily tested,and the photocurrent caused by different angles of polarized light did not change much.The above experimental results provide valuable information for further understanding of the anisotropic properties of 2D tellurium nanosheets.