Study On The Growth Mechanism And The Properties Of Two-dimensional Symmetric Tellurene

Since the successful synthesis of two-dimensional tellurene materials,due to its excellent properties,such as p-type conductivity,ferroelectric,piezoelectric,pyroelectric,environmental stability,high room temperature mobility and light response,etc.,it has been used in photodetectors,biosensors,gas Sensors,biomedicine,field effect transistors,piezoelectric devices,energy equipment,optical modulation devices,etc.have shown great application potential.At present,the synthesis methods of two-dimensional tellurium nanostructures include liquid-phase exfoliation(LPE),chemical vapor deposition(CVD),epitaxial growth,and hydrothermal method,in which the hydrothermal method can be prepared at low temperature to obtain high-quality tellurium with various nanostructures and is independent of the selection of substrate materials.This thesis focuses on the preparation and characterization of two-dimensional tellurium nanosheets with symmetrical structures by hydrothermal growth assisted by polyvinylpyrrolidone(PVP).In this thesis,we investigated the effect of PVP as a surface blocking ligand during the growth process in the hydrothermal synthesis experiment and discussed the influence of PVP chain length,namely PVP molecular weight(the larger molecular weight of PVP,the longer chain length),on the properties of tellurium nanostructures.By changing the molecular weight of PVP from 10 K,24 K to 58 K,130 K,the changes of yield and thickness of two-dimensional tellurium nanosheets were studied.We found that the yield of two-dimensional tellurene nanosheets is higher and the thickness is thinner for the PVP with medium-chain length at certain growth temperatures and times.Typically,when the molecular weight of PVP is 58 K,the yield of tellurium nanostructures is the highest of about 42.6%,and the thickness distribution of tellurium nanosheets is 20-40 nanometers.Meanwhile,interestingly,three kinds of tellurene nanosheets with special symmetrical morphologies were obtained,which were marked as "V-shaped","heart-shaped" and "paper airplane-shaped".The high-resolution transmission electron microscope was used to systematically study the morphology and structure of the nanosheets with three special morphologies.It was found that moiré fringes appeared in the symmetrical area,and the growth and evolution mechanism of the symmetrical nanosheets was proposed.The research of nonlinear optics plays an important role in the development of material crystal structure analysis,laser technology,and spectroscopy.The second harmonic generation(SHG)effect is used as strong evidence of the potential applications of materials in nonlinear optics.In this thesis,the optical signal intensity of two-dimensional tellurene nanosheets at different excitation wavelengths was investigated.It has been found that the optical signal intensity was the highest at the excitation wavelength of 920 nanometers.By fitting analysis of the influence of different excitation power on the intensity of the optical signal under a specific excitation wavelength,it is confirmed that the generation of the optical signal is caused by the second harmonic generation effect,and the intensity of the optical signal was quadratically dependent on the excitation power.Based on the analysis of the polarization-resolved SHG signal,the tellurium crystal orientation is identified and presents a dumbbell shape in polar coordinates.It shows that two-dimensional tellurium materials have great development potential in the field of nonlinear optics technology.