Preparation And Properties Of Several Group ? And Group ? Elements And Compound Quantum Dots

Since the discovery of low-dimensional materials,a large number of studies have shown that the nature of the material changes with the nanometer size.For example,quantum dot materials may exhibit different physical,electrical,or optical properties from corresponding large particle materials.These unique properties have great application value in optoelectronic nanomaterial devices,functional materials,and biomedical applications.This dissertation is devoted to exploring the preparation methods and optical properties of IV and VI quantum dots and compounds,laying the theoretical and experimental foundation for the future industrial semiconductor quantum dot device manufacturing.The transition metal dichalcogenide is another research hotspot following graphene.Its unique layered structure can be used as an excellent solid lubricant.At the same time,its excellent activity makes it potentially useful in the field of electrocatalytic hydrogen evolution.Therefore,it is of great significance to study the quantum dots of Group IV and Group VI quantum dots and transition metal dichalcogenides in both academic research and engineering applications.The research of this dissertation includes two aspects.On the one hand,the ultrasonic method was used to prepare elemental Ge QDs,Te QDs,Se QDs,transition metal dichalcogenide MoSe₂ QDs,and MoTe₂ QDs.The method of preparing quantum dots by using the ultrasonic method has the advantages of low cost,simple operation,mild reaction conditions and the like,and the quantum dots with small size are prepared.on the other hand,the optical properties of the group IV and the group VI elements and the quantum dots of the compound are studied.The morphology of Ge QDs was characterized by AFM and TEM.The results showed that Ge QDs with a radius of about 3.00 nm were prepared and had good crystallinity.The calculated Ge QDs band gap was 1.48 eV.Raman,XPS studied the vibration of Ge QDs sample functional groups.UV-Vis,PL and PLE spectra have been used to study the optical properties of Ge QDs and their excitation wavelength dependence in fluorescence spectroscopy,which shows similarities with graphene quantum dots optical characteristics.The structural characteristics of Te QDs were characterized by XRD,SEM,TEM and AFM.The results showed that the uniform size and the good crystallinity of Te QDs were successfully prepared and the particle size was about 4 nm.XPS and EDS were used to study the vibrational bond and elemental composition in the sample of the Te QDs.The Raman peaks of the Te QDs appeared at the positions of 122.6 cm^-1and 142.2 cm^-1.The bandgap of Te QDs was 1.34 eV based on UV-Vis Tauc pattern analysis.Photoluminescence spectroscopy shows that there are discrete levels of Te QDs.The PL peak shows a redshift due to defects or surface states of the Te QDs;in addition,the two excitation peaks are caused by two electron transition channels and the maximum PLE peak?except for the 263 nm peak?Also red-shifted.XRD,SEM and AFM showed that the prepared quantum dots of selenium have a size of about 3 nm and have the characteristics of uniform size,good crystallinity and good dispersibility.The functional group composition and vibrational spectra of selenium quantum dots were studied by XPS,FTIR,Raman and EDS.there are two transitions from the ground state to the excited state in the UV-Vis quantum dots,which are 3.54 eV?350 nm?and 4.43 eV?280 nm?,respectively.Optical Properties of Selenium Quantum Dots by PL and PLE Spectra.The surface morphology of the samples was characterized by SEM,TEM and AFM.The results showed that MoSe₂,MoTe₂ QDs and nanosheets coexisted in the sample.We prepared quantum dots with MoSe₂ and MoTe₂ QDs diameters of 3.7 nm and 3.5nm,respectively.Quantum dots have good crystallinity.XPS investigates the elemental composition and orbital state of MoSe₂,MoTe₂ samples.For the Raman spectroscopy study,we found that the Raman peaks of the two features of the MoSe₂sample moved 0.1 cm^-1 and 1.1 cm^-1 respectively towards the high-energy direction with respect to the bulk material.The characteristic Raman peak of the MoTe₂ sample moves 1 cm^-1 in the low energy direction relative to the bulk material.Photoluminescence?PL?and PLE spectra were used to study the optical properties of MoSe₂,MoTe₂ QDs,and nanosheets,demonstrating that there are four electronic transition levels for MoSe₂ QDs and nanosheets.For MoTe₂ QDs and nanosheets,PL and PLE peaks are red-shifted when there is an energy level that matches the absorption wavelength.