Chemical syntheses and characterizations of II-VI semiconductor nanocrystals

This thesis is mainly concerned with the chemical syntheses and property characterizations of a series of II-VI semiconductor nanocrystals. We have synthesized ZnTe and Cr doped ZnTe nanoparticles, ZnSe:en precursor nanoribbon arrays, and CdSe quantum dots (QDs) and tetrapods nanocrystals using simple chemical methods. Structures and compositions of the obtained nanocrystals are characterized by the high-resolution transmission electron microscopy (HRTEM), energy dispersion spectrometry (EDS), X-ray diffraction (XRD), X-ray fluorescence (XRF), and high-resolution scanning electron microscopy (HRSEM). The optical absorption spectroscopy, photoluminescence (PL), and field emission experiments are also carried out.;ZnTe and Cr doped ZnTe nanoparticles (ZnTe:Cr) have been synthesized by colloidal nano-synthesis method. The characterizations by XRD, XRF, and TEM show that the synthesized ZnTe and ZnTe:Cr crystals have cubic structure and Cr is incorporated into ZnTe. The magnetic properties of ZnTe:Cr are investigated. A hysteresis is observed in the magnetization versus magnetic field measurements below 10K. The detailed analyses suggest that the hysteresis is possibly originated from the magnetic short-range orders of Zn1-xCrxTe compounds in the sample.;Vertically-aligned ZnSe:en precursor nanoribbon arrays have been grown directly on Zn foil via a simple solvothermal procedure. The nanoribbons are 100 ∼ 300 nm in width and several nanometers in thickness. Their lengths are about a few micrometers. The conversion of the ZnSe:en nanoribbon arrays to ZnSe nanoribbon arrays with wurtzite structures is achieved by a thermal annealing in N2 atmosphere. Upon the release of en, the morphology of the ZnSe nanoribbon arrays is well-preserved and the crystallinity of the ribbons is improved. Uniform field emissions were observed from ZnSe nanoribbons. Results show that the ZnSe nanoribbon arrays are good field emitters in comparison with many other nanostructures as having a low turn-on field of 5 V/mum and a high field enhancement factor of 1382.;CdSe tetrapod nanocrystals have been synthesized using a simple nano-synthesis method by controlling the protonic acidity of the cadmium OA-TOP precursor. The crystal structure of the tetrapods is determined by HRTEM analysis, which shows that the tetrapod has a zinc blende core and four wurtzite arms. The possible growth mechanism in our experiment is discussed. The electronic structure of tetrapod is calculated in comparison with the spherical dot, indicating that for our tetrapod samples the ground state is dominated by the core diameter and only for the states above the fourth excited state the "arm diameter" effect becomes important.;The steady state and transient optical properties of CdSe tetrapods and QDs are studied in details. No qualitative differences are detected. The field emission properties of CdSe tetrapods with different arm lengths are also investigated in details, showing that the longer the arm length, the lower the turn-on field, and the higher the enhancement factor.