Controllable Synthesis And Vibrating Properties Of CdSe Based Heterostructure Nanowires

In recent years, one dimensional semiconductor nanowire heterostructures has become a hot spot in the research of nano-materials science. One dimensional semiconductor heterostructures play an important role in "tailor" homogeneous physical property of semiconductor nanowires and have potential application in many fields. In this dissertation, we mainly focus on the controllable synthesis and vibrating properties of CdSe based nanowire heterostructures. The results are introduced as following:1. CdSe-Ge, CdSe-Ge-CdSe heterostructure nanowires, CdSe-Ge biaxial nanowire core /polycrystalline Ge sheath heterostructures, Ge-GeSe biaxial nanowires and GeSe nanobelts were grown via a simply one-step thermal evaporation of CdSe and Ge with the molar ratio of 1:1,1:2,1:3 and 1:4, respectively. The as-synthesized products were characterized by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscope (HRTEM) equipped with an energy dispersive spectrometer (EDS) and micro-Raman spectrometer for chemical compounds and structure study. The CdSe and Ge subnanowires in CdSe-Ge biaxial nanowires (or triaxial nanowires) and the Ge and GeSe subnanowires in Ge-GeSe biaxial nanowires are single crystalline. A good epitaxial relationship exists in the interface between CdSe and Ge in CdSe-Ge biaxial nanowires and in the interface between Ge and GeSe in Ge-GeSe biaxial nanowires. Two sides of CdSe subnanowires in the CdSe-Ge-CdSe triaxial nanowire both contain a high density of stacking defects along its entire length. The obvious differential in microstructure is just induced by the view angle.2. The CdSe-Ge biaxial (CdSe-Ge-CdSe triaxial) nanowire heterostructures, and Ge-GeSe biaxial nanowire are grown via a co-growth vapor-transfer mechanism based on VLS. The formation of the CdSe-Ge core/polycrystalline Ge shell nanowires also could reasonably be attributed to the co-growth mechanism. As the deficiency of CdSe, the co-growth process terminated, and a secondary growth of the extra Ge started on the primary bi-coaxial nanowires. During the CVD method, the GeSe vapor formed from the reaction of Ge and CdSe. Then the CdSe, Ge and GeSe vapors would deposit in the suitable temperature region according to their different melt point and different nanostructures could be obtained in the corresponding position.3. We study the vibrating properties of 4 samples using Room temperature Raman spectrum, and observe LO mode of CdSe, LO (TO) mode of Ge, LO and TO mode of GeSe in the four different nanostructures have a different wave-number shift in comparison with that of the responding bulk counterpart, respectively. The LO mode of CdSe in CdSe-Ge nanowires have 20 cm-1 blue-shift in comparison with that of the corresponding bulk counterpart CdSe, which may be caused by tensile stress or high density of stacking defects in CdSe subnanowires in CdSe-Ge-CdSe triaxial nanowires. The LO mode of Ge in CdSe-Ge (or CdSe-Ge-CdSe), CdSe-Ge core/polycrystalline Ge sheath and Ge-GeSe heterostructural nanowires have a downshift by 8,5 and 2 cm-1 in comparison with that of the bulk counterpart Ge, respectively. The TO mode of GeSe in Ge-GeSe herterostructure nanowires and GeSe nanobelts have a downshift by 8 and 5 cm-1 in comparison with that of bulk GeSe. The LO mode of GeSe in Ge-GeSe herterostructure nanowires has 3 cm-1 redshift in comparison with that of bulk GeSe. With regards to the microstructure of heterostructure nanowires, the downshift of LO mode may be caused by tensile stress, which affects the Raman line by a downshift. And the different shift scales are attracted by the different size of the Ge subnanowires, Ge nanocrystalline and GeSe sub-nanowires. In addition, the possibility of the vibrating modes introduced by the doping effect can be ruled out.