Metal chalcogenide, silicide, and oxide nanostructures

Abstract

In this study, we investigate the synthesis, stress-dependent phase transitions and Raman analysis of metal chalcogenide $(Ag_2Te)$, silicide $(V_5Si_3)$ and oxide $(Bi_2O_3)$ nanostructures. First, we prepare the cross-section of laterally epitaxial single crystalline $Ag_2Te$ nanowires (NWs) and observe that stress energy can assist the phase transition by stabilizing the high temperature, without going through the intermediate-temperature phase. Second, we control the reaction time and temperature gradient in chemical vapor deposition and obtain the $V_5Si_3$ nanotubes (NTs) and NWs. Third, we employ the polarized Raman single NW spectroscopy for identifying the growth direction and shape effect in the α and $β-Bi_2O_3$ NWs, respectively In chapter 1, we demonstrate synthesis and new phase transitions of single crystalline $Ag_2Te$ NWs. Laterally epitaxial single crystalline $Ag_2Te$ NWs are synthesized on sapphire substrates by the vapor transport method. We observed the phase transitions of these $Ag_2Te$ NWs via in-situ transmission electron microscopy (TEM) after covering them with Pt layers. The constrained NW shows phase transition from monoclinic to a body-centered cubic (BCC) structure near the interfaces, which is ascribed to the thermal stress caused by differences in the thermal expansion coefficients. Furthermore, we observed the nucleation and growth of BCC phase penetrating into the face-centered cubic matrix at 200 ℃ by high resolution TEM in real time. Our results would provide valuable insight into how compressive stresses imposed by overlayers affect behaviors of nanodevices. In chapter 2, we report the morphology control of vanadium silicide nanostructures. Free-standing hexagonal $V_5Si_3$ nanostructures are grown on a vanadium foil substrate placed on Si powder by a chemical vapor deposition. By controlling the reaction time and substrate temperature, we have succeeded in morphology tuning of $V_5Si_3$ NTs and NWs. When the downstr...