Synthesis and Characterization of One and Two - Dimensional Nanostructures and Their Potential Applications in Nanoelectronics and Nanodevices

Abstract

One-dimensional (1-D) nanostructures have been gaining attraction in the research community as the ideal systems for exploring novel phenomena at the nanoscale due to the simple synthesis techniques and unique optical, mechanical and electrical properties. Electronic devices such as photodetectors based on such nanostructures have rapidly advanced in recent years by integration of various materials and structures into functional devices. Specifically, nanowire-based photodetectors exhibit numerous opportunities for nanoscale optoelectronics due to their physics and technologies. As tellurides of Zn and Cd among II-VI compound semiconductors have become popular for nanoelectronics applications, detailed characterization of their thermal and electrical properties is required. We synthesized different types of II-VI nanowires (NWs) including ZnTe, CdTe and CdxZn¬1-xTe after optimizing the relevant growth conditions for each of them via the vapor-liquid-solid technique. The effect of various growth parameters on the morphologies of NWs were investigated by advanced characterization techniques and microscopy methods throughout this work. For thermal stability characterization of nanowires, the melting behavior of ZnTe nanowires has been investigated at a wide temperature range from room temperature up to 520 oC using a transmission electron microscope (TEM) in both bright field and high-angle annular dark field (HAADF-STEM) modes. Using in-situ TEM measurements the ZnTe nanowires (having a diameter of 30-160 nm) were heated in a controllable heating system and their melting temperature was studied. The results showed a significant reduction of the melting temperature compared to bulk (from 1300 to 450 oC). These results should be considered in further treatments on ZnTe nanowire-based devices such as an annealing treatment to anticipate the possible failure of the devices due to the melting of the nanowire. Following the understanding of thermal properties of nanowires, the effect of thermal annealing on the thermal conductivity of ZnTe nanowires was studied on a microfabricated suspended device. A decrease in thermal conductivity was observed after each thermal annealing step at all the measured temperatures. Thermal annealing can be a potential method to improve the thermoelectric efficiency of nanowires, not only by enhancing the electrical conduction as demonstrated in the present work, but also by suppressing the thermal transport at the same time. For photodetection characterization of nanowires, a MEMS device was made to study the photoresponse of CdTe nanowires to the incident light. The electronic, transport and for the first time photoconductive characteristics of the CdTe nanowire field effect transistors were studied systematically. The electrical characterization of a single CdTe nanowire verifies p-type semiconductor behavior. The CdTe NW FET responses to visible-NIR (400-800 nm) incident light with a fast, reversible and stable response depicting a high responsivity (81 AW-1), photoconductive gain (~2.5×104%) and reasonable response (0.9 s and 1 s for both response and decay time, respectively). As one of the promising two-dimensional nanostructures, vertical graphene or carbon nanowalls (CNWs) were synthesized via radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) system. Different substrates including Cu, Si, Si/Ni with a 150 nm film of Ni, Si/Au with a 150 nm film of Au were used and CNWs were grown on all of them successfully. Transmission electron microscopy (TEM) images illustrated a high concentration of graphitized planes. The graphene sheets had approximately ten layers near the base but the number of layers reduced to a few layers on the top. It was concluded that CNWs could be grown without any catalysts and independent of the type of the substrates. Magnetic hysteresis loops and weak ferromagnetic responses were observed from the samples in room temperature and below. In order to understand the growth mechanism of synthesized vertical graphene, a detailed mechanism enabled by TEM observations was proposed. Among the growth steps including nucleation, growth and completion of the free-standing graphene, the nucleation occurs at the mismatch of graphitic carbon layers (surface at buffer layer on the substrate as well as carbon onions) and the active sites for growth can be the surface steps as well as the edges at the top. The diffusion of carbon adatoms on the surfaces of vertical graphene supplies the materials needed for the growth while the rate controlling step is the attachment of carbon atoms at the active sites. The vertical graphene grown here also can complement one-dimensional structures in nanoelectronics applications and IR detector development.