Abstract
Synthesis, characterizations, and physical properties of CrSi2 and CdTe-based nanostructures have been investigated. Both materials have attracted intensive research effort in recent years, owing to their semiconducting properties and various practical applications. The design and development in functional nanodevices based on these two materials are of current interest. Furthermore, the possible further advanced applications of these structures are also proposed. Free-standing CrSi2/SiO2 nanocables have been synthesized via a simple atmospheric pressure chemical vapor deposition (APCVD) method. High quality nanocables were produced by a chemical vapor transport based method without using metal catalysts. The nanocables were formed by a direct reaction of CrCl2 vapor and a Si substrate in one single step. Structural characterization confirms the core of these nanocables to be hexagonal CrSi2, grown in the [001] direction. The room temperature ferromagnetism in CrSi2/SiO2 nanocables is discovered for the first time and the hysteresis loops show shape anisotropy effect when the applied magnetic field is perpendicular or parallel to the substrate. Through first-principles calculation, we found that the Cr atoms at the interface between CrSi2 and SiO2 layers possess a significant high saturation magnetization up to 2.202 B, while the Cr atoms in the middle of CrSi2 layer have a negligible magnetic moment (0.002 B). The calculated total saturation magnetization of the measured sample is close to the measured value obtained by superconducting quantum interference device (SQUID). The room temperature ferromagnetism in CrSi2/SiO2 nanocables is attributed to unpaired Cr atoms at the interface and high surface-to-volume ratio of these 1D nanostructures. Surface spins of pure CrSi2 nanowires after the removal of outer SiO2 layer also contribute large magnetic moment. Moreover, a comparison of measured values for CrSi2/SiO2 nanocables with different aspect ratios indicates that the magnetization is indeed proportional to interface area. The results obtained from the present fundamental studies shall lend substantial support to the development of future dilute magnetic semiconductor devices. Thermoelectric and mechanical properties of an individual CrSi2 nanowiwre were investigated. ZT value ~0.30+/-0.01 for a single CrSi2 NW with ~70 nm in diameter can be directly assessed by employing Harmon method. A remarkable enhancement of ~15 % for ZT value up to ~0.35+/-0.01 can be achieved due to reduced thermal conductivity by roughening the surface of NW because of higher surface-to-volume ratio. ZT value was found to increase with decreasing diameter. On the other hand, elastic modulus of CrSi2 NWs is first investigated in the present study. The elastic modulus was found to be independent of diameter and the averaged modulus value of ~225 GPa is obtained. The enhancement of the thermoelectric properties of CrSi2 nanowires with robust mechanical properties may lead to their practical applications in advanced thermoelectric devices in the future. A zinc blende structure based nanogenerator were developed. Here, free-standing CdTe microwires/nanowires (MWs/NWs) have been synthesized with a facile one-step hydrothermal method. The structural analysis shows that the synthesized materials (which consist of wires and particles) are composed of multiple phases with much more zinc blende CdTe than wurtzite CdTe. This coexistence of two phases was also confirmed using high resolution transmission electron microscopy (HRTEM). A laterally packaged nanogenerator (NG) can generate up to 0.3 V and 40 nA when strain is applied on the individual MW. Due to the high stability, the MW can be used in piezoelectric applications under various circumstances. Through these combined properties, zinc blende-based CdTe material appears to be promising for application in self-powered system in energy harvesting.