Synthesis, Characterization And Properties Of Germanium And Germanium Oxide Nanowires

One-dimensional (1D) nanostructures such as wires, rods, belts and tubes have become the focus of research not only because of their interesting electronic and optical properties intrinsically associated with their low dimensionality and the quantum confinement effect, but also because of their unique applications in mesoscopic physics and fabrication of nanoscale devices. Among them, one-dimensional germanium and germanium oxide nanowires exhibit potential applications in nanoelectronics and optoelectronics due to their unique electronic and photonic properties. In this paper, germanium and germanium oxide nanowires have been systematically studied. The main research works are listed as following:First, high quality single crystalline germanium nanowires (GeNWs) and germanium oxide nanowires (GeONWs) have been synthesized via thermal evaporation method under vapor-liquid-solid mechanism. The morphology and nanostructure of the as-synthesized GeNWs and GeONWs have been characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM). The result shows that the growth direction of the as-prepared GeNWs are along [111] and [110], the GeNWs have a 0.33 nm spacing of Ge (111) planes with diamond structure. The diameter of GeONWs is in the range of 50 to 100 nm, and the nanowire is a single crystal with hexagonal structureSecond, electronic structure and photoluminescence property of single crystalline germanium oxide nanowires with green light emission have been investigated. The electronic structure of the as-prepared GeNWs have been studied by X-ray photoelectron spectroscopy (XPS). The EDS and XPS of GeONWs show the substoichiometric oxygen in the GeONWs with an atomic ratio close to 1:1.8. The electronic structure and local structure of GeONWs have also been investigated by X-ray absorption fine structure (XAFS), which shows disorder and degradation of long range order in the nanowires due to the nanosize in one-dimension and oxygen vacancy in the nanowires. Synchrotron radiation based X-ray excited optical luminescence (XEOL) from GeONWs exhibits strong green light at 2.3 eV (540 nm), which is corresponding with the individual GeONWs photoluminescence (PL) spectra excited by laser. The strong emission is attributed to the oxygen related defect states in oxygen vacancies confirmed by X-ray absorption near-edge structure (XANES) and XEOL.Third, surface functionalization and application of germanium nanowires have been studied. In this part, copper nanoparticles (CuNPs) and silver nanoparticles (AgNPs) have been reductively fabricated on the hydrogen-terminated surface of germanium nanowires (GeNWs), which exhibited moderate reactivity toward the direct reduction of Cu (II) and Ag (I) ion to metal nanoparticles in aqueous solution at room temperature. The density of CuNPs and AgNPs increased with increasing Cu2+ and Ag+ concentration, while the particle size decreased with decreasing Cu2+ and Ag+ concentration, respectively. The average sizes of CuNPs and AgNPs are 5, 10 and 15 nm for 10-5 M, 10-4 M and 10-3 M Cu²⁺ and Ag⁺ concentration, respectively. The electronic properties of the CuNPs/GeNWs and AgNPs/GeNWs system have been studied by X-ray photoelectron spectroscopy. The Ag nanoparticles-embedded germanium nanowires have been used as a unique surface-enhanced Raman scattering substrate, which could achieve the single molecule detection. The observed enhancement factor of the fabricated AgNPs/GeNWs substrate was estimated to be 3.8×10⁷and 4.3×10⁷, respectively.