Germanium (Ge)-carbon (C) core-shell nanowires (NWs), 15 - 80 nm thick and <1 μm long, were grown using continuous-wave laser vaporization of Ge-graphite composite targets in high pressure (0.1 - 0.9 MPa) Ar gas. The NW core was crystalline Ge and the shell was amorphous C. The fraction of the NWs in deposits was changed significantly by the Ge content in the targets and had a maximum at the Ge content of 40 atomic %. With increasing Ar pressure, thicker NWs were grown. A strong correlation was evident between the two diameters of the NW and nanoparticle (NP) attached with the tip of the NW. The growth of the NWs can be explained by the formation of Ge-C liquid-like molten NPs having a specific range of size and composition and precipitation of Ge and C followed by phase separation.
One-dimensional semiconductor nanowires (NWs) have attracted much interest due to their size effects and novel properties. Among various NWs, silicon (Si) NWs have been preferentially studied since Si is of great technological importance in a wide range of applications such as electronic and optoelectronic devices and chemical biological sensors. Germanium (Ge) NWs have also been studied since Ge provides properties that are superior to those of Si in device applications such as a higher carrier mobility and a larger Bohr exciton radius [
More recently, Ge NWs have also gained attention in the research and development of lithium-ion secondary batteries having an alternative anode material of graphite [
We have succeeded in forming some one-dimensional nanostructures, such as carbon nanotubes (CNTs) filled with Cu [
Laser vaporization of Ge-graphite composite (Ge content: 5 - 100 atomic (at.) %) targets was carried out in the presence of Ar gas as reported in previous studies [
After laser vaporization using a single laser shot, the deposits in the chamber were collected and examined with a scanning electron microscope (SEM, Hitachi, S-4800) and a transmission electron microscope (TEM) operating at 100 kV (Hitachi, H-7000). Raman spectra of the deposits were taken using a Raman spectrometer (Jobin Yvon, T-640000M1) with excitation by means of the 488-nm line of an Ar+ laser. Deposits obtained after 20 laser shots were collected and used to measure the powder x-ray diffraction (XRD) patterns on an x-ray diffractometer (Rigaku, Ultima IV) or the x-ray photoelectron (XPS) spectra on a spectrometer (Shimadu, ESCA-3400) using a MgKα x-ray source. The peak position of the main component in the C 1s core level spectrum was assumed to be 284.6 eV for calculating the binding energies of each peak.
Depending on the Ge content, deposits of different morphologies were obtained.
For deposits obtained at a Ge content of 40 at.%, XRD, XPS, and Raman spectrum measurements were performed.
(311), (440), and (331) reflections of a face-centered cubic Ge crystal. Ge, C, and O atoms were detected from XPS measurements. Their peak positions in the XPS spectrum were very close to those already reported in the literature [
signed to Ge atoms bonded to C and to oxidized Ge atoms, respectively. The presence of the major metallic Ge-Ge component was consistent with the SAED and XRD patterns. The low-intensity Ge-O2 component indicates that the C layer is effective to prevent the oxidation of the Ge NWs. In the C 1s spectrum, a major component at 284.6 eV was assigned to C atoms bonded to C. At lower energy, a minor component at 283.3 eV was assigned to C atoms bonded to Ge. At higher energy, two other components at 285.9 and 287.7 eV were assigned to C atoms bonded to O. The atomic ratio of O/Ge was estimated to be approximately 0.18 from the XPS spectra. The O atoms bonded to the Ge and C atoms were suggested to be 37% and 63%, respectively.
Thicker NWs were obtained under higher Ar gas pressure conditions.
core-shell NWs attached with NPs were observed, similar to those in
Each of the diameters of NWs and NPs attached with tips of the NWs were measured for more than 400 samples obtained at 0.1, 0.5, and 0.9 MPa.
Because of the presence of NPs attached with NWs and the strong correlations of the diameters with those of NWs, we believe that liquid-like molten NPs act as seeds for the growth of NWs at high temperature. Ge-filled CNTs have been
formed by arc discharge [
Successive anisotropic precipitation of Ge and C from the molten Ge-C NP is required for the NW growth. A higher level in the supersaturation of Si, compared to that for the stage of successive Si NW growth, was reported for the nucleation of Si NWs from Au-Si molten NPs [
A simple method of forming Ge-C core-shell NWs was presented by means of CW laser irradiation onto Ge-graphite targets in the presence of high-pressure Ar gas. SEM and TEM examinations indicated that deposits with different morphologies such as graphitic polyhedra, Ge NPs, and core-shell NWs were formed depending on the Ge content in the targets. The fraction of the NWs in the deposits maximized at a Ge content of ~40 at.%. The higher magnification TEM image, SAED pattern, XRD pattern, XPS spectra, and Raman spectra of the deposits indicated that the NWs consisted of crystalline Ge cores and amorphous C shells. The low-intensity Ge-O2 component in the XPS spectrum indicates that the C layer is effective to prevent the oxidation of the Ge NWs. As the Ar gas pressure increased, thicker Ge-C core-shell NWs were obtained. The growth of the NWs was explained by the formation of liquid-like molten Ge-C NPs having a specific range of size and composition acting as seeds to nucleate and grow the NWs in a space confined by Ar gas. Further work is needed to clarify the nucleation and growth processes of the NWs and the conductive properties of the C shells.
The authors are grateful for the “Kakenhi (15K04606)” Grant-in-Aid for Scientific Research provided by the Japan Society for the Promotion of Science in support of this work.
Hatano, K., Asano, Y., Kameda, Y., Koshio, A. and Kokai, F. (2017) Formation of Germanium-Carbon Core-Shell Nanowires by Laser Vaporization in High-Pressure Ar Gas without the Addition of Other Metal Catalysts. Materials Sciences and Applications, 8, 838-847. https://doi.org/10.4236/msa.2017.812061