Abstract
Since the discovery of carbon nanotubes (CNTs), one-dimensional (1-D) nanomaterials have attracted a lot of attention due to not only fundamental scientific interests but also their potential applications in molecular optoelectronic devices. In this research field of 1-D nanomaterials, CNT and 1-D oxide nanomaterials are no doubt two of the most popular topics of investigation. The present study is in association with the above two subject materials, and can be divided into two major parts. The first part deals with the electrical transport properties of individual multi-walled CNTs (MWCNTs), and the second part focuses on the study of 1-D Ge-based ternary oxide nanostructures. In the investigation of electrical transport of individual MWCNTs, two kinds of MWCNTs were studied: one is disordered MWCNT, and the other is defective MWCNT. In the case of disordered MWCNTs, the intrinsic tube resistivity increased with decreasing temperature, and showed a T½ dependence in the temperature range of 4.2-295 K. The experimental finding can be well interpreted in terms of Al'tshuler-Aronov model in which strong electron-electron interaction leads to a singular negative correction to the single particle density of electronic states near the Fermi level for a disordered system, thus resulting in increased resistivity at low temperatures. Such a wide fitting range of temperature for T½ dependence has never been reported for other materials, implying an extremely short carrier scattering time in the order of fentosecond. As for the defective MWCNTs, the contact resistance was in the order of several kΩ using Nb leads, and the temperature dependence of tube conductance could be explained using a simple two band model. An unexpected electrical hysteresis was observed in the I-V curves of most of these MWCNT devices at low temperatures as well as room temperature, and it was generally observed that the resistance increased after hysteresis. Such a phenomenon has not been reported for CNTs in the literature so far. With both low inducing current and power consumption, this hysteresis is quite suitable for application in molecular memory devices. In the second part of the study on 1-D Ge-based ternary oxide nanostructures, we first demonstrated a novel growth phenomenon of Zn2GeO4 nanorods from Zn-containing Ge nanoparticles (ZCGNs) prepared by a vapor condensation technique. Zn2GeO4 nanorods were formed by aging these ZCGNs in water at room temperature. Due to the poor chemical stability of Ge surface in water, the ZCGNs first underwent a structural transformation into wrinkled amorphous membranes composed of Zn, Ge, and oxygen. After further aging, single-crystalline Zn2GeO4 nanorods nucleated directly from the amorphous membranes, and then continued to grow in the aqueous environment. These nanorods with the diameter ranging from several tens to more than 100 nm exhibited a blue-green luminescence peaked at 450 nm. This unique transformation route may provide a new thinking for preparing similar 1-D Ge-based ternary oxides or other 1-D nanomaterials. Besides the aforementioned experimental finding, it was further found that hydrated Ca5Ge2O9 nanowires could be synthesized by either immersing pure Ge nanoparticles or adding GeO2 aqueous solution into Ca(OH)2 aqueous solution at a stoichiometry of Ca:Ge = 5:2. In the first case, the Ge nanoparticles dissolved gradually in the solution, and the released Ge ions reacted rapidly with the calcium ions to form hydrated Ca5Ge2O9 nanowires. The reaction was completed in 10 min. In the second case, the reaction rate was increased due to the Ge ions already present in the aqueous solution. The diameter of these hydrated nanowires varied from several tens to more than 100 nm. After dehydrating the nanowires at 400oC, amorphous Ca-Ge-O nanowires were obtained. Hydrated strontium germanate nanowires with the atomic ratio of Sr to Ge at 1 could also be synthesized by the same approach in which the Ca(OH)2 aqueous solution was replaced by Sr(OH)2•8H2O aqueous solution. Both the formation process and chemistry of the hydrated strontium germanate nanowires were very similar to those of hydrated Ca5Ge2O9 ones. With subsequent dehydrating these nanowires at 400oC, amorphous Sr-Ge-O nanowires could also be obtained. Both amorphous Ca-Ge-O and Sr-Ge-O nanowires exhibited a very similar blue-violet luminescence. The emission band distributed from 300 to 550 nm, with the main peak locating at 380 nm. Ge-associated luminescence centers are proposed to be responsible for this emission. The formation of these amorphous nanowires served as a new approach to prepare amorphous one-dimensional nanomaterials.