Plasma is a unique state of matter for chemical reaction and materials fabrications, which also finds its application in nanoscience and nanofabrication. In the work described in this thesis, three different research topics on fabrication and modification of the nanomaterials are studied based on the microwave plasma technique. In first part, plasma-assisted chemical vapor deposition and reactive ion etching (RIE) methods were used to structure diamond and cubic boron nitride films. The films were constructed to different surface nanostructures, including nanocones, nanopillars, and nanowhiskers. The electrochemical and hydrophobic properties of the prepared nanostructured films were studied. In second part, high-quality adherent diamond films were deposited on gallium nitride (GaN) substrated by a microwave plasma system. In third part, silicon nanowires (Si NWs) were carbonized in the microwave plasma, and the cycling performance of the lithium ion battery anode from carbon coated Si NWs was studied. The main results were summarized.
First, high-density uniform diamond pillars were fabricated by a plasma-assisted RIE method from both nanodiamond (ND) and microcrystalline diamond (MD) films using gold etching masks. The structuring processes were based on three fundamental steps: (i) deposition of ND or MD films, (ii) predeposition of a thin gold layer, and (iii) bias-assisted RIE in hydrogen and argon plasmas. The thin gold layer aggregated to nanodots, which served as masks in the subsequent etching process. The formation of the diamond nanopillars involved the combined action of physical etching via sputtering and chemical etching by reactive hydrogen atoms/ions. The method was proven to be simple, low-cost, versatile, and applicable for obtaining diamond nanostructures with a high areal density, high uniformity, and controlled geometry. The method can also be applied to the design and make other materials comprising nanostructures.
The electrochemical properties of the nanostructured diamond films were studied. The diamond nanocone and nanopillar arrays with high aspect ratios were expected to have improved performances in electrochemical analysis compared with their flat thin film counterparts, and could be considered as competitive alternatives to carbon nanotubes in electrochemical sensors. In the current study, uniform, high-density, and vertically aligned boron-doped diamond nanocone and nanopillar arrays were constructed on heavily boron-doped ND films. The electrochemical behavior of the flat and surface nanostructured ND films was evaluated by analyzing redox couples of [Fe(CN)6]3-/4- at a scan rate of 100 mV s-1. The nanostructured ND films were found to exhibit great improved sensitivity as electrodes due to their enlarged electroactive surface areas. The combination of the intrinsic outstanding properties of diamond and the versatile construction method for diamond nanostructures suggests that the boron-doped diamond nanocone and nanopillar arrays may be applied in electrochemical analysis, especially in corrosive solutions, as an alternative electrode with high stability, sensitivity, and reproducibility.
The plasma-assisted RIE method was applied in the nanoconstruction of films of cubic boron nitride (cBN), an analog material of diamond. Compare with diamond, a gold dot mask is necessary in the etching process of the cBN nanostructures. In RIE of cBN films at a low bias voltage, the lower etching rate cause the formation of a conical nanostructure as a result of the gradual shrinkage of the mask dots and the concurrent etching of the fresh cBN surfaces surround the mask dots. In RIE of cBN films at a high bias voltage, ions of higher kinetic energy increase the etching rate of cBN with respect to mask, leading to columnar nanopillars instead of nanocones in cBN films surface. Hence, the geometric configurations of cBN surface nanostructures can be tailored by controlling the RIE conditions.
By a series of nanoconstruction processes of diamond and cBN films via a plasma-assisted RIE method, different kinds of nanostructures of diamond and cBN were fabricated. The nanostructures include arrays of nanocone, nanopillar, and nanowhisker. The wettability of the films with different surface nanostructures was studied. Surface nanostructuring could dramatically improve the hydrophobicity of ND and cBN films. With further enhancement by surface fluorination, superhydrophobic ND and cBN surfaces with contact angles greater than 150° and sliding angles smaller than 10° were achieved. The results are consistent with the Cassie model. The nanocone and nanowhisker array structure, which have sharp tips, were more efficient in enhancing the hydrophobic behavior.
Diamond film deposition on a patterned GaN substrate in a microwave plasma chemical vapor deposition system was also studied. The deposition process included three approaches: i) a two-step process involving an initial rapid growth stage, ii) introduction of nitrogen into the hydrogen-based plasma to suppress the reactions between GaN and hydrogen, and iii) deposition in argon-based plasmas. All these approaches were demonstrated to be successful in restraining the decomposition and etching of GaN substrates in the plasmas. No crack or delamination was observed. The integration of diamond with GaN enabled the improvement in the heat dissipation, and subsequently, of the performance and lifetime of the GaN-based devices.
Lastly, CH4/H2 microwave plasma was used to carbonize silicon nanowires to increase their stability as anode materials in lithium ion batteries. The β-silicon carbide phase existed in the fabricated nanowires, as confirmed by high-resolution transmission electron microscopy and selected area electron diffraction results. These coated silicon nanowires (C@SiNWs) anodes inherited the good electrical conductive properties of the electroless-etched Si NWs, as well as exhibited remarkably improved lithium storage performance in stability and cycle life compared with Si NWs. After 50 cycles, the C@SiNWs anode still had 30% capacity, whereas the Si NW anode was already decayed to zero. The cycling result indicated that the prepared nanowires are promising anode materials for high-performance rechargeable lithium batteries.
| Date of Award | 16 Jul 2012 |
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| Original language | English |
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| Awarding Institution | - City University of Hong Kong
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| Supervisor | Wenjun ZHANG (Supervisor) |
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