| Summary: | 博士 === 國立交通大學 === 材料科學與工程系 === 88 === Using the research roadmap to reveal an integrated view on the research and development of carbon nitrides, and to highlight a key direction, carbonitro-compound precursors combined with due kinetic control approaches to synthesize the carbon nitride films were proposed and conducted. These carbonitro-compound precursors possess high N/C ratios and contain C-N single bonds and C=N double bonds as well as exhibit a six-fold ring structure quite similar to that in the hypothetical b-C3N4 phase. The latter feature of these compounds are expected to be a good sputtering target materials to enhance the nucleation and growth processes and improve the crystallinity of carbon nitrides. This is a fundamental revolution on raw material selections to the synthesis of carbon nitrides. In the first effort, a novel target material composed of carbon nitride was prepared by an UV photo-assisted synthesis method to replace the traditional graphite target. This method involves UV photochemical reaction of alkali amides and alkyl halides as carbon and nitrogen sources, respectively, followed by high temperature sintering under flowing nitrogen. Elemental analyses indicate the presence of C, N and H in the target material with a nitrogen to carbon ratio of 0.23. The infrared spectroscopy reveals a mixture of sp3 and sp2 carbon and sp2 nitrogen in the sintered powders. Six diffraction peaks attributable to a-C3N4 are observed in XRD pattern. The grains of ~ 0.3 mm in diameter in an extended network are also observed in the SEM micrographs. The sintered carbon nitride powder is stable up to 800℃.
However, this method involves a very high possibility of explosive free radical reaction and low conversion yield. Hence, in the second effort, three series of organics, namely, azaadenine, adenine and melamine, were adopted as target materials for synthesizing hydrogenated carbon nitride films by ion beam sputtering method. This method is the first innovative attempt in the syntheses of carbon nitrides. These three targets are also anticipated to provide abundant hydrogenated carbonitro-species as intermediate states to effectively reduce the high activation energy barrier for the formation of carbon nitrides. Hence, this innovation of using these organics is obviously a great progress than the traditional methods of using methane, nitrogen gas, graphite or ammonia as raw materials. Without extra nitrogen sources, the azaadenine and adenine targets with pre-existing carbonitro-bonds, high N/C ratio as well as six-fold carbonitro-ring structure similar to that in the hypothetical C3N4 were successfully developed to deposit the crystalline carbon nitride films. The resultant films via azaadenine and adenine targets contain high nitrogen content with an average N/C ratio of about 0.5. The chemical bonding structure consists of sp3 hybridization of the carbon and sp2 hybridization of the nitrogen and carbon. Furthermore, the dense and uniformly distributed nano-sized crystalline carbon nitride films can be achieved. While the diffraction peak positions are in close proximity with some theoretical values for the b-C3N4 phase, the crystal structure of the films is yet to be determined. Although the deposited films via melamine targets have the highest N/C ratio of 1.57, these deposited films retain the bonding characters and crystal structure of melamine. A blue light emission at 418.2 nm is observed on the cathodoluminescent spectrum of the deposited film via melamine targets.
Moreover, the specific chemical information introduced by carbonitro-organics may provide excellent probes on the formation mechanisms of carbon nitride. It''s intriguing to note that the structures including bonding, compositions and crystal structure of the deposited films at 1000 eV argon ion beam sputtering energy via either azaadenine or adenine targets are nearly the same, but are different from the films via melamine target. The seven mutual hydrogenated carbonitro-species in the mass spectra of azaadenine and adenine targets suggest the resembling predominant mechanism during deposition. This may also account for the similarity in the general characteristics, such as bonding, compositions and crystal structure, of the crystalline carbon nitride films deposited by ion beam sputtering method via azaadenine and adenine targets. The existence of a weak N=N bond in the chemical structure of azaadenine, but not in that of adenine may lead to a difference in the growth rate and some minor deviations in their structures.
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