新的寬能隙與超硬之氮化碳材料合成方法

Abstract

經由氮化碳研究地圖的製作，提供了對氮化碳研究發展之全面性的整合思維; 並揭示出從基礎的氮、碳複合前驅物原料的開發，來搭配合適的製程方法，將是成功合成氮化碳的關鍵性契機。 由於本研究所開發的氮、碳複合前驅物原料，具有已經現存的氮碳鍵結、高氮含量，與類氮化碳的六員雜氮環結構的優勢，將能有助於降低合成氮化碳的所需極高的活化能，並提升所合成之氮化碳的結晶性。 故此法相較十多年來，傳統用於合成氮化碳所採用的分離式碳源（例如甲烷）與氮源（例如氮氣）的方式，具有顯著的創新與進步性，以生化分子複合氮、碳前驅物原料來合成氮化碳為一前所未有的革命性新構想。 在第一階段的實驗中，一種平均氮碳比23~15%、具有氮、碳單鍵與雙鍵的組態，與含有類α相氮化碳結構的粉末，成功的以鹼金屬胺鹽和烷基鹵化物為原料，經紫外光子輔助化學合成法製造出來。 經由熱重分析顯示，此氮、碳有機物耐熱性可達800℃，遠超過一般的有機物; 然而，因為本反應具有高爆炸性的危險與產率低的缺點，無法繼續供應量產，製成供濺鍍製程所需之氮、碳複合靶材。 所以在第二階段的實驗，分別使用兩種生化有機分子(azaadenine & adenine線縹呤)與一種商用美耐皿塑膠所用的有機分子(melamine)做為原料，並配合離子束沉積製程，來合成氮化碳薄膜。 在azaadenine與 adenine為原料的實驗，經X光光電子光譜及掃瞄式歐傑電子顯微鏡分析顯示所得氮化碳之氮/碳比在0.50+0.04 間; 霍氏轉換紅外光光譜、XPS及拉曼光譜發現有C-N與C=N化學鍵結狀態存在; 且X光繞射圖譜與穿透式電子顯微鏡顯示此為均勻、緻密、高純度之具結晶性奈米級氮化碳薄膜。 但是，在第三系列的melamine靶材之離子束沉積實驗，雖然成長出的薄膜具有接近氮化碳理想計量57%氮含量，但因melamine本身具有從固體直接昇華成氣體的特性，導致所沉積的薄膜具有與原來相同的鍵結與晶體結構，不符理論預測之氮化碳特性。 然而，經由陰極射線螢光光譜儀研究，意外的發現經由離子束沉積所製備的melamine薄膜，其發光特性較原本melamine 的原料有明顯的往高波段偏移的趨勢，而在418.2 nm有明顯的發光訊號，為一新發現的發藍光有機光電材料。 除了在合成氮化碳具有優點外，氮、碳複合前驅物原料所具有已經現存氮碳鍵結的化學資訊，更有助於沉積過程中反應機構的探討。 經過質譜儀分析azaadenine與 adenine這兩種不同分子結構的有機物，發現他們具有七種相同的氮碳複合解離物種與共通的主要分解機構，順利的解釋了azaadenine與 adenine這兩種不同分子結構的有機物，所成長之薄膜不論在鍵結、組成與晶體結構上具有雷同的品質特性。

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.