The substrate effect for CVD growth WS2

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题名:	The substrate effect for CVD growth WS2
作者:	張紘翔;Zhang, Hong-Xiang
贡献者:	物理學系
关键词:	二硫化鎢;化學氣象沉積;基板效應;SEM;TEM;WS2;CVD;substrate effect;SEM;TEM
日期:	2023-07-18
上传时间:	2023-10-04 14:54:02 (UTC+8)
出版者:	國立中央大學
摘要:	目前利用化學氣象成績(CVD)進行大面積生長的二硫化鎢(WS_2)多為多晶，因此形成較多的晶界，而阻礙電子傳輸。在CVD中成核晶像主要是由生長基板這個參數決定，因此了解利用CVD生長WS_2時，基板對成核過程的影響是重要的，將有助於優化製程。在我們的研究當中，利用SEM量測WS_2在高定像熱解石磨(HOPG)、剝離的六方氮化硼(exfoliated h-BN)上生長的情形。發現具有兩種不同的成核趨勢: 模式1.因基板表面缺陷處降低了成核所需形成能而在缺陷處進行成核。模式2.利用表面形貌造成前軀體在表面具有濃度分布，而在皺褶(wrinkle)、摺疊(folding)等高度變化處進行成核。接著發現在使用CVD-石墨烯(Gr)當作生長基板時，利用氯化鈉(NaCl)是否參與生長WS_2時，將會影響CVD-Gr的表面缺陷狀況，而導致在有NaCl參與反應時是以模式1成核，反之則是模式2。接著在SEM以及TEM的量測分析中，確定WS_2在以模式1進行成核時並不會與CVD-Gr以固定晶像進行成核生長，但是在以模式2時，則會以固定晶像進行成長並且會依表面形貌決定主導成核的前驅體大小，使的在表面形貌為較平緩起伏時夾角為0°，也就是在單層石墨烯波紋(ripple)處，而在劇烈起伏處也就雙層石墨烯wrinkle或bubble處，夾角則為19°。;Recently, the large area WS_2 is prepared by utilizing CVD growth. However, the growth result is mostly polycrystalline, which leads to the formation of grain boundary, and the grain boundaries block electronic transmission. In CVD, the growth substrate is usually the primary parameter that affects the grain orientation, so it is important to understand the role of the substrate in CVD in order to achieve optimizing the fabrication. Thus, we utilized SEM to analyze the growth result for CVD growth WS_2 under different growth substrate. Then, we found that the nucleation mechanism of WS_2 on HOPG and exfoliated-h-BN can be divided into 1. Nucleation in the defect region due to the surface defected state reduces the formation energy. 2. Nucleation in the wrinkle or bubble region because the surface morphology leads to a higher local precursor concentration and accumulates into a larger cluster of precursor. In addition, we used NaCl to participate in the CVD growth of WS_2, and succeeded in switching the nucleation mechanism of WS_2 between above two modes when CVD growth graphene was used as the substrate, and as observed in the subsequent TEM measurements that WS_2 nucleated at the wrinkle and bubble had a tendency to grow in the same crystal orientation as the growth substrate. Furthermore, the degree of surface undulation determines the size of the precursor cluster that dominates the nucleation, making the angle between WS_2 and graphene are 0° or 19°.
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