Synthesis and Optical Spectroscopy Study of Two-Dimensional Crystals: Graphene and Beyond



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The discovery of graphene, a single atomic layer of hexagonally packed carbons, can be considered as the opening of an epic of two-dimensional (2D) materials. With the restriction in dimensions and the weak interlayer reactions, these isolated layer crystals have exhibited distinctive properties from their bulk forms. Moreover, integrating heterogeneous layered crystals is expected as a promising approach to produce materials with complex structures and well-engineered properties. Therefore, this dissertation emphasizes on synthesizing ideal 2D building blocks, graphene and transition metal dichalcogenides (TMDCs) crystals via chemical vapor deposition (CVD), exploring their optical spectroscopic signatures, and verifying the practicability of artificial stacking with the simplest case, twisted bilayer graphene (tBLG). After evaluating several key factors, millimeter size of monolayer hexagon graphene crystals have been harvested with designed recipe. Synthesis of bilayer hexagon graphene domains, monolayer and few-layer MoS2 and WS2 crystals in micrometer size have also been demonstrated. In addition, hundreds-of-micrometer sized twisted bilayer graphene domains have been achieved by artificial staking of two monolayer graphene hexagons, of which crystal orientation mismatch can be directly estimated by the misalignment of edges. Raman and photoluminescence spectroscopy are utilized not only to determine the number of layers and the quality of as-grown samples, but also to characterize the underlying crystalline and even electronic structures. This study provides valuable insight of synthesizing, processing, and characterizing 2D crystals in engineered approaches and paves a way for introducing them into electronic and photonic application.



Two-dimensional Materials, Graphene, Transition metal dichalcogenides, Chemical vapor depostion, Raman spectroscopy, Photoluminescence Spectroscopy


Portions of this document appear in: Wang, Yanan, Zhihua Su, Wei Wu, Shu Nie, Xinghua Lu, Haiyan Wang, Kevin McCarty et al. "Four-fold Raman enhancement of 2D band in twisted bilayer graphene: evidence for a doubly degenerate Dirac band and quantum interference." Nanotechnology 25, no. 33 (2014): 335201. And in: Wang, Yanan, Zhihua Su, Wei Wu, Shu Nie, Nan Xie, Huiqi Gong, Yang Guo et al. "Resonance Raman spectroscopy of G-line and folded phonons in twisted bilayer graphene with large rotation angles." Applied Physics Letters 103, no. 12 (2013): 123101. And in: Xing, Sirui, Wei Wu, Yanan Wang, Jiming Bao, and Shin-Shem Pei. "Kinetic study of graphene growth: Temperature perspective on growth rate and film thickness by chemical vapor deposition." Chemical Physics Letters 580 (2013): 62-66.