Chemical vapor deposition of graphene: synthesis, characterization, and applications

In this dissertation I discuss the synthesis of graphene using chemical vapor deposition on Ni and Cu substrate, as well as various applications using CVD graphene. ❧ Chapter 1 gives a brief introduction of graphene, the electrical properties of graphene, and chemical vapor deposition method of graphene synthesis. ❧ Chapter 2 discusses a simple, scalable and cost-efficient method to prepare graphene using methane-based chemical vapor deposition on nickel films deposited over complete Si/SiO2 wafers. By using highly diluted methane, single- and few-layer graphene were obtained, as confirmed by micro Raman spectroscopy. In addition, a transfer technique has been applied to transfer the graphene film to target substrates via nickel etching. Field-effect transistors based on the graphene films transferred to Si/SiO₂ substrates revealed a weak p-type gate dependence, while transferring of the graphene films to glass substrate allowed its characterization as transparent conductive films, exhibiting transmittance of 80% in the visible wavelength range. ❧ In chapter 3, continuous, highly flexible, and transparent few-layer graphene films synthesized from Ni film were implemented as transparent conductive electrodes (TCE) in organic photovoltaic cells. Graphene films were synthesized by CVD, transferred to transparent substrates, and evaluated in organic solar cell heterojunctions (TCE/poly-3,4-ethylenedioxythiophene:poly styrenesulfonate (PEDOT:PSS)/copper phthalocyanine/fullerene/bathocuproine/aluminum). Key to our success is the continuous nature of the CVD graphene films, which led to minimal surface roughness (~0.9 nm) and offered sheet resistance down to 230 Ω/sq (at 72% transparency), much lower than stacked graphene flakes at similar transparency. In addition, solar cells with CVD graphene and indium tin oxide (ITO) electrodes were fabricated side-by-side on flexible polyethylene terephthalate (PET) substrates and were confirmed to offer comparable performance, with power conversion efficiencies (η) of 1.18 and 1.27%, respectively. Furthermore, CVD graphene solar cells demonstrated outstanding capability to operate under bending conditions up to 138°, whereas the ITO-based devices displayed cracks and irreversible failure under bending of 60°. Our work indicates the great potential of CVD graphene films for flexible photovoltaic applications. ❧ In chapter 4, we discuss comparative study and Raman characterization on the formation of graphene on single crystal Ni (111) and polycrystalline Ni substrates using chemical vapor deposition. Preferential formation of monolayer/bilayer graphene on the single crystal surface is attributed to its atomically smooth surface and the absence of grain boundaries. In contrast, CVD graphene formed on polycrystalline Ni leads to higher percentage of multilayer graphene (≥3 layers), which is attributed to the presence of grain boundaries in Ni that can serve as nucleation sites for multilayer growth. Micro-Raman surface mapping reveals that the area percentages of monolayer/bilayer graphene are 91.4% for the Ni (111) substrate and 72.8% for the polycrystalline Ni substrate under comparable CVD conditions. The use of single crystal substrates for graphene growth may open ways for uniform high-quality graphene over large areas. ❧ Chapter 5 discusses a vapor trapping method for the growth of large-grain, single-crystalline graphene flowers with grain size up to 100 μm. Controlled growth of graphene flowers with four lobes and six lobes has been achieved by varying the growth pressure and the methane to hydrogen ratio. Surprisingly, electron backscatter diffraction study revealed that the graphene morphology had little correlation with the crystalline orientation of underlying copper substrate. Field effect transistors were fabricated based on graphene flowers and the fitted device mobility could achieve ~ 4,200 cm²V⁻¹s⁻¹ on Si/SiO₂ and ~ 20,000 cm²V⁻¹s⁻¹ on hexagonal boron nitride (h-BN). Our vapor trapping method provides a viable way for large-grain single-crystalline graphene synthesis for potential high-performance graphene-based electronics. ❧ In chapter 6, a simple, clean, and highly anisotropic hydrogen etching method was developed for chemical vapor deposited graphene catalyzed by the copper substrate. By exposing CVD graphene on copper foil to hydrogen flow around 800 °C, we observed that the initially continuous graphene can be etched to have many hexagonal openings. In addition, we found that the etching is temperature dependent. Compared to other temperatures (700, 900, and 1000 °C), etching of graphene at 800 °C is most efficient and anisotropic. Of the angles of graphene edges after etching, 80% are 120°, indicating the etching is highly anisotropic. No increase of the D band along the etched edges indicates that the crystallographic orientation of etching is in the zigzag direction. Furthermore, we observed that copper played an important role in catalyzing the etching reaction, as no etching was observed for graphene transferred to Si/SiO₂ under similar conditions. This highly anisotropic hydrogen etching technology may work as a simple and convenient way to determine graphene crystal orientation and grain size and may enable the etching of graphene into nanoribbons for electronic applications. ❧ Brief conclusions are drawn in chapter 7. Future directions of graphene are also discussed in chapter 7. ❧ In summary, this dissertation starts from CVD graphene synthesis and fulfills with various applications using as-synthesized graphene material, which proves the potential of CVD graphene for device application, OPV cells, and other possible applications. With the continuous improvement of graphene quality, as well as the promising application results shown in this dissertation, we should expect many more applications that exploit all kinds of unique properties of graphene in near future.

本博士论文探讨了以镍（Ni）与铜（Cu）为衬底的化学气相沉积（Chemical Vapor Deposition，CVD）法制备石墨烯的工艺，以及化学气相沉积石墨烯的各类应用。
第1章简要介绍了石墨烯、石墨烯的电学特性，以及石墨烯合成的化学气相沉积法。
第2章论述了一种简便、可规模化且成本高效的石墨烯制备方法：以沉积于完整Si/SiO₂晶圆表面的镍薄膜为衬底，采用甲烷基化学气相沉积工艺制备石墨烯。通过使用高稀释度甲烷，成功获得单层及少层石墨烯，经显微拉曼光谱（Micro Raman Spectroscopy）表征得以确认。此外，本研究采用转移工艺，通过刻蚀镍层将石墨烯薄膜转移至目标衬底。将石墨烯薄膜转移至Si/SiO₂衬底后制备的场效应晶体管（Field-Effect Transistor, FET）展现出弱p型栅极调控特性；而将石墨烯薄膜转移至玻璃衬底后，可作为透明导电薄膜进行表征，在可见光波段的透光率可达80%。
第3章将以镍薄膜为衬底制备的连续、高柔性且透明的少层石墨烯薄膜用作有机光伏电池的透明导电电极（Transparent Conductive Electrode, TCE）。石墨烯薄膜通过化学气相沉积法合成，随后转移至透明衬底，并在有机太阳能电池异质结结构（TCE/聚(3,4-乙烯二氧噻吩):聚(苯乙烯磺酸)（PEDOT:PSS）/酞菁铜（copper phthalocyanine）/富勒烯（fullerene）/浴铜灵（bathocuproine）/铝）中进行性能评估。本研究的关键在于化学气相沉积石墨烯薄膜的连续特性，这使得薄膜表面粗糙度极低（约0.9 nm），并可实现低至230 Ω/sq的方块电阻（透光率为72%时），远低于同等透光率下堆叠石墨烯薄片的性能。此外，本研究在柔性聚对苯二甲酸乙二醇酯（Polyethylene Terephthalate, PET）衬底上并行制备了搭载化学气相沉积石墨烯与氧化铟锡（Indium Tin Oxide, ITO）电极的太阳能电池，经测试二者性能相当，功率转换效率（η）分别为1.18%与1.27%。进一步测试表明，化学气相沉积石墨烯基太阳能电池可在高达138°的弯折条件下正常工作，而搭载氧化铟锡电极的器件在弯折至60°时即出现裂纹并发生不可逆失效。本研究证实了化学气相沉积石墨烯薄膜在柔性光伏领域的巨大应用潜力。
第4章针对以单晶Ni(111)与多晶镍为衬底的化学气相沉积石墨烯生长过程，开展了对比研究与拉曼表征。单晶衬底表面优先生长单层/双层石墨烯，这归因于其原子级平整的表面与无晶界的特性。与之相对，多晶镍衬底上生长的化学气相沉积石墨烯中，多层石墨烯（≥3层）占比更高，这源于多晶镍存在晶界，可作为多层石墨烯生长的形核位点。显微拉曼表面成像表征显示，在相近的化学气相沉积工艺条件下，Ni(111)单晶衬底上单层/双层石墨烯的面积占比为91.4%，多晶镍衬底上则为72.8%。采用单晶衬底进行石墨烯生长，可为大面积制备均匀高质量石墨烯提供新的途径。
第5章提出了一种气相捕获法，用于合成晶粒尺寸可达100 μm的大晶粒单晶石墨烯花。通过调控生长压力与甲烷-氢气流量比，可可控制备四瓣与六瓣结构的石墨烯花。令人意外的是，电子背散射衍射（Electron Backscatter Diffraction, EBSD）研究表明，石墨烯的形貌与下方铜衬底的晶体取向几乎无关。基于石墨烯花制备的场效应晶体管，在Si/SiO₂衬底上的载流子迁移率可达~4200 cm²V⁻¹s⁻¹，在六方氮化硼（hexagonal boron nitride, h-BN）衬底上则可达~20000 cm²V⁻¹s⁻¹。本研究提出的气相捕获法，可为合成大晶粒单晶石墨烯以制备高性能石墨烯基电子器件提供可行方案。
第6章开发了一种简便、洁净且高度各向异性的氢刻蚀工艺，用于铜衬底上化学气相沉积石墨烯的刻蚀改性。将铜箔上的化学气相沉积石墨烯置于约800 ℃的氢气氛围中，可将原本连续的石墨烯薄膜刻蚀出大量六边形孔洞。此外，本研究发现该刻蚀过程具有温度依赖性：相较于700 ℃、900 ℃与1000 ℃的刻蚀温度，800 ℃下的刻蚀效率最高且各向异性最强。刻蚀后石墨烯边缘的角度中，80%为120°，证实该刻蚀过程具有高度各向异性。刻蚀边缘处未出现D峰（D band）增强，表明刻蚀的晶体学取向为锯齿型方向。进一步研究发现，铜衬底在刻蚀反应中起到了催化作用：将石墨烯转移至Si/SiO₂衬底后，在相同条件下未观察到明显刻蚀现象。这种高度各向异性的氢刻蚀技术，可作为一种简便可靠的石墨烯晶体取向与晶粒尺寸表征手段，也可用于将石墨烯刻蚀为纳米带以满足电子应用需求。
第7章对本研究进行了简要总结，并展望了石墨烯领域的未来发展方向。
综上，本博士论文从化学气相沉积石墨烯的合成研究出发，拓展了所制备石墨烯材料的各类应用，证实了化学气相沉积石墨烯在器件制备、有机光伏电池及其他潜在领域的应用价值。随着石墨烯质量的持续提升，结合本研究中展现出的优异应用成果，未来有望开发出更多利用石墨烯独特性能的实际应用场景。