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

2012-08-22In 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