Controllable Synthesis And Properties Of Mono- And Bi-layer Graphene Films

Graphene is a two-dimensional carbon-based new material, which is formed by closed packing carbon atoms with honeycomb structure. Due to its excellent electrical and optical properties, graphene films have wide applications in microelectronic and photoelectronic devices. Graphene grown by chemical vapor deposition is considered as one of the most perspective methods due to its large area, low cost and high quality. It is known that the physical properties of graphene would be obviously affected by its structure, thus it is significant to investigate the controllable synthesis of CVD graphene, and further understand the effect mechanism of structures on the electrical and photoelectrical properties of graphene for its application in the electrical and photoelectrical devices. Although much progress has been made in the investigations of synthesis and property of CVD graphene, many problems are still with us, including not only the fundamental research, but also the device application. In this work, we focused on the CVD monolayer and bilayer graphene. Based on the understanding the formation mechanism of domain, layer number, stacking order and defect, the controllable growth of CVD graphene was achieved, and then the effect mechanism of structure on the properties of graphene was studied, and finally the THz modulator based on graphene field effect transistor was investigated. The main results are as follows.1. For solving the problem for the poor uniformity of CVD monolayer graphene due to the unexpected formation bilayer regions, the facile method for growing large-area, high uniform an quality monolayer graphene has been demonstrated by controlling the dynamic balance process: graphene formation on the surface of the copper atoms, Cu atoms/graphene sublimation, and graphene formation on the new copper surface. The results show that, due to the uneven defects on the polycrystalline copper foil substrate surface, methane tends to be decomposed at the defects to form bilayer graphene regions. Bilayer graphene regions not only affect the uniformity of the large area graphene film, but also make the surface scattering enhancement, which leads to the decrease of the electrical properties of graphene. In order to suppress the formation of bilayer graphene regions, a long time dynamic balance process is proposed under the high temperature of 1000 ℃ by more than 120 minutes graphene growth time with the process of graphene formed on the surface of the copper atoms, Cu atoms/graphene sublimation, graphene formed on the new copper surface. After that, the copper foil surface tends to be flat(uneven defects decrease or disappear). It effectively inhibits the formation of bilayer graphene regions. Finally, large area and uniform monolayer graphene film without bilayer regions are obtained on the surface of copper foil. The carrier mobility of graphene is increased from ~1000 cm2/V-s to ~2500 cm2/V-s by improving the uniformity of graphene films.2. For solving the problem for the large-area monolayer film with small domains, a facile method for growing large-area monolayer film with large domains has been demonstrated by controlling the graphene domain nucleation density. The results show that the nucleation density of graphene domain can be controlled by the concentration and flow rate of the diluted methane. Through the copper foil substrate temperature and growth time can modulate individual graphene crystal domain growth size and merging process of the graphene crystal domains. Through the process optimization, the controllable growth of the large domain of millimeter scale graphene is realized, and the large area monolayer graphene film formed by large domain merging is prepared. The carrier mobility of the continuous graphene film is up to 4542.5 cm2/V-s, and the on/off ratio is up to 13.7. This is the material basis for the development of high mobility, high switching ratio of graphene transistor devices.3. A facile CVD method is proposed to synthesize the twisted bilayer graphene film by introducing decaborane as cocatalyst, and the experimental results show that the small angle twisted bilayer graphene has the electrical transmission band gap. By introducing decaborane as cocatalyst, it effectively promotes the catalytic pyrolysis of methane on copper. Twisted bilayer graphene is obtained under the seed limitation and self limiting growth mechanism. For the first time, a tunable electrical transmission band gap is observed on twisted bilayer graphene with a small rotation angle. Under the vertical electric field, the Dirac resistance of the bilayer graphene increases with the increasing of the average potential field, which indicates that the bilayer graphene has an electrical transmission band gap and can be modulated by the applied electric field.4. It is the first time to prepare twisted bilayer graphene domain with the same size of the top and bottom layers, which could solved the problem of the existing double-layer graphene domain with different shape and size of the top and bottom graphene layer. By introducing decaborane as cocatalyst and controlling the growth temperature, growth time and the flow of the diluted methane. The domain nucleation of top and bottom graphene layer formed at the the same time in the initial stage. And then the twisted bilayer graphene domain with the same size of the top and bottom layers is obtained after the growth process from the initial bilayer domain nucleation. By optimizing the process, millimetre-sized twisted bilayer graphene domain under 29.8° rotation angle with the same size of top and bottom graphene layer is achieved.5. For the first time to the best of our knowledge, we demonstrated the feasibility of flexible THz modulators based on the coplanar-gate field-effect transistor structure. By using PET as the substrate, graphen as the channel, ion-gel as the gate dielectric, the THz transmittance through this THz graphene modulator can be well controlled with a modulation depth up to 22% by tuning the carrier concentration of graphene via electrical gating. Due to the unique structure of ion-gel/graphene/PET, the flexible THz graphene modulator has a low insertion loss(1.2 dB). In addition, the proposed THz graphene modulator shows superior flexible performance, where the modulation properties can be maintained almost unchanged, not only under bending deformations, but also before and after bending 1000 times. It indicates that the flexible THz modulator is expected to be applied in the terahertz system with a variety of complex surfaces.