Study On Fabrication Of Graphene-based Transparent Conductive Films By Direct Contact Transfer

Graphene, a two-dimensional atomically thick carbon layer with a hexagonal lattice structure, has attracted worldwide interest due to its extraordinary properties. Its fascinating electrical, mechanical, optical and chemical properties are important for practical applications, including field-effect transistors (FET), solar cells, transparent conductive films and so on. The demonstration of a large-area, low-cost synthesis of high quality graphene films on metallic catalytic substrates also opens up the enormous possibility for a lot of important applications in flexible and transparent conductive films instead of ITO. However, the research in transparent conductive films is still faced with several issues. Firstly, the electronic devices require the graphene to exist on dielectric substrates, but the graphene grown directly on dielectric substrates is too small and not in high quality, which has several defects. Hence, it is essential to transfer the graphene generated via chemical vapor deposition (CVD) from metallic catalytic substrates to dielectric substrates. Secondly, at present, the most popular transfer method is using a carrier material such as polymethyl methacrylate (PMMA) or thermal release tape (TRT) to transfer graphene onto target substrates. However, each method has its own drawbacks. PMMA and TRT organic adhesive residues can easily contaminate the transferred graphene films. It is likely to cause cracks and tears during the transfer process. In the thesis, we mainly focused on the 4-layer graphene grown directly via CVD and improved graphene transfer method to obtain high-performance graphene-based transparent conductive films. The work involved is as following:(1) It is difficult to acquire single-crystal graphene via atmospheric pressure chemical vapor deposition (APCVD), of which its transmittance can reach 97.7%, but the sheet resistance is about 300Ω/sq generally. When applied to transparent conductive films, the sheet resistance is too high. In order to achieve 90% transmittance and dozens of sheet resistance, we used CVD method to grow graphene of about 4 layers on Cu foil, and then transferred it to dielectric substrates. The graphene grown directly with 4 layers has two advantages:firstly, the graphene of multilayer is easier to grow than monolayer via APCVD; secondly, during the transfer process, the 4-layer graphene can be transferred successfully at one batch rather than multiple times layer by layer, which effectively reduces the residues of PMMA or TRT. We conducted comparative experiments to study growth temperature, substrate, carbon source and growth time, and we obtained the optimal growth conditions of 4-layer graphene.(2) We developed a simple and efficient method for graphene transfer-direct contact transfer method. Since the new method doesn’t involve any organic polymers as carrier materials, the transferred graphene surface is clean and flat without any residues of PMMA or TRT. Compared with traditional transfer methods, we found that the new transfer method showed significant improvements in transfer quality, transmittance and conductivity. This new transfer method has effectively solved the issues of the PMMA or TRT residues, and it doesn’t require high-temperature annealing process. It is a simple and efficient way to achieve clean and flat surfaces. Furthermore the direct contact method can transfer graphene to any other substrates.(3) We used APCVD method under the conditions that growth substrate is Cu, growth temperature is 1000℃, gas flow ratio of Ar/H2/CH4 is 80/20/15 and growth time is 10min, and we obtained the 4-layer graphene with uniform grain size about 348.72nm. Then we transferred it to polyethylene terephthalate (PET) substrate by direct contact transfer method, subsequently we obtained the graphene-based transparent conductive films with 90.4% transmittance and the sheet resistance of about 64Ω/sq.