||Graphene is a versatile material that has been applied in many applications. Transparent conductor is one promising field that graphene is viewed as a possible candidate to replace indium tin oxide (ITO). There are many methods to produce graphene. In the process, reduced graphene oxide (rGO) is an important precursor to generate graphene. Due to the low production cost, rGO is a popular candidate to replace ITO. For rGO to achieve good electric and optical properties, the dispersion and percolation is essential for rGO as a good transparent conductor. Many factors affect the degree of dispersion of rGO, e.g. surfactant, post deposition process and surface condition. In this thesis, we investigate how surfactants, annealing and plasma treatment affect the electrical and optical properties of rGO and the associated mechanism.
The role of surfactants is carried out by adding two different surfactants Sodium Dodecyl Benzene Sulfonate (SDBS) and Sodium Dodecyl Sulfate (SDS). The results show that when the surfactant exceeds its critical micelle concentration (CMC), the electrical conductivity increases significantly. For SDBS and SDS, the CMC is estimated as 70 mg and 200 mg, respectively. The other factor also affect the dispersion is the ultrasonication time. These two factors are correlated: longer ultrasonication seems to reduce the amount of CMC.
To investigate how annealing affect the rGO properties, 200 mg SDBS and 300 mg SDS dispersed rGO thin film were annealed at 250℃,300 ℃, 350 ℃, 400 ℃. The results indicate that as the annealing temperature increases, the sheet resistance decreases. As for optical transmittance, 200 mg SDBS dispersed rGO thin film show little variation while for 300 mg SDS dispersed rGO thin film, the annealing rGO has better transmittance property. The burning of surfactants which are electrical insulators leads to the reduction of sheet resistance. The different variation in optical transmittance might be the different chemical reaction in these surfactants to annealing process.
The role of surface wettability is investigated by O2 plasma treatment on glass substrate. Different power 25W and 50W O2 plasma were applied to generate hydrophilic surface. The results show that the surface wettability has no distinct effect for SDBS dispersed rGO thin films. The hydrophilic surface is advantageous for sheet resistance reduction only when the amount of surfactant of SDBS is over 90 mg. However, for SDS dispersion rGO thin film, the hydrophilic surface shows monotonic trend in sheet resistance reduction when the surfactant of SDS is over 200 mg. This implies that different surfactant has different response to not only the annealing temperature but the surface wettability as well.