Graphene: Synthesis by Chemical Vapor Deposition, its Transfers and Characterizations, and Electro-optic Application in the Mid-infrared Region
Authors: Akpan Usenobong B. , Emah Joseph B. , Ekong Sylvester A. 
Published: February 2025

This study investigates the synthesis, transfer, and characteriza­tion of graphene using Chemical Vapor Deposition (CVD) for tunable applications in electro-optic devices and transparent electrodes in the mid- infrared region. The research aimed to synthesize con­ductive graphene at the laboratory scale and fabricate a graphene field-effect transistor (GFET) to explore its electro-optic tunability in the mid- infrared spectrum, targeting applications such as transparent but conducting electrodes in smartphone displays, holographic displays, and nondestructive internal imaging. Graphene was synthesized on a 25 micron smooth copper substrate using a custom-built CVD system, with growth parameters optimized at 1000 ° C in a methane and hydrogen-argon atmosphere. The transfer processes utilized Poly(methyl methacrylate) PMMA, and Polyethylene tereph-thalate (PET)/silicone materials, and the resulting bilayer graphene was characterized using Raman spectroscopy, scanning electron microscopy, and resistance measurements. Despite achieving tunable layer control, the synthesized graphene exhibited challenges in conductivity. Comparative analysis of fast and slow cooling processes revealed differences in graphene coverage but did not resolve conductivity issues. Although challenges remain in custom-made CVD graphene, which was aimed at producing robust and less expensive conductive graphene, the successful fabrication of a short-circuit-free GFET with tunable optical properties, using a simple but single-ebeam evaporation technique and the expensive commercial CVD graphene underscores the potential of CVD graphene in electro-optic applica­tions and transparent electronics. Electro-optically, the GFET device composed of commercial graphene/SiO2(10nm)/ α-Ge(650nm) fabricated on patterned NiCr(0.7nm)/Au(125nm)/glass substrate at a source voltage of 0.1V and at zero gate voltage, at a fixed mid-infrared wavelength of 10.6-µm of CO2 laser, exhibited a 4 % fractional change (or contrast or visibility) in graphene’s reflectance with a 15 % in absorbance, implying a 85 % high transparency of graphene at 10.6-µm wavelength. At the same zero gate voltage but with a source voltage of 0.5V, a 2.4% visibility was achieved in the graphene reflectance with a 9 % absorbance, implying a high transparency of 91 %. The tuning of the reflectance visibility of graphene as a function of the gate bias at 0.1V and 0.5V source biases showed a combined increasing and decreasing linear relationship with the increasing gate bias. Electrical measurements on the GFET show the following results: the transconductance gave a strange step-like increasing modulation of the drain current with increasing positive gate bias at a zero source voltage. The sheet resistance (source-drain resistance) and gate- source resistance gave 8.9 kΩ/sq and 5.1 MΩ, respectively, which meets the megaohm requirement for metal-oxide-semiconductor-FET devices. However, for a GFET device composed of commercial graphene/SiO2(10nm)/α-Ge(650nm) fabricated on patterned NiCr(0.7nm)/Au(125nm)/Si(525µm doped) substrate, a linear decrease in drain current transitioning from positive through zero to negative values was achieved with increasing positive gate bias at zero drain voltage, but with a 11.5 kΩ in gate-source resistance. This is promising for epsilon-near-zero metamaterial property investigation for application. These results may be useful in the study of voltage-biased graphene-based multilayered ENZ metamaterial for tunable display, transparent electrodes and imaging application