UCLA

Fabrication of electronic materials typically require intensive energy, even when its predominant use is heating. Such energy is usually derived from local utilities that harness fossil fuels, nuclear power, wind, and solar photovoltaics, among other resources. Thus, direct solar-thermal capture provides a compelling option to drive heating processes, reducing greenhouse gas emissions from the industrial sector. Graphene is one of numerous materials of heightened interest in the semiconductor and energy conversion industries. It consists of carbon atoms arranged in a two-dimensional hexagonal structure. Graphene's exceptional properties make it applicable to many photonic and electronic devices. Several approaches have been employed to synthesize graphene; however, chemical vapor deposition (CVD) is mostly used, consuming high energy using plasma or electric heating sources. Nevertheless, CVD techniques provide an effective methodology for mass production by roll-to-roll mechanisms and control of graphene's number of layers and quality through growth kinetics.

Here, a high-flux solar simulator (HFSS) that mimics the sun's spectrum was built to study solar-thermal energy in graphene fabrication. A simple, accurate, and inexpensive methodology is necessary to characterize the HFSS radiative flux. In this work, an inverse mapping technique that uses a custom radiometer and infrared camera, validated by a direct characterization method (heat flux gauge), is used to characterize the output from a 10 kWe xenon lamp solar simulator. The heat flux profile is determined in a vacuum chamber using a readily available graphite target and an inverse numerical heat transfer model. Such an approach allows characterization in practical conditions, such as inside the reactor and including effects from the viewport. Results indicate that the solar simulator produces peak fluxes in the 1.5-4.5 MW/m^2 range, and its output can be controlled using a variable power supply. The HFSS is then integrated into a cold-wall CVD reactor equipped with a gas supply and vacuum auxiliaries, automated with precise controls and safety interlocks to monitor graphene growth parameters. A related numerical heat transfer model of a copper substrate atop a tungsten mount is derived and validated to predict the peak temperature at various HFSS supply currents and vacuum pressures, facilitating graphene growth under different conditions.

A parametric study of graphene growth parameters was conducted using a probabilistic Bayesian regression model. The regression model utilizes Gaussian processes and an information acquisition function to find conditions that yield high-quality graphene products. Characterization tools such as backscattered electron images and Raman mapping were employed to assess the effects of growth conditions on graphene film quality, uniformity, and preferential graphene growth characteristic size. We report the synthesis of high-quality single-layer graphene (SLG) and AB-stacked bilayer graphene films with I_D/I_G of 0.21 and 0.14, respectively. Synthesis was carried out in a one-step and short-time process of 5 min. Electron diffraction analysis illustrates peak intensities resembling SLG and AB-bilayer graphene with up to 5 and 20 μm grain sizes, respectively. The measured optical transmissivity of SLG and AB-bilayer graphene in this work falls in the 0.959-0.977 and 0.929-0.953 ranges. Additionally, the sheet resistances by a 4-point probe with 1 mm spacing were measured as 15.5 � 4.6 and 3.4 �1.5 kΩ/sq, respectively.

To exploit graphene's extraordinary properties, mass production methods are warranted. By flattening the heat flux profile, a larger area on the copper substrate reaches a temperature of 1060℃, enabling larger-area graphene synthesis. Synthesis of high-quality (I_D/I_G = 0.13) AB-stacked bilayer graphene has been achieved with greater than 90% coverage in a one-step and short time process of 5 min. Synthesized graphene exhibits spatial uniformity over a large area up to 20 mm in radius with large grain sizes up to 20 μm. Graphene film transmissivity and sheet resistance fall in the 92.8-95.3% and 2-4 kΩ/sq range, respectively. However, a roll-to-roll mechanism in solar-thermal chemical vapor deposition is necessary for scaling-up graphene growth. Moderate-quality (I_D/I_G of 0.7) films were achieved at a speed of 200 mm/min with at least 72% SLG coverage suitable as transparent conductive electrodes. Lower-quality graphene synthesized in this work can still be applied as an oxidation barrier for metals. Therefore, direct-solar capture provides a compelling option to harvest a renewable energy resource and drive graphene synthesis.