ICMCTF 2021 Virtual Conference Session F3: 2D Materials: Synthesis, Characterization, and Applications

Graphene is the most commonly studied 2D material due to its exceptional physical and chemical properties, originating from its atomic structure. However, the successful graphene applications are driven by the ability to synthesise it at high growth rates and low temperatures, which enable large-scale production on a variety of substrates. The synthesis of vertically standing graphene (VSG) is of particular interest due to its exposed sharp edges, non-stacking morphology and large surface-to-volume ratio, leading to advanced technological applications including sensors, flexible electronic devices and fuel cells. Plasma-enhanced chemical vapour deposition (PE-CVD) has emerged as a promising technique to synthesise graphene at lower temperatures. The plasma energy drives the CVD precursor decomposition and reaction kinetics, allowing better control over the deposition parameters that tailor graphene properties.

It is generally assumed, and often true, that homoepitaxy yields higher crystalline quality thin films than heteroepitaxy. Studies conducted nearly three decades ago have shown that layered materials, owing to weak van der Waals (vdW) bonding across the layers, can aid in heteroepitaxial growth of layered as well as non-layered materials. In the recent years, two-dimensional (2D) layered materials have been shown to promote 'remote epitaxy', where the 2D layer present at the substrate-film interface does not hinder the epitaxial registry between the film and the substrate. Here, we demonstrate that 2D hexagonal boron nitride (hBN, a = 0.250 nm and c = 0.667 nm) buffer layers improves the crystallinity of sputter-deposited thin films. We provide evidence for this phenomenon via heteroepitaxial growth of body centered cubic metal (Mo), hexagonal MoS₂, and trigonal Ta₂C thin films on hBN-covered Al₂O₃(0001) substrates. Furthermore, our studies indicate that inserting hBN layers at regular intervals results in highly-0002-orientated growth and suppression of polycrystallinity in thicker Ta₂C films.

Manufacturing processes are often highly energy-intensive, even when the energy is primarily used for simple heating processes. This energy tends to derive from local utilities, which currently employ a blend of sources ranging from fossil fuels to renewable wind and solar photovoltaics, among others. When the end manufacturing need is thermal energy, direct solar-thermal capture provides a compelling option, but one that has rarely been employed to date. Here, we report a solar-thermal process using a simulated solar concentrator to demonstrate the ability of such a source to produce a high-value product, namely graphene on copper. Material deposition occurs at a surface and requires knowledge of material science, manufacturing, and heat transfer modeling. In this study, we employ a 10 kW_e concentrated solar source (solar simulator) capable of producing an adjustable high heat flux distribution (up to 4.5 MW/m², or 4,500 suns) in order to produce graphene rapidly on copper foil by chemical vapor deposition. The custom-built reactor consists of a xenon short arc lamp (that closely approximates the solar spectrum) placed at a truncated reflector’s first focal point to concentrate source radiation with a Lorentzian-like heat flux distribution on the reflector’s second focal point. Through the use of a controllable DC power supply and shutter, incident heat flux can be controlled and varied. Copper substrates are placed on a well-insulated mount that allows for varying the substrate’s focal position, and hence heat flux distribution. We use the concentrated solar source to study the effect of heating and photocatalysis on the deposition product, and we begin to optimize the process by modeling substrate heat transfer processes that depend highly on optical and local thermal conditions. The process is monitored by optical emission spectroscopy, including an IR camera, pyrometer, and near-IR spectrometer, to determine appropriate gas recipes (flowrate and relative concentrations of methane and hydrogen) and other operating conditions, such as vacuum pressure, that yield high-quality product. The graphene produced through this process is further analyzed with scanning electron and Raman microscopy to assess the uniformity of graphene deposition as well as its quality, which is associated with the intensity ratio between Raman peaks of C-C in-plane vibrations and graphene lattice defects. Upon optimizing the operating conditions, graphene deposition will be extended to a larger and continuous scale through the use of a roll-to-roll solar chemical vapor deposition.