Chinnasamy Sengottaiyan

Chemical vapor deposition (CVD) is a promising method to grow high-quality, large-area graphene, yet it requires transferring graphene to target substrates without compromising its quality. The poly(methyl methacrylate)-assisted graphene transfer method is the most commonly used for transferring graphene grown by CVD. However, the quality of graphene is often degraded by polymer residues remaining after transfer and by wrinkles or cracks on the graphene. Paraffin-assisted graphene transfer has emerged as a more practical method, with the advantages of reducing wrinkles and contamination. However, complete removal of paraffin without introducing new contaminants or mechanical damage remains a significant challenge that requires meticulous process control. This work presents a paraffin-assisted transfer for large-area graphene grown by CVD, emphasizing not only its merits but also the challenges and optimization strategies for improving the transfer process. Our findings will contribute to extending paraffin-assisted CVD graphene transfer to achieve high-quality graphene.

The strategic design of nanoarchitecture in two-dimensional (2D) material is a key challenge for the development of proton exchange membranes. Although graphene oxide (GO) has a high intrinsic proton conductivity (PC), its practical application is restricted by its stability. Here, we show that the ordered layered structure of GO and montmorillonite (MMT) exhibits enhanced properties compared to the typical disordered mixed structure, leading to improved PC (2.71 vs. 0.636 mS/cm). This enhancement is directly associated with optimized degree of hydration, indicated by higher water uptake capacity (64.12% vs. 24.71%) and swelling ratio (53.8% vs. 19.43%), which facilitate continuous proton-conducting pathways. Contact angle (CA) measurements further substantiate this disparity in hydrophilicity. The layered membrane exhibits strongly hydrophilic surfaces (15.8° on the MMT side and 54.1° on the GO side), whereas both sides of the mixed membrane present consistently hydrophobic characteristics (69.5° and 65.5°). The present work presents an optimized strategy for engineering 2D material re-stacking to achieve enhanced proton conductivity, paving the way for next-generation energy devices.

Large single-crystal graphene is an important material that holds great promise for applications in graphene electronics, whereas twisted bilayer graphene exhibits breakthrough structural properties and revolutionizes diverse future devices. Herein, we synthesized millimeter scale high-quality graphene stacked in a few layers on copper (Cu) foil by atmospheric pressure chemical vapor deposition (APCVD). The maximum size of hexagonal graphene was found to be around 5.5 mm. Raman spectroscopy and mapping analysis specified the quality and clear structural changes in the graphene lattice consisting of monolayer to twisted few-layers. Especially, using nongap mode tip-enhanced Raman spectroscopy (TERS), a significant enhancement of the Raman signal was observed on the twisted graphene. Using Kelvin force probe microscopy (KPFM), we confirmed that the surface potential shifts more negatively with an increase in the number of layers. This is due to the lattice misalignment in the twisted structure, changing the charge distribution and Fermi level. The present findings of the physical properties of large-scale and twisted few-layer graphene are expected to contribute to the development of twistronics devices.

Layer-dependent physical properties of exfoliated 2D materials are critical for both fundamental studies and technological applications. However, our understanding of local electrical conductivity on an atomic scale remains limited. In this study, we employ polydimethylsiloxane (PDMS)-assisted mechanical exfoliation followed by a micromanipulation technique to achieve a large-sized monolayer MoS₂ (∼30 μm) for investigating its layer-dependent physical properties. Raman spectroscopy revealed the distinct structure of monolayer MoS₂, with increased peak intensity and a narrower separation between the A_1g and E_2g peaks. Photoluminescence (PL) spectra showed increased intensity and higher photon energy levels in monolayer MoS₂ compared to bilayer and multilayer MoS₂. Raman and PL mapping of monolayer and bilayer MoS₂ provides valuable insights into their structural and optical characteristics. Atomic force microscopy (AFM) measurements indicated thicknesses of approximately 0.7 nm for monolayer MoS₂ and 0.4 nm for reduced graphene oxide (RGO). Kelvin probe force microscopy revealed a contact potential difference between the monolayer and few-layer MoS₂ and the MoS₂/RGO surface. Conductive AFM (CAFM) measurements demonstrated consistent and reproducible electrical characteristics of the monolayer MoS₂/RGO surface. Moreover, the obtained work function (WF) and Schottky barrier height (Φ_B) of MoS₂/RGO exhibited layer-dependent behavior: increasing layer thickness reduced the Φ_B due to enhanced charge transfer while raising the WF, indicating shifts in Fermi level alignment and the interfacial electronic structure. These findings offer a deeper understanding of the optical, structural, and electrical properties of the MoS₂/RGO heterostructures.