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1. Electrochemical Bubble Delamination and Transfer of CVD Graphene from Copper to Si/SiO2 Utilizing Mechanical Assistance

   Perrico, Zane; Shen, Jianhao; Perera, Asela; Chakravarty, Swapnajit (October 2023, Bulletin of the American Physical Society)

   Chair: Petru Fodor, Department of (Ed.)

   High quality graphene can efficiently be grown on large surface areas of copper foil through chemical vapor deposition (CVD). To transfer CVD graphene onto a substrate for use in nanoscale photonic devices a process called electrochemical bubble delamination is utilized. During the delamination and transfer procedure the CVD graphene is at its most susceptible. Therefore, the incentive to develop a minimal-contact and replicable process is high. The use of a mechanical stage controlled by an actuator is a promising method of avoiding significant mechanical defects like folding or tearing and is capable of ensuring the film is delaminated at the right speed and from bottom to top. The quality of the transferred graphene is varied with regions of high-quality graphene up to 80x80µm while the typical transfer region has a large presence of gaps, cracks, and PMMA residues. It is evident that extending the mechanical assistance to other parts of the transfer process may be valuable, however, the occurrence of mechanical and chemical defects in the transferred graphene is still a limiting factor in the use of electrochemical bubble delamination. 

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2. The Line Speed Effect in Roll-to-Roll Dry Transfer of Chemical Vapor Deposition Graphene,

   Hong, N. (December 2021, 2021 International Conference on Micro- and Nano-devices Enabled by R2R Manufacturing)

   Sreenivasan, S.V. (Ed.)

   A roll-to-roll (R2R) technique is especially desirable for transfer of chemical vapor deposition (CVD) graphene towards high-speed, low-cost, renewable, and environmentally friendly manufacturing of graphene-based electronic devices, such as flexible touchscreens, field effect transistors and organic solar cells. A R2R graphene dry transfer system is recently developed. Monolayer graphene is transferred from a copper growth substrate to a polymer backing layer by mechanical peeling. In this work, we present an experimental study to examine the effects of line speed of the mechanical peeling process on the transferred graphene quality. It is shown that the effect of line speed is not monotonic, and an optimal speed exists to yield the highest and most consistent electrical conductivity of transferred graphene among the process conditions studied. This study provides understanding of process parameter effects and demonstrates the potential of the R2R dry transfer process for large-scale CVD graphene toward industrial applications. 

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3. Deconstructing proton transport through atomically thin monolayer CVD graphene membranes

   https://doi.org/10.1039/D2TA01737G  

   Chaturvedi, Pavan; Moehring, Nicole K.; Cheng, Peifu; Vlassiouk, Ivan; Boutilier, Michael S.; Kidambi, Piran R. (January 2022, Journal of Materials Chemistry A)

   Selective proton (H + ) permeation through the atomically thin lattice of graphene and other 2D materials offers new opportunities for energy conversion/storage and novel separations. Practical applications necessitate scalable synthesis via approaches such as chemical vapor deposition (CVD) that inevitably introduce sub-nanometer defects, grain boundaries and wrinkles, and understanding their influence on H + transport and selectivity for large-area membranes is imperative but remains elusive. Using electrically driven transport of H + and potassium ions (K + ) we probe the influence of intrinsic sub-nanometer defects in monolayer CVD graphene across length-scales for the first time. At the micron scale, the areal H + conductance of CVD graphene (∼4.5–6 mS cm −2 ) is comparable to that of mechanically exfoliated graphene indicating similarly high crystalline quality within a domain, albeit with K + transport (∼1.7 mS cm −2 ). However, centimeter-scale Nafion|graphene|Nafion devices with several graphene domains show areal H + conductance of ∼339 mS cm −2 and K + conductance of ∼23.8 mS cm −2 (graphene conductance for H + is ∼1735 mS cm −2 and for K + it is ∼47.6 mS cm −2 ). Using a mathematical-transport-model and Nafion filled polycarbonate track etched supports, we systematically deconstruct the observed orders of magnitude increase in H + conductance for centimeter-scale CVD graphene. The mitigation of defects (>1.6 nm), wrinkles and tears via interfacial polymerization results in a conductance of ∼1848 mS cm −2 for H + and ∼75.3 mS cm −2 for K + (H + /K + selectivity of ∼24.5) via intrinsic sub-nanometer proton selective defects in CVD graphene. We demonstrate atomically thin membranes with significantly higher ionic selectivity than state-of-the-art proton exchange membranes while maintaining comparable H + conductance. Our work provides a new framework to assess H + conductance and selectivity of large-area 2D membranes and highlights the role of intrinsic sub-nanometer proton selective defects for practical applications. 

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4. Advances in transferring chemical vapour deposition graphene: a review

   https://doi.org/10.1039/C7MH00485K  

   Chen, Mingguang; Haddon, Robert C.; Yan, Ruoxue; Bekyarova, Elena (January 2017, Materials Horizons)

   The unique two-dimensional structure and outstanding electronic, thermal, and mechanical properties of graphene have attracted the interest of scientists and engineers from various fields. The first step in translating the excellent properties of graphene into practical applications is the preparation of large area, continuous graphene films. Chemical vapour deposition (CVD) graphene has received increasing attention because it provides access to large-area, uniform, and continuous films of high quality. However, current CVD synthetic techniques utilize metal substrates (Cu or Ni) to catalyse the growth of graphene and post-growth transfer of the graphene film to a substrate of interest is critical for most applications such as electronics, photonics, and spintronics. Here we discuss recent advances in the transfer of as-grown CVD graphene to target substrates. The methods that afford CVD graphene on a target substrate are summarized under three categories: transfer with a support layer, transfer without a support layer, and direct growth on target substrates. At present the first two groups dominate the field and research efforts are directed towards refining the choice of the support layer. The support layer plays a vital role in the transfer process because it has direct contact with the atomically thin graphene surface, affecting its properties and determining the quality of the transferred graphene. 

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5. Assessing the quality of large-area monolayer graphene grown on liquid copper for size-selective ionic/molecular membrane separations

   https://doi.org/10.1088/2053-1591/acefb2  

   Romaniak, Grzegorz; Cheng, Peifu; Dybowski, Konrad; Kula, Piotr; Kidambi, Piran R (October 2023, Materials Research Express)

   Abstract Monolayer graphene growth on liquid copper (Cu) has attracted attention due to advantages of a flat/smooth catalytic growth surface, high synthesis temperature (>1080 °C) as well as the possibility of forming graphene domains that are mobile on the liquid Cu with potential to minimize grain boundary defects and self-assemble into a continuous monolayer film. However, the quality of monolayer graphene grown on liquid copper and its suitability for size-selective ionic/molecular membrane separations has not been evaluated/studied. Here, we probe the quality of monolayer graphene grown on liquid Cu (via a metallurgical process, HSMG^®) using Scanning Electron Microscope (SEM), High-resolution transmission electron microscope (HR-TEM), Raman spectroscopy and report on a facile approach to assess intrinsic sub-nanometer to nanometer-scale defects over centimeter-scale areas. We demonstrate high transfer yields of monolayer graphene (>93% coverage) from the growth substrate to polyimide track etched membrane (PITEM, pore diameter ∼200 nm) supports to form centimeter-scale atomically thin membranes. Next, we use pressure-driven transport of ethanol to probe defects > 60 nm and diffusion-driven transport of analytes (KCl ∼0.66 nm, L-Tryptophan ∼0.7–0.9 nm, Vitamin B12 ∼1–1.5 nm and Lysozyme ∼3.8–4 nm) to probe nanoscale and sub-nanometer scale defects. Diffusive transport confirms the presence of intrinsic sub-nanometer to nanometer scale defects in monolayer graphene grown on liquid Cu are no less than that in high-quality graphene synthesized via chemical vapor deposition (CVD) on solid Cu. Our work not only benchmarks quality of graphene grown on liquid copper for membrane applications but also provides fundamental insights into the origin of intrinsic defects in large-area graphene synthesized via bottom-up processes for membrane applications. 

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