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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. Synthesis and Characterization of Free-Stand Graphene/Silver Nanowire/Graphene Nano Composite as Transparent Conductive Film with Enhanced Stiffness

   https://doi.org/10.3390/app10144802  

   Guo, Chuanrui ; Li, Yanxiao ; Zhu, Yanping ; Wu, Chenglin ; Chen, Genda ( July 2020 , Applied Sciences)

   As-grown graphene via chemical vapor deposition (CVD) has potential defects, cracks, and disordered grain boundaries induced by the synthesis and transfer process. Graphene/silver nanowire/graphene (Gr/AgNW/Gr) sandwich composite has been proposed to overcome these drawbacks significantly as the AgNW network can provide extra connections on graphene layers to enhance the stiffness and electrical conductivity. However, the existing substrate (polyethylene terephthalate (PET), glass, silicon, and so on) for composite production limits its application and mechanics behavior study. In this work, a vacuum annealing method is proposed and validated to synthesize the free-stand Gr/AgNW/Gr nanocomposite film on transmission electron microscopy (TEM) grids. AgNW average spacing, optical transmittance, and electrical conductivity are characterized and correlated with different AgNW concentrations. Atomic force microscope (AFM) indentation on the free-stand composite indicates that the AgNW network can increase the composite film stiffness by approximately 460% with the AgNW concentration higher than 0.6 mg/mL. Raman spectroscopy shows the existence of a graphene layer and the disturbance of the AgNW network. The proposed method provides a robust way to synthesize free-stand Gr/AgNW/Gr nanocomposite and the characterization results can be utilized to optimize the nanocomposite design for future applications. 

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3. 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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4. 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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5. Facile Fabrication of Large‐Area Atomically Thin Membranes by Direct Synthesis of Graphene with Nanoscale Porosity

   https://doi.org/10.1002/adma.201804977  

   Kidambi, Piran R. ; Nguyen, Giang D. ; Zhang, Sui ; Chen, Qu ; Kong, Jing ; Warner, Jamie ; Li, An‐Ping ; Karnik, Rohit ( October 2018 , Advanced Materials)

   Direct synthesis of graphene with well‐defined nanoscale pores over large areas can transform the fabrication of nanoporous atomically thin membranes (NATMs) and greatly enhance their potential for practical applications. However, scalable bottom‐up synthesis of continuous sheets of nanoporous graphene that maintain integrity over large areas has not been demonstrated. Here, it is shown that a simple reduction in temperature during chemical vapor deposition (CVD) on Cu induces in‐situ formation of nanoscale defects (≤2–3 nm) in the graphene lattice, enabling direct and scalable synthesis of nanoporous monolayer graphene. By solution‐casting of hierarchically porous polyether sulfone supports on the as‐grown nanoporous CVD graphene, large‐area (>5 cm²) NATMs for dialysis applications are demonstrated. The synthesized NATMs show size‐selective diffusive transport and effective separation of small molecules and salts from a model protein, with ≈2–100× increase in permeance along with selectivity better than or comparable to state‐of‐the‐art commercially available polymeric dialysis membranes. The membranes constitute the largest fully functional NATMs fabricated via bottom‐up nanopore formation, and can be easily scaled up to larger sizes permitted by CVD synthesis. The results highlight synergistic benefits in blending traditional membrane casting with bottom‐up pore creation during graphene CVD for advancing NATMs toward practical applications.

    

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