The process of oxidation
of a copper surface coated by a layer
of graphene in water-saturated air at 50 °C was studied where
it was observed that oxidation started at the graphene edge and was
complete after 24 h. Isotope labeling of the oxygen gas and water
showed that the oxygen in the formed copper oxides originated from
water and not from the oxygen in air for both Cu and graphene-coated
Cu, and this has interesting potential implications for graphene as
a protective coating for Cu in dry air conditions. We propose a reaction
pathway where surface hydroxyl groups formed at graphene edges and
defects induce the oxidation of Cu. DFT simulation shows that the
binding energy between graphene and the oxidized Cu substrate is smaller
than that for the bare Cu substrate, which facilitates delamination
of the graphene. Using this process, dry transfer is demonstrated
using poly(bisphenol A carbonate) (PC) as the support layer. The high
quality of the transferred graphene is demonstrated from Raman maps,
XPS, STM, TEM, and sheet resistance measurements. The copper foil
substrate was reused without substantial weight loss to grow graphene
(up to 3 cycles) of equal quality to the first growth after each cycle.
It was found that dry transfer yielded graphene with less Cu impurities
as compared to methods using etching of the Cu substrate. Using PC
yielded graphene with less polymeric residue after transfer than the
use of poly(methyl methacrylate) (PMMA) as the supporting layer. Hence,
this dry and clean delamination technique for CVD graphene grown on
copper substrates is highly advantageous for the cost-effective large-scale
production of graphene, where the Cu substrate can be reused after
each growth.
We study the electrochemistry of single layer graphene edges using a nanopore-based structure consisting of stacked graphene and Al2O3 dielectric layers. Nanopores, with diameters ranging from 5 to 20 nm, are formed by an electron beam sculpting process on the stacked layers. This leads to unique edge structure which, along with the atomically thin nature of the embedded graphene electrode, demonstrates electrochemical current densities as high as 1.2 × 104 A/cm2. The graphene edge embedded structure offers a unique capability to study the electrochemical exchange at an individual graphene edge, isolated from the basal plane electrochemical activity. We also report ionic current modulation in the nanopore by biasing the embedded graphene terminal with respect to the electrodes in the fluid. The high electrochemical specific current density for a graphene nanopore-based device can have many applications in sensitive chemical and biological sensing, and energy storage devices.
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