713 research outputs found
High quality graphene synthesized by atmospheric pressure CVD on copper foil
Graphene was synthesized at 1000°C by Atmospheric Pressure Chemical Vapor Deposition on copper foil from methane diluted in argon and hydrogen. The influence of the main synthesis parameters was studied on 2x2 cm2 foils in order to obtain continuous monolayer graphene without crystalline defect. The uniformity, crystal quality and number of layers of graphene were analyzed by Raman spectroscopy and Scanning Electronic Microscopy. First, an increase of the annealing pre-treatment duration induced an increase of the average size of copper grains, leading to larger graphene flakes of higher crystallinity presenting a lower number of layers. Similar evolutions of graphene characteristics were observed when decreasing the methane concentration to 20 ppm, whereas an increase of run duration led to a loss of graphene quality and to a higher number of graphene layers, confirming that graphene formation is not self-limiting on copper. An optimum hydrogen/methane ratio was found, quite different from other results of the literature, probably due to differences in the copper pre-treatment step. Finally, an optimized three steps process was developed to form monolayer continuous graphene of high quality, successfully transposed to 7x7 cm2 substrates after a reactor scale-up
Hidden area and mechanical nonlinearities in freestanding graphene
We investigated the effect of out-of-plane crumpling on the mechanical
response of graphene membranes. In our experiments, stress was applied to
graphene membranes using pressurized gas while the strain state was monitored
through two complementary techniques: interferometric profilometry and Raman
spectroscopy. By comparing the data obtained through these two techniques, we
determined the geometric hidden area which quantifies the crumpling strength.
While the devices with hidden area obeyed linear mechanics with
biaxial stiffness N/m, specimens with hidden area in the range
were found to obey an anomalous Hooke's law with an exponent
Methane as an effective hydrogen source for single-layer graphene synthesis on Cu foil by plasma enhanced chemical vapor deposition
A single-layer graphene is synthesized on Cu foil in the absence of H2 flow
by plasma enhanced chemical vapor deposition (PECVD). In lieu of an explicit H2
flow, hydrogen species are produced during methane decomposition process into
their active species (CHx<4), assisted by the plasma. Notably, the early stage
of growth depends strongly on the plasma power. The resulting grain size (the
nucleation density) has a maximum (minimum) at 50 W and saturates when the
plasma power is higher than 120 W because hydrogen partial pressures are
effectively tuned by a simple control of the plasma power. Raman spectroscopy
and transport measurements show that decomposed methane alone can provide
sufficient amount of hydrogen species for high-quality graphene synthesis by
PECVD.Comment: 22 pages, 6 figure
Field-effect transistors made of graphene grown on recycled copper foils
In this paper, recycled low purity copper foils (98%) are tested after a multiple-use process for obtaining singlelayer graphene (SLG). The graphene transfer technique based on water electrolysis (bubbling) is used for the preservation of the Cu foils in multiple graphene deposition cycles. Preliminary cleaning by O2 plasma remove carbon residues from the copper surface. The Cu foils are then cleaned with hydrochloric (HCl) or acetic acid
(CH3COOH) solutions, used as chemical baths, and the results compared. Atomic force microscopy used to check the Cu foil roughness, a critical parameter for the growth of SLG, shows root-mean-square roughness of 4.61, 28.00, 3.50 nm, for new Cu foil, after acetic acid, and after hydrochloric acid treatments, respectively. Full
coverage of SLG was obtained only during the third usage of Cu foils, i.e., after two recycling cycles. Carrier
mobility measured on graphene field-effect transistors fabricated after each recycling cycle, show values of 814
(1728) and 1847 cm2/V (1147 cm2/V), for electrons (holes) after one and two recycling cycles, respectively, thus demonstrating the improvement of the quality of the graphene with the number of Cu recycling cycles. Devices fabricated with graphene grown on the initial, low-purity, Cu foil did not show transistor behavior.FCT in the framework of the Strategic Funding UIDB/04650/2020, POCI-01-0145-FEDER-028114 (GRAPHSENS). NPq – Brazil. Work supported by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding UIDB/04650/2020 and by the project POCI-01-0145-FEDER-028114 (GRAPHSENS
Grain Boundaries in Graphene on SiC(000) Substrate
Grain boundaries in epitaxial graphene on the SiC(000) substrate are
studied using scanning tunneling microscopy and spectroscopy. All investigated
small-angle grain boundaries show pronounced out-of-plane buckling induced by
the strain fields of constituent dislocations. The ensemble of observations
allows to determine the critical misorientation angle of buckling transition
. Periodic structures are found among the flat
large-angle grain boundaries. In particular, the observed highly ordered grain boundary is assigned to the previously
proposed lowest formation energy structural motif composed of a continuous
chain of edge-sharing alternating pentagons and heptagons. This periodic grain
boundary defect is predicted to exhibit strong valley filtering of charge
carriers thus promising the practical realization of all-electric valleytronic
devices
The control of graphene double-layer formation in copper-catalyzed chemical vapor deposition
The growth of graphene during Cu-catalyzed chemical vapor deposition was
studied using 12CH4 and 13CH4 precursor gasses. We suggest that the growth
begins by the formation of a multilayer cluster. This seed increases its size
but the growth speed of a particular layer depends on its proximity to the
copper surface. The layer closest to the substrate grows fastest and thus
further limits the growth rate of the upper layers. Nevertheless, the growth of
the upper layers continues until the copper surface is completely blocked. It
is shown that the upper layers can be removed by modification of the conditions
of the growth by hydrogen etching.Comment: 17 pages, 4 figure
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