190 research outputs found
Kinetic control of catalytic CVD for high-quality graphene at low temperatures.
Low-temperature (∼600 °C), scalable chemical vapor deposition of high-quality, uniform monolayer graphene is demonstrated with a mapped Raman 2D/G ratio of >3.2, D/G ratio ≤0.08, and carrier mobilities of ≥3000 cm(2) V(-1) s(-1) on SiO(2) support. A kinetic growth model for graphene CVD based on flux balances is established, which is well supported by a systematic study of Ni-based polycrystalline catalysts. A finite carbon solubility of the catalyst is thereby a key advantage, as it allows the catalyst bulk to act as a mediating carbon sink while optimized graphene growth occurs by only locally saturating the catalyst surface with carbon. This also enables a route to the controlled formation of Bernal stacked bi- and few-layered graphene. The model is relevant to all catalyst materials and can readily serve as a general process rationale for optimized graphene CVD.Acknowledgment. R.S.W. acknowledges funding from
EPSRC (Doctoral Training Award). S.H. acknowledges funding
from ERC grant InsituNANO (no. 279342). This research was
partially supported by the EU FP7 Work Programme under grant
GRAFOL (project reference 285275).This is the accepted manuscript. The final version is available from ACS at http://pubs.acs.org/doi/abs/10.1021/nn303674g
Substrate-assisted nucleation of ultra-thin dielectric layers on graphene by atomic layer deposition
We report on a large improvement in the wetting of Al2O3 thin films grown by
un-seeded atomic layer deposition on monolayer graphene, without creating point
defects. This enhanced wetting is achieved by greatly increasing the nucleation
density through the use of polar traps induced on the graphene surface by an
underlying metallic substrate. The resulting Al2O3/graphene stack is then
transferred to SiO2 by standard methods.P.R.K. acknowledges funding from Cambridge Commonwealth
Trust. R.S.W. acknowledges funding from EPSRC (Doctoral
training award). S.H. acknowledges funding from ERC Grant
InsituNANO (No. 279342) and EPSRC (Grant No. EP/
H047565/1).This is the accepted manuscript. The final version is available from AIP from http://scitation.aip.org/content/aip/journal/apl/100/17/10.1063/1.4707376
The parameter space of graphene chemical vapor deposition on polycrystalline Cu
A systematic study on the parameter space of graphene CVD on polycrystalline Cu foils is
presented, aiming at a more fundamental process rationale in particular regarding the choice
of carbon precursor and mitigation of Cu sublimation. CH4 as precursor requires H2 dilution
and temperatures ≥1000°C to keep the Cu surface reduced and yield a high quality, complete
monolayer graphene coverage. The H2 atmosphere etches as-grown graphene, hence
maintaining a balanced CH4/H2 ratio is critical. Such balance is more easily achieved at low
pressure conditions, at which however Cu sublimation reaches deleterious levels. In contrast,
C6H6 as precursor requires no reactive diluent and consistently gives similar graphene quality
at 100-150°C lower temperatures. The lower process temperature and more robust processing
conditions allow the problem of Cu sublimation to be effectively addressed. Graphene
formation is not inherently self-limited to a monolayer for any of the precursors. Rather, the
higher the supplied carbon chemical potential the higher the likelihood of film inhomogeneity
and primary and secondary multilayer graphene nucleation. For the latter, domain boundaries
of the inherently polycrystalline CVD graphene offer pathways for a continued carbon supply
to the catalyst. Graphene formation is significantly affected by the Cu crystallography, i.e. the
evolution of microstructure and texture of the catalyst template form an integral part of the
CVD process.S.H. acknowledges funding from ERC grant InsituNANO (n°279342) and from EPSRC
(Grant Nr. EP/H047565/1). P.R.K. acknowledges funding from the Cambridge
Commonwealth Trust and C.D. acknowledges funding from Royal Society.This is the accepted manuscript. The final version is available from ACS at http://pubs.acs.org/doi/abs/10.1021/jp303597m
Quantum well confinement and competitive radiative pathways in the luminescence of black phosphorus layers
Black phosphorus (BP) stands out from other 2D materials by the wide
amplitude of the band-gap energy (Delta(Eg)) that sweeps an optical window from
Visible (VIS) to Infrared (IR) wavelengths, depending on the layer thickness.
This singularity made the optical and excitonic properties of BP difficult to
map. Specifically, the literature lacks in presenting experimental and
theoretical data on the optical properties of BP on an extended thickness
range. Here we report the study of an ensemble of photoluminescence spectra
from 79 passivated BP flakes recorded at 4 K with thicknesses ranging from 4 nm
to 700 nm, obtained by mechanical exfoliation. We observe that the exfoliation
steps induce additional defects states that compete the radiative recombination
from bound excitons observed in the crystal. We also show that the evolution of
the photoluminescence energy versus thickness follows a quantum well
confinement model appreciable from a thickness predicted and probed at 25 nm.
The BP slabs placed in different 2D heterostructures show that the emission
energy is not significantly modulated by the dielectric environment.
Introduction Confinement effectsComment: 11 pages, 3 figures - Main text 12 pages, 5 figures - Supporting
informatio
Measuring the nonlinear refractive index of graphene using the optical Kerr effect method
© 2016 Optical Society of America.By means of the ultrafast optical Kerr effect method coupled to optical heterodyne detection (OHD-OKE), we characterize the third-order nonlinear response of graphene and compare it to experimental values obtained by the Z-scan method on the same samples. From these measurements, we estimate a negative nonlinear refractive index for monolayer graphene, n2 = -1.1 × 10-13 m2/W. This is in contradiction to previously reported values, which leads us to compare our experimental measurements obtained by the OHD-OKE and the Z-scan method with theoretical and experimental values found in the literature and to discuss the discrepancies, taking into account parameters such as doping
Graphene-passivated nickel as an oxidation-resistant electrode for spintronics.
We report on graphene-passivated ferromagnetic electrodes (GPFE) for spin devices. GPFE are shown to act as spin-polarized oxidation-resistant electrodes. The direct coating of nickel with few layer graphene through a readily scalable chemical vapor deposition (CVD) process allows the preservation of an unoxidized nickel surface upon air exposure. Fabrication and measurement of complete reference tunneling spin valve structures demonstrate that the GPFE is maintained as a spin polarizer and also that the presence of the graphene coating leads to a specific sign reversal of the magneto-resistance. Hence, this work highlights a novel oxidation-resistant spin source which further unlocks low cost wet chemistry processes for spintronics devices.R.S.W. acknowledges funding from EPSRC
(Doctoral training award). S.H. acknowledges funding from ERC
Grant InsituNANO (Project Reference 279342). P.S. acknowledges
the Institut Universitaire de France for junior fellowship
support. This research was partially supported by the EU FP7
work programme under Grant GRAFOL (Project Reference
285275).This is the accepted manuscript. The final version is available from ACS at http://pubs.acs.org/doi/abs/10.1021/nn304424x
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