163 research outputs found
Edge chirality determination of graphene by Raman spectroscopy
Raman imaging on the edges of single layer micromechanical cleavage graphene
(MCG) was carried out. The intensity of disorder-induced Raman feature (D band
at ~1350 cm-1) was found to be correlated to the edge chirality: it is stronger
at the armchair edge and weaker at the zigzag edge. This shows that Raman
spectroscopy is a reliable and practical method to identify the chirality of
graphene edge and to help in determination of the crystal orientation. The
determination of graphene chirality is critically important for fundamental
study as well as for applications.Comment: 14 pages, 3 figures, 1 tabl
Interaction between graphene and SiO2 surface
With first-principles DFT calculations, the interaction between graphene and
SiO2 surface has been analyzed by constructing the different configurations
based on {\alpha}-quartz and cristobalite structures. The single layer graphene
can stay stably on SiO2 surface is explained based on the general consideration
of configuration structures of SiO2 surface. It is also found that the oxygen
defect in SiO2 surface can shift the Fermi level of graphene down which opens
out the mechanism of hole-doping effect of graphene absorbed on SiO2 surface
observed in experiments.Comment: 17 pages, 7 figure
Plasma-Assisted Synthesis of Carbon Nanotubes
The application of plasma-enhanced chemical vapour deposition (PECVD) in the production and modification of carbon nanotubes (CNTs) will be reviewed. The challenges of PECVD methods to grow CNTs include low temperature synthesis, ion bombardment effects and directional growth of CNT within the plasma sheath. New strategies have been developed for low temperature synthesis of single-walled CNTs based the understanding of plasma chemistry and modelling. The modification of CNT surface properties and synthesis of CNT hybrid materials are possible with the utilization of plasma
Interference enhancement of Raman signal of graphene
Raman spectroscopic studies of graphene have attracted much interest. The
G-band Raman intensity of a single layer graphene on Si substrate with 300 nm
SiO2 capping layer is surprisingly strong and is comparable to that of bulk
graphite. To explain this Raman intensity anomaly, we show that in addition to
the interference due to multiple reflection of the incident laser, the multiple
reflection of the Raman signal inside the graphene layer must be also accounted
for. Further studies of the role of SiO2 layer in the enhancement Raman signal
of graphene are carried out and an enhancement factor of ~30 is achievable,
which is very significant for the Raman studies. Finally, we discuss the
potential application of this enhancement effect on other ultra-thin films and
nanoflakes and a general selection criterion of capping layer and substrate is
given.Comment: 13 pages, 3 figures to be published in Applied Physics Letter
Stacking Dependent Optical Conductivity of Bilayer Graphene
The optical conductivities of graphene layers are strongly dependent on their
stacking orders. Our first-principle calculations show that while the optical
conductivities of single layer graphene (SLG) and bilayer graphene (BLG) with
Bernal stacking are almost frequency independent in the visible region, the
optical conductivity of twisted bilayer graphene (TBG) is frequency dependent,
giving rise to additional absorption features due to the band folding effect.
Experimentally, we obtain from contrast spectra the optical conductivity
profiles of BLG with different stacking geometries. Some TBG samples show
additional features in their conductivity spectra in full agreement with our
calculation results, while a few samples give universal conductivity values
similar to that of SLG. We propose those variations of optical conductivity
spectra of TBG samples originate from the difference between the commensurate
and incommensurate stackings. Our results reveal that the optical conductivity
measurements of graphene layers indeed provide an efficient way to select
graphene films with desirable electronic and optical properties, which would
great help the future application of those large scale misoriented graphene
films in photonic devices.Comment: 20 pages, 5 figures, accepted by ACS Nan
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