2 research outputs found
Graphene CVD: Interplay Between Growth and Etching on Morphology and Stacking by Hydrogen and Oxidizing Impurities
The growth of high quality graphene
layers by chemical vapor deposition
(CVD) has been found to strongly depend on growth conditions with
results varying greatly from one laboratory to another for nominally
identical conditions. We report the results of a systematic investigation
of the role of hydrogen and oxidizing impurities present in the gas
feedstock during the growth and cooling stages in low-pressure CVD.
First, we show that for a partial pressure of oxidizing impurities
below 1 ppb, hydrogen is not required for graphene growth from methane.
Second, we demonstrate that purified hydrogen does not etch graphene
films at typical growth temperatures. Third, a flow of purified hydrogen
during cooling counterbalances graphene etching by oxygen, thus protecting
the films. Films grown under high purity conditions (low level of
oxidizing impurities) exhibit a higher bilayer and multilayer coverage;
Surprisingly some of these bi- and multilayer graphene islands are
twisted with respect to the first graphene layer as revealed by hyperspectral
Raman imaging. Overall, this growth behavior suggests a competitive
action between film growth from the carbon precursors and etching
by the oxidative species. Our results provide new fundamental insights
on the graphene CVD growth, highlighting the important yet indirect
role of hydrogen and its major influence on controlling the action
of oxidizing impurities on nucleation and etching during the growth
process
Antiresonances in the Mid-Infrared Vibrational Spectrum of Functionalized Graphene
We
report anomalous antiresonances in the infrared spectra of doped
and disordered single layer graphene. Measurements in both reflection
microscopy and transmission configurations of samples grafted with
halogenophenyl moieties are presented. Asymmetric transparency windows
at energies corresponding to phonon modes near the Γ and K points
are observed, in contrast to the featureless spectrum of pristine
graphene. These asymmetric antiresonances are demonstrated to vary
as a function of the chemical potential and defect density. We propose
a model that involves coherent intraband scattering with defects and
phonons, thus relaxing the optical selection rule forbidding access
to <b>q</b> ≠ Γ phonons. This interpretation of
the new phenomenon is supported by our numerical simulations that
reproduce the experimental features