172 research outputs found
Raman Spectroscopy Study of Annealing-Induced Effects on Graphene Prepared by Micromechanical Exfoliation
In this note, we report a Raman spectroscopy study of annealing-induced
effects on graphene samples prepared by the microexfoliation method. It was
shown that randomly located adhesive residues often contaminate nearby graphene
sheets during thermal annealing. The contamination on graphene can be as thin
as ~1 nm, but gives several new Raman bands of unusually strong intensity. We
also find that their intensity is strongly dependent on the excitation
wavelength implying that graphene-induced Raman enhancement may be operative.
The current study also suggests that graphene can be selectively sensitive
towards certain molecular species in binding, which can be exploited for
interesting application.Comment: 8 pages, 4 figure
Probing the Intrinsic Properties of Exfoliated Graphene: Raman Spectroscopy of Free-Standing Monolayers
The properties of pristine, free-standing graphene monolayers prepared by
mechanical exfoliation of graphite are investigated. The graphene monolayers,
suspended over open trenches, are examined by means of spatially resolved Raman
spectroscopy of the G-, D-, and 2D-phonon modes. The G-mode phonons exhibit
reduced energies (1580 cm-1) and increased widths (14 cm-1) compared to the
response of graphene monolayers supported on the SiO2 covered substrate. From
analysis of the G-mode Raman spectra, we deduce that the free-standing graphene
monolayers are essentially undoped, with an upper bound of 2x10^11 cm-2 for the
residual carrier concentration. On the supported regions, significantly higher
and spatially inhomogeneous doping is observed. The free-standing graphene
monolayers show little local disorder, based on the very weak Raman D-mode
response. The two-phonon 2D mode of the free-standing graphene monolayers is
downshifted in frequency compared to that of the supported region of the
samples and exhibits a narrowed, positively skewed line shape
Optical Probing of Electronic Interaction between Graphene and Hexagonal Boron Nitride
Even weak van der Waals (vdW) adhesion between two-dimensional solids may
perturb their various materials properties owing to their low dimensionality.
Although the electronic structure of graphene has been predicted to be modified
by the vdW interaction with other materials, its optical characterization has
not been successful. In this report, we demonstrate that Raman spectroscopy can
be utilized to detect a few % decrease in the Fermi velocity (vF) of graphene
caused by the vdW interaction with underlying hexagonal boron nitride (hBN).
Our study also establishes Raman spectroscopic analysis which enables
separation of the effects by the vdW interaction from those by mechanical
strain or extra charge carriers. The analysis reveals that spectral features of
graphene on hBN are mainly affected by change in vF and mechanical strain, but
not by charge doping unlike graphene supported on SiO2 substrates. Graphene on
hBN was also found to be less susceptible to thermally induced hole doping.Comment: 19 pages, 4 figure
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