Graphene-enhanced Raman spectroscopy of thymine adsorbed on single-layer graphene
Fesenko et al. Nanoscale Research Letters
Graphene-enhanced Raman spectroscopy of thymine adsorbed on single-layer graphene
Olena Fesenko 0
Galyna Dovbeshko 0
Andrej Dementjev 2
Renata Karpicz 2
Tommi Kaplas 1
Yuri Svirko 1
0 Institute of Physics, National Academy of Sciences of Ukraine , 46 Nauki Ave., Kyiv 03680 , Ukraine
1 Institute of Photonics, University of Eastern Finland , Yliopistokatu 7, Joensuu FI-80101 , Finland
2 Center for Physical Sciences and Technology, Institute of Physics , A. Gostauto 11, Vilnius LT-01108 , Lithuania
Graphene-enhanced Raman scattering (GERS) spectra and coherent anti-Stokes Raman scattering (CARS) of thymine molecules adsorbed on a single-layer graphene were studied. The enhancement factor was shown to depend on the molecular groups of thymine. In the GERS spectra of thymine, the main bands are shifted with respect to those for molecules adsorbed on a glass surface, indicating charge transfer for thymine on graphene. The probable mechanism of the GERS enhancement is discussed. CARS spectra are in accord with the GERS results, which indicates similar benefit from the chemical enhancement.
Single-layer graphene; Thymine; Surface-enhanced Raman spectroscopy (SERS); Graphene-enhanced Raman scattering (GERS); Coherent anti-Stokes Raman scattering (CARS); Graphene-enhanced coherent anti-Stokes Raman scattering (GECARS)
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Background
Surface-enhanced Raman spectroscopy (SERS) has
become an efficient technique that enables detection and
study of an extremely small amount of biochemical
materials and single-molecule detection [1-4]. SERS is
based on the enhancement of the local optical field by
several orders of magnitude in the vicinity of a rough
metal surface or metal island film due to excitation of
the collective oscillations of conduction electrons at the
metal surface (surface plasmons). However, investigation
of the biological and biochemical species often requires
substrates of higher chemical inertness. Such substrates
can be based in particular on carbon allotropes, e.g., on
graphene or carbon nanotubes. But in graphitic
materials, the surface plasmon resonance is found in the THz
range [5,6], i.e., plasmon-based local field enhancement
can hardly be employed in optical spectroscopy with
such carbon-based substrates. Nevertheless, it has been
recently demonstrated [7-10] that the Raman signal of
molecules deposited on graphene and graphene oxides is
enhanced by several orders of magnitude, which is likely
to be caused by the so-called chemical mechanism [11],
i.e., chemical interaction of deposited molecules and
carbon atoms of the substrate. This phenomenon is called
graphene-enhanced Raman scattering (GERS) [12] and
may become important for spectroscopy of certain
biological and biochemical species. In particular, we have
recently reported the enhancement for Raman and coherent
anti-Stokes Raman scattering of thymine adsorbed on
graphene oxide [10].
In the present paper, we report on a comparative study
of the surface-enhanced Raman scattering and
surfaceenhanced coherent anti-Stokes Raman scattering for
thymine (Thy) adsorbed on graphene layers.
Samples
In the Raman and coherent anti-Stokes Raman scattering
(CARS) measurements, we use aqueous 1 mg/ml and 10
g/ml solutions of commercially available thymine
(SigmaAldrich, St. Louis, MO, USA). The samples for optical
experiments were prepared by depositing a drop of Thy
solution on graphene-on-silica or glass substrates,
respectively. The average surface density of Thy after water
evaporation was either 200 ng/cm2 or 20 g/cm2.
The graphene-on-silica substrates were fabricated by
depositing a graphene sheet on fused silica. Single-layer
graphene was prepared by using chemical vapor deposition
(CVD) of graphene on a copper foil described elsewhere
[13,14]. Before the start of the graphitization process, the
copper substrate was annealed for 1 h at a temperature of
1,000C in 15-mbar hydrogen atmosphere. After the
annealing, the CVD chamber was pumped down and filled
with 1:1 H2:CH4 gas mixture for 10 min (15 mbar). Because
of the self-limiting graphene growth on a copper substrate,
almost single-layer graphene [15] was deposited on both
surfaces of the copper foil. The templated graphene growth
was suppressed by short hydrogen etching at a temperature
of 1,000C and pressure of 50 mbar [16]. After the etching,
the CVD chamber was cooled down to room temperature
in hydrogen atmosphere (15 mbar). The graphene
deposited on the backside of the copper was etched away in
harsh oxygen plasma (100 W/20 sccm/2 min).
The graphene sheet was spin coated by a 1-m-thick
polymethyl methacrylate (PMMA) layer, and then the
copper foil was removed by wet etching in FeCl3. The
remaining PMMA/graphene stack was rinsed in deionized
water for 30 min and then placed on a silica substrate in
such a way that the graphene was facing the silica. In
order to relax internal stress in the stack, another PMMA
layer was deposited on top of the e (...truncated)