Electromagnetic Enhancement of Graphene Raman Spectroscopy by Ordered and Size-Tunable Au Nanostructures
© Zhang et al. 2015
Received: 30 August 2015
Accepted: 27 September 2015
Published: 6 October 2015
The size-controllable and ordered Au nanostructures were achieved by applying the self-assembled monolayer of polystyrene microspheres. Few-layer graphene was transferred directly on top of Au nanostructures, and the coupling between graphene and the localized surface plasmons (LSPs) of Au was investigated. We found that the LSP resonance spectra of ordered Au exhibited a redshift of ~20 nm and broadening simultaneously by the presence of graphene. Meanwhile, the surface-enhanced Raman spectroscopy (SERS) of graphene was distinctly observed; both the graphene G and 2D peaks increased induced by local electric fields of plasmonic Au nanostructures, and the enhancement factor of graphene increased with the particle size, which can be ascribed to the plasmonic coupling between the ordered Au LSPs and graphene.
Graphene is the first two-dimensional carbon atomic crystal which is constructed by several layers of honeycomb-arrayed carbon atoms. This promising material is highly attractive for the fabrication of high-frequency nanoelectronic and optoelectronic devices due to its exceptional optical and electrical properties, such as extreme mechanical strength, ultrahigh electrical carrier mobility, and very high light transmittance [1–3]. Unfortunately, the graphene of only one-atomic-layer thickness exhibits lower light absorption (only ~2.3 % for a single layer) originating from the weak light-graphene interaction, which is unfavorable for high-performance graphene-based optoelectronic devices. Several approaches have been proposed to enhance the absorption of graphene, including using one-dimensional photonic crystal and localized surface plasmons (LSPs) [4, 5]. LSPs in conventional systems are the collective oscillations of conduction electrons in the metal nanoparticles when illuminated and excited by light with appropriate wavelength, and the resonance excitation of the LSPs induces a large enhancement and confinement of the local electric field in the vicinity of the metal nanostructures. Generation of LSPs stimulates a wide range of applications such as ultratrace biochemical sensing, enhanced absorption in photovoltaic cells, surface plasmon-enhanced fluorescence, and Raman scattering. From a spectroscopic point of view, surface-enhanced Raman spectroscopy (SERS) has become a promising spectacular application of plasmonics especially for the graphene-LSP hybrid system. On the other hand, the two-dimensional nature of graphene and its well-known Raman spectrum make it a favorable test bed for investigating the mechanisms of SERS, and various nanoparticle geometries have proven to deliver a considerable Raman enhancement in the case of graphene [6–11]. A Raman enhancement of 103 times had been detected for graphene from the dimer cavity between two closely packed Au nanodisks. However, fabrication and space control of the Au nanodisks are very complex and costly . Sun et al. deposited Ag on the surface of a graphene film, and distinct Raman enhancement had been achieved. However, the quality of graphene significantly deteriorated after deposition of metal nanoparticles which limits the further application of graphene .
Except for enhancing intensity of Raman scattering of graphene by LSPs, graphene had also been adopted to tune the surface plasmons resonance wavelength of metal nanostructures. For instance, the plasmonic behavior of Au nanoparticles can be tuned by varying the thickness of the Al2O3 spacer layer inserted between the graphene and nanoparticles . Nevertheless, the Au nanoparticles are randomly distributed on the surface of the Al2O3 layer which is unfavorable for the precise controllability and investigation of the inter-coupling of the graphene-metal hybrid system. Obviously, combination of enhanced near-fields of ordered plasmonic nanostructures with unusual optoelectronic properties of graphene will provide a more promising application for novel graphene-based optoelectronic devices, and thus, research on the plasmonic coupling between graphene and plasmonic ordered nanoparticles is highly desirable.
In this paper, ordered and size-controlled Au nanostructures were fabricated using the inverted self-assembled monolayer template of polystyrene microspheres. A chemical vapor deposition (CVD) graphene was transferred directly on top of Au nanostructures, and the interaction between graphene and LSPs of Au has been systematically investigated. We found that the SERS of graphene was apparently observed and Raman intensities of both the graphene G and 2D peaks increased with the size of Au which was induced by the local electric field of plasmonic Au nanostructures. On the other hand, the absorption spectra of Au nanostructures exhibited a redshift of ~20 nm and a slight broadening by the presence of graphene, which was due to the inter-coupling between Au LSPs and graphene.
Surface morphologies of the ordered Au nanostructures with various shapes and sizes were characterized by atomic force microscopy (AFM, NT-MDT solver P47). The Au nanostructures with graphene coverage were investigated by field-emission scanning electron microscopy (SEM, Hitachi FE-S4800). Raman spectra of graphene were recorded with a Horiba LabRAM HR800 spectrometer using the 514-nm excitation line from an Ar ion laser. For both SEM and Raman measurements, the SiO2/Si substrates were adopted. The ultraviolet (UV)-visible absorption spectra of the ordered Au nanostructures were measured as a function of the incident wavelength using a Varian Cary 5000 spectrophotometer in a double-beam mode. The quartz substrate was adopted for the UV-vis measurement of the Au nanostructures with and without graphene, respectively.
Results and Discussion
Circularity, width, and resonance position of Au LSP with and without graphene coverage as a function of the initial thicknesses of the Au film
Initial thickness of Au film (nm)
LSPR without graphene (nm)
LSPR with graphene (nm)
1.09 ± 0.02
100 ± 4.5
1.21 ± 0.01
120 ± 5.4
1.13 ± 0.02
128 ± 6.7
1.06 ± 0.04
140 ± 7.8
The above results also indicate that the Au LSPs are also strongly affected by the presence of graphene, which can be attributed to the coupling between the graphene film and the localized electromagnetic field of the Au nanostructures. As the incident light is perpendicular to the surface of the Au nanostructures, the incident electric field is parallel to the sample surface and has no vertical component, and only the lateral electron oscillations within the Au nanostructures can be induced. When the LSPs of Au nanostructures with graphene coating are excited, the image dipoles or quadrupoles within the graphene sheet which are antiparallel to the dipoles or quadrupoles in Au will be formed [5, 12]. The presence of the antiparallel image dipoles and quadrupoles can reduce the internal electric field in the Au nanostructures, which results in the redshift and broadening of LSP resonance peaks for the Au nanostructures with graphene.
Now we turn towards the nature of the chemical interaction between the ordered Au nanostructures and graphene, which is another physical mechanism for the SERS especially for the graphene-nanoparticle hybrid system [3, 19]. From the Raman spectrum, we find that the G peak position spectrally shifts from 1350 to 1360 cm−1 for graphene that was transferred on the surface of the ordered Au nanostructures while the 2D peak position almost remains constant. It has been reported that the G peak of graphene is blueshifted for both electron and hole doping, while the 2D peak is redshifted for electron doping and blueshifted for hole doping [18, 21]. In this case, graphene is in direct contact with the ordered Au nanostructures, and the work function of Au (5.0 eV) is nearly the same with that of graphene (4.8 eV). Considering that there are huge amounts of electrons for the Au nanostructures, electron transfer from Au to graphene will occur, leading to an electron doping for graphene; thus, the G peak is blueshifted and the 2D peak is slightly redshifted. On the other hand, we deduce that graphene is under compressive strain when it is directly transferred on top of Au as some ridges appear on the surface of graphene (clearly in Fig. 3); hence, both the G and 2D peaks exhibit a blueshift trend . As a result, both strain and doping effects lead to a slight blueshift for the G peak and negligible shift for the 2D peak position. However, we consider that charge transfer is not the dominant mechanism for the enhanced Raman intensity since the Raman intensity is increased with the size of Au nanostructures and the charge transfer effect should be independent on the Au size.
In summary, the coupling between graphene and LSPs of ordered and size-controllable Au nanostructures has been investigated systematically by directly transferring graphene on the surface of the Au nanostructures. The absorption spectra of Au exhibit a redshift of ~20 nm after graphene coverage, which can be ascribed to the plasmonic coupling between the Au LSPs and graphene. On the other hand, the graphene SERS is significantly observed, and intensities of the G and 2D peaks increase with increasing size of the ordered Au nanostructures. The electromagnetic plasmonic effect rather than the charge transfer mechanism is considered to be the dominant mechanism for the SERS effect of graphene. We believe the results are beneficial not only for further understanding the coupling mechanism between graphene and the ordered metallic nanostructures, but also for developing plasmonic graphene-based optoelectronic devices.
This work was financially supported by the National Natural Science Foundation of China (Grant No. 61404051) and Fundamental Research Funds for the Central Universities (2014ZB0016).
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