Surface-enhanced Raman scattering of suspended monolayer graphene
© Huang et al.; licensee Springer. 2013
Received: 19 July 2013
Accepted: 30 October 2013
Published: 15 November 2013
The interactions between phonons and electrons induced by the dopants or the substrate of graphene in spectroscopic investigation reveal a rich source of interesting physics. Raman spectra and surface-enhanced Raman spectra of supported and suspended monolayer graphenes were measured and analyzed systemically with different approaches. The weak Raman signals are greatly enhanced by the ability of surface-enhanced Raman spectroscopy which has attracted considerable interests. The technique is regarded as wonderful and useful tool, but the dopants that are produced by depositing metallic nanoparticles may affect the electron scattering processes of graphene. Therefore, the doping and substrate influences on graphene are also important issues to be investigated. In this work, the peak positions of G peak and 2D peak, the I2D/IG ratios, and enhancements of G and 2D bands with suspended and supported graphene flakes were measured and analyzed. The peak shifts of G and 2D bands between the Raman and SERS signals demonstrate the doping effect induced by silver nanoparticles by n-doping. The I2D/IG ratio can provide a more sensitive method to carry out the doping effect on the graphene surface than the peak shifts of G and 2D bands. The enhancements of 2D band of suspended and supported graphenes reached 138, and those of G band reached at least 169. Their good enhancements are helpful to measure the optical properties of graphene. The different substrates that covered the graphene surface with doping effect are more sensitive to the enhancements of G band with respect to 2D band. It provides us a new method to distinguish the substrate and doping effect on graphene.
78.67.Wj (optical properties of graphene); 74.25.nd (Raman and optical spectroscopy); 63.22.Rc (phonons in graphene)
Graphene, the thinnest sp2 allotrope of carbon arranged in a honeycomb lattice, has attracted many attentions because its unique and novel electrical and optical properties [1–3]. The wonderful and remarkable carrier transport properties of suspended graphene compared with supported graphene have been studied [4–9]. The performances of dopants, the effects of defects in graphene, and the phonon modes of suspended and supported graphenes vary but can be well understood using Raman spectroscopy [10–12]. Raman spectroscopy and surface-enhanced Raman spectroscopy (SERS) have been extensively applied to understand the vibration properties of materials [13–18], and they are regarded as powerful techniques in characterizing the band structure and detail of phonon graphene interaction [19–24]. The ability of SERS, a wonderful and useful technique used to enhance weak Raman signals, has attracted considerable attention. In previous SERS measurements, however, the doping induced by metallic nanoparticles on graphene deposition may affect the electron scattering processes of graphene. Otherwise, the metallic nanoparticles on graphene are also used as an electrode in graphene-based electronic devices [25, 26]. Therefore, the effect of charged dopants and the substrate which affected graphene are both important issues to be investigated. In this work, the supported and suspended monolayer graphene samples were fabricated by micromechanical cleavage method. They were identified as the monolayer graphene by the optical microscopy and Raman spectroscopy because of the color contrast, various bandwidth, and peak position of 2D band with different graphene layers [27, 28]. The Raman and SERS signals of suspended and supported graphenes can be measured and analyzed systematically. The peak positions of G and 2D bands, the I2D/IG ratio, and enhancements of G and 2D bands were obtained, respectively. With our analysis, details about the effects of charged impurities and substrate can be realized. The peak shift of G and 2D bands and the I2D/IG ratio are useful to demonstrate the dopants and substrate effects on the graphene. The well-enhanced G and 2D bands are obtained to enhance the weak Raman signals. Moreover, the enhancements of G band with respect to 2D band are found to be more sensitive to various substrate influences on the graphene surface. This paper provides a new approach to investigate the substrate and doping effect on graphene.
Results and discussion
In order to minimize the random errors, each Raman spectra data point was obtained by five-time repetitions. As presented in Figure 3c, the I2D/IG ratio of suspended graphene under Raman signals is 4.1 ± 0.1 and larger than supported graphene which is 3.6 ± 0.5, while the I2D/IG ratio of suspended graphene on the SERS signals is around 2.9 ± 0.1 and smaller than supported graphene which is 3.0 ± 0.2. The result disclosed the substrate effect on the supported graphene is stronger than the suspended graphene. To our understanding, this above effect can be reasonable because SiO2/Si substrate has a direct contact on the graphene surface so the silver nanoparticles deposition causes stronger doping effect on the suspended graphene than supported graphene.
To understand the Raman and SERS signals, the enhancements of G and 2D bands with the suspended and supported graphenes are shown in Figure 3d, respectively. The enhancement is defined as the integrated intensities of SERS over Raman signals for the G and 2D bands, respectively. In our analysis, the enhancement of G band on supported graphene is 169.3 ± 20.1 and smaller than suspended graphene which is 196.2 ± 8.3, while the enhancement of 2D band with supported graphene which is 141.1 ± 4.3 is similar with suspended graphene which is 138.6 ± 1.6. The high enhancements of G and 2D bands are useful to enhance weak Raman signals, and the enhancements of G band with suspended and supported graphenes are both stronger than those of 2D band. Otherwise, the enhancement of G band is reduced obviously as silver nanoparticles deposited on suspended graphene, revealing that the enhancement of G band is sensitive to substrate effect on graphene with respect to 2D band. Based on the results, the doping effects with various substrates are obviously related to the enhancement of G band.
In our work, Raman and SERS signals of supported and suspended monolayer graphenes were measured systematically. The peak positions of G and 2D bands and the I2D/IG ratios were varied. The enhancement effect of suspended and supported graphenes was calculated and analyzed. The peak shifts of G and 2D bands and their Raman spectra and I2D/IG of SERS signals are found very useful in the investigation of the substrate and doping effect on the optical properties of graphene. The enhancements of G and 2D bands have been found to cause the great improvement of weak Raman signals. Otherwise, the more sensitive enhancements of G band with respect to 2D band are related to the doping effect with various substrates that covered the graphene surface. The optical emission spectra of suspended and supported graphenes have provided us a with new identification approach to understand the substrate and doping effect on graphene.
We wish to acknowledge the support of this work by the National Science Council, Taiwan under contact nos. NSC 98-2112-M-006-004-MY3, NSC 101-2112-M-006-006, and NSC 102-2622-E-006-030-CC3.
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