اثر سوبسترا یا مواد معدنی و آلی طلا در طیف رامان گرافن Effect of Gold Substrates on the Raman Spectra of Graphene

Raman spectra of a single layer graphene sheet placed in different gold substrates were obtained and are discussed in the context of surface enhanced Raman scattering (SERS). The gold substrates were composed of a combination of a thermally deposited gold film and a close-packed gold nanosphere layer. The SERS effects were negligible when the excitation wavelength was 514 nm, while the Raman signals were enhanced 3- to 50-fold when the excitation wavelength was 633 nm. The large SERS enhancement accompanied a spectral distortion with appearance of several unidentifiable peaks, as well as enhancement of a broadened D peak. These phenomena are interpreted as the local field enhancement in the nanostructure of the gold substrates. The difference in the enhancement factors among the various gold substrates is explained with a model in which the spatial distribution and polarization of the local field and the orientation of the inserted graphene sheet are considered important. Key Words: Graphene, Raman, Gold, Nanoparticles, Surface Enhanced Raman Scattering Introduction Since micromechanical cleavage of graphite crystals provided the first isolated graphene in 2004,1 graphene has been one of the most intensively studied materials for the past few years owing to its unique physical properties and potential applications. The methods of preparing a monolayer or a few layers of graphene in large sizes or in defect-free states are under current development.2-4 One of the most popular nondestructive methods to analyze for synthesized graphene is Raman spectroscopy. Due to the unique electronic band structure,5,6 the Raman scattering of graphene is a resonance process sensitive to the degree of disorder and the number of layers. In the graphene Raman spectra in visible wavelengths, three characteristic peaks have been well studied.7-13 The peak near 1580 cm‒۱ , or the G band, is assigned to an in-plane asymmetric translational motion of two nearby carbon atoms (E2gmode). This is a degenerated optical phonon mode at the Brillouin zone center (the Γ point of the reciprocal lattice space), and is induced by a single resonance process. The peak near 1300 ~ 1400 cm‒۱ is denoted with the D band, an inplane carbon ring breathing mode (A1g mode), forbidden in perfect graphite. The peak position of the D band is dependent on the excitation wavelength, which is explained by a double resonance process at the K point of the reciprocal lattice space. This process requires a scattering at defect sites in order to conserve the momentum. For this reason, the D band has been considered a measure of disorder in graphite crystals and has been found dominant at the edge sites of a single layer graphene.11 The double resonance process also induces an activation of two phonons, whose peak appears between 2600 ~ 2800 cm‒۱ . Since this peak frequency is close to double the frequency of the D band, it is denoted with the 2D, while other authors refer to it with the oldfashioned G’ band. The resonance process for the 2D band is momentum-conserved and does not require the scattering at defect sites. The 2D peak position is sensitive to the excitation wavelength and the graphene layer, and therefore its band shape can be used to identify the number of graphene layers.3,4,12,13 When graphene is placed on a substrate, the substrate-graphene interaction may modify the band structure. In the case of weak interactions with substrates such glass, sapphire and ITO, the graphene Raman spectra show slight peak shifts.14-19 In this report, a case of a metal substrate was investigated with a view to study metal-graphene hybrid structures. Substrates made of gold were chosen and discussed in the context of surface enhanced Raman scattering (SERS). In general, two mechanisms are considered to be important in the SERS phenomena.20 The first is a chemical mechanism (CM) where a new resonance state is generated through a charge transfer between the substrate and adsorbate. The second is an electromagnetic mechanism (EM) in which a local electric field is greatly enhanced in certain noble metal nanostructures, such as sharp shapes or gaps (so-called “hot spots”). Since the graphene is in the form of a 2-dimensional sheet, the metal substrate in the form of a 2-dimensional film is first considered. In a flat gold film, one may expect a charge transfer between graphene-gold and an enhancement of Raman peaks through the CM. In contrast, the EM is expected to be relatively small because of lack of hot spots. The EM can be activated when noble metal nanoparticles are placed on the flat gold film. In this case, the enhanced local field is generated at the contact areas of the two structures by coupling the localized surface plasmon of the nanoparticles with the surface plasmon polariton of the flat film.21-24 Another variety of 2-dimensional structure is a close-packed nanoparticle layer, which can be obtained by compressing nanoparticles, after collected at the water-nonpolar liquid interface, with Langmuir-Blodget type barriers. The substrates produced in this way have proven effective SERS substrates,25-28 as a high density of enhanced local electric field is generated at the contact 1000 Bull. Korean Chem. Soc. 2010, Vol. 31, No. 4 Nayoung Kim et al. 2D G D 2D D NS-g-NS g-NS NS-g-Film g-Film g-SI (a) λex = 514 nm 5000 counts G (b) λex = 633 nm 50000 counts 1000 1500 2000 2500 3000 Raman Shift (cm‒۱ ) Intensity (a) λex = 514 nm (b) λex = 633 nm Figure 1. Raman spectra of a graphene sheet in five different substrates. (a) and (b) are the cases for the two excitation wavelengths. All spectra are displayed without any data manipulation, such as background subtraction or intensity attenuation. The sample identity of each spectrum is indicated in the inserted window. Note that the order of the samples in the inserted window is the same as the order of background plateaus in the spectral series. points between the nanoparticles. Interestingly, the SERS intensity increased as the number of closed-packed nanoparticle layers increased, which has been interpreted as an effective interlayer plasmon coupling, in addition to the intralayer plasmon coupling