Polarization dependence of double resonant Raman scattering band in bilayer graphene

The polarization dependence of the double resonant Raman scattering (2D) band in bilayer graphene (BLG) is studied as a function of the excitation laser energy. It has been known that the complex shape of the 2D band of BLG can be decomposed into four Lorentzian peaks with different Raman frequency shifts attributable to four individual scattering paths in the energy–momentum space. From our polarization dependence study, however, we reveal that each of the four different peaks is actually doubly degenerate in its scattering channels, i.e., two different scattering paths with similar Raman frequency shifts for each peak. We find theoretically that one of these two paths, ignored for a long time, has a small contribution to their scattering intensities but are critical in understanding their polarization dependences. Because of this, the maximum-to-minimum intensity ratios of the four peaks show a strong dependence on the excitation energy, unlike the case of single-layer graphene (SLG). Our findings thus reveal another interesting aspect of electron–phonon interactions in graphitic systems.     

The electrical properties of graphene modified by bromophenyl groups derived from a diazonium compound

Graphene field-effect transistors were fabricated with mechanically exfoliated single-layer graphene (SLG) and bilayer graphene (BLG) sheets and the functionalization effects of bromophenyl groups derived from a diazonium compound on its transfer properties were explored. Spectroscopic and electrical studies reveal that the bromophenyl grafting imposes p-doping to both SLG and BLG. The modification of SLG by bromophenyl groups significantly reduces the hole carrier mobility and the saturation current in SLG transistors, suggesting an increase in both long-range impurity and short-range defect scattering. Unexpectedly, the bromophenyl group functionalization on BLG does not obviously increase both types of scattering, indicating that the BLG is relatively more resistant to charge- or defect-induced scattering. The results indicate that chemical modification is a simple approach to tailor the electrical properties of graphene sheets with different numbers of layers.     

The origin of sub-bands in the Raman D-band of graphene

In Raman spectroscopy investigations of defective suspended graphene, splitting in the D band is observed. Four double resonance Raman scattering processes: the outer and inner scattering processes, as well as the scattering processes with electrons first scattered by phonons (“phonon-first”) or by defects (“defect-first”), are found to be responsible for these features of the D band. The D sub-bands associated with the outer and inner processes merge with increasing defect concentration. However a Stokes/anti-Stokes Raman study indicates that the splitting of the D band due to the separate “phonon-first” and “defect-first” processes is valid for suspended graphene. For graphene samples on a SiO₂/Si substrate, the sub-bands of D band merge due to the increased Raman broadening parameter resulting from the substrate doping. Moreover, the merging of the sub-bands shows excitation energy dependence, which can be understood by considering the energy dependent lifetime and/or scattering rate of photo-excited carriers in the Raman scattering process.     

Surface-enhanced Raman scattering of suspended monolayer graphene

Abstract

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 I_2D/I_G 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 I_2D/I_G 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.

PACS

78.67.Wj (optical properties of graphene); 74.25.nd (Raman and optical spectroscopy); 63.22.Rc (phonons in graphene)     

Role of residual polymer on chemical vapor grown graphene by Raman spectroscopy

Although various applications extensively utilize polymer-assisted graphene transfer step, the role of residual polymer on graphene was not well-understood. Here, we report the effect of poly (methyl methacrylate) (PMMA) on chemical vapor deposition-grown hexagonal graphene via Raman spectroscopy. Analysis of bare-, PMMA-covered supported, and PMMA-covered suspended graphene exhibits that their G and 2D band positions are progressively downshifted in that order. Mapping of spatial G and 2D band shifts into doping and strain contributions shows that PMMA residue exerts moderate 0.15% tensile strain on graphene/substrate, as compared to that of bare graphene. During this tensile strain, residual PMMA-covered graphene maintains its doping level as much as bare graphene does.     

Behavior of the high frequency Raman modes of double-wall carbon nanotubes after doping with bromine or iodine vapors

The effects of two different halogen dopants (bromine and iodine) at different concentrations on the higher frequency modes (the so-called G and G′ bands) of the Raman spectra of double-wall carbon nanotube (DWCNT) “buckypaper” are investigated. The effects of dopants on different DWCNT configurations (metallic inner/semiconducting outer and vice versa) are studied by changing the laser excitation energy. The doping causes the loss of the Breit–Wigner–Fano line shape and the appearance of less metallic behavior. An increase of the relative intensity of the G⁺ band, which is more sensitive for the outer metallic tubes, is clearly observed with increasing Br₂ concentration in the sample. By analysis of the G⁺ band and the G′ band it is possible to measure the changes in the electron–phonon coupling, due to the charge-transfer between the dopant (Br₂ or I₂) and the tubes in the DWCNT. The doping effect causes an upshift of the G⁺ band and a suppression of the contribution of the inner tubes to the G′ band signal and as a consequence, the observed G′ band is dominated by the contribution from the outer tubes.     

Layer-dependent nanoscale electrical properties of graphene studied by conductive scanning probe microscopy

The nanoscale electrical properties of single-layer graphene (SLG), bilayer graphene (BLG) and multilayer graphene (MLG) are studied by scanning capacitance microscopy (SCM) and electrostatic force microscopy (EFM). The quantum capacitance of graphene deduced from SCM results is found to increase with the layer number (n) at the sample bias of 0 V but decreases with n at -3 V. Furthermore, the quantum capacitance increases very rapidly with the gate voltage for SLG, but this increase is much slowed down when n becomes greater. On the other hand, the magnitude of the EFM phase shift with respect to the SiO₂ substrate increases with n at the sample bias of +2 V but decreases with n at -2 V. The difference in both quantum capacitance and EFM phase shift is significant between SLG and BLG but becomes much weaker between MLGs with a different n. The layer-dependent quantum capacitance behaviors of graphene could be attributed to their layer-dependent electronic structure as well as the layer-varied dependence on gate voltage, while the layer-dependent EFM phase shift is caused by not only the layer-dependent surface potential but also the layer-dependent capacitance derivation.     

Probing substrate influence on graphene by analyzing Raman lineshapes

We provide a new approach to identify the substrate influence on graphene surface. Distinguishing the substrate influences or the doping effects of charged impurities on graphene can be realized by optically probing the graphene surfaces, included the suspended and supported graphene. In this work, the line scan of Raman spectroscopy was performed across the graphene surface on the ordered square hole. Then, the bandwidths of G-band and 2D-band were fitted into the Voigt profile, a convolution of Gaussian and Lorentzian profiles. The bandwidths of Lorentzian parts were kept as constant whether it is the suspended and supported graphene. For the Gaussian part, the suspended graphene exhibits much greater Gaussian bandwidths than those of the supported graphene. It reveals that the doping effect on supported graphene is stronger than that of suspended graphene. Compared with the previous studies, we also used the peak positions of G bands, and I_2D/I_G ratios to confirm that our method really works. For the suspended graphene, the peak positions of G band are downshifted with respect to supported graphene, and the I_2D/I_G ratios of suspended graphene are larger than those of supported graphene. With data fitting into Voigt profile, one can find out the information behind the lineshapes.     

Electronic structure study of ordering and interfacial interaction in graphene/Cu composites

Graphene CVD-grown on Cu has been studied using Raman spectroscopy, X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES). Raman data indicate the presence of weak compressive strain at the interface of graphene/Cu. Compared with highly ordered pyrolytic graphite (HOPG), new electronic states in the conduction band are observed for graphene/Cu, which are mainly ascribed to the defect states and interfacial interaction between the single graphene layer and Cu surface. Moreover, polarization dependent XAS measurements demonstrate that the graphene/Cu exhibits a high degree of alignment and weak corrugation on the surface. Significant intensity modulation in the resonant XES spectral shape upon different excitation energies near the C K-edge indicates that graphene layer preserves an intrinsic momentum as that of HOPG and the interaction between graphene and Cu shows weak influence on the valence band structure of graphene. However, broad inelastic features and subtle peak shifts are observed in the resonant XES spectra of graphene/Cu in comparison of HOPG, which can be mainly attributed to the electron–phonon scattering and charge transfer from the interfacial interaction of graphene and Cu substrate.     

Raman spectroscopy of morphology-controlled deposition of Au on graphene

By tuning substrate temperatures in a thermal deposition process, Au nanostructures with different morphologies, such as polygons, dendrites, irregular islands and dense clusters have been obtained on graphene surface. The surface-enhanced Raman scattering (SERS) of graphene caused by gold decoration is systematically investigated. The enhancement factor of graphene G band and the extent of G band splitting are found to be dependent on the morphologies of gold clusters. A maximum enhancement factor as high as ∼270 is obtained for polygonal gold film deposited on monolayer graphene. Furthermore, as a SERS substrate, graphene combined with polygonal gold shows the highest Raman enhancing efficiency for crystal violet (CV) molecules. The mechanisms for the above results are discussed.     

Evolution of Raman spectra in nitrogen doped graphene

We present systematical Raman studies of nitrogen doped graphene (NG). Defective graphene by Ar⁺ ion bombardment was also studied for comparison. It was found that the defects/nitrogen dopants in NG are not homogenous. Our results also suggest that the G peak position and I_2D/I_G ratio cannot be simply used as fingerprint of doping concentration in NG. Both doping and compressive strain (as verified by transmission electron microscope) contribute to the shift of Raman peaks, while both doping and lattice defects contribute to the attenuation of 2D peak. Finally, the nature of defects in NG was probed and found that they are boundary defects. The detail analysis of the evolution of Raman spectra in NG would greatly help on the characterization and future application of this novel material.     

Influence of N-doping on the structural and photoluminescence properties of graphene oxide films

Nitrogen (N) was doped into graphene oxide (GO) films at temperatures of 600–900 °C under the flow of a mixture of NH₃ and Ar. The N (atomic) concentration was varied in the range of 3.63–7.45%. XPS and FTIR spectra show that there are mainly single C–N and double CN bonds in the GO sheet. Raman spectra indicate that the G band becomes closer to the position of the G band of graphite with increasing doping temperature, and thus reveal that N doping produces a blue-shift of the G-band. In room-temperature photoluminescence (PL) spectra, N-doping produces an increase not only in the overall PL intensity, but also in the wavelength of the peak maxima. The shift of the induced PL of N-doped graphene is attributed mainly to the increased number of graphitic (or quaternary) N.     

Doublet of D and 2D bands in graphene deposited with Ag nanoparticles by surface enhanced Raman spectroscopy

Surface enhanced Raman spectrum of graphene was obtained by modifying graphene with Ag nano-particles. The doublet of D band and 2D band was observed due to surface enhanced Raman scattering (SERS) enhancement and relaxation of the selection rules originated from the interaction of Ag nano-particles and graphene. The difference in the doublet of D band in which the separation between the two peaks is 11 cm⁻¹ is close to the theoretical value of 9 cm⁻¹ and can be attributed to the disorder and edge effect. The doublet of 2D band can be assigned to the asymmetry of dispersion relation along KM and KГ respectively. The phonon mode at 1510 cm⁻¹ can be associated with the iTO phonon near ГK/4. This confirms the phonon dispersion based on double resonance (DR) theory in iTO branch.     

Characterizing intrinsic charges in top gated bilayer graphene device by Raman spectroscopy

In this work we study the behavior of the optical phonon modes in bilayer graphene devices by applying top gate voltage, using Raman scattering. We observe the splitting of the Raman G band as we tune the Fermi level of the sample, which is explained in terms of mixing of the Raman (E_g) and infrared (E_u) phonon modes, due to different doping in the two layers. We theoretically analyze our data in terms of the bilayer graphene phonon self-energy which includes non-homogeneous charge carrier doping between the graphene layers. We show that the comparison between the experiment and theoretical model not only gives information about the total charge concentration in the bilayer graphene device, but also allows to separately quantify the amount of unintentional charge coming from the top and the bottom of the system, and therefore to characterize the intrinsic charges of bilayer graphene with its surrounding environment.     

Synthesis of sulfur-doped p-type graphene by annealing with hydrogen sulfide

Doping is an important method to modulate the electronic properties of graphene. Among various types of doped graphene, sulfur-doped graphene is expected to have a wider band gap due to the electron-withdrawing character of sulfur. However, it is difficult to dope graphene with S because S atom is much larger than C atom. In this paper, S-doped graphene is synthesized by a simple method with hydrogen sulfide annealing. It is confirmed by high-resolution transmission electron microscopy diffraction and Raman spectra that S-doping in graphene is surface adsorption doping forming carbon–sulfur compound crystal domains. We are also noted that the doping intensity is affected by annealing time indicating the doping process is controllable. Electrical measurements show that sulfur plays an acceptor role in S-doped graphene leading to a p-type behavior, and after sulfur-doping, graphene exhibits higher resistance and larger on/off ratio.     

Enhanced Fluorescence and Local Vibrational Mode in Near‐White‐Light‐Emitting ZnO:Mg Nanorods System

We report exciton and phonon properties of ZnO:Mg nanorods of different Mg doping concentration. X‐ray diffraction studies (XRD) confirm the growth of wurtzite phase ZnO nanostructures. XRD reveals doping‐induced shift in peaks and formation of secondary phase related to Mg. Optical properties of the prepared nanorods are investigated by using UV‐Visible absorption and photoluminescence spectroscopic techniques. Optical absorption studies show strong free excitonic absorption of ZnO and extra absorption bands related to the defect centers of the secondary phase (MgO) formed after Mg doping. Photoluminescence studies show sharp band in UV region and defects‐related broad band emission in the visible range. Gaussian‐fitted photoluminescence spectra show that the emission is composed of free exciton recombination and its longitudinal optical (LO) phonon replica. In addition, Mg‐related local vibrational mode observed in Raman and FTIR spectra after Mg doping, indicates the incorporation of Mg into the lattice positions of wurtzite ZnO.     

Role of extended defected SiC interface layer on the growth of epitaxial graphene on SiC

An extended layer of defected SiC has been observed in SiC subjected to heat treatments at 850 and 1050 °C prior to growth of graphene by thermal decomposition. This layer is found to strongly affect the graphene thickness, surface morphology, and Raman spectrum of graphene grown on it. By comparing the strength of the XPS signal associated with this layer it was found that the samples with stronger defected layer signal had the least number of surface pits but also showed the increase in Raman D to G band ratio. The shifts in 2D and G peaks are associated with varying amounts of strain and unintentional doping induced by the SiC defected interface layer, respectively.     

The transformation of a gold film on few-layer graphene to produce either hexagonal or triangular nanoparticles during annealing

The shape transformation of gold directly on graphene has been well studied by thermally annealing gold-deposited graphene samples at the temperature range from 600 to 800 °C. We find that few-layer graphene can be served as a platform to transform a gold film into mainly hexagonal gold nanoparticles (AuNPs) at 600 or 700 °C, or coexistence of hexagonal and triangular AuNPs at 800 °C. Especially, the size and density of these AuNPs are dependent on the number of graphene layers, indicating the strong relationship between gold shape transformation and the number of graphene layers on the substrate. We propose that annealing-induced growth of gold islands and the layer-dependent interactions among Au and n-layer graphene are the two main causes for this shape transformation. Meanwhile, Raman enhancing effects of these AuNPs are also investigated. These faceted AuNPs exhibit excellent SERS effects on Raman spectra of few-layer graphene with the enhancement factors up to several hundreds. Combined with n-layer graphenes, these faceted AuNPs can be used as graphene-based SERS substrates for increasing Raman signals of adsorbed rhodamine 6G molecules with a larger scale than those based on fresh graphenes.     

Directly observable G band splitting in Raman spectra from individual tubular graphite cones

Individual tubular graphite cones (TGCs) are characterized using Raman spectroscopy at room temperature. A split G band at 1569–1587 cm⁻¹ is directly observed from the root to the tip of TGCs without requiring surface electromagnetic and chemical enhancement effects. The G band can be deconvoluted into two Lorentzian peaks at 1571 ± 2 and 1584 ± 3 cm⁻¹, which may be attributed to the resonance enhancement of a single chirality excitation in the innermost tubes and all constituent tubes of the TGCs, respectively. Results suggest that the splitting of G band can be envisaged as a fingerprint for monochirality of tubular carbon nanomaterials.     

Raman spectroscopy and band structure of Pd-hybridized multilayer graphene

Graphene sheets prepared through liquid exfoliation of expanded graphite were hybridized with Pd nanoparticles. The impact of these particles on the electronic and physical structure of the graphene is determined through transmission electron microscopy and Raman spectroscopy using 532 and 325 nm excitation wavelengths. Based on the changes to the Raman D and G peaks, insights are provided concerning the deposition mechanism at pristine and defective lattice sites, as well as electronic scattering. These data are compared to ab initio band structure computations. For purposes of the model, the graphene/Pd hybrid was approximated by a charged graphene sheet. The resulting structure exhibited π–π¹ expansion approaching the Γ point of the Brillouin zone which was validated by tracking the Raman D band dispersion.