Cheng Wen Huang

Revealing anisotropic strain in exfoliated graphene by polarized Raman spectroscopy

We report on a polarized Raman study on mechanically cleaved single-layer graphene films. Under a specific orientation of scattering measurement, the width and position of the G peak change with the incident polarization direction, while the integrated intensity of that is unaltered. This phenomenon is explained by a proposed mode in which the peak is contributed by a mixture of un-, compressive-, and tensile-strained G sub-modes. The compression and tension are both uniaxial and approximately perpendicular to each other. They are undesigned and located in either separated or overlapped sub-areas within the probed local region. Compared to the unstrained wavenumber of 1580 cm(-1), compression induces a blue shift while tension causes a red one. The sub-modes correlated with the light polarization through different relationships split the G peak into three sub-ones. We develop a method to quantitatively analyze the positions, widths, intensities, and polarization dependences of sub-peaks. This analysis quantitatively reveals local strain, which changes with the detected area of a graphene film. The method presented here can be extended to detect the strain distribution in the film and thus is a promising technology for graphene characterization.

Layer-dependent morphologies of silver on n-layer graphene.

The distributions of sizes of silver nanoparticles that were deposited on monolayer, bilayer, and trilayer graphene films were observed. Deposition was carried out by thermal evaporation and the graphene films, placed on SiO2/Si substrates, were obtained by the mechanical splitting of graphite. Before the deposition, optical microscopy and Raman spectroscopy were utilized to identify the number of the graphene layers. After the deposition, scanning electron microscopy was used to observe the morphologies of the particles. Systematic analysis revealed that the average sizes of the nanoparticles increased with the number of graphene layers. The density of nanoparticles decreased as the number of graphene layers increased, revealing a large variation in the surface diffusion strength of nanoparticles on the different substrates. The mechanisms of formation of these layer-dependent morphologies of silver on n-layer graphene are related to the surface free energy and surface diffusion of the n-layer graphene. The effect of the substrate such as SiO2/Si was investigated by fabricating suspended graphene, and the size and density were similar to those of supported graphene. Based on a comparison of the results, the different morphologies of the silver nanoparticles on different graphene layers were theorized to be caused only by the variation of the diffusion barriers with the number of layers of graphene.

Observation of strain effect on the suspended graphene by polarized Raman spectroscopy

We report the strain effect of suspended graphene prepared by micromechanical method. Under a fixed measurement orientation of scattered light, the position of the 2D peaks changes with incident polarization directions. This phenomenon is explained by a proposed mode in which the peak is effectively contributed by an unstrained and two uniaxial-strained sub-areas. The two axes are tensile strain. Compared to the unstrained sub-mode frequency of 2,672 cm−1, the tension causes a red shift. The 2D peak variation originates in that the three effective sub-modes correlate with the light polarization through different relations. We develop a method to quantitatively analyze the positions, intensities, and polarization dependences of the three sub-peaks. The analysis reflects the local strain, which changes with detected area of the graphene film. The measurement can be extended to detect the strain distribution of the film and, thus, is a promising technology on graphene characterization.

Fluorescence quenching due to sliver nanoparticles covered by graphene and hydrogen-terminated graphene

Fluorescence quenching effects on graphene or hydrogen-terminated graphene covered sliver nanoparticles are studied and the results are explained with energy transfer models. The fluorescence signal of R6G is suppressed by the graphene flakes via Forster resonance energy transfer and by the silver nanoparticles via surface energy transfer. The relative fluorescence intensities of R6G are reduced to 28% and 69% on the single-atom-thick graphene flake and the hydrogen-terminated graphene covered silver film, respectively. The mechanism of the quenching effect is illustrated by the energy diagram of electron transition.

Surface-enhanced Raman scattering of suspended monolayer graphene

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.PACS78.67.Wj (optical properties of graphene); 74.25.nd (Raman and optical spectroscopy); 63.22.Rc (phonons in graphene)

Probing 2D sub-bands of bi-layer graphene

Investigations of Raman spectra and surface enhanced Raman spectra (SERS) of supported and suspended bilayer graphene were realized. The ability of SERS to greatly enhance the Raman signals is regarded as useful tools, but the dopants induced by the metallic nanoparticle deposition may affect the electron scattering processes. The four 2D sub-bands of bilayer graphene are associated with four-step Stokes–Stokes double-resonance Raman electron scattering processes and can be analyzed by fitting the corresponding Raman peak with multi-Lorentzian functions. To extricate the dopant effect of the substrate from one of metallic nanoparticles, suspended graphene is adopted here. The enhancements of SERS over Raman spectra are also calculated. For the supported graphene, the SERS enhancement factors of the sub-bands obey 2D22 < 2D11 < 2D21 < 2D12 and exhibit an integrated intensity that is proportional to the Raman cross-section. For suspended graphene, such factors obey 2D11 < 2D12 < 2D21 < 2D22, with 2D12, 2D21, and 2D22 being close to each other because the rate of the scattering for some processes decreases and contributes to another process in the integrated area of the Raman signals when the decays happen. The reason for the factor difference is discussed through the presented analysis of the Raman and SERS signals of supported and suspended bilayer graphene.

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 I2D/IG 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 I2D/IG 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.

The localized surface plasmon resonance sensor based on tapered optical fiber modified with gold nanoparticles

LSPR sensor based on tapered optical fiber for refractive index sensing has been analyzed. The result of index sensitivity is about 5×10-5 RIU. Such sensor resolution is comparable to those of the current LSPR sensor.

Scrutinizing graphene with polarized Raman spectroscopy

The authors report polarized Raman measurement of single-layer graphene. The G peak position shows polarization dependence, because of the stress on the sample. The relation between the local stress and the polarization will be discussed.

Optical switching of porous anodic aluminum oxide films embedded with silver nanoparticles

Dielectric films embedded with two-dimensional (2-D) arrays of metal nanoparticles have been fabricated by electro deposition of silver into nanopores of anodic aluminum oxide (AAO) films. Their pore diameter and inter-pore spacing are 80 nm and 20 nm, respectively. With its photoconductivity, the specimen has a higher conductance when being illuminated by a 514 nm wavelength laser. This photoconductivity can be explained by improved electron transport due to plasmonic coupling among neighboring silver nanoparticles induced by light illumination. The 2-D photoconductivity is expected to enable applications which are not suitable for one-dimensional photoconductivity induced by plasmonic coupling of nanoparticles embedded in nanowires. The 2-D photoconductivity of metal-nanoparticle-embedded dielectric films are expected to facilitate optoelectronic integrated circuits in the future.