Interactions Between Epitaxial Graphene Grown on the Si- and C-Faces of 4H-SiC Investigated Using Raman Imaging and Tip-Enhanced Raman Scattering.

Interactions between epitaxial graphene grown on Si- and C-faces were investigated using Raman imaging and tip-enhanced Raman scattering (TERS). In the TERS spectrum, which has a spatial resolution exceeding the diffraction limit, a D band was observed not from graphene surface, but from the edges of the epitaxial graphene ribbons without a buffer layer, which interacts with SiC on the Si-face. In contrast, for a graphene micro-island on the C-face, the D band disappeared even on the edges where the C atoms were arranged in armchair configurations. The disappearance of the edge chirality via combination between the C atoms and SiC on the C-face is responsible for this phenomenon. The TERS signals from the C-face were weaker than those from the Si-face without the buffer layer. On the Si-face with a buffer layer, the graphene TERS signal was hardly observed. TERS enhancement was suppressed by interactions on the edges or by the buffer layer between the SiC and graphene on the C- or Si-face, respectively.

Local structural changes in graphene oxide layers induced by silver nanoparticles.

Structural changes of graphene oxide (GO) in silver/graphene oxide (AGO) nanocomposites are investigated using tip-enhanced Raman scattering (TERS). Because of markedly high spatial resolution of the TERS technique, the measurements of molecular information at specific nano-scaled positions can be achieved by constructing line-profile TERS spectra straight from the center of silver nanoparticles (AgNPs) on GO layers. The results show evidences that AgNPs cause shortening of C-C bonds beneath AgNPs, flattening of GO layers, and critical bending on GO layers. Additionally, a connection of carbon atoms via a C-C network subsequently expands structural changes with a distance of 200-250 nm from the center of AgNPs, even though this distance is larger than the size of AgNPs. The proposed model of GO structural changes unveils new understanding about changes in properties from GO to AGO nanocomposites, which will contribute to a development of advanced nanostructures/nanocomposites in the near future.

3D SERS Imaging Using Chemically Synthesized Highly Symmetric Nanoporous Silver Microparticles.

3D surface-enhanced Raman scattering (SERS) imaging with highly symmetric 3D silver microparticles as a SERS substrate was developed. Although the synthesis method is purely chemical and does not involve lithography, the synthesized nanoporous silver microparticles possess a regular hexapod shape and octahedral symmetry. By using p-aminothiophenol (PATP) as a probe molecule, the 3D enhancement patterns of the particles were shown to be very regular and predictable, resembling the particle shape and exhibiting symmetry. An application to the detection of 3D inhomogeneity in a polymer blend, which relies on the predictable enhancement pattern of the substrate, is presented. 3D SERS imaging using the substrate also provides an improvement in spatial resolution along the Z axis, which is a challenge for Raman measurement in polymers, especially layered polymeric systems.

Characterization of SiC-grown epitaxial graphene microislands using tip-enhanced Raman spectroscopy.

Single-layer graphene microislands with smooth edges and no visible grain boundary were epitaxially grown on the C-face of 4H-SiC and then characterized at the nanoscale using tip-enhanced Raman spectroscopy (TERS). Although these graphene islands appear highly homogeneous in micro-Raman imaging, TERS reveals the nanoscale strain variation caused by ridge nanostructures. A G' band position shift up to 9 cm(-1) and a band broadening up to 30 cm(-1) are found in TERS spectra obtained from nanoridges, which is explained by the compressive strain relaxation mechanism. The small size and refined nature of the graphene islands help in minimizing the inhomogeneity caused by macroscale factors, and allow a comparative discussion of proposed mechanisms of nanoridge formation.

Tip-enhanced Raman spectroscopic measurement of stress change in the local domain of epitaxial graphene on the carbon face of 4H-SiC(000-1).

We develop a bulk silver tip for tip-enhanced Raman scattering (TERS) and obtain TERS spectra of epitaxial graphene on the carbon face of 4H-SiC(000-1) with a high signal-to-noise ratio. Thanks to the high quality of TERS spectra we firstly find that the G band in the TERS spectra exhibits position-by-position variations in both lower wavenumber shifts and spectral broadening. The analysis of the variations reveals that the shifts and broadenings have a linear correlation between each other, indicating that the variations are induced by the position dependent local stress on graphene based on a uniaxial strain model.