ABSTRACT
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.
ABSTRACT
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.
ABSTRACT
A simple one-step process was adopted to fabricate anisotropic gold nanoassemblies. Evaporation-assisted nanoparticle assembly at the meniscus was maintained in this regard. Scanning electron microscopy confirmed that the constituent nanoparticles of the anisotropic gold nanoassembly are neither in physical contact nor agglomerated; instead, they are separated by a small interparticle gap. Crystal violet was adsorbed on the anisotropic gold nanoassembly, and a large enhancement (several orders of magnitude) in surface-enhanced Raman scattering (SERS) was observed. The assembly was thus proved to be highly SERS-active. Such an anisotropic gold nanoassembly allowed polarization dependent and polarization selective SERS experiments to be carried out. Inhomogeneous SERS and surface plasmon resonance distribution were observed along the assembly. Polarization-dependent SERS enhancement reached its highest value at in-plane polarization to the long axis, whereas polarization-selective SERS characteristics at the same spatial position showed uniform enhancement. Fluorescence emission accompanied with SERS signals was also characterized. Polarization-dependent fluorescence was enhanced at in-plane polarization to the long axis, whereas polarization-selective fluorescence was not enhanced. The experimental results were correlated and explained with three-dimensional finite definite time domain simulations as well. Such interstitial-limited gold nanoassembly provides means to realize polarized SERS characteristics for ensemble SERS measurements, which are important not only for the application-oriented fabrication of SERS-active substrates but also for understanding their feasibility in those applications.
ABSTRACT
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.
Subject(s)
Carbon Compounds, Inorganic/chemistry , Graphite/chemistry , Silicon Compounds/chemistry , Microscopy, Atomic Force , Signal-To-Noise Ratio , Silver/chemistry , Spectrum Analysis, RamanABSTRACT
This paper reports the characteristics of surface-enhanced Raman scattering (SERS) and surface-enhanced fluorescence (SEF) using a unique SERS-active substrate comprised of a single layer and a double layer of two-dimensional (2D) gold nanostructure. Colloidal gold nanoparticles were immobilized on a glass substrate and a multi-purpose experimental setup was adopted to obtain surface plasmon resonance (SPR), SERS and SEF on a single platform. Inhomogeneous intensity distribution was observed in correlated images of SPR and SERS. Several laser lines were used as excitation sources for further SERS measurements. Higher SERS intensities were observed with longer wavelength excitations at the same spatial position. Fluorescence measurements were carried out using 514 nm line and SEF images were obtained using the same sample. Fluorescence emissions were found to be enhanced in the presence of 2D gold nanostructure. A series of SERS spectra were recorded by conducting ensemble SERS measurements at a periodic interval of 2 microm, crossing bare substrates, the single layer and the double layer of gold nanostructure. The double layer provides higher enhancement in SERS than that of the single layer. Polarization-selective SERS measurements obtained at the single layer and double layer showed a clear difference in their dispersions. SERS intensities of the analytes adsorbed at the single layer were fitted well with cos(4)theta dependence; however, for the double layer, the relationship was quite uncertain.
