In this article, we use a combination of micro-Raman and microphotoluminescence measurements to demonstrate that rubrene single crystals, which appear as yellow platelets and red needles, are two distinct orientations of the same crystallographic unit cell. As confirmed by X-ray diffraction and polarization microscopy, we show that red and yellow crystals represent the crystallographic orientations ( 020) and ( 002), respectively, with the crystallographic c-axis being the optically activated orientation in red crystals and the b-axis in yellow crystals. Raman measurements of red crystals are characterized by a pronounced Raman shift at 217 cm −1 , which to the best of our knowledge is the first report of this peak at its predicted spectral position. The Raman and photoluminescence spectra obtained from yellow crystals change abruptly at their borders into the spectra of red crystals, confirming the distinct spectroscopic properties of each crystal orientation. The combination of Raman and photoluminescence measurements applied at different focal planes within red crystals reveals that the 217 cm −1 peak can be classified as a surface active mode.

Understanding the mechanisms of surface‐enhanced Raman scattering (SERS) phenomena is essential for advancing next‐generation Raman‐based chemical and biological sensors. SERS studies typically examine the excitation wavelength dependence for a given nanostructure morphology. However, more comprehension of the SERS mechanisms can be gained by investigating the combined excitation wavelength and metal morphology dependencies. Here, using a thin Au film with tapered boundaries as the Raman enhancing surface, we investigate the SERS mechanisms exhibited along the sloped region of the sample. The tapered boundaries provide controllably different Au morphologies from densified thin film to nano‐island to nanoparticle structures. A layer of Fullerene C60 was thermally evaporated uniformly across the Au film. The excitation wavelength of 532 nm generated similar SERS spectra across the Au surface, which closely matched the regular Raman scattering spectrum, thus indicating enhancement of the normal Raman modes of C60. In contrast, an excitation wavelength of 784 nm generated different SERS spectra across the boundary whose intensity from the tapered Au film surface was 10 times higher than the SERS signal from the Au thin film region. The spectral differences observed with 784‐nm excitation are indicative of an electromagnetic mechanism controlled by near field interactions of local surface plasmon resonances. Using the configuration of tapered Au films can enable fast determination of the most enhancing Au nanostructure for a given laser excitation and provide an in‐depth insight into the enhancement mechanisms associated with a particular analyte molecule, as shown here for C60. Copyright © 2017 John Wiley & Sons, Ltd.

Raman spectroscopy is a widely used characterization technique in material science. It is a non-destructive tool with relatively simple instrumentation, and provides intrinsic qualitative information of analytes by probing their vibrational modes. In many cases, Raman enhancement is essential for detecting low-intensity signals in high-noise environments, spectrally unresolved features, and hidden modes. Here we present optical and Raman spectroscopic characterization of fullerene C 60 in a gold microcavity. The fabrication of single-layered gold mirrors is facile, low cost and direct but was proven to give considerably significant enhancement. The findings of this work demonstrate the cavity resonance as a powerful tool in obtaining tunability over individual peak for selective enhancement in the tuned spectral range. The PL of the material within the cavity has demonstrated a red shift assumed to be caused by the low-energy transitions. These transitions are induced by virtual low-energy states generated by the cavity. We further observe that adopting this principle enables resolution of active Raman modes that until now were unobserved. Finally, we assigned the new experimentally observed modes to the corresponding motions calculated by DFT.

Phase separation occurs whenever a solvent leaves a solution of strongly incompatible polymers. This can happen in bulk and in films. Films can be tailored as substrates for multiple applications such as solar cells, surface catalysis, and antireflection coatings. In this study, polystyrene (PS) was dissolved with polyvinyl acetate (PVAc) in different ratios using chlorobenzene as the solvent. Thin films of different ratios of PS and PVAc were deposited on glass via spin coating. The deposited films were investigated for their morphology, strain, surface area, and Raman scattering. The incompatibility between the two polymers leads to the growth of roughened PVAc islands supported by the PS matrix. A down shift in the Raman PVAc signal was observed in the combined film as compared with a 100% PVAc film, which was attributed to the high strain of PVAc that grew as tips. As the PVAc concentration in the polymer blend increases, the porous regions in the film expand and the amount and height of PVAc tips increase as well, up to the point where the pores merge to create a uniform surface. The optimal ratio for the deposition of a uniformly roughened surface is 75% PVAc and 25% PS. For demonstrating a possible application, we applied the partially roughened surface as a substrate for surface-enhanced Raman scattering and demonstrated at least 500% increase in the signal intensity measured in roughened areas. This is explained by the rod effect from the PVAc tips.