Optical film-thinning of few-layer graphene epitaxially grown on 4H-SiC(0001) surface : Robustness of monolayer-graphene against the photoexcitation.

As the properties of graphene films depend on their stacked atomic layers, their thickness should be accurately controlled to improve their specific properties. However, by existing methods, controlling the homogeneity of graphene films at the atomic level remains difficult. In this work, photo-stimulated structural modifications of few-layer graphene epitaxially grown on 4H-SiC(0001) were studied using Raman scattering spectroscopy and core-level photoemission spectroscopy. Iterative excitation with laser pulses (800nm, 100fs, p-polarized, 250mJ/cm2) changed the graphene-related 2D Raman line, which is composed of three components characterized by their different responses upon photoexcitation: two components decaying at fast and slow rates, and a component highly resistant to excitation. Core-level photoemission spectroscopy revealed that the observed decay of the 2D line was associated with the elimination of carbon atoms from the graphene layers, finally leaving the robust thin film of single-layer graphene by prolonged excitation. Therefore, this work clearly demonstrates the thickness-dependent structural stability of graphene to optical excitation and opens a promising new method for thinning graphene. An underlying mechanism for the photo-stimulated modifications was also proposed.

Chiral sum frequency spectroscopy of thin films of porphyrin J-aggregates.

Thin films of chiral porphyrin J-aggregates have been studied by vibrationally and electronically doubly resonant sum frequency generation (SFG) spectroscopy. It was revealed that the chiral supramolecular structures of porphyrin aggregates in solutions were retained in the thin film samples, and their chirality was determined by using chiral vibrational SFG spectroscopy. Electronic resonance profiles of some vibrational bands in achiral and chiral SFG were different from each other, and both were distinct from electronic absorption spectra. To account for these peculiar profiles, we have proposed interference effects of Raman tensor components in achiral and chiral SFG susceptibilities, which is analogous to that of resonance Raman scattering.