Raman and photoluminescence properties of red and yellow rubrene crystals

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.