Polarized photoluminescence characteristics of rubrene sheets with nanometer thickness

We report on the temperature dependence of the polarized photoluminescence (PL) exhibited by crystalline organic rubrene sheets with a thickness of ∼60 nm. To probe the correlations between the electronic states and the molecular stacking of rubrene molecules, the polarized PL spectrum of a single rubrene sheet was analyzed as a function of temperature from 10 to 250 K. Drastic changes were observed in PL peak positions and intensities ∼100 and ∼170 K, which were likely related to variations in molecular ordering with temperature. The changes in the PL characteristics of the sheet ∼170 K are similar to those observed for rubrene bulk crystals, in terms of the enhancement of mobility and crystallinity. In addition, ∼100 K, the reversal of the relative ratio of PL intensities corresponding to two crystalline axes, a and b, in the rubrene sheets suggested an anisotropic variation in the molecular ordering along these two crystalline axes.