Size-dependent cathodoluminescence properties of 5,5′-di(4-biphenylyl)-2,2′-bithiophene nanocrystals

Abstract

Bulk and nano crystals of 5,5′-di(4-biphenylyl)-2,2′-bithiophene (BP2T) were prepared using solution-grown and improved reprecipitation methods. Agreement between the peak energies of the cathodoluminescence (CL) and photoluminescence spectra of the BP2T bulk crystal demonstrated that optical transitions occur from the lowest energy level of the singlet excited state to the singlet ground state. A high degree of crystallinity (81.0%) was observed from X-ray diffraction (XRD) measurements of the BP2T nanocrystals. Selected area electron diffraction and XRD patterns of the nanocrystals revealed that the BP2T molecules were almost upright against the crystal basal plane ((006) plane). The crystal size dependence of the CL spectra demonstrated that the 0–1 band peak energy of the CL spectrum gradually blue-shifted compared to that of the BP2T bulk crystal as the nanocrystal size decreased below 560 nm. We demonstrated that the corresponding relation between the BP2T nanocrystal sizes and CL spectra can be precisely determined using CL measurements.