Observation of Size-Dependent Optical Properties Based on Surface and Quantum Effects in Nanocrystals of 5,5′-Bis(4-Biphenylyl)-2,2′-Bithiophene

Herein, 5,5 '-bis(4-biphenylyl)-2,2 '-bithiophene (BP2T or PPTTPP) nanocrystals are prepared via a modified miniemulsion technique. X-ray diffraction and selected area electron diffraction analyses reveal that the BP2T molecules stand almost upright against the crystal basal plane. With decreasing nanocrystal size, the peak energies of photoluminescence (PL) spectra are blue-shifted in the photon energy range from the bulk crystal to the monomer state. Energy-wavevector dispersion plots reproduced using the data of size-dependent PL spectra indicate that the size dependence of optical properties cannot be explained by the conventional quantum confinement effect used in inorganic nanocrystals. The micro-PL and time-resolved PL measurements of the BP2T nanocrystals reveal that the size-dependent optical properties can be attributed to a combined effect based on the surface effects as well as lattice softening, and quantum effects involving specific surface area-dependent site shift effect and exciton confinement. Moreover, amplified spontaneous emission is observed from the nanocrystals approximate to 600-1000 nm in size, which is smaller than the size previously reported for organic systems.