The thermodynamic characteristics of organic crystal growth by physical vapor transport: towards high-quality and color-tunable crystal preparation
In this work, several organic crystals of anthracene (Ac), tetracene (Tc), pentacene (Pc) and 2,5-diphenyl-1,4-distyrylbenzene with two trans double bonds (DPDSB) have been prepared by the physical vapor transport (PVT) method. The structural characteristics of these crystals are investigated by atomic force microscopy (AFM) and X-ray diffraction (XRD). In the PVT method, the growth temperature influences the kinetic energy of molecules and the disturbance of crystal lattices caused by intermolecular interactions during the crystal formation process. The crystal grown at the lower temperature has a smaller monolayer thickness and a narrower full width at half maximum (FWHM) calculated from diffraction peak degrees given by XRD data, which indicates that the molecules are more orderly arranged, compared with the crystal grown at the higher temperature. Based on the crystal growth thermodynamic characteristics in vapor, Tc doped Ac crystals with partial energy transfer (sky-blue emission) and nearly complete energy transfer (green emission) have been prepared corresponding to the low temperature and high temperature growth conditions, respectively. The photoluminescence efficiencies of Tc doped Ac crystals with sky-blue emission and green emission are ~40% and ~74%, the energy transfer efficiencies are 63.8% and 98.9%, respectively. Our results are relevant to the understanding of organic crystal growth thermodynamic behaviors and guide us to the design and preparation of color-tunable emission and high-luminescence efficiency organic crystals that are expected to be of interest for organic light emitting transistors, diodes and lasers.