Abstract
Coumarin-6 polycrystalline films were fabricated from vacuum deposition at various substrate temperatures Tsub from 106 to 178 °C with a fixed source temperature of 185 °C. Because of its slenderer and more asymmetric structure, the adhered coumarin-6 molecule on top of the growing interface encounters a larger steric energetic barrier of 0.92 eV as estimated from the Arrhenius plot of growth rate versus 1/Tsub. From top-view SEM pictures, the as-deposited coumarin-6 thin films exhibit a twisted pattern and a kinematic roughness for Tsub < 150 °C; while clear facets emerge for Tsub ≥ 150 °C due to the increase of surface diffusion energy of the adhered molecules. From XRD analysis, besides the confirmation of the triclinic structure two anomalous peaks observed at 2θ ~ 9.007° and 7.260° are explained due to the co-existence of N- and S-coumarin-6-isomers within the crystalline grains. Furthermore, for coumarin-6 polycrystalline films deposited at Tsub = 150 °C with high crystallinity of the constituent grains, the bandgap determined from optical transmission is around 2.392 eV; and from photoluminescence spectra, the fitted four emission components are assigned to the Frenkel and charge transfer excitons recombination with participation of molecular vibrational states.
Highlights
The small organic molecule of 3-(2-benzothiazolyl)-7-(diethylamino)coumarin (C20H18N2O2S, named as coumarin-6 or coumarin-540)[1] has been known and utilized as one of the fluorescent dyes for staining organelles or materials used in medicine[2,3], and for high-gain medium in tunable and amplifier lasers[4,5,6]
Vacuum deposition technique was adopted to deposit coumarin-6 polycrystalline films with Tsou determined by a conventional thermogravimetric analysis (TGA) and Tsub chosen from a constitutional supersaturation (CSS) method
Because of its slenderer and more asymmetric structure accompanied by a larger permanent dipole, the adhered coumarin-6 molecule on top of the growing interface encounters a higher steric hindrance to occupy the appropriate location and correct orientation, and possesses a larger steric energetic barrier of 0.92 eV as estimated from the Arrhenius plot of growth rate versus 1/Tsub
Summary
The small organic molecule of 3-(2-benzothiazolyl)-7-(diethylamino)coumarin (C20H18N2O2S, named as coumarin-6 or coumarin-540)[1] has been known and utilized as one of the fluorescent dyes for staining organelles or materials used in medicine[2,3], and for high-gain medium in tunable and amplifier lasers[4,5,6]. Coumarin-6 molecular crystal possesses a relatively high melting point around 220 °C, a high stability of surface morphology, and an efficient light emission, and can be considered as an important organic material for optoelectronic devices. The proper values of Tsub corresponding to a fixed Tsou can be experimentally determined from the concept of constitutional supersaturation (CSS), and the effects of Tsub on the size, orientation, and crystallinity of the constituent grains and on the growth rate of various small organic molecular polycrystalline films were reported[21,22,23,24,25]. This work, though of technological importance to the fabrication of coumarin-6 thin films for applications in organic multilayer devices, is expected to clarify some fundamental problems of crystallization mechanisms, weak intermolecular interactions, and surface kinematic effects for the production of organic molecular solids
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
