title: Role of molecular conformations in rubrene polycrystalline films growth from vacuum deposition at various substrate temperatures.
abstract: We report on the optical and structural characterization of rubrene polycrystalline films fabricated from vacuum deposition with various substrate temperatures (Tsub). Depending on Tsub, the role of twisted and planar rubrene conformational isomers on the properties of rubrene films is focused. The temperature (T)-dependent inverse optical transmission (IOT) and photoluminescence (PL) spectra were performed on these rubrene films. The origins of these IOT and PL peaks are explained in terms of the features from twisted and planar rubrene molecules and of the band characteristics from rubrene molecular solid films. Here, two rarely reported weak-peaks at 2.431 and 2.605 eV were observed from IOT spectra, which are associated with planar rubrene. Besides, the T-dependence of optical bandgap deduced from IOT spectra is discussed with respect to Tsub. Together with IOT and PL spectra, for Tsub> 170 °C, the changes in surface morphology and unit cell volume were observed for the first time, and are attributed to the isomeric transformation from twisted to planar rubrenes during the deposition processes. Furthermore, a unified schematic diagram in terms of Frenkel exciton recombination is suggested to explain the origins of the dominant PL peaks performed on these rubrene films at 15 K.
authors.affiliation: Department of Physics and Center for Nanotechnology, Chung Yuan Christian University, Chungli District, Taoyuan City, 32023, Taiwan.
authors.forename: Ku-Yen
authors.initials: KY
authors.lastname: Lin
authors.affiliation: Department of Mechanical Engineering, Chung Yuan Christian University, Chungli District, Taoyuan City, 32023, Taiwan.
authors.forename: Yan-Jun
authors.initials: YJ
authors.lastname: Wang
authors.affiliation: Department of Physics and Center for Nanotechnology, Chung Yuan Christian University, Chungli District, Taoyuan City, 32023, Taiwan.
authors.forename: Ko-Lun
authors.initials: KL
authors.lastname: Chen
authors.affiliation: Department of Mechanical Engineering, Chung Yuan Christian University, Chungli District, Taoyuan City, 32023, Taiwan.
authors.forename: Ching-Yuan
authors.initials: CY
authors.lastname: Ho
authors.affiliation: Department of Physics and Center for Nanotechnology, Chung Yuan Christian University, Chungli District, Taoyuan City, 32023, Taiwan.
authors.forename: Chun-Chuen
authors.initials: CC
authors.lastname: Yang
authors.affiliation: Department of Physics and Center for Nanotechnology, Chung Yuan Christian University, Chungli District, Taoyuan City, 32023, Taiwan.
authors.forename: Ji-Lin
authors.initials: JL
authors.lastname: Shen
authors.affiliation: Department of Physics and Center for Nanotechnology, Chung Yuan Christian University, Chungli District, Taoyuan City, 32023, Taiwan.
authors.forename: Kuan-Cheng
authors.initials: KC
authors.lastname: Chiu