MODULATION OF PTCDA NANOSTRUCTURE AND OPTICAL PROPERTY: DEPENDENCE ON GROWTH TEMPERATURE.

3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) nanostructures with different morphologies are prepared on glass substrates at different substrate temperature (Ts) in a molecular beam epitaxy (MBE) system. Scanning/transmission/scanning transmission electron microscopy (SEM/TEM/STEM), X-ray diffraction (XRD), selected area electron diffraction (SAED) and nanobeam diffraction (NBD) techniques are employed in the systematical characterizations of the nanostructures. It is found that the PTCDA nanosheets (NSs), nanowires and nanorods are facile to produce at Ts = 350°, 330°C and 240°C, respectively; the continuous films are obtained at 180°C and 50°C. XRD studies indicate that only the α-phase polymorph is formed regardless of the Ts. SAED and NBD results show that the nanowire and NS are single crystalline. The optical properties of the prepared PTCDA nanostructures are also found to be influenced by Ts and are correlated with the crystal quality and size. PTCDA nanowires and NSs exhibit an obvious redshift and broadening in the adsorption spectra, and enhanced emission intensity. The improved optical properties facilitate potential applications of these nanostructures in organic optoelectronic devices. With the transformation from multi-crystalline film to 1D nanostructures, the crystal texture of PTCDA enhances greatly. The improved structural quality leads to a reduced distance between the oppositely charged stacked molecules on the microscopic scale, and thus a favorite formation of the self-trapped charge-transfer (CTST) excitons. Simultaneously, the transition probabilities from the ground state to CT and CTST states are enhanced, while the mixed F-CT states are attenuated with the evolutions of PTCDA morphology. The observed redshift and broadening of the optical adsorption band as well as its intensified emission accompanying the crystalline quality increasing of PTCDA nanostructures suggest potential applications of these nanostructures in relevant organic optoelectronic devices. [ABSTRACT FROM AUTHOR]