| Over the last several years, organic functional molecules, especially their nanomaterials have been attracting many attentions due to the significant advantages in the area of the controllable synthesis, devices, and processes which made them be an important part in building the flexible, low-cost, huge area of optoelectronic devices, for example, OLED, OSC, sensor and so on. Among the variety of candidate unit molecules for organic semiconductors materials, organic conjugated small molecules possessing π-conjugated electronic systems, easy modification, and good chemical stabilities have captured more and more interests. In particular, highly crystalline organic crystals with ideal molecular arrangement obtained from organic small conjugated molecules offer high mobility and excellent intrinsic charge-transport properties. Considering that all of the properties could be adjusted, it may be expected that concisely controllable synthesis of organic nanostructures could optimize the relevant device performance. Therefore, the research on the synthesis and property modulation of organic single crystals could occupy a significant place in designing and optimizing the nanoscale devices. Different morphologies of PTCDA,3,4,9,10perylenetetracarboxylic-dianhydride, nanostructures are prepared on AAO with ordered nanoholes and glass substrates at different substrate temperature (Ts) in a high vacuum molecular beam epitaxy (MBE) system by the organic molecule beam deposition (OMBD) method. Scanning/Transmission/Scanning transmission electron microscopy (SEM/TEM/STEM), selected area electron diffraction (SAED), powerful X-ray diffraction (XRD) and nano beam diffraction (NBD) UV-vis absorption, PL and so on techniques are employed in the systematical characterizations of the morphology, construction and optical property of the nanostructures. The results are following below:1. The different morphologies of PTCDA nanostructures prepared on AAO substrate was mainly affected by the surface curvature of AAO and the substrate temperature. The increasing diameter of the hole of AAO led to the edge and cutting-edge of the hole becomes smoother, which mean the surface curvature of AAO is decreased. The SEM results indicated that the PTCDA were not easy to nucleate on the substrate in this situation which led to the lower nucleus number. It was found that the PTCDA nanowires and nanorods were facile to produce at Ts= 380, and325℃, respectively; the continuous films were obtained at50℃.2. An L-shape PTCDA nanosheet was first produced by modulating the substrate temperature. According to the studies of sample morphologies, we found that the morphologies of PTCDA nanostructures were mainly affected by substrate temperature (Ts). As Ts increased from50to350℃, different PTCDA films and nanostructures were synthetized, including nanofilms in the low Ts range, nanorods in the middle range, nanowires and L-shape nanosheets at the high Ts. Moreover, the formation of PTCDA single-crystals nanosheets and nanowires were correlated well with the conventional VS nucleation mechanism.3. XRD studies indicated that only the a-phase polymorph is formed regardless of the Ts. SAED and NBD results showed that the nanowires and nanosheets were single crystalline. The studies of UV-vis and PL shown that the optical properties of the as-prepared PTCDA nanostructures were also affected by the substrate temperature and were correlated with the crystal size and quality. PTCDA nanowires and nanosheets exhibited an obviously redshift and broadening in the adsorption spectra, and increased emission intension of PL bands. The improved optical properties facilitated potential applications of these nanostructures in organic optoelectronic devices. |
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