Orienting Semiconducting Nanocrystals on Nanostructured Polycarbonate Substrates: Impact of Substrate Temperature on Polymorphism and In-Plane Orientation

摘要

This paper reports a systematic investigation of the growth mechanism of zinc phthalocyanine, a p-type semi-conductor, on oriented polymer substrates of bisphenol A polycarbonate (PC). It focuses on the impact of the substrate temperature on the polymorphism and the in-plane orientation of the ZnPc nanocrystals. The study combines transmission electron microscopy, X-ray diffraction, atomic force microscopy, UV-vis absorption spectroscopy, and force field-based molecular calculations. Large areas of oriented PC substrates are prepared by a simple process that combines mechanical rubbing and solvent-induced crystallization. The PC substrates show a smooth semicrystalline morphology with a preferential (a,c) surface of crystalline lamellae with a high in-plane orientation of the polymer chains. These substrates induce a unidirectional orientation of ZnPc nanocrystals with a preferred contact plane. The selection of the preferred ZnPc in-plane growth direction relies on a "self-amplified" mechanism whereby a fraction of ZnPc nanocrystals oriented by the polycarbonate substrate enforces the orientation of neighbouring domains during film growth. Regarding polymorphism, two domains of substrate temperature (T(s)) are evidenced. For 33 degrees C <= T(s) = 115 degrees C, ZnPc nanocrystals grow exclusively in the alpha form. The determined crystal structure of alpha-ZnPc (a = 1.23 nm, b = 0.38 nm, c = 1.28 nm and gamma = 96 +/- 1 degrees, Z = 1) is isomorphous to the recently refined structures of CuPc and CoPc. For 115 degrees C <= T(s) <= 200 degrees C, ZnPc films consist of both alpha and beta nanocrystals with a gradual increase of the proportion of beta-form with increasing T(s). The onset of the beta polymorph growth coincides with a marked change in the nanocrystal size. The beta phase appears when the ZnPc nanocrystals reach some critical dimensions which can be estimated from the T(s)-dependence of the nanocrystal size. The relative stability of the alpha and beta polymorphs is explained by the balance of the bulk and the surface energy of the nanocrystals. A simple model is developed to predict and force-field based molecular simulations are used to estimate the critical dimensions of the ZnPc nanocrystals.