para-Sexiphenyl Ultrathin Film and Weak Epitaxy Growth

有机半导体薄膜的形态结构对器件性能具有非常重要的影响，近年来高质量有机半导体薄膜的制备成为有机半导体薄膜器件研究的核心内容之一。弱取向外延生长能够获得类单晶迁移率的高质量有机半导体薄膜。而制备出高取向、大尺寸、低缺陷、连续的高质量外延生长基底是获得类单晶外延薄膜的前提条件，这样就需要深入了解它们的生长行为、生长机理及薄膜相态特性。因此，高质量外延基底生长和弱取向外延生长行为及机理的研究就具有十分重要的理论价值和实用价值，是选择和扩展弱取向外延生长材料体系的基础。由此，本论文中工作主要分为两部分，第一部分工作是对外延基底六联苯超薄膜生长的研究：一方面生长出大尺寸、高取向、连续的超薄膜为弱取向外延生长提供高质量的外延基底，另一方面丰富和发展了有机半导体薄膜生长理论。第二部分工作是对酞菁化合物在六联苯超薄膜上弱取向外延生长行为和机理的研究，为弱取向外延材料体系的选择与扩展提供实验依据和理论指导。 首先，研究了六联苯超薄膜生长行为。研究结果表明：(1) 六联苯超薄膜在高温(＞60 oC)和低温(≤60 oC)的二氧化硅(SiO2)基底上生长具有不同的生长机理：高温时薄膜生长符合扩散受限凝聚生长(DLA)机理，低温时薄膜生长是由低有序薄膜经过亚稳分解后重组向团状岛转变。(2) 六联苯超薄膜的生长行为和相结构表明单层和双层薄膜是两种不同的相态：单层薄膜是高取向的具有液晶特性的薄膜相态，双层薄膜是长程有序的近似体相β-phase结构的结晶相。(3) 通过优化基底温度和生长参数，可以制备出高取向、大尺寸、连续的六联苯超薄膜，即可以为酞菁化合物的弱取向外延生长提供高质量的外延基底。 然后，以平面型自由酞菁(H2Pc)及酞菁锌(ZnPc)和非平面型酞菁氧钒(VOPc)为例，深入研究了酞菁化合物在六联苯超薄膜上的弱取向外延生长行为及机理。研究结果表明：(1) 弱取向外延生长的酞菁分子在六联苯超薄膜上立着生长，p-p共轭的方向平行于基底，同时酞菁分子在薄膜平面内具有规则的取向织构。这种高取向的酞菁化合物薄膜有利于载流子在薄膜平面内的传输，其迁移率达到了相应的单晶水平。(2) 由于六联苯双层及单层薄膜结构和相态的差别，平面型酞菁化合物表现出不同的外延生长行为：在六联苯双层薄膜上生长的酞菁化合物薄膜在薄膜平面内只有一种取向，对应于有公度外延生长(Commensurate Epitaxy)；在六联苯单层薄膜上生长的酞菁化合物薄膜在薄膜平面内有三种取向，同时兼具有公度外延生长和无公度外延生长(Incommensurate Epitaxy)。但非平面型VOPc由于分子排列方式及三斜晶体结构的本质，在六联苯单层及双层薄膜上都只表现出无公度外延生长。(3) 六联苯(001)晶面上突起的氢原子所形成的[110]、[1-10]和[010]沟道对酞菁分子具有强烈的预取向作用，从而形成取向的单分子柱晶核。然后在晶格匹配效应和基底沟道效应下，分别形成有公度外延生长和无公度外延生长。即晶格匹配关系和基底沟道效应为寻找弱取向外延生长有机半导体材料体系提供了理论指导依据。The morphology and structure of organic semiconductor thin film have important influence on device properties. The growth of high-quality organic semiconductor thin film is one of the core subjects in organic semiconductor area. High-quality organic semiconductor thin film, which the field-effect mobilities are up to the same level as the corresponding single crystals, can be fabricated by weak epitaxy growth (WEG). Hence, it is very valuable to study the growth of high-quality epitaxial substrate and the WEG behavior and mechanism in both fundamental and applied perspectives. In this thesis, we focused on the growth behavior of para-sexiphenyl (p-6P) ultrathin film and the weak epitaxy growth (WEG) of phthalocyanine compounds on p-6P ultrathin film. Firstly, the growth behavior of p-6P ultrathin film was studied. The growth of p-6P ultrathin film at high temperature (﹥60 oC) and low temperature (≤60 oC) on silicon dioxide (SiO2) substrate exhibits different mechanism, respectively. At high substrate temperature, the mechanism of diffusion-limited aggregation (DLA) controls the growth of submonolayer thin film with fractal islands. While at low substrate temperature, the submonolayer thin film is composed of the compact islands. Its growth exhibits another growth mechanism that the stable compact islands are formed by dissociation and reorganization of the metastable disordered film. The growth behaviors and structures suggest that mono-layer and double-layer p-6P films are two different phases. The mono-layer p-6P film is highly oriented film-phase of liquid-like crystal, while the double-layer p-6P film is a long-range ordered crystal phase which is similar to β-phase. Through optimizing substrate temperature and growth parameters, highly oriented, large size domain, and continuous p-6P ultrathin films are fabricated successfully on amorphous SiO2 substrate. They supply high-quality epitaxial substrates for WEG of phthalocyanine compounds. Then, the WEG behaviors and mechanism were studied in the model system of metal-free phthalocyanine (H2Pc), zinc phthalocyanine (ZnPc), and vanadyl- phthalocyanine (VOPc) grown on p-6P ultrathin films (mono- and double-layers). Through WEG, phthalocyanine molecules are standing-up growth, and the π-π conjugated direction is parallel to the film plane. Meanwhile, Phthalocyanine films also exhibits regular in-plane orientations. The highly oriented phthalocyanine films in favor of transmitting of carriers in the film plane, and the field-effect mobilities with phthalocyanine compounds on p-6P as active layers are up to the same level as the corresponding single crystals. Because of the different phases of mono-layer and double-layer p-6P films, the planar phthalocyanine compound H2Pc and ZnPc exhibited dissimilar WEG behaviors. They show single in-plane orientation which corresponds to commensurate epitaxy on double-layer p-6P film. While three in-plane orientations are observed on mono-layer p-6P film, which take on incommensurate epitaxy as well as commensurate epitaxy. But for non-planar VOPc, it only shows incommensurate epitaxy on p-6P films because of its molecular array mode and triclinic crystal structure. The [110], [1-10] and [010] geometrical channels of p-6P (001) plane formed by protuberant hydrogen atoms have strong influence on the preparative orientation of phthalocyanine molecules, and they support to nucleate by single molecular column. Then, under the effects of lattice match and geometrical channels, commensurate epitaxy and incommensurate epitaxy can be formed, respectively. Therefore, based on the relationship of lattice match and the effect of geometrical channels, many other organic semiconductor material systems that can be WEG will be discovered