Solution Cast Phenybiphenyloxdiazole(PBD) Single Crystals Induced by External Field and Micro-electrical Properties

本论文以2-（4-叔丁基苯基）-5-（4-二联苯基）-1，3，4-呃唑（PBD）单晶为研究对象，在对溶液浇铸的PBD多晶薄膜的晶体结构的研究基础上，通过调控溶液浇铸PBD单晶生长过程中的溶剂挥发和外加静电场，获得了晶胞轴轴和c轴垂直基板取向的不同的PBD单晶。利用导电探针原子力显微镜考察了与之相对应的电学性能及其各向异性。在空气中溶液浇铸的PBD多晶薄膜表现出c轴垂直基板取向的特征。基于这一现象，我们通过控制溶剂场中溶剂挥发的时间，即控制溶剂挥发的动力学过程，得到了大面积有序排列的c轴垂直基板取向的PBD单晶。在溶剂挥发的过程中，通过原子力显微镜观察到薄膜的形貌发生明显的改变，此现象表明PBD由亚稳态的多形态（Polymorphs）转变为单晶。当在这一过程中施加一个场强为5kV／cm的静电场时，可以得到b轴垂直基板取向的PBD单晶。我们利用导电探针原子力显微镜测量了制备在Au／S基板上这两种不同取向结构的PBD单晶在纳米尺度上的电学性能，观察到了纳米尺度上的PBD单晶的电学各向异性。电荷的传输性能是在导电金探针和Au／S基板之间测量的。在电场诱导作用下得到的b轴垂直基板取向的PBD单晶测量得到其电子迁移率为3*10-3cm2/Vs。而通过动力学控制溶剂挥发得到的c轴垂直基板取向的PBD单晶，在低电压域内测量得到的电子迁移率比电场诱导作用下得到的轴垂直基板取向的PBD单晶要高两个数量级。这么高的电荷各向异性是由于相邻分子之间兀轨道重叠的程度不同而决定的。In this thesis, solution cast 2-(4-tert-butylphenyl)-5-(4-biphenyl)-l,3,4-oxdiazole (PBD) single crystals with different crystallographic axes (b, c) orientation have been manipulated under electrostatic field and by solvent evaporation. The electrical properties and electrical anisotropy of these single crystals were investigated by Conducting Probe Atomic Force Microscopy. The solution cast polycrystalline films of PBD with c-orientation were observed without controlling the rate of solvent evaporation. Based on this phenomena, large area of aligned solution cast PBD single crystals with c-orientation were obtained by controlling the time of solvent evaporation or the kinetic process of solvent evaporation in solvent field. During the process of solvent evaporation, metastable polymorphs converted to single crystals and obvious morphological changes were observed. Another oriented PBD single ciystal with 6-orientation was achieved by applying an electrostatic field with an intensity of 5 kV/cm during the whole process of solvent evaporation. Conducting probe atomic force microscopy (CP-AFM) was used to measure the local electrical properties of these two oriented PBD single crystals grown on Au/S substrates in nanoscale and electrical anisotropy of single crystals over nanoscopic length scales was observed. Charge transport was measured perpendicular to the substrate between the conducting probe atomic force microscopy Au tip and the Au/S substrate. The electron mobility of 3 x 10"3 cm2/Vs for PBD single crystal with 6-orientation obtained by applying an electrostatic field was determined. And the electron mobility of PBD single crystal with c-orientation achieved by controlling kinetic process of solvent evaporation was about two orders of magnitude higher than that with 6-orientation induced by electrostatic field due to the anisotropic charge transport at the low voltage region. The higher anistropic charge transport is due to different degree of 7c-orbital overlap between adjacent molecules