【作者】 窦卫东；

【导师】 鲍世宁；

【作者基本信息】 浙江大学 ， 物理学， 2009， 博士

【摘要】 在过去的几十年里,有机半导体作为以Si和Ge为代表的常规半导体材料可能的替代材料受到了工业界的广泛关注,并极大的激励了基础领域对有机半导体的研究。对有机薄膜的生长特性,有机分子的吸附特性和扩散特性、电荷输运特性等方面的研究受到了人们广泛的关注。另外有机半导体材料还具有良好的光电特性,在有机光电器件方面有潜在的应用。基于有机半导体材料的器件多数为薄膜结构,制约有机半导体器件发展的一个关键问题是其载流子传输效率较低。而研究发现,通过提高薄膜结构的有序度可以大大提高载流子的传输效率。为了控制薄膜的生长结构,有必要深入认识其自组装过程的机理,即在这个过程中分子-衬底相互作用和分子-分子相互作用到底扮演着怎样的角色,二者如何分别对分子的自组装结构产生影响。另外,在有机半导体器件中,将不可避免的涉及金属(电极)与半导体薄膜之间的接触问题,二者之间的界面电子态结构决定着电极-有机薄膜之间载流子(电子或空穴)注入壁垒大小,因此深入认识二者界面电子结构对有机半导体设计和加工具有指导意义。基于以上两个影响有机半导体器件性能的问题,本论文研究了具有平面结构的芳香族有机半导体分子在过渡金属表面上的薄膜结构和分子-金属界面电子结构。同时,我们从基于密度泛函理论的第一性原理计算出发,研究了分子间的相互作用在自组装过程中的角色。由于分子的自组装过程同时受到分子间作用力和分子-衬底之间相互作用力的共同制约,因此不可避免的也会讨论到后一种相互作用在分子薄膜自组装中的影响。尤其是在强相互作用系统中,后一种相互作用的影响更加明显。我们通过对比分子在不同衬底上的结构来认识分子间相互作用对薄膜结构的影响。本论文第一首先简要介绍了有机半导体分子薄膜结构和电子态研究的发展和现状。同时,以有机分子在金属单晶表面的生长为主线,介绍了有机薄膜的生长模式、结构分析方法以及影响分子-金属界面电子结构的主要因素。第二章主要介绍了研究工作涉及的实验手段,包括STM、LEED、UPS等,介绍了实验的工作原理,以及对某些实验设备进行的改造。同时还简要介绍了密度泛函理论以及基于密度泛函理论的一些商用计算软件包。第三章至第六章介绍了本论文的主要工作。论文主要着眼与两个方面：分子自组装结构和界面电子结构。本论文选择了三种具有平面结构的芳香族分子(并四苯、花和单氟取代的花酰亚胺分子)为研究对象,研究了它们在Cu、Ag单晶表面的吸附单层结构和界面电子结构。根据紫外光电子能谱,在Cu(110)衬底上,随着并四苯分子在衬底表面上覆盖度的逐渐增大,具有π特性的分子轨道具有较大的能量位移,表明分子与衬底之间主要通过π-d相互作用联系。衬底d电子谱峰随覆盖度的变化表明分子在覆盖度大于一个单层时其吸附结构发生了相变,从平面吸附模式转变为直立(或倾斜)吸附模式。这一结论与前人的研究结果不同,以往认为并四苯分子在Cu(110)表面上,单层时就是直立结构。实验和理论研究结果表明：并四苯分子在四度对称的Cu(100)表面上,在室温下,单层条件下的并四苯分子也是平面吸附结构,而不是以前认为的直立吸附模式。在饱和单层覆盖度下,分子沿两个等价的高对称度方向<01 1>方向吸附,两个方向吸附的分子数目比例接近1：1。这一特点是由衬底表面的对称性决定的。对于这一系统,由于分子间的相互作用相对于分子-衬底间的相互作用而言比较弱,因此在室温下生长,不易形成大范围的有序结构,但在局域仍可看到有序的结构。实验研究结果表明：茈分子在Ag(110)表面上,在不同覆盖度条件下,其自组装结构是不同的。同时从密度泛函理论计算角度解释了覆盖度增大的过程中自组装结构相变的原因,发现其结构的相变为两种相互作用(分子间的相互作用和分子-衬底之间的相互作用)竞争的结果。稳定的吸附结构是通过两种相互作用的平衡来实现。通过紫外光电子能谱并结合密度泛函理论研究表明：三种单氟取代的花酰亚胺分子(n型有机半导体分子)在Cu(110)表面的吸附结构是不同的。由于氟原子的不同取代位置对分子结构的影响,造成吸附结构中分子-衬底之间不同强度的相互作用。吸附结构的不同反映了氟原子不同取代位置的影响。实验和理论同时研究了三种单氟取代的花酰亚胺分子在Cu(110)表面的不同电子态结构。更多还原

【Abstract】 The past decades witness the fast developing in the study of organic materials, which are believed to bear potential applications in electronic devices that are presently dominated by their inorganic counterpart, Si and Ge. Presently, the organic materials have been used in field effect transistor (OFET) and light-emitting device (OLED) as the active layer due to their unique electronic characters. One problem that hinders the application of organic materials in electronic devices is the low mobility of charge carriers in the film as well as between film and metal contact. It has been widely accepted that the mobility of charge carrier in the film can be significantly improved by improving the ordering of the film. It is therefore urgently important to get deeper understanding about the factors that affecting the self-assembly (SA) structure of organic molecules on metal surfaces, namely intermolecular interaction and molecule-substrate interaction. What are their roles and how they driving the SA process. In addition, the interface electronic structures of organic/metal system were also studied due to their importance in determining the injection barrier (hole and electron injection barrier) between organic film and metal contact.The main effort of this thesis is focused on the growth properties of planar aromatic molecules on metal surfaces and their interfacial electronic structures, aiming to understand the roles of interactions mentioned above in the film growth process. Especially, the intermolecular interaction was emphasized. However, the molecule-substrate interaction was also studied since the SA process is driven by both interactions. The role and its importance of intermolecular interaction in the SA process were investigated by series competitive studies by depositing molecules onto surfaces with different surface reaction activities.The first chapter is an introduction about the organic film growth and the interfacial electronic structures between adsorbed molecules and metal substrate. Except the general review regarding to the organic film growth on metal surface, the methods to determine the adsorbed structures and the factor affecting the interfacial electronic structures are also presented. The second chapter is a detailed description about the experimental instruction and calculation method, including scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), ultraviolet photoelectron spectroscopy (UPS) and the first-principle calculation based on density functional theory (DFT).The experimental and theoretical results were presented in the following four chapters, in which two aspects regarding with the film growth process were discussed, namely SA structures and interfacial electronic structures. Three kinds of molecules were selected as candidates for these purposes, which are tetracene, perylene and fluorinated derivatives of perylene diimides.In the third chapter, the electronic structure of tetracene film on Cu (110) surface was investigated by a combination study of UPS and DFT calculation. The photoemission features originated fromπcharacterized molecular orbitals suffered obvious shift in energy position, indicating the strongπ-d interaction between tetracene and Cu (110) surface. The growth beyond the first layer undergoes a structural phase transition from flat-lying mode to upright-standing mode, due the weakening of molecule-substrate interaction and gradually dominating intermolecular interaction in the multilayer. This work showed strong evidence that the previous conclusion about tetracene aligning geometry on the Cu (110) surface is wrong, in which the structure with molecular plane perpendicular to the substrate surface is actually multilayer structure.The fourth chapter focuses on the structural study of tetracene adsorbed on Cu (100) surface, which is a four-fold symmetry surface. The molecule was observed to adopt lying-down mode rather than the upright-standing mode at monolayer regime if the molecules were deposited by keeping the substrate at room temperature. The long axis of the molecules were found to be aligned randomly along two identical high symmetry direction of<011> with the counting ratio of the molecules aligning along the two directions closes to unity. Since the intermolecular interaction in this system is much weaker with respect to the molecule-substrate interaction, no long-range ordered structures were observed. However, the ordered structure in some local region was observed showing the possibility of ordering growth motivated by intermolecular interaction.In the fifth chapter, the weak interacting system was concerned, namely perylene/Ag (110), which is turned out to be a proper candidate to study the role of intermolecular interaction in the SA process due the moderate molecule-substrate interaction. By a combination study of STM and DFT calculation, the competition between intermolecular interaction and molecule-substrate interaction was found to be dominated in the SA process, and the stable structure is a balance between these interactions.Finally in the sixth chapter, the electronic structure of three kinds of Fluorinated Derivatives of Perylene Diimides on Cu (110) surface had been studied by using UPS as well as DFT calculations. The difference in electronic structures of these molecules on metal surface was found to be attributed to the different aligning geometry with respect to the substrate surface, which is determined by different intensity of molecule-substrate interaction for these molecules.更多还原