【作者】 张静；

【导师】 侯建国； 王兵；

【作者基本信息】 中国科学技术大学 ， 凝聚态物理， 2010， 博士

【摘要】 二维表面吸附体系作为表面科学的一个重要研究领域,在分子电子、分子识别、催化等方面有着广泛的应用潜质。但是作为一个较新的研究领域,还有非常多的问题有待解决,特别是分子间相互作用和分子-衬底相互作用如何对自组装过程产生影响,只有解决了这个问题,才有可能根据需要设计分子和选择体系。分子．间相互作用的决定因素主要是分子上的官能团和分子的对称性,分子-衬底相互作用很大程度上依赖于衬底的性质。吡啶环能够参与形成弱氢键,因此含有吡啶环的分子是自组装形成超分子结构的重要结构单元之一。在本论文中,我们以金属衬底上吸附的多吡啶环分子为研究对象,重点研究了三重对称的TPTZ分子和二重对称的DPTZ分子在Au(111)和Ag(111)表面的自组装。在第一章中,我们简要介绍了观察二维表面强有力的实验工具——扫描隧道显微镜(STM)以及它的诞生、发展和应用。随后简单介绍了表面分子自组装体系的基本概念、研究意义和研究现状。最后介绍了本论文实验中所使用的低温超高真空扫描隧道显微镜系统。在第二章中,我们利用扫描隧道显微镜系统地研究了2,4,6-tris(2-pyridyl)-s-triazine分子在Au(111)表面的自组装行为。通过改变TPTZ分子的覆盖度,能够操控该分子在衬底上形成周期可调的、手性单一的、由菱形单胞组成的二维超结构。在低覆盖度下,TPTZ分子形成了‘1×1’和‘2×2’结构,而在较高覆盖度下,形成了‘6×6’、‘7×7’和‘8×8’结构,且每种结构都有镜像对称的两种畴。任何一种结构的单胞都由中心对称的两个三角形半单胞组成。半单胞内的分子之间通过单个-CH…N-氢键相结合,而半单胞边界上的分子则通过双-CH…N-氢键相连接。结合高分辨的STM图像和基于第一性原理的计算,我们发现任何一个手性畴都是由一种手性分子组成的,说明在分子组装的过程中发生了手性分离,其根源是分子之间具有手性选择的氢键相互作用。我们提出了一种能量密度分析法,发现分子间氢键相互作用和分子-衬底之间由非公度到公度的转变调节着组装结构的周期,定量地解决了TPTZ自组装过程中的驱动力问题,该方法可推广应用到其他自组装体系中,有助于理解自组装过程中各种相互作用的影响。第三章中,我们在上一章研究TPTZ分子自组装的基础上,深入地研究了TPTZ分子在外电场作用下的场致异构现象。发现无论是较高偏压的脉冲还是较低偏压的长时间扫描带来的电场激励都能使边界上TPTZ分子的一个吡啶环变亮。理论模拟显示这是由于在电场的作用下,吡啶环发生倾斜所导致的。边界分子之间较强的相互作用能够将吡啶环稳定在这种倾斜状态,而内部分子之间则没有能完成这种任务的相互作用。另外,内部分子间紧密的排布导致的较大的空间位阻也阻碍了吡啶环的倾斜。该实验结果为通过分子间相互作用调制功能分子提供了一种新的思路。在第四章中,我们观察了DPTZ分子在Ag(111)表面和Au(111)表面的自组装来研究分子对称性和衬底对自组装过程的影响。发现DPTZ分子在两种表面上、低覆盖度下都形成均匀分布的一维分子链结构,在高覆盖度下,则形成了较为复杂的多种二维网格结构。结合高分辨的STM图像和DFT理论计算,我们发现分子间具有方向性的氢键作用和分子-衬底之间的电荷转移带来的分子链间斥力的共同作用促成了一维分子链的形成。吸附在250 K的Ag(111)衬底上的DPTZ分子形成的蜂窝结构则说明了衬底温度过低会将分子局域在动力受限状态。另外,Au(111)表面的再构对分子链的延伸有一定的限制作用。更多还原

【Abstract】 Two-dimensional adsorbed system has attracted extensive attentions because of its potential applications in molecular electronics, molecular recognition and catalysis, etc. As a hot research field, there still exist a lot of open questions, especially, the influence of the intermolecular interaction and the molecule-substrate interaction. Molecules containing pyridyl group is important in construction of diverse supramolecular structures with hydrogen bond, which are kinds of important building blocks in self-assemly of molecular patterns. This thesis focuses on the self-assembly of the molecules with multi-pyridyl groups on metal surfaces.In chapter I, we briefly introduce the powerful instrument used in surface science:scanning tunneling microscopy (STM). In the following part, we give the overview of the self-assembly systems on solid surfaces, including the basic principles, significance and present progresses. In the end, we briefly describe the ultra-high vacuum low temperature STM system used in this work.In chapter II, we present the study on the self-assembly of 2,4,6-tris(2-pyridyl)-s-triazine (TPTZ) on Au (111) surface using STM. By systematically changing the coverage of the TPTZ molecules, the self-assemblies of TPTZ form homochiral suprastructures with tunable period undergoes spontaneous chiral resolution. The TPTZ molecules form enantiomorphous domains composed of rhombic supercells with various periods depending on the coverage of molecules, that is,’1×1’and’2 x 2’structures at low coverages, and’6×6’,’7×7’，and’8×8’structures at higher coverages. In a unit cell of a certain enantiomer, the two triangular half-unit cells, consisting of adsorbed TPTZ molecules, are centrosymmetric to each other. The molecules inside each half-unit cell are bound to each other through a single-CH···N-hydrogen bond, while the molecules at the boundaries between half-unit cells are bound through double-CH···N-hydrogen bonds. The STM images and the DFT calculations reveal that the molecules in an enantiomorphous domain adopt the same adsorption orientation of either R-TPTZ or L-TPTZ, which indicates that the adsorbed TPTZ molecules on Au(111) undergo spontaneous chiral resolution. The subtle balance between the intermolecular interaction and the molecule-substrate interaction tunes the period of the superstructure. The total interaction energy densities obtained from the DFT calculations explain the experimental observations quantitatively.In chapterⅢ, following the study of TPTZ, we present the further investigation of functional properties of TPTZ assemblies. By applying proper bias voltage, we found the electric-field causes one of the three pyridyl groups of the TPTZ molecules becoming bright at the domain boundary of the supramolecular pattern. We suggest that in this process the pyridyl group undergoes a conformation change. The theoretical simulations indicate such conformation change is because of the rotation of pyridyl group with N tilting to the Au substrate. The molecules at the boundary may have some free space for the rotation of the pyridyl group, and the intermolecular interaction between the molecules at different domains may help to stabilize such conformation. However, the intermolecular interaction of inner molecules of the same domain has much larger steric hindrance, preventing the rotation of the molecules.In chapterⅣ, we investigated the self-assembly of 3,6-Di-2-yridyl-1,2,4,5-tetrazine (DPTZ) molecules on Ag (111) and Au (111) surfaces. On both of the surfaces, one dimensional chain is formed at low coverage and the structure of the SAM become much complex with the coverage increasing. The STM experiments combined with DFT calculations reveals that the directional hydrogen bond and charge transfer between molecule and substrate induced dipole-dipole repulsion is responsible for the formation of one dimensional chain. The honeycomb network formed on Ag (111) surface kept on 250 K indicates that the low temperature of substrate will trap the molecules in the kinetic limited state. Taking account of the influence of substrate, the chains on Ag (111) surface behave quite different from those on Au (111) surface更多还原