掃瞄式電子穿隧顯微鏡對有機分子在Ag/Ge(111)-(r3xr3)R30°表面的自組裝與光反應行為的研究
Abstract
有機分子在無機材料上所形成的自組裝單層吸附層行為近年來被研究的相當廣泛；特別是在有機發光材料(OLED)這方面，更是蓬勃發展，其中共軛高分子poly(para-phenylene vinlyene) (PPV)以及其寡鏈分子oligo-para-phenylenevinylene (OPV)更是研究的重心所在，主要是這類分子的發光效率很高。但其在表面上的自組裝行為與表面異構化光反應機制尚未非常清楚，引起我們想作進一步的探討。
本論文主要利用掃瞄式電子穿隧顯微鏡(STM)，來研究這類有機發光分子在Ag/Ge(111)-(r3xr3)R30°表面上自我組裝與光化學異構化行為。而有機分子則是選用不同的OPV的最小單元來作為研究的有機分子，分別是：順式,與反式-二苯乙烯(trans-, cis-, stilbene)、順,順式,與反,反式-對二苯乙烯基苯(trans,tran-, cis,csi-, distyrylbenzene)、順式,與反式-偶氮苯(trans-, cis-, azobenzene)等。利用這些最小的單元分子來研究其在表面上的分子自我組裝跟表面異構化光反應機制。
經研究結果發現，三個有機分子均可以在表面上形成整齊規則排列，主要原因是因為我們選用的Ag/Ge(111)-(r3xr3)R30°表面，其單位晶格在表面銀原子所聚集形成的載體距離(6.7 Å)幾乎等於這些多苯環有機分子的兩個苯環中心間距離，因此才可以使這些有機分子在表面上自組裝成整齊的排列。而在表面異構化光反應機制部分，順式與反式的二苯乙烯，可以經由暴露在不同波長的光下，通過雙激子(biexcition)的方式，在表面上成對翻轉(pairwise)，來控制其順反式結構。但是對於相同的碳碳雙鍵分子，順,順式與,反,反式-對二苯乙烯基苯(trans,tran-,cis,csi-, distyrylbenzene)而言，因為一次要三個激子(triexcition)聚在一起機率太低，另外即使相聚在一起後要進行三分子翻轉時，也會因為空間立體障礙過大而無法進行異構化反應。而對於順式與反式-偶氮苯而言，則是因為氮氮雙鍵跟表面的作用力太強，使得吸收光子後，電子組態由基態躍遷至激發態，也會很快的釋出能量而回到基態，而無法進行異構化反應。
Self-ordered monolayers formed by organic molecules on semiconductor substrates are of considerable value in various technical applications. In particular, the ordering organic molecules on surface is crucial to charge transport between molecules, charge injection, wetting, friction, and optical properties. The surface-induced order of organic molecules can also affect the bulk over comparatively large distances, influencing their structure and electrooptical properties. Thus, understanding how functionalized organic molecules order at surfaces is crucial for optimizing their use in the application. Scanning tunneling microscopy (STM) is a powerful method to resolve these issues, because it allows sub-molecular resolution imaging of molecules adsorbed on a surface. This tactics permits an insight into the conformational, mechanical and electronic structure and thus functionalities of the molecules. In this thesis, the specially chosen shortest oligo-p-phenylenevinylene (OPV) molecules such as stilbene, distyrylbenzene, and azobenzene, acting as model systems for molecular nanotechnology, are reviewed. The presented studies focus on self-ordered behavior, and the photoisomerization mechanism on the Ag/Ge(111)-(r3xr3)R30° (referred to as Ag/Ge(111)-r3 hereafter) surface. The atomic structure of the Ag/Ge(111)-r3, called inequivalent triangles (IET) model, has been found in our STM images which is different from the widely accepted honeycomb chained triangle (HCT) model before. Furthermore, the result of the density functional theory (DFT) calculation also shows that the IET structure is 0.35 eV energetically more stable than the HCT model. The adsorption and desorption of stilbene on Ag/Ge(111)-r3 were investigated by using low energy electron diffraction (LEED), temperature programmed desorption (TPD), STM, and DFT. cis- stilbenes form a (2×1) overlayer structure on Ag/Ge(111)-r3 at a coverage of ~ 1 ML. The STM images show parallel strips with three equivalent directions, indicating a self-ordered molecular structure. The TPD traces fit the half-order kinetics for molecular desorption of stilbene from Ag/Ge(111)-r3 with desorption energies of 20.1 (cis-) and 21.3 kcal/mol (trans-), which are comparable with the calculated values using the DFT method. A plausible explanation for the stilbene desorption process on Ag/Ge(111)-r3 is proposed and discussed. The adsorption and self-organized monolayer of trans, trans-distyrylbenzene (tt-DSB) and cis, cis-distyrylbenzene (cc-DSB) on Ag/Ge(111)-r3 were studied by STM. High-resolution images allow identification of internal structure of individual tt-DSB molecules with three phenyl rings and their molecular arrangements on the Ag/Ge(111)-r3 surface. The lattice match makes Ag/Ge(111)-r3 an ideal substrate for tt-DSB self-organization and formation of (3 × 1) overlayer unit cell. The structural model and the molecule registry, corresponding to STM images for the adlayers of tt-DSB on Ag/Ge(111)-r3, are proposed and discussed. For cc-DSB adsorption on Ag/Ge(111)-r3, uniform molecular overlayers with two discernible molecular images corresponding to two major types of cc-DSB conformers were observed. The photoisomerization of DSB is not observed to take place at this surface because of two reasons as follows：the triexciton-assisted photoisomerization is hard to happen, and the molecule is constrained by the steric hindrance imposed by neighboring molecules. The interfacial structures of azobenzene on Ag/Ge(111)-r3 were studied by STM. High-resolution images allow the identification of individual molecules, with trans-azobenzene (TAB) appearing with a distinctive dumbbell shape. From in situ observation of the substrate lattice, the TAB monolayers were found to form a (2 × 1) structure. The interaction between TAB and the substrate plays the controlling role in influencing the structure of the TAB overlayers. A model for the unit cell of TAB monolayers is proposed and discussed. In the case of the photoisomerization, the surface quenching lifetime is too short so that the photoisomerization is hard to proceed at this highly coupling adsorbate-surface system.
Self-ordered monolayers formed by organic molecules on semiconductor substrates are of considerable value in various technical applications. In particular, the ordering organic molecules on surface is crucial to charge transport between molecules, charge injection, wetting, friction, and optical properties. The surface-induced order of organic molecules can also affect the bulk over comparatively large distances, influencing their structure and electrooptical properties. Thus, understanding how functionalized organic molecules order at surfaces is crucial for optimizing their use in the application. Scanning tunneling microscopy (STM) is a powerful method to resolve these issues, because it allows sub-molecular resolution imaging of molecules adsorbed on a surface. This tactics permits an insight into the conformational, mechanical and electronic structure and thus functionalities of the molecules. In this thesis, the specially chosen shortest oligo-p-phenylenevinylene (OPV) molecules such as stilbene, distyrylbenzene, and azobenzene, acting as model systems for molecular nanotechnology, are reviewed. The presented studies focus on self-ordered behavior, and the photoisomerization mechanism on the Ag/Ge(111)-(r3xr3)R30° (referred to as Ag/Ge(111)-r3 hereafter) surface. The atomic structure of the Ag/Ge(111)-r3, called inequivalent triangles (IET) model, has been found in our STM images which is different from the widely accepted honeycomb chained triangle (HCT) model before. Furthermore, the result of the density functional theory (DFT) calculation also shows that the IET structure is 0.35 eV energetically more stable than the HCT model. The adsorption and desorption of stilbene on Ag/Ge(111)-r3 were investigated by using low energy electron diffraction (LEED), temperature programmed desorption (TPD), STM, and DFT. cis- stilbenes form a (2×1) overlayer structure on Ag/Ge(111)-r3 at a coverage of ~ 1 ML. The STM images show parallel strips with three equivalent directions, indicating a self-ordered molecular structure. The TPD traces fit the half-order kinetics for molecular desorption of stilbene from Ag/Ge(111)-r3 with desorption energies of 20.1 (cis-) and 21.3 kcal/mol (trans-), which are comparable with the calculated values using the DFT method. A plausible explanation for the stilbene desorption process on Ag/Ge(111)-r3 is proposed and discussed. The adsorption and self-organized monolayer of trans, trans-distyrylbenzene (tt-DSB) and cis, cis-distyrylbenzene (cc-DSB) on Ag/Ge(111)-r3 were studied by STM. High-resolution images allow identification of internal structure of individual tt-DSB molecules with three phenyl rings and their molecular arrangements on the Ag/Ge(111)-r3 surface. The lattice match makes Ag/Ge(111)-r3 an ideal substrate for tt-DSB self-organization and formation of (3 × 1) overlayer unit cell. The structural model and the molecule registry, corresponding to STM images for the adlayers of tt-DSB on Ag/Ge(111)-r3, are proposed and discussed. For cc-DSB adsorption on Ag/Ge(111)-r3, uniform molecular overlayers with two discernible molecular images corresponding to two major types of cc-DSB conformers were observed. The photoisomerization of DSB is not observed to take place at this surface because of two reasons as follows：the triexciton-assisted photoisomerization is hard to happen, and the molecule is constrained by the steric hindrance imposed by neighboring molecules. The interfacial structures of azobenzene on Ag/Ge(111)-r3 were studied by STM. High-resolution images allow the identification of individual molecules, with trans-azobenzene (TAB) appearing with a distinctive dumbbell shape. From in situ observation of the substrate lattice, the TAB monolayers were found to form a (2 × 1) structure. The interaction between TAB and the substrate plays the controlling role in influencing the structure of the TAB overlayers. A model for the unit cell of TAB monolayers is proposed and discussed. In the case of the photoisomerization, the surface quenching lifetime is too short so that the photoisomerization is hard to proceed at this highly coupling adsorbate-surface system.
Description
Keywords
掃瞄式電子穿隧顯微鏡, 有機發光分子, 自我組裝, 光化學異構化, oligo-p-phenylenevinylene, Scanning tunneling microscopy, photoisomerization, Self-ordered