摘要
随着信息工业的高速发展，光电子材料和器件也步入了纳米尺度，纳米光电子学应运而生。它结合了光电子技术与纳米技术，主要研究纳米结构中电子与光子相互作用，具有划时代的意义。而纳米等离子体激元光子学则是近年来纳米光电子学领域中研究十分活跃的一个新兴分支。在本论文中，我们主要介绍基于扫描隧道显微镜针尖诱导发光技术(STML)研究纳米等离子体激元光子学中亚波长尺度内电子与光子、表面等离子体激元与分子偶极之间的相互耦合作用。 扫描隧道显微镜诱导发光技术是通过扫描隧道显微镜(STM)针尖高度局域的隧穿电子非弹性隧穿激发隧道结发光，并利用高效率的光子收集系统与高灵敏的光子检测系统来测量隧道结发射出的光子信息。它具有超高的空间、时间、能量分辨率，从而揭示了亚波长尺度隧道结内所发生的各种电光转换过程，有助我们理解纳米级尺度载流子输运与复合特征、能量转移与传递过程。
在第一章中，我们主要介绍扫描隧道显微镜针尖诱导发光技术和纳米等离子体激元光子学的背景。其中，我们将突出介绍STML技术应用于研究纳米等离子激元光子学的独特优势以及其发展潜力。通过扫描隧道显微镜技术与纳米等离子体激元技术的完美结合，我们力求在单分子尺度上对电子与声子、电子与光子、光子与纳米环境、等离子激元与分子偶极之间的相互作用给出清晰的物理图像，并最终实现纳米环境中单量子体系光子态的精确控制。
在第二章中，我们将介绍金属表面多层卟啉分子薄膜的扫描隧道显微镜诱导发光研究。首先，我们在原子级平整的金属表面，热蒸发沉积逐层生长多层卟啉分子超薄膜，然后利用扫描隧道显微镜针尖诱导发光技术研究金属针尖－分子－金属衬底隧道节的发光现象。实验中，我们发现针尖－衬底组成的纳米谐振腔中针尖诱导等离子体激元发光模式会随针尖状态变化而发生改变。通过脉冲修饰针尖或者选择不同的针尖，我们可以获得不同等离子激元模式的纳米谐振腔。然后，在谐振腔中引入卟啉荧光分子。通常情况下，由于金属表面的荧光萃灭效应，卟啉分子偶极需要一定的脱耦合作用(多层膜中，卟啉分子自身做脱耦合层)才能实现中性分子偶极发光。
石云生实验发现，在亚波长尺度的纳米谐振腔中等离子体激元与卟啉分子偶极之间存在较强的相互耦合作用。当卟啉分子偶极的跃迁能级与等离子体激元的模式相匹配时，会发生明显的卟啉分子荧光共振增强，甚至会使得一些原本不发光的卟啉分子高振动激发态能级发光(即热荧光现象)。进一步，我们还发现了受到等离子体激元模式调制的分子发光能量高于激发电子能量的分子上转换荧光
现象，进而提出了等离子体激元辅助的分子上转换荧光机制。
在第三章中，我们主要介绍了STM研究金属表面卟啉衍生物超薄膜的结构与组装，并尝试了化学修饰卟啉衍生物分子STM诱导发光研究。在实验中，我们首先探索了卟啉分子通过溶液滴分子自组装制备方式在原子级平整金属表面生长卟啉分子超薄膜的可行性。从而实现溶液法在金属表面制备难以热蒸
发沉积生长的大卟啉衍生物分子 Bz2OxP, NaPh2OxP, Pyr2OxP, Porph2OxP超薄膜。通过STM图像，我们观察得到母体OxP卟啉内环N键上不同取代基的化学修饰对卟啉分子结构与金属表面分子自组装的影响。
切削机
通常情况下，没有任何取代基时，卟啉分子不仅非常稳定，适合热蒸发，而且金属表面组装非常有序平整。但当内环N键上添加较小取代基苯甲基(又叫苄基，用Bz 表示)时，分子热稳定性差，不适合热蒸发，只能溶液组装方式制备超薄膜。随着取代基团进一步增大(萘甲基NaPh→芘甲基Pyr→卟啉环甲基Porph)，其金属表面有序组装难度逐渐加大，且往往需要低温退火才能观察其表面小范围有序结构。实验发现，不同的取代基会修饰并改变金属表面分子构型以及其自组装结构。但从另一个角度，随着取代基的增大，分子跟衬底间的萃灭效应减弱，有助于实现单个分子荧光，这将是实现单分子发光的一个重要途径。由于该卟啉衍生物无法制备金属表面多层膜脱耦合层，我们始终无法获得卟啉类衍生物分子发光。我们所获得的针尖-卟啉衍生物-衬底隧道节发光更多的反映了隧道节中纳米等离子体激元发光的特征，分子的引入是作为间隔层减弱了等离子体激元发光。
在第四章中，我们探讨了分子寿命对于隧道节内分子发光的影响，并在此基础上展开了金属表面长寿命(～μs)三重态分子(PtOEP，Eu(dpm)3和Ir(ppy)3)多层膜的扫描隧道显微镜诱导发光研究。实验证实，有机发光二极管(OLED)中常见的三重态分子PtOEP可以在金属表面热蒸发沉积生长成平整有序的多层超薄膜结构，但即使在光致金属表面超薄膜磷光信号非常强的时候，仍然没有观察到隧道节电致
PtOEP分子三重态发光。进一步, 我们还研究了稀土离子类三重态分子(Eu(dpm)3和Ir(ppy)3)的组装与STML发光特征。由于Eu(dpm)3和Ir(ppy)3分子均为极性分子，分子间相互作用较强，所以其在金属表面无法形成有序结构。但对于Eu(dpm)3分子，其在多层膜之后则逐渐展现部分分子有序结构，无疑体现了Eu(dpm)3分子的独特性，即分子间相互作用中排斥力与吸引力的一种平衡，而其中衬底起的作用至今仍是一个迷。与PtOEP分子类似，稀土离子类分子多层膜STM诱导发光也没有观察到分子发光信号。
造成这一现象的根本原因，可能在于两方面因素。一方面，扫描隧道显微镜针尖高度局域的电子注入激发(可达到单分子激发水平)与分子三重态长寿命共
同导致光子发射过程的动力学速率过低。另一方面，三重态分子能级中缺少能够与纳腔等离子体激元发生耦合作用的电子态，即纳腔等离子体激元只能泵浦激发增强分子单重态之间的跃迁，而无法直接泵浦激发基态分子进入三重态能级。
最后，我们将初探在隧道节内引入复杂OLED结构(电子传输层－发光层－空穴传输层结构)的扫描隧道显微镜诱导发光研究。首先我们设计了针尖－多层PtOEP－7层TPP分子－金属衬底样品的隧道节发光，并获得了TPP分子发光信息，而同时测得的光致发光信息则同时包含PtOEP和TPP分子的信息。实验表明，对于隧道节内复杂分结构是可以实现分子发光的，同时隧道节内发光的分子也可以并非最
外层分子发光。更深入的，我们还设计了针尖－Eu(dpm)3分子－TPD分子－衬底以及针尖－Alq３分子－Eu(dpm)3分子－TPD分子－衬底体系的扫描隧道显微镜诱导发光实验。但其电致发光结果却始终表现为纳腔等离子体激元发光的信息，无分子发光信息。更没有观察到TPD分子作为Matrix，向三重态分子发生的共振能量转移。这种传统OLED结构在隧道节中之所以并没有观察到分子发光，究其原因，可以归结为两类：第一，电子传输层或者空穴传输层厚度不够，不足以起到控制电子传输或者空穴传输的作用；第二，扫描隧道显微镜针尖的高度局域注入，不能形成有效的截面电流，不利于OLED结构的发光。
总结，我们通过利用扫描隧道显微镜针尖诱导发光技术，研究了隧道节内纳米尺度上的电子与光子、等离子体激元与分子偶极、分子偶极与分子偶极之间的耦合与转换相互作用，同时还探讨了隧道节内载流子输运控制对于隧道节发光的影响。这对于推动纳米等离子体激元技术在分子尺度上的发展，起到了积极的作用。
关键词：扫描隧道显微镜针尖诱导发光 表面等离子体激元 分子偶极 纳米谐振腔等离子激元 荧光萃灭 脱耦合 荧光共振增强 热荧光 上转换荧光 分子三重态 分子间相互作用 泵浦增强
ABSTRACT
With the rapid development of electronic information industry, the photoelectron materials and device
s were all stepping into sub-wavelength nanoscale, and the nanoscale photoelectronics was born. It combined photoelectron technology and nano-technology, aiming to the interactions between electron and photon in nanoscale. One recently hot topic in nanoscale photoelectronics is nanoplasmonics. In this thesis, we would mainly discuss the investigation of coupling between electron and photon, surface plasmon and molecular dipole in sub-wavelength scale by Scanning tunneling microscope induced luminescence(STML) technology.
白刚玉
Scanning tunneling microscope induced luminescence was based on that the tip’s highly localized tunneling current could excite photon emission by inelastic tunneling process. By the highly efficient collection and detection of photons, we could investigate the informations of photons from tunneling junction. It could reveal the electron-photon transitions in sub-wavelength tunneling junction with ultra-high spatial、time and energy resolution by STML technology. It would be helpful for us to comprehend the transfer and combination of carriers、the energy transition and transfer in nanoscale.
In chapter one, we introduced the background of both STML and nanoplasmonics. Inside this chapter, we would lay emphasis on the introduction to particular advantages of STML technology, when it was applied into nanoplasmonics. By the perfect combination between STML and nanoplas
monics, we were aiming to give clear physical pictures of couplings between electron and phonon, electron and photon, photon and nano-enviroment, plasmon and molecular dipole, etc in nanoscale, and finally to get the precise control of quantum system’s photon state.
In chapter two, we investigated the STML of multi-layer porphyrin (TPP) molecules on metal substrate. Firstly, we prepared the atomic flat metal surface by sputtering and annealing in ultra-high vacuum. Then porphyrin molecules were deposited one by one layer on metal surface. Finally we studied STM tip induced luminescence from tip-molecule-substrate junction. In experiments, we found that the plasmon mode of nano-cavity between tip and metal substrate could be changed by the tip’s status which could be mediated by pulse. Taking the porphyrin molecule into account, usually it must be decoupled to realize molecular photon emission by a
decoupling layer on metal surface.
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We also found that there was strong coupling between nanocavity plasmon and molecular dipole in sub-wavelength scale junction. When the nanocavity plasmon mode matched the molecular dipole transition, the porphyrin’s transition would be enhanced strongly by Resonance, even hot electroluminescence from high vibrational excited state could also be observed. Further more, due to
strong coupling, the molecular up-conversion luminescence assisted by nanocavity plasmon was also be mediated by the nanocavity plasmon mode.
负债率
In chapter three, we did the investigation of porphyrin derivates molecular ultra-thin film on metal substrate. Since the porphyrin derivates were not steady in thermal deposition, we developed the new method of self assembled in liquid on metal surface by several droplets, which was firstly found proper for porphyrin molecules. Then we prepared the Bz2OxP, NaPh2OxP, Pyr2OxP, Porph2OxP ultra-thin films on metal surface individually by this way. By STM image, we found the molecule Bz2OxP, NaPh2OxP, Pyr2OxP, Porph2OxP had different molecular structures and assemble structures, all due to the different chemical group mediated to the N bonds of porphyrin inner backbone ring.
Usually, the porphyrin molecules could be assembled on metal surface with good order by thermal deposition. But for these porphyrin derivates(Bz2OxP, NaPh2OxP, Pyr2OxP, Porph2OxP), only the self assembled in liquid by droplets would be suitable. With the size increasing of chemical group, it was more and more difficult to get high ordered structures in monolayer film, even after the annealing in ultra high vacuum. On the other side, the increase of chemical group was helpful to decoupling effect, thus it might be an important way to realize single molecule fluorescence on metal
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surface by STML technology. In our experiments, the chemical groups were not enough for decoupling and the self assembled method could not prepare multi-layer film, so we still observed the nanocavity plasmon photon emission from the tip-porphyrin derivate-substrate systems, in which the molecule was just recognized as a dielectric layer.
In chapter four, we discussed the roles of lifetime to the molecular fluorescence in tunneling junction. Based on these discussions, we continued the STML research of long lifetime triplet molecules such as PtOEP, Eu(dpm)3 and Ir(ppy)3 on metal surface. As proved, PtOEP molecule, which was widely used in organic OLED, could be deposited one by one layer on metal surface with fine structure. But we didn’t observe
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