【作者】 张杨；

【导师】 侯建国； 董振超；

【作者基本信息】 中国科学技术大学 ， 凝聚态物理， 2010， 博士

【摘要】 分子尺度上的光电集成器件是未来信息能源技术中的一个重要发展方向,其科学基础是在纳米尺度上对光电相互作用以及光子态的调控。当一个分子靠近金属纳米结构的时候,分子的荧光强度、峰型及角分布等性质会受到纳腔等离激元(nanocavity plasmons, NCP)的极大调制。通过控制金属纳米结构的尺寸、材料与形状,纳腔等离激元的共振峰位会有明显的变化,进而可实现对分子发光性能的控制。分子发光能够直观地反映分子光学跃迁与能量转移本质,而要得到分子发光,首先需要避免金属纳腔对分子荧光的淬灭。本论文利用高光学灵敏度和高空间分辨率的扫描隧道显微镜(STM)诱导发光技术,研究了单个分子在隧道结纳腔中的发光性能以及介电脱耦合层对实现分子荧光的重要作用,并将我们在STM诱导发光中得到的规律拓展到普通的金属纳腔体系,实现了卟啉分子的光致特殊荧光。我们的工作有助于揭示纳米尺度下,电子、激子、等离激元与光子之间的耦合与转化机制,可以为分子光电器件的研究提供重要的科学依据。本论文主要分为以下四个部分。在第一章中,我们首先介绍了表面等离激元的定义、特性以及表面等离激元学的在各方面的应用。由于扫描隧道显微镜诱导发光是我们的一个主要探测手段,我们随后介绍STM诱导发光技术。在简要介绍光子收集与检测等实验技术后,我们较为系统的阐述了STM诱导发光在金属表面、半导体表面的研究,以及荧光分子等体系的研究历史与现状。最后,我们还简要介绍了我们所采用的实验仪器。在第二章中,我们以扫描探针-CoOEP分子-金属衬底体系为主要研究对象,利用STM,通过对吸附在Au表面上的单个CoOEP分子进行切割操纵,比较了操纵前后在CoOEP分子上所测量的光谱,进而分析了分子对探针诱导的隧道结纳腔等离激元发光的影响。我们提出分子扮演着间隔层、能量耗散体以及电偶极振荡体三种作用,详细分析了吸附分子对纳腔表面等离激元发光的调制作用。与此同时,我们发现STM诱导的隧道结纳腔等离激元发光对局域环境的变化非常敏感,在分子的不同位置以及临近分子的金表面上,发光强度都会所有不同。第三章详细地研究了Al2O3/NiAl表面上两种Zn卟啉分子(ZnTPP和ZnTBPP分子)的STM电致发光现象。结合STS谱,我们认为直接吸附在薄层氧化铝表面上的Zn卟啉分子仍与NiAl衬底存在较强的相互作用,无法得到来自中性分子HOMO-LUMO的荧光,而小分子团簇上的第二层Zn卟啉分子利用第一层的分子做进一步的脱耦合,就能够实现中性分子荧光。通过改变STM纳腔等离激元共振模式,我们实现了来自高振动激发态的热荧光,以及发射光子能量高于激发电子能量的上转换发光。结合我们在Au上多层TPP分子的STM诱导发光结果,我们讨论了探针激发纳腔等离激元对分子荧光的三个方面的作用:提高分子的激发速率、提高分子的辐射速率、决定哪些光子能够耦合到远场。第四章在前面的工作基础上,进一步拓展了我们对于分子光学频谱的调控工作。前期通过修饰STM探针的方法,难于可控的实现表面等离激元共振峰位的调控。在本章中,我们通过化学合成的方法来制备具有特定等离激元共振的金银纳米结构。将单分子层的卟啉分子蒸发到Si片表面的金银纳米颗粒膜、金银纳米颗粒“阵列”上,通过表面等离激元与分子荧光的共振激发,提高了卟啉分子对应特殊荧光峰的激发速率,我们实现了平时难以观察到的卟啉分子热荧光,实现了对分子的发光频谱调控。更多还原

【Abstract】 Molecular optoelectronics is one of the important research directions for future developments in information technology and energy technology. Its scientific basis lies in the control of electron-photon interaction and tuning of molecular photonic states. Close to a metallic nanostructure, molecular fluorescence spectra will be greatly modified in intensity, spectral shape, and angular distribution, due to the coupling of molecular emitters with surface plasmons. The surface plasmon resonance mode of a metallic nanostructure could be modulated through the change of its material, size, and shape as well as the surrounding dielectric media, which offers flexible means to modulate and control molecular fluorescence. In order to gain insights into molecular optical transitions and related energy transfer processes, it is crucial to generate molecular specific photon signals, and to that end, a decoupling structure should be used to avoid the fluorescence quenching caused by the nearby metallic nanostructures. In this dissertation, we first investigated tunneling electron induced molecular fluorescence in a scanning tunneling microscope (STM) junction nano-cavity, and then extended the frequency tuning obtained in STM induced luminescence (STML) to more general situations of molecular photoluminescence (PL) near metallic nano-cavities. Through surface plasmon resonance excitation, special fluorescence is generated from high excited states of porphyrin molecules. This work provides important information on the coupling and interconversion mechanism among electrons, excitons, plasmons, and photons, which may pave the way for the development of nanoscale molecular optoelectronic devices. This dissertation is mainly composed of the following four chapters.In chapter one, we start with a brief introduction of the definition, characterizations, and applications of surface plasmons. As a major technique in this work to explore the interplay between molecules and surface plasmons, a relative comprehensive introduction is presented on STML research, from experimental setup and measurements, to the highlighted STML research on metals, semiconductors, and organic molecules. At the end of the chapter, the STML experimental instruments in our lab are briefly introduced.In chapter two, the modulation effect of adsorbed molecules on nanocavity surface plasmon (NSP) emissions in STM junctions were investigated through single molecular manipulation. Tunneling electron excited luminescence spectra indicate an overall suppression of NSP emissions when an intact or excised CoOEP molecule is inserted into the junction. However, the NSP emissions for the excised molecule are found to be stronger than that of the intact molecule, somewhat unexpected for a slightly larger molecule-substrate separation. We attribute such enhancement to a competing result of three different kinds of roles that an adsorbed molecule can play in NSP emissions: a geometric dielectric spacer, an energy dissipater, and a dynamic dipole oscillator. The NSP emission spectral intensity varies in the different part of an excised CoOEP, due to different local environment, which suggests the potential of NSPE mapping to reveal the fine structures inside an adsorbed molecule.In chapter three, we studied the electroluminescence properties of Zn porphyrin molecules (ZnTPP and ZnTBPP) adsorbed on the ultrathin Al2O3 decoupling layer grown on the NiAl surface. Combined with STS, we find that the neutral molecular fluorescence could not be generated from the Zn porphyrins directly adsorbed on alumina, due to strong hybridization between the molecules and NiAl substrate. Through further decoupling of the first layer of Zn porphyrins on alumina, neutral molecular fluorescence is observed on the second layer of porphyrin molecular clusters. Hot-luminescence and up-conversion fluorescence are also generated through the modulation of surface plasmon resonance modes in STM nano-junction (or, NSP resonance). These observations, combined with our previous studies on electroluminescence of TPP multi-layers on metals, lead us to speculate the three major role of NSP resonance in molecular fluorescence from nano-cavites: 1) Excitation enhancement on certain energy-matching optical transitions, 2) emission enhancement through the increase of the radiative decay rate, and 3) selection of photonic modes that can be coupled to far-field.In chapter four, we extend our frequency-tuning research of molecular fluorescence from STML to molecular photoluminescence (PL) near metallic nano-cavities. The NSP resonance in STM junctions is difficult to control due to the lack of a“controlled”STM tip. In this chapter, both metallic thin films of noble metals (Au, Ag) and“arrays”of metallic nano-particles on Si substrates are fabricated, with desired surface plasmon resonance modes. Through mutual resonance excitation between adsorbed porphyrin molecules and surface plasmons in nano-cavites, the excitation rates of the special optical transitions are greatly increased. hot-luminescence, usually unobservable in normal spectral measurements, are generated and detected, extending the frequency tuning range of porphyrin molecules from the normal red to green regime.更多还原