【作者】 张晓磊；

【导师】 董振超；

【作者基本信息】 中国科学技术大学 ， 凝聚态物理， 2009， 博士

【摘要】 随着半导体微电子器件在材料尺寸和运作性能上趋于极限,基于分子和纳米结构的器件研究已是历史所趋、势在必行。未来信息和能源技术的一个重要发展方向就是分子尺度上的光电集成技术,而其物理基础便是分子尺度上的光电相互作用和光子态调控。扫描隧道显微镜(STM)不仅可以用来观察和操纵纳米世界的一个个原子和分子,而且其高度局域化的隧穿电流还可以用来激发隧道结发光,能提供隧道结纳腔内与各种激发及其衰变有关的局域电磁场响应和跃迁本质等信息。这种利用STM隧穿电流的激发来研究隧道结发光特性的技术称为STM诱导发光技术(STML),是一种融合高分辨扫描隧道显微镜和高灵敏单光子检测技术优点的集成系统。这种联用技术为我们提供了一种新的联系单分子电子学和单分子光电子学的实验手段,不仅利用隧穿电流的输运信息,而且通过非弹性过程所产生的光子信息去研究分子-固体界面的性质以及分子在纳米环境中的光电行为,特别是与隧道结中分子或纳米结构的电子输运和能量转移有关的现象与机制。要了解光学跃迁和能量转移信息,首先就需要让隧道结中的分子发光。然而,隧道结中的分子由于离金属表面很近,其发光受到荧光淬灭作用的强烈制约。如何通过分子光子态调控来实现纳米隧道结中的分子发光,就成为我们的研究重点。本论文的工作集中在金属表面的有机分子发光体系,通过采用分子脱耦合技术,利用光致激发和STM电致激发的方法研究了Au(111)表面上TPP卟啉分子的荧光现象,并提出一些新的机制来解释实验中观察到的新的光电效应。本论文主要按照以下三个部分进行讨论。在第一章中,我们介绍了扫描隧道显微镜诱导发光的研究背景。我们首先简要介绍了扫描隧道显微镜的诞生、发展、构造、原理以及在原子级成像和操纵领域的应用;然后系统介绍了STM诱导发光在金属表面、半导体表面、纳米微结构以及有机分子等体系的研究历史和现状,并给出了文献报道的一些理论模型;最后我们对有机分子发光的光子态调控作了简介。在第二章中,我们研究了硫醇自组装膜(SAM)脱耦合层对金属表面亚单层卟啉分子的荧光衰变特性的影响。我们通过精确调控短链硫醇分子的长度,来研究SAM间隔层厚度很小(1–2.5 nm)的情况下,TPP/SAM/Au体系的荧光光谱和寿命变化规律。我们发现,硫醇自组装膜作为电子脱耦合层有效地阻隔了分子和衬底之间的电荷传输,削弱了荧光淬灭效应,使得顶层的TPP分子发出特征的Q带荧光,表明TPP/SAM/Au体系的STM诱导分子发光具有一定的可行性。此外我们还发现,非辐射能量传输的淬灭效应在TPP/SAM/Au三明治结构中依然占据主导作用,衰减速率与间隔距离之间存在1/d3的依赖关系,表明即便在分子和衬底之间的距离小到1 nm的情况下,经典的CPS镜像偶极理论依然适用,而且激发态分子和金属衬底之间的能量传输是通过块体衰减机制来实现的。第三章是本论文工作的重点。我们详细研究了金属表面上超薄TPP卟啉分子膜的STM电致发光现象,实现了频率可调的分子电致发光,并发现了令人惊奇的来自高振动激发态的无驰豫热荧光现象,以及发射光子能量比激发电子能量还要高的上转换发光现象。为了避免分子和金属衬底之间距离太近导致非辐射的能量传输,引起强烈的荧光淬灭,我们采用多层分子脱耦合(5ML～3 nm)的途径来调控分子光子态,使得处于STM隧道结中的顶层分子发光。高分辨的STM图像表明,TPP分子逐层生长,并形成高度有序、平整的堆积结构,保证了样品的质量和实验的可控性。我们发现,在高度限域的STM纳腔环境中,分子的电致发光光谱形状不再简单地遵循在孤立分子那样的Franck-Condon分布,而是依赖于纳腔等离激元(NCP)的模式。当我们把STM电场诱导的NCP共振频率调控到与特定的分子光学跃迁[S1(v’≥0)→S0(v≥0)]的频率相匹配时,该(v′,v)辐射跃迁就会得到显著的共振增强,导致光谱形状发生巨大变化,并拓宽发光能量范围达4倍之多(即将TPP分子荧光范围从650?730 nm拓展到560?780 nm)。不仅如此,当激发电压对应的等离激元的能量低于(0,0)或(0,1)这样的光学跃迁能量时,分子依然能够通过多步非弹性散射的过程被有效激发,发射出能量上转换的光子;这些光子会反过来诱导更高能量的等离激元,并被纳腔进一步放大,从而引发新的分子共振增强荧光。我们进一步提出等离激元辅助的“反斯托克斯”拉曼散射机制,来解释STM诱导分子发光中的上转换现象。我们的研究结果表明,高度局域的纳腔等离激元场可以看作是能量可调的近场相干光源,可以对分子的激发和发射产生很强的共振增强作用,进而控制分子的辐射衰变通道,实现频率可调的分子电致发光。这些现象和认识有助于揭示纳米等离激元环境中电子、激子、等离激元、声子、以及光子等基本量子之间的耦合和转化机制,对于纳米光源的研制、分子等离激元学和单分子光电子学的发展具有崭新而深远的意义。更多还原

【Abstract】 As the downsizing trend of the nowaday microelectronic semiconductor device will soon reach its physical limit, there is a demand for exploring new technologies and one of the alternatives is molecule-based nanodevices. Molecular-scale optoelectronic integration is one of the important research directions for future Information and energy technology and its physical basis lies in electron-photon interaction and controlled tuning of photonic states. A scanning tunneling microscope (STM) can go beyond imaging and manipulation with atomic resolution; the highly localized tunneling current from the nanoprobe can also be used for excitation of light emission, the so-called STM induced luminescence (STML). Such combined technique of STM with single-photon detectors can provide additional information on local electromagnetic properties pertaining to the excitation and decay of various excited states in the tunnel junction and offers a new experimental tool to link the study of single-molecule electronics with single-molecular optoelectronics. Apart from the transport property from tunneling currents, the photon signals excited by inelastic tunneling processes can reveal further information of charge transport at the molecule-solid interface and optoelectronic behavior of molecules in a nanoenvironment, especially the phenomena and mechanisms on the electron transport and energy transfer in the tunnel junction.In order to gain insights into the nature of molecular optical transition and energy transfer at the nanoscale, the first step is to generate molecular-based photon emission from the junction. However, light emission from molecules near metals is challenging due to the fluorescence quenching effect. Its occurrence requires strategies to tune the molecular photonic states so that the nonradiative damping of molecular excited states is suppressed. In this dissertation, we focused on STM induced molecular fluorescence from organic molecules on metal surfaces using molecules as a decoupling layer. After a comprehensive background introduction of the STML field, we investigated the decoupling effect of short-chain alkanethiols on the photoluminescence behavior of quasimonolayered porphyrin molecules near metals, and then moved to the description of the major part of the thesis, the STM induced molecular fluorescence from multi-monolayer porphyrin films. The dissertation is composed of the following three chapters. Chapter one starts with a literature survey in the field of STM-induced luminescence. After a short briefing on STM about its operational principle and applications in atomic-scale imaging and manipulation, we present a relatively comprehensive introduction on the past history and present status of the STML research, from experimental setup to modes of measurements, with highlighted STML examples from metal and semiconducting surfaces to organic molecules and nanostructures. This is followed by a short description of some theoretical models on STML. The chapter concludes with a brief introduction on the tuning of photonic states for molecular fluorescence near surfaces.Chapter two deals with the decoupling effect of short-chain alkanethiols on the photoluminescence property of quasi-monolayered porphyrin molecules near metals. We investigate the spectral feature and fluorescence decay of porphyrin?alkanethiol?metal sandwich structures at very small separations (1–2.5 nm) through fine-tuning the length of alkanethiols. The self-assembled monolayer (SAM) formed by alkanethiols on Au(111) acts as an efficient electronic decoupling layer and suppresses the interface quenching via direct charge transfer. Clear Q-band emissions are observed for the tetraphenyl porphyrins (TPP) in the sandwich structures, which implies that the TPP/SAM/Au structure may be feasible for the generation of STM induced molecular fluorescence. However, the fluorescence quenching via nonradiative energy transfer to the metal still prevails in the porphyrin-alkanethiol-metal sandwich structures. The decay rates are found to follow a 1/d3 dependency on spacer thickness, which suggests that the classical electromagnetic theory appears still valid at distance down to 1 nm through volume damping.Chapter three presents detailed studies on the STM induced molecular fluorescence from an ultrathin TPP film (<3 nm) and demonstrates the color-tuning of electrically driven molecular fluorescence of porphyrins not only into the hitherto unseen low-energy region of relaxed fluorescence, but also into the striking hot-luminescence regime emitting directly from highly excited vibronic states (S1(v′>0). We use a multimonolayer decoupling approach in which the emitting molecules on the top are positioned within the localized cavity plasmonic field, but not too close to the surface to avoid otherwise fast nonradiative damping. High-resolution STM imaging indicates that they grow layer by layer and form highly ordered patterns up to several monolayers, which provides a clear justification for controlled experiments on a well-defined sample. We find that in a highly confined STM nanocavity, molecular electroluminescence no longer follows the Franck-Condon distribution for free molecules but is governed by the frequency dependent nanocavity plamon mode. A particular S1(v’≥0)→S0(v≥0) vibronic transition is found to be strongly enhanced when the resonance frequency of electric-field induced plasmons is tuned to match the molecular transition, which yields dramatic spectral profile modifications. Furthermore, when the plasmon energy is lower than and off resonant with electronic excitations such as (0,0) and (0,1) transitions, the molecule can still be effectively excited to emit upconversion photons through multi-step inelastic scattering processes. The emitted photon in turn induces new plasmons with higher energy that can be further amplified by the nanocavity, leading to new resonance enhanced fluorescence of the molecules. We propose a plasmon assisted“anti-Stoke”Raman scattering mechanism to explain the upconversion molecular electroluminescence. Our observations demonstrate that, the strong near fields of local nanocavity plasmons behaves like a strong coherent optical source with tunable energy and can be used to control the radiative channels of molecular emitters via intense resonance enhancement on both excitation and emission. Our results shed new light on how electrons, excitons, plasmons, and photons are coupled and interconverted in a nanoscale plasmonic environment.更多还原