Exploring The Interplay Between Molecular Fluorescence And Nanocavity Plasmon Resonance

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.