Decoupling Studies In Scanning Tunneling Microscope Induced Single Molecule Fluorescence

Nanoscale optoelectronic integration is one of the important directions for future information and energy technologies. The underlying physical basis lies in the control of opto-electronic interaction and photonic states at the single molecular scale. Scanning tunneling microscope induced luminescence (STML) technique, combining the scanning tunneling microscope (STM) with the sensitive single photon optical detection, offers a comprehensive way to investigate the opto-electronic effects at single-molecule level, which can deepen our understanding on the interaction and conversion among electrons, photons, excitons and plasmons and thus lay foundation for the development of electrically driven nanoscale photon-sources and optoelectronic integration devices.The molecular fluorescence quenching is a serious problem in STM junctions due to the conducting substrates required for STM studies. For molecules directly absorbed on metallic substrates, the molecular fluorescence is completely quenched because of the ultra-fast electron transfer and the energy transfer between the emitters and the substrates. Therefore, how to avoid the fluorescence quenching is a key issue in STML studies. There are two methods to isolate the emitters from the metallic substrates:the physical decoupling method and the chemical decoupling method. The most common method is the physical decoupling method, which uses a thin insulating spacer between the emitters and the substrate to suppress the quenching. Meanwhile, the chemical decoupling method refers to the chemical synthesis of some super-molecular structures which employ certain functional groups to isolate the chromophores. Furthermore, the chemical decoupling method also enable us to achieve enhanced fluorescence intensity by modifying the dipole orientation of the chromophores with respect to the nanocavity plasmon (NCP) in STM junction.In this dissertation, focusing on the electronic decoupling issue in STML, we aim to realize single-molecule electroluminescence through both physical and chemical decoupling approaches. A configuration based on "tip-emitter-spacer-substrate" is adopted to investigate the electron-photon conversion process and the interaction between the nanocavity plasmon and moleculear excitations, with the spacer either as a physical self-assembled monolayer (SAM) or built-in within a multi-functionalized optoelectronic molecule. This dissertation is composed of the following four chapters.In chapter one, we first briefly introduce the STML technique and its research history and current status, then present a brief overview of nanoplasmonics and its application, finally we introduce the characteristics of the porphyrins and experimental instruments used in this work.In chapter two, molecule-specific fluorescence of single porphyrin molecules is realized using tunneling electron excitations with alkanethiols SAM as the physical decoupling layer to suppress the fluorescence quenching. The distance (d) from the emitter to metal substrates can be controlled through fine-tuning the length of alkanethiols. With increased chain lengths of alkanethiols from C4S-C8S (0.81-1.25nm), the Qx(0,0) emission band become narrowed. The full width at half maximum (FWHM, w) of this emission band is found to follow the relation w∝1/d2.9(7) which is close to the1/d3dependency predicted by the classical dipole theory and suggests that the classical dipole model appears still valid down to sub-nanometer scale.In chapter three, we study the chemical decoupling approach via using a self-decoupled porphyrin molecule with a tripodal anchor based on the design concept of an emitter-spacer-anchor configuration. We use different wet-chemistry methods to disperse these multi-functionalized optoelectronic molecules on Au(111). Nanoscale electroluminescence from single porphyrin molecules or aggregates on Au(111) has been realized by tunneling electron excitation. The rigid tripodal anchor not only acts as a decoupling spacer but also controls the orientation of the molecule. Intense molecular electroluminescence can be obtained from the emission enhancement provided by a good coupling between the molecular transition dipole and the axial nanocavity plasmon.In chapter four, we study two kinds of organic molecules with tetrapod structures in order to have better control on the dipole orientation of the molecules, since tripodal molecules do not always stand up on the substrate. For the tetrapod structure, since each pod has an emitter, there is always a fluorophore with the dipole orientation aligned along the axial direction of the probe, so it is possible to produce larger perpendicular dipole component and stronger plasmonic enhancement. thus results in big perpendicular dipole component. We use flash desorption technique to disperse tetrapod perylene and tetrapod porphyrin (ZnP4TPM) on Au(111), but molecule-specific fluorescence is also realized from tetrapod porphyrin molecules. From the STML studies it was observed that the quantum efficiency can be enhanced up to10-4, which is about1-2order of magnitude stronger than previous fluorescence from the emitters decupled by molecular multilayers (with a quantum efficiency of10-5～10-6).In summary, the fluorescence from single molecules has been realized using physical and chemical decoupling approaches to suppress the quenching. Of particular interest is the chemical decoupling approach, which can not only offer self-decoupling function, but also enable the control of transition dipole orientation. Stronger molecular electroluminescence may thus be expected due to the presence of larger axial dipole component and stronger plasmonic enhancement. These results open up a new route for generating electrically driven nanoscale light sources.