Decoupling Control In Scanning Tunneling Microscope Induced Molecular Luminescence

With the device size down to the nanoscale, semiconductor microelectronics technology will face great challenge due to the bottleneck issues related to the quantum leakage, heat dissipation and RC-deday. In comparison with electrons, photons as an information carrier are advantageous in terms of wide bandwidth, large capacity, high speed and low power consumption. However, conventional photonic technology is subjected to the Abbe diffraction limit and is difficult to realize high density of integration. How to drive down the size of the photonic element and merge the electronics and photonics at the nanaoscale has become one of the primary issues in the development of nanophotonics and nanoscale optoelectronic integration. Electrically driven nano-sources of photons provide a possibility for manufacturing nano-optoelectronic devices, and the physical basis of which is the interaction and conversion between electrons and photons at the nanometer scale. Scanning tunneling microscope (STM) not only can be used to characterize materials structures down to the atomic and molecular scale, the tip of the STM can also be used as an atomic-size source of tunnel electrons to electrically excite quantum emitters. The combination of STM with highly sensitive photon detectors provides us a unique means to probe electron-photon coupling and optoelectronic effects at the nanoscale.Previous results of STM induced luminescence from molecules have shown that electronic decoupling and nano-cavity plasmon are the two key factors for getting molecule-specific luminescence. To avoid quenching of the molecular radiative transitions, strategies have been used by inserting insulating film or multilayer molecules as a spacer for the fluorescent molecules. However, to our knowledge, chemical decoupling inside a single organic molecule have not yet been reported, probably due to the complexities of molecule design and synthesis as well as the difficulties in sample purification and fabrication on the surface. In this dissertation, in addition to the physical decoupling strategy, we investigate STM induced molecular luminescence through chemical modification of molecules such as adding spacer groups as a decoupling layer to separate the fluorescent emitter from the metal substrate.This dissertation is composed of the following four chapters.In chapter one, we first briefly introduce the research background of STM and nanoplasmonics. Then we present a broad review of the STM induced luminescence (STML) technique, particularly on its research history and current status. The chapter is concluded with a brief introduction of the experimental instruments used in this work.In chapter two, we investigate the modulation of C6o molecules on the nanocavity plasmonic (NCP) emission from the Ag tip-C60-Au(111) system. Through combined analyses of bias dependent STML spectra, photon maps, and differential conductance data, we not only justify the plasmonic nature of emission in this system, but also point out that the observed emission suppression is mainly a result of reduced local field strength due to the increase of gap distance. More importantly, the NCP emission is found not only redshifted significantly at a given voltage, but also strongly coverage dependent in terms of the onset voltage and energy cutoff. These observations indicate that the C60molecules act beyond a pure dielectric spacer, their electronic states are actively involved in the NCP emission process. Furthermore, the energy of the NCP emission follows qualitatively a modified quantum cutoff relation, hv≤|eV-EM|, with the molecular state serving as an initial or final state in the inelastic tunneling (IET) process. In this chapter, we also demonstrate molecularly resolved photon maps on the C60monolayer and discuss the mechanism of the optical contrast briefly.In chapter three, we investigated the influence of dipole orientation on the STM induced luminescence from standing pentacene molecules that are packed on the C6o/Au(111) surface, where the C60monolayer is used as a template for the growth of pentacene and also serves as a bottom decoupling layer, separating pentacene molecules from metal substrate to avoid quenching effect. Electroluminescence from the free excitons of pentacene crystals was obtained in the multilayer-pentacene/C6o/Au(111) system for a layer thickness of pentacene around5-6ML, which indicates again that only a few monolayers of pentacene molecules is sufficient to avoid fluorescence quenching even for highly conducting molecules like pentacene. We also find that the molecular luminescence appears only at positive sample biases, which might be a combined result of the junction asymmetry of the molecular states, including the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), with respect to the Fermi level of the metal substrate. In addition, the transition dipole of pentacene molecules can interact with the nanocavity plasmon, as revealed by the selected enhancement on the emission band that spectrally matches to the NCP resonance. However, we find that the emission intensity in the present pentacene-C60-Au system is relatively weak, which suggests that the vertical component of the molecular transition dipole is probably not large enough to invoke a strong interaction with the NCP field. The reason for such weak coupling is probably because the dipole orientation of the lowest transition mode associated with the HOMO-LUMO transition is parallel to the molecular plane, but along the short molecular axis. Although the tilted standing configuration offers a certain degree of vertical components that can resonate with the NCP field, the plasmon-exciton coupling is still not very intense. Therefore, the design of functional optoelectronic molecules that can produce large vertical dipole components along the tip axis direction is important to obtain plasmon enhanced molecular luminescence.In chapter four, we investigate such chemical decoupling approach for STM induced molecular luminescence via the design of functional organic molecules with a built-in spacer group. There are two purposes for designing and synthesizing the functional molecules. One is to use a chemical group as a decoupling unit that separates the fluorescent group from the metal substrate to avoid the quenching effect. The other is to align the transition dipole moment of molecules along the direction of the NCP field to obtain plasmon enhanced fluorescence. In this chapter, we investigate three types of porphyrin derivatives. The first type is oxporphyrinogen derivatives with N-substituted porphyrins (Porph2OxP). Porph2OxP adsorbs on Au(111) with the OxP unit interacting strongly with Au(111) and N-substituted porphyrins staying away from the Au(111) surface. However, the fluorescence from N-substituted porphyrins is still quenched because of the energy transfer between porphyrins and OxP and the strong interaction between OxP and Au(111). The other two types of porphyrin derivatives are tripod porphyrin and tetrapod porphyrin (ZnP4TPM). Our experiments show that the top fluorophore of the tripod and tetrapod porphyrins can be effectively decoupled from the Au(111) substrate and our STML results for these two molecules exhibit molecule-specific fluorescence from top porphyrin group. This is probably because the transition dipole of the top porphyrin group in both tripod and tetrapod porphyrins has a significant vertical component that can couple strongly with the STM nanocavity plasmon, thus resulting in strong plasmon enhanced fluorescence.In summary, the research results demonstrated in this work provide a strong message that, the control on the molecular configuration with decoupling function and preferred dipole orientation is crucial to obtain plasmon enhanced molecular fluorescence. These findings are important for the development of electrically driven nanoscale light sources and nano-optoelectronic integration.