Scanning Tunneling Microscope Induced Molecular Luminescence From Multimonolayer Porphyrin Films

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.