Theoretical Investigations Of Scanning Tunneling Microscope Induced Light Emission

A scanning tunneling microscope (STM) is capable of more than just observing and manipulating the nanoworld with atomic resolution, the tunneling current can also be used as a local source of excitation to produce light from the junction, which can provide additional information on local electromagnetic properties pertaining to the decay of various excitations. Moreover, STM induced luminescence (STML) links the study of molecule electronics with molecular optoelectronics, which may realize the ultrahigh spatial resolution with chemical recognition. In order to gain insights into optical transitions of molecules and related energy transfer, the first step is to obtain STM induced molecular luminescence. However, molecular fluorescence is quenched when emitting molecules are close to a metal surface. So, a decoupling layer should be inserted between the emitting molecule and the metal substrate to reduce the nonradiative energy transfer from excited molecules to the metal substrate. Moreover, the electromagnetic interaction between the molecules and the metal substrate will modify the spectra profile and intensity of STM induced molecular luminescence. In this dissertation, we first investigate the influence of a dielectric layer on photon emission induced by a STM, and then extend our theories to STM induced luminescence from the vibronic transitions of tetraphenyl porphyrin (TPP) molecules. We find that the resonant excitation by nanocavity plasmons will greatly modify the emitting spectra, even generate hot luminescence from higher vibronic state S₁(1) or S₁(2). In order to generate strong nanocavity plasmons and realize strong resonant excitation, we investigate the optimization of nanocavity field enhancement by two-dimensional plasmonic photonic crystals (PPC). The dissertation is mainly composed of the following four chapters.In chapter one, we first briefly introduce the definition and basic principles of surface plasmons and some related plasmonic effects and potential applications. We also introduce the basic concepts of STM and some theoretical models that are often used in STM related problems. Then, we present a relatively comprehensive introduction to the history and status of theoretical studies on STM induced light emission. The recent research progress on STM induced molecular luminescence is also introduced briefly. In chapter two, we propose a combined approach of first-principles calculations with classical electrodynamics to elucidate the effect of a dielectric layer on STM induced light emission. First, we use density functional theory (DFT) to calculate the effective potential along the surface normal for a preset gap distance and then solve the Schr?dinger equation to obtain the wave functions and their corresponding tunnel currents. Then, we use realistic hyperbolic tip shape and the boundary element method (BEM) to calculate the local electric field enhancement at the STM cavity. The optical properties of light emission are calculated using the reciprocity theorem of electrodynamics in the nonretarded limit. We investigate two typical systems, one with a molecular dielectric layer (W-tip/C₆₀/Au(111)) and the other with an oxide dielectric layer (W-tip/Al₂O₃/NiAl(110)). We find that in both systems the radiated power is reduced considerably in comparison with the emission from the pristine metal surface but the spectral profile remains very similar without substantial peak shifts. We demonstrate that the suppression of the radiated power is mainly due to the increase of the tip-metal separation and resultantly the reduction of the electromagnetic coupling between the tip and metal substrate. The absence of substantial peak shifts is a competing result of the blueshift because of the increased tip-metal separation by the redshift of screening of the dielectric layer in the STM cavity. It is noteworthy that, the agreement of our calculated results with experiments is owing to the exact description of the surface potential by the DFT when a dielectric layer is inserted, so that the distance-current dependency can be properly evaluated. The vacuum gap distance between the tip and layer surface is found to be slightly shortened in spite of the overall increase of the tip-metal separation.In chapter three, we investigate the modification effect of the STM induced molecular luminescence by nanocavity plasmons (NCP). Each vibronic transition of the molecule is modeled as an independent damped oscillator. The electromagnetic interaction between the molecule and the metal substrate is treated by the effective medium theory. The dielectric function of the effective medium ((ε|-) ) is defined by the contribution from both the molecule layer and the metal substrate. The typical system we investigated is the STML from five-monolayer TPP on Au(111). We find that when the resonant frequency of the nanocavity plasmons is tuned to match with certain vibronic transition of the TPP molecule, the emission intensity from this transition is strongly enhanced. By tuning the nanocavity plasmons, we even obtain resonant hot electroluminescence from higher vibronic state S₁(1) or S₁(2) for TPP molecules. Moreover, we find that the shift of resonant wavelength of the nanocavity plasmons will lead to a small shift of emission peak from the TPP molecules in the same direction. Our calculated results can produce nicely the experimental observations. We attribute the spectra profile modifications to the strong resonant excitation by nanocavity plasmons, followed by resonant fluorescence from non-equilibrium excited states.In chapter four, in order to further discuss the way to generate strong nanocavity plasmons, so that a high spontaneous emission rate of molecules can be obtained, we investigate the influence of the Ag nanorod radius (r) on the resonant modes of a two-dimensional plasmonic photonic crystal (PPC) with dipole sources embedded into the central vacancy area, by using finite-difference time-domain (FDTD) methods. Both the localized surface plasmon (LSP) mode of individual Ag nanorods and the resonant cavity mode of PPC are found to vary as a function of r. The resonant cavity mode is strongly enhanced as r is increased, while the LSP signal will eventually become no longer discernable in the Fourier spectrum of the time-evolved field. Furthermore, an optimized condition for the nanocavity field enhancement has been found for a given PPC periodicity (e.g., d=375 nm) with the critical nanorod radius r_c=d/3, at which the resonant cavity mode has the strongest field enhancement, best field confinement, and largest Q-factor. We attribute such field enhancement optimization to the competition between the blocking of cavity confined light to radiate out when the cavity resonant frequency falls inside the opened photonic stopband as r reaches rc and the transfer of cavity mode energy to the inter-particle plasmons when r is further increased. Moreover, we find that the resonant frequency of the cavity mode has a maximum resonant energy at the critical radius r_c, and can be tuned over a relatively wide energy range by changing the nanorod radii, e.g., from～526 nm at r=62.5 nm to～488 nm at r=125 nm. Our calculation results indicate a possibility not only to tune the maximum field enhancement, but also to tune the energy of resonant cavity mode and the position of photonic stopband through the nanorod radii so that strong resonant excitation can be realized. These results may provide new routes to the resonant enhancement of spontaneous emission rates of quantum emitters in the cavity and to the quality improvement of the PPC plasmonic nanolasers.