Optical Detection And Single Molecular Electroluminescence In Scanning Tunneling Microscopy

As the device size is driven down to the nanoscale,the Moore’s law that governs the development of microelectronic technology is going to reach its limit in the near future. Nanoscale optoelectronic integration is an emerging field that is aimed at the merging of nanoclectronics with nano-photonics and will provide a possibility to extend the Moore’s law to the post-Moore era. Therefore,understanding the molecular optoelectronic behavior in a nano-environment, especially at single molecule level, is crucial for the development of nano optoelectronic devices.A single molecule can act as a sensor for its nano-environment because of either its own optical features or its influence on the local optical properties. In addition, single molecular fluorescence reveals single-photon emission feature and may be a potential single-photon source for attractive applications in quantum information, quantum communication and quantum cryptography. However, the traditional excitation method based on lasers suffers from the diffraction limit,which can work only on samples that has a very dilute concentration.Tunneling electron excitation, by contrast, can excite a molecule selectively due to the highly localized nature of tunneling currents. The combined technique of a scanning tunneling microscope (STM) with highly sensitive optical detectors is capable of studying the optical and electronic characteristics of single molecules with high spatial resolution, high time resolution and high energy resolution.The signal of single molecular fluorescence is extremely weak. In order to reduce the difficulties of experiments,on one hand from the viewpoint of instrumentation, one has to combine efficient photon collectors and detectors with the STM, on the other hand from the tuning aspect of photonic states, one also has to optimize the sample structure so as to raise the quantum yield. In this dissertation,we focused on the optimization of the optics and sample structures to increase the photon signal intensity. We also developed different measurement methods for STM induced luminescence(STML).This dissertation is composed the following five chapters.In chapter one, we briefly introduced the background of STML.After the introduction of the history, basic principles,and applications of surface plasmons(SP), we presented an overview of the status of STML research on metals,semiconductors and nanostructurcs.The chapter is ended with a brief introduction on the principle and methods of single-photon detection.In chapter two,we first provided a comparison over the various types of photon collectors and detectors used in previous STML measurements.Then we used the Zcmax software to design a double-aspherical-lens photon collection setup.Both our simulation and experimental results indicated that the hemisphere solid angle coverage is up to13%for photon collection, so far the highest for the double-lens system.We also designed and built up a number of different photon detection modes for STML, including the photon mapping function based on avalanched photodiodes, the color mapping function by a charge couple device (CCD),tip enhanced Raman spectroscopy (TERS),and single photon correlation measurements.In chapter three, by using the STML technique optimized above, we demonstrated submolecular resolution photon maps for tert-butyl tetraphenyl porphyrin molecules on Ag(111)under small tunneling currents down to pico-ampere and short exposure time down to milliseconds per pixel.The molecules are found to modulate the emission intensity of tip induced plasmons from the substrate, with suppressed emission on the molecular island but enhanced emission at the island boundary. The contrast mechanism is further analyzed via an electrodynamic simulation that models the intramolecular structure as an effective dielectric medium. We attribute the sub-molecular contrast to the co-operative effect of gap-distance dependent plasmonic field strength with highly localized electrical excitations down to the sub-nanometer scale.In the first part of chapter four.we chose a-sexithiophene (a-6T) as a fluorescence molecule, which is known to behave as a good hole transport agent. We also studied the self-assembled growth of multilayer α-6T molecules and related optical properties. For5ML a-6T, we have obtained instinct molecular fluorescence through laser excitation, but no intrinsic molecular fluorescence could be observed by using tunneling electron excitation. We provide a qualitative explanation of these results in terms of both classical electrodynamic and quantum picture. These observations indicate that, in addition to the electronic decoupling from the metal substrate, the electronic structure and energy alignment of molecules should also be taken into consideration for generating STM induced molecular fluorescence. In the second part of this chapter, STML was carried out on porphyrin molecules by using the a-6T multilayer as a decoupling layer. We obtained a high quantum efficiency (QE) that reached10-3photons per electron, a value unobserved before for porphyrin systems. Such a high QE was not due to the tip-molecule contact but arises from the efficient recombination of electrons injected from the tip and abundant holes supplied by the α-6T. The plasmon-exciton coupling may also play an important role in the photon emission process.In the fifth chapter,we used an ultrathin NaCl as a decoupling layer and obtained STML from a single porphyrin molecule. We observed spectral shift and splitting phenomena on molecular clusters. The spectral splitting is considered to stem from the co-excitation of several molecules, while the spectral shift is believed to arise from the polar charge effect of the supported NaCl substrate.The calculated excitation energy by the time-dependent density functional theory (TD-FDT) showed a similar trend with the experimental result. Finally we perform a preliminary photon correlation measurement for STM induced luminescence from porphyrin molecules. We observed a small dip at the zero delay time, which may provide a hint of single-photon emission for the electron driven porphyrin electroluminescence.