Nanoscale Characterizations By Scanning Tunneling Microscope Induced Luminescence

How to characterize the optical and electrical properties of nanostructures at the single molecular scale is important for the development of nano-optoelectronics. Being advantageous in achieving atomic resolution in real space, scanning tunneling microscopy ?STM? have significantly extended our ability to understand and control the microscopic world to a brand new level. STM induced luminescence ?STML?technology, which combines the ultrahigh spatial resolution of STM with the time and energy resolution of optical detection, can not only improve the chemical analysis ability of STM, but also open up new ways to study the local optoelectronic behavior of nanostructures in a nano-environment and related decay kinetics. This dissertation focuses on the instrumental development of the STML technique and its applications in both achieving single-molecule electroluminescence and identifying intermediate states of a H-tautomerization process. Using STML, we demonstrate that the realization of STM induced hot luminescence from well-isolated single molecules. Furthermore,through spectrally and spatially resolved photon imaging for the relaxationless luminescence, we observe the intermediate state in the H-tautomerization process at the single-molecule scale for the first time. Our works lay the foundation for the applications of STML in the areas of electrically driven nanoscale light sources, nano-optoelectronic integration and reaction dynamics.The dissertation is composed of the following four chapters.In the first chapter, we give a brief introduction of the concept and principle of STM and plasmonics. Then we present an overview of STML, describing its history, status and development. The chapter is concluded with an introduction of the experimental setups used in the present work.In the second chapter, we investigate the electroluminescence from an isolated single free-base phthalocyanine ?H₂Pc? molecule induced by STM. By using an ultrathin NaCl island as a decoupling layer on Ag?100?, we demonstrate the realization of single-molecule electroluminescence that originates from intramolecular HOMO-LUMO transitions of H2PC. By analyzing electroluminescent spectra and scanning tunneling spectra, we discuss the mechanism of single-molecule electroluminescence and the important role of nanocavity plasmon in STML. The chapter is concluded by a brief description on how single-molecule electroluminescence can be modulated by STM manipulation.In the third chapter, we investigate the reaction process of current induced H-tautomerization in a single H₂Pc. By exploiting the resonant enhancement of nanocavity plasmon on the spontaneous emission rate, the relaxationless hot electroluminescence is generated and the spectral peaks are found to be split. Through spectrally and spatially resolved photon imaging for the split peaks, each sub-peak is found to correspond to either a trans-isomer or a cis-isomer by combining with theoretical simulations. That is to say, our STM induced molecular luminescence technique allows us to directly identify the presence of an intermediate state between two stable trans-isomers in the intramolecular proton transfer process. Our results not only deepen our knowledge on the proton transfer phenomena in single molecules, but also open up new routes to explore ultrafast molecular dynamics through spectral information in the energy and space domains.In the fourth chapter, we show our preliminary effort to extend the instrumental function of STML to include the near-field detection by a fiber tip and back-focal-plane imaging from the backside of a transparent sample. Using a fiber tip coated with indium tip oxides ?ITO? as a STM tip, we have successfully detected the near-field emission from the GaAs?111? surface and the second-layer H2TBPP molecules adsorbed, which demonstrate the feasibility of the near-field detection in our current system. We have also started to develop the back-focal-plane detection through the optical collection systems at the backside of a transparent sample in an ultrahigh-vacuum and low-temperature STM. Now we have successfully detected the back-focal-plane patterns of electrically excited nano-cavity plasmon emission from STM junctions, which may pave the way for our future studies on the emission properties such as the angular distributions, polarizations, dipole orientations, etc.