Scanning Tunneling Microscopy Induced Luminescence Of Molecules On Different Substrates

The technological progress in the semiconductor industry has driven thecomponents of microelectronics down to the nanoscale in dimensions. Suchdownsizing trend imposes on us finding not only innovative fabrication technologiesbut also novel operational principles. Molecule-based nanodevices have become anactive forefront during the search for promising candidates of new electronic devicesbecause molecules are nanometer in size, highly functional, and can bemass-produced. One important research direction for future information and energytechnologies is nanoscale optoelectronic integration, whose scientific basis lies in thecontrol of the interaction between photons and electrons and the use of specificphotonic states for particular optoelectronic functions.The quest to investigate nanoscale quantum objects down to atomic andmolecular scale has promoted the development of a variety of experimentaltechniques for studying nanoscale optoelectronics. A scanning tunneling microscope(STM) can do more than atomic imaging and manipulation; its tunneling current canalso be used as a local source of excitation to produce light from the junction. Thistechnique, usually called STM induced luminescence (STML), can offer additionalinformation on local electromagnetic coupling associated with the decay of variousexcitations and has become an increasingly important tool for exploring theoptoelectronic behavior of single molecules in a nano-environment, in particularregarding the interaction of molecules with surroundings and the energy decaykinetics of excited states.Molecular fluorescence from tunnel junctions is one of the important researchdirections in the STML field. Single molecular electroluminescence can not onlyprovide fundamental understanding to the interfacial optoelectronic properties oforganic devices, but also, as a potential single-photon source, offer promisingapplications to quantum information processing. The realization of STM inducedmolecular luminescence and particularly single-molecule electroluminescence onsurfaces depend on how effectively the luminescent core can be decoupled from thesubstrate to avoid fluorescence quenching and how well a localized excitation sourcecan be applied to individual molecules. Depending on the type of substrates, the approach to making a decoupling layer and the effect of electronic decoupling aredifferent. On the other hand, although the nanometer size of a STM tip apex providesa natural excitation source that can be highly localized onto a single molecule, itremains unclear how molecules can be effectively excited and then decay radiativelyback to the ground state. One heavily debated issue there is the role of nanocavityplasmons in the light emission process of molecules. It is well known that substrateswith different materials can have very different plasmonic fields. Noble metalsubstrates feature strong plasmonic fields in the visible range, while the semimetallicHOPG substrate and the semiconducting Si(100) substrate show negligible plasmonicfields in the same optical range. Therefore, substrates play an important role in thephenomena and nature of STM induced fluorescence, and the research carried out inthis work using different substrates can help to clarify the mechanism of STMinduced molecular fluorescence. In addition, the mainstream materials for the currentand future information technology are silicon and graphene, the STML research onthe HOPG and Si substrates may provide useful information to the future molecularoptoelectronic integration at the nanoscale.In this dissertation, we focus on STM induced luminescence from moleculesthat are deposited on different substrates (namely, metal, HOPG and Si(100)). Suchresearch will help to reveal the important information on the coupling andinterconversion mechanism among electrons, photons, excitons and plasmons insidethe nanoscale tunnel junction and can provide useful scientific basis for thedevelopment of nanoscale molecular optoelectronic devices. The dissertation iscomposed of the following five chapters.In chapter one, we present an overview of the history and status of STM inducedluminescence. After a brief introduction of the plasmon concept and its application,we proceed to describe the basic principle of STM and STM induced luminescence.The chapter was concluded with a relatively comprehensive introduction of the STMLresearch on metals, semiconductors, and fluorescent molecules.In chapter two, we describe a reliable fabrication procedure of silver tips forSTML experiments. The tip was first etched electrochemically to yield a sharp coneshape and then sputter cleaned in ultrahigh vacuum to remove surface oxidation. Thequality of silver tips thus fabricated not only offers atomically resolved STM imaging,but more importantly, also allows us to perform challenging color photon mappingwith emission spectra taken at each pixel simultaneously during the STM scan and under relatively small tunnel currents and relative short exposure time.In chapter three, we investigate the STML of molecules on the metal substrates.The sample structure is composed of the perylene derivative (PTCDA) moleculesabsorbed on the surface of Ag(111) or Au(111). Noble metal substrates are known tofeature strong plasmonic fields. Through the comparison of the STML spectraacquired on different metal substrates, we are able to investigate the interactionbetween the molecule and the metal substrate and its modulation effect on thenanocavity plasmonic (NCP) emission from the STM junction. We believe that themolecules directly adsorbed on metal surfaces serve beyond as a geometric spacer, themolecule-substrate interaction could change the local density of final states and canthus modify the NCP emission through the variation of inelastic tunneling rates. Inaddition, the generation of molecular electroluminescence is found to depend on notonly the decoupling effect, but also the electronic structure and energy level alignmentof molecules at the interface.In chapter four, we study the electroluminescence properties of TPP porphyrinmolecules adsorbed on the HOPG surface. HOPG is a semimetal that featuresnegligible plasmonic modes in the visible range. By the virtue of this property, we canpractically rule out the influence of surface plasmons from the substrate on themolecular electroluminescence, which enables us to gain insights into the mechanismof STM induced molecular fluorescence. Photoluminescence spectra from a samplewith 5 monolayers (MLs) of TPP molecules on HOPG show intrinsic molecularfluorescence and thus indicate that at least the top molecular layer is well decoupledfrom the substrate. Upon fabricating a good emitting tip, we performed the STMLexperiments on the same sample and detected similar molecule-specific emission.Through both the comparison of different STML features using"bright"and"dark"tips and the selected enhancement of molecule-specific emission bands, we are lead toconclude that the near-field excitation from the tip plasmons play a decisive role ingenerating molecular electroluminescence. The mechanism invoked in the organiclight emitting diodes, either the intrinsic electroluminescence via impact ionization orthe inject-type electron-hole recombination, is unlikely in the STM tunnel junction.In chapter five, we carry out preliminary study of the STML of molecules on thehydrogen-terminated Si(100)-2×1 surface and demonstrate for the first time theHOMO-LUMO radiative transition from porphyrin molecular clusters on the Sisubstrate. The plasmonic field strength of semiconducting Si substrates is also very weak in the visible range, whose effect on the molecules can thus be ignored. Ifmolecules are directly adsorbed on the Si(100) surface, the molecules tend to bemodified or even destroyed due to the strong bonding with the Si dangling bonds.However, the hydrogen-terminated Si(100)-2×1 surface can act as the (thinnest)decoupling layer and prevent the interaction between the molecules and the Si(100)surface. The STML properties of several molecules (PTCDA, TPP, ZnTPP andH2TBPP) on the H-Si(100)-2×1 reconstructed surface were examined. We failed todetect molecule-specific emission from the former three molecules (PTCDA, TPP andZnTPP), probably duo to either strong interaction of the molecules with the substrateor packing configurations. However, we succeeded in detecting the intrinsic Q-bandemission from the H2TBPP molecules, probably due to the better decoupling functionof the eight tertiary butyl groups decorating the perimeter of the H2TBPP core.