Scanning tunneling spectroscopy

About: Scanning tunneling spectroscopy is a research topic. Over the lifetime, 7886 publications have been published within this topic receiving 213828 citations.

Electronic structure of a polydiacetylene nanowire fabricated on highly ordered pyrolytic graphite.

Abstract: A molecular wire candidate, the polydiacetylene chain, fabricated in a substantial support layer of monomers self-assembled on a highly ordered pyrolytic graphite surface, was studied using scanning tunneling microscopy and spectroscopy. The density of states of individual polymers and constituent monomers were observed on the same surface, and then compared with the calculated results. The spectrum delineating the density of states of the polydiacetylene wire clearly reveals the theoretically predicted � -band and band edge singularities of the one-dimensional polymer. The nanoscale fabrication of organic molecular layers on solid surfaces is an attractive subject with respect to future applications of nanotechnology. The use of organic molecules as molecular switches and wire connections is viewed by many as a possible way of overcoming the size constraints imposed by silicon-based technologies. We have succeeded in controlling the fabrication of linear polydiacetylene (PDA) wire by using a scanning tunneling microscopy (STM) probe tip on a self-assembled monomolecular (SAM) layer [1]. The fabricated PDA wire grows out in a straight line and extends to a submicrometer length on the surface. The fully extended conjugated backbone of the PDA is not only expected to function as an electrically conducting nanowire but is also interesting in terms of exploring physics in onedimensional (1D) systems. It is well known that the conjugated conducting polymers, e.g., polyacetylene, achieve extraordinarily high conductivity upon doping. PDA is already known as one of the conjugated conducting polymers [2]. The high conductivity of PDA, which is 8 orders of magnitude higher than that of its pristine crystal, has been attained upon elaborate iodine doping [3]. Furthermore, the values of the carrier mobility of PDA are higher than or comparable to those of the leading conductive polymers [2,4]. In the conjugated conducting polymers, the charges generated upon doping or photoexcitation are stored in localized defects, which are solitons, polarons, or bipolarons. They function as charge carriers and their electronic states appear within the band gap. They degenerate upon further doping and promote the formation of a halffilled band consisting of the polaron lattice [5]. However, in an individual polymer backbone, i.e., an isolated polymer wire, the capability and mechanisms of electrical conduction are still controversial. In this Letter, we report the results of scanning tunneling spectroscopy (STS) of an individual PDAwire. The results could provide a key to

Interpretation and theory of tunneling experiments on single nanostructures

Abstract: We discuss the interpretation of tunneling experiments on single molecules or semiconductor quantum dots weakly coupled to metallic electrodes. We identify the main features in the current-voltage curves and in the conductance using an extension of the theory of single charge tunneling. We analyze important quantities, such as the charging energy and the quasiparticle gap, providing simple rules to interpret the experiments. We discuss the limitations of the capacitance model to describe the system. We show that at a bias larger than the band-gap energy of the nanostructure the tunneling of both electrons and holes must be taken into account. We use self-consistent tight-binding calculations to illustrate these points and provide a comparison with recent experimental results on InAs nanocrystals.

Charging and Chemical Reactivity of Gold Nanoparticles and Adatoms on the (111) Surface of Single-Crystal Magnetite: A Scanning Tunneling Microscopy/Spectroscopy Study

Abstract: We present a scanning tunneling microscopy (STM)/scanning tunneling spectroscopy (STS) study of a model catalyst system consisting of supported gold nanoparticles on a reduced Fe3O4(111) surface in ultrahigh vacuum. Gold forms two electrically distinct nanoparticles on an iron oxide surface upon annealing multilayer Au/Fe3O4(111) at 500 °C for 15 min. I (V) curves taken via STS measurements show that large gold nanoparticles (∼8 nm) exhibit a metallic electronic structure and, thus, are likely neutral. Single gold adatoms appear to be strongly bonded to the oxygen sites of the Fe3O4(111) surface, and tunneling electrons are observed to flow predominantly from the STM tip to the Au adatoms and into the oxygen sites of the surface. The site-specific adsorption of the gold adatoms on oxygen surface atoms and the size-sensitive nature of the electronic structure suggest that Au adatoms are likely positively charged. When this Au/Fe3O4(111) system is dosed with CO at 260 K, adsorption of CO molecules normal to...

Scanning probe microscopy of oxide surfaces: atomic structure and properties

Abstract: The intersection of two fields, oxide surface science and scanning probe microscopy (SPM), has yielded considerable insight on atomic processes at surfaces. Oxide surfaces, especially those containing transition metals, offer a rich variety of structures and localized physical phenomena that are exploited in a wide range of applications. Nonlinear optics, superconductivity, ferroelectricity and chemical catalytic activity are but a few. Furthermore, the challenges and solutions associated with the chemistry of these surfaces and particularly the solutions to these problems have led to important understanding of tip–surface interactions that can inform SPM studies of all materials. Here, the development of understanding of the model systems TiO2 and SrTiO3 are considered in detail, to demonstrate the role of nonstoichiometry in surface structure evolution and the approach to interpreting structure at the atomic level. Then a combination of scanning tunneling microscopy, noncontact atomic force microscopy and theory are applied to a variety of oxide systems including Al2O3, NiO, ferroelectric BaTiO3, tungstates and molybdates. Recently developed sophisticated probes of local properties include spin-polarized tunneling, Fourier mapping of charge density waves, band gap mapping of superconductors and ultra fast imaging of atomic diffusion. The impact of these studies on our understanding of the behavior of oxides and of tip–surface interactions is summarized.

Femtosecond laser assisted scanning tunneling microscopy

Abstract: The excitation of the tunneling junction of a scanning tunneling microscope using ultrashort laser pulses combined with detection of a tunneling current component which depends nonlinearly on the laser intensity allows, in principle, to simultaneously obtain ultimate spatial and temporal resolution To achieve this goal, a laser system that produces ultrashort laser pulses is combined with an ultrahigh vacuum scanning tunneling microscope The basic technical considerations are discussed and it is shown that atomic resolution can be achieved under pulsed laser excitation of the tunneling junction The pulsed illumination gives rise to several contributions to the measured total current Experimental evidence for signal contributions due to thermal expansion, transient surface potentials and multiphoton photoemission are presented