Inelastic electron tunneling spectroscopy

Inelastic electron tunneling spectroscopy is a useful technique for the study of vibrational modes of molecules adsorbed on the surface of oxide layers in a metal-insulator-metal tunnel junction. The technique involves studying the effects of adsorbed molecules on the tunneling spectrum of such junctions. The data give useful information about the structure, bonding, and orientation of adsorbed molecules. One of the major advantages of inelastic electron tunneling spectroscopy is its sensitivity. It is capable of detecting on the order of 10(10) molecules (a fraction of a monolayer) on a 1-square-millimeter junction. It has been successfully used in studies of catalysis, biology, trace impurity detection, and electronic excitations. Because of its high sensitivity, this technique shows great promise in the area of solid-state electronic chemical sensing.     

Rapid electron tunneling through oligophenylenevinylene bridges

We measured rate constants of thermal, interfacial electron transfer through oligophenylenevinylene bridges between a gold electrode and a tethered redox species in contact with an aqueous electrolyte using the indirect laser-induced temperature jump technique. Analysis of the distance dependence indicates that, unlike other bridges studied to date, the rate constants are not limited by electronic coupling for bridges up to 28 angstroms long. The energy levels of the bridges relative to those of the redox species rule out hopping through the bridge. We conclude that, out to 28 angstroms, the transfer is limited by structural reorganization and that electron tunneling occurs in less than 20 picoseconds, suggesting that oligophenylenevinylene bridges could be useful for wiring molecular electronic elements.     

Intermolecular electron transfer by quantum mechanical tunneling

The transfer of electrons from one molecule to another by quantum mechanical tunneling has recently been implicated in biological electron transport. This report describes observations of electron transfer between aromatic molecules in a rigid matrix, in which electrons apparently tunnel through tens of angstroms of inert solvent. The kinetics tend to confirm the tunneling process, which is likely to be an important means of electron transfer when diffusion is blocked by steric factors or immobilization of the reactants.     

Spectroscopy of biological compounds with inelastic electron tunneling

Metal-insulator-metal electron tunnel junctions can be doped with a solution of an organic compound by placing a drop of the solution on the insulator and spinning of the excess. Electrical measurement of the second derivative of voltage with respect to current, as a function of applied voltage, then gives a spectrum of vibrational modes equivalent to an infrared or Raman spectrum, but with the use of only micrograms of sample.     

Molecular light emission induced by inelastic electron tunneling

Light emission from molecular layers has been induced by inelastically tunneling electrons in a tunneling junction. The fast quenching of molecular emission on metal surfaces was suppressed by use of the "transparent conductor" indium-tin-oxide for the junction electrodes. The emission measurements have been made in squeezable tunneling junctions as small as 10(-9) square centimeters, coated with 9-10 dichloro-anthracene layers. At a bias of 2.5 to 3.5 volts, yields of 5000 photons per microcoulomb were observed. Evidence for the molecular origin of the emission is given. This method shows good prospects for use in the imaging of chromophores on surfaces with atomic resolution.     

Montmorillonite: electron diffraction from two-dimensional single crystals

Some Selected area electron diffraction patterns of isolated monocrystalline particles of both Wyoming and Camp Berteaux sodiumti-monfmorillonites fail to show the mirror platne symmetry required for the accepted structure. A triclinic model deserves consideraction.     

High-resolution electron microscopy and scanning tunneling microscopy of native oxides on silicon

High-resolution transmission electron microscopy and scanning tunneling microscopy have been combined to examine the structure of the thin "native" oxide that forms on silicon surfaces at room temperature. Differences in the cleaning procedures for silicon wafers may affect the morphology of this oxide and critically influence further processing on the silicon substrates. An etch that ended with a dip in hydrofluoric acid provided a thinner oxide and a lower interface step density than did a sulfuric peroxide treatment. The availability of complementary information from high-resolution transmission electron microscopy and scanning tunneling microscopy is discussed.     

X-ray and electron diffraction of ocular and bone marrow crystals in paraproteinemia

Crystals in the cornea, conjunctiva, and bone marrow of a patient with a monoclonal gammopathy were analyzed by x-ray and selective-area electron diffraction. X-ray diffraction rings obtained from the abnormal crystalline deposits closely matched cholesteryl stearate patterns, and electron diffraction findings were suggestive of this lipid.     

Surface structure of polymers: glancing-angle electron diffraction study of polyethylene

The surface regions of polyethylene nucleated and crystallized in contact with both a high-energy surface (gold) and a low-energy surface (fluorinated ethylene-propylene copolymer Teflon) have been examined by means of glancing-angle electron-diffraction techniques. Examination of these surfaces has been confined to a maximum depth of 120 angstroms. The surface region of the polyethylene generated in contact with the gold is considerably more crystalline than the surface generated in contact with the fluorinated ethylene-propylene copolymer Teflon. These results tend to corroborate recent wettability and infrared studies. Apparently, the surface structure of polyethylene is highly dependent upon the method of preparation.     

4D visualization of transitional structures in phase transformations by electron diffraction

Complex systems in condensed phases involve a multidimensional energy landscape, and knowledge of transitional structures and separation of time scales for atomic movements is critical to understanding their dynamical behavior. Here, we report, using four-dimensional (4D) femtosecond electron diffraction, the visualization of transitional structures from the initial monoclinic to the final tetragonal phase in crystalline vanadium dioxide; the change was initiated by a near-infrared excitation. By revealing the spatiotemporal behavior from all observed Bragg diffractions in 3D, the femtosecond primary vanadium-vanadium bond dilation, the displacements of atoms in picoseconds, and the sound wave shear motion on hundreds of picoseconds were resolved, elucidating the nature of the structural pathways and the nonconcerted mechanism of the transformation.     

4D electron diffraction reveals correlated unidirectional behavior in zinc oxide nanowires

The confined electronic structure of nanoscale materials has increasingly been shown to induce behavior quite distinct from that of bulk analogs. Direct atomic-scale visualization of nanowires of zinc oxide was achieved through their unique pancake-type diffraction by using four-dimensional (4D) ultrafast electron crystallography. After electronic excitation of this wide-gap photonic material, the wires were found to exhibit colossal expansions, two orders of magnitude higher than that expected at thermal equilibrium; the expansion is highly anisotropic, a quasi-one-dimensional behavior, and is facilitated by the induced antibonding character. By reducing the density of nanowires, the expansions reach even larger values and occur at shorter times, suggesting a decrease of the structural constraint in transient atomic motions. This unanticipated ultrafast carrier-driven expansion highlights the optoelectronic consequences of nanoscale morphologies.     

Laser-induced chemistry for microelectronics

Laser-controlled chemical reactions are being explored for use in all phases of the processing of semiconductor devices. Laser-induced chemical processing can produce submicrometer features without the aid of photolithography. Research is also providing new information on light-excited and light-enhanced interface reactions.     

Laser-induced fluorescence and bonding of metal dimers

This article describes a technique for the spectroscopic of metal clusters and intermetallic compounds. First, metallic samples are vaporized by a pulsed YAG (yttrium-aluminum garnet) laser, and then the gaseous products are excited with a pulsed-dye laser until they fluoresce. A time-resolved, fluorescence spectrum is then measured by the product. The application of this technique to the study of metal dimers is reviewed, with emphasis on recent results from Be(2) and Cr(2). Studies of such species often yield insights into the chemistry of metals and metal-metal bonding.     

Bond lengths in free molecules of buckminsterfullerene, c60, from gas-phase electron diffraction

Electron diffraction patterns of the fullerene C(60) in the gaseous state have been obtained by volatilizing it from a newly designed oven-nozzle at 730 degrees C. The many peaks of the experimental radial distribution curve calculated from the scattered intensity are completely consistent with icosahedral symmetry for the free molecule. On the basis of this symmetry assumption, least-squares refinement of a model incorporating all possible interatomic distances led to the values r(g)(C(1)-C(2)) = 1.458(6) angstroms (A) for the thermal average bond length within the five-member ring (that is, for the bond fusing five- and six-member rings) and r(g)(C(1)-C(6)) = 1.401(10) A for that connecting five-member rings (the bond fusing six-member rings). The weighted average of the two bond lengths and the difference between them are the values 1.439(2) A and 0.057(6) A, respectively. The diameter of the icosahedral sphere is 7.113(10) A. The uncertainties in parentheses are estimated 2sigma values.     

Experimental tunneling ratchets

Adiabatically rocked electron ratchets, defined by quantum confinement in semiconductor heterostructures, were experimentally studied in a regime where tunneling contributed to the particle flow. The rocking-induced electron flow reverses direction as a function of temperature. This result confirms a recent prediction of fundamentally different behavior of classical versus quantum ratchets. A wave-mechanical model reproduced the temperature-induced current reversal and provides an intuitive explanation.     

Long journey into tunneling

I am, of course, deeply aware of important contributions made by many colleagues and my friends throughout this long journey. The subject of the second section was carried out when I was in Japan and all the rest (35) has been performed in the United States of America. Since my journey into tunneling is still continuing, I do not come to any conclusions in this talk. However, I would like to point out that many high barriers exist in this world: barriers between nations, races, and creeds. Unfortunately, some barriers are thick and strong. But I hope, with determination, we will find a way to tunnel through these barriers easily and freely, to bring the world together so that everyone can share in the legacy of Alfred Nobel.