SINCE its invention in the early 1980s by Binnig and Rohrer1,2, the scanning tunnelling microscope (STM) has provided images of surfaces and adsorbed atoms and molecules with unprecedented resolution The STM has also been used to modify surfaces, for example by locally pinning molecules to a surface3 and by transfer of an atom from the STM tip to the surface4 Here we report the use of the STM at low temperatures (4 K) to position individual xenon atoms on a single-crystal nickel surface with atomic pre-cision This capacity has allowed us to fabricate rudimentary structures of our own design, atom by atom The processes we describe are in principle applicable to molecules also In view of the device-like characteristics reported for single atoms on surfaces5,6, the possibilities for perhaps the ultimate in device miniaturization are evident

Positioning single atoms with a scanning tunnelling microscope

The legacy of Gilbert Newton Lewis (1875-1946) pervades the lexicon of chemical bonding and reactivity. The power of his concept of donor-acceptor bonding is evident in the eponymous foundations of electron-pair acceptors (Lewis acids) and donors (Lewis bases). Lewis recognized that acids are not restricted to those substances that contain hydrogen (Bronsted acids), and helped overthrow the "modern cult of the proton". His discovery ushered in the use of Lewis acids as reagents and catalysts for organic reactions. However, in recent years, the recognition that Lewis bases can also serve in this capacity has grown enormously. Most importantly, it has become increasingly apparent that the behavior of Lewis bases as agents for promoting chemical reactions is not merely as an electronic complement of the cognate Lewis acids: in fact Lewis bases are capable of enhancing both the electrophilic and nucleophilic character of molecules to which they are bound. This diversity of behavior leads to a remarkable versatility for the catalysis of reactions by Lewis bases.

Lewis Base Catalysis in Organic Synthesis

Reference LPI-ARTICLE-1999-017View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

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Metal Oxide Surfaces and Their Interactions with Aqueous Solutions and Microbial Organisms.

THE scanning tunnelling microscope1 (STM) has been employed in recent years in attempts to develop atomic-scale electronic devices, both by examining device-like characteristics in preexisting structures2,3 and by creating new structures by the precise manipulation of atoms and molecules with the STM tip4–6. Here we report the operation of a bistable switch that derives its function from the motion of a single atom. A xenon atom is moved reversibly between stable positions on each of two stationary conducting 'leads', corresponding to the STM tip and a nickel surface. The state of the switch is set (that is, the xenon atom is moved to the desired location) by the application of a voltage pulse of the appropriate sign across the leads. The state of the switch is identified by measuring the conductance across the leads. This switch is a prototype of a new class of potentially very small electronic devices which we will call atom switches.

An atomic switch realized with the scanning tunnelling microscope

Electrical bistability is a phenomenon in which a device exhibits two states of different conductivities, at the same applied voltage. We report an organic electrical bistable device (OBD) comprising of a thin metal layer embedded within the organic material, as the active medium [L. P. Ma, J. Liu, and Y. Yang, US Patent Pending, (2001)]. The performance of this device makes it attractive for memory-cell type of applications. The two states of the OBD differ in their conductivity by several orders in magnitude and show remarkable stability, i.e., once the device reaches either state, it tends to remain in that state for a prolonged period of time. More importantly, the high and low conductivity states of an OBD can be precisely controlled by the application of a positive voltage pulse (to write) or a negative voltage pulse (to erase), respectively. One million writing-erasing cycles for the OBD have been tested in ambient conditions without significant device degradation. These discoveries pave the way for newer applications, such as low-cost, large-area, flexible, high-density, electrically addressable data storage devices.

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Organic electrical bistable devices and rewritable memory cells

We have investigated the incorporation of boron into the Si(111) (\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3)R30\ifmmode^\circ\else\textdegree\fi{} surface from low boron concentration up to (1/3 monolayer, using tunneling microscopy and spectroscopy and first-principles total-energy calculations. Surprisingly, we find that a (\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3)R30\ifmmode^\circ\else\textdegree\fi{} structure composed almost entirely of Si adatoms on a Si double layer can be stabilized by boron doping in near surface layers. Moreover, when boron atoms are in the surface layers, the adatom site is unfavorable compared with the site underneath the adatom, unlike other group-III elements adsorbed on the Si(111) surface.

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Surface doping and stabilization of Si(111) with boron.

Using molecular beam epitaxy we have fabricated a quantum wedge: a nanoscale flat-top lead island on a stepped Si(111) surface. Imaging the top surface of the wedge with a scanning tunneling microscopy reveals the phenomenon of electron interference fringes: a discrete periodic spatial variation of the tunnel current originating from the quantization of electron states in the wedge.

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Electron Fringes on a Quantum Wedge

Identifying Molecular Orientation of Individual C 60 on a Si\(111\)-\(7×7\) Surface

THE tunnel diode1, which is widely used in high-speed electronics applications2, depends on the property of negative differential conductivity, that is, a negative slope in the current–voltage curve. The mechanism underlying the tunnel diode's behaviour, namely the existence of a range of biases for which tunnelling is forbidden or suppressed following a bias for which tunnelling is strongly favoured, has been employed subsequently in the design of new devices that also display the conductance anomaly, such as the double-barrier resonant-tunnelling device3. It has been predicted4 that the conductance anomaly could result from a similar mechanism at the tunnel junction of the scanning tunnelling microscope (STM), where localized states on adsorbate and tip atoms give rise to allowed and suppressed energies for tunnelling. The STM has the capability to image regions of negative differential conductivity induced by individual atoms on a surface. Here we report the observation of negative differential conductivity on particular binding sites of a Si (111) surface doped with boron. Specific current–voltage characteristics are shown to be related to the presence or absence of the dopant at individual atomic sites, and negative differential conductivity is observed at –1.4 V tip bias at a specific type of site. Tunnelling spectroscopy indicates that the effect results from a tunnel-diode mechanism.

Demonstration of the tunnel-diode effect on an atomic scale

Two-dimensional systems possess a unique topological ordering that is not found in either three- or one-dimensional systems1. Using high-resolution scanning tunnelling microscopy, we show here that a 60-carbon-atom (C60) array on a self-assembled monolayer of an alkylthiol forms an ideal two-dimensional system which has another novel topological order originating from the orientational degrees of freedom. At a temperature of 5 K, the two-dimensional C60 forms a domain structure in which the correlation function of the molecular orientation within a domain is constant anywhere (so every C60 has the same orientation) but changes abruptly at domain boundaries. Remarkably, the positional order and the bond-orientational order are both fully preserved across domain boundaries.

Dongmin Chen

Papers

Surface doping and stabilization of Si(111) with boron.

Electron Fringes on a Quantum Wedge

Identifying Molecular Orientation of Individual C 60 on a Si\(111\)-\(7×7\) Surface

Demonstration of the tunnel-diode effect on an atomic scale

Topology of two-dimensional C60 domains.