Cl insertion on Si(100)-(2x1): etching under conditions of supersaturation.

We use scanning tunneling microscopy to show that Cl2 dosing of Cl-saturated Si(100)-(2x1) surfaces at elevated temperature leads to uptake beyond "saturation" and allows access to a new etching pathway. This process involves Cl insertion in Si-Si dimer bonds or backbonds, diffusion of the inserted Cl, and ultimately desorption of SiCl2. Investigations into the etch kinetics reveal that insertion occurs via a novel form of Cl2 dissociative chemisorption that is mediated by dangling bond sites. Upon dissociation, one Cl atom adsorbs at the dangling bond while the other can insert.

Tunneling electron induced bromine hopping on Si(100)-(2 x 1).

Tunneling electrons from the tip of a scanning tunneling microscope can be used to induce adatom hopping on Br-terminated Si(100)-(2x1) at low current and without voltage pulses. Hopping does not occur when electrons tunnel from a sample to a tip. The threshold energy is +0.8 V, and tunneling spectroscopy shows antibonding Si-Br states 0.8 eV above the Fermi level. Electron capture in these states is a necessary condition for hopping, but repulsive adsorbate interactions that lower the activation barrier are also required. Such interactions are strong near saturation for Br but are insufficient when the coverage is low or when Br is replaced by Cl.

Surface modification without desorption: recycling of Cl on Si(100)-(2 x 1).

We demonstrate chlorine-induced modification of Si(100)-(2 x 1) under conditions where Cl is recycled rather than desorbed as SiCl2. A dimer with 2 Cl atoms, 2SiCl, converts to SiCl2+Si, allowing the bare Si atom to escape onto the terrace. At temperatures below the desorption threshold, the SiCl2 unit decays through Cl diffusion, allowing the second Si atom to escape. The result is a dimer vacancy, terrace regrowth structures, and Cl that is able to participate in another pitting event. Access to this unexpected roughening pathway is controlled by the Cl concentration and temperature. This previously overlooked process represents an important component of Si(100) surface processing.

Growth and sintering of fullerene nanotubes.

Carbon nanotubes produced in arcs have been found to have the form of multiwalled fullerenes, at least over short lengths. Sintering of the tubes to each other is the predominant source of defects that limit the utility of these otherwise perfect fullerene structures. The use of a water-cooled copper cathode minimized such defects, permitting nanotubes longer than 40 micrometers to be attached to macroscopic electrodes and extracted from the bulk deposit. A detailed mechanism that features the high electric field at (and field-emission from) open nanotube tips exposed to the arc plasma, and consequent positive feedback effects from the neutral gas and plasma, is proposed for tube growth in such arcs.

Uranium stabilization of c28: a tetravalent fullerene.

Laser vaporization experiments with graphite in a supersonic cluster beam apparatus indicate that the smallest fullerene to form in substantial abundance is C(28). Although ab initio quantum chemical calculations predict that this cluster will favor a tetrahedral cage structure, it is electronically open shell. Further calculations reveal that C(28) in this structure should behave as a sort of hollow superatom with an effective valence of 4. This tetravalence should be exhibited toward chemical bonding both on the outside and on the inside of the cage. Thus, stable closed-shell derivatives of C(28) with large highest occupied molecular orbital-lowest unoccupied molecular orbital gaps should be attainable either by reacting at the four tetrahedral vertices on the outside of the C(28) cage to make, for example, C(28)H(4), or by trapping a tetravalent atom inside the cage to make endothedral fullerenes such as Ti@C(28). An example of this second, inside route to C(28) stabilization is reported here: the laser and carbon-arc production of U@C(28).

Formation of fullerides and fullerene-based heterostructures.

Two potassium fulleride phases, metallic K(3)C(60) and nonmetallic K(6)C(60), are formed when potassium is incorporated into thin C(60) films under ultrahigh vacuum conditions. Phase separation is observed for intermediate stoichiometries. Results obtained for the C(60)-K(3)C(60) heterostructure demonstrate that it is stable against potassium migration from the K(3)C(60) phase. In contrast, the C(60)-K(6)C(60) interface is not stable and K(3)C(60) is formed.

Order and Disorder in C60 and KxC60 Multilayers: Direct Imaging with Scanning Tunneling Microscopy.

Monolayer and multilayer structures of C(60), a high temperature van der Waals solid, have been studied with scanning tunneling microscopy. Structures grown on GaAs(110) at 300 kelvin and at elevated temperatures show significantly different morphologies because of balances between thermodynamics and kinetics. Condensation onto stepped surfaces demonstrates preferred bonding and nucleation at step edges. Detailed studies of potassium incorporation in crystalline C(60) show highly ordered structures in the K(3)C(60) metallic state but disordered non-metallic structures for high potassium concentrations.

Electronic States of KxC60: Insulating, Metallic, and Superconducting Character.

The recent report of electrical conductivity in the alkali metal fullerides and the discovery of superconductivity at 18 K for KxC(60) has raised fundamental questions about the electronic states on either side of the Fermi level, their occupancy with K intercalation, and the mechanism of superconductivity. Direct photoemission evidence is presented of filling of bands derived from the lowest unoccupied molecular orbital as a function of K incorporation for the metallic and insulating phases. This filling is not rigid band-like, and it reflects disorder in the K sites. Theoretical analysis indicates that KxC(60) is a strong coupling superconductor, and we suggest that the enhanced electron-phonon interaction is related to the unique hybridization of the C sp-derived states.

Ordered Overlayers of C60 on GaAs(110) Studied with Scanning Tunneling Microscopy.

Studies of C(60) overlayer growth on GaAs(110) with scanning tunneling microscopy show large first monolayer islands that are locally well ordered, structurally stable, and commensurate with the GaAs surface owing to molecule-substrate interactions. Within the distorted close-packed structure, two distinct adsorption sites were identified, one of them being elevated because of stress in the C(60) monolayer.

Cluster assembly of interfaces: nanoscale engineering.

Overlayer structures can be formed on surfaces by the deposition of clusters containing hundreds or thousands of atoms. Cluster assembly alters the reaction pathway at the surface so that novel structures with unique chemical and physical properties can be stabilized. This article discusses the process of cluster assembly. Cluster-assembled interfaces are compared to those obtained by conventional techniques, and examples are given for metal-semiconductor and semiconductor-high temperature superconductor systems.

An outbreak of acute hemorrhagic conjunctivitis in central Minnesota.

The only confirmed outbreak of acute hemorrhagic conjunctivitis in the continental United States during 1982 occurred in Brainerd, Minnesota. The disease first appeared in a 27-year-old woman during a trip to Tahiti. The patient's two daughters (8 months old and 3 years old) developed mild conjunctivitis two days later. Although the symptoms of all three had completely resolved before they arrived in Brainerd, illness compatible with acute hemorrhagic conjunctivitis developed in five other individuals who shared a house with them there. Symptoms appeared within two days in four of the five and within three days in the fifth. All five had high neutralizing antibody titers to enterovirus 70 (1:128, 1:32, 1:32, 1:128, and 1:256) and one had high antibody titers to coxsackievirus A24 which may cause similar symptoms (less than 1:4 in all cases). The symptoms resolved without sequelae in all five patients within five days. These cases demonstrated that a person who is no longer symptomatic can still infect others and that an infected person can transmit enterovirus 70 for at least eight days after the onset of symptoms.

Monitoring hearing loss at United Airlines.

The airlines are a highly diversified industry. Their major concerns are in the safe transportation of passengers and cargo with on time arrival and departure of its aircraft at various locations throughout the world. The challenge the airlines face is in the development and administration of an effective hearing conservation program that will produce valid audiometry at all of its logistic operations and yet be economically feasible and practical. The methods and techniques used by United Airlines to develop and administer an effective hearing conservation program are presented here. Guidelines employing the OSHA Noise Exposure Standard were incorporated in the program requirements as set forth by the medical department.

Solid-state photoelectron spectroscopy with synchrotron radiation.

Synchrotron radiation sources, providing intense, tunable, polarized, and stable beams of ultraviolet and x-ray photons, are having a great impact on biology, physics, chemistry, materials science, and other areas of research. Synchrotron radiation has revolutionized solid-state photoelectron spectroscopy by enhancing its capabilities for investigating the electronic behavior of solids and solid surfaces. Several fundamental photoemission techniques that rely on synchrotron radiation are discussed in this article. These techniques have an adjustable tunable surface sensitivity and provide the first direct mapping of the electronic band structure. Recent applications of photoelectron spectroscopy for studies of chemisorption geometries and surface structures are discussed.