Martin S. Leung

Unpinning of the Fermi level on GaAs by flowing water

Unpinning of the Fermi level on GaAs (100) surfaces by photochemical reactions resulting from simultaneous exposure of specimens to flowing water and light was recently reported. We discuss here a series of experiments carried out to provide further information on the changes in surface electronic structure responsible for unpinning of the Fermi level under these conditions. The present work supports the conclusion that the surface states which pin the Fermi level are associated with elemental arsenic and arsenic sesquioxide (As2O3). Effects of each of these two species on pinning are distinguished experimentally. We find that, in addition to photochemical reactions, exposure to flowing water alone can result in Fermi level unpinning under certain conditions. The oxygen content of the wash water and the specimen preparation are shown to be important variables.

Imaging of MoS2 by scanning tunneling microscopy

We report imaging of molybdenum disulfide by scanning tunneling microscopy (STM) in air. MoS2, a layer lattice material, is an interesting compound scientifically and is technologically important as a solid lubricant and as a catalyst. Images with atomic spatial resolution were formed only at negative (−0.8 to −1.9 V) sample bias, i.e., by electrons tunneling from the sample into the tip. Our observations are consistent with an electronic model in which the valence electrons are located in bonding or nonbonding orbitals largely confined within the S‐Mo‐S ‘‘sandwich’’ layers which make up the MoS2 lattice. There are no sigma or pi sulfur bonding orbitals in the basal plane from which electrons can tunnel. The STM image is produced by electrons originating from the filled molybdenum dz2 orbitals. These orbitals do not participate significantly in the formation of chemical bonds. They do project sufficiently far spatially above the basal plane in the presence of an applied electric field to permit formation ...

In/Pt ohmic contacts to GaAs

Graded heterojunction InGaAs ohmic contacts to GaAs have been prepared which show improved electrical and mechanical properties. The improvements result from the use of a thin Pt layer between the In layer and the substrate which controls the reaction of the In and the GaAs. Evidence is also offered that the InAs heterojunction regions are epitaxial.

Transport study of a single bismuth nanowire fabricated by the silver and silicon nanowire shadow masks

The authors have carried out measurements of the electrical conductivity of single bismuth nanowires fabricated by the low energy electron beam lithography using the silver/silicon nanowire shadow masks. The examined nanowires had cross-sectional dimensions of 40×30 and 40×50nm2. The chosen nanowire sizes had been slightly below the critical diameter D (∼50nm) at which a semimetal to semiconductor phase transition was predicted to occur. The results reveal a semiconductorlike temperature dependence of the electrical conductivity of a bismuth nanowire, which is strikingly different from that of the bulk bismuth.

Effects of Hydrolysis On Metallo-Organic Solution Deposition of PZT FILMS

The effects of hydrolysis on the degree of polymerization during metallo-organic solution deposition of lead zirconate titanate (PZT) films have been investigated. The reaction of lead 2-ethylhexanoate, zirconium n-tetrapropoxide, and titanium tetrabutoxide in isopropanol with water were studied using thermogravimetry, specular reflectance Fourier transform infrared spectroscopy (FTIR) and optical and electron microscopy. Films prepared from coating solutions having varying amounts of water exhibited dramatic differences in morphology. The films were spin-coated on platinum coated fused silica substrates and annealed at 525°C for 30 minutes. Unhydrolyzed coating solutions and solutions with a mole ratio of water to total metal of 0.5 yielded perovskite films with 0.5–5μm grains. A mole ratio of 1.5 (the amount of water required to completely hydrolyze the metallo–organics in the solution) formed amorphous, porous films. The stability of the prepolymerized films inhibits crystallization and densification at moderate temperatures.

Fabrication of bismuth nanowires with a silver nanocrystal shadowmask

We fabricated bismuth (Bi) nanowires with low energy electron beam lithography using silver (Ag) nanocrystal shadowmasks and a subsequent chlorine reactive ion etching. Submicron-size metal contacts on the single Bi nanowire were successfully prepared by in situ focused ion beam metal deposition for transport measurements. The temperature dependent resistance measurements on the 50 nm wide Bi nanowires showed that the resistance increased with decreasing temperature, which is characteristic of semiconductors and insulators.

A scanning tunneling microscope based on a motorized micrometer

A scanning tunneling microscope (STM) that uses a motorized micrometer to provide for coarse mechanical motion of the specimen relative to the tunneling tip is described. The specimen is attached directly to the micrometer spindle. No additional motion reduction mechanisms, such as levers, are necessary. After positioning the tip optically about 0.2–0.3 mm from the specimen, tunneling current is established automatically under the control of the system computer by a sequence of mechanical motions of the micrometer combined with extensions of a tubular piezoelectric scanner. Tunneling current is typically acquired in 10–15 min.

First results of material charging in the space environment

A satellite experiment, designed to measure potential charging of typical thermal‐control materials at near‐geosynchronous altitude, was flown as part of the SCATHA program. Direct observations of charging of typical satellite materials in a natural charging event (⩾ 5 keV) are presented. The results show some features which differ significantly from previous laboratory simulations of the environment.

Effect of probe geometry on the Hall response in an inhomogeneous magnetic field: A numerical study

The effect of probe geometry on the classical Hall response to a weak perpendicular inhomogeneous magnetic field is studied numerically. An electric potential equation based on a classical model of the two-dimensional Hall effect is solved numerically for a generalized flux distribution to find the Hall response function. We find that the magnitude and shape of this response function is strongly affected by probe geometry. Asymmetric cross-shaped Hall probes, with one narrow voltage lead, have a strongly peaked response more localized than in symmetric probe arrangements. This suggests novel lithographic patterns that may improve the spatial resolution of Hall magnetometry and scanning Hall probe microscopy.

Imaging of polydiacetylene on graphite by scanning tunneling microscopy

Scanning tunneling microscopy has been applied to obtain atomically resolved images of polydiacetylene on graphite and to observe hydrogen bonding between polymers directly. Polydiacetylenes have a pseudo‐one‐dimensional conjugated pi‐electron system which gives rise to nonlinear optical behavior of both fundamental and practical significance. Under low surface coverage, the images show a single polymer of CH3(CH2)11—C≡C—C≡C—(CH2)8—COOH. At high coverage, the images show a well‐ordered monolayer of polymers with the nonpolar side groups pointing toward the hydrophobic graphite surface. The spatial electron distribution indicates substantial hydrogen bonding between polymers in the plane containing the polar side groups.