Gary W. Stupian

Three-Dimensional Macroporous Silicon Photonic Crystal with Large Photonic Band Gap

Three-dimensional photonic crystals based on macroporous silicon are fabricated by photoelectrochemical etching and subsequent focused-ion-beam drilling. Reflection measurements show a high reflection in the range of the stopgap and indicate the spectral position of the complete photonic band gap. The onset of diffraction which might influence the measurement is discussed.

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 ...

Convenient optical pressure gauge for multimegabar pressures calibrated to 300GPa

The accurate measurement of pressure by a straightforward and inexpensive optical procedure has been needed in the multimegabar region since static pressures over 216GPa, 361GPa, 420GPa and 560GPa were obtained in the diamond anvil cell. Here, a simple optical pressure gauge based on the Raman shift of the diamond at the center of a diamond tip at the diamond–sample interface is calibrated against a primary gauge (Pt isotherm at 300K from shock data) to 300GPa, thus enabling researchers who do not have a synchrotron to conveniently measure pressure with an optical scale from 50to300GPa.

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.

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.

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.

Auger spectroscopy of silicones

Silicon LMM Auger spectra arising from a variety of silicon compounds, mostly silicones, have been studied. Silicones may be regarded, in a sense, as compounds intermediate between elemental silicon and silicon dioxide. The observed silicon Auger peak energies of silicones are found to lie between the Si (LMM) peak energies of Si and SiO2, and are correlated with the net electronic charge on silicon as estimated from a very simple model of charge transfer based on Pauling electronegativities.