Stephen P. Kelty

(RbxK1–x)3C60 Superconductors: Formation of a Continuous Series of Solid Solutions

By means of an approach that employs alkali-metal alloys, bulk single-phase (RbxK1–x)3C6O superconductors have been prepared for all x between 0 and 1. For x = 1 it is shown that the maximum superconducting fraction, which approaches 100% in sintered pellets, occurs at a Rb to C60 ratio of 3:1. More importantly, single-phase superconductors are formed at all intermediate values of x, and it is shown that the transition temperature (Tc) increases linearly with x in this series of materials. The formation of a continuous range of solid solutions demonstrates that the rubidium- and potassium-doped C60 superconducting phases must be isostructural, and furthermore, suggests that the linear increase in Tc with x results from a chemical pressure effect.

Clusters in solution: growth and optical properties of layered semiconductors with hexagonal and honeycombed structures

Transmission electron microscopy and optical absorption were used to examine small clusters of the layered semiconductors, PbI2 and BiI3. In both systems, a layer of metal is sandwiched between two hexagonally closed‐packed layers of iodine. We describe a simple solution preparation which gives rise to clusters corresponding to single layer sandwiches, roughly 7 A thick, whose lateral dimensions vary from 12 to 90 A depending on the solvent and the nature of the growing cluster interface. The cluster size distributions are markedly different for PbI2 and BiI3 reflecting the different structure in the metal planes of these systems. PbI2 cluster sizes are determined by hexagonal symmetry, with cluster growth achieved by placement of lead atoms symmetrically about a smaller cluster. In BiI3, whose metal plane has a honeycombed structure like graphite, clusters grow to be much larger with their sizes determined by the closure of six‐membered rings. The optical absorption spectra of PbI2 and BiI3 can be quanti...

Direct observation of graphite layer edge states by scanning tunneling microscopy

The electronic and structural properties of two different edge terminations in normal graphite layer edges were investigated at atomic resolution using scanning tunneling microscopy (STM) and electronic band structure calculations. An emphasis is placed on distinguishing the electronic structure of [1,1,2,0] (“armchair”) and [1,0,1,0] (“zigzag”) edge terminations. Experimental STM investigations reveal that the [1,1,2,0] termination exhibits a (3×3)R30° superstructure in the local electron density of states profile superimposed on the normal graphite lattice. The [1,0,1,0] edge shows a different superstructure typified by two to three atomic lattice cells of high electron density parallel to the edge. The armchair edge superstructure persists for 2–3 nm (equivalent to about 10 unit cells) into the bulk layer. By modeling these edge types using extended Huckel tight binding calculations, we show that the results are consistent with an unreconstructed edge geometry. The details of the edge states do not...

NEXAFS determination of electronic and catalytic properties of transition metal carbides and nitrides: From single crystal surfaces to powder catalysts

Abstract In this paper we will provide a brief review of our recent investigations of the physical and chemical properties of transition metal carbides and nitrides using the near-edge X-ray absorption fine structure (NEXAFS) technique. We will first use single crystal model surfaces to demonstrate the capabilities of NEXAFS for determining fundamental electronic and structural properties of transition metal carbides and nitrides, such as the ionicity of the metal–nonmetal bonds, the p-projected density of unoccupied states, and the location of carbon and nitrogen atoms on the surface or interstitial subsurface sites. We will use several examples to correlate these electronic and structural properties to the chemical reactivities of the transition metal carbide and nitride model surfaces. Furthermore, we will compare the general similarities of the NEXAFS investigations between model surfaces and powder catalysts. We will also provide several examples to demonstrate how the fundamental learnings from the model surfaces can be used to understand the catalytic properties of powder materials with complicated catalytic formulations, such as the bimetallic oxycarbide and oxynitride catalysts.

Low-Signal-Level Raman Spectroscopy with an Intensified Optical Multichannel Array Detector

A conventional intensified optical multichannel array detector (I-OMA) with minor modifications can detect organic monolayers at “counting rates” as low as 0.05 photoelectrons per second per pixel. Both hardware modifications and data analysis techniques are presented that allow one to obtain maximum performance from the conventional I-OMA.

Correlation of local structure and electrical activation in arsenic ultrashallow junctions in silicon

The understanding of the behavior of arsenic in highly doped near surface silicon layers is of crucial importance for the formation of N-type ultrashallow junctions in current and future very large scale integrated technology. This is of particular relevance when studying recently developed implantation and annealing methods. Past theoretical as well as experimental investigations have suggested that the increase in As concentration, and therefore the reciprocal proximity of several As atoms, leads to a drastic increase in electrically inactive defects giving only marginal reduction in sheet resistance. Monoclinic SiAs aggregates as well as various arsenic-vacancy clusters contribute to the deactivation of arsenic. This study aims to correlate between the results of electrical activation measurements and x-ray absorption fine structure measurements. Samples were doped with a nominal fluence of 1×1015–3×1015 atoms/cm2, implanted at 2 keV, and annealed by rapid thermal treatments, laser submelt treatments, ...

A Stable, Multi-Subunit Complex of β2Glycoprotein I

Beta2glycoprotein I (beta2GPI) is a 54-kDa plasma protein which is recognized as an autoantigen for antibodies from patients with antiphospholipid syndrome (APS). SDS-PAGE (under reducing conditions) of beta2GPI from three sources indicates that the 54-kDa beta2GPI band is accompanied by a band corresponding to an 8-kDa protein. In the absence of detergent and reducing agents (native PAGE), beta2GPI demonstrated a large complex (molecular mass approximately 320 kDa) which is dissociable by boiling in 6-8 M urea, yielding several lower molecular mass bands, one of which corresponds to the 8-kDa protein observed in SDS-PAGE. Sera from five healthy adults demonstrated native beta2GPI migration equivalent to the commercially purified protein. Atomic force microscopy (AFM) images of native beta2GPI show aggregrates of particles each having a diameter of 30-35 nm. This is consistent with a globular unit the size of which would be substantially larger than that expected for a 54-kDa protein. These experiments suggest that the 54-kDa beta2GPI monomer subunits exist as a multimeric complex with the 8-kDa protein.

All-Atom CHARMM Force Field and Bulk Properties of Perfluorozinc Phthalocyanines

Classical force fields within the CHARMM parametrized model are developed for zinc phthalocyanines including the parent per-hydro molecule and per-fluoro-alkyl substituted derivatives. Partial atomic charges, 2-body bond lengths, and 3-body angle parameters were obtained from B3LYP-level density functional calculations. Force constants for 2-, 3-, and 4-body interactions were derived from existing fluoroalkane models and incorporated assuming transferability. The force fields were validated by comparing equilibrium molecular geometries from molecular dynamics simulations with density functional theory (DFT) calculations and, where available, published experimental XRD refinements. The models produce molecular geometries for the target materials within 1-2% of expected values. Intermolecular interaction geometries were also investigated using molecular dynamics simulations. The results provide new insight and predictions of the equilibrium stacking and orientational intermolecular interactions of this novel class of modified phthalocyanines.

Scanning tunneling microscopy investigations of the surface structure and electronic properties of ternary graphite intercalation compounds

Scanning tunneling microscopy has been used to characterize the surface structure of the KHgC4 (stage 1) and KHgC8 (stage 2) graphite intercalation compounds. Images of the stage 1 and stage 2 materials exhibit a new commensurate 2×2 superlattice in addition to the centered hexagonal lattice observed in images of pristine graphite. Consideration of the intercalant layer structure and previous studies of stage 1 (MC8) and stage 2 (MC24) alkali metal graphite intercalation compounds (GICs) indicate that the 2×2 superlattice is due to a modulation of the surface carbon layer density of states by the periodic (2×2) potential of the potassium ions. In addition, a new orthorhombic superlattice, a=b=0.89 nm and <a−b≂89°, has been observed in images of the stage 1 KHgC4 material. Possible origins of this novel superstructure are discussed.

Deactivation of submelt laser annealed arsenic ultrashallow junctions in silicon during subsequent thermal treatment

The use of nonequilibrium annealing approaches can produce very high levels of arsenic electrical activation in Si. However, subsequent thermal treatments between 500 and 800°C easily deactivate the dopant to a level one order of magnitude below the solid solubility. In this work, the authors study the deactivation of laser annealed (LA) ultrashallow arsenic distributions in silicon using Hall effect measurements, extended x-ray absorption fine structure spectroscopy, and secondary ion mass spectrometry. Single crystal Si (100) wafers implanted with As ions at 2keV energy and different doses were activated with a millisecond LA at 1300°C using a scanning diode laser annealing system under nonmelt conditions. The samples were then thermally treated in a furnace at 300–900°C in a N2 atmosphere for 10min. Electrical deactivation has been observed for all the implanted doses but for the lowest one. In particular, it was observed that the higher the As dose the easier the deactivation, in particular, after the...