Thomas J. Kistenmacher

High quality self‐nucleated AlxGa1−x N layers on (00.1) sapphire by low‐pressure metalorganic chemical vapor deposition

High quality AlxGa1−xN alloy films with x<0.4 have been prepared on self‐nucleated (00.1) sapphire substrates by low‐pressure metalorganic chemical vapor deposition. It has been shown that the lattice constant of the films varies linearly with alloy composition x (Vegard’s law is obeyed) and that homogeneous and inhomogeneous strain and alloy clustering are minimized in these self‐nucleated AlxGa1−xN films. Consistent with their reduced strain and chemical uniformity, the derived optical band gaps of these AlxGa1−xN films also show a linear dependence on alloy composition x, yielding a bowing parameter b≊0 eV.

The effect of thermal annealing on GaN nucleation layers deposited on (0001) sapphire by metalorganic chemical vapor deposition

It has been shown by optical and x‐ray measurements that GaN nucleation layers deposited at 540 °C on (0001)‐oriented sapphire substrates have a measurable crystalline component, although the x‐ray data and the lack of absorbance features near the direct band gap of GaN suggest that the crystallite size is very small. Upon annealing to higher temperatures, the crystallite size increases and the crystal perfection improves markedly, until at temperatures near those empirically determined to be optimum for growth of an epitaxial overlayer, it approaches that of good quality single‐crystal material. Most of the recrystallization of the nucleation layer occurs during the ramp from its deposition temperature to the growth temperature of the GaN overlayer, and there appears to be no advantage to prolonged annealing at high temperatures prior to epitaxial growth. In fact, x‐ray diffractometer results suggest that the nucleation layer deteriorates after 20 min at temperatures above 1015 °C, under the conditions u...

Solution and Solid State Studies of Tetrafluoro-7,7,8,8-Tetracyano-p-Quinodimethane, TCNQF4. Evidence for Long-Range Amphoteric Intermolecular Interactions and Low-Dimensionality in the Solid State Structure

Solution and solid state studies of TCNQF4 are reported. The electron affinity of TCNQF4 has been derived from spectral observations of the visible-near IR charge-transfer band for the pyrene compl...

Crystal Structures for the Electron Donor Dibenzotetrathiafulavalene, DBTTF, and Its Mixed-stack Charge-transfer Salts with the Electron Acceptors 7,7,8,8-tetracyano-p-quinodimethane, TCNQ, and 2,5-difluoro-7,7,8,8-tetracyano-p-quinodimethane, 2,5-TCNQF2

Crystal structures for the electron donor DBTTF and its charge-transfer salts with the acceptors TCNQ and 2,5-TCNQF2 are reported. Crystal data for the three systems are as follows: (a) neutral DBT...

Dipeptide-metal-nucleoside complexes as models for enzyme-metal-nucleic acid ternary species. synthesis and molecular structure of the cytidine complex of glycylglycinatocopper(II)

The cytidine complex of glycylglycinatocopper(II) has been synthesized, and its molecular structure determined by single-crystal X-ray diffraction methods. The complex crystallizes in the monoclinic system, space group P21, with a = 4.716(3)A, b = 26.86(6)A, c = 14.761(14)A, β = 90.63(6)°, Z = 4. The two independent complexes in the asymmetric unit have nearly identical molecular conformations. The coordination geometry about the copper is approximately square planar with the tridentate glycylglycine dianion and N(3) of cytidine occupying the four coordination sites. The binding of the nucleoside to the dipeptide complex is further enhanced by a weak, axial Cu⋯O(2)[cytidi

The crystal and molecular structure of an organic conductor from 4,4',5,5'-tetramethyl-Δ2,2'-bis-1,3-diselenole and 7,7,8,8-tetracyano-p-quinodimethane [TMTSF–TCNQ]

This report relates the synthesis of two salts, one conducting and one insulating, and the crystal structure of the conducting salt formed from TMTSF and TCNQ. The conducting salt crystallized in the triclinic system, space group P]-, with cell data: a=3.883 (1), b= 7.645 (5), c= 18.846(9) /~,, a = 77.34(5), fl = 89.67 (3), F = 94-63 (3) °, V = 543.97/~3, Z = 1, Dm 1.98 (1), De = 1.99 g cm -3. Intensities for 2216 non-zero reflections were collected by counter methods on an automated diffractometer. The structure was solved by standard heavy-atom methods and has been refined by full-matrix least squares to a final R value of 0.107. The TMTSF cations and TCNQ anions form separate, homologous stacks in the crystal with interplanar spacings of 3.60 and 3-26 ,~, respectively. The planes of the molecular ions are tilted in opposite directions relative to the short a axis to form a dihedral angle of 54.9 °. The intrachain stacking and molecular overlaps are close to those observed in the chemically similar salt HMTSF-TCNQ, but the interchain coupling in TMTSF-TCNQ appears to be substantially weaker. Since HMTSF-TCNQ remains conducting at low temperature whereas TMTSF-TCNQ does not, we infer that interchain interactions play a crucial role in the evolution of the metal-to-insulator transition in TMTSF-TCNQ. 417

Solution and solid state studies on the interactions of protonated cytosine salts. III. Interpyrimidine base stacking and asymmetric interbase hydrogen bonding in the structure of 1-methylcytosine hemihydroiodide hemihydrate

Abstract Structural and spectroscopic data are presented on the compound 1-methylcytosine hemihydroiodide hemihydrate. In the solid, a 1:1 triply hydrogenbonded complex consisting of one protonated and one neutral 1-methylcytosine base is observed. The hydrogen bonding in this complex is asymmetric, and the asymmetry in the interbase hydrogen bonding is stimulated, at least in part, by base stacking considerations. The hydrogen-bonded base pairs associate into dimers about a crystallographic center of symmetry; the base-base stacking mode is strong [mean stacking distance = 3.22A] and is such that the molecular overlap is between protonated and neutral 1-methylcytosine molecules. The asymmetric, interbase hydrogen bonding and the protonated over neutral base stacking mode coexist in a synergistic interrelationship which maximizes molecular association and crystal packing. Intermolecular hydrogen bonds involving the 1-methylcytosine bases, the water of crystallization and the iodide anion also contribute to the overall crystal stability. Infra-red and 1H nmr data are also presented.

Crystal structure of the radical-cation radical-anion salt from 2,2′-bi-1,3-dithiole and 7,7,8,8-tetracyanoquinodimethane

The three-dimensional structure of the radical-cation radical-anion salt from 2,2′-bi-1,3-dithiole (1) and 7,7,8,8-tetracyanoquinodimethane (2) has been determined by X-ray diffraction methods; the structure is composed of segregated columnar stacks of cations and anions.

A microelectromechanical‐based magnetostrictive magnetometer

The principles of operation of a microelectromechanical (MEMS)‐based magnetometer designed on the magnetoelastic effect are described. The active transduction element is a commercial (001) silicon microcantilever coated with an amorphous thin film of the giant magnetostrictive alloy Terfenol‐D [(Dy0.7Te0.3)Fe2]. In addition to the magnetostrictive transducer, basic components of the magnetometer include: (a) mechanical resonance of the coated‐microcantilever through coupling to an ac magnetic field; and (b) detection by optical beam deflection of the microcantilever motion utilizing a laser diode source and a position‐sensitive detector. Currently, the sensitivity of this MEMS‐based magnetostrictive magnetometer is ∼1μT.

A high sensitivity, wide dynamic range magnetometer designed on a xylophone resonator

A novel magnetometer based on a classical xylophone resonator is described. The device consists of an aluminum bar supported by two wires placed at the nodal points of the fundamental resonance frequency. The wires also supply current of this frequency to the bar. In the presence of a magnetic field, the Lorentz force causes the resonator to vibrate. The amplitude of this vibration is proportional to a vector component of the magnetic field. The device is intrinsically linear, and by altering the drive current the sensitivity can range from nanoteslas to teslas.