Jody G. Redepenning

Characterization of electrolytically prepared brushite and hydroxyapatite coatings on orthopedic alloys

Details of a procedure for electrolytically preparing highly pure brushite (CaHPO4.2H2O) coatings on high surface area metal substrates are provided. The influence of current density and deposition time on coating morphology is described. After a discussion of procedures used to convert the brushite coatings to hydroxyapatite, results from a preliminary animal study that demonstrate the propensity of these coatings to stimulate bony ingrowth into porous plugs are presented.

Quantized magnetoresistance in atomic-size contacts.

When the dimensions of a metallic conductor are reduced so that they become comparable to the de Broglie wavelengths of the conduction electrons, the absence of scattering results in ballistic electron transport and the conductance becomes quantized. In ferromagnetic metals, the spin angular momentum of the electrons results in spin-dependent conductance quantization and various unusual magnetoresistive phenomena. Theorists have predicted a related phenomenon known as ballistic anisotropic magnetoresistance (BAMR). Here we report the first experimental evidence for BAMR by observing a stepwise variation in the ballistic conductance of cobalt nanocontacts as the direction of an applied magnetic field is varied. Our results show that BAMR can be positive and negative, and exhibits symmetric and asymmetric angular dependences, consistent with theoretical predictions.

MEASUREMENT OF FORMAL POTENTIALS FOR HYPERICIN IN DIMETHYLSULFOXIDE

The formal potentials for hypericin are measured in dimethylsulfoxide using cyclic voltammetry. Two reversible one‐electron reductions and a more poorly defined multielectron oxidation are observed. The formal potentials for these processes are consistent with the visible spectrum of hypericin and they indicated that hypericin is both a good oxidizing agent and a good reducing agent in the excited state.

In situ magnetoresistance of Ni nanocontacts

Magnetoresistance properties of Ni nanocontacts in the ballistic quantum regime are investigated in situ during closure and opening of electrochemically grown planar electrodes. The magnitude of conductance change when sweeping the magnetic field is of the order of the quantum conductance e2/h for conductance values spanning 1–100 quanta. The relative orientation of electric current and applied magnetic field changes the magnetoresistance sign, with symmetry properties reminiscent of bulk anisotropy magnetoresistance. Ex situ investigations of samples of higher conductance values, of the order of 1000 quanta, unambiguously show the analogy with bulk anisotropy magnetoresistance.

Orientation and bonding of biphenyldimethyldithiol

The molecular orbitals of 1,1 � -biphenyl-4,4 � -dimethyldithiol (HS–CH2– C6H4–C6H4–CH2–SH) are identified from combined photoemission and inverse photoemission studies and compared with theory for several different surfaces, molecular conformations and molecular orientations. The preferential molecular orientations of biphenyldimethyldithiol, on both Au(111) and polycrystalline Co, are identified from polarization resolved photoemission studies. Two different molecular orientations are adopted by biphenyldimethyldithiol on gold depending on adsorption conditions. Biphenyldimethyldithiol is observed to bond more strongly to cobalt than gold surfaces. (Some figures in this article are in colour only in the electronic version)

Resonant tunneling in magnetoresistive Ni/NiO/ Co nanowire junctions

Magnetotransport studies performed on electrodeposited Ni/NiO/Co nanojunctions show a broad distribution of magnetoresistance values spanning from +40% to −25%, with an average of about 2%, corresponding to observations on large-area junctions. The dispersion in the results can be understood in terms of tunneling via localized states in the barrier. Calculations based on Landauer–Buttiker theory explain this behavior in terms of disorder-driven statistical variations in magnetoresistance with a finite probability of the inversion of tunnel magnetoresistance sign due to resonant tunneling.

Influence of supporting electrolyte activity on formal potentials measured for dissolved internal standards in acetonitrile

Osmotic coefficients are determined for tetra-n-butylammonium perchorate (TBAClO4) in acetonitrile. Pitzer equations are fit to the data, and the resulting fitting parameters are used to calculate activity coefficients for TBAClO4. The activity coefficients are used to control electrolyte activities so that the junction potential across a permselective anion exchanger is near zero. An electrochemical cell containing this junction is used to examine the influence of electrolyte activity on the formal potentials measured for ferrocene/ferrocenium and decamethylferrocene/decamethylferrocenium. It is clear that the oxidized forms of both redox couples are ion-paired with perchlorate anions in the supporting electrolyte and that this ion-pairing has a significant influence on the formal potentials.

ELECTROCHEMICAL CLEAVAGE OF THE NITROGEN-NITROGEN BOND IN HYDRAZIDO(2-) COMPLEXES OF MOLYBDENUM : FORMATION OF AMMONIA

Abstract The controlled potential electrolysis of a series of hydrazido(2-) complexes of molybdenum, [MoX(NNH2)(TR1)(PPh3)]Y[TR1 = PhP(CH2CH2PPh2)2; 1, X = Y = Br; 2, X = Y = Cl; 3, X = F, Y = BF4] in tetrahydrofuran (thf) at a mercury-pool electrode with 85% phosphoric acid as the proton source generates ammonia. The yield of ammonia is quantitative for 2. For 1 and 3 the yields are 0.32 and 0.24 mol of ammonia per mole of complex, respectively. These data are compared with yields of ammonia obtained from non-electrolysis methods.

The anomalous "stiffness" of biphenydimethyldithiol

Although organic adsorbates and thin films are generally regarded as “soft” materials, the effective Debye temperature, indicative of the dynamic motion of lattice normal to the surface, can be very high. For biphenyldimethyldithiol, the effective Debye temperature, determined from core level photoemission from the all carbon arene rings, is comparable to that of graphite. We associate this rigidity to the stiffness of the benzene rings, and the ordering in the molecular thin film. Measurements on the sulfur of poly(hexylthiophene) show that sulfur in an arene ring is less dominated by soft modes than the pendant sulfur of biphenyldimethyldithiol on the time scale of photoemission.

In-Situ Characterization of Ultra-Small Magnetic Junctions Made by Electrochemical Techniques

Abstract : Electrochemical impedance spectroscopy is used to characterize the growth of NiO over Ni electrodes. We find a limited increase of thickness and a significant increase of porosity of the oxide as a function of time and anodization potential. Conductance measurements performed on Ni/NiO/Co junctions of 30 nm diameters indicate the presence of a Coulomb blockade at low temperatures and small bias. Tunneling is observed at higher bias. Small magnetoresistance ratios (1%) are found.