Robert M. Metzger

Theory of electrical rectification in a molecular monolayer

The current-voltage characteristics in Langmuir-Blodgett monolayers of \ensuremath{\gamma}-hexadecylquinolinium tricyanoquinodimethanide

Why are some oxides metallic, while most are insulating?

({\mathrm{C}}_{16}{\mathrm{H}}_{33}\mathrm{Q}\ensuremath{-}3\mathrm{CNQ})

Rectification by a Monolayer of Hexadecylquinolinium Tricyanoquinodimethanide between Gold Electrodes

sandwiched between Al or Au electrodes is calculated, combining ab initio and self-consistent tight-binding techniques. The rectification current depends not only on the position of the LUMO and HOMO relative to the Fermi levels of the electrodes as in the Aviram-Ratner mechanism, but also on the profile of the electrostatic potential which is extremely sensitive to where the electroactive part of the molecule lies in the monolayer. This second effect can produce rectification in the direction opposite to the Aviram-Ratner prediction.

Electrodeposition of highly uniform magnetic nanoparticle arrays in ordered alumite

Abstract A large variety of undoped binary and ternary transition metal oxides, including formally divalent, trivalent and tetravalent metal cations, have been examined. These 76 compounds are classified as either “metals”, “insulators”, or having a “metal-to-insulator” transition. In an attempt to understand these variations, the Zaanen-Sawatzky-Allen framework was used in which each compound can be characterized by three parameters: the Coulomb correlation or disproportionation energy (U′), the charge-transfer energy (Δ) and the bandwidth (W). Assuming W is constant, we have calculated U′ and Δ using a simple ionic model, which includes only the gas phase ionization potentials and the bare electrostatic Coulomb interactions between the ions. With this model, the occurence of metallic conductivity is remarkably well accounted for in these oxides.

Influence of particle size on the magnetic viscosity and activation volume of /spl alpha/-Fe nanowires in alumite films

The asymmetry can be ascribed to the intervalencetransfer band between the HOMO and LUMO, appropriatelyshifted by the applied electrical potential, and coupled tolevels in the Al electrodes. The optical absorption at 570 nm(2.17 eV) in films of 1 can be ascribed to the HOMO–LUMOgap, or in slightly different language, to the transition from theground state (dipole moment 43 8D)

Simple and perovskite oxides of transition-metals: Why some are metallic, while most are insulating

We report the fabrication of nanometer scale ordered arrays of magnetic cylindrical nanoparticles with low aspect ratio (height/radius a=0.2–7) and ultrahigh uniformity. Anodization and electrochemical deposition are employed for template synthesis and metal particle growth, respectively. Particle uniformity is achieved by an electrodeposition scheme, utilizing pulse reverse voltage wave forms to control nucleation and growth of the particles. The resulting nanoparticles are polycrystalline and grains are randomly oriented. The magnetic properties of the array are dominated by particle shape and by interparticle magnetostatic interactions. A very clear transition of the anisotropy from perpendicular to in plane is observed at an aspect ratio a of about two. The arrays exhibit good thermal stability, demonstrating a great potential of these structures as future recording media in a patterned scheme. The pulse reverse electrodeposition technique shows great promise for the synthesis of nanostructures of var...

Synthesis and properties of α″‐Fe16N2 in magnetic particles

Particle size effects on the magnetic viscosity and activation volume were studied at room temperature for elongated /spl alpha/-Fe particles in alumite (Fe nanowires in an anodized aluminum oxide film). Both magnetic viscosity and activation volume are strongly dependent on the particle diameter, but independent of the particle length. The magnetization reversal mechanism of elongated /spl alpha/-Fe particles in alumite films is discussed. The activation volume may represent the size of the magnetic switching unit propagating along the particle length during the non-uniform magnetization reversal.

Magnetism of α ″‐Fe16N2 (invited)

Abstract Some 76 simple and perovskite transition-metal oxides are classified as “metals,” “insulators,” and those exhibiting metal-insulator transitions. Using the framework of Zaanen, Sawatzky, and Allen and a simple ionic model to estimate the two relevant energies ( Δ 0 and U ′ 0 ), we can find boundaries which separate the insulating oxides from two types of metals: low Δ 0 metals and low U ′ 0 metals. In addition, compounds with metal-insulator transitions are found to be on (or near) these boundaries. It is concluded that the large differences in conductivity behavior of oxides are largely due to differences in the ionization potentials of the transition metal cations.

Synthesis of high moment and high coercivity iron nitride particles

The α‘‐Fe16N2 phase was synthesized by a nitriding, quenching, and tempering process starting from Fe2O3. Acicular γ‐Fe2O3 was first reduced to α‐Fe under H2, then it was converted to γ‐austenite at 650–700 °C by nitriding using a NH3‐H2 mixed gas. A martensitic transformation to α’‐martensite occurred when the sample was quenched in liquid nitrogen. Finally, the α‘‐Fe16N2 phase was formed from the α’‐martensite by tempering in the temperature range 120–200 °C. In order to increase the extent of the martensitic transformation, acicular Fe2O3 particles with 10–15 at % MnO2 were prepared by oxidizing Fe++ and Mn++ in alkaline aqueous solution. When a quenched sample, which contains martensite, α‐Fe, and austenite, was tempered at 200 °C for different times, the magnetic moment first increased (transformation of α’‐martensite to α‘‐Fe16N2); the highest magnetic moment was obtained at about 60 min of tempering. Using even longer tempering times, the magnetic moment decreased (decomposition of α‘‐Fe16N2 to α‐F...

The unimolecular rectifier: unimolecular electronic devices are coming …

The metastable α‘‐Fe16N2 phase may have a magnetic moment up to 50% higher than that of pure bulk α‐Fe. This article addresses the following issues. (i) Can epitaxial films of α‘‐Fe16N2 be prepared phase pure? Yes, but there are some doubts. (ii) Can powders of α‘‐Fe16N2 be prepared phase pure? Not yet. (iii) Is the Mossbauer spectrum due to α‘‐Fe16N2, to martensite, or to something else? Most assign it to α‘‐Fe16N2. (iv) What is the specific saturation magnetic moment of α‘‐Fe16N2? Some claim it is close to that of α‐Fe, most claim that it is much larger. (v) Is the high moment due to α‘‐Fe16N2, or to some other phase?