Malcolm J. Skove

Origin of FM Ordering in Pristine Micro- and Nanostructured ZnO

An unexpected presence of ferromagnetic (FM) ordering in nanostructured nonmagnetic metal oxides has been reported previously. Though this property was attributed to the presence of defects, systematic experimental and theoretical studies to pinpoint its origin and mechanism are lacking. While it is widely believed that oxygen vacancies are responsible for FM ordering, surprisingly we find that annealing as-prepared samples at low temperature (high temperature) in flowing oxygen actually enhances (diminishes) the FM ordering. For these reasons, we have prepared, annealed in different environments, and measured the ensuing magnetization in micrometer and nanoscale ZnO with varying crystallinity. We further find from our magnetization measurements and ab initio calculations that a range of magnetic properties in ZnO can result, depending on the sample preparation and annealing conditions. For example, within the same ZnO sample we have observed ferro- to para- and diamagnetic responses depending on the annealing conditions. We also explored the effects of surface states on the magnetic behavior of nanoscale ZnO through detailed calculations.

Mechanical properties of chemical vapor deposition-grown multiwalled carbon nanotubes

The bending modulus (Young’s modulus) of several chemical vapor deposition-grown multiwalled nanotubes (MWNTs) have been measured using a vibrating reed technique. Three different precursors were used to produce MWNTs with differing densities of defects in the tube walls. Individual MWNTs were electrostatically driven in air over a dark-field light microscope and the bending modulus of the nanotubes was determined from the frequency of the first vibrational resonance. A correlation between the defect density and the bending modulus was found which implies that the bending modulus is relatively more sensitive to wall defects than the nanotube diameter.

Tuning electrical and thermal connectivity in multiwalled carbon nanotube buckypaper

We find that the electrical and thermal connectivity in multiwalled carbon nanotube buckypaper can be tuned using a spark plasma sintering (SPS) technique. Elevated SPS temperatures promote the formation of inter-tube connections and consequently impact the electrical resistivity, thermoelectric power and thermal conductivity of the buckypaper. In particular, the electrical resistivity as a function of SPS temperature exhibits a percolation-type behavior while the low temperature lattice thermal conductivity shows a crossover behavior in the sample dimensionality. The results are discussed in terms of the quasi-one-dimensional metallic nature of multiwalled carbon nanotubes, the packing density and the electron-phonon coupling.

Electrical detection of oscillations in microcantilevers and nanocantilevers

Precise determination of the resonant frequency, phase, and quality factor in micromechanical and nanomechanical oscillators would permit, among other things, (i) the detection of trace amounts of adsorbed molecules through a shift in the resonant frequency, and (ii) pressure variations in the environment which affect the mechanical damping of the oscillator. The major difficulty in making these measurements in many cases is the ancillary equipment such as lasers or high magnetic fields that must be used. Being able to make precise measurements with a fully electrical actuation and detection method would greatly extend the usefulness of these oscillators. Detecting the oscillation through changes in the capacitance between the oscillator and a counter electrode is difficult because the static capacitance between them as well as the parasitic capacitance of the rest of the circuitry overwhelm the detection. We have found that the charge on a microcantilever or nanocantilever when driven by a nearby counter...

Evidence for surface states in pristine and Co-doped ZnO nanostructures: magnetization and nonlinear optical studies

An unexpected presence of ferromagnetic (FM) ordering in nanostructured ZnO has been reported previously. Recently, from our detailed magnetization studies and ab initio calculations, we attributed this FM ordering in nanostructured ZnO to the presence of surface states, and a direct correlation between the magnetic properties and crystallinity of ZnO was observed. In this study, through a systematic sample preparation of both pristine and Co-doped ZnO nanostructures, and detailed magnetization and nonlinear optical (NLO) measurements, we confirm that the observed FM ordering is due to the presence of surface states.

Linear and volume compressibilities and isothermal third‐order elastic constants

Bridgman measured the changes in the dimensions or volume of many crystals as a function of hydrostatic pressure P. He expressed the change in volume ΔV from the original volume V as ΔV/V=−a′P+b′P2, where a′ and b′ are elastic constants. Murnaghan’s theory of finite deformations relates the constant b′ to combinations of second‐ and third‐order elastic constants. Bridgman’s experimental work may be compared to values computed from published values of the third‐order elastic constants. While the agreement is not close, Bridgman’s work may be useful for materials for which no other experimental values exist.

Ultra-Sensitive Duffing Behavior of a Microcantilever

We investigate the properties of an electrostatically driven microcantilever exhibiting duffing-like behavior using harmonic detection of resonance. Its potential use as a highly sensitive sensing platform is discussed. We find high sensitivity of this duffing system near its bistability point in a gaseous environment. The response of the higher harmonics of the measured charge on the cantilever induced by an ac voltage that drives the counter electrode is investigated. In particular, we follow the duffing behavior at the higher harmonics (up to the sixth harmonic) as a function of gap distance between the cantilever and counter electrode. To our knowledge, this work represents the first experimental demonstration of sensing a pressure change using the duffing behavior.

Deformation twinning in Zn, Sn and Bi single crystal whiskers

Abstract In ‘whiskers’ of Zn, Sn and Bi it is possible, by application of axial tension, to nucleate a twinned region and pass the twin boundaries the length of the crystal. Nucleation and propagation stresses for twinning in whiskers are found. The completely twinned crystal will withstand the large stresses characteristic of perfect whiskers.

Direct measurement of shear properties of microfibers.

As novel fibers with enhanced mechanical properties continue to be synthesized and developed, the ability to easily and accurately characterize these materials becomes increasingly important. Here we present a design for an inexpensive tabletop instrument to measure shear modulus (G) and other longitudinal shear properties of a micrometer-sized monofilament fiber sample, such as nonlinearities and hysteresis. This automated system applies twist to the sample and measures the resulting torque using a sensitive optical detector that tracks a torsion reference. The accuracy of the instrument was verified by measuring G for high purity copper and tungsten fibers, for which G is well known. Two industrially important fibers, IM7 carbon fiber and Kevlar(®) 119, were also characterized with this system and were found to have G = 16.5 ± 2.1 and 2.42 ± 0.32 GPa, respectively.

An analytic characterization of the harmonic detection of resonance method

While it has proven useful as a sensor and as a system for exploring and examining nonlinear oscillation, the harmonic detection of resonance (HDR) method has not been fully derived and explained analytically. We develop the equation of motion for the oscillation of a cantilever which is electrostatically driven into resonance and compared to experiment. The resonance signal is measured both by a photodetector (mechanical signal) and a charge amplifier (electrical signal) and is found to be in good agreement with the derived equation of motion. Finally, a few nonlinear phenomena observed in our HDR experiments will be examined analytically.