Stephan von Molnar

Detection of single magnetic bead for biological applications using an InAs quantum-well micro-Hall sensor

Room-temperature detection of a single commercial superparamagnetic bead (1.2μm in diameter) suitable for biological applications has been realized using an InAs quantum-well micro-Hall sensor. The detection was demonstrated using phase-sensitive detection on a single Hall cross as well as in a Hall gradiometry setup. The high signal to noise ratio, obtained in both configurations, promises detection of single nanometer-size particles by further miniaturization of the device to submicron dimensions.

Hall magnetometry on a single iron nanoparticle

High-sensitivity magnetometry over a wide temperature range has been achieved using submicron GaAs/GaAlAs Hall gradiometry. The sensitivity and versatility of the technique was demonstrated by the successful measurement of the magnetization switching of a single Fe nanoparticle with m∼5×105 μB (∼5×10−15 emu) at temperatures as high as 75 K.

Controlled Synthesis of Phase-Pure InAs Nanowires on Si(111) by Diminishing the Diameter to 10 nm

Here we report the growth of phase-pure InAs nanowires on Si (111) substrates by molecular-beam epitaxy using Ag catalysts. A conventional one-step catalyst annealing process is found to give rise to InAs nanowires with diameters ranging from 4.5 to 81 nm due to the varying sizes of the Ag droplets, which reveal strong diameter dependence of the crystal structure. In contrast, a novel two-step catalyst annealing procedure yields vertical growth of highly uniform InAs nanowires ∼10 nm in diameter. Significantly, these ultrathin nanowires exhibit a perfect wurtzite crystal structure, free of stacking faults and twin defects. Using these high-quality ultrathin InAs nanowires as the channel material of metal-oxide-semiconductor field-effect transistor, we have obtained a high ION/IOFF ratio of ∼10(6), which shows great potential for application in future nanodevices with low power dissipation.

Magnetic characterization of a single superparamagnetic bead by phase-sensitive micro-Hall magnetometry

Employing phase sensitive micro-Hall magnetometry at room temperature, we map the susceptibility of a single superparamagnetic bead, 1.2μm in diameter, as a function of magnetic field. We find that the dependence can be explained by modeling the bead as an ensemble of noninteracting superparamagnetic nanoparticles with log-normal distribution of magnetic moments. We also discuss the effect of possible dipolar interactions between the nanoparticles on the obtained results.

All-electrical switching and control mechanism for actomyosin-powered nanoactuators

A fast all-electrical activation and control mechanism for biomolecular motor-powered nanoactuators has been developed. Rapid and reversible on–off control of actomyosin biomolecular motors was experimentally demonstrated using in vitro motility assays. The results show that the motility of the actin filaments can be cycled repeatedly by electrically controlled thermal activation in the temperature range from 10°C to 50°C without functional loss. The fast response of the filaments upon rapid temperature switching suggests that thermal activation provides an effective method for turning actomyosin-powered nanoactuators on and off.

All Zinc-Blende GaAs/(Ga,Mn)As Core–Shell Nanowires with Ferromagnetic Ordering

Combining self-catalyzed vapor-liquid-solid growth of GaAs nanowires and low-temperature molecular-beam epitaxy of (Ga,Mn)As, we successfully synthesized all zinc-blende (ZB) GaAs/(Ga,Mn)As core-shell nanowires on Si(111) substrates. The ZB GaAs nanowire cores are first fabricated at high temperature by utilizing the Ga droplets as the catalyst and controlling the triple phase line nucleation, then the (Ga,Mn)As shells are epitaxially grown on the side facets of the GaAs core at low temperature. The growth window for the pure phase GaAs/(Ga,Mn)As core-shell nanowires is found to be very narrow. Both high-resolution transmission electron microscopy and scanning electron microscopy observations confirm that all-ZB GaAs/(Ga,Mn)As core-shell nanowires with smooth side surface are obtained when the Mn concentration is not more than 2% and the growth temperature is 245 °C or below. Magnetic measurements with different applied field directions provide strong evidence for ferromagnetic ordering in the all-ZB GaAs/(Ga,Mn)As nanowires. The hybrid nanowires offer an attractive platform to explore spin transport and device concepts in fully epitaxial all-semiconductor nanospintronic structures.

Hopping transport in TiO2:Co: A signature of multiphase behavior

TiO2:Co films have been grown in the rutile phase under varying oxygen growth pressures. Detailed microstructural analysis using transmission electron microscopy shows that reduced growth pressure increases the presence of cobalt clusters. The resistivity is found to follow a log(ρ)∼T−1/2 dependence over a wide temperature range. This behavior is characteristic of hopping transport in multiphase systems and implies the presence of cobalt clustering. Thus, transport measurements are shown to be sensitive to the appearance of metallic clusters in these systems and suggest a rapid method for determing the presence of clustering in these and other magnetically doped semiconductor systems.

The detection of specific biomolecular interactions with micro-Hall magnetic sensors

The detection of reagent-free specific biomolecular interactions through sensing of nanoscopic magnetic labels provides one of the most promising routes to biosensing with solid-state devices. In particular, Hall sensors based on semiconductor heterostructures have shown exceptional magnetic moment sensitivity over a large dynamic field range suitable for magnetic biosensing using superparamagnetic labels. Here we demonstrate the capability of such micro-Hall sensors to detect specific molecular binding using biotin-streptavidin as a model system. We apply dip-pen nanolithography to selectively biotinylate the active areas of InAs micro-Hall devices with nanoscale precision. Specific binding of complementarily functionalized streptavidin-coated superparamagnetic beads to the Hall crosses occurs via molecular recognition, and magnetic detection of the assembled beads is achieved at room temperature using phase sensitive micro-Hall magnetometry. The experiment constitutes the first unambiguous demonstration of magnetic detection of specific biomolecular interactions with semiconductor micro-Hall sensors, and the selective molecular functionalization and resulting localized bead assembly demonstrate the possibility of multiplexed sensing of multiple target molecules using a single device with an array of micro-Hall sensors.

Modulation of noise in submicron GaAs/AlGaAs hall devices by gating.

We present a systematic characterization of fluctuations in submicron Hall devices based on GaAs/AlGaAs two-dimensional electron gas heterostructures at temperatures between 1.5 to 60 K. A large variety of noise spectra, from 1/f to Lorentzian, are obtained by gating the Hall devices. The noise level can be reduced by up to several orders of magnitude with a moderate gate voltage of 0.2 V, whereas the carrier density increases less than 60% in the same range. The significant dependence of the Hall noise spectra on temperature and gate voltage is explained in terms of the switching processes related to impurities in n-AlGaAs.

Magneto-transport in magnetic compound semiconductors and metals

This paper reviews the physics of magnetic and dilute magnetic semiconductors. Their electrical transport properties are discussed with reference to the important role played by bound magnetic polarons in these systems. We also discuss the magneto-transport properties of the binary oxide CrO2, which is purported to be a half-metallic ferromagnet. These materials are of significant importance for spintronics applications.