Our controlled growth of Ni/ZnO nanorod heterostructures by MOVPE opens up significant opportunities for the fabrication of spintronic device structures on a single nanorod. The simple yet accurate thickness control allows tunable magnetic properties in nanosized magnetic layers on individual nanorods due to a crossover from superparamagnetism to ferromagnetism. These magnetic building blocks may be used as components for nanoscale spin-valve transistors, spin lightemitting diodes, and nonvolatile storage and logic devices. More generally, we believe that the simple ªbottom±upº heterostructural approach might readily be expanded to create many other magnetic-semiconductor nanorod heterostructures. ExperimentalFor the fabrication of magnetic-metal/ZnO nanorod heterostructures, ZnO nanorods were prepared on Al 2 O 3 (0001) substrates using a low-pressure MOVPE system [11]. No metal impurity catalyst was deposited on the substrates. For ZnO nanorod growth, diethylzinc, and oxygen were employed as reactants with argon as the carrier gas. For magnetic metal evaporation on the ZnO nanorods, Ni was evaporated on vertically aligned ZnO nanorods at room temperature using an electron-gun evaporation system with a base pressure 7±8 10 ±8 torr [12]. During metal evaporation, metal flux was incident on the top surfaces of the ZnO nanorods. The deposition rate was monitored using a thickness monitor and controlled to be 0.2 s ±1 . Metal layer thickness was in the range of 50±400 , as determined by TEM [12].The crystal orientation of nanorod heterostructures was characterized using SR-XRD. The SR-XRD measurements of Ni/ZnO nanorod heterostructures were performed using a four-circle diffractometer of the 3C2 synchrotron X-ray diffraction beam line at the Pohang Accelerator Laboratory [17]. The beam size at the focal point was typically less than 1 mm 2 and a fixed-exit double-crystal monochromator was used. The diffracted beam intensity was measured using a scintillation detector. High X-ray flux yielded a significant signal-to-noise ratio to measure very thin Ni layers on the nanorods. The crystal orientation of Ni layers was confirmed using TEM. Details in the SEM and TEM measurements have been reported elsewhere [11,12].The topography and magnetic images of Ni/ZnO nanorods were measured using a MFM that was modified from a commercial AFM. The MFM was operated in amplitude detection mode. MFM tips are AFM cantilevers coated with 30 nm of cobalt and have a resonance frequency of 18 kHz. Before measuring the magnetic properties of samples, they were saturated in a fixed direction along their easy axis under the applied magnetic field of 3000 Oe, after which an MFM image was taken to determine its magnetization. The AFM and MFM images were measured simultaneously on the same region.Further magnetic properties of Ni/ZnO nanorod heterostructures were measured using both SQUID and a high-sensitivity AGM at room temperature in a field of up to 60 kOe. For the data presented here, the diamagnetic background from Al 2 O 3 (0001) substr...
Chip-based biosensor arrays for label-free and high-throughput detection were fabricated and tested. The sensor array was composed of a 150-nm-thick, 50-nm-gap, and 600-nm-period gold nanoslits. Each array size was 100 mumx100 mum. A transverse-magnetic polarized wave in these metallic nanostructures generated resonant surface plasmons at a wavelength of about 800 nm in a water environment. Using the resonant wavelength shift in the nanoslit array, we achieved detection sensitivity up to 668 nm per refractive index unit, about 1.7 times larger than that reported on an array of nanoholes. An antigen-antibody interaction experiment in an aqueous environment verified the sensitivity in a surface binding event.
The wavelength sensitivities of three kinds of nanostructures (nanoslits, nanoholes, and concentric circles) with various aperture sizes were compared in water environment. These nanostructures were made on a 110-nm-thick gold film with a period of 600 nm. Surface plasmon resonances in these nanostructures produce transmission dips near the phase-matching conditions while peaks at longer wavelengths. The wavelength sensitivities measured at dips are close to theoretical predictions and about 1.5 times larger than those measured at peaks. Such sensitivity difference is attributed to various surface plasmon distributions, as illustrated by the finite-difference time-domain calculations. In addition, the sensitivity decreases with the increase of aperture size. The nanoslit array and concentric circles have better sensitivities than the nanohole array due to the no cut-off transmission.
The nanoscaled tris-͑8-hydroxyquinoline͒ aluminum (AlQ 3) crystalline film was synthesized by vapor condensation. It was stacked with nanometer-sized rods, approximately 100 nm wide and 1 m long, and had a surface roughness of about 100 nm. The vibronic progression with several separated peaks was observed in the photoluminescence spectrum at room temperature. It is attributed to the crystallinity of AlQ 3 and the coupling of vibrations of the individual ligands to the fluorescence transition. The emission current was also observed with a turn-on field of 12.0 V/m, and a current density of about 0.8 mA/cm 2 at 22 V/m. Therefore, the AlQ 3 crystalline film provides a choice for field emission.
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