D. AlMawlawi

Magnetic properties of Fe deposited into anodic aluminum oxide pores as a function of particle size

A combination of magnetic and electron microscopic measurements on iron deposited in chemically widened anodic aluminum oxide films has allowed us to relate the coercivity of these materials to the dimensions of the metal particles. The coercivity is found to be highly anisotropic and to depend on the aspect ratio of the metallic particles. The functional dependence of Hc on the aspect ratio fits the expression reported by Jacobs and Bean [Phys. Rev. 100, 1060 (1955)] for magnetization reversal by symmetric fanning. This is likely due to the fact that in our unannealed samples the metal deposit consists of a cylindrical assembly of fused single‐domain particles. Pore widening by chemical dissolution of the anodic oxide is found to depend quadratically on the time that the sample is soaked in the acid bath.

Nonlithographic nano-wire arrays: fabrication, physics, and device applications

A novel system of nanostructures is described consisting of nonlithographically produced arrays of nano-wires directly electrodeposited into porous anodic aluminum oxide templates. Using this method regular and uniform arrays of metal or semiconductor nano-wires or nano-dots can be created with diameters ranging from /spl sim/5 nm to several hundred nanometers and with areal pore densities in the /spl sim/10/sup 9/-10/sup 11/ cm/sup -2/ range. We report on the present state of their fabrication, properties, and prospective device applications. Results of X-ray diffraction, Raman and magnetic measurements on metal (Ni, Fe) and semiconductor (CdS, CdSe, CdS/sub x/Se/sub 1-x/, Cd/sub x/Zn/sub 1-x/S and GaAs) wires are presented. The I-V characteristics of two terminal devices made from the nano-arrays are found to exhibit room temperature periodic conductance oscillations and Coulomb-blockade like current staircases. These observations are likely associated with the ultra-small tunnel junctions that are formed naturally in the arrays. Single-electron tunneling (SET) In the presence of interwire coupling in these arrays is shown to lead to the spontaneous electrostatic polarization of the wires. Possible device applications such as magnetic memory or sensors, electroluminescent flat-panel displays, and nanoelectronic and single-electronic devices are also discussed.

Nanowires formed in anodic oxide nanotemplates

A simple electrochemical method is described for producing metal or semiconductor nanowires with diameters in the continuous range 10 to 200 nm. The technique involves a three-step process that begins with the electrochemical generation of an aluminum oxide template with uniform nanometer-sized pores, followed by the deposition of metal or semiconductor in them. The nanowires are then exposed for study or device fabrication by etching back the oxide matrix. Examples of cadmium nanowires fabricated by this technique are shown.

FIELD EMITTERS BASED ON POROUS ALUMINUM OXIDE TEMPLATES

The field-emission properties of cold cathodes produced using nano-porous anodic aluminum oxide (AAO) templates are reported. Several types of field emitters were fabricated: aligned copper nanowires grown halfway up the parallel nano-pores of the AAO; aligned multiwalled carbon nanotubes grown to the top of the pores; surfaces overgrown with random tangles of carbon nanotubes; and empty AAO templates. Significant field-emission currents (field enhancement values ∼ 1800) were obtained at threshold voltages as low as 80 V (corresponding to fields of 3–4 V/μm) on samples of nanotube tangles. Perfectly aligned carbon nanotubes were less efficient field emitters and had lower field enhancement values. These observations are explained in terms of the mean separation of active tips in the two sets of samples. Empty templates and metal nanowire arrays show lower field enhancements and higher threshold electric fields (40–70 V/μm). In these samples significant field-emission currents are produced at relatively lo...

Highly regular anatase nanotubule arrays fabricated in porous anodic templates

Abstract Compact, continuous and uniform anatase nanotubules with diameters in the range 50–70 nm were produced inside PAO nano-templates by pressure impregnating the PAO pores with titanium isopropoxide then oxidatively decomposing the reagent at 500 °C. Cleaning the surface of the template and repeating the process several times produced titania nanotubules with a wall thickness ∼3 nm per impregnation. The tube exteriors appeared to be faithful replicas of the pores in which they were formed. The crystallinity and form (anatase) of the titania were determined using XRD and Raman spectroscopy. Tubules in this low diameter range could not be produced using sol–gel.

Magnetic properties of arrays of oriented iron particles as a function of particle size, shape and spacing

Abstract Hysteresis and remanence properties are reported for elongated iron particles deposited electrochemically in cylindrical pores of diameter d and spacing dc oriented perpendicular to the plane of an aluminium oxide substrate. Particle size, spacing and axial ratio were in the ranges d = 17–32 nm, dc = 26.5–64.5 nm and l/d = 3–20 in three series of samples. For d = 17 nm and l/d ≥ 10, hysteresis loops measured parallel to particle axes were almost rectangular and had parameters characteristic of single-domain (SD) particles reversing incoherently by fanning: M rs M s = 0.75–0.95, H cr ≈ H c ≈ 2 kOe , H sat ≈ 3 kOe . Perpendicular to particle axes, there was little hysteresis and Hsat ≈ 6 kOe, typical of coherent reversals. Larger and less elongated particles had reduced parallel coercive forces (Hc = 1.4–1.7 kOe) and remanence ratios ( M rs M s = 0.3–0.4 ). Anhysteretic remanent magnetization produced in a weak field (10 Oe) had an inflected alternating field (a.f.) demagnetization curve characteristic of SD behaviour. The a.f. decay curve for 0.9 kOe isothermal remanent magnetization (IRM) was more exponential in form. The effect of interactions was tested by varying particle spacing dc. The distribution of critical fields for IRM acquisition was insensitive to both particle interaction and orientation of the field parallel or perpendicular to particle axes. However, each sample had a critical field distribution for a.f. demagnetization that was different from that for acquisition of IRM. For the most strongly interacting sample (d = 17 nm, dc = 26.5 nm), median magnetizing fields for 0.9 kOe IRM were H| ≈ H⊥ ≈ 450 Oe, whereas median a.f. destructive fields were H⊥ ≈ 200 Oe, H| ≈ 400 Oe. Strong negative interactions are indicated by this observed deviation from the Wohlfarth relations.

Nonlithographic Fabrication and Physics of Nanowire and Nanodot Array Devices — Present and Future

It has been the “proven truth” for some decades that the performance of electronic devices should rise with decreasing dimensions. The continuos advance of lithographic technology in the past made possible serious exploration of the fascinating world of low-dimensional structures and was in turn propelled by the results of this exploration. Today, the submicron lithography in combination with different growth techniques such as MBE and MOCVD is routinely used for fabrication of quantum-well based devices with superior characteristics.