Latika Menon

Magnetic properties of self-assembled Co nanowires of varying length and diameter

Ferromagnetic Co nanowires have been electrodeposited into self-assembled porous anodic alumina arrays. Due to their cylindrical shape, the nanowires exhibit perpendicular anisotropy. The coercivity, remanence ratio, and activation volumes of Co nanowires depend strongly on the length, diameter, and spacing of the nanowires. Both coercivity and thermal activation volume increase with increasing wire length, while for constant center-to-center spacing, coercivity decreases and thermal activation volume increases with increasing wire diameter. The behavior of the nanowires is explained qualitatively in terms of localized magnetization reversal.

Super-resolution imaging using a three-dimensional metamaterials nanolens

Super-resolution imaging beyond Abbe’s diffraction limit can be achieved by utilizing an optical medium or “metamaterial” that can either amplify or transport the decaying near-field evanescent waves that carry subwavelength features of objects. Earlier approaches at optical frequencies mostly utilized the amplification of evanescent waves in thin metallic films or metal-dielectric multilayers, but were restricted to very small thicknesses (⪡λ, wavelength) and accordingly short object-image distances, due to losses in the material. Here, we present an experimental demonstration of super-resolution imaging by a low-loss three-dimensional metamaterial nanolens consisting of aligned gold nanowires embedded in a porous alumina matrix. This composite medium possesses strongly anisotropic optical properties with negative permittivity in the nanowire axis direction, which enables the transport of both far-field and near-field components with low-loss over significant distances (>6λ), and over a broad spectral ra...

Giant photoresistivity and optically controlled switching in self-assembled nanowires

We report the observation of giant photoresistivity in electrochemically self-assembled CdS and ZnSe nanowires electrodeposited in a porous alumina film. The resistance of these nanowires increases by one to two orders of magnitude when exposed to infrared radiation, possibly because of real-space transfer of electrons from the nanowires into the surrounding alumina by photon absorption. This phenomenon has potential applications in “normally on” infrared photodetectors and optically controlled switches.

A Study of Anodization Process during Pore Formation in Nanoporous Alumina Templates

We have carried out a systematic investigation of the anodization procedure in order to determine the exact chemical mechanism of the dissolution process responsible for pore formation in nanoporous alumina templates. We measured the anodization current as a function of time and compared it with the thickness of porous aluminum oxide layer obtained from cross-section scanning electron microscopy images. From this, we calculated the number of moles of electrons generated per mole of porous alumina grown. This analysis is consistent with a reaction mechanism in which aluminum is converted to aluminum oxide in addition with the direct transfer of aluminum ions across the thin barrier aluminum oxide into the electrolyte. Hence, experimental evidence is reported that indicate that the dissolution process observed is that of Al 3+ ions migrating across the barrier layer and dissolving into the solution. Contrary to the suggestion of some authors, these experiments indicate that at most a negligible amount of oxide dissolves during the anodization of aluminum.

Ignition studies of Al/Fe2O3 energetic nanocomposites

We prepare energetic nanocomposites, which undergo an exothermic reaction when ignited at moderate temperature. The nanocomposites are a mixture of Al fuel and Fe2O3 oxidizer where Fe2O3 is in the form of an array of nanowires embedded in the thin Al film. We achieve a very high packing density of the nanocomposites, precise control of oxidizer–fuel sizes at the nanoscale level, and direct contact between oxidizer and fuel. We find that the flame temperature does not depend on ignition temperature.

Possible room-temperature ferromagnetism in hydrogenated carbon nanotubes

We find that ferromagnetism can be induced in carbon nanotubes (CNTs) by introducing hydrogen. Multiwalled CNTs grown inside porous alumina templates contain a large density of defects resulting in significant hydrogen uptake when annealed at high temperatures. This hydrogen incorporation produces H-complex and adatom magnetism which generates a sizable ferromagnetic moment and a Curie temperature near T(C)=1000 K. We studied the conditions for the incorporation of hydrogen, the temperature-dependent magnetic behavior, and the dependence of the ferromagnetism on the size of the nanotubes.

Engineering Low-Aspect Ratio Carbon Nanostructures: Nanocups, Nanorings, and Nanocontainers

The synthesis of carbon nanostructures, with interesting morphologies, has created a revolution in nanotechnology; carbon nanotube is a case in point, but other nanoscale morphologies of graphitic carbon could provide compelling uses. In particular short structures, including very short nanotubes, have proven impossible to be grown by existing techniques due to the difficulty in controlling and terminating growth during initial stages. Here we present architectures engineered from graphitic carbon, having up to 10(5) times smaller length/diameter (L/D) ratios compared to conventional nanotubes, revealing unique morphologies of nanocups, nanorings, and large area connected nanocup arrays. Such highly engineered hollow nanostructures were fabricated using precisely controlled short nanopores inside anodic aluminum oxide templates. The nanocups were effectively used to hold and contain other nanomaterials, for example, metal nanoparticles, leading to the formation of multicomponent hybrid nanostructures with unusual morphologies. The results reported here open up possibilities to integrate new morphologies of graphitic carbon in nanotechnology applications.

Negative index metamaterials based on metal-dielectric nanocomposites for imaging applications

Negative index metamaterials are demonstrated based on metal-dielectric nanocomposites prepared using a versatile bottom-up nanofabrication approach. The method involves the incorporation of vertically aligned metal nanowires such as Au and Ag inside dielectric aluminum oxide nanotemplates. Optical absorbance measurements show resonance peaks corresponding to the transverse and longitudinal surface plasmon modes. A quantitative model based on effective medium theory is in excellent agreement with experimental data, and points to specific composite configurations and wavelength regimes where such structures can have applications as negative refraction media for imaging.

Roles of pH and acid type in the anodic growth of porous alumina.

Several theoretical models have been formulated to explain the growth of porous structures in anodized alumina. Using some basic assumptions, these models predict the size and shape of the pores in the anodic porous alumina as functions of pH and voltage. Additionally, they address issues of stability in the pore growth. In this work, we have carried out a systematic experimental investigation to study the stability phase diagram as a function of pH and applied voltage. We also obtain the dependence of pore dimensions on the pH, voltage, and acid type. Based on our results, and insight gained from recent chemical analysis of the porous alumina anodization process, we conclude that the models must include an appropriate weighting factor to account for the oxidation and dissolution mechanism during the pore formation.

Effects of surface morphology on magnetic properties of Ni nanowire arrays in self-ordered porous alumina

Ni nanowire arrays were produced by electrodeposition of Ni into self-assembled porous anodic alumina templates. With different anodization voltages, the wires show different surface morphologies. The temperature dependence of the coercivity and activation volume of wires with regular shape can be explained by thermal activation over an energy barrier with a 3/2 power dependence on the field. The wires with irregular surface morphologies show abnormal temperature dependences of the coercivity and activation volume. Possible mechanisms for these behaviours are discussed.