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A.           Nanofabrication Based on Nanoporous Alumina Templates

We are developing non-lithographic, nanofabrication methods based on porous alumina templates. These are aluminum oxide membranes consisting of nano-sized, cylindrical holes arranged in a parallel manner. The membranes are prepared by electrochemical oxidation of aluminum film in an acid under dc conditions. The pore parameters, such as pore diameter, pore length, can be controlled by adjusting the fabrication conditions. The critical parameters are the acid used and the applied dc voltage. Typical pore diameters range from 8-200nm, pore length is of the order of several microns. Pores are quasi-ordered and the packing density is about 1010-1011pores/cm2. Long-range ordering of the pores can be improved by means of ordered pretexturing of the aluminum foil prior to anodization, either by means of a multi-step anodization process or by means of a nanoindentation process.


Once the templates are synthesized, we use several nanofabrication methods to prepare nanostructures. The simplest method is electrodeposition of material inside the pores. This method is particularly useful in preparing metallic nanowires. For example, we have demonstrated Au and Ag nanowires in porous alumina templates. Such materials have important applications in metamaterials, for example in negative refraction.

We have also used the electrodeposition method to fabricate magnetic nanowires, such as Fe, Co, Ni and magnetic alloys such as FeCo. We are also using these membranes as a deposition mask or an etch mask to prepare nanodots, nanopillars and nanoporous structures. This method is not restrictive with respect to the kind of nanostructures that can be fabricated. For example, we have already demonstrated semiconductor nanopillar and nanoporous structures, magnetic-semiconductor heterostructures. Such materials have tremendous potential applications in nano, opto and spintronic devices. In contrast to existing nanofabrication methods, these nanofabrication methods are versatile, inexpensive and efficient and allow fabrication of self-assembled nanoarrays with feature size down to 8-10nm.

Research Highlights/Major Findings

(1) Experimentally, we showed that the dissolution process responsible for the formation of pores is due to the Al3+ ions migrating across the barrier layer and dissolving into the solution. Contrary to the suggestion of some authors, our experiments conclusively proved that the oxide dissolution process (chemical dissolution) is negligible (Wu et al. JES, 2007)
(2) Current models to describe pore growth assume an equal weighting factor for the competing oxidation process and the oxide dissolution process. In accordance with finding (1) and a systematic investigation by our group on pore growth as a function of fabrication parameters, we showed that pore growth models do not consistently fit experimental results. Hence, the models must be modified to by adjusting the weighting factor to account for this mechanism as shown in (1) (Friedman et al. JCP, 2007).
(3) We showed the formation of high-aspect ratio rod-like structures or ‘nano-noodles’ of alumina or titania. Such structures are formed either by rapid anodization of aluminum/titanium foil or by long-term post-anodization treatment of porous templates. Specifically, the nano-noodle structures form during anodization in highly acidic electrolytes and/or at high anodization voltages, or when porous templates are etched for long periods of time in an acid (Friedman et al. JNN, 2008).
(4) We showed that a vertically aligned Au nanowire array embedded in porous aluminum oxide can behave like a negative index material in specific wavelength regime, higher than the longitudinal resonance. The main requirement is that the aspect ratio of the nanowires must be very large ~103. Thus by adjusting the wire dimensions and interwire spacing (in effect controlling the fill factor, ratio of metal versus dielectric), one can in principle fabricate a negative index material that will operate in the visible region (Menon et al. accepted in APL). This project is funded by AFRL: “Metamaterials for conformal antenna technologies” (PI: Sridhar).
 
B.           Titania Nanotubes

Similar to nanoporous alumina templates, titania nanotemplates can be fabricated by anodization in fluorine containing electrolytes. Typically, the templates consist of an array of nanotubes though under certain electrochemical conditions, an array of nanopores can also be developed.
It has been hypothesized that the presence of fluorine ions in the electrolyte is essential for the formation of nanoporous or nanotubular TiO2 because of fluorine’s unique ability to ‘dissolve’ TiO2 and form the complex TiF62– in solution. Typical electrolytes used are solutions of hydrofluoric acid or acids such as sulfuric or phosphoric acid containing fluorine salts. When using acidic electrolytes the anodic oxidation of titanium reaches a ‘steady state’ after 20–40 min after which time the rate of titanium oxidation at the nanotube floor and the rate of dissolution of nanotube walls become equal. Consequently, tube growth in acidic solutions is limited and no tubes longer than ~500 nm could be grown (with diameters as small as 25 nm). One of the main applications of titania nanotubes is in the solar production of hydrogen. For the efficient solar production of hydrogen, it is estimated that significantly longer nanotubes would be needed. Use of more alkaline electrolytes can lead to longer titania nanotubes, when grown for very long anodization times of the order of 24 h. The longest tubes thus grown have lengths of ~7mm, diameters of ~100 nm, and have shown the highest solar water splitting efficiencies.

Research Highlights/Major Findings

For the first time, we demonstrated the fabrication of titania nanotubes electrochemically in an organic acid containing chlorine salts (without fluorine salts). Such nantoubes have very large aspect ratios: they have lengths of the order of 50mm and diameters of the order of 25nm. The process is extremely rapid, that is the tube length reaches 50 mm over a very short anodization time of around 10min (C. Richter et al., Adv. Mater. 2007; Panaitescu et al. JES 2007; Richter et al. JMR 2007).
              
 
C.                      Magnetic and Semiconductor Nanostructures

We are preparing semiconductor and magnetic semiconductor nanostructures using two different methods. In the first method, the nanostructures are etched through the use of porous alumina templates as masks. In the second method, nanowires of GaN and GaMnN are being fabricated by means of chemical vapor deposition method using Ni catalysts. The use of porous alumina templates allows for the controlled deposition (uniform diameter and spacing) of Ni catalyst which in turn produces a uniform GaN wire growth.

Research Highlights/Major findings

(1) Using nanoporous alumina templates as etch mask, we have demonstrated semiconductor (GaN, Si, GaAs, etc.) nanowire, nanopillar and nanoporous structures (Menon et al. JES, 2004). This work was carried out at Texas Tech under a funded NSF NIRT grant (ECCS 0304224) titled “Nano-Arrays of Large Bandgap Semiconductors for Light Emitting and Spintronic Devices” (PI: Temkin, TTU). Recently, in continuation with this work, we have also demonstrated magnetic semiconductor GaMnAs nanodots (Bennett et al. JAP 2008)
(2) Epitaxially grown GaN nanowire networks have been demonstrated on c-plane sapphire for a low density of Ni catalyst. The network exhibits a hexagonal symmetry which coincides with the m-axis of c-plane sapphire substrate. Raman spectroscopy and photoluminescence studies confirm that the crystal structure is of the zinc blende type in contrast with the typical wurtzite structure of the vertically grown GaN nanowires (Wu et al., accepted in JMC). Such an epitaxial network has tremendous application due to its compatibility with micro and nanoscale engineering processes. Also, the zinc blende structure has superior transport and optical properties with respect to the wurtzite structure. We will be exploring these applications in future projects (papers/proposals under preparation).
 
D.           Novel Energetic Nanocomposites
 
We have fabricated a unique nanocomposite material, which exhibits energetic behavior at high temperature. Our nanocomposite consists of two components, oxidizer and fuel, which react exothermically at high temperature to release a large amount of energy. Fabrication of such composites is based on the use of nanoporous alumina membranes. In the final product, an array of nanowires (either oxidizer or fuel) is partially embedded in a thin film (corresponding fuel or oxidizer). The wires are arranged perpendicular to the film like the bristles of a brush, allowing for maximum area density.
In our demonstration, Fe nanowires, are created in a regular array by means of electrodeposition inside nanoporous alumina membranes. This is followed by several processing steps, including wet and dry etching, annealing and thin film deposition techniques to create the final product, namely Al film (fuel) attached to an array of Fe2O3 (oxidizer) nanowires. While the nanocomposites, as-synthesized, are stable at room temperature, at high temperature (>410oC), they are found to ignite. We have demonstrated ignition in these nanocomposites using several different methods such as butane flame, resistive heating element, and laser ignition. We are currently investigating the ignition properties of these materials and learning about the reaction mechanism, burn velocity and other fundamental quantities as a function of composite dimensions.

Ideally, for maximum energy release, the composites must satisfy two conditions, nanoscaled (diffusion length) size of fuel and oxidizer, and intimate physical contact between fuel and oxidizer. Both these conditions are fully satisfied using our approach. In addition, in our materials, the nanocomposites are highly ordered unlike the random mixing of components commonly achieved in other fabrication approaches, such as sol-gel and ultrasonic mixing. This makes our nanocomposite, unique. Our nanocomposite is in effect a “nanoexplosive”. Such a “nanoexplosive” can be directly integrated into novel nanodevices for applications involving triggering of explosives (as in MEMS-based fuzes), applications involving large thermal amplification and in any other application requiring light-weight, single-use energy sources.

Research Highlight/Major finding:
This is the first demonstration of a nanowire-based energetic material (Menon et al. APL, 2004).
 
E.           Nanowires for Neuronal Recording Applications
 
Nanostructures are expected to play a key role in the investigation of neuronal activity. Neurons are complex structures with feature sizes ranging from the millimeter to micron to nanoscale regime. Single nanowires and/or nanowire arrays can therefore be effectively used to investigate live neural network functions in precise detail, tracking small changes in cell structure and electrical activity in a minimally invasive fashion and at the same time with excellent spatial and temporal resolution. Such studies will significantly improve our understanding of brain functions and will also have significant clinical applications. Electrical stimulation of neuronal cells is considered one of the possible treatments for rehabilitation of persons with spinal cord injuries, Parkinson’s disease and Alzheimer’s disease. A reliable electrical-neuronal interface is critical to this effort. Currently, microelectrode arrays are being used with some success, though their applications are limited due to small number of implanted electrodes leading to absence of single-cell resolution. Use of nanowires will greatly advance this application.

We are using Au nanowire arrays embedded in nanoporous alumina templates to investigate nano-neuro interactions through morphological and optical studies. We are also testing our nanowire arrays to understand how effectively they can stimulate neurons and finally, how effectively they can record electrical activity from them. Two proposals based on these ideas have been funded by NSF (ECCS 0738294) SGER: “Nanowire Array Technologies for Neural Recording Applications”, PI: Menon, (CBET 0608892) NER: “Nano-Biodevices for Reliable, Long-term Stimulation and Recording of Neural Activity”, PI: Menon. As a result of this funding, we have been able to put together a cell culture facility in my laboratory. We have obtained preliminary results on mouse neuroblastoma and rat hippocampal neurons on the surface of Au nanowire arrays. It may also be added that our research and critical literature survey in this area will soon result in a book titled “Nanotechnology in Neuroscience” (editor L. Menon) to be published by Pan Stanford Publishing, Singapore (to be completed in 2009). The book is expected to have contributions by several authors working in similar areas of research.

Research Highlight/Major finding:

Arrays of gold nanowires have been synthesized inside nanoporous aluminum oxide templates. The electrochemical fabrication approach allows for good control over wire dimensions and interwire spacing for optimal recording. The wires have been successfully integrated with gold electrodes at one end by means of electron beam lithography. Primary hippocampal neuronal cells have been successfully cultured at the other end having coated the nanowire array surface with an appropriate coating such as a collagen-laminin matrix. Calcium signals from stimulated neuronal cells have also been demonstrated pointing to healthy cultures (presented at MRS, NSTI conference, 2007-2008).