Track-etched templates

There has been extensive recent use of track-etched membranes as templates. As will be discussed in detail below, these membranes are ideal for producing parallel arrays of metal nanowires or nanotubules. This is usually done via electroless metal deposition [25], but many metals have also been deposited electrochemically [26]. For example, several groups have used track-etched templates for deposition of nanowires and segmented nanowires, which they then examined for giant magnetoresistance [27-29]. Other materials templated in the pores of track etch membranes include conducting polymers [30] and polymer-metal composites [31]. 

Symmetric membranes and asymmetric membranes are two basic types of membrane based on their structure. Symmetric membranes include non-porous (dense) symmetric membranes and porous symmetric membranes, while asymmetric membranes include integrally skinned asymmetric membranes, coated asymmetric membranes, and composite membranes. A number of different methods are used to prepare these membranes. The most important techniques are sintering, stretching, track-etching, template leaching, phase inversion, and coating (13,33). 

Dauginet-De Pra, L., Ferain, E., Legras, R., and S. Demoustier-Champagne. 2002. Fabrication of a new generation of track-etched templates and their use for the synthesis of metalhc and organic nanostructures. Nuclear Instruments and Methods in Physics Research B 196 81-88. 

De Leo M, Pereira FC, Moretto LM, Scopece P, Polizzi S, Ugo P (2007) Towards a better understanding of gold electroless deposition in track-etched templates. Chem Mater 19 5955-5964... 

Investigations have been conducted with polycarbonate track-etched templates, aluminum oxide membranes and nonporous mica [344-352], and in all cases the nanomaterial preparation consists primarily of a controlled electrodeposition of copper from a precursor solution (usually CUSO4) inside the nanochannels of the template. In a second step, the template is removed, thus releasing the elongated nanostructures. By following this approach, Gao and coworkers [353] prepared dense and continuous copper nanowires which were 30 xm long and had a uniform... 

Within the scope of thermoelectric nanostructures, Sima et al. [161] prepared nanorod (fibril) and microtube (tubule) arrays of PbSei. , Tej by potentiostatic electrodeposition from nitric acid solutions of Pb(N03)2, H2Se03, and Te02, using a 30 fim thick polycarbonate track-etch membrane, with pores 100-2,000 nm in diameter, as template (Cu supported). After electrodeposition the polymer membrane was dissolved in CH2CI2. Solid rods were obtained in membranes with small pores, and hollow tubes in those with large pores. The formation of microtubes rather than nanorods in the larger pores was attributed to the higher deposition current. 

Sima M, Enculescu I, Visan T, Spohr R, Trautmann C (2004) Electrochemical deposition of PbSei j Tej nanorod arrays using ion track etched membranes as template. Mol Cryst Liq Cryst 418 749-755... 

Possin, in 1970, was the first to use the pores in track-etched mica membranes as templates to make nanomaterials [23]. This was accom-... 

An alternate method to produce templated electrodes is the use of chemical reduction of the monomer in the presence of a track-etched or alumina membrane. Parthasarathy et al. [46] have produced enzyme-loaded nanotubules by a combination of both electrochemical and chemical deposition. Initially, the alumina membrane was sealed at one end with a thick Au film (Figure 1.9a), after which the membrane was placed into a mixture of pyrrole and Et4NBF4. The pyrrole was then electropolymerized to form a small plug of polypyrrole at the closed end of the alumina membrane (Figure 1.9b). Subsequently, the membrane was placed into a... 

The most commonly used hard templates are anodic aluminum oxide (AAO) and track-etched polycarbonate membranes, both of which are porous structured and commercially available. The pore size and thickness of the membranes can be well controlled, which then determine the dimension of the products templated by them. The pores in the AAO films prepared electrochemically from aluminum metals form a regular hexagonal array, with diameters of 200 nm commercially available. Smaller pore diameters down to 5 nm have also been reported (Martin 1995). Without external influences, capillary force is the main driving force for the Ti-precursor species to enter the pores of the templates. When the pore size is very small, electrochemical techniques have been employed to enhance the mass transfer into the nanopores (Limmer et al. 2002). 

The requirement for use of these membranes as templates is to have high track etch ratios. For PC, track etch ratios above ten thousand have been observed. At the other extreme, addition of solvents, such as methanol, ethanol or propanol can dramatically decrease the track etch ratio, down to 2 to 4 in the case of PET. Although they accelerate the etching process, polymers like PMMA are generally etched with an acidic medium leading to very low etch track ratios 1-10. In a recent study on the polymer polyallyl diglycol carbonate (PADC) [9] it was found that etch-rate values of the PADC increase nearly fourfold if the polymer is irradiated with 100 Mrad dose of electrons at 2 MeV prior to the heavy ion irradiation. The etch tracks were created by 140 MeV Si ions. 

Figure 6.5 fJ-type CuSCN nanowires grown in polymer foil templates prepared by ion track etching. 

Martin and coworkers176 177 178 have used controlled pore-size membranes as templates to electrochemically grow fibrillar mats of CEPs. Similar structures have also been produced using nanoporous particle track-etched polycarbonate membranes with both ppyl79,180,181,182 and PAn183 via chemical and electrochemical techniques. The approach involves the oxidation of the monomer within the pores of a template. This is achieved electrochemically as illustrated in Figure 2.17. The electrode sub-... 

Hollow CEP cigar-shaped nanotubes with sealed ends, synthesized via the track-etched polycarbonate template route, have also been reported by Mativetsky and Datars.222 These materials exhibited a small drop in conductivity as the diameter decreased from 400 to 50 nm, contrary to previous reports.223 224The small decrease... 

Martin and coworkers have demonstrated the use of cylindrical-pored templates for the preparation of tubes and fibers composed of metal oxides, metals and polymers [35,36]. Track-etching of polycarbonate films gives membranes with cylindrical pores that are randomly distributed across the membrane. The pore diameters are monodisperse and, in the example described here, are 600 nm. Lak-shmi et al. have used these membranes for the preparation of vanadium oxide fibers [36]. The pores of the organic filter were filled with vanadium(V) triiso-propoxy oxide in an argon atmosphere. Exposure to air at 60°C induces hydrolysis of the precursor before an oxygen plasma is used to remove the polycarbonate. The crystalline alpha phase vanadium oxide (V2O5) fibers obtained by this... 

One of the challenges in the synthesis of nanorods is to produce nanorods of uniform size and distribution in an easy and reproducible manner. If a template is provided for the synthesis of nanorods, the dimension/geometry of the template dictates the dimension of the nanorods. Templates have been commonly used to produce isolated oriented nanorods. The most commonly used templates are anodic aluminum oxide (AAO) or polycarbonate membrane (ion-track etched). Other templates include three-dimensional microporous materials, such as zeohtes, mica, glass, block-copolymer, and even carbon nanotubes. A template should contain uniform sized pores, be... 

Another way to template thin films of nano-sized cylinders perpendicular to the surface is to start with a preformed membrane of track-etched polycarbonate or nanoporous alumina. A fiuid dispersion of a filler material can be drawn into the pores. Anodized aluminum oxide was the template for construction of lithium ion nanobatteries having many parallel cells filled with the solid state electrolyte PEO-LiOTf (poly(ethylene oxide)-lithium trifluoromethanesulfonate) and the electrodes coated on the top and bottom surfaces of the film (41). 

Ion track-etched membranes are an ideal template for preparation of metallic and organic nanostructures of desired shapes, which have a great potential for industrial applications (Dauginet-De Pra et al. 2002). The hollow track space of membranes can be easily filled with atoms of practically any kind by means of the electrochemical deposition from the electrolyte solution, the chemical polymerization from solution of the desired monomer and initiator, or the capillary action of the pores that enables the solution of the dissolved material entering into the pores followed by slow evaporation of the solvent (Chakarvarti 2009). 

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