Polycarbonate Etched Track Templates

During irradiation by swift ions, latent ion tracks are formed along the path of the ions. After irradiation, the material is subjected to chemical treatment leading to formation of the hollow channel from the latent ion track. The size and shape of the etched ion track is determined by the chemical processing. The etch process depends on the energy deposition density of the ion along its path, on the radiation 

Selective etching of ion tracks leading to array of pores of size 10-15 nm. 

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. 

Generally the highest density of such pores in these polymers is 10 cm . Commercially available membranes are etched polycarbonate or polyester of thickness typically in the range 10 to 30 pm and with pore sizes down to 20 nm. 

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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... 

Figure 16.2.8 Scheme of an Au-NEE prepared using a track-etched polycarbonate membrane as template (A). Particular of the section of the active area (B) top view (C) section of the aU NEE ready for use as working electrode, (a) Track-etched golden membrane (b) copper adhesive tape with conductive glue to connect to instrumentation (c) aluminum adhesive foil with non-conductive glue (d) insulating tape. Note Some dimensions are only indicative and not in scale. 

Endres, F. Liu, Z. Shapouri, M. El Abedin, S. Z. Electrochemical synthesis of vertically aligned zinc nanowires using track etched polycarbonate membranes as templates. Phys. Chem. Chem. Phys. 2013, 15, 11362-11367. 

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. 

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). 

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... 

Although the template-based synthetic methods offer many advantages, there are some disadvantages. In all cases, the templates have to be removed and so these methods may not be suitable for making large quantities of nanorods. Also, a track-ion etched polycarbonate membrane may possess intersecting pores that will affect the homogeneity of the rods produced. 

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). 

Polypyrrole nanotubes synthesized at a constant potential using the pores of nanoporous polycarbonate (PC) particle track-etched membranes as a template [63,64] and polypyrrole nanowires synthesized at a constant potential of 0.9 V (vs. SCE) using nanochannels of proton-modified natural zeolite clinoptilolite [65] have been reported. Their electrochemical properties were improved compared to the conventional polypyrrole. 

Semiconducting one-dimensional (ID) nanolibers or nanowires are of interest for a wide variety of applications including interconnects, functional devices, and molecular sensors as well as for fundamental physics studies. Devices have been fabricated fi om semiconductor, and carbon nanotubes, and more recently from ICP nanofibers. It has been predicted that ICP nanofibers will have unique electrical, optical, and magnetic properties [134]. Several different methods for producing these ICP nanofibers have been developed with or without the aid of a template. The template-based methods involve synthesizing a tubular structure of the ICP within the pores of a support membrane, such as an alumina membrane [135] or a track-etched polycarbonate membrane [136]. However, more recent work has... 

In general, template method is classified by soft and hard templates. Whereas anodic aluminum oxide (AAO) membrane, track-etched polycarbonate (PC) and zeolite can be used as hard templates, soft templates include surfactant, cyclodextrin, liquid crystal, etc. Compared with soft and hard templates, template-free method represents the fabrication technique of conducting polymer nanomaterials without the template, which is discussed in this section [115]. 

The formation of nanostructured arrays of conjugated polymers by the utilization of nanoporous templates has been reported. The deposition of the polymer inside the pores can be achieved by filling the pores with a solution of polymer and evaporation of the solvent or by the direct synthesis of conjugated polymer inside the pores by chemical or electrochemical approaches. Porous templates were based on track-etched polycarbonate membranes [106-108] or alumina that is obtained by anodic aluminum oxidation (AAO) [109-lllj. Thus, periodic vertical channels with diameters between 20 and 120 nm are formed by first electrochemical oxidation and etching and then subsequent etching for pore widening (Figure 13.16). 

One of the earliest applications of the template method was to prepare ensanble miCTo-scopic (7, 18) and nanoscopic electrodes (116, 141). Such electrodes were prepared by electrochemically depositing noble metals within the pores of the commercially available polymeric filtration membranes. The fabrication of a microelectrode ensemble based on the electrochemical deposition of platinum into the pores of a track-etched microporous polycarbonate host membrane was first shown in 1987 by Charles Martin (7). The word ensemble was used to describe the final device because the elements in the device are not evenly spaced. The procedure is simple, and requires only routine and inexpensive electrochemical instrumentation. It was ultimately found that electroless plating allowed for more uniform metal deposition (116). Both plating methods are important for the fabrication of the array, and further considerations continue in the following. 

Nanometer and micrometer sized tubes have been prepared from polymeric materials using suitably shaped templates to ensure the final shape/ These templates can typically be divided into several categories. Hard templates consist of thin porous membranes, often alumina ° or track-etched polycarbonate where the nanometer-sized pores are partially backfilled (wetted) with polymeric material. Removal of the template then yields the polymer tubes. The outer tube diameter is defined by the template pore sizes while the inner (open) diameter of the tubes can often be controlled by the amount of polymer deposited into the template pores. The length of the tubes typically corresponds to the thickness of the porous membrane. 

Most of the work that has been carried out using porous membranes as hard templates was done on porous alumina (anodic aluminium oxide, AAO) or track-etched polycarbonate membranes. The general principle of nanotube formation relies on a continuous polymer film being formed on the inside of the pores that remains as a tubular structure once the template is removed. There are several possibilities as to how this polymer film can be formed inside the pore. A film can be obtained by wetting the pore with a polymer melt or a polymer solution. Alternatively, the polymerization of monomers... 

In the past few years there has been a real surge of new techniques for the preparation of porous materials that are characterized by well-defined cylindrical pores of sizes from a few micrometers, down to the nanometer range. Most notably, porous anodic alumina (PAA) [17] and porous silicon (p-Si) [18,19] that are prepared by electrochemical anodization, and track-etched polymer membranes (polycarbonate, polyimide, polyethylene terephtalate, etc.), represent the most well-known cases of porous membranes that are candidates for filtration applications and also for their use as templates in nanotechnology (nanowire fabrication [20]). The pore diameter range of these membranes is comparable to the typical thickness of polymer brushes that are usually prepared in the laboratory. 

In the formation of lead zirconate titanate (PZT) nanorods, lead acetate, titanium isopropoxide, zirconium w-propoxide, glacial acetic acid, lactic acid, ethylene glycol, and glycerol were used for the PZT sols. The template membranes used for the growth of nanorods were track-etched hydrophilic polycarbonate (PC) with pore diameters of 100 and 200 mn and a thickness of 10 /xm. The PC membrane attached to Al cathode is placed... 

The first procedure reported for the production of metal nanorods is called template synthesis.This method entails the preparation or deposition of the desired material within the cylindrical and monodisperse pores of a nanopore membrane. Martin and coworkers used polycarbonate filters, prepared by the track-etch method, and nanopore aluminas prepared electrochemically from Al foil, as template materials. This method allows the preparation of cylindrical nanostructures with monodisperse diameters and lengths, and depending on the nature of the membrane and the synthetic method used, these may be solid nanowires or hollow nanotubes. 

In developing these template synthetic methods, we made an interesting discovery. When these polymers are synthesized (either chemically or electrochemically) within the pores of the track-etched polycarbonate membranes, the polymer preferentially nucleates and grows on the pore walls [11,14,46]. As a result, polymeric tubules are obtained at short polymerization times (Fig. 16.2A). These tubular structures have been quite useful in our fundamental investigations of electronic conductivity in the template-synthesized materials (see below). In addition, tubular structures of this type have a number... 

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