Nanopores etching

FIGURE 3.6. (a) Cross-sectional schematics of a silicon wafer with a nanopore etched through a suspended silicon nitride membrance. SAM is formed between sandwiched Au eletrodes in the pore area (circled), (b) I(V) characteristics of a Au-2 -amino-4-ethynylphenyl-4-ethynylphenyl-5 -nitro-1 -benzenethiolate-Au (chemical structure shown below) molecular junction device at 60 K. The peak current density is 50 A/cm2, the NDR is 2400 pQ. cm2, the peak-to-valley ratio is 1030 1. [Adapted from Ref.30 Chen el al., Science 286, 1550-1552 (1999).]... 

Keywords Block copolymers Etching Nanolithography Nanoporous Self-assembly... 

Fig. 4 Nanoporous membrane from an etched gyroid structure. Reproduced from [21]... 

Similar approach has also been taken by Ferain and Legras [133,137,138] and De Pra et al. [139] to produce nanostructured materials based on the template of the membrane with etched pores. Polycarbonate film was also of use as the base membrane of the template, and micro- and nanopores were formed by precise control of the etching procedure. Their most resent report showed the successful formation of ultrasmall pores and electrodeposited materials of which sizes were as much as 20 nm [139]. Another attractive point of these studies is the deposited materials in the etched pores. Electrochemical polymerization of conjugated polymer materials was demonstrated in these studies, and the nanowires based on polypyrrole or polyaniline were formed with a fairly cylindrical shape reflecting the side wall structure of the etched pores. Figure 10 indicates the shape of the polypyrrole microwires with their dimension changes by the limitation of the thickness of the template. 

While the cell efficiencies of these films were not specifically investigated, best parameters of 2 mAcm (ca. AMI illumination quantum efficiencies increased with decreasing illumination intensity due to diffusion limitations in the nanoporous film) 0.5 V and ca. 50% fill factor were obtained. However, great variation in these parameters were obtained one reason for this can be seen from a consideration of Figure 9.9. If a CdSe film is etched, but less than optimally (shorter time, more dilute HCl), it is clear that after a certain, unique etch treatment, zero net photocurrent will be obtained. The actual photocurrent (and other ontpnt parameters) of the film is a balance between photoanodic and photoca-thodic cnrrents. 

Nanoporous materials are of great interest for example for their photoluminescent properties (silicon) or possible use as templates (alumina). The types of synthesis of these materials are numerous and the etching/ anodization influences the results [27 - 29]. A detailed studied of Cherenkov radiation in nanoporous alumina membranes has been done which explains the existence of low energy peaks around 7-8 eV [30] (Figure 9 left). Whereas for a cylindrical hole in alumina the simulation does not show a peak, a cylindrical shell of alumina does. The Cherenkov radiation is confined in the shell, as in a wave guide, and a peak appears. If an effective... 

Nanopore junctions, shown in Fig. 10.10, were investigated by Reed and coworkers [43-45, 71]. A small hole, 30-50 nm in diameter, was etched in a silicon nitride membrane by e-beam lithography. One side of the hole was filled with evaporated gold and a SAM (consisting of —1000 molecules) was formed inside the hole when placed in a solution. A second Au electrode was then deposited on the other side of the hole by evaporation. 

More effective nanoporous carbons have been obtained by the template technique. A nitrogenated precursor is introduced in a nanoporous scaffold and subsequently pyrolyzed then the nitrogen-enriched replica is obtained by etching the host with hydrofluoric acid. The first materials of this... 

Of special interest is impedance of nanoporous honeycomb diamond electrodes [81] whose fabrication was briefly described in Section 2. By oxygen-plasma etching, an ordered structure of vertical pores (60 to 80 mn in diameter, 500 mn deep) was... 

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). 

One possible approach is based on the aspiration of the dilute polymer inside a nanopore (Fig. 1). The nanopores are produced easily by particle-track etching [8], and can be bought commercially. Three requirements must be imposed ... 

One of the potential applications of these ABC triblock copolymers was explored by Hillmyer and coworkers in 2005 [118]. They have prepared nanoporous membranes of polystyrene with controlled pore wall functionality from the selective degradation of ordered ABC triblock copolymers. By using a combination of controlled ring-opening and free-radical polymerizations, a triblock copolymer polylactide-/j-poly(A,/V-dimethylacrylamide)-ib-polystyrene (PLA-h-PDMA-h-PS) has been prepared. Following the self-assembly in bulk, cylinders of PLA are dispersed into a matrix of PS and the central PDMA block localized at the PS-PLA interface. After a selective etching of the PLA cylinders, a nanoporous PS monolith is formed with pore walls coated with hydrophilic PDMA. 

Xu et al. [40] have very recently prepared alumina with U-shaped pores of a uniform diameter (40-50 nm) whose bottoms were opened in a controlled way by Ar -ion milling. Fig. 4 shows an AFM image of the resulting pores after short-term ion milling, which produced pores 8-14 nm wide. By contrast, long-term ion milling led to a hole diameter equal to nanopore diameter, indicating that the U-shaped bottom cap was etched away completely. 

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