Nanopore junctions

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. 

A sharp peak at —2 V, demonstrating a dear NDR effect, was observed in the /(V) characteristics of a nanopore junction containing substituted OPE molecule 84d when measured at 60 K [44]. Later studies, involving nitro- and amino-substituted OPE-based nanopores, also demonstrated a controllable memory effect of the junction [71b]. This effect was based on the switching between a high and a low conductivity state of the OPE, which could be controlled by the bias voltage. 

An early nanopore study focused on an asymmetric Au-molecule-Ti junction based on thiol end-capped biphenyl 81b molecules [45], The asymmetry of the structure led to the observation of a prominent rectifying behavior with larger current when the Ti electrode was negatively biased. Recent work by Bao and coworkers [72] has shown that vapor deposition of Ti on SAMs results in penetration of the monolayer, thus destroying it. Similar observations were made using Au and A1 deposition. However, destruction of the monolayer could in this case be prevented if SAMs of dithiols were used, since the Au or A1 would react with the free thiol end. 

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To avoid penetration and filament formation via static and randomly scattered pinholes, one approach is to diminish the area of the junction until statistically the presence of a pinhole defect is near vanishing, as might be calculated by a Poisson distribution. An example of this strategy is the use of a nanopore junction of 50 nm diameter, though in this case the device fabrication yields were still reported to be quite low, down to a few percent [16]. 

FIG. 11 Schematic illustration of the electric potential profiles inside and outside a nanopore with lipid bilayer membranes separating the internal and external electrolyte solutions. The dotted line is a junction potential representation where the internal potential is shifted. 

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).]... 

A method to form metal-SAM-metal nanowires with a diameter < 40 nm was developed by Mallouk and coworkers [51, 76]. The nanowires were produced by electrodeposition of Au or Pd into the nanopores of a polycarbonate membrane. A SAM was formed at the end of the wire and a second metal contact (Au, Ag or Pd) was deposited on top of this. The polycarbonate was subsequently dissolved in dichloromethane, which released a large quantity (1011 cm-2) of nanowires that could be aligned individually between pairs of lithographically fabricated metal electrodes. A schematic illustration of the nanowire molecular junctions is shown in Fig. 10.14. 

Figure 6.14 Current-voltage characteristics of an extended p-n junction between CuSCN and nanoporous Xi02 (a) linear scale (b) logarithmic scale.

Konenkamp R., Ernst K., Rost C., Moller J., Fischer C.-H. and Lux-Steiner M. C. (2000c), Semiconductor growth and junction formation within nanoporous oxides , Physica Status Solidi (a) 182, 151-155. 

Gary Hodes received his BSc and PhD from Queen s University of Belfast in 1968 and 1971 respectively, and has been at the Weizmann Institute of Science, Rehovot, Israel since 1972. His research has focused on semiconductor film deposition from solutions (initially electrochemical and later chemical bath deposition) and on various types of solar cells (liquid junction, thin film, polycrystalline and nanoporous) and quantum dots using these films. Throughout his career, he has also studied various aspects of semiconductor surface treatments. More recently, he is continuing work on various aspects of chemical bath deposition mechanisms and also increasingly concentrating on nanocrystalline, semiconductor-sensitised solar cells. 

Scheme 3.2 (Top and middle) General schematic for the generation of nanoporous PS film. (Bottom) Atomic force microscopy (AFM) phase images of (a) PS375-tR.il]-PE0225 thin film, and (b) nanoporous PS film from oxidation-induced cleavage of the [Ru] junction.

The simplest photoelectrochemical cells consist of a semiconductor working electrode and a metal counter electrode, both of which are in contact with a redox electrolyte. In the dark, the potential difference between the two electrodes is zero. The open circuit potential difference between the two electrodes that arises from illumination of the semiconductor electrode is referred to as the photovoltage. When the semiconductor and counter electrode are short circuited, a light induced photocurrent can be measured in the external circuit. These phenomena originate from the effective separation of photogenerated electron-hole pairs in the semiconductor. In conventional photoelectrochemical studies, the interface between the flat surface of a bulk single crystalline semiconductor and the electrolyte is two dimensional, and the electrode is illuminated from the electrolyte side. However, in the last decade, research into the properties of nanoporous semiconductor electrodes interpenetrated by an electrolyte solution has expanded substantially. If a nanocrystalline electrode is prepared as a film on a transparent conducting substrate, it can be illuminated from either side. The obvious differences between a flat (two dimensional) semiconductor/ electrolyte junction and the (three dimensional) interface in a nanoporous electrode justify a separate treatment of the two cases. 

The abihty to measure and to control charge transport across nanometer-scale metal-molecule-metal junctions represents a key step toward the realization of molecular-based electronics [190-192]. Various experimental approaches have been employed to study molecular junctions in two- and three-terminal configurations. These include scanning probe microscopies (STM, STS, CP-AFM) [193-208], crossed-wire junctions [209], mechanical [210-215] and electromigration [216,217] break junctions, nanopores [218] and mercury drop electrodes [219]. Approaches in condensed media, and in par-... 

The selection made covers the first efficient and stable system based on the ternary chalcopyrite CulnSe2, an electrochemical treatment to avoid a toxic etching step in solid-state CIS device fabrication, the first stable and efficient liquid-junction solar cell (InP), and a novel concept where nanoemitters, interspersed in a nanoporous passivating film, are used to scavenge excess minority carriers. 

FIGURE 54.10 SEM image of a polymer matrix formed by infusing the monomers into a coUoidal crystal, polymerizing the monomers, and then etching away the crystals. The nanopores between the cavities are the result of incomplete wetting of the colloids at their junctions. (Reprinted from Jiang, P., et al., J. Am. Chetn. Soc., 121, 11630, 1999. With permission.)... 

One of the first examples of a nanoporous polymer made from an ordered block copolymer was reported in 1988 by Lee et al. [34]. In recent decades a number of strategies to remove the minor phase have been published [12]. This is mainly done by degradation of one of the blocks or cleavage of the block junction. Those strategies generally require the use of harsh conditions such as etching [35-37], ozonlysis [38, 39], hydrolysis [40], or depolymerization [41]. One appealing approach is the use of noncovalent H-bonds, which can be broken easily under mild conditions. 

Nonlinear Electrokinetic Phenomena, Fig. 4 Sketch of space charge formation and second-kind flow at the junction between a nanopore and a micropore in a microfluidic device or porous material (From Leinweber and Tallarek [15]). (a) In equilibrium, the micropore contains neutral electrolyte, while the nanopore has overlapping double layers containing mostly counterions. 

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