Numerical Simulation of Single-Electron Tunneling in Random Arrays of Small Tunnel Junctions Formed by Percolation of Conductive Nanoparticles

We numerically simulated electrical properties, i.e., the resistance and Coulomb blockade threshold, of randomly-placed conductive nanoparticles. In simulation, tunnel junctions were assumed to be formed between neighboring particle-particle and particle-electrode connections. On a plane of triangle 100×100 grids, three electrodes, the drain, source, and gate, were defined. After random placements of conductive particles, the connection between the drain and source electrodes were evaluated with keeping the gate electrode disconnected. The resistance was obtained by use of a SPICE-like simulator, whereas the Coulomb blockade threshold was determined from the current-voltage characteristics simulated using a Monte-Carlo simulator. Strong linear correlation between the resistance and threshold voltage was confirmed, which agreed with results for uniform one-dimensional arrays

One-dimensional array of small tunnel junctions fabricated using 30-nm-diameter gold nanoparticles placed in a 140-nm-wide resist groove

We present percolative arrays of gold nanoparticles (NPs) formed in a resist groove. To enhance the con nection probability, the width of the resist groove (140 nm) was designed to be approximately five times larger than the diameter of gold NPs (30 nm). Two-stage deposition of gold NPs was employed to form bridge connections between the source and drain electrodes. Dithiol molecules coated on surfaces of gold NPs worked as tunnel barriers. 5 of 12 samples exhibited Coulomb blockade characteristics, in one of which the gate response was confirmed

Adsorption density control of N719 on TiO2 electrodes for highly efficient dye-sensitized solar cells

N719 dye adsorption on anatase TiO 2 surfaces with different adsorption densities was investigated by IR-absorption spectroscopy using a multiple internal reflection technique. When the N719 dye was adsorbed on the TiO 2 surface by immersing it in an N719 solution consisting of acetonitrile and tertiary-butyl alcohol, chemisorbed and physisorbed N719 dyes coexisted on the surface. The ratio of chemisorbed and physisorbed dye densities was dependent on the N719 exposure, and excessive exposure led to the generation of the physisorbed dye. Test fabrications of the dye-sensitized solar cells ͑DSCs͒ with various N719 exposures suggested that we need to optimize the dye adsorption density to achieve a higher power conversion efficiency in DSCs. Ever since O'Regan and Grätzel reported highly efficient, low cost dye-sensitized solar cells ͑DSCs͒ using cisdi͑thiocyanato͒bis͑2,2Ј-bipyridyl-4,4Љ-dicarboxylate͒ruthenium͑II͒ ͑N719͒ with a photovoltaic efficiency of 11%, 1 many researchers have tried to reproduce the solar cell. Unfortunately, the attempts have all ended in failure as O'Regan and Grätzel did not release detailed information on the preparation of the TiO 2 electrode, which is the most important aspect of their innovation. Many papers have been published regarding TiO 2 fabrication to achieve larger surface areas for dye adsorption, in which nanoparticle TiO 2 films, 1-3 nanoparticle TiO 2 films mixed with relatively larger TiO 2 particles, 4 particle TiO 2 with a high pore density, 8 They proposed several adsorption models in which the N719 molecule anchors on the TiO 2 surface through one or two of its carboxyl groups. Leon et al. speculated on N719 anchors with a carboxylic moiety with a coordination of bridging or bidentate linkages. 9 They found that the vibration frequency of COO − changes, depending on whether the N719 is connected on the TiO 2 surface or physisorbed. These studies were helpful in understanding the adsorption mechanism of N719, although the discussion on the impact of physisorbed dyes was insufficient. In this paper, we observed the adsorption states of the N719-sensitizing dye on anatase TiO 2 surfaces with a variety of adsorption densities using IRAS with a multiple-internal-reflection ͑MIR͒ geometry. Chemisorbed and physisorbed dyes coexisted on the dyeadsorbed surface. The ratio of the chemisorbed and the physisorbed dye densities was dependent on the exposure of the TiO 2 surfaces to the N719 dye, and overexposure generated a physisorbed state. Test fabrications of DSCs with various N719 exposures suggested that the degeneration of the physisorbed N719 is effective in enhancing power generation. We discuss how to promote the power conversion efficiency of DSCs without interference of the physisorbed N719 dye in the photovoltaic operation. Experimental We investigated N719 adsorption by a TiO 2 -coated GaAs prism using MIR-IRAS. The anatase TiO 2 was fabricated by metallorganic chemical vapor deposition ͑CVD͒ described elsewhere. 10,11 The deposited TiO 2 had preferential orientations of ͑101͒, ͑200͒, and ͑211͒, which were confirmed by X-ray diffraction. The average thickness of the TiO 2 film was 0.1 m, with an average roughness of less than 10 nm. The photocatalytic reaction of the deposited TiO 2 film was confirmed using MIR-IRAS, and the results were released elsewhere. The N719 adsorption for the IRAS measurement was performed by dropping the N719 solution onto the TiO 2 surface. The N719 dye solution was a mixture of 10 mL acetonitrile and 10 mL tertiarybutyl alcohol containing N719 at a concentration of 0.36 mg/mL. When we dropped the dye onto the surface, we controlled the wet area so it was constant by the casting method. The wet surface was then air-dried at room temperature. The adsorption density of N719 was calculated by assuming that all N719 included in the dye solution remained uniformly on the surface. In this study, we fabricated DSCs based on investigations made with MIR-IRAS. In the fabrication, TiO 2 films with a thickness of 56 m with an area of 0.5 ϫ 0.5 cm were formed on SnO 2 -coated glass by drawing and extending TiO 2 paste using a glass rod and a plastic tape spacer. The TiO 2 paste was made from TiO 2 powder manufactured by Ishihara Sangyo ͑ST-01͒, deionized water, acetic acid solution with pH of 3.5, and poly͑ethylene glycol͒. The TiO 2 films were baked in air at a temperature of 500°C for 1 h. The N719 adsorption was performed by dropping N719 solution onto the TiO 2 surface. The redox electrolyte was 0.1 M LiI, 0.05 M I 2 , 0.6 M dimethyl propyl imidazolium iodide, and 0.5 M tert-butylpyridine in dried acetonitrile. The photovoltaic properties of the DSCs were measured using a current-voltage measurement system, and a 5000 K, 300 W xenon lamp was used as a light source. The light * Electrochemical Society Active Member.

Capacitance extraction method for a free-standing bilayer lipid membrane formed over an aperture in a nanofabricated silicon chip

A bilayer lipid membrane (BLM) is the main component of the cell membrane of living organisms, which can be formed artificially. Although the specific capacitance of a BLM is known to be in the range of 0.4–1.0 μF cm^–2, many previous works that formed free-standing BLMs over an aperture in silicon chips reported larger values beyond this typical range, which suggests that equivalent-circuit models are not adequate. In this work, we modified the equivalent-circuit model by adding a resistance element of silicon. To evaluate the validity of the modified model, we applied the model to the results of electrochemical impedance spectroscopy for free-standing BLMs formed over an aperture in nanofabricated silicon chips. The derived specific capacitance values were 0.57 ± 0.08 μF cm^–2, which settles in the typical range

Equivalent circuit model modified for free-standing bilayer lipid membranes beyond 1 TΩ

A cell is the basic functional unit of living organisms. Bilayer lipid membranes (BLMs), which form cell membranes can be assembled by using artificial methods. The electrochemical characteristics of BLMs are normally investigated using electrochemical impedance spectroscopy (EIS); however, the equivalent circuit need to be modified by the experimental conditions. In this study, we formed plain BLMs to determine the underlying equivalent circuit model of free-standing BLMs, and we measured the electrical characteristics using EIS. To analyze the results of EIS, we proposed equivalent circuit models including electrical double layer (EDL) effects on both sides of a BLM. We also extracted and evaluated the electrochemical parameters; the aperture-suspended BLMs using an Si chip having tapered edge recorded TΩ-order membrane resistances, which were one order higher than those reported in most previous studies. Regarding the capacitances of EDL, we compared the extracted values and the calculated results

Dielectrophoretic Assembly of Gold Nanoparticle Arrays Evaluated in Terms of Room-Temperature Resistance

Gold nanoparticles (GNPs) are often used as island electrodes of single-electron (SE) devices. One of technical challenges in fabrication of SE devices with GNPs is the placement of GNPs in a nanogap between two lead electrodes. Utilization of dielectrophoresis (DEP) phenomena is one of possible solutions for this challenge, whereas the fabrication process with DEP includes stochastic aspects. In this brief paper, we present our experimental results on electric resistance of GNP arrays assembled by DEP. More than 300 pairs of electrodes were investigated under various DEP conditions by trial and error approach. We evaluated the relationship between the DEP conditions and the electric resistance of assembled GNP arrays, which would indicate possible DEP conditions for fabrication of SE devices

Evaluation of the inter-particle distance of gold nanoparticles dispersed on silane-treated substrates to fabricate dithiol-connected arrays

Small tunnel junctions using gold nanoparticles (GNPs) as electrodes have been studied to fabricate single-electron devices. GNPs connected via dithiol molecules have been used as small tunnel junctions, and a two-stage dispersion method was used to fabricate dithiol-connected GNP arrays. In this process, the GNPs were fixed on silane-treated substrates by immersing the substrate in a colloidal gold solution. For fabricating dithiol-connected arrays, the inter-particle distance of the dispersed GNPs must be smaller than the GNP diameter. Consequently, the inter-particle distance controlled by the immersion time (T IM1) was evaluated. For T IM1 values exceeding 8 h, the inter-particle distance was less than the GNP diameter. A second dispersion of GNPs after treating samples with dithiol realized particle connections. For the GNP arrays produced with T IM1 values greater than 8 h, the I–V characteristics were measured at 77 K, and the yield of devices exhibiting nonlinear I–V curves was 23%