Conductivity of 2-D Ag Quantum Dot Arrays: Computational Study of the Role of Size and Packing Disorder at Low Temperatures

The crossover from two dimensions to one dimension in granular electronic materials

Granular conductors are solids comprising densely packed nanoparticles, and have electrical properties that are determined by the size, composition and packing of the composite nanoparticles. The ability to control these properties in two- and three-dimensional granular conductors has made such systems appropriate for use as prototypes for investigating new physics. However, the fabrication of strictly one-dimensional granular conductors remains challenging. Here, we describe a method for the assembly of nanoparticles into granular solids that can be tuned continuously from two to one dimension, and establish how electron transport evolves between these limits. We find that the energy barriers to transport increase in the one-dimensional limit, in both the variable-range-hopping (low-voltage) and sequential-tunnelling (high-voltage) regimes. Furthermore, in the sequential-tunnelling regime we find an unexpected relationship between the temperature and the voltage at which the conductance becomes appreciable - a relationship that appears peculiar to one-dimensional systems. These results are explained by extrapolating existing granular conductor theories to one dimension.

Charge transport behavior of N-(2-Mercaptopropionyl glycine)-protected gold clusters with temperature

N-(2-Mercaptopropionyl glycine)-passivated gold clusters (Au-MPG) represent a unique networked system of monolayer-protected clusters (MPCs) due to inter- and/or intramolecular hydrogen bonding between the MPC units thanks to the terminal carboxylic acid groups of the monolayers. We have investigated the size-dependent electric charge transport in well-dried thin films of Au-MPG MPCs by a four-probe methodology as a function of temperature. The MPCs display a novel behavior of metallic-like-to-semiconductor crossover with increase of temperature. The MPCs having average cluster core sizes of approximately 1.6, approximately 3, and approximately 4 nm displayed a metallic-like nature (a linear increase of resistance of the films with increase of temperature) at low temperatures ( approximately 20-60 K), which crosses over to semiconductor behavior (decrease of resistance with increase of temperature) at high temperatures.

Inkjet-printed gold nanoparticle chemiresistors: influence of film morphology and ionic strength on the detection of organics dissolved in aqueous solution

The influence of film morphology on the performance of inkjet-printed gold nanoparticle chemiresistors has been investigated. Nanoparticles deposited from a single-solvent system resulted in a "coffee ring"-like structure with most of the materials deposited at the edge. It was shown that the uniformity of the film could be improved if the nanoparticles were deposited from a mixture of solvents comprising N-methyl-2-pyrrolidone and water. Electrical conductivity measurements showed that both "coffee ring" and "flat" films were qualitatively similar suggesting that the films have similar nanoscale structures. To form the functional chemiresistor device, the 4-(dimethylamino)pyridine coating on the nanoparticle was exchanged with 1-hexanethiol to provide a hydrophobic sensing layer. The performance of 1-hexanethiol coated gold nanoparticle chemiresistors to small organic molecules, toluene, dichloromethane and ethanol dissolved in 1 M KCl in regard to changes in impedance and response times was unaffected by the film morphology. For larger hydrocarbons such as octane, the rate of uptake of the analyte into the film was significantly faster when the flatter nanoparticle film was used as opposed to the "coffee ring" film which has a thicker edge. Furthermore, the presence of potassium and chloride ions in the solution media does not significantly affect the impedance of the nanoparticle film at 1 Hz (<2% variation in film impedance over more than four orders of magnitude change in ionic strength). However, the ionic strength of the media affected the partitioning of the analyte into the hydrophobic nanoparticle film. The response of the sensor was found to increase with an increased salt concentration due to a salting-out of the analyte from the solution.

Synthesis of dendron-protected porphyrin wires and preparation of a one-dimensional assembly of gold nanoparticles chemically linked to the pi-conjugated wires

A one-dimensional assembly of gold nanoparticles chemically bonded to pi-conjugated porphyrin polymers was prepared on a chemically modified glass surface and on an undoped naturally oxidized silicon surface by the following methods: pi-conjugated porphyrin polymers were prepared by oxidative coupling of 5,15-diethynyl-10,20-bis-((4-dendron)phenyl) porphyrin (6), and its homologues (larger than 40-mer) were collected by analytical gel permeation chromatography (GPC). The porphyrin polymers (>40-mer) were deposited using the Langmuir-Blodgett (LB) method on substrate surfaces, which were then soaked in a solution of gold nanoparticles (2.7 +/- 0.8 nm) protected with t-dodecanethiol and 4-pyridineethanethiol. The topographical images of the surface observed by tapping mode atomic force microscopy (AFM) showed that the polymers could be dispersed on both substrates, with a height of 2.8 +/- 0.5 nm on the modified glass and 3.1 +/- 0.5 nm on silicon. The height clearly increased after soaking in the gold nanoparticle solution, to 5.3 +/- 0.5 nm on glass and 5.4 +/- 0.7 nm on silicon. The differences in height (2.5 nm on glass and 2.3 nm on silicon) corresponded to the diameter of the gold nanoparticles bonded to the porphyrin polymers. The distance between gold nanoparticles observed in scanning electron microscopic images was ca. 5 nm, indicating that they were bonded at every four or five porphyrin units.

Electron Transport in Two-Dimensional Arrays of Gold Nanocrystals Investigated by Scanning Electrochemical Microscopy

This article reports the use of the scanning electrochemical microscope (SECM) to investigate the electronic properties of Langmuir monolayers of alkane thiol protected gold nanocrystals (NCs). A substantial increase in monolayer conductivity upon mechanical compression of the Au NC monolayer is reported for the first time. This may be the room temperature signature of the insulator to metal transition previously reported for comparable silver NC monolayers. Factors influencing the conductivity of the monolayer NC array are discussed.

Cu nanocrystal growth on peptide nanotubes by biomineralization: size control of Cu nanocrystals by tuning peptide conformation

With recent interest in seeking new biologically inspired device-fabrication methods in nanotechnology, a new biological approach was examined to fabricate Cu nanotubes by using sequenced histidine-rich peptide nanotubes as templates. The sequenced histidine-rich peptide molecules were assembled as nanotubes, and the biological recognition of the specific sequence toward Cu lead to efficient Cu coating on the nanotubes. Cu nanocrystals were uniformly coated on the histidine-incorporated nanotubes with high packing density. In addition, the diameter of Cu nanocrystal was controlled between 10 and 30 nm on the nanotube by controlling the conformation of histidine-rich peptide by means of pH changes. Those nanotubes showed significant change in electronic structure by varying the nanocrystal diameter; therefore, this system may be developed to a conductivity-tunable building block for microelectronics and biological sensors. This simple biomineralization method can be applied to fabricate various metallic and semiconductor nanotubes with peptides whose sequences are known to mineralize specific ions.

Au nanocrystal growth on nanotubes controlled by conformations and charges of sequenced peptide templates

A new biological approach to fabricate Au nanowires was examined by using sequenced peptide nanotubes as templates. The sequenced histidine-rich peptide molecules were assembled on nanotubes, and the biological recognition of the sequenced peptide selectively trapped Au ions for the nucleation of Au nanocrystals. After Au ions were reduced, highly monodisperse Au nanocrystals were grown on nanotubes. The conformations and the charge distributions of the histidine-rich peptide, determined by pH and Au ion concentration in the growth solution, control the size and the packing density of Au nanocrystals. The diameter of Au nanocrystal was limited by the spacing between the neighboring histidine-rich peptides on nanotubes. A series of TEM images of Au nanocrystals on nanotubes in the shorter Au ion incubation time periods reveal that Au nanocrystals grow inside the nanotubes first and then cover the outer surfaces of nanotubes. Therefore, multiple materials will be coated inside and outside the nanotubes respectively by controlling doping ion concentrations and their deposition sequences. It should be noted that metallic nanocrystals in diameter around 6 nm are in the size domain to observe a significant conductivity change by changing the packing density, and therefore this system may be developed into a conductivity-tunable building block.