Electron transport within transparent assemblies of tin-doped indium oxide colloidal nanocrystals.

Stripe-like compact assemblies of tin-doped indium oxide (ITO) colloidal nanocrystals (NCs) are fabricated by stop-and-go convective self-assembly (CSA). Systematic evaluation of the electron transport mechanisms in these systems is carried out by varying the length of carboxylate ligands protecting the NCs: butanoate (C4), octanoate (C8) and oleate (C18). The interparticle edge-to-edge distance L0, along with a number of carbon atoms in the alkyl chain of the coating ligand, are deduced from small-angle x-ray scattering (SAXS) measurements and exhibit a linear relationship with a slope of 0.11 nm per carbon pair unit. Temperature-dependent resistance characteristics are analyzed using several electron transport models: Efros-Shklovskii variable range hopping (ES-VRH), inelastic cotunneling (IC), regular island array and percolation. The analysis indicated that the first two models (ES-VRH and IC) fail to explain the observed behavior, and that only simple activated transport takes place in these systems under the experimental conditions studied (T = 300 K to 77 K). Related transport parameters were then extracted using the regular island array and percolation models. The effective tunneling decay constant ßeff of the ligands and the Coulomb charging energy EC are found to be around 5.5 nm(-1) and 25 meV, respectively, irrespective of ligand lengths. The theoretical tunneling decay constant ß calculated using the percolation model is in the range 9 nm(-1). Electromechanical tests on the ITO nanoparticle assemblies indicate that their sensitivities are as high as ∼30 and remain the same regardless of ligand lengths, which is in agreement with the constant effective ßeff extracted from regular island array and percolation models.

A transparent flexible z-axis sensitive multi-touch panel based on colloidal ITO nanocrystals.

Bottom-up fabrication of a flexible multi-touch panel prototype based on transparent colloidal indium tin oxide (ITO) nanocrystal (NC) films is presented. A series of 7% Sn(4+) doped ITO NCs protected by oleate, octanoate and butanoate ligands are synthesized and characterized by a battery of techniques including, high resolution transmission electron microscopy, X-ray diffraction, (1)H, (13)C and (119)Sn nuclear magnetic resonance spectroscopy, and the related diffusion ordered spectroscopy. Electrical resistivities of transparent films of these NCs assembled on flexible polyethylene terephthalate substrates by convective self-assembly from their suspension in toluene decrease with the ligand length, from 220 × 10(3) for oleate ITO to 13 × 10(3)Ω cm for butanoate ITO NC films. A highly transparent, flexible touch panel based on a matrix of strain gauges derived from the least resistive film of 17 nm butanoate ITO NCs sensitively detects the lateral position (x, y) of the touch as well as its intensity over the z-axis. Being compatible with a stylus or bare/gloved finger, a larger version of this module may be readily implemented in upcoming flexible screens, enabling navigation capabilities over all three axes, a feature highly desired by the display industry.

High-throughput fabrication of anti-counterfeiting colloid-based photoluminescent microtags using electrical nanoimprint lithography.

This work demonstrates the excellent capability of the recently developed electrical nanoimprint lithography (e-NIL) technique for quick, high-throughput production of well-defined colloid assemblies on surfaces. This is shown by fabricating micron-sized photoluminescent quick response (QR) codes based on the electrostatic directed trapping (so called nanoxerography process) of 28 nm colloidal lanthanide-doped upconverting NaYF4 nanocrystals. Influencing experimental parameters have been optimized and the contribution of triboelectrification in e-NIL was evidenced. Under the chosen conditions, more than 300 000 nanocrystal-based QR codes were fabricated on a 4 inch silicon wafer, in less than 15 min. These microtags were then transferred to transparent flexible films, to be easily integrated onto desired products. Invisible to the naked eye, they can be decoded and authenticated using an optical microscopy image of their specific photoluminescence mapping. Beyond this very promising application for product tracking and the anti-counterfeiting strategies, e-NIL nanoxerography, potentially applicable to any types of charged and/or polarizable colloids and pattern geometries opens up tremendous opportunities for industrial scale production of various other kinds of colloid-based devices and sensors.

Quantification of the electrostatic forces involved in the directed assembly of colloidal nanoparticles by AFM nanoxerography.

Directed assembly of 10 nm dodecanethiol stabilized silver nanoparticles in hexane and 14 nm citrate stabilized gold nanoparticles in ethanol was performed by AFM nanoxerography onto charge patterns of both polarities written into poly(methylmethacrylate) thin films. The quasi-neutral silver nanoparticles were grafted on both positive and negative charge patterns while the negatively charged gold nanoparticles were selectively deposited on positive charge patterns only. Numerical simulations were conducted to quantify the magnitude, direction and spatial range of the electrophoretic and dielectrophoretic forces exerted by the charge patterns on these two types of nanoparticles in suspension taken as models. The simulations indicate that the directed assembly of silver nanoparticles on both charge patterns is due to the predominant dielectrophoretic forces, while the selective assembly of gold nanoparticles only on positive charge patterns is due to the predominant electrophoretic forces. The study also suggests that the minimum surface potential of charge patterns required for obtaining effective nanoparticle assembly depends strongly on the charge and polarizability of the nanoparticles and also on the nature of the dispersing solvent. Attractive electrostatic forces of about 2 × 10( - 2) pN in magnitude just above the charged surface appear to be sufficient to trap silver nanoparticles in hexane onto charge patterns and the value is about 2 pN for gold nanoparticles in ethanol, under the present experimental conditions. The numerical simulations used in this work to quantify the electrostatic forces operating in the directed assembly of nanoparticles from suspensions onto charge patterns can easily be extended to any kind of colloid and serve as an effective tool for a better comprehension and prediction of liquid-phase nanoxerography processes.

Pyrene-derived novel one- and two-component organogelators.

A new class of alkyl-chain-appended pyrene derivatives 4-14 were synthesized and evaluated for their gelation abilities. Depending on the nature of the linking group, these compounds gelated a number of organic solvents, either in the presence or in the absence of the acceptor molecule 2,4,7-trinitrofluorenone (TNF). Compounds with ester, ether, or alkyl linkages gelated a number of hydroxylic and hydrocarbon solvents by means of a charge-transfer interaction with TNF, while compounds with amide, urethane and urea linkers formed gels on their own in a variety of solvents by means of pi-pi stacking and hydrogen-bonding interactions. The Xray crystal structure of urethane (S)-12 showed hydrogen-bonding and stacking features, as suggested by the model. The gels obtained were investigated by spectroscopic and electron microscopic techniques which provided structural insights.