Nanoparticle and sol-gel inks for direct-write assembly of functional metallic and metal oxide materials

Concentrated nanoparticle and sol-gel inks have been created for the direct-write assembly of functional metallic and metal oxide materials. Direct-write assembly is an innovative, low-cost patterning technique in which specialized inks are printed in arbitrary planar and three-dimensional (3D) forms with lateral dimensions that are an order of magnitude lower than those achieved by inkjet printing. Paramount to this approach is the design of concentrated inks that can be extruded through fine deposition nozzles as filament(s), which then undergo rapid solidification in air to maintain their shape even as they span unsupported gaps or are patterned out-of-plane.Concentrated silver nanoparticle inks are synthesized for direct-write assembly of flexible, stretchable, and spanning microelectrodes (2--30 mum in diameter). By carefully controlling the silver nanoparticle concentration, size, and distribution, inks with high solids loading (&ge 70 wt%) are produced that are ideally suited for omnidirectional printing. Low temperature annealing (typically 200--250°C) yields highly conductive microelectrodes. Self-supporting microelectrodes in either planar or 3D forms of arbitrary complexity are patterned on a wide variety of substrates, including plastic, glass, and semiconductors. The patterned microelectrodes can withstand repeated bending and stretching to large levels of strain with minimal degradation of their electrical properties. Using this technique, wire-bonding to fragile devices and patterning of complex interconnects for solar cell and LED arrays are demonstrated.Concentrated titania-based, sol-gel inks have been created for direct-write assembly of functional oxides in both planar and 3D configurations. The sol-gel ink is tailored to facilitate flow through fine deposition nozzles (typically 1 in diameter). During heat treatment, the structures are converted into the desired oxide phase, while simultaneously maintaining their structural fidelity even as they experience significant volumetric shrinkage (&sim75%). After heat treatment, titania (TiO2) structures exhibit characteristic feature sizes that range from 0.2--1 mum. Both 1D and 3D micro-periodic structures are patterned. 3D micro-periodic structures composed of orthogonally-stacked layers of parallel oxide rods are produced in a woodpile architecture. These 3D structures exhibit a partial photonic bandgap with a reflectivity of up to 92% in the near IR and a broad stop-peak width of &sim26%. Scanning electron microscopy, atomic force microscopy, and optical reflectivity measurements acquired on 1D and 3D micro-periodic structures reveal their high degree of structural uniformity.The ability to pattern 1D arrays of TiO2 microwires offers precise control of filament diameter and spatial location, enabling a systematic study of microwire TiO2 gas sensors. A model gas sensor consisting of a single layer of parallel microwires is printed with the TiO2-based sol-gel ink in a well-defined, programmable pattern. The as-printed structure is heat treated in air to 600°C to form anatase TiO2. After heat treatment, the TiO2 wire diameter is measured as (628 +/- 13 nm). Gas sensing measurements on the TiO2 microwire array performed at elevated temperatures (200--300°C) indicate high sensitivity towards NO2 and CO gases, with estimated sensitivity limits in the sub-ppm range for NO2 and single ppm range for CO. Under ambient conditions, the TiO2 microwire array responds quite significantly and reversibly to low NO2 concentrations (down to 0.5 ppm). This is a highly promising result for the creation of low-power, gas sensor devices based upon direct-write assembled TiO2 microwire arrays.