All-printed paper memory.

We report the memory device on paper by means of an all-printing approach. Using a sequence of inkjet and screen-printing techniques, a simple metal­insulator­metal device structure is fabricated on paper as a resistive random access memory with a potential to reach gigabyte capacities on an A4 paper. The printed-paper-based memory devices (PPMDs) exhibit reproducible switching endurance, reliable retention, tunable memory window, and the capability to operate under extreme bending conditions. In addition, the PBMD can be labeled on electronics or living objects for multifunctional, wearable, on-skin, and biocompatible applications. The disposability and the high-security data storage of the paper-based memory are also demonstrated to show the ease of data handling, which are not achievable for regular silicon-based electronic devices. We envision that the PPMDs manufactured by this cost-effective and time-efficient all-printing approach would be a key electronic component to fully activate a paper-based circuit and can be directly implemented in medical biosensors, multifunctional devices, and self-powered systems.

Flexible miniaturized nickel oxide thermistor arrays via inkjet printing technology.

In this study, an inkjet printing process was developed to produce thermistor arrays for temperature sensing applications. First, a formulation process was carefully performed to generate a stable nanoparticle ink for nickel oxide, a material with a large temperature coefficient of resistance. The thermistor was then fabricated by printing a square NiO thin film in between two parallel silver conductive tracks on either glass plates or polyimide films. The printed thermistor, which has an adjustable dimension with a sub-millimeter scale, can operate over a wide range from room temperature to 200 °C with great sensitivity (B values ~4300 K) without hysteretic effects. When printed on polyimide films, the thermistors can also be bent or attached to curved surfaces to provide accurate and reliable temperature measurements. Moreover, the thermistor responds quickly to small temperature changes and provides an effective tool for transient temperature measurements. Finally, a thermistor array was fabricated to show the flexibility of this inkjet printing process and to demonstrate the applicability of the printed devices for temperature sensing applications.

Preserving precision of inkjet-printed features with solvents of different volatilities.

For printed micropatterns on plastic substrates, the decreasing volume because of solvent evaporation frequently leads to contact line receding and changes the original printed pattern. To prevent printing quality deterioration caused by contact line motions, an ink formulation method was developed. A nearly non-volatile solvent [polyethylene glycol (PEG)] with a low receding angle on polyethylene terephthalate (PET) sheets was added in water to hold the contact line. To understand the dewetting phenomena of inks, the geometrical evolution of circular liquid films under evaporation was recorded and analyzed. The results showed that the contact line receded as water evaporated for inks with low PEG concentrations but remained pinned at a moderate PEG concentration (~10 wt %). A simple model was proposed to explain the dewetting phenomena and can successfully predict the critical PEG concentration, beyond which the contact lines will be unconditionally pinned. The optimized water/PEG solvent can then be used to prepare dye- or particle-based inks, which preserved accurate features after solvent evaporation.

Self-assembled mesoporous silica nanoparticles in controlled patterns produced by soft-lithography and ink-jet printing.

This study demonstrate assembly of mesoporous silica nanoparticles (MSNs) into various patterns by soft-lithography and ink-jet printing techniques. A clear suspension containing MSNs with a particle size around 58 nm is firstly synthesized. Then, soft-lithographic techniques (i.e., MIMIC, impression with PDMS) and an ink-jet printing technique are applied to create various patterns assembled by MSNs. The MIMIC method results in a high density of MSNs, but is limited to linear patterns due to the capillary principle. The impression method led to MSN colloids in various patterns, but the MSNs assembled in low density due to the lack of the colloidal supplements. The ink-jet technique can create various patterns more conveniently, and the final patterns are generated after a de-wetting process. During the de-wetting process, the MSN concentrations and the jetted times are related only to the final number of particles dispersed in patterns. Comparison of different patterning techniques will be helpful towards creation of patterned assembly with MSNs.

Silver conductive features on flexible substrates from a thermally accelerated chain reaction at low sintering temperatures.

In this study, a simple and effective silver ink formulation was developed to generate silver tracks with high electrical conductivity on flexible substrates at low sintering temperatures. Diethanolamine (DEA), a self-oxidizing compound at moderate temperatures, was mixed with a silver ammonia solution to form a clear and stable solution. After inkjet-printed or pen-written on plastic sheets, DEA in the silver ink decomposes at temperatures higher than 50 °C and generates formaldehyde, which reacts spontaneously with silver ammonia ions to form silver thin films. The electrical conductivity of the inkjet-printed silver films can be 26% of the bulk silver after heating at 75 °C for 20 min and show great adhesion on plastic sheets.

Direct writing patterns for electroless plated copper thin film on plastic substrates.

A simple and efficient method is developed to create conductive copper thin films on polymer surfaces. Instead of regular palladium colloid inks, micropatterns of silver nitrate inks, which serve as an activating agent for copper plating, were printed and dried on flexible plastic substrates. The printed plastic sheets were then immersed in an electroless copper plating bath at 55 °C for 2 min to create copper thin films on the printed patterns. The prepared copper films have an electrical conductivity as high as 83% of bulk copper and show good adhesion on PET or PI substrates.