Preparation of Solid Silver Nanoparticles for Inkjet Printed Flexible Electronics with High Conductivity Paper Written By Wenfeng Shen, Xianpeng Zhang, Qijin Huang, Qingsong Xu, Weijie Song Presented By Gabe Shindnes, Liza Ross, Bill Shi

Purpose Utilize Ag nanoparticles to develop a method to print flexible electrical circuits. Flexible printed LED array

Inkjet Printing Applications Inkjet printing technology has been used for LED’s, photovoltaic cells, and sensors. Smart Clothing

Remaining solution: Nanoparticles Possible Conductors Conductive Polymers, Graphene, Carbon Organo-metallic compounds Metal precursors Metal nanoparticles Why they are not suitable for application: Polymers, graphene and carbon have too low conductivity Organo-metallic compounds and metal precursors require too high temperatures Remaining solution: Nanoparticles

TEM Image of Ag Nanoparticles Silver Nanoparticles Chosen for their good conductivity, low resistivity, reasonable cost and resistance to oxidation. TEM Image of Ag Nanoparticles

Synthesis of Poly(acrylic acid) (PAA)-coated AgNP Silver source: silver nitrate (AgNO3) Reducing agent: Monoethanolamine (MEA) Capping molecule: Poly(acrylic acid) Medium: deionized water AgNO3 --->PAA coated, charged nanoparticles dispersed in aqueous solution 1. Undergo vigorous magnetic stirring for an hour 2. Heated up to 65 degrees Celsius 3. Colors vary in the sequence of yellowish, brown, orange, and dark red. 4. A black precipitate of PAA coated AgNP is collected after adding ethanol 5. Collected through funnel and washed with ethanol to remove residual MEA and PAA

Inkjet Printing and Heat Treatment of Silver Patterns Silver ink lines were printed using a common color printer AgNP inks printed on Kodak photo paper, and polyethylene terephthalate (PET) PET substrate was precoated with Poly(diallyldimethylammonium) chloride (PDAC) so the ink would sinter at room temperature Kodak paper already had PDAC-like molecules After printing, samples are heat-treated at temperatures ranging from 50-280 ℃. Then: The structural and electric properties of silver patterns are investigated

Dependent Variables and Measurement Methods: Structural characterizations (electron density, atom positions) - X ray diffraction Particle size distribution and morphology- Transmission electron microscope Viscosity of AgNP- Viscometer Surface tension- Drop weight method Electrical resistivity- 4 point probe system

Results and Discussion Properties of Poly Acrylic Acid The powder obtained from grinding solid silver NPs was observed with an X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM- results not shown with SEM), Thermogravimetric/Differential Thermal Analysis (TG/DTA) XRD: results demonstrate that the powder was composed of well-crystallized silver NPs, and that the PAA-coated silver particles did not change after being held in the room for 3 months Scherrer Formula ( ; formula that determines the size of particles in the form of powder) Showed that all particles were approximately 30nm Poly Acrylic Acid

Results and Discussion TG/DTA: data showed a miniscule weight loss from room temperature to 300℃ of 0.7% caused by the release of organic molecules from the surface of the silver NPs 0.1% weight loss from 955℃ to 1000 ℃; caused by temperature disturbance 99.2 % of weight was pure silver Benefit of using silver NPs is that they can be stored at room temperature can also be prepared as printing material by dispersing NPs with water Silver NPs ranged from 20nm to 230nm, with mean being somewhere between 30 to 50 nm

Results and Discussion TEM (Transmission Electron Microscope) Data Data obtained from TEM on Silver NPs: Particle size ranged from 10 - 100nm The majority of the particles were less than 40nm Data agrees with the data obtained from the X-Ray Diffraction (XRD) analysis Transmission Electron Microscope

Results and Discussion Silver NP Ink Properties Purpose: Produce small liquid silver droplets and to deposit them in a specific location on a substrate Most important properties of droplets for this : Viscosity and Surface Tension Increasing the density of Silver NPs in the liquid showed a positive correlation in both Viscosity and Surface Tension Other important properties to consider when working with non- Newtonian fluids: Jet Velocity, Characteristic radius (radius of the orifice), and Density Dimensionless numbers that describe these properties Reynolds, Weber (measures spreading behavior)

Results and Discussion Properties of Silver NPs The Ohnesorge number describes the physical properties of the ink and the size of the scale Oh ranges from 0.1 to 1.0 since Z ranges from 1 to 10 A high Z (larger than 10) will create a large number of viscous satellite droplets A low Z (smaller than 1) will prevent the separation of the droplets due to the viscous forces The Z value of the silver droplets that were created (varying in weight from 10 to 25 percent) ranged from 9.94 to 7.00 which made these inks printable

Results and Discussion The printed line width was set at 1pt, equating to approximately 530μm. If the silver nanoparticles were heated at higher temperatures, the more conductive the lines became At higher temperature, there were less cracks, helping induce electrical conductivity Lines were printed 14 times, with each print decreasing the resistance at an exponential rate a) Room Temperature b) 50oC c) 80oC d) 140oC

Results and Discussion Greatest resistivity difference between temperatures with only one print cycle, but decreases significantly with additional print cycles 8μΩ cm when sintered at room temperature compared to 3.7μΩ cm at 180oC Allows high conductivity at low temperatures This is important because it allows for inkjet printing for paper and plastic products

Conclusion: Synthesis of PAA coated AgNPs kept at room temp. without degeneration Low cost reaction and environmentally friendly No centrifugation—Ideal for low cost and mass production Lowest electrical resistivity measured was 8.0 µΩ cm → high conductivity