Nanowire Ink

January 30, 2017

The materials used in today's physics laboratory are quite different from those of the past. Ernest Rutherford (1871-1937), a physicist who was awarded the 1908 Nobel Prize in Chemistry, was famous for building experiments from simple items such a piece of a bicycle handlebar, string, and sealing wax.[1] Natural leather was often used as a gasket material, since it remains pliable at low temperatures.

Ernest Rutherford (1871-1937). Rutherford is famous for the discovery of the proton, but he was also Director of the Cavendish Laboratory of Cambridge University when James Chadwick discovered the neutron in 1932. (Wikimedia Commons image, modified for artistic effect.)

Aquadag was another common material in physics laboratories of yesteryear. Aquadag, a colloid of water and graphite is applied like a paint, and it's a reasonable conductor of electricity when it dries. Solutions diluted 1:1 with water produce coatings with a sheet resistance of about 800 ohms/square when air dried, but this decreases to just 20-30 ohms/square after heating at 300 °C.[2] For comparison, copper has a sheet resistance of less than a milliohm per square.

The main industrial application of aquadag was as an electrode material for cathode ray tubes where the high resistance isn't a problem since the currents are low. Since graphite is a good dry lubricant, aquadag and a variant containing molybdenum disulfide are used to lubricate. India ink, another common laboratory item in the era when graphs and diagrams for publications were drawn by hand and not generated by a computer, is a colloid of fine carbon soot in water that also acts as a conductive paint.

Staedtler Mars pen set, circa 1980, that I used to draw many of my early data graphs. The ink in this set is India ink. Photograph by the author.

Today, silver in lacquer or epoxy is a common substitute for aquadag and India ink when making connections to a specimen for electrical measurement. Such silver-filled inks and pastes produce layers with a sheet resistance of a few tens of milliohms per square. A common use for these, other than wire connection, is to repair broken conductors in printed circuit boards.

This leads to the idea that it might be possible to actually "print" your printed circuit board. I wrote in an earlier article (Silver Ink, January 19, 2012) how scientists from the Department of Materials Science and Engineering and the Frederick Seitz Materials Research Laboratory at the University of Illinois at Urbana-Champaign have developed a process for this application.[3-4]

The limitation of inkjet printing a colloid of solid silver particles is that the smallest conductor width is the particle diameter, which is generally several tens of micrometers.[3] To overcome this problem, the Illinois research team deposited liquid precursors for silver based on silver acetate that yielded 22 wt-% silver.[3] The one disadvantage of the precursor technique is that a decomposition reaction is required at a temperature that's too high for most polymeric substrates.

Microstructure of printed silver ink after annealing. The electrical conductivity is the same as for bulk silver.[3] (Courtesy of S. Brett Walker. Used with permission.)

In research to produce conductive silver ink that doesn't require a high temperature processing step, chemists at Duke University have experimented with a colloid in which the solid silver is in the form of nanowires.[5-6] The conductivity of printed circuit traces made from this ink far exceeds that for inks made from nanospheres or microflakes when deposited without a high temperature processing step, thus enabling printed circuitry on paper and plastic substrates.[6]

In their experiments, the Duke chemists examined the morphology and resistivity of thick films of silver nanowires of two different lengths, silver nanoparticles, and silver microflakes when processed at temperatures from 70-400 °C.[5] Distilled water was used as the ink vehicle, and the tests were facilitated with a simple sample preparation method. A punch was used to create holes in double-sided tape, which was applied to glass slides. A precise volume of the silver nanoparticle ink was injected into these tape wells, the water was evaporated, and the various temperatures were applied.[6]

With heating to just 70 °C, films of the long silver nanowires had a resistivity of just 1.8 x 10^-5 ohm-cm, so they were 4000 times more conductive than films made from silver nanoparticles.[5] After sintering at 300 °C, the resistivity of all the silver materials, except microflakes, converged to a value of about 2.5 x 10^-5 ohm-cm, while films of silver microflakes were about 10 times less conductive, having a value of about 4 x 10^-4 ohm-cm.[5] When ten wt-% silver nanowires were added to silver nanoparticle ink, there was a 400-fold improvement in conductivity.[5]

Top row, material processed at 70°C; bottom row, material processed at 300 °C. These images show regions of about two micrometers square in all cases, with significant melting observed at 300°C. (Duke University image by Ian Stewart and Benjamin Wiley.)

These results are significant, and they would allow wide application in solar cells, flexible displays, touchscreens, batteries and implantable bio-electronic devices.[6] Says Benjamin Wiley, an assistant professor of chemistry at Duke University and corresponding author on the paper describing these results,

"The nanowires had a 4,000 times higher conductivity than the more commonly used silver nanoparticles that you would find in printed antennas for RFID tags... So if you use nanowires, then you don't have to heat the printed circuits up to such high temperature and you can use cheaper plastics or paper... There is really nothing else I can think of besides these silver nanowires that you can just print and it's simply conductive, without any post-processing."[6]

Significantly, the required quantity of silver needed to make these conductors is small.[6] The Duke research team is experimenting with inkjet printing of conductive circuit traces. Also, silver-coated copper nanowires, which would be less expensive than the pure silver nanowires, might display the same conductivities.[6] This research was supported by the National Science Foundation.[6]

References:

1. Ashutosh Jogalekar, "Ernest Rutherford, master of simplicity," Scientific American Blogs, August 30, 2013.
2. Data Sheet AGG303: Colloidal Graphite - "Aquadag," Agar Scientific (PDF File).
3. S. Brett Walker and Jennifer A. Lewis, "Reactive Silver Inks for Patterning High-Conductivity Features at Mild Temperatures," J. Am. Chem. Soc., (January 5, 2012), DOI: 10.1021/ja209267c.
4. Liz Ahlberg, "Particle-free silver ink prints small, high-performance electronics," University of Illinois Press Release, January 12, 2012.
5. Ian E. Stewart, Myung Jun Kim, and Benjamin J. Wiley, "Effect of Morphology on the Electrical Resistivity of Silver Nanostructure Films," ACS Appl. Mater. Interfaces, Article ASAP (December 16, 2016), DOI: 10.1021/acsami.6b12289.
6. Kara Manke, "Nanowire 'Inks' Enable Paper-Based Printable Electronics," Duke University Press Release, January 3, 2017.