Lead author of the paper Professor Mark Hersam is Professor of Materials Science and Engineering at the McCormick School of Engineering and Applied Science, one of 12 schools that make up Northwestern.
“We have been studying graphene in my laboratory for the past five years,” he told ScienceOmega.com. “However, our specific efforts to develop printable graphene inks have occurred primarily in the past three years.”
The properties of graphene are attractive and intriguing to engineers, and in particular to designers and developers of next-generation electronic devices. It is highly conductive, chemically very stable and mechanically flexible. The opportunity to combine it with inkjet printing opens exciting avenues of possibility.
“Inkjet printing is among the most mature printing methods in industry, which means that it is relatively inexpensive and highly scalable,” explained Professor Hersam.
It is also capable of printing over large areas and on a variety of substrates, leading researchers in the past to explore the use of inkjet printing to produce transistors and solar cells, among other components. Enabling the use of graphene in inkjet printing, however, has proved problematic. For instance, it is difficult to harvest enough graphene without affecting the electronic properties of the material.
Challenges have been overcome
“It is challenging to achieve sufficiently high graphene concentration in a solvent that has the appropriate viscosity for inkjet printing,” Professor Hersam noted. “In addition, it is difficult to achieve high electrical conductivity after printing.”
This is due to the fact that the breaking up – or exfoliation – of graphite requires oxidising conditions which result in the production of graphene oxide, a much less conductive substance than pure graphene. To achieve an unoxidised product certain solvents are required, the residues of which also have a negative effect on conductivity. However, as Professor Hersam affirmed, all of these challenges have now been overcome.
The process developed by the professor and his colleagues at McCormick involves the use of ethanol and ethyl cellulose to exfoliate graphite at room temperature; it can be utilised to mass-produce graphene without compromising the substance’s electrical conductivity. Residues are kept to minimum, and the resulting powder has a relatively high concentration of nanometre-sized graphene flakes, which are subsequently mixed with a solvent to produce ink.
The researchers went on to demonstrate the use of the ink by printing complex patterns in numerous 14-nanometre-thick layers. Even when bent, the conductivity of the ink is hardly affected, suggesting that foldable electronics could be a possible application, but further obstacles will need to be overcome.
“While an increasing number of distinct flexible/foldable materials have been developed, the next challenge is to seamlessly integrate them together into fully-fabricated electronic devices and circuits,” concluded Professor Hersam. “We are actively working on this problem and hope to solve these remaining challenges over the next couple of years.”