A new type of inkjet printer has been developed that can accurately print dots of various materials as small as 250 nanometers in diameter. The inkjet printer could enable the rapid synthesis of complex nanoscale structures from various materials.
“The goal is to do manufacturing,” says John roger, professor of engineering at the University of Illinois, Urbana Champaign. New printers can use a wide range of materials to make new devices, from plastic electronics and flexible displays to photovoltaic cells and new biomedical sensors, Rogers explains.
Researchers have shown that the new ink jets can print very precise patterns of electrically conductive polymers and carbon nanotubes; they have also shown that DNA can be printed without damaging it. âIt’s hard to do with traditional silicon manufacturing techniques,â says Rogers.
Often, the nanomaterials needed to fabricate ultra-small biomedical devices and nanoscale polymer electronics are in solution, meaning they do not lend themselves to traditional microfabrication techniques. For this reason, printing is an attractive alternative, both in terms of cost and complexity, says Heiko Wolf of IBM Zurich Research Laboratories‘Structures and devices group at the nanometric scale, in Switzerland.
But structuring structures at the nanoscale has so far proved difficult. âConventional ink jets are limited to resolutions of around 25 microns,â says Rogers.
Traditional ink jets work by pushing ink out of a nozzle to form droplets, either heating the ink or applying physical pressure to force it out. While it works well at the micrometer scale, surface tension and fluid flow issues start to become a hindrance when researchers try to downsize. âThe smaller the size of the nozzle, the harder it is to move fluid through it,â says Rogers. âSo the amount of force you have to apply disproportionately increases. “
To overcome this, Rogers and his colleagues use a different approach, called electrohydrodynamic inkjet (or e-jet) printing. âWe are pulling fluids rather than pushing them,â he says.
This involves the use of electric fields to create the droplets and relies on the presence of a certain amount of electrically charged particles, or ions, in the fluid. Capillary forces pull the fluid from its reservoir to form a semi-spherical droplet suspended from its edge, like a drop of water on a faucet.
By using electrodes to create an electric field between the tip of the nozzle and the substrate you want to print the material on, it’s possible to make the droplet conical, says Rogers. âThe ions build up on the surface of the fluid, at the top of the cone,â he says. This concentration of ions allows the tip of the cone to detach and form a droplet which is only a fraction of the volume of the cone.
âYou can generate droplets that are smaller than the diameter of the nozzle,â says Rogers. âYou’re just pinching droplets. It is only at the end of the cone that the droplets form.
Using this approach, Rogers and his colleagues showed that they could print lines of material 700 nanometers wide or individual dots only 250 nanometers in diameter.
In addition to the size of the droplets, spatial accuracy is also improved, says Rogers. He and his team discovered quite fortuitously that the field used to create the droplet also helps guide the charged droplet to the target substrate. âIt was kind of a bonus,â says Rogers.
Electrohydrodynamic printers have been used in the past, says Howard taub, associate director of HP Labs, in Palo Alto, Calif. The novelty here is the high resolution, he says.
But, says Taub, what these new e-jets make up for in resolution, they lack in speed. The high voltages required to generate the fields can be difficult to pulse in order to print quickly. Ordinary printers can eject droplets in the range of 10,000 to 100,000 times per second. Rogers e-jets, on the other hand, operate at around 1,000 times per second.
One solution is to use inkjet head arrays, explains Taub. But that can lead to other problems, he says: âThe droplets will interact with each other because they are charged. They should therefore be kept spaced.
Rogers says his group is working on the issue of speed. He and his colleagues have already shown that nozzles can be placed as close as 250 microns without the droplets interacting. They are now working with several manufacturers to bring the technology to market.