Nanoxerography sets the pattern
Researchers at the University of Minnesota, US, have used nanoxerography to print patterns of nanoparticles onto charged surfaces with a resolution as good as 100 nm. The team used a liquid-phase technique to assemble particles of iron, iron oxide and carbon and a gas-phase technique to handle silver and gold nanoparticles.
“Our goal was to develop a form of ‘directed self-assembly’ that would enable parallel positioning of nanoparticles to overcome the speed limitations of serial techniques,” Heiko Jacobs told nanotechweb.org. “Nanoxerography has the advantage that it enables the positioning of any material that can hold charge.”
Nanoxerography uses electrostatic forces to attract nanoparticles to the desired location. To create the charged areas required, Jacobs and colleagues used flexible patterned electrodes that they brought into contact with a thin-film electret on a silicon substrate. Applying a voltage pulse between the electrode and silicon transferred the charge into the electret.
The team tried two types of electrode. The first consisted of a patterned 5 mm-thick poly(dimethylsiloxane) (PDMS) stamp supported on a copper plate and coated with a 60 nm-thick gold film on top and InGa alloy down the sides, to provide a good contact to the copper plate. These electrodes incorporated patterns of parallel lines less than 200 nm wide or arrays of 30 nm-high circular posts with diameters of 200 nm. The second type of electrode consisted of a 3 inch diameter, 10 micron-thick n-doped silicon wafer containing a pattern of lines 450 nm wide and 200 nm deep.
In the liquid-phase assembly process, the scientists put the chip carrying the charge pattern into a vial containing non-polar solvent in an ultrasonic bath. Then they added an aggregate of nanoparticles to the solvent. The aggregate broke up under sonication and the nanoparticles aligned on the charged surface within seconds. In this way, the team made patterns of 30 nm-diameter graphitized carbon nanoparticles, red iron-oxide particles smaller than 500 nm and iron beads less than 2 microns in size.
In the gas-phase process, a tube furnace generated nanoparticles by evaporation and condensation. The nanoparticles then travelled in a stream of nitrogen gas to a particle assembly module. Here, two electrodes generated an electric field that directed the nanoparticles towards the charged sample surface. Jacobs and colleagues used the gas-phase technique to assemble patterns of silver and gold nanoparticles.
The team produced patterns covering areas of up to 5 x 5 mm. The gas-phase technique achieved a resolution of 100 nm, while the liquid-phase technique produced a resolution of 200 nm; values that are 500-1000 times better than those of traditional xerographic printers. According to the researchers, the current limit on resolution is the size of the features that they can make on the electrodes. For the PDMS electrode this limit is about 100 nm as smaller features tend to collapse, but silicon-based electrodes should be able to support feature sizes of about 10 nm.
“Our results pose a promising method of future nanotechnological device fabrication by positioning specific particles onto specific locations in a relatively short amount of time,” said Jacobs.
The researchers reported their work in Nanotechnology.
Liz Kalaugher is editor of nanotechweb.org.