The nanoscale
titania pattern before and after heat-treatment
A new lithography
technique enables the production of nanoscale patterns of titania for high-tech
applications
Titanium dioxide, or titania, is an inorganic material
commonly used as a whitening agent in food and toothpaste. It is also used as
one of the main active ingredients in sunscreens. The properties that make
titania useful in commercial applications — namely its whitening ability and
high refractive index — are now being exploited in a wide range of
technological applications.
One particular area of interest has been the application
of titania in dye-sensitized solar cells — devices that can be used to convert
sunlight into electricity. Such application often requires the formation of
intricate surface patterns, with the key limiting factors for development being
cost and speed of processing. Now, Ramakrishnan Ganesan, Mohammad Saifullah and
co-workers1 at the A*STAR Institute of Materials Research and Engineering have
described the use of a technique called step-and-flash imprint lithography
(SFIL) to produce such patterns on the nanoscale.
“The precursor method to SFIL is thermal nanoimprint
lithography, which is extremely time-consuming as it requires
temperature-cycling processes to form a pattern,” explains Saifullah. “A mold
could be pressed into a heated (and softened) resist material or a liquid
precursor could be forced into a mold and then hardened upon heating.”
Newer processes eliminate the need for heating by using
irradiation with ultraviolet (UV) light to harden the polymer. Although this
process may be ideal for organic polymer materials, it is more problematic when
using inorganic materials such as titania as the liquid precursor materials are
highly viscous and do not spread easily. As a result, the dispensing nozzle may
sometimes become blocked.
The chemicals used to make titania can also be unstable
in solution, so the team had to identify a mixture of components that offered a
combination of stability and low viscosity. “We found that an allyl
functionalized titanium complex was stable in combination with other polymer
precursors,” explains Saifullah. The final component of the mixture is a
photoinitiator — which starts the polymerization process upon irradiation with
UV light.
The mixture was dispensed onto the surface in the form of
droplets, and the mold pressed into place to help the liquid spread.
Irradiation with UV light results in hardening of the pattern, after which the
mold can be removed. A final heating step burns away the organic material,
leaving behind a shrunken version of the original pattern made from titania
(see image). Significantly, the aspect ratio of the pattern is maintained after
the heat-treatment process.
“Our current method is quite specific to titania, but
after tackling this most important material, we hope to develop similar
procedures for other inorganic materials,” says Saifullah.
The A*STAR-affiliated researchers contributing to this
research are from the Institute
of Materials Research and Engineering
References
- Ganesan,
R. et al. Direct patterning of TiO2 using
step-and-flash imprint lithography. ACS Nano 6,
1494–1502 (2012). | article
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