|
Fabricating precise biomolecular structures at extremely small
scales is critical to the progress of nanotechnology and related fields.
Traditionally, one of the ways this has been accomplished has
been through the use of rubber stamps with tiny features which are covered with
molecular “inks” and then stamped onto substrate surfaces, creating molecular
patterns. But when using this technique at the nanoscale, molecules tend to
diffuse on the surface both during and after stamping, blurring the patterns.
To address this problem, researchers at the University of
California, Los Angeles (UCLA) have turned this conventional “soft
lithography” process on its head: Instead of using a stamp to transfer
molecules onto bare surfaces, they have used chemically treated stamps to remove
molecules already in place on gold substrates, essentially peeling away select
molecules through chemical bonds to create precise patterns measuring just a
few molecules across.
The new process, called chemical lift-off lithography (CLL),
results in higher-resolution patterning and avoids the blurring problems of
earlier techniques.
The findings are published in Science. The research was supported by the U.S. Department of
Energy and the Kavli Foundation.
The stamp used in the new process is molded by using a
“master” made with more sophisticated and expensive tools than those
used in making rubber stamps for offices and children, but the stamps can be
used over and over again. Between each use, they are simply reactivated by an
oxygen plasma.
The chemical bonds formed at the stamp—substrate interface are
sufficiently strong to remove not only molecules in the monolayers but also one
layer of gold atoms from the substrate. This observation settled a long-running
discussion over whether, for such monolayers, gold–gold bonds break more easily
than molecule–gold bonds—they do, the researchers found.
The research team was able to fabricate a variety of
high-resolution patterned features, and stamps were cleaned and reused many
times with little feature deterioration. The remaining monolayer, they found,
can act as a resist for etching exposed gold features. The backfilling of new
molecules into the lifted-off areas enabled patterned protein capture, and
sharp 40 nm chemical patterns were achieved.
Led by Anne Andrews and Paul Weiss, the UCLA team represents a
collaboration among researchers from UCLA’s California NanoSystems
Institute, the Semel Institute for Neuroscience and Human Behavior at UCLA, and
the departments of chemistry and biochemistry, materials
science and engineering, and psychiatry and biobehavioral sciences.
Conventional nanolithographic patterning techniques, such as
photolithography and electron-beam lithography, are expensive, time consuming,
require specialized equipment and instrumentation, and have limited
capabilities for chemical patterning; here, they only need to be used for the
fabrication of stamp master molds.
Once individual masters are produced, CLL is used for
high-resolution, high-throughput, low-cost pattern fabrication. This method
enables patterns to be transferred to underlying substrates, and
multiple-stamping strategies can be used to produce high-fidelity
nanometer-scale patterns on gold substrates, with the additional possibility of
patterning different materials, such as silicon, germanium, platinum, and
graphene.
The patterns fabricated demonstrate that CLL not only transfers
large-area, high-fidelity patterns, but the post–lift-off exposed gold areas
are advantageous for producing multiplexed patterned surfaces for selective
capture of biomolecules from complex mixtures.
