Researchers from Cornell University have devised a simple and efficient means of creating highly ordered arrays of nanoparticles without the aid of a surface-modified substrate or self-assembled monolayer acting as a template [W. Cheng et al., Nature Nano. (2008) doi: 10.1038/nnano.2008.279].
Ordered arrays of nanoparticles have unique properties that hold promise for optical and electronic applications. But fabricating these arrays or superlattices presents a challenge. The traditional approach relies on evaporating a drop of solution containing the desired nanoparticles – a far-from-equilibrium dewetting process that can produce irregular patterns. Alternatively, lithography techniques produce highly regular patterns but are very expensive.
Dan Luo and coworkers have instead come up with a new method that controls the local nucleation and growth of superlattices. Like traditional dewetting techniques, the researchers use microdroplets of solution containing nanoparticles. But they use a polydimethylsiloxane (PDMS) mold or stamp to control the shape and location of droplets containing DNA-capped Au nanoparticles on a Si substrate.
The pressure exerted on the PDMS stamp and its geometry control the dewetting dynamics of the solution. At high pressures, a microdroplet forms on the substrate in the middle of the stamp – ‘center’ dewetting – and a three-dimensional ‘supra-crystal’ forms. Conversely, under lower pressures ‘edge’ dewetting occurs and rings – or ‘corrals’ – form.
The simple method can produce one-, two- or three-dimensional structures on the same substrate in one go. Despite its lack of sophistication, the approach can produce nanoscale features (down to 12 nm) from microscale molds.
“We can rationally control nanoparticle internal structures and overall shapes with a simple and inexpensive method,” says Luo. “We can pattern nanoparticles into versatile nanoscale features while still maintaining their ordering.”
It is possible to pattern large areas in this way – Luo and his team report patterned superlattices on a Si substrate 20 × 25 mm2. And despite its lack of sophistication, the patterns are remarkably uniform. However, defects do remain a limitation, admits Luo.
“The work is a very important and novel demonstration of fabricating bottom-up devices from a simple micromolding method,” says Alejandro L. Briseno of the University of Massachusetts, Amherst.
One of the unique features of the approach is that it can be easily applied to a variety of materials, says Briseno. As well as DNA-capped Au nanoparticles, the researchers have also used the technique with CdSe/ZnS quantum dots and conducting polymers.
“The large-area patterning of superlattices may overcome technical barriers of current device fabrication,” says Briseno. “It should be possible to blend several components to produce objects with novel properties.”