From 2-D blueprint, material assembles into novel 3-D nanostructures
An international team of scientists affiliated with the University of Wisconsin Nanoscale Science and Engineering Center has coaxed a self-assembling material into forming never-before-seen, three-dimensional nanoscale structures, with potential applications ranging from catalysis and chemical separation to semiconductor manufacturing.
Led by UW-Madison Chemical and Biological Engineering Professors Paul Nealey and Juan de Pablo and colleagues at Georg-August University in Germany and the Paul Scherrer Institute in Switzerland, the team has discovered that materials known as block copolymers will spontaneously assemble into intricate 3-D shapes when deposited onto particular 2-D surface patterns created with photolithography.
The result, published in the Jan. 27 issue of Physical Review Letters, demonstrates a promising strategy for building complex, 3-D nanostructures by using standard tools of the semiconductor industry, says Nealey. Those tools, particularly lithography, already allow the making of devices with dimensions substantially smaller than 100 nanometers, or a hundred-thousandth of a centimeter.
But photolithography is also limited, he says, because as practiced today it is essentially a two-dimensional process.
“What we've done by using self-assembling block copolymers is to extend photolithography to three dimensions,” says Nealey. “And the structures we've fabricated are completely different from the same block copolymer materials in the bulk.”
Also important to manufacturing, the new 3-D nanostructures are stable, well defined and nearly defect-free over large areas. They also align perfectly with the underlying lithographic pattern — a key requirement for any device or application based on them.
“This research shows that lithography combined with block copolymers is more versatile and powerful than we thought. We can now create completely new structures that will no doubt have new properties and new applications,” says de Pablo. “Exactly what those structures will be is anybody's guess; here we demonstrate a complicated one. But the important thing is they open up a new field of exploration, both for these materials and this technology.”
The specific structures the team produced consist of two tightly interwoven, yet completely independent, networks of channels and passages — all at the scale of atoms. “What we have are two interpenetrating meshes both of which are completely continuous. And yet you could travel through one from end to end without ever entering the other,” says de Pablo.
The networks also register perfectly with the photolithographic pattern underneath, which tells scientists exactly where each channel ends and gives them ready access to channel openings. A gas, for example, might be introduced through the openings to react with a catalyst deposited on the walls of the network. Nanoscale materials have massive surface areas compared to their volumes; thus, catalysis would be extremely efficient.
Another use would be chemical separation of substances of different sizes. “This process gives us exquisite control over the dimensions of pores,” says de Pablo. “So, we could easily make membranes that are permeable to substances smaller than the length scale of the material.”
The researchers study specific block copolymers consisting of long chains of two different types of molecules, which alternate with each other in blocks. At high temperature, block copolymers are molten and randomly mixed. But when cooled down, the material spontaneously assembles into alternating layers of molecules.
In previous studies, the authors showed how block copolymers could be directed to arrange themselves into an underlying photolithographic pattern with absolute perfection. They also discovered how to pattern irregular structures, like those found in nanoelectronic devices, by this approach. The key advantage to using self-assembling materials to create structures, rather than traditional semiconductor technology, says Nealey, is they provide much more precise control over the dimensions and shapes of features as small as 10 nanometers.
The team followed a similar approach in the current research. But instead of employing a lithographic pattern of stripes similar to the naturally layered structure of bulk block copolymers, they created a square array of dots on which to deposit the block copolymer film.
“Throughout all the previous work by ourselves and others, what people have been doing, perhaps without thinking too much, was to make a surface pattern with lithography that matched the geometry of the bulk copolymers,” says de Pablo. “What we've demonstrated in this work is that by combining complex, nonobvious patterns with the well known bulk morphologies, we can get the system to behave in a completely new, yet controllable and predictable, way.”
This work was supported by the National Science Foundation Nanoscale Science and Engineering Center at the University of Wisconsin and the Semiconductor Research Corporation.