Gwen Wright and Aaron Stein at the electron beam lithography writer in the CFN cleanroom. Photo: Brookhaven National Laboratory.Next-generation electronic devices will need to exploit the nanoscale, where materials span just billionths of a meter. But balancing complexity, precision and manufacturing scalability on such fantastically small scales is inevitably difficult. Fortunately, some nanomaterials can be coaxed into snapping themselves into desired formations, a process known as self-assembly.
Scientists at the US Department of Energy's (DOE) Brookhaven National Laboratory have just developed a way to direct the self-assembly of multiple molecular patterns within a single material, producing new nanoscale architectures. The results are published in a paper in Nature Communications.
"This is a significant conceptual leap in self-assembly," said Brookhaven Lab physicist Aaron Stein, lead author on the study. "In the past, we were limited to a single emergent pattern, but this technique breaks that barrier with relative ease. This is significant for basic research, certainly, but it could also change the way we design and manufacture electronics."
The current process for creating microchips, for example, uses meticulously patterned templates to produce the nanoscale structures that process and store information. Through self-assembly, however, these structures could spontaneously form without the need for exhaustive preliminary patterning. The new self-assembly technique represents a step towards this goal, by offering a way to generate multiple distinct patterns, greatly increasing the complexity of nanostructures that can be formed in a single step.
"This technique fits quite easily into existing microchip fabrication workflows," said study co-author Kevin Yager, also a Brookhaven physicist. "It's exciting to make a fundamental discovery that could one day find its way into our computers."
The experimental work was conducted entirely at Brookhaven Lab's Center for Functional Nanomaterials (CFN), a DOE Office of Science User Facility, leveraging in-house expertise and instrumentation.
The technique uses block copolymers – chains of two distinct molecules linked together – because of their intrinsic ability to self-assemble. "As powerful as self-assembly is, we suspected that guiding the process would enhance it to create truly 'responsive' self-assembly," said study co-author Greg Doerk, also at Brookhaven. "That's exactly where we pushed it."
To guide self-assembly, scientists create precise but simple substrate templates. This involves using a method called electron beam lithography to etch patterns that are thousands of times thinner than a human hair on to the template surface. They then add a solution containing a set of block copolymers onto the template, spin the substrate to create a thin coating and ‘bake’ it all in an oven to kick the molecules into formation. Thermal energy drives interaction between the block copolymers and the template, setting the final configuration, which could be parallel lines or dots in a grid.
"In conventional self-assembly, the final nanostructures follow the template's guiding lines, but are of a single pattern type," Stein said. "But that all just changed."
The scientists had previously discovered that mixing together different block copolymers allowed multiple, co-existing line and dot nanostructures to form.
"We had discovered an exciting phenomenon, but couldn't select which morphology would emerge," Yager said. But then the team found that tweaking the substrate changed the structures that emerged. By simply adjusting the spacing and thickness of the lithographic line patterns, which are easy to fabricate using modern tools, the self-assembling blocks can be locally converted into ultra-thin lines or a high-density arrays of nano-dots.
"We realized that combining our self-assembling materials with nanofabricated guides gave us that elusive control. And, of course, these new geometries are achieved on an incredibly small scale," said Yager.
"In essence," said Stein, "we've created 'smart' templates for nanomaterial self-assembly. How far we can push the technique remains to be seen, but it opens some very promising pathways."
"Many nano-fabrication labs should be able to do this tomorrow with their in-house tools – the trick was discovering it was even possible," added Gwen Wright, another CFN co-author.
The scientists now plan to increase the sophistication of the process, using more complex materials in order to move toward more device-like architectures.
"The ongoing and open collaboration within the CFN made this possible," said Charles Black, director of the CFN. "We had experts in self-assembly, electron beam lithography and even electron microscopy to characterize the materials, all under one roof, all pushing the limits of nanoscience."
This story is adapted from material from Brookhaven National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.