Portland, Ore. A novel processing technique that combines known molecules to realize a new class of synthesized material has enabled 10-nanometer precision lithography. The invention enables the lithographic-like self-assembly of molecules into one-, two- or three-dimensional nanoscale structures by combining a block copolymer with a dendrimer. The latter is a "cascade molecule" in which the atoms are arrayed along a backbone of carbon.
"In our experiment we demonstrated 10-nm feature sizes, but we envision our invention working with traditional lithography to encode information into a material that enables it to self-assemble into domains with angstrom-scale precision," said Ulrich Wiesner, professor of materials science and engineering at Cornell University. He performed the work at the university with the help of physics professor Sol Gruner, director of the Cornell High Energy Synchrotron Source; postdoctoral researcher Byoung-Ki Cho; and doctoral candidate Anurag Jain.
The researchers said their invention could lead to ultraprecise nanoscale features that improve the efficiency of batteries, solar cells and fuel cells.
In living organisms, molecules begin with individual one-dimensional properties that spontaneously self-assemble into 2- and then 3-D structures. By a similar careful crafting of "seed" molecules, the final two- and three-dimensional characteristics of a material can be preordained to self-assemble with nanoscale precision.
The researchers demonstrated the process by combining a block copolymer molecule with a dendrimer molecule. The result was a new class of synthetic macromolecule dubbed an extended amphiphilic dendron by its inventors. Amphiphilic dendrons have a polar, water-soluble group attached to a nonpolar, water-insoluble hydrocarbon chain. As a result, they exhibit a combination of behaviors that has been widely sought after worldwide for over a decade.
"The first block copolymers were synthesized in the 1950s, and the treelike macromolecules called dendrimers were synthesized in the 1980s and 1990s. But we are the first group to put them together so that the best characteristics of each are combined," said Wiesner.
The researchers demonstrated a range of nanoscale structures that were precise to 10 nm and were preordained to self-assemble from amphiphilic dendrons. Among them were nanoscale-precise continuous three-dimensional cubes, double sandwiches and cylinders.
"In essence, we can program amphiphilic dendrons encode information into them that determines how they will self-assemble into precise two- and three-dimensional nanoscale structures," said Wiesner.
In general, the dendrons begin as individual molecules that change shape as their temperature is elevated. At each phase, a stable, predictable structural change results, enabling self-assembly steps to be programmed that successively build up to three dimensions. This is done without layering two-dimensional structures to achieve 3-D, as is normal with traditional lithographic techniques for chip making.
In the demonstration, the researchers showed cylinders that phase-transitioned into two-dimensional lamellae and then into three-dimensional cubes. When doped with lithium salts, an ion-transport mechanism was enhanced to propel charge carriers along nanometer-sized, self-assembled channels.
"We think that batteries, fuel cells and solar cells will all be able to refine their nanoscale features to enhance the conductivity of their underlying materials," said Wiesner.
Next, the team will investigate using induced self-assembly to create a "supramolecular switch" that dramatically changes conductivity in response to slight changes in temperature, thereby enabling a supersensitive temperature sensor.
Wiesner's work was supported by the National Science Foundation's Cornell Center for Materials Research.