Team creates LEDs, photovoltaic cells, and light detectors using novel one-molecule-thick material.
CAMBRIDGE, Mass. -- Researchers at MIT have developed an extremely fast way to put tiny, regular nanoscale patterns on the surfaces of thin films.
The chemical process, akin to instantaneously printing very precise miniature wallpaper, was described in an article by Cheolmin Park, graduate student in MIT's Program in Polymer Science and Technology; Edwin Thomas, Morris Cohen Professor of Materials Science; and colleagues in the May 25 issue of Nature.
These nanostructures may be used in the electronics industry in manufacturing patterning processes, or to generate integrated circuits and component structures in the completed devices.
Nanoscale patterned materials play an important role in photonics, biology and in creating high-density magnetic recording devices. Rectangular arrays of cylindrical polymer posts -- a sort of a forest of poles -- have been used as synthetic media for separation processes involving DNA.
The MIT researchers have found that by carefully controlling the molecular weight and volume fraction of block copolymers (a type of polymer made up of two or more different monomer units linked together in blocks), nanostructures with highly ordered features can be produced.
Polymers are widely used because they can form films at low cost, are easy to fabricate and can be synthesized and modified for specific functions.
The key for engineering applications of these materials is fast, simple processing methods that induce large areas on the films to line up in well-ordered two-dimensional microstructures.
The researchers aimed to create a thin film with periodically spread, vertically oriented cylindrical shapes. The goal is to end up with a chemically patterned surface that is virtually defect-free over large areas.
Block copolymers consist of chemically distinct macromolecules in a single chain. Dissimilar blocks repulse each other and tend to segregate. Different molecular weights of the blocks result in different size and spacing of shapes, resulting in periodic arrangements of spheres, cylinders and wall-like plates on the films.
Another advantage to block copolymers over standard lithographic techniques is that their patterns form by self-assembly, reducing the number of process steps.
Patterning with block copolymers is not new, but to get a well-ordered structure, growth must be exceedingly slow, on the order of millimeters a day. Electric fields, temperature gradients and patterned substrates have been used to get this kind of control.
In the past, another stumbling block to the use of block copolymers for these applications was the lack of control over the orientation of the tiny structures that form within the films. In contrast, the MIT process, developed with researchers Claudio DeRosa from the University of Naples, Italy, and Bernard Lotz from Centre National de la Recherche Scientifique (CNRS) in France, quickly results in a film that is both chemically and structurally regular.
"The new method is based on the formation of a semicrystalline block copolymer film with crystalline organic solvent," Thomas said. "These methods have been applied in the metals processing industry for more than 50 years, but have until now been overlooked by polymer processors.
"The crystalline organic solvent dissolves the block copolymer above its melting temperature. When the temperature decreases below the crystallization temperature of the organic solvent, the organic solvent becomes a crystalline substrate onto which the block copolymer forms well-aligned domains. This simple and fast process creates the orientation of block copolymer microdomains over a large area within a few seconds."
This work was funded by the National Science Foundation, the American Chemical Society-Petroleum Research Fund and the US-France NSF-CNRS.