Manufacturing Bits: April 29

Silky e-beam lithography; making exotic nanowires; new self-assembly materials.

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Silky e-beam lithography
Tufts University has put a soft and silky spin on direct-write electron-beam lithography. Researchers used common silk as the resist material, enabling the production of photonic lattices, quantum dots and other structures.

This approach is a green alternative to traditional and toxic resists. The silk-based resist is developed using a water-based process. It starts with the silk aqueous solution and ends with the development of the exposed silk film in water.

Optical-grade silk fibroin aqueous solution, obtained from the cocoons of the Bombyx mori caterpillar, was placed on a substrate and spin-coated to form a silk film. (Source: Tufts)

Optical-grade silk fibroin aqueous solution, obtained from the cocoons of the Bombyx mori caterpillar, was placed on a substrate and spin-coated to form a silk film. (Source: Tufts)

Silk has a polymorphic crystalline structure, which enables the material to be used as either as a positive or negative resist. Silk can be modified, enabling a variety of functional resists, such as biologically active versions.

Using water-based silk e-beam lithography, researchers fabricated nanoscale photonic lattices using both neat silk and silk doped with quantum dots, green fluorescent proteins and horseradish peroxidase (HRP). Found in the roots of plant horseradish, HRP is used in biochemistry applications. It is used to amplify a weak signal and increase detectability of a target molecule.

In the past, researchers showed that silk could be nanofabricated, but it required other nano materials. This is the first time that silk has been fabricated to begin the fabrication manufacturing chain.

“In a world that strives to reduce toxic footprints associated with manufacturing, our laboratory is exploring biopolymers, and silk in particular, as a candidate material to replace plastics in many high-technology applications,” said Fiorenzo Omenetto, the Frank C. Doble Professor of Biomedical Engineering at Tufts, on the university’s Web site.

“By showing that biomolecules of the enzyme HRP remained active after the electron beam nanofabrication process, we demonstrated the feasibility of fabricating biologically active silk sensing devices, something not currently available,” added Benedetto Marelli, a post-doctoral associate in Omenetto’s laboratory.

Making exotic interconnects
Two-dimensional materials continue to generate interest in R&D labs. The 2D materials include graphene, boron nitride (BN) and the transition-metal dichalcogenides (TMDs).

Two materials, molybdenum diselenide and molybdenum disulfide, belong to a class of TMD materials. Both are attractive materials for use in future field-effect transistors (FETs).

Using a beam of electrons, Vanderbilt University has developed flexible metallic nanowires that are only three atoms wide. The wires are made of molybdenum disulfide, which is a strong material with a high electron mobility.

Series of still scanning electron micrographs (a to d) show how the electron beam is used to create nanowires. (Source:  Vanderbilt)

Series of still scanning electron micrographs (a to d) show how the electron beam is used to create nanowires. (Source: Vanderbilt)

Others have already created functioning FETs based on TMD materials. Vanderbilt has provided another piece of the puzzle by developing the interconnects with the materials.

Researchers from the university have devised a way to create 3D circuits by stacking monolayers of these materials. They showed that in-situ scanning transmission electron microscopy (TEM) can be used to follow the structural transformation between semiconducting (2H) and metallic (1T) phases in single-layered molybdenum disulfide with atomic resolution.

They also showed that areas of the 1T phase can be controllably grown in a layer of the 2H phase using an electron beam. “This will likely stimulate a huge research interest in monolayer circuit design,” said Junhao Lin, a Vanderbilt Ph.D. student, on the university’s site. “Because this technique uses electron irradiation, it can in principle be applicable to any kind of electron-based instrument, such as electron-beam lithography.”

New self-assembly materials
Brookhaven National Laboratory has devised a class of self-assembling materials, which can organize themselves at feature sizes below 10nm.

The materials are known as giant surfactants. These large molecules mimic the features of small surfactants, which are compounds that lower the surface tension between two liquids or between a liquid and a solid.

The larger structures have been transformed into functional molecular nanoparticles. This is done by clicking them with other polymer chains. The resulting materials bridge the gap between small molecule surfactants and traditional block copolymers.

“The controlled structural variations of these giant surfactants through precision synthesis further reveal that their self-assemblies are remarkably sensitive to primary chemical structures, leading to highly diverse, thermodynamically stable nanostructures with feature sizes around 10nm or smaller in the bulk, thin-film, and solution states, as dictated by the collective physical interactions and geometric constraints,” according to researchers.

These findings could propel the understanding the chemical and physical principles of self-assembly. They could also pave the way for technology in applications such as nanopatterning technology and microelectronics.