System Bits: July 30

Controlling nanomaterials; water to nanoribbons.


Controlling nanomaterials
To find out why some sets of flat nanocrystals arrange themselves in an alternating, herringbone style even though it wasn’t the simplest pattern, University of Pennsylvania researchers turned to experts in computer simulation at the University of Michigan and the Massachusetts Institute of Technology.

The result of the collaboration gives nanotechnology researchers a new tool for controlling how objects one-millionth the size of a grain of sand arrange themselves into useful materials, it gives a means to discover the rules for “programming” them into desired configurations.

The excitement in this is not in the herringbone pattern, it’s about the coupling of experiment and modeling and how that approach lets researchers take on a very hard problem. Previous work in the University of Pennsylvania’s research group has been focused on creating nanocrystals and arranging them into larger crystal superstructures. Ultimately, researchers want to modify patches on nanoparticles in different ways to coax them into more complex patterns. The goal is developing “programming matter,” that is, a method for designing novel materials based on the properties needed for a particular job.

By engineering interactions at the nanoscale, the researchers believe they can begin to assemble target structures of great complexity and functionality on the macroscale.

The MIT researchers introduced the concept of nanoparticle “patchiness” in 2004 and use computer simulations to understand and design the patches.

Recently, the University of Pennsylvania researchers made patterns with flat nanocrystals made of heavy metals, known to chemists as lanthanides, and fluorine atoms. Lanthanides have valuable properties for solar energy and medical imaging, such as the ability to convert between high- and low-energy light.

The patterns did not line up as expected, which is why they turned to MIT for assistance, which built a computer model that could recreate the self-assembly of the same range of shapes that the UPenn team had produced. The resulting simulations gave them valuable data at a quantum mechanical level.

These transmission electron microscope images show the two different patterns the nanocrystals could be made to pack in. (Source: University of Pennsylvania)

The study showed a way forward making very subtle changes in building block architecture and getting a very profound change in the larger self-assembled pattern. The goal is to have knobs that you can be adjusted to get a big change in structure, and this is one of the first efforts that shows a way forward for how to do that.

Graphene nanoribbons
With the potential for application in future semiconductor manufacturing below the level that current lithography can take us today, researchers at Rice University have shown how water makes it practical to form long graphene nanoribbons less than 10nm wide.

A bit of water adsorbed from the atmosphere was found to act as a mask in a process that begins with the creation of patterns via lithography and ends with very long, very thin graphene nanoribbons. The ribbons form wherever water gathers at the wedge between the raised pattern and the graphene surface.

The water formation is called a meniscus; it is created when the surface tension of a liquid causes it to curve. In the Rice process, the meniscus mask protects a tiny ribbon of graphene from being etched away when the pattern is removed.

The researchers said any method to form long wires only a few nanometers wide should catch the interest of microelectronics manufacturers as they approach the limits of their ability to miniaturize circuitry. And to be able to pattern a line this thin right where it is wanted is a big deal because it permits chip makers to take advantage of the smallness in size of nanoscale devices.

Water’s tendency to adhere to surfaces is often annoying, but in this case it’s essential to the process. There are big machines that are used in electronics research that are often heated to hundreds of degrees under ultrahigh vacuum to drive off all the water that adheres to the inside surfaces, otherwise there’s always going to be a layer of water. In these experiments, water accumulates at the edge of the structure and protects the graphene from the reactive ion etching (RIE). So in this case, that residual water is the key to success.

~Ann Steffora Mutschler

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