System Bits: Sept. 22

Spintronics materials; invisibility cloak; pillared graphene.

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Scaling up production of thin electronic materials
With potential application in future spintronics applications, among other things, a team led by MIT researchers have developed a way to make large sheets of molybdenum telluride (MoTe2) and other materials like graphene that hold promise for electronic, optical, and other high-tech applications.

The team — which includes MIT postdoc Lin Zhou; professors Mildred Dresselhaus, Jing Kong, and Tomás Palacios; and eight others at MIT, the China University of Petroleum, Central South University in China, the National Tsing-hua University in Taiwan, and Saitama University and Tohoku University in Japan — said their method is also likely to work for many similar 2D materials, and could make widespread applications feasible.

Mildred Dresselhaus and Lin Zhou of MIT (Source: MIT)

Mildred Dresselhaus and Lin Zhou of MIT (Source: MIT)

The material has a similar bandgap to silicon and in single-layer form has a direct bandgap that allows better light emission as well as having strong absorption for solar radiation, which is key to making practical solar cells.

They said molybdenum telluride can exist in two different forms: one is metallic, meaning it conducts electricity well; the other is a natural semiconductor, lending itself to applications in electronics. Controlling how the material is made allows the researchers to create whichever form is needed for a particular use.

The method is based on chemical vapor deposition (CVD), and the researchers said this makes it possible to create sheets of any thickness, and of a size limited only by the dimensions of the CVD chamber used for deposition.

Invisibility cloak
Most of the time, invisibility cloaks are associated with Star Trek and Harry Potter, but now, researchers at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) Berkeley have devised an ultra-thin invisibility “skin” cloak that can conform to the shape of an object and conceal it from detection with visible light.

The researchers pointed out that while this cloak is only microscopic in size, the principles behind the technology should enable it to be scaled-up to conceal macroscopic items as well.

A 3D illustration of a metasurface skin cloak made from an ultrathin layer of nanoantennas (gold blocks) covering an arbitrarily shaped object. Light reflects off the cloak (red arrows) as if it were reflecting off a flat mirror. (Source: Berkeley Lab)

A 3D illustration of a metasurface skin cloak made from an ultrathin layer of nanoantennas (gold blocks) covering an arbitrarily shaped object. Light reflects off the cloak (red arrows) as if it were reflecting off a flat mirror. (Source: Berkeley Lab)

To do this, the Berkeley researchers worked with brick-like blocks of gold nanoantennas to fashion a “skin cloak” barely 80nm thick, then wrapped around a 3D object about the size of a few biological cells and arbitrarily shaped with multiple bumps and dents. The surface of the skin cloak was meta-engineered to reroute reflected light waves so that the object was rendered invisible to optical detection when the cloak is activated.

The team believes this is the first time a 3D object of arbitrary shape has been cloaked from visible light, and given that it is easy to design and implement, it is potentially scalable for hiding macroscopic objects.

The ability to manipulate the interactions between light and metamaterials offers tantalizing future prospects for technologies such as high resolution optical microscopes and superfast optical computers. Invisibility skin cloaks on the microscopic scale might prove valuable for hiding the detailed layout of microelectronic components or for security encryption purposes. At the macroscale, among other applications, invisibility cloaks could prove useful for 3D displays.

Pillared graphene
Rice University researchers have discovered that putting nanotube pillars between sheets of graphene could create hybrid structures with a unique balance of strength, toughness and ductility — throughout all three dimensions.

While carbon nanomaterials are common now as flat sheets, nanotubes and spheres, they are now being eyed for use as building blocks in hybrid structures with unique properties for electronics, heat transport and strength. The team said it is laying a theoretical foundation for such structures by analyzing how the blocks’ junctions influence the properties of the desired materials.

Rice materials scientist Rouzbeh Shahsavari and alumnus Navid Sakhavand calculated how various links, particularly between carbon nanotubes and graphene, would affect the final hybrid’s properties in all directions. They found that introducing junctions would add extra flexibility while maintaining almost the same strength when compared with materials made of layered graphene.

Carbon nanotube pillars between sheets of graphene may create hybrid structures with a unique balance of strength, toughness and ductility throughout all three dimensions, according to Rice University scientists. Five, seven or eight-atom rings at the junctions can force the graphene to wrinkle. (Source: Rice University)