Tailored flexible illusion coatings hide objects from detection

Antenna covered with copper patterned dielectric substrate creates a flexible metasurface that acts as an illusion coating, cloaking the antenna or making it appear to be something entirely different.Image: Zhihao Jiang/Penn State

By A'ndrea Elyse Messer

UNIVERSITY PARK, Pa. -- Developing the cloak of invisibility would be wonderful, but sometimes simply making an object appear to be something else will do the trick, according to Penn State electrical engineers.

"Previous attempts at cloaking using a single metasurface layer were restricted to very small-sized objects," said Zhi Hao Jiang, postdoctoral fellow in electrical engineering, Penn State. "Also, the act of cloaking would prevent an enclosed antenna or sensor from communicating with the outside world."

Jiang and Douglas H. Werner, John L. and Genevieve H. McCain Chair Professor of Electrical Engineering, developed a metamaterial coating with a negligible thickness that allows coated objects to function normally while appearing as something other than what they really are, or even completely disappearing. They report their research in Advanced Functional Materials.

The researchers employ what they call "illusion coatings," coatings made up of a thin flexible substrate with copper patterns designed to create the desired result. They can take a practical size metal antenna or sensor, coat it with the patterned film and when the device is probed by a radio frequency source, the scattering signature of the enclosed object will appear to be that of a prescribed dielectric material like silicon or Teflon. Conversely, with the proper pattern, they can coat a dielectric and it will scatter electromagnetic waves the same as if it were a metal object.

"The demonstrated illusion/cloaking coating is a lightweight two-dimensional metasurface, not a bulky three-dimensional metamaterial," said Werner.

The researchers take the object they want cloaked and surround it with a spacer, either air or foam. They then apply the ultrathin layer of dielectric with copper patterning designed for the wavelengths they wish to cloak. In this way, antennae and sensors could be made invisible or deceptive to remote inspection.

Another application of this material would be to protect objects from other emitting objects nearby while still allowing electromagnetic communication between them. This was not possible with the conventional transformation optics-based cloaking method because the cloaking mechanism electromagnetically blocked the cloaked object from the outside, but this new coating allows the object surrounded to continue working while being protected. In an array of antennae, for example, interference from the nearby antennas can be suppressed.

The metasurface coating consists of a series of copper, geometric patterns placed on a flexible substrate using standard lithographic methods currently used to create printed circuit boards. Each illusion coating must be designed for the specific application, but the designs are optimized mathematically. This method of manufacture is low cost and well established.

Another advantage of this method is that it works not only for direct hits by radio frequency waves incident normally on the coated object, but also continues to operate properly within a 20 degree field of view, making it a better angle-tolerant shield than previous attempts that employed bulky metamaterials. Currently, the metasurface coatings only work on narrow bands of the spectrum for any application, but can be adapted to work in other bands of the electromagnetic spectrum including the visible spectrum.

"We haven't tried expanding the bandwidth yet," said Werner. "But the theory suggests that it should be possible and it will probably require multiple layers with different patterns to do that."

Illusion coatings could be used for things other than hiding. They could enhance the way radio frequency ID tags work or could redistribute energy in different, controlled patterns making things more visible rather than less visible. The materials shielding ability can also be used to protect any type of equipment from stray or intentional electromagnetic interference.

The National Science Foundation supported this work through a Materials Research Science and Engineering Center at Penn State.

Process for Producing Layered 2-D Materials Determines Their Electronic Properties

By Dexter Johnson

http://spectrum.ieee.org/nanoclast/semiconductors/materials/process-for-producing-layered-2d-materials-determines-their-electronic-properties

This year, we’ve seen the emergence of different types of transistors being produced entirely from layered two-dimensional (2-D) materials featuring the dichalcogenides, tungsten diselenide (WSe2) and molybdenum disulfide(MoS2). Researchers at Argonne National Laboratory in Illinois produced a transparent thin-film transistor (TFT) with WSe2 as the semiconducting layer, graphene for the electrodes, and hexagonal boron nitride as the insulator. At about the same time, researchers at Lawrence Berkeley National Laboratory in California built an all 2-D transistor that took the shape of a field emission transistor (FET), with MoS2 as the semiconducting layer.

Now, in a collaborative effort, researchers at Rice University, Oak Ridge National Laboratory, Vanderbilt University, and Pennsylvania State University have developed a novel method for producing these hybrid layered 2-D structures. Their technique, they report, provides a high degree of control on how the resulting devices perform.

In research published in the journal Nature Materials, the researchers demonstrated how, by altering the temperature the materials are exposed to during the chemical vapor deposition (CVD) process used to produce these 2-D layered devices, they could yield either an in-plane monolayer composite, which has a small but stable band gap, or a stacked layered hybrid, which exhibits enhanced photoluminescence. At high temperatures, the researchers got vertically stacked bilayers of MoS2 and WSe2, with the tungsten on top. At lower temperatures, the two 2-D materials grew side by side.

“With the advent of 2-D layered materials, people are trying to build artificial structures using graphene and now dichalcogenides as building blocks,” said Pulickel Ajayan of Rice University in a press release. “We show that depending on the conditions, we can combine two dichalcogenides to grow either in-plane hybrid or in stacks.”

“What’s even more interesting is that the layered structure has a particular lock-in stacking order,” said Wu Zhou of Oak Ridge National Laboratory in the news release. “When you stack 2-D materials by transferring layers, there’s no way to control their orientation to one another. That impacts their electronic properties. In this paper, we demonstrate that in a certain window, we can get a particular stacking order during growth, with a particular orientation.”

Ajayan has characterized the development as “pixel engineering” because atomically thin semiconductors could be manipulated in production so that their potential uses in optoelectronics are almost limitless.

“We should be able to tweak certain regions to control certain functions, like light or terahertz emission,” said Rice's Robert Vajtai, another of the study's coauthors, in the release. “The whole idea, really, is to create domains with different electronic characters within a single layer.”

‘Pixel’ engineered electronics have growth potential

This atomic-resolution image shows the sharp line between tungsten disulfide (brighter) and molybdenum disulfide (darker) in a new material made in a collaboration led by Rice University. The two-dimensional material shows promise for nanoelectronic applications. Courtesy of Oak Ridge National Laboratory - 
http://news.rice.edu/2014/09/29/pixel-engineered-electronics-have-growth-potential-2/
Mike Williams

Rice, Oak Ridge, Vanderbilt, Penn scientists lead creation of atom-scale semiconducting composites

A little change in temperature makes a big difference for growing a new generation of hybrid atomic-layer structures, according to scientists at Rice University, Oak Ridge National Laboratory, Vanderbilt University and Pennsylvania State University.

Rice scientists led the first single-step growth of self-assembled hybrid layers made of two elements that can either be side by side and one-atom thick or stacked atop each other. The structure’s final form can be tuned by changing the growth temperature.

Stacked (top) and in-plane nanomaterials self-assemble in two ways, depending on the temperature at which they're grown, according to Rice University researchers who led the project. The semiconducting materials show promise for a new generation of "pixel" electronics. In the illustration, green spheres are tungsten, purple are molybdenum and yellow are sulfur. Courtesy of the Ajayan Group

The discovery reported online this week in Nature Materials could lead to what Rice materials scientist Pulickel Ajayan calls “pixel engineering”: atomically thin semiconductors with no limit to their potential for use in optoelectronic devices.

The researchers led by Ajayan and Wu Zhou, a materials scientist at Oak Ridge, discovered the interesting new composites when they combined the growth of two-dimensional molybdenum disulfide and tungsten disulfide through chemical vapor deposition. In this process, specific gases are heated in a furnace, where their atoms gather in an orderly fashion around a catalyst to form the crystalline material.

High-temperature growth – about 850 degrees Celsius (1,563 degrees Fahrenheit) – yielded vertically stacked bilayers, with tungsten on top. At lower temperatures, about 650 degrees C (1,202 degrees F), the crystal lattices preferred to grow side by side. The interfaces in either material are sharp and clean, as seen under a scanning electron microscope and in spectroscopic studies.

“With the advent of 2-D layered materials, people are trying to build artificial structures using graphene and now dichalcogenides as building blocks,” Ajayan said. Because graphene is atomically thin and flat and dichalcogenides like molybdenum disulfide are not quite that flat, there is some incompatibility when these are grown together — but two dichalcogenides with different compositions could be compatible. “We show that depending on the conditions, we can combine two dichalcogenides to grow either in-plane hybrid or in stacks.”

An image of a vertically stacked heterostructure of tungsten disulfide (bottom) and molybdenum disulfide, created in an experiment at Rice University. The structures self assemble in a furnace at 850 degrees Celsius. Lower temperatures formed single-layer heterostructures. The structure is about 20 microns across. Courtesy of the Ajayan Group

The monolayer composites have small but stable band gaps, while the stacked composite layers show modified electronic properties such as enhanced photoluminescence, which will be useful for electronics that rely on optical signals.

“What’s even more interesting is that the layered structure has a particular lock-in stacking order,” Zhou said. “When you stack 2-D materials by transferring layers, there’s no way to control their orientation to one another. That impacts their electronic properties. In this paper, we demonstrate that in a certain window, we can get a particular stacking order during growth, with a particular orientation.”

The new materials could be used for vertically stacked field-effect transistors as well as electronic devices only a few atoms thick, he said.

“We should be able to tweak certain regions to control certain functions, like light or terahertz emission,” said Robert Vajtai of Rice, a co-author of the study. “The whole idea, really, is to create domains with different electronic characters within a single layer.”

“Our goal is to build fully functional electronic devices on a single plane, or maybe a few layers,” added Mauricio Terrones, a co-author from Penn State. “What we’ve accomplished means that pretty much any architecture for devices is now possible on a single atomic layer. And that’s remarkable.”

An image of an in-plane heterostructure of tungsten disulfide and molybdenum disulfide, created in an experiment at Rice University. The structures self assemble in a furnace at 650 degrees Celsius. Higher temperatures formed stacked heterostructures. The structure is about 20 microns across. Courtesy of the Ajayan Group

Co-authors are graduate students Yongji Gong, Gang Shi, Sidong Lei and Gonglan Ye and postdoctoral researcher Xiaolong Zou; Jun Lou, an associate professor and associate department chair of materials science and nanoengineering; and Boris Yakobson, the Karl F. Hasselmann Professor of Materials Science and NanoEngineering and a professor of chemistry, all of Rice; Junhao Lin and Sokrates Pantelides of Oak Ridge and Vanderbilt University; Xingli Wang, Beng Kang Tay and Zheng Liu of Nanyang Technological University, Singapore; graduate student Zhong Lin of Pennsylvania State University; and Humberto Terrones, the Rayleigh Endowed Chair Professor of Physics at Rensselaer Polytechnic Institute.

Vajtai is a faculty fellow at Rice. Mauricio Terrones is a professor of physics, chemistry, materials science and engineering at Penn State. Ajayan is Rice’s Benjamin M. and Mary Greenwood Anderson Professor in Mechanical Engineering and Materials Science and of chemistry and chair of the Department of Materials Science and NanoEngineering.

The Army Research Office, the Department of Energy, the National Science Foundation, the Microelectronics Advanced Research Corp., the Defense Advanced Research Projects Agency, the U.S. Office of Naval Research and the Ministry of Education Academic Research Fund and Silicon Technologies Center of Excellence, Singapore, supported the research.