OT-SpectroscopyNOW blog-Phosphorene: Two-dimensional Raman

 Phosphorene: Two-dimensional Raman

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http://www.spectroscopynow.com/raman/details/highlight/14de3301024/Last-Months-Most-Accessed-Feature-Phosphorene-Two-dimensional-Raman.html

Flat out phosphorus

Raman spectroscopy and transmission electron microscopy have been used by an international team to investigate the phosphorus analogue of graphene, the two-dimensional phosphane, known as phosphorene.

Phosphorene has potential applications in a new class of semiconducting transistor for that perennial aspiration, the ever faster and more powerful computer of the future. Unfortunately, while phosphorene can conduct electrons its ability to do so is anisotropic, meaning it depends on which way you orient it relative to the system as to whether it does so or not. Thus, a quick and simple way to determine the orientation of the material was needed for experimental setups and now, a team comprising researchers from the Massachusetts Institute of Technology, the Rensselaer Polytechnic Institute (RPI) in Troy, New York state, Tohoku University in Japan, Oak Ridge National Laboratory, Tennessee and the University of Pennsylvania, has done just that. There approach accurately determines orientation by examining the interaction between light and electrons within phosphorene or other thin layers of black phosphorus.

Calculated approach

Materials scientists have been studying phosphorene intently since it was first isolated in 2014. RPI's Vincent Meunier and his team confirmed the structure of phosphorene that same year. "This is a really interesting material because, depending on which direction you do things, you have completely different properties," explains Meunier, a phenomenon that might of course be exploited in devices. "But because it's such a new material, it's essential that we begin to understand and predict its intrinsic properties."

Meunier and colleagues have now built on the theoretical modelling and prediction of the properties of phosphorene using Rensselaer's supercomputer in the Center for Computational Innovations (CCI). On the basis of their calculations, they have home in on certain features of this novel material that will ultimately help physicists and materials scientists better understand it and thence technologists make use of those properties.

Writing in the journal ACS Nano Letters, the team initially set out to refine an existing technique for determining the orientation of the crystal using Raman spectroscopy. The team were reviewing their Raman data and spotted a few unexplained inconsistencies. So, they next turned to obtaining images of the orientation of their crystalline samples using Transmission Electron Microscopy (TEM), and lined these up with the "images" gleaned from the Raman results. As a topographic technique, TEM offers a definitive determination of the orientation of a crystal, but takes a lot more effort than recording a Raman spectrum. Nevertheless, the comparison revealed that electron-phonon interactions alone did not account for the orientation of the crystal. And the reason why led the way to yet another anisotropy of phosphorene - that of interactions between photons of light and electrons in the crystal.

Intrinsic anisotropy

The Raman spectrum should be intrinsic to the material and thus show the anisotropy of phosphorene. "But, it turns out that if you shine the light in different directions, you get different results, because the interaction between the light and the electrons in the material - the electron-photon interaction - is also anisotropic, but in a non-commensurate way," explains Meunier. The team suspected that phosphorene was anisotropic with respect to electron-photon interactions, but hadn't quite anticipated the significance of the property. "Usually electron-photon anisotropy doesn’t make such a big difference, but here, because we have such a particular chemistry on the surface and such a strong anisotropy, it's one of those materials where it makes a huge difference," Meunier adds.

Fundamentally, the discovery reveals a limitation in what current interpretation of Raman spectra can achieve in studying these materials. "It turns out that it's not so easy to use Raman vibrations to find out the direction of the crystal," Meunier explains. "But, and this is the beautiful thing, what we found is that the electron-photon interaction (which can be measured by recording the amount of light absorbed) - the interaction between the electrons and the laser - is a good predictor of the direction. Now you can really predict how the material will behave as a function of excitement with an outside stimulus."

Meunier worked with Mildred Dresselhaus of the Massachusetts Institute of Technology, as well as colleagues at Tohoku University in Japan, Oak Ridge National Laboratory, Tennessee and the University of Pennsylvania.

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.

Development of 'Slater Insulator' That Rapidly Changes from Conductor to Insulator at Room Temperature may lead to new Terahertz devices

(Left) Photograph of a crystal of Perovskite type osmium oxide and (right) schematic diagram of its crystal structure. White circles: sodium ions, red circles: oxygen ions. Osmium ions exist in the central part of the octahedron. (Credit: Copyright NIMS)

http://www.sciencedaily.com/releases/2012/07/120711134534.htm

ScienceDaily (July 11, 2012) — Dr. Kazunari Yamaura, a Principal Researcher of the  Strongly Correlated Materials Group, Superconducting Properties Unit, in joint work with a researchNIMS group at the Oak Ridge National Laboratory (United States), has succeeded in developing a Slater insulator which functions at room temperature.

Dr. Kazunari Yamaura, a Principal Researcher of the Strongly Correlated Materials Group, Superconducting Properties Unit, National Institute for Materials Science (NIMS; President: Sukekatsu Ushioda), in joint work with a research group at the Oak Ridge National Laboratory in the United States, succeeded in development of a Slater insulator which functions at room temperature.
Slater insulators have been studied for more than 50 years as insulators with special properties. Although Slater insulators display the properties of metals at a sufficiently high temperature, they become insulators when cooled to a certain temperature (transition temperature) peculiar to the substance concerned. Because this transition temperature was conventionally far lower than room temperature, study had been limited to scientific research, and virtually no research had been done aiming at development to applications.
This research clarified the fact that a new material (Perovskite type osmium oxide), which was synthesized for the first time by NIMS in 2009, is the Slater insulator with the highest transition temperature to date. This result was verified through joint experimental research with a research group at the Oak Ridge National Laboratory in the United States using the neutron diffraction method.
Because this new material displays the characteristics of a Slater insulator at room temperature without requiring cooling, it is not only scientifically interesting, but also has the potential for development to application as a new material. If further progress can be achieved in research with this new material as a starting point, there is a possibility that new materials and devices with unprecedented functions can be developed. Concretely, application to solid state devices for detecting signals in the terahertz region, new thermoelectric conversion materials, etc. is considered possible. In the future, research will be carried out aiming at development of new materials with possible practical applications.