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.

Nanoscale optical switch breaks miniaturization barrier

by David Salisbury | Posted on Thursday, Mar. 13, 2014 — 2:40 PM

http://news.vanderbilt.edu/2014/03/nanoscale-optical-switch/

Graduate student Kent Hallman checking the sample alignment the vapor deposition machine located in Vanderbilt Institute for Nanoscale Science and Engineering's clean room. (Joe Howell / Vanderbilt)

An ultra-fast and ultra-small optical switch has been invented that could advance the day when photons replace electrons in the innards of consumer products ranging from cell phones to automobiles.

The new optical device can turn on and off trillions of times per second. It consists of individual switches that are only one five-hundredth the width of a human hair (200 nanometers) in diameter. This size is much smaller than the current generation of optical switches and it easily breaks one of the major technical barriers to the spread of electronic devices that detect and control light: miniaturizing the size of ultrafast optical switches.

Physicist Richard Haglund has been studying the properties of vanadium dioxide for more than 20 years. (Joe Howell / Vanderbilt)

The new device was developed by a team of scientists from Vanderbilt University, University of Alabama-Birmingham, and Los Alamos National Laboratory and is described in the March 12 issue of the journal Nano Letters.

The ultrafast switch is made out of an artificial material engineered to have properties that are not found in nature. In this case, the “metamaterial” consists of nanoscale particles of vanadium dioxide (VO2) – a crystalline solid that can rapidly switch back and forth between an opaque, metallic phase and a transparent, semiconducting phase – which are deposited on a glass substrate and coated with a “nanomesh” of tiny gold nanoparticles.

The scientists report that bathing these gilded nanoparticles with brief pulses from an ultrafast laser generates hot electrons in the gold nanomesh that jump into the vanadium dioxide and cause it to undergo its phase change in a few trillionths of a second.

“We had previously triggered this transition in vanadium dioxide nanoparticles directly with lasers and we wanted to see if we could do it with electrons as well,” said Richard Haglund, Stevenson Professor of Physics at Vanderbilt, who led the study. “Not only does it work, but the injection of hot electrons from the gold nanoparticles also triggers the transformation with one fifth to one tenth as much energy input required by shining the laser directly on the bare VO2.”

Left: Illustration of terahertz optical switches shows the vanadium dioxide nanoparticles coated with a "nanomesh" of smaller gold particles. Right: Scanning electron microscope image of the switches at two resolutions. (Haglund Lab / Vanderbilt)

Both industry and government are investing heavily in efforts to integrate optics and electronics, because it is generally considered to be the next step in the evolution of information and communications technology. Intel, Hewlett-Packard and IBM have been building chips with increasing optical functionality for the last five years that operate at gigahertz speeds, one thousandth that of the VO2 switch.

“Vanadium dioxide switches have a number of characteristics that make them ideal for optoelectronics applications,” said Haglund. In addition to their fast speed and small size, they:

• Are completely compatible with current integrated circuit technology, both silicon-based chips and the new “high-K dielectric” materials that the semiconductor industry is developing to continue the miniaturization process that has been a major aspect of microelectronics technology development;
• Operate in the visible and near-infrared region of the spectrum that is optimal for telecommunications applications;
• Generate an amount of heat per operation that is low enough so that the switches can be packed tightly enough to make practical devices: about ten trillionths of a calorie (100 femtojoules) per bit.

“Vanadium dioxide’s amazing properties have been known for more than half a century. At Vanderbilt, we have been studying VO2 nanoparticles for the last ten years, but the material has been remarkably successfully at resisting theoretical explanations,” said Haglund. “It is only in the last few years that intensive computational studies have illuminated the physics that underlies its semiconductor-to-metal transition.”

Graduate student Christina McGahan holding a disk on which centimeter square samples are grown. (Joe Howell / Vanderbilt)

Vanderbilt graduate students Kannatassen Appavoo and Joyeeta Nag fabricated the metamaterial at Vanderbilt; Appavoo joined forces with University of Alabama, Birmingham graduate student Nathaniel Brady and Professor David Hilton to carry out the ultrafast laser experiments with the guidance of Los Alamos National Laboratory staff scientist Rohit Prasankumar and postdoctoral scholar Minah Seo. The theoretical and computational studies that helped to unravel the complex mechanism of the phase transition at the nanoscale were carried out by postdoctoral student Bin Wang and Sokrates Pantelides, University Distinguished Professor of Physics and Engineering at Vanderbilt.

The university researchers were supported by Defense Threat-Reduction Agency grant HDTRA1-0047, U.S. Department of Energy grant DE-FG02-01ER45916, U.S. Department of Education GAANN Fellowship P200A090143 and National Science Foundation grant DMR-1207241. Portions of the research were performed at the Vanderbilt Institute of Nanoscale Science and Engineering in facilities renovated with NSF grant ARI-R2 DMR-0963361, at the Center for Integrated Nanotechnologies at Los Alamos National Laboratory under USDOE contract DE-AC52-06NA25396) and at Sandia National Laboratories under USDOE contract DE-AC04-94AL85000).