‘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.

Every atom counts in graphene formation

Mike Williams http://news.rice.edu/2012/09/04/every-atom-counts-in-graphene-formation-2/

Rice University lab’s nanoreactor theory could advance quality of material’s growth

Like tiny ships finding port in a storm, carbon atoms dock with the greater island of graphene in a predictable manner. But until recent research by scientists at Rice University, nobody had the tools to make that kind of prediction.

Rice University researchers have come up with a set of calculations to predict how graphene grows in the process known as chemical vapor deposition. The graph set against an illustration of graphene growing on a nickel catalyst shows the initial energy barrier a carbon atom must overcome to join the bloom; subsequent atoms face an ever-smaller energy barrier until the process begins again for the next line. Vasilii Artyukhov/Rice University

Electric current shoots straight across a sheet of defect-free graphene with almost no resistance, a feature that makes the material highly attractive to engineers who would use it in things like touchscreens and other electronics, said Rice theoretical physicist Boris Yakobson. He is co-author of a new paper about graphene formation to appear this week in the Proceedings of the National Academy of Sciences.
To examine exactly what happens at the atomic level, Yakobson and his Rice colleagues took a close look at the now-common process called chemical vapor deposition (CVD), in which a carbon source heated in a furnace is exposed to a metal catalyst to form graphene, a single-atom layer of pure carbon.
Yakobson, Rice’s Karl F. Hasselmann Professor of Mechanical Engineering and Materials Science and professor of chemistry, and his colleagues calculated the energies of individual atoms as they accrete to form graphene at the “nanoreactor” dock where the carbon vapor and catalyst meet. With the help of theories long applied to crystal growth, they determined that, at equilibrium, some patterns of graphene are more likely to form than others depending on the catalyst used.
One hitch has been that the edge of a graphene sheet dictates how — or even if — current may proceed to an electrode. Grain boundaries — transitions in the hexagons’ angles that appear when islands of graphene merge during growth — can also derail electrons. Yakobson said these edges and boundaries determine the sheet’s overall electronic, mechanical and magnetic properties, so knowing the conditions under which graphene would favor edges that look like zigzags or armchairs – or some angle in between – is important to researchers who want to grow the material for use in electronic components.
Yakobson and his co-authors, research associate Vasilii Artyukhov and graduate student Yuanyue Liu, drew upon their knowledge of crystal growth for their nanoreactor theory. They present a comprehensive model of how atoms migrate from the feedstock – usually a carbon-rich mist in a CVD furnace (and sometimes, famously, a cookie) – to the catalyst and finally to the graphene lattice.

A graphic by Rice researchers shows graphene growth via open-pentagon armchair edges, with atoms joining one by one to form the material’s familiar hexagonal lattice. The researchers analyzed the energies involved in graphene creation in a study that may help experimentalists grow better-quality graphene via chemical vapor deposition. Yakobson Lab/Rice University

“Owing to the talents and efforts of materials chemists, graphene now grows like mold on almost anything, and from almost any feedstock,” Yakobson said. “But how it looks and the shape it takes is hard to understand or predict.
“If you spill a little water on a flat, horizontal table, it will form a little puddle of circular shape, because water is isotropic – all directions are identical, and a circle has the smallest perimeter and therefore the lowest-energy shape,” he said.
But on the nanoscale, carbon atoms don’t always act like water. “When carbon is ‘spilled’ on metal, things get more complicated,” he said. “Different directions dictate different physical properties, and as a result, graphene’s shape can be a polygon or a star or a flower.”
That sounds like the way a crystal grows, a property not lost on the researchers.
“Despite the huge amount of research being done on graphene all over the world, almost nobody so far has treated graphene synthesis as a crystal growth process and taken advantage of the rich theoretical tool set developed in mid-20th century for semiconductor technology,” Artyukhov said. “Crystal growth theory is a large and established field of science, and there are many more concepts that can be applied to graphene synthesis beyond the first steps outlined in our work.”
The ultimate shape of graphene depends on the subtle interplay of energies and speed of growth. Like water, atoms take the path of least resistance, and that path can change due to slight temperature changes and variations in the carbon vapor density.
“As carbon is added in CVD growth, different sides advance with different speeds,” Yakobson said.
The team used density functional theory to calculate the formation of graphene for all possible edge orientations on various catalysts, including nickel, iron, copper and cobalt. They found the energy levels of atoms can be mapped, step by step, as they leave the vapor and join the lattice at a nanoreactor.
A sheet of graphene starts to form when the first few carbon atoms attach to the catalyst and establish a nucleus around which atoms continue to settle. The graphene grows in rows as new atoms are added, but the rows don’t have straight edges. Some have a zigzag pattern, others form a more complex shape that scientists call armchair. The shape of the edge pattern is dictated by the most efficient use of energy. The Rice team found that zigzag edges face a high-energy barrier at the start of a new row, but the rest of the row’s atoms fall into line quickly and easily. For armchairs, the initial barrier is smaller but remains the same for every subsequent atom that docks.
Skewed edges – in between zigzag and armchair – grow fastest of all, because they have the smallest energy barrier to overcome to start or complete a row, Liu said. Also interesting, he said, is the finding that carbon vapor with atom pairs called dimers might prompt faster and better-quality graphene growth.
The researchers found the lagging zigzag edges are a bottleneck that, independent of the metal substrate, helps determine the overall shape of a graphene bloom. Other kinetic factors can also lead to variations that produce stars, flowers or asymmetric shapes.
The researchers were surprised to find that open-pentagon armchair edges are the most likely growth pattern under equilibrium on iron, cobalt and nickel, while zigzag edges were especially pronounced on a copper catalyst. They also found mathematical evidence that certain defects, in which five- and seven-atom polygon pairs replace adjacent hexagons, are unlikely to form except in a vacuum, an unrealistic scenario for graphene growth. That puts the new theory in line with Yakobson’s previous work to show how unlikely defects are to form when growing carbon nanotubes.
Yakobson said the theory advances on one the carbon-growth community considers canonical — thevapor-liquid-solid paradigm — by getting right down to the smallest details.
The work was supported by the Office of Naval Research, the Air Force Office of Scientific Research and the Robert Welch Foundation. Computations were performed with National Science Foundation support using Rice’s Data Analysis and Visualization Cyberinfrastructure (DAVinCI), the Kraken supercomputer at the National Institute for Computational Sciences and the Hopper supercomputer at the National Energy Research Scientific Computing Lab at Lawrence Berkeley National Lab.

Researchers at Rice University and Hong Kong Polytechnic University Demonstrate that Graphene Walls could make Powerful Electronics

(PhysOrg.com) -- To stand a ribbon of graphene upright, it needs diamond on the soles of its shoes

http://www.physorg.com/news/2011-09-graphene-walls-powerful-electronics.html
A new paper by collaborators at Rice University and Hong Kong Polytechnic University demonstrates the possibility that tiny strips of graphene -- one-atom-thick sheets of carbon -- can stand tall on a substrate with a little support. This leads to the possibility that arrays of graphene walls could become ultrahigh density components of electronic or spintronic devices.
The work was published this month in the online edition of the Journal of the American Chemical Society.
Calculations by Rice theoretical physicist Boris Yakobson, Assistant Professor Feng Ding of Hong Kong Polytechnic and their collaborators showed substrates not only of diamond but also nickel could chemically bind the edge of a strip of a graphene nanoribbon. Because the contact is so slight, the graphene walls retain nearly all of their inherent electrical or magnetic properties.
And because they're so thin, Yakobson and Ding calculated a theoretical potential of putting 100 trillion graphene wall field-effect transistors (FETs) on a square-centimeter chip.
That potential alone may make it possible to blow past the limits implied by Moore's Law -- something Yakobson once discussed with Intel founder Gordon Moore himself.
"We met in Montreal, when nano was a new kid on the block, and had a good conversation," said Yakobson, Rice's Karl F. Hasselmann Chair in Engineering and a professor of materials science and mechanical engineering and of chemistry. "Moore liked to talk about silicon wafers in terms of real estate. Following his metaphor, an upright architecture would increase the density of circuits on a chip -- like going from ranch-style houses in Texas to skyscraper condos in Hong Kong.
"This kind of strategy may help sustain Moore's Law for an extra decade," he said.

A sheet of material a fraction of a nanometer wide is pretty pliable, he said, but the laws of physics are on its side. Binding energies between carbon in the diamond matrix and carbon in graphene are maximized at the edge, and the molecules bind strongly at a 90-degree angle. Minimal energy is required for the graphene to stand upright, which is its preferred state. (Walls on a nickel substrate would be angled at about 30 degrees, the researchers found.)

Yakobson said the walls could be as close to each other as 7/10ths of a nanometer, which would maintain the independent electronic properties of individual nanoribbons. They could potentially be grown on silicon, silicon dioxide, aluminum oxide or silicon carbide.
The research illustrated differences between walls made of two distinct types of graphene, zigzag and armchair, so-called because of the way their edges are shaped.
Sheets of graphene are considered semimetals that have limited use in electronics because electrical current shoots straight through without resistance. However, armchair nanoribbons can become semiconductors; the thinner the ribbon, the larger the band gap, which is essential for transistors.
Zigzag nanoribbons are magnetic. Electrons at their opposing edges spin in opposite directions, a characteristic that can be controlled by an electric current; this makes them suitable for spintronic devices.
In both cases, the electronic properties of the walls can be tuned by changing their height.
The researchers also suggested nanowalls could become nanoarches by attaching opposing ends of a graphene ribbon to the substrate. Rather than lie flat on the diamond or nickel surface, the energies at play along the binding edges would naturally force the graphene strip to rise in the middle. It would essentially become a half-nanotube with its own set of potentially useful properties.
Precisely how to turn these two-dimensional building blocks into a three-dimensional device presents challenges, but the payoff is great, Yakobson said. He noted that the research lays the groundwork for subnanometer electronic technology.
More information: Read the abstract at http://pubs.acs.or … 21/ja2037854
Provided by Rice University (news : web)