Researchers develop new technique for making graphene competitor molybdenum disulfide

"Seeding" the growth of molybdenum disulfide flakes gave the researches enough control over their location to spell a message.

by Evan Lerner

 http://phys.org/news/2015-02-technique-graphene-competitor-molybdenum-disulfide.html#jCp

Graphene, a single-atom-thick lattice of carbon atoms, is often touted as a replacement for silicon in electronic devices due to its extremely high conductivity and unbeatable thinness. But graphene is not the only two-dimensional material that could play such a role.

University of Pennsylvania researchers have made an advance in manufacturing one such material, molybdenum disulfide. By growing flakes of the material around "seeds" of molybdenum oxide, they have made it easier to control the size, thickness and location of the material.

Unlike graphene, molybdenum disulfide has an energy band gap, meaning its conductivity can be turned on and off. Such a trait is critical for semiconductor devices used in computing. Another difference is that molybdenum disulfide emits light, meaning it could be used in applications like LEDs, self-reporting sensors and optoelectronics.

The study was led by A. T. Charlie Johnson, professor in the Department of Physics & Astronomy in Penn's School of Arts & Sciences, and includes members of his lab, Gang Hee Han, Nicholas Kybert, Carl Naylor and Jinglei Ping. Also contributing to the study was Ritesh Agarwal, professor of materials science and engineering in Penn's School of Engineering and Applied Science; members of his lab, Bumsu Lee and Joohee Park; and Jisoo Kang, a master's student in Penn's nanotechnology program. They collaborated with researchers from South Korea's Sungkyunkwan University, Si Young Lee and Young Hee Lee.       

Their study was published in the journal Nature Communications.

"Everything we do with regular electronics we'd like to be able to do with two-dimensional materials," Johnson said. "Graphene has one set of properties that make it very attractive for electronics, but it lacks this critical property, being able to turn on and off. Molybedenum disulfide gives you that."

Graphene's ultra-high conductivity means that it can move electrons more quickly than any known material, but that is not the only quality that matters for electronics. For the transistors that form the basis for modern computing technology, being able to stop the flow of electrons is also critical.    

"Molybedenum disulfide is not as conductive as graphene," Naylor said, "but it has a very high on/off ratio. We need 1's and 0's to do computation; graphene can only give us 1's and .5's."  

Other research groups have been able to make small flakes of molybdenum disulfide the same way graphene was first made, by exfoliating it, or peeling off atomically thin layers from the bulk material. More recently, other researchers have adopted another technique from graphene manufacture, chemical vapor deposition, where the molybdenum and sulfur are heated into gasses and left to settle and crystalize on a substrate.  

The problem with these methods is that the resulting flakes form in a scattershot way.

"Between hunting down the flakes," said Kybert, "and making sure they're the right size and thickness, it would take days to make a single measurement of their properties"

The Penn team's advance was in developing a way to control where the flakes form in the chemical vapor deposition method, by "seeding" the substrate with a precursor.

"We start by placing down a small amount of molybdenum oxide in the locations we want," Naylor said, "then we flow in sulfur gas. Under the right conditions, those seeds react with sulfur and flakes of molybdenum disulfide being to grow."

"There's finesse involved in optimizing the growth conditions," Johnson said, "but we're exerting more control, moving the material in the direction of being able to make complicated systems. Because we grow it where we want it, we can make devices more easily. We have all of the other parts of the transistors in a separate layer that we snap down on top of the flakes, making dozens and potentially even hundreds, of devices at once. Then we were able to observe that we made transistors that turned on and off like they were supposed to and devices that emit light like they are supposed to."

Being able to match up the location of the molybdenum disulfide flakes with corresponding electronics allowed the researchers to skip a step they must take when making graphene-based devices. There, graphene is grown in large sheets and then cut down to size, a process that adds to the risk of damaging contamination.  

Future work on these molybdenum disulfide devices will complement the research team's research on graphene-based biosensors; rather than outputting the detection of some molecule to a computer, molybdenum disulfide-based sensors could directly report a binding event through a change in the light they emit.

This research also represents first steps that can be applied toward fabricating a new family of two-dimensional materials.

"We can replace the molybdenum with tungsten and the sulfur with selenium," Naylor said, "and just go down the periodic table from there. We can imagine growing all of these different materials in the places we choose and taking advantages of all of their different properties."

 Explore further: Molybdenum disulfide may find new application for thin-film transistors in extremely high-temperature electronics

More information: "Seeded growth of highly crystalline ​molybdenum disulphide monolayers at controlled locations." Nature Communications 6, Article number: 6128DOI: 10.1038/ncomms7128

Journal reference: Nature Communications  

Provided by University of Pennsylvania  

Abstract-Tailoring the Spectroscopic Properties of Semiconductor Nanowires via Surface-Plasmon-Based Optical Engineering

Carlos O. Aspetti and Ritesh Agarwal *

Department of Materials Science and Engineering,University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States

J. Phys. Chem. Lett., 2014, 5, pp 3768–3780

DOI: 10.1021/jz501823d

Publication Date (Web): October 10, 2014

Copyright © 2014 American Chemical Society

*E-mail: riteshag@seas.upenn.edu

http://pubs.acs.org/doi/abs/10.1021/jz501823d

Semiconductor nanowires, due to their unique electronic, optical, and chemical properties, are firmly placed at the forefront of nanotechnology research. The rich physics of semiconductor nanowire optics arises due to the enhanced light–matter interactions at the nanoscale and coupling of optical modes to electronic resonances. Furthermore, confinement of light can be taken to new extremes via coupling to the surface plasmon modes of metal nanostructures integrated with nanowires, leading to interesting physical phenomena. This Perspective will examine how the optical properties of semiconductor nanowires can be altered via their integration with highly confined plasmonic nanocavities that have resulted in properties such as orders of magnitude faster and more efficient light emission and lasing. The use of plasmonic nanocavities for tailored optical absorption will also be discussed in order to understand and engineer fundamental optical properties of these hybrid systems along with their potential for novel applications, which may not be possible with purely dielectric cavities.

Nanoplasmonic 'whispering gallery' breaks emission time record in semiconductors

Image via Wikipedia

http://www.physorg.com/news/2011-07-nanoplasmonic-gallery-emission-semiconductors.html
Renaissance architects demonstrated their understanding of geometry and physics when they built whispering galleries into their cathedrals. These circular chambers were designed to amplify and direct sound waves so that, when standing in the right spot, a whisper could be heard from across the room. Now, scientists at the University of Pennsylvania have applied the same principle on the nanoscale to drastically reduce emission lifetime, a key property of semiconductors, which can lead to the development of new ultrafast photonic devices.
The research was conducted by associate professor Ritesh Agarwal, postdoctoral fellows Chang-Hee Cho and Sung-Wook Nam and graduate student Carlos O. Aspetti, all of the Department of Materials Science and Engineering in Penn's School of Engineering and Applied Science. Michael E. Turk and James M. Kikkawa of the Department of Physics and Astronomy in the School of Arts and Sciences also contributed to the study.
Their research was published in the journal Nature Materials.
"When you excite a semiconductor, then it takes a few nanoseconds to get back to the ground state accompanied by emission of light," Agarwal said. "That's the emission lifetime. It's roughly the amount of time the light is on, and hence is the amount of time it takes for it to be ready to be turned on again.

"The previous state of the art was taking a nanowire, just like ours, and laying it on a metal surface," Agarwal said. "We curved the metal surface around the wire, making a complete nanoscale plasmonic cavity and the whispering gallery effect."
For certain nanowire sizes, the silver coating creates pockets of resonance and hence highly confined electromagnetic fields within the nanostructure. Emission lifetime can then be engineered by precisely controlling high intensity electromagnetic fields inside the light-emitting medium, which is the cadmium sulfide core.
To reach an emission lifetime measured in femtoseconds, the researchers needed to optimally balance this high-confinement electromagnetic field with an appropriate "quality factor," the measurement of how good a cavity is at storing energy. To complicate matters, quality factor and confinement have an inverse relationship; the higher the quality-factor a cavity has the bigger it is and the smaller its confinement. However, by opting for a reasonable quality factor, the researchers could vastly increase the confinement of the electric field inside the nanowire by using resonant surface plasmons and get the record-breaking emission lifetime.
This many-orders-of-magnitude improvement could find a home in a variety of applications such as LEDs, detectors and other nanophotonic devices with novel properties.
"Plasmonic computers could make good use of these nanowires," Cho said. "We could increase modulation speed into the terahertz range whereas electronic computers are limited to a few gigahertz range."
"The same physics governs emission and absorption, so these nanowires could also be used for increasing efficiency of absorption in solar cells," Agarwal said.
Provided by University of Pennsylvania (news : web)

"If you're making a modulator, something that switches back and forth, you're limited by this time constant. What we've done is reduced it to less than a picosecond. It's more than a thousand times faster than anything currently available."
In semiconductors, the excited state is when energy is present in the system, and the ground state is when there is none. Normally, the semiconductor must first "cool down" in the excited state, releasing energy as heat, before "jumping" back to the ground state, releasing the remaining energy as light. The Penn team's semiconductor nanowires, however, can jump directly from a high-energy excited state to the ground, all but eliminating the cool-down period.
The advancement in emission lifetime is due to the unique construction of the team's nanowires. At their core, they are cadmium sulfide, a common nanowire material. But they are also wrapped in a buffer layer of silicon dioxide, and, critically, an outer layer of silver. The silver coating supports what are known as surface plasmons, unique waves that are a combination of oscillating metal electrons and of light. These surface plasmons are highly confined to the surface the silicon dioxide and silver layers meet.