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2-D Nanomaterials Put Photovoltaics on a Diet

Soon after graphene was discovered back in 2004, a number of other two-dimensional (2-D) materials appeared, vying for the same attention. By now, the list has grown to a veritable catalogue of 2-D materials.

The story of these 2-D materials will ultimately be about how well they can adapt to different applications that we consider for them today, or may discover in the future. Of them, perhaps no area has been as hotly pursued as photovoltaics (PVs).

One interesting new potential application of graphene is in creating what is known as “hot carrier” cells in which the graphene produces, after absorbing one photon, is capable of generating multiple electrons instead of just a single one. But the main focus of applying graphene to PVs has been as a replacement for indium tin oxide (ITO) used in the electrodes of organic solar cells.

Now researchers at MIT are looking at 2-D materials, such as molybdenum disulfide and molybdenum diselenide, to make the thinnest and lightest PVs ever made. While this may not produce the highest energy conversion efficiency or be the cheapest material for PVs—the two typical metrics most sought after—they do expect that their lightness should create some possibilities in this application.

Graphene's own energy conversion capabilities are not what you would call impressive, and more generally, two-dimensional materials don’t really compete with the 18-19 percent conversion efficiencies of standard silicon cells already on the market. But in the research, which was published in the ACS journal Nano Letters (“Extraordinary Sunlight Absorption and 1 nm-Thick Photovoltaics using Two-Dimensional Monolayer Materials”),  the MIT team demonstrated that if you stacked just three sheets on top of each other, a 1nm-thick stack can absorb up to 10 percent of incident sunlight, which is one order of magnitude higher than gallium arsenide and silicon. While the actual conversion efficiency is still pretty low at 1 percent, it does correspond to power densities being 100 to 1000 times higher than the best existing ultrathin solar cells.

“Stacking a few layers could allow for higher efficiency, one that competes with other well-established solar cell technologies,” said Marco Bernardi, a postdoc in MIT’s Department of Materials Science, in a press release.

Jeffrey Grossman, Associate Professor of Power Engineering at MIT and the paper's senior author, added in the release: “It’s 20 to 50 times thinner than the thinnest solar cell that can be made today. You couldn’t make a solar cell any thinner.”

Whether thinner and lightness will really translate into something that the market demands remains to be seen. The problem, as the researchers seem to concede, is that at this point there is no way to produce molybdenum disulfide and molybdenum diselenide in bulk. Until manufacturing techniques are developed that make that possible—and economical—the thinnest solar cells will likely remain laboratory curiosities.

Illustration: Jeffrey Grossman and Marco Bernard/MIT News

Is Putin Pulling the Plug on Russian Nanotechnology?

Russia’s generously funded and much ballyhooed nanotechnology initiative, Rusnano, has had its share of intrigue and certainly many detractors since its launch, not the least of which have been the leaders of the government, such as former president Dmitry Medvedev. But still it managed to continue on and seemed to be tracking fairly well with reported revenues of $300 million for 2011.

Just when it seemed Russia had found a shortcut into the nanotechnology arms race that has developed over the last decade and was sweeping up all the discarded nanotechnology companies that had run aground on the rocks of capitalism, Russian President Vladimir Putin last month looked to be sacrificing both Rusnano and another technology project Skolkovo—an attempt to build a Silicon Valley outside of Moscow—to  solidify his political aims.

As reported in last month’s Bloomberg, Putin was coming down hard on these two technology initiatives to project that he was tough on corruption and mismanagement of public funds.

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Nanomaterials Go Beyond Post-Silicon to Post-Semiconductor

Yesterday, IEEE Spectrum published a feature “Changing the Transistor Channel” that chronicles the laborious migration from the ubiquitous silicon in transistors to new materials, primarily compound semiconductors known as III-Vs.

These efforts to replace the semiconducting silicon in the channels of transistors is being pursued by all the big chip manufacturers and international research labs.. Various nanomaterials from graphene to nanowires made from III-V materials are being experimented with to help achieve that aim.

As momentum builds in this field, researchers at Michigan Technological University (MTU) are looking ahead not only beyond silicon but also to when semiconductors will not even be needed for transistors.

Yoke Khin Yap, a physicist at MTU, and his colleagues, including those at Oak Ridge National Laboratory (ORNL), have developed a method by which they use an insulator—boron nitride nanotubes—coupled with quantum dots to create a path for electrons to travel between electrodes in a transistor. No semiconductor material is used in the design.

“The idea was to make a transistor using a nanoscale insulator with nanoscale metals on top,” Yap said in a press release. “In principle, you could get a piece of plastic and spread a handful of metal powders on top to make the devices, if you do it right. But we were trying to create it in nanoscale, so we chose a nanoscale insulator, boron nitride nanotubes, or BNNTs for the substrate.”

Two years ago, Yap and his team developed a way to make a virtual carpet out of BNNTs. In this latest research, which was published in the journal Advanced Materials (“Room Temperature Tunneling Behavior of Boron Nitride Nanotubes Functionalized with Gold Quantum Dots”),  the MTU team devised a method for depositing gold quantum dots on the BNNT carpet using a laser. The BNNTs turn out to be perfect for the job. They have controllable and uniform diameters so they can confine the size of the quantum dots.

When Yap and his colleagues, along with scientists at ORNL, put a voltage on the electrodes, they observed that the electrons jumped from one gold quantum dot to the next in an orderly fashion. This phenomenon is known as quantum tunneling. One benefit of this device is that the quantum tunneling effect is achieved at room temperature conditions.

“Imagine that the nanotubes are a river, with an electrode on each bank. Now imagine some very tiny stepping stones across the river,” said Yap in a press release. “The electrons hopped between the gold stepping stones. The stones are so small, you can only get one electron on the stone at a time. Every electron is passing the same way, so the device is always stable.”

This design allowed for the creation of a transistor that did not require a semiconductor. When sufficient charge was applied, the material was in a conducting state. When the charge was removed, it reverted back to being an insulator. An additional benefit to the design was that it didn’t suffer any “leakage” of electrons that plagues silicon, creating overheating problems and wasted energy.

Yap notes: “Theoretically, these tunneling channels can be miniaturized into virtually zero dimension when the distance between electrodes is reduced to a small fraction of a micron.”

Image: Yoke Khin Yap

Carbon Nanotubes Capture Electrical Signals Between Neurons

President Obama’s BRAIN initiative, which was launched back in April, may already have a new tool for mapping the human brain in its arsenal . Researchers at Duke University have used a carbon nanotube to capture electrical signals from individual neurons.

With a complete 3-D digital map of the human brain now available as part of the European Human Brain Project, brain research is gaining a lot of momentum. The carbon nanotube probe developed by the Duke team, which acts like a sort of harpoon, first spearing the neurons and then collecting the electrical signals they send to communicate with other neurons, is expected to provide a new level of insight into the human brain.

“To our knowledge, this is the first time scientists have used carbon nanotubes to record signals from individual neurons, what we call intracellular recordings, in brain slices or intact brains of vertebrates," said Bruce Donald, a professor of computer science and biochemistry at Duke University, in a press release.

The research (“Intracellular Neural Recording with Pure Carbon Nanotube Probes”), which was published in the journal PLoS ONE, overcame the shortcomings (literally) of other attempts to use carbon nanotubes (CNTs) as neuron probes. Previously, CNTs have proven to be too short or too thick for the job. But the Duke team was able to make their CNT probe one millimeter long (quite long for CNTs) and capable of monitoring the electrical signals between neurons more precisely than the glass or metallic electrodes that are typically used.

The researchers were able to achieve these unique CNT characteristics with a specially devised technique. They accumulated carbon nanotubes at the tip of a tungsten wire until the tubes took the shape of a needle-like probe. Next, they coated the probe with an insulating material and then removed the insulating material with a focused ion beam. This process of applying, then removing the insulating material gave the probe an extremely fine point.

"The results are a good proof of principle that carbon nanotubes could be used for studying signals from individual nerve cells," said Duke neurobiologist Richard Mooney, a study co-author, in press release. "If the technology continues to develop, it could be quite helpful for studying the brain."

While the researchers concede that more research needs to be done to improve the electrical recording capabilities of the probes—even as improvements are made to their geometry and the insulating layers—the Duke team has applied for a patent on the probe. The researchers expect that the technology could not only prove useful for mapping the brain but for creating brain-computer interfaces.

Photo: Inho Yoon and Bruce Donald, Duke

 

Graphene Comes to the Rescue of Molecular Electronics

Molecular electronics are a long-proposed—and sometimes forgotten—aim for electronics. The field promises a time when the basic building blocks of electronics are individual molecules. But a reliable method for testing these molecular components has remained elusive.

Now a joint research team comprising chemists and physicists from the Department of Chemistry Nano-Science Center at the University of Copenhagen and the Chinese Academy of Sciences in Beijing has developed a graphene-based chip whose initial application could be testing the molecular chips researchers envision.

The research (“Ultrathin Reduced Graphene Oxide Films as Transparent Top-Contacts for Light Switchable Solid-State Molecular Junctions”), which was published in the Wiley journal Advanced Materials, claims to be the first time that a transistor composed of just one molecular monolayer functioned on a chip.

While Kasper Nørgaard, an associate professor of chemistry at the University of Copenhagen, believes that the first applications for the graphene-based chip will be in testing future molecular electronics, the chip itself represents a first step towards integrated molecular circuits.

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Spot Welding Graphene Transistors on the Atomic Scale

With researchers still struggling to open a band gap in graphene at room temperature sufficient for transistor applications, it’s sometimes good to remember what makes graphene such an appealing material that they think it's worth the struggle—after all, an awful lot of effort has gone into engineering a band gap into a material that intrinsically doesn’t have one. High among those benefits is the promise of high electron mobility and simpler chemical doping techniques—with a far easier path to interconnection than its cousin, carbon nanotubes.

To buoy hopes of graphene in transistor applications, researchers at Aalto University in Finland and Utrecht University in the Netherlands have demonstrated the ability to create single atom contacts between gold and graphene nanoribbons.

The research ("Suppression of electron–vibron coupling in graphene nanoribbons contacted via a single atom"), which was published in the journal Nature Communications, showed that contacts between graphene and gold could be established without significantly modifying the very electrical properties of graphene’s honeycomb lattice that make it so attractive in the first place.

The process starts with an atomic-scale mapping of the graphene using atomic-force microscopy (AFM) and a scanning tunneling microscope (STM). Then a chemical bond is achieved by sending voltage pulses from the tip of a STM to create single bonds to the graphene nanoribbons at precisely determined locations. The pulse from the STM removes one hydrogen atom from the end of the graphene nanoribbon, initiating the bond formation.

"The edges of the chemically synthesized ribbons that we use are hydrogen terminated just as you would have in a molecule (e.g., pentacene)," Professor Peter Liljeroth, who heads the Atomic Scale Physics group at Aalto University, explained to me in an e-mail. "We can use bias voltage pulses from the STM tip to knock off the hydrogen atoms one-by-one. When you remove a single hydrogen, you form what a chemist would call a radical and a physicist would call a dangling bond: the carbon atom without the hydrogen has an unpaired electron that would like to form a bond with something. It does this with one of the atoms of the underlying gold substrate. So we remove the hydrogen, the carbon atom becomes more reactive and forms a bond spontaneously with one of the gold atoms."

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Graphene Nanoribbons Bring New Twist to Li-ion Batteries

More than four years ago, James Tour at Rice University developed a method by which cylindrical carbon nanotubes could be unzipped to form graphene nanoribbons (GNR). About 18 months after making that discovery, Tour described his work here on the pages of IEEE Spectrum.

Today, Tour and his colleagues have found an application for their GNR material that could increase the storage capacity of lithium ion (Li-ion) batteries.

The research, which is described in the journal ACS Nano ("Graphene Nanoribbon and Nanostructured SnO2 Composite Anodes for Lithium Ion Batteries"), has developed a method by which the GNR can be combined with tin oxide in a way that gives it greater storage capacity than the theoretical maximum of tin oxide alone. The prototype device that the Rice team developed still managed to maintain a storage capacity more than twice that of traditional graphite after 50 charge-discharge cycles.

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Nanoparticles Promise to Make LEDs Cheaper

Light-emitting diode (LED) light sources have a lot going for them. They have longer life spans than their incandescent rivals and better luminous efficiency, and they’re environmentally friendlier. But those benefits come at a high cost—literally.

There are a number of points in the production of LEDs worthy of attack, such as the bases on which they're grown. Another involves scarce rare-earth metals, a problem endemic to high-tech manufacturing. Now researchers at the University of Washington (UW) have come up with a nanoparticle that could replace the rare-earth-element phosphors currently used in LEDs to soften the harsh blue light they emit.

Chang-Ching Tu, a post-doctoral researcher at UW, has launched a new company, LumiSands, to market the nanoparticles. The technique for producing them involves etching off the material from wafers of silicon. While silicon does not typically emit light, when it is in crystalline form at dimensions below five nanometers it can begin to glow.

The silicon-based nanoparticles emit a red light that, when combined with part of the harsh blue light of the LEDs, produces greens, yellows, and reds that resemble sunlight.

“The beauty of our technology is to create a highly efficient fluorescent material by using silicon rather than rare-earth elements or other types of heavy-metal compound semiconductors,” Tu said in a UW press release. “The manufacturing process can be performed in a basic laboratory setting and is easy to scale up.”

The technology, though still evolving, is far enough along to launch a company, a prototype of the devices has been made, and Tu believes LumiSands could start manufacturing devices based on the technology within a year. He will continue to work on the red-light-emitting technology and then move on to other colors so that LEDs equipped with them will give off a white light with no rare-earth elements.

Image: Mary Levin, UW

Trilogy of 2-D Materials Could Constitute Future Electronics

Researchers at Rice University and Oak Ridge National Laboratory (ORNL) are aiming to remake the world of two-dimensional materials, including graphene, molybdenum disulfide (MDS) and hexagonal boron nitride (hBN), so that together they constitute a trilogy of materials for the next generation of electronics.

The ultimate goal of their research is to combine these three 2D materials: a semiconductor, insulator and conductor (MBS, hBN and graphene, respectively) to create a range of electronic devices, such as field-effect transistors, integrated logic circuits, photodetectors and flexible optoelectronics. To get there the researchers have come up with a better way of producing MDS.

When research into using MDS as a 2D material for electronics first started to gain notice, scientists suggested that it would serve as a compliment to graphene, especially in applications that require a transparent semiconductor. While some have seen a rivalry between the two 2D materials developing,  research has continued to pursue them as compliments to one another. In fact, recently graphene and MDS have been mated to create a new flash memory. And earlier this year, some of the same Rice University researchers in this current work showed that graphene could be weaved together with hBN to create nanoscale patterns.

While a trilogy of 2D materials might be the long-range aim, the team of researchers started their work by seeing if they could produce large, high-quality sheets of MDS through chemical vapor deposition (CVD) instead of employing the so-called “Scotch Tape” method in which layers of the material are peeled off from bulk samples.

In the research, which was published in the journal Nature Materials (“Vapour phase growth and grain boundary structure of molybdenum disulphide atomic layers”),  the team discovered that they could improve the CVD process by adding artificial edges to the substrate.

“The material is difficult to nucleate, unlike hBN or graphene,” said Sina Najmaei. co-author of the paper, in a press release. “We started learning that we could control that nucleation by adding artificial edges to the substrate, and now it’s growing a lot better between these structures.”

The final material built up through CVD was sent over to ORNL where microscopy tools were used to characterize it. Among the properties they discovered in the material is what they believe to be the potential for the atoms in the MDS to bind with carbon atoms in the graphene.

“We’re working on it,” said Zheng Liu, a researcher at Rice, in the press release. “We would like to stick graphene and MDS together (with hBN) into what would be a novel, 2-D semiconductor component.”

“These are very different materials, with different electronic properties and band gaps. Putting one on top of the other would give us a new type of material that we call van der Waals solids,” said Pulickel Ajayan, an engineering professor at Rice. “We could put them together in whatever stacking order we need, which would be an interesting new approach in materials science.”

Image: Oak Ridge National Laboratory

Angela Belcher: The Consummate Nanotechnologist

The first I heard about Angela Belcher, someone explained to me her work using genetically engineered viruses to build electronic circuits through self assembly. That was ten years ago and in the ensuing decade she has, to the fascination of everyone, mixed the biological with the electrical in ways that alter entire industries, including solar power, batteries, fuel cells, and fuel production.

So consistently inventive has been her work that I was surprised she had not won the US $500,000 Lemelson-MIT Prize until she was given it this week.

Belcher is the consummate nanotechnologist because she has never been tied down to one discipline, instead moving freely between biology, physics, and chemistry. This multidisciplinary approach has allowed her to consider ideas that are readily dismissed by those tied to just one of these disciplines.

Two years ago I had the occasion to speak to Michael Grätzel, himself a winner, in 2010, of the similarly-prestigious Millennium Technology Prize. I asked him about Belcher’s work in using viruses to manipulate carbon nanotubes for use in dye-sensitized solar cells. Grätzel, who invented the dye-sensitized solar cell, said, “That’s a real breakthrough. We can learn a lot from her fascinating experiment.”

Belcher’s research is a rare combination of the visionary and the practical—consistently groundbreaking, yet there have always been commercial implications. In 2002, along with Evelyn Hu of Harvard University, Belcher set up Cambrios Technologies Corporation to commercialize the use of genetically modified viruses to create transparent coatings made of silver nanowires for touch screen displays. Then in 2007 she and Hu co-founded another company, Siluria Technologies, to use the viruses to produce clean fuel.

It’s clear that when Belcher develops a method for using viruses to create a new generation of lithium-ion batteries, she is doing it with the expectation that it will someday be used in the real world.

“The full implications of Angela Belcher’s work are only beginning to be realized, and yet the applications already appear to be far-reaching,” says Hu in a press release covering Belcher’s award. “Her inventions are always linked back to her profound passion and compassion for society, and her desire to improve the quality of life for others.”

Image: Dominick Reuter/MIT

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Nanoclast

IEEE Spectrum’s nanotechnology blog, featuring news and analysis about the development, applications, and future of science and technology at the nanoscale.

 
Editor
Dexter Johnson
Madrid, Spain
 
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Rachel Courtland
Associate Editor, IEEE Spectrum
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