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Could Borophene Rival Graphene?

Future electronic devices and batteries could become faster and more energy efficient by harnessing new 2-D sheets of electrically-conductive materials. The latest example of such materials comes in the form of newly-created sheets of boron atoms, called borophene, that could outperform even graphene as an electrical conductor in 2-D form.

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Thin Films of Correlated Metals Could Replace ITO

It’s become somewhat of a rite of passage: Any new nanomaterial is eventually offered as an alternative to indium tin oxide (ITO) in displays. We’ve seen the standard offerings of carbon nanotubes and graphene trotted out fairly regularly, and we’ve even seen some pretty exotic offerings like those based on inorganic materials patterned by man-made viruses.

This effort to replace ITO makes sense because it is a relatively scarce resource. And with the explosion of handheld electronic devices, the demand for ITO as a transparent conductor for controlling display pixels has become increasingly acute.

Now there’s a new pretender to ITO’s throne: correlated metals. Researchers at the Penn State Materials Research Institute believe that by using 10-nanometer-thick films of these materials, it’s possible to make large screen displays, touch screens, and even photovoltaics more affordable and efficient.

In this new class of materials, which includes strontium vanadate and calcium vanadate, electrons flow like a liquid rather than like a gas as they do in conventional metals. The Penn State researchers wanted to see if they could make these correlated metals demonstrate a high optical transparency despite possessing metal-like conductivity.

“We are trying to make metals transparent by changing the effective mass of their electrons,” said Roman Engel-Herbert, assistant professor at Penn State, in a press release. Engel-Herbert explains that the team has chosen materials in which the electrostatic interaction between electrons is very large compared with their kinetic energy.  The result, he says, is that:

Electrons ‘feel’ each other and behave like a liquid rather than a gas of non-interacting particles. This electron liquid is still highly conductive, but when you shine light on it, it becomes less reflective, thus much more transparent.”

Even while they were conducting the correlated metals research—which is explained in a paper published in the journal Nature MaterialsLei Zhang, the paper’s lead of author, knew that the team had struck upon something significant.

“I came from Silicon Valley, where I worked for two years as an engineer before I joined the group,” said Zhang, who is a graduate student in Engel-Herbert’s group.  “I was aware that there were many companies trying hard to optimize those ITO materials and looking for other possible replacements, but they had been studied for many decades and there just wasn’t much room for improvement,” said Zhang in the press release. “When we made the electrical measurements on our correlated metals, I knew we had something that looked really good compared to standard ITO.”

With ITO selling for about $750 per kilogram, while vanadium goes for only around $25/kg (and strontium for even less), it’s pretty clear that the correlated metals will make for much cheaper transparent conductors. The question is whether it will be possible to integrate these materials into existing large-scale manufacturing processes for displays.

Engel-Herbert added: “Our correlated metals work really well compared to ITO...From what we understand right now, there is no reason that strontium vanadate could not replace ITO in the same equipment currently used in industry.”

Nanopillars Are Becoming the New Black in Photovoltaics

Less than two weeks ago, we reported on research out of Stanford University in which a process was developed for creating nanopillars on the metallic surface of a solar cell. These nanopillars would negate the reflective properties of the metal wires needed for shunting power to and from the device, thus allowing more photons to pass through the surface.

Now, a collaboration among researchers at the University of Illinois at Urbana Champaign and the University of Massachusetts at Lowell has yielded another approach to producing nanopillars on the surface of solar cells that promises to allow more photons through so more electricity is generated.

The Stanford researchers presented a one-step chemical process in which silicon and a perforated gold film are placed together in a solution of hydrofluoric acid and hydrogen peroxide. The Illinois and Massachusetts-based researchers employ a metal-assisted chemical etch process (MacEtch) to produce their pillars.

In a paper published in the journal Advanced Materials, the latter team explains how the MacEtch process, which is a patented method developed at the University of Illinois, was used to create tiny nanopillars that rise above the metal film.

“The nanopillars enhance the optical transmission while the metal film offers electrical contact. Remarkably, we can improve our optical transmission and electrical access simultaneously,” said Runyu Liu, a graduate researcher at Illinois and a coauthor of the paper, in a press release.

The sudden popularity of these nanopillars is explained somewhat in the press release. Coauthor Viktor Podolskiy, a professor at the University of Massachusetts at Lowell, says the aim has been to develop nanostructures with holes that can find a way around the model known as Fresnel’s equations, which describe the reflection and transmission of light at the interface between two materials.

“It has been long known that structuring the surface of a material can increase light transmission,” said Podolskiy in the press release. “Among such structures, one of the more interesting is similar to structures found in nature, and is referred to as a ‘moth-eye’ pattern: tiny nanopillars which can ‘beat’ the Fresnel equations at certain wavelengths and angles.”

While the two research groups created their nanopillars using different processes, both groups claim to have achieved largely similar improvements to the solar cells’ light-absorbing efficiency. Both the Stanford and the Illinois-Massachusetts teams reported that even when more than half of a solar cell’s surface is covered in metal, 90 percent of the incident light can make it past that layer.

For both of these research projects, the challenge will be to build a silicon-based solar cell with a greater energy conversion ratio than what is currently state-of-the-art, or even currently available on the market.

Daniel Wasserman, a professor at University of Illinois, added: “We are looking to integrate these nanostructured films with optoelectronic devices to demonstrate that we can simultaneously improve both the optical and electronic properties of devices operating at wavelengths from the visible all the way to the far infrared.”

A New, Bottom-up Process for Tunable Nanostructured Germanium

Researchers at the Technische Universität München (TUM) and the Ludwig Maximilian University of Munich have developed a new type of porous germanium with 200-nanometer pores giving it a large active surface area compared to its volume. This property is important for the exchange of electrical charges via a conducting surface in applications such as photovoltaic cells and batteries. The researchers, led by Thomas Fässler, a chemist researching novel materials at TUM, published their research in Angewandte Chemie Online earlier this month. 

Currently, scientists use etching techniques to create nanostructured surfaces in germanium or silicon—a top down approach. The Munich researchers however used a bottom-up method based on the use of so-called Zintl clusters—charged structures of identical atoms discovered by the German chemist Eduard Zintl in the 1930s. 

The germanium Zintl clusters, consisting of nine germanium atoms, were dissolved in a solution, and because they are charged, they repelled each other. Next, the researchers poured the solution over a template comprising a layer of closely packed polymer beads, 50 to 200 nm in diameter. The solution filled the spaces between the beads; when the solution was subsequently evaporated by heating, the germanium atoms merged and formed pure germanium—but studded with the beads. By dissolving the polymer beads, a pure germanium, spongy and lightweight, was left—with pores where the polymer beads had been.

Fässler notes that this is the first time such a porous material that is entirely inorganic has been created in this fashion. Further, he says, this porous semiconductor matrix can be doped by conventional means or by introducing dopants in the porous network, making it easy to tune its electrical properties.

Highly porous materials have been studied for their capability to transport charge carriers more efficiently in photovoltaic cells and batteries. Says Fässler:

Our overall goal is the creation of a hybrid solar cell; this means, the combination of an organic conductive polymer and a classical material—in our case, germanium. If you fill the pores with a conductive polymer, you can produce charge separation with sunlight, where the polymer produces a hole and the germanium uptakes an electron. 

There are still hurdles. “The specific size of the pores is now 200 nm,” says Fässler. “In the long term we want to make those pores smaller. We built a first prototype, but the performance is low." He adds that he expects it to take a year to get a prototype with a new material to work.

Still, the German team’s research is on a fast track with a lithium-ion battery featuring a porous semiconductor electrode. Fässler explains that while other groups have used nanosize silicon or germanium particles, his group uses the inverse approach: producing nanoporous germanium instead of nanoparticles. 

Currently, for the prototype, we use a germanium half cell on one side, and a pure lithium cell on the other side. This is our benchmark. The lithium will react with the walls of the pores and we have a much higher surface exposed to the lithium. Generally, if you use silicon or germanium in an anode,  you need additional carbon with a high surface area to get a better conductivity.  In our case, we can make a half cell without using any carbon. 

Fässler reports that he and his collaborators are now doing cycling with this battery.  

Silicon is the next major step in their research, but there are tradeoffs. “Actually, germanium is a better conductor,” says Fässler, “but silicon is much cheaper.” And although Zintl clusters for silicon exist, they are more difficult to make and to manipulate.

Spy Agency Bets on IBM for Universal Quantum Computing

A real-life U.S. version of “Q Branch” from the James Bond films has greater ambitions than creating personal spy gadgets such as exploding watches or weaponized Aston Martins. It’s betting on an IBM team to develop the first logical qubits as crucial building blocks for universal quantum computers capable of outperforming today’s classical computers.

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Small Tweaks to Its Recipe and "White Graphene" Could Change Electronics

Strictly speaking, hexagonal boron nitride is a semiconductor, but its band gap is so big that, for all practical purposes, it behaves like an insulator.

It’s because of this pseudo-insulator characteristic that researchers have been interested in combining boron nitride with other two-dimensional materials such as graphene to create hybrid materials capable of doing what each constituent can’t do on its own.

Now researchers at the Department of Energy’s Oak Ridge National Laboratory (ORNL) have developed a process for producing a nearly perfect single layer of hexagonal boron nitride—dubbed “white graphene”—that the researchers believe could be a game changer for the use of the material in electronic applications.

In research published in the journal Chemistry of Materials, the ORNL researchers followed traditional chemical vapor deposition (CVD) steps, in which gaseous reactants are introduced into a furnace to form a film on a metal substrate that’s usually made of copper. However, they added a little something to the recipe that provided a more gentle and controllable way to introduce the reactants into the furnace and took better advantage of the conditions inside the furnace.

“I just thought carefully beforehand and was curious. For example, I remind myself that there are many conditions in this experiment that can be adjusted and could make a difference,” said ORNL’s Yijing Stehle, postdoctoral associate and lead author of a paper, in a press release. “Whenever I see non-perfect results, I do not count them as another failure but, instead, another condition adjustment to be made. This ‘failure’ may become valuable.”

The result of finding value in failure was a “white graphene” that has lived up to the material’s previously unachieved theoretical performance potential. What this means is that if white graphene were used as a substrate material for its carbon analogue, the electron mobility of the combined materials would be a thousand times higher than that of graphene on other substrate materials.

“Imagine batteries, capacitors, solar cells, video screens and fuel cells as thin as a piece of paper,” said Stehle in the press release. “Imagine your message being sent thousands of times faster.”

Laser-Induced Graphene Looks to Displace Batteries With Supercapacitors

Almost exactly a year ago, we first got word that researchers at Rice University had developed a method for producing graphene that features a computer-controlled laser. They dubbed the result laser-induced graphene (LIG).

Since then, LIG has been proposed for flexible supercapacitors that could power wearable electronics.

The researchers at Rice have continued to pursue supercapacitors for this new form of graphene and have continued to refine the LIG process to the point where they now believe it may be capable of moving energy storage away from batteries and towards supercapacitors.

The key attribute of LIG is how comparatively easy it is to produce as opposed to graphene made via chemical vapor deposition. For LIG, all that is needed is a commercial polyimide plastic sheet and a computer-controlled laser. The Rice researchers discovered that the laser would burn everything on the polyimide except the carbon from the top layer. What remains is a form of graphene.

You can see a description and demonstration of the process in the video below.

The researchers think that this process will ultimately lend itself to roll-to-roll production. That will eliminate complex manufacturing conditions that have thus far limited the widespread application of microsupercapacitors.

“It’s a pain in the neck to build microsupercapacitors now,” said James Tour, who has been leading this line of research at Rice since the beginning, in a press release. “They require a lot of lithographic steps. But these we can make in minutes: We burn the patterns, add electrolyte and cover them.”

The researchers claim that the microsupercapacitors they have fabricated using LIG have demonstrated an energy density that is on par with thin-film lithium-ion batteries. The microsupercapitors’ capacitance was measured at 934 microfarads per square centimeter; they boast an energy density of 3.2 milliwatt-hours per cubic centimeter. As these are supercapacitors, their power density far exceeds that of batteries. Perhaps most importantly, the devices did not exhibit any degradation over time, maintaining mechanical stability even after being bent 10,000 times.

Encouraging as these numbers are, there yet remains some work to be done before supercapacitors displace batteries.

We’re not quite there yet, but we’re getting closer all the time,” said Tour in the press release. “In the interim, they’re able to supplement batteries with high power. What we have now is as good as some commercial supercapacitors. And they’re just plastic.”

Light-scattering Nanoparticles Could Lead to Invisbility Cloaks and Smaller Optical Antennas

A research team from A*STAR Data Storage Institute in Singapore and St. Petersburg University in Russia has discovered certain light-scattering properties of nanoparticles that could lead to smaller and more effective optical nanoantennas and even invisibility cloaks.

The research, which was published in the journal ACS Photonics, took the form of numerical calculations of the light-scattering properties of dielectric nanoparticles, which are nanoparticles that are electrical insulators and can be polarized by an applied electric field.

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Nanopillars Hide Solar Wiring

Photovoltaics convert the photons from light into a voltage—thus the name. And anything that reduces the number of photons that strike the electric field of the semiconducting P-N junction inside the solar cell will reduce the voltage it generates.

One of things that curtail the amount of photons interacting with the solar cell is the design of the device’s outer layer. In many silicon solar cell designs, a grid of metal wires is employed in order to get the electricity to and from the device. The problem: the wires reflect light away from the surface, reducing the amount of photons that are absorbed. (It should be noted that in some photovoltaics, indium tin oxide is used to create transparent conducting films that stand in for the metal wire grids.)

Researchers at Stanford University have taken a novel approach to addressing this problem by creating a nanostructure comprising so-called nanopillars that rise above the metal surfaces. The nanopillars direct the light into the solar cell in a way that avoids reflection by the metal, effectively making the metal invisible to the incoming light.

In research published in the journal ACS Nano, the researchers were able to fabricate the nanopillars in a one-step chemical process. They started with a 16-nanometer-thick film of gold that had been layered onto a flat sheet of silicon. The gold film was covered with an array of nanoscale holes.

“We immersed the silicon and the perforated gold film together in a solution of hydrofluoric acid and hydrogen peroxide,” said Thomas Hymel, a Stanford graduate student who co-authored the ACS Nano article, in a press release. “The gold film immediately began sinking into the silicon substrate, and silicon nanopillars began popping up through the holes in the film.”

Once the gold-coated silicon is dipped in the chemicals, the metal is transformed from a shiny gold to a dark red, which indicates that the material will no longer reflect light. You can watch a demonstration of this process in the video below:

The results of this change are dramatic. The Stanford researchers estimate that this could increase the efficiency of some silicon solar cells by as much as 22 percent.

“Solar cells are typically shaded by metal wires that cover 5 to 10 percent of the top surface,” said lead author Vijay Narasimhan in a press release. “In our best design, nearly two-thirds of the surface can be covered with metal, yet the reflection loss is only 3 percent. Having that much metal could increase conductivity and make the cell far more efficient at converting light to electricity.”

This nanopillar architecture is not limited to use with gold; it can also function with contacts made of silver, platinum, nickel and other metals. Using the technique with other semiconductor materials could make it useful in applications such as photosensors, light-emitting diodes and displays, and transparent batteries.

New Method for Producing Nanowires Could Offer a Commercial Avenue

Nanowires are developing quite the reputation in photonics. But even that growing status is dwarfed by the amount of research that has gone into developing semiconductor nanowires since the late 1990s. Research teams like those run by Charles Lieber at Harvard University, Zhong Lin Wang at Georgia Tech, and Lars Samuelson at Lund University are a few of the leading groups that have been spearheading this research over the last decade.

Now a research group at the University of Wollongong in Australia claims to have made a breakthrough in the production of semiconductor nanowires that could have an enormous impact on photonics applications such as telecommunications.

In a paper published in the journal Nanoscale, the researchers say that the key to the breakthrough was the addition of the element bismuth to the elements arsenic and gallium. The heavier element bismuth resists joining the arsenic-gallium crystal, and as a result, forms a coating on the surface of the crystal that is an array of small droplets.

“These droplets act as a catalyst for the growth of nanostructures, which in this case turned out to self-assemble in the form of tracks,” said Julian Steele, a coauthor of the research, in a press release. The nanotracks themselves were grown by the Australian team’s UK and U.S. collaborators; they were actually trying to grow solid thin-film materials.

“We were able to add to the work in understanding what we were seeing and why the tracks formed,” says Steele. “The problem with trying to understand how the nanotrack shape is formed is the fact that only a handful of theoretical models exist to describe how they grow, and none that explains our unusual shapes.”

The Australian researchers demonstrated in simulations a new self-assembly new growth model for the nanowires that could overcome the prohibitive costs associated with using nanowires in a range of electronic applications.

“Because of the price tag currently attached with their fabrication, the science of nanowires still remains in the world of laboratories,” said Steele in the press release. He added:

In the same way that the development of new materials late in the 20th century helped to realize our current tech-age—from smartphones to driverless cars—the next frontier is how to assemble these materials at the nanoscale in order to exploit small-scale physics (quantum mechanics) for enhanced efficiency and function.

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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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