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Nanostructured Silicon Solar Cells Achieve High Conversion Efficiency Without Antireflective Coatings

The economics of solar cells always involves striking a balance between conversion efficiency and manufacturing costs.

Researchers at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) believe that they have struck a balance between both of these factors by developing a nanotechnology-enabled silicon solar cell that boasts 18.2 percent conversion efficiency and that should be cheaper to produce.

The research, which was published in the journal Nature Nanotechnology (“An 18.2%-efficient black-silicon solar cell achieved through control of carrier recombination in nanostructures”), was able to create a solar cell without the use of anti-reflective coatings that are typically required to reach that conversion efficiency.

The NREL team was able to achieve this high efficiency without anti-reflection coatings by making billions of nano-sized holes in silicon. Since the holes are actually smaller than the light wavelengths striking the silicon, there is no sudden change in the light density and the light doesn’t reflect back off the surface of the silicon.

While the NREL researchers had previously demonstrated that these nanostructures in silicon reflected less light off the surface, this latest research was the first time they were able to use the technique to achieve high conversion efficiency with the nanostructure silicon cells.

“This work can have a big impact on both conventional and emerging solar cell based on nanowires and nanospheres. For the first time it shows that really great solar cells can be made from nanostructured semiconductors,” Howard Branz, the principal investigator of the research says in an NREL press release.

While attaining high energy conversion efficiencies for an individual cell—and even doing it with a cheaper manufacturing process that eliminates the need for antireflective coating—is an achievement, a big obstacle remains getting those individual cells into a module without significant losses in efficiency.

This may be an area that will be addressed in further research. Branz adds: “The next challenges are to translate these results to common industrial practice and then get the efficiency over 20 percent. After that, I hope to see these kinds of nanostructuring techniques used on far thinner cells to use less semiconductor material.”

European Commission Pulls Back on New Nano Regulations

Last year the European Commission (EC) was eager to show its proactive approach to regulating nanomaterials when—after a protracted process--the Commission arrived at a definition for nanomaterials.

While the EC achieved its goal of a definition, the definition itself came under some pretty pointed criticisms for being so broad as to include the incidental nanoparticles that are produced when a car’s tires roll on pavement.

“We’ve met people recently who work on the legal side within large chemical organizations, who up until October last year didn’t know anything about nanotechnology and suddenly they’re going to be caught within legislation, which is quite a shock to them,” said Jeremy Warren, CEO of Nanosight, in a webinar he produced this year to explain the impact of the new definition.

When any company—European or otherwise—believes that it has been swept up into a regulatory framework for a material that they had no intention of making or using, or perhaps even knew existed, government bureaucrats are certainly going to hear about it. It didn’t take long for industry ministers of European countries to start to take heed.

Last week we began to see how the EC was trying to reel in their proactive approach when it released its “Communication on the Second Regulatory Review on Nanomaterials.” One of the points of the position paper was: “that current legislation covers to a large extent risks in relation to nanomaterials and that risks can be dealt with under the current legislative framework”.

All of that work to develop a definition of nanomaterials so as to create a class of materials that are not currently known to be hazardous (but might be someday) seemed to be all for naught. Instead the EC seems to have taken the position that current laws governing run-of-the-mill materials pretty much handle a large majority of nanomaterials out there.

The reaction of NGO’s like Greenpeace and Friends of the Earth was swift and angry. The NGOs trotted out the term “precautionary principle,” which seems to have come to mean an absolute moratorium on all nanomaterials rather than producers taking on the burden of proof regarding the level of risk of their products.

Another pervasive sentiment among the NGOs is that the EC is stalling. If the EC were indeed stalling, one possible explanation would be that they want scientific data to be gathered proving nanomaterials safe and until then still promote new companies and new products by delaying the imposition of regulations. I suppose that’s what the NGOs believe is happening in this case.

To me, it’s a bit too conspiratorial an explanation. I am more likely to believe this long process stems from the way bureaucracies operate, especially the European variety. They love to commission reports and studies and then hold meetings on the results. The European Union’s approach to the specific issue of nanosilver’s risk has driven some scientists to such levels of frustration they felt compelled to write an article for Nature Nanotechnology, decrying the situation.

Bureaucratic dithering aside, the real obstacle to arriving at a swift resolution about the risk of nanomaterials is that the science takes a long time. As I’ve commented before, we are asking for an almost complete overhaul of the periodic table in terms of toxicity to determine the risk of nanomaterials. Let’s keep the length of time at resolving these issues in that context.

Plasmonics Used to Dope Graphene

The big push in graphene research for electronics has been overcoming its lack of an inherent band gap.  But silicon has another leg up on graphene when it comes to electronics applications: it can comparatively easily be p- and n-doped (positive and negative).

While there have been a number of approaches taken for doping graphene, researchers at Rice University believe that the idea of plasmon-induced doping of graphene could be ideal for this purpose.

The research (“Plasmon-Induced Doping of Graphene”), which was published in the journal ACS Nano, looks to use plasomonics, which exploits the fact that “photons striking small, metallic structures can create plasmons, which are oscillations of electron density in the metal.”

The Rice team placed nanoscale plasmoic antennas—dubbed nonamers—on the graphene to manipulate light in such a way that they inject electrons into the graphene, changing its conductivity. The nonamers tooks the form of eight nanoscale gold discs that encircled one large gold disc, and were placed on the graphene with electron beam lithography.

When the graphene and nonamers are exposed to light, the incident light is converted into hot electrons that transform those portions of the graphene where the nonamers are located from a conductor to an n-doped semiconductor.

“Quantum dot and plasmonic nanoparticle antennas can be tuned to respond to pretty much any color in the visible spectrum,” says Rice professor Peter Nordlander, one of the authors of the paper, in the university's press release about the research. “We can even tune them to different polarization states, or the shape of a wavefront."

Nordlander adds: “That’s the magic of plasmonics. We can tune the plasmon resonance any way we want. In this case, we decided to do it at 825 nanometers because that is in the middle of the spectral range of our available light sources. We wanted to know that we could send light at different colors and see no effect, and at that particular color see a big effect.”

While the possibility of a process that simply uses light for doping graphene seems pretty amazing, the researchers are looking ahead to a day when a flashlight in a particular pattern would replace a key for unlocking a door by triggering the circuitry of the lock to open it. “Opening a lock becomes a direct event because we are sending the right lights toward the substrate and creating the integrated circuits. It will only answer to my call,” Norlander suggests in the release.

Graphene-based Gas Membranes Promise Reduced Carbon Dioxide Emissions

Researchers at the University of Colorado Boulder have achieved the first experimental results of using graphene as a membrane to separate gases. While still a long way off from industrial use, the membranes do possess mechanical properties that should prove beneficial in  natural gas production. The ultra-thin, graphene-based membranes' highly selective pores increase flux through the membrane making the process more energy efficient, thereby reducing the plant's production of carbon dioxide.

The Univ. of Colorado research builds on work that was completed last year at Boulder that showed that graphene possessed extraordinary adhesion capabilities. That work demonstrated that if graphene were used in a multi-layer membrane, the adhesion between the layers of the membrane would be extremely strong. 

This most recent work also seems to go one step beyond research at MIT from earlier this year. In that work, scientists used computer simulation to show that nanoporous graphene could replace membrane materials currently used in the reverse osmosis water desalination processes. At the time, the MIT researchers said they expected that turning their computer models into real world membranes would be daunting. According to the researchers, getting the pores size precisely right would be difficult to do on a large scale.

While the Boulder researchers have not attempted to scale up their experimental results yet, they have managed to get the pore size correct on the graphene so it can separate a variety of gases.

The research, which was published in the journal Nature Nanotechnology (“Selective molecular sieving through porous graphene”),  was able to achieve its precisely sized pores by etching them into graphene sheets with a process involving ultraviolet light-induced oxidation. The resulting porous graphene was then tested on a range of gases including “hydrogen, carbon dioxide, argon, nitrogen, methane and sulphur hexaflouride -- which range in size from 0.29 to 0.49 nanometers.”

“These atomically thin, porous graphene membranes represent a new class of ideal molecular sieves, where gas transport occurs through pores which have a thickness and diameter on the atomic scale,” says Colorado mechanical engineering professor Scott Bunch  in a university press release.

The main technical challenge, according to the researchers, will be bringing these results up to an industrial scale. In particular, they will need to find a process by which they can create large enough sheets of graphene.  The researchers even concede that getting the pores precisely defined still needs further development.

Nanowires Show the Strain Limit of Silicon

Most nanotechnology developments targeted at electronics look ahead to a post-silicon world.  But silicon is still firmly with us and every attempt is being made to wring that last drop of capability out of the material, sometimes with the help of nanotechnology.

For the last decade, researchers have been pushing silicon's limits by straining it. Whether it be more recently the organic semiconductor variety, or just the run-of-the-mill, non-organic variety, strained silicon has been the mainstay of pushing silicon to the very edge of its capabilities. The question is how far can strained-silicon electronics take us?

Swiss researchers at Paul Scherrer Institute and the ETH Zurich may have an answer to that question. With their most recent research, they have strained silicon nanowires right up to their breaking point and still managed to integrate it into an electronic component.

Renato Minamisawa from the Paul Scherrer Institute describes the research in a press release as “"the strongest tension ever generated in silicon; probably even the strongest obtainable before the material breaks."

To accomplish this, the Swiss researchers have turned to the tried-and-true method of top-down manufacturing: etching a substrate with a silicon layer that is already under some strain. In the research, which was published in the journal Nature Communications (“Top-down fabricated silicon nanowires under tensile elastic strain up to 4.5%”), the Swiss team etched dumbbell-shaped bridges into the strained silicon, which exploit the phenomenon of strain accumulation mechanisms.

Basically, the silicon is initially strained in all directions. As you etch away the material into narrow bridges, the thin material left is pulled in just two directions. "Since all the force which was distributed over a larger area before the etching now has to concentrate in the wire, a high tension is created within it", says Minamisawa.

But just straining silicon to its maximum before it breaks is fairly straightforward process and not that noteworthy on its own.

"There is actually no magic behind building up tension in a wire - you just have to pull strongly on both ends", explains Hans Sigg of the Laboratory for Micro- and Nanotechnology at the Paul Scherrer Institute in the same press release. "The challenge is to implement such a wire in a stressed state into an electronic component."

In the method that the researchers developed, the thin wires that remain after the etching of the silicon layer are attached to the rest of the material only at its endpoints, and most importantly are perfectly uniform in dimension and strain. It would be possible to produce thousands of such wires all within a very precise strained state. "And it is even scalable, meaning that the wires can be fabricated as small as you want," Sigg points out.

Despite making every effort to ensure that the process is perfectly compatible with current fabrication methods and materials, the researchers seem curiously unconcerned whether the process ever makes it into industry. As Minamisawa notes about the silicon nanowires: "But even if they do not end up in microelectronic applications, our research could show what the limits of silicon electronics really are."

Silicon Oxide as Resistive Memory Goes Transparent

Since 2008, James Tours’ research group at Rice University has been championing its discovery that silicon oxide can form the basis of resistive memory. In the past four years, there have been a fair number of doubters who just couldn’t believe that silicon oxide could switch its resistance, but there has also been a growing body of research from other labs that corroborates that four-year old work.

Now Tour and his team--with Jun Yao at the fore—have pushed their research one step further with their latest paper in the journal Nature Communications (“Highly transparent nonvolatile resistive memory devices from silicon oxide and graphene”) in which they demonstrate a non-volatile, transparent memory chip resistant to both heat and radiation.

The core of the new memory chip is the use of silicon oxide that has been sandwiched between layers of graphene and placed on flexible plastic sheets. While the Rice researchers have been looking at the use of graphene in flexible displays of late, in that work nanowires were sandwiched between the layers of graphene. In this latest research, silicon oxide is sandwiched between the graphene layers.

In fact, it has been the Rice team’s nanowire-and-graphene research—and its dogged belief in silicon oxide’s ability to switch its resistance and serve as a resistive memory—that has made this latest development possible. In the 2010 paper that followed their 2008 discovery of silicon oxide’s capabilities, Yao observed that running a voltage through silicon oxide removed oxygen atoms from the material creating a 5-nanometer-wide channel, and turned it into pure silicon filaments.

When Yao reversed the voltage, he discovered that the silicon filaments could be broken and reformed, creating a “0” for a broken circuit and a “1” for the healed circuit—the basis of computer memory. Since then Yao has been attempting to build up evidence that the switching effect he witnessed was not a result of the breaking graphite but because of the underlying crystalline silicon.

“Jun was the first to recognize what he was seeing. Nobody believed him,” says Tour in the press release. “Jun quietly continued his work and stacked up evidence, eventually building a working device with no graphite.”

To quiet the doubters, who believed that the switching had to be due to some carbon in the system, Yao created a device that had no exposure to the carbon at all. The devices that the Rice team is producing now contain no metals except for the leads attached to the graphene electrodes.

Tour further notes: “Now we’re making these memories with about an 80 percent yield of working devices, which is pretty good for a non-industrial lab When you get these ideas into industries’ hands, they really sharpen it up.”

The history of nano-enabled, non-volatile memory's challenges to flash memory have proven to be less than successful in the past, but this could be an architecture that changes that trend.

New Study Indicates Nanoparticles Do Not Pass Through Skin

For at least the last several years, NGO’s like Friends of the Earth (FoE) have been leveraging preliminary studies that indicated that nanoparticles might pass right through our skin to call for a complete moratorium on the use of any nanomaterials in sunscreens and cosmetics.

Even if you’re a world-renowned expert on assessing the risk of nanomaterials, you had better not challenge the orthodoxy of that line of thinking.  Or if you’re an NGO that investigated the issue of nanoparticles in sunscreens and found no risk, you should be prepared to be so marginalized by other NGOs that you will hardly warrant a mention.

On the contrary, the prevailing argument was that if you’re a company using nanoparticles in your sunscreen you had to prove your product was safe, even if there was no conclusive evidence that your product was risky and a fair amount of evidence that it was safe. So frightening and compelling was this scare screed that in a poll of Australians they said they would prefer to risk getting skin cancer rather than use a sunscreen that might contain nanoparticles. Fear almost always wins out over reason.

Unfortunately for the fear mongers, the evidence is mounting that nanoparticles cannot penetrate the skin. Researchers at the University of Bath in the UK found that even the smallest nanoparticles are not capable of passing through the skin barrier.

The research, which was published in the Journal of Controlled Release (“Objective assessment of nanoparticle disposition in mammalian skin after topical exposure”),  employed laser scanning confocal microscopy to see whether fluorescently tagged polystyrene beads on the scale of 20 to 200 nanometers were absorbed into the skin.

“Previous studies have reached conflicting conclusions over whether nanoparticles can penetrate the skin or not,” says Professor Richard Guy from the University’s Department of Pharmacy & Pharmacology, in the press release. “Using confocal microscopy has allowed us to unambiguously visualize and objectively assess what happens to nanoparticles on an uneven skin surface. Whereas earlier work has suggested that nanoparticles appear to penetrate the skin, our results indicate that they may in fact have simply been deposited into a deep crease within the skin sample.”

This latest UK research certainly won’t put this issue to rest. These experiments will need to be repeated and the results duplicated. That’s how science works. We should not be jumping to any conclusions that this research proves nanoparticles are absolutely safe any more than we should be jumping to the conclusion that they are a risk. Science cuts both ways.

 

Graphene Replaces Traditional Silicon Substrates in Future Devices

Researchers at the Norwegian University of Science and Technology (NTNU) have patented and are commercializing a method by which gallium arsenide (GaAs) nanowires are grown on graphene.

The method, which was described and published in the journal Nano Letters (“Vertically Aligned GaAs Nanowires on Graphite and Few-Layer Graphene: Generic Model and Epitaxial Growth”), employs Molecular Beam Epitaxy (MBE) to grow the GaAs nanowires layer by layer. A video describing the process can be seen below.

"We do not see this as a new product," says Professor Helge Weman, a professor at NTNU's Department of Electronics and Telecommunications in the press release. "This is a template for a new production method for semiconductor devices. We expect solar cells and light emitting diodes to be first in line when future applications are planned."

Whether it is a method or a product, Weman and his colleagues have launched a new company called Crayonano that will be commercializing the hybrid material that the researchers developed.

The researchers contend that replacing traditional semiconductor materials as a substrate will reduce material costs. The silicon materials are fairly expensive and are usually over 500µm thick for 100mm wafers. As the video explains, using graphene reduces the substrate thickness to the width of one atom. Obviously reduction in material is really only a side benefit to the use of graphene. The real advantage is that the electrode is transparent and flexible, thus its targeting for solar cells and LEDs.

Interestingly Weman sees his team's work as a compliment to the work of companies like IBM that have used graphene “to make integrated circuits on 200-mm wafers coated with a continuous layer of the atom-thick material.”

Weman notes: "Companies like IBM and Samsung are driving this development in the search for a replacement for silicon in electronics as well as for new applications, such as flexible touch screens for mobile phones. Well, they need not wait any more. Our invention fits perfectly with the production machinery they already have. We make it easy for them to upgrade consumer electronics to a level where design has no limits."

As magnanimous as Weman’s invitation sounds, one can’t help but think it comes from concern. The prospect of a five-year-development period before a product gets to market might be somewhat worrying for a group of scientists who just launched a new startup. A nice licensing agreement from one of the big electronics companies must look appealing right about now.

Graphene Proves To Be One Hundred Times Better at Rustproofing Metals

While the wonder material graphene continues to come under pressure from other two-dimensional materials in electronics applications, it has continued to build up applications far afield from electronics.

One of the applications that has opened up over the last year is rustproofing. In May of this year, researchers from the University of Buffalo demonstrated that they could use graphene in rustproofing steel.

Now researchers at Monash University in Australia and Rice University in the USA have used graphene to rustproof copper.

The research, which was published in the journal Carbon (“Protecting copper from electrochemical degradation by graphene coating”), claims that the graphene-based coating renders copper nearly 100 times more resistant to corrosion than if left unprotected.

“We have obtained one of the best improvements that has been reported so far,” says study co-author Dr Mainak Majumder in the university press release. “At this point we are almost 100 times better than untreated copper. Other people are maybe five or six times better, so it’s a pretty big jump.”

To achieve the atomic-scale rustproof coating, the researchers simply heated the graphehe to temperatures between 800 and 900 degrees celcisus and then applied the graphene to the copper through chemical vapor deposition. Seeing whether they can apply the graphene coating at a lower temperature will be a focus for future research.

The University of Buffalo researchers explained that their research into rustproofing steel was in part motivated by a desire to find a more environmentally friendly method than the chrome electroplating that is typically used. But the Monash and Rice team see their graphene film replacing polymer coatings used in metals, so the environmental aspect is less acute in this case.

Nonetheless, the Australian-U.S. research team believes that this use of graphene could change the rustproofing methods for products as varied as ocean-going vessels and electronics.

Carbon Nanotubes Form Smallest Pixels for 3D Holographic Imaging

The holograms we have seen for the past 50 years have at once fascinated and disappointed us. If we had been hoping to see something along the lines of the projected image of Princess Leia from Star Wars, or the holodeck from Star Trek Next Generation, disappointment would likely have overwhelmed our sense of fascination.

Two years ago researchers at the University of Arizona for the first time “demonstrated an optical material that can display "holographic video," as opposed to static holograms found in credit cards and product packages.” Since then it seemed our hopes for holograms have been getting brighter.

Now researchers at the University of Cambridge’s Centre of Molecular Materials for Photonics and Electronics (CMMPE) have used carbon nanotubes to create 3D hologram images with an extremely wide field of view and the highest possible resolution.

The research, which was published in the journal Advanced Materials (“Carbon Nanotube Based High Resolution Holograms”), essentially used the carbon nanotubes as diffractive elements that turn the carbon nanotubes into optical projectors. The small size of the carbon nanotubes created smaller pixels thus boosting the resolution of the image.

“Smaller pixels allow the diffraction of light at larger angles – increasing the field of view. Essentially, the smaller the pixel, the higher the resolution of the hologram,” says Dr. Haider Butt from CMMPE in a press release.

The demonstration of their new carbon nanotube-based pixels involved spelling out the name “Cambridge” using various colors of laser light that had been scattered through the carbon nanotube pixels. While initially a fairly modest display and dependent on the prohibitively expensive carbon nanotubes, Butt believes that some kind of nanomaterials will form the basis of a new approach to holographic images.

Butt adds in the release: “A new class of highly sensitive holographic sensors can be developed that could sense distance, motion, tilt, temperature and density of biological materials. What’s certain is that these results pave the way towards utilizing nanostructures to producing 3D holograms with wide field of view and the very highest resolution.”

To replace the carbon nanotubes, the researchers are looking at the prospect of using zinc oxide nanowires, which Zhong Lin Wang at Georgia Tech has been using over the years for its of piezoelectric qualities.

The other big issue that the researchers still we need to address is investigating “holographic video” because currently the carbon nanotube pixels can only project static holograms. Looks like there’s still some work to be done before Princess Leia holograms are projected, at least with a nanomaterial as the pixel.

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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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