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A current area of intense interest in nanotechnology is van der Waals heterostructures, which are assemblies of atomically thin two-dimensional (2D) crystalline materials that display attractive conduction properties for use in advanced electronic devices.

A representative 2D semiconductor is graphene, which consists of a honeycomb lattice of carbon atoms that is just one atom thick. The development of van der Waals heterostructures has been restricted by the complicated and time-consuming manual operations required to produce them. That is, the 2D crystals typically obtained by exfoliation of a bulk material need to be manually identified, collected, and then stacked by a researcher to form a van der Waals heterostructure. Such a manual process is clearly unsuitable for industrial production of electronic devices containing van der Waals heterostructures

Now, a Japanese research team led by the Institute of Industrial Science at The University of Tokyo has solved this issue by developing an automated robot that greatly speeds up the collection of 2D crystals and their assembly to form van der Waals heterostructures. The robot consists of an automated high-speed optical microscope that detects crystals, the positions and parameters of which are then recorded in a computer database. Customized software is used to design heterostructures using the information in the database. The heterostructure is then assembled layer by layer by a robotic equipment directed by the designed computer algorithm. The findings were reported in Nature Communications.

Robot developed for automated assembly of designer nanomaterials. Credit: 2018 SATORU MASUBUCHI, INSTITUTE OF INDUSTRIAL SCIENCE, THE UNIVERSITY OF TOKYO

Robot developed for automated assembly of designer nanomaterials. Credit: 2018 SATORU MASUBUCHI, INSTITUTE OF INDUSTRIAL SCIENCE, THE UNIVERSITY OF TOKYO

“The robot can find, collect, and assemble 2D crystals in a glove box,” study first author Satoru Masubuchi says. “It can detect 400 graphene flakes an hour, which is much faster than the rate achieved by manual operations.”

When the robot was used to assemble graphene flakes into van der Waals heterostructures, it could stack up to four layers an hour with just a few minutes of human input required for each layer. The robot was used to produce a van der Waals heterostructure consisting of 29 alternating layers of graphene and hexagonal boron nitride (another common 2D semiconductor). The record layer number of a van der Waals heterostructure produced by manual operations is 13, so the robot has greatly increased our ability to access complex van der Waals heterostructures.

“A wide range of materials can be collected and assembled using our robot,” co-author Tomoki Machida explains. “This system provides the potential to fully explore van der Waals heterostructures.”

The development of this robot will greatly facilitate production of van der Waals heterostructures and their use in electronic devices, taking us a step closer to realizing devices containing atomic-level designer materials.

Physicists at the University of Warwick have today, Thursday 19th April 2018, published new research in the fournal Science today 19th April 2018 (via the Journal’s First Release pages) that could literally squeeze more power out of solar cells by physically deforming each of the crystals in the semiconductors used by photovoltaic cells.

This is an artists impression of squeezing more power out of solar cells by physically deforming each of the crystals in the semiconductors used by photovoltaic cells. Credit: University of Warwick/Mark Garlick

This is an artists impression of squeezing more power out of solar cells by physically deforming each of the crystals in the semiconductors used by photovoltaic cells. Credit: University of Warwick/Mark Garlick

The paper entitled the “Flexo-Photovoltaic Effect” was written by Professor Marin Alexe, Ming-Min Yang, and Dong Jik Kim who are all based in the University of Warwick’s Department of Physics.

The Warwick researchers looked at the physical constraints on the current design of most commercial solar cells which place an absolute limit on their efficiency. Most commercial solar cells are formed of two layers creating at their boundary a junction between two kinds of semiconductors, p-type with positive charge carriers (holes which can be filled by electrons) and n-type with negative charge carriers (electrons).

When light is absorbed, the junction of the two semiconductors sustains an internal field splitting the photo-excited carriers in opposite directions, generating a current and voltage across the junction. Without such junctions the energy cannot be harvested and the photo-exited carriers will simply quickly recombine eliminating any electrical charge.

That junction between the two semiconductors is fundamental to getting power out of such a solar cell but it comes with an efficiency limit. This Shockley-Queisser Limit means that of all the power contained in sunlight falling on an ideal solar cell in ideal conditions only a maximum of 33.7% can ever be turned into electricity.

There is however another way that some materials can collect charges produced by the photons of the sun or from elsewhere. The bulk photovoltaic effect occurs in certain semiconductors and insulators where their lack of perfect symmetry around their central point (their non-centrosymmetric structure) allows generation of voltage that can be actually larger than the band gap of that material (the band gap being the gap between the valence band highest range of electron energies in which electrons are normally present at absolute zero temperature and the conduction band where electricity can flow).

Unfortunately the materials that are known to exhibit the anomalous photovoltaic effect have very low power generation efficiencies, and are never used in practical power-generation systems.

The Warwick team wondered if it was possible to take the semiconductors that are effective in commercial solar cells and manipulate or push them in some way so that they too could be forced into a non-centrosymmetric structure and possibly therefore also benefit from the bulk photovoltaic effect.

For this paper they decided to try literally pushing such semiconductors into shape using conductive tips from atomic force microscopy devices to a “nano-indenter” which they then used to squeeze and deform individual crystals of Strontium Titanate (SrTiO3), Titanium Dioxide (TiO2), and Silicon (Si).

They found that all three could be deformed in this way to also give them a non-centrosymmetric structure and that they were indeed then able to give the bulk photovoltaic effect.

Professor Marin Alexe from the University of Warwick said:

“Extending the range of materials that can benefit from the bulk photovoltaic effect has several advantages: it is not necessary to form any kind of junction; any semiconductor with better light absorption can be selected for solar cells, and finally, the ultimate thermodynamic limit of the power conversion efficiency, so-called Shockley-Queisser Limit, can be overcome. There are engineering challenges but it should be possible to create solar cells where a field of simple glass based tips (a hundred million per cm2) could be held in tension to sufficiently de-form each semiconductor crystal. If such future engineering could add even a single percentage point of efficiency it would be of immense commercial value to solar cell manufacturers and power suppliers.”

A chemical reactor that operates at extremely high temperatures is being developed by KAUST and could improve the efficiency and economy of a commonly used process in the semiconductor industry, with flow-on benefits for Saudi Arabia’s chemical industry.

The production of semiconductors relies on epitaxy: a process that creates high-quality single-crystal materials by depositing atoms on to a wafer layer by layer, controlling thickness with atomic precision.

The most common method of epitaxy is metalorganic chemical vapor deposition, or MOCVD. Pure vapors of organic molecules containing the desired atoms–for example, boron and nitrogen in the case of boron nitride–are injected into a reaction chamber. The molecules decompose on a heated wafer to leave the semiconductor’s atoms behind on the surface, which bond both to each other and the wafer to form a crystal layer.

Ph.D. student Kuang-Hui Li and a team led by Xiaohang Li at KAUST are developing an MOCVD reactor that can efficiently operate at extremely high temperatures to create high-quality boron nitride and aluminum nitride materials and devices particularly promising for flexible electronics, ultraviolet optoelectronics and power electronics.

The epitaxy of high-quality boron nitride and aluminum nitride have been a huge challenge for the conventional MOCVD process, which usually operates below 1200 degrees Celsius. Epitaxy of these materials responds best to temperatures over 1600 degrees Celsius; however, the most common resistant heaters are not reliable at these temperatures.

Although induction heaters can reach these temperatures, the heating efficiency of the conventional design is low. Because the wasted energy can overheat the gas inlet, it must be placed far away from the wafer, which is problematic for high-quality boron nitride and aluminum nitride due to particle generation and low utilization of organic molecules.

The KAUST team has developed an innovative and low-cost induction heating structure to solve these problems. “Our design can help greatly improve uniformity for up to 12-inch wafers and reduce particle generation, which is crucial for high-quality material and device fabrication,” says Kuang-Hui. “It also allows us to discover new materials.”

The results show significant increase in heating efficiency and reduction in wasted energy. “This equipment research involves many disciplines and is highly complex. However, history has shown that equipment innovation is the key to scientific breakthroughs and industrial revolution,” says Xiaohang Li. “A goal of the research is to set up MOCVD manufacturing activities that can be integrated into the huge chemical industry of Saudi Arabia.”

Flexible televisions, tablets and phones as well as ‘truly wearable’ smart tech are a step closer thanks to a nanoscale transistor created by researchers at The University of Manchester and Shandong University in China.

The international team has developed an ultrafast, nanoscale transistor – known as a thin film transistor, or TFT, – made out of an oxide semiconductor. The TFT is the first oxide-semiconductor based transistor that is capable of operating at a benchmark speed of 1 GHz. This could make the next generation electronic gadgets even faster, brighter and more flexible than ever before.

A TFT is a type of transistor usually used in a liquid crystal display (LCD). These can be found in most modern gadgets with LCD screens such as smart phones, tablets and high-definition televisions.

How do they work? LCD features a TFT behind each individual pixel and they act as individual switches that allow the pixels to change state rapidly, making them turn on and off much more quickly.

But most current TFTs are silicon-based which are opaque, rigid and expensive in comparison to the oxide semiconductor family of transistors which the team from the UK and China are developing. Whilst oxide TFTs will improve picture on LCD displays, it is their flexibility that is even more impressive.

Aimin Song, Professor of Nanoelectronics in the School of Electrical & Electronic Engineering, The University of Manchester, explains: “TVs can already be made extremely thin and bright. Our work may help make TV more mechanically flexible and even cheaper to produce.

“But, perhaps even more importantly, our GHz transistors may enable medium or even high performance flexible electronic circuits, such as truly wearable electronics. Wearable electronics requires flexibility and in many cases transparency, too. This would be the perfect application for our research.

“Plus, there is a trend in developing smart homes, smart hospitals and smart cities – in all of which oxide semiconductor TFTs will play a key role.”

Oxide-based technology has seen rapid development when compared to its silicon counterpart which is increasingly close to some fundamental limitations. Prof Song says there has been fast progress in oxide-semiconductors in recent years and extensive efforts have been made in order to improve the speed of oxide-semiconductor-based TFTs.

So much so some oxide-based technology has already started replacing amorphous silicon in some gadgets. Prof Song thinks these latest developments have brought commercialisation much closer.

He added: “To commercialise oxide-based electronics there is still a range of research and development that has to be carried out on materials, lithography, device design, testing, and last but not the least, large-area manufacturing. It took many decades for silicon technology to get this far, and oxides are progressing at a much faster pace.

“Making a high performance device, like our GHz IGZO transistor, is challenging because not only do materials need to be optimised, a range of issues regarding device design, fabrication and tests also have to be investigated. In 2015, we were able to demonstrate the fastest flexible diodes using oxide semiconductors, reaching 6.3 GHz, and it is still the world record to date. So we’re confident in oxide-semiconductor based technologies. ”

 

Collaborative research team of Prof. Jun Takeda and Associate Prof. Ikufumi Katayama in the laboratory of Yokohama National University (YNU) and Nippon Telegraph and Telephone (NTT) successfully observed petahertz (PHz: 1015of a hertz) electron oscillation. The periodic electron oscillations of 667-383 attoseconds (as: 10-18 of a second) is the fastest that has ever been measured in the direct time-dependent spectroscopy in solid-state material.

NIR femtosecond pulse (pump pulse) induces the electron oscillation, which is monitored by the extreme ultraviolet IAP (probe pulse) based on the transient absorption spectroscopy. Credit: Nippon Telegraph and Telephone (NTT)

NIR femtosecond pulse (pump pulse) induces the electron oscillation, which is monitored by the extreme ultraviolet IAP (probe pulse) based on the transient absorption spectroscopy. Credit: Nippon Telegraph and Telephone (NTT)

As high-speed shutter cameras capture motions of fast-moving objects, researchers generally use laser (pulse) like instantaneous strobe light in order to observe the ultrafast motion of an electron underlying a physical phenomenon. The shorter the pulse duration, the faster the electron oscillation can be observed. The frequency of the lightwave-field in the visible and ultraviolet region can reach the petahertz (PHz: 1015 of a hertz), which means that the oscillation periodicity can achieve attosecond (as: 10-18 of a second) duration.

In previous studies, NTT researchers of the team generated an isolated attosecond pulse (IAP) [H. Mashiko et al., Nature commun. 5, 5599 (2014)] and monitored the electron oscillation with 1.2-PHz frequency using gallium-nitride (GaN) semiconductor [H. Mashiko et al., Nature Phys. 5, 741 (2016)]. The next challenges are the observation of faster electron oscillation in the chromium doped sapphire (Cr:Al2O3) insulator and the characterization of the ultrafast electron dephasing.

The paper, published in the journal Nature communications reports a successful observation of the near-infrared (NIR) pulse-induced multiple electronic dipole oscillations (periodicities of 667-383 as) in the Cr:Al2O3 solid-state material. The measurement is realized by the extreme short IAP (192-as duration) and the use of stable pump (NIR pulse) and probe (IAP) system (timing jitter of ~23 as). The characterized electron oscillations are the fastest that has ever been measured in the direct time-dependent spectroscopy. In addition, the individual dephasing times in the Cr donor-like intermediate level and the Al2O3 CB state are revealed.

Dr. Hiroki Mashiko, a NTT scientist of the team, said, “We contrived the robust pump-probe system with an extremely short isolated attosecond pulse, which led to the observation of the fastest electron oscillation in solid-state material in recorded history. The benefits of this study are directly related to the control of various optical phenomena through the dielectric polarization, and the results will help the development of future electronic and photonic devices.”

In a new paper published by Nano Energy, experts from the Advanced Technology Institute (ATI) at the University of Surrey detail a new methodology that allows designers of smart-wearables to better understand and predict how their products would perform once manufactured and in use.

The technology is centred on materials that become electrically charged after they come into contact with each other, known as ‘triboelectric materials’ – for example, a comb through hair can create an electrical charge. ‘Triboelectric Nanogenerators (TENGs)’ use this static charge to harvest energy from movement through a process called electrostatic induction. Over the years, a variety of TENGs have been designed which can convert almost any type of movement into electricity. The University of Surrey’s tool gives manufacturers an accurate understanding of the output power their design would create once produced.

This follows the news earlier this year of the ATI announcing the creation of its £4million state-of-the-art Nano-Manufacturing Hub. The new facility will produce plastic nanoscale electronics for wearable sensors, electronic tags and other electronic devices.

Ishara Dharmasena, lead scientist on this project from the University of Surrey, said: “The future global energy mix will depend on renewable energy sources such as solar power, wind, motion, vibrations and tidal. TENGs are a leading technology to capture and convert motion energy into electricity, extremely useful in small scale energy harvesting applications. Our work will, for the first time, provide universal guidance to develop, compare and improve various TENG designs. We expect this technology in household and industrial electronic products, catering to a new generation of mobile and autonomous energy requirements.”

Professor Ravi Silva, Director of the Advanced Technology Institute, said: “This is truly an exciting area of research for our team – an area we have been working on over a number of years. We believe that our new tool will be of great help to a lot of researchers and designers who are investigating these materials.

“The world urgently needs new forms of affordable and renewable energy sources. TENGs not only present a wonderful opportunity for the consumer electronics industry, but they are an incredibly exciting material group that could be used in all countries and remote locations where the nation grid does not extend, particularly for radios, wireless communication devices and medical equipment.”

Over the past decades, computers have become faster and faster and hard disks and storage chips have reached enormous capacities. But this trend cannot continue forever: we are already running up against physical limits that will prevent silicon-based computer technology from attaining any impressive speed gains from this point on. Researchers are particularly optimistic that the next era of technological advancements will start with the development of novel information-processing materials and technologies that combine electrical circuits with optical ones. Using short laser pulses, a research team led by Misha Ivanov of the Max Born Institute in Berlin together with scientists from the Russian Quantum Center in Moscow have now shed light on the extremely rapid processes taking place within these novel materials. Their results have appeared in the prestigious journal Nature Photonics.

Of particular interest for modern material research in solid state physics are “strongly correlated systems”, so called for the strong interactions between the electrons in these materials. Magnets are a good example of this: the electrons in magnets align themselves in a preferred direction of spin inside the material, and it is this that produces the magnetic field. But there are other, entirely different structural orders that deserve attention. In so-called Mott insulators for example, a class of materials now being intensively researched, the electrons ought to flow freely and the materials should therefore be able to conduct electricity as well as metals. But the mutual interaction between electrons in these strongly correlated materials impedes their flow and so the materials behave as insulators instead.

By disrupting this order with a strong laser pulse, the physical properties can be made to change dramatically. This can be likened to a phase transition from solid to liquid: as ice melts, for example, rigid ice crystals transform into free-flowing water molecules. Very similarly, the electrons in a strongly correlated material become free to flow when an external laser pulse forces a phase transition in their structural order. Such phase transitions should allow us to develop entirely new switching elements for next-generation electronics that are faster and potentially more energy efficient than present-day transistors. In theory, computers could be made around a thousand times faster by “turbo-charging” their electrical components with light pulses.

The problem with studying these phase transitions is that they are extremely fast and it is therefore very difficult to “catch them in the act”. So far, scientists have had to content themselves with characterising the state of a material before and after a phase transition of this kind. Researchers Rui E. F. Silva, Olga Smirnova, and Misha Ivanov of the Berlin Max Born Institute, however, have now devised a method that will, in the truest sense, shed light on the process. Their theory involves firing extremely short, tailored laser pulses at a material – pulses that can only recently be produced in the appropriate quality given the latest developments in lasers. One then observes the material’s reaction to these pulses to see how the electrons in the material are excited into motion and, like a bell, emit resonant vibrations at specific frequencies, as harmonics of the incident light.

“By analysing this high harmonic spectrum, we can observe the change in the structural order in these strongly correlated materials ‘live’ for the first time,” says first author of the paper Rui Silva of the Max Born Institute. Laser sources capable of targetedly triggering these transitions have only been available since very recently. The laser pulses namely have to be amply strong and extremely short – on the order of femtoseconds in duration (millionths of a billionth of a second).

In some cases, it takes only a single oscillation of light to disrupt the electronic order of a material and turn an insulator into a metal-like conductor. The scientists at the Berlin Max Born Institute are among the world’s leading experts in the field of ultrashort laser pulses.

“If we want to use light to control the properties of electrons in a material, then we need to know exactly how the electrons will react to light pulses,” Ivanov explains. With the latest-generation laser sources, which allow full control over the electromagnetic field even down to a single oscillation, the newly published method will allow deep insights into the materials of the future.

Researchers from Tomsk Polytechnic University together with their international colleagues have discovered a method to modify and use the one-atom thin conductor of current and heat, graphene without destroying it. Thanks to the novel method, the researchers were able to synthesize on single-layer graphene a well-structured polymer with a strong covalent bond, which they called ‘polymer carpets’. The entire structure is highly stable; it is less prone to degradation over time that makes the study promising for the development of flexible organic electronics. Also, if a layer of molybdenum disulfide is added over the ‘nanocarpet’, the resulting structure generates current under exposure to light. The study results were published in Journal of Materials Chemistry C.

This is the scheme for obtaining a hybrid structure of 'graphene-polymer'. Credit: Tomsk Polytechnic University

This is the scheme for obtaining a hybrid structure of ‘graphene-polymer’. Credit: Tomsk Polytechnic University

Graphene is simultaneously the most durable, light and an electrically conductive carbon material. It can be used for manufacturing solar batteries, smartphone screens, thin and flexible electronics, and even in water filters since graphene films pass water molecules and stop all other compounds. Graphene should be integrated into complex structures to be used successfully. However, it is a challenge to do that. According to scientists, graphene itself is stable enough and reacts poorly with other compounds. In order to make it react with other elements, i.e. to modify it, graphene is usually at least partially destroyed. This modification degrades the properties of the materials obtained.

Professor Raul D. Rodriguez from the Research School for Chemistry & Applied Biomedical Sciences says: ‘When functionalizing graphene, you should be very careful. If you overdo it, the unique properties of graphene are lost. Therefore, we decided to follow a different path.

In graphene, there are inevitable nanodefects, for example, at the edges of graphene and wrinkles in the plane. Hydrogen atoms are often attached to such defects. It is this hydrogen that can interact with other chemicals.’

To modify graphene, the authors use a thin metal substrate on which a graphene single-layer is placed. Then graphene is covered with a solution of bromine-polystyrene molecules. The molecules interact with hydrogen and are attached to the existing defects, resulting in polyhexylthiophene (P3HT). Further exposed to light during the photocatalysis, a polymer begins to ‘grow’.

‘In the result, we obtained the samples which structure resembles ‘polymer carpets’ as we call them in the paper. Above such a ‘polymer carpet’ we place molybdenum disulfide. Due to a unique combination of materials, we obtain a ‘sandwich’ structure’ that functions like a solar battery. That is, it generates current when exposed to light. In our experiments a strong covalent bond is established between the molecules of the polymer and graphene, that is critical for the stability of the material obtained,’ notes Rodriguez.

According to the researcher, the method for graphene modification, on the one hand, enables obtaining a very sturdy compound; on the other hand, it is rather simple and cheap as affordable materials are used. The method is versatile because it makes growing very different polymers directly on graphene possible.

‘The strength of the obtained hybrid material is achieved additionally because we do not destroy graphene itself but use pre-existing defects, and a strong covalent bond to polymer molecules. This allows us to consider the study as promising for the development of thin and flexible electronics when solar batteries can be attached to clothes, and when deformed they will not break,’ the professor explains.

Toyoda Gosei Co., Ltd. has achieved state-of-the-art high current operation1 in a vertical GaN power semiconductor developed using gallium nitride (GaN), a main material in blue LEDs.

Power semiconductors are widely used in power converters2 such as power sources and adaptors for electronic devices. However, simultaneous achievement of both high breakdown voltage3 and low loss4 (low conduction loss and switching loss) at high levels has been difficult with conventional silicon due to its material properties.

In its power semiconductors, Toyoda Gosei uses GaN, which has material properties of high breakdown voltage and low loss, and employs a vertical device structure in which electrical current flows vertically from or to a substrate. These changes have enabled a GaN power transistor chip with operating current of over 50A, highest ever reported for vertical GaN transistors2, and high-frequency (several megahertz) operation. Some prospective applications are shown below.

Promising areas of use (examples)

Power converters
More compact & lighter weight, higher efficiency

 Power control units (PCUs) for automobiles, etc.
DC-DC converters

High frequency power sources
Higher output

 Wireless power supply

Toyoda Gosei will continue development of these power semiconductors for improved reliability, aiming to achieve practical applications in cooperation with semiconductor and electronics manufacturers.

The newly developed vertical GaN power transistors (MOSFET)5 and Schottky barrier diodes6 will be presented on panel displays at the Techno-Frontier 2018 Advanced Electronic & Mechatronic Devices and Components Exhibition, held at Makuhari Messe, Chiba, Japan from April 18 to April 20. The world’s first full vertical-GaN DC-DC converter equipped with these devices will also be demonstrated at the company’s booth (6F-11, Hall 6).

1 According to internal Toyoda Gosei survey (as of April 2018).
2 Power conversion refers to conversion between direct and alternating current, direct current transformation, alternating current frequency conversion, etc.
3 The property of withstanding the high breakdown voltage during power conversion and not allowing current flow during off operation (non-conductance).
4 Heat loss generated by electrical resistance during electric conduction or when switching on/off.
5 Semiconductor used in power on/off.
MOSFET: Metal-oxide-semiconductor field-effect-transistor.
6 Semiconductor used in converting (rectification) from alternating current to direct current. Toyoda Gosei uses a trench MOS structure, in which trenches are formed at fixed intervals in the chip surface of the diode, achieving low leakage current operation at high temperatures.

Trapping light with an optical version of a whispering gallery, researchers at the National Institute of Standards and Technology (NIST) have developed a nanoscale coating for solar cells that enables them to absorb about 20 percent more sunlight than uncoated devices. The coating, applied with a technique that could be incorporated into manufacturing, opens a new path for developing low-cost, high-efficiency solar cells with abundant, renewable and environmentally friendly materials.

This is illustration shows the nanoresonator coating, consisting of thousands of tiny glass beads, deposited on solar cells. The coating enhances both the absorption of sunlight and the amount of current produced by the solar cells. Credit: K. Dill, D. Ha, G. Holland/NIST

This is illustration shows the nanoresonator coating, consisting of thousands of tiny glass beads, deposited on solar cells. The coating enhances both the absorption of sunlight and the amount of current produced by the solar cells. Credit: K. Dill, D. Ha, G. Holland/NIST

The coating consists of thousands of tiny glass beads, only about one-hundredth the width of a human hair. When sunlight hits the coating, the light waves are steered around the nanoscale bead, similar to the way sound waves travel around a curved wall such as the dome in St. Paul’s Cathedral in London. At such curved structures, known as acoustic whispering galleries, a person standing near one part of the wall easily hears a faint sound originating at any other part of the wall.

Whispering galleries for light were developed about a decade ago, but researchers have only recently explored their use in solar-cell coatings. In the experimental set up devised by a team including Dongheon Ha of NIST and the University of Maryland’s NanoCenter, the light captured by the nanoresonator coating eventually leaks out and is absorbed by an underlying solar cell made of gallium arsenide.

Using a laser as a light source to excite individual nanoresonators in the coating, the team found that the coated solar cells absorbed, on average, 20 percent more visible light than bare cells. The measurements also revealed that the coated cells produced about 20 percent more current.

The study is the first to demonstrate the efficiency of the coatings using precision nanoscale measurements, said Ha. “Although calculations had suggested the coatings would enhance the solar cells, we could not prove this was the case until we had developed the nanoscale measurement technologies that were needed,” he noted.

This work was described in a recent issue of Nanotechnology by Ha, collaborator Yohan Yoon of NIST and Maryland’s NanoCenter, and NIST physicist Nikolai Zhitenev.

The team also devised a rapid, less-costly method of applying the nanoresonator coating. Researchers had previously coated semiconductor material by dipping it in a tub of the nanoresonator solution. The dipping method takes time and coats both sides of the semiconductor even though only one side requires the treatment.

In the team’s method, droplets of the nanoresonator solution are placed on just one side of the solar cell. A wire-wound metal rod is then pulled across the cell, spreading out the solution and forming a coating made of closely packed nanoresonators. This is the first time that researchers have applied the rod method, used for more than a century to coat material in a factory setting, to a gallium arsenide solar cell.

“This is an inexpensive process and is compatible with mass production,” said Ha.