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As consumers around the world have become increasingly dependent on electronics, the transistor, a semiconductor component central to the operation of these devices, has become a critical subject of scientific research. Over the last several decades, scientists and engineers have been able to both shrink the average transistor size and dramatically reduce its production costs. The current generation of smartphones, for example, relies on chips that each feature over 3.3 billion transistors.

Most transistors are silicon-based and silicon technology has driven the computer revolution. In some applications, however, silicon has significant limitations. These include use in high power electronic devices and in harsh environments like the engine of a car or under cosmic ray bombardment in space. Silicon devices are prone to faltering and failing in difficult environments.

Addressing these challenges, Jiangwei Liu, from Japan’s National Institute for Materials Sciences, and his colleagues describe new work developing diamond-based transistors this week in the journal Applied Physics Letters, from AIP Publishing.

“Silicon-based transistors often suffer from high switching loss during power transmission and fail when exposed to extremely high temperatures or levels of radiation,” Liu said. “Given the importance of developing devices that use less power and perform under harsh conditions, there has been a lot of interest within the broader scientific community in determining a way to build transistors that utilizes manufactured diamonds, which are a very durable material.”

And with this very interest in mind, the team developed a new fabrication process involving diamond, bringing “hardened electronics” closer to realization.

“Manufactured diamonds have a number of physical properties that make them very interesting to researchers working with transistors,” said Yasuo Koide, a professor and senior scientist at the National Institute for Materials Science leading the research group. “Not only are they physically hard materials, they also conduct heat well which means that they can cope with high levels of power and operate in hotter temperatures. In addition, they can endure larger voltages than existing semiconductor materials before breaking down.”

The research group focused their work on enhancement-mode metal-oxide-semiconductor field-effect transistors (MOSFETs), a type of transistor that is commonly used in electronics. One of the distinguishing features of transistors is inclusion of an insulated terminal called a “gate” whose input voltage determines whether the transistor will conduct electricity or not.

“One of the developments that makes our fabrication process innovative is that we deposited yttrium oxide (Y2O3) insulator directly onto the surface of the diamond [to form the gate],” said Liu. “We added the yttrium oxide to the diamond with a technique known as electron beam evaporation, which involves using a beam of electrons to transform molecules of yttrium oxide from the solid state to the gaseous state so that they can be made to cover a surface and solidify on it.”

According to Liu, yttrium oxide has many desirable qualities, including high thermal stability, strong affinity to oxygen and wide band gap energy, which contributes to its capabilities as an insulator.

“Another innovation was that the yttrium oxide was deposited as a single layer,” Liu said. “In our previous work, we have created oxide bi-layers, but a single layer is appealing because it’s less difficult and less expensive to manufacture.”

Liu and his colleagues hope to refine their understanding of electron movement through the diamond transistor with future research projects.

“We work with a type of manufactured diamond that has a hydrogen layer on its surface. One of the important challenges going forward will be to understand the mechanism of electron conduction through this carbon-hydrogen layer,” said Liu.

“Ultimately, our team’s goal is to build integrated circuits with diamonds,” Koide said. “With this in mind, we hope our work can support the development of energy-efficient devices that can function in conditions of extreme heat or radiation.”

Hafnia dons a new face


May 12, 2017

It’s a material world, and an extremely versatile one at that, considering its most basic building blocks — atoms — can be connected together to form different structures that retain the same composition.

Diamond and graphite, for example, are but two of the many polymorphs of carbon, meaning that both have the same chemical composition and differ only in the manner in which their atoms are connected. But what a world of difference that connectivity makes: The former goes into a ring and costs thousands of dollars, while the latter has to sit content within a humble pencil.

The inorganic compound hafnium dioxide commonly used in optical coatings likewise has several polymorphs, including a tetragonal form with highly attractive properties for computer chips and other optical elements. However, because this form is stable only at temperatures above 3100 degrees Fahrenheit — think blazing inferno — scientists have had to make do with its more limited monoclinic polymorph. Until now.

A team of researchers led by University of Kentucky chemist Beth Guiton and Texas A&M University chemist Sarbajit Banerjee in collaboration with Texas A&M materials science engineer Raymundo Arroyave has found a way to achieve this highly sought-after tetragonal phase at 1100 degrees Fahrenheit — think near-room-temperature and potential holy grail for the computing industry, along with countless other sectors and applications.

The team’s research, published today in Nature Communications, details their observation of this spectacular atom-by-atom transformation, witnessed with the help of incredibly powerful microscopes at Oak Ridge National Laboratory. After first shrinking monoclinic hafnium dioxide particles down to the size of tiny crystal nanorods, they gradually heated them, paying close attention to the barcode-like structure characterizing each nanorod and, in particular, its pair of nanoscale, fault-forming stripes that seem to function as ground zero for the transition.

“In this study we are watching a tiny metal oxide rod transform from one structure, which is the typical material found at room temperature, into a different, related structure not usually stable below 3100 degrees Fahrenheit,” said Guiton, who is an associate professor of chemistry in the UK College of Arts & Sciences. “This is significant because the high-temperature material has amazing properties that make it a candidate to replace silicon dioxide in the semiconductor industry, which is built on silicon.”

Watch through the microscope’s lens as hafnia atoms rearrange themselves at nanoscale levels in this video showing the same raw data seen by the team, courtesy of the UK Guiton Group.

The semiconductor industry has long relied on silicon dioxide as its thin, non-conductive layer of choice in the critical gap between the gate electrode — the valve that turns a transistor on and off — and the silicon transistor. Consistently thinning this non-conductive layer is what allows transistors to become smaller and faster, but Guiton points out there is such a thing as too thin — the point at which electrons start sloshing across the barrier, thereby heating their surroundings and draining power. She says most of us have seen and felt this scenario to some degree (pun intended), for instance, while watching videos on our phones and the battery simultaneously drain as the device in our palm noticeably begins to warm.

As computer chips become smaller, faster and more powerful, their insulating layers must also be much more robust — currently a limiting factor for semiconductor technology. Guiton says this new phase of hafnia is an order of magnitude better at withstanding applied fields.

When it comes to watching hafnia’s structural transition between its traditional monoclinic state and this commercially desirable tetragonal phase at near-room temperature, Banerjee says it’s not unlike popular television — specifically, the “Hall of Faces” in the HBO show “Game of Thrones.”

“In essence, we have been able to watch in real time, on an atom by atom basis, as hafnia is transformed to a new phase, much like Arya Stark donning a new face,” Banerjee said. “The new phase of hafnia has a much higher ‘k’ value representing its ability to store charge, which would allow transistors to work really quickly while merely sipping on power instead of sapping it. The stripes turn out to be really important, since that is where the transition starts as the hafnia loses its stripes.”

Arroyave credits real-time atomic-scale information for enabling the group to figure out that the transformation occurs in a very different way at nanoscale levels than it does within the macroscopic particles that result in hafnia’s monoclinic form. The fact that it is nanoscale in the first place is why he says the transition occurs at, or much closer to, room temperature.

“Through synthesis at the nanoscale, the ‘height’ of the energy barrier separating the two forms has been shrunk, making it possible to observe tetragonal hafnia at much lower temperatures than usual,” Arroyave said. “This points toward strategies that could be used to stabilize a host of useful forms of materials that can enable a wide range of functionalities and associated technologies. This is just one example of the vast possibilities that exist when we start to explore the ‘metastable’ materials space.”

Banerjee says this study suggests one way to stabilize the tetragonal phase at actual room temperature — which he notes that his group previously accomplished via a different method last year — and big implications for fast, low-power-consumption transistors capable of controlling current without drawing power, reducing speed or producing heat.

“The possibilities are endless, including even more powerful laptops that don’t heat up and sip on power from their batteries and smart phones that ‘keep calm and carry on,'” Banerjee said. “We are trying to apply these same tricks to other polymorphs of hafnium dioxide and other materials — isolating other phases that are not readily stabilized at room temperature but may also have strange and desirable properties.”

Scientists have greatly expanded the range of functional temperatures for ferroelectrics, a key material used in a variety of everyday applications, by creating the first-ever polarization gradient in a thin film.

The achievement, reported May 10 in Nature Communications by researchers at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), paves the way for developing devices capable of supporting wireless communications in extreme environments, from inside nuclear reactors to Earth’s polar regions.

Ferroelectric materials are prized for having a spontaneous polarization that is reversible by an applied electric field and for the ability to produce electric charges in response to physical pressure. They can function as capacitors, transducers, and oscillators, and they can be found in applications such as transit cards, ultrasound imaging, and push-button ignition systems.

Berkeley Lab scientists created a strain and chemical gradient in a 150-nanometer-thin film of barium strontium titanate, a widely used ferroelectric material. The researchers were able to directly measure the tiny atomic displacements in the material using cutting-edge advanced microscopy at Berkeley Lab, finding gradients in the polarization. The polarization varied from 0 to 35 microcoulombs per centimeter squared across the thickness of the thin-film material.

On the left is a low-resolution scanning transmission electron microscopy (STEM) image of a ferroelectric material that is continuously graded from barium strontium titanate (BSTO, top) to barium titanate (BTO, bottom). The material is grown on a gadolinium scandate (GSO) substrate buffered by a strontium ruthenate (SRO) bottom electrode. To the right are local nanobeam diffraction-based 2D maps of a-axis and c-axis lattice parameters that confirm large strain gradients in the ferroelectric material. The material is promising as electrically-tunable capacitors with extreme temperature stability. Credit: Anoop Damodaran/Berkeley Lab

On the left is a low-resolution scanning transmission electron microscopy (STEM) image of a ferroelectric material that is continuously graded from barium strontium titanate (BSTO, top) to barium titanate (BTO, bottom). The material is grown on a gadolinium scandate (GSO) substrate buffered by a strontium ruthenate (SRO) bottom electrode. To the right are local nanobeam diffraction-based 2D maps of a-axis and c-axis lattice parameters that confirm large strain gradients in the ferroelectric material. The material is promising as electrically-tunable capacitors with extreme temperature stability. Credit: Anoop Damodaran/Berkeley Lab

Tossing out textbook predictions

“Traditional physics and engineering textbooks wouldn’t have predicted this observation,” said study principal investigator Lane Martin, faculty scientist at Berkeley Lab’s Materials Sciences Division and UC Berkeley associate professor of materials and engineering. “Creating gradients in materials costs a lot of energy–Mother Nature doesn’t like them–and the material works to level out such imbalances in whatever way possible. In order for a large gradient like the one we have here to occur, we needed something else in the material to compensate for this unfavorable structure. In this case, the key is the material’s naturally occurring defects, such as charges and vacancies of atoms, that accommodate the imbalance and stabilize the gradient in polarization.”

Creating a polarization gradient had the beneficial effect of expanding the temperature range for optimal performance by the ferroelectric material. Barium titanate’s function is strongly temperature-dependent with relatively small effects near room temperature and a large, sharp peak in response at around 120 degrees Celsius. This makes it hard to achieve well-controlled, reliable function as the temperature varies beyond a rather narrow window. To adapt the material to work for applications at and around room temperature, engineers tune the chemistry of the material, but the range of temperatures where the materials are useful remains relatively narrow.

“The new polarization profile we have created gives rise to a nearly temperature-insensitive dielectric response, which is not common in ferroelectric materials,” said Martin. “By making a gradient in the polarization, the ferroelectric simultaneously operates like a range or continuum of materials, giving us high-performance results across a 500-degree Celsius window. In comparison, standard, off-the-shelf materials today would give the same responses across a much smaller 50-degree Celsius window.”

Beyond the obvious expansions to hotter and colder environments, the researchers noted that this wider temperature range could shrink the number of components needed in electronic devices and potentially reduce the power draw of wireless phones.

“The smartphone I’m holding in my hand right now has dielectric resonators, phase shifters, oscillators–more than 200 elements altogether–based on similar materials to what we studied in this paper,” said Martin. “About 45 of those elements are needed to filter the signals coming to and from your cell phone to make sure you have a clear signal. That’s a huge amount of real estate to dedicate to one function.”

Because changes in temperature alter the resonance of the ferroelectric materials, there are constant adjustments being made to match the materials to the wavelength of the signals sent from cell towers. Power is needed to tune the signal, and the more out of tune it is, the more power the phone needs to use to get a clear signal for the caller. A material with a polarization gradient capable operating over large temperatures regimes could reduce the power needed to tune the signal.

Faster detectors enable new imaging techniques

Understanding the polarization gradient entailed the use of epitaxial strain, a strategy in which a crystalline overlayer is grown on a substrate, but with a mismatch in the lattice structure. This strain engineering technique, commonly employed in semiconductor manufacturing, helps control the structure and enhance performance in materials.

Recent advances in electron microscopy have allowed researchers to obtain atomic-scale structural data of the strained barium strontium titanate, and to directly measure the strain and polarization gradient.

“We have established a way to use nanobeam scanning diffraction to record diffraction patterns from each point, and afterwards analyze the datasets for strain and polarization data,” said study co-author Andrew Minor, director of the National Center for Electron Microscopy at Berkeley Lab’s Molecular Foundry, a DOE Office of Science User Facility. “This type of mapping, pioneered at Berkeley Lab, is both new and very powerful.”

Another key factor was the speed of the detector, Minor added. For this paper, data was obtained at a rate of 400 frames per second, an order of magnitude faster than the 30-frame-per-second rate from just a few years ago. This technique is now available for users at the Foundry.

“We’re seeing a revolution in microscopy related to the use of direct electron detectors that is changing many fields of research,” said Minor, who also holds an appointment as a UC Berkeley professor of materials science and engineering. “We’re able to both see and measure things at a scale that was hard to imagine until recently.”

A team of researchers, led by the University of Minnesota, have discovered a new nano-scale thin film material with the highest-ever conductivity in its class. The new material could lead to smaller, faster, and more powerful electronics, as well as more efficient solar cells.

The discovery is being published today in Nature Communications, an open access journal that publishes high-quality research from all areas of the natural sciences.

Researchers say that what makes this new material so unique is that it has a high conductivity, which helps electronics conduct more electricity and become more powerful. But the material also has a wide bandgap, which means light can easily pass through the material making it optically transparent. In most cases, materials with wide bandgap, usually have either low conductivity or poor transparency.

“The high conductivity and wide bandgap make this an ideal material for making optically transparent conducting films which could be used in a wide variety of electronic devices, including high power electronics, electronic displays, touchscreens and even solar cells in which light needs to pass through the device,” said Bharat Jalan, a University of Minnesota chemical engineering and materials science professor and the lead researcher on the study.

Currently, most of the transparent conductors in our electronics use a chemical element called indium. The price of indium has generally gone up over the last two decades, which has added to the cost of current display technology. As a result, there has been tremendous effort to find alternative materials that work as well, or even better, than indium-based transparent conductors.

In this study, researchers found a solution. They developed a new transparent conducting thin film using a novel synthesis method, in which they grew a BaSnO3 thin film (a combination of barium, tin and oxygen, called barium stannate), but replaced elemental tin source with a chemical precursor of tin. The chemical precursor of tin has unique, radical properties that enhanced the chemical reactivity and greatly improved the metal oxide formation process. Both barium and tin are significantly cheaper than indium and are abundantly available.

“We were quite surprised at how well this unconventional approach worked the very first time we used the tin chemical precursor,” said University of Minnesota chemical engineering and materials science graduate student Abhinav Prakash, the first author of the paper. “It was a big risk, but it was quite a big breakthrough for us.”

Jalan and Prakash said this new process allowed them to create this material with unprecedented control over thickness, composition, and defect concentration and that this process should be highly suitable for a number of other material systems where the element is hard to oxidize. The new process is also reproducible and scalable.

They further added that it was the structurally superior quality with improved defect concentration that allowed them to discover high conductivity in the material. They said the next step is to continue to reduce the defects at the atomic scale.

“Even though this material has the highest conductivity within the same materials class, there is much room for improvement in addition, to the outstanding potential for discovering new physics if we decrease the defects. That’s our next goal,” Jalan said.

Researchers at North Carolina State University have developed a new approach for manipulating the behavior of cells on semiconductor materials, using light to alter the conductivity of the material itself.

“There’s a great deal of interest in being able to control cell behavior in relation to semiconductors – that’s the underlying idea behind bioelectronics,” says Albena Ivanisevic, a professor of materials science and engineering at NC State and corresponding author of a paper on the work. “Our work here effectively adds another tool to the toolbox for the development of new bioelectronic devices.”

The new approach makes use of a phenomenon called persistent photoconductivity. Materials that exhibit persistent photoconductivity become much more conductive when you shine a light on them. When the light is removed, it takes the material a long time to return to its original conductivity.

When conductivity is elevated, the charge at the surface of the material increases. And that increased surface charge can be used to direct cells to adhere to the surface.

“This is only one way to control the adhesion of cells to the surface of a material,” Ivanisevic says. “But it can be used in conjunction with others, such as engineering the roughness of the material’s surface or chemically modifying the material.”

For this study, the researchers demonstrated that all three characteristics can be used together, working with a gallium nitride substrate and PC12 cells – a line of model cells used widely in bioelectronics testing.

The researchers tested two groups of gallium nitride substrates that were identical, except that one group was exposed to UV light – triggering its persistent photoconductivity properties – while the second group was not.

“There was a clear, quantitative difference between the two groups – more cells adhered to the materials that had been exposed to light,” Ivanisevic says.

“This is a proof-of-concept paper,” Ivanisevic says. “We now need to explore how to engineer the topography and thickness of the semiconductor material in order to influence the persistent photoconductivity and roughness of the material. Ultimately, we want to provide better control of cell adhesion and behavior.”

Researchers at the Institute for Molecular Science, National Institutes of Natural Sciences (Japan) have developed a method for high performance doping of organic single crystal. Furthermore, they succeeded in the Hall effect measurement of the crystal — the world’s first case. The research has been published in the Advanced Materials.

Controlling “holes” and “electrons” responsible for electric conduction of p-type and n-type semiconductors by doping — adding a trace amount of impurity — had been the central technology in the 20th century’s inorganic single crystal electronics represented by silicon chips, solar cells, and light emitting diodes. The number of carriers (holes and electrons) created by doping and their moving speed (mobility) can be freely evaluated by “Hall effect measurement” using a magnetic field. However, in the field of organic electronics emerging in the 21th century, no one has ever attempted to dope impurities into an organic single crystal itself nor measure its Hall effect.

Researchers at the Institute for Molecular Science, National Institutes of Natural Sciences (Japan) have developed a method for high performance doping of organic single crystal. Furthermore, they succeeded in the Hall effect measurement of the crystal -- the world's first case. The research has been published in the Advanced Materials. Credit: Institute for Molecular Science

Researchers at the Institute for Molecular Science, National Institutes of Natural Sciences (Japan) have developed a method for high performance doping of organic single crystal. Furthermore, they succeeded in the Hall effect measurement of the crystal — the world’s first case. The research has been published in the Advanced Materials. Credit: Institute for Molecular Science

“We have combined the rubrene organic single crystal growth technique with our original ultra-slow deposition technique of one billionth of a nanometer (10- 9 nm) per second, which includes a rotating shutter having aperture.” explains Chika Ohashi, a PhD student, SOKENDAI in the group. “For the first time, we have succeeded in producing the 1 ppm doped organic single crystal and have detected its Hall effect signal.” The doping efficiency of the organic single crystal was 24%, which is a much higher performance compared to 1% for the vacuum deposited amorphous film of the same material.

Lab head Prof. Masahiro Hiramoto sees the present results have the meaning of dawn of organic single crystal electronics similar to the silicon single crystal electronics. In future, devices such as high performance organic single crystal solar cells may be developed.

Everything we experience is made of light and matter. And the interaction between the two can bring about fascinating effects. For example, it can result in the formation of special quasiparticles, called polaritons, which are a combination of light and matter. A team at the Center for Theoretical Physics of Complex Systems, within the Institute for Basic Science (IBS), modeled the behavior of polaritons in microcavities, nanostructures made of a semiconductor material sandwiched between special mirrors (Bragg mirrors). Published in Scientific Reports, this research brings new ideas to the emerging valleytronics field.

Minimal energy locations, called valleys, are shown with white crosses. Credit: IBS

Minimal energy locations, called valleys, are shown with white crosses. Credit: IBS

Emerging from the coupling of light (photons) and matter (bound state of electrons and holes known as excitons), polaritons have characteristics of each. They are formed when a light beam of a certain frequency bounces back and forth inside microcavities, causing the rapid interconversion between light and matter and resulting in polaritons with a short lifetime. “You can imagine these quasiparticles as waves that you make in water, they move together harmoniously, but they do not last very long. The short lifetime of polaritons in this system is due to the properties of the photons,” explains Mr Meng Sun, first author of the study.

Researchers are studying polaritons in microcavities to understand how their characteristics could be exploited to outperform the present semiconductor technologies. Modern optoelectronics read, process, and store information by controlling the flow of particles, but looking for new more efficient alternatives, other parameters, like the so-called ‘valleys’ could be considered. Valleys can be visualized by plotting the energy of the polaritons to their momentum. Valleytronics aims to control the properties of the valleys in some materials, like transition metal dichalcogenides (TMDCs), indium gallium aluminum arsenide (InGaAlAs), and graphene.

Being able to manipulate their features would lead to tunable valleys with two clearly different states, corresponding for example to 1 bit and 0 bit, like on-off states in computing and digital communications. A way to distinguish valleys with the same energy level is to obtain valleys with different polarization, so that electrons (or polaritons) would preferentially occupy one valley over the others. IBS scientists have generated a theoretical model for valley polarization that could be useful for valleytronics.

Although polaritons are formed by the coupling of photons and excitons, the research team modeled the two components independently. “Modeling potential profiles of photons and excitons separately is the key to find where they overlap, and then determine the minimal energy positions where valleys occur,” points out Sun.

A crucial feature of this system is that polaritons can inherit some properties, like polarization. Valleys with different polarization form spontaneously when the splitting of the transverse (i.e. perpendicular) electronic and magnetic modes of the light beam is taken into consideration (TE-TM splitting).

Since this theoretical model predicts that valleys with opposite polarization can be distinguished and tuned, in principle, different valleys could be selectively excited by a polarized laser light, leading to a possible application in valleytronics.

ClassOne Technology (classone.com), manufacturer of budget-friendly Solstice plating systems, announced it’s new CopperMax chamber — a design that is demonstrating major copper plating cost reductions for users of ≤200mm wafers.

ClassOne cited actual performance data from a CopperMax pilot installation on a Solstice tool at a Fortune 100 customer. Over a six-month period the customer tracked their actual production operating costs while using the new chamber for copper TSV, Damascene and high-rate copper plating. For the three processes with CopperMax they reported that operating costs were reduced between 95.8% and 98.4% compared with previously used conventional plating chambers.

“Many of our emerging market customers are starting to do copper plating,” said Kevin Witt, President of ClassOne Technology. “So we’ve spent a lot of time on the process, working to reduce customer costs and also increase performance. And the new CopperMax chamber is proving to do both.”

ClassOne pointed out that consumables are the largest cost factor in copper plating. Optimizing copper plating generally requires the use of expensive organic additives — which are consumed very rapidly and need to be replenished frequently.

CopperMax chamber

“We learned, however, that over 97% of those expensive additives were not being consumed by the actual plating process,” said Witt. “Most were being used up simply by contact with the anode throughout the process! So, we designed our new copper chamber specifically to keep additives away from the anode — and the results are pretty dramatic. Significant savings can be realized by high- and medium-volume users with high throughputs as well as by lower-volume and R&D users that have long idle times.”

The company explained that the CopperMax chamber employs a cation-exchange semipermeable membrane to divide the copper bath into two sections. The upper section contains all of the additives, and it actively plates the wafer. The lower section of the bath contains the anode that supplies elemental copper — which is able to travel through the membrane and into the upper section to ultimately plate the wafer. However, the membrane prevents additives from traveling down to the anode, where they would break down and form process-damaging waste products.

As a result, the CopperMax bath remains much cleaner, and bath life is extended by over 20x. This increases uptime, enables higher-quality, higher-rate Cu plating, and it reduces cost of ownership very substantially.

For example, a customer using a Solstice system with six CopperMax chambers and running TSV and high-rate copper plating will save over $300,000 per year just from additive use reductions.

In addition, the CopperMax also reduces Cu anode expenses. The chamber is designed to use inexpensive bulk anode pellets instead of solid machined Cu material, which cuts anode costs by over 50%. And since the pellets have 10x greater surface area they also increase the allowable plating rates.

“Like the rest of our equipment, this new chamber aims to serve all those smaller wafer users who have limited budgets,” said Witt. “Simply stated, CopperMax is going to give them a lot more copper plating performance for a lot less.”

Solstice plating system

Scientists at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) have developed a proof-of-principle photoelectrochemical cell capable of capturing excess photon energy normally lost to generating heat.

Using quantum dots (QD) and a process called Multiple Exciton Generation (MEG), the NREL researchers were able to push the peak external quantum efficiency for hydrogen generation to 114 percent. The advancement could significantly boost the production of hydrogen from sunlight by using the cell to split water at a higher efficiency and lower cost than current photoelectrochemical approaches.

Details of the research are outlined in the Nature Energy paper Multiple exciton generation for photoelectrochemical hydrogen evolution reactions with quantum yields exceeding 100%, co-authored by Matthew Beard, Yong Yan, Ryan Crisp, Jing Gu, Boris Chernomordik, Gregory Pach, Ashley Marshall, and John Turner. All are from NREL; Crisp also is affiliated with the Colorado School of Mines, and Pach and Marshall are affiliated with the University of Colorado, Boulder.

Beard and other NREL scientists in 2011 published a paper in Science that showed for the first time how MEG allowed a solar cell to exceed 100 percent quantum efficiency by producing more electrons in the electrical current than the amount of photons entering the solar cell.

“The major difference here is that we captured that MEG enhancement in a chemical bond rather than just in the electrical current,” Beard said. “We demonstrated that the same process that produces extra current in a solar cell can also be applied to produce extra chemical reactions or stored energy in chemical bonds.”

The maximum theoretical efficiency of a solar cell is limited by how much photon energy can be converted into usable electrical energy, with photon energy in excess of the semiconductor absorption bandedge lost to heat. The MEG process takes advantages of the additional photon energy to generate more electrons and thus additional chemical or electrical potential, rather than generating heat. QDs, which are spherical semiconductor nanocrystals (2-10 nm in diameter), enhance the MEG process.

In current report, the multiple electrons, or charge carriers, that are generated through the MEG process within the QDs are captured and stored within the chemical bonds of a H2 molecule.

NREL researchers devised a cell based upon a lead sulfide (PbS) QD photoanode. The photoanode involves a layer of PbS quantum dots deposited on top of a titanium dioxide/fluorine-doped tin oxide dielectric stack. The chemical reaction driven by the extra electrons demonstrated a new direction in exploring high-efficiency approaches for solar fuels.

Two-dimensional materials, or 2D materials for short, are extremely versatile, although – or often more precisely because – they are made up of just one or a few layers of atoms. Graphene is the best-known 2D material. Molybdenum disulphide (a layer consisting of molybdenum and sulphur atoms that is three-atoms thick) also falls in this category, although, unlike graphene, it has semiconductor properties. With his team, Dr Thomas Mueller from the Photonics Institute at TU Wien is conducting research into 2D materials, viewing them as a promising alternative for the future production of microprocessors and other integrated circuits.

Stefan Wachter, Dmitry K. Polyushkin and Thomas Mueller (f.l.t.r.). Credit: TU Wien, Marco Furchi

Stefan Wachter, Dmitry K. Polyushkin and Thomas Mueller (f.l.t.r.). Credit: TU Wien, Marco Furchi

The whole and the sum of its parts

Microprocessors are an indispensable and ubiquitous component in the modern world. Without their continued development, many of the things we take for granted these days, such as computers, mobile phones and the internet, would not be possible at all. However, while silicon has always been used in the production of microprocessors, it is now slowly but surely approaching its physical limits. 2D materials, including molybdenum disulphide, are showing promise as potential replacements. Although research into individual transistors – the most basic components of every digital circuit – made of 2D materials has been under way since graphene was first discovered back in 2004, success in creating more complex structures has been very limited. To date, it has only been possible to produce individual digital components using a few transistors. In order to achieve a microprocessor that operates independently, however, much more complex circuits are required which, in addition also need to interact flawlessly.

Thomas Mueller and his team have now managed to achieve this for the first time. The result is a 1-bit microprocessor consisting of 115 transistors over a surface area of around 0.6 mm2 that can run simple programs. “Although, this does of course seem modest when compared to the industry standards based on silicon, this is still a major breakthrough within this field of research. Now that we have a proof of concept, in principle there is no reason that further developments can’t be made,” says Stefan Wachter, a doctoral student in Dr Mueller’s research group. However, it was not just the choice of material that resulted in the success of the research project. “We also gave careful consideration to the dimensions of the individual transistors,” explains Mueller. “The exact relationships between the transistor geometries within a basic circuit component are a critical factor in being able to create and cascade more complex units.”

Future prospects

It goes without saying that much more powerful and complex circuits with thousands or even millions of transistors will be required for this technology to have a practical application. Reproducibility continues to be one of the biggest challenges currently being faced within this field of research along with the yield in the production of the transistors used. After all, both the production of 2D materials in the first place as well as the methods for processing them further are still at the very early stages. “As our circuits were made more or less by hand in the lab, such complex designs are of course pretty much beyond our capability. Every single one of the transistors has to function as planned in order for the processor to work as a whole,” explains Mueller, stressing the huge demands placed on state-of-the-art electronics. However, the researchers are convinced that industrial methods could open up new fields of application for this technology over the next few years. One such example might be flexible electronics, which are required for medical sensors and flexible displays. In this case, 2D materials are much more suitable than the silicon traditionally used owing to their significantly greater mechanical flexibility.