Tag Archives: letter-pulse-tech

Researchers have uncovered the exact mechanism that causes new solar cells to break down in air, paving the way for a solution.

Solar cells harness energy from the Sun and provide an alternative to non-renewable energy sources like fossil fuels. However, they face challenges from costly manufacturing processes and poor efficiency – the amount of sunlight converted to useable energy.

Light-absorbing materials called organic lead halide perovskites are used in a new type of solar cells that have shown great promise, as they are more flexible and cheaper to manufacture than traditional solar cells constructed of silicon.

However, perovskite cells degrade rapidly in natural conditions, greatly decreasing their performance in a matter of days. This is one reason they are not currently widely used.

Previously, a team led by scientists from the Department of Chemistry at Imperial discovered that this breakdown is due to the formation of ‘superoxides’ that attack the perovskite material. These superoxides are formed when light hitting the cells releases electrons, which react with the oxygen in the air.

Now, in a study published in Nature Communications, the team have determined how the superoxides form and how they attack the perovskite material, and have proposed possible solutions.

Working with Dr Christopher Eames and Professor Saiful Islam at the University of Bath, the team found that superoxide formation is helped by spaces in the structure of the perovskite normally taken up by molecules of iodide. Although iodide is a component of the perovskite material itself, there are defects where iodide is missing. These vacant spots are then used in the formation of superoxides.

The team found that dosing the material with extra iodide after manufacturing did improve the stability, but that a more permanent solution could be to engineer the iodide defects out.

Lead author of the new study, Nicholas Aristidou from the Department of Chemistry at Imperial, said: “After identifying the role of iodide defects in generating superoxide, we could successfully improve the material stability by filling the vacancies with additional iodide ions. This open up a new way of optimising the material for enhanced stability by controlling the type and density of defects present.”

Lead researcher Dr Saif Haque from the Department of Chemistry at Imperial added: “We have now provided a pathway to understand this process at the atomic scale and allow the design of devices with improved stability.”

Currently, the only way of protecting perovskite cells from degradation by air and light is to encase them in glass. However, perovskite solar cells are made from flexible material designed to be used in a range of settings, so the glass encasement severely limits their function.

Dr Haque said: “Glass encasement restricts movement and adds weight and cost to the cells. Improving the perovskite cell material itself is the best solution.”

The team hope to next test the stability of the cells in real-world settings. The cells would be exposed to a combination of both oxygen and moisture, testing the cells in more relevant scenarios.

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.”

Chemists have tried to synthesize carbon nanobelts for more than 60 years, but none have succeeded until now. A team at Nagoya University reported the first organic synthesis of a carbon nanobelt in Science. Carbon nanobelts are expected to serve as a useful template for building carbon nanotubes and open a new field of nanocarbon science.

The new nanobelt, measuring 0.83 nanometer (nm) in diameter, was developed by researchers at Nagoya University’s JST-ERATO Itami Molecular Nanocarbon Project, and the Institute of Transformative Bio-Molecules (ITbM). Scientists around the world have tried to synthesize carbon nanobelts since the 1950s and Professor Kenichiro Itami’s group has worked on its synthesis for 12 years.

“Nobody knew whether its organic synthesis was even possible or not,” says Segawa, one of the leaders of this study who had been involved in its synthesis for 7 and a half years. “However, I had my mind set on the synthesis of this beautiful molecule.”

Carbon nanobelts are belt-shaped molecules composed of fused benzene rings, which are aromatic rings consisting of six carbon atoms. Carbon nanobelts are a segment of carbon nanotubes, which have various applications in electronics and photonics due to their unique physical characteristics.

Current synthetic methods produce carbon nanotubes with inconsistent diameters and sidewall structures, which changes their electrical and optical properties. This makes it extremely difficult to isolate and purify a single carbon nanotube that has a specific diameter, length and sidewall structure. Therefore, being able to precisely control the synthesis of structurally uniform carbon nanotubes will help develop novel and highly functional materials.

Carbon nanobelts have been identified as a way to build structurally uniform carbon nanotubes. However, synthesizing carbon nanobelts is challenging due to their extremely high strain energies. This is because benzene is stable when flat, but becomes unstable when they are distorted by fusion of the rings.

To overcome this problem, Guillaume Povie, a postdoctoral researcher of the JST-ERATO project, Yasutomo Segawa, a group leader of the JST-ERATO project, and Kenichiro Itami, the director of JST-ERATO project and the center director of ITbM, have succeeded in the first chemical synthesis of a carbon nanobelt from a readily available precursor, p-xylene (a benzene molecule with two methyl groups in the 1,4- (para-) position) in 11 steps.

The key to this success is their synthetic strategy based on the belt-shaped formation from a macrocycle precursor with relatively low ring strain. In their strategy, the team prepared a macrocycle precursor from p-xylene in 10 steps, and formed the belt-shaped aromatic compound by a coupling reaction. Nickel was essential to mediate the coupling process.

“The most difficult part of this research was this key coupling reaction of the macrocycle precursor,” says Povie. “The reaction did not proceed well day after day and it took me three to four months for testing various conditions. I have always believed where there’s a will, there’s a way.”

In 2015, Itami launched a new initiative in his ERATO project to focus particularly on the synthesis of the carbon nanobelt. At the so-called “belt festival,” various new synthetic routes for the carbon nanobelt were proposed and more than 10 researchers were involved in the project. On September 28, 2016, exactly a year after the start of the festival, the carbon nanobelt structure was finally revealed by X-ray crystallography in front of the Itami group members. Everyone held their breath while staring at the screen during X-ray analysis, and cheered when the cylindrical shape image of the carbon nanobelt appeared on the screen. Itami, Segawa and Povie expressed their joy with a high five (movie: https://www.youtube.com/watch?v=cABZla9w0uo).

“It was one of the most exciting moments in my life and I will never forget it,” says Itami. “Since this is the result of a decade-long study, I greatly appreciate all the past and current members of my group for their support and encouragement. Thanks to their skill, toughness, sense and strong will of all members, we achieved this successful result.”

The synthesized carbon nanobelt is a red-colored solid and exhibits deep red fluorescence. Analysis by X-ray crystallography revealed that the carbon nanobelt has a cylindrical shape in the same manner as carbon nanotubes. The researchers also measured its light absorption and emission, electric conductivity and structural rigidity by ultraviolet-visible absorption fluorescence, and Raman spectroscopic studies, as well as theoretical calculations.

“Actually, the synthesis part was finished last August but I could not rest until I was able to confirm the X-ray structure of the carbon nanobelt,” says Povie. “I was really happy when I saw the X-ray structure.”

The carbon nanobelt will be released to the market in the future. “We are looking forward to discovering new properties and functionalities of the carbon nanobelt with researchers from all over the world,” say Segawa and Itami.

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.”

Two-dimensional graphene consists of single layers of carbon atoms and exhibits intriguing properties. The transparent material conducts electricity and heat extremely well. It is at the same time flexible and solid. Additionally, the electrical conductivity can be continuously varied between a metal and a semiconductor by, e.g., inserting chemically bound atoms and molecules into the graphene structure – the so-called functional groups. These unique properties offer a wide range of future applications as e.g. for new developments in optoelectronics or ultrafast components in the semiconductor industry. However, a successful use of graphene in the semiconductor industry can only be achieved if properties such as the conductivity, the size and the defects of the graphene structure induced by the functional groups can already be modulated during the synthesis of graphene.

In an international collaboration scientists led by Andreas Hirsch from the Friedrich-Alexander-Universität Erlangen-Nürnberg in close cooperation with Thomas Pichler from the University of Vienna accomplished a crucial breakthrough: using the latter’s newly developed experimental set-up they were able to identify, for the first time, vibrational spectra as the specific fingerprints of step-by-step chemically modified graphene by means of light scattering. This spectral signature, which was also theoretically attested, allows to determine the type and the number of functional groups in a fast and precise way. Among the reactions they examined, was the chemical binding of hydrogen to graphene. This was implemented by a controlled chemical reaction between water and particular compounds in which ions are inserted in graphite, a crystalline form of carbon.

This is a section of a graphene network with chemically bound hydrogen atom: the spectral vibrational signature of the single carbon-carbon bonds adjacent to the bound hydrogen atom is highlighted in different colors. Copyright: Frank Hauke, FAU

This is a section of a graphene network with chemically bound hydrogen atom: the spectral vibrational signature of the single carbon-carbon bonds adjacent to the bound hydrogen atom is highlighted in different colors. Copyright: Frank Hauke, FAU

Additional benefits

“This method of the in-situ Raman spectroscopy is a highly effective technique which allows controlling the function of graphene in a fast, contact-free and extensive way already during the production of the material,” says J. Chacon from Yachay Tech, one of the two lead authors of the study. This enables the production of tailored graphene-based materials with controlled electronic transport properties and their utilisation in semiconductor industry.

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.

No more error-prone evaporation deposition, drop casting or printing: Scientists at Ludwig-Maximilians-Universitaet (LMU) in Munich and FSU Jena have developed organic semiconductor nanosheets, which can easily be removed from a growth substrate and placed on other substrates.

Today’s computer processors are composed of billions of transistors. These electronic components normally consist of semiconductor material, insulator, substrate, and electrode. A dream of many scientists is to have each of these elements available as transferable sheets, which would allow them to design new electronic devices simply by stacking.

This has now become a reality for the organic semiconductor material pentacene: Dr. Bert Nickel, a physicist at LMU Munich, and Professor Andrey Turchanin (Friedrich Schiller University Jena), together with their teams, have, for the first time, managed to create mechanically stable pentacene nanosheets.

The researchers describe their method in the journal Advanced Materials. They first cover a small silicon wafer with a thin layer of a water-soluble organic film and deposit pentacene molecules upon it until a layer roughly 50 nanometers thick has formed. The next step is crucial: by irradiation with low-energy electrons, the topmost three to four levels of pentacene molecular layers are crosslinked, forming a “skin” that is only about five nanometers thick. This crosslinked layer stabilizes the entire pentacene film so well that it can be removed as a sheet from a silicon wafer in water and transferred to another surface using ordinary tweezers.

Apart from the ability to transfer them, the new semiconductor nanosheets have other advantages. The new method does not require any potentially interfering solvents, for example. In addition, after deposition, the nanosheet sticks firmly to the electrical contacts by van der Waals forces, resulting in a low contact resistance of the final electronic devices. Last but not least, organic semiconductor nanosheets can now be deposited onto significantly more technologically relevant substrates than hitherto.

Of particular interest is the extremely high mechanical stability of the newly developed pentacene nanosheets, which enables them to be applied as free-standing nanomembranes to perforated substrates with dimensions of tens of micrometers. That is equivalent to spanning a 25-meter pool with plastic wrap. “These virtually freely suspended semiconductors have great potential,” explains Nickel. “They can be accessed from two sides and could be connected through an electrolyte, which would make them ideal as biosensors, for example”. “Another promising application is their implementation in flexible electronics for manufacturing of devices for vital data acquisition or production of displays and solar cells,” Turchanin says.

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.”

As electronics become increasingly pervasive in our lives – from smart phones to wearable sensors – so too does the ever rising amount of electronic waste they create. A United Nations Environment Program report found that almost 50 million tons of electronic waste were thrown out in 2017–more than 20 percent higher than waste in 2015.

Troubled by this mounting waste, Stanford engineer Zhenan Bao and her team are rethinking electronics. “In my group, we have been trying to mimic the function of human skin to think about how to develop future electronic devices,” Bao said. She described how skin is stretchable, self-healable and also biodegradable – an attractive list of characteristics for electronics. “We have achieved the first two [flexible and self-healing], so the biodegradability was something we wanted to tackle.”

The team created a flexible electronic device that can easily degrade just by adding a weak acid like vinegar. The results were published May 1 in the Proceedings of the National Academy of Sciences.

A newly developed flexible, biodegradable semiconductor developed by Stanford engineers shown on a human hair. Credit: Bao Lab

A newly developed flexible, biodegradable semiconductor developed by Stanford engineers shown on a human hair. Credit: Bao Lab

“This is the first example of a semiconductive polymer that can decompose,” said lead author Ting Lei, a postdoctoral fellow working with Bao.

In addition to the polymer – essentially a flexible, conductive plastic – the team developed a degradable electronic circuit and a new biodegradable substrate material for mounting the electrical components. This substrate supports the electrical components, flexing and molding to rough and smooth surfaces alike. When the electronic device is no longer needed, the whole thing can biodegrade into nontoxic components.

Biodegradable bits

Bao, a professor of chemical engineering and materials science and engineering, had previously created a stretchable electrode modeled on human skin. That material could bend and twist in a way that could allow it to interface with the skin or brain, but it couldn’t degrade. That limited its application for implantable devices and – important to Bao – contributed to waste.

Bao said that creating a robust material that is both a good electrical conductor and biodegradable was a challenge, considering traditional polymer chemistry. “We have been trying to think how we can achieve both great electronic property but also have the biodegradability,” Bao said.

Eventually, the team found that by tweaking the chemical structure of the flexible material it would break apart under mild stressors. “We came up with an idea of making these molecules using a special type of chemical linkage that can retain the ability for the electron to smoothly transport along the molecule,” Bao said. “But also this chemical bond is sensitive to weak acid – even weaker than pure vinegar.” The result was a material that could carry an electronic signal but break down without requiring extreme measures.

In addition to the biodegradable polymer, the team developed a new type of electrical component and a substrate material that attaches to the entire electronic component. Electronic components are usually made of gold. But for this device, the researchers crafted components from iron. Bao noted that iron is a very environmentally friendly product and is nontoxic to humans.

The researchers created the substrate, which carries the electronic circuit and the polymer, from cellulose. Cellulose is the same substance that makes up paper. But unlike paper, the team altered cellulose fibers so the “paper” is transparent and flexible, while still breaking down easily. The thin film substrate allows the electronics to be worn on the skin or even implanted inside the body.

From implants to plants

The combination of a biodegradable conductive polymer and substrate makes the electronic device useful in a plethora of settings – from wearable electronics to large-scale environmental surveys with sensor dusts.

“We envision these soft patches that are very thin and conformable to the skin that can measure blood pressure, glucose value, sweat content,” Bao said. A person could wear a specifically designed patch for a day or week, then download the data. According to Bao, this short-term use of disposable electronics seems a perfect fit for a degradable, flexible design.

And it’s not just for skin surveys: the biodegradable substrate, polymers and iron electrodes make the entire component compatible with insertion into the human body. The polymer breaks down to product concentrations much lower than the published acceptable levels found in drinking water. Although the polymer was found to be biocompatible, Bao said that more studies would need to be done before implants are a regular occurrence.

Biodegradable electronics have the potential to go far beyond collecting heart disease and glucose data. These components could be used in places where surveys cover large areas in remote locations. Lei described a research scenario where biodegradable electronics are dropped by airplane over a forest to survey the landscape. “It’s a very large area and very hard for people to spread the sensors,” he said. “Also, if you spread the sensors, it’s very hard to gather them back. You don’t want to contaminate the environment so we need something that can be decomposed.” Instead of plastic littering the forest floor, the sensors would biodegrade away.

As the number of electronics increase, biodegradability will become more important. Lei is excited by their advancements and wants to keep improving performance of biodegradable electronics. “We currently have computers and cell phones and we generate millions and billions of cell phones, and it’s hard to decompose,” he said. “We hope we can develop some materials that can be decomposed so there is less waste.”

In the world of semiconductor physics, the goal is to devise more efficient and microscopic ways to control and keep track of 0 and 1, the binary codes that all information storage and logic functions in computers are based on.

A new field of physics seeking such advancements is called valleytronics, which exploits the electron’s “valley degree of freedom” for data storage and logic applications. Simply put, valleys are maxima and minima of electron energies in a crystalline solid. A method to control electrons in different valleys could yield new, super-efficient computer chips.

A University at Buffalo team, led by Hao Zeng, PhD, professor in the Department of Physics, worked with scientists around the world to discover a new way to split the energy levels between the valleys in a two-dimensional semiconductor.

The work is described in a study published online today (May 1, 2017) in the journal Nature Nanotechnology.

The key to Zeng’s discovery is the use of a ferromagnetic compound to pull the valleys apart and keep them at different energy levels. This leads to an increase in the separation of valley energies by a factor of 10 more than the one obtained by applying an external magnetic field.

“Normally there are two valleys in these atomically thin semiconductors with exactly the same energy. These are called ‘degenerate energy levels’ in quantum mechanics terms. This limits our ability to control individual valleys. An external magnetic field can be used to break this degeneracy. However, the splitting is so small that you would have to go to the National High Magnetic Field Laboratories to measure a sizable energy difference. Our new approach makes the valleys more accessible and easier to control, and this could allow valleys to be useful for future information storage and processing,” Zeng said.

The simplest way to understand how valleys could be used in processing data may be to think of two valleys side by side. When one valley is occupied by electrons, the switch is “on.” When the other valley is occupied, the switch is “off.” Zeng’s work shows that the valleys can be positioned in such a way that a device can be turned “on” and “off,” with a tiny amount of electricity.

Microscopic ingredients

Zeng and his colleagues created a two-layered heterostructure, with a 10 nanometer thick film of magnetic EuS (europium sulfide) on the bottom and a single layer (less than 1 nanometer) of the transition metal dichalcogenide WSe2 (tungsten diselenide) on top. The magnetic field of the bottom layer forced the energy separation of the valleys in the WSe2.

Previous attempts to separate the valleys involved the application of very large magnetic fields from outside. Zeng’s experiment is believed to be the first time a ferromagnetic material has been used in conjunction with an atomically thin semiconductor material to split its valley energy levels.

“As long as we have the magnetic material there, the valleys will stay apart,” he said. “This makes it valuable for nonvolatile memory applications.”

Athos Petrou, a UB Distinguished Professor in the Department of Physics, measured the energy difference between the separated valleys by bouncing light off the material and measuring the energy of reflected light.

“We typically get this type of results only once every five or 10 years,” Petrou said.

Extending Moore’s law

The experiment was conducted at 7 degrees Kelvin (-447 Fahrenheit), so any everyday use of the process is far in the future. However, proving it possible is a first step.

“The reason people are really excited about this, is that Moore’s law [which says the number of transistors in an integrated circuit doubles every two years] is predicted to end soon. It no longer works because it has hit its fundamental limit,” Zeng said.

“Current computer chips rely on the movement of electrical charges, and that generates an enormous amount of heat as computers get more powerful. Our work has really pushed valleytronics a step closer in getting over that challenge.”