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Common sense might dictate that for an object to move from one point to another, it must go through all the points on the path.

“Imagine someone driving from Kansas City to Topeka on I-70 — it’s safe to say that he must be in Lawrence at some point during the trip,” said Hui Zhao, associate professor of physics & astronomy at the University of Kansas. “Or in basketball, when KU’s Josh Jackson receives an alley-oop pass from Frank Mason III and dunks the ball from above to below the rim, the ball must be in the hoop at some point in time.”

Not so for electrons in the quantum world, which don’t follow such common-sense rules for the most part.

“Electrons can show up on the first floor, then the third floor, without ever having been on the second floor,” Zhao said.

Zhao, along with KU physics graduate student Frank Ceballos and Self Graduate Fellow Samuel Lane, has just observed the counterintuitive motion of electrons during experiments in KU’s Ultrafast Laser Lab.

“In a sample made of three atomic layers, electrons in the top layer move to the bottom layer, without ever being spotted in the middle layer,” said the KU researcher.

Because this sort of “quantum” transport is very efficient, Zhao said it can play a key role in a new type of manmade material called “van der Waals materials” that could be used someday in solar cells and electronics.

Their findings were just published in Nano Letters, a premier journal on nanoscience and nanotechnology.

The KU research team fabricated the sample by using the “Scotch tape” method, where single-molecule layers are lifted from a crystal with tape, then verified under an optical microscope. The sample contains layers of MoS2, WS2 and MoSe2 — each layer thinner than one nanometer. All three are semiconductor materials and respond to light with different colors. Based on that, the KU researchers used a laser pulse of 100 femtosecond duration to liberate some of the electrons in the top MoSe2 layer so they could move freely.

“The color of the laser pulse was chosen so that only electrons in the top layer can be liberated,” Zhao said. “We then used another laser pulse with the ‘right’ color for the bottom MoS2 layer to detect the appearance of these electrons in that layer. The second pulse was purposely arranged to arrive at the sample after the first pulse by about 1 picosecond, by letting it travel a distance 0.3 mm longer than the first.”

The team found electrons move from the top to the bottom layer in about one picosecond on average.

“If electrons were things that followed ‘common sense,’ like so-called classical particles, they’d be in the middle layer at some point during this one picosecond,” Zhao said.

The researchers used a third pulse with another color to monitor the middle layer and found no electrons. The experimental discovery of the counterintuitive transport of electrons in the stack of atomic layers was further confirmed by simulations performed by theorists Ming-Gang Ju and Xiao Cheng Zeng at the University of Nebraska-Lincoln, who co-authored the paper. According to Zhao, the verification of quantum transport of electrons between atomic layers connected by van der Waals force is encouraging news for researchers developing new materials.

“The Stone Age, Bronze Age and Iron Age — materials have been the defining element of human history,” he said. “The modern information-technology age is largely based on silicon, which is a result of many decades of material research focused on finding new materials and developing better techniques to make them with high quality and low cost.”

Zhao said in recent decades researchers have learned to tune properties of materials by changing their size and shape on a nanometer scale. A new form of nanomaterials, known as two-dimensional materials, was discovered about a decade ago. “They are formed by single layers of atoms or molecules,” he said. “The most well-known example is graphene, a single layer of carbon atoms. So far, about 100 types of two-dimensional materials have been discovered, such as the three used in this study. Because these atomic layers can be stacked by using van der Waals force, they opened up an entirely new route to make new functional materials.”

The researcher said his team’s work focused on a key requirement for such materials to be ideal for electronic and optical applications: Electrons must be able to move between these atomic layers efficiently.

“This study showed electrons can transfer between these layers in a quantum fashion, just like in other conductors and semiconductors,” he said.

Quantum dots are very small particles that exhibit luminescence and electronic properties different from those of their bulk materials. As a result, they are attractive for use in solar cells, optoelectronics, and quantum computing. Quantum computing involves applying a small voltage to quantum dots to regulate their electron spin state, thus encoding information. While traditional computing is based on a binary information system, electron spin states in quantum dots can display further degrees of freedom because of the possibility of superposition of both states at the same time. This feature could increase the density of encoded information.

Readout of the electron spin of quantum dots is necessary to realize quantum computing. Single-shot spin readout has been used to detect spin-dependent single-electron tunneling events in real time. The performance of quantum computing could be improved considerably by single-shot readout of multiple spin states.

A Japanese research collaboration based at Osaka University has now achieved the first successful detection of multiple spin states through single-shot readout of three two-electron spin states of a single quantum dot. They reported their findings in Physical Review Letters.

To read out multiple spin states simultaneously, the researchers used a quantum point contact charge sensor positioned near a gallium arsenide quantum dot. The change in current of the charge sensor depended on the spin state of the quantum dot and was used to distinguish between singlet and two types of triplet spin states.

“We obtained single-shot ternary readout of two-electron spin states using edge-state spin filtering and the orbital effect,” study first author Haruki Kiyama says.

That is, the rate of tunneling between the quantum dot and electron reservoir depended on both the spin state of the electrons and the interaction between electron spin and the orbitals of the quantum dot. The team identified one ground state and two excited states in the quantum dot using their setup.

The researchers then used their ternary readout setup to investigate the spin relaxation behavior of the three detected spin states.

“To confirm the validity of our readout system, we measured the spin relaxation of two of the states,” Kiyama explains. “Measurement of the dynamics between the spin states in a quantum dot is an important application of the ternary spin readout setup.”

The spin relaxation times for the quantum dot measured using the ternary readout system agreed with those reported, providing evidence that the system yielded reliable measurements.

This ternary readout system can be extended to quantum dots composed of other materials, revealing a new approach to examine the spin dynamics of quantum dots and representing an advance in quantum information processing.

Imec, the research and innovation hub in nano-electronics and digital technologies, today announced that their 200mm gallium nitride-on-silicon (GaN-on-Si) e-mode power devices with a pGaN gate architecture showed no degradation after heavy ion and neutron irradiation. The irradiation tests were performed in collaboration with Thales Alenia Space, a leader in innovative space systems. The results demonstrate that imec’s 200mm GaN-on-Si platform delivers state-of-the-art GaN-based power devices for earth as well as for space applications.

GaN-on-silicon transistors operate at higher voltages, frequencies and temperatures than their silicon counterparts. This makes them the ideal candidates for power conversion devices as they show less power losses in electricity conversion. First-generation GaN-based power devices are used today and will play a key role in the power conversion of future electronic devices such as battery chargers, smartphones, computers, servers, automotive, lighting systems and photovoltaics.

Imec has been  developing the next-generation of GaN-based power devices with improved performance and reliability. Imec’s latest 200mm GaN-on-Si platform shows good  wafer-to-wafer reproducibility and low dynamic Rdson. The platform is currently available for dedicated development or technology transfer to imec’s current and future partners.

imec Ron

Imec’s latest generation of  200mm GaN-on-Si e-mode pGaN devices were irradiated with heavy ions (Xenon) and neutrons. Pre and post irradiation tests revealed that there was no permanent degradation of transistor characteristics: no shifts in threshold voltage nor gate rupture. The excellent radiation hardness of imec’s devices is important, as it enables applications in space, where fluxes of heavy ions and neutrons can damage electronic circuits in satellites and space stations.

Thales Alenia Space Belgium has surveyed, since many years, the evolution in the field of wide band gap devices. These family of components is promising for a significant increase in performances. But, robustness to space radiation is mandatory for electronic devices in our equipment’s. The result obtained with Imec’s GaN-on-Si devices is an important step in the way to space based power conversion applications.

“These results are important to start using this promising technology for space applications. Also, it demonstrates that our 200mm GaN-on-Si platform has reached a high level of technology readiness and can be adopted by industry,” stated Rudi Cartuyvels, Executive Vice President at imec. “At imec, we use 200mm silicon substrates for GaN epitaxy and this technology can be used on 200mm CMOS-compatible infrastructure. Thanks to innovations in transistor architecture and substrate technology, we’ve succeeded in making GaN devices on larger wafer diameters than used today, which brings lower cost perspectives for the second generation of GaN-on-Si power devices. Imec is also looking beyond today’s technology, exploring novel substrates, higher level of integrations and novel devices.”

These results were achieved in the framework of the European Space Agency (ESA) project “ESA AO/1-7688/13/NL/RA”, GaN devices for space based DC-DC power conversion applications.

Andrew Barnes ESA Technical Officer overseeing the project stated: “GaN is a critical technology for future space missions with a wide range of potential applications, including smaller size, higher efficiency DC-DC power conversion subsystems. These results, obtained from the first phase of an ESA GSTP project, are important and show that the p-GaN devices developed by imec offer excellent radiation robustness for operation in space. In the second phase of the project it is planned to industrialize this technology in readiness for a future space qualification program”. The European Space Agency (ESA) is Europe’s gateway to space. Its mission is to shape the development of Europe’s space capability and ensure that investment in space continues to deliver benefits to the citizens of Europe and the world.

A chance observation of crystals forming a mark that resembled the stain of a coffee cup left on a table has led to the growth of customized polycrystals with implications for faster and more versatile semiconductors.

Thin-film semiconductors are the foundation of a vast array of electronic and optoelectronic devices. They are generally fabricated by crystallization processes that yield polycrystals with a chaotic mix of individual crystals of different orientations and sizes.

Significant advances in controlling crystallization has been made by a team led by Professor Aram Amassian of Material Science and Engineering at KAUST. The group included individuals from the KAUST Solar Center and others from the University’s Physical Science and Engineering Division in collaboration with Cornell University. Amassian said, “There is no longer a need to settle for random and incoherent crystallization.”

Crystallization behavior can be controlled locally, creating regions with different crystal patterns. Credit: KAUST 2017

Crystallization behavior can be controlled locally, creating regions with different crystal patterns. Credit: KAUST 2017

The team’s recent discovery began when Dr. Liyang Yu of the KAUST team noticed that a droplet of liquid semiconductor material dried to form an outer coffee-ring shape that was much thicker than the material at the center. When he induced the material to crystallize, the outer ring crystallized first.

“This hinted that local thickness matters for initiating crystallization,” said Amassian, which went against the prevailing understanding of how polycrystal films form.

This anomaly led the researchers to delve deeper. They found that the thickness of the crystallizing film could be used to manipulate the crystallization of many materials (see top image). Most crucially, tinkering with the thickness also allowed fine control over the position and orientation of the crystals in different regions of a semiconductor.

“We discovered how to achieve excellent semiconductor properties everywhere in a polycrystal film,” said Amassian. He explained that seeding different patterns of crystallization at different locations also allowed the researchers to create bespoke arrays that can now be used in electronic circuits (see bottom image).

This is a huge improvement to the conventional practice of making do with materials whose good properties are not sustained throughout the entire polycrystal nor whose functions at different regions can be controlled.

“We can now make customized polycrystals on demand,” Amassian said.

Amassian hopes that this development will lead to high-quality, tailored polycrystal semiconductors to promote advances in optoelectronics, photovoltaics and printed electronic components. The method has the potential to bring more efficient consumer electronic devices, some with flexible and lightweight parts, new solar power generating systems and advances in medical electronics. And all thanks to the chance observation of an odd pattern in a semiconductor droplet.

The team will now explore ways to move their work beyond the laboratory through industry partnerships and research collaborations.

In cooperation with Okmetic Oy and the Polish ITME, researchers at Aalto University have studied the application of SOI (Silicon On Insulator) wafers, which are used as a platform for manufacturing different microelectronics components, as a substrate for producing gallium nitride crystals. The researchers compared the characteristics of gallium nitride (GaN) layers grown on SOI wafers to those grown on silicon substrates more commonly used for the process. In addition to high-performance silicon wafers, Okmetic also manufactures SOI wafers, in which a layer of silicon dioxide insulator is sandwiched between two silicon layers. The objective of the SOI technology is to improve the capacitive and insulating characteristics of the wafer.

The researchers used Micronova's cleanrooms and, in particular, a reactor designed for gallium nitride manufacturing. The image shows a six-inch substrate in the MOVPE reactor before manufacturing. Credit: Aalto University / Jori Lemettinen

The researchers used Micronova’s cleanrooms and, in particular, a reactor designed for gallium nitride manufacturing. The image shows a six-inch substrate in the MOVPE reactor before manufacturing. Credit: Aalto University / Jori Lemettinen

“We used a standardised manufacturing process for comparing the wafer characteristics. GaN growth on SOI wafers produced a higher crystalline quality layer than on silicon wafers. In addition, the insulating layer in the SOI wafer improves breakdown characteristics, enabling the use of clearly higher voltages in power electronics. Similarly, in high frequency applications, the losses and crosstalk can be reduced”, explains Jori Lemettinen, a doctoral candidate from the Department of Electronics and Nanoengineering.

‘GaN based components are becoming more common in power electronics and radio applications. The performance of GaN based devices can be improved by using a SOI wafer as the substrate’, adds Academy Research Fellow Sami Suihkonen.

SOI wafers reduce the challenges of crystal growth

Growth of GaN on a silicon substrate is challenging. GaN layers and devices can be grown on substrate material using metalorganic vapor phase epitaxy (MOVPE). When using silicon as a substrate the grown compound semiconductor materials have different coefficients of thermal expansion and lattice constants than a silicon wafer. These differences in their characteristics limit the crystalline quality that can be achieved and the maximum possible thickness of the produced layer.

‘The research showed that the layered structure of an SOI wafer can act as a compliant substrate during gallium nitride layer growth and thus reduce defects and strain in the grown layers”, Lemettinen notes. GaN based components are commonly used in blue and white LEDs. In power electronics applications, GaN diodes and transistors, in particular, have received interest, for example in frequency converters or electric cars. It is believed that in radio applications, 5G network base stations will use GaN based power amplifiers in the future. In electronics applications, a GaN transistor offers low resistance and enables high frequencies and power densities.

Ionotronic devices rely on charge effects based on ions, instead of electrons or in addition to electrons. These devices open new opportunities for creating electrically switchable memories. However, there are still many technical challenges to overcome before this new kind of memories can be produced.

Researchers at Aalto University in Finland have visualized how oxygen ion migration in a complex oxide material causes the material to alter its crystal structure in a uniform and reversible fashion, prompting large modulations of electrical resistance. They performed simultaneous imaging and resistance measurements in a transmission electron microscope using a sample holder with a nanoscale electrical probe. Resistance-switching random access memories could utilize this effect.

Researchers performed imaging and resistance measurements in a transmission electron microscope using a sample holder with a nanoscale electrical probe. Credit: Mikko Raskinen / Aalto University

Researchers performed imaging and resistance measurements in a transmission electron microscope using a sample holder with a nanoscale electrical probe. Credit: Mikko Raskinen / Aalto University

Sample holder helps control migration of ions 

“In a transmission electron microscope, a beam of high-energy electrons is transmitted through a very thin specimen. Various detectors collect the electrons after their interaction with the sample, providing detailed information about the atomic structure and composition of the material. The technique is extremely powerful for nanomaterials characterization, but if used conventionally, it does not allow for active material manipulation inside the microscope. In our study, we utilized a special sample holder with a piezo-controlled metallic probe to make an electrical nanocontact. This in situ method allowed us to apply short voltage pulses and thereby control the migration of oxygen ions in our sample,” explains Academy of Finland Research Fellow Lide Yao.

The researchers found that migration of oxygen ions away from the contact area results in an abrupt change in the oxide lattice structure and an increase of electrical resistance. Reversal of the voltage polarity fully restores the original material properties. Electro-thermal simulations, performed by PhD candidate Sampo Inkinen, showed that a combination of current-induced sample heating and electric-field-directed ion migration causes the switching effect.

Ionotronic concept for manipulation of several material properties

“The material that we investigated in this study is a complex oxide. Complex oxides can exhibit many interesting physical properties including magnetism, ferroelectricity, and superconductivity, and all these properties vary sensitively with the oxidation state of the material. Voltage-induced migration of oxygen ions does change the amount of oxidation, triggering strong material responses. While we have demonstrated direct correlations between oxygen content, crystal structure, and electrical resistance, the same ionotronic concept could be utilized to control other material properties,” says Professor Sebastiaan van Dijken, who is a coauthor on the paper with Yao.

“In the current study, we employed a special sample holder for simultaneous measurements of the atomic-scale structure and electrical resistance. We are now developing an entirely new and unique holder that would allow for transmission electron microscopy measurements while the specimen is irradiated by intense light. We plan to investigate atomic scale processes in perovskite solar cells and other optoelectronic materials with this setup in the future,” adds Yao.

Quantum mechanics, the physics that governs nature at the atomic and subatomic scale, contains a host of new physical phenomena to explore quantum states at the nanoscale. Though tricky, there are ways to exploit these inherently fragile and sensitive systems for quantum sensing. One nascent technology in particular makes use of point defects, or single-atom misplacements, in nanoscale materials, such as diamond nanoparticles, to measure electromagnetic fields, temperature, pressure, frequency and other variables with unprecedented precision and accuracy.

Quantum sensing could revolutionize medical diagnostics, enable new drug development, improve the design of electronic devices and more.

For use in quantum sensing, the bulk nanodiamond crystal surrounding the point defect must be highly perfect. Any deviation from perfection, such as additional missing atoms, strain in the crystalline lattice of the diamond, or the presence of other impurities, will adversely affect the quantum behavior of the material. Highly perfect nanodiamonds are also quite expensive and difficult to make.

A cheaper alternative, say researchers at Argonne National Laboratory and the University of Chicago, is to take defect-ridden, low-quality, commercially manufactured diamonds, and then “heal” them.

In a paper published this week in APL Materials, from AIP Publishing, the researchers describe a method to heal diamond nanocrystals under high-temperature conditions, while visualizing the crystals in three dimensions using an X-ray imaging technique.

“Quantum sensing is based on the unique properties of certain optically active point defects in semiconductor nanostructures,” said F. Joseph Heremans, an Argonne National Laboratory staff scientist and co-author on the paper.

These defects, such as the nitrogen-vacancy (NV) centers in diamond, are created when a nitrogen atom replaces a carbon atom adjacent to a vacancy in the diamond lattice structure. They are extremely sensitive to their environment, making them useful probes of local temperatures, as well as electric and magnetic fields, with a spatial resolution more than 100 times smaller than the thickness of a human hair.

Because diamonds are biologically inert, quantum sensors based on diamond nanoparticles, which can operate at room temperature and detect several factors simultaneously, could even be placed within living cells, where they could, according to Heremans, “image systems from the inside out.”

Heremans and his colleagues, including Argonne’s Wonsuk Cha and Paul Fuoss, as well as David Awschalom of the University of Chicago, set out to map the distribution of the crystal strain in nanodiamonds and to track the healing of these imperfections by subjecting them to high temperatures, up to 800 degrees Celsius in an inert helium environment.

“Our idea of the ‘healing’ process is that gaps in the lattice are filled as the atoms move around when the crystal is heated to high temperatures, thereby improving the homogeneity of the crystal lattice,” said Stephan Hruszkewycz, also a staff scientist at Argonne and lead author on the paper.

This nanodiamond healing was monitored with a 3-D microscopy method called Bragg coherent diffraction imaging, performed by subjecting the crystals to a coherent X-ray beam at the Advanced Photon Source at Argonne. The X-ray beam that scatters off the nanodiamonds was detected and used to reconstruct the 3-D shape of the nanocrystal, “and, more importantly, the strain state of the crystal,” Hruszkewycz said.

The researchers found that nanodiamonds “shrink” during the high-temperature annealing process, and surmise that this occurs because of a phenomenon called graphitization. This phenomenon occurs when the surface of the material is converted from the normal diamond lattice arrangement into graphite, a single layer of chicken-wire-like arranged carbon atoms.

The study marks the first time that Bragg coherent diffraction imaging has been shown to be useful at such high temperatures, a capability that, Hruszkewycz said, “enables the exploration of structural changes in important nanocrystalline materials at high temperatures that are difficult to access with other microscopy techniques.”

Hruszkewycz added that the research represents “a significant step towards developing scalable methods of processing inexpensive, commercial nanodiamonds for quantum sensing and information processing.”

Graphene’s unusual electronic structure enables this extraordinary material to break many records of strength, electricity and heat conduction. Physicists at the Center for Theoretical Physics of Complex Systems (PCS), in collaboration with the Research Institute for Standards and Science (KRISS), used a model to explain the electronic structure of graphene measured by a new spectroscopic platform. These techniques, published in the journal Nano Letters, could promote future research on stable and accurate quantum measurements for new 2D electronics.

(Left) Nanodevice structure to measure graphene's electronic properties. Graphene is sandwiched between two hBTN layers and the two electrodes (graphite and silicon). (Right) Conductance of single layer graphene at different voltages, showing the dip at around 350 mV. Credit: IBS

(Left) Nanodevice structure to measure graphene’s electronic properties. Graphene is sandwiched between two hBTN layers and the two electrodes (graphite and silicon). (Right) Conductance of single layer graphene at different voltages, showing the dip at around 350 mV. Credit: IBS

Recently, interest in 2D materials has risen exponentially in both academia and industry. These materials are made by extremely thin sheets, which have different physical properties compared to conventional 3D materials. Moreover, when different 2D sheets are stacked on the top of each other, new electrical, optical, and thermal properties emerge. One of the most promising and much studied 2D materials is graphene: a single sheet of carbon atoms. In order to study the electronic properties of both single and double layer graphene, the team constructed a nanodevice with graphene sandwiched between two layers of an insulating material known as hexagonal boron nitride (hBN). On top of this device they placed graphite as electrode. Graphite is essentially made up of hundreds of thousands of layers of graphene. The bottom layer consisted of one layer of silicon and one of silica.

By tuning the voltages applied via the graphite and the silicon, the scientists measured the changes in the conductance of graphene, which reflects its electronic properties. The electrons of graphene have a particular energy structure, represented by the so-called Dirac cone, which is actually made by two cones that look like a sandglass, with only an infinitesimally small point in between (Dirac Point). You can think of it as an unusual cocktail glass shaped liked a sandglass, where the drink plays the function of the graphene’s electrons. At temperature close to zero Kelvin (-273 degrees Celsius), the electrons pack into the lowest available energy states and fill up the double-cone glass from the bottom up, until a certain energy level, called Fermi level, is reached. Applying a negative voltage via the silicon and graphite layers is equivalent to drinking from the glass, while a positive voltage has the same effect as adding liquid to the glass. Through modulating the applied voltages, the scientists could deduce the electronic structure of graphene by following the Fermi level. In particular, they noticed that when the voltage applied to graphite is around 350 milliVolts, there is a dip in the conductance measurement, by which the Fermi level matches with the Dirac point. This is a well-known property of single layer graphene.

Finally, the electrical properties change again when a magnetic field is applied to the single layer graphene. In this case, instead of a sandglass cocktail glass, the energy of the electrons is more similar to a ladder where electrons of increasing energies can be found on the higher rungs. Gaps between the ladder rungs are devoid of electrons, while the steps fill with electrons from the bottom upwards. Interestingly, the data obtained by the scientists of KRISS was successfully reproduced by the theoretical physicists at IBS showed more than 40 rungs, technically known as Landau levels. Each level clearly distinguished because of the low background noise.

Indeed, the scientists could also match the theoretical and experimental data relative to the electronic properties of bilayer graphene. Double layer graphene, has a different conductance behavior with a broader dip, better known as an energy gap. In the presence of an electric field perpendicular to it, this energy gap makes double layer graphene more similar to the current tunable semiconductors. “We used an intuitive model to reproduce the experimental measurement and we gave a theoretical explanation to why these energy configurations form with single and double layer graphene,” explains MYOUNG Nojoon, first co-author of this study. “This model provides a gauge between voltages and energy in spectroscopic measurements, and we believe that this is a fundamental step to study graphene’s electronic properties further.”

Researchers in Singapore and China have collaborated to develop a self-powered photodetector that can be used in a wide range of applications such as chemical analysis, communications, astronomical investigations and much more.

Typically, photodetectors require an external voltage to provide the driving force for separating and measuring photo-generated electrons that comprise the detection. To eliminate this need, the research team led by Junling Wang and Le Wang at Nanyang Technological University in Singapore developed a novel, sensitive and stable photodetector based on a semiconducting junction called a GdNiO3/Nb-doped SrTiO3 (GNO/NSTO) p-n heterojunction. An inherent electric field at the GNO/NSTO interface provides the driving force for efficient separation of photo-generated carriers, eliminating the need for an external power source.

In addition to its self-powered feature, Wang and his team report tuning the material properties to achieve broad sensitivity. For these compounds, most research work thus far has focused on studying the origin of metal-insulator transition, but this team took a different approach.

The properties of perovskite nickelates, the category of solar cell materials in which this structure falls, are very sensitive to oxygen content. This sensitivity enables fine tuning of the final electronic structures by varying the oxygen environment during film deposition (constructing the heterojunction).

“Our work is novel and confirms that nickelates films have tunable band gaps with changing of the oxygen vacancy concentration, which makes them ideal as light absorbing materials in optoelectronic devices,” said Wang. “Using the self-powered photodetector we designed, we study its photo responsivity using light sources with different wavelengths, with significant photo-response appearing when the light wavelength decreases to 650 nanometers.” Wang said.

A significant challenge in developing this photodetector was determining the correct band structure, or energy structure available to electrons, of the 10 nanometer thick GNO films.

“To obtain the band structures, we used both spectroscopic ellipsometry measurements and ultraviolet photoelectron spectroscopy (UPS) measurements,” said Wang. Using the deduced values for the optical bandgap from these measurements, along with known limits and values for GNO films, they could plot the energy levels and work functions of the various components in the devices.

The team hopes to explore more materials with similar features. “One of the remarkable features of nickelates […] is the dependence of their physical properties on the chosen rare earth element,” said Wang. “Thus far, we have only studied GdNiO3 film, but besides that we can also investigate other “R”-NiO3 films where “R” can be Nd (neodymium), Sm (animony), Er (erbium) and Lu (lutetium) and study their potential applications in the photodetector.”

The team also plans to improve the performance of the photodetector by adding an insulating SrTiO3 (STO) layer sandwiched between the GdNiO3 film and NSTO substrate.

This novel work has great potential for applications using optoelectronic devices. “We believe that this paper will stimulate further studies and enlarge the potential applications of systems based on nickelates,” said Wang.

A team of scientists from the Energy Department’s National Renewable Energy Laboratory (NREL) determined that surface recombination limits the performance of polycrystalline perovskite solar cells.

Considerable research into perovskites at NREL and elsewhere has proved the material’s effectiveness at converting sunlight into electricity, routinely topping 20 percent efficiency. The sunlight creates mobile electrons whose movement generates the power but upon encountering defects can slip into a non-productive process. Known as a recombination, this process reduces the efficiency of a solar cell. For the cell to be the most efficient, the recombination must occur slowly.

With prior studies into perovskites focusing on bulk recombination, one area left unexamined until now concerned the surface recombination in lead iodide perovskites. NREL’s scientists determined recombination in other parts of a methylammonium perovskite film isn’t as important as what’s happening on the surface, both the top and bottom.

Matthew Beard and his colleagues within NREL’s Chemistry and Nanoscience Center studied surface recombination in single-crystal and polycrystalline films using transient reflection spectroscopy. Their findings, Top and bottom surfaces limit carrier lifetime in lead iodide perovskite films, appear in Nature Energy.

“What’s important is to know where the recombination is coming from,” said Beard, lead author of the research paper. “There are multiple sources of possible recombination. In order to improve your device, you’re asked to get rid of all non-radiative recombination. Typically people forget about surfaces. They think about grain boundaries. They think about bulk defects and so forth.”

Beard’s co-authors are all from NREL: Ye Yang, Mengjin Yang, David T. Moore, Yong Yan, Elisa M. Miller, and Kai Zhu.

Beard said the research determined surface recombination emerged as an obstacle to overcome. Surface recombination directly affects the performance of a photovoltaic device. The ability to engineer surfaces stands poised to benefit perovskite-based optoelectronic applications. A fast surface recombination can be used to design photodetectors, while lasers and light-emitting diodes require a slower speed.

A second study that concurrently appeared in the journal Physical Chemistry Chemical Physics was authored by Mengjin Yang, Yining Zeng, Zhen Li, DongHoe Kim, Chun-Sheng Jiang, Jao van de Lagemaat, and Kai Zhu further strengthened the conclusions of the paper. This study, using high-resolution fluorescence-lifetime imaging, also showed that surface recombination is the determining factor instead of grain boundary recombination.

The researchers compared two types of samples: single crystals and polycrystalline films. Surprisingly surface recombination is worse for single crystalline samples compared to the polycrystalline samples found in solar cell devices. Chemically, excess methylammonium iodide was present on the surface of the polycrystalline film but absent on the single-crystal sample.

“That seems to help,” Beard said. “The single crystal has a lead-rich surface and a faster surface recombination.”

The research suggested a light coating of a protective material on the surface of the polycrystalline thin films could further improve the performance of perovskite solar cells.