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Imec, a research and innovation hub in nanoelectronics and digital technologies, announces that Niels Verellen, one of its young scientists, has been awarded an ERC Starting Grant. The grant of 1.5 million euros (for 5 years) will be used to enable high-resolution, fast, robust, zero-maintenance, inexpensive and ultra-compact microscopy technology based on on-chip photonics and CMOS image sensors. The technology paves the way for multiple applications of cell imaging in life sciences, biology, and medicine and compact, cost-effective DNA sequencing instruments.

Microscopy is an indispensable tool in biology and medicine that has fueled many breakthroughs. Recently the world of microscopy has witnessed a true revolution in terms of increased resolution of fluorescent imaging techniques, including a Nobel Prize in 2014. Yet, these techniques remain largely locked-up in specialized laboratories as they require bulky, expensive instrumentation and highly skilled operators.

The next big push in microscopy with a large societal impact will come from extremely compact and robust optical systems that will make high-resolution microscopy highly accessible and as such facilitate the diagnosis and treatment of diseases or disorders caused by problems at the cell or molecular level, such as meningitis, malaria, diabetes, cancer, and Alzheimer’s disease. Moreover, it will pave the way to DNA analysis as a more standard procedure, not only for the diagnosis of genomic disorders or in forensics, but also in cancer treatment, follow-up of transplants, the microbiome, pre-natal tests, and even agriculture, and archeology.

Niels Verellen, Senior Photonics Researcher & project leader at imec: “Compact, high-resolution and high-throughput microscopy devices will induce a profound change in the way cell biologists do research, in the way DNA sequencing becomes more and more accessible, in the way certain diseases can be diagnosed, new drugs are screened in the pharma industry, and healthcare workers can diagnose patients in remote areas.”

The topic of Verellen’s ERC grant is the development of Integrated high-Resolution On-Chip Structured Illumination Microscopy (IROCSIM). This new technology is based on a novel imaging platform that integrates active on-chip photonics and CMOS image sensors. “Whereas existing microscopy techniques today suffer from a trade-off between equipment size, field-of-view, and resolution, the IROCSIM solution will eliminate the need for bulky optical components and enable microscopy in the smallest possible form-factor, with a scalable field-of-view and without compromising the resolution,” continues Verellen.

The European Research Council (ERC) is a pan European funding body designed to support investigator-driven frontier research and stimulate scientific excellence across Europe. The ERC aims to support the best and most creative scientists to identify and explore new opportunities and directions in any field of research. ERC Starting grants in particular are designed to support outstanding researchers with 2 to 7 years postdoctoral experience.

Jo De Boeck, imec’s CTO says: “We are very proud that young researchers such as Niels Verellen are awarded an ERC Starting Grant and as such get a unique opportunity to fulfill their ambitions and creative ideas in research. At imec, we select and foster our young scientists and provide them with a world-class infrastructure. These ERC Starting Grants show that their work indeed meets the highest standards.”

Growing a batch of carbon nanotubes that are all the same may not be as simple as researchers had hoped, according to Rice University scientists.

Rice materials theorist Boris Yakobson and his team bucked a theory that when growing nanotubes in a furnace, a catalyst with a specific atomic arrangement and symmetry would reliably make carbon nanotubes of like chirality, the angle of its carbon-atom lattice.

Rice University scientists have decoded the unusual growth characteristic of carbon nanotubes that start out as one chirality but switch to another, resulting in nearly homogenous batches of single-walled nanotubes. The nanotubes grow via chemical vapor deposition with a carbon-tungsten alloy catalyst. Credit: Evgeni Penev/Rice University

Instead, they found the catalyst in question starts nanotubes with a variety of chiral angles but redirects almost all of them toward a fast-growing variant known as (12,6). The cause appears to be a Janus-like interface that is composed of armchair and zigzag segments – and ultimately changes how nanotubes grow.

Because chirality determines a nanotube’s electrical properties, the ability to grow chiral-specific batches is a nanotechnology holy grail. It could lead to wires that, unlike copper or aluminum, transmit energy without loss. Nanotubes generally grow in random chiralities.

The Rice theoretical study detailed in the American Chemical Society journal Nano Letterscould be a step toward catalysts that produce homogenous batches of nanotubes, Yakobson said.

Yakobson and colleagues Evgeni Penev and Ksenia Bets and graduate student Nitant Gupta tackled a conundrum presented by other experimentalists at a 2013 workshop who used an alloy of cobalt and tungsten to catalyze single-walled nanotubes. In that lab’s batch, more than 90 percent of the nanotubes had a chirality of (12,6).

The numbers (12,6) are coordinates that refer to a nanotube’s chiral vector. Carbon nanotubes are rolled-up sheets of two-dimensional graphene. Graphene is highly conductive, but when it is rolled into a tube, its conductivity depends on the angle — or chirality — of its hexagonal lattice.

Armchair nanotubes — so called because of the armchair-like shape of their edges — have identical chiral indices, like (9,9), and are highly desired for their perfect conductivity. They are unlike zigzag nanotubes, such as (16,0), which may be semiconductors. Turning a graphene sheet a mere 30 degrees will change the nanotube it forms from armchair to zigzag or vice versa.

Penev said the experimentalists explained their work “in a way which was puzzling from the very beginning. They said this catalyst has a specific symmetry that matches the (12,6) edge, so these nanotubes preferentially nucleate and grow. This was the emergence of the so-called symmetry matching idea of carbon nanotube selective growth.

“We read and digested that, but we still couldn’t wrap our minds around it,” he said.

Shortly after the 2013 conference, the Yakobson lab published its own theory of nanotube growth, which showed that the balance between two opposing forces — the energy of the catalyst-nanotube contact and the speed at which atoms attach themselves to the growing tube at the interface — are responsible for chirality.

Five years later, that turns out to be just as true in their new paper, though with a twist. The Rice calculations show that the alloy Co7W6 promotes the formation of the Janus-like interface that ensures the necessary kink at the edge and allows carbon atoms to attach themselves to the nanotube’s foundation. But the catalyst also forces the nanotube to incorporate defects that alter its initial chirality midstream.

“We uncovered two things,” Yakobson said. “One is that the carbon atom types at the base of the nanotube separate into armchair and zigzag segments. The second is the tendency for the formation of defects that drive the chirality, or helicity, change. That makes (12,6) a sort of transient attractor, at least during short experiments. If they were able to grow forever, (12,6) nanotubes would eventually switch to armchairs.”

The unusual growth pattern might have been diagnosed much earlier if it weren’t for an age-old typo that required some dogged detective work.

“The trouble was in a standard online database that gives the crystal structure of this cobalt-tungsten alloy,” said Bets, co-lead author of the paper with Penev. “One entry was wrong. That messed up the structure so badly that we couldn’t use it in our density functional theory calculations.”

Once they found the error, Bets and co-author Gupta went back to the 1938 German paper that was first to correctly detail the structure of Co7W6. Even with that in hand, the team’s calculations used every bit of computing power they could find to simulate the energetic connections between each atom in the catalyst and carbon feedstock.

“We figured out that if we had run the calculations in series instead of in parallel, they would have taken the equivalent of at least 2,000 years of computer time,” Bets said.

“This paper is remarkable in many aspects: in the timing, the amount of detail and the surprises we found,” Penev said. “We’ve never had a project like this. We don’t yet know how this will be applicable to other materials, but we’re working on it.”

“There are four or five experimental papers, pretty recent ones, that also show a change of chirality during growth,” Bets said. “In fact, because it’s a probabilistic process, it’s essentially unavoidable. But until now it’s never been considered in the theoretical investigation of growth.”

Solar cells need to slim down.

Solar cells are devices that absorb photons from sunlight and convert their energy to move electrons — enabling the production of clean energy and providing a dependable route to help combat climate change. But most solar cells used widely today are thick, fragile and stiff, which limits their application to flat surfaces and increases the cost to make the solar cell.

“Thin-film solar cells” could be 1/100th the thickness of a piece of paper and flexible enough to festoon surfaces ranging from an aerodynamically sleek car to clothing. To make thin-film solar cells, scientists are moving beyond the “classic” semiconductor compounds, such as gallium arsenide or silicon, and working instead with other light-harvesting compounds that have the potential to be cheaper and easier to mass produce. The compounds could be widely adopted if they could perform as well as today’s technology.

In a paper published online this spring in the journal Nature Photonics, scientists at the University of Washington report that a prototype semiconductor thin-film has performed even better than today’s best solar cell materials at emitting light.

“It may sound odd since solar cells absorb light and turn it into electricity, but the best solar cell materials are also great at emitting light,” said co-author and UW chemical engineering professor Hugh Hillhouse, who is also a faculty member with both the UW’s Clean Energy Institute and Molecular Engineering & Sciences Institute. “In fact, typically the more efficiently they emit light, the more voltage they generate.”

The UW team achieved a record performance in this material, known as a lead-halide perovskite, by chemically treating it through a process known as “surface passivation,” which treats imperfections and reduces the likelihood that the absorbed photons will end up wasted rather than converted to useful energy.

“One large problem with perovskite solar cells is that too much absorbed sunlight was ending up as wasted heat, not useful electricity,” said co-author David Ginger, a UW professor of chemistry and chief scientist at the CEI. “We are hopeful that surface passivation strategies like this will help improve the performance and stability of perovskite solar cells.”

Ginger’s and Hillhouse’s teams worked together to demonstrate that surface passivation of perovskites sharply boosted performance to levels that would make this material among the best for thin-film solar cells. They experimented with a variety of chemicals for surface passivation before finding one, an organic compound known by its acronym TOPO, that boosted perovskite performance to levels approaching the best gallium arsenide semiconductors.

“Our team at the UW was one of the first to identify performance-limiting defects at the surfaces of perovskite materials, and now we are excited to have discovered an effective way to chemically engineer these surfaces with TOPO molecules,” said co-lead author Dane deQuilettes, a postdoctoral researcher at the Massachusetts Institute of Technology who conducted this research as a UW chemistry doctoral student. “At first, we were really surprised to find that the passivated materials seemed to be just as good as gallium arsenide, which holds the solar cell efficiency record. So to double-check our results, we devised a few different approaches to confirm the improvements in perovskite material quality.”

DeQuilettes and co-lead author Ian Braly, who conducted this research as a doctoral student in chemical engineering, showed that TOPO-treating a perovskite semiconductor significantly impacted both its internal and external photoluminescence quantum efficiencies — metrics used to determine how good a semiconducting material is at utilizing an absorbed photon’s energy rather than losing it as heat. TOPO-treating the perovskite increased the internal photoluminescence quantum efficiencies by tenfold — from 9.4 percent to nearly 92 percent.

“Our measurements observing the efficiency with which passivated hybrid perovskites absorb and emit light show that there are no inherent material flaws preventing further solar cell improvements,” said Braly. “Further, by fitting the emission spectra to a theoretical model, we showed that these materials could generate voltages 97 percent of the theoretical maximum, equal to the world record gallium arsenide solar cell and much higher than record silicon cells that only reach 84 percent.”

These improvements in material quality are theoretically predicted to enable the light-to-electricity power conversion efficiency to reach 27.9 percent under regular sunlight levels, which would push the perovskite-based photovoltaic record past the best silicon devices.

The next step for perovskites, the researchers said, is to demonstrate a similar chemical passivation that is compatible with easily manufactured electrodes — as well as to experiment with other types of surface passivation.

“Perovskites have already demonstrated unprecedented success in photovoltaic devices, but there is so much room for further improvement,” said deQuilettes. “Here we think we have provided a path forward for the community to better harness the sun’s energy.”

Imec, a research and innovation hub in nanoelectronics, energy and digital technology, within the partnership of EnergyVille, today announced a record result for its 4-terminal Perovskite/silicon tandem photovoltaic cell. With a power conversion efficiency of 27.1 percent, the new imec tandem cell beats the most efficient standalone silicon solar cell. Further careful engineering of the Perovskite material will bring efficiencies over 30% in reach.

Perovskite microcrystals are a promising material system to make high-performance thin-film solar cells. They can be processed into thin, light, semitransparent modules that can achieve a high power conversion efficiency, are inexpensive to produce, and have a high absorption efficiency for sunlight. Because they can be made semitransparent, perovskite solar cells and modules can also be used on top of silicon solar cells. When the Perovskite is carefully engineered, the absorbance in the Perovskite minimizes the thermal losses that occur in the silicon cell. As a result, a Perovskite-silicon tandem solar cell can potentially reach power conversion efficiencies above 30 percent.

Imec’s new record tandem cell uses a 0.13 cm² spin-coated Perovskite cell developed within our Solliance cooperation stacked on top of a 4 cm² industrial interdigitated back-contact (IBC) silicon cell in a 4-terminal configuration, which is known to have a higher annual energy yield compared to a 2-terminal configuration. Additionally, scaling up the tandem device by using a 4 cm2 perovskite module on a 4 cm2 IBC silicon cell, a tandem efficiency of 25.3% was achieved, surpassing the stand-alone efficiency of the silicon cell.

Manoj Jaysankar, doctoral researcher at imec/EnergyVille, adds: “We have been working on this tandem technology for two years now, and the biggest difference with previous versions is in the engineering and processing of the Perovskite absorber, tuning its bandgap to optimize the efficiency for tandem configuration with silicon.”

“Adding Perovskite on top of industrial silicon PV may prove to be the most cost-effective approach to further improve the efficiency of photovoltaics,” concludes Tom Aernouts, group leader for thin-film photovoltaics at imec/EnergyVille. “Therefore, we invite all companies in the PV value chain that are looking into higher efficiencies, to partner with us and explore this promising path.”

Taking a multiband approach explains ‘electron-hole reverse drag’ and exciton formation

Mystifying experimental results obtained independently by two research groups in the USA seemed to show coupled holes and electrons moving in the opposite direction to theory.

Now, a new theoretical study has explained the previously mysterious result, by showing that this apparently contradictory phenomenon is associated with the bandgap in dual-layer graphene structures, a bandgap which is very much smaller than in conventional semiconductors.

The study authors, which included FLEET collaborator David Neilson at the University of Camerino and FLEET CI Alex Hamilton at the University of New South Wales, found that the new multiband theory fully explained the previously inexplicable experimental results.

Excitons travel across an ultra-low energy transistor without wasted dissipation of energy. Credit: FLEET: ARC Centre of Excellence in Future Low Energy Electronics Technologies

Exciton transport

Exciton transport offers great promise to researchers, including the potential for ultra-low dissipation future electronics.

An exciton is a composite particle: an electron and a ‘hole’ (a positively charged ‘quasiparticle’ caused by the absence of an electron) bound together by their opposite electrical charges.

In an indirect exciton, free electrons in one 2D sheet can be electrostatically bound to holes that are free to travel in the neighbouring 2D sheet.

Because the electrons and holes are each confined to their own 2D sheets, they cannot recombine, but they can electrically bind together if the two 2D sheets are very close (a few nanometres).

If electrons in the top (‘drive’) sheet are accelerated by an applied voltage, then each partnering hole in the lower (‘drag’) sheet can be ‘dragged’ by its electron.

This ‘drag’ on the hole can be measured as an induced voltage across the drag sheet, and is referred to as Coulomb drag.

A goal in such a mechanism is for the exciton to remain bound, and to travel as a superfluid, a quantum state with zero viscosity, and thus without wasted dissipation of energy.

To achieve this superfluid state, precisely engineered 2D materials must be kept only a few nanometres apart, such that the bound electron and hole are much closer to each other than they are to their neighbours in the same sheet.

In the device studied, a sheet of hexagonal-boron-nitride (hBN) separates two sheets of atomically-thin (2D) bilayer graphene, with the insulating hBN preventing recombination of electrons and holes.

Passing a current through one sheet and measuring the drag signal in the other sheet allows experimenters to measure the interactions between electrons in one sheet and holes in the other, and to ultimately detect a clear signature of superfluid formation.

Only recently, new, 2D heterostructures with sufficiently thin insulating barriers have been developed that allow us to observe features brought by strong electron-hole interactions.

Explaining the inexplicable: negative drag

However, experiments published in 2016 showed extremely puzzling results. Under certain experimental conditions, the Coulomb drag was found to be negative – i.e. moving an electron in one direction caused the hole in the other sheet to move in the opposite direction!

These results could not be explained by existing theories.

In this new study, these puzzling results are explained using crucial multi-band processes that had not previously been considered in theoretical models.

Previous experimental studies of Coulomb drag had been performed in conventional semiconductor systems, which have much larger bandgaps.

However bilayer graphene has a very small bandgap, and it can be changed by the perpendicular electric fields from the metal gates positioned above and below the sample.

The calculation of transport in both conduction and valence bands in each of the graphene bilayers was the ‘missing link’ that marries theory to experimental results. The strange negative drag happens when the thermal energy approaches the bandgap energy.

The strong multiband effects also affect the formation of exciton superfluids in bilayer graphene, so this work opens up new possibilities for exploration in exciton superfluids.

The study Multiband Mechanism for the Sign Reversal of Coulomb Drag Observed in Double Bilayer Graphene Heterostructures by M. Zarenia, A.R. Hamilton, F.M. Peeters and D. Neilson was published in Physical Review Letters in July 2018.

Acknowledgement: The study was led by David Neilson, and by Mohammad Zarenia while at the University of Antwerp, Belgium. The authors of the theoretical study worked with data provided by experimentalists from the two US groups: Cory Dean (Columbia University) and Emanuel Tutuc (University of Texas at Austin) who discovered the original puzzling results. The research was supported by the Flemish government (Belgium), the University of New South Wales, the University of Camerino and by the Australian Research Council via FLEET.

Superfluids and FLEET

Exciton superfluids are studied within FLEET’s Research theme 2 for their potential to carry zero-dissipation electronic current, and thus allow the design of ultra-low energy exciton transistors.

The use of twin atomically-thin (2D) sheets to carry the excitons will allow for room-temperature superfluid flow, which is key if the new technology is to become a viable ‘beyond CMOS’ technology. A bilayer-exciton transistor would be a dissipationless switch for information processing.

In a superfluid, scattering is prohibited by quantum statistics, which means that electrons and holes can flow without resistance.

In this single, pure quantum state, all particles flow with the same momentum, so that no energy can be lost through dissipation.

FLEET (the Australian Research Council Centre of Excellence in Future Low-Energy Electronics Technologies) brings together over a hundred Australian and international experts, with the shared mission to develop a new generation of ultra-low energy electronics.

The impetus behind such work is the increasing challenge of energy used in computation, which uses 5-8% of global electricity and is doubling every decade.

A key challenge of such ultra-miniature devices is overheating – their ultra-small surfaces seriously limit the ways for the heat from electrical currents to escape.

Working to address “hotspots” in computer chips that degrade their performance, UCLA engineers have developed a new semiconductor material, defect-free boron arsenide, that is more effective at drawing and dissipating waste heat than any other known semiconductor or metal materials.

This could potentially revolutionize thermal management designs for computer processors and other electronics, or for light-based devices like LEDs.

Illustration showing a schematic of a computer chip with a hotspot (bottom); an electron microscope image of defect-free boron arsenide (middle); and an image showing electron diffraction patterns in boron arsenide. Credit: Hu Research Lab / UCLA Samueli

The study was recently published in Science and was led by Yongjie Hu, UCLA assistant professor of mechanical and aerospace engineering.

Computer processors have continued to shrink down to nanometer sizes where today there can be billions of transistors on a single chip. This phenomenon is described under Moore’s Law, which predicts that the number of transistors on a chip will double about every two years. Each smaller generation of chips helps make computers faster, more powerful and able to do more work. But doing more work also means they’re generating more heat.

Managing heat in electronics has increasingly become one of the biggest challenges in optimizing performance. High heat is an issue for two reasons. First, as transistors shrink in size, more heat is generated within the same footprint. This high heat slows down processor speeds, in particular at “hotspots” on chips where heat concentrates and temperatures soar. Second, a lot of energy is used to keep those processors cool. If CPUs did not get as hot in the first place, then they could work faster and much less energy would be needed to keep them cool.

The UCLA study was the culmination of several years of research by Hu and his students that included designing and making the materials, predictive modeling, and precision measurements of temperatures.

The defect-free boron arsenide, which was made for the first time by the UCLA team, has a record-high thermal conductivity, more than three-times faster at conducting heat than currently used materials, such as silicon carbide and copper, so that heat that would otherwise concentrate in hotspots is quickly flushed away.

“This material could help greatly improve performance and reduce energy demand in all kinds of electronics, from small devices to the most advanced computer data center equipment,” Hu said. “It has excellent potential to be integrated into current manufacturing processes because of its semiconductor properties and the demonstrated capability to scale-up this technology. It could replace current state-of-the-art semiconductor materials for computers and revolutionize the electronics industry.”

The study’s other authors are UCLA graduate students in Hu’s research group: Joonsang Kang, Man Li, Huan Wu, and Huuduy Nguyen.

In addition to the impact for electronic and photonics devices, the study also revealed new fundamental insights into the physics of how heat flows through a material.

“This success exemplifies the power of combining experiments and theory in new materials discovery, and I believe this approach will continue to push the scientific frontiers in many areas, including energy, electronics, and photonics applications,” Hu said.

The international team of scientist of Peter the Great St. Petersburg Polytechnic University (SPbPU), Leibniz University Hannover (Leibniz Universität Hannover) and the Ioffe Institute found a way to improve nanocomposite material which opens a new opportunities to use it in hydrogen economy and other industries. The obtained results are explained in the academic article “The mechanism of charge carrier generation at the TiO2–n-Si heterojunction activated by gold nanoparticles” published in journal Semiconductor Science and Technology.

The study is dedicated to the composite material, a semiconductor based on titanium dioxide. Its applications are widely studied by the researchers all over the world. But the processes which take place in this material are very complex. Therefore, to use the semiconductor more effectively, it is necessary to ensure that the energy enclosed between its layers can be released and transmitted.

In framework of the experiments the researchers of SPbPU, Leibniz University Hannover and Ioffe Institute propose a qualitative model to explain the complex processes.

The scientific group used a composite material consisting of a silicon wafer (standard silicon wafer used in electronic devices), gold nanoparticles and a thin layer of titanium dioxide. In the framework of the experiment to transfer the energy inside the material, the researchers intended to isolate nanoparticles from silicon. If nanoparticles are not isolated from the silicon wafer, then the energy can’t be transmitted neither to the silicon nor to the titanium dioxide. It leads to the energy loss.

“The obtained material was a silicon wafer with pillar-like structures grown on its surface. It was used as a substrate for the sample. Gold nanoparticles were situated on top of these pillars and the whole structure was coated with titanium oxide. Thus, nanoparticles contacted only titanium dioxide, and simultaneously were isolated from silicon. The number of boundaries between the layers decreased, we tried to describe the processes in the material. In addition, we assumed that this structure would increase the efficiency of using the energy of light illuminating the surface of our material”, says Dr. Maxim Mishin, professor of Physics, Chemistry, and Technology of Microsystems Equipment Department of SPbPU.

In St. Petersburg, an international scientific group established a model of a new structure, then the main part of the structure was created in Hannover: a silicon wafer with pillars and gold nanoparticles situated on top of it.

The experiment was performed as follows. At first, the wafer was oxidized, i.e. it was covered with a layer of the substrate, and gold nanoparticles were put on top of it.

“After that, we faced the next task: to create pillars and to perform the etching of the substrate so that it is remained under the particles and not and in between them. Considering that we are dealing with nanosizes, the diameter of gold nanoparticles is about 10 nanometers, and the height of the pillar is 80 nanometers, this is not a trivial task. The development of modern nanoelectronics makes it possible to use the so-called “dry” etching methods such as reactive ion etching”, adds Dr. Marc Christopher Wurz from the Institute of Micro Production Technology at Leibniz University Hannover.

According to scientists, the process of technology development had not been rapid: at the first stages of the experiment, while using the ion etching, all gold nanoparticles were simply demolished from the oxidized wafer. In the course of one week, the researchers were selecting the parameters for etching plasma system, so that the gold nanoparticles remained on the surface. The whole experiment was conducted within 10 days.

This scientific project is ongoing. The researchers mention that this nanocomposite material can be used in optical devices operating in the visible light spectrum. In addition, it can be used as a catalyst to produce hydrogen from water, or, for example, to purify water by stimulating the decomposition of complex molecules. In addition, this material may be useful as an element of a sensor which detects a gas leak or increased concentration of harmful substances in the air.

Electrical circuits are constantly being scaled down and extended with specific functions. A new method now allows electrical contact to be established with simple molecules on a conventional silicon chip. The technique promises to bring advances in sensor technology and medicine, as reported in the journal Nature by chemists from the University of Basel and researchers from IBM Research – Zurich in Rüschlikon.

To further develop semiconductor technology, the field of molecular electronics is seeking to manufacture circuit components from individual molecules instead of silicon. Because of their unique electronic properties, molecules are suited to applications that cannot be implemented using conventional silicon technology. However, this requires reliable and inexpensive methods for creating electrical contacts at the two ends of a molecule.

The ability to produce thousands of elements

Researchers from the University of Basel and IBM Research – Zurich have now developed a technique that allows electrical contact to individual molecules to be established. Thousands of stable metal-molecule-metal components can be produced simultaneously by depositing a film of nanoparticles onto the molecules, without compromising the properties of the molecules. This approach was demonstrated using alkane-dithiol compounds, which are made up of carbon, hydrogen, and sulfur.

Tiny pores were filled with molecules and brought into electrical contact via a platinum electrode from below and a gold nanoparticle electrode from above. Credit: IBM Research – Zurich

The researchers used a type of sandwich construction in which an interlayer of molecules is brought into contact with metallic electrodes from above and below. The lower electrode consists of a layer of platinum, which is coated with a layer of non-conducting material. Tiny pores are then etched into this layer to produce arbitrary patterns of compartments of different sizes, inside which there is an electrical contact with the platinum electrode.

Self-assembled monolayers

The researchers then took advantage of the ability of certain molecules to self-assemble. Onto the pattern of pores, they applied a solution containing alkane-dithiol molecules, which self-assemble into the pores, forminga densely packed monolayer film. Within this film, the individual molecules exhibit a regular arrangement and an electrical connection with the lower platinum electrode. Electrical contact with the molecular layer is established via an upper electrode made of gold nanoparticles.

The new technique largely resolves the issues that previously hampered the creation of electrical contacts to molecules – such as high contact resistance or short circuits by filaments penetrating the film. Building blocks fabricated by this method can be operated under standard conditions and provide long-term stability. Moreover, the method can be applied to a variety of other molecular systems and opens up new avenues for integrating molecular compounds into solid-state devices. Its applications could include new types of instruments in the fields of sensor technology and medicine.

“Our approach will help speed up the development of chemically fabricated and controllable electronic and sensor components,” says Professor Marcel Mayor of the Department of Chemistry at the University of Basel. The project received significant funding from the National Center of Competence in Research (NCCR) for Molecular Systems Engineering, in which the University of Basel and ETH Zurich are leading houses.

Researchers have demonstrated nanomaterial-based white-light-emitting diodes (LEDs) that exhibit a record luminous efficiency of 105 lumens per watt. Luminous efficiency is a measure of how well a light source uses power to generate light. With further development, the new LEDs could reach efficiencies over 200 lumens per watt, making them a promising energy-efficient lighting source for homes, offices and televisions.

Researchers created nanomaterial-based white LEDs that exhibit a record high efficiency thanks to quantum dots that are suspended in solution rather than embedded in a solid. The new LEDs could offer an energy-efficient lighting source for homes, offices and televisions. Credit: Sedat Nizamoglu, Koç University

“Efficient LEDs have strong potential for saving energy and protecting the environment,” said research leader Sedat Nizamoglu, Koç University, Turkey. “Replacing conventional lighting sources with LEDs with an efficiency of 200 lumens per watt would decrease the global electricity consumed for lighting by more than half. That reduction is equal to the electricity created by 230 typical 500-megawatt coal plants and would reduce greenhouse gas emissions by 200 million tons.”

The researchers describe how they created the high-efficiency white LEDs in Optica, The Optical Society’s journal for high impact research. The new LEDs use commercially available blue LEDs combined with flexible lenses filled with a solution of nano-sized semiconductor particles called quantum dots. Light from the blue LED causes the quantum dots to emit green and red, which combines with the blue emission to create white light.

“Our new LEDs reached a higher efficiency level than other quantum dot-based white LEDs,” said Nizamoglu. “The synthesis and fabrication methods for making the quantum dots and the new LEDs are easy, inexpensive and applicable for mass production.”

Advantages of quantum dots

To create white light with today’s LEDs, blue and yellow light are combined by adding a yellowish phosphor-based coating to blue LEDs. Because phosphors have a broad emission range, from blue to red, it is difficult to sensitively tune the properties of the generated white light.

Unlike phosphors, quantum dots generate pure colors because they emit only in a narrow portion of the spectrum. This narrow emission makes it possible to create high-quality white light with precise color temperatures and optical properties by combining quantum dots that generate different colors with a blue LED. Quantum dots also bring the advantage of being easy to make and the color of their emission can be easily changed by increasing the size of the semiconductor particle. Moreover, quantum dots can be advantageously used to generate warm white light sources like incandescent light bulbs or cool white sources like typical fluorescent lamps by changing the concentration of incorporated quantum dots.

Although quantum dots embedded in a film are currently used in LED televisions, this lighting approach is not suitable for widespread use in general lighting applications. Transferring the quantum dots in a liquid allowed the researchers to overcome the problematic drop in efficiency that occurs when nanomaterials are embedded into solid polymers.

Making efficient white LEDs requires quantum dots that efficiently convert blue light to red or green. The researchers carried out more than 300 synthesis reactions to identify the best conditions, such as temperature and time of the reaction, for making quantum dots that emit at different colors while exhibiting optimal efficiency.

“Creating white light requires integrating the appropriate amount of quantum dots, and even if that is accomplished, there are an infinite number of blue, green and red combinations that can lead to white,” said Nizamoglu. “We developed a simulation based on a theoretical approach we recently reported and used it to determine the appropriate amounts and best combinations of quantum dot colors for efficient white light generation.”

To make the new LEDs, the researchers filled the space between a polymer lens and LED chip with a solution of quantum dots that were synthesized by mixing cadmium, selenium, zinc and sulfur at high temperatures. The researchers used a type of silicone to make the lens because its elasticity allowed them to inject solutions into the lens without any solution leaking out, and the material’s transparency enabled the necessary light transmission.

The researchers showed that their liquid-based white LEDs could achieve an efficiency double that of LEDs that incorporate quantum dots in solid films. They also demonstrated their white LEDs by using them to illuminate a 7-inch display.

“Quantum dots hold great promise for efficient lighting applications,” said Nizamoglu. “There is still significant room for technology development that would generate more efficient approaches to lighting.”

As a next step, the researchers are working to increase the efficiency of the LEDs and want to reach high efficiency levels using environmentally friendly materials that are cadmium- and lead-free. They also plan to study the liquid LEDs under different conditions to ensure they are stable for long-term application.

KLA-Tencor Corporation announced two new defect inspection products at SEMICON West this week, addressing two key challenges in tool and process monitoring during silicon wafer and chip manufacturing at the leading-edge logic and memory nodes. The VoyagerTM1015 system offers new capability to inspect patterned wafers, including inspection in the lithography cell immediately after development of the photoresist, when the wafer can be reworked. The Surfscan SP7 system delivers unprecedented defect detection sensitivity on bare wafers, smooth and rough films—essential for manufacturing silicon substrates intended for the 7nm logic and advanced memory device nodes, and equally critical for earliest detection of process issues during chip manufacturing. Together the two new inspection systems are designed to accelerate time-to-market for innovative electronic devices by capturing defect excursions at their source.

“With leading IC technologies, wafer and chip manufacturers have very little room for error,” said Oreste Donzella, Senior Vice President and Chief Marketing Officer at KLA-Tencor. “Critical dimensions of next-generation chips are so small that the minimum size of a yield-killing defect on bare silicon wafers or blanket-film monitor wafers has shrunk below the detection limit of available tool monitoring systems. A second key gap in the defect detection space has been reliably detecting yield-killing defects introduced early in the lithography process, whether 193i or EUV. Our engineering teams have developed two new defect inspection systems—one for unpatterned/monitor wafers and one for patterned wafers—that provide key capability for engineers to address these difficult defect issues rapidly and accurately.”

The Surfscan SP7 unpatterned wafer defect inspection system achieves its high sensitivity through innovations in illumination and sensor architecture that produce decades of improvement in resolution over that of the previous-generation Surfscan tool. This leap in resolution is the key to detection of the smallest killer defects. The new resolution realm also enables real-time classification of many defect types, such as particles, scratches, slip lines and stacking faults—without removing the wafer from the Surfscan tool or affecting the system throughput. At the same time, control over peak power density allows the Surfscan SP7 to inspect thin, delicate EUV photoresist materials.

The Voyager 1015 patterned wafer defect inspection system closes a long-standing industry gap in after-develop inspection (ADI), leveraging novel illumination, collection and sensor architecture. This revolutionary laser scattering inspection system drives sensitivity forward while reducing nuisance signals—and delivers results substantially sooner than the next-best alternatives. Like the new Surfscan SP7, the Voyager system features exceptional control of power density, allowing inline inspection of delicate photoresist materials after develop. High throughput capture of critical defects in the litho cell and other modules of the fab allows process issues to be identified and rectified rapidly.