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Organic semiconductors are lightweight, flexible and easy to manufacture. But they often fail to meet expectations regarding efficiency and stability. Researchers at the Technical University of Munich (TUM) are now deploying data mining approaches to identify promising organic compounds for the electronics of the future.

Producing traditional solar cells made of silicon is very energy intensive. On top of that, they are rigid and brittle. Organic semiconductor materials, on the other hand, are flexible and lightweight. They would be a promising alternative, if only their efficiency and stability were on par with traditional cells.

Together with his team, Karsten Reuter, Professor of Theoretical Chemistry at the Technical University of Munich, is looking for novel substances for photovoltaics applications, as well as for displays and light-emitting diodes – OLEDs. The researchers have set their sights on organic compounds that build on frameworks of carbon atoms.

Contenders for the electronics of tomorrow

Depending on their structure and composition, these molecules, and the materials formed from them, display a wide variety of physical properties, providing a host of promising candidates for the electronics of the future.

“To date, a major problem has been tracking them down: It takes weeks to months to synthesize, test and optimize new materials in the laboratory,” says Reuter. “Using computational screening, we can accelerate this process immensely.”

Computers instead of test tubes

The researcher needs neither test tubes nor Bunsen burners to search for promising organic semiconductors. Using a powerful computer, he and his team analyze existing databases. This virtual search for relationships and patterns is known as data mining.

“Knowing what you are looking for is crucial in data mining,” says PD Dr. Harald Oberhofer, who heads the project. “In our case, it is electrical conductivity. High conductivity ensures, for example, that a lot of current flows in photovoltaic cells when sunlight excites the molecules.”

Algorithms identify key parameters

Using his algorithms, he can search for very specific physical parameters: An important one is, for example, the “coupling parameter.” The larger it is, the faster electrons move from one molecule to the next.

A further parameter is the “reorganization energy”: It defines how costly it is for a molecule to adapt its structure to the new charge following a charge transfer – the less energy required, the better the conductivity.

The research team analyzed the structural data of 64,000 organic compounds using the algorithms and grouped them into clusters. The result: Both the carbon-based molecular frameworks and the “functional groups”, i.e. the compounds attached laterally to the central framework, decisively influence the conductivity.

Identifying molecules using artificial intelligence

The clusters highlight structural frameworks and functional groups that facilitate favorable charge transport, making them particularly suitable for the development of electronic components.

“We can now use this to not only predict the properties of a molecule, but using artificial intelligence we can also design new compounds in which both the structural framework and the functional groups promise very good conductivity,” explains Reuter.

Publication:

Finding the Right Bricks for Molecular Lego: A Data Mining Approach to Organic Semiconductor Design
Christian Kunkel, Christoph Schober, Johannes T. Margraf, Karsten Reuter, Harald Oberhofer
Chem. Mater. 2019, 31, 3, 969-978 – DOI: 10.1021/acs.chemmater.8b04436
https://pubs.acs.org/doi/10.1021/acs.chemmater.8b04436

Scientists at the U.S. Naval Research Laboratory (NRL) and the Air Force Research Laboratory (AFRL) have developed a way to directly write quantum light sources, which emit a single photon of light at a time, into monolayer semiconductors such as tungsten diselenide (WSe2). Single photon emitters (SPEs), or quantum emitters, are key components in a wide range of nascent quantum-based technologies, including computing, secure communications, sensing and metrology.

(a) Illustration showing an AFM tip indenting the TMD/polymer structure to introduce local strain. (b) Patterned single photon emission in WSe2 induced by AFM indentation of the letters ‘NRL’ and ‘AFRL’. (c) AFM indents produce single photon emitter ‘ornaments’ on a monolayer WSe2 ‘Christmas tree.’ Credit: US Naval Research Laboratory

In contrast with conventional light emitting diodes which emit billions of photons simultaneously to form a steady stream of light, an ideal SPE generates exactly one photon on demand, with each photon indistinguishable from another. These characteristics are essential for photon-based quantum technologies under development. In addition, such capabilities should be realized in a material platform which enables precise, repeatable placement of SPEs in a fully scalable fashion compatible with existing semiconductor chip manufacturing.

NRL scientists used an atomic force microscope (AFM) to create nanoscale depressions or indents in a single monolayer of WSe2 on a polymer film substrate. A highly localized strain field is produced around the nano-indent which creates the single photon emitter state in the WSe2. Time correlated measurements performed at AFRL of this light emission confirmed the true single photon nature of these states. These emitters are bright, producing high rates of single photons, and spectrally stable, key requirements for emerging applications.

“This quantum calligraphy allows deterministic placement and real time design of arbitrary patterns of SPEs for facile coupling with photonic waveguides, cavities and plasmonic structures,” said Berend Jonker, Ph.D., senior scientist and principal investigator. “Our results also indicate that a nano-imprinting approach will be effective in creating large arrays or patterns of quantum emitters for wafer scale manufacturing of quantum photonic systems.”

Dr. Matthew Rosenberger, lead author of the study, points out the importance of this discovery stating, “In addition to enabling versatile placement of SPEs, these results present a general methodology for imparting strain into two dimensional (2D) materials with nanometer-scale precision, providing an invaluable tool for further investigations and future applications of strain engineering of 2D devices.”

The results of this study pave the way for the use of 2D materials as solid state hosts for single photon emitters in applications relevant to the Department of Defense (DoD) mission, such as secure communications, sensing and quantum computation. Such applications enable communication between distant DoD forces which is not vulnerable to eavesdropping or decryption, an essential requirement to insure the safety of the warfighter.

Quantum computation on a chip provides onboard capability to rapidly analyze very large data sets acquired by sensor arrays, so that the entire data set does not have to be transmitted, reducing bandwidth requirements. The research results are reported in the January 2019 ACS Nano (DOI: 10.1021/acsnano.8b08730).

The research team included Dr. Matthew Rosenberger, Dr. Hsun-Jen Chuang, Dr. Saujan Sivaram, Dr. Kathleen McCreary, and Dr. Berend Jonker from the NRL Materials Science and Technology Division; and Dr. Chandriker Kavir Dass and Dr. Joshua R. Hendrickson from the AFRL Sensors Directorate. Both Rosenberger and Sivaram hold National Research Council (NRC) fellowships at NRL, and Chuang holds an American Society for Engineering Education (ASEE) fellowship at NRL.

Metal halide perovskites are regarded as next generation materials for light emitting devices (LEDs). A recent joint-research co-led by the scientist from City University of Hong Kong (CityU) has developed a new and efficient fabrication approach to produce all-inorganic perovskite films with better optical properties and stability, enabling the development of high colour-purity and low-cost perovskite LEDs with a high operational lifetime.

a) Device structure and a corresponding cross-sectional TEM image of the multi-layer PeLEDs; b) Schematic flat-band energy diagram of the PeLED; c) Normalized photoluminescence spectrum of the CsPbBr3 film, and electroluminescence spectrum of the PeLED at an applied voltage of 5.5 V Credit: City University of Hong Kong

Perovskite LEDs (PeLEDs) are an emerging light-emitting technology with advantages of low manufacturing cost, high light quality and energy efficiency. Metal halide (meaning compounds of metals with chlorine, bromine or iodine) perovskites have recently attracted a lot of attention as promising materials for solution-processed LEDs, owing to their excellent optical properties, such as saturated emission colors and easy color tunability.

In particular, perovskites based on inorganic cesium cations, namely CsPbX3 (where X can be chlorine, bromine and iodine), exhibit better thermal and chemical stability compared to the organic-inorganic ‘hybrid’ metal halide perovskites, and may thus provide the base for high-performance LEDs with reasonable operational stability. But the previous inorganic PeLEDs exhibited relatively poor electro-luminescence performance due to their large perovskite grain sizes.

Now a team of researchers at CityU and at Shanghai University in mainland China has developed an efficient fabrication approach to make smooth inorganic perovskite films with substantially enhanced performance and stability. Their findings appear in the latest issue (2019, 10, 665) of the scientific journal Nature Communications, titled “Trifluoroacetate induced small-grained CsPbBr3 perovskite films result in efficient and stable light-emitting devices “.

The team has found that using cesium trifluoroacetate (TFA) as the cesium source in the one-step solution coating, instead of the commonly used cesium bromide (CsBr), enables fast crystallization of small-grained CsPbBr3 perovskite crystals, forming the smooth and pinhole-free perovskite films. This is because the interaction of TFA- anions with Pb2+ cations in the CsPbX3 precursor solution greatly improves the crystallization rate of perovskite films and suppresses surface defects.

As a result, the team has managed to make efficient and stable green PeLEDs based on these films, with a maximum current efficient of 32.0 cd A-1 corresponding to an external quantum efficiency of 10.5% – a level generally considered as satisfactory in existing PeLEDs.

More importantly, the all-inorganic perovskite LEDs based on these films demonstrated a record operational lifetime. They have a half-lifetime of over 250 hours at an initial luminance of 100 cd m-2, which is a 17 times improvement in operational lifetime compared with CsBr-derived PeLED.

“Our study suggests that the high color-purity and low-cost all-inorganic lead halide perovskite films can be developed into highly efficient and stable LEDs via a simple optimization of the grain boundaries,” says Andrey Rogach, Chair Professor of Photonics Materials at CityU, who is one of the correspondence authors of the paper.

“I foresee significant application potential of such films, as they are easy to fabricate and can be easily deposited by printing to realise various optoelectronic devices,” he adds.

Another correspondence author of the paper is Professor Yang Xuyong from Shanghai University. The first authors are Wang Haoran at Shanghai University and Zhang Xiaoyu, a former visiting research student at CityU, now working as a postdoc at Jilin University.

Quantum computers promise to be a revolutionary technology because their elementary building blocks, qubits, can hold more information than the binary, 0-or-1 bits of classical computers. But to harness this capability, hardware must be developed that can access, measure and manipulate individual quantum states.

Researchers at the University of Pennsylvania’s School of Engineering and Applied Science have now demonstrated a new hardware platform based on isolated electron spins in a two-dimensional material. The electrons are trapped by defects in sheets of hexagonal boron nitride, a one-atom-thick semiconductor material, and the researchers were able to optically detect the system’s quantum states.

Researchers at the University of Pennsylvania’s School of Engineering and Applied Science have now demonstrated a new hardware platform based on isolated electron spins in a two-dimensional material. The electrons are trapped by defects in sheets of hexagonal boron nitride, a one-atom-thick semiconductor material, and the researchers were able to optically detect the system’s quantum states. Credit: Ann Sizemore Blevins

The study was led by Lee Bassett, assistant professor in the Department of Electrical and Systems Engineering, and Annemarie Exarhos, then a postdoctoral researcher in his lab.

Fellow Bassett Lab members David Hopper and Raj Patel, along with Marcus Doherty of the Australian National University, also contributed to the study.

It was published in the journal Nature Communications, where it was selected as an Editor’s Highlight.

There are number of potential architectures for building quantum technology. One promising system involves electron spins in diamonds: these spins are also trapped at defects in diamond’s regular crystalline pattern where carbon atoms are missing or replaced by other elements. The defects act like isolated atoms or molecules, and they interact with light in a way that enables their spin to be measured and used as a qubit.

These systems are attractive for quantum technology because they can operate at room temperatures, unlike other prototypes based on ultra-cold superconductors or ions trapped in vacuum, but working with bulk diamond presents its own challenges.

“One disadvantage of using spins in 3D materials is that we can’t control exactly where they are relative to the surface” Bassett says. “Having that level of atomic scale control is one reason to work in 2D. Maybe you want to place one spin here and one spin there and have them talk them to each other. Or if you want to have a spin in a layer of one material and plop a 2D magnet layer on top and have them interact. When the spins are confined to a single atomic plane, you enable a host of new functionalities.”

With nanotechnological advances producing an expanding library of 2D materials to choose from, Bassett and his colleagues sought the one that would be most like a flat analog of bulk diamond.

“You might think the analog would be graphene, which is just a honeycomb lattice of carbon atoms, but here we care more about the electronic properties of the crystal than what type of atoms it’s made of,” says Exarhos, who is now an assistant professor of Physics at Lafayette University. “Graphene behaves like a metal, whereas diamond is a wide-bandgap semiconductor and thus acts like an insulator. Hexagonal boron nitride, on the other hand, has the same honeycomb structure as graphene, but, like diamond, it is also a wide-bandgap semiconductor and is already widely used as a dielectric layer in 2D electronics.”

With hexagonal boron nitride, or h-BN, widely available and well characterized, Bassett and his colleagues focused on one of its less well-understood aspects: defects in its honeycomb lattice that can emit light.

That the average piece of h-BN contains defects that emit light had previously been known. Bassett’s group is the first to show that, for some of those defects, the intensity of the emitted light changes in response to a magnetic field.

“We shine light of one color on the material and we get photons of another color back,” Bassett says. “The magnet controls the spin and the spin controls the number of photons that the defects in the h-BN emit. That’s a signal that you can potentially use as a qubit.”

Beyond computation, having the building block of a quantum machine’s qubits on a 2D surface enables other potential applications that depend on proximity.

“Quantum systems are super sensitive to their environments, which is why they’re so hard to isolate and control,” Bassett says. “But the flip side is that you can use that sensitivity to make new types of sensors. In principle, these little spins can be miniature nuclear magnetic resonance detectors, like the kind used in MRIs, but with the ability to operate on a single molecule.

Nuclear magnetic resonance is currently used to learn about molecular structure, but it requires millions or billions of the target molecule to be assembled into a crystal. In contrast, 2D quantum sensors could measure the structure and internal dynamics of individual molecules, for example to study chemical reactions and protein folding.

While the researchers conducted an extensive survey of h-BN defects to discover ones that have special spin-dependent optical properties, the exact nature of those defects is still unknown. Next steps for the team include understanding what makes some, but not all, defects responsive to magnetic fields, and then recreating those useful defects.

Some of that work will be enabled by Penn’s Singh Center for Nanotechnology and its new JEOL NEOARM microscope. The only transmission electron microscope of its kind in the United States, the NEOARM is capable of resolving single atoms and potentially even creating the kinds of defects the researchers want to work with.

“This study is bringing together two major areas of scientific research,” Bassett says. “On one hand, there’s been a tremendous amount of work in expanding the library of 2D materials and understanding the physics that they exhibit and the devices they can make. On the other hand, there’s the development of these different quantum architectures. And this is one of the first to bring them together to say ‘here’s a potentially room-temperature quantum architecture in a 2D material.'”

Two-dimensional transition metal dichalcogenides (2D-TMDs) such as monolayer molybdenum disulphide (MoS2) are atomically thin semiconductors in which a layer of transition metal atom is sandwiched between two layers of chalcogen atoms, in the form MX2. They can exist in both a semiconducting 1H-phase and a quasi-metallic 1T’-phase, with each having a different crystal structure. The 1T’-phase is particularly interesting as theoretical predictions show that it has potential to be used in less conventional applications, such as super capacitor electrodes and hydrogen evolution reaction catalysts. However, the quantity of 1T’-phase 2D-TMDs that can be obtained by converting them from the 1H-phase through a phase transition process is low. This potentially limits the use of such novel materials for a wide range of applications.

Molecules of monolayer molybdenum disulphide (MoS2) and tungsten diselenide (WSe2) on top of a metal substrate. Credit: National University of Singapore

A research team led by Professor Andrew Wee from the Department of Physics at the National University of Singapore’s (NUS) Faculty of Science has discovered that while different 2D-TMD materials have their own intrinsic energy barriers when transiting from the 1H to the 1T’ structural phase, the use of a metallic substrate with higher chemical reactivity can significantly increase the 1H- to 1T’- phase transition yield. This is a convenient and high-yielding method to obtain 2D-TMD materials in their 1T’ metallic phase. When the 2D-TMD material is placed in contact with the metal substrate, such as gold, silver and copper, electric charges are transferred from the metal substrate to the 2D-TMD material. Furthermore, it weakens the bond strength of the 2D-TMD structure significantly, and increases the magnitude of the interfacial binding energy. This in turn increases the susceptibility of the 1H-1T’ structural phase transition. As a result, this enhanced interfacial hybridisation at the interface of the two materials makes the 1H-1T’ structural phase transition much easier to achieve.

The NUS research team combined multiple experimental techniques and first-principles calculations in their research work. These includes optical spectroscopies, high resolution transmission electron microscopy and density functional theory based first-principles calculations to identify the phase changes – both 1H- and 1T’-phases – of the 2D-TMDs in the samples.

This study provides new insights on the influence of interfacial hybridisation affecting the phase transition dynamics of 2D-TMDs. The findings can potentially be used in a model system for the controlled growth of 2D-TMDs on metallic substrates, creating possibilities for new 2D-TMDs-based device applications.

Prof Wee said, “The controllability of the semiconductor to metal phase transition at the 2D-TMD and metal interfaces can enable new device applications such as low contact resistance electrodes.”

Optical circuits are set to revolutionize the performance of many devices. Not only are they 10-100 times faster than electronic circuits, but they also consume a lot less power. Within these circuits, light waves are controlled by extremely thin surfaces called metasurfaces that concentrate the waves and guide them as needed. The metasurfaces contain regularly spaced nanoparticles that can modulate electromagnetic waves over sub-micrometer wavelength scales.

The new method employs a natural process already used in fluid mechanics: dewetting. CREDIT © Vytautas Navikas / 2019 EPFL

Metasurfaces could enable engineers to make flexible photonic circuits and ultra-thin optics for a host of applications, ranging from flexible tablet computers to solar panels with enhanced light-absorption characteristics. They could also be used to create flexible sensors to be placed directly on a patient’s skin, for example, in order to measure things like pulse and blood pressure or to detect specific chemical compounds.

The catch is that creating metasurfaces using the conventional method, lithography, is a fastidious, several-hour-long process that must be done in a clean room. But EPFL engineers from the Laboratory of Photonic Materials and Fiber Devices (FIMAP) have now developed a simple method for making them in just a few minutes at low temperatures – or sometimes even at room temperature – with no need for a clean room. The EPFL’s School of Engineering method produces dielectric glass metasurfaces that can be either rigid or flexible. The results of their research appear in Nature Nanotechnology.

Turning a weakness into a strength

The new method employs a natural process already used in fluid mechanics: dewetting. This occurs when a thin film of material is deposited on a substrate and then heated. The heat causes the film to retract and break apart into tiny nanoparticles. “Dewetting is seen as a problem in manufacturing – but we decided to use it to our advantage,” says Fabien Sorin, the study’s lead author and the head of FIMAP.

With their method, the engineers were able to create dielectric glass metasurfaces – rather than metallic metasurfaces – for the first time. The advantage of dielectric metasurfaces is that they absorb very little light and have a high refractive index, making it possible to effectively modulate the light that propagates through them.

To construct these metasurfaces, the engineers first created a substrate textured with the desired architecture. Then they deposited a material – in this case, chalcogenide glass – in thin films just tens of nanometers thick. The substrate was subsequently heated for a couple of minutes until the glass became more fluid and nanoparticles began to form in the sizes and positions dictated by the substrate’s texture.

The engineers’ method is so efficient that it can produce highly sophisticated metasurfaces with several levels of nanoparticles or with arrays of nanoparticles spaced 10 nm apart. That makes the metasurfaces highly sensitive to changes in ambient conditions – such as to detect the presence of even very low concentrations of bioparticles. “This is the first time dewetting has been used to create glass metasurfaces. The advantage is that our metasurfaces are smooth and regular, and can be easily produced on large surfaces and flexible substrates,” says Sorin.

Noting the startling advances in semiconductor technology, Intel co-founder Gordon Moore proposed that the number of transistors on a chip will double each year, an observation that has been born out since he made the claim in 1965. Still, it’s unlikely Moore could have foreseen the extent of the electronics revolution currently underway.

Today, a new breed of devices, bearing unique properties, is being developed. As ultra-miniaturization continues apace, researchers have begun to explore the intersection of physical and chemical properties occurring at the molecular scale.

Advances in this fast-paced domain could improve devices for data storage and information processing and aid in the development of molecular switches, among other innovations.

Nongjian “NJ” Tao and his collaborators recently described a series of studies into electrical conductance through single molecules. Creating electronics at this infinitesimal scale presents many challenges. In the world of the ultra-tiny, the peculiar properties of the quantum world hold sway. Here, electrons flowing as current behave like waves and are subject to a phenomenon known as quantum interference. The ability to manipulate this quantum phenomenon could help open the door to new nanoelectronic devices with unusual properties.

“We are interested in not only measuring quantum phenomena in single molecules, but also controlling them. This allows us to understand the basic charge transport in molecular systems and study new device functions,” Tao says.

Tao is the director of the Biodesign Center for Bioelectronics and Biosensors. In research appearing in the journal Nature Materials, Tao and colleagues from Japan, China and the UK outline experiments in which a single organic molecule is suspended between a pair of electrodes as a current is passed through the tiny structure.

The researchers explore the charge transport properties through the molecules. They demonstrated that a ghostly wavelike property of electrons–known as quantum interference– can be precisely modulated in two different configurations of the molecule, known as Para and Meta.

It turns out that quantum interference effects can cause substantial variation in the conductance properties of molecule-scale devices. By controlling the quantum interference, the group showed that electrical conductance of a single molecule can be fine-tuned over two orders of magnitude. Precisely and continuously controlling quantum interference is seen as a key ingredient in the future development of wide-ranging molecular-scale electronics, operating at high speed and low power.

Such single-molecule devices could potentially act as transistors, wires, rectifiers, switches or logic gates and may find their way into futuristic applications including superconducting quantum interference devices (SQUID), quantum cryptography, and quantum computing.

For the current study, the molecules–ring-shaped hydrocarbons that can appear in different configurations–were used, as they are among the simplest and most versatile candidates for modeling the behavior of molecular electronics and are ideal for observing quantum interference effects at the nanoscale.

In order to probe the way charge moves through a single molecule, so-called break junction measurements were made. The tests involve the use of a scanning tunneling microscope or STM. The molecule under study is poised between a gold substrate and gold tip of the STM device. The tip of the STM is repeatedly brought in and out of contact with the molecule, breaking and reforming the junction while the current passes through each terminal.

Thousands of conductance versus distance traces were recorded, with the particular molecular properties of the two molecules used for the experiments altering the electron flow through the junction. Molecules in the ‘Para’ configuration showed higher conductance values than molecules of the ‘Meta’ form, indicating constructive vs destructive quantum interference in the molecules.

Using a technique known as electrochemical gating, the researchers were able to continuously control the conductance over two orders of magnitude. In the past, altering quantum interference properties required modifications to the charge-carrying molecule used for the device. The current study marks the first occasion of conductance regulation in a single molecule.

As the authors note, conductance at the molecular scale is sensitively affected by quantum interference involving the electron orbitals of the molecule. Specifically, interference between the highest occupied molecular orbital or HOMO and lowest unoccupied molecular orbital or LUMO appears to be the dominant determinant of conductance in single molecules. Using an electrochemical gate voltage, quantum interference in the molecules could be delicately tuned.

The researchers were able to demonstrate good agreement between theoretical calculations and experimental results, indicating that the HOMO and LUMO contributions to the conductance were additive for Para molecules, resulting in constructive interference, and subtractive for Meta, leading to destructive interference, much as waves in water can combine to form a larger wave or cancel one another out, depending on their phase.

While previous theoretical calculations of charge transport through single molecules had been carried out, experimental verification has had to wait for a number of advances in nanotechnology, scanning probe microscopy, and methods to form electrically functional connections of molecules to metal surfaces. Now, with the ability to subtly alter conductance through the manipulation of quantum interference, the field of molecular electronics is open to a broad range of innovations.

Germanene is a 2D material that derives from germanium and is related to graphene. As it is not stable outside the vacuum chambers in which is it produced, no real measurements of its electronic properties have been made. Scientists led by Prof. Justin Ye of the University of Groningen have now managed to produce devices with stable germanene. The material is an insulator, and it becomes a semiconductor after moderate heating and a very good metallic conductor after stronger heating. The results were published in the journal Nano Letters.

Germanane is converted into germanene by thermal annealing, which removes the hydrogen (red). Credit: Ye Lab / University of Groningen

Materials of just one atomic layer are of interest in the construction of new types of microelectronics. The best known of these, graphene, is an excellent conductor. Materials like silicon and germanium could be interesting as well, as they are fully compatible with well-established protocols for device fabrication, and could be seamlessly integrated into the present semiconductor technology.

Unstable

‘But the 2D version of germanium, germanene, is very unstable’, explains University of Groningen Associate Professor of Device Physics Justin Ye. Germanene is made from germanium by adding calcium. The calcium ions create 2D layers from a 3D crystal and are then replaced by hydrogen. These 2D layers of germanium and hydrogen are called germanane. But once the hydrogen is removed to form germanene, the material becomes unstable.

Ye and his colleagues solved this in a remarkably simple way. They made devices with the stable germanane, and then heated the material to remove the hydrogen. This resulted in stable devices with germanene, which allowed the scientists to study its electronic properties.

Hydrogen

‘The initial material was an insulator’, says Ye. A Ph.D. student from his group heated these devices, which is a tried and tested method to increase conductivity. He noted that the material became very conductive, and its resistance was just one order of magnitude above that of graphene. ‘So it became an excellent metallic conductor.’ Further experiments showed that moderate heating (up to 200°C) produced semiconducting germanane.

Germanene can, therefore, be an insulator, a semiconductor or a metallic conductor, depending on the heat treatment with which it is processed. It remains stable after being cooled to room temperature. The heating causes multilayer flakes of germanene to become thinner – confirmation that the change in conductivity is most likely caused by the disappearance of hydrogen.

Spintronic device

Germanene could be of interest in the construction of spintronic devices. These devices use a current of electron spins. This is a quantum mechanical property of electrons, which can best be imagined as electrons spinning around their own axis, causing them to behave like small compass needles. Graphene is an excellent conductor of electron spins, but it is hard to control spins in this material because of their weak interaction with the carbon atoms (spin-orbit coupling).

‘The germanium atoms are heavier, which means there is a stronger spin-orbit coupling’, says Ye. This would provide better control of spins. Being able to construct metallic germanene with both excellent conductivity and strong spin-orbit coupling should therefore pave the way to spintronic devices.

Researchers from the University of Houston have reported significant advances in stretchable electronics, moving the field closer to commercialization.

Researchers from the University of Houston have reported significant advances in the field of stretchable, rubbery electronics. Credit: University of Houston

In a paper published Friday, Feb. 1, in Science Advances, they outlined advances in creating stretchable rubbery semiconductors, including rubbery integrated electronics, logic circuits and arrayed sensory skins fully based on rubber materials.

Cunjiang Yu, Bill D. Cook Assistant Professor of mechanical engineering at the University of Houston and corresponding author on the paper, said the work could lead to important advances in smart devices such as robotic skins, implantable bioelectronics and human-machine interfaces.

Yu previously reported a breakthrough in semiconductors with instilled mechanical stretchability, much like a rubber band, in 2017.

This work, he said, takes the concept further with improved carrier mobility and integrated electronics.

“We report fully rubbery integrated electronics from a rubbery semiconductor with a high effective mobility … obtained by introducing metallic carbon nanotubes into a rubbery semiconductor with organic semiconductor nanofibrils percolated,” the researchers wrote. “This enhancement in carrier mobility is enabled by providing fast paths and, therefore, a shortened carrier transport distance.”

Carrier mobility, or the speed at which electrons can move through a material, is critical for an electronic device to work successfully, because it governs the ability of the semiconductor transistors to amplify the current.

Previous stretchable semiconductors have been hampered by low carrier mobility, along with complex fabrication requirements. For this work, the researchers discovered that adding minute amounts of metallic carbon nanotubes to the rubbery semiconductor of P3HT – polydimethylsiloxane composite – leads to improved carrier mobility by providing what Yu described as “a highway” to speed up the carrier transport across the semiconductor.

By bombarding an ultrathin semiconductor sandwich with powerful laser pulses, physicists at the University of California, Riverside, have created the first “electron liquid” at room temperature.

The achievement opens a pathway for development of the first practical and efficient devices to generate and detect light at terahertz wavelengths — between infrared light and microwaves. Such devices could be used in applications as diverse as communications in outer space, cancer detection, and scanning for concealed weapons.

The research could also enable exploration of the basic physics of matter at infinitesimally small scales and help usher in an era of quantum metamaterials, whose structures are engineered at atomic dimensions.

The UCR physicists published their findings online Feb. 4 in the journal Nature Photonics. They were led by Associate Professor of Physics Nathaniel Gabor, who directs the UCR Quantum Materials Optoelectronics Lab. Other co-authors were lab members Trevor Arp and Dennis Pleskot, and Associate Professor of Physics and Astronomy Vivek Aji.

A video depicting the research is available here.

In their experiments, the scientists constructed an ultrathin sandwich of the semiconductor molybdenum ditelluride between layers of carbon graphene. The layered structure was just slightly thicker than the width of a single DNA molecule. They then bombarded the material with superfast laser pulses, measured in quadrillionths of a second.

“Normally, with such semiconductors as silicon, laser excitation creates electrons and their positively charged holes that diffuse and drift around in the material, which is how you define a gas,” Gabor said. However, in their experiments, the researchers detected evidence of condensation into the equivalent of a liquid. Such a liquid would have properties resembling common liquids such as water, except that it would consist, not of molecules, but of electrons and holes within the semiconductor.

“We were turning up the amount of energy being dumped into the system, and we saw nothing, nothing, nothing — then suddenly we saw the formation of what we called an ‘anomalous photocurrent ring’ in the material,” Gabor said. “We realized it was a liquid because it grew like a droplet, rather than behaving like a gas.”

“What really surprised us, though, was that it happened at room temperature,” he said. “Previously, researchers who had created such electron-hole liquids had only been able to do so at temperatures colder than even in deep space.”

The electronic properties of such droplets would enable development of optoelectronic devices that operate with unprecedented efficiency in the terahertz region of the spectrum, Gabor said. Terahertz wavelengths are longer than infrared waves but shorter than microwaves, and there has existed a “terahertz gap” in the technology for utilizing such waves. Terahertz waves could be used to detect skin cancers and dental cavities because of their limited penetration and ability to resolve density differences. Similarly, the waves could be used to detect defects in products such as drug tablets and to discover weapons concealed beneath clothing.

Terahertz transmitters and receivers could also be used for faster communication systems in outer space. And, the electron-hole liquid could be the basis for quantum computers, which offer the potential to be far smaller than silicon-based circuitry now in use, Gabor said.

More generally, Gabor said, the technology used in his laboratory could be the basis for engineering “quantum metamaterials,” with atom-scale dimensions that enable precise manipulation of electrons to cause them to behave in new ways.

In further studies of the electron-hole “nanopuddles,” the scientists will explore their liquid properties such as surface tension.

“Right now, we don’t have any idea how liquidy this liquid is, and it would be important to find out,” Gabor said.

Gabor also plans to use the technology to explore basic physical phenomena. For example, cooling the electron-hole liquid to ultra-low temperatures could cause it to transform into a “quantum fluid” with exotic physical properties that could reveal new fundamental principles of matter.

In their experiments, the researchers used two key technologies. To construct the ultrathin sandwiches of molybdenum ditelluride and carbon graphene, they used a technique called “elastic stamping.” In this method, a sticky polymer film is used to pick up and stack atom-thick layers of graphene and semiconductor.

And to both pump energy into the semiconductor sandwich and image the effects, they used “multi-parameter dynamic photoresponse microscopy” developed by Gabor and Arp. In this technique, beams of ultrafast laser pulses are manipulated to scan a sample to optically map the current generated.