Category Archives: Process Materials

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

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.

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.

The intelliFLEX Innovation Alliance announced today that Mark Majewski, a 30-year veteran of the Canadian technology industry and former geographic director at a major semiconductor company, has succeeded Peter Kallai as CEO.

Mr. Majewski has extensive experience in the electronics and technology industries in Canada, having overseen the generation of hundreds of millions of dollars at STMicroelectronics while running its East Central U.S. and Canada regions. He’s also been a key leader at several startups, volunteers as a mentor at the RIC Centre and Haltech, and most recently was the technology lead for business development at Ontario Centres of Excellence (OCE).

Mr. Majewski’s goal as CEO is to unite the growing critical mass of Canadian printable, flexible and hybrid electronics (FHE) companies and research with the country’s electronics and semiconductor industries. With his decades of technology experience, Mr. Majewski has the breadth of contacts, experience, and knowledge to successfully position intelliFLEX and its members alongside this massive industry.

“I’m honoured to have been named the next intelliFLEX CEO. I’ve taken this role because I believe in FHE and its future,” says Mr. Majewski. “All electronics players in Canada who want to expand their capabilities should be looking at this technology as it goes mainstream. Not only does FHE open the doors to new products and applications, it also has incredible value in augmenting and improving everyday electronics products that already exist.”

Indeed, as microelectronics and semiconductor companies hit the limits of Moore’s Law for integrated circuits, mainstream companies are searching for new ways to produce electronic components more efficiently for new and existing applications.

That’s where printable, flexible and hybrid electronics come in: FHE, which represents a $31.6B global market opportunity, uses next-generation additive and manufacturing electronics technologies that can help all electronics players in Canada. This strategy has already been embraced in the U.S. where a cross-pollination of mainstream electronics, FHE, and semiconductors is occurring.

“I’ve cherished the opportunity to work with intelliFLEX,” said outgoing CEO Peter Kallai, who founded intelliFLEX and will remain involved by supporting Mr. Majewski during the transition period and sitting on the board of directors. “However, what we need to do is move the organization into the mainstream electronics industry and be the rising tide of the ecosystem that lets all our members sail further, faster and easier.

“We needed a professional from that industry, with the right background, to do that. And I strongly believe Mark will take intelliFLEX to the next level.”

At the same time, intelliFLEX will also move its head office from Ottawa to the Greater Toronto Area. This will help the organization be physically closer to the heart of Canada’s electronics industry, of which the majority is located in Toronto. Seventy-five per cent of intelliFLEX members are in either Ontario or Quebec.

A team of researchers from Lehigh University, Oak Ridge National Laboratory, Lebanon Valley College and Corning Inc. has demonstrated, for the first time, that crystals manufactured by lasers within a glass matrix maintain full ferroelectric functionality.

Ferroelectric single-crystal-architecture-in-glass is a new class of metamaterials that would enable active integrated optics if the ferroelectric behavior is preserved within the confines of glass. We demonstrate using lithium niobate crystals fabricated in lithium niobosilicate glass by femtosecond laser irradiation that not only such behavior is preserved, the ferroelectric domains can be engineered with a DC bias. A piezoresponse force microscope is used to characterize the piezoelectric and ferroelectric behavior. The piezoresponse correlates with the orientation of the crystal lattice as expected for unconfined crystal, and a complex micro- and nano-scale ferroelectric domain structure of the as-grown crystals is revealed. Credit: Keith Veenhuizen, Sean McAnany, Rama Vasudevan, Daniel Nolan, Bruce Aitken, Stephen Jesse, Sergei V. Kalinin, Himanshu Jain and Volkmar Dierolf

“This includes the ability to uniformly orient and reverse orient the ferroelectric domains with an electric field?despite the fact that the crystal is strongly confined by the surrounding glass,” says Volkmar Dierolf, Chair of Lehigh University’s Department of Physics and one of the scientists who worked on the experiments that resulted in these findings.

Dierolf, who holds a joint appointment with Lehigh’s Department of Materials Science and Engineering part of the P.C. Rossin College of Engineering and Applied Science, is co-Principal Investigator on a National Science Foundation (NSF)-funded project, Crystal in Glass, along with Principal Investigator Himanshu Jain, Diamond Distinguished Chair of Lehigh’s Department of Materials Science and Engineering. The group has become a world leader in producing single crystals in glass by localized laser irradiation. Read more about their work: “Crossing a critical threshold” and “Lehigh scientists fabricate a new class of crystalline solid.”

The team conducted the first detailed examination of the piezoelectric and ferroelectric properties of laser induced crystals confined in glass. They found that the as-grown crystals possess a complex ferroelectric domain structure that can be manipulated via the application of a DC bias. The findings have been published online today in MRS Communications in a paper called “Ferroelectric domain engineering of lithium niobate single crystal confined in glass.”

“The findings open up the possibility of a new collection of optical devices that use fully functional laser-fabricated crystals in glass which rely on the precise control of the ferroelectric domain structure of the crystal,” said Keith Veenhuizen, currently Assistant Professor, Department of Physics at Lebanon Valley College and the lead author of the paper, which builds on the work he did as a graduate student at Lehigh.

Applications for such technology include use in modern fiber optic technology used for data transmission.

“Being able to embed such functional single crystal architectures within a glass enables high efficiency coupling to existing glass fiber networks,” says Dierolf. “Such low loss links?that maximize performance?are of particular importance for future quantum information transfer system that are projected to take over the current schemes for optical communication,” adds Dierolf.

Water molecules distort the electrical resistance of graphene, but a team of European researchers has discovered that when this two-dimensional material is integrated with the metal of a circuit, contact resistance is not impaired by humidity. This finding will help to develop new sensors -the interface between circuits and the real world- with a significant cost reduction.

The many applications of graphene, an atomically-thin sheet of carbon atoms with extraordinary conductivity and mechanical properties, include the manufacture of sensors. These transform environmental parameters into electrical signals that can be processed and measured with a computer.

Due to their two-dimensional structure, graphene-based sensors are extremely sensitive and promise good performance at low manufacturing cost in the next years.

To achieve this, graphene needs to make efficient electrical contacts when integrated with a conventional electronic circuit. Such proper contacts are crucial in any sensor and significantly affect its performance.

But a problem arises: graphene is sensitive to humidity, to the water molecules in the surrounding air that are adsorbed onto its surface. H2O molecules change the electrical resistance of this carbon material, which introduces a false signal into the sensor.

However, Swedish scientists have found that when graphene binds to the metal of electronic circuits, the contact resistance (the part of a material’s total resistance due to imperfect contact at the interface) is not affected by moisture.

“This will make life easier for sensor designers, since they won’t have to worry about humidity influencing the contacts, just the influence on the graphene itself,” explains Arne Quellmalz, a PhD student at KTH Royal Institute of Technology (Sweden) and the main researcher of the research.

The study, published in the journal ACS Applied Materials & Interfaces, has been carried out experimentally using graphene together with gold metallization and silica substrates in transmission line model test structures, as well as computer simulations.

“By combining graphene with conventional electronics, you can take advantage of both the unique properties of graphene and the low cost of conventional integrated circuits.” says Quellmalz, “One way of combining these two technologies is to place the graphene on top of finished electronics, rather than depositing the metal on top the graphene sheet.”

As part of the European CO2-DETECT project, the authors are applying this new approach to create the first prototypes of graphene-based sensors. More specifically, the purpose is to measure carbon dioxide (CO2), the main greenhouse gas, by means of optical detection of mid-infrared light and at lower costs than with other technologies.

In addition to the KTH Royal Institute of Technology, the companies SenseAir AB from Sweden and Amo GmbH from Germany are likewise participants in the CO2-DETECT project, as is the Catalan Institute of Nanotechnology (ICN) from Barcelona.

Organic semiconductors enable the fabrication of large-scale printed and mechanically flexible electronic applications, and have already successfully established themselves on the market for displays in the form of organic light-emitting diodes (OLEDs). In order to break into further market segments, however, improvements in performance are still needed. Doping is the answer. In semiconductor technology, doping refers to the targeted introduction of impurities (also called dopants) into the semiconductor material of an integrated circuit. These dopants function as intentional “disturbances” in the semiconductor that can be used to specifically control the behaviour of the charge carriers and thus the electrical conductivity of the original material. Even the smallest amounts of these can have a very strong influence on electrical conductivity. Molecular doping is an integral part of the majority of commercial organic electronics applications. Until now, however, an insufficient fundamental physical understanding of the transport mechanisms of charges in doped organic semiconductors has prevented a further increase in conductivity to match the best inorganic semiconductors such as silicon.

Researchers from the Dresden Integrated Center for Applied Physics and Photonic Materials (IAPP) and the Center for Advancing Electronics Dresden (cfaed) at TU Dresden, in cooperation with Stanford University and the Institute for Molecular Science in Okazaki, have now identified key parameters that influence electrical conductivity in doped organic conductors. The combination of experimental investigations and simulations has revealed that introducing dopant molecules into organic semiconductors creates complexes of two oppositely charged molecules. The properties of these complexes like the Coulomb attraction and the density of the complexes significantly determine the energy barriers for the transport of charge carriers and thus the level of electrical conductivity. The identification of important molecular parameters constitutes an important foundation for the development of new materials with even higher conductivity.

The results of this study have just been published in the renowned journal Nature Materials. While the experimental work and a part of the simulations were conducted at the IAPP, the Computational Nanoelectronics Group at the cfaed under the leadership of Dr. Frank Ortmann verified the theoretical explanations for the observations by means of simulations at the molecular level. In doing so, a comprehensive foundation for new applications for organic semiconductor technology has been created.

An international team of researchers has reported a breakthrough in fabricating atom-thin processors – a discovery that could have far-reaching impacts on nanoscale chip production and in labs across the globe where scientists are exploring 2D materials for ever-smaller and -faster semiconductors.

The team, headed by New York University Tandon School of Engineering Professor of Chemical and Biomolecular Engineering Elisa Riedo, outlined the research results in the latest issue of Nature Electronics.

They demonstrated that lithography using a probe heated above 100 degrees Celsius outperformed standard methods for fabricating metal electrodes on 2D semiconductors such as molybdenum disulfide (MoS2). Such transitional metals are among the materials that scientists believe may supplant silicon for atomically small chips. The team’s new fabrication method – called thermal scanning probe lithography (t-SPL) – offers a number of advantages over today’s electron beam lithography (EBL).

First, thermal lithography significantly improves the quality of the 2D transistors, offsetting the Schottky barrier, which hampers the flow of electrons at the intersection of metal and the 2D substrate. Also, unlike EBL, the thermal lithography allows chip designers to easily image the 2D semiconductor and then pattern the electrodes where desired. Also, t-SPL fabrication systems promise significant initial savings as well as operational costs: They dramatically reduce power consumption by operating in ambient conditions, eliminating the need to produce high-energy electrons and to generate an ultra-high vacuum. Finally, this thermal fabrication method can be easily scaled up for industrial production by using parallel thermal probes.

Riedo expressed hope that t-SPL will take most fabrication out of scarce clean rooms – where researchers must compete for time with the expensive equipment – and into individual laboratories, where they might rapidly advance materials science and chip design. The precedent of 3D printers is an apt analogy: Someday these t-SPL tools with sub-10 nanometer resolution, running on standard 120-volt power in ambient conditions, could become similarly ubiquitous in research labs like hers.

“Patterning Metal Contacts on Monolayer MoS2 with Vanishing Schottky Barriers Using Thermal Nanolithography” appears in the January 2019 edition of Nature Electronics and can be accessed at http://dx.doi.org/10.1038/s41928-018-0191-0 with a “News & Views” analysis at https://www.nature.com/articles/s41928-018-0197-7.

Riedo’s work on thermal probes dates back more than a decade, first with IBM Research – Zurich and subsequently SwissLitho, founded by former IBM researchers. A process based on a SwissLitho system was developed and used for the current research. She began exploring thermal lithography for metal nanomanufacturing at the City University of New York (CUNY) Graduate Center Advanced Science Research Center (ASRC), working alongside co-first-authors of the paper, Xiaorui Zheng and Annalisa Calò, who are now post-doctoral researchers at NYU Tandon; and Edoardo Albisetti, who worked on the Riedo team with a Marie Curie Fellowship.