Category Archives: Process Materials

Researchers at the Center for Multidimensional Carbon Materials (CMCM), within the Institute for Basic Science (IBS) have demonstrated graphene coating protects glass from corrosion. Their research, published in ACS Nano, can contribute to solving problems related to glass corrosion in several industries. Glass has a high degree of both corrosion and chemical resistance. For this reason it is the primary packaging material to preserve medicines and chemicals. However, over time at high humidity and pH, some glass types corrode. Corroded glass loses its transparency and its strength is reduced. As a result, the corrosion of silicate glass, the most common and oldest form of glass, by water is a serious problem especially for the pharmaceutical, environmental and optical industries, and in particular in hot and humid climates.

Although there are different types of glass, ordinary glazing and containers are made of silicon dioxide (SiO2), sodium oxide (Na2O) along with minor additives. Glass corrosion begins with the adsorption of water on the glass surface. Hydrogen ions from water then diffuse into the glass and exchange with the sodium ions present on the glass surface. The pH of the water near the glass surface increases, allowing the silicate structure to dissolve.

Scientists have been looking at how to coat glass to protect it from damage. An ideal protective coating should be thin, transparent, and provide a good diffusion barrier to chemical attack. Graphene with its chemical inertness, thinness, and high transparency makes it very promising as a coating material. Moreover, owing to its excellent chemical barrier properties it blocks helium atoms from penetrating through it. The use of graphene coating is being explored as a protective layer for other materials requiring resistance to corrosion, oxidation, friction, bacterial infection, electromagnetic radiation, etc.

IBS scientists grew graphene on copper using a technique previously invented by Prof. Rodney S. Ruoff and collaborators, and transferred either one or two atom-thick layers of graphene onto both sides of rectangular pieces of glass. The effectiveness of the graphene coating was evaluated by water immersion testing and observing the differences between uncoated and coated glass. After 120 days of immersion in water at 60°C, uncoated glass samples had significantly increased in surface roughness and defects, and reduced in fracture strength. In contrast, both the single and double layer graphene-coated glasses had essentially no change in both fracture strength and surface roughness.

“The purpose of the study was to determine whether graphene grown by chemical vapor deposition on copper foils, a now established method, could be transferred onto glass, and protect the glass from corrosion. Our study shows that even one atom-thick layer of graphene does the trick,” explains Prof. Ruoff, director of the CMCM and Professor at the Ulsan National Institute of Science and Technology (UNIST). “In the future, when it is possible to produce larger and yet higher-quality graphene sheets and to optimize the transfer on glass, it seems reasonably likely that graphene coating on glass will be used on an industrial scale.”

Researchers from MIPT’s Laboratory of 2D Materials’ Optoelectronics, Institute of Radioengineering and Electronics, and Tohoku University (Japan) have theoretically demonstrated the possibility of creating compact sources of coherent plasmons, which are the basic building blocks for future optoelectronic circuits. The way in which the device would operate is based on the unique properties of van der Waals heterostructures — composites of graphene and related layered materials. A paper detailing the study has been published in the Physical Review B journal.

The plasmon is a quasi-particle that is a “mixture” of oscillating electrons and the electromagnetic field coupled with them. Plasmons can be used to generate, transmit, and receive signals in integrated circuits. Plasmons can act as mediators between electrons and light waves in highly efficient photodetectors and sources, particularly in the actively explored terahertz range. It is interesting to note that plasmon energy can be stored at a length scale much smaller than the wavelength of light. This means that plasmonic devices can be far more compact than their photonic counterparts. The most “compressed” plasmons are those that are bound to the conducting planes, and these plasmons can be used to make the most compact optoelectronic devices.

But where can one find a conducting plane that supports ultra-confined plasmons? For more than forty years, such objects have been created by sequential growth on nanometer-thin semiconductors with affine crystal structures. In this process, certain layers are enriched with electrons and obtain good electrical conductivity. These “layer-cakes” are called heterostructures — Russian physicist Zhores Alferov was awarded the 2000 Nobel Prize in Physics for their development.

However, growing nanoscale layers is not the only way of obtaining flat semiconductors. During the last decade, researchers’ attention has been focused on a different, intrinsically two-dimensional material — graphene. Graphene is a one-atom-thick layer of carbon, and it can be obtained by simply slicing a graphite crystal. The study of the unique electronic properties of graphene (which are radically different from those of classical heterostructures) was marked by another Nobel prize awarded to the MIPT alumni Andre Geim and Konstantin Novoselov (2010). A great number of graphene-based devices have already been created, including transistors receiving high-frequency signals, ultrafast photodetectors and even the first prototypes of lasers. The properties of graphene can be further enhanced by placing it on another material with a similar crystal structure. Materials similar to graphene can essentially be used to create the “layer-cake” heterostructures mentioned above. In this case, however, the building blocks of the structures are joined by van der Waals forces, which is why they are called van der Waals heterostructures.

Band diagram of the graphene -- tungsten disulphide -- graphene structure explaining the principle of plasmon generation. The application of interlayer voltage V results in the enrichment of one layer by electrons (blue), and the emergence of free states (called holes) in the opposite layer (red). An electron can tunnel from an occupied state to an empty state (dashed line), and its excess energy can be spent to excite a plasmon (red wavy line). CREDIT © MIPT

Band diagram of the graphene — tungsten disulphide — graphene structure explaining the principle of plasmon generation. The application of interlayer voltage V results in the enrichment of one layer by electrons (blue), and the emergence of free states (called holes) in the opposite layer (red). An electron can tunnel from an occupied state to an empty state (dashed line), and its excess energy can be spent to excite a plasmon (red wavy line). CREDIT © MIPT

In their work, the researchers show that a heterostructure comprising two graphene layers separated by a thin layer of tungsten disulphide not only supports the compact two-dimensional plasmons, but can also generate them upon the application of interlayer voltage.

“The structure we are modeling is essentially the gain medium for plasmons,” explains Dmitry Svintsov, the first author of the research. “More common examples of gain media are the neon-helium mixture in a gas laser, or a semiconductor diode in a laser pointer. When passing through such a medium, the light is amplified, and if the medium is placed between two mirrors, the medium will generate the light by itself. The combination ‘gain medium plus mirrors’ is at the heart of any laser, while the gain medium for plasmons is a necessary element of a plasmonic laser, or spaser. If the gain medium is switched on and off, the plasmonic pulses can be obtained on demand, which could be used for signal transmission in integrated circuits. The plasmons generated in the gain medium can also be uncoupled from the graphene layers and propagate as photons in free space. This allows one to create tunable sources of terahertz and far infrared radiation.”

<> Band diagram of the graphene — tungsten disulphide — graphene structure explaining the principle of plasmon generation. The application of interlayer voltage V results in the enrichment of one layer by electrons (blue), and the emergence of free states (called holes) in the opposite layer (red). An electron can tunnel from an occupied state to an empty state (dashed line), and its excess energy can be spent to excite a plasmon (red wavy line).

Apparently, the gain medium is not a perpetuum mobile, and the particles created by it — either photons or plasmons–must get their energy from a certain source. In neon-helium lasers, this energy is taken from an electron thrown onto a high atomic orbital by the electric discharge. In semiconductor lasers, the photon takes its energy from collapsing positive and negative charge carriers — electrons and holes, which are supplied by the current source. In the proposed double graphene layer structure, the plasmon takes its energy from an electron hopping from a layer with high potential energy to a layer with low potential energy, as shown in the figure. The creation of a plasmon as a result of this jump is similar to the way in which waves form as a diver enters the water.

To be more precise, the electron transition from one layer to another is more like soaking through the barrier rather than jumping over it. This phenomenon is called tunneling, and typically the probability of tunneling is very low already for nanometer-thin barriers. One exception is the case of resonant tunneling, when each electron from one layer has a “well-prepared” place in the opposite layer.

“The principle of plasmon generation studied by our group is similar to the principle of the quantum cascade laser proposed by the Russian scientists Kazarinov and Suris and realized in the USA (Faist and Capasso) more than twenty years afterwards. In this laser, the photons take energy from electrons tunneling between gallium arsenide layers through the AlGaAs barriers. Our calculations show that in this principal scheme, one can profitably replace gallium arsenide with graphene, while tungsten disulphide can act as a barrier material. This structure is able to generate not only photons, but also their compressed counterparts–plasmons. The generation and amplification of plasmons was previously thought to be a very challenging problem, but the structure we have proposed brings us one step closer to the solution,” says Dmitry Svintsov.

A research team at Worcester Polytechnic Institute (WPI) has developed a revolutionary, light-activated semiconductor nanocomposite material that can be used in a variety of applications, including microscopic actuators and grippers for surgical robots, light-powered micro-mirrors for optical telecommunications systems, and more efficient solar cells and photodetectors.

“This is a new area of science,” said Balaji Panchapakesan, associate professor of mechanical engineering at WPI and lead author of a paper about the new material published in Scientific Reports, an open access journal from the publishers of Nature. “Very few materials are able to convert photons directly into mechanical motion. In this paper, we present the first semiconductor nanocomposite material known to do so. It is a fascinating material that is also distinguished by its high strength and its enhanced optical absorption when placed under mechanical stress.

“Tiny grippers and actuators made with this material could be used on Mars rovers to capture fine dust particles.” Panchapakesan noted. “They could travel through the bloodstream on tiny robots to capture cancer cells or take minute tissue samples. The material could be used to make micro-actuators for rotating mirrors in optical telecommunications systems; they would operate strictly with light, and would require no other power source.”

Like other semiconductor materials, molybdenum disulfide, the material described in the Scientific Reports paper (“Chromatic Mechanical Response in 2-D Layered Transition Metal Dichalcogenide (TMDs)-based Nanocomposites”), is characterized by the way electrons are arranged and move about within its atoms. In particular, electrons in semiconductors are able to move from a group of outer orbitals called the valence band to another group of orbitals known as the conduction band only when adequately excited by an energy source, like an electromagnetic field or the photons in a beam of light. Crossing the “band gap,” the electrons create a flow of electricity, which is the principal that makes computer chips and solar cells possible.

When the negatively-charged electrons move between orbitals, they leave behind positively charged voids known as holes. A pair of a bound electron and an electron hole is called an exciton.

In their experiments, Panchapakesan and his team, which included graduate students Vahid Rahneshin and Farhad Khosravi, as well as colleagues at the University of Louisville and the University of Warsaw Pasteura, observed that the atomic orbitals of the molybdenum and sulfur atoms in molybdenum disulfide are arranged in a unique way that permits excitons within the conduction band to interact with what are known as the p-orbitals of the sulfur atoms. This “exciton resonance” contributes to the strong sigma bonds that give the two dimensional array of atoms in molybdenum sulfide its extraordinary strength. The strength of this resonance is also responsible for a unique effect that can generate heat within the material. It is the heat that gives rise to the material’s chromatic (light-induced) mechanical response.

To take advantage of the later phenomenon, Panchapakesan’s team created thin films made up of just one to three layers of molybdenum disulfide encased in layers of a rubber-like polymer. They exposed these nanocomposites to various wavelengths of light and found that the heat generated as a result of the exciton resonance caused the polymer to expand and contract, depending on the wavelength of the light used. In previous work, Panchapakesan’s team harnessed this photo-mechanical response by fabricating tiny grippers that open and close in response to light pulses. The grippers can capture plastic beads the size of a single human cell.

In further testing, Panchapakesan and his team discovered another unique behavior of the molybdenum disulfide composite that opens the door to a different set of applications. Employing what is known as strain engineering, they stretched the material and discovered that mechanical stresses increased its ability to absorb light.

“This is something that cannot be done with conventional thin-film semiconductors,” Panchapakesan said, “because when you stretch them, they will prematurely break. But with its unique material strength, molybdenum disulfide can be stretched. And its increased optical absorption under strain makes it a good candidate for more efficient solar cells, photodetectors, and detectors for thermal and infrared cameras.

“The exciton resonance, photomechanical response, and increased optical absorption under strain make this an extraordinary material and an intriguing subject for further investigation,” he added.

Nature has inspired generations of people, offering a plethora of different materials for innovations. One such material is the molecule of the heritage, or DNA, thanks to its unique self-assembling properties. Researchers at the Nanoscience Center (NSC) of the University of Jyväskylä and BioMediTech (BMT) of the University of Tampere have now demonstrated a method to fabricate electronic devices by using DNA. The DNA itself has no part in the electrical function, but acts as a scaffold for forming a linear, pearl-necklace-like nanostructure consisting of three gold nanoparticles. The research was funded by the Academy of Finland.

The DNA itself has no part in the electrical function, but acts as a scaffold for forming a linear, pearl-necklace-like nanostructure consisting of three gold nanoparticles. Credit: the University of Jyväskylä

The DNA itself has no part in the electrical function, but acts as a scaffold for forming a linear, pearl-necklace-like nanostructure consisting of three gold nanoparticles. Credit: The University of Jyväskylä

The nature of electrical conduction in nanoscale materials can differ vastly from regular, macroscale metallic structures, which have countless free electrons forming the current, thus making any effect by a single electron negligible. However, even the addition of a single electron into a nanoscale piece of metal can increase its energy enough to prevent conduction. This kind of addition of electrons usually happens via a quantum-mechanical effect called tunnelling, where electrons tunnel through an energy barrier. In this study, the electrons tunnelled from the electrode connected to a voltage source, to the first nanoparticle and onwards to the next particle and so on, through the gaps between them.

“Such single-electron devices have been fabricated within the scale of tens of nanometres by using conventional micro- and nanofabrication methods for more than two decades,” says Senior Lecturer Jussi Toppari from the NSC. Toppari has studied these structures already in his PhD work.

“The weakness of these structures has been the cryogenic temperatures needed for them to work. Usually, the operation temperature of these devices scales up as the size of the components decreases. Our ultimate aim is to have the devices working at room temperature, which is hardly possible for conventional nanofabrication methods – so new venues need to be found.”

Modern nanotechnology provides tools to fabricate metallic nanoparticles with the size of only a few nanometres. Single-electron devices fabricated from these metallic nanoparticles could function all the way up to room temperature. The NSC has long experience of fabricating such nanoparticles.

“After fabrication, the nanoparticles float in an aqueous solution and need to be organised into the desired form and connected to the auxiliary circuitry,” explains researcher Kosti Tapio. “DNA-based self-assembly together with its ability to be linked with nanoparticles offer a very suitable toolkit for this purpose.”

Gold nanoparticles are attached directly within the aqueous solution onto a DNA structure designed and previously tested by the involved groups. The whole process is based on DNA self-assembly, and yields countless of structures within a single patch. Ready structures are further trapped for measurements by electric fields.

“The superior self-assembly properties of the DNA, together with its mature fabrication and modification techniques, offer a vast variety of possibilities,” says Associate Professor Vesa Hytönen.

Electrical measurements carried out in this study demonstrated for the first time that these scalable fabrication methods based on DNA self-assembly can be efficiently utilised to fabricate single-electron devices that work at room temperature.

The research builds on a long-term multidisciplinary collaboration between the research groups involved. In addition to the above persons, Dr Jenni Leppiniemi (BMT), Boxuan Shen (NSC), and Dr Wolfgang Fritzsche (IPHT, Jena, Germany) contributed to the research. The study was published on 13 October 2016 in Nano Letters. Collaborative travel funding was obtained from DAAD in Germany.

A team of Shanghai Jiao Tong University researchers has used the shape of cicada wings as a template to create antireflective structures fabricated with one of the most intriguing semiconductor materials, titanium dioxide (TiO2). The antireflective structures they produced are capable of suppressing visible light — 450 to 750 nanometers — at different angles of incidence.

I. Photograph and scanning electron microscope characterizations of a black cicada wing (Cryptympana atrata Fabricius). II. Synthesis process of biomorphic TiO2 with ordered nano-nipple array structures. III. Counter map angle-dependent antireflection of biomorphic TiO2 and non-templated TiO2, respectively. Credit: Shanghai Jiao Tong University

I. Photograph and scanning electron microscope characterizations of a black cicada wing (Cryptympana atrata Fabricius). II. Synthesis process of biomorphic TiO2 with ordered nano-nipple array structures. III. Counter map angle-dependent antireflection of biomorphic TiO2 and non-templated TiO2, respectively. Credit: Shanghai Jiao Tong University

Why cicada wings? The surfaces of the insect’s wings are composed of highly ordered, tiny vertical “nano-nipple” arrays, according to the researchers. As they report this week in Applied Physics Letters, from AIP Publishing, the resulting biomorphic TiO2 surface they created with antireflective structures shows a significant decrease in reflectivity.

“This can be attributed to an optimally graded refractive index profile between air and the TiO2 via antireflective structures on the surface,” explained Wang Zhang, associate professor at State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University in China.

Small spaces between the ordered nano-antireflective structures “can be thought of as a light-transfer path that let incident light rays into the interior surface of the biomorphic TiO2 — allowing the incident light rays to completely enter the structure,” Zhang continued. “The multiple reflective and scattering effects of the antireflective structures prevented the incident light from returning to the outside atmosphere.”

Significantly, the team’s work relies on “a simple and low-cost sol-gel (wet chemical) method to fabricate biomorphic TiO2 with precise subwavelength antireflective surfaces,” Zhang pointed out. “The TiO2 was a purely anatase phase (a mineral form of TiO2), which has unique antireflective surfaces. This led to an optimally graded refractive index and, ultimately, to angle-dependent antireflective properties within the visible light range.”

In terms of applications, the team’s biomorphic TiO2 antireflective structures “show great potential for photovoltaic devices such as solar cells,” Zhang said. “We expect our work to inspire and motivate engineers to develop antireflective surfaces with unique structures for various practical applications.”

Even after high calcination at 500 C, the antireflective structures retain their morphology and high-performance antireflection properties. These qualities should enable the coatings to withstand harsh environments and make them suitable for long-term applications.

In the future, the team plans “to reduce the optical losses in solar cells by using materials with a higher refractive index such as tantalum pentoxide or any other semiconductor materials,” Zhang said.

Scientists with the Energy Department’s National Renewable Energy Laboratory (NREL) for the first time discovered how to make perovskite solar cells out of quantum dots and used the new material to convert sunlight to electricity with 10.77 percent efficiency.

The research, Quantum dot-induced phase stabilization of a-CsPbI3 perovskite for high-efficiency photovoltaics, appears in the journal Science. The authors are Abhishek Swarnkar, Ashley Marshall, Erin Sanehira, Boris Chernomordik, David Moore, Jeffrey Christians, and Joseph Luther from NREL. Tamoghna Chakrabarti from the Colorado School of Mines also is a co-author.

In addition to developing quantum dot perovskite solar cells, the researchers discovered a method to stabilize a crystal structure in an all-inorganic perovskite material at room temperature that was previously only favorable at high temperatures. The crystal phase of the inorganic material is more stable in quantum dots.

Most research into perovskites has centered on a hybrid organic-inorganic structure. Since research into perovskites for photovoltaics began in 2009, their efficiency of converting sunlight into electricity has climbed steadily and now shows greater than 22 percent power conversion efficiency. However, the organic component hasn’t been durable enough for the long-term use of perovskites as a solar cell.

NREL scientists turned to quantum dots-which are essentially nanocrystals-of cesium lead iodide (CsPbI3) to remove the unstable organic component and open the door to high-efficiency quantum dot optoelectronics that can be used in LED lights and photovoltaics.

The nanocrystals of CsPbI3 were synthesized through the addition of a Cs-oleate solution to a flask containing PbI2 precursor. The NREL researchers purified the nanocrystals using methyl acetate as an anti-solvent that removed excess unreacted precursors. This step turned out to be critical to increasing their stability.

Contrary to the bulk version of CsPbI3, the nanocrystals were found to be stable not only at temperatures exceeding 600 degrees Fahrenheit but also at room temperatures and at hundreds of degrees below zero. The bulk version of this material is unstable at room temperature, where photovoltaics normally operate and convert very quickly to an undesired crystal structure.

NREL scientists were able to transform the nanocrystals into a thin film by repeatedly dipping them into a methyl acetate solution, yielding a thickness between 100 and 400 nanometers. Used in a solar cell, the CsPbI3 nanocrystal film proved efficient at converting 10.77 percent of sunlight into electricity at an extraordinary high open circuit voltage. The efficiency is similar to record quantum dot solar cells of other materials and surpasses other reported all-inorganic perovskite solar cells.

The research was funded in part by the Energy Department’s Office of Science and by the SunShot Initiative.

NREL is the U.S. Department of Energy’s primary national laboratory for renewable energy and energy efficiency research and development. NREL is operated for the Energy Department by The Alliance for Sustainable Energy, LLC.

The SunShot Initiative is a collaborative national effort that aggressively drives innovation to make solar energy fully cost-competitive with traditional energy sources before the end of the decade. Through SunShot, the Energy Department supports efforts by private companies, universities, and national laboratories to drive down the cost of solar electricity to $0.06 per kilowatt-hour. Learn more at energy.gov/sunshot.

Researchers have designed a device that uses light to manipulate its mechanical properties. The device, which was fabricated using a plasmomechanical metamaterial, operates through a unique mechanism that couples its optical and mechanical resonances, enabling it to oscillate indefinitely using energy absorbed from light.

This is an optically-driven mechanical oscillator fabricated using a plasmomechanical metamaterial. Credit: UC San Diego Jacobs School of Engineering

This is an optically-driven mechanical oscillator fabricated using a plasmomechanical metamaterial. Credit: UC San Diego Jacobs School of Engineering

This work demonstrates a metamaterial-based approach to develop an optically-driven mechanical oscillator. The device can potentially be used as a new frequency reference to accurately keep time in GPS, computers, wristwatches and other devices, researchers said. Other potential applications that could be derived from this metamaterial-based platform include high precision sensors and quantum transducers. The research was published Oct. 10 in the journal Nature Photonics.

Researchers engineered the metamaterial-based device by integrating tiny light absorbing nanoantennas onto nanomechanical oscillators. The study was led by Ertugrul Cubukcu, a professor of nanoengineering and electrical engineering at the University of California San Diego. The work, which Cubukcu started as a faculty member at the University of Pennsylvania and is continuing at the Jacobs School of Engineering at UC San Diego, demonstrates how efficient light-matter interactions can be utilized for applications in novel nanoscale devices.

Metamaterials are artificial materials that are engineered to exhibit exotic properties not found in nature. For example, metamaterials can be designed to manipulate light, sound and heat waves in ways that can’t typically be done with conventional materials.

Metamaterials are generally considered “lossy” because their metal components absorb light very efficiently. “The lossy trait of metamaterials is considered a nuisance in photonics applications and telecommunications systems, where you have to transmit a lot of power. We’re presenting a unique metamaterials approach by taking advantage of this lossy feature,” Cubukcu said.

The device in this study resembles a tiny capacitor–roughly the size of a quarter–consisting of two square plates measuring 500 microns by 500 microns. The top plate is a bilayer gold/silicon nitride membrane containing an array of cross-shaped slits–the nanoantennas–etched into the gold layer. The bottom plate is a metal reflector that is separated from the gold/silicon nitride bilayer by a three-micron-wide air gap.

When light is shined upon the device, the nanoantennas absorb all of the incoming radiation from light and convert that optical energy into heat. In response, the gold/silicon nitride bilayer bends because gold expands more than silicon nitride when heated. The bending of the bilayer alters the width of the air gap separating it from the metal reflector. This change in spacing causes the bilayer to absorb less light and as a result, the bilayer bends back to its original position. The bilayer can once again absorb all of the incoming light and the cycle repeats over and over again.

The device relies on a unique hybrid optical resonance known as the Fano resonance, which emerges as a result of the coupling between two distinct optical resonances of the metamaterial. The optical resonance can be tuned “at will” by applying a voltage.

The researchers also point out that because the plasmomechanical metamaterial can efficiently absorb light, it can function under a broad optical resonance. That means this metamaterial can potentially respond to a light source like an LED and won’t need a strong laser to provide the energy.

“Using plasmonic metamaterials, we were able to design and fabricate a device that can utilize light to amplify or dampen microscopic mechanical motion more powerfully than other devices that demonstrate these effects. Even a non-laser light source could still work on this device,” said Hai Zhu, a former graduate student in Cubukcu’s lab and first author of the study.

“Optical metamaterials enable the chip-level integration of functionalities such as light-focusing, spectral selectivity and polarization control that are usually performed by conventional optical components such as lenses, optical filters and polarizers. Our particular metamaterial-based approach can extend these effects across the electromagnetic spectrum,” said Fei Yi, a postdoctoral researcher who worked in Cubukcu’s lab.

Dr. Lingkui Meng, Dr. Yasutomo Segawa, Professor Kenichiro Itami of the JST-ERATO Itami Molecular Nanocarbon Project, Institute of Transformative Bio-Molecules (ITbM) of Nagoya University and Integrated Research Consortium on Chemical Sciences, and their colleagues have reported in the Journal of the American Chemical Society, on the development of a simple and effective method for the synthesis of thiophene-fused PAHs.

Thiophene-fused PAHs are organic molecules composed of multiple aromatic rings including thiophene. Thiophene is a five-membered aromatic ring containing four carbon atoms and a sulfur atom. Thiophene-fused PAHs are known to be one of the most common organic semiconductors and are used in various electronic materials, such as in transistors, organic thin-film solar cells, organic electro-luminescent diodes and electronic devices. More recently, they have found use in wearable devices due to their lightweight and flexibility.

Yellow and gray colors on the molecule represent sulfur and carbon atoms respectively. Thiophene-fused PAHs have found uses as transistors. Credit: ITbM, Nagoya University

Yellow and gray colors on the molecule represent sulfur and carbon atoms respectively. Thiophene-fused PAHs have found uses as transistors. Credit: ITbM, Nagoya University

Thienannulation (thiophene-annulation) reactions, a transformation that makes new thiophene rings via cyclization, leads to various thiophene-fused PAHs. Most conventional thienannulation methods require the introduction of two functional groups adjacent to each other to form two reactive sites on PAHs before the cyclization can take place. Thus, multiple steps are required for the preparation of the substrates. As a consequence, a more simple method to access thiophene-fused PAHs is desirable.

A team led by Yasutomo Segawa, a group leader of the JST-ERATO project, and Kenichiro Itami, the director of the JST-ERATO project and the center director of ITbM, has succeeded in developing a simple and effective method for the formation of various thiophene-fused PAHs. They have managed to start from PAHs that have only one functional group, which saves the effort of installing another functional group, and have performed the thienannulation reactions using elemental sulfur, a readily available low cost reagent. The reactions can be carried out on a multigram scale and can be conducted in a one-pot two-step reaction sequence starting from an unfunctionalized PAH. This new approach can also generate multiple thiophene moieties in a single reaction. Hence, this method has the advantage of offering a significant reduction in the number of required steps and in the reagent costs for thiophene-fused PAH synthesis compared to conventional methods.

The researchers have shown that upon heating and stirring the dimethylformamide solution of arylethynyl group-substituted PAHs and elemental sulfur in air, they were able to obtain the corresponding thiophene-fused PAHs. The arylethynyl group consists of an alkyne (a moiety with a carbon-carbon triple bond) bonded to an aromatic ring. The reaction proceeds via a carbon-hydrogen (C-H) bond cleavage at the position next to the arylethynyl group (called the ortho-position) on PAHs, in the presence of sulfur. As the ortho-C-H bond on the PAH can be cleaved under the reaction conditions, prior functionalization (installation of a functional group) becomes unnecessary.

Arylethynyl-substituted PAHs are readily accessible by the Sonogashira coupling, which is a cross-coupling reaction to form carbon-carbon bonds between an alkyne and a halogen-substituted aromatic compound. The synthesis of thiophene-fused PAHs can also be carried out in one-pot, in which PAHs are subjected to a Sonogashira coupling to form arylethynyl-substituted PAHs, followed by direct treatment of the alkyne with elemental sulfur to induce thienannulation.

“Actually, we coincidentally discovered this reaction when we were testing different chemical reactions to synthesize a new molecule for the Itami ERATO project,” says Yasutomo Segawa, one of the leaders of this study. “At first, most members including myself felt that the reaction may have already been reported because it is indeed a very simple reaction. Therefore, the most difficult part of this research was to clarify the novelty of this reaction. We put in a significant amount of effort to investigate previous reports, including textbooks from more than 50 years ago as well as various Internet sources, to make sure that our reaction conditions had not been disclosed before,” he continues.

The team succeeded in synthesizing more than 20 thiophene-fused PAHs. They also revealed that multiple formations of thiophene rings of PAHs substituted with multiple arylethynyl groups could be carried out all at once. Multiple thiophene-fused PAHs were generated from three-fold and five-fold thienannulations, which generated triple thia[5]helicene (containing three thiophenes) and pentathienocorannulene (containing five thiophenes), respectively. The pentathienocorannulene was an unprecedented molecule that was synthesized for the first time.

“I was extremely happy when I was able to obtain the propeller-shaped triple thia[5]helicene and hat-shaped pentathienocorannulene, because I have always been aiming to synthesize exciting new molecules since I joined Professor Itami’s group,” says Lingkui Meng, a postdoctoral researcher who mainly conducted the experiments. “We had some problems in purifying the compounds but we were delighted when we obtained the crystal structures of the thiophene compounds, which proved that the desired reactions had taken place.”

“The best part of this research for me is to discover that our C-H functionalization strategy on PAHs could be applied to synthesize structurally beautiful molecules with high functionalities,” says Segawa. “The successful synthesis of a known high-performance organic semiconductive molecule, (2,6-bis(4-n-octylphenyl)- dithieno[3,2-b:2?,3?-d]thiophene, from a relatively cheap substrate opens doors to access useful thiophene compounds in a rapid and cost-effective manner.”

“We hope that ongoing advances in our method may lead to the development of new organic electronic devices, including semiconductor and luminescent materials,” say Segawa and Itami. “We are considering the possibilities to make this reaction applicable for making useful thiophene-fused PAHs, which would lead to the rapid discovery and optimization of key molecules that would advance the field of materials science.”

For more than a decade, engineers have been eyeing the finish line in the race to shrink the size of components in integrated circuits. They knew that the laws of physics had set a 5-nanometer threshold on the size of transistor gates among conventional semiconductors, about one-quarter the size of high-end 20-nanometer-gate transistors now on the market.

Some laws are made to be broken, or at least challenged.

A research team led by faculty scientist Ali Javey at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) has done just that by creating a transistor with a working 1-nanometer gate. For comparison, a strand of human hair is about 50,000 nanometers thick.

This is a schematic of a transistor with a molybdenum disulfide channel and 1-nanometer carbon nanotube gate. Credit: Sujay Desai/Berkeley Lab

This is a schematic of a transistor with a molybdenum disulfide channel and 1-nanometer carbon nanotube gate. Credit: Sujay Desai/Berkeley Lab

“We made the smallest transistor reported to date,” said Javey, a lead principal investigator of the Electronic Materials program in Berkeley Lab’s Materials Science Division. “The gate length is considered a defining dimension of the transistor. We demonstrated a 1-nanometer-gate transistor, showing that with the choice of proper materials, there is a lot more room to shrink our electronics.”

The key was to use carbon nanotubes and molybdenum disulfide (MoS2), an engine lubricant commonly sold in auto parts shops. MoS2 is part of a family of materials with immense potential for applications in LEDs, lasers, nanoscale transistors, solar cells, and more.

The findings will appear in the Oct. 7 issue of the journal Science. Other investigators on this paper include Jeff Bokor, a faculty senior scientist at Berkeley Lab and a professor at UC Berkeley; Chenming Hu, a professor at UC Berkeley; Moon Kim, a professor at the University of Texas at Dallas; and H.S. Philip Wong, a professor at Stanford University.

The development could be key to keeping alive Intel co-founder Gordon Moore’s prediction that the density of transistors on integrated circuits would double every two years, enabling the increased performance of our laptops, mobile phones, televisions, and other electronics.

“The semiconductor industry has long assumed that any gate below 5 nanometers wouldn’t work, so anything below that was not even considered,” said study lead author Sujay Desai, a graduate student in Javey’s lab. “This research shows that sub-5-nanometer gates should not be discounted. Industry has been squeezing every last bit of capability out of silicon. By changing the material from silicon to MoS2, we can make a transistor with a gate that is just 1 nanometer in length, and operate it like a switch.”

When ‘electrons are out of control’

Transistors consist of three terminals: a source, a drain, and a gate. Current flows from the source to the drain, and that flow is controlled by the gate, which switches on and off in response to the voltage applied.

Both silicon and MoS2 have a crystalline lattice structure, but electrons flowing through silicon are lighter and encounter less resistance compared with MoS2. That is a boon when the gate is 5 nanometers or longer. But below that length, a quantum mechanical phenomenon called tunneling kicks in, and the gate barrier is no longer able to keep the electrons from barging through from the source to the drain terminals.

“This means we can’t turn off the transistors,” said Desai. “The electrons are out of control.”

Because electrons flowing through MoS2 are heavier, their flow can be controlled with smaller gate lengths. MoS2 can also be scaled down to atomically thin sheets, about 0.65 nanometers thick, with a lower dielectric constant, a measure reflecting the ability of a material to store energy in an electric field. Both of these properties, in addition to the mass of the electron, help improve the control of the flow of current inside the transistor when the gate length is reduced to 1 nanometer.

Once they settled on MoS2 as the semiconductor material, it was time to construct the gate. Making a 1-nanometer structure, it turns out, is no small feat. Conventional lithography techniques don’t work well at that scale, so the researchers turned to carbon nanotubes, hollow cylindrical tubes with diameters as small as 1 nanometer.

They then measured the electrical properties of the devices to show that the MoS2 transistor with the carbon nanotube gate effectively controlled the flow of electrons.

“This work demonstrated the shortest transistor ever,” said Javey, who is also a UC Berkeley professor of electrical engineering and computer sciences. “However, it’s a proof of concept. We have not yet packed these transistors onto a chip, and we haven’t done this billions of times over. We also have not developed self-aligned fabrication schemes for reducing parasitic resistances in the device. But this work is important to show that we are no longer limited to a 5-nanometer gate for our transistors. Moore’s Law can continue a while longer by proper engineering of the semiconductor material and device architecture.”

In the quest for faster and more powerful computers and consumer electronics, big advances come in small packages.

The high-performance, silicon-based transistors that control today’s electronic devices have been getting smaller and smaller, allowing those devices to perform faster while consuming less power.

But even silicon has its limits, so researchers at The University of Texas at Dallas and elsewhere are looking for better-performing alternatives.

In a new study published Oct. 7 in the journal Science, UT Dallas engineers and their colleagues describe a novel transistor made with a new combination of materials that is even smaller than the smallest possible silicon-based transistor.

“Silicon transistors are approaching their size limit,” said Dr. Moon Kim, professor of materials science and engineering at UT Dallas and an author of the study. “Our research provides new insight into the feasibility to go beyond the ultimate scaling limit of silicon-based transistor technology.”

The study authors also included Kim’s graduate student Qingxiao Wang, and collaborators at the University of California, Berkeley, Stanford University and the Lawrence Berkeley National Laboratory, which led the project. Researchers in California fabricated the transistor and performed theoretical simulations, while the UT Dallas team physically characterized the device using an atomic resolution electron microscope on campus.

As current flows through a transistor, the stream of electrons travels through a channel, like tap water flowing through a faucet out into a sink. A “gate” in the transistor controls the flow of electrons, shutting the flow off and on in a fraction of second.

“As of today, the best/smallest silicon transistor devices commercially available have a gate length larger than 10 nanometers,” said Kim, the Louis Beecherl Jr. Distinguished Professor in the Erik Jonsson School of Engineering and Computer Science. “The theoretical lower limit for silicon transistors is about 5 nanometers. The device we demonstrate in this article has a gate size of 1 nanometer, about one order of magnitude smaller. It should be possible to reduce the size of a computer chip significantly utilizing this configuration.”

One of the challenges in designing such small transistors is that electrons can randomly tunnel through a gate when the current is supposed to be shut off. Reducing this current leakage is a priority.

“The device we demonstrated shows more than two orders of magnitude reduction in leakage current compared to its silicon counterpart, which results in reduced power consumption,” Kim said. “What this means, for example, is that a cellphone with this technology built in would not have to be recharged as often.”

Instead of using silicon, the researchers built their prototype device with a class of semiconductor materials called transition metal dichalcogenides, or TMDs. Specifically, their experimental device structure used molybdenum disulfide for the channel material and a single-walled carbon nanotube for the gate.

Kim said there are many technical challenges before large-scale manufacturing of the new transistor is practical or even possible.

“Large-scale processing and manufacturing of TMD devices down to such small gate lengths will require future innovations,” he said.