Lawrence Berkley & DOE: Surprising Discoveries about 2D Molybdenum Disulfide

Scientists with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) have used a unique nano-optical probe to study the effects of illumination on two-dimensional semiconductors at the molecular level. Working at the Molecular Foundry, a DOE Office of Science User Facility, the scientific team used the “Campanile” probe they developed to make some surprising discoveries about molybdenum disulfide, a member of a family of semiconductors, called “transition metal dichalcogenides (TMDCs), whose optoelectronic properties hold great promise for future nanoelectronic and photonic devices.

“The Campanile probe’s remarkable resolution enabled us to identify significant nanoscale optoelectronic heterogeneity in the interior regions of monolayer crystals of molybdenum disulfide, and an unexpected, approximately 300 nanometer wide, energetically disordered edge region,” says James Schuck, a staff scientist with Berkeley Lab’s Materials Sciences Division. Schuck led this study as well as the team that created the Campanile probe, which won a prestigious R&D 100 Award in 2013 for combining the advantages of scan/probe microscopy and optical spectroscopy.

“This disordered edge region, which has never been seen before, could be extremely important for any devices in which one wants to make electrical contacts,” Schuck says. “It might also prove critical to photocatalytic and nonlinear optical conversion applications.”

Schuck, who directs the Imaging and Manipulation of Nanostructures Facility at the Molecular Foundry, is the corresponding author of a paper describing this research in Nature Communications. The paper is titled “Visualizing nanoscale excitonic relaxation properties of disordered edges and grain boundaries in monolayer molybdenum disulfide.” The co-lead authors are Wei Bao and Nicholas Borys. (See below for a complete list of authors.)

2D-TMDCs rival graphene as potential successors to silicon for the next generation of high-speed electronics. Only a single molecule in thickness, 2D-TMDC materials boast superior energy efficiencies and a capacity to carry much higher current densities than silicon. However, since their experimental “discovery” in 2010, the performance of 2D-TMDC materials has lagged far behind theoretical expectations primarily because of a lack of understanding of 2D-TMDC properties at the nanoscale, particularly their excitonic properties. Excitons are bound pairs of excited electrons and holes that enable semiconductors to function in devices.

“The poor understanding of 2D-TMDC excitonic and other properties at the nanoscale is rooted in large part to the existing constraints on nanospectroscopic imaging,” Schuck says. “With our Campanile probe, we overcome nearly all previous limitations of near-field microscopy and are able to map critical chemical and optical properties and processes at their native length scales.”

The Campanile probe, which draws its name from the landmark “Campanile” clock tower on the campus of the University of California at Berkeley, features a tapered, four-sided microscopic tip that is mounted on the end of an optical fiber. Two of the Campanile probe’s sides are coated with gold and the two gold layers are separated by just a few nanometers at the tip. The tapered design enables the Campanile probe to channel light of all wavelengths down into an enhanced field at the apex of the tip. The size of the gap between the gold layers determines the resolution, which can be below the diffraction optical limit.

In their new study, Schuck, Bao, Borys and their co-authors used the Campanile probe to spectroscopically map nanoscale excited-state/relaxation processes in monolayer crystals of molybdenum disulfide that were grown by chemical vapor deposition (CVD). Molybdenum disulfide is a 2D semiconductor that features high electrical conductance comparable to that of graphene, but, unlike graphene, has natural energy band-gaps, which means its conductance can be switched off.

“Our study revealed significant nanoscale optoelectronic heterogeneity and allowed us to quantify exciton-quenching phenomena at crystal grain boundaries,” Schuck said. “The discovery of the disordered edge region constitutes a paradigm shift from the idea that only a 1D metallic edge state is responsible for all the edge-related physics and photochemistry being observed in 2D-TMDCs. What’s happening at the edges of 2D-TMDC crystals is clearly more complicated than that. There’s a   mesoscopic disordered region that likely dominates most transport, nonlinear optical, and photocatalytic behavior near the edges of CVD-grown 2D-TMDCs.”

In this study, Schuck and his colleagues also discovered that the disordered edge region in molybdenum disulfide crystals harbors a sulfur deficiency that holds implications for future optoelectronic applications of this 2D-TMDC.

“Less sulfur means more free electrons are present in that edge region, which could lead to enhanced non-radiative recombination,” Schuck says. “Enhanced non-radiative recombination means that excitons created near a sulfur vacancy would live for a much shorter period of time.”

Schuck and his colleagues plan to next study the excitonic and electronic properties that may arise, as well as the creation of p-n junctions and quantum wells, when two disparate types of TMDCs are connected.

“We are also combining 2D-TMDC materials with so-called meta surfaces for controlling and manipulating the valley states and circular emitters that exist within these systems, as well as exploring localized quantum states that could act as near-ideal single-photon emitters and quantum-entangled Qubit states,” Schuck says.

In addition to Schuck, Bao, Borys and Weber-Bargioni, other co-authors of the Nature Communications paper are Changhyun Ko, Joonki Suh, Wen Fan, Andrew Thron, Yingjie Zhang, Alexander Buyanin, Jie Zhang, Stefano Cabrini, Paul Ashby, Alexander Weber-Bargioni, Sefaattin Tongay, Shaul Aloni, Frank Ogletree, Junqiao Wu and Miquel Salmeron.
This research was supported by the DOE Office of Science.

Additional Information

For more about the research of James Schuck and his group go here

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Lawrence Berkeley National Laboratory addresses the world’s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab’s scientific expertise has been recognized with 13 Nobel prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy’s Office of Science. For more, visit www.lbl.gov.

Consortium including MIT awarded $110M national grant to promote photonics manufacturing

Partnership of government, industry, and academia will pursue integration of optical devices with electronics.

MIT is a key player in a new $600 million public-private partnership announced today by the Obama administration to help strengthen high-tech U.S.-based manufacturing.

Physically headquartered in New York state and led by the State University of New York Polytechnic Institute (SUNY Poly), the American Institute for Manufacturing Integrated Photonics (AIM Photonics) will bring government, industry, and academia together to advance domestic capabilities in integrated photonic technology and better position the U.S. relative to global competition.

Federal funding of $110 million will be combined with some $500 million from AIM Photonics’ consortium of state and local governments, manufacturing firms, universities, community colleges, and nonprofit organizations across the country.

Technologies that can help to integrate photonics, or light-based communications and computation, with existing electronic systems are seen as a crucial growth area as the world moves toward ever-greater reliance on more powerful high-tech systems. What’s more, many analysts say this is an area that could help breathe new life into a U.S. manufacturing base that has been in decline in recent years.

The public-private partnership announced today aims to spur these twin goals, improving integration of photonic systems while revitalizing U.S. manufacturing. The consortium includes universities, community colleges, and businesses in 20 states. Six state governments, including that of Massachusetts, are also supporting the project.

MIT faculty will manage important parts of the program: Michael Watts, an associate professor of electrical engineering and computer science, will lead the technological innovation in silicon photonics. Lionel Kimerling, the Thomas Lord Professor in Materials Science and Engineering, will lead a program in education and workforce development.

“This is great news on a number of fronts,” MIT Provost Martin Schmidt says. “Photonics holds the key to advances in computing, and its pursuit will engage and energize research and economic activity from Rochester, New York, to Cambridge, Massachusetts, and beyond. MIT faculty are excited to contribute to this effort.”

An ongoing partnership

MIT’s existing collaboration with SUNY Poly led to the first complete 300-millimeter silicon photonics platform, Watts says. That effort has led to numerous subsequent advances in silicon photonics technology, with MIT developing photonic designs that SUNY Poly has then built in its state-of-the-art fabrication facility.

Photonic devices are seen as key to continuing the advances in computing speed and efficiency described by Moore’s Law — which may have reached their theoretical limits in existing silicon-based electronics, Kimerling says. The integration of photonics with electronics promises not only to boost the performance of systems in data centers and high-performance computing, but also to reduce their energy consumption — which already accounts for more than 2 percent of all electricity use in the U.S.

Kimerling points out that a single new high-performance computer installation can contain more than 1 million photonic connections between hundreds of thousands of computer processor units (CPUs). “That’s more than the entire telecommunications industry,” he says — so creating new, inexpensive, and energy-efficient connection systems at scale is a major need.

The integration of such systems has been progressing in stages, Kimerling says. Initially, the conversion from optical to electronic signals became pervasive at the network level to support long-distance telecommunication, but it is now moving to circuit boards, and will ultimately go to the level of individual integrated-circuit chips.

“Europe is ahead in industry coordination right now,” following a decade of government investment, Kimerling says. This new U.S. initiative, he says, is “one of the first of this kind in the U.S., and the bet is that the innovation and research here, combined with the manufacturing capability, will allow our companies to really take off.”

Leadership in technological innovation

Within the new alliance, MIT will lead technological innovation in silicon photonics. That task will be managed by Watts.

The evolving integration of photonics and electronics will have a great impact on many different technologies, Watts says. For example, LIDAR systems — similar to radar, but using light beams instead of radio waves — have great potential for collision-avoidance systems in cars, since they can provide greater detail than radar or sonar. Watts has worked to develop single-chip LIDAR devices, which could eliminate the moving parts in existing devices — such as tiny gimbaled mirrors used to direct the light beams in a scanning pattern — replacing them with fixed, electrically steerable phased-array systems, like those now used for cellphone tower antennas.

“LIDAR systems that exist today are both bulky and expensive, because they use mechanically scanned lasers,” Watts says. But doing the same thing at the nanoscale, using phased-array systems on a chip, could drastically reduce size and cost, providing high-resolution, chip-scale, 3-D imaging capabilities that do not exist today, he says.

There are many other areas where integration of photonics and electronics could lead to big advances, including in biological and chemical sensors that could have greater sensitivity than existing electronic versions, and in new kinds of medical imaging systems, such as optical coherent tomography.

“The goal of this initiative is to lower the barriers to entry in this field for U.S. companies,” Watts says. It is intended to function much like a major public-private initiative that helped pave the way, decades ago, for the development of electronic chip manufacturing in the U.S.

Significant photonic chip manufacturing capabilities have been developed at SUNY Poly, in Albany, New York. That facility has already made the world’s largest silicon-based photonic circuit, a chip designed at MIT, and built using industry-standard 300-millimeter-wide silicon wafers, Watts says.

Contributions in education and training

MIT will also host AIM Photonics’ program in education and workforce development, which Kimerling will direct. This will include developing educational materials — ranging from K-12 through continuing education — to prepare future employees for this emerging industry, including teaching on the design of integrated photonic devices. MIT will also lead workforce development, with an emphasis on including veterans, underrepresented minorities, and other students, by developing a variety of materials to teach about the new technologies.

MIT will work to support internships, apprenticeships, and other forms of hands-on training in a national network of industry and university partners. The effort will also support an industry-wide roadmap to help align the technology supply chain with new manufacturing platforms.

Kimerling says that a significant issue in developing a robust photonics industry is the need to develop a trained workforce of people who are familiar with both electronics and optical technology — two very different fields. Educational programs that encompass these disparate fields “are important, and don’t exist today in one organization,” he says.

One expected impact of the new initiative is the development of a corridor along Interstate 90, from Boston to Rochester, New York, of industrial firms building on the base of new technology to develop related products and services, much as Silicon Valley emerged in California around companies such as Intel and their chip-making technology.

Other major members of AIM Photonics include the University of Arizona, the University of Rochester, and the University of California at Santa Barbara. In addition to the Department of Defense, federal funding for the project will come from the National Science Foundation, the Department of Energy, the National Institute for Standards and Technology, and NASA.

Roots in the Advanced Manufacturing Partnership

Today’s news flows from the work of the Advanced Manufacturing Partnership (AMP), a White House-led effort begun in 2011 with the aim of bringing together industry, universities, and the federal government to identify and invest in key emerging technologies, with the idea of stoking a “renaissance in American manufacturing.”

AMP was inaugurated with former MIT President Susan Hockfield as co-chair; MIT President L. Rafael Reif subsequently served in that same capacity as part of “AMP 2.0.” Those groups’ work led to President Barack Obama’s commitment to establish a National Network of Manufacturing Innovation, to consist of linked institutes such as the one announced today.

“Massachusetts’ strong role in the AIM Photonics team stems from a collaboration involving MIT and many other partner organizations across the Commonwealth: universities, community colleges, and large and small manufacturers throughout the integrated photonics supply chain,” says Krystyn Van Vliet, a professor of materials science and engineering and biological engineering, and MIT faculty lead for AMP 2.0. “The support of Gov. Charlie Baker and Secretary of Housing and Economic Development Jay Ash was key to the success of the AIM Photonics team, and we appreciate their efforts. This manufacturing institute will help Massachusetts inspire and prepare the next generation of integrated photonics manufacturing careers, businesses, and leaders.”

“Today’s announcement is a testament to the outstanding team of industrial and academic leaders assembled by AIM Photonics and its plan to establish the U.S. as a global leader in this emerging technology,” says Michael Liehr, AIM CEO and SUNY Poly executive vice president of innovation and technology and vice president of research. “This would not have been possible without the critical support of Gov. Andrew Cuomo, whose pioneering leadership in establishing New York state’s globally recognized, high-tech R&D ecosystem has enabled historic economic growth and innovation and secured our partnership with the state of Massachusetts. SUNY Poly is excited to be working with partners such as MIT on this initiative, which will be truly transformational for both the industry and the nation.”

University of Rochester: Defects on an atomically thin semiconductor can produce light-emitting quantum dots: Applications in Integrated Photonics

Researchers at the University of Rochester have shown that defects on an atomically thin semiconductor can produce light-emitting quantum dots. The quantum dots serve as a source of single photons and could be useful for the integration of quantum photonics with solid-state electronics — a combination known as integrated photonics.

Scientists have become interested in integrated solid-state devices for quantum information processing uses. Quantum dots in atomically thin semiconductors could not only provide a framework to explore the fundamental physics of how they interact, but also enable nanophotonics applications, the researchers say.

Quantum dots are often referred to as artificial atoms. They are artificially engineered or naturally occurring defects in solids that are being studied for a wide range of applications. Nick Vamivakas, assistant professor of optics at the University of Rochester and senior author on the paper, adds that atomically thin, 2D materials, such as graphene, have also generated interest among scientists who want to explore their potential for optoelectronics. However, until now, optically active quantum dots have not been observed in 2D materials.

In a paper published in Nature Nanotechnology this week, the Rochester researchers show how tungsten diselenide (WSe2) can be fashioned into an atomically thin semiconductor that serves as a platform for solid-state quantum dots. Perhaps most importantly the defects that create the dots do not inhibit the electrical or optical performance of the semiconductor and they can be controlled by applying electric and magnetic fields.

Vamivakas explains that the brightness of the quantum dot emission can be controlled by applying the voltage. He adds that the next step is to use voltage to “tune the color” of the emitted photons, which can make it possible to integrate these quantum dots with nanophotonic devices.

A key advantage is how much easier it is to create quantum dots in atomically thin tungsten diselenide compared to producing quantum dots in more traditional materials like indium arsenide.

“We start with a black crystal and then we peel layers of it off until we have an extremely thin later left, an atomically thin sheet of tungsten diselenide,” said Vamivakas.

The researchers take two of these atomically thin sheets and lay one over the other one. At the point where they overlap, a quantum dot is created. The overlap creates a defect in the otherwise smooth 2D sheet of semiconductor material. The extremely thin semiconductors are much easier to integrate with other electronics.

The quantum dots in tungsten diselenide also possess an intrinsic quantum degree of freedom — the electron spin. This is a desirable property as the spin can both act as a store of quantum information as well as provide a probe of the local quantum dot environment.

“What makes tungsten diselenide extremely versatile is that the color of the single photons emitted by the quantum dots is correlated with the quantum dot spin,” said first author Chitraleema Chakraborty. Chakraborty added that the ease with which the spins and photons interact with one another should make these systems ideal for quantum information applications as well as nanoscale metrology.

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The above story is based on materials provided by University of Rochester. Note: Materials may be edited for content and length.

Silicon Photonics: The next step to a High-Bandwidth Future?

The computing and telecommunications industries have ambitious plans for the future: Systems that will store information in the cloud, analyze enormous amounts of data, and think more like a brain than a standard computer. Such systems are already being developed, and scientists at IBM Research have now demonstrated what may be an important step toward commercializing this next generation of computing technology. They established a method to integrate silicon photonic chips with the processor in the same package, avoiding the need for transceiver assemblies.

CMOS silicon photonics chip.

The new technique, which will be presented 25 March at this year’s OFC Conference and Exposition in Los Angeles, California, USA, should lower the cost and increase the performance, energy efficiency and size of future data centers, supercomputers and cloud systems.

Photonic devices, which use photons instead of electrons to transport and manipulate information, offer many advantages compared to traditional electronic links found in today’s computers. Optical links can transmit more information over larger distances and are more energy efficient than copper-based links. To optimally benefit from this technology, a tight integration of the electrical logic and optical transmission functions is required. The optical chip needs to be as close to the electrical chip as possible to minimize the distance of electrical connection between them. This can only be accomplished if they are packaged together.

“IBM has been a pioneer in the area of CMOS integrated silicon photonics for more than 12 years, a technology that integrates functions for optical communications on a silicon chip,” said Bert Offrein, manager of the photonics group at IBM Research – Zurich. “In addition to the silicon technology advancements at the chip-level, novel system-level integration concepts are also required to fully profit from the new capabilities silicon photonics will bring,” he continued.

Optical interconnect technology is currently incorporated into data centers by attaching discrete transceivers or active optical cables, which come in pre-assembled building blocks. The pre-packaged transceivers are large and expensive, limiting their large-scale use, Offrein said. Furthermore, such transceivers are mounted at the edge of the board, resulting in a large distance between the processor chip and the optical components.

Offrein and his IBM colleagues from Europe, the United States and Japan instead proposed an integration scheme in which the silicon photonic chips are treated similarly to ordinary silicon processor chips and are directly attached to the processor package without pre-assembling them into standard transceiver housings. This improves the performance and power efficiency of the optical interconnects while reducing the cost of assembly. Challenges arise because alignment tolerances in photonics are critical (sub-micron range) and optical interfaces are sensitive to debris and imperfections, thus requiring the best in packaging technology.

IBM scientist Roger Dangel holds a thin film polymer waveguide. IBM is experimenting with waveguides as a way to integrate silicon photonic chips into data center systems.

The team demonstrated efficient optical coupling of an array of silicon waveguides to a substrate containing an array of polymer waveguides. The significant size difference between the silicon waveguides and the polymer waveguides originally presented a major challenge. The researchers overcame this obstacle by gradually tapering the silicon waveguide, leading to an efficient transfer of the optical signal to the polymer waveguide.

The method is scalable and enables the simultaneous interfacing of many optical connections between a silicon photonic chip and the system. The optical coupling is also wavelength and polarization insensitive and tolerant to alignment offsets of a few micrometers, Offrein said.

“This integration scheme has the potential to massively reduce the cost of applying silicon photonics optical interconnects in computing systems,” Offrein said. Cheaper photonic technology enables its deployment at a large scale, which will lead to computing systems that can process more information at higher performance levels and with better energy efficiency, he explained.

“Such systems will be key for future applications in the field of cloud-computing, big data, analytics and cognitive computing. In addition, it will enable novel architectures requiring high communication bandwidth, as for example in disaggregated systems,” Offrein said.

About the presentation

Presentation W3A.4, titled “Silicon Photonics for the Data Center,” will take place Wednesday 25 March at 14:00 PDT in room 402AB at the Los Angeles Convention Center.

Source: IBM

Scientists Develop a Nanolamp with a Lightning-Fast Switch

Scientists have developed a light source which converts an electrical voltage pulse into a light pulse by means of a single molecule, possibly serving as a prototype for nano-components that convert electrical into optical signals with gigahertz frequencies.

Information is processed and transmitted by ever-smaller components, sometimes with electrons and sometimes with light. Scientists at the Max Planck Institute for Solid State Research in Stuttgart have now developed a light source which converts an electrical voltage pulse into a light pulse by means of a single molecule. Here the molecule functions as a transistor-controlled light switch which even allows the intensity of the light to be regulated. Since the molecular switch allows the light to be switched on and off extremely fast, the light source could serve as a prototype for nano-components that convert electrical into optical signals with gigahertz frequencies.

Researchers at the Max Planck Institute for Solid State Research apply a voltage between a gold surface coated with a layer of spherical carbon molecules and the tip of a scanning tunneling microscope. The resulting electric field (indicated by the grey arrows in the diagram) can be regulated by the level of the voltage and the distance between the tip and the metal surface. With a particular field strength, the single molecule (in magenta) becomes electrically charged, which immediately leads to electrical energy being converted to light (the yellow wave). Credit: MPI for Solid State Research

Today, organic dyes do not just provide color for carpets, newspapers or clothing when light shines on them. Now they themselves shine in electric light sources, in organic light-emitting diodes (OLEDs), like those in smartphone screens. However, the displays still contain transistors for regulating the brightness alongside the actual light sources (pixels). A team from the Max Planck Institute for Solid State Research, the Max Planck EPFL Center and the Karlsruhe Institute of Technology, has now combined the two functions in a single molecule.

The researchers working with Klaus Kern, Director of the Stuttgart-based Max Planck Institute, construct their nanolamp with integrated transistor control by placing a dye molecule on a layer of Buckminsterfullerenes – which are spherical carbon molecules. The layer of carbon spheres coats a metal carrier, in this case of gold, which serves as an electrode. “As the second electrode above the dye molecule, we use the tip of a scanning tunneling microscope,” says Klaus Kuhnke. “But a second metal layer would also be suitable.” However, the researchers were only able to discover the astonishing properties of the individual molecule because they used a movable tip for their investigation. What they actually did was to scan the surface with the tip, measuring the light emitted at the same time. “In the process we observed that light is produced on the dye molecules,” according to Kuhnke.

The voltage first produces light waves which are trapped on the metal surface

The researchers now regulate the electric field on the molecule with an electrical voltage between the gold carrier and the tip of the scanning tunneling microscope (STM), as well as the distance between the two electrical contacts. If this exceeds 2.5 volts per nanometer, the lamp is switched on. The molecule, however, does not just switch the light on and off. It actually allows continuous regulation of the light intensity, getting brighter and darker over a very narrow band of a few millivolts. It thus functions in this range similarly to a light-emitting transistor.

The electrical energy is not converted directly into light energy in the switching process, but indirectly via “plasmons”. These can be imagined as light waves that are trapped on the metallic surface and may be radiated by such things as surface irregularities. With their help, more information can be transmitted or processed in a small space in the form of light than with light alone: plasmons can run along metal tracks which are narrower than 100 nanometers, whereas optical fibers, for instance, must be at least half as wide as the wavelength of the light they transmit.

The switching process takes less than a billionth of a second

The organic molecule plays a decisive role in the generation of the trapped and radiated light waves on the metal surface: a minimal change in the electric field at the location of the molecule decides whether light is produced or not. This makes the nanolamp interesting for the transfer of digital information with light, where “light on” stands for the one of a data bit and “light off” for the zero. “A small modulation of the electric field at the molecule produces a bit stream that is emitted as light and can transfer a message,” says Klaus Kuhnke. And since a light source above the threshold value turns on with a tiny change in voltage, the switching process takes place extremely quickly: It takes less than a billionth of a second, and so may eventually permit data transfers with bit rates in the gigahertz range.

The control of the intensity of the light by a single molecule is decisive for the speed of the light switch. Mechanical light switches are operated by a lever, and the heavier this lever is the more effort it takes to move the switch from one switching position to another. These clumsy levers correspond in electronics to unavoidable capacitances which swallow a part of the current without producing any light. The larger the light-switching element is, the more time and energy is required to charge the “parasitic” capacitors. Here the minute size of the molecule helps: It costs hardly any additional energy to charge the environment of a single molecule the size of a millionth of a millimeter with a tiny voltage of a few millivolts – the switching process is correspondingly fast. “Such a molecular light source thus promises to become a new, efficient component for information transmission – especially as the light produced may still be weak, but is clearly perceptible with the naked eye,” says Klaus Kuhnke.

Publication: Christoph Große, et al., “Dynamic Control of Plasmon Generation by an Individual Quantum System,” Nano Letters, 2014, 14 (10), pp 5693–5697; DOI: 10.1021/nl502413k

Source: Max Planck Institute

Image: MPI for Solid State Research

2D molybdenum disulfide: A Promising New Optical Material for Ultra-Fast Photonics

Inspired by the unique optical and electronic property of graphene, two-dimensional layered materials have been intensively investigated in recent years, driven by their potential applications for future high speed and broadband electronic and optoelectronic devices. Layers of molybdenum disulfide (MoS2), one kind of transition metals chalcogenides, have been proven to be a very interesting material with the semiconducting property.

The basic infrastructure of molybdenum disulfide is a single-atomic layer of molybdenum sandwiched between two adjacent atomic layers of sulfide. This compound exists in nature as molybdenite, a crystal material found in rocks around the world, frequently taking the characteristic form of silver-colored hexagonal plates. For decades, molybdenite has been used in the manufacturing of lubricants and metal alloys. Like in the case of graphite, the properties of single-atom sheets of MoS2 long went unnoticed.

From the view point of applications in electronics, molybdenum disulfide sheets exhibit a significant advantage over graphene: they have an energy gap – an energy range within which no electron states can exist. By applying an electric field, the sheets can be switched between a state that conducts electricity and one that behaves like an insulator. Theoretically, a switched-off molybdenum disulfide transistor would consume even as little as several hundred thousand times less energy than a silicon transistor.

Graphene, on the other hand, has no energy gap and transistors made of graphene cannot be fully switched off. More importantly, the relatively weak absorption co-efficiency of graphene (2.3 % of incident light per layer) might significantly delimit its light modulation ability for optical communication devices such as light detector, modulator and absorber.

Molybdenum disulfide’s semiconducting ability, strong light-matter interaction and similarity to the carbon-based graphene makes it of interest to scientists as a viable alternative to graphene in the manufacture of electronics, particularly photoelectronics. Scientists have found that the physical properties of two-dimensional (2D) MoS2 change markedly when it has nanoscale properties.

A slab of MoS2 that is even a micron thick has an “indirect” bandgap while a two-dimensional sheet of molybdenum disulfide has a “direct” bandgap. It shows thickness dependent band-gap properties, allowing for the production of tunable optoelectronic devices with diversified spectral operation. In pushing towards practical optical applications of 2D MoS2, an essential gap on understanding the nonlinear optical response of 2D MoS2 and how it interacts with light, must be filled. Now, one research group on photonics based on 2D materials, from Shenzhen University, reports a breakthrough in the light-matter interaction of 2D MoS2 and fabricating a novel optical device using few layers of molybdenum disulfide (see paper in Optics Express: “Molybdenum disulfide (MoS2) as a broadband saturable absorber for ultra-fast photonics”).

Thanks to the direct-band and ultrafast response in few layer MoS2, its optical absorbance can become saturated if under high power excitation, as a result of the band filling effect in conduction band. A saturable absorber is an important element for pulse operation in a laser cavity which absorb weaker energy of light modes while get across higher energy. After millions of circulation in laser cavity, ultra-short (ps or fs in temporal duration) pulses with a high concentration power could be generated. MoS2 has indirect bandgap in bulk material with a band gap of ∼1.2 eV and direct band-gap in monolayer structure with a broader band gap of ∼1.9 eV. Although it seems that few-layer MoS2 might have limited operation bandwidth and fails to operate as a broadband saturable absorber.

However, according to their careful experimental studies, the team found that few-layer MoS2 could still possess wavelength insensitive saturable absorption responses, which is caused by the special molecular structures in few-layer MoS2. It is worth commenting on the broadband performance of graphene and MoS2. The broadband performance of graphene is intrinsic, due to its gapless nature. However, it is more complex in the exfoliated MoS2 nanoparticle sample they used (see paper in Scientific Reports: “Ytterbium-doped fiber laser passively mode locked by few-layer Molybdenum Disulfide (MoS2) saturable absorber functioned with evanescent field interaction”) due to the mixture of 1T (metallic) and 2H (semiconducting) phases present. The 1T phases usually predominate in as-exfoliated samples due to doping by impurities, giving rise to similar broadband performance as graphene. If the MoS2 can be rendered predominantly 2H, its absorption at resonance energy will be stronger.

This means that at specific wavelength that is in resonance with the band gap, we expect that MoS2 saturable absorber can potentially give stronger saturable absorption response than graphene in view of its strong bulk-like photon absorption and exciton generation owing to Van Hove singularities.

Fig. 1: The broadband saturable absorption of few-layer MoS2 and the performance of mode locked operation. (click on image to enlarge)

The enhanced, broadband and ultra-fast nonlinear optical response in 2D semiconducting transition metal disulfides (TMDs) indicates unprecedented potential for ultra-fast photonics, ranging from high speed light modulation, ultra-short pulse generation to ultra-fast optical switching. However, the stability and robustness issues of TMDs turns out to be a significant problem if exposed to high power laser illumination. Unlike graphene that has extremely high thermal conductivity, flexibility and mechanical stability, TMDs may show much lower optical damage threshold than graphene because of their poorer thermal and mechanical property, although explorations on the photonic applications are being fueled by their advantages.

It is worth mentioning that polymethacrylate (PMMA) is indispensable for protecting few-layer MoS2 from vertical transmission if under strong optical power density. In principle, MoS2 couldn’t afford even higher laser illumination than 100 mW (pure material) and 500 mW (with PMMA protection) adheres to a fiber tail with mode field diameter of several micrometers in our experiment, which might seriously limit its potential applications in practical optical devices. Taper fibers inspired us to solve this challenge, schematically shown in Fig. 2. Few layer MoS2 was coupled on the waist of the taper fiber and interacted with an evanescent field of laser illumination. In this approach, the material doesn’t need to bear high optical power.

This optical device could bear 1 W laser injection without damage and also could achieve mode locked operation in a fiber laser as a saturable absorber.

Fig. 2: Schematic diagram of the taper fiber and the ytterbium-doped fiber laser passively mode locked by the MoS2-taper-fiber-saturable absorber.

“By depositing few-layer MoS2 upon the tapered fiber, we can employ a ‘lateral interaction scheme’ of utilizing the strong optical response of 2D MoS2, through which not only the light-matter interaction can be significantly enhanced owing to the long interaction distance, but also the drawback of optical damage of MoS2 can be mitigated. This MoS2-taper-fiber device can withstand strong laser illumination up to 1 W. Considering that layered TMDs hold similar problems as MoS2, our findings may provide an effective approach to solve the optical damage problem on those layered semiconductor materials,” Prof. Han Zhang from the Key Laboratory for Micro-Nano Optoelectronic Devices at Hunan University, concludes.

“Beyond MoS2, we anticipated that a number of MoS2-like layered TMDs (such as, WSe2, MoSe2, TaS2 etc) can also be developed as promising optoelectronic devices with high power tolerance, offering inroads for more practical applications, such as large energy laser mode-locking, nonlinear optical modulation and signal processing etc.”

This work provides a very convenient but practical way to overcome the disadvantages (very low optical damage threshold) of 2D semiconducting TMDs, simply by adopting a ‘lateral interaction scheme’. Stimulated by this technological innovation, we anticipate that researcher might propose new types of light interaction modes with 2D materials, particularly, the integration of 2D materials with various waveguide structures, such as Silicon waveguide. It will definitely not only solve the problems concerning easily optical damage, but also lead to new physics on how light propagates along and interacts with the 2D semiconducting surface, in the present of waveguides. Eventually, it might revolutionize our viewpoints on 2D optoelectronics, and open up a new test-bed with unprecedented chances for conceptually new optoelectronic devices.

By Dr. Feng Luan, Assistant Professor, Division of Communication Engineering, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore

Read more: 2D molybdenum disulfide: a promising new optical material for ultra-fast photonics

A New Way to Convert Light to Electrical Energy

Abstract:
The conversion of optical power to an electrical potential is of general interest for energy applications, and is typically accomplished by optical excitation of semiconductor materials. A research team has developed a new method for this conversion, using an all-metal structure, based on the plasmon resonance in metal nanostructures.

Pasadena, CA | Posted on November 1st, 2014

 

Plasmoelectric potentials occur when metal nanostructures are excited by light at wavelengths near their resonant wavelengths, and may someday enable development of entirely new types of all-metal optoelectronic devices that can convert light into electrical energy.

This new finding could have a significant impact on the understanding of the electrochemical energy landscapes for photovoltaic, photoelectrochemical and optoelectronic devices. According to Dr. Harry Atwater, who led the study, “This work illustrates that electrical potentials can arise in metallic nanostructures in surprising ways. Although it is not clear how applications might develop from this finding, whenever you can design a optical material to produce potentials, it points toward possibilities for sensors and power converters.”

The findings are published today in the journal Science.

Follow Related Links Here:

http://daedalus.caltech.edu/research/plasmonics.php

Quantum Dots: Global Market Growth and Future Commercial Prospects 2014

LONDON, Oct. 14, 2014 /PRNewswire/ — REPORT HIGHLIGHTS

The global market for quantum dots (QDs) was estimated to generate $121 million in revenues in 2013. This market is expected to reach about $1.1 billion in 2016 and about $3.1 billion by 2018, at a compound annual growth rate (CAGR) of 90.8% for the five-year period, 2013 to 2018.
An overview of the global markets for quantum dots and their future commercial prospects.

Report Information is especially valuable to individuals and organizations seeking more insight into the current status of QDs, their stand-alone capabilities within the spectrum of nanomaterials, as well as to nanomaterials manufacturers, investors seeking near-term commercialization opportunities, and technologists confronted with nanomaterial device integration issues. Exploiting the use of quantum dots in the biological, biomedical, electronics, energy, optics, optoelectronics, and security applications industries as well as the Evaluation of key and relevant patents.

INTRODUCTION

Among the many subsets of nanomaterials, quantum dots (QDs) are like no other. At dimensions typically below 10 nanometers (nm), nanocrystalline (nc) semiconductors (SC), metals and magnetic materials can all exhibit extraordinary quantum confinement phenomenon. Basically, at these dimensions, their physical size encroaches upon the fundamental quantum confinement dimensions of orbiting electrons that are uniquely prescribed by their atomic nucleus. Within the regime of these critical dimensions, QDs exhibit distinctly different behavior from their bulk form, which manifests itself, for example, in distinctly different optical, electronic and magnetic properties.

Today, scientists can precisely synthesize nanocrystalline materials at these critical dimensions and thereby systematically tune their quantum confining behavior. As a result there is currently enormous interest to exploit and capitalize on the unique properties exhibited by QD materials.

As a harbinger for future business developments, colloidal QD-bioconjugates are among the first wave of commercial product applications stimulating market interest. Primarily, these have quickly established a niche market in the life sciences and biomedical communities, where they provide unrivalled cellular imaging and therapeutic detection capabilities.

Other promising prototype developments of SC QDs now on the commercial-horizon range include: a new generation of flash memory devices; nanomaterial enhancements for improving the performance of flexible organic light-emitting diodes (LEDs), as well as solid-state white-LED lighting; and a core technology used in flexible solar panel coatings.
With these impending commercial developments and their enormous business potential, this report provides a timely assessment of quantum dot materials—where they are currently at, and where they might be in the foreseeable future.

STUDY GOAL AND OBJECTIVES

The primary objective of this report is threefold: to assess the current state-of-the-art in synthesizing QDs; to identify the current market players seeking to exploit QD behavior; and to evaluate actual or potential markets in terms of application, type and projected commercial market revenues.

SCOPE AND FORMAT

Since their parallel discovery in Russia and the U.S. almost 30 years ago, SC QDs, until quite recently, have resided exclusively in the domain of solid-state physics, where they have been fabricated using expensive and sophisticated molecular beam epitaxy (MBE) or chemical vapor deposition (CVD) equipment.

However, in a relatively short time frame, this situation has changed dramatically with the recent commercial availability of colloidal QDs synthesized by less expensive wet-chemical processes. Practically, the availability of QDs in a colloidally dispersed form will help demystify these somewhat esoteric materials. Most importantly, colloidal QDs now provide access to a much broader audience, which promises to further widen their potential market exploitation.

Current and future applications of QDs impact a broad range of industrial markets. These include, for example:

• biology and biomedicine;
• computing and memory;
• electronics and displays;
• optoelectronic devices such as LEDs, lighting and lasers;
• optical components in telecommunications and image sensors;
• security applications such as covert identification tagging or biowarfare detection sensors;
• solar energy generation and storage.

This report probes in considerable depth the early pioneers and champions in this field in industry, government and academic laboratories. The most active organizations, promising technical applications, and developments realizable within the next five years, will all be highlighted.

CONTRIBUTIONS OF THE STUDY AND TARGET AUDIENCE

This report represents a major update of the BCC Research report (NAN027C) Quantum Dots: Global Market Growth and Future Commercial Prospects, published in February 2011. The most significant revisions in the new edition include:
An extensive updated patent analysis (2011 to 2013).
An in-depth assessment of the unfolding commercial markets.
Progress in the synthesis and commercial scale up by QD producers.
Updated company profiles of the producers and end users dictating market development.
Updated five-year market projection analysis of the emerging QD market.

The report’s comprehensive technical and business assessment on the current status of the QD-based industry should prove informative to nanomaterials manufacturers, investors seeking near-term commercialization opportunities, technologists confronted with nanomaterial device integration issues and companies specifically interested in exploiting QDs in biological, biomedical, electronics, energy, optics, optoelectronics and security applications.

derived from the enormous amount of patent and technical literature relating to QDs disclosed in the public domain. In addition, complementary information has also been drawn from the business community, such as company investment news, company profiles, press releases and personal telephone interviews with selected companies.

Hybrid graphene–giant nanocrystal quantum dot assemblies

September 24, 2014

Coupling two distinct nanostructures often leads to a new class of hybrid material with unique properties and functionalities. On the other hand, it could produce worthless hybrids, as one nanostructure could neutralize the useful properties of the other.

Graphene, an atomically thin sheet of carbon, exhibits unique electrical and optical properties that are highly beneficial for a wide range of electronic, optoelectronic, and photovoltaic applications. Semiconductor nanocrystals (NCs) that exhibit exceptional optical properties derived from 3D quantum confinement of their charge carriers (i.e., electrons and holes), have also inspired many applications spanning from biomedical imaging to solid state lighting and quantum communication. When NCs are coupled to graphene, excitons (i.e., electron–hole pairs) created in the NCs by the absorption of light can recombine nonradiatively, subsequently transferring their energy to graphene via near-field interactions (i.e, Förster energy transfer).

As a result, the NC’s photoluminescence (PL) is quenched strongly. While such an effect may be useful for energy harvesting and sensing applications, it neutralizes the exceptional light emission properties of the NCs and therefore severely inhibits the use of NC–graphene hybrid systems in applications such as light-emitting diodes, lasers, and as single photon sources.

In contrast to the above scenario, scientists from the Center for Integrated Nanotechnologies, Los Alamos National Laboratory, show that when a new class of nanocrystals called giant-NQDs (g-NQDs) are coupled to graphene, bi-excitons (BX), i.e., pairs of excitons, optically excited in g-NQDs, experience efficient recombination and subsequent simultaneous emission of photon pairs. A joint effort with scientists from the Theoretical Division, (Los Alamos National Laboratory) resulted in a model providing an insight into this interesting phenomenon. The model predicts the formation of excess charge density, named a ‘charge puddle’, within the graphene sheet right underneath a photocharged g-NQD. In this case, the g-NQD acts as a gate electrode, controlling the amount of charge in the puddle. Collective oscillations of the puddle’s charges results in a new localized plasmon mode interacting with the excitons and bi-excitons of the g-NQD. This interaction gives rise to the observed enhancement in photon emission.

These findings reveal a tremendous potential for graphene-g-NQD hybrids in applications requiring highly efficient single-photon and photon-pair emission, including lasing and entangled photon sources. The discovery of a new plasmon mode also has several important implications in the fields of plasmonics, photonics, and quantum optics. Specifically, extending the tunability of graphene plasmon modes to the visible spectral range should dramatically expand the optical functionality of graphene plasmonics. Furthermore, the formation of this new plasmon mode directly underneath the g-NQD provides a perfect solution to the general problem of quantum emitter–plasmonic cavity alignment hindering a realization of strong plasmon–exciton coupling. Access to such a coupling regime may ultimately open a new route towards quantum plasmonics.

MIT & U of Manchester Researchers Discover Potential New Types of Transistors and Electronic Circuits

When moving through a conductive material in an electric field, electrons tend to follow the path of least resistance—which runs in the direction of that field.

But now physicists at MIT and the University of Manchester have found an unexpectedly different behavior under very specialized conditions—one that might lead to new types of transistors and electronic circuits that could prove highly energy-efficient.

They’ve found that when a sheet of graphene—a two-dimensional array of pure carbon—is placed atop another two-dimensional material, electrons instead move sideways, perpendicular to the electric field. This happens even without the influence of a magnetic field—the only other known way of inducing such a sideways flow.

What’s more, two separate streams of electrons would flow in opposite directions, both crosswise to the field, canceling out each other’s electrical charge to produce a “neutral, chargeless current,” explains Leonid Levitov, an MIT professor of physics and a senior author of a paper describing these findings this week in the journal Science.

Credit: Christine Daniloff/MIT

The exact angle of this current relative to the electric field can be precisely controlled, Levitov says. He compares it to a sailboat sailing perpendicular to the wind, its angle of motion controlled by adjusting the position of the sail.

Levitov and co-author Andre Geim at Manchester say this flow could be altered by applying a minute voltage on the gate, allowing the material to function as a transistor. Currents in these materials, being neutral, might not waste much of their energy as heat, as occurs in conventional semiconductors—potentially making the new materials a more efficient basis for computer chips.

“It is widely believed that new, unconventional approaches to information processing are key for the future of hardware,” Levitov says. “This belief has been the driving force behind a number of important recent developments, in particular spintronics”—in which the spin of electrons, not their electric charge, carries information.

The MIT and Manchester researchers have demonstrated a simple transistor based on the new material, Levitov says.

“It is quite a fascinating effect, and it hits a very soft spot in our understanding of complex, so-called topological materials,” Geim says. “It is very rare to come across a phenomenon that bridges materials science, particle physics, relativity, and topology.”

In their experiments, Levitov, Geim, and their colleagues overlaid the graphene on a layer of boron nitride—a two-dimensional material that forms a hexagonal lattice structure, as graphene does. Together, the two materials form a superlattice that behaves as a semiconductor.

This superlattice causes electrons to acquire an unexpected twist—which Levitov describes as “a built-in vorticity”—that changes their direction of motion, much as the spin of a ball can curve its trajectory.

Electrons in graphene behave like massless relativistic particles. The observed effect, however, has no known analog in particle physics, and extends our understanding of how the universe works, the researchers say.

Whether or not this effect can be harnessed to reduce the energy used by computer chips remains an open question, Levitov concedes. This is an early finding, and while there is clearly an opportunity to reduce energy loss to heat locally, other parts of such a system may counterbalance those gains. “This is a fascinating question that remains to be resolved,” Levitov says.

Francisco Guinea, a research professor at Spain’s Instituto de Ciencia de Materiales de Madrid, who was not connected with this research, calls the approach taken by this team “novel and imaginative. … The characterization of these currents in graphene is a very important advance in the understanding of two-dimensional materials.”

The work has great potential, Guinea adds, because “two-dimensional materials with special topological properties are the basis of new technologies for the manipulation of quantum information.”

Explore further: New species of electrons can lead to better computing

More information: ‘Detecting topological current in graphene superlattices” by R. V. Gorbachev, J. C. W. Song, G. L. Yu, A. V. Kretinin, F. Withers, Y. Cao, A. Mishchenko, I. V. Grigorieva, K. S. Novoselov, L. S. Levitov, A. K. Geim’, Science Sep 11, 2014.

Journal reference: Science

Provided by Massachusetts Institute of Technology