Yearly Archives: 2016

With an eye to the next generation of tech gadgetry, a team of physicists at The University of Texas at Austin has had the first-ever glimpse into what happens inside an atomically thin semiconductor device. In doing so, they discovered that an essential function for computing may be possible within a space so small that it’s effectively one-dimensional.

In a paper published July 18 in the Proceedings of the National Academy of Sciences, the researchers describe seeing the detailed inner workings of a new type of transistor that is two-dimensional.

Transistors act as the building blocks for computer chips, sending the electrons on and off switches required for computer processing. Future tech innovations will require finding a way to fit more transistors on computer chips, so experts have begun exploring new semiconducting materials including one called molybdenum disulfide (MoS2). Unlike today’s silicon-based devices, transistors made from the new material allow for on-off signaling on a single flat plane.

Keji Lai, an assistant professor of physics, and a team found that with this new material, the conductive signaling happens much differently than with silicon, in a way that could promote future energy savings in devices. Think of silicon transistors as light bulbs: The whole device is either turned on or off at once. With 2-D transistors, by contrast, Lai and the team found that electric currents move in a more phased way, beginning first at the edges before appearing in the interior. Lai says this suggests the same current could be sent with less power and in an even tinier space, using a one-dimensional edge instead of the two-dimensional plane.

“In physics, edge states often carry a lot of interesting phenomenon, and here, they are the first to turn on. In the future, if we can engineer this material very carefully, then these edges can carry the full current,” Lai says. “We don’t really need the entire thing, because the interior is useless. Just having the edges running to get a current working would substantially reduce the power loss.”

Researchers have been working to get a view into what happens inside a 2-D transistor for years to better understand both the potential and the limitations of the new materials. Getting 2-D transistors ready for commercial devices, such as paper-thin computers and cellphones, is expected to take several more years. Lai says scientists need more information about what interferes with performance in devices made from the new materials.

“These transistors are perfectly two-dimensional,” Lai says. “That means they don’t have some of the defects that occur in a silicon device. On the other hand, that doesn’t mean the new material is perfect.”

Lai and his team used a microscope that he invented and that points microwaves at the 2-D device. Using a tip only 100 nanometers wide, the microwave microscope allowed the scientists to see conductivity changes inside the transistor. Besides seeing the currents’ motion, the scientists found thread-like defects in the middle of the transistors. Lai says this suggests the new material will need to be made cleaner to function optimally.

“If we could make the material clean enough, the edges will be carrying even more current, and the interior won’t have as many defects,” Lai says.

The paper’s other authors are postdoctoral researchers Di Wu and Xiao Li; research scientist Lan Luan, and graduate students Xiaoyu Wu and Zhaodong Chu, and professor Qian Niu in UT Austin’s Department of Physics; and graduate student Wei Li, former graduate student Maruthi N. Yogeesh, postdoctoral researcher Rudresh Ghosh, and associate professor Deji Akinwande of UT Austin’s Department of Electrical and Computer Engineering.

Earlier this year, both Lai and Akinwande won Presidential Early Career Awards for Scientists and Engineers, the U.S. government’s highest honor for early-stage scientists and engineers.

By Pete Singer, Editor-in-Chief

The semiconductor industry is moving quickly to adopt a variety of new materials in an effort to increase chip performance. These new materials can create a host of safety concerns that must be addressed. Many of the new process chemicals have low vapor pressures, are highly reactive and present serious hazards to personnel and equipment. Many new CVD precursors and their associated reaction by-products are flammable, pyrophoric, toxic, corrosive or otherwise hazardous to personnel or destructive to equipment. “The problem’s always been there. It’s just becoming more acute as new processes emerge,” said Andrew Chambers, Senior Product Manager at Edwards Ltd., Clevedon, UK.

The Danger That Lurks Figure

Process byproducts are pumped through exhaust lines to a gas abatement system. Residual precursor materials or reaction byproducts often have a tendency to condense in pipe-work, including process exhausts. These exhaust pipes must be cleaned regularly, since condensed material will block the pipe, reduce its conductance and cause process problems.

Epitaxial silicon (“epi”) deposition processes, for example, are particularly notorious for the process decomposition products condensing in exhaust pipes or in the foreline of the pump. The hazard is greatest when the exhaust system is dismantled for cleaning. “The condensed material can react violently when it’s exposed to air and will burn vigorously or even explode. That presents a pretty serious hazard to the service engineers who are charged with taking the pipe apart and cleaning it,” Chambers said.

These problems can largely be avoided, however, by keeping the exhaust pipe or the pump foreline at a high temperature to avoid condensation of the material. If the surfaces in the exhaust system are warm enough, the processed products transit through the exhaust pipe and into the abatement system, where they can be combusted and dealt with in a safe fashion.

In many fabs, the heating is done with heating tape, but that’s not always the best (or safest) way to go. “In principle, that works up to a point but it’s quite difficult to apply that kind of technique when you’ve got accessories like a large ball valve in the line, where there are brackets attaching the exhaust pipe to the wall or there’s a system for injecting nitrogen dilution gas into the exhaust. The idea of heater tapes is convenient but not a very effective fix,” Chambers said. “Furthermore, removing heater tape to dismantle and clean the exhaust pipe can be inconvenient and time consuming” he added.

What’s really required is an approach that involves heating the pipe in a uniform fashion so that the pipe is universally at a high temperature to avoid the condensation. “You can’t afford to have cold spots in the pipe where there’s no heater or there’s no insulation because the moment you have a cold spot in the pipe, then material’s going to condense there and cause a local blockage,” he said.

Edwards offers a new Temperature Management System (SMART TMS) that ensures these compounds remain volatile until they enter the abatement device. SMART TMS is designed to heat both forelines and pump exhaust lines uniformly as far as the inlet of the abatement device. Molded high surface area heaters maximize contact with pipes and are designed to maintain them at a constant temperature between ambient and 180°C, recognizing that when choosing the temperature set point, knowledge of what process materials and byproducts are going to be in the exhaust pipe is invaluable.

Chambers said this approach is also superior to other heating methods using custom heater mats with integral insulation. “The difficulty you encounter with those kinds of systems is that the heater mat and jacket tend to be custom-designed to suit the particular installation. You spend a lot of time designing stuff, placing orders and waiting for it to be manufactured. Once it is manufactured and installed, there’s no flexibility. If you change the configuration of the exhaust pipe, you’ve got to go buy a whole set of new pipeline heating components,” he said.

With the Edwards SMART TMS approach, heater mats are provided separately from the insulation. “The heater mats are provided in standard lengths and as shaped components too, for elbows, valves, T-pieces and so on. You basically assemble the heater mats of sufficient length to heat your pipe from one end to the other. Then, since they’re made from low-particulate material, the insulation jackets can be cut to shape on assembly to fit the exhaust pipe. They’re all basically reusable,” Chambers said.

Success in thermal management goes well beyond mechanical considerations, however. “A lot of the skill and judgment in temperature management of exhaust pipes is knowing what factors you need to take care of to get decent temperature control throughout the system,” Chambers said.

When handling flammable gases, for example, nitrogen is often used to dilute them below their lower flammable limit to make them safe. “Typically, you pour a whole lot of nitrogen dilution gas into your exhaust pipe. The way in which you do that has a very significant impact on the temperature of the gas and the temperature of the exhaust pipe,” Chambers said. “If you’ve heated your exhaust pipe up to a temperature based on the process gases flowing through it and then you flow into it a couple of hundred liters per minute of cold nitrogen, then your heating system is no longer going to be fully effective. You start to run into the kind of condensation problem you were trying to avoid in the first place.”

The Edwards solution to that problem is to employ a system to heat up the nitrogen dilution gas. “Providing a nitrogen heater system as an accompaniment to a temperature management system for the exhaust pipe is sometimes a desirable thing to do,” Chambers added.

SMART TMS includes a sophisticated control system. “In our system, we have a controller which takes care of exhaust pipes on a zone by zone basis. The controller can control nine zones. All of those nine zones may be nine separate pipes. It may be one long pipe with nine zones in it over a long distance,” Chambers said.

The controller has useful operational features such as the ability to set and log different control temperatures and user-defined limits in each zone. If a temperature strays outside the user-defined band, an alert is transmitted from the controller to the process tool, the fab central monitoring system or other fault reporting system, depending on the nature of the fault. Furthermore, recognizing that some processes can cause very hazardous byproduct build-up in cold exhausts, SMART TMS includes a “fail-on” function to ensure that in the event of a component failure or loss of temperature indication a high pipeline temperature is maintained until servicing can be scheduled. In these cases the integrated health-check function provides an alert, while dual safety devices in each heater provide intrinsic safety and protection against thermal runaway.

“In the future, we can imagine the process which is running in the tool can be used to inform the set-up of the sub-fab equipment, the dry pumps or even the temperature management system,” Chambers said. “We’ve come a long way from relatively simple electrical heaters installed on an ad hoc basis to a sophisticated combination of process knowledge, a wide range of heater mats and shaped heaters, very efficient insulation materials and intelligent controllers with data acquisition capability.”

By Pete Singer, Editor-in-Chief

Last year, Rudolph Technologies, Inc. announced the widespread adoption and success of its newest macro defect inspection tool, the NSX® 330 Series. The NSX 330 Series provides high-speed macro defect inspection with 2D\3D metrology for advanced packaging applications, which are being developed primarily to support mobility. The company said it had been “quickly and enthusiastically adopted,” garnering repeat orders from top foundries, integrated device manufacturers (IDMs) and outsourced assembly and test (OSAT) manufacturers.

The NSX 330 Series offers an array of metrology capabilities for both 2D and 3D metrology applications, including 100 percent bump height and coplanarity measurements. The NSX 330 series has now been further improved by incorporating a high speed bump laser triangulation sensor and the highly accurate VT-SS distance and thickness sensor. “We specifically offer these capabilities on a single platform because they improve total measurement accuracy on complex materials which have troubled the industry for some time,” said Scott Balak, director, inspection product management, Rudolph Technologies Inc. (Bloomington, MN).

Figure 1 illustrates the problem. The goal is to measure the actual bump height and overall coplanarity from bump top to polyimide (PI) surface. If one or more bumps are too high or too low, the other bumps won’t connect. A high-speed laser triangulation sensor attempts to see through the polyimide (PI) layer, which is typically 3-6 microns thick. “The problem is that polyimide isn’t completely transparent, so when the triangulation sensor attempts to detect the bottom of this PI layer, it is actually finding it somewhere in the middle. The current industry’s work around is to assume a PI thickness and apply a PI layer offset; however, PI thickness variation limits the accuracy of this approach,” Balak explained.

Figure 1

Figure 1

Inaccurate measurements create unnecessary review work. Because bumps may have acceptable coplanarity, but they are incorrectly flagged for further evaluation. “Customers use the review mode to determine if the bump is actually too big, or too small” Balak said.

Enter Rudolph’s Visible Thickness and Shape Sensor (VT-SS) sensor, which can concurrently measure the transparent layer thickness as well as the metal feature step height above the surface of the transparent layer. This is achieved through the integration of reflectometry and visible light interferometry principles. The direct reflection from the transparent layer provides direct thickness measurement of the transparent material, while the interferometry captures topography (distance from the sensor), allowing the system to measure the thickness of the opaque metals by scanning over the edge of the feature.

“Rudolph samples multiple bumps with both the laser triangulation and VT-SS sensors to accurately obtain a measurement average of wafer PI thickness while simultaneously calibrating the triangulation sensor with an accurate PI offset for the specific wafer being measured.,” Balak said. “The properly calibrated triangulation sensor then quickly and accurately measures millions of bumps per wafer correctly flagging bad bumps and eliminating the need to review good product. Wafer results are then sent to our Discover Analysis solution where customers can analyze correlations between defectivity and process metrology to improve the overall process. Whether it is understanding wafer and lot level trends or specific individual bumps; Discover provides the drill down capability required for root cause analysis.”

Silicon Valley specialty semiconductor foundry Noel Technologies, a provider of process development and substrate fabrication for a variety of high-technology industries, celebrates its 20th anniversary this month. According to market data from the industry trade association SEMI, Noel Technologies is one of only two companies still offering foundry services in Silicon Valley, where there were once dozens of wafer-fabrication facilities.

Using industry-standard process flows and materials, Noel Technologies develops and perfects semiconductor-manufacturing recipes for customers in the IC, renewable energy, automotive electronics, LED lighting, optoelectronics, MEMS and other nanoelectronics industries. The foundry can work with traditional silicon wafers up to 450mm as well as non-standard substrates including III-V compound materials, glass and fused silica.

The company provides chip makers with a bridge from IC development work to volume production, a much-needed service in proving the viability of new devices and innovative manufacturing processes. The multi-billion-dollar cost of building today’s wafer fabs has led many semiconductor companies to adopt a “fabless” strategy by outsourcing chip manufacturing to a foundry, many of which are located in Asia.

“While other companies have moved their fabrication operations out of Silicon Valley – whether in pursuit of lower labor costs, tax holidays or other financial incentives – we are dedicated to working with local and far-off semiconductor companies on their prototyping, pilot manufacturing and production needs,” said Leon Pearce, founder and chief technical officer of Noel Technologies.

To deliver short cycle times and maximize the utility of its installed equipment base, Noel Technologies operates seven days a week, 20 hours per day. Projects vary in size from single wafers to thousands per month, depending upon each customer’s unique needs.

Pearce and his daughter Kristin Boyce, president of Noel Technologies, co-founded the company in 1996 with three employees. Through strategic technology additions, tool acquisitions and facility expansions, they have grown the staff to 50 personnel. Hill joined the company 12 years ago, leveraging her extensive semiconductor experience to expand Noel Technologies’ foundry services and better serve its broad customer base. Together, the three senior executives focus on customer needs, emerging market requirements and new applications.

The company continues to operate at its original location and has no corporate debt, both extreme rarities in the semiconductor industry. Noel Technologies owns and operates a Class 100 cleanroom facility equipped with tools that support its process-driven services model and run by a well-trained engineering department.

A novel three-dimensional solar cell design developed at Georgia Tech will soon get its first testing in space aboard the International Space Station. An experimental module containing 18 test cells was launched to the ISS on July 18, and will be installed on the exterior of the station to study the cells’ performance and their ability to withstand the rigors of space.

solar6_1

In addition to testing the three-dimensional format, the module will also study a low-cost copper-zinc-tin-sulfide (CZTS) solar cell formulation. In all, the module launched to the ISS contains four types of PV devices: 3-D cells based on conventional cadmium telluride, 3-D cells based on CZTS materials, traditional planar solar cells produced at Georgia Tech, and planar cells based on CZTS.

The experiment was aboard SpaceX’s Falcon 9 rocket that blasted off at 12:45 a.m. EDT from Cape Canaveral Air Force Station in Florida.

“We want to see both the light-trapping performance of our 3-D solar cells and how they are going to respond to the harshness of space,” said Jud Ready, a principal research engineer at the Georgia Tech Research Institute (GTRI) and an adjunct professor in the Georgia Tech School of Materials Science and Engineering. “We will also measure performance against temperature, because temperature has an influence on the performance of a solar cell.”

Built by coating miniature carbon nanotube “towers” with a photo-absorber that captures sunlight from all angles, the 3-D cells developed by Ready’s lab could boost the amount of power obtained from the small surface areas many spacecraft have. The cells would absorb light from any direction, eliminating the need for mechanical devices to aim PV modules toward the sun.

The PV cell experiment will be installed on the NanoRacks External Platform (NREP), where robustness of the solar cells will be studied under harsh space conditions for six months. The project is sponsored by the Center for the Advancement of Science in Space (CASIS), and the Space Station opportunity was provided by NanoRacks via its Space Act Agreement with NASA’s U.S. National Labs.

“The CZTS photovoltaic arrays were built using the readily available elements copper, zinc, tin and sulfur to replace rarer CIGS – copper, indium, gallium and selenium – which are used in similar thin-film solar cells,” said Ready. “The CZTS approach produces an efficient photo-absorber using earth-abundant materials that cost around a thousand times less than rare-earth elements like indium, gallium and selenium.”

One virtue of CZTS photovoltaic material is its electron band structure, Ready explained. Like CIGS, CZTS is a direct band gap material. In semiconductor physics, this means incoming solar photons are able to emit current-producing electrons directly, rather than moving through power-robbing intermediate states as indirect band gap materials, like silicon, require.

Moreover, Ready said, direct band gap materials have good resistance to the powerful ionizing radiation encountered in space. That’s because direct band gaps are larger than indirect band gaps; it’s harder for radiation to damage these larger gaps so severely that functionality is seriously impaired.

The 3-D capability could prove especially valuable on the International Space Station, which is exposed daily to 15-16 sunrises and sunsets as it orbits Earth every 92 minutes at 17,150 m.p.h. The 3-D towers can exploit the sun’s rays for longer periods than conventional 2-D planar – or flat – designs, which work most efficiently only when the sun is directly overhead.

“With our 3-D design, as the sun’s angle increases more surface is exposed and there’s a growing chance that photons will enter,” Ready said. “Also, 3-D technology provides more opportunity for photons to bounce around between the towers, increasing the likelihood they will be converted to electron hole pairs and produce mobile charge carriers.”

As the ISS orbits, the 3-D arrays’ performance will be compared to a high quality commercial 2-D planar cell array installed nearby. If things go as expected, GTRI’s cells will provide relatively better performance than the other cells as they move away from high noon. The new CZTS 3-D arrays will also be tested in space against an older 3-D design made by GTRI using cadmium telluride.

One of the GTRI development team’s key achievements to date has been identifying the best ways to manufacture CZTS solar cells. The team has pinpointed techniques for successfully processing the four Earth-abundant elements into an efficient photo absorber.

“In manufacturing you have to heat these elements, and one major issue is that they evaporate at different rates,” Ready explained. “Getting them to blend in the desired ratios, so that the stoichiometry is retained and electron levels of the constituent elements match up as they should, has been a challenge.”

GTRI’s photovoltaic arrays will be encased in Lexan containers aboard the ISS. Lexan, a clear yet strong polymer, produces minimal interference with incoming solar rays but can protect the delicate arrays from astronauts and space debris – and also protect the crew from any pieces of the arrays that might separate.

After the six-month mission, the solar cells will be sent back to Earth via a cargo ship. The research team will assess the cells’ post-mission performance and look for damage from radiation and other space hazards.

“If it can survive in space, which is the harshest of environments from the standpoint of wide temperature swings, radiation and numerous other factors, then we can be confident it will work well down on Earth,” Ready said.

In a recent work published in Nature Communications, the research group led by ICREA Professor at ICFO Frank Koppens demonstrate a novel way to detect low-energy photons using vertical heterostructures made by stacking graphene and other 2D semiconducting materials. By studying the photoresponse of these atomically thin sandwiches, the researchers have shown that it is possible to generate a current by heating electrons in graphene with infrared light and extracting the hottest electrons over a vertical energy barrier.

This ingenious mechanism, named photo-thermionic effect, takes advantage of the unique optical properties of graphene such as its broadband absorption, ultrafast response and gate-tunability. Moreover, owing to their vertical geometry, devices relying on this effect make use of the entire surface of graphene and can be potentially scaled up and integrated with flexible or rigid platforms.

More generally, this study reveals once again the amazing properties of these man-made heterostructures. According to Prof. Frank Koppens “this is just the tip of the iceberg, these 2D sandwiches still have a lot to reveal”. ICFO researcher Mathieu Massicotte, first author of this study, emphasizes the new possibilities opened up by these new materials: “Everyone knows it is possible to detect light with graphene using in-plane geometries, but what about the out-of-plane direction? To answer, you need to think outside the 2D box!”

The results obtained from this study have shown that heterostructures made of 2D materials and graphene can be used to detect low-energy photons which could lead to new, fast and efficient optoelectronic applications, such as high-speed integrated communication systems and infrared energy harvesting. In addition, it demonstrates the compatibility of 2D materials with the digital chips currently utilized in cameras, paving the way for low cost infrared spectrometers and imaging systems.

tsu-jae

Dr. Tsu-Jae King Liu

Intel Corporation today announced that Dr. Tsu-Jae King Liu has been elected to serve on Intel’s board of directors.

“We are very pleased to have Dr. Liu join the Intel board and look forward to her contributions,” said Intel Chairman Andy Bryant. “She brings a wealth of expertise in silicon technology and innovation that will be valuable for Intel in many areas as we navigate a significant business transition while continuing to lead in advancing Moore’s Law and harnessing its economic value.”

Liu, 53, holds a distinguished professorship endowed by TSMC in the Department of Electrical Engineering and Computer Sciences (EECS), in the College of Engineering at the University of California, Berkeley where she also serves as associate dean for Academic Planning and Development. Liu’s previous administrative positions within the College of Engineering include associate dean for research and EECS department chair. She has also held research and engineering positions at the Xerox Palo Alto Research Center and Synopsys Inc.

Liu holds over 90 patents and has received numerous awards for her research, including the Intel Outstanding Researcher in Nanotechnology Award (2012) and the SIA University Researcher Award (2014). Currently, her research is focused on nanometer-scale logic and memory devices, and advanced materials, process technology and devices for energy-efficient electronics. She received B.S., M.S. and Ph.D. degrees in electrical engineering from Stanford University in 1984, 1986 and 1994, respectively.

Toshiba Corporation (TOKYO:6502) and Western Digital Corporation (NASDAQ:WDC) today celebrated the opening of the New Fab 2 semiconductor fabrication facility located in Yokkaichi, Mie Prefecture, Japan.

Expanded use of flash memory in smartphones, SSDs, and other applications is driving continued growth of the global flash memory market. The New Fab 2 facility will support the conversion of the companies’ 2D NAND capacity to 3D flash memory, allowing realization of solutions offering higher densities and better device performance.

Construction of New Fab 2 began in September 2014. Following partial completion of the facility in October 2015, Toshiba and SanDisk (acquired in May 2016 by Western Digital Technologies Inc., a wholly owned subsidiary of Western Digital Corporation) worked together to implement leading-edge manufacturing capabilities for mass production of 3D flash memory, and first-phase production started in March of this year. The parties intend to further invest to expand production capacity over time, depending on market conditions.

In addition, Yokkaichi operations will leverage the site-wide integrated production system, which employs big data processing to analyze over 1.6 billion data points each day, to further improve manufacturing efficiency and the quality of 3D flash memory.

The parties are committed to working together to enhance the value they offer to customers and to continue innovation as market leaders.

Satoshi Tsunakawa, President and CEO of Toshiba Corporation, said, “Advanced technologies underline our commitment to respond to continued demand as an innovator in flash memory. We are enhancing manufacturing efficiency and the quality of our world-class facility. Building on that, we also plan investments of as much as 860 billion yen by FY2018, in line with market situation. Our commitment is firm, and we are confident that our joint venture with Western Digital will produce cost competitive next generation memories at Yokkaichi.”

Steve Milligan, Chief Executive Officer of Western Digital, said, “As a leader in non-volatile memory products and solutions, we are excited to be entering the 3D NAND era with our partner Toshiba. The New Fab 2 enables us to begin the conversion of our existing 2D NAND capacity to 3D NAND and continues our long-standing presence in Yokkaichi, Mie Prefecture, and Japan.”

Just as many of us might be resigned to clogged salt shakers or rush-hour traffic, those working to exploit the special properties of carbon nanotubes have typically shrugged their shoulders when these tiniest of cylinders fill with water during processing. But for nanotube practitioners who have reached their Popeye threshold and “can’t stands no more,” the National Institute of Standards and Technology (NIST) has devised a cheap, quick and effective strategy that reliably enhances the quality and consistency of the materials–important for using them effectively in applications such as new computing technologies.

To prevent filling of the cores of single-wall carbon nanotubes with water or other detrimental substances, the NIST researchers advise intentionally prefilling them with a desired chemical of known properties. Taking this step before separating and dispersing the materials, usually done in water, yields a consistently uniform collection of nanotubes. In quantity and quality, the results are superior to water-filled nanotubes, especially for optical applications such as sensors and photodetectors.

To prevent cores of single-wall carbon nanotubes from filling with water or other detrimental substances, the NIST researchers advise intentionally prefilling them with a desired chemical of known properties. Taking this step before separating and dispersing the materials, usually done in water, yields a consistently uniform collection of nanotubes, especially important for optical applications. Credit: Fagan/NIST

To prevent cores of single-wall carbon nanotubes from filling with water or other detrimental substances, the NIST researchers advise intentionally prefilling them with a desired chemical of known properties. Taking this step before separating and dispersing the materials, usually done in water, yields a consistently uniform collection of nanotubes, especially important for optical applications. Credit: Fagan/NIST

The approach opens a straightforward route for engineering the properties of single-wall carbon nanotubes–rolled up sheets of carbon atoms arranged like chicken wire or honey combs–with improved or new properties.

“This approach is so easy, inexpensive and broadly useful that I can’t think of a reason not to use it,” said NIST chemical engineer Jeffrey Fagan.

In their proof-of-concept experiments, the NIST team inserted more than 20 different compounds into an assortment of single-wall carbon nanotubes with an interior diameter that ranged from more than 2 down to about 0.5 nanometers. Led by visiting researcher Jochen Campo, the scientists tested their strategy by using hydrocarbons called alkanes as fillers.

The alkanes, which include such familiar compounds as propane and butane, served to render the nanotube interiors unreactive. In other words, the alkane-filled nanotubes behaved almost as if they were empty–precisely the goal of Campo, Fagan and colleagues.

Compared with nanotubes filled with water and possibly ions, acids and other unwanted chemicals encountered during processing, empty nanotubes possess far superior properties. For example, when stimulated by light, empty carbon nanotubes fluoresce far brighter and with sharper signals.

Yet, “spontaneous ingestion” of water or other solvents by the nanotubes during processing is an “endemic but often neglected phenomenon with strong implications for the development of nanotube applications,” the NIST team wrote in a recent article in Nanoscale Horizons.

Perhaps because of the additional cost and effort required to filter out and gather nanotubes, researchers tend to tolerate mixed batches of unfilled (empty) and mostly filled single-wall carbon nanotubes. Separating unfilled nanotubes from these mixtures requires expensive ultracentrifuge equipment and, even then, the yield is only about 10 percent, Campo estimates.

“If your goal is to use nanotubes for electronic circuits, for example, or for fluorescent anti-cancer image contrast agents, then you require much greater quantities of materials of consistent composition and quality,” Campo explained, who was exploring these applications while doing postdoctoral research at the University of Antwerp. “This particular need inspired development of the new prefilling method by asking the question, can we put some passive chemical into the nanotube instead to keep the water out.”

From the very first simple experiments, the answer was yes. And the benefits can be significant. In fluorescence experiments, alkane-filled nanotubes emitted signals two to three times stronger than those emitted by water-filled nanotubes. Performance approached that of empty nanotubes–the gold standard for these comparisons.

As important, the NIST-developed prefilling strategy is controllable, versatile and easily incorporated into existing methods for processing single-wall carbon nanotubes, according to the researchers.

A collaboration of researchers from Kumamoto, Yamaguchi, and Osaka Universities in Japan have discovered a new method of drastically changing the color and fluorescence of a particular compound using only oxygen (O2) and hydrogen (H2) gases. The fully reversible reaction is environmentally friendly since it produces only water as a byproduct. Rather than using electrical or photo energy, the discovery uses energy from the gases themselves, which is expected to become a future trend, to switch the color and fluorescence properties. The technique could be used as a detection sensor for hydrogen or oxygen gases as well as for property controls of organic semiconductors and organic light emitting diodes (OLEDs).

An efficient chemical synthesis method for picene-13, 14-dione. Credit: Dr. Hayato Ishikawa

An efficient chemical synthesis method for picene-13, 14-dione. Credit: Dr. Hayato Ishikawa

Polyaromatic compounds (PACs) are widely used in fluorescent materials, semiconductor materials, organic EL devices, and organic solar-cell devices. The research performed at Kumamoto University focused on using energy from gases to trigger a molecular switch in a PAC. In particular, focus was placed on H2 as a reductant and O2 as an oxidant.

“We tried to determine the most attractive compounds that could freely and dramatically change the optical properties of the PAC with a redox reaction,” said Associate Professor Hayato Ishikawa from Kumamoto University. “Specifically, we introduced an orthoquinone moiety to the PAC that possessed the most ideal switching properties under a redox reaction with hydrogen and oxygen gases.”

To determine the candidates with the best switching properties, researchers screened several orthoquinone-containing aromatic compounds in a computational study. The ideal molecules clearly showed switching between fluorescence emission and quenching, and between a colored and colorless state.

Picene-13, 14-dione was nominated as the most promising candidate from the computational analysis. The researchers then developed an original protocol to efficiently synthesize the compound from commercially available petroleum raw materials. The key steps for the synthesis were the transition metal-catalyzed coupling reaction and the ring construction reaction by an organocatalyst. This synthetic methodology is also applicable to the synthesis of various other similar compounds or derivatives.

A palladium nanoparticle catalyst was added to the synthesized picene-13, 14-dione and then H2 gas was bubbled into the solution. As predicted by the computational study, a dramatic change in color and fluorescence of the solution was observed; its color and fluorescence changed from yellow to colorless, and from non-fluorescent to blue fluorescent respectively. The subsequent reverse oxidation proceeded smoothly when H2 gas was exchanged for O2 gas, and the solution reverted back to its original state.

“When we performed a detailed analysis, it was revealed that the resultant changes in color and fluorescence were caused by two different molecular states. The prediction of these states, and our ideas about this phenomenon, were strongly supported by both the computational analysis and the experimental results,” said Associate Professor Ishikawa. “This molecular switching technology of an aromatic compound using an orthoquinone moiety is a new insight that appears to have been reported first by our research team.”

An important advantage of this technology is that it is environmentally friendly since the byproduct of the reaction is simply water. Additionally, the synthetic PACs don’t experience very much damage after each reaction meaning that the molecular switch has excellent reusability.

“We have considered a wide range of future applications for this molecular technique,” said Associate Professor Masaki Matsuda, a research collaborator from Kumamoto University. “For example, we can put this molecular sheet into a package of food filled with an inert gas to check whether oxygen, which promotes the spoilage of food, has entered the package. All that would be required is a simple check under a UV light; the package wouldn’t even have to be opened. Organic semiconductors and OLEDs could also benefit from the ability to control optical properties using energy from gases. For example, organic semiconductors could be made to change their electrical properties, and OLEDs could show on/off switching characteristics by using the energy from gas that is supplied to it. The applications for this technology are numerous.”

The findings of this research were published in the Angewandte Chemie International Edition, online edition, on May 4th, 2016.