Meeting Day One: Monday, July 1, 2013
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With a mighty strike of the golden gong onstage at the Suntec Convention Center in Singapore, Nobel Laureate Professor Yuan-Tseh Lee of Academia Sinica in Taiwan, and President of the International Council for Science, declared the official opening of the 2013 ICMAT on Monday morning. At this signal, the Lion Dancers, a tradition in Singapore, streamed into the auditorium and danced to ward off evil spirits, and bring good fortune, prosperity, and happiness to the proceedings.
What a way to start a scientific convention! With nine plenary lectures, four theme lectures, two public lectures, and 30 symposia covering the broad range of cutting edge materials science topics, the 7th International Conference on Materials for Advanced Technologies (ICMAT) is sure to provide a full week for all in attendance. That includes approximately 2,500 people from 55 countries who gathered here to discuss the latest developments in materials science. Under the inspired guidance of Professor B.V.R. Chowdari, who has led the organization of this meeting since its inception in July 2001, the attendees are guaranteed to have a great week of science and international fellowship. The Meeting Scene® will be here to report on the highlights every day, bringing the best of ICMAT2013 right to your email inbox. Stay tuned!
Professor Yuan-Tseh Lee sounds the gong to start the 2013 ICMAT while
Professor B.V.R. Chowdari (left) and Professor Eng Chye Tan look on.
Ceremonial Lion Dancer
Professor B.V.R Chowdari, Organizing Chairman of ICMAT 2013 and President of MRS Singapore welcomed guest of honor Yuan-Tseh Lee and Plenary Speaker Alan Heeger before extending his greetings and thanks to all those in attendance. He noted the major contributions to the organizing of ICMAT 2013 from the National University of Sinagpore, Nanyang Technological University, and the Agency for Science, Technology, and Research.
Chowdari then introduced Eng Chye Tan, Deputy President and Provost of the National University of Singapore, to give the Welcoming Address. Professor Tan recalled the year 2000, when Professor Chowdari first spoke about plans to hold a one-time ICMAT meeting in Singapore. Tan thought it was good timing because materials science was beginning to take off in Singapore, and is now proud to say that this “one-time” meeting is in its seventh incarnation as one of the largest conferences of its kind outside of the United States.
Guest of Honor Professor Yuan –Tseh Lee of Academia Sinica in Taiwan, and President of the International Council for Science-- a Nobel Laureate in Chemistry, no less--spoke next on a variety of themes, both whimsical and dire.
He began by observing the changes in the focus of materials scientists over the years. A while back the focus was on entrepreneurialism, before it switched to innovation. The current focus is on multidisciplinarity, according to Lee, and recently policy makers around the world are taking notice.
On the light side, he related his love of playing tennis into his seventies, and how materials science has made him better than he was in his sixties. “Nowhere does the development of materials science appear more quickly than in the sporting goods,” he said. From the wooden racket he started with in his younger years, to the aluminum alloy and carbon composite rackets of his middle years, he now uses a racket made of a piezoelectric material that gives him better control over both hard and soft shots. “Since my sixties, my tennis skills have gone down, but new tennis rackets compensate for it,” Lee said.
Finally, he turned to the topics of overpopulation and climate change that are doing so much damage to our planet. “We are stepping away from sunshine, stepping away from nature,” he said.
When he was young, “Earth was not too bad," Lee said. "Our generation will hand earth to the next generation and I’ll have to tell them that Earth is sick. It’s going down. I’m sorry. We didn’t take care of it.” He called for a change in consumption and production patterns, and urged us to be happy with less. A low carbon society could help take us back to the sunshine, back to nature. “Materials scientists can play an important role in connecting us to the sunshine again,” Lee concluded.
First Plenary Lecture
The Role of the Heisenberg Uncertainty Principle in Bulk Heterojunction Solar Cells
Alan Heeger, Nobel Laureate in Chemistry, University of California Santa Barbara
Professor Alan Heeger, winner of the 2000 Nobel Prize in Chemistry together with Alan MacDiarmid and Hideki Shirakawa for their discovery and development of conductive polymers, provided interesting insight into how the Heisenberg Uncertainty Principle can enable ultrafast charge transfer over distances of 10-20 nm in a nanoscopically textured material in bulk heterojunction solar cells. (Heeger is shown here with his wife Ruth.)
But before he got to that insight, he related some of the history that brought him and his colleagues to this discovery. They had been studying plastic solar cells, or “organic photovoltaics” (OPVs), for some time when the announcement that fullerenes had been discovered came in 1985. What if they mixed their semiconducting polymers with these fullerenes to see what happened?
The result was “a very exciting and interesting ride,” Heeger said, “and it’s not finished.”
It turns out that the fullerenes act as acceptors and the semiconducting polymers as donors in this system. In the 1990s and beyond, data obtained by various research groups provided a better picture of what was needed to produce viable OPVs. One group measured the electron transfer time at approximately 50 femtoseconds. Regarding the open circuit voltage (Voc), lots of electrons in the lowest unoccupied molecular orbital (LUMO) leave behind a lot of holes; to achieve a large Voc you need a deep highest occupied molecular orbital (HOMO) in the donor material. It became clear that self-assembly was needed to create a phase separation morphology with highly connective pathways throughout the OPV sample at the 10-20 nm length scale.
Of perhaps the most importance was the constant competition between sweeping the carriers out of the system and the recombination of electrons and holes. The sweeping out process gives energy to the external circuit while recombination quenches it. Ultrafast transfer of carriers is required.
The Heisenberg Uncertainty Principle provides the mechanism for ultrafast transfer. In the photogeneration process, the Uncertainty Principle generates a coherent, delocalized wavefunction, which lives long enough before collapsing that it profoundly affects the charge transfer process. We also know from Heisenberg’s Principle that we cannot know the location of the photoexcitation to an accuracy greater than about λ/4π ˃ 30 nm. The coherent quantum state given by the Uncertainty Principle yields immediate probabilities of ultrafast electron transfer to a nearby fullerene domain. Thus, ultrafast electron transfer is enabled by the Uncertainty Principle.
“That the Uncertainty Principle is responsible for ultrafast charge transfer is very, very pleasing to me,” Heeger said,” because the Uncertainty Principle is the cornerstone of quantum theory.”
First Theme Lecture
Semiconductor Nanowires for Optoelectronic Device Applications
Chennupati Jagadish, Australian National University, Canberra, Australia
Nanowires are the building blocks of the next generation of electronics and photonics, Professor Chennupati Jagadish contended in his Theme Lecture, “because nanowires overcome the fundamental limitation of lattice mismatch. We should be able to grow nanowires of any material on any substrate without lattice mismatch.”
Investigating III-V semiconductors, Jagadish and his colleagues used metalorganic chemical vapor deposition (MOCVD) to form nanowires. Given that there is always a competition between axial and radial growth of nanowires, sometimes the samples looked more like “nano-needles” with a broader base tapering to a point.
The researchers solved this problem by developing a two temperature process. First, they held the temperature at 450°C for one minute, then decreased the temperature to between 350 - 390°C to obtain prolonged growth. They formed cylindrical, atomically perfect nanowires using this method.
Taking this a step further, Jagadish next investigated core-shell nanowires consisting of an atomically perfect GaAs core clad in AlGaAs. By repeating the process to form a GaAs core with alternating layers of AlGaAs, GaAs, and another layer of AlGaAs, the researchers formed quantum well tubes. Samples of these tubes with 4-nm and 8-nm diameters produced light emission at 1.7 eV and 1.57 eV, respectively.
In terms of potential devices, Jagadish demonstrated optically pumped GaAs nanowire lasers with good photoluminescent intensity, making them “the smallest lasers in the world,” according to Jagadish. Nanowire solar cells offer the unique opportunity to decouple carrier collection and light absorption by controlling the height of the nanowires. For the flexible nanowire organic solar cells they are investigating, doping studies will be critical, Jagadish concluded.
Symposium B1: Emerging Topics
Organic Semiconductor LEDs and Solar Cells: the Role of Spin
Richard Friend, Cavendish Laboratory, University of Cambridge
Richard Friend discussed the role of spin in organic semiconductor materials. In organic light emitting diodes (OLEDs), both triplet and singlet excitons are formed upon charge injection. The singlet state is emissive, creating the characteristic glow, while the triplet state undergoes non-radiative relaxation. The triplet state is lower in energy and accounts for 75% of excited states; however, when there is a build-up of triplet states, two carriers can collide to create one singlet excited state, which then undergoes radiative decay. A positive relationship is seen between the build-up of triplet states and luminescence, and once the cell is saturated with triplet states a significant portion of emitted light is from triplet-triplet collisions. This is a common strategy for creating luminescence in commercial blue OLEDs.
For organic photovoltaics (OPVs) the reverse process is desired. While singlet states still undergo radiative decay, which in some cases can be reabsorbed by the cell, triplet charge-transfer states can relax to bound triplets on the donor, followed by facile charge separation and ending in photocurrent generation. In successful solar cells, triplet-triplet collisions are avoided. The singlet exciton state can break down to two triplet states, essentially generating two charge carriers for one photon. This happens extremely fast, in approximately 80 femtoseconds, and gives more than 100% charge collection.
A 3:1pentecene:PCBM blend was found to be ideal for limiting unfavorable triplet-triplet collisions. Building upon this charge-doubling process, a second semiconductor with half the band gap, lead selenide, was added to the blend to generate two photons from visible light and a third from infrared. The multiplying layer could potentially be incorporated into a silicon-based solar cell to improve absorption in the visible region, where silicon doesn’t fully absorb.
Symposium B2: Device & Materials Concepts I
Improved Materials and Device Architectures for Polymer Solar Cells
René Janssen, Technische Universiteit Eindhoven
In an invited talk, René Janssen discussed several methods his group is using to improve the efficiency of polymer-based organic photovoltaics (OPVs). The first method is to increase the amount of absorbed sunlight by adding a retroreflective foil over the cell. This foil is simply a textured PDMS surface with 1000 nm relief structures that look a little like sideways cubes. PDMS is optically clear, initially allowing light to pass through to the cell, and the 3D-structure refracts the reflected light back into the cell. By just “putting a piece of plastic in front of the cell,” as Janssen says, a 20% improvement in current and a 1% increase in power conversion efficiency (PCE) was achieved.
Controlling the active layer morphology is critical to achieve high quantum efficiency, or conversion of photons to excitons, and increase dissociation of charge transfer states into free carriers, needed to produce photocurrent. Nanocrystalline domains help in this effect; however, impurities caused by slight synthetic differences, such as a change in catalyst, ligand-to-catalyst ratio, temperature, or concentration, can have severe consequences in the performance of an OPV cell. The size of fibrils, formed by stacking of the conjugated material, plays a role in device efficiency, where thin fibrils on the order of 5 nm gave devices with PCEs above 7%, while thicker fibrils suffered low PCEs.
Tandem cells also increase OPV performance. In general, a wide band gap cell is followed by a small band gap cell to capture more of the incoming photons. Using PC60BM as the acceptor in the small band gap cell and PC70BM in the wide band gap cell, a PCE of 8.9% was achieved. They went a step farther and fabricated a pseudo-triple junction cell by duplicating the low band gap cell to get a PCE of 9.6%.
Symposium W1: Novel Materials and a Special Tribute to Professor Ma Jan
Professor Ma Jan was a beloved and dynamic young professor at Nanyang Technological University who died suddenly last June. Professor Freddy Boey, Deputy President and Provost of Nanyang Technological University, a close friend and colleague who watched Professor Jan's meteoric rise in the university, paid tribute to him with a warm and touching speech before this symposium, which was dedicated to Jan's memory.
Shape Memory and Superelastic Ceramics
Christopher Schuh, MIT
Christopher Schuh, a metallurgist, freely admitted that the words in the title of his talk didn’t normally go together, but then proceeded to tell the story of how he got interested in ceramics research, and the unexpected results he and his colleagues at MIT and Nanyang Technological University had achieved.
Not surprisingly, it was Professor Ma Jan, whose inspiring though too short life was being remembered in this special symposium, who continually posed the question to Schuh: What about ceramics? For a metallurgist working on shape memory alloys, the question seemed bizarre—how could brittle ceramics have anything to do with these shape restoring alloys? And why should a metallurgist care? But the more Schuh thought about it, the more intrigued he became, to the benefit of ceramics research everywhere.
Schuh knew this at the outset: the shape-memory process was a thermally driven, reversible martensite-to-austenite phase change that occurs in a surprisingly large number of metals—hundreds of them. Superelasticity is a similar phenomenon in which a load is applied at a constant temperature to obtain the same phase transformation. Strangely, of the hundreds of shape-memory alloys known, all are brittle except one. Polycrystalline NiTi, also known as Nitinol, is ductile enough to be used commercially as stents to open clogged arteries. The rest are brittle intermetallic alloys.
The brittleness comes from fracture at the grain boundaries, which place a significant constraint on phase transformations. So the researchers developed the idea to minimize the number of grain boundaries, creating what Schuh calls an “oligocrystal” with a few grain boundaries, as opposed to the many grain boundaries in polycrystals. The structure of a bamboo rod, with its circumferential boundaries placed far apart, is a demonstration of an oligo-type structure.
By reducing the number of grain boundaries, Schuh and colleagues were able to demonstrate shape-memory properties in previously brittle compounds such as CuZn and CuAlNi. “We took brittle metals and unlocked the shape-memory properties,” Schuh said.
But could this be done for ceramics? They began their investigation using brittle zirconia as a starting point. At low doping levels, zirconia undergoes a martensitic phase change from monoclinic to tetragonal, which causes cracking. So the researchers fabricated zirconia doped with ceria or yttria, with plans to control the transformation temperature by controlling the doping level. They then used focused ion beam milling to cut a pillar-shaped, oligocrystalline grain out of the bulk. The pillar bent during the austenite to martensite transformation; upon heating to the 450°C transformation temperature, the sample recovered 90% of its original shape, thereby demonstrating shape-memory properties. Superlasticity up to 3% strain was also observed on a different sample, with elastic properties surviving 60 or more cycles.
“A shape-memory ceramic is a new material in its own right,” Schuh concluded.
Symposium X3.2: MOFs & CPs – Properties & Applications -2
Metal Organic Frameworks for Clean Energy Applications
George Shimizu, University of Calgary
Proton exchange membrane (PEM) fuel cells have the potential to replace current alkaline fuel cells. George Shimizu from the University of Calgary is developing a metal-organic framework (MOF) proton exchange membrane that operates above 100°C, which enhances electrode kinetics and decreases electrode poisoning. Typically, water is used as the guest molecule for transport of a proton through the membrane, but this limits the operating temperature to under 100°C. Shimizu has focused on alternative carriers with higher boiling points, such as pyrazole and 1,2,4-triazole. Previously, his group developed an MOF called PCMOF-2, in which the pores are lined with sulfonate groups, which act as proton traps, with proton conductivities around 10-4 S/cm. For these systems to be viable for commercial applications, the conductivity must approach 10-2 S/cm. Incorporating a benzene triphosphonate bridging ligand into PCMOF-2 allowed more protons to be present in the pores and brought the conductivity up to 2.1x10-2 S/cm.
However, the altered PCMOF-2 is not very water stable, which is essential for fuel cell applications. So Shimizu’s next task was to develop a water stable MOF-based proton exchange membrane. To accomplish this, a benzene phosphonate bridging ligand was used to link Li3+ metal centers, called PCMOF-5. Under acidic conditions, protons can replace the metal centers to get a proton conductivity of 10-3 S/cm. No structure changes occurred after isolating the system in boiling water for a week, proving PCMOF-5 to be stable in aqueous conditions. The next challenge will be to combine the high conductivity of the altered PCMOF-2 with the water stability of MOF-5 to form a water stable proton exchange membrane with high conductivity.
Symposium AA1: Multifunctional Lab-on-a-chip Systems for Biological Imaging and Analysis
Integrated Microfluidic Platform for Multiplexed Enzymatic Bioassay with Serial Reagent Controls
Chia-Hung Chen, National University of Singapore
When epithelial ovarian cancer is detected in the first year, the survival rate is around 70%, but if detection doesn’t occur until the fifth year, the survival rate plummets to below 40%. Therefore, early detection is key to getting successful treatment. Chia-Hung Chen has designed a microfluidic device capable of diagnosing this disease without the need of a doctor, a so-called “clinical lab at home.” Patient samples are added to aqueous droplets that contain different reagents, each drop a kind of micro-test tube, which flow though the microfluidic cell in an oil carrier fluid. Certain proteases, released by the migration of cancer cells, are screened to create a fingerprint, warning of possible cancer. The matrix-like fingerprint is evolved based on the occurrence of a reaction within the droplet. By concentrating the droplets, high sensitivity can be achieved with greater than 90% accuracy.
Yuanjun Yan, National University of Singapore
Using microfluidics, Yuanjun Yan has developed an optical sorting method to separate fluorescent particles from non-fluorescent particles with 100% accuracy. The radiative pressure given off by fluorescing particles is capable of deflecting 10 µm, tagged polystyrene spheres at a 20° angle into a collection channel, while non-tagged particles are not deflected and continue past the collection channel and into the waste. To induce fluorescence and steer particle flow, a 352 nm optical fiber was integrated into the microfluidic device. Researchers plan to use this sorting mechanism with living cells, namely C. elegans, to increase the control and efficiency of numerous biological and behavioral studies.
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This work was partially supported by the IMI Program of the National Science Foundation under Award No. DMR 08-43934. Specifically, the work of Apprentice Science Reporter Jenna Bilbrey was funded under this NSF award, which is managed by the International Center for Materials Research (ICMR), University of California, Santa Barbara, USA. MRS thanks the NSF and the ICMR for their continued support, without which the Meeting Scene would not be possible.
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