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Meeting Day Two: Tuesday, July 2, 2013

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While the rain fell off and on in Singapore on Tuesday, the 2,500 delegates of the 2013 ICMAT were snug inside the Suntec Convention Center, with plenty to keep their interest. Two Plenary Lectures in the morning were followed by a Theme Lecture in the afternoon. The 30 symposia continued to provide forums for discussion of the latest scientific developments in many areas of materials science. Meanwhile, the exhibitors demonstrated their latest equipment and proud students showed off the posters describing their research.

But that was just during the day. The evening found busloads of scientists being carried to the National University of Singapore campus to hear public lectures by Yuan-Tseh Lee and Alan Heeger, the two Nobel Laureates honoring us by sharing their experience at ICMAT 2013. Joined by college students and other members of the public in the University Cultural Center, the scientists enjoyed Dr. Lee’s talk on the sustainability of the planet, and Dr. Heeger’s lecture on the creativity, discovery, and risk that can make scientific research so exciting.



Large video images of the 2013 ICMAT Plenary and Theme Lecturers
grace the wall outside the Suntec Convention Center--the "Wall of Fame"


Meeting Highlights

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Second Plenary Lecture

From Molecular Switches to Molecular Machines

J. Fraser Stoddart, Northwestern University

 

Stoddart is well-known in both the fields of supramolecular chemistry and nanotechnology. Currently, he is combining the two to create molecular switches based on molecular shuttles. The rotaxane shuttle contains tetrathiafulvalene (ttf) and 1,5-dioxynaphthalene (dnp) stations along the backbone that interact with a cyclobisparaquat(p-phenylene) macrocycle. Movement of the macrocycle ring depends on the relative stability of reversible, non-covalent interactions of the ring with the ttf and dnp units. By oxidizing the system, the ring moves from the ttf, the more stable state, to the dnp. In solution, the equilibrium of ttf:dnp binding is 150:1. Once oxidized to the dnp-bound state, the system quickly decays back to the ttf-bound state.

 

In order for these molecular switches to make useful devices, the switch needs to move from solution to metal-bound systems. For this, the group attached the rotaxane wires to a flat gold surface, and, through cyclic voltammetry, determined the activation energy for shuttling the ring from ttf to dnp to be 17.7 kcal mol-1. Next, molecular tunnel junctions were assembled to form molecular RAM by attaching Ti and polysilicon electrodes to either side of the rotaxane. The standard 0 and 1 bits were determined by the ground state (ttf-bound) and metastable state (dnp-bound) conformations, respectively. A large density of 1011 bits per cm2 was achieved. The main drawback is the non-robustness of these systems, which die between 25 and 100 cycles. To overcome this, the group experimented with polymer rotaxanes, but encountered the same problem. Now they are looking into incorporating the molecular switches into metal organic frameworks, known for being extremely sturdy.

 Third Plenary Lecture

Pulsed Laser Deposition: God’s Gift to Complex Oxides Creating New States of Matter with Oxide Heteroepitaxy
Ramamoorthy Ramesh, Associate Director of Oak Ridge National Laboratory

In the area of complex oxides research, “pulsed laser deposition is the key—the hero in the movie,” said Ramamoorthy Ramesh at the start of his talk. Complex oxides come in the form of pyrochlores, layered structures, spinels, rock salt, and other configurations, and act as high-temperature superconductors, ferroelectrics, and colossal magnetoresistance materials, to name a few. They have a huge number of degrees of freedom, including charge, lattice, orbital, and spin; Ramesh and his colleagues are trying to add epitaxy to this list.

Using pulsed laser deposition, the researchers hope to be able to switch ferromagnetic domains using electricity instead of magnetism.They are focusing on multiferroics, which combine magnetic and ferroelectric properties, to do the job. Bismuth ferrite, BiFeO3, is the main target material. It is a classic perovskite that has been highly studied. In the 111 plane, BiFeO3 is magnetically coupled; adjacent parallel planes are antiferromagnetic. Spin orbit coupling leads to a slight canted moment of the planes, skewing  the planes from a parallel orientation by just a few degrees.

Pulsed laser deposition allows them grow a material with great control at the half-unit-cell level, showing “just how profound epitaxy can be,” Ramesh said. “We can play with interface chemistry at the 2 angstrom scale.” Depositing a 3-nm-thick layer of lanthanum strontium manganite (LSMO) on a BiFeO3 substrate and hitting it with a laser beam can reveal what happens to the magnetic state of Fe at the interface. The researchers found that the BiFeO3 at the interface has a magnetic moment that is 20 times larger than the canted moment of BiFeO3. The conclusion: interface magnetism is quite different!

Depositing Co0.90Fe0.10 on BiFeO3 has enabled Ramesh and his colleagues to examine the magnetic structure domain by domain. In each domain, the moment in Co0.90Fe0.10 is parallel to the projection of the canted moment of BiFeO3. SEM polarization analysis (SEMPA), which provides a vector analysis of CoFe magnetization, shows that the magnetic domain structure and the ferroelectric domain structure are the same. This makes out-of-plane, reversible switching of magnetism possible, as is needed for a spin valve. Switching the ferroelectric field produces changes in the resonance field due to the magnetic properties of Co. The researchers believe that the canted moment in each domain changes by approximately 180 degrees as the polarization switches by 180 degrees. “If we can use this coupling,” Ramesh said, “we can get true electric field control of magnets.”


Second Theme Lecture

Nanogenerators as New Energy Technology and Piezotronics for Functional Systems
Zhong Lin Wang, Georgia Institute of Technology and Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences

Zhong Lin Wang defines nanoenergy as “the energy required for the sustainable, maintenance-free, and self-powered operation of micro/nano electronics.” He and his team are investigating many different ways to produce and use nanoenergy for implantable sensors, remote patient monitoring, and environmental monitoring, among other applications.

In 2004-2005, the first paper on using single-nanowire ZnO as a self-powered system was published. The paper showed that the non-central symmetry of ZnO produces a piezopotential along the axis of the nanowire. Under strain, electrons flow in ZnO nanowires in the form of alternating current.

Much of Wang’s research involves making devices that take advantage of the triboelectric effect, a type of contact electrification. In one example, he worked with polymer systems such as PET/PMMA and Kapton to obtain an open circuit voltage (Voc). Wang showed a series of videos in which nanogenrators produced a voltage through a variety of simple processes. These included an LED in a shoe that is self-powered by the motion of the walking foot; a series of LEDs that light up with the pressure of air passing through a channel (demonstrated onstage by Wang blowing into a small device); and a foot tapping on a pedal to produce small amounts of electricity. The vibration of a car engine, a finger tapping on a sofa, and the vibration of a table when it is kicked are other examples Wang noted in which energy is not being captured.

“We have many activities like this in our daily lives,” Wang said, "and we should take advantage of the potential for energy generation.”

He gave many more examples where contact, sliding, or rotation could be used to generate electricity. He also showed how nanogenarators could be used a self-powered, active sensors that could detect the landing of a feather, the rolling of an eyeball in its socket, or for mercury ion detection.

Finally, Wang discussed “piezotronics,” a term he coined to described electronic devices powered by the piezopotential of certain non-central symmetric materials—ZnO, GaN, InN, etc.— under strain. Piezotronics have the advantages of miniaturization, portability, and functionality. As an example, he showed a rod that was half metal and half ZnO that acts as a strain-gated transistor. In tensile conditions the transistor is on; under compression it is off. Similarly, CdSe has been used in a vertical piezotronic transistor controlled by individual gates. This two-terminal transistor is individually addressable in a large array of transistors. Wang showed a 92 x 92 pixel device made of these transistors that can detect local applied pressure. He hopes eventually to be able to use piezotronic devices as embedded sensors for health monitoring, among many other applications.


First Public Lecture

Sustainable Development of Human Society

Yuan-Tseh Lee, 1986 Nobel Laureate in Chemistry

Lee started his public lecture by stressing the importance of the sun. “The burning sun brought about life on Earth,” he said. There was a time, only 300 years ago, when humans were a part of nature and “everything came from the sun.” But the advancement of science and technology has caused a “great divergence” that has led us away from nature. The Industrial Revolution brought machinery that lessened our reliance on the sun for fuel. Now the majority of our energy comes from fossil fuels.

With the explosion of production came added consumption, and with fervent consumerism came waste. Humans are leaving a tremendous footprint on Earth, as seen by the increase of drastic weather events, climate change, and fading ecosystems. The younger generation will be faced with attending to a failing Earth depleted of resources.

“Scientists alone can’t solve it,” Lee said. “A single nation alone can’t solve it.” He implored everyone to participate in finding a global solution. Our society must become sustainable for next generations to have a bright future. He reminded us that environmental problems don’t respect national borders and that all parts of our planet are intimately connected. He appealed to the scientists attending ICMAT 2013 to form global collaborations to develop new sources of energy, such as wind and solar. He finished by asserting that to sustain the human race we must get “back to nature, back to sunshine.”


Second Public Lecture

Creativity, Discovery, and Risk: Nobel Prizes Past and Future
Alan Heeger, 2000 Nobel Laureate in Chemistry

Alan Heeger started his public lecture by relating how he became a physical scientist at the age of four. Armed with a set of toy soldiers and a miniature cannon, he soon got tired of firing the flat stick projectiles that came with the kit, and switched to matches instead. He remembers the first one bursting into flame when the match hit the cement sidewalk, and his mother quickly putting an end to the fun. But not before he learned how tipping the cannon up at an angle changed the trajectory of the projectile—his first spark of scientific creativity.

“Perhaps the greatest pleasure of being a scientist,” he said, “is to have an abstract idea and then do a series of experiments which demonstrates that the idea was correct—to demonstrate that nature actually behaves as conceived in the mind of a scientist.”

Expounding on the theme of his talk, Heeger was quick to point out that creativity and discovery are related, but not the same. He gave a few examples:

  • Watson and Crick’s creativity was uncovering the secret of life; their discovery was the double helix structure of DNA.
  • Arno Penzias and Robert Wilson’s creativity was trying to make a low noise microwave amplifier; their discovery was the blackbody background radiation left over from the Big Bang.
  • Einstein’s creativity was the idea that curved space-time was equivalent to, and the origin of, gravity; his discovery came when light was actually observed to bend around the sun’s gravitational field during the solar eclipse of 1919.

Relating this concept to his own life, Heeger described how he and Alan MacDiarmid had sat around his office at the University of Pennsylvania years ago, speculating about how they might make a polymer that would conduct electricity. This was their creativity. Their discovery was the new field of semiconducting and metallic polymers that revolutionized the world of plastics.

In addition to creativity and discovery, risk is an integral part of the scientific process, Heeger said. In the beginning of the conductive polymer investigations, his physicist friends thought he was crazy to be working on polymers that were considered complex, impure materials that were poorly characterized. But it was the assumption of this risk that led to the Nobel Prize in Chemistry in 2000.

Conducting plastic substrates were the first step toward inexpensive, roll-to-roll fabrication of flexible electronic films. Heeger's passion to this day is trying to figure out how to makeinexpensive, printed, flexible solar cells to replace silicon-based photovoltaics. He also has started a medical device business that markets a microfluidics device call LiquidBiopsyTM for the early detection of certain cancer cells in the bloodstream.

Heeger closed by encouraging the young scientists in the audience to cherish creativity; to be bold and have the audacity to seek to discover; and to remember that creativity and discovery necessarily involve risk. “Dealing with that risk is part of the thrill of science,” he concluded.


Technical Program

Symposium C5.2: Hybrid Systems and Bioseparation

Hybrid Polymer Membrane Using Click Chemistry

Namita Roy Choudhury, University of South Australia

 

The incorporation of ionic liquids into polymeric membranes can result in membranes with high ionic conductivity, thermal stability, flexibility, and increased proton transport. To synthesize these materials, Choudhury uses thiol-ene click chemistry, which involves reaction of a carbon-carbon double bond with a thiol to form a carbon-carbon single bond with bound thiol groups. Typical ionic liquids, such as imidazolium, thiazolium, and pyridinium, are functionalized with a vinyl group and then photopolymerized in the presence of thiol to create rubbery cross-linked films. The ionic liquid is bound to the polymer membrane and does not leach out, as with physically mixed ionic liquid membranes.

 

  

 

 

Symposium H4: Metallic Nanostructures


Electrochemistry of Gold Nanoclusters and Their Use in Electrochemical Sensing

Dongil Lee, Yonsei University

 

Ultra-small Au nanoclusters, under 2 nm in diameter, exhibit molecular-like redox behavior. For typical sensing applications Au nanoparticles must be used along with a small-molecule redox mediator, but, in contrast to larger clusters, small nanoclusters exhibit their own trackable redox. Lee has taken advantage of this phenomenon to detect small biological molecules, such as ascorbic acid, uric acid, and dopamine, with sensitivities around 1.5 µA/µM. Clusters of just 25 Au atoms, passivated by thiol capping ligands, were enmeshed in a sol gel matrix and deposited on an Au electrode. In the presence of the analyte, the anodic current shifted by a detectable amount. However, the presence of ascorbic acid interfered with the detection of dopamine. But by changing the neutral thiol ligands to charged ligands, selective interaction with the dopamine cation was achieved.

 

  

 

 

Symposium J5.1: Low-Dimensional Nanomaterials II

Experimental Realization and Characterization of Silicene

Lan Chen, Chinese Academy of Sciences

 

Using molecular beam epitaxy, Chen and his group have grown silicene, the Si analogue of graphene, onto a Ag(111) surface. Although silicene has the characteristic honeycomb pattern of graphene, there are several distinguishing features. The most notable is the sp3 hybridization which gives silicene a slight buckled structure with bond angles of 101.7°. Because of this unique structure the Dirac cone, or the electronic band structure, is not circular as in graphene; the Dirac cone of silicene is warped into a hexagon. Silicene is expected to have a large spin-orbit coupling and display a quantum Hall effect. Chen also hinted at possible superconductivity, though he was clear to say more experimental work needs to be done on silicene.

 

Scanning the Meeting

 


  

  

  
  

  



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The Meeting Scene e-newsletter of the Materials Research Society (MRS) presents news from MRS and other conferences directly from the conference venue.

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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