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December 2015

Symposium CC: Organic Bioelectronics—From Biosensing Platforms to Implantable Nanodevices

Molly Stevens, Imperial College London

Materials-Based Approaches for Regenerative Medicine

Written by Maria Torres Arango

With cancer diagnosis and treatment a current major concern, characterization approaches in regenerative medicine—led by Molly Stevens (Imperial College London)—shows promising routes to diagnose, monitor, and do treatment of specific affected tissues. Nanoneedle patches are devices that that can sense the pH based on the fluorescence levels displayed by the needles in contact with healthy or tumorous cells. Moreover, through surface modification of the patches, protease enzyme mapping is also possible. In vivo delivery studies of organic molecules and inorganic nanoparticles into cells show very high transfer rates, and although the mechanistic studies are at an early stage, the high resolution patches prove to be effective. “I think what’s happening is that the cell is actually trying to fagocytose it, it is sort of trying to it up, which is why they [the molecules or nanoparticles] are going in so well, spontaneously,” is Stevens’ hypothesis. Regardless of what the mechanism may be, this research is of enormous importance not only in oncology but in many other biological sciences to study cell behavior.


Materials Theory Award

Mat-theory-lgSteven G. Louie, University of California, Berkeley

Understanding Excited-State Phenomena in Quasi-2D Materials

Written by Arthur L. Robinson

For a solid material, Steven G. Louie said, the quantum mechanics equation for the N-particle problem not only cannot be solved, its solution with 1023 coordinates would not be useful. As a theorist interested in ab initio calculations, Louie turned his attention to properties that can be measured. The spectroscopic properties as determined by excited-state behavior of a material, rather than the ground-state properties for which density functional theory is appropriate, typically give rise to a material’s defining attributes and determine its usefulness. These include electronic, optical, and transport characteristics that are essential in applications.

For his presentation, Louie divided spectroscopic properties into “N+1 particle” phenomena such as photoemission and tunneling that involve a single excited electron for which the electron self-energy is important (quasiparticles) and “N+2 particle” phenomena such as optical absorption for which electron–hole interactions must be included (excitons). He illustrated how these apply to typical devices like a photovoltaic cell which involves electron–hole creation by photon absorption, thermalization of the electron and holes by scattering, electron–hole pair transport by diffusion, and electron and hole separation that involve both exciton and quasiparticle properties.

Stepping deeper into theory, Louie discussed quasiparticle excitation, the problem of a single excited electron in a cloud of electrons. The energy spectrum (dispersion) now includes the electron self-energy, which includes contributions from electron–electron, electron–phonon, and possibly other interactions. A spectral function (probability of finding an electron with a given momentum and energy) is determined by the self-energy, which shifts and broadens the narrow energy-distribution peak obtained for non-interacting electrons. Calculating the spectral function involves interacting single-particle Green’s functions and what is called the GW approximation (keeping the first term in a perturbation theory expansion). Excitonic effects involving electron–hole interactions can be obtained using the GW approximation of the interacting two-particle Green’s function (GW-BSE approach).

With this background, Louie shifted to illustrating the necessity of including these many-particle effects in calculations of quasiparticle dispersion and lifetimes; optical absorption; exciton binding energies, wavefunctions, and radiative lifetimes; and forces in photo-excited states. For example, the theoretical bandgap in a large number of semiconductors does not agree with the experimental value unless the electron-self energy is included. Similarly, calculating the absorption spectrum of silicon requires electron–hole interactions to match experiment.

Nowadays, materials with small sizes and restricted geometry are of great interest owing to novel and useful properties that are either absent or not prominent in bulk materials and that arise from quantum confinement, enhanced many-electron interactions, reduced dimensionality, and symmetry effects. While graphene is the prototypical example of an atomically thin two-dimensional (2D) system, there are actually a large number of materials in which to study the effects of reduced dimensionality. Louie emphasized theoretical studies of quasi-2D systems, such as monolayer and few-layer transition metal dichalcogenides (e.g., MoS2, MoSe2, WS2, and WSe2) and metal monochalcogenides (such as GaSe) and mentioned the possibility of building van der Waals heterostructures from such materials. He concentrated on the electrical and optical spectra of MoS2. While it has the honeycomb structure of graphene, its lack of inversion symmetry combined with a strong spin-orbit coupling gives rise to new charge, spin, and valley degrees of freedom. Valley refers to energy minima at the vertices of the hexagonal Brillouin zone, which are no longer equivalent in MoS2.

As we by now expected, the self-energy and excitonic effects alter the band structure and absorption spectrum considerably with eV-level energy shifts. Large exciton binding energies also give rise to new exciton physics, such as quasi-2D screening that reverses the ordering of exciton energy levels relative to the hydrogenic model. Substrate screening is likewise important, shifting the exciton binding energy. Wanting his talk not to go overtime, Louie finished with a quick look at the results of excitons with a finite center of mass, in which electron–hole exchange interactions mix excitons in inequivalent Brillouin-zone minima, giving rise to what he called “massless” excitons!

The final message was that one size does not fit all; one must apply the appropriate theoretical treatment for the specific situations and properties.

[The Materials Theory Award, endowed by Toh-Ming Lu and Gwo-Ching Wang, recognizes exceptional advances made by materials theory to the fundamental understanding of the structure and behavior of materials. Steven G. Louie is honored "for his seminal contributions to the development of ab initio methods for and the elucidation of many-electron effects in electronic excitations and optical properties of solids and nanostructures."]


Science as Art Winners

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Science As Art Winners

FIRST PLACE

1    #55, Aleks Labuda, Asylum Research

2    #30, Liang Gong, University of Delaware

3    #43, Yi-Yeoun Kim, University of Leeds

4    #52, Bin Liu, Pacific Northwest National Laboratory

SECOND PLACE

1    #16, Tanja  Joerg, Montan Universitat Leoben

2    #54, Chenxi Qian, University of Toronto

3    #17,  Ali Jawaid, Air Force Research Laboratory

4    #24, Guillaume Schweicher, University of Cambridge

5    #38, Sana Mzali, Thales Research & Technology

6     #22, Hyosung An, Texas A&M University

7     #1, Jun Young Lee, Yonsei University

8    #11, Myfanwy Evans, TU Berlin

9     #39, Anna Maria Pappa, Ecole des Mines  St. Etienne

10    #48, Yixiao Zhang, Rutgers


Symposium X—Frontiers of Materials Research

Symp-x-lgChristopher Ober, Cornell University

Fifty Years of Moore’s Law: Towards Fabrication at Molecular Dimensions

Written by Arthur L. Robinson

Christopher Ober completed the week-long series of Symposium X presentations with a look at options for continuing the so-far remarkable adherence of the microelectronics industry to Intel co-founder Gordon Moore’s famous 1965 prediction that the number of transistors on a chip would roughly double every two years. Currently the state-of-the-art is 1.3 trillion transistors with 14-nm features. The projection photolithography process used to generate microchip circuit patterns now touches many areas of science beyond electronics, such as biology and the life sciences.

Two of the key aspects of photolithography are the source, currently a 193-nn argon fluoride laser, and the photoresist that is exposed by the laser shining through a mask with the desired pattern. Ober began with a discussion of current photoresists that change from being oil- to water-soluble when exposed, involving a process called chemical amplification because it so efficiently uses a single photon to drive many reactions. Structures made in this way today are the size of the machinery in living cells, but diffusion of protons generated in the resist during exposure is one factor limiting the resolution of the pattern.

To keep Moore’s Law intact and to approach molecular-scale pattern formation, Ober said, researchers are rethinking every aspect of the patterning process. The first serious alternative, directed self-assembly (DSA) puts lithography aside and instead harnesses the phase-separation behavior of block copolymers to create patterns defined by the nanostructure of the polymer. Ideal copolymers strongly phase separate and combine cleavable and crosslinkable blocks, such as polystyrene–polydmethlysiloxane (PS–PDMS), and photoactive compounds. While these can be used as photoresists, a technique called mixed solvent vapor annealing generates a variety of much higher-resolution self-assembled patterns from phase-separated block copolymer systems, but the kinetics are too slow for commercial processes. Laser spike annealing in which a PS–PDMS-coated wafer moves under a focused laser beam can drive directed self-assembly in about 10 ms. While there is progress toward making semiconductor patterns, the inability to make arbitrary patterns can be a disadvantage.

Extreme ultraviolet (EUV) lithography enables the production of arbitrary patterns at similar length scales. The process is similar to conventional projection photolithography except for the much shorter wavelength (currently expected to be 13.5 nm), the vacuum environment required, the need for reflective rather than refractive optics to avoid damage due to the high absorption at EUV wavelengths, and the need for new resists. At this short wavelength, absorption in the resist is at the atomic rather than the functional level, and usual resists are transparent. Ober described the possibility of a resist based on hybrid nanoparticles comprising metal (halfnium, zirconium, titanium, for example) oxide cores coated with carboxyl acid ligands. The metal oxide cores absorb the EUV light, the ligands enhance solubility, and the cores transfer the energy to the photoactive compounds in the resist. Chemical amplification is not involved in the patterning mechanism. Ober concluded that between directed self-assembly and EUV resists, Moore’s Law is safe for the next few years.

Chemical amplification is not dead, however. Ober observed that organic electronics do not currently require the ultrahigh resolution of the integrated-circuit industry. For these materials, resists with aqueous base “orthogonal” solvents, in contrast to the usual resist solvents, such as hydroxylic or non-polar organic solvents, provide a way to fabricate flexible, organic electronic and photonic devices employing chemically benign, environmentally-friendly process solvents. In a patterning demonstration experiment at a Cornell spin-off company with red, green, and blue light organic emitting diodes, the resolution exceeded that of commercial smart-phone displays and exhibited a longer lifetime.

Another instance of the usefulness of chemical amplifications is what Ober called vanishing or transient electronics, which function for a fixed time before dissolving. One application would be a network of disposable sensors to monitor toxic spills. To accomplish the disappearing act, a substrate containing arrays of cells, one type with rubidium spheres in a polymer and the other with packets of sodium hydrofluoride salt. When a valve that lets in air is opened, the rubidium oxidizes, releasing heat. The salt decomposes, releasing hydrofluoric acid that etches the electronics. Finally, the electronics and the substrate vaporize, leaving a thin-film nitride and salt residue, all in about one minute. Experiments with a polycarbonate polymer have demonstrated polymer evaporation.

Concluding his talk, Ober discussed the use of the laser annealing with block copolymers to give new life to chemically amplified systems by enhancing image formation. Experiments comparing hot-plate and laser heating of lithographically produced patterns indicated the latter enhances the quality of the pattern and uses less energy than the former, depending on the resist details, in particular the “leaving groups” they contain.


Thursday Best Poster Award Winners

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Paresh Parmar, Imperial College London
Collagen – Mimetic Peptide-Modifiable Hydrogels for Articular Cartilage Regeneration

Poster-q13-15-lgQ13.15
Manual Schweiger, University of Heidelber/University of Erlangen
Hundred Fold Enhancement of Photolumiescence from Aligned Single-walled Carbon Nanotubes by Polymer Transfer

 Poster-jj12-15-lgJJ12.15
Hee Taek Yi, Rutgers University
Reversible Tuning of Metal-Insulator Crossovers and Magnetism in SrRuO3 by Ionic Gating

Poster-rr9-03-lgRR9.03
Zhibo Zhao, MIT
Cathodoluminescence of High Indium Content InGan/GaN Quantum Wells


Government Agencies Presentations

In European-based government agencies as well as US-government-based agencies, funding opportunities are available for international collaboration in materials research. While science researchers might not typically look to NATO for funding, Michael Switkes—the science advisor for NATO’s Science for Peace & Security Program—said science actually serves as a diplomatic tool to forge partnerships between countries. Key priorities include emerging security challenges (e.g., counter-terrorism and cyber defense) and new developments and crisis prevention (e.g., security-related advanced technology). Switkes showed examples of a few projects such as a sensor system in the city that can detect explosives in order to identify suicide bombers. An international review panel consisting of members across disciplines evaluate project proposals. This unique panel will include scientists who may determine that the technology is feasible while a social scientist on the panel may point out the right-to-privacy issues that need to be resolved before giving the green light. International conferences are held, Switkes said, to determine emphases for research projects.

From the European Commission, James Gavigan walked the audience through the Horizon 2020 maze. He clarified the various programs within Horizon 2020 and pointed out a number of materials-related research projects that fall under the different programs, including advanced materials and nanotechnology for health and for energy, for example. The three top priorities are “excellent science,” “industrial leadership,” and “social challenges.” In the 2016–2017 budget, €16 billion will be available, with approximately 30% of all topics flagged for international collaboration, including US partnerships. See video for more information.

Thomas Christian from the US Air Force Office of Scientific Research (AFOSR) also talked about international collaborations. AFOSR has international offices in London, Tokyo, and Santiago, Chile. The agency’s mission is to probe today’s technology limits and develop future technologies with relevance to the US Department of Defense. Christian said AFOSR wants to identify breakthrough research opportunities in the States and abroad. AFOSR has supported research resulting in more than 200 patents since 2009, stating that Chad Mirkin (Northwestern University)—plenary speaker at this year’s MRS Meeting—had work funded by AFOSR.

Other US agencies represented at this Meeting were ARPA-E, Department of Energy’s Office of Basic Sciences, Office of Naval Research, and the National Science Foundation.


Engaged Learning of Materials Science and Engineering in the 21st Century (Symposium A)

Nobel laureate Harry Kroto, who is Professor Emeritus at the University of Sussex, opened Symposium A on materials education via Skype! Kroto spoke about a revolution in science education. The first one, he said, came with the printing press in the 15th century which increased the number of books available in Europe from tens of thousands to millions. Information became much more widely available. So now we’re experiencing another revolution due to development of the digital age, he said. A number of educators in Symposium A addressed how they are changing the education paradigm with use of the internet and social media.

In one example, Eric Mazur (Harvard University) is using a team-based, project-based approach in an introductory course to physics for pre-med and engineering students. The project assigned to each team of students, he said, has to be too difficult for any one student to do alone so that everyone feels a “social responsibility” to contribute. Instead of lectures, students receive reading assignments online where they use social media to annotate and discuss the text with one another. The students’ ownership of learning, Mazur said, is key to this paradigm.

Mazur’s approach of peer-instruction had inspired Steve Yalisove who has been developing this approach in his courses at the University of Michigan. Early this week, Yalisove led an interactive tutorial on student-centered learning (versus lectures). Yalisove also utilizes online communication and videos (including projects in which the students need to develop videos) in the coursework. Likewise, Jerry Floro (University of Virginia) assigns his students to develop videos that connect materials science to societal issues. His students’ videos have to include the structure-properties-processing component at the heart of materials research. In this way, Floro said, his non-science majors will still be able to differentiate materials science from other science disciplines years later.

While a good number of participants in the symposium are very excited about this student-centered approach, the question remains: How do we get department heads or the university to endorse this new paradigm and provide the necessary resources. Amy Moll of Boise State University described how she engages faculty as they work toward “creating a culture” that re-envisions a new approach to education.

Speakers at the symposium mentioned a number of books and web pages in their talks. A few are captured here:

Books

How learning works: Seven research-based principles for smart teaching, by S.A. Ambrose et al.

Chocolate model of change, by Diane Dormant

Creating significant learning experiences, by L. Dee Fink

Who owns the learning?, Alan November

Web pages

Geoset

Perusall

POGIL

Intro to Materials Science – Guided Inquiry


Color Changing Clothes

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Clothes that change color as you move seems like something out of an 80’s sci-fi movie. This idea is being revisited by Mathias Kolle at MIT. More than just being iridescent, the fibers he produces actually change color when stretched. Made of rolled up multilayers of polymer, the fibers reflect a small range of wavelengths based on how thick they are at the moment. Stretching them or bending them changes the thickness which produces a color that can range over the entire visual spectrum. These can be used for large, easily read strain gauges or as threads for sutures made by robots to give tension feedback to the surgeon, and of course they can make one fine looking shirt.

D8.11/E16.11 Biologically Inspired, Mechano-Sensitive, Colortunable Photonic Fibers