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Sheng W.,Materials Science and Engineering Program | Dore K.,Center for Neural Circuits and Behavior | Alhasan A.H.,University of California at San Diego | Grossman M.,University of California at San Diego | And 2 more authors.
ACS Nano | Year: 2014

Near-infrared (NIR) light-triggered release from polymeric capsules could make a major impact on biological research by enabling remote and spatiotemporal control over the release of encapsulated cargo. The few existing mechanisms for NIR-triggered release have not been widely applied because they require custom synthesis of designer polymers, high-powered lasers to drive inefficient two-photon processes, and/or coencapsulation of bulky inorganic particles. In search of a simpler mechanism, we found that exposure to laser light resonant with the vibrational absorption of water (980 nm) in the NIR region can induce release of payloads encapsulated in particles made from inherently non-photo-responsive polymers. We hypothesize that confined water pockets present in hydrated polymer particles absorb electromagnetic energy and transfer it to the polymer matrix, inducing a thermal phase change. In this study, we show that this simple and highly universal strategy enables instantaneous and controlled release of payloads in aqueous environments as well as in living cells using both pulsed and continuous wavelength lasers without significant heating of the surrounding aqueous solution. © 2014 American Chemical Society.

Park H.J.,Chonnam National University | Park H.J.,Korea Photonics Technology Institute | Bae H.J.,Chonnam National University | Park J.B.,Chonnam National University | And 7 more authors.
Journal of Vacuum Science and Technology B: Nanotechnology and Microelectronics | Year: 2016

This work presents the enhancement of the wall-plug efficiency in GaN-based vertical light-emitting diodes (LEDs) by splitting into nine LED cells. The cells are monolithically integrated and connected in series to develop high-voltage vertical LEDs (HV-VLEDs). Wall-plug efficiency at an input power of 1 W for the optimum-design HV-VLEDs is improved to 43.5% from the value of 40.6%, as compared to that of conventional VLED with the same active device area. The result indicates that the HV-VLEDs with uniform current spreading improve the conversion efficiencies of VLEDs and are expected to be more beneficial in larger-chip-size and higher-power LEDs. © 2016 American Vacuum Society.

McKittrick J.,State University of New York at Buffalo | Chen P.-Y.,Materials Science and Engineering Program | Tombolato L.,State University of New York at Buffalo | Novitskaya E.E.,Materials Science and Engineering Program | And 5 more authors.
Materials Science and Engineering C | Year: 2010

Some of the most remarkable materials in terms of energy absorption and impact resistance are not found through human processing but in nature. Solutions to the continuing problems of improved composite technologies may lie in replicating naturally occurring systems. In this review, we examine several mammalian structural materials: bones (bovine femur and elk antler), teeth and tusks from various taxa, horns from the desert big horn sheep, and equine hooves. We establish the relationships between structural and mechanical properties for these materials, with an emphasis on energy absorption mechanisms. We also identify the energy absorbing strategies utilized in these materials. Implementation of these bioinspired design strategies can serve as a basis for the design of new energy absorbent synthetic composite materials. Synthetic constituent materials arranged according to the principles outlined in this work will achieve the same synergistic effects as nature and no longer be confined to the limitations imposed by a mixture law. © 2010 Elsevier B.V.

Kumar B.,Materials Science and Engineering Program | Kubiak C.P.,Materials Science and Engineering Program
Journal of Physical Chemistry C | Year: 2010

Hydrogen-terminated p-type silicon was used as a photocathode for the selective photoreduction of CO2 to CO in the presence of Re(bipy-But)(CO)3Cl (bipy-But = 4,4′-di-tert-butyl-2,2′-bipyridine) as an electrocatalyst. The reduction of CO2 to CO on p-type silicon was achieved at a potential more than 600 mV lower than that required with a Pt electrode. A Faradaic efficiency of 97 ± 3% and an overall efficiency of 9.3% and 10% for the conversion of monochromatic and polychromatic light, respectively, to electricity were observed for the CO2 photoreduction process. A short-circuit quantum efficiency of 61% for light-to-chemical energy conversion was observed for the conversion of CO2 to CO. © 2010 American Chemical Society.

Abstract: Magnetic field-induced helical mode and topological transitions in a topological insulator nanoribbon Luis A. Jauregui1,2, Michael T. Pettes3†, Leonid P. Rokhinson1,2,4, Li Shi3,5 and Yong P. Chen1,2,4* 1 Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907, USA. 2 School of Electrical and Computer Engineering, Purdue University, West Lafayette, Indiana 47907, USA. 3 Department of Mechanical Engineering, University of Texas at Austin, Austin, Texas 78712, USA. 4 Department of Physics and Astronomy, Purdue University, West Lafayette, Indiana 47907, USA. 5 Materials Science and Engineering Program, University of Texas at Austin, Austin, Texas 78712, USA. †Present address: Department of Mechanical Engineering, University of Connecticut, Storrs, Connecticut 06269, USA. *e-mail: The spin-helical Dirac fermion topological surface states in a topological insulator nanowire or nanoribbon promise novel topological devices and exotic physics such as Majorana fermions. Here, we report local and non-local transport measurements in Bi2Te3 topological insulator nanoribbons that exhibit quasi-ballistic transport over ?2μm. The conductance versus axial magnetic flux Φ exhibits Aharonov-Bohm oscillations with maxima occurring alternately at half- integer or integer flux quanta (Φ0 = h/e, where h is Planck's constant and e is the electron charge) depending periodically on the gate-tuned Fermi wavevector (kF) with period 2π/C (where C is the nanoribbon circumference). The conductance versus gate voltage also exhibits kF-periodic oscillations, anti-correlated between Φ=0 and Φ0/2. These oscillations enable us to probe the Bi2Te3 band structure, and are consistent with the circumferentially quantized topological surface states forming a series of one-dimensional subbands, which undergo periodic magnetic field-induced topological transitions with the disappearance/appearance of the gapless Dirac point with a one-dimensional spin helical mode. Researchers have created nanoribbons of an emerging class of materials called topological insulators and used a magnetic field to control their semiconductor properties, a step toward harnessing the technology to study exotic physics and building new spintronic devices or quantum computers. Unlike ordinary materials that are either insulators or conductors, topological insulators are paradoxically both at the same time - they are insulators inside but conduct electricity on the surface, said Yong P. Chen, a Purdue University associate professor of physics and astronomy and electrical and computer engineering who worked with doctoral student Luis A. Jauregui and other researchers. The materials might be used for "spintronic" devices and practical quantum computers far more powerful than today's technologies. In the new findings, the researchers used a magnetic field to induce a so-called "helical mode" of electrons, a capability that could make it possible to control the spin state of electrons. The findings are detailed in a research paper that appeared in the advance online publication of the journal Nature Nanotechnology on Jan. 18 and showed that a magnetic field can be used to induce the nanoribbons to undergo a "topological transition," switching between a material possessing a band gap on the surface and one that does not. "Silicon is a semiconductor, meaning it has a band gap, a trait that is needed to switch on and off the conduction, the basis for silicon-based digital transistors to store and process information in binary code," Chen said. "Copper is a metal, meaning it has no band gap and is always a good conductor. In both cases the presence or absence of a band gap is a fixed property. What is weird about the surface of these materials is that you can control whether it has a band gap or not just by applying a magnetic field, so it's kind of tunable, and this transition is periodic in the magnetic field, so you can drive it through many 'gapped' and 'gapless' states." The nanoribbons are made of bismuth telluride, the material behind solid-state cooling technologies such as commercial thermoelectric refrigerators. "Bismuth telluride has been the workhorse material of thermoelectric cooling for decades, but just in the last few years people found this material and related materials have this amazing additional property of being topological insulators," he said. The paper was authored by Jauregui; Michael T. Pettes, a former postdoctoral researcher at the University of Texas at Austin and now an assistant professor in the Department of Mechanical Engineering at the University of Connecticut; Leonid P. Rokhinson, a Purdue professor of physics and astronomy and electrical and computer engineering; Li Shi, BF Goodrich Endowed Professor in Materials Engineering at the University of Texas at Austin; and Chen A key finding was that the researchers documented the use of nanoribbons to measure so-called Aharonov-Bohm oscillations, which is possible by conducting electrons in opposite directions in ring-like paths around the nanoribbons. The structure of the nanoribbon - a nanowire that is topologically the same as a cylinder - is key to the discovery because it allows the study of electrons as they travel in a circular direction around the ribbon. The electrons conduct only on the surface of the nanowires, tracing out a cylindrical circulation. "If you let electrons travel in two paths around a ring, in left and right paths, and they meet at the other end of the ring then they will interfere either constructively or destructively depending on the phase difference created by a magnetic field, resulting in either high or low conductivity, respectively, showing the quantum nature of electrons behaving as waves," Jauregui said. The researchers demonstrated a new variation on this oscillation in topological insulator surfaces by inducing the spin helical mode of the electrons. The result is the ability to flip from constructive to destructive interference and back. "This provides very definitive evidence that we are measuring the spin helical electrons," Jauregui said. "We are measuring these topological surface states. This effect really hasn't been seen very convincingly until recently, so now this experiment really provides clear evidence that we are talking about these spin helical electrons propagating on the cylinder, so this is one aspect of this oscillation." Findings also showed this oscillation as a function of "gate voltage," representing another way to switch conduction from high to low. "The switch occurs whenever the circumference of the nanoribbon contains just an integer number of the quantum mechanical wavelength, or 'fermi wavelength,' which is tuned by the gate voltage of the electrons wrapping around the surface," Chen said. It was the first time researchers have seen this kind of gate-dependent oscillation in nanoribbons and further correlates it to the topological insulator band structure of bismuth telluride. The nanoribbons are said to possess "topological protection," preventing electrons on the surface from back scattering and enabling high conductivity, a quality not found in metals and conventional semiconductors. They were fabricated by researchers at the UT Austin. The measurements were performed while the nanoribbons were chilled to about minus 273 degrees Celsius (nearly minus 460 degrees Fahrenheit). "We have to operate at low temperatures to observe the quantum mechanical nature of the electrons," Chen said. Future research will include work to further investigate the nanowires as a platform to study the exotic physics needed for topological quantum computations. Researchers will aim to connect the nanowires with superconductors, which conduct electricity with no resistance, for hybrid topological insulator-superconducting devices. By further combining topological insulators with a superconductor, researchers may be able to build a practical quantum computer that is less susceptible to the environmental impurities and perturbations that have presented challenges thus far. Such a technology would perform calculations using the laws of quantum mechanics, making for computers much faster than conventional computers at certain tasks such as database searches and code breaking. ### Note to Journalists: A copy of the research paper is available from Emil Venere, Purdue News Service, at 765-494-4709, The research and the team have been supported with funding from the U.S. Defense Advanced Research Projects Agency, Intel Corp., National Science Foundation, Department of Energy and the Purdue Center for Topological Materials. For more information, please click If you have a comment, please us. Issuers of news releases, not 7th Wave, Inc. or Nanotechnology Now, are solely responsible for the accuracy of the content.

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