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Bangalore, India

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News Article | February 28, 2017
Site: phys.org

A team of engineers and scientists from the NYU Tandon School of Engineering Department of Chemical and Biomolecular Engineering, the NYU Center for Soft Matter Research, and Sungkyunkwan University School of Chemical Engineering in the Republic of Korea report they have found a pathway toward the self-assembly of these elusive photonic crystal structures never assembled before on the sub-micrometer scale (one micrometer is about 100 times smaller than the diameter of a strand of human hair). The research, which appears in the journal Nature Materials, introduces a new design principle based on preassembled components of the desired superstructure, much as a prefabricated house begins as a collection of pre-built sections. The researchers report they were able to assemble the colloidal spheres into diamond and pyrochlore crystal structures - a particularly difficult challenge because so much space is left unoccupied. The team, comprising Etienne Ducrot, a post-doctoral researcher at the NYU Center for Soft Matter Research; Mingxin He, a doctoral student in chemical and biomolecular engineering at NYU Tandon; Gi-Ra Yi of Sungkyunkwan University; and David J. Pine, chair of the Department of Chemical and Biomolecular Engineering at NYU Tandon School of Engineering and a NYU professor of physics in the NYU College of Arts and Science, took inspiration from a metal alloy of magnesium and copper that occurs naturally in diamond and pyrochlore structures as sub-lattices. They saw that these complex structures could be decomposed into single spheres and tetrahedral clusters (four spheres permanently bound). To realize this in the lab, they prepared sub-micron plastic colloidal clusters and spheres, and employed DNA segments bound to their surface to direct the self-assembly into the desired superstructure. "We are able to build those complex structures because we are not starting with single spheres as building blocks, but with pre-assembled parts already 'glued' together," Ducrot said. "We fill the structural voids of the diamond lattice with an interpenetrated structure, the pyrochlore, that happens to be as valuable as the diamond lattice for future photonic applications." Ducrot said open colloidal crystals, such as those with diamond and pyrochlore configurations, are desirable because, when composed of the right material, they may possess photonic band gaps—ranges of light frequency that cannot propagate through the structure—meaning that they could be for light what semiconductors are for electrons. "This story has been a long time in the making as those material properties have been predicted 26 years ago but until now, there was no practical pathway to build them," he said. "To achieve a band gap in the visible part of the electromagnetic spectrum, the particles need to be on the order of 150 nanometers, which is in the colloidal range. In such a material, light should travel with no dissipation along a defect, making possible the construction of chips based on light." Pine said that self-assembly technology is critical to making production of these crystals economically feasible because creating bulk quantities of crystals with lithography techniques at the correct scale would be extremely costly and very challenging. "Self-assembly is therefore a very appealing way to inexpensively create crystals with a photonic band gap in bulk quantities," Pine said. More information: Colloidal alloys with preassembled clusters and spheres, Nature Materials, DOI: 10.1038/nmat4869 , http://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat4869.html


News Article | February 28, 2017
Site: www.eurekalert.org

BROOKLYN, New York - Colloidal particles, used in a range of technical applications including foods, inks, paints, and cosmetics, can self-assemble into a remarkable variety of densely-packed crystalline structures. For decades, though, researchers have been trying to coax colloidal spheres to arranging themselves into much more sparsely populated lattices in order to unleash potentially valuable optical properties. These structures, called photonic crystals, could increase the efficiency of lasers, further miniaturize optical components, and vastly increase engineers' ability to control the flow of light. A team of engineers and scientists from the NYU Tandon School of Engineering Department of Chemical and Biomolecular Engineering, the NYU Center for Soft Matter Research, and Sungkyunkwan University School of Chemical Engineering in the Republic of Korea report they have found a pathway toward the self-assembly of these elusive photonic crystal structures never assembled before on the sub-micrometer scale (one micrometer is about 100 times smaller than the diameter of a strand of human hair). The research, which appears in the journal Nature Materials, introduces a new design principle based on preassembled components of the desired superstructure, much as a prefabricated house begins as a collection of pre-built sections. The researchers report they were able to assemble the colloidal spheres into diamond and pyrochlore crystal structures - a particularly difficult challenge because so much space is left unoccupied. The team, comprising Etienne Ducrot, a post-doctoral researcher at the NYU Center for Soft Matter Research; Mingxin He, a doctoral student in chemical and biomolecular engineering at NYU Tandon; Gi-Ra Yi of Sungkyunkwan University; and David J. Pine, chair of the Department of Chemical and Biomolecular Engineering at NYU Tandon School of Engineering and a NYU professor of physics in the NYU College of Arts and Science, took inspiration from a metal alloy of magnesium and copper that occurs naturally in diamond and pyrochlore structures as sub-lattices. They saw that these complex structures could be decomposed into single spheres and tetrahedral clusters (four spheres permanently bound). To realize this in the lab, they prepared sub-micron plastic colloidal clusters and spheres, and employed DNA segments bound to their surface to direct the self-assembly into the desired superstructure. "We are able to build those complex structures because we are not starting with single spheres as building blocks, but with pre-assembled parts already 'glued' together," Ducrot said. "We fill the structural voids of the diamond lattice with an interpenetrated structure, the pyrochlore, that happens to be as valuable as the diamond lattice for future photonic applications." Ducrot said open colloidal crystals, such as those with diamond and pyrochlore configurations, are desirable because, when composed of the right material, they may possess photonic band gaps -- ranges of light frequency that cannot propagate through the structure -- meaning that they could be for light what semiconductors are for electrons. "This story has been a long time in the making as those material properties have been predicted 26 years ago but until now, there was no practical pathway to build them," he said. "To achieve a band gap in the visible part of the electromagnetic spectrum, the particles need to be on the order of 150 nanometers, which is in the colloidal range. In such a material, light should travel with no dissipation along a defect, making possible the construction of chips based on light." Pine said that self-assembly technology is critical to making production of these crystals economically feasible because creating bulk quantities of crystals with lithography techniques at the correct scale would be extremely costly and very challenging. "Self-assembly is therefore a very appealing way to inexpensively create crystals with a photonic band gap in bulk quantities," Pine said. This research was funded by the U.S. Army Research Office under a Multidisciplinary University Research Initiative (MURI) grant. About the New York University Tandon School of Engineering The NYU Tandon School of Engineering dates to 1854, the founding date for both the New York University School of Civil Engineering and Architecture and the Brooklyn Collegiate and Polytechnic Institute (widely known as Brooklyn Poly). A January 2014 merger created a comprehensive school of education and research in engineering and applied sciences, rooted in a tradition of invention and entrepreneurship and dedicated to furthering technology in service to society. In addition to its main location in Brooklyn, NYU Tandon collaborates with other schools within NYU, the country's largest private research university, and is closely connected to engineering programs at NYU Abu Dhabi and NYU Shanghai. It operates Future Labs focused on start-up businesses in downtown Manhattan and Brooklyn and an award-winning online graduate program. For more information, visit http://engineering. .


Hiremath U.S.,Center for Soft Matter Research
Tetrahedron Letters | Year: 2013

The first examples of mesogenic bis(N-salicylideneaniline)s (BSANs), wherein two lipophilic (half-disk shaped) entities are interlinked through the dihydroxydiformylbenzene core, were synthesized and characterized. In particular, three constitutional (positional) isomeric BSANs were prepared by the facile twofold condensation of 3,4,5-tris(alkoxy)anilines with 2,3-dihydroxyterephthalaldehyde, 4,6-dihydroxyisophthalaldehyde, and 2,5-dihydroxyterephthalaldehyde and their structures were established by elemental analyses, FT-IR, 1H NMR, and 13C NMR. Proton NMR experiments demonstrated their existence in enol-imine (OH) form solely. Polarizing optical microscopic, differential scanning calorimetric, and powder X-ray diffraction studies evidenced the occurrence of columnar mesomorphism in two sets of isomers. © 2013 Elsevier Ltd. All rights reserved.


Varshney S.K.,Center for Soft Matter Research
Liquid Crystals | Year: 2013

Exclusive synthesis and characterisation of new dinitrohexa(alkoxy)triphenylene compounds were carried out. Nitration of 2,3,6,7,10,11-hexa(alkoxy)triphenylene with fuming nitric acid yielded 2,3,6,7,10,11-hexa(alkoxy)-1,5-dinitrotriphenylene with regioisomer 2,3,6,7,10,11-hexa(alkoxy)-1,8-dinitrotriphenylene and that of 1-chloro-2,3,6,7,10,11-hexa(alkoxy)triphenylene yielded 1-chloro-2,3,6,7,10,11-hexa(alkoxy)-5,12-dinitrotriphenylene with regioisomer 1-chloro-2,3,6,7,10,11-hexa(alkoxy)-5,9-dinitrotriphenylene. These isomers were separated by column chromatography and the molecular structures were confirmed by spectroscopic and spectrometric analyses. The liquid crystalline properties were investigated by optical polarising microscopy, differential scanning calorimetry (DSC) and X-ray analysis. The compound 2,3,6,7,10,11-hexa(butyloxy)-1,5-dinitrotriphenylene is a crystalline solid, whereas 2,3,6,7,10,11-hexa(butyloxy)-1,8-dinitrotriphenylene exhibits columnar phases. The higher homologues exhibit hexagonal columnar mesophase with a wide thermal range. Interestingly, 2,3,6,7,10,11-hexa(alkoxy)-1,8-dinitrotriphenylene has the higher clearing temperature compared to its 2,3,6,7,10,11-hexa(alkoxy)-1,5-dinitrotriphenylene isomer. Similarly, 1-chloro-2,3,6,7,10,11-hexa(alkoxy)-5,9-dinitrotriphenylene is liquid and 1-chloro-2,3,6,7,10,11-hexa(alkoxy)-5,12-dinitrotriphenylene exhibits hexagonal columnar mesophase at room temperature. © 2013 Taylor & Francis.


Hiremath U.S.,Center for Soft Matter Research
Tetrahedron | Year: 2014

Two novel series of optically active dimers comprising cholesterol and biphenyl-4-yl 4-(n-alkoxy)benzoate cores interlinked though either an odd-parity/even-parity spacer have been prepared and characterized. They stabilize an extremely complex, frustrated liquid crystalline state viz., the twist grain boundary (TGB) phase with chiral smectic C structure, denoted as TGBC- phase, over a wide (50-110 °C) temperature range. Notably, the dimers with an odd-parity spacer show an additional frustrated liquid crystal phase namely, the blue phase (BP). The presence of such frustrated phases suggests that the synthesized dimers are characterized by high enantiomeric excess and strong molecular chirality. Thus, 12 new optically active, nonsymmetric dimers reported herein constitute new examples of rarely found strongly chiral, optically pure dimers showing frustrated liquid crystal phase over an adequately wide thermal range. © 2014 Elsevier Ltd. All rights reserved.


Shanker G.,Martin Luther University of Halle Wittenberg | V. Yelamaggad C.,Center for Soft Matter Research
Journal of Physical Chemistry B | Year: 2011

The results of our detailed studies pertaining to a relatively new class of low molar mass mesogens are presented. Four homologous series of optically active dimer-like mesogens, comprising cholesterol as the conventional pro-mesogenic core covalently tethered to a nonmesogenic salicylaldimine segment through a flexible spacer of varying length and parity, have been synthesized and evaluated for their thermal, electrical switching, electrochemical, and gelation properties. The thermal behavior, being the prime focus of this study, has been characterized by optical, calorimetric, X-ray diffraction, and electrical switching studies. In each series, the length of the even/odd-parity spacer is held constant, while the length of the terminal N-alkyl tail attached to nonmesogenic salicylaldimine core has been varied to gain insight into the fundamental correlation between structure and phase transitional properties. These compounds, with a few exceptions, exhibit liquid crystal phase(s); the identified phases are chiral nematic (N*), twist grain boundary (TGB), chiral smectic A (SmA), and chiral smectic C (SmC*) phases. The selective reflection property of the N* phase and ferroelectric behavior of the SmC* phase have been ascertained for some selected members. In general, the phase behavior shows a dependence on the length and parity of the central spacer as well as the length of the terminal tail. The odd-even effect has been prominently found in the clearing temperatures; the even-parity dimer-like compounds belonging to three different series exhibit higher values when compared to members of an odd-parity series. Thus, our study demonstrates that these new class of low molar mass materials behave analogous to liquid crystal dimers. Electrochemical behavior and gelation ability have been demonstrated for some selected materials. © 2011 American Chemical Society.


Prasad S.K.,Center for Soft Matter Research
Angewandte Chemie - International Edition | Year: 2012

Shape matters: Changes, which are brought about by irradiation, to the properties of a medium have been of immense interest not only in terms of basic science, but also for applications such as data storage media and molecular devices. In the light of a recent publication reporting a photodriven liquid to liquid crystalline transformation (see figure), an overview of similar transitions is presented. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


Krishnamurthy K.S.,Center for Soft Matter Research
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics | Year: 2013

The observation of two polarity-sensitive electrical responses found in the low-frequency (<1 Hz) regime of a square wave field is reported for an achiral rodlike smectic-C liquid crystal with negative dielectric and conductivity anisotropies and in the 90 -twisted configuration. The first involves a transient director modulation appearing at each polarity reversal and vanishing under steady field conditions. The instability is polarity sensitive, with the maximum distortion localized near the negative electrode instead of the sample midplane. This is inferred from the wave-vector orientation alternating in the two halves of the driving cycle between the alignment directions at the two substrates. Various electro-optic characteristics of this temporal phenomenon are also described. Following a similar observation in nematic liquid crystals, we associate the transient periodic order with the Carr-Helfrich mechanism assisted by quadrupolar flexoelectric polarization obtaining under electric field gradients. The second polarity-sensitive effect manifests in the relative shift of the periodic Fréedericksz pattern upon field reversal. The shift, which is linear in field for low fields, tends to saturate for large fields. It is interpreted as due to flexoelectric polarization associated primarily with the c director twist about the layer normal. A model involving a periodic wedgelike band, which has the twist localized within it and is flanked by two uniformly and transversely aligned regions, accounts for the flexoelectric shift of the optical pattern. © 2013 American Physical Society.


Hiremath U.S.,Center for Soft Matter Research
Liquid Crystals | Year: 2014

Twelve new constitutional bis(N-salicylideneaniline)s (BSANs) belonging to three different series have been prepared and characterised by the two-fold reaction of alkoxyanilines with 2,3-dihydroxyterephthalaldehyde, 4,6-dihydroxyisophthalaldehyde and 2,5-dihydroxyterephthalaldehyde. Their thermal behaviour and liquid crystallinity has been established with the help of polarising optical microscope, differential scanning calorimeter (DSC) and powder X-ray diffraction (XRD). Our study reveals this behaviour as determined by the nature of central core and the number of alkoxy tails. For example, the three series of compounds prepared are either non-mesomorphic or stabilised calamitic and discotic mesophases, such as nematic/smectic C and columnar (Col) phases. These materials represent the first examples of mesogenic BSANs exhibiting mesomorphism. © 2014 © Taylor & Francis.


Bramhaiah K.,Center for Soft Matter Research | John N.S.,Center for Soft Matter Research
RSC Advances | Year: 2013

Free-standing ultra-thin hybrid films of reduced graphene oxide (rGO) with Au, Ag and Pd nanoparticles are generated at an aqueous/organic interface by in situ chemical reduction and spontaneous assembly. The reduction is initiated at a 'bare' interface or a 'modified' interface in a single step or two-step synthetic strategy. The hybrid materials are characterized by UV-visible, infra-red and Raman spectroscopies, X-ray diffraction, scanning electron (SEM), transmission electron (TEM) and atomic force microscopies (AFM). UV-visible spectra confirm the presence of isolated metal nanoparticles grafted on to rGO layers and Raman spectra signal a charge transfer across the constituent metal nanoparticles and rGO in the hybrid material. SEM and AFM studies show that the morphology of the hybrid films constitutes a homogeneous dispersion of metal nanoparticles and rGO for reduction at the 'bare' interface, and a random grafting of metal nanoparticles on rGO for reduction at the 'modified' interface. A mechanism for the formation of the films is proposed that involves a simultaneous transport and reduction of GO sheets and metal precursor at the interface or a directed reduction of metal precursor on rGO surface, facilitated by external aids. The utility of these hybrid films as catalysts is exemplified in p-nitrophenol reduction. Our method provides a fast, simple and inexpensive route to obtain free-standing hybrid films of rGO with metal nanoparticles for various applications. © 2013 The Royal Society of Chemistry.

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