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      Optical determination of crystal phase in semiconductor nanocrystals

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          Abstract

          Optical, electronic and structural properties of nanocrystals fundamentally derive from crystal phase. This is especially important for polymorphic II–VI, III–V and I-III-VI 2 semiconductor materials such as cadmium selenide, which exist as two stable phases, cubic and hexagonal, each with distinct properties. However, standard crystallographic characterization through diffraction yields ambiguous phase signatures when nanocrystals are small or polytypic. Moreover, diffraction methods are low-throughput, incompatible with solution samples and require large sample quantities. Here we report the identification of unambiguous optical signatures of cubic and hexagonal phases in II–VI nanocrystals using absorption spectroscopy and first-principles electronic-structure theory. High-energy spectral features allow rapid identification of phase, even in small nanocrystals (∼2 nm), and may help predict polytypic nanocrystals from differential phase contributions. These theoretical and experimental insights provide simple and accurate optical crystallographic analysis for liquid-dispersed nanomaterials, to improve the precision of nanocrystal engineering and improve our understanding of nanocrystal reactions.

          Abstract

          Identifying crystallographic phases in solution is not possible with standard diffraction methods. Here, Lim et al. demonstrate the in situ identification of cubic and hexagonal phases of cadmium selenide nanocrystals using optical methods based on first-principles electronic theory.

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          Most cited references 65

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          Generalized Gradient Approximation Made Simple.

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            Experimental Determination of the Extinction Coefficient of CdTe, CdSe, and CdS Nanocrystals

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              Shape control of CdSe nanocrystals

              Nanometre-size inorganic dots, tubes and wires exhibit a wide range of electrical and optical properties that depend sensitively on both size and shape, and are of both fundamental and technological interest. In contrast to the syntheses of zero-dimensional systems, existing preparations of one-dimensional systems often yield networks of tubes or rods which are difficult to separate. And, in the case of optically active II-VI and III-V semiconductors, the resulting rod diameters are too large to exhibit quantum confinement effects. Thus, except for some metal nanocrystals, there are no methods of preparation that yield soluble and monodisperse particles that are quantum-confined in two of their dimensions. For semiconductors, a benchmark preparation is the growth of nearly spherical II-VI and III-V nanocrystals by injection of precursor molecules into a hot surfactant. Here we demonstrate that control of the growth kinetics of the II-VI semiconductor cadmium selenide can be used to vary the shapes of the resulting particles from a nearly spherical morphology to a rod-like one, with aspect ratios as large as ten to one. This method should be useful, not only for testing theories of quantum confinement, but also for obtaining particles with spectroscopic properties that could prove advantageous in biological labelling experiments and as chromophores in light-emitting diodes.
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                Author and article information

                Journal
                Nat Commun
                Nat Commun
                Nature Communications
                Nature Publishing Group
                2041-1723
                17 May 2017
                2017
                : 8
                Affiliations
                [1 ]Department of Bioengineering, University of Illinois at Urbana-Champaign , 1270 Digital Computer Laboratory MC-278, Urbana, Illinois 61801, USA
                [2 ]Micro and Nanotechnology Laboratory, University of Illinois at Urbana-Champaign , 208 North Wright Street MC-249, Urbana, Illinois 61801, USA
                [3 ]Intelligent Devices and Systems Research Group, DGIST , 333 Techno Jungang-Daero, Hyeonpung, Daegu 42988, Republic of Korea
                [4 ]Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign , Materials Science and Engineering Building MC-201, 1304 West Green Street, Urbana, Illinois 61801, USA
                Author notes
                Article
                ncomms14849
                10.1038/ncomms14849
                5442309
                28513577
                Copyright © 2017, The Author(s)

                This work is licensed under a Creative Commons Attribution 4.0 International License. The images or other third party material in this article are included in the article's Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder to reproduce the material. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/

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