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Article 38
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Photoluminescence Origin of Au (SR) and Au (SR) Nanoparticles: A Theoretical Perspective 22
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K. L. Dimuthu M. Weerawardene, Emilie Brigitte Guidez, and Christine M. Aikens J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b01958 • Publication Date (Web): 27 Jun 2017 Downloaded from http://pubs.acs.org on June 27, 2017
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The Journal of Physical Chemistry
Photoluminescence Origin of Au38(SR)24 and Au22(SR)18 Nanoparticles: A Theoretical Perspective K. L. Dimuthu M. Weerawardene,† Emilie B. Guidez,‡ and Christine M. Aikens†*
†
Department of Chemistry, Kansas State University, Manhattan, KS 66506, USA ‡
Department of Chemistry, Iowa State University, Ames, IA, 50011, USA
*
[email protected], 1-785-532-0954, fax: 1-785-532-6666
Abstract Photoluminescence of metal nanoparticles has drawn considerable research interest due to their potential fundamental and industrial applications in optoelectronics and biomedicine. However, the origin and underlying mechanism of photoluminescence in these clusters still need to be explored. Herein, the geometrical and electronic structural changes upon photoexcitation in the Au38(SH)24 and Au22(SH)18 nanoclusters are discussed using time-dependent density functional theory (TD-DFT) methods. Geometric relaxations in the Au23 core of Au38(SH)24 up to a maximum of 0.05 Å lead to slight electronic structure changes in the optimized singlet excited states with different state symmetries. The observed geometric and electronic structure variations upon photoexcitation are minor compared to the previously studied Au25(SH)18– nanoparticle. These small distortions can be correlated with small Stokes shifts calculated in the range of 0.06-0.09 eV, in comparison to 0.49 eV for the Au25(SH)18– nanoparticle. Compared to Au38(SH)24, the optimized first singlet and triplet excited states of Au22(SH)18 nanoparticle show larger structural flexibility in the Au7 core, which leads to significant electronic structure modifications and large Stokes shifts. These states are predicted to have microsecond-scale lifetimes, in agreement with available experimental data.
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Introduction Luminescence properties of gold nanoclusters (NCs) have drawn increasing research interest during the past decade. Their ultrafine size, good biocompatibility, and excellent stability can replace semiconductor quantum dots and organic dye molecules in a variety of biomedical applications including bio-imaging, sensing, and cancer therapy.1-7 Initially, atomically precise thiolate-protected gold NCs (Aun(SR)m) showed very weak visible to near-infrared (NIR) luminescence with a typical quantum yield (QY) of
