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Letter
Evolution of Excited State Dynamics in Periodic Au28, Au36, Au44 and Au52 Nanoclusters Meng Zhou, Chenjie Zeng, Matthew Y. Sfeir, Mircea Cotlet, Kenji Iida, Katsuyuki Nobusada, and Rongchao Jin J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b01597 • Publication Date (Web): 10 Aug 2017 Downloaded from http://pubs.acs.org on August 13, 2017
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Evolution of Excited State Dynamics in Periodic Au28, Au36, Au44 and Au52 Nanoclusters Meng Zhou,1 Chenjie Zeng,1 Matthew Y. Sfeir,2 Mircea Cotlet,2 Kenji Iida,3 Katsuyuki Nobusada,3 Rongchao Jin1* 1
Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA
2
Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, USA
3
Department of Theoretical and Computational Molecular Science, Institute for Molecular Science, Myodaiji, Okazaki,
444-8585, Japan, and Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University Katsura, Kyoto 615-8520, Japan *To whom correspondence should be addressed. Email:
[email protected].
Abstract: Understanding the correlation between the atomic structure and optical properties of gold nanoclusters is essential for exploration of their functionalities and applications involving light harvesting and electron transfer. Here, we report the femto-nanosecond excited state dynamics of a periodic series of face-centered cubic (FCC) gold nanoclusters (including Au28, Au36, Au44, and Au52), which exhibit a set of unique features compared to other similar sized clusters. Molecular-like ultrafast Sn→S1 internal conversions (i.e., radiationless electronic transitions) are observed in the relaxation dynamics of FCC periodic series. Excited state dynamics with near-HOMO-LUMO gap excitation lacks ultrafast decay component and only the structural relaxation dominates in the dynamical process, which proves the absence of core-shell relaxation. Interestingly, both the relaxation of the hot carriers and the band-edge carrier recombination become slower as the size increases. The evolution in excited state properties of this FCC series offers new insight into the structure-dependent properties of metal nanoclusters, which will benefit their optical energy harvesting and photocatalytic applications. TOC Graphic
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Atomically precise metal nanoclusters have emerged as a new class of material and attracted considerable research interests.1,2 The new optical and electronic properties have led to a number of exploratory studies for applications such as catalysis,3 optics,4-6 solar energy harvesting for photovoltaics7-9 and photocatalysis10-12. In terms of fundamental studies, correlating the optical/electronic properties with the nanocluster structures still remains challenging.13-20 In the research of structural control over thiolate-protected gold nanoclusters, different types of crystal structures have been obtained, such as face-centered cubic (FCC),21 body-centered cubic (BCC),22 and hexagonal-close packed (HCP) structures in gold nanoclusters of comparable sizes.23 Among them, FCC is a particularly interesting type owing to its exclusiveness in bulk gold and the emergence of much larger HOMO-LUMO gap (Eg)21 than the icosahedral counterpart1 (c.f. Eg=1.7 eV of FCC Au36(SR)24 vs. 0.9 eV of icosahedral Au38(SR)24). The FCC structure was previously predicted to be unstable in the cluster regime,1 thus the attainment of Au36(SR)24 indeed came as a surprise.21 With more FCC structures solved,24-26 there have been some studies to investigate their unique structure-dependent properties including the UV-vis absorption property and the Eg scaling law,25 DFT simulations.27,28 and X-ray absorption spectroscopic analyses.29,30 The FCC periodic series of nanoclusters, including Au28(SR)20, Au36(SR)24, Au44(SR)28 and Au52(SR)32 with a uniform spacing of Au8(SR)4, constitute an interesting system25 and a full understanding of their optical properties is of particular importance for applications in optics and solar energy harvesting. Unlike conventional gold nanoparticles in which surface plasmon resonance dominates the optical response,31 gold nanoclusters (e.g. < 2 nm) shows strong quantum confinement effect with manifestations in the discrete UV-Vis absorption spectra,13,14 and both the metal core and the surface motifs contribute to the absorption spectra.20 Due to their discrete absorption and photoluminescence,6,14,20 gold nanoclusters may be treated as semiconductors (e.g. the size-dependent Eg).32 For thiolate-protected gold nanoclusters, the inner gold atoms form a highly symmetric stable core, whereas the surface motifs (e.g. dimeric or trimeric staples containing both gold and thiolate) are defined as the shell.1 Femtosecond time-resolved transient absorption and fluorescence up-conversion experiments have shown that Au25(SR)18, Au38(SR)24 and doped nanoclusters of similar sizes exhibit strong core-shell coupling during their excited state deactivation.33-38 Under both near HOMO-LUMO gap (H-L gap) and higher energy excitations, one can observe core to shell charge transfer for Au25(SR)18 nanoclusters. Fluorescence upconversion experiments on Au25 and Au38 indicated that for both nanoclusters, ultrafast visible luminescence originates from the metal core while long-lived near infrared luminescence arises from the staple motif.34,35 For rod-shaped Au25 nanoclusters, no such core-shell relaxation was observed due to the absence of Aux(SR)x+1 staple motifs.35,39 In typical small molecules, relaxation from higher excited states to lower excited states occurs through ultrafast internal conversion (IC), i.e. Sn→S1.40,41 The relatively long lived higher excited state (lifetime ~1 ps) for Au25 suggests that they differ from both small molecules and nanoparticles.36 On the other hand, the FCC gold nanoclusters have shown different structural and optical properties compared to similar sized icosahedral nanoclusters, such as the much larger H-L gaps in FCC counterparts,25 segregation of Au4 tetrahedra in FCC nanoclusters,26 and smaller Au-Au bond length in Au4 units (e.g. 2.79 Å) than the 2.88 Å bond length in bulk gold.26,29,30 Here, we probe the evolution in the excited state behavior of the periodic series of FCC gold nanoclusters by both nanosecond and femtosecond transient absorption spectroscopy. The photophysics of the FCC nanoclusters has not been elucidated in previous studies and is of major importance if the potential of such materials is to be fulfilled in optoelectronics and light harvesting applications. The Au28(SR)20,
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Au36(SR)24, Au44(SR)28, and Au52(SR)32 (SR= SPh-tBu) nanoclusters indeed exhibit some unique features of photophysics. Unlike the similar sized Au25(SR)18 and Au38(SR)24 nanoclusters which show core-shell type relaxations,33-38 the FCC Au28 and Au36 nanoclusters instead exhibit molecular-like ultrafast internal conversions from Sn to S1, and that Au44 and Au52 nanoclusters show similar photophysics but with the internal conversions slowing down with increasing size. Ultrafast transient absorption spectroscopy suggests that the primary photophysics of FCC nanoclusters derives from the metal core. Such insight helps to understand the electron energy flow in these FCC nanoclusters, which will benefit further work of advancing the applications of such nanoclusters in solar energy harvesting, optical sensing and imaging, and other applications. By looking into the photophysics of a periodic series of gold nanoclusters with the same FCC structure, we hope to i) seek an understanding of how the excited state dynamics evolves as a function of size and ii) how the atomic structure affects the dynamics. As shown in Figure 1A, the absorption spectra of all the four FCC gold nanoclusters exhibit discrete peaks. Uniform evolution can be clearly observed: as the size increases from Au28, Au36, Au44 to Au52, the lowest-energy absorption peak in the visible range shifts from 600 nm to 800 nm, while the peak around 400 nm remains almost constant. From the UV-vis spectra, the optical gap of Au28, Au36, Au44 and Au52 are determined to be Eg~1.77, 1.76, 1.51, and 1.39 eV, respectively. As the size of the metal core increases, the H-L gap decreases slowly and the absorption spectrum changes smoothly (Figure 1A), in contrast to other gold nanoclusters32 such as Au25 (1.3 eV) and Au38 (0.9 eV) in which the H-L gap evolves more rapidly with size. Theoretical calculations based on density function theory (DFT) gave the Kohn-Sham (KS) orbitals of these FCC nanoclusters (Figure 1B). The distributions of KS orbitals near the HOMO-LUMO become denser for larger nanoclusters, and the calculated trend of Eg agrees with the redshift of the absorption spectra. Moreover, the K-S orbitals of the FCC series (Figure 1B) near HOMO and LUMO are contributed by considerable amounts of S and other atoms on the surface, which indicates that the absorption spectra have both contributions from the core gold and surface shell. The same trend can be observed in the local density of states (LDOS) distribution of Au and S atoms (see Figure S1).
Figure 1. (A) UV-Vis absorption spectra of the periodic FCC nanoclusters (SR stands for SPh-tBu), and (B) Kohn-Sham
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orbitals of the series.
Similar to the trend in absorption spectra, the photoluminescence (PL) spectra of the FCC series show uniform evolution (Figure 2). Au28 and Au36 show PL peaks around 760 nm and 770 nm, respectively (Figure 2A, B); note: no PL was observed in the 850-1700 nm region with an InGaAs detector. Both Au44 and Au52 exhibit their PL peak around 950 nm (Figure 2C, D), while their PL bands below 850 nm (measured by a PMT detector) are much weaker compared to Au28 and Au36, thus, in the plots we make the visible bands merge with the NIR bands. As the size of the metal core increases from Au28/36 to Au44/52, the PL peak is redshifted, which agrees with the decrease of the H-L gap shown in the absorption spectra (Figure 1). The single fluorescence peak in the FCC nanoclusters is quite different from that of Au25(SR)18 nanoclusters in which multiple PL bands were observed.1,6,35
Figure 2. Photoluminescence spectra of (A) Au28, (B) Au36, (C) Au44, (D) Au52 nanoclusters with excitation at 360 nm (3.4 eV). For Au28 and Au36, a photomultiplier (PMT) was used as the detector. For Au44 and Au52, both PMT (
