Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX
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Electronic and Geometric Structure, Optical Properties, and Excited State Behavior in Atomically Precise Thiolate-Stabilized Noble Metal Nanoclusters Published as part of the Accounts of Chemical Research special issue “Toward Atomic Precision in Nanoscience”. Christine M. Aikens*
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Department of Chemistry, Kansas State University, Manhattan, Kansas 66506, United States CONSPECTUS: Ligand-protected noble metal nanoclusters are of interest for their potential applications in areas such as bioimaging, catalysis, photocatalysis, and solar energy harvesting. These nanoclusters can be prepared with atomic precision, which means that their stoichiometries can be ascertained; the properties of these nanoclusters can vary significantly depending on the exact stoichiometry and geometric structure of the system. This leads to important questions such as: What are the general principles that underlie the physical properties of these nanoclusters? Do these principles hold for all systems? What properties can be “tuned” by varying the size and composition of the system? In this Account, we describe research that has been performed to analyze the electronic structure, linear optical absorption, and excited state dynamics of thiolate-stabilized noble metal nanoclusters. We focus primarily on two systems, Au25(SR)18− and Au38(SR)24, as models for understanding the principles underlying the electronic structure, optical properties, luminescence, and transient absorption in these systems. In these nanoclusters, the orbitals near the HOMO−LUMO gap primarily arise from atomic 6sp orbitals located on Au atoms in the gold core. The resulting nanocluster orbitals are delocalized throughout the core of these systems. Below the core-based orbitals lies a set of orbitals that are primarily composed of Au 5d and S 3p atomic orbitals from atoms located around the exterior gold−thiolate oligomer motifs. This set of orbitals has a higher density of states than the set arising from the core 6sp orbitals. Optical absorption peaks in the near-infrared and visible regions of the absorption spectrum arise from excitations between core orbitals (lowest energy peaks) and excitations from oligomer-based orbitals to core-based orbitals (higher energy peaks). Nanoclusters with different stoichiometries have varying gaps between the core orbitals themselves as well as between the band of oligomer-based orbitals and the band of core orbitals. These gaps can slow down nonradiative electron transfer between excited states that have different character; the excited state electron and hole dynamics depend on these gaps. Nanoclusters with different stoichiometries also exhibit different luminescence properties. Depending on factors that may include the symmetry of the system and the rigidity of the core, the nanocluster can undergo large or small nuclear changes upon photoexcitation, which affects the observed Stokes shift in these systems. This dependence on stoichiometry and composition suggests that the size and the corresponding geometry of the nanocluster is an important variable that can be used to tune the properties of interest. How does doping affect these principles? Replacement of gold atoms with silver atoms changes the energetics of the sp and d atomic orbitals that make up the nanocluster orbitals. Silver atoms have higher energy sp orbitals, and the resulting nanocluster orbitals are shifted in energy as well. This affects the HOMO−LUMO gap, the oscillator strength for transitions, the spacings between the different bands of orbitals, and, as a consequence, the Stokes shift and excited state dynamics of these systems. This suggests that nanocluster doping is one way to control and tune properties for use in potential applications.
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This Account will first describe a number of initial studies that led to our current understanding of the electronic structure of thiolate-protected noble metal nanoclusters, then summarize recent breakthroughs in understanding the effects of doping on nanocluster properties and elucidating the excited state behavior in nanoclusters. This Account will primarily focus on two gold−thiolate nanoclusters, Au25(SR)18− and Au38(SR)24, and their relatives that have been widely studied due to their high stability and intriguing properties. The
INTRODUCTION
Ligand-protected noble metal nanoclusters have numerous applications that range from bioimaging to catalysis to light harvesting.1−4 These nanoclusters are an important class of nanoparticles that can be prepared with atomic precision,1,2 which enables the close comparison of experimental properties with those predicted from theoretical computation, 5,6 especially when the geometric structure of the nanoclusters can be determined. Theory can then provide insight into the origins of the chemical reactivity or physical properties of the nanocluster. © XXXX American Chemical Society
Received: July 23, 2018
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DOI: 10.1021/acs.accounts.8b00364 Acc. Chem. Res. XXXX, XXX, XXX−XXX
Article
Accounts of Chemical Research principles learned from the study of these systems can be applied to other ligand-protected nanoclusters.
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BACKGROUND: GEOMETRIC AND ELECTRONIC STRUCTURE
Au25(SR)18−
One of the first nanoparticles whose X-ray crystal structure was determined was the Au25(SR)18− nanocluster. Its geometrical structure was independently determined by two experimental studies7,8 and one density functional theory (DFT) investigation.9 The nanocluster consists of a 13-atom, approximately icosahedral gold core covered by six −SR−Au−SR−Au−SR− gold−thiolate oligomer motifs that are often referred to as “staples” or “semirings” (Figure 1). Overall, the nanocluster possesses approximate Th point group symmetry although the SR groups lead to a reduction in symmetry to Ci.7,8
Figure 2. Kohn−Sham orbitals and orbital energies for Au25(SH)18−. Reproduced with permission from ref 11. Copyright 2008 American Chemical Society. Figure 1. Single crystal structure of Au25(SR)18− (R = CH2CH2Ph). (A) Approximately icosahedral Au13 core. (B) Au13 core with 12 exterior Au atoms. (C) Au25(SR)18− structure showing the oligomeric S−Au−S−Au−S units. Magenta, Au; yellow, S. Reproduced with permission from ref 7. Copyright 2008 American Chemical Society.
Akola et al. showed using mathematical projection techniques that the HOMO and LUMO orbitals could be considered to be P-like and D-like orbitals, respectively, due to their similarities with the usual spherical harmonics for p and d functions, where the capital letter indicates a “superatom” orbital that is delocalized throughout the nanoparticle core.9,10 Shortly thereafter, Aikens presented the Kohn−Sham orbitals of these systems (Figure 2), in which the P and D character could be visualized in the nanocluster core.11 The three highest occupied orbitals were shown to form a nearly triply degenerate set of P orbitals. The five lowest unoccupied orbitals all possess D character; these are split into a lowerenergy, nearly doubly degenerate set and a higher-energy, almost triply degenerate set as a result of ligand-field splitting (LFS) by the SR−Au−SR−Au−SR oligomers. The first two peaks in the absorption spectrum (Figure 3) arise primarily from transitions from the occupied P orbitals into the first and second sets of D orbitals. The strength of LFS depends on the geometry of the nanocluster. Initial time-dependent DFT (TDDFT) calculations on Au25(SR)18− using a BP86/TZP.4f optimized geometry predicted a difference in energy of 1.11 eV between peaks a and b (Figure 3) in the Au25(SR)18− absorption spectrum, whereas the experimentally determined splitting is 0.95 eV.7 Aikens found that DFT functionals that predict geometries in better agreement with the experimental crystal structure (in this case, LDA functionals rather than GGA or hybrid functionals) lead to a better prediction of the optical absorption spectrum.12 Thus, it is important to have a reasonable geometry for the nanocluster when comparing optical properties. For example, Aikens showed that a strategy of geometry optimization using the Xalpha functional (or other
Figure 3. TDDFT absorption spectrum for Au25(SH)18−. Reproduced with permission from ref 7. Copyright 2008 American Chemical Society.
LDA functional) followed by calculation of TDDFT using an asymptotically correct functional such as SAOP, yielded results in good agreement with experiment (with ∼0.15 eV underestimation of peak positions).12 Additional groups have also considered the appropriate level of theory required to treat gold nanoparticles (see, e.g., ref 13). Using a relatively simple charge-perturbed particle-in-asphere model, Guidez and Aikens later showed that this geometrical dependence of the peak splitting in the absorption spectrum can be understood to arise from changes in the radius of the core, which affects the electron density in the nanoparticle.14 Moreover, they demonstrated that the degree of splitting is expected to decrease as the nanoparticle size increases and the overall oligomer arrangement on the nanoparticle becomes more spherically symmetric.14 Another way of visualizing the electronic structure of the Au25(SR)18− system is using a molecular orbital diagram. For Au25(SR)18−, the P orbitals primarily arise from 6sp atomic orbitals from the Au core with other contributions from other atoms (Figure 4).7,11 Below this set, the orbitals primarily arise B
DOI: 10.1021/acs.accounts.8b00364 Acc. Chem. Res. XXXX, XXX, XXX−XXX
Article
Accounts of Chemical Research
Figure 6. Calculated TDDFT absorption spectrum of the D3 isomer of Au38(SCH3)24. Adapted with permission from ref 16. Copyright 2010 American Chemical Society. Figure 4. Kohn−Sham orbital energy level diagram for Au25(SH)18− indicating the relative contributions of the atomic orbitals. Reproduced with permission from ref 7. Copyright 2008 American Chemical Society.
were primarily composed of oligomer-based (Au 5d and S 3p) atomic orbitals. The first two peaks in the absorption spectrum of Au38(SR)24 (Figure 6, peaks a and b) were found to arise from intraband transitions among the delocalized 6sp orbitals, whereas peaks c and d had significant interband transitions arising from oligomer-based orbitals; peak d was predicted to be the strongest peak in the low energy (
