Atomically precise large-sized Au_n(SR)_m nanoclusters and scaling relationships among size, bandgap and excited-state lifetime

Abstract

The synthesis of atomically precise, large-sized Au_n(SR)_m nanoclusters (SR = thiolate) with n of 100 or more atoms has long been a challenge. In this work, we report the synthesis of Au₁₀₀(Napt)₄₂ and Au₁₀₂(IPBT)₄₄ nanoclusters (where Napt stands for 1-naphthalenethiolate and IPBT for 4-isopropylbenzenethiolate). Their optical absorption, single-electron charging, and femtosecond electron dynamics are investigated. Both Au₁₀₀(Napt)₄₂ and Au₁₀₂(IPBT)₄₄ exhibit multiple discrete absorption bands in their optical spectra owing to a quantized electronic structure in ultrasmall metal nanoclusters. Electrochemical analysis determines the precise HOMO–LUMO gaps of Au₁₀₀(Napt)₄₂ and Au₁₀₂(IPBT)₄₄ by single-electron charging. The electronic excited-state lifetimes of Au₁₀₀(Napt)₄₂ and Au₁₀₂(IPBT)₄₄ are determined by femtosecond transient absorption spectroscopy. The scaling relationship between the E_g and size (n) and the relationship of the excited state lifetime versus the E_g of large-sized Au_n(SR)_m are discussed. The findings provide a fundamental understanding of large-sized metal nanoclusters, which will benefit their future applications in optics, electronics, and other fields.

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Introduction

When the size of metal nanoparticles is reduced to ∼2.2 nanometers, distinct quantum confinement effects come into play, manifested in the emergence of a sizable HOMO–LUMO gap (E_g) and the disappearance of plasmon resonance.^1–3 Fundamental studies of the electronic and optical properties of such quantum-sized nanoclusters (NCs) require precise size control at the single-atom level.^4–8 In recent years, breakthroughs in the synthesis have led to the establishment of a library of atomically precise Au_n(SR)_m NCs.⁹ These Au NCs can be protected by ligands such as thiolate,¹ phosphine,² alkynyl,^6,10 N-heterocyclic carbene,^7,11 and halide.¹² The structural diversity of the Au kernel and Au–ligand interface, as well as the organic outer shell, provides great tunability of the functionality of NCs for diverse applications.^13–18

In the quantum-size regime (e.g., tens to hundreds of atoms per core), the material properties of gold NCs become very sensitive even to a small variation; for example, a single-atom variation can trigger structural transformation, and single-atom doping may induce significant changes to the optical absorption, photoluminescence, catalytic activity, and magnetic properties.^19–21 The same adamantanethiolate-protected Au₂₁(SR)₁₅, Au₂₂(SR)₁₆, Au₂₂Cd(SR)₁₆, and Au₂₄(SR)₁₆ series offers a neat platform for atom-by-atom evolution of small-sized NCs.²² Structurally, a single cadmium addition to Au₂₂(SR)₁₆ induces a surface reconstruction in Au₂₂Cd₁(SR)₁₆ as well as a kernel reconstruction from the Au₁₀ bioctahedron in Au₂₂(SR)₁₆ to the Au₁₃ cuboctahedron in Au₂₂Cd₁(SR)₁₆.²² The addition of metal atoms to the NC in an atom-by-atom manner also causes dramatic changes to the optical absorption, reflected in the distinct color changes to the NCs.²² Such a high sensitivity of the structural and optical properties to single-atom variations is due to the strong quantum confinement effect in small NCs. In addition to the optical effects, recent work on various crystalline phases of Au NCs also found that the electronic excited-state relaxation can be drastically varied by the metal core's phase,²³ with the hexagonal close-packed Au₃₀(SR)₁₈ and body-centered cubic Au₃₈S₂(SR)₂₀ NCs protected by the same type of ligand showing a three-orders-of-magnitude variation of the photoexcited carrier lifetime. On the other hand, for larger sized Au_n(SR)_m NCs, the sensitivity of properties to the variation in size (n) remains less studied, as more efforts on the synthesis of large-sized NCs are still needed.

In the investigation of how the optical properties of gold NCs evolve with increasing size (n, the number of gold atoms), Negishi et al. previously reported a series of Au_n(SC₁₂)_m NCs with n ranging from 38 to ∼520, in which the critical region—where a molecule-like electronic structure and a non-bulk geometric structure emerge—was determined to be between Au₁₄₄ and Au₁₈₇.²⁴ Higaki et al. determined the transition from nonmetallic Au₂₄₆ to metallic Au₂₇₉.^25,26 When taking the shape factor into consideration, Pei's group carried out theoretical work and reported that Au₂₆₄(SH)₉₆ is a critical size for the transition in cuboidal NCs.²⁷ Zhou et al. drew a grand picture of the three-stage evolution from non-scalable to scalable optical properties of Au_n(SR)_m NCs.²⁸ Nevertheless, the large-sized NCs with n ≥ 100 atoms still remain scarce and synthetically challenging. To further investigate the scaling relationship of the optical and electronic properties with the number of Au atoms in the NCs, more large-sized NCs should be synthesized and their electronic and optical properties should be investigated.

Herein, we report the synthesis, optical absorption and electronic properties of Au₁₀₀(Napt)₄₂ and Au₁₀₂(IPBT)₄₄ NCs. These NCs exhibit ultrasmall gaps (E_g < 0.5 eV), and precise determination of such gaps is not trivial.² Their steady-state and transient optical absorption properties are studied. Furthermore, we summarize the scaling relationships of the gap E_g vs. size (n), as well as the carrier lifetime vs. E_g in the large-size regime. The findings of this work provide a deeper understanding of the size evolution behavior of the electronic properties of NCs in the large-size regime.

Results and discussion

Synthesis and characterization of Au₁₀₀(Napt)₄₂ and Au₁₀₂(IPBT)₄₄

Au₁₀₀(Napt)₄₂ (Napt = 1-naphthalenethiolate) was prepared by a ligand-exchange reaction from polydispersed Au_x(2,4-DMBT)_y nanoclusters (2,4-DMBT = 2,4-dimethylbenzenethiol) at 80 °C overnight; of note, the Au_x(2,4-DMBT)_y crude product was prepared by NaBH₄ reduction of HAuCl₄·3H₂O in a two-phase solution (toluene/water). Separation of Au₁₀₀(Napt)₄₂ from side products was achieved by thin-layer chromatography (TLC); further synthetic details are provided in the ESI.† The high purity and molecular formula of Au₁₀₀(Napt)₄₂ are confirmed by matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ESI) mass spectrometry analyses. As can be seen in Fig. 1(a), the MALDI spectrum shows a very narrow peak at ∼25 kDa for Au₁₀₀(Napt)₄₂, indicating its high purity. Its precise mass was determined by ESI-MS (Fig. 1a, inset). A dominant peak of the molecular ion at m/z 8794.09 (M³⁺) was observed, along with a minor peak at 13 192.75 (not shown) corresponding to the 2+ ion. These ions are formed under the ESI conditions, as the native state of the NC is charge neutral. Both peaks match well with the calculated mass of Au₁₀₀(Napt)₄₂ (FW = 26 384.20 Da). The UV–vis spectrum of Au₁₀₀(Napt)₄₂ shows multiple absorption bands at 430, 520, 610, and 730 nm in the visible range (Fig. 1b) due to ultrasmall size-induced quantization of the electronic structure, as opposed to the quasi-continuous conduction band in metallic-state nanoparticles. All these bands are highly congested, which is typical of large-sized NCs due to their much denser electronic states compared to small sized NCs such as Au₂₅(SR)₁₈, where the peaks are well separated.⁹ The highly structured, molecular-like optical features in gold nanoclusters often indicate high purity, as size-mixed nanoclusters readily wash out such features and give rise to a featureless, decay-like spectrum.

Fig. 1 (a) MALDI and ESI (inset) mass spectra of Au₁₀₀(Napt)₄₂; (b) absorption spectrum of Au₁₀₀(Napt)₄₂.

We next discuss the case of Au₁₀₂(IPBT)₄₄, which was synthesized by a two-step, size-focusing method. Briefly, a poly-dispersed Au_x(IPBT)_y mixture was first obtained by the reactions of HAuCl₄·3H₂O with 4-isopropylbenzenethiol (IPBT) and reduction by NaBH₄. Further thermal etching of the poly-dispersed Au_x(IPBT)_y was conducted in toluene with 0.5 mL IPBT at 80 °C. After washing with methanol, pure Au₁₀₂(IPBT)₄₄ was collected. In MALDI-MS analysis, a single peak was observed at ∼25 kDa (Fig. 2a), demonstrating the high purity of the sample. The ESI spectrum gives two sets of peaks at m/z 8915.59 (3+) and 13 373.20 (2+), corresponding to the 3+ and 2+ ions, respectively. Both peaks are consistent with the expected formula weight of Au₁₀₂(IPBT)₄₄ (FW = 26 746.38 Da). Of note, the slight fragmentation within each set (Fig. 2a inset) is caused by the subtraction or addition of one IPBT ligand. The optical absorption spectrum of Au₁₀₂(IPBT)₄₄ shows peaks at 420, 505, 600, and 730 nm (Fig. 2b), which are comparable to the bands of Au₁₀₀(Napt)₄₂, indicating an intrinsic relationship (see below), in particular, given the fact that these two NCs differ by only two gold atoms and two ligands.

Fig. 2 (a) MALDI and ESI (inset) mass spectra of Au₁₀₂(IPBT)₄₄; (b) absorption spectrum of Au₁₀₂(IPBT)₄₄.

Structural insights

The highly structured optical absorption spectra of Au_n(SR)_m NCs reflect the metal core structures and can indeed serve as “fingerprints”.⁹ We find that the optical spectra of both Au₁₀₀(Napt)₄₂ and Au₁₀₂(IPBT)₄₄ are quite similar to those of previously reported Au₁₀₂(p-MBA)₄₄ and Au₁₀₃S₂(Napt)₄₁ (Fig. S1†) having solved structures.^29,30 This can be explained by the fact that all these four NCs (charge-neutral) possess the same number of nominal free-electron count, i.e., 58e, e.g., 100 (Au 6s¹) − 42 (monovalent SR ligand) = 58e. It should be noted that the S species in the Au₁₀₃S₂(Napt)₄₁ nanocluster localizes two electrons per S atom due to its divalent nature (S²⁻), so the free electron number = 103 − 2 × 2 − 41 = 58e. It is worth noting that the Au₁₀₂(IPBT)₄₄ nanocluster from the current work shares the same number of gold atoms and ligands (though hydrophobic) with the previously reported aqueous phase Au₁₀₂(p-MBA)₄₄ (where p-MBA = SPh-p-COOH), with the only difference being in the para-substitution on the benzene ring (i.e., isopropyl vs. –COOH). Based on the empirical rule that the kernel structure plays a decisive role in the optical absorption spectra,⁹ plus the fact that the four abovementioned NCs share the same 58e count, we deduce that Au₁₀₂(IPBT)₄₄ should have the same Au₇₉ Marks decahedral kernel, i.e., a shell-by-shell Au₇@Au₃₂@Au₄₀ arrangement, and the surface should be composed of 19 monomeric (SR–Au–SR) and two dimeric (SR–Au–SR–Au–SR) staple-like motifs as in the structure of its aqueous soluble Au₁₀₂(p-MBA)₄₄ counterpart.²⁹ With respect to Au₁₀₀(Napt)₄₂, we also deduce that the same Au₇₉ decahedron should be adopted, but the surface should comprise 21 monomeric (SR–Au–SR) staple motifs protecting the Au₇₉ kernel, similar to the pair of Au₁₀₂(p-MBA)₄₄ and Au₁₀₃S₂(Napt)₄₁ NCs, which share the same Au₇₉ kernel but slightly different surface motifs.^29,30

Compared to the previously reported aqueous Au₁₀₂(p-MBA)₄₄,²⁹ the attainment of organic soluble Au₁₀₂(IPBT)₄₄ in the current work allows for a precise determination of E_g and a detailed study on the single-electron charging energy, which was previously inaccessible with the aqueous Au₁₀₂(p-MBA)₄₄ (vide infra).

HOMO–LUMO gap determination and the scaling relationship with size

The 58e system with close sizes (n = 100, 102 and 103) and various ligands is a great platform for a comparative study of their electronic properties. Here, we carried out further electrochemical measurements. Differential pulse voltammetry (DPV) analysis indicates that both Au₁₀₀(Napt)₄₂ and Au₁₀₂(IPBT)₄₄ exhibit discrete electronic energy levels and rich charge states (Fig. 3). Following a meticulous method established by Wang and coworkers,³¹ the E_g of Au₁₀₀(Napt)₄₂ was deduced to be 0.40 eV from the electrochemical gap between the first oxidation and the first reduction potentials after subtracting the charging energy (i.e., 0.61₂–0.21₆ = 0.40 eV; the subscripts indicate single-meV accuracy³¹). Similarly, the E_g of Au₁₀₂(IPBT)₄₄ is 0.33 eV (i.e., 0.57₂–0.24₄ = 0.33 eV). The gaps of these two 58e NCs are similar to those of Au₁₀₂(p-MBA)₄₄ (0.45 eV)²⁹ and Au₁₀₃S₂(Napt)₄₁ (0.38 eV).³⁰ The similar E_g values of these four 58e NCs support the abovementioned rationale for using the same Au₇₉ kernel.

Fig. 3 Differential pulse voltammograms of (a) Au₁₀₀(Napt)₄₂ and (b) Au₁₀₂(IPBT)₄₄. The gaps of 0.612 and 0.572 eV are without subtraction of charging energy.

It is worth comparing the determined E_g of Au₁₀₀ and Au₁₀₂ NCs (quasi-spherical) with other NCs of slightly smaller or larger sizes, such as Au₉₂(TBBT)₄₄³² and Au₉₉(SR)₄₂,^33,34 and two rod-shaped NCs of Au₉₆(PET)₆₈ and Au₁₁₄(PET)₈₀.³⁵ Fig. 4 shows a plot of E_g versus the number of gold atoms (n) in the size range between 92 and 114, as well as those with larger sizes, including Au₁₃₀(SR)₅₀ (various types of ligands),^36–39 Au₁₃₃(SR)₅₂,^40,41 and Au₁₄₄(SR)₆₀.^42–44 One can see that the spherical (or nearly spherical) NCs exhibit a trend (Fig. 4, the black solid line is a guide). Interestingly, their E_g is generally smaller than that of the rod-shaped Au₉₆(PET)₆₈ and Au₁₁₄(PET)₈₀ by ∼0.27 eV (i.e., the gap between the top and lower lines, Fig. 4) at comparable sizes. We attribute this to two factors: (i) the shape-induced quantum effect on the electronic structure, i.e., the lower symmetry of quantum rods³⁵ leads to a stronger quantum confinement effect in the smaller dimension of the rods, and (ii) the higher ligand-to-gold ratio in rod-shaped NCs, e.g., the Au₉₆(PET)₆₈ rods possess many more ligands (∼43% higher) than Au₁₀₀(Napt)₄₂ and Au₁₀₂(IPBT)₄₄. The latter factor is expected to enlarge the E_g because more valence electrons (6s of gold atoms) are localized in the covalent bonds of Au–SR motifs.

Fig. 4 Scaling relationship between the E_g of large-sized Au_n(SR)_m NCs and the number of gold atoms (n).

Ultrafast electron dynamics in Au₁₀₀ and Au₁₀₂ NCs

Gold NCs with 100 atoms or more have ultrasmall energy gaps (E_g < 0.5 eV), which are hard to reach for other molecular materials; thus, such NCs are quite unique and worthy of investigation due to their electron dynamics. Here, we probe the excited-state dynamics of Au₁₀₀(Napt)₄₂ and Au₁₀₂(IPBT)₄₄ by performing femtosecond transient absorption (fs-TA for short) measurements. After photoexcitation at 400 nm, the NCs exhibit significant fast and slow decays in the relaxation process (Fig. 5a and b). Au₁₀₀(Napt)₄₂ and Au₁₀₂(IPBT)₄₄ show very similar TA spectra and relaxation dynamics, which further indicates that they should share a similar atomic structure (Au₇₉ kernel). Specifically, a distinct ground-state bleaching (GSB) signal is seen at ∼525 nm, which matches with the ground state absorption band in both NCs. Strong excited-state absorption (ESA) signals are observed at 580 and 675 nm. Overall, the similar GSB and ESA signals for both NCs also indicate their similar electronic structures.

Fig. 5 Transient absorption data map of (a) Au₁₀₀(Napt)₄₂ and (b) Au₁₀₂(IPBT)₄₄ pumped at 400 nm. Evolution associated spectra (EAS) obtained from global fitting of (c) Au₁₀₀(Napt)₄₂ and (d) Au₁₀₂(IPBT)₄₄.

Global analysis on the fs-TA data gives three decay components for both NCs (Fig. 5c and d). The fast relaxation can be fitted by two processes: the <1 ps process is assigned to the relaxation from the higher excited state to the lower excited state, while the few-picosecond process should be considered as the energy rearrangement process around the lowest excited state (ESI Fig. S2†). The slow process takes 266 ps for Au₁₀₀(Napt)₄₂ and 466 ps for Au₁₀₂(IPBT)₄₄, which are the relaxation from the lowest excited state to the ground state (i.e. exciton recombination). Based on the above observations, we can deduce that upon photoexcitation, transitions from the ground state to the excited state occur predominantly through core-based orbitals, and the excited core then relaxes via core phonons and hence exhibits similar relaxation dynamics.

Among the 58e NCs (organic soluble ones, including Au₁₀₀(Napt)₄₂ (E_g = 0.40 eV), Au₁₀₂(IPBT)₄₄ (E_g = 0.33 eV), and Au₁₀₃S₂(Napt)₄₁ (E_g = 0.4 eV)), a variation of ∼1.8 times (266 vs. 466 ps) can be seen in their excited-state lifetimes (ESI Fig. S3†). This is attributed to the surface effect because the excited state relaxation also involves the surface, while the E_g value and optical absorption peaks are primarily dictated by the kernel. When compared to the water phase Au₁₀₂(p-MBA)₄₄ (E_g = 0.45 eV), a 10-time increase in the excited lifetime is observed in Au₁₀₂(p-MBA)₄₄ (ESI Fig. S3†). Such a large effect was also found in the case of organic soluble Au₁₃₀(SR)₅₀ versus water soluble Au₁₃₀(SR)₅₀.^37,38 This distinct difference may be caused by the –COOH perturbation to the surface and/or the hydrogen-bonding effect in aqueous NCs.

Scaling relationship of the excited state lifetime with the energy gap

We further discuss the scaling relationship of the excited-state lifetime (τ) with E_g for large-sized Au_n(SR)_m NCs. Here, we focus on (quasi)spherical NCs since more sizes have become available.² The lifetimes of Au₁₀₀(Napt)₄₂ and Au₁₀₂(IPBT)₄₄ are much higher than that of the well-known Au₁₄₄(SR)₆₀ (a few ps)^42–44 due to their E_g difference as well as the ligand effects. Our results suggest that when E_g decreases (i.e., approaching the metallic state), the ratio of the fast decay process (on the ps scale) increases in the whole decay process in large-sized NCs such as Au₁₃₀, Au₁₃₃, Au₁₄₄ and Au₂₄₆ NCs,²⁸ which may be ascribed to the difficulty in separation between the fast decay (the relaxation from the higher excited state to the lowest excited state) and slow decay (the relaxation from the lowest excited state to the ground state) when the energy gap is so small. Thus, the excited-state relaxation cannot be treated as a sequential model since there is no population accumulation in the lower excited state for those Au NCs with very small E_g. By fitting the band gaps and excited-state lifetimes of Au₁₀₀(Napt)₄₂, Au₁₀₂(IPBT)₄₄ and other large-sized NCs,²⁸ we observed a quantitative relationship in which the excited-state lifetimes of Au NCs decrease exponentially with the energy gaps (ESI Fig. S4a†). Here, we fit the data with the energy gap law,

k_nr = Ae^{−γE_g/ħω} (1)

where k_nr is the non-radiative recombination rate, γ is a molecular parameter, and ω is the highest vibrational frequency involved in non-radiative decay. In large-sized NCs (n > 100), photoluminescence is no longer observable; thus, the electron relaxation dynamics is dominated by the non-radiative decay. Accordingly, k_nr can be obtained by the inverse of the excited-state lifetime (1/τ) from fs-TA measurements. Using the linear form of eqn (1),we obtained a good linear relationship between ln k_nr and E_g (Fig. S4b†), with the fitted intercept (ln A) being 27.5 and the slope (−γ/ħω) being −15 eV⁻¹.

Conclusions

In summary, we devised synthetic routes for two large-sized nanoclusters Au₁₀₀(Napt)₄₂ and Au₁₀₂(IPBT)₄₄ and characterized their optical and electronic properties. Together with Au₁₀₂(p-MBA)₄₄ and Au₁₀₃S₂(Napt)₄₁, a family of four 58e NCs is formed, which allows us to compare their atomic and electronic structures, as well as the electron relaxation dynamics. For the atomic structure, the two new NCs are rationalized to possess the same Au₇₉ decahedral kernel as that observed in the crystallographically characterized Au₁₀₂(p-MBA)₄₄ and Au₁₀₃S₂(Napt)₄₁ NCs, evidenced by the similarity in their steady-state absorption and transient absorption spectral features (e.g., common GSB signals at 525 nm and ESA signals at 580 and 675 nm), as well as comparable ultrafast dynamics. The determination of ultrasmall HOMO–LUMO gaps of Au₁₀₀(Napt)₄₂ and Au₁₀₂(IPBT)₄₄ is nontrivial but is successfully done by meticulous DPV analysis. The E_g values (∼0.4 eV) of Au₁₀₀(Napt)₄₂ and Au₁₀₂(IPBT)₄₄ are similar to those of their crystallographically characterized counterparts, indicating the decisive role of the kernel in the ground-state electronic properties, while the excited-state dynamics involves both the kernel and surface (i.e., the ligand dependence). Finally, extensive analyses done by summarizing the NCs from Au₉₂(SR)₄₄ to Au₂₇₉(SR)₈₄ give rise to the scaling relationships between the E_g and size (n), as well as the relationship between the excited-state lifetime and E_g, from which we identified a strong shape effect on E_g and also strong nonradiative decay. Such findings greatly deepen our fundamental understanding of the optical and electronic properties of Au_n(SR)_m NCs in the large-size regime, which will benefit the future development of applications of such atomically precise materials.

Data availability

All the data are available and reported in the manuscript and its ESI.†

Conflicts of interest

The authors have no conflicts of interest to declare.

Acknowledgements

We thank Dr Meng Zhou for helpful discussions. This material is based upon the work supported as part of the Atomic-C2E project by the U.S. Department of Energy, Office of Science under award number DE-SC-0024716. H. W. acknowledges financial support by the Air Force Office of Scientific Research (AFOSR) under award number FA9550-21-1-0192.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5nr01244a

‡ These authors contributed equally.

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