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Origin of the bright photoluminescence of few-atom silver clusters confined in LTA zeolites

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Science  17 Aug 2018:
Vol. 361, Issue 6403, pp. 686-690
DOI: 10.1126/science.aaq1308

Unmasking the glow of silver clusters

Small silver clusters stabilized by organic materials or inorganic surfaces can exhibit bright photoluminescence, but the origin of this effect has been difficult to establish, in part because the materials are heterogeneous and contain many larger but inactive clusters. Grandjean et al. studied silver clusters in zeolites, using x-ray excited optical luminescence to monitor only the emissive structures (see the Perspective by Quintanilla and Liz-Marzán). Aided by theoretical calculations, they identified the electronic states of four-atom silver clusters bound with water molecules that produce bright green emission—thus identifying candidate materials for application in lighting, imaging, and therapeutics.

Science, this issue p. 686; see also p. 645

Abstract

Silver (Ag) clusters confined in matrices possess remarkable luminescence properties, but little is known about their structural and electronic properties. We characterized the bright green luminescence of Ag clusters confined in partially exchanged Ag–Linde Type A (LTA) zeolites by means of a combination of x-ray excited optical luminescence-extended x-ray absorption fine structure, time-dependent–density functional theory calculations, and time-resolved spectroscopy. A mixture of tetrahedral Ag4(H2O)x2+ (x = 2 and x = 4) clusters occupies the center of a fraction of the sodalite cages. Their optical properties originate from a confined two-electron superatom quantum system with hybridized Ag and water O orbitals delocalized over the cluster. Upon excitation, one electron of the s-type highest occupied molecular orbital is promoted to the p-type lowest unoccupied molecular orbitals and relaxes through enhanced intersystem crossing into long-lived triplet states.

Few-atom luminescent silver clusters (AgCLs) (1) stabilized through organic (such as peptides, proteins, polymers, and DNA) (26) or inorganic (such as glasses and zeolites) (79) templates have emerged as promising candidates for a broad range of applications in lighting, imaging, sensing, and therapeutics (4). Compared with conventional quantum dots, few-atom AgCLs combine an ultrasmall size with excellent size-dependent photoluminescence (PL) spanning the ultraviolet to near-infrared spectrum. Strong quantum confinement of Ag valence electrons appears to break up the continuous density of states into discrete energy levels and confer molecular-like properties to the AgCLs. Nevertheless, the lack of a detailed understanding of the fundamental photophysical mechanisms underlying their emissions is hampering the rational design of AgCLs with improved and tailored optical properties. Atomic structures for AgCLs have not been determined unambiguously because of their vast distribution of size, environment, and template interactions, as well as the presence of a large fraction of nonluminescent Ag species.

The AgCLs that self-assemble in the cavities of the rigid aluminosilicate crystalline framework of zeolites have the most homogeneous and efficient emissions. The PL of AgCLs confined in thermally activated Ag-loaded zeolite structures of faujasite (FAU) and Linde Type A (LTA) topologies features tunable absorption and emission, large Stokes shifts, and exceptionally high external luminescence quantum efficiencies reaching unity (8, 10). However, the structure of these AgCLs has not been fully elucidated yet because of the complexity of Ag-zeolite host-guest interactions and the sensitivity of Ag-zeolite composites toward radiation (electrons and photons) used in structural characterization techniques (11). We present a detailed investigation of the structural and electronic properties of partially Ag-exchanged Ag3K9-LTA zeolite by use of three complementary techniques. This system was selected for its green PL featuring an excellent external luminescence quantum yield of 23% among the Ag-LTA zeolites, its good stability toward x-ray irradiation (11), and its simpler crystallographic structure than that of FAU. With x-ray excited optical luminescence (XEOL), the x-ray absorption fine structure (EXAFS) signal is detected exclusively from the Ag atom fraction involved in the PL process at the Ag K-edge, thus selectively determining the structure of the emitting Ag species (12). This univocal assignment could not be made in earlier work (10); hence, approach may provide more detailed understanding of luminescent properties for a variety of few-atom Ag clusters. The structures obtained experimentally were confirmed computationally with geometry optimizations by using density functional theory (DFT) methods, while time-dependent DFT (TD-DFT) was applied to determine the electronic transitions responsible for the absorption and emission spectra of the stable isomers. Last, to confirm the electronic structure of the theoretically modeled AgCLs, we identified the relevant decay modes and time scales involved in the absorption and luminescence processes of Ag3K9-LTA using a combination of femto- to millisecond time-resolved spectroscopies.

The large number of structural characterizations of AgCLs stabilized in LTA zeolites, often performed by means of x-ray diffraction and electron spin resonance (1316), have led to numerous incomplete and often contradictory structural models. Ag3-4 clusters were tentatively related to partially Ag-exchanged LTA zeolites, whereas Ag6 clusters were associated to fully exchanged samples. High-resolution transmission electron microscopy (HRTEM) revealed octahedral Ag6 clusters in the sodalite cages of fully exchanged Ag-LTA zeolites (17), but no evidence linking this structure to the PL was given. Similar analysis of partially exchanged Ag-LTA zeolites remained unsuccessful because of the strong influence of electron-beam irradiation (17, 18).

EXAFS (19) provides information on cluster size and atom bonding both for the emissive and nonemissive clusters (20). By contrast, XEOL exclusively detects the XAFS signal from the atoms constituting the emissive species (12, 21, 22). Also, although x-ray irradiation can affect the structure of few-atom clusters (11), XEOL would monitor any beam degradation effect (supplementary materials). The three-dimensional (3D) structures of the AgCLs were determined by combining the fitting results of the XEOL and transmission-detected EXAFS collected simultaneously (table S1).

We primarily analyzed the XEOL-detected EXAFS of Ag3K9-LTA. The χ(k) k3-weighted EXAFS data and the corresponding phase-corrected Fourier transform (FT) best fits are shown in Fig. 1, A and B.

Fig. 1 Ag K-edge XEOL and transmission-detected EXAFS and FTs of heat-treated Ag3K9-LTA and derived structures.

(A) XEOL-detected and (C) transmission-detected k3-weighted Ag K-edge EXAFS with the (B) phase-corrected XEOL-detected FT and (D) transmission-detected FT best fits. (E to J) Structures of (E) Ag4(H2O)4 and (H) Ag4(H2O)2, including [(F) and (I)] AgR cations and [(G) and (J)] embedded in the sodalite cage (~0.66 nm free diameter).

The first and second peaks of the FT were fitted with, respectively, two oxygen (O) atoms at 2.36 Å (N1) and three Ag atoms at 2.82 Å (N2). Additionally, the fit was completed with two longer shells consisting of 0.4 K at 3.05 Å (N4) and 1.1 Ag at 3.3 Å (N5). The Ag atoms coordinated to three other Ag atoms (AgC) form tetrahedra-like Ag4 clusters located inside the sodalite cage, as was shown with TEM (17). The AgC atoms are each coordinated to two O atoms likely corresponding to extra-framework water molecules because of the short Ag-O distances and the fact that these O atoms are removed with a concomitant loss of the sample PL upon dehydration of the material (supplementary materials). The absence of contributions from the sodalite atoms [O, silicon (Si), and aluminum (Al)] embedding AgCLs in the EXAFS signal is discussed in the supplementary materials.

The analysis of the XEOL-detected signal shows that the species at the origin of the bright-green PL observed in Ag3K9-LTA are Ag4 clusters with short Ag-Ag distances of 2.82 Å, in which each Ag atom is bound to two water molecules at 2.36 Å. They are further surrounded by isolated K and Ag cations likely positioned in the single six-membered rings (S6Rs) of the same sodalite cage. A twofold coordination of AgC atoms, however, does not correspond to an integer number of water molecules but rather to two different stoichiometries of x = 2 and x = 4, corresponding to a water coordination per AgC of 1.5 and 3, respectively. This analysis suggests the presence of a mixture of Ag4(H2O)4 and Ag4(H2O)2 with a ~34/66 ratio (3 × 0.34 + 1.5 × 0.66 = 2). Attempts to use other models for analyzing the XEOL-detected signal, including the structure used for the analysis of the transmission-detected EXAFS presented below, were either not successful or led to unrealistic fitting parameters (supplementary materials).

We also analyzed the transmission-detected EXAFS collected simultaneously with the XEOL-detected data. The χ(k) k3-weighted EXAFS data and the phase-corrected FT best fits of heat-treated Ag3K9-LTA are shown in Fig. 1, C and D. The distinct profiles compared with those collected with XEOL detection indicate that two different average local environments of Ag atoms were measured simultaneously, which is consistent with the x-ray absorption near-edge structure analysis (supplementary materials).

The first peak in the FT was fit with 2.5 O at 2.34 Å (N1) corresponding to the combination of the framework O (OF) from the S6Rs rings and the H2O ligands, as shown in the XEOL analysis. The second peak in the FT is a multipeak composed of 2.6 Si/Al atoms (N2) at 3.26 to 3.30 Å corresponding to a fraction of nonluminescent Ag cations located near the center of the S6Rs (AgR) (fig. S13), not detected in XEOL-EXAFS. The second peak in the FT analysis also contained a weaker Ag-Ag contribution (N3) of 1.7 Ag at unusually short distances of 2.70 Å. This feature corresponds to the remaining part of the silver atoms AgC (~57%) that are coordinated to ~3 (1.7/0.57) silver neighbors and are forming Ag4 clusters inside the sodalite cage. The 4% discrepancy between the AgC-AgC (2.70 to 2.82 Å) distances found by the two detection approaches suggests that XEOL measured preferentially the excited state structure of the clusters (supplementary materials). AgC in Ag4 clusters are coordinated to 2.1 O (1.2/0.57) from water molecules. Additionally, four shells (N4 to N7) consisting of K and Ag were detected at liquid-nitrogen temperatures between 2.97 and 4.49 Å corresponding to AgC-K or AgC-AgR distances from basal AgC in Ag4 tetrahedra associated with the absence or presence, respectively, of a water molecule sandwiched between the two atoms (fig. S13). These shells are complemented by two long-distance contributions (N8 and N9) corresponding to AgC-AgR from apical AgC in Ag4 tetrahedra and AgR-AgR (fig. S7a) detected at 5.24 and 6.13 Å at LN. The distinct combination of distances of N4 to N9 shells closely fit the Ag-LTA sodalite crystallographic model (fig. S7B), fully supporting the AgCL local structures proposed.

Fig. 2 Frontier orbitals of [Ag4(H2O)4(Si24H24O36)]2+ and energy level diagram of Ag4(H2O)22+ and Ag4(H2O)42+ clusters in Ag3K9-LTA.

(A) Frontier orbitals consist of one single symmetric s-type HOMO (1S0) and three singlet one-node p-type 1P (ml = –1, +1, or 0) LUMOs (px, py, pz) delocalized over all the Ag and O atoms of the cluster. Atom colors are Si, gray; O, red; Ag, blue; hydrogen, white. (B) Energy level diagram showing the ground-state 1S0 and the excited states 3P and 1P of water-free unperturbed Ag42+ clusters and the ground-state 1S0 and the six singlet 1P and triplet 3P excited states of Ag4(H2O)22+ and Ag4(H2O)42+ perturbed by means of water ligand field interaction. Blue arrows represent the allowed transitions, and the green arrows represent the luminescent transitions between the relaxed states.

The EXAFS investigation shows that the emitters in Ag3K9-LTA consist of ~40% of Ag4(H2O)4 and 60% of Ag4(H2O)2 tetrahedra-like clusters located at the center of the sodalite cage. These structures are presented in Fig. 1, E to J. The clusters are coordinated at their faces by two or four water molecules located near the center of the S6Rs and sandwiched between three AgC and one AgR (or K cation). AgCLs consisting of 57% of the total number of exchanged Ag atoms are mostly surrounded by the remaining 43% isolated AgR cations plus some additional K cations in the S6Rs. This indicates that Ag cations in partially exchanged Ag3K9-LTA concentrate (six to seven Ag atoms instead of three expected from the Ag stoichiometry) in a limited fraction of the sodalite cages (~45%). (23).

We used a combination of DFT and TD-DFT to model AgCLs and probed their charges with the natural bonding orbital approach. Two stable isomers—[Ag4(H2O)x(Si24H24O36)], x = 2 and x = 4, showing the best agreement between calculated and measured structures and absorption spectra—were obtained when applying a +2 charge preferentially localized on the Ag4CLs but extending toward the cluster surrounding (tables S4 and S5). The doubly charged Ag4CLs exhibit a closed-shell electronic configuration, in which the Ag 4d shell is completely filled, and the two remaining valence 5s electrons are delocalized over the cluster. Within a superatom model, two electrons associated to a metal cluster correspond to the smallest magic number with enhanced stability (24, 25), which is consistent with analogous theoretical work on Ag4CLs (26, 27).

Both isomers [Ag4(H2O)x, x = 2 and 4] consist of pseudotetrahedral Ag4 clusters located, unlike the experimental structures, in an off-centered position in the sodalite cage, with one or two Ag atoms coordinated directly to OF (supplementary materials). In Ag4(H2O)4, each Ag atom is coordinated on average to two O atoms (water plus OF) with an average Ag-Ag bond distance of ~2.79 and 2.92 Å in the ground and excited state, respectively (fig. S14). In Ag4(H2O)2, an average Ag-Ag distance of 2.87 Å in the ground state and a mean O coordination close to 1.3 were obtained (fig. S15). These calculated structures featuring a 5% increase in the AgC-AgC distances from the ground to the excited states are in good agreement with the experimental results (supplementary materials).

The calculated frontier orbitals for both Ag4(H2O)42+ and Ag4(H2O)22+ isomers (Fig. 2A and fig. S16, respectively) are composed of a contribution of ~50% from Ag 5s atomic orbitals and of up to 25% from the O states of the surrounding OF and H2O, as shown in the density of states curves (fig. S18). A doubly occupied HOMO of totally symmetric s-type orbital forms the ground state (1S0) and two sets of three singlet (1P) and three triplet (3P) LUMOs consisting of one-node p-type orbitals form the expected lowest-lying cluster orbitals for a cluster system with two skeleton electrons. Both states in each isomer have similar atomic orbital compositions (fig. S18), suggesting that the excitations are mainly localized on the cluster.

The key role of water ligands in AgCLs’ electronic properties is highlighted in the energy level diagram (Fig. 2B and fig. S17) of Ag4(H2O)42+ and Ag4(H2O)22+ along with water-free Ag42+ clusters as reference. In water-free unperturbed Ag42+, the ground state is the HOMO 1S0, and the LUMOs consist of threefold degenerated singlet 1P and triplet 3P excited states. Upon coordination with water, the ligand field lifts the degeneracy of the LUMOs into six excited states: three singlet 1P [total spin quantum number (S) = 0; total orbital angular momentum quantum number (L) = 1; magnetic quantum number (ml) = –1, +1, or 0) and three triplet 3P (S = 1; L = 1; ml = –1, +1, or 0) states. The absorption occurs from the ground state corresponding to two electrons of opposite spins on a HOMO s-like orbital (1S0) to the singlet excited states 1P (3.5 and 3.7 eV) that correspond to one electron on a s-like orbital and one electron on LUMO, LUMO+1, or LUMO+2 p-like orbitals, with the two electrons having opposite spins.

These transitions feature large oscillator strengths f because they are allowed by spin and angular momentum selection rules. The strong overlap of the high-energy triplet 3P (S = 1, L = 1, ml = 0) state with the 1P singlet states ensures, in combination with large spin-orbit coupling expected for Ag, an enhanced intersystem crossing. Upon light excitation, a fraction of 1P singlet states population is transferred to the high-energy triplet state that finally decays into the low-lying 3P (S = 1, L = 1, ml = –1 or +1) triplet state. As shown in Fig. 2B, the intensity of the ligand-field splitting of the p-like LUMOs is directly proportional to the number of water molecules coordinating AgCLs, decreasing the band gap when going from Ag4(H2O)22+ to Ag4(H2O)42+ isomers. The bright-green emission of Ag4(H2O)42+ then occurs from the lowest-lying 3P triplet excited state 3P (S = 1, L = 1, ml = –1) to the ground state 1S0.

The modeled absorption spectra for the Ag4(H2O)42+ and Ag4(H2O)2 2+ isomers (Fig. 3) are in excellent agreement with the steady-state optical experimental data. Calculated absorption peaks found at 343 and 320 nm in Ag4(H2O)42+ and Ag4(H2O)22+, respectively, closely match the experimental excitation peaks at 340 nm (3.64 eV) and 310 nm (4.00 eV). These results also confirm the assumption that the 37/63 intensity ratio of the two main emission peaks is directly related to the 34/66 fraction ratio of Ag4(H2O)4 and Ag4(H2O)2 present in Ag3K9-LTA composites. Last, the green PL energy obtained experimentally at 550 nm (2.25 eV) fits the transition energy of 556 nm (2.23 eV) of the modeled transition from the relaxed lowest-lying 3P triplet excited state to the 1S0 ground state (Fig. 2B).

Fig. 3 Steady-state excitation-emission of Ag3K9-LTA.

(A) 2D excitation–emission plot. (Inset) The picture of an x-ray–irradiated sample under 366 nm illumination. (B) Excitation spectrum λdetection = 555 nm of as-prepared Ag3K9-LTA. Calculated 1S0 HOMO to 1P (S = 0; L = 1; ml = –1, +1) LUMOs absorption spectra of x = 2 and x = 4 [Ag4(H2O)x]2+ isomers showing a good agreement with experiments.

To determine the decay time scale and energies of Ag3K9-LTA modeled optical transitions and to verify the triplet nature of its bright green PL, we performed a global analysis of the decays obtained with femtosecond fluorescence up-conversion, time-correlated single-photon counting (TCSPC) and nanosecond luminescence time-resolved spectroscopies. Decays obtained with femtosecond fluorescence up-conversion in the range of 410 to 570 nm reveal three time components of 0.5 and 2.6 ps related to relaxation processes and >50 ps attributed to the 1P-to-1S0 transition (Fig. 4, A and B; fig. S24, decay traces; and table S7). The amplitude-to-wavelength dependence (AWD) of the first two ultrafast components (maxima centered at 510 and 530 nm) are slightly blue-shifted relative to the maximum of the steady-state emission spectrum (550 nm), suggesting the rapid depopulation of 1P (S = 0; L = 1; ml = –1, +1) singlet excited states. These ultrafast components attributed to nonfluorescent short-lived intermediate species formed after the relaxation of 1P Franck-Condon states match closely the energy of the 1P (S = 0, L = 1, ml = –1)–to-1S0 transition predicted with TD-DFT (Fig. 2B). These intermediate states rapidly convert into fluorescent 3P triplet states lying at similar energies via intersystem crossing (Fig. 4, B and D).

Fig. 4 Time-resolved spectroscopy of Ag3K9-LTA.

(A and B) AWD 3D time-resolved fluorescence emission spectra in 50-ps time window obtained with (C) femtosecond fluorescence up-conversion, in 1-ms time window through nanosecond luminescence. (D) Schematic illustration of the main electronic states involved as a function of energy (on the vertical axis).

This model is confirmed through the analysis of the PL decays in the micro- to millisecond range that reveal three time constants of 423 ns, 10.6 μs, and 116 μs. On the basis of the AWD presented in Fig. 4C, the 423-ns component is attributed to an excited state with an intense emission peaking at 520 to 540 nm (2.38 to 2.30 eV), with estimated radiative and nonradiative rates of 5.43 × 105 s−1 and 18.2 × 105 s−1, respectively. The close resemblance of the AWD of this state with the stationary emission spectrum indicates that these long-lived species are at the origin of the bright-green emission observed in Ag3K9-LTA. This emission occurring from long-lived states characteristic for spin-forbidden transitions points toward their peculiar triplet nature. The other decay times of 10.6 and 118 μs are attributed to two different triplet excited states, with weak emissions centered at 630 and 680 nm (1.97 and 1.82 eV), respectively (Fig. 2B; fig. S25, decay traces; and table S8), which are associated with the presence of residual amounts of emissive species such as AgCLs with different size and/or water coordination (10).

The triplet nature of Ag3K9-LTA bright-green emission is further corroborated by the remarkable enhancement of the PL accompanied by a dramatic increase of the decay time from 423 ns to 106 μs for the 540-nm main emission at low temperature (77 K), whereas the faster components remained mostly unaffected (figs. S26 to S33). This result shows that excited-state kinetics, in which nonradiative decay channels are hindered at low temperature and characteristic of triplet-state emissions, is involved. Hence, given the close correspondence between the computational and the photophysics results, the cluster-ligand interaction as included in the model of a two-electron doubly charged AgCL, possibly with a fraction of the charge located on the sodalite cage, explains the occurrence of long-lived bright luminescent states in Ag3K9-LTA.

By measuring exclusively the local structure of the emissive Ag clusters in partially exchanged LTA zeolites, XEOL-XAFS has allowed for the first time the unambiguous identification of their functional structures. DFT modeling based on these detailed structures showed that the double positively charged Ag4(H2O)4 and Ag4(H2O)2 clusters, in which water ligand molecules modulate the HOMO-LUMO gap, behave as confined two-electron helium or alkaline earth–like superatom quantum systems that mainly emit via their long-lived lowest-lying 3P triplet excited state, as confirmed with time-resolved optical spectroscopy. We anticipate that similar photophysical properties may also apply to luminescent AgCLs confined in other inorganic and organic scaffolds. This is likely the case for AgCLs confined in fully exchanged LTA or in FAU zeolites that possess very similar structural and luminescent properties (10). This new understanding of the mechanism of AgCLs’ bright luminescence and its expected dependence on the interaction with oxygen ligands, electron confinement, electrostatic interaction, and charge transfer to the surrounding silver atoms, which differ from the single Ag cations emission model proposed earlier (2830), should lead to substantial improvements in the rational design of AgCLs optical properties.

Supplementary Materials

www.sciencemag.org/content/361/6403/686/suppl/DC1

Materials and Methods

Supplementary Text

Figs. S1 to S35

Tables S1 to S8

References (3145)

References and Notes

Acknowledgments: We thank B. Dieu for the graphical material. Funding: We acknowledge the IAP-7 (Belspo), the EU (FP7/2007-2013 and GA nos. 310651-SACS and 607417-Catsense), the Flemish government “Methusalem” (CASAS, Meth/08/04, CASAS2, and Meth/15/04), the SoPPoM program, the FWO (G.0990.11, G.0197.11, G.0962.13, G.0B39.15, and ZW15_09 GOH6316N), and KU Leuven (GOA/14 and IDO/07/011). We thank the ESRF (CH-4207) and the staff of LISA-BM08 and DUBBLE-BM26A (26-01-865) beamlines. Author contributions: D.G. and P.L. conceived and directed the study. E.C.-G., W.B., and M.B.J.R. prepared the samples and performed the steady-state optical, elemental, and thermogravimetric analysis characterization. D.G., E.C.-G., F.D., D.B., W.B., and S.A. performed the XEOL-XAFS and Tr-XAFS measurements. D.G. and S.A. analyzed the XAFS data. N.T.C., P.S., and M.T.N. performed the DFT calculations. E.F. and J.H. designed, performed, and analyzed the time-resolved optical experiments. All authors contributed to discussions and interpretations of the combined experimental and theoretical results. D.G., P.L., E.F., and S.A. prepared the manuscript with contributions from all coauthors. All authors have approved the final version of the manuscript. Competing interests: None declared. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper or the supplementary materials.
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