Excited-State Behaviors of M1Au24(SR)18 Nanoclusters: The Number

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Excited State Behaviors of MAu (SR) Nanoclusters: The Number of Valence Electrons Matters Meng Zhou, Chuanhao Yao, Matthew Y. Sfeir, Tatsuya Higaki, Zhikun Wu, and Rongchao Jin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11057 • Publication Date (Web): 20 Feb 2018 Downloaded from http://pubs.acs.org on February 26, 2018

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The Journal of Physical Chemistry

Excited State Behaviors of M1Au24(SR)18 Nanoclusters: The Number of Valence Electrons Matters Meng Zhou,1 Chuanhao Yao,2 Matthew Y. Sfeir,3 Tatsuya Higaki,1 Zhikun Wu,2 Rongchao Jin1* 1

Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania15213, USA Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanotechnology, CAS Center for Excellence in Nanoscience, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, Anhui 230031, China. 3 Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York11973, USA 2

*To whom correspondence should be addressed. Email: [email protected]. Abstract: Doping is a quite useful strategy for probing the structure and properties of metal nanoclusters, but the effect of doping on the photodynamical properties is still not fully understood. Here, we reveal that the number of valence electrons plays a major role in determining the photodynamics of M1Au24(SR)18 nanoclusters. By carrying out temperature-dependent optical absorption, it is found that Cd doping enhances electron-phonon coupling while Hg doping does not significantly alter the coupling. Moreover, the relaxation dynamics of [M1Au24(SR)18]0 (M=Hg/Cd) nanoclusters show similar features to that of the negatively charged Au25 nanocluster. Specifically, the 8-electron M1Au24 (M = Cd/Hg) nanoclusters show a long excited-state lifetime (~100 ns) and a weak picosecond relaxation, similar to the case of anionic [Au25]- nanocluster. On the other hand, the non 8-electron MAu24 (M = Pd/Pt) nanoclusters show much more significant picosecond relaxation and thus much shorter excited state lifetimes, which resembles the case of neutral [Au25]0. The picosecond relaxation in all the six cases can be explained by core-shell charge transfer or relaxation to the surface trap state. These results are of great importance for fundamental understanding of the interplay between the valence electrons and the optical properties of metal nanoclusters.

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Introduction Atomically precise metal nanoclusters offer exciting opportunities in fundamental research owing to their well defined structures and unique properties.1-4 Significant advances in gold nanocluster research have made it possible to correlate the optical properties with structures,5-7 which allows for rational design and functionalization of these ultrasmall gold nanoparticles for specific applications. The optical properties of metal nanoclusters, including photoluminescence,8,9 nonlinear absorption,2-4 and long-lived excited-state lifetime,10,11 differentiate such nanoclusters from those larger counterparts ⎯ plasmonic nanoparticles.12 Understanding the optical properties, especially the excited state dynamics of gold nanoclusters, will help to develop new applications in energy storage and conversion.13-15 In recent research, ultrafast spectroscopy has been employed to (i) probe the evolution from metallic to molecular behavior;16-19 (ii) correlate the electron dynamics with the structure of nanoclusters;20-23 and (iii) understand the excited state interactions between clusters and dye molecules as well as solvents.24-26 Nevertheless, understanding of the excited state process as well as the correlation with structures of metal nanoclusters is still far from complete; for instance, it still remains elusive how the electron and phonon dynamics are affected by the type of crystal structure, metal composition, and surface-protecting ligands of metal nanoclusters. Doping and alloying foreign atoms into metal nanoclusters has been an effective strategy to probe the electronic properties of ligand-protected nanoclusters and broaden their applications.27,28 For instance, alloying and doping has been found to increase the photoluminescence quantum yield,8,29 enhance the catalytic performance and stabilize the metal nanoclusters.30,31 Among the nanoclusters, [Au25(SR)18]- has long been an attractive platform for investigating the effect of doping on the optical properties and structure.32,33 The structure of [Au25(SR)18]- consists of an icosahedral Au13 core protected by six Au2(SR)3 dimeric staple motifs. It has been found that heterometals with different activity (such as Cu, Hg, Ag, Pt, Cd, and Pd) can be doped into the Au25 nanocluster via substitution for gold,32,33 which modifies the optical absorption and other properties of the nanocluster. Both light doping and heavy doping in Au25 and related systems have been reported,34-37 which demonstrates that it is now possible to precisely control the composition of nanoclusters through “molecular surgery”,36 much like the methodologies in organic chemistry. In this work, we probe the effects of Cd and Hg doping on the Au25 optical properties by temperature dependent absorption and time-resolved spectroscopy. Previous work on the [M1Au24(SR)18]0 (M=Hg/Cd) 2


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nanoclusters has determined both structures,32,33 and ultrafast spectroscopic studies also revealed that Pd and Pt doping can change the electron dynamics of Au25.22,23 In the current work, the temperature dependent absorption spectroscopy analysis reveals that the Cd-doped 25-atom nanocluster shows a significantly larger electron-phonon coupling constant and a higher phonon frequency than that of the parent Au25 nanocluster. Electron dynamics of both doped nanoclusters show an ultrafast decay ( 600 nm) are primarily contributed by the MAu24 metal core while those at shorter wavelength (< 400 nm) have considerable contributions from staple motifs.5,27

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Figure 1. UV-vis absorption spectra of [Au25(SR)18]-, CdAu24(SR)18 and HgAu24(SR)18 nanoclusters at room temperature in (A) wavelength and (B) energy scales. Inset in (A): the atomic structure of the 25-atom nanoclusters (the R groups omitted). To probe the effect of Cd- and Hg-doping on the optical properties of anionic Au25, we performed temperature-dependent UV-vis absorption spectroscopy on [Au25]-, CdAu24 and HgAu24 nanoclusters. For both doped and undoped clusters, the absorption peaks become sharpened and stronger at low temperatures (Figure 2). In the parent anionic Au25 nanocluster, all the absorption peaks are blueshifted (i.e. toward higher energy) in comparison to the case of room temperature (Figure 2A), similar to the observations first reported by Ramakrishna and coworkers.22,40 In the case of CdAu24, as temperature decreases from 300 K to 120 K, the absorption peaks around 1.68 eV and 1.8 eV are blueshifted prominently while those at higher energy are much less shifted (Figure 2B). The case of HgAu24 is particularly interesting in that the room temperature single peak around 1.74 eV is split into three peaks at 1.6 eV, 1.79 eV and 1.9 eV at 80 K (Figure 2C), similar to the results reported recently by Ramakrishna’s group.22 The different behaviors of Cd- and Hg-doping implies that these dopants have different effects on the valence 1P orbital (quasi-degenerate) of [Au25]-,5,41 with CdAu24 showing a similar splitting into P3/2 (doubly degenerate) and P1/2 as in anionic Au25 and therefore two peaks between 1.5-2 eV

39,42

but HgAu24 giving rise to three

non-degenerate orbitals of P character and therefore three peaks between 1.5-2 eV.

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Figure 2. Temperature-dependent absorption spectra of [Au25(SR)18]-, [CdAu24(SR)18]0 and [HgAu24(SR)18]0 nanoclusters. (A-C) UV-vis absorption spectra as a function of temperature; (D-F) Absorption maxima as a function of temperature and the corresponding fits with equation (1) (see main text). The transition from liquid to glass phase occurred around 140 K, which is the freezing point of the solvent (2-methyltetrahydrofuran). The spikes around 1.37 eV (900 nm) is an instrument artifact. To model the temperature dependent absorption spectra, we fitted the temperature dependent maxima of the three nanoclusters (Figure 2 D-F) using the O’Donnell-Chen equation43 as follows:

⎡ ⎛ =ω ⎞ ⎤ E (T ) = E(0) − C hυ ⎢coth ⎜ ⎟ − 1⎥ ⎜ ⎢⎣ ⎝ 2kT ⎠ ⎥⎦

(1)

where, E(0) is the absorption peak position at 0 K, C is the dimensionless e-p coupling constant, and ω is the average energy of the low-frequency vibration which is responsible for the electron-phonon interaction. The fitting parameters of the two doped nanoclusters and the parent Au25 nanocluster are listed in Table 1. It is found that CdAu24, HgAu24 and anionic Au25 show different electron-phonon coupling constants and frequencies. Our results on the anionic Au25(SR)18 nanocluster matches well with previous results reported by Ramakrishna’s group.40 The phonon frequency of anionic Au25 is ~43 meV (350 cm-1), which was assigned to the vibration of the surface staple motif40 (-S-Au-S-Au-S-). Here, in CdAu24 and HgAu24, the phonon frequencies are 65 meV (472 cm-1) and 28 meV (224 cm-1), respectively, which should also be 6


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ascribed to the vibration of the staple motif. We note that the phonon frequency and coupling constant of HgAu24 is different from the results reported by Ramakrishna’s group, which might be to the different protecting ligands (i.e., 1-hexanethiolate22 vs 2-phenylethanethiolate in the current work). The change in phonon frequency in both doped nanoclusters could be related to the slight change in bond-length and geometrical structure. From the crystal structure of CdAu24,32,34 it can be found that the Au-S bond length is shortened compared to that of anionic Au25, which agrees with the higher vibration frequency. Thus, Cd doping would increase the electron-phonon coupling strength and give rise to higher phonon frequency because of its shrunk structure.

Table 1. Fitting parameters of temperature-dependent peak maxima using the O’Donnell-Chen equation. E(0), eV

C

ω, meV

[Au25(SR)18]-

1.87±0.006 3.7±0.17

43±1

[CdAu24(SR)18]0

1.9±0.002

9.0±1.3

65±5

[HgAu24(SR)18]0

1.8±0.001

2.2±0.12

28±3

Nanosecond and femtosecond time-resolved transient absorption spectra were measured for both doped and undoped nanoclusters to probe the effects of doping on the photophysics behavior. It can be seen that both doped and undoped 25-atom nanoclusters show similar transient absorption spectral profiles (Figure 3). After photoexcitation, strong excited-state absorption (ESA) can be observed between 500 nm and 600 nm for all three nanoclusters. In ~1 ps, the strong ESA in all cases decays significantly and a long lived decay can be observed between 10 ps and over 100 ns. In the nanosecond range (1 ns to 1000 ns), one can observe ESAs at 500-600 nm and 700-820 nm ranges in all nanoclusters. From the transient absorption data map (Figure 3), one can observe that HgAu24 has a relatively shorter excited state lifetime compared to those of CdAu24 and anionic Au25.

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Figure 3. Femtosecond and nanosecond transient absorption spectra data map of (A) [Au25]-, (B) [CdAu24]0 and (C) [HgAu24]0 nanoclusters pumped at 360 nm at all time delays. Data prior to ~1 ns were collected using a mechanical delay. An electronically controlled probe was used to generate data after ~1 ns with the same pump pulse. For quantitative analysis, we first performed global fitting on the nanosecond transient absorption (ns-TA for short) data (Figure 4). During the global fitting, we used the same time constants to fit kinetic traces at all probe wavelengths. The spectral features at nanosecond timescale are similar for the three nanoclusters, broad excited state absorptions overlapped with ground state bleachings which correspond to the steady state absorptions (Figure 4A-C). The excited state lifetime is around 100 ns, 200 ns and 50 ns, for [Au25]-, CdAu24 and HgAu24 nanoclusters, respectively (Figure 4D). Interestingly, the optical bandgaps for the three nanoclusters are 1.3 eV, 1.4 eV and 1.2 eV, respectively.33,34 Therefore, the trend of excited state lifetimes of the three nanoclusters follows the bandgap law, i.e. the larger bandgap, the longer excited state lifetime.44,45

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Figure 4. (A-C) Decay associated spectra obtained from global fitting on the nanosecond transient absorption spectra of anionic Au25, CdAu24 and HgAu24 nanoclusters, respectively. (D) Normalized kinetic traces monitored around 600 nm of the three nanoclusters. Global fitting on the femtosecond TA data gives the decay associated spectra of the two doped and the undoped nanoclusters (Figures 5 and 6). In global fitting, we fixed the long-lived components in all three clusters according to the above nanosecond measurements. For the anionic Au25 nanocluster, previous work used two decay components to fit the relaxation dynamics.46 Here, after a closer look at the fitting quality, we found that three decay components indeed give a better fitting quality than two components (Figure 5B), especially for those kinetic traces probed at higher energy (500-600 nm). The first sub-picosecond component should be an internal conversion process (i.e., from a higher excited state to a lower excited state). The second decay component (5.6 ps) is found to be dependent on the solvent polarity (Figure S1 in Supporting Information). Upon photoexcitation, the solvent will reorganize around the cluster surface and stabilize the excited state. As solvents with higher polarity lead to faster decay, the excited state should have a charge transfer character. However, the amplitude of the 5.6 ps component in the anionic Au25 nanocluster is much weaker compared to that of the neutral Au25 nanocluster.46

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Figure 5. (A) Decay associated spectra (DAS) obtained from global fitting on the TA spectra of [Au25]-, (B) 3-component fitting vs 2-component fitting probed at 510 nm. In both the doped nanoclusters, with 360 nm excitation, three decay components are required to obtain the best fitting quality (Figure 6 A and B). Similar to the case of anionic Au25, the second decay component (few picoseconds) has a relatively low amplitude. The third DAS component for both doped clusters from the fs-TA measurements has the same spectral features to that of the nanosecond TA measurements (c.f. Figure 3), which indicates that there are indeed only three decay components in both doped cases. With excitation at 730 nm, the ultrafast component is absent, i.e. a 2 picosecond decay followed by a nanosecond decay (Figure 6 B, D). Thus, the sub-picosecond component is confirmed to be an internal conversion process.

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Figure 6. Decay associated spectra (DAS) obtained from global fit on the TA spectra of (A-B) CdAu24 and (C-D) HgAu24 with respective excitation at 360 nm and 730 nm. By combining the relaxation dynamics data of Cd/Hg doped clusters with the data of [Au25]-,0 and Pd/Pt doped ones,23,46 we can now make a systematic comparison. Figure 7 compares the kinetic traces of all six nanoclusters probed at ESA around 600 nm. Table 2 lists their HOMO-LUMO gaps, valence electron numbers, and relaxation decay time constants. One can clearly see that these six nanoclusters can be divided into two groups. Group I, including [Au25]-, CdAu24 and HgAu24, show a subpicosecond decay followed by a non-decay plateau, and the picosecond relaxation is weak. Group II, including [Au25]0, PdAu24 and PtAu24, one can observe that the 3~5 ps decay component dominates the relaxation dynamics, which significantly shortens the excited state lifetime. The >1 ns component observed in group II nanoclusters should be assigned to the charge transfer state (or trap state) arising from the shell gold. Moreover, the short-lived lifetimes in PtAu24 and PdAu24 can also be caused by the small bandgap (about 0.3 eV).22 Previously, we investigated the effect of charge state on the dynamics of Au25 nanoclusters46 and concluded that the charge state affected the dynamics. Here, it is found that, despite the fact that all the four doped nanoclusters are neutral (CdAu24, HgAu24, PdAu24, PtAu24), the Cd/Hg doped ones are similar to the

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anionic Au25 while the Pd/Pt doped ones are more like the neutral Au25. These results indicate that the electron dynamics of the 25-atom nanoclusters is closely related to the number of valence electrons.

Figure 7. Kinetic traces from TA measurements probed at 600 nm and the corresponding fits of (A) [Au25]-, CdAu25, and HgAu24; (B) [Au25]0, PdAu25, and PtAu24 nanoclusters. Table 2. Time constants obtained from global fitting on TA spectra of Au25 and doped nanoclusters with excitation at 360 nm. HOMO-LUMOGap Valence (eV) electrons

Relaxation time constants

Group I [Au25(SR)18]-

1.3±0.1

8e

600 fs, 5.6 ps, 100 ns

[CdAu24(SR)18]0

1.4±0.1

8e

450 fs, 1.8 ps, 200 ns

[HgAu24(SR)18]0

1.2±0.1

8e

450 fs, 2.8 ps, 50 ns

[Au25(SR)18]0 [ref. 48]

1.3

7e

1000 fs, 5 ps, >1 ns

[PdAu24(SR)18]0 [ref. 23,49]

0.3

6e

600 fs, 5 ps, >1 ns

[PtAu24(SR)18]0 [ref. 22,23]

0.3

6e

600 fs, 3.5 ps, >1 ns

Group II

Our data suggests that the difference in relaxation dynamics of MAu24 nanoclusters can be categorized and explained by the number of valence electrons. In group I, all the three nanoclusters have the 8-electron structure.47 In group II, the neutral Au25 has a 7-electron configuration48 while the Pd/Pt doped nanoclusters 12


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have the 6-electron configuration.49,50 Previous work ascribed the short excited state lifetime of [PtAu24(SR)18]0 to its smaller HOMO-LUMO gap.22 After comparing the energy gaps, one can observe that, although [Au25(SR)18]0 has a comparable energy gap to that of [Au25(SR)18]-, [Au25(SR)18]0 exhibits a much shorter excited state lifetime. Therefore, the relatively short lifetime of [PtAu24(SR)18]0 is also largely owing to its non-eight-electron configuration. Very recently, Jiang and coworkers investigated the interaction between hydrogen and [Au25(SR)18]q nanoclusters and found that hydrogen acts as a “metal” by releasing its 1s electron.51 They found that after doping the non-eight-electron [MAu24(SR)18]q with hydrogen and making the nanocluster a 8-electron configuration, the nanocluster exhibits a similar HOMO-LUMO gap and frontier orbitals as those of other non-doped, 8-electron nanoclusters.51 In the current work, our data implies that the number of valence electrons largely affect the electronic structure as well as the electron dynamics of MAu24 systems. It would then be helpful to discuss the assignment of the fast decay component in the 25-atom series of nanoclusters. The sub-ps component in group I can be explained by internal conversion and the 2-5 ps decay can be explained by core-shell charge transfer or relaxation to the surface trap state. It has been concluded that the absorption peaks (within the 400-800 nm range) of 25-atom nanoclusters are mainly contributed by the MAu12 (M=Au/Pd/Pt) core.27 As the ground state bleaching (GSB) in group I is long-lived, the nanosecond relaxation dynamics should mainly arise from the metal core. The picosecond component should only dissipate a minor part of the excited state energy by either charge transfer or energy re-distribution. In group II, the majority of the GSB signal disappears within 30 ps, which suggests that the core should not contribute to the TA signal after 30 ps. The 3~5 ps decay should thus be a charge transfer from the metal core to the surface states, which dominates the relaxation dynamics (see Scheme 1).

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Scheme 1. Relaxation dynamics of eight-electron and non-eight-electron configuration [MAu24]q nanoclusters, the dominated excited state relaxation pathways are indicated by block arrows in red in both cases. Conclusion In summary, we have investigated the temperature-dependent absorption and fs/ns relaxation dynamics of [CdAu24(SR)18]0 and [HgAu24(SR)18]0 nanoclusters and compared them with the previously reported [Au25(SR)18]q and [PdAu24(SR)18]0 and [PtAu24(SR)18]0 nanoclusters. It is found that Cd doping increases the electron-phonon coupling strength and phonon frequency in the CdAu24 nanocluster, while Hg doping shows an opposite trend. Interestingly, the relaxation dynamics behavior of the 25-atom series can be divided into two groups by the number of nominal valence electrons. Those with a 8-electron configuration (CdAu24, HgAu24 and negatively charged Au25) show a strong internal conversion (

Excited-State Behaviors of M1Au24(SR)18 Nanoclusters: The Number

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