Temperature-Dependent Absorption and Ultrafast Exciton Relaxation

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Article

Temperature-Dependent Absorption and Ultrafast Exciton Relaxation Dynamics in MAu (SR) Clusters (M = Pt, Hg): Role of the Central Metal Atom 24

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Viraj Dhanushka Thanthirige, Minseok Kim, Woojun Choi, Kyuju Kwak, Dongil Lee, and Guda Ramakrishna J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09386 • Publication Date (Web): 22 Sep 2016 Downloaded from http://pubs.acs.org on September 27, 2016

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The Journal of Physical Chemistry

Temperature-Dependent Absorption and Ultrafast Exciton Relaxation Dynamics in MAu24(SR)18 Clusters (M = Pt, Hg): Role of the Central Metal Atom Viraj Dhanushka Thanthirige,§† Minseok Kim,§‡ Woojun Choi,‡ Kyuju Kwak,‡ Dongil Lee*,‡ and Guda Ramakrishna*,† †

Department of Chemistry, Western Michigan University, Kalamazoo MI 49008, USA ‡

Department of Chemistry, Yonsei University, Seoul 120-749, Korea

ACS Paragon Plus Environment

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Abstract

Temperature-dependent and ultrafast transient absorption measurements were carried out to probe the optical properties and exciton relaxation dynamics in metal-doped (Pt and Hg) Au25 clusters. Optical absorption and electrochemistry results have shown that the Pt-doped cluster has distinctly different HOMO-LUMO gap than that of Au25 while the gap did not change much for Hg-doped Au25. A decrease in temperature had resulted in much sharper absorption features as well as increased number of absorption peaks, enhanced oscillator strength and a shift in the energy maximum to higher energies for all metal-doped Au25 clusters. Interestingly, the peaks observed for Pt and Hg-doped clusters are very different from that of undoped Au25 cluster suggesting that the altered structures play a crucial role on their optical properties. From the analysis of absorption peak shifts, higher phonon energies of 67 ± 8 meV were determined for Pt and Hg-doped Au25 clusters when compared to 43 ± 6 meV for undoped Au25. The larger phonon energies suggest stronger coupling of core-gold and shell-gold and are explained by contraction of metal-doped clusters. Ultrafast transient absorption results have shown that Pt-doping lead to faster excited state relaxation, where more than 70% of the created electron-hole pairs recombine within 20 picoseconds. However, Hg-doping and undoped Au25 relax to shell gold and recombination takes much longer time. The results are consistent with energy gap law where the smaller energy gap for PtAu24 led to faster exciton relaxation. An interesting correlation between the spin-orbit coupled transitions and bleach maximum was observed, which can be ascribed to exciton localization in Au12-icosahedron.


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Introduction Atomically precise gold clusters with their unique optical, electrochemical, magnetic and catalytic properties have captured the imagination of scientists for over a decade.1-9 The interest is fueled from both fundamental science as well as their applications in catalysis, sensors and solar cells.10-12 Among the gold clusters with known structures, most studied system is Au25(thiolate)18 which is composed of a center Au, Au12 icosahedron, and 12 Au atoms in the shell (Figure 1A).13-15 In recent years, doping one or more foreign metals into the Au25 clusters have been extensively explored as it is a powerful way to tune their electronic, optical and electrochemical properties.16-31 Several research groups have studied both mono and multi doped Au clusters and they have shown higher catalytic activity and structural stability when compared with undoped clusters. Whereas multiply doped clusters are typically resulted when the dopant is Ag or Cu, mono-metal doped MAu24 (M = Pd, Pt, Cd, Hg) clusters are obtained from a variety of synthetic methods.26-31 Optical properties of Au25 clusters were investigated by various spectroscopic techniques and wealth of information on the influence of the crystal structure and electronic properties was obtained.32-40 Temperature-dependent absorption properties of Au clusters were reported by our group where the decrease in temperature led to sharper and prominent peaks, increased oscillator strengths and shift of the absorption to higher energies.32,33 The results were interpreted by invoking electron-phonon interactions of core-gold comprising of center Au and Au12 icosahedron and the shell-gold made up of –S-Au-S-Au-S staple motifs.32 In addition, ultrafast spectroscopy has been employed to understand the exciton relaxation dynamics in Au25 clusters by various groups. Ultrafast luminescence measurements34,35 have shown cascade relaxation in the excited states while transient absorption measurements have shown ultrafast core-gold to


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shell-gold relaxation followed by long-lived excited states.36-41 The influence of charge-state on the cluster was studied by transient absorption spectroscopy and interesting correlation between the core-gold and shell-gold relaxation was observed.40 In a recent study, 2-D electronic spectroscopy was carried out to understand the hot electron relaxation dynamics in Au25 clusters.41 However, much less is known about the optical properties of metal-doped Au25 clusters when compared to undoped Au25. In a recent study, Jin and co-workers42 have shown with ultrafast transient absorption measurements that the central metal atom plays a major role in the excited spectral behavior of Pt- and Pd-doped Au25 clusters. For a better understanding of metal atom doping in Au25 clusters, more optical measurements are needed that can address the interactions of core-gold and shell-gold as well as the origin of their electronic transitions. It was shown recently that the stability of the metal-doped Au25 appears to arise from the closure of super atom electron shells.29 Replacing the central Au with Hg or Cd has led to neutral MAu24 clusters with superatomic 8-electron (1S21P6) configuration that is typically observed for anionic Au25 clusters.29 By contrast, 6-electron (1S21P4) MAu24 (M = Pt, Pd) clusters are obtained as a result of the structural distortion that accompanies 1P orbital splitting.30 It is of interest, therefore, to study how altering the central metal atom changes their electronic and optical properties. In this study, we have used the combined voltammetry, temperaturedependent absorption and ultrafast transient absorption measurements to monitor the coupling of core-gold and shell-gold and subsequent exciton relaxation dynamics.


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Experimental Methods Chemicals Hydrogen tetrachloroaurate trihydrate (HAuCl4·3H2O, reagent grade), mercury chloride (HgCl2, >99.5%), 1-hexanethiol (98%), sodium borohydride (NaBH4, 99%), hydrogen peroxide solution

(H2O2,

34

wt%),

tetraoctylammonium

bromide

((C8H17)4NBr,

98%),

and

tetrabutylammonim hexafluorophosphate (Bu4NPF6,> 99%) were purchased from Sigma-Adrich. Extrapure

grade

dichloromethane,

toluene,

acetone,

acetonitrile,

ethanol,

methanol,

tetrachloroethylene and tetrahydrofuran (THF) were used. Water was purified by using a Millipore Milli-Q system (18.2 MΩ·cm). All chemicals were used as received without further purification.

Synthesis of Au25(SR)18 , PtAu24(SR)18 and HgAu24(SR)18. Au25(SR)18 and PtAu24(SR)18 clusters were synthesized according to a procedure reported by Kwak et al.30 To synthesize HgAu24(SR)18, HgCl2·3H2O (0.013 g, 0.05 mmol), HAuCl4·3H2O (0.176 g, 0.45 mmol) and ((C8H17)4NBr (0.317 g, 0.58 mmol) were dissolved in 15 mL of THF in a 100 mL vial. After being stirred for 5 min at room temperature, the solution color changed from orange to dark red. 1-hexanethiol (0.320 mL, 2.5 mmol) was then added to the solution at room temperature. After 5 min, NaBH4 (0.190 g, 5.0 mmol, 5 mL) dissolved in an ice-cold water was added to the mixture all at once. After being stirred for additional 5 h, the aqueous phase was decanted and the remaining organic phase was washes with copious amounts of water to remove water soluble impurities. The remaining product is subsequently washed with methanol to remove excess thiol (Au25 clusters are removed during this process). After thoroughly washing the product with methanol, the product solution was subsequently rotary evaporated to near


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dryness. Highly pure HgAu24(SR)18 clusters were then extracted from the cluster product with 5:10 (v/v) dichloromethane/acetonitrile mixture. The composition of the cluster product was confirmed by MALDI mass spectrometry. In addition, the metal ratio determined by the inductively coupled plasma mass spectrometry was found to be Hg1.35 Au23.65, matching well with the MALDI result. (See supporting information S1, S2)

Methods Matrix-assisted laser desorption ionization (MALDI) mass spectra were obtained using an AB Sciex MALDI-TOF mass spectrometer (4800 plus) equipped with a standard UV nitrogen laser (337 nm). Samples were prepared by drop casting a mixture of a cluster solution (0.7 mM CH2Cl2 ) and a matrix, trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene] malononitrile (DCTB), saturated in CH2Cl2 on a sample plate. X-ray photoelectron spectroscopy (XPS) measurements were performed on an XPS system (K-alpha, Thermo UK) using a monochromatic Al Kα X-ray source (1486.6 eV). Samples were loaded onto a glass plate and binding energies were calibrated to the C1s peak at 284.8 eV. Surface chemical analysis software (Thermo Avantage version 5.35) was used to curve fit the peak position and integrated intensities. Absorption spectrometry was carried out on a Shimadzu UV-Vis-NIR spectrophotometer (UV-3600) using cluster solutions in tetrachloroethylene. 1H-NMR spectrometry was conducted on a 250 MHz FT-NMR spectrometer (DPX 250, Bruker Biospin). Voltammetry was carried out with an electrochemical workstation (model 660 B, CH Instruments) in CH2Cl2, containing 0.1 M tetrabutylammonium hexafluorophosphate as a supporting electrolyte that was degassed and blanketed with a high-purity Argon gas. Square wave voltammetry was conducted at 100 mV/s with a pulse height and a width of 20 mV and 20


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ms, respectively. A Pt disk (0.4 mm diameter) was used as the working electrode, a Pt disk (0.4 mm diameter) as the counter electrode, and a Ag wire quasi-reference electrode as the reference electrode for the voltammetric measurements. Ferrocene (Fc+/0) was added as an internal reference for AgQRE. In this paper, potentials are reported versus Fc+/0. Ground state electronic absorption and temperature-dependent absorption measurements were carried out using a Shimadzu UV 2101 PC absorption spectrometer. An Optistat DN cryostat (Oxford instruments), temperature controller and a pressure gauge were used to conduct the temperature-dependent experiments from 77 to 303 K. Methylcyclohexane: methylcyclopentane (1:1 v/v forms a clear glass at low temperature) were used as the solvents for temperaturedependence measurements. Femtosecond transient absorption measurements were carried out at the Center for Nanoscale Materials, Argonne National Laboratory. S1 Briefly, a Spectra Physics Tsunami Ti:Sapphire, 75 MHz oscillator was used to seed a 1.66 KHz Spectra-physics Spit-Fire Pro regenerative amplifier. 95% of the output from the amplifier is used to pump a TOPAS optical parametric amplifier, which is used to provide the pump beam in a Helios transient absorption setup (Ultrafast Systems Inc.). A pump beam of 370 nm was used for the measurements. The remaining 5% of the amplifier is focused onto a sapphire crystal to create a white light continuum that serves as the probe beam in our measurements (440 to 780 nm). The pump beam was depolarized and chopped at 833 Hz, and both pump and probe beams were overlapped in the sample. Little degradation of the samples was observed after the measurements. All the analysis was carried out in Surface Xplorer Pro.

Results and Discussion


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Parts A to D of Figure 1 show the crystal structure, MALDI-MS, optical absorption and SWV data of the synthesized metal-doped Au25 clusters. As shown in the structure of the doped cluster (Figure 1A), both Pt and Hg dopants are preferably located at the center of the core, but the overall crystal structure seems to be unaffected with similar Au12 icosahedron covered with –SAu-S-Au-S staple motifs comprising of shell-gold. Matrix-assisted laser desorption ionization (MALDI) mass spectra in Figure 1B shows that there is only a single peak observed for each cluster, verifying the high purity of the synthesized clusters. Although the mass differences between the doped and undoped clusters are very small, the isotope patterns of Pt- and Hg-doped clusters can clearly be distinguished from that of Au25(SR)18 and match well with the simulated isotope patterns of PtAu24(SR)18 (green) and HgAu24(SR)18 (purple), respectively, as can be seen in the insets of Figure 1B and Figure S1A. These clusters are abbreviated as Au25, PtAu24, and HgAu24. The fragment peaks of the HgAu24 in Figure S1 shows sequential loss of Au(SR) units and all contain Hg, indicating that Hg dopant is likely to be in the core. Additionally, the X-ray photoelectron spectrum (XPS) of HgAu24 shows that the Hg 4f7/2 peak is observed at 99.8 eV, indicating that the doped Hg is Hg0. This strongly suggests that Hg atom is at the center of the HgAu24 cluster; a larger binding energy would be expected if it was located at the surface of the core or in the ligand shell.28 As reported before,30 the charge-state of the isolated Au25 and PtAu24 were found to be, respectively, -1 and 0 (i.e., [Au25]- and [PtAu24]0). In parts B and C of Figure S2, the XPS analysis of the isolated HgAu24 indicates that there is no nitrogen (from N(C8H17)4+) nor Cl- associated with the cluster. Additional NMR analysis in Figure S3 shows that there is no N(C8H17)4+ peak found, suggesting that the synthesized HgAu24 is charge neutral. This is in good agreement with the center-doped HgAu24 cluster prepared by a metal exchange


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method.29 Taken together, these results undoubtedly confirm that the isolated Hg-doped cluster is [HgAu24]0, where the Hg atom is located at the center. In a recent study, it was found that replacing the central gold atom of the Au25 cluster with Pt leads to a drastic change in absorption spectral features as shown in Figure 1C.30 Most important among them is the appearance of the NIR band centered near 1.05 eV. The change was ascribed to Jahn-Teller-like distortion occurring when the superatomic electron configuration changes from 8-electron Au25- (1S21P6) to 6-electron PtAu240 (1S21P4). The structural distortion accompanies the 1P orbital splitting, giving rise to a new NIR band. By contrast, HgAu240 keeps the same 8-electron configuration and thus its absorption spectrum is quite similar to that of Au25-. There have been two doping locations identified for Hg-doped clusters; one is centerdoped and the other is outer-shell-doped HgAu24, prepared by metal exchange methods.28, 29 In Figure 1C, the first absorption peak observed at 1.69 eV (734 nm) matches reasonably with the simulated optical spectrum and clearly different from that of the outer-shell-doped HgAu24 (~697 nm).28 To gain further insight into the electronic structures of the doped clusters, we carried out square-wave voltammetry (SWV) measurements for Au25, PtAu24 and HgAu24 and corresponding curves are shown in Figure 1D. As can be seen in the figure, all voltammograms exhibit well-resolved current peaks that lie at the formal potentials of the charge-state couples of the clusters. The open-circuit potentials are found to be -0.49, -0.49, and -0.48 V for Au25, PtAu24, and HgAu24, respectively. Accordingly, O1, O2, and O3 denote the first, second and third oxidation couples, and R1 and R2 denote the first and second reduction couples of the clusters. The HOMO-LUMO gap of each cluster can then be obtained by subtracting the charging energy term (O1-O2) from the electrochemical gap (O1-R1).30 The formal potentials


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and predicted HOMO-LUMO gaps are summarized in Table S1. Whereas the small HOMOLUMO gap (0.29 V) observed for PtAu24 suggests the 1P orbital splitting, the HOMO-LUMO gaps of Au25 (1.29 V) and HgAu24 (1.20 V) are quite similar. These results again point to that Hg-doping has smaller effect on both the ground state optical and electrochemical properties. To further probe the influence of central metal on the coupling of core-gold/shell-gold and exciton relaxation, temperature-dependent absorption and ultrafast transient absorption measurements were carried out.

Figure 1. (A) Structures of Au25(SR)18 (left) and doped MAu24(SR)18 clusters (M = Pt, Hg) (right). Color codes: golden, gold atoms of the core; olive, gold atoms of the shell; green, sulfur atoms (the rest of the ligand is omitted for clarity). (B) MALDI mass spectra of Au25(SR)18,


10

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PtAu24(SR)18, and HgAu24(SR)18. The insets show the comparison between the experimental data (lines) and the simulated isotope patterns (black bars). (C) UV-vis-NIR absorption spectra of Au25, PtAu24, and HgAu24 in trichloroethylene. The energy-scale absorption spectrum, Abs(E), was converted from the wavelength-scale, Abs(), according to the relation Abs(E)  Abs()2. (D) SWVs of Au25, PtAu24, and HgAu24 obtained in CH2Cl2 containing 0.1 M Bu4NPF6.

In a previous study, we have shown that the core-gold/shell-gold electron phonon interactions dominate the temperature-dependent absorption in Au25 clusters.32,33 At low temperatures, transitions that are sharp with increased optical densities were observed for Au25. These are assigned to electron-phonon and exciton-phonon interactions with a phonon energy of 45 meV that was assigned to the bending mode of the shell gold.43 Temperature-dependent absorption measurements were carried out for HgAu24 and PtAu24 clusters and are shown in Figure 2. The results have shown changes similar to Au25 with a shift of absorption to higher energies, appearance of new vibronic peaks and increased absorption at low temperatures. Closer analysis of temperature-dependent absorption has revealed interesting features for PtAu24 (Figure 2B) and HgAu24 (Figure 2C) when compared to Au25 (Figure 2A). A clear shift in absorption to higher energies was observed with a decrease in temperature as well as an increase in oscillator strength for both doped and undoped clusters.


11


A

25

1.90 eV 2.57 eV

3

PtAu24 2.16 eV

78 K 2

1

3.34 eV

3.06 eV

77 K

10

1.67 eV

3.34 eV

15

4

3.07 eV 3.24 eV

4

4

B

2.87 eV

Au

Abs Intensity (10 )

Abs. Intensity (10 )

5

1.93 eV

5 305 K

303 K

0

0 1.5

1.8

2.1

2.4

2.7

3.0

3.3

1.5

2.0

8

C

2.5

3.0

3.5

4.0

Energy (eV)

Energy (eV) 2.88 eV

HgAu24

3.11 eV

2.72 eV 2.60 eV

4

Abs Intensity (10 )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.79 eV

6

4

1.66 eV

1.95 eV 77 K 2.36 eV

2 303 K 0 1.5

2.0

2.5

3.0

Energy (eV)

Figure 2. Temperature-dependent absorption intensity (Abs()2) spectra at different temperatures for (A) Au25, (B) PtAu24 and (C) HgAu24.

The shift in absorption can be attributed to electron-phonon interactions as in the case of semiconductors and similar treatment was applied in our previous study.32 Knappenberger and co-workers34 have observed similar shifts in their luminescence measurements and a phonon energy of 50 meV was determined from their analysis. To understand the influence of central metal atom on the phonon energy and the coupling of core-gold and shell-gold, similar analysis as described previously for Au25 was performed for metal-doped Au25. Parts A and B of Figure 3 show the plot of energy gap versus temperature for PtAu24 and HgAu24, respectively. The


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obtained data for PtAu24 (Figure 3A) and HgAu24 (Figure 3B) were fitted using O’Donnell-Chen relationship.44

0

〈 〉〈hυ〉 coth

〈

〉

1

(eq 1)

where, 〈 〉 is a coupling constant, E(0) is the energy gap at 0 K, and 〈hυ〉 is the average phonon mode responsible for electron-phonon interaction. The magnitude of coth

〈

〉

1

represents the effective number of available phonons. The obtained phonon energies as well as the coupling terms are provided in Table 1. A comparison of undoped Au25 is also provided in table 1. Note two major differences between undoped and metal-doped Au25. Firstly, the phonon energy is around 67 ± 8 meV for metal-doped Au25 when compared to 43 ± 6 meV for undoped Au25. Secondly, the coupling constant is significantly larger (6.0 for metal-doped Au25 compared to 3.6 for Au25) for metal-doped Au25 indicating the strong coupling of core-gold and shell-gold. Larger phonon energy suggests that the –S-Au-S-Au-S bending mode is stronger in the case of metal-doped Au25 clusters when compared to undoped Au25. These results can be explained by considering the differences in their structures. In an extended X-ray absorption fine-structure (EXAFS) study of PtAu24,16 Zhang et al. have found that replacing the central Au with Pt in the Au25 cluster causes contraction of the cluster; the average M-Au (Au12-icosahedron) bond length as well as the Au-S bond length in the shell are shortened compared to those of undoped Au25. X-ray crystallography studies also revealed that the Pt-doped cluster are indeed shrunk.17 Structural data are not available for the center-doped HgAu24, but comparison with the similar center-doped CdAu24 suggests that the HgAu24 is likely to be shrunk.29 The shorter bond of shellgold increase the phonon energy of bending mode that is responsible for core-gold and shell-gold


13


coupling. The higher interaction energy between the central dopant and Au24 frame calculated for PtAu24 also supports the stronger core-shell coupling in metal-doped Au25 clusters.20 These measurements were able to clearly prove the role of central metal atom in altering the phonon energy that couples the core-gold and shell-gold.

PtAu24

HgAu24

1.80

2.16

1.79

A

Energy Gap (eV)

Energy Gap (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2.14

2.12

1.78

B

1.77 1.76 1.75 1.74

2.10

1.73

100

150

200

250

300

100

150

Temperature (K)

200

250

300

Temperature (K)

Figure 3. Plots of energy-gap versus temperature for (A) PtAu24 and (B) HgAu24. Red curves are obtained by fitting the data to O’Donnell-Chen relationship.

Table 1. Parameters obtained from fitting the energy gap versus temperature to O’Donnell-Chen relationship. Sample

Eg (0) (eV)

El-ph energy (meV)

Au25

1.893 ± 0.003

43±6

3.6±0.4

PtAu24

2.167 ± 0.003

67±6

6.4±0.8

HgAu24

1.796 ± 0.002

68±9

6±2


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One of the interesting features with regard to the absorption bands for Au25 at low temperatures is the splitting of the lowest energy absorption into two different bands separated by 230 meV. This splitting was not reproduced by conventional theoretical calculations but Jiang and co-workers45 were able to replicate them with the incorporation of spin-orbit coupling in their calculations. Interestingly for HgAu24, with a decrease in temperature, the lowest energy absorption has split into three vibronic bands each separated by an average energy of ~140 meV that is quite different from that of Au25. This result shows that even though the HOMO-LUMO gaps are similar for Au25 and HgAu24, the transitions that are responsible for the observed electronic transitions might be different. Interestingly for HgAu24, higher energy absorption centered at 2.5 eV (Figure 2C) has shown a split similar to that of Au25 with a 240 meV separation. Also, temperature-dependent absorption spectra of PtAu24 shown in Figure 2B show the emergence of two peak split at around 2 eV with an energy separation of 230 meV quite similar to Au25. These interesting features are not observed at higher temperatures and become prominent at only low temperatures. Further computer simulations are necessary to clarify the observed splitting in HgAu24 and PtAu24 clusters. Ground state optical and electrochemical properties have revealed that the energy gap is significantly altered for Pt-doped Au25 and little change was observed for Hg-doped Au25. On the other hand, temperature-dependent absorption measurements have shown that the coupling of core-gold and shell-gold has increased with metal atom doping. In addition, the phonon energy has become significantly larger with central metal atom doping. Previous studies of femtosecond transient absorption for Au25 and related clusters revealed faster relaxation of core-gold and shell-gold followed by long-lived state.36-40 In a recent study, Jin and co-workers42 have investigated the ultrafast relaxation dynamics of doped MAu24 clusters (M = Pd, Pt) using


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ultrafast transient absorption measurements. However, the exciton relaxation dynamics and the origin of transitions in these metal-doped clusters are largely unexplored. In this study, we have carried out femtosecond transient absorption measurements on undoped Au25, HgAu24 and PtAu24 after exciting at 370 nm to better understand the influence of the central metal atom on their optical properties. Excited state absorption (ESA) and kinetics measurements were carried out for all clusters and complete spectra are shown in Figure S4. Main differences between Au25 and metal-doped (Pt, Hg) Au25 were revealed in their ESA spectral features and kinetic decay constants. For the comparison of the influence of the central metal atom on ultrafast exciton relaxation dynamics, species associated spectra obtained from global fit analysis for Au25, PtAu24 and HgAu24 are shown in parts A, B and C of Figure 4. As discussed above, the lowest energy absorption in the case of Au25 arises from superatom transition between P1/2, P3/2 and D type orbitals as explained by Jiang et al.45 Interestingly, the bleach is localized on P1/2D transition for Au25 as revealed in the component’s bleach maximum (C3, long-lived). A recent study of 2D electronic spectroscopy by Knappenberger and co-workers41 has shown that the ground state bleach arises from the Px,Py to D orbitals which is same as the P1/2D transition. Theoretical calculations by Chen et al. have also shown that the exciton is localized on the orbitals related to Au13 core.46 This result confirms that the excitons in Au25 do not recombine within 1 ns. Longer time scale measurements show that the lifetimes of Au25 clusters are in 100s of nanoseconds.47,48


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3

C1 (630 fs) C2 (2.3 ps) C3 (long)

Au25

A Amplitude

2

1

0

-1

500

550

600

650

700

750

Wavelength (nm)

1.0

PtAu24

B

Amplitude

0.5

0.0

-0.5

C1(1 ps) C2 (3.5 ps) C3 (long)

-1.0

-1.5 450

500

550

600

650

700

750

Wavelength (nm)

0.6

C

HgAu24

C1 (720 fs) C2 (140 ps) C3 (long)

0.4

Amplitude

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.2

0.0

-0.2 500

550

600

650

700

750

Wavelength (nm)

Figure 4. Species-associated spectra obtained from global fit analysis for (A) Au25, (B) PtAu24 and (C) HgAu24.


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Page 18 of 30

On the other hand, the ESA spectra of PtAu24 (see Supporting Information, Figure S4) are quite similar to what was reported by Jin and co-workers42 and the species associated spectra obtained for PtAu24 (Figure 4B) matched well with the reported data. However, in that study, the relaxation was assigned to faster core-gold to shell-gold relaxation (1 ps and 3.5 ps) followed by long-lived excited states for PtAu24 clusters. We interpret the results differently as the bleach observed in the global fit analysis arises from the disappearance of the ground state absorption. The bleach recovery has to be attributed to exciton relaxation dynamics as the absorption in the visible region arises from the transitions involving core-gold comprising of Au12 icosahedron and central metal atom.46 The species associated spectrum of PtAu24 (Figure 4B) shows a bleach with component C2 (3.5 ps) that is centered around 590 nm and this bleach is similar to the component C3 observed for Au25. The position of the bleach maximum for PtAu24 is around 2.10 eV, which is where the room temperature absorption spectrum of PtAu24 was split into two peaks at low temperatures. (Figure 2B) Coincidentally, similar split absorption (P1/2D) was responsible for the bleach in Au25. From these results, the exciton relaxation in PtAu24 can be explained as the relaxation of core-gold (1 ps) to give rise to the bleach where the exciton is localized. This exciton recombines quickly (3.5 ps) to populate the ground state of PtAu24. The longer component C3 (long) observed for PtAu24 is assigned to ligand centered transitions. This result is quite different from that of Au25 where the exciton relaxation takes a long time (~100s of nanoseconds). Similar transient measurements carried out for HgAu24 revealed interesting features. The species associated spectrum of HgAu24 (Figure 4C) shows a core-gold to shell-gold relaxation of 700 fs followed by thermalization of the lowest ESA (component C2 ~140 ps) to give rise to a


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bleach centered on 495 nm. This bleach does not recombine within 1 ns (C3). The exciton recombination observed for HgAu24 is similar to Au25 but different from PtAu24. A comparison of bleach recovery kinetics that represents the exciton relaxation is provided in Figure S5 that shows Pt-doped cluster behaves differently than that of undoped and Hg-doped Au25. Optical absorption and electrochemistry measurements have shown that the HOMO-LUMO gap is around 0.29 eV for PtAu24 (Table S1) while it is much larger for Au25 and HgAu24. Faster recombination observed for PtAu24 can be attributed to its low HOMO-LUMO gap that is consistent with the energy gap law.49 Here again, one interesting observation can be made with respect to the bleach maximum observed for HgAu24. (Figure 4C) The bleach maximum arose around the 2.50 eV, where the room temperature absorption of HgAu24 was split into two peaks with a 240 meV separation (Figure 2C). This is coincidentally similar to un-doped Au25, where the spin-orbit split transition (P1/2D) was responsible for observed bleach. Further computer calculations are needed to prove the presence of spin-orbit split transition in HgAu24. From the combined temperature-dependent absorption and femtosecond transient absorption measurements, three interesting features with respect to the role of the central metal atom on the optical properties of Au25 clusters were revealed. Firstly, the phonon energy associated with the shell-gold has become higher for metal-doped clusters. Secondly, the exciton recombination has become shorter for PtAu24. Finally, an interesting observation was made that P1/2D transition of Au12-icosahedron is the dominant transition for both the undoped and doped Au25 clusters and is responsible for the observed ground state bleach of clusters. Higher phonon energies and stronger coupling of core-gold and shell-gold for metal-doped Au25 clusters was a result of the contraction of Au25 crystal structure upon metal atom doping. The shorter exciton recombination


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observed for PtAu24 was assigned to smaller HOMO-LUMO gap observed for PtAu24 (0.29 eV compared to 1.29 eV for Au25 and 1.20 eV for HgAu24). This result is in accordance with energy-gap law, where smaller energy gap leads to efficient coupling of excited state and ground state leading to faster exciton recombination. Figure 5 depicts a representative picture of involved electronic transitions in Au25 and metal-doped Au25 clusters. With Hg-doping, the HOMO-LUMO energy gap is not altered significantly while with Pt-doping, the HOMO of Au25 was split into two, and creating a lower energy LUMO and a shorter energy gap of 0.29 eV though it is not an optically allowed transition. However, this shorter energy gap causes faster non-radiative deactivation in PtAu24. Lastly, the bleach seems to arise from P1/2D transition for both metal-doped and un-doped Au25 clusters. However, further theoretical calculations are needed to further probe the origin of this transition in metal-doped clusters.

LUMO + 1

e-

Relaxation (~700 fs) LUMO +1 (D)

LUMO (D)

Relaxation (~1 ps)

> 100 ns M=Au, Hg

eHOMO (P)

h+

HOMO (P)

h+

HOMO - 1

LUMO

3.5 ps M= Pt HOMO - 1

Figure 5. Schematic diagram depicting the exciton relaxation in metal-doped Au25 clusters.


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Conclusions Combined

temperature-dependent

absorption

and

femtosecond

transient

absorption

measurements were carried out to understand the influence of central metal atom on the optical properties and electron relaxation dynamics. Steady-state absorption and voltammetry measurements show that Hg-doping of Au25 does not alter the HOMO-LUMO gap significantly whereas Pt-doping drastically decreases the gap by virtue of being a 6 electron system. Interestingly, temperature-dependent absorption measurements reveal interesting properties where in both Hg- and Pt-doping influence the core-gold/shell-gold coupling and the phonons responsible for electron-phonon interaction increases to 67 meV when compared to 43 meV for the undoped cluster. Increased electron-phonon energies can be explained by the contraction of metal-doped clusters. Femtosecond transient analysis has shown the evidence for strong coregold/shell-gold coupling as well as shorter exciton lifetime for PtAu24 consistent with energy-gap law of smaller the energy gap, faster the exciton recombination. In addition, the P1/2D transition was found to be responsible for the ground state bleach for all clusters. All the results point to the importance of central metal atom in dictating the optical properties of quantum-sized Au25 clusters.

Supporting Information Available MALDI mass spectra for Au25(SR)18 and HgAu24(SR)18, XPS spectra of HgAu24(SR)18 along with 1H-NMR data are provided. Supporting information related to voltammetry, formal potential and potential gaps and detailed transient absorption results are provided. This material is available free of charge via the Internet at http://pubs.acs.org.


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Acknowledgements D.L. acknowledges support by the Korea CCS R&D Center (KCRC) grant (NRF2014M1A8A1074219), the NRF grant (NRF-2014R1A2A1A11051032 and 2009-0093823), and the Yonsei University Future-leading Research Initiative of 2014. G.R. acknowledges the support of ACS-PRF #53999-ND5. We would like to thank Dr. Gary Wiederrecht for help with transient absorption measurements at Argonne National Laboratory. Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the U.S. Department of Energy, Office of science, office of Basic Energy Sciences, under contract no: DE-AC0206CH11357. Author Information *E-mail: [email protected], [email protected] Phone: +1 2693872854; +822-21235638. §

denotes equal contribution Notes The authors declare no competing financial interest

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TOC Graphic

Increased core-shell coupling

e-

h+

M

Modified exciton recombination

MAu24(SR)18 (M = Pt, Hg)


30

Temperature-Dependent Absorption and Ultrafast Exciton Relaxation

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