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Temperature-Dependent Optical Absorption Properties of Monolayer-Protected Au25 and Au38 Clusters Mary Sajini Devadas,† Semere Bairu,† Huifeng Qian,‡ Ekkehard Sinn,† Rongchao Jin,‡ and Guda Ramakrishna*,† † ‡
Department of Chemistry, Western Michigan University, Kalamazoo, Michigan 49008, United States Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States
bS Supporting Information ABSTRACT: The temperature dependence of electronic absorption is reported for quantum-sized monolayer-protected gold clusters (MPCs). The investigations were carried out on Au25L18 (L = SC6H13) and Au38L24 (L = SC2H4Ph) clusters, which show discrete absorption bands in the visible and near-infrared region at room temperature and with a decrease in temperature: (i) the optical absorption peaks become sharper with the appearance of vibronic structure, (ii) the absorption maximum is shifted to higher energies, and (iii) the oscillator strengths of transitions increased. Smaller temperature dependence of absorption is observed for plasmonic gold nanoparticles. The results of the band gap shifts are analyzed by incorporating electronphonon interactions using the O’DonnellChen model. An average phonon energy of ∼400 cm1 is determined, and is attributed to the phonons of semiring gold. The unique property of decreasing oscillator strength with increasing temperature is modeled in the DebyeWaller equation, which relates oscillator strength to the excitonphonon interaction. SECTION: Nanoparticles and Nanostructures
Q
uantum-sized monolayer-protected gold clusters (MPCs) have received enormous research attention because of their exciting optical, electrochemical, and catalytic properties.111 MPCs with a size less than 2.2 nm12 show prominent quantum confinement effects with an energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The advent of efficient synthetic methodologies to prepare and characterize monodispersed gold clusters has enabled researchers to probe their unique physicochemical properties.3,7,8 These measurements can provide indepth understanding of the evolution of molecular gold to nanoparticle gold electronic transitions. Among several MPCs investigated, Au25L18 and Au38L24 clusters are under the spotlight of research interest for both theoreticians and experimentalists as these clusters show discrete electronic absorption and electrochemical properties.1331 Recently, the crystal structures of these clusters have been determined.1315,18 Both structures feature a gold coreshell system (Scheme 1). The core is (Au13 for Au25L18 clusters1315 and Au23 for Au38L24 clusters18) passivated by a semiring gold shell (consisting of six dimeric staple motifs (SAuSAuS) for Au25L18 and six dimeric plus three monomeric staples for Au38L24). Density functional theory (DFT) calculations performed based on the structures have successfully modeled the experimentally observed absorption spectra of Au25L18 and Au38L24 at room temperature.1924 Ultrafast optical measurements of MPCs demonstrate the existence of the gold-core and semiring gold states and possible relaxation between them.2529 In a recent study, we have shown r 2011 American Chemical Society
Scheme 1. Crystal Structure of a Au25L18 Cluster15 (Magenta: Au; Yellow: S) and Corresponding Cartoon Diagram Depicting Core States and Semiring States
unique ultrafast visible luminescence from Au25L18 clusters and a stepwise relaxation from gold core-states to semiring states.29 Although several investigations have focused on the optical properties of MPCs, its understanding is far from complete. Several questions remain to be answered, such as the nature of excitons (Wannier or Frenkel) in these MPCs, the electron phonon interactions, and the coupling of the gold-core states Received: September 21, 2011 Accepted: October 14, 2011 Published: October 14, 2011 2752
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Figure 1. (A) Optical absorption spectra of Au25(C6S)18 and (B) optical absorption spectra of Au38(PhC2)24 clusters dissolved in a methyl cyclohexane:methyl cyclopentane mixture at two different temperatures.
with semiring gold states. Temperature-dependent optical measurements can answer some of these questions, as they are often used to understand the electronphonon and excitonphonon interactions in semiconductor structures. Although there have been some steady state and time-resolved advanced optical measurements on the MPCs, the temperature dependence is sparsely investigated. Miles and Murray31 have reported on the temperature dependence of electrochemical properties in quantum-size clusters and showed that the energy gap increases with a decrease in temperature. Pradeep and co-workers32,33 have measured the temperature dependence of luminescence in MPCs and have shown that the intensity of near-infrared luminescence34 decreases with decreasing temperature. Apart from these studies, very few theoretical and experimental studies have focused on the temperature-dependent optical properties of MPCs. In the present investigation, systematic temperature-dependent optical absorption measurements are carried out on Au25L18 and Au38L24 clusters. The measurements are performed to understand the variation in the energy gap, electronphonon and exciton-phonon interactions, and the coupling of gold-core states with semiring states as a function of temperature. A comparative study was also carried out with the gold nanoparticles showing surface plasmon absorption. The measurements have shown interesting trends of optical absorption as a function of temperature in quantum-sized clusters (as opposed to plasmonic nanoparticles), such as shift to higher energies, appearance of fine structure, and increased oscillator strengths with decreasing temperature. The observed temperature-dependent electronic absorption is modeled with the electronphonon and excitonphonon interactions where the core-gold electrons couple with semiring gold phonons. The average energy of phonon modes, 0 K energy maximum, and excitonphonon coupling strengths are obtained from the analysis. Electronic absorption spectra of the Au25L18 and Au38L24 clusters dissolved in methyl cyclohexane:methyl cyclopentane (1:1 v/v forms a clear glass at low temperature) are shown in Figure 1. At room temperature, Au25L18 shows absorption centered at 675 nm with additional peaks in the visible region. Similar optical absorption features are observed for Au25L18 in other solvents at room temperature, hence, they are essentially solvent independent.15 The transitions are ascribed to anionic Au25L18 clusters, and the electrochemical measurements concur with it.11 For Au38L24 clusters, the electronic absorption spectra
were taken in a wider wavelength window from 350 to 1200 nm. A broad absorption maximum at 1055 nm is observed along with other spectral features in the visible region. The samples are of molecular purity and the absorption spectra match well with earlier results.17,18 Interestingly, the optical absorption at low temperature for Au25L18 clusters shows two clearly resolved lowenergy absorption peaks with maxima around 658 and 730 nm, together with other sharp features in the 400 to 480 nm region. The spectrum for Au25L18 at 78 K is very striking and systematic temperature absorption studies were performed to probe further. Similarly, the Au38L24 clusters have shown unique features at 90 K when compared to the room-temperature absorption spectrum. Three features can be noticed with decreasing temperatures: (i) the absorption features became sharper and new peaks emerged, (ii) the absorption maximum shifted to higher energies, and (iii) the oscillator strengths increased significantly. Since the absorption features are quite broad for Au25L18 and Au38L24 clusters at higher temperatures, absorption intensity (A*λ2) analysis was performed.6 Figure 2A,B shows the plots of absorption intensity versus energy at different temperatures for Au25L18 and Au38L24, respectively. The absorption intensity plots show well-resolved peaks, which are used to analyze the temperature dependence of the spectral energies and oscillator strengths. The temperature-dependent absorption analysis (Figure 2) provides important information on the trends of absorption maxima, vibronic features, and corresponding oscillator strengths. First, the absorption maxima are shifted to higher energies with a decrease in temperature (for example, for Au25L18 clusters, the low-energy maximum of 1.81 eV is shifted to 1.90 eV, and the 2.79 eV peak shifts to 2.87 eV). Similar shifts to higher energy are observed in the Au38L24 clusters. The temperature-dependent absorption shift to higher energy correlates well with the increase in the electrochemical energy gap with decreasing temperature.31 Further analysis of the shift of absorption is pursued in the context of band gap variation in semiconductor structures. It should be noted that the gold nanoparticles with surface plasmon absorption did not show significant change in the absorption maximum with temperature (see Supporting Information). Second, the absorption features became very sharp, and new vibronic features are observed with decreasing temperature. For example, the 1.81 eV (HOMO f LUMO) peak of Au25L18 clusters resolves into two peaks with maxima around 1.67 and 1.90 eV, respectively. The observed 1.67 eV peak is assigned to the PzfLUMO transition,23 which 2753
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Figure 2. Intensity spectra at different temperatures57 for (A) Au25(C6S)18 and (B) Au38(PhC2)24 clusters.
was predicted theoretically and had been observed as a shoulder in room-temperature experiments. Similar sharp vibronic features are resolved for Au38L24 clusters. The sharp electronic transitions and new vibronic features observed in MPCs are analyzed by incorporating electronphonon interactions (vide infra). Finally, an interesting trend of increased absorbance or absorption intensity (i.e., enhanced oscillator strength) is observed for the investigated MPCs with a decrease in the temperature, which is quite unusual for either semiconductors or organic molecules. The most notable result of the temperature-dependent electronic absorption spectra is the shift of the absorption maximum to higher energies with decreasing temperature. This result is the first report of the variation of optical absorption maximum with temperature for metal clusters. The understanding of temperature-dependent optical properties of metal clusters is quite limited compared to that of their magnetic and electrical properties.35,36 Kreibig previously reported the temperature dependence of the surface plasmon resonance of small silver and gold nanoparticles embedded in a glass matrix and found that the spectrum decreases in intensity and broadens as temperature rose from 1.5 to 300 K.3739 The results were explained based on the electron phonon scattering and thermal lattice contraction with decreasing temperature.37 Doremus investigated the surface plasmon absorption in 12-nm gold nanoparticles embedded in a glass matrix and varied the temperature from 469 to 767 K.40 He observed broadening of the surface plasmon absorption and a decrease in intensity with a red shift of absorption, attributed to the decreased electron concentration with increasing temperature.40 It should be stressed that in both cases, the decrease in surface plasmon absorption intensity was less than 10% over a wider range. More recently, Link and El-Sayed41 have shown a small temperature dependence of the surface plasmon absorption of 22-nm particles in water (in the range of 291345 K) and negligible temperature dependence of the plasmon absorption band for other sizes. They correlated the small temperature dependence to electron electron scattering rather than electronphonon interactions, which can lead to significant temperature dependence.41 The assignment is consistent with the lower Debye temperature of bulk gold (170 K),42 suggesting that the contribution of phonons to the absorption spectra of plasmonic metal particles is quite limited. However, Kreibig38 has shown that size quantization of metal particles can have significant impact on the electron phonon interactions. He reported that gold nanoparticles with diameters less than 5 nm do not show any temperature dependence and attributed this to the quenching of electronphonon scattering by the level quantization.38 In this scenario, electronic
absorption spectra for the investigated quantum-size MPCs should be temperature-independent as the electronphonon interaction is negligible due to the level quantization. By contrast, our results on MPCs show drastic temperature-dependent electronic absorption. However unlike the small metallic particles studied by Kreibig,3739 our systems involve the additional SAuSAuS shell on the core-gold, which can alter the electronphonon interactions and possibly lead to additional scattering. The temperature-dependent shift of the energy gaps is a wellestablished phenomenon for semiconductor structures.4349 The shift in the semiconductor band gaps with temperature is often attributed to electronphonon and lattice expansion interactions. They are modeled with empirical relationships (Varshini relationship46 and BoseEinstein equation47), which relate the phonon occupation number with temperature, and satisfactory fits can be obtained. As the quantum-sized MPCs possess discrete electronic absorption and behave more like molecules, we invoke electronphonon interactions to model the temperature dependence of band gaps observed in quantumsized MPCs. We use a modified version of the BoseEinstein relationship that was developed by O’Donnell and Chen44 to model the temperature-dependent absorption maximum of the MPCs. The O’DonnellChen equation relates the energy (E(T)) to temperature by the following expression:44 Æhυæ EðTÞ ¼ Eð0Þ ÆCæÆhυæ coth 1 ð1Þ 2kT where ÆCæ is a coupling constant, E(0) is the energy gap at 0 K, and Æhυæ is the average phonon mode responsible for electron phonon interaction. The magnitude of [coth [Æhυæ/2kT] 1] represents the effective number of available phonons. Shown in Figure 3A is a plot of the energy maximum as a function of the temperature for Au25L18 clusters. We have used the peak with a maximum at 1.89 eV in the plot, as the peak at 1.67 eV is not wellresolved at higher temperatures. An average electronphonon energy of 43 ( 6 meV is obtained with a 0 K energy of 1.893 ( 0.003 eV and ÆCæ of 3.6 ( 0.4. Similar analysis of the lower energy maximum for Au38L24 clusters was carried out (Figure 3B), which yielded 54 ( 7 meV phonon energy and 0 K-band gap of 1.675 ( 0.001 eV. The parameters obtained from the fit are provided in Table 1. The ÆCæ value obtained for Au25 clusters is significantly larger than that for Au38, suggesting stronger electronphonon interaction. The observed ÆCæ values for the investigated MPCs are comparable to the values reported for 2754
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Figure 3. Variation of the absorption maximum with temperature for (A) Au25(C6S)18 and (B) Au38(PhC2)24. The data is fit with the O’DonnellChen relationship (eq 1).
Table 1. Parameters Obtained from the Fitting of Temperature-Dependent Absorption Spectra sample Au25(C6S)18
Eg (0)
Æhυæ
(eV)
(meV)
ÆCæ
ÆSæ
f0
1.893 ( 0.003 43 ( 6 3.6 ( 0.4 2.53 ( 0.08 0.93 ( 0.08
Au38(PhC2)24 1.675 ( 0.001 54 ( 7 1.8 ( 0.3 3.2 ( 0.2
1.1 ( 0.2
conventional semiconductors (for example, ÆCæ = 1.47 for Si and ÆCæ = 2.94 for GaAs),44 indicating the effectiveness of electron phonon interaction. Also, the average energy of phonon modes for MPCs (∼50 meV) is significantly larger than the Debye temperature of bulk gold (15 meV),42 suggesting that the phonons responsible for the electronphonon interactions are not from the gold core (Au13 for Au25L18 or Au23 for Au38L24), but rather from the shell-gold composed of staples. As noted earlier, the increase in the oscillator strength with a decrease in temperature is one of the surprising results of our study. Within the Condon approximation, the oscillator strength of any electronic transition is independent of the temperature.50,51 This is true for most complex organic molecules. However, temperature-dependent oscillator strength is observed for some organic molecules, which was ascribed to the presence or absence of HertzbergTeller vibronic coupling or conformational relaxation or weak hydrogen bonding interactions.5153 However, such parameters cannot explain the observed temperaturedependent oscillator strength of the MPCs. It was also shown in the case of semiconductors that the oscillator strengths of the semiconductor transitions are invariant with temperature. In most cases, increases in the band widths are observed, but the oscillator strength remains constant. However, we observe a significant increase in the oscillator strength (close to 300%), apart from the bandwidth changes, with decreasing temperature. This striking result cannot be explained based on a simple increase in the solvent density or change in the refractive index, which can only account for a maximum 10% increase in the oscillator strength. One major difference between the gold nanoparticles possessing surface plasmon and the quantum-size MPCs in this work is the discrete absorption spectra for the MPCs; the latter indicates the presence of excitonic states. Thus, we use excitonphonon interactions to account for the changes in oscillator strengths.48,49 The temperature-dependent oscillator strengths (f) of the electronic transitions are modeled using
the DebyeWaller expression,48,54 which correlates the oscillator strength with excitonphonon interactions. Æhυæ f ≈ f0 exp ÆSæ coth ð2Þ 2kT where ÆSæ is a HughRhyns factor that describes the coupling strength between the exciton and average phonon mode Æhυæ,54 and f0 is the oscillator strength at 0 K. The oscillator strengths of the transitions are obtained from the electronic absorption 9 Rspectrum using the following expression: fabs ≈ 4.3 10 ε(v) dv. The oscillator strengths of the Au25L18 and Au38L24 are obtained from the analysis, and are plotted as a function of temperature (Figure 4). The fits are performed using eq 2 to obtain excitonphonon coupling strength (ÆSæ) and 0 K oscillator strengths. For the fitting, the average phonon mode obtained from the energy gap analysis was used. The corresponding fit parameters are provided in Table 1. Similar analysis was performed by Zhang et al.48,49 to fit the temperature-dependent oscillator strengths in InGaAs/GaAs quantum-well structures. It should be noted (Table 1) that ÆSæ values, which signify the excitonphonon coupling strength, are almost 10 times larger when compared to the semiconductor quantum-well structures. The discrepancy can be ascribed to the nature of excitonphonon interactions in the semiconductor quantum wells and that of quantum-size MPCs. The phonons involved in semiconductor quantum-well structures correspond to either InGaAs or GaAs, while the phonons in MPCs are of the semiring gold and might possess significant ligand contribution. Three distinct features emerged from the electronic absorption measurements with a decrease in the temperature, viz., (i) shift of optical transitions to higher energies, (ii) sharper absorption peaks with the emergence of new vibronic structure, and (iii) ∼3-fold increase in the oscillator strength. The first two features are explained on the basis of decreased electron phonon interactions at lower temperatures. The emergence of vibronic features can be explained on the basis of decreased electronphonon interactions, which sharpen the absorption peaks and resolve the vibronic features. The temperaturedependent energy maxima were fitted with the electron phonon interactions yielding average phonon energies of close to 400 cm1(∼50 meV) for the MPCs. So, which phonons are responsible for the observed stronger electronphonon or excitonphonon interactions? The AuAu core gold 2755
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Figure 4. Plots of oscillator strength versus temperature for (A) Au25(C6S)18 (1.9 eV peak) and (B) Au38(PhC2)24 (1.67 eV) clusters.
vibrations55 occur at 8090 cm1, and ultrafast coherent vibrations of similar frequency were observed for MPCs.12,28 Hence, core AuAu vibrations are probably not responsible for the phonon interactions. Recent far-infrared experiments carried out on gold clusters55,56 and MPCs have shown that the AuS stretching frequencies fall in the range of 250 cm1. However, multiple vibrations and broader features were observed for MPCs with staple motifs that can shift the vibrations up to 400 cm1.55 Thus, we attribute the phonons to the semiring goldligand staple motif vibrations. This assignment is supported by crystal structures1315,18 and ultrafast time-resolved measurements,2529 which show the presence of core-gold states and semiring states corresponding to staple motifs. It has to be pointed out that the influence of AuAu core gold phonons (80 cm1) might have some contribution to the excitonphonon interactions at much lower temperatures. However, current measurements show that the shell phonons are responsible for the observed temperature-dependent absorption. It is shown both theoretically and experimentally that the electronic transitions are dominated by the gold core states absorption, suggesting that the excitons primarily arise from the core-gold. The semiring gold states do influence the core-gold electronic transitions at higher temperatures by borrowing their oscillator strengths, similar to the HertzbergerTeller vibronic coupling, which disappears at lower temperature. From the results, it is found that the phonons of semirings dictate the temperaturedependent electronic absorption spectra in quantum-size MPCs, and theoretical calculations must include additional electron phonon interactions while addressing the electronic transitions in gold clusters with a definite core. In summary, temperature-dependent electronic absorption properties of the Au25L18 and Au38L24 clusters are reported. Interesting absorption features such as shift of the absorption maxima to higher energies, sharper absorption peaks along with additional vibronic peaks, and increased oscillator strengths of the electronic transitions are observed with decreasing temperatures. The results are explained on the basis of electronphonon interactions involving core-gold electrons or excitons with semiring gold phonons. The temperature-dependent absorption maxima and oscillator strengths are fitted with models that describe the energy gaps in semiconductor quantum dots and quantum wells. Significantly larger phonon energies of ∼400 cm1 are observed for the MPCs, suggesting that they arise out of the semiring gold. Also, increased excitonphonon coupling strengths are observed for MPCs, which reduces the oscillator strength of electronic transitions at higher temperatures.
’ ASSOCIATED CONTENT
bS
Supporting Information. Synthesis and experimental details. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We acknowledge the support from U.S Army Grant No W911QY-07-1-0003 and W911NF-09-C-0135 and AFOSR Award No. FA9550-11-1-9999 (FA9550-11-1-0147). G.R. acknowledges the support of the Western Michigan University Research Development Award. ’ REFERENCES (1) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. MonolayerProtected Cluster Molecules. Acc. Chem. Res. 2000, 33, 27–36. (2) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. Nanocrystal Gold Molecules. Adv. Mater. 1996, 8, 428–433. (3) Jin, R. Quantum Sized, Thiolate-Protected Gold Nanoclusters. Nanoscale 2010, 2, 343–362. (4) Zhu, Y.; Qian, H.; Drake, B. A.; Jin, R. Atomically Precise Au25(SR)18 Nanoparticles as Catalysts for the Selective Hydrogenation of α,β-Unsaturated Ketones and Aldehydes. Angew. Chem., Int. Ed. 2010, 49, 1295–1298. (5) Lopez-Acevedo, O.; Kacprzak, K. A.; Akola, J.; Hakkinen, H. Quantum Size Effects in Ambient CO Oxidation Catalysed by LigandProtected Gold Clusters. Nat. Chem. 2010, 2, 29–334. (6) Walter, M.; Akola, J.; Lopez-Acevedo, O.; Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Whetten, R. L.; Gr€onbeck, H.; H€akkinen, H. A Unified View of Ligand-Protected Gold Clusters as Superatom Complexes. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 9157–9162. (7) Jin, R.; Qian, H.; Wu, Z.; Zhu, Y.; Zhu, M.; Mohanty, A.; Garg, N. Size Focusing: A Methodology for Synthesizing Atomically Precise Gold Nanoclusters. J. Phys. Chem. Lett. 2010, 1, 2903–2910. (8) Wu, Z.; MacDonald, M.; Chen, J.; Zhang, P.; Jin, R. Kinetic Control and Thermodynamic Selection in the Synthesis of Atomically Precise Gold Nanoclusters. J. Am. Chem. Soc. 2011, 133, 9670–9673. (9) Ramakrishna, G.; Varnavski, O.; Kim, J.; Lee, D.; Goodson, T., III. Quantum-Sized Gold Clusters as Efficient Two-Photon Absorbers. J. Am. Chem. Soc. 2008, 130, 5032–5033. 2756
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Monolayers on Gold Cluster Compounds. Langmuir 1996, 12, 3604– 3612. (57) UV/Visible absorption spectra are measured at the following temperatures for Au25L18: 78, 93, 113, 133, 153,173,193, 213, 233, 253, 263, 273, 283, 293, 303, 313, and 323 K. Temperatures of 90, 105, 120, 135, 150, 165, 180, 195, 210, 225, 240, 255, 285, and 303 K were used for Au38 L24.
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