Temperature-Dependent Photoluminescence of Structurally-Precise

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Article

Temperature-Dependent Photoluminescence of Structurally-Precise Quantum-Confined Au (SCH) and Au (SC H ) Metal Nanoparticles 25

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Thomas D. Green, Chongyue Yi, Chenjie Zeng, Rongchao Jin, Stephen McGill, and Kenneth L. Knappenberger J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp505913j • Publication Date (Web): 07 Sep 2014 Downloaded from http://pubs.acs.org on September 14, 2014

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The Journal of Physical Chemistry

Temperature-Dependent Photoluminescence of Structurally-Precise Quantum-Confined Au25(SC8H9)18 and Au38(SC12H25)24 Metal Nanoparticles Thomas D. Green,a Chongyue Yi,a Chenjie Zeng,b Rongchao Jin,b Stephen McGill,c and Kenneth L. Knappenberger, Jr.a* a

Department of Chemistry and Biochemistry, Florida State University, Tallahassee, FL 323064390 b

Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213 c

National High Magnetic Field Laboratory, Tallahassee, FL 32310

KEYWORDS . Metal nanoparticles, photoluminescence, monolayer-protected clusters, temperature-dependent optical properties, electron-phonon coupling.

ACS Paragon Plus Environment

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Abstract. Temperature-dependent photoluminescence of structurally precise Au25(SC8H9)18 and Au38(SC12H25)24 monolayer-protected cluster (MPC) nanoparticles were studied using energyresolved, intensity-integrated, and time-resolved spectroscopy. Measurements were carried out at sample temperatures spanning the range from 4.5 K to 200 K following electronic excitation using 3.1-eV pulsed lasers. The integrated PL intensity for Au25(SC8H9)18 increased sharply by 70% as the sample temperature was increased from 4.5 K to 45 K. The PL intensity was statistically invariant for temperatures between 45 K and 65 K, but was quenched when the sample temperature was raised above 65 K. For both MPC samples, the global PL emission included several components. Each PL component exhibited an increase in emission energy when the sample temperature was increased from 4.5 K to 40 K. This unexpected behavior may imply that MPCs in the 1 nm domain have negative expansion coefficients. Quantitative analysis of PL emission energies and peak widths obtained at sample temperatures greater than 45 K indicated MPC non-radiative relaxation dynamics are mediated by coupling to low-frequency vibrations associated with the ligand shell that passivated the nanoclusters, which accounted for the low emission yields at high sample temperatures. Contributions from two different vibrational modes were identified: Au(I)-S stretching (200 cm-1) and Au(0)-Au(I) stretching (90 cm-1). Analysis of each PL component revealed that the magnitude of electronic-vibration coupling was state specific, and consistently larger for the high-energy portions of the PL spectra. The total integrated PL intensity of the Au25(SC8H9)18 MPC was correlated to the relative branching ratios of the emission components, which confirmed decreased emission for recombination channels associated with strong electron-vibration coupling and high emission yields for low emission energies at low temperature. The efficient low-energy emission was attributed to a charge-transfer PL transition. This conclusion was reached based on the strong


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correlation between temperature-dependent intensity-integrated and time-resolved emission measurements that revealed an ≈ 3.5 to 5.5 meV activation barrier to non-radiative decay. These findings suggest that nanoscale structure and composition can be modified to tailor the optical and mechanical properties and electronic relaxation dynamics of MPC nanostructures.


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I. Introduction. Colloidal metal nanoparticles represent a promising class of catalytic and light-harvesting nanomaterials. These opportunities depend on the unique optical and electronic properties of metals confined to nanoscale dimensions.1 For colloidal metal nanoparticles, size-dependent properties, such as the surface plasmon resonance (SPR) in nanoparticles > 3-5 nm, vary widely with particle shape and size. The SPR produces large local surface fields that amplify molecular spectroscopy signals2-5 and enhance photocatalytic processes.6-11 In contrast, quantum-confined gold nanoclusters (< 2 nm) exhibit discrete electronic states resulting in structure-specific catalytic12-21 and optical properties.22-31 It has been shown that these non-plasmonic subnanometer metal systems can also lead to photocatalytic enhancement upon visible irradiation.3235

Since these small nanoclusters cannot induce strong local electric fields, these observations

suggest that injection arising from excited charge carriers within the nanocluster may be necessary for photocatalysis. Understanding the relaxation dynamics of photo-excited nanoclusters is a critical step toward achieving a full understanding of photocatalysis mediated by nanoscale metal systems. Another possible contribution to nanoparticle-mediated photocatalysis involves thermal activation,36 in which hot carriers couple efficiently to the nanoparticle through electron-phonon interactions prior to energy transfer across the metal/environment interface. Challenges in providing predictive structure-property correlations for nanoscale metal domains result from the inherently large size distributions of colloidal gold nanoparticles. Monolayer-protected gold nanoclusters (MPCs) represent an emerging class of nanomaterials for understanding the photochemical and photophysical properties of nanometer gold domains. Much like gas-phase clusters, specific “magic” sizes of MPCs with a combination


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of electronic and geometric shell closings can be formed.18,22-25,37-48 Moreover, synthetic and electrochemical methods exist for tailoring these materials’ optical, electronic, and capacitive properties.49-50 Although Superatom models have been used to describe the electronic structure and extinction spectra of ~1-2 nm nanoclusters,22,25,37-42 electronic relaxation dynamics (including electron-phonon coupling and photoluminescence) of these intermediate nanometals following photoexcitation are poorly understood. One

of

the

more

fascinating

features

of

MPCs

is

near-infrared

(NIR)

photoluminescence.51 Based on the Jellium free-electron model, size-dependent visible emission is expected for gold domains in the 1-2 nm size range.52 However, broad emission ranging from approximately 1.8 eV to 1.4 eV are reported for Au25(SR)18 and Au25(SG)18 MPCs, where SR represents an alkanethiol and SG is glutathione.51,53-54 Broad emission spanning the range of 1.6 eV to 1.0 eV is reported for 38-atom gold nanoclusters.55 It is likely that NIR emission by MPCs arises from a hybrid of electronic states that incorporate the protecting units. This hypothesis is corroborated by ultrafast spectroscopy measurements, which suggest that electron relaxation from the core to ligand states precedes NIR radiative recombination.56-58 Recent liganddependent emission studies indicate ligand-to-gold charge transfer is important for efficient PL.54 In this manuscript, we report temperature-dependent energy-resolved, intensityintegrated, and time-resolved PL studies for Au25(SC8H9)18 and Au38(SC12H25)24 MPCs for sample temperatures ranging from 4.5 K to 200 K. These data provide direct evidence for the importance of electron-vibration coupling in ligand states for mediating non-radiative energy relaxation for both nanocluster species. The vibrations are assigned to stretching modes corresponding to the nanocluster’s protecting ligand shell incorporating surface Au atoms. These data also indicate that the magnitude of electronic-vibration coupling influences the PL emission


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yield. The broad near infrared PL of nanoclusters was assigned to charge transfer emission involving the ligand band of the MPCs. II. Experimental Section The Au25(SC8H9)18 nanoclusters (where C8H9 stands for SCH2CH2Ph) were synthesized based on a previously reported one-pot method.59 The synthesis of Au38(C12H25)24 followed a previously reported protocol.60 MPCs were prepared for temperature-dependent PL measurements by dispersing nanoclusters into a 25% solution of polystyrene in toluene. This solution was dropcast onto a clean quartz cover-slip, and allowed to dry. The sample was then mounted on the sample probe and lowered into the cryostat. A small amount of helium exchange gas was introduced into the evacuated sample chamber to bring the sample into thermal equilibrium with the liquid helium chamber (4.5 K). Control of the temperature above 4.5 K was achieved via a resistive heater mounted in the probe. Electronic excitation was achieved using a 1 kHz regeneratively amplified Ti:Sapphire laser system producing laser pulses centered at 800 nm. The output beam was attenuated and frequency doubled to produce a 400 nm excitation source. After passing through a series of dichroic mirrors and filters to isolate the second harmonic, the energy of the beam was adjusted to 1 µJ/pulse, and directed into the sample space of the cryostat. The resulting PL was collected in transmission geometry using an optical fiber.

Spectrally resolved PL experiments were

carried out using a 0.75 m spectrometer (McPherson) coupled to a liquid nitrogen-cooled CCD. Time-resolved experiments were performed using an avalanche photodiode (Quantique) coupled to a time-correlated single photon counting unit (Becker & Hickl). Long pass filters (Semrock) were used to isolate the PL from residual excitation pulses.


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III. Results and Discussion The linear absorbance spectra of Au25(SC8H9)18 and Au38(SC12H25)24 are shown in Figure 1. For both samples, the spectra consisted of multiple discrete transitions superimposed on a continuous absorption profile that increased exponentially in intensity as energy increased. The HOMO-LUMO transition for Au25(SC8H9)18 was observed at 1.8 eV, and the shoulder on the high-energy side of this peak was characteristic of the neutral charge state of Au25(SR)18.37 Computational work by Aikens and coworkers indicates that low-energy (2.5 eV) arise from electronic transitions between HOMO-n states localized on the protecting semi-rings and superatom D LUMO and LUMO+1 states.37

Figure 1. Linear absorption spectrum of Au25(SC8H9)18 (black) and Au38(SC12H25)24 (blue). The spectrum of Au38(SC12H25)24 is offset for clarity.

For Au38(SC12H25)24, the HOMO-LUMO energy gap was detected at 1.18 eV. The Au38(SC12H25)24 optical spectrum is accurately described by treating the elongated nanocluster as a nanorod.61 Theoretical descriptions using a particle-in-a-cylinder model attribute the HOMOLUMO electronic transitions to metal-based orbitals of the twenty-three-atom core. Absorption peaks arising at energies greater than 1.6 eV involve contributions from both metal- and ligand-


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based electronic transitions. In the work presented here, samples were excited using 3.1 eV (400 nm) photon energies, resulting in electronic excitation involving both ligand- and core-based orbitals. Specifically for Au25(SC8H9)18, interband excitation promoted an electron from the ligand band to the Superatom D band. The Au25(SC8H9)18 and Au38(SC12H25)24 intensity-integrated PL spectra obtained at several sample temperatures following 3.1-eV excitation are shown in Figure 2. Both samples exhibited broad near-infrared global PL that consisted of multiple peaks. Although the low-temperature PL

Figure 2. Photoluminescence spectrum of Au25(SC8H9)18 and Au38(SC12H25)24 as a function of temperature following excitation at 3.1 eV. (A) Increasing temperature from 4.5 K to 50 K led to a significant increase in PL intensity from Au25(SC8H9)18. (B) Above 50 K, the PL intensity of Au25(SC8H9)18 decreased with increasing temperature. (C) No significant change in the PL intensity from Au38(SC12H25)24 was detected over the temperature


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range of 4.5 K to 50 K. (D) As the sample temperature was increased from 50 K to 200 K, the PL intensity of Au38(SC12H25)24 decreased.

spectra of the two MPCs were similar, the samples exhibited distinct temperature dependences. Analysis of the Au25(SC8H9)18 PL spectrum as a function of temperature revealed two temperature ranges where the response of the PL was markedly different. The PL intensity increased as the sample temperature was raised from 4.5 K to 50 K (Figure 2A). This type of temperature response was unexpected, since increasing the sample temperature of nanoparticles typically activates nonradiative processes that compete with radiative relaxation.62 As the temperature was increased from 50 K to 200 K, the PL intensity decreased, and a red shift in the energy of the global PL band was observed (Figure 2B). This type of temperature dependence is common in semiconductor photophysics.62 In addition to the effect of temperature on the integrated-PL intensity, the profile of the Au25(SC8H9)18 global PL emission was also temperature dependent. As the sample temperature was raised from 4.5 K to 200 K, a lowenergy component in the spectrum appeared to become more prominent, while the high-energy region broadened and decreased in height. The temperature-dependent PL spectra of Au38(SC12H25)24 are given in Figures 2C and 2D. The spectral profile is similar to previous reports on Au38 nanocluster PL, which showed a broad emission band spanning the range from 1.6 eV to 1.0 eV.55 We note that due to the low quantum efficiencies of our sensor for NIR detection, the current study is limited to the high-energy portion of the Au38(SC12H25)24 PL spectrum. In contrast to the PL from Au25(SC8H9)18 (Figures 2A and 2B), no significant increase in integrated emission intensity was observed for Au38(SC12H25)24 in the 4.5 K to 50 K temperature range. Increasing sample temperature from 50 K to 200 K led to a decrease in integrated PL intensity. Similar to Au25(SC8H9)18, the emission


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profile obtained for Au38(SC12H25)24 was also temperature sensitive. At 4.5 K, the high-energy portion of PL was more intense than the low-energy component. However, as sample temperature was increased to 50 K, the low-energy component became the dominant feature. In order to understand the nature of the MPC-specific PL results, the temperature dependence of each component contributing to nanocluster emission was examined. For Au25(SC8H9)18, the global PL spectrum was fit using three Gaussian components, referred to hereafter as P1Au25 (1.51 eV), P2Au25 (1.57 eV), and P3Au25 (1.72 eV) (Figure 3a). The Au38(SC12H25)24 global PL was fit using two Gaussian components, P1Au38 (1.58 eV) and P2Au38 (1.74 eV) (Figure 3b).


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Figure 3. (A) 4.5 K PL spectrum of Au25(SC8H9)18 fit to three Gaussian peaks at 1.51 eV, 1.57 eV, and 1.72 eV. (B) 4.5 K PL spectrum of Au38(SC12H25)24 fit to two Gaussian peaks at 1.58 eV and 1.74 eV. The artifacts from fiber absorption lines are shown in gray.

In addition to the PL components, each spectrum contained two small, negative-amplitude components. These components arose from PL absorption by the optical fiber used to collect MPC emission. These absorption features were accurately accounted for by including two negative amplitude Gaussian peaks in the fits (gray peaks in Figure 3). The energies of these absorption peaks (1.41 eV, 1.71 eV) were independent of MPC identity, insensitive to sample temperature, and matched the energies of known absorption bands for the optical fiber used for these measurements.63 Because the global PL for both MPCs spanned a range of energies less than that predicted by free electron models, we attributed these results to radiative relaxation via transitions that included the ligand shell. Consistent with these results, van Wijngaarden et al.55 have shown that low-temperature PL of sub-nm MPCs included multiple emission peaks. In order to understand better the nature of the complex near infrared emission spectra of MPCs, we examined the temperature-dependent PL intensities, energies and lifetimes. Analysis of each individual component of MPC emission indicated that the extent of temperature-dependent intensity change, energy shift, and peak broadening were state specific. The total integrated PL intensity and the relative branching ratios of the fit components for Au25(SC8H9)18 are plotted versus sample temperature in Figure 4. The integrated PL intensity increased by 70% as the sample temperature was increased from 4.5 K to 40 K. The integrated intensity did not change (i.e. was statistically invariant) when the sample temperature was increased from 40 K to 65 K. However, when the sample temperature was raised over 65 K, the emission quenched significantly; the integrated PL intensity decreased by 65% in going from a sample temperature of 65 K to 200 K. Examination of the branching ratios for Au25(SC8H9)18


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emission (Figure 4) indicated significant amplitude redistribution between the three PL components over the temperature range studied. P1Au25 increased in relative intensity by 20% as the sample temperature was raised from 4.5 K to 50 K, at which point it saturated. From 50 K to 85 K, the relative intensity of P1Au25 was statistically invariant, but decreased by 43% as the sample temperature was raised from 85 K to 200 K. P3Au25 exhibited behavior opposite to that of P1Au25. Over the temperature range of 4.5 K to 45 K, the relative intensity of P3Au25 decreased with increasing sample temperature, maintaining at the minimum up to approximately 65 K. Increasing sample temperature from 65 K to 200 K led to an increase in the relative intensity of P3Au25. The contribution of P2Au25 to Au25(SC8H9)18 PL decreased slightly over the investigated range, but was much less sensitive to sample temperature than either P1Au25 or P2Au25. The strong correlation between integrated PL intensity and the branching ratios revealed that MPC emission resulted from at least two distinguishable relaxation channels. In particular, relaxation proceeding by the low-energy configuration (low temperature) resulted in greater luminescence yields, whereas relaxation via the high-energy transition (high temperature) yielded weaker emission. Similar temperature dependences were observed for Au38(SC12H25)24 (Figures 4C & 4D). The integrated PL intensity was reduced for sample temperatures > 65 K (Figure 4C), and this reduction corresponded to a sharp increase in the branching ratio for the high-energy component (Figure 4D). Taken together, the integrated intensity (i.e. emission yield) and branching ratio data implied an activation barrier of approximately 3.5 meV – 5.5 meV existed for accessing competitive non-radiative processes.


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Figure 4. (A) Integrated intensity of the PL spectrum of Au25(SC8H9)18 as a function of temperature normalized, at 4.5 K. Increasing the sample temperature from 4.5 K to 50 K led to strong increase in the total PL. As the temperature was increased from 50 K to 200 K the PL intensity decreased. (B) Temperature-dependent branching ratios of the three PL bands of Au25(SC8H9)18. As temperature was increased from 4.5 K to 50 K, the contribution of P3Au25 is reduced (black solid spheres), while that of P1Au25 increases (red solid spheres). At higher temperatures P3Au25 broadens significantly, causing its branching ratio to increase. P1Au25 becomes less significant as temperature is increased above 65 K. The branching ratio of P2Au25 (blue solid spheres) decreased slightly over the entire temperature range investigated; however it was much less sensitive than the other peaks. (C) Integrated intensity of Au38(SC12H25)24 plotted versus sample temperature, normalized at 4.5 K. Increasing the sample temperature led to an overall reduction in PL intensity, with the strongest sensitivity occurring above 65 K. (B) Temperature-dependent branching ratios of the two PL bands of Au38(SC12H25)24. The relative contribution of P2Au38 (black solid spheres) increased sharply at the expense of P1Au38 (red solid spheres) when the sample temperature was raised above 65 K.


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The increase in P2Au38 was correlated to the overall reduction in integrated PL for sample temperatures exceeding 65 K.

In order to understand the non-radiative contributions to MPC PL, we examined the temperature-dependent emission energies and peak widths for each component. Over the temperature range studied, the peak width of the high-energy PL component increased significantly for both Au25(SC8H9)18 and Au38(SC12H25)24. In fact, the PL peak width of the highenergy component increased by 170% and 160% for Au25(SC8H9)18 and Au38(SC12H25)24, respectively. The lower-energy components did not show comparable temperature-induced broadening. The differences in line broadening for the high- and low-energy peaks of the MPCs again suggested that nanocluster relaxation dynamics proceeded by multiple relaxation channels with state-specific electron-vibration coupling. Taken together, the integrated PL intensities, branching ratio analysis, and peak width data implied that radiative recombination by the highenergy mechanism was inefficient with respect to the low-energy process because of stronger coupling to non-radiative electron-vibration channels. In order to examine state-specific electronic-vibration coupling, the emission energy shift of each peak (referenced to 4.5 K) is plotted as a function of temperature in Figure 5. It was readily observed from these data that the temperature-dependent energy shift was distinct for each emission component, although some general similarities were observed. As the sample temperature was increased from 4.5 K to 40 K, an unexpected blue shift was observed for each PL component for both MPCs. The observed blue shift will be discussed later in this section. As temperature was increased from 40 K to 200 K, all three peaks underwent a significant red shift, the magnitude of which was peak specific, with the high-energy peak exhibiting the largest energy shift for both MPCs. This red shift with


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increasing temperature is a signature of phonon (or vibrational) modes contributing to the electronic relaxation dynamics.

Figure 5. (A) Temperature-dependent energy shift of PL peaks of Au25(SC8H9)18. Fitting these data to EQ 1 revealed that all three peaks couple to a vibrational mode corresponding to ~21 meV, however the extent of coupling was state specific. P3Au25 coupled most strongly to the mode (S = 7.8 ± 0.4), P2Au25 exhibited the weakest coupling (S = 2.3 ± 0.1), and P1Au25 exhibited intermediate coupling (S = 3.6 ± 0.3). (B) Temperature-dependent energy shift of the two PL peaks of Au38(SC12H25)24. Fitting to EQ 1 revealed that again, both peaks couple to a similar vibrational mode (~22 meV). P1Au38 was found to have a coupling constant of 3.2 ± 0.1, while P2Au38 coupled more strongly (S = 7.0 ± 0.2).


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In order to quantify the vibrational modes that mediate photoluminescence, we have modeled the temperature-dependent energy shifts of each peak using the relationship developed by O’Donnell and Chen (Eqn. 1) 〈ℏ〉 − 1 = 0 − 〈ℏ 〉 coth

(1)

where is the temperature-dependent energy gap, 0 is the extrapolated energy gap for 0 K, 〈ℏω〉 is the average vibrational energy, is a dimensionless coupling constant, and is the Boltzmann constant.64 Fitting the data for each peak to Eqn. (1) revealed that the different relaxation channels giving rise each peaks coupled to the same vibrational mode energy (∼20 meV), but the coupling constants were component specific. The results of the fits are summarized in Table 1. We note that Eqn. 1 was only accurate for fitting data obtained at sample temperatures greater than 40 K. This result was consistent with the intensity-integrated data, which were suggestive of an ≈ 3.5 meV to 5.5 meV activation barrier for accessing efficient non-radiative relaxation channels. Table 1. Au25(SC8H9)25 and Au38(SC12H25)24 emission peak energies, electronic-vibrational coupling constants (S), and vibrational energies obtained from temperature-dependent photoluminescence fitting using Equation 1. Nanocluster

Peak (eV)

S

〈ℏω〉 (meV)

Au25(SC8H9)18

1.512

3.6 ± 0.3

21 ± 0.2

1.574

2.3 ± 0.1

22 ± 0.1

1.717

7.8 ± 0.4

19 ± 0.1

1.58

3.2 ± 0.1

19 ± 3

1.74

7.0 ± 0.2

25 ± 4

Au38(SC12H25)24


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For Au25(SC8H9)18, the coupling constant for the high energy peak (S=7.8) was more than twice that for the low energy peak (S=3.6), and the intermediate component exhibited the weakest coupling (S=2.3). Similar results were obtained for Au38(SC12H25)24; the high-energy peak yielded a coupling constant of S=7.0 and the value for the low-energy one was S=3.2. Clearly, the dynamics revealed through the high-energy PL component exhibited the strongest coupling to the vibrational mode for both MPCs. This result is consistent with the temperature-dependent global intensity behavior (Figures 2-5). The high-energy peak reflected relaxation by a manifold of electronic states that coupled strongly with the vibrational mode, which resulted in decreased PL emission intensity. Analysis of the temperature dependence of the low-energy peak revealed weak vibrational coupling, which led to greater PL intensity. The temperature-dependent peak widths were analyzed using Eqn. 2,

= !" + $ +

%&'( *&'( + ,)

(2)

where is the temperature-dependent peak width, Γ/01 is the inhomogenous peak width, σ is a coupling coefficient, Γ4/5 represents the electronic-vibrational coupling strength, and E4/5 is the vibrational-mode energy.65 The peak width of the Au25(SC8H9)18 1.72 eV peak is shown as a function of temperature in figure 6.


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Figure 6.

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Temperature-dependent peak width of P3Au25 from Au25(SC8H9)18 (black circles) and P2Au38 from

Au38(SC12H25)24 (red circles). The fit to EQ 2 for each species is represented by the solid lines. From the fit of the Au25(SC8H9)18 data it was determined that the energy of the vibrational mode was 12.9 ± 0.2 meV, while the mode extracted from the fit of the Au38(SC12H25)24 data was 24 ± 1 meV.

Increasing temperature from 30 K to 200 K causes the line to broaden by approximately 150 meV. In the range of 4.5 K to 30 K, the line appeared to narrow slightly, ~10 meV. Although this is a small effect, this behavior is not consistent with electron-phonon-coupling models. For this reason, only data in the range of 30 – 200 K were included in the fit to Eq. (2). The best fit to the Au25(SC8H9)18 peak width data yielded a vibrational mode energy of 12.9 ± 0.2 meV (Table 2). Analysis of the Au38(SC12H25)24 data resulted in a vibrational mode energy of 24 ± 1 meV. Table 2. Au25(SC8H9)25 and Au38(SC12H25)24 inhomogeneous peak widths (Γ) and vibrational energies obtained from temperature-dependent photoluminescence fitting using Equation 2. Nanocluster

Peak (eV)

!" (meV)

7 8 (meV)

Au25(SC8H9)18

1.72

192 ± 5

12.9 ± 0.2

Au38(SC12H25)24

1.74

190 ± 3

24 ± 1


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The cumulative results of the energy shift and peak width analyses identify two distinguishable vibrational modes that mediated non-radiative decay in Au25(SC8H9)18 MPCs. The energies of these vibrations are ∼22 meV and ~13 meV, which correspond to frequencies of 5.3 THz (180 cm-1) and 3.1 THz (105 cm-1), respectively. These values are in good agreement with Raman scattering energies previously observed by Murray and coworkers on Au25(SC8H9)18 and in far-IR absorption studies of ~2 nm thiolate-protected nanoparticles by Creutz and coworkers.66-67 Our temperature-dependent PL measurements also match well with Raman studies on polymeric Au-S chains and temperature-dependent absorption analysis for thiolateprotected MPCs.68,69 The 3.1 THz mode is assigned to the Au(0)–Au(I) stretching vibration, and the 5.3 THz mode to the Au(I)–S stretch of the protecting staple units.68 As noted above, and shown in Figure 5, all PL components exhibited an energetic blue shift when the sample temperature is increased from 4.5 K to 40 K. For most materials, a red shift is expected with increasing temperature because of temperature-activated vibrational modes and particle expansion; the latter effect leads to reduced spatial confinement of the nanoparticle.70 Although more research is necessary to resolve the exact origin of the observed temperaturedependent blue shift, one possible explanation is that gold MPCs have a negative expansion coefficient in the temperature range between 4.5 K and 40 K. We note that previous studies point to negative expansion coefficient to account for pressure-dependent properties of assemblies of ultrasmall (∼ 1 nm) metal nanoparticles.71 MPCs smaller than three nanometers exhibit sizeindependent acoustic vibrations with frequencies ranging between 2 THz and 3 THz when excited with femtosecond laser pulses.72 The observed acoustic vibrations are inconsistent with expectations based on electronic relaxation that couples to the expansion coordinate of fcc gold.


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These discrepancies could be accounted for if the expansion coefficients of sub-nanometer MPCs are distinct from those of larger gold nanoparticles. Taken together, the results from analyzing the PL integrated intensity, emission energies, and peak widths indicated that electronically excited Au25(SC8H9)18 and Au38(SC12H25)24 MPCs relax by at least two distinguishable channels: 1) an efficient low-energy radiative recombination path, and 2) a comparatively high-energy configuration that is dominated by efficient nonradiative electron-vibration coupling in the ligand shell. The Au25(SC8H9)18 and Au38(SC12H25)24 data are similar in nature to the well-known temperature dependence of metal-to-ligand-chargetransfer (MLCT) emission of transition metal-ligand complexes (MLC). For MLCs, rapid (microsecond) phosphorescence due to MLCT states dominates the low-temperature PL spectra.73 Increased sample temperatures result in reduced PL quantum yields because of enhanced non-radiative decay mediated by vibronic coupling between metal d states and ligand orbitals.74,75 In the case of the gold nanoclusters considered here, similar excited state processes could occur assuming low-energy emission proceeded via charge transfer states associated with the Au(I)-SR ligand band. Increased sample temperatures would be expected to result in quenched PL when the activation barrier to accessing to gold-localized states is exceeded. Previously, Jin and co-workers attributed ligand-dependent PL of Au25 to a ligand-to-nanocluster charge-transfer process,54 in agreement with reports of ligand-to-metal charge transfer emission for Au(I) thiolate complexes.76 In order to more fully characterize the electronic relaxation mechanisms of gold MPCs, we carried out temperature-dependent time-resolved PL measurements on the Au25(SC8H9)18 and Au38(SC12H25)24 nanoclusters. The time-resolved PL data obtained for Au25(SC8H9)18 and Au38(SC12H25)24 obtained at several sample temperatures are given in Figure 7. These data were


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obtained by collecting PL signal over the entire global emission profile. For both samples, the time-dependent PL signal exhibited strong temperature sensitivity. The time-domain data for both MPCs were fit to a bi-exponential decay function; a stretch parameter was included for one component of the Au25(SC8H9)18 decay. Fitting of the 4.5 K Au25(SC8H9)18 data yielded τ1 = 47 ± 2 µs, τ2 = 140 ± 26 µs. The stretch parameter for τ2 was 0.79 ± 0.02 at 4.5 K. Upon increasing sample temperature, both τ1 and τ2 decreased to 7.8 ± 0.4 µs and 48 ± 3 µs, respectively. The stretch parameter for τ2 was 0.76 ± 0.01 at 200 K. A similar temperature dependence was recorded for Au38(SC12H25)24, which showed τ1 decreasing from 11.1 ± 0.1 µs to 1.76 ± 0.03 µs and τ2 decreasing from 40.5 ± 0.2 µs to 7.75 ± 0.04 µs when sample temperature was raised from 4.5 K to 200 K. These data (Figure 7) were used to calculate average PL decay rates, which are reported versus sample temperature in Figure 8a.


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Figure 7. (A) Temperature-dependent and time-resolved PL emission of Au25(SC8H9)18 obtained at several sample temperatures. (B) Temperature-dependent and time-resolved PL emission of Au38(SC12H25)24 obtained at several sample temperatures. All data were collected by detecting the global PL band for the specified MPC following 3.1 eV excitation.


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Figure 8. (A) Radiative decay rates obtained for Au25(SC8H9)18 (black) and Au38(SC12H25)24 (red) plotted versus sample temperature.

(B) Plot of ln(kavg) vs. inverse sample temperature for Au25(SC8H9)18 (black) and

Au38(SC12H25)24 (red) plotted in the low-temperature range (4.5 K – 35 K). The linear trend lines represent fits to a first-order Arrhenius model.


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The average PL decay rates shown in Figure 8a, plotted over the temperature range from 4.5 K to 200 K, exhibited two distinct regions of temperature sensitivity, common to both MPC samples. In the low-temperature region (4.5 K to 40 K), the average PL decay rate of both Au25(SC8H9)18 and Au38(SC12H25)24 increased sharply with increasing sample temperature. The change in average decay was much less pronounced for Au38(SC12H25)24 when the sample temperature was increased from 40 K to 200 K, and appeared to saturate for Au25(SC8H9)18 at sample temperatures greater than 40 K. These data were reminiscent of the temperaturedependent decay rates for metal-ligand complex charge-transfer emission.74,79 The temperaturedependent time-resolved PL, emission yield, and peak width data were all consistent with the decay rate results, and implied an activation barrier of approximately 3.5 eV that affected MPC relaxation dynamics and optical properties. In order to quantify the strong temperature dependence observed at low sample temperatures, ln(kavg) was plotted verses inverse sample temperature over the range from 4.5 K to 30 K in Figure 8B. For both samples, these data provided a linear relationship implying a firstorder Arrhenius model accounted for the strong temperature sensitivity of the PL rate constants. Analysis of the data in Figure 8B yielded activation barriers of 0.95 ± 0.03 meV and 0.79 ± 0.02 meV for Au25(SC8H9)18 and Au38(SC12H25)24, respectively. We attribute these small activation energies to energy gaps between hole fine structure states in the ligand band. Time-dependent density functional calculations predict multiple fine structure components for states in the MPC ligand band.22,25,37 This model is similar in nature to the existence of bright and dark fine structure states, common to colloidal semiconductors and organometallic compounds.77-79


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The remainder of the temperature-dependent PL data can be reconciled by invoking a model based on charge transfer emission. The microsecond emission time constants are consistent with those expected for MLCT phosphorescence transitions involving a variety of metal and ligand charge transfer interactions.73 Previous femtosecond transient absorption measurements from our group revealed an ultrafast intersystem crossing process for Au25(SC8H9)18, which could mediate relaxation to a high-spin CT state.80 Once the electron is in the CT state it can relax by competitive radiative and non-radiative channels. For metal-ligand complexes, the temperature-dependent PL decay rates and yields depend on the electronic energy gap separating the high-spin CT state and higher-energy metal d orbitals.74,75,81 When sufficient thermal energy is supplied to the complex, reduced emission yields result due to the efficient coupling of d orbitals to non-radiative vibrational relaxation channels. The combination of temperature-dependent PL emission yields, electron-vibration analysis, and emission rates provide strong evidence that near infrared MPC emission proceeds by a manifold of charge transfer states, as described in the model given in Figure 9. Electronic excitation via LMCT transitions generates a hot hole state, which relaxes internally via rapid ISC forming high-spin holes states.80 At low temperature, microsecond radiative recombination of an electron in a metal orbital with the high-spin hole state in the ligand band yields efficient PL. Reduced PL yields (Φ) observed for increased sample temperatures can be understood from temperature-dependent emission decay rates (kD). The emission decay rate is the sum of the nonradiative rate (kNR) and the radiative decay rate (kR), as shown in Eqn. 3. kD = kNR + kR

(3)

When non-radiative channels are very efficient (kNR >> kR), kD ≈ kNR and non-radiative decay processes dominate the decay rate. This expectation is consistent with the near-saturation


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behavior of temperature-dependent kD values plotted in Figure 8A. The relation between the decay rates and the PL yields is given in Eqn. 4 as: Φ =

:;

:
:;

(4)

Again, considering the case where kNR >> kR, activation of efficient non-radiative channels will result in quenched PL (i.e. reduced Φ), as reported in Figure 4. Based on the strong correlation between temperature-dependent branching ratios, emission yield, energy shifts, peak widths, and decay rates, raising the sample temperature above 40 K surmounted the activation barrier to nonradiative decay. The branching ratio analysis (Figure 4) shows that the high-energy relaxation channel is the dominant contribution at elevated temperatures. The temperature-dependent peak width and energy shift analysis showed that coupling to non-radiative decay channels was significantly stronger for the high-energy configuration than it was for the low-energy emission configuration that yielded efficient PL at low sample temperatures. Moreover, our results indicated that non-radiative relaxation was mediated by electronic-vibrational coupling involving stretching modes associated with the Au(I) species of the ligand shell. An alternative mechanism for PL of amine-passivated nanoclusters has been proposed, for which microsecond emission proceeds by delayed fluorescence.82 Based on the comprehensive temperature-dependent data presented herein, the PL mechanism for Au25(SC8H9)18 and Au38(SC12H25)24 is most consistently reconciled using a charge transfer emission model.


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Figure 9. Relative energy-level diagram depicting the nanocluster charge transfer emission model. The relative ordering of the ligand band with respect to the HOMO state was adopted from reference 22. Electronic excitation occurs though absorption (kABS) by the HOMO-5 ligand-band state, promoting an electron to the primarily metalcentered LUMO, which is followed by rapid intersystem crossing (ISC). Competitive radiative (kR) and nonradiative (KNR) relaxation proceeds to a manifold of states in the ligand band.

IV. Conclusions Temperature-dependent photoluminescence measurements were performed on colloidal Au25(SC8H9)18 and Au38(SC12H25)24 monolayer-protected nanoclusters. The global PL emission of Au25(SC8H9)18 was composed of three distinguishable components centered at 1.72 eV, 1.57 eV, and 1.51 eV, which reflected different relaxation pathways. For the Au38(SC12H25)24 sample, only the high-energy portion of the global PL emission could be measured. For Au25(SC8H9)18, an approximate 70% increase in the total integrated photoluminescence emission intensity was observed when the sample temperature was increased from 4.5 K to 45 K. However, increasing the sample temperature to values exceeding 65 K resulted in a reduction in PL emission intensity. The intensity of the high-energy component of the Au38(SC12H25)24 PL emission exhibited weak temperature dependence for sample temperatures between 4.5 K and 60 K, but


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decreased significantly as sample temperature was increased from 60 K to 200 K. The data were described using a charge-transfer model. The complicated temperature dependence of the Au25(SC8H9)18 and Au38(SC12H25)24 PL intensity was attributed to thermal activation of specific radiative and non-radiative recombination paths that resulted in reduced emission yields. This conclusion was reached by direct correlation of the total integrated PL intensity and trends in the relative branching ratios of the highest- and lowest- energy components of the global emission peak. Quantitative analysis of temperature-dependent PL emission energies and peak widths revealed that nanocluster dynamics for both samples were mediated by vibrations that correspond to stretching vibrations involving the Au(I) species of the protecting shell of the nanocluster. These results indicate that the electronic relaxation dynamics of monolayerprotected nanoclusters are sensitive to the structure of the outer ligand shell. Therefore, there is great potential for tailoring these nanomaterials for use as functional components in photochemical devices.


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TABLE OF CONTENTS GRAPHIC.

AUTHOR INFORMATION Corresponding Author K. L. K.: [email protected] The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was supported by a National Science Foundation (NSF) award to K.L.K., Grant Number CHE-1150249. R.J. acknowledges financial support by the Air Force Office of Scientific Research under AFOSR Award No. FA9550-11-1-0147. A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by National Science Foundation Cooperative Agreement No. DMR-1157490, the State of Florida, and the U.S. Department of Energy. We thank Professor Ramakrishna Guda for helpful discussions.


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