Evolution of Nonlinear Optical Properties: From Gold Atomic Clusters

Jul 30, 2012 - NanoScience Technology Center, University of Central Florida, Orlando, Florida 32826, United States. ‡College of Optics and Photonics...
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Evolution of Nonlinear Optical Properties: From Gold Atomic Clusters to Plasmonic Nanocrystals Reji Philip,†,∥ Panit Chantharasupawong,‡ Huifeng Qian,§ Rongchao Jin,§ and Jayan Thomas*,†,‡ †

NanoScience Technology Center, University of Central Florida, Orlando, Florida 32826, United States College of Optics and Photonics, CREOL, University of Central Florida, Orlando, Florida 32826, United States § Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States ‡

S Supporting Information *

ABSTRACT: Atomic clusters of metals are an emerging class of extremely interesting materials occupying the intermediate size regime between atoms and nanoparticles. Here we report the nonlinear optical (NLO) characteristics of ultrasmall, atomically precise clusters of gold, which are smaller than the critical size for electronic energy quantization (∼2 nm). Our studies reveal remarkable features of the distinct evolution of the optical nonlinearity as the clusters progress in size from the nonplasmonic regime to the plasmonic regime. We ascertain that the smallest atomic clusters do not show saturable absorption at the surface plasmon wavelength of larger gold nanocrystals (>2 nm). Consequently, the third-order optical nonlinearity in these ultrasmall gold clusters exhibits a significantly lower threshold for optical power limiting. This limiting efficiency, which is superior to that of plasmonic nanocrystals, is highly beneficial for optical limiting applications. KEYWORDS: Gold nanoclusters, plasmonic nanocrystals, third-order optical nonlinearity, optical limiting nm and is referred to as the plasmonic “nanocrystals” or “nanoparticles” regime. The number density is 59 atoms/nm3 in both regimes for gold, owing to the similar atomic packing densities. Au clusters with size of the order of the de Broglie wavelength of conduction electrons (∼ 0.5 nm) exhibit discrete energy levels and molecule-like HOMO−LUMO transitions, while larger Au nanoparticles (>5 nm) exhibit quasi-continuous electronic bands. In general, gold clusters of less than 2 nm size lose their bulk-like electronic properties and are considered not to support collective plasmon excitation.16,17 The evolution of the optical spectrum of gold clusters in the quantum size regime (up to ∼300 atoms and ∼2 nm cluster diameter) is a strong function of size, and therefore, nonlinear optical properties in this size regime are worthy of investigation. Ultrasmall clusters exhibit a spectacular optical behavior which is fundamentally different from that of larger plasmonic nanocrystals. The transition from the cluster to the nanocrystalline state is significant, as it raises fundamental questions regarding the evolution of discrete electronic states toward a band structure. Recently, considerable progress has been achieved in the study of the linear and nonlinear optical properties of gold nanoparticles and clusters of varying size and

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oble metal nanoparticles have stimulated considerable interest since medieval ages because of the beautiful sizedependent colors arising from their plasmon resonances. The preparation of gold colloid by Michael Faraday in 18571 inspired many scientists to work in this field, and the interaction of light with gold particles was explained by Gustav Mie in 1908.2 However, finer details of collective excitation of conduction band electrons (i.e., plasmon resonance) were not fully understood before the establishment of the band theory of solids. The physical and chemical stability of nanomaterials has been investigated in great detail recently, and thiol-stabilized gold clusters and nanoparticles have attracted significant attention in this regard. These materials are important in both fundamental science3 and technological applications such as catalysis,4 optics,5 biomedicine,6,7 chemical sensing,8 and optical limiting.9 From large-scale density functional theory calculations, the superatom model has been proposed to account for the unusual stability of a few of the thiolateprotected gold clusters.10,11 The synthesis, separation, and characterization of thiolate-protected Au clusters were first carried out by the Whetten group and extended later by others.12−15 It is found that metal nanoparticles exhibit two interesting size regimes, which are separated by the critical size for electronic energy quantization. The first is from subnanometer to about 2 nm and is referred to as the “nanoclusters” (or “clusters”) regime, while the second is from about 2 to ∼100 © 2012 American Chemical Society

Received: May 25, 2012 Revised: July 17, 2012 Published: July 30, 2012 4661

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shape.9,18−21 For instance, by measuring two-photon fluorescence excited at 800 nm, Goodson et al. have shown that two-photon absorption cross-section increases as the size of ultrasmall gold clusters increases from 1.1 to 4.0 nm.22 Further studies by the same group on fluorescence upconversion revealed an abrupt change in the optical properties (fluorescence, transient absorption, two-photon absorption) around 2.2 nm, in agreement with the calculated critical size for quantization.23 In view of this size-dependent variation in cluster properties, in the present work we have carried out a systematic investigation of optical nonlinearity in the recently developed atomically precise gold clusters Au25(SR)18, Au38(SR)24, and Au144(SR)60, (where R = CH2CH2Ph)24 and compared it with the nonlinearity measured in larger Au nanocrystals of 4 nm size. Our studies reveal that the sizedependent onset of plasmonic nature can be observed in clusters as small as Au144 from nonlinear transmission measurements. Preparation and Characterization of Gold Nanoclusters. We synthesized the nanoclusters used in this study according to a size-focusing methodology.24 This method consists of two primary steps: (i) preparation of polydisperse Aun(SR)m nanoclusters with a controlled proper size distribution by kinetic control over the early stage reaction,25 and (ii) size focusing of the Aun(SR)m nanoclusters into a specific size.24,25 By controlling the initial size distribution range of the polydisperse Aun(SR)m nanoclusters, molecularly pure nanoclusters of various sizes such as Au25(SR)18, Au38(SR)24, and Au144(SR)60, (where R = CH2CH2Ph) have been obtained after size focusing. These size-specific nanoclusters are the most robust species among the respective size distributions, and hence, they survived the size-focusing process while the other sizes were decomposed and/or converted to the most stable sizes in the process.24,25 For experimental details, the Au25(SR)18 synthesis and crystal structure have been reported in refs 26 and 27, Au38(SR)24 in refs 28 and 29, and Au144(SR)60 in ref 30. The samples were tested for purity first by matrixassisted laser desorption ionization (MALDI) and then by electrospray ionization (ESI) mass spectrometric analyses. Electrospray mass spectrometry data of the samples (Figure 1) reveal that the clusters are monodisperse, with a high precision in the number of atoms in each cluster.31 The absorption spectra of Au25 and Au38 exhibit clear molecular features, while these are less distinct in Au144. The atomic structures of Au25 and Au38 have been attained by X-ray crystallography (Figure 2), in which Au25 exhibits a nearly spherical structure (1 nm diameter of metal core, Figure 2a), while Au38 exhibits a rod-like structure (Figure 2b). The structure of Au144 has not been determined by X-ray crystallography, though a structural model was proposed from density functional theory (DFT) by Häkkinen and colleagues;32 their calculated X-ray diffraction (XRD) pattern agreed with earlier XRD measurements in 29 kDa thiolate-protected gold clusters.33 The absorption spectrum of the Au nanocrystals (∼4 nm, purchased from Fluka) is shown in Figure 3, with the TEM image and polydispersity given as insets. The absorption peak around 530 nm clearly indicates the presence of the SPR band in the nanocrystal sample. Nonlinear Optical Measurements Using z-Scan Technique. Nonlinear transmission measurements were carried out using the open aperture z-scan technique,34 which is essentially a measurement of the optical transmission of the sample as a function of input light fluence. A schematic of the z-scan

Figure 1. UV−vis absorption spectra and electrospray mass spectrometry (ESI-MS) data (insets) of Au25(SR)18 (counterion: tetraoctyammonium, TOA+), charge-neutral Au38(SR)24, and chargeneutral Au144(SR)60, respectively.31 In all cases, R = CH2CH2Ph.

experimental setup is given in Figure 4. The basic setup typically consists of a pulsed laser which is set to run at a given energy, a converging lens which focuses the laser beam and a laser energy detector. The beam is considered to propagate along the “z” axis. For any given energy setting of the laser, the fluence will be a maximum at the focal point (z = 0), and will decrease toward either direction on the z-axis. Therefore, light fluence falling on a sample can be easily varied by translating the sample through the z-axis, towards or away from the focal point (hence the name “z-scan”). The detector measures the transmitted energy for different sample positions, from which a position versus transmission curve (z-scan curve) can be drawn. Optical nonlinearity coefficients can then be calculated by numerically fitting the measured z-scan curve to standard nonlinear transmission equations. Samples for measurements were prepared by dispersing the nanoclusters and nanocrystals in toluene. A Nd:YAG laser (Minilite I, Continuum) emitting 5 ns laser pulses at the second harmonic wavelength (532 nm) was used for excitation. Each sample was so prepared that it had a linear transmission of 25% at this wavelength, when taken in 1 mm path length quartz cuvettes. The laser output was passed through a spatial filter to remove the higher spatial modes and give a clean Gaussian beam. The beam was focused using a plano-convex lens (f = 100 mm), and the beam radius at the focus (ω0) was measured to be 13 μm by using the knife-edge method. The laser pulse 4662

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Figure 2. Atomic structure of (a) Au25(SR)18 and (b) Au38(SR)24. Color labels: magenta = Au, yellow = S, gray = C. For clarity, hydrogen atoms are not shown.

Scans of Au25 and Au38 indicate nonlinear absorption throughout the z-scan range, while that of the nanocrystals indicates significant absorption saturation occurring in addition to nonlinear absorption.9 It is possible to plot the sample transmission against input laser fluence using the z-scan data. For this purpose, we note that for a spatially Gaussian beam, the light fluence Fin(z) at any position z can be calculated from the corresponding beam radius ω(z) and the input laser pulse energy Ein. The beam radius is given by ω(z) = ω(0)[1 + (z /z 0)2 ]1/2

(1)

and the position-dependent fluence can be calculated from the expression

Figure 3. Optical absorption spectrum of Au nanocrystals showing the SPR band around 530 nm. Insets show the TEM image and polydispersity histogram.

Fin(z) = 4(ln 2)1/2 E in /π 3/2w(z)2

(2)

Figure 6a−d depicts the nonlinear transmission of the samples plotted against Fin(z). From Figures 5 and 6, it is obvious that in Au25 and Au38 an optical limiting behavior is seen throughout the incident fluence range, while in Au144 the limiting is preceded by weak absorption saturation in the lower fluence region. On the other hand in the Au nanocrystals the limiting is preceded by prominent absorption saturation. A nonlinear absorption coefficient α(I) given by α0 + βeff I α (I ) = 1 + (I /Is) (3)

Figure 4. Representative schematic of the open aperture z-scan setup used for nonlinear optical measurements. A laser beam is focused using a converging lens. The light energy transmitted by the sample (taken in a 1 mm cuvette) at different positions is monitored as the sample is translated along the z-axis, through the focal point.

can be considered for modeling this behavior, where α0 is the unsaturated linear absorption coefficient at the excitation wavelength, I is the input laser intensity (fluence divided by laser pulse width), and Is is the saturation intensity (intensity at which the linear absorption drops to half of its original value). βeff is the effective two-photon absorption coefficient,35 which accounts for two-photon and two-step excitations taking place in the medium. The transmitted intensity for a given input intensity can be calculated by numerically solving the corresponding nonlinear propagation equation, given by

energy was 15 μJ. The sample cuvette was mounted on a precision linear translation stage (Newport, ILS150PP), using which the cuvette could be moved to any desired position with respect to the beam focus. For each z-scan we translated the sample through a distance of 40 mm with a step size of 500 μm or lower. At each position a laser pulse was fired, and the incident and transmitted pulse energies were recorded using pyroelectric probes (LaserProbe, RjP-735). Stage movement, laser firing, and data acquisition were synchronized using a LabView program. The z-scan curves measured in the samples are presented in Figure 5a−d. In open-aperture z-scan, valleys indicate a decrease in transmission, while peaks indicate an increase in transmission. Au25 and Au38 exhibit valley-shaped curves, whereas Au nanocrystals show a central valley flanked by two symmetric peaks on either side. The onset of these peaks is visible in the form of two humps flanking the valley in Au144. z-

⎡⎛ α ⎤ ⎛ I ⎞⎞ dI = ⎢⎜⎜ 0 + ⎜ ⎟⎟⎟ + βeff I ⎥I dz′ ⎢⎣⎝ 1 ⎥⎦ ⎝ Is ⎠⎠

(4)

where z′ indicates the propagation distance within the sample. From the numerically calculated best-fit curves to the experimental data the nonlinear parameters Is and β can be estimated. These values, given in Table 1, show that βeff 4663

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Figure 5. Open-aperture z-scans measured in the Au clusters and nanocrystals. (a) Au25, (b) Au38, (c) Au144, and (d) Au nanocrystals (∼4 nm). Samples are excited using 5 ns laser pulses at 532 nm. Linear transmission of all samples is 25% at this wavelength. As seen from the figures, optical transmission is a function of sample position (z = 0 is the beam focus). Tnorm is the measured transmission normalized by the linear transmission of the sample. Solid curves correspond to numerical fits to the data using eq 4. The valley-shaped curves of Au25 and Au38 indicate pure optical limiting behavior, while the humps flanking the valley in Au144 signify the onset of saturable absorption. The absorption saturation is significant in the Au nanocrystals, as indicated by the strong peaks.

Figure 6. Nonlinear transmission in the Au clusters and nanocrystals, calculated from the z-scan data using eqs 1 and 2. (a) Au25, (b) Au38, (c) Au144, and (d) Au nanocrystals (∼4 nm). Saturable absorption sets in as the cluster size increases and becomes prominent in the nanocrystals. Solid curves are numerical fits to the data obtained using eq 4, from which the nonlinear parameters are calculated.

about the same, but the βeff value drops by a factor of 5, which results in the prominent absorption saturation exhibited by the Au nanocrystals. The optical limiting thresholds Ft (input

increases with size in the cluster regime. Is is too high to be effective in Au25 and Au38 but falls to a significant value of 1.5 × 1012 W/m2 in Au144. In the nanocrystal regime, the Is value is 4664

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at 400 nm (2.91 eV) arises mainly from an interband transition (d→sp). The multiband absorption spectrum of Au25 clusters is indeed different when compared to the SPR dominated spectrum of larger Au nanocrystals. This spectral structure is usually attributed to quantum confinement effects. However, it may be noted that from time-dependent DFT calculations Aikens37 has recently suggested ligand-field splitting as a cause for the multiple peaks. According to this study the absorption spectra are not separable into core and ligand contributions; rather, geometric and electronic interactions between the two are responsible for the resultant complex spectra. The present results can be explained on the basis of the occurrence and mutual interaction of interband, intraband, and SPR transitions in Au clusters and Au nanocrystals (Figure 8).

Table 1. Effective Two-Photon Absorption Coefficient (βeff) and Saturation Intensity (Is) Calculated for the Samples sample

linear transmission

Is (W/m2)

βeff (m/W)

Au25 Au38 Au144 Au NCs (5 nm)

25% 25% 25% 25%

NA NA 1.5 × 1012 2.2 × 1012

2.0 3.5 7.5 1.5

× × × ×

10−10 10−10 10−10 10−10

fluence at which the transmission drops to 50% of the linear transmission) are found to be 4.0 J/cm2 for Au25 and 3.0 J/cm2 for both Au38 and Au144. It may be noted that optical nonlinearity in metal nanoparticles is ultrafast in nature, occurring in the range of picoseconds/femtoseconds.36 Moreover, the optical limiting property is wavelength-tunable as well. Measurements in Au25 and Au38 have given z-scan curves similar in shape to those previously measured in molecules like C60, which fit well to an effective two-photon absorption mechanism. Results given in Table 1 are in agreement with the two-photon fluorescence studies of Goodson and colleagues,10 in which an increase of two-photon cross section with cluster size was observed. Progression from Nanoclusters to Plasmonic Nanocrystals. The Au25(SR)18 and Au38(SR)24 (R = CH2CH2Ph) nanoclusters exhibit unique optical properties, as manifested in their highly structured multiple-band optical absorption spectra. Unlike metallic Au nanocrystals with a quasi-continuous band structure, Au25(SR)18 and Au38(SR)24 exhibit discrete electronic energy levels and possess HOMO−LUMO gaps which are quite large, for example, 1.3 eV in Au25(SR)18 and 0.9 eV in Au38(SR)24. Density functional theory calculations on these nanoclusters have offered a deeper understanding of their optical properties. For instance, a precise correlation of the Au25 structure with its calculated optical absorption properties has been obtained, and the theoretical spectrum, especially the spectral shape, agrees well with measurement.27 The energy level diagram for Au25 is shown in Figure 7. The first excited transition occurring at 670 nm (1.52 eV) is the HOMO→ LUMO transition, which is essentially an intraband (sp→sp) transition. The peak at 450 nm (2.63 eV) is caused by mixed intraband (sp→sp) and interband (d→sp) transitions, and that

Figure 8. Optical excitation and subsequent energy relaxation in Au clusters and nanocrystals. Excitation of Au clusters at 532 nm results in intraband (sp→sp) and interband (d→sp) transitions. Even though surface plasmon resonance (SPR) will be prominent in plasmonic nanocrystals, the decay of SPR is through interband, intraband, and radiative transitions. Interband and intraband transitions generate free carriers which absorb further, resulting in optical limiting.

Intraband excitation is the excitation of free valence electrons near the Fermi surface, which is energetically permissible even with low-energy photons, while interband transitions occur from the d-band to the conduction band and require higher photon energies. Therefore, when Au clusters are excited at 532 nm, both intraband and interband excitations will occur, generating free carriers in the conduction band. These free carriers can absorb additional photons from the laser pulse by phonon assistance, which results in the strong optical limiting action seen in the Au clusters. On the other hand, in plasmonic Au nanocrystals the SPR response will be prominent, which is the collective excitation of electrons near the Fermi surface. This strong excitation results in an absorption saturation at the SPR wavelengths (i.e., bleaching of the SPR spectrum).38 It is known that SPR will decay by three different routes, namely, radiative emission, interband excitation, and intraband excitation.39 Since the luminescence efficiency of Au nanocrystals is generally low, SPR decay will occur mostly through interband and intraband transitions, leading to the generation of free carriers. Therefore there are two opposing effects at work in the Au nanocrystals: depletion of electrons near the Fermi surface by SPR, causing saturation of absorption (SA), and absorption

Figure 7. Energy level diagram of the model compound Au25(SH)18−. The absorption at 670 nm is the HOMO→LUMO (sp→sp) intraband transition, while that at 450 nm is due to mixed intraband (sp→sp) and interband (d→sp) transitions. Absorption at 400 nm is due to an interband transition (d→sp). 4665

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for administering the project, project planning, and project execution.

by free carriers in the conduction band, causing reverse saturable absorption (RSA). However, Au nanocrystals show the lowest βeff value (Table 1), which indicates that free carrier absorption is weaker in the nanocrystals compared to the nanoclusters. The substantial occurrence of absorption saturation along with reduced free carrier absorption results in the peculiar intensity-dependent nonlinear transmission behavior exhibited by the Au nanocrystals in our measurements. Our studies prove that ultrasmall Au clusters are good optical limiters, with a limiting threshold close to that of the benchmark limiter C60. This is due to the fact that, unlike Au nanocrystals, the molecular Au clusters (e.g., Au25 and Au38) do not possess a SPR band, and therefore do not suffer absorption saturation when excited at the SPR wavelength region. It also turns out that, under similar conditions, the expected indirect excitation of interband and intraband transitions through SPR decay in nanocrystals contributes less to RSA, compared to the direct excitation in the nanoclusters. The absence of absorption saturation in the Au clusters makes them more useful for optical power limiting applications, in comparison to the larger Au nanocrystals. Moreover, Au144 is in the transition regime between molecular clusters and nanocrystals, as revealed by the weak plasmonic response affecting its nonlinear transmission behavior. In summary, from nonlinear transmission measurements we have established that atomically precise nonplasmonic gold quantum clusters exhibit an exclusive optical power limiting behavior for nanosecond laser pulses at 532 nm. Transition to the plasmonic regime sets in progressively with size, and at 144 atoms (Au144) weak absorption saturation manifests in the zscan curve. Larger Au nanoclusters (∼4 nm) display a strong SPR band and substantial absorption saturation. On the application side, it is shown that nonplasmonic quantum clusters are superior candidates for optical power limiting applications at the SPR wavelengths of noble metals, due to the unique absence of absorption saturation effects in them at these excitation wavelengths.

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ASSOCIATED CONTENT

S Supporting Information *

Absorption spectrum of Au144. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address ∥

Raman Research Institute, C.V. Raman Avenue, Sadashivanagar, Bangalore 560080, India. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.T acknowledges University of Central Florida start-up funding for the financial support. Authors thank Dr. Binh Duong and Material Characterization Facility at UCF for the assistance in taking TEM images. R.J. acknowledges research support by the Air Force Office of Scientific Research under AFOSR Award No. FA9550-11-1-9999 (FA9550-11-1-0147) and the Camille Dreyfus Teacher-Scholar Awards Program. J.T. was responsible 4666

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(36) Tokizaki, T.; Nakamura, A.; Kaneko, S.; Uchida, K.; Omi, S.; Tanji, H.; Asahara, Y. Appl. Phys. Lett. 1994, 65, 941. (37) Aikens, C. M. J. Phys. Chem. Lett. 2008, 112, 19797. (38) Ahmadi, T. S.; Logunov, S. L.; El-Sayed, M. A. J. Phys. Chem. 1996, 100, 8053. (39) Sonnichsen, C.; Franzl, T.; Wilk, T.; von Plessen, G.; Feldmann, J. Phys. Rev. Lett. 2002, 88, 077402.

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