Femtosecond Laser-Induced Size Reduction of Aqueous Gold

ARTICLE pubs.acs.org/JPCC

Femtosecond Laser-Induced Size Reduction of Aqueous Gold Nanoparticles: In Situ and Pump
Probe Spectroscopy Investigations Revealing Coulomb Explosion Daniel Werner,† Akihiro Furube,‡ Toshihiro Okamoto,§ and Shuichi Hashimoto*,† †

Department of Ecosystem Engineering, The University of Tokushima, Tokushima 770-8506, Japan National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8565, Japan § Department of Optical Science and Technology, The University of Tokushima, Tokushima 770-8506, Japan ‡

bS Supporting Information ABSTRACT: In situ extinction spectroscopy and transient absorption spectroscopy of the femtosecond laser-induced fragmentation of 60 nm diameter aqueous gold nanoparticles were performed. The threshold laser fluences of fragmentation determined by in situ spectroscopy and transmission electron microscopy, (7.3 ( 1.5) mJ 3 cm
2 for excitation at 400 nm and (3.6 ( 0.5) mJ 3 cm
2 at 532 nm, agreed well with the values of 6.0
7.4 and 3.4
4.1 mJ 3 cm
2 calculated by our simulation based on the two—temperature and liquid drop models. The transient absorption study revealed that real-time observation of fragmentation is possible at picosecond time scales. When monitored at 490 nm, at which the effect of fast relaxation dynamics is minimal, excitation at 400 nm afforded a reduced extinction signal of the localized surface plasmon resonance (LSPR) band of gold nanoparticles at laser fluences greater than or equal to (6.1 ( 1) mJ 3 cm
2. The reduction can be ascribed to nanoparticle fragmentation because the intensity (I) of the LSPR band depends on particle radius (R), I µ R3. The signal reduction occurred not instantaneously but gradually within 100 ps, suggesting separation of initial densely packed small clusters during the observation period. The onset of the size reduction was laser-fluence-dependent, and it occurred earlier at higher fluences. This fluence dependence was explained well within the framework of our model: fragmentation occurs for liquid rather than solid gold, and the onset suggests the initiation of particle melting. The present result demonstrated that femtosecond laser-induced fragmentation is dominated by the Coulomb explosion mechanism, discussed many times without experimental verification. We believe we can provide information long needed in the field.

’ INTRODUCTION Nanoparticles (NPs) of gold and silver are characterized by extremely high absorption and scattering cross sections in the visible and near
IR regions owing to the localized surface plasmon resonance (LSPR) band, whose intensity and peak position depend critically on the particle size and geometry, spacing, and dielectric functions of the surrounding medium.1 This optical property allows noble metal NPs to exhibit intriguing time- and space-dependent responses to interaction with pulsed lasers, giving rise to a rich physics and chemistry. For instance, laser-induced relocation and release of gold NPs (Au NPs) or nanostructures from a substrate surface have been demonstrated for excitation by both femtosecond and nanosecond lasers.2
4 Nanoholes can be prepared on Au NP-modified glass substrates by exposure to single-shot nanosecond pulses of intensities far less than the breakdown threshold of the substrate.5,6 Moreover, nanobubbles capable of killing tumor cells are formed on the surfaces of aqueous Au NPs where water molecules are heated to a critical temperature caused by heat r 2011 American Chemical Society

transfer from the particle.7,8 Interestingly, pulsed laser irradiation of gold nanorods induced transformations into spherical and small particles.9 These phenomena are closely associated with a photothermal response in which metallic NPs can be efficiently heated by illuminating them at their LSPR bands. A photothermal mechanism is believed to play a critical role in the laser-induced size reduction of noble metal NPs.10
12 This size reduction constitutes the core technology behind the ablation-based generation of NPs in liquids by a top-down approach13
16 and thus has attracted a great deal of attention. Nanosecond and picosecond laser excitation of aqueous Au NPs have been observed to cause shape changes and fragmentation depending on the excitation intensity.10
12,17,18 This is because effective absorption of the laser energy can change a NP's shape by surface melting and eventual fragmentation because of the Received: December 25, 2010 Revised: March 17, 2011 Published: April 08, 2011 8503

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The Journal of Physical Chemistry C evaporation of an entire particle heated above the boiling point of 3129 K. The Coulomb explosion model has been proposed as an alternative mechanism for size reduction.19
24 This model assumes the ejection of many electrons to generate multiply ionized NPs that undergo spontaneous fission owing to charge repulsion. Thermionic emission due to heating of the particles to high temperatures is considered to be the electron source.21 Additionally, a near-field ablation mechanism that assumes the spallation of solid Au NPs by the action of intense laser field caused by the plasmonic near-field enhancement was postulated for femtosecond laser excitation.25 Despite the enduring efforts of previous investigations, no clear classification has been made as to which mechanism prevails depending on parameters: laser pulse duration, laser energy, and optical and thermophysical properties of particles. The energy deposition resulting from the laser interaction of the metal NP occurs in the following three stages.26,27 First, the laser light is absorbed by the electronic system, creating nascent nonthermal electrons. After a few hundred femtoseconds, a thermal equilibrium is reached corresponding to the Fermi distribution as a result of electron
electron scattering. Subsequently, the electron energy is transferred to the lattice via electron
phonon coupling (7200 K is met because TL cannot exceed the boiling point of Au NPs.39 We examine the experimental results given below on the basis of the simulation result. In Situ Spectroscopy Coupled with TEM Acquisition. The extinction (scattering and absorption) of the LSPR band for spherical Au NPs is strongly size dependent, and spectral measurement of the extinction can directly determine the particle diameter for a low-concentration solution having uniform size distribution. At the low laser powers (peak intensity Pmax < 1013
1015 W 3 cm
2) employed here, two-photon excitationinduced fragmentation of Au NPs is unlikely.42 In addition, the repetition rate should be low to avoid heat accumulation.43 These are our basic assumptions for conducting the in situ measurements. The thresholds of reshaping and size reduction depending on the laser fluence were measured by continuous irradiation of femtosecond lasers. Aqueous Au NPs (60 nm in diameter) exhibit a distinct extinction spectrum having a peak position of 534 nm in the visible region. To observe continuous morphological changes such as reshaping and fragmentation, the simultaneous acquisition of UV
vis spectra by irradiation was conducted for 60 min at various laser fluences. Aqueous Au NP solutions contained in a quartz cell were irradiated by femtosecond laser pulses having various laser fluences at both 400 and 532 nm. An example of the observed in situ spectral changes is given in Figure 2. 8505

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Figure 2. Typical time sequence of in situ spectra for 60 nm Au NPs in aqueous solution during femtosecond pulsed-laser irradiation at 400 nm, laser fluence 10.6 mJ 3 cm
2. The spectra were recorded at 1, 3, 7, 15, 25, 40, and 60 min. The repetition rate was 1 kHz. The symbol ΔA represents the change in LSPR band peak intensity after 60 min of irradiation.

Figure 3 shows the experimental plots of ΔA, i.e., the change in LSPR-band peak extinction intensity as a function of the laser fluence after 3 600 000 shots (60 min at 1 kHz repetition). The curve of ΔA vs fluence gives a measure of the size reduction at two different excitation wavelengths, 400 nm (a) and 532 nm (b). In Figure 3a, a slightly decreased ΔA of approximately
0.1 occurs at a laser fluence of 2.5 mJ 3 cm
2, and ΔA remains constant up to 5.0 mJ 3 cm
2. When the laser fluence is increased to 7.3 mJ 3 cm
2, a greater decrease in ΔA occurs and continues at greater fluences. Thus, we tentatively assign 7.3 mJ 3 cm
2 to the threshold energy of size reduction.44 The TEM images and resultant size distributions of the samples after 60 min of laser irradiation are shown in Figure 4 at both 1 kHz and 100 Hz of pulse repetition. The TEM results reveal that the initial slight change in the LSPR-band peak intensity can be attributed to the reshaping of Au NPs from faceted to spherical, affording a slightly reduced mean diameter of 55 nm from the initial 60 nm. However, still some faceted Au NPs were present in the TEM micrographs (see Supporting Information, Figures S3 and S4) because of the large ratio of the solution volume to the irradiated volume. Mixing of unchanged particles to the reshaped NPs was unavoidable in our spatially unresolved experiment. In addition, small NPs having a mean diameter of ∼3 nm were found next to unchanged or reshaped NPs at laser fluences above 7.3 mJ 3 cm
2, which is consistent with the in situ observations. The reshaping took place even at high fluences owing to the exposure of intact NPs to the wings of a spatial Gaussian pulse, not to the peak intensity position; this side effect was inevitable in our experiment. An important consequence that should be drawn from the TEM observation is that the mean diameters of the reshaped Au NPs exhibit practically no volume reduction within an experimental error regardless of laser fluence. This was clearly observed at 100 Hz repetition (Figure 4 X
Z). This means that only the particles exposed to the peak intensity of a laser pulse undergo complete fragmentation (Supporting Information, Figure S5; time-dependent concentration change of 60 nm particles). This picture is in marked contrast to previous size reduction observations using picosecond and nanosecond pulsed lasers, where a gradual size reduction due to a surface evaporation mechanism was observed with increasing laser fluences and irradiation periods (pulse numbers).10
12,18 In the present study, exposure

Figure 3. Plots of experimental ΔA, the change in LSPR-band peak intensity, after 3 600 000 shots (1 kHz, 60 min) vs laser fluence (scale on the left) upon excitation at 400 nm (a) and 532 nm (b). Calculated laserfluence-dependent temperature evolution of Te (dashed red line) and TL (solid black line) for a 60 nm aqueous Au NP (scale on the right) is also included. The vertical dashed
dotted lines (q1 = 6.0, q2 = 7.4 mJ 3 cm
2 in (a) and q1 = 3.4, q2 = 4.1 mJ 3 cm
2 in (b)) indicate the range of the calculated Coulomb explosion threshold for liquid gold nanodroplets. The dashed line shows the surface melting threshold at 700 K. The two steps within the temperature curves of the lattice TL (solid black line) indicate the melting point (1337 K) and boiling point (3129 K) of bulk gold. Temperatures remain constant, while the enthalpies are consumed. Error bars in measured fluences originate from the uncertainty in estimating laser spot sizes.

to the laser fluence of 7.3 mJ 3 cm
2 gives similar average NP diameters of 57 nm after 360 000 and 3 600 000 shots (10 times different, see Figure 4 C and 4 X). Furthermore, a much higher concentration of fragmented 3 nm Au NPs is observed at greater repetition rate by comparing 1 kHz and 100 Hz (Figure 4 D and 4 Y). Here, the mean diameters of the bigger NPs gave a difference of 4 nm, which means a reduced layer of only 2 nm at 1 kHz (typical error (8 nm). Note that a remarkable reduction of the 60 nm Au NPs was observed only at a repetition rate of 1 kHz when relatively large fluences of >10 mJ 3 cm
2 were applied. In this instance, small NPs of approximately 4 nm were obtained at the expense of reduction in the mean diameters of the original Au NPs. This observation correlates well with the photothermal surface evaporation mechanism11,12 because high repetition rate may bring about heat accumulation.45 The present observation of invariant mean diameter of the parent NPs, obtained regardless of the fluence (Figure 4) and irradiation time (Supporting Information, Figure S6), with the formation of small particles, cannot be explained by the photothermal mechanism. Instead, this may suggest the occurrence of Coulomb explosion that leads to the exclusive formation of fragments from particles exposed to 8506

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Figure 4. TEM images and corresponding size distributions of 60 nm Au NPs after 60 min of laser irradiation at 1 kHz (A
E) and 100 Hz (X
Z) at an excitation wavelength of 400 nm. Notations A
E in the TEM images correspond to those of the data points in Figure 3, and X
Z correspond to Figure S4, Supporting Information. 200
500 particles were examined to construct the histograms.

the center of the laser spot while leaving unexposed or slightly exposed parent particles without undergoing fragmentation. In addition to the experimental fluence-dependent ΔA data points, Figure 3 shows the calculated maximum Te and TL curves depending on the laser fluence. The thermal nonequilibrium established between Te and TL is observed. The Coulomb

explosion threshold according to the liquid drop model for 60 nm Au NPs found by applying our model is also indicated. The threshold for reshaping, indicated by the vertical dashed line p, appears at TL ≈ 700 K, which is 52% of the melting point of bulk gold (1337 K). Previous studies46,47 have found that surface melting of Au NPs occurs at lattice temperatures lower than the 8507

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The Journal of Physical Chemistry C melting point of bulk gold because of the large surface-area-tovolume ratio. A further increase in the laser fluence allows the maximum TL to reach the bulk melting point of gold, where the temperature remains constant, while the melting enthalpy is consumed. Thus, the inner core of Au NP starts to melt until the particle is completely transferred to the liquid phase. Importantly, the size reduction threshold of 60 nm Au NPs was observed at a temperature corresponding to the melting point and thus cannot be explained by the surface evaporation mechanism. Inspection of the temperature curve of Te suggests that a fragmentation temperature (Tefrag) of 7260
7640 K is necessary to eject a sufficient number of electrons (∼630) through thermionic emission.21,48,49 This high Te is necessary to exceed the Rayleigh instability for 60 nm gold liquid droplets. The simulation results for Te, which yield a fragmentation threshold of 6.0
7.4 mJ 3 cm
2, are in good agreement with the experimental size reduction threshold of (7.3 ( 1.5) mJ 3 cm
2. To check the validity of the calculated Coulomb explosion threshold’s dependence on the excitation wavelength, the experiment was also performed at an excitation wavelength of 532 nm. As shown in Figure 3b, at a laser fluence of (3.6 ( 0.5) mJ 3 cm
2, an appreciably decreased ΔA was observed. This value is within the range of the theoretical Coulomb explosion threshold of 3.4
4.1 mJ 3 cm
2. For excitation at 532 nm, a laser fluence lower than that of a 400 nm laser is sufficient to initiate the fragmentation of 60 nm Au NPs. This is because the LSPR band has a greater absorption cross section than the interband transition at 400 nm. Notably, our previous nanosecond pulsed-laser size reduction experiments demonstrated less efficient fragmentation upon excitation at 532 nm.18,39 This difference is because of the explosive evaporation of the surrounding water occurring on nanosecond time scales. The evaporation allows a significant depression of the LSPR band owing to the reduction in the refractive index of water from 1.33 to 1.07 for a long pulse duration of 5 ns. In femtosecond laser excitation of metal NPs, bubble formation has no effect on the absorption behavior of Au NPs because it occurs at much longer time delays (several tens of picoseconds) than the excitation pulse duration. Femtosecond Transient Absorption Spectroscopy. We selected transient absorption spectroscopy to confirm the Coulomb explosion events directly. The basic idea of the experiments is that a single-shot laser-induced fragmentation can cause a small depression of the LSPR band analogous to the appreciable signal reduction caused by multiple laser shots in the in situ experiment. Moreover, fragmentation can be an instantaneous event that can be monitored spectroscopically with femtosecond time resolution. Transient absorption spectroscopy has already been successfully applied to study the electron and phonon dynamics in metal NPs.28
34 Figure 5 shows typical transient spectra at different delay times using a 150 fs laser pulse at 400 nm having a fluence below the fragmentation threshold, i.e., 3.7 mJ 3 cm
2. The observation of instantaneous bleaching and subsequent bleaching decay of the LSPR band with two positive wings, caused by the depression and broadening of the band, has been ascribed to the heating and cooling dynamics of electron systems.28
34 The LSPR-band depression occurs because the solution transmits more light. This implies that hot electrons do not absorb further incoming photons before they relax to the ground state. Transient spectra at a laser fluence near the fragmentation threshold are given in the Supporting Information, Figure S7, and those at a fluence above the threshold are given in Figure S8.

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Figure 5. Typical femtosecond transient absorption spectra of 60 nm Au NPs upon excitation by a laser fluence of 3.7 mJ 3 cm
2, which is below the fragmentation threshold. Red circle indicates the intersection of the spectra with a zero absorption line at (490 ( 5) nm, at which the pump
probe signal of the electron
phonon dynamics is minimal.

To distinguish morphological changes in Au NPs caused by reshaping and size reduction from the strong bleaching signal of the electron
phonon dynamics, we selected a monitor wavelength of (490 ( 5) nm, at which the effect of electron thermalization and cooling is minimal. As in the typical example shown in Figure 5, virtually no transient absorption was recorded at delays from 0 to 2 ns at this wavelength.50 Figure 6a shows the time curves at (490 ( 5) nm up to 300 ps. Below 3.7 mJ 3 cm
2, no remarkable signal changes were recorded, indicating that the laser fluence falls short of the size reduction threshold. At laser fluences greater than or equal to 6.1 mJ 3 cm
2, however, the extinction signal decreased continuously until it reached a plateau value at delays between 100 and 300 ps. The transient signals ΔAbs exhibited higher bleaching rates with increasing fluence. This is reasonable because higher fluences can cause greater degrees of fragmentation. To show clearly the onset of bleaching, an expanded view of Figure 6a at shorter time scales is given in the Supporting Information (Figure S9). A close scrutiny of Figure 6a reveals that the red curve corresponding to 3.7 mJ 3 cm
2 shows a slightly reduced transient signal ΔAbs of
0.0006 up to 300 ps. This weak signal change is ascribed to the surface-melting-induced reshaping of Au NPs, occurring at all laser fluences above the surface melting threshold. By considering this signal level of
0.0006 (dashed
dotted line m in Figure 6a) as a new reference at which no size reduction occurs, a time-dependent signal reduction below this line observed at higher laser fluences was ascribed to the occurrence of fragmentation. For instance, at a fluence of 8.6 mJ 3 cm
2, the delay time before the signal starts to fall below line m is about 9.3 ps (Figure S9, Supporting Information). Further increases in laser fluence to 12.2 and to 17.2 mJ 3 cm
2 yield shorter delays of 3.7 and 3.2 ps, respectively. Our simulation predicts that the onset time for fragmentation due to the Coulomb explosion mechanism is governed by the time at which TL reaches the melting temperature, while Te exceeds the fragmentation temperature. Thus, the onset should be earlier at higher fluences. The calculated delay time to reach the melting point of Au NPs at each laser fluence is 5.7
13.8 ps for 8.6 mJ 3 cm
2, 3.4
5.7 ps for 12.2 mJ 3 cm
2, and 2.4
3.2 ps for 17.2 mJ 3 cm
2. The experimentally observed onset time of the size reduction depending on laser fluence falls inside the range of the calculated delays. The experimental result 8508

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Figure 6. (a) Femtosecond transient absorption bleaching signals at various fluences vs time for 60 nm aqueous Au NPs measured at (490 ( 5) nm. Line m shows the signal level at which surface melting occurs. (b) Femtosecond transient absorption signals at various fluences vs time for 60 nm aqueous Au NPs at delays of up to 2 ns.

suggests that no instantaneous Coulomb explosion occurs on excitation. Even at high laser fluences, at which Tefrag exceeds the Rayleigh instability limit for solid 60 nm Au NPs (>8300 K), size reduction occurs in the liquid phase. Unexpectedly, the bleaching signal at longer time scales exhibited a recovery trend. Figure 6b depicts the time curves of ΔAbs at three different laser fluences at long delays of up to 2 ns. At a delay time of 500
700 ps, time-dependent increase in the ΔAbs signals at 6.1 and 17.2 mJ 3 cm
2 is observed. This absorption recovery can be indicative of nanobubble formation originating from hot fragmented 3 nm NPs. The bubbles can cause a decreased light transmission due to the scattered light at the water
vapor interface, thus contributing to an increased light extinction. This observation of absorption signal recovery requires further scrutiny through additional experiments. The transient bleaching cannot be ascribed to the bubble formation.51 Importantly, the lifetime of the bubbles is dependent on laser fluence; higher fluences give longer lifetimes and bigger bubble sizes.8 This typical behavior was unobserved in Figure 6b, where the transient signals at 6.1 and 17.2 mJ 3 cm
2 (∼3 times increase) show similar bleaching and recovery curves. Additionally, the initiation of the transient bleaching takes place within less than 10 ps in our experiment. The time delay for bubble formations is at least several tens of picoseconds (∼40 to 50 ps), even if we assume that explosive boiling occurs at lower

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temperatures, i.e., ∼500 K, due to the spinodal process investigated by Neumann and co-workers.52 Coulomb Explosion Mechanism. Here, we consider the possible Coulomb explosion dynamics revealed by the singleshot experiment. Scheme 1 illustrates the size reduction process of Au NPs. After the electron
electron thermalization time of e500 fs, a Fermi distribution at the electron temperature Te is established, and thermionic electron emission occurs. Simultaneously, the hot electrons relax through electron
phonon scattering and contribute to increasing TL up to 700 K, where reshaping occurs as a result of surface melting. A further increase in TL to 1337 K (the melting point) causes a phase transformation from solid to liquid, reducing the surface tension (surface energy) of Au NPs. When the critical charge of a liquid Au NP exceeds the Rayleigh instability threshold owing to the thermionic emission of conduction electrons, the liquid droplet breaks up into many smaller nanodroplets. These nanodroplets move away from each other because of the repulsive positive force. At the same time, the explosion may grant the fragments a certain kinetic energy, whose exact value is difficult to estimate. The time scale to reach the melting point is 3.2
9.3 ps, depending on the laser fluence, according to the present transient absorption data given in Figure 6a. The entire fragmentation process can take place in ∼100 ps, as observed by the maximum bleaching of the ΔAbs signal, until the fragmented Au NPs remain far apart owing to the kinetic energy. We numerically simulated the separation process of the fragments. A hexagonal close-packed geometry was tentatively assumed for uniformly sized spheres of 3 nm diameter. Upon electrostatic break-up of the original liquid nanodroplets, the fragments initially form densely packed clusters. They later fall apart to generate separate particles. The local cluster concentration within the probe area decreases depending on the time delay. This causes the bleaching of the transient absorption signal observed in Figure 6. We simulated this process by applying the Maxwell Garnett effective medium theory given in eq 1.1 1 þ 2f Λ 1
fΛ

ð1aÞ

1 εðω, RÞ
εm εm εðω, RÞ þ 2εm

ð1bÞ

εeff ðω, RÞ ¼ εm

Λ¼

Here, ε(ω,R) denotes the size-dependent dielectric function of the Au NPs, and εm represents the dielectric constant of the surrounding water. The calculation was performed by considering values at room temperature because the transient absorption signal at (490 ( 5) nm does not exhibit temperature dependence. The filling factor f is defined as f = N 3 VP, where N is the local NP concentration and VP is the average NP volume. The filling factor f is a measure of the concentration of one species in a medium associated with the sample volume and takes values between 0 and 1. Finally, εeff describes the effective dielectric function, which expresses the linear response of aggregates of clusters to the external field in terms of the dielectric functions of Au NPs and the surrounding medium. The substitution of the response of Au NPs and the surrounding water by the macroscopic εeff represents the replacement of the composite inhomogeneous material by fictitious homogeneous materials of εeff, characterized by the same macroscopic optical properties. The symbol Λ follows from Mie theory in the quasi-static limit, which 8509

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Scheme 1. Schematic Illustration Representing the Femtosecond-Laser-Induced Fragmentation Process Proposed on the Basis of Our Experimental Data

Figure 7. Relative absorption signal of 3 nm Au NPs at 496 nm as a function of local concentration. Highest concentration of 5.23  1025 m
3 corresponds to the hexagonal close-packing geometry for uniformly sized spheres (f = 0.74), and the lowest corresponds to separate particles (f = 0.001).

is satisfactory for our problem because of the extremely small 3 nm diameter Au NPs, for which only dipole oscillations of the conduction electrons occur. The absorption spectrum is represented by the absorption constant γ(λ) given in eq 2.1 vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u u ε21, eff þ ε22, eff t ε 4πk 4π 1, eff ¼ þ γðλÞ ¼
ð2Þ λ λ 2 2 Here, κ stands for the absorption coefficient, and λ denotes the wavelength of the probe beam. The indices 1 and 2 of the effective dielectric function stand for the real and imaginary part. The calculated UV
vis spectral change in the course of separation is shown in the Supporting Information (Figure S10
11), and the resulting ΔAbs at 496 nm is shown in Figure 7. Figure 7 simulates well the time-dependent change in the local concentration of clusters, which can qualitatively reproduce the time-dependent bleaching behavior within 100 ps. Comparison with Other Studies. The previous study demonstrated the lattice expansion and melting of 38 nm diameter Au NPs in water by looking at the Bragg reflection intensity on irradiation of 100 fs pulsed laser (λ = 400 nm).25 It was reported that the surface ablation of Au NPs occurs at 9
12 mJ 3 cm
2 before the complete melting of the particle that takes place at a fluence of 15 mJ 3 cm
2. A fragmentation model was proposed that assumes the spallation of gold ions from the surface of the Au

NPs caused by enhanced near-field of the incident laser light. To check this possibility in our system, we performed the Mie calculation53,54 to acquire electric field distribution for a single 60 nm gold particle surrounded by water. The distribution of electric field intensity |E| with respect to the incident light intensity |Ei|, |E|/|Ei| was obtained (Supporting Information, Figure S12), and the maximum |E|/|Ei| of 3.02 afforded the electric field of 1.6 V 3 nm
1 at our fragmentation threshold fluence of 7.3 mJ 3 cm
2. According to the previous studies, a much higher field is necessary to induce surface evaporation or ion emission. For instance, the threshold values of E = 6 V 3 nm
1 were reported55 to produce a field emission current enough to heat the sharp section of a gold tip to induce surface evaporation, and E = 45 V 3 nm
1 is required for ion emission from a tungsten tip in ultrahigh vacuum (UHV).56 The threshold of electron field emission was calculated using the Fowler
Nordheim equation57 for a 60 nm gold particle. Assuming that the electron field emission occurs from the whole particle surface, which is actually an overestimation because of a strong electric field occurring only at the poles, the field required to eject one electron within the electron thermalization time of ∼500 fs (the Fermi distribution is re-established) is estimated to be 6.1 V 3 nm
1 for electrons at the Fermi level. For electrons that are excited to the energy state of 3.1 eV (400 nm) above the Fermi level, the electric field required for field emission is still 2.5 V 3 nm
1. Here, it should be noted that 400 nm laser light mainly contributes to the excitation of interband (5d to 6sp) rather than intraband (within 6sp). Furthermore, it is necessary to eject several hundreds of electrons (∼630) to exceed the Rayleigh instability limit of a 60 nm liquid nanodroplet, which is unrealistic for the field electron emission. Field strengths necessary for initiating ion emission are much higher than those for electron emission. Thus, in our case, the occurrence of near-field ablation is unlikely. This is consistent with our observation: the near-field ablation would lead to a gradual size reduction that was not observed at all in our TEM photographs at 100 Hz repetition rate. It is pertinent to note that a large electric field enhancement factor of 11.3 obtained previously25 can be ascribed to the quadratic field enhancement factor (E2/Ei2) because we obtained a factor (|E|/|Ei|) of only 3.08 for a 38 nm Au NPs. This value leads to a corrected electric field of 2.2
2.5 V 3 nm
1 instead of 9.6
12.3 V 3 nm
1 at the threshold fluence. The detail is given in the Supporting Information (sections 8 and 9). The shape modification of silver nanoparticles (Ag NPs) embedded in a soda-lime glass was observed by irradiating 8510

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The Journal of Physical Chemistry C femtosecond pulsed-laser light.58,59 The work function of silver in vacuum is 4.3 eV, 0.8 eV less than that of gold. By using an electron affinity value of 1.2 eV for silica glass,60 the photon energy necessary for ejecting an electron from Ag NPs to the conduction band of the glass is calculated to be only 3.1 eV, which is compared with the energy of 400 nm laser light (3.1 eV). Accordingly, electrons slightly above the Fermi level can directly be ejected from the NPs and transferred to the conduction band of glass. Electron ejection was reported to take place also from Au NPs embedded in silica glass exposed to intense laser pulses (50 ps, 351 nm).60 Nevertheless, it was concluded that the main ejection process is ascribable to thermionic emission, and the direct photoemission of hot electrons is less favored. At this moment, we find neither the evidence of near-field ablation through the field ion emission nor that of fragmentation due to direct photoionization on femtosecond laser excitation of Au NPs in solution.

’ CONCLUSION The Coulomb explosion mechanism through thermionic emission for the laser-induced size reduction of aqueous Au NPs has been a subject of debate for many years, and no definitive experimental proofs have been given thus far. Our present femtosecond transient absorption experiment offers direct spectroscopic observation of size reduction that is ascribable to this mechanism, and the size reduction occurs within 100 ps. The result was well-supported by a numerical simulation of Te (electron temperature) and TL (lattice temperature), the temperature parameters that constitute a measure of whether Coulomb explosion or photothermal evaporation prevails. Importantly, we determined the fragmentation threshold energy, which coincides with the melting temperature of liquid Au NPs derived from the liquid drop model. The time delay of the onset observed for transient absorption bleaching correlated well with the time to reach the fragmentation temperature in our simulation. This also supports the reliability of the result. Likelihood of near-field ablation was also examined but enjoyed little support from the calculated electric field near the surface of Au NP at the fragmentation laser fluence. The near-field ablation mechanism or Coulomb explosion through photoelectric effect can operate, but at much higher laser intensities that give the near-field values from 40 to 50 V 3 nm
1 or cause multiphoton excitation-assisted direct electron ejection. The present study revealed that a femtosecond study is a straightforward method to observe Coulomb-explosion-based fragmentation without disturbance by the photothermal response. However, we also found that certain precautions, such as a low repetition rate at low laser fluences, are necessary to avoid the photothermal effect. Although Au NPs are known to efficiently convert electromagnetic radiation into heat and have been used in photothermal applications, we find a new dimension, distinct from photothermal events, by femtosecond laser irradiation at the low intensity limit. The Coulomb explosion of Au NPs may find potential applications in nanofabrication and nanopatterning on substrates. We expect that nanobullets rich in kinetic energy can effectively damage material surfaces. Such an attempt is now underway. ’ ASSOCIATED CONTENT

bS

Supporting Information. (1) Time profile of electron
phonon dynamics to determine the exact time delays, (2)

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fragmentation threshold determination by single-shot femtosecond laser irradiation, (3) TEM images of initial 60 nm aqueous Au NPs subjected to various fluencies of femtosecond laser irradiation for in situ spectroscopic measurements, (4) concentration changes as a function of irradiation time for initial 60 nm Au NPs, (5) ΔA, Te, and TL vs laser fluence curves obtained for 400 nm pulsed-laser excitation operated at a 100 Hz repetition rate, (6) femtosecond transient absorption spectra and time curves at (490 ( 5) nm showing an expanded view of Figure 6a, (7) calculated UV
vis spectra for various densities of 3 nm Au NPs in water, (8) electric near-field distribution calculated by Mie theory and the electric field strength at the threshold laser fluence of Au NPs in water, and (9) estimation of field electron emission threshold. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT Financial support from KAKENHI (no. 21020025 on Priority Area on Strong Photon
Molecule Coupling Field (no. 470) and no. 22655043) and the Tokyo Ohka Foundation for the Promotion of Science and Technology (research grant 2010) is gratefully acknowledged. Mr. M. Tagami and Mr. T. Ueki are acknowledged for TEM photograph acquisition. ’ REFERENCES (1) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Berlin, 1995. (2) Habenicht, A.; Olapinski, M.; Burmeister, F.; Leiderer, P.; Boneberg, J. Science 2005, 309, 2043–2045. (3) Huang, W; Qian, W.; El-Sayed, M. A. J. Am. Chem. Soc. 2006, 128, 13330–13331. (4) Tabor, C.; Qian, W.; El-Sayed, M. A. J. Phys. Chem. C 2007, 111, 8934–8941. (5) Hashimoto, S.; Uwada, T.; Hagiri, M.; Takai, H.; Ueki, T. J. Phys. Chem. C 2009, 113, 20640–20647. (6) Hashimoto, S.; Uwada, T.; Hagiri, M.; Shiraishi, R. J. Phys. Chem. C 2011, 115, 4986–4993. (7) Kotaidis, V.; Dahmen, C.; Von Plessen, G.; Springer, F.; Plech, A. J. Chem. Phys. 2006, 124, 184702. (8) Lukaianova-Hleb, E.; Hu, Y.; Latterini, L.; Tarpani, L.; Lee, S.; Drezek, R. A.; Hafner, J. H.; Lapotko, D. O. ACS Nano 2010, 4, 2109–2123. (9) Link, S.; Burda, C.; Mohamed, M. B.; Nikoobakht, B.; El-Sayed, M. A. J. Phys. Chem. A 1999, 103, 1167–1170. (10) Takami, A.; Kurita, H.; Koda, S. J. Phys. Chem. B 1999, 103, 1226–1232. (11) Inasawa, S.; Sugiyama, M.; Yamaguchi, Y. J. Phys. Chem. B 2005, 109, 9404–9410. (12) Inasawa, S.; Sugiyama, M.; Noda, S.; Yamaguchi, Y. J. Phys. Chem. B 2006, 110, 3114–3119. (13) Amendola, V.; Meneghetti, M. Phys. Chem. Chem. Phys. 2009, 11, 3805–3821. (14) Wang, G. W. Prog. Mater. Sci. 2007, 52, 648–698. (15) Sasaki, K.; Takada, N. Pure Appl. Chem. 2010, 82, 1317–1327. (16) Asahi, T.; Sugiyama, T; Masuhara, H. Acc. Chem. Res. 2008, 41, 1790–1798. (17) Pyatenko, A.; Yamaguchi, M.; Suzuki, M. J. Phys. Chem. C 2009, 113, 9078–9085. 8511

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