Supporting Information

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1 Supporting Information Picosecond-to-Nanosecond Dynamics of Plasmonic Nanobubbles from Pump-Probe Spectral Measurements of Aqueous Colloidal Gold Nanoparticles Tetsuro Katayama, Kenji Setoura, Daniel Werner, Hiroshi Miyasaka *, Shuichi Hashimoto * Department of Optical Science and Technology, The University of Tokushima, Tokushima , Japan, Division of Frontier Materials Science and Center for Quantum Materials Science and Technology under Extreme Conditions, Osaka University, Toyonaka, Osaka , Japan, PRESTO, Japan Science and Technology Agency, Kawaguchi , Japan. * corresponding author, hashichem@tokushima-u.ac.jp, miyasaka@chem.es.osaka-u.ac.jp 1

2 S1. Particle images (TEM micrographs) and their size distributions. diameter: (19±3) nm Figure S1 (a) TEM image of Au NPs (BBI, EMGC2, nominal 2-nm diameter) and a histogram of particle sizes for 35 particles. The average size and the dispersion were estimated to be 19 and 3 nm. 1 nm Figure S1 (b) TEM image of Au NPs (BBI, EMGC6, nominal 6-nm diameter) and a histogram of particle sizes for 918 particles. The solid line in the figure is the Gaussian curve. The average size and the dispersion were estimated to be 58 and 5 nm. 2

3 2 nm Figure S1 (c) TEM image of Au NPs (BBI, EMGC1, nominal 1-nm diameter) and a histogram of particle sizes for 4 particles. The solid line in the figure (left) is the Gaussian curve. The average size and the dispersion were estimated to be 1 and 8 nm. 2nm nm 1 frequency diameter (nm) Figure S1 (d) TEM image of Au NPs (BBI, EMGC15, nominal 15-nm diameter) and a histogram of particle sizes for 1 particles. The average size and the dispersion were estimated to be 155 and 18 nm. 3

4 S2. Comparison of steady-state extinction spectra of Au NPs before and after the picosecond 355-nm laser irradiation. (a) (b) (c) Figure S2 Extinction spectra of Au NPs in aqueous solution. 2 nm-diameter for (a), 6-nm diameter for (b), and 1-nm diameter for (c), before and after 1 shots (15 ps, 355 nm laser) at the threshold fluences of bubble formation. 4

5 S3. Experimental configuration and the method of calibration for the excitation laser intensity Figure S3-1 shows the experimental setup for picosecond pump-probe spectroscopy. Excitation and probe beams were introduced collinearly to the sample cuvette with a 1-cm path length through a 2-mm diameter pinhole. The pinhole that adjusts the diameter of excitation laser pulse was placed in front of the cuvette. The probe light (picosecond white light continuum) with a diameter of less than 1 mm was passed through the center of excitation laser. A portion of the excitation beam was reflected by a cover slip and the intensity of the reflected light was measured by a photodiode monitor. Figure S3-2 shows the excitation laser energy measured by a power meter (OPHIR, 2A-SH) placed at the sample position as a function of the output of a photodiode signal. The linear relationship was obtained between the photodiode output signal and the laser energy in the experimental intensity range: the solid line represents a linear least-squares fitting to the experimental data points. Optical delay line Probe light (Supercontinuum) Sample Short-cut filter Monochromator Pump light (3 = 355 nm) Nd 3+ :YAG laser+kd*p 2 mm Pinhole Signal Intensity (a.u.) ps -4 4 Time / ps Multichannel Photodiode Time profile of the excitation laser pulse Figure S3-1 Experimental configuration and the time profile of the excitation laser pulse. Figure S3-3 shows the transient absorption spectrum of pyrene in cyclohexane solution at 1 ps after photoexcitation. The transient absorption spectrum that is quite similar to the ones given in the previous measurements. 1 The excitation intensity-dependent absorbance of pyrene in cyclohexane was used for the calibration of excitation laser fluence. This is because the method has been shown to be reliable free from the spatial inhomogeneity of excitation pulses and small fluctuations in overlap between the excitation and probe beams. Figure S3-4 shows the absorbance change at 468 nm at a delay of 1 ps plotted against the excitation laser intensity. A gradual saturation behavior was observed with increasing excitation intensity. The saturation occurred as a result of absorption due to a S n S 1 transition at 355 nm. The solid curve in Figure S3-4 was drawn 5

6 Pulse energy ( mj ) including a transient inner filter effect and also a depletion of the ground state molecules. The detail of the calculation method with rigorous examination of the method were given in a previous paper. 1 The intensity of a reflected excitation light was recorded for the transient absorption measurement of Au NPs, and it was converted to the excitation laser fluence. The excitation laser intensity calibration with pyrene in cyclohexane was performed each time before the experiment. Reference: 1. Miyasaka, H.; Masuhara, H.; Mataga, N. Laser Chem., 1983, 1, Photodiode signal counts Figure S3-2. The relationship between the excitation intensity measured by a power meter (OPHIR, 2A-SH) at the sample position and the intensity of a reflected excitation light. The red line represents a least-squares fitting by a linear function, y = ( ) x. 6

7 Absorbance Absorbance ps Wavelength / nm Figure S3-3. Transient absorption spectrum of pyrene in cyclohexane at 1 ps after the photoexcitation Pyrene in cyclohexane. Mon. at 468 nm (at 1 ps) Calculated curve 4 8 mj cm Figure S3-4. Excitation laser intensity dependent S n S 1 absorbance of pyrene in cyclohexane at 468nm at a time delay of 1 ps. The red curve represents a fitting based on simulation given in the literature. 1 7

8 Absorbance After sample ( J / pulse) Time / ps 8 Figure S3-5. The S n S 1 absorption of pyrene in cyclohexane as a function of the relative time interval between the pump and probe pulses monitored at 468nm. The time origin of ps was chosen by determining the time at which the transient absorbance of the S n S 1 transition was half that obtained at a time delay that was sufficiently long to ensure that the transient absorbance had saturated. Beam splitter 3 2 Photodiode sample solution Power meter Before sample (photodiode signal counts) Figure S3-6. Schematic view of the set up and the relationship between the excitation intensity measured by a power meter (OPHIR, 2A-SH) at the sample position and the intensity of a reflected excitation light before the sample position. Excitation wavelength was at 355 nm. The red line represents a least-squares fitting by a linear function, y = (2.8 ± ) x. Laser fulences were less than 1 mj cm -2. 8

9 S4. Temperature- and medium refractive index-dependent absorption cross section spectra of a 6-nm diameter Au NP x1-15 (a) x (b) d = 6 nm 283 K 583 K 843 K 1193 K 7 6 d = 6 nm 283 K 583 K 843 K 1193 K 1 5 Cabs (m 2 ) 8 6 Cabs (m 2 ) wavelength (nm) wavelength (nm) x1-15 (c) x (d) d = 6 nm 283 K 583 K 843 K 12 K 7 6 d = 6 nm 283 K 583 K 843 K 12 K 1 5 Cext (m 2 ) 8 6 Cext (m 2 ) wavelength (nm) wavelength (nm) Figure S4. Temperature-dependent absorption cross section, C abs as a function of wavelength calculated for 6-nm diameter Au NP in water (n=1.33) (a) and in air (n=1.) (b) by applying Mie theory. The particle temperatures were shown in the figures. To calculate the temperature-dependent C ext, the temperature dependent dielectric functions of bulk gold that were determined experimentally by Otter (ref. 34) were used. For comparison, extinction cross section (C ext ) spectra in water (c) and in air (d) were shown. Here, C ext = C abs + C sca, with C sca, scattering cross section. 9

10 Extinction S5. Transient bleaching recovery observed at 532 nm beyond the timescale of the pump-probe measurements mj cm Figure S5 (a). Extinction change observed at 532 nm when excited by a 355 nm, 15 ps laser with a fluence of 4.6 mj cm -2, (probe light: CW laser (PHOTOP, DPGL-25F), detector: photodiode (Thorlabs, DET1A/M, rise time < 1 ns)) at a time scale longer than that of the pump-probe measurement, showing the indication of permanent bleach. The extinction signals were 19-times accumulated. The permanent bleach was not observed at 4. mj cm -2. 1

11 1 nm Figure S5 (b). TEM micrographs of 6-nm diameter Au NPs before (upper panel) and after (lower panel) irradiation at 4.6 mj cm -2 for 5 shots by a 355 nm, 15 ps laser. The red arrow indicates a spherical particle generated by the laser irradiation among the original faceted particles. 11

12 (a) (b) (c) Figure S5 (c). TEM micrographs of 6-nm diameter Au NPs under a pressure of 6 MPa before (a), after irradiation at 4.9 mj cm -2 (b) and after irradiation at 9.9 mj cm -2 for 1 shots by a 355 nm, 15 ps laser. 12

13 . Extinction Time / s nm Figure S5 (d). Extinction time curve (upper panel) observed at 532 nm for 6 nm diameter reshaped Au NPs (lower panel: TEM image) when excited by a 355 nm, 15 ps laser with a fluence of 5. mj cm

14 Extinction Extinction (.2 / div.) S6. Threshold behavior of bubble signals. Extinction (.2 / div.) Wavelength / nm (a) 9 (b) 44.8 mj / cm mj / cm mj / cm mj / cm 2 Figure S6-1. Transient extinction spectra of Au NPs with different sizes in aqueous solution monitored at 2 ns following the excitation; (a) for 2 nm, (b) for 6 nm, (c) for 1 nm, and (d) for 15 nm in diameter. The fluence of the excitation laser pulse is shown in each of the figures mj / cm mj / cm mj / cm mj / cm Wavelength / nm 9 Extinction (.2 / div.) Wavelength / nm (c) 9 2. mj / cm mj / cm mj / cm (d) Wavelength / nm 59.9 mj / cm mj / cm mj / cm mj / cm 2 14

15 Extinction.4 Au 2 nm.8 Au 6 nm.3 45 nm 6 nm 65 nm 7 nm.6 45 nm 6 nm 65 nm 7 nm Extinction Fluence ( mj cm -2 ) Fluence ( mj cm -2 ) Au 1 nm Au 15 nm.3 7 nm 75 nm 8 nm.6 43 nm 48 nm Extinction.2 Extinction Fluence (mj / cm 2 ) Fluence ( mj / cm 2 ) Figure S6-2. Fluence-dependent extinction changes at different wavelengths for aqueous colloidal Au NPs (2-, 6-, 1-, and 15-nm diameters) measured at the time delay of 2 ns following the excitation with a 15 ps, 355-nm laser pulse. 15

16 S7. Bubble formation threshold-related behavior. Figure S7-1. Simulated threshold laser fuence vs. Au NP diameter, calculated assuming the temperature of a water layer 2 nm from the NP interface reaches 55 K, after 4 ps of the laser excitation (355 nm, 15 ps). Figure S7-2. Diameter-dependence of a quantity, (C abs /V) / G c (G c : critical thermal interface conductance, C abs : absorption cross section, V: particle volume) that represents the medium heating efficiency. 16

17 (a) (b) (c) (d) (e) Figure S7-3. (a), (b) Particle temperature for the excitation of Au NPs (2-nm diameter, 6-nm diameter, 1-nm diameter) in water (n = 1.33) by a 15 ps FWHM, 355 nm pulse was plotted as a function of delay time. The laser intensity was given in the figure. (c) (e) Temperature profiles as a function of a distance from the particle center at two time delays for 2-nm (c), 6-nm (d), and 1-nm (e) Au NPs. The time-dependent temperatures show that the particle cooling is strongly dependent on the particle diameter: the cooling rate is much greater for smaller particle diameter. The temperature profiles suggest that medium cooling is remarkable for 2-nm-diameter Au NP. The calculation was performed using COMSOL Multiphysics v.4.3a ( 17

18 S8. Temperature simulations To gain an insight into the temperature behavior of Au NPs initiated by laser excitation, we calculated the electron temperature T e, the lattice temperature T L, and the medium temperature T m at the particle interface for a 6 nm diameter Au NP in water. Figure S8 shows T e, T L, and T m for two excitation energies of 3., 5. and 1 mj cm 2 (the experimental bubble formation threshold is 5.2 mj cm 2 ) at various delay times. The temperatures were calculated by applying the two-temperature model. 1 We used the Gaussian temporal profile of excitation laser pulse (FWHM: 15 ps) for the calculation. Note here that the calculated temperatures at the various excitation energies were not exact, but they provided a good indication of the experimental temperature at a given laser fluence. In this regard, one major challenge is the determination of the interfacial heat conductance. 2 We used a value of W m 2 K 1 for the Au NPs in water. 3 As expected, a very high T e was realized instantaneously (T e : 326 K (3. mj cm 2 ), T e : 426 K (5. mj cm 2 ), and 545 K (1 mj cm 2 )), which contributed to the strong initial LSPR bleaching. 4 Subsequently, T L and T m exhibited rising and decaying behavior. The maximum T L values of 83 K (3. mj cm 2 ), 12 K (5. mj cm 2 ) and 162 K (1 mj cm 2 ) were reached when the attainable T m values were 583 K (3. mj cm 2 ), 683 K (5. mj cm 2 ) and 768 K (1 mj cm 2 ), as shown in Figure S8a, S8c and S8e. Figure S8b and S8d represent the temperature profiles as a function of distance from the particle center. Here, typical temperature profiles are shown for two simulated time delays. Notably, the medium temperatures were distance- and time-dependent. Also, a large temperature gap was observed at the NP-medium interface in the early stages but it disappeared at later stages. It has been postulated that at ambient pressure, superheated water molecules are formed directly around the Au NPs, undergoing vaporization as a result of explosive evaporation when the water medium is heated close to the spinodal temperature of ~573 K (~3 C). 5 References: 1. Hartland, G. V. Chem. Rev. 211, 111, Wilson, O. M.; Hu, X.; Cahill, D. G.; Braun, P. V. Phys. Rev. B, 22, 66, Plech, A.; Kotaidis, V.; Grésillon, S.; Dahmen, C.; Von Plessen, G. Phys. Rev. B 24, 7, Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. Rev. 25, 15, Caupin, F.; Herbert, E. C. R. Phys. 26, 7,

19 (a) (b) T e after 21ps after 41ps T /K 1 T L T /K T m time /ns (c) distance from NP center /nm (d) T e 12 1 after 24ps after 1.37ns T /K 1 T L T /K T m time /ns distance from NP center /nm (e) (f) T e after 24ps after 5.4ns T /K 1 T L T /K 1 8 T m time /ns distance from NP center /nm Figure S8. The simulated temporal evolution of the electron temperature T e (dotted black curve), the lattice temperature T L (dashed red curve), and the maximum water temperature T m at the NP water interface (solid blue curve, 2 nm from the particle surface) for a 6-nm-diameter gold sphere absorbing a 15 ps laser pulse (the FWHM of the Gaussian time profile) at 355 nm and at laser energy densities of 3. mj cm 2, where P max = W cm 2 (a), and 5. mj cm 2, where P max = W cm 2 (c), and 1 mj cm 2, where P max = W cm 2 (e). The radial temperature distributions at two delay times are shown for frame a (b), for frame c (d), and for frame e (f). Note that the melting point of bulk gold is 1337 K, and that the boiling point of bulk gold is ~31 K. 19

20 S9. Particle temperature-dependent C ext spectra of a Au NP surrounded by a bubble. Figure S9. Calculated particle temperature-dependent C ext spectra based on equation 4, C C ( T, bubble) C ( RT ) ) for 6-nm diameter Au NP with a bubble diameter of 24 nm in ext ext L ext water, showing that the transient extinction spectra are insensitive to the particle temperature once a bubble with a sufficient diameter is formed. 2

21 S 1. Calculation of heat accumulation. We estimated the increase in bulk temperature of 5 ml aqueous solution contained in a cell (1 1 5 mm) when the solution was irradiated with a single laser pulse (wavelength: pulse width: 355 nm, 15 ps FWHM, beam diameter: 2 mm). The extinction of the solution at 355 nm was.73. 2D temperature distribution was calculated using COMSOL Multiphysics for the geometric configuration given above. The result was represented by temperatures, with and without forced convection, at the laser beam center as a function of time delay after the irradiation. The repetition rate of the laser irradiation was.5 Hz. When forced convection was applied as in the experiment, the original solution temperature, 293 K, recovered within 1s with a maximum temperature increase of 4K. Figure S1. Time evolution of bulk temperature estimated by calculation. 21

22 S11. The method of converting extinction vs. time curve to bubble diameter vs. time curve. x1-15 C ext /m nm 49nm 535nm 6nm 65nm bubble diameter /nm Figure S11 (a). C ext (1193 K, bubble) as a function of bubble diameter obtained by the Mie calculation. The corresponding C ext (1193 K, bubble) spectra dependent on bubble diameter were shown in Figure 6b. 22

23 (b) (c) Extinction MPa 4.9 mj cm -2.1 MPa 5.2 mj cm -2-8x nm 2. Extinction MPa 4.9 mj cm -2.1 MPa 5.2 mj cm nm.8 1. (d1) (d2) Extinction Extinction x Figure S11 (b). The transient extinction ( extinction) as a function of time at 45 nm at different external pressures,.1 MPa (in the presence of bubbles) and 6 MPa (in the absence of bubbles); (c) the transient extinction ( extinction) as a function of time at.1 MPa (in the presence of bubbles) and 6 MPa (in the absence of bubbles); (d1) shows Ext (45 nm) = Ext (45 nm,.1 MPa) Ext (45 nm, 6 MPa) and (d2) Ext (535 nm) = Ext (535 nm,.1 MPa) Ext (535 nm, 6 MPa) at various time delays. 23

24 (e1) Ext. (X nm) / Ext. (535 nm) (e2) 1 15 X = 7 nm X = 65 nm X = 6 nm X = 45 nm Diameter / nm (e3) 7 nm 65 nm 6 nm 45 nm Figure S11 (e). Conversion from extinction vs. time curve to bubble dynamics: (e1) C ext (X nm)/c ext (535 nm) vs. bubble diameter obtained from the spectral simulation described in the text; (e2) Ext (X nm)/ Ext (535 nm) vs. time curve obtained from the experimental Ext vs. time curves (errors: < 1%, estimated by the standard deviation); (e3) bubble diameter vs. time curve. The red line represents a fitting by 24

25 t t Dt ( ) ( ) exp ( ) exp ns ns Errors in estimating diameters: 2% by the standard deviation of the fitting analysis. 25

26 Appendixes Appendix 1 (a) (d) Extinction x nm Au 6 nm Extinction -2-4x nm Au 2 nm (b) (e) 3x1-2 2 Extinction nm Au 6 nm (c) 2.x1-2 Extinction nm Au 2 nm nm Au 6 nm Appendix Figure 1. Time profiles of the transient extinction signals of 6 nm Au NPs in water excited with a laser intensity of 1.1 mj cm (355 nm, 15 ps) (a) (c), and 2 nm diameter Au NPs in water excited at 1.1 mj cm (355 nm, 15 ps) (d) and (e). (a) time profile at 515 nm with a bleaching recovery time constant of 2 ns, (b) time profile at 55 nm, corresponding to the fast decay of the positive wing; (c) acoustic phonon vibration observed at 55 nm in the time scales of less than 3 ps; (d), time profile at 51 nm with a bleaching recovery time constant of 12 ps (which is much faster than that for the 6 nm particles), (e) time profile at 56 nm, corresponding to the fast decay of the positive wing. These spectra suggest the absence of bubble formation at a given laser 26

27 fluence. The red line in (a) represents a fitting by A( t) A1 exp t 2 ns And that in (c) represents a fitting by A( t) A1 exp t 2 ps 27

28 Appendix 2 (a) (b) Extinction nm -.1 Au 6 nm -.2 E Extinction nm Au 6 nm -5x1-2 E Extinction nm Au 6 nm. E (c) Extinction nm Au 6 nm (d) E Appendix Figure 2. Time evolution of transient extinction at a laser intensity of 5.2 mj cm 2 for 6-nm diameter Au NPs. Monitoring wavelengths: (a) 49 nm the isosbetic point of the plasmon band bleaching caused by temperature-induced broadening shortly after excitation; (b) 45 nm the left-hand wing of the plasmon band bleaching; (c) 63 nm the right-hand wing of the plasmon band bleaching; and (d) 535 nm the plasmon band bleaching minimum. The outer frames represent short time scales up to 2 ns, while the insets represent longer time scales up to 18 ns. The red lines represent a fitting by applying a following equation: t t t A( t) A1 exp A2 exp A3 exp 15 ps 1.2ns 2.3ns 28

29 Appendix 3 (a) (b) x1-15 x nm 1. 63nm C ext C ext time /ns time /ns x1-15 x nm 1. 63nm C ext C ext time /ns time /ns Appendix Figure 3. Simulated transient extinction time curves for a single 6-nm diameter Au NP in water in the absence of vapor bubble formation at two wavelengths: 45 nm (a) and 63 nm (b). Upper frames: up to 2 ns, lower frames: up to 15 ns. Simulation parameters: excitation laser wavelength, 355 nm; laser intensity, 1 mj cm 2 ; laser pulse duration, 15 ps. Here, the time evolution of C ext is caused by the temperature evolution of the Au NP accompanied by the time-dependent spatial temperature distributions of the medium water surrounding the NP. 29

30 Appendix 4 Thermal expansion of gold Linear expansion: r / r T (K) Volume expansion: V / V T (K) References: [1] Paradis, P.F.; Ishikaa, T.; Koike, N. Gold Bulletin 28, 41, (for temperatures above melting point) [2] Nix, F. C.; MacNair, D. Phys. Rev. 1941, 6, (for temperatures below melting point) 3

31 Appendix 5 Parameters used for calculation item symbol solid liquid vapor specific heat capacity of C m [J kg K -1 ] water density of water ρ m [kg m -3 ] T m T m T m T m T 5 m T m.59 heat k m [W m -1 conductivity of K -1 ] water lattice heat capacity of gold C l [J g K -1 ] K -1 (1) T l.149 (1) density of gold ρ m [kg m -3 ] m P /V P (T l ) K -1 (T l K) (2) thermal expansion coefficient of gold surface tension of gold α l 3 ( K -1 T l ) (3) K -1 T l σ [N m -1 ] 8.78 (4) (T l 1337) (5) melting enthalpy of gold lattice constant of fcc unit cell work function of gold in vacuum H melt [J/m 3 ] a fcc [m] W [ev] 5.1 Fermi energy E F [ev] 5.51 Reference [1] Green, D. W.; Perry, R. H. book: Perry s Chemical Engineers Handbook ; McGraw-Hill: New York 27, 8 th edition. [2] Paradis, P. F.; Ishikawa, T.; Koike, N. Gold Bulletin 28, 41, 3, [3] Nix, F. C.; MacNair, D. Phys. Rev. 1941, 6, [4] Nanda, K. K; Maisels, A.; Kruis, F. E. J. Phys. Chem. C 28, 112, [5] Egry, I.; Lohoefer, G.; Jacobs, G. Phys. Rev. Lett. 1995, 75, 22,

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