Computational Modeling of Pulsed Laser-Induced Heating and Evaporation of Gold Nanoparticles

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1 pubs.acs.org/jpcc Computational Modeling of Pulsed Laser-Induced Heating and Evaporation of Gold Nanoparticles Michael Strasser,, Kenji Setoura, Uwe Langbein, and Shuichi Hashimoto*, Department of Optical Science and Technology, The University of Tokushima, 2-1 Minami-Josanjima, Tokushima , Japan Department of Physics, RheinMain University of Applied Sciences, Am Bru ckweg 26, D Ru sselsheim, Germany *S Supporting Information ABSTRACT: Pulsed-laser-induced size reduction of plasmonic nanoparticles in solution has long been known for a drawback resulting from polydispersed products. Recently, by adjusting external pressure, laser intensity, and excitation wavelength, the nanosecond pulsed-laser excitations of colloidal gold nanoparticles in pressurized aqueous solution were found to enable tuning of the particle size and size distribution. Nevertheless, the mechanism underlying the phenomenon is poorly understood. Here we propose a model based on temperature calculations via the two-temperature model coupled to a surface evaporation mechanism. Incorporating the temperature-induced plasmon band bleaching during the excitation is crucial. Our computational result indicated that the photothermal evaporation of gold nanoparticles of a given size occurred at temperatures below the boiling point of bulk gold, leading to a smaller particle diameter with increasing laser fluence; the result qualitatively explains the experiment. The method developed here to calculate temperature is applicable to various nanoscale experiments including surfaceenhanced Raman spectroscopy, where a proper assessment is indispensable when treating photothermal effects of plasmonic nanoparticles under illumination by pulsed and focused continuous lasers. INTRODUCTION Pulsed-laser irradiation of noble-metal nanoparticles achieves shape changes, size reduction, and particle growth. 1 4 The technique attracts growing interest because of its potential to control particle size and shape, which is important in various applications of nanomaterials. Previously, nanosecond and picosecond pulsed-laser-induced size reduction of colloidal gold nanoparticles (Au NPs) was observed in aqueous solution by exciting the localized surface plasmon resonance (LSPR) band at 532 nm or the interband transition at 355 nm On the basis of observations at threshold laser fluences that give particle temperatures close to the boiling point (b.p.) of gold, the mechanism behind size reduction has been ascribed to photothermal evaporation at particle temperatures above b.p. In practice, laser-induced size reduction in solution has one drawback in that products are polydispersed because of the poor temperature control of NPs. To overcome the drawback, Werner and coworkers applied high pressures exceeding the critical pressure of water (22.1 MPa), which resulted in a remarkably narrow-sized distribution of Au NPs with controlled particle diameters depending on the laser fluence applied. 10 Recent studies of homogeneous heating revealed that besides boiling or explosive evaporation at temperatures exceeding the b.p., surface evaporation can occur at temperatures below the b.p. of bulk gold. 11,12 The surface evaporation mechanism enables gradual particle size reduction that was observed under high pressures. The failure to observe such a surface evaporation in pulsed laser-heating at ambient pressure is ascribed to two factors: experimental deficiencies and limitations in temperature calculation of the system. Experimentally, gaining control over the temperature of particles undergoing evaporation is difficult in view of excitation laser wavelengths and intensities. In particular, difficulties in controlling size could arise when bubble formation of surrounding water molecules results from heat transfer from the particle Once the particle is surrounded by a bubble, the particle temperature becomes uncontrollable because heat dissipation from the particle to the medium is strongly suppressed, effectively resulting in particle boiling. To summarize, Scheme 1 illustrates laser-induced size reduction that depends on particle temperature. Note that if the particle temperature is below the evaporation threshold, surface melting is known to generate spherical particles with smooth surfaces. 16 The previous temperature calculations for laser heating of Au NPs contained a number of limitations. Early studies by the Koda group and the Inasawa group ignored the heat loss to the surrounding medium to simplify the temperature calculations. 5 7 To obtain particle (lattice) temperature, T l, and radialdependent medium temperature, T m (r), the Plech group worked in the Laplace domain by applying an inverse Laplace transformation instead of numerically solving the heat equation 13,14 Received: August 18, 2014 Revised: October 9, 2014 Published: October 10, American Chemical Society 25748

2 Scheme 1. Scheme for Laser-Induced Size Reduction Depending on Particle Temperature, T l ρc Tr (, t ) = ( k T( r, t)) + S( t) t Here ρ is the mass density, c is the specific heat capacity, k is the thermal conductivity of the system at the position r, and S(t) is the energy deposition term. In their approach, approximate equations were derived for two limiting cases, short-pulse and CW laser excitations. In the short-pulse regime, the time evolution of the particle temperature is determined after energy deposition. Hence, any temperature-induced effects including plasmon band bleaching that should occur at high temperature, 15 were not taken into account. Moreover, the approximation is only valid when particle cooling is negligible during energy deposition. In the CW excitation, limitations similar to the short-pulse regime can occur because energy deposition and particle cooling are in energetic equilibrium. Volkov and coworkers performed the temperature calculations using the two-temperature model (TTM) described via coupled heat equations for T e (electron temperature) and for T l (lattice temperature). 17 However, they treated energy deposition onto NPs rather coarsely by replacing the absorption cross-section, C abs, of a NP with a constant physical cross-section. Werner and coworkers considered heat dissipation by including a heat-transfer equation at the particle medium interface into the TTM for electron and lattice (particle) temperatures for femtosecond (fs) and nanosecond (ns) excitations. 18 They showed that the calculated laser fluence that produces temperatures near the bp of gold can explain the experimental threshold in size reduction of Au NPs. This work is aimed at performing the computational temperature analysis of Au NPs under pulsed-laser irradiation by applying the TTM to interpret through the surface evaporation mechanism the size reductions of Au NPs in pressurized aqueous solutions. We included in the calculation temperature-dependent changes in physical properties (e.g., thermal conductivity, density, and dielectric functions) of the medium and related effects. At the same time, we calculated the amount of surface evaporation during particle heating by laser illumination of an Au NP. Notably, estimating temperatures of laser-heated Au NPs under atmospheric pressure is difficult because of bubbles forming around hot NPs, causing sudden changes in thermal conductivity, heat capacity, and density of water. Nevertheless, the temperature calculation (1) under a high pressure such as 100 MPa can be performed because such a bubble formation is negligible, and physical properties change gradually with increasing temperature. Therefore, we can give a significantly improved explanation for laser fluence-dependent size reduction of Au NPs observed under high pressures. RESULTS AND DISCUSSION The energy deposition into Au NPs is governed by the laser pulse power density S(t) represented by a Gaussian function, the absorption cross section C abs, and the particle volume, V (Supporting Information, S1). The illustration of particle heating modes and the corresponding optical spectra calculated by applying Mie theory (Figure 1) 19 reveal that particle heating results in the gradual bleaching of the LSPR band. Additionally, medium heating causes further decreases in absorption. In nanosecond pulsed-laser heating, LSPR bleaching is dependent on time delay, laser fluence, and particle diameter, Figure 2. Figure 2 reveals that the maximum particle temperature occurs at a diameter of 80 nm. If the temperature-dependent C abs is not considered, the excitation at the wavelength of 532 nm, which is located near the peak position of a LSPR band of Au NP, would give the highest heating efficiency for a 50 nm diameter particle for which the ratio C abs /V is the highest (Supporting Information, S1). However, this picture is greatly affected by the pulse width of the excitation laser and the particle temperature. Our result suggested that the heating efficiency declines for such a particle size at high particle temperatures because plasmon-band bleaching dominates. (See Figure 2d,f for the decrease in C abs.) Here a much higher heating efficiency was gained instead with an 80 nm diameter NP. Although the particle and medium temperatures greatly affect the extinction spectra of Au NPs, such effects were neglected in a number of studies For a pulsed laser excitation at the LSPR band position, calculations yield a sudden decrease in C abs at the excitation wavelength during energy deposition, (Figure 3a), resulting in a time-dependent decrease in energy absorption. For pulse durations in the nanosecond regime, T e and T l increase in quasi-thermal equilibrium; T m (medium temperature at the NP surface) increases as a result of heat transfer across the NP water interface (Figure 3b). The efficiency of energy exchange is limited by a finite thermal conductance at the NP water interface. 23 Because of the high pressure of 100 MPa at high temperatures, the medium thermal conductivity is reduced by about one to two orders of magnitude compared with that at 300 K, resulting in slow medium heating (or particle cooling). The spatiotemporal profile of water temperature is given in the Supporting Information, S2. Under thermal equilibrium, one can simplify the ordinary differential equations (ODEs) of electron and lattice systems (see the Methods section) to the one-temperature model (OTM) 4,18 C( T) dt l l dt = St () F where C l (T l ) is the sum of electron and lattice heat capacities, S is the energy input to the free electron gas (FEG) by the laser pulse, and F is heat loss at the NP water interface. Given that C l (T l ) is approximately two orders of magnitude greater than C e (T e ), the electron heat capacity is negligible. 24 This approximation is valid for pulse widths much longer than the electron phonon coupling time (1.7 ps). 25 In contrast, when (2) 25749

3 Figure 1. Illustration of heating modes (left: top, without particle heating; middle: only particle heating; bottom: particle and medium heating) and the corresponding optical spectra (right) of 50 nm diameter Au NP in water at 100 MPa. (a) Extinction cross-section, C ext, absorption cross-section, C abs, and scattering cross-section, C sca, as a function of wavelength at 300 K. (b) C abs spectra at various particle temperatures given in the figure. (c) C abs spectra at various particle and medium temperatures. Medium temperature gradients are given in the Supporting Information, S5. the pulse duration is in the picosecond or femtosecond regime, the equilibrium condition is not reached. For femtosecond to picosecond time regimes, the heat transfer to the surrounding medium is negligible within energy deposition time scales. In this case, our calculations are in good agreement with a previous model that neglected LSPR band bleaching. 18 The previous experiment under high pressures revealed that after a sufficient number of laser pulses surface evaporation stopped, thereby leaving cores and fragments. 10 Most importantly, the minimum core size was clearly dependent on the laser fluence applied at the excitation wavelength of 532 nm, near the LSPR peak position. Starting from 100 nm diameter Au NPs and by applying laser pulses of to , a gradual controlled size reduction in Au NPs from a core diameter of 90 nm to that of 40 nm was achieved with <5% diameter distributions by increasing the laser fluence from 43 to 136 mj cm 2. A further increase in the number of laser shots did not influence the final diameter. The present task is to give a satisfactory explanation for the laser fluence-dependent size control based on the precise temperature calculation. Let us start our temperature simulation from a 100 nm diameter Au NP that is subjected to continuous 10 Hz irradiation of 5 ns fwhm Gaussian pulses. From the temperature time profile for each pulse applied every 100 ms (Figure 3b), the particle completely cools within 100 ms when the next pulse arrives. The maximum particle temperature, T max, and cooling time for each excitation depend on particle diameter and laser fluence. Here cooling time is defined 25750

4 The Journal of Physical Chemistry C Figure 2. Particle temperature (a,c,e) and ΔCabs (b,d,f) as functions of laser fluence and particle diameter on excitation with a 5 ns fwhm laser pulse at a wavelength of 532 nm under a pressure of 100 MPa (medium: water). The Cabs values were taken at 532 nm. (Cabs values decreased with temperature increase.) The time delays were set with reference to the peak position of a Gaussian laser pulse as 10 ns. Panel pairings (a,b), (c,d), and (e,f) correspond to time delays of 5, 10, and 30 ns, respectively. as the time interval over which the particle temperature, Tl, stays above the melting point (m.p.). Obviously, higher laser fluences result in increased Tmax and cooling times. In contrast, the particle diameter dependence of calculated Tmax was not straightforward. The diameter-dependent Tmax and cooling time for three different laser fluences are depicted in Figure 4. Whereas the cooling time decreases monotonically with decreasing particle diameter (Figure 4a), Tmax at first rises to a maximum and then decreases with decreasing particle diameter (Figure 4b). The account of diameter-dependent behavior of Tmax is given in the Supporting Information, S3. To estimate the surface evaporation of Au NPs that leads to size reduction, we applied the Kelvin equation coupled to the kinetic gas theory and calculated the amount of evaporated atoms per pulse (Supporting Information, S4).26,27 The vapor pressure over the curved surfaces can be evaluated from the equilibrium vapor pressure28 and the temperature-dependent surface energy of Au.29 The amount of evaporated atoms per pulse, θ, depends on the evaporation coefficient, αs, defined by the ratio of adsorbed to desorbed atoms; values of αs range between unity for complete evaporation and zero for no evaporation. Because particle temperature increases with laser energy absorption and decreases in time following heat transfer to the surroundings, a differential method was applied by dividing the time-dependent particle temperature curve, as typified in Figure 3b, into 10 ps steps from which the values of 25751

5 Figure 3. (a) Absorption cross-section (red solid line, left axis) at 532 nm of a 50 nm diameter Au sphere in water at 100 MPa as a function of time delay. A laser fluence of 50 mj cm 2, excitation wavelength of 532 nm, and pulse width of 5 ns (fwhm) were used for the simulation. For the temporal intensity profile of a Gaussian laser pulse (dashed black line with scale along the right axis), the peak position is set at 10 ns. The jump and drop in the C abs curve are caused by NP volume expansion following melting and shrinkage during the liquid solid phase transition of Au NPs. (b) Calculated temperatures of FEG (T e, red dashed line), lattice (T l, black solid line), and medium water (T m, blue dashed-dotted line) as a function of time delay. Because FEG and lattice temperatures are in quasi-equilibrium and the two curves of T e and T l overlap, the curve of T e was drawn by shifting 0.3 ns. Simulation conditions are the same as panel a. The horizontal line indicates the melting point of gold. Figure 4. Cooling time (a) and maximum particle temperature (b) depicted as a function of Au NP diameter calculated at three different fluences. Here the cooling time is defined by the time duration for which the particle temperature is above the mp of gold. The dots in panel a indicate the experimental final diameters (blue dot: 69 nm; black dot: 61 nm; red dot, 52 nm) and corresponding cooling times depending on the applied laser fluence as indicated in the key legend. In panel b, T max for experimental final particle diameters (T final ) is marked by arrows. θ are calculated (Supporting Information, Scheme S4). Within one step, the temperature is almost constant (±1 K), and hence θ is given as the sum of the overall steps. We assumed that surface evaporation is possible at temperatures above m.p. This assumption is based on a recent study by the Gordel group, who determined the onset of surface evaporation of Au NP to be at the m.p., in addition to the observation of surface melting at temperatures below m.p. under continuous heating in an oven. 11 In our model, the effect of surface melting was not taken into account for simplicity. This is acceptable because the time duration during which Au is in the liquid state is much longer than the period of surface melting. A quantitative analysis of θ was not possible because of insufficient data for α s at high pressures. The continuous size reduction by repeated irradiation with laser pulses is visualized by plotting θ as a function of particle diameter (Figure 5). The graph reveals that θ decreases continuously with decreasing particle diameter, implying that the evaporation continuously slowed down. Once T max reaches a certain point because of fast cooling, no appreciable evaporation can take place, that is, size reduction stops. The final diameter depends on the laser fluence applied. With increasing laser fluence, the final size is shifted toward smaller diameters. The computational result is consistent with experimental data and the most important finding we obtained. Remarkably, θ 0, at which the evaporation stops, was found to be nearly the same for the laser fluences specified in Figure 5 with an evaluated θ 0 being 150 ± 30 atoms (equivalent to 0.001% change in thickness for a 50 nm diameter Au NP) per pulse. Ideally, θ 0 should be close to zero. However, our method is still incomplete because we cannot predict the final particle diameter for a given laser fluence

6 Figure 5. Progress in surface evaporation represented by θ as a function of Au NP diameter calculated at three different laser fluences (90 mj cm 2, red dashed line; 78 mj cm 2, black solid line; and 60 mj cm 2, blue dashed-dotted line) along with particle size limits estimated from the value of θ at which the evaporation is nearly negligible. In the calculation, the evaporation coefficient was set to The calculations were performed for a pulse width of 5 ns (fwhm), an external pressure of 100 MPa, and an excitation wavelength of 532 nm. Previously, the photothermal size reduction of Au NPs under atmospheric pressure was considered to take place when the particle temperature was raised above the b.p. of gold However, because of the bubble forming around the particle, estimating the temperature during laser heating is difficult; the difficulty arises in establishing a sudden temperature change from bubble generation surrounding the particle. The advantage of the pressurized solution is that tracking of the continuous temperature change can be made. At 100 MPa, the T max values of approximately 5160 (60 mj cm 2 ), 5790 (78 mj cm 2 ), and 5900 K (90 mj cm 2 ) were estimated in the final stage, where the size reduction is expected to stop. These temperatures were considered below the b.p. of gold at 100 MPa (6800 K) due to the rise in b.p. A factor decisive for the final particle temperature is the cooling period after pulsed-laser irradiation. When the particle cooling period is reduced to a few tens of nanoseconds, the final T max should be raised close to the b.p. of gold for size reduction to take place. In contrast, in continuous heating, a long cooling period reduces the T max to near the m.p. of gold; this is based on experiments performed by Gordel and coworkers on the onset of surface evaporation of Au NP occurring at m.p. 11 At this point, we comment on the effect of ambient pressure. In the simplest case, the ambient pressure changes the b.p. of Au NP in accordance with the Clausius Clapeyron equation. 30 Moreover, the interface thermal conductance, which strongly dictates the heat transfer across the NP water interface, is determined by the heat capacity and thermal conductivity of water, both of which are pressure-dependent. 23 Under high pressures above the critical pressure of water, the supercritical water layer should be formed surrounding the Au NPs. 31 Higher pressures result in shorter cooling times because of greater densities and subsequent decrease in the surface evaporation rate. In addition, the evaporation coefficient should decrease with increasing pressure. Thus, we can state that higher laser intensities are needed to achieve size reduction at higher pressures. Moreover, high pressure enables tuning of the particle diameter to be more precise. Finally, we briefly mention the fate of evaporated species. The laser-induced evaporation can occur by emitting atoms and clusters into solution. The Au atomic and clustered ions were identified by mass spectrometric detection. 32,33 These atoms and clusters are believed to unite in solution, forming small NPs (2 to 3 nm) that were detected by TEM observations. 5 10,31,34 Small NPs potentially further aggregate, but when they are subjected to laser illumination, they can also undergo evaporation. Thus, one infers that the processes related to size increase and size reduction continue. Recall, however, that smaller particles have smaller C abs values, allowing slower temperature increase. Additionally, the particle cooling rate is size-dependent; the cooling rate is much faster for smaller particles. 4 These may allow small particles to grow very slowly. CONCLUSIONS We revealed the mechanism behind the surface evaporation of Au NPs leading to a reduced core diameter through rigorous calculations of the temperature by applying the TTM. Our simulation considered LSPR band bleaching caused by particle and medium local heating by applying Mie theory with two important modifications: temperature-dependent dielectric functions of Au and a multicore shell model of a medium refractive index gradient. Both affected the value of C abs significantly for an Au NP under laser illumination, although these effects were neglected in the past. Furthermore, the temperature-dependent heat capacity, density, and thermal expansion of Au NPs were considered in the temperature calculation. Despite these elaborations, we met difficulty in that the physical behavior of, for example, the dielectric functions of Au and water was only accurately known over limited temperature ranges and high temperature values had to be assumed. To determine the lower limit of evaporation, we used the kinetic gas theory coupled to the Kelvin equation. We assumed that the nature of surface evaporation is described by both T max and cooling time during laser illumination. For cooling times of a few tens of nanoseconds, T max is required to increase as high as the NP b.p. In contrast, long cooling times demand T max as low as the NP m.p. Consequently, although we could not predict the exact final particle diameter, we can explain qualitatively the laser fluence-dependent decrease in particle diameter at 100 MPa based on the calculation of the time-dependent temperature profile. The temperature-dependent changes of optical and thermophysical properties are of significant importance in describing Au NPs under optical excitation using focused CW lasers as well as intense pulsed lasers. Thus, photothermal effects on the NP physical properties should always be considered in cases such as surface-enhanced Raman spectroscopy (SERS) at single particle levels. The method proposed here to calculate temperature reveals clues to the nanoscale photothermal response of metal nanoparticles. METHODS SECTION The temperature evolution of a FEG and lattice systems is wellmodeled by the TTM with its two coupled partial differential equations (PDEs). 1,2,35 For nanosecond pulse durations, the heat flux into the surrounding medium leads to particle cooling during energy deposition. Hence, the model was extended to media in which radial heat conduction is assumed to occur; the equation for the medium coupling the electron and lattice 25753

7 systems is the boundary condition at the NP water interface. 18 Given that heat conduction in an Au NP is much faster than that in water, the electron and lattice equations can be simplified to ODEs. Accordingly, the TTM in the NP medium systems is given by C( T) dt e e dt C( T) dt l l dt l e = GT ( ) [ T T] + St ( ) e e l = GT ( ) [ T T] F e e l Tm 1 C T = t r r k T r T m( m) ( ) r (3) (4) 2 m m 2 m (5) Here C denotes heat capacity, k is thermal conductivity, S is energy input to the FEG by the laser pulse, and F is heat loss at the NP water interface. The indices e,l, and m represent the electron, lattice, and medium. The ODEs and PDE are solved in temporal, t, and radial, r, coordinates. Details of the TTM are described in the Supporting Information, S1. In the nanosecond time regime, heat transfer to the surrounding medium creates a local temperature gradient in water adjacent to the Au NP. Under high pressures, the refractive index of water coupled to the temperature gradient changes. Hence, a multicore shell extension of the Mie theory was applied to calculate the absorption cross-section, C abs In the model, an Au NP with homogeneous temperature is surrounded by a number of water layers, each of which is characterized by a shell thickness and constant refractive index (Supporting Information, S5). Furthermore, the dielectric function of gold is described by the sum of the intraband contribution (Drude term) and the 5d to 6sp interband transition. 35,39 The collective oscillation of the FEG through the interaction with the electromagnetic field of incident light gives an appreciable peak in the extinction spectrum. The extinction intensity of the LSPR band undergoes bleaching when the FEG temperature is increased. 40,41 In particular, C abs of Au NPs < 100 nm diameter at the excitation wavelength near the LSPR band ( 520 nm) is strongly affected by bleaching. In this work, we obtained fittings using Otter s experimental temperature-dependent complex refractive indices to obtain temperature-dependent dielectric functions used in simulating the temperature-induced LSPR band bleaching (Supporting Information, S6). 42,43 ASSOCIATED CONTENT *S Supporting Information Two-temperature model, spatiotemporal evolution of water temperature, diameter-dependent behavior of T max, particle evaporation under pulsed-laser-irradiation, refractive index gradient, and dielectric function of gold. This material is available free of charge via the Internet at AUTHOR INFORMATION Corresponding Author * hashichem@tokushima-u.ac.jp. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS Financial support from KAKENHI (nos and ) is gratefully acknowledged. REFERENCES (1) Link, S.; El-Sayed, M. A. Shape and Size Dependence of Radiative, Non-radiative and Photothermal Properties of Gold Nanocrystals. Int. Rev. Phys. Chem. 2000, 19, (2) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chemistry and Properties of Nanocrystals of Different Shapes. Chem. Rev. 2005, 105, (3) Matsuo, N.; Muto, H.; Miyajima, K.; Mafune, F. Single Laser Pulse Induced Aggregation of Gold Nanoparticles. Phys. Chem. Chem. Phys. 2007, 9, (4) Hashimoto, S.; Werner, D.; Uwada, T. Studies on the Interaction of Pulsed Lasers with Plasmonic Gold Nanoparticles toward Light Manipulation, Heat Management, and Nanofabrication. J. Photochem. Photobiol., C 2012, 13, (5) Takami, A.; Kurita, H.; Koda, S. Laser-induced Size Reduction of Noble Metal Particles. J. Phys. Chem. B 1999, 103, (6) Inasawa, S.; Sugiyama, M.; Yamaguchi, Y. Laser-Induced Shape Transformation of Gold Nanoparticles Below the Melting Point: The Effect of Surface Melting. J. Phys. Chem. B 2005, 109, (7) Inasawa, S.; Sugiyama, M.; Yamaguchi, Y. Bimodal Size Distribution of Gold Nanoparticles under Picosecond Laser Pulses. J. Phys. Chem. B 2005, 109, (8) Yamada, K.; Miyajima, K.; Mafune, F. Thermionic Emission of Electrons from Gold. Nanoparticles by Nanosecond Pulse-Laser Excitation of Interband. J. Phys. Chem. C 2007, 111, (9) Werner, D.; Hashimoto, S.; Uwada, T. Remarkable Photothermal Effect of Interband Excitation on Nanosecond Laser-induced Reshaping and Size Reduction of Pseudospherical Gold Nanoparticles in Aqueous Solution. Langmuir 2010, 26, (10) Werner, D.; Hashimoto, S. Controlling the Pulsed-Laser- Induced Size Reduction of Au and Ag Nanoparticles via Changes in the External Pressure, Laser Intensity, and Excitation Wavelength. Langmuir 2013, 29, (11) Gordel, M.; Olesiak-Banska, J.; Matczyszyn, K.; Nogues, C.; Buckle, M.; Samoc, M. Post-synthesis Reshaping of Gold Nanorods using a Femtosecond Laser. Phys. Chem. Chem. Phys. 2014, 16, (12) Meng, G.; Yanagida, T.; Kanai, M.; Suzuki, M.; Nagashima, K.; Xu, Bo; Zhuge, F.; Klamchuen, A.; He, Y.; Rahong, S.; Kai, S.; Kawai, T. Pressure-induced Evaporation Dynamics of Gold Nanoparticles on Oxide Substrate. Phys. Rev. E 2013, 87, (13) Plech, A.; Kotaidis, V.; Gresillon, S.; Dahmen, C.; von Plessen, G. Laser-induced Heating and Melting of Gold Nanoparticles Studied by Time-resolved X-ray Scattering. Phys. Rev. B 2004, 70, (14) Siems, A.; Weber, S. A.L.; Boneburg, J.; Plech, A. Thermodynamics of Nanosecond Nanobubble Formation at Laser- Induced Metal Nanoparticles. New J. Phys. 2011, 13, (15) Lapotko, D. O. Optical Excitation and Detection of Vapor Bubbles around Plasmonic Nanoparticles. Opt. Express. 2009, 17, (16) Cavicchi, R. E.; Meier, D. C.; Presser, C.; Prabhu, V. M.; Guha, S. Single Laser Pulse Effects on Suspended-Au-Nanoparticle Size Distributions and Morphology. J. Phys. Chem. C 2013, 117, (17) Volkov, A. N.; Svilla, C.; Zhigilei, L. V. Numerical Modeling of Short Pulse Laser Interaction with Au Nanoparticle Surrounded by Water. Appl. Surf. Sci. 2007, 253, (18) Werner, D.; Hashimoto, S. Improved Working Model for Interpreting the Excitation Wavelength- and Fluence-dependent Response in Pulsed Laser-induced Size Reduction of Aqueous Gold Nanoparticles. J. Phys. Chem. C 2011, 115, (19) Bohren, C. F.; Huffman, D. R. Absorption and Scattering of Light by Small Particles; Wiley: New York, (20) Fang, Z.; Zhen, Y.-R.; Neumann, O.; Polman, A.; Garcia de Abajo, F. J.; Nordlander, P.; Halas, N. J. Evolution of Light-Induced Vapor Generation at a Liquid-Immersed Metallic Nanoparticle. Nano Lett. 2013, 13, (21) Carlson, M. T.; Khan, A.; Richardson, H. H. Local Temperature Determination of Optically Excited Nanoparticles and Nanodots. Nano Lett. 2011, 11,

8 (22) Fedoruk, M.; Meixner, M.; Carretero-Palacios, S.; Lohmuller, T.; Feldmann, J. Nanolithography by Plasmonic Heating and Optical Manipulation of Gold Nanoparticles. ACS Nano 2013, 7, (23) Ge, Z.; Cahill, D. G.; Braun, P. V. AuPd Metal Nanoparticles as Probes of Nanoscale Thermal Transport in Aqueous Solution. J. Phys. Chem. B 2004, 108, (24) Ashcroft, N. W.; Mermin, N. D. Solid State Physics; Harcourt Brace: Orlando, FL, (25) Baffou, G.; Quidant, R. Thermo-plasmonics: using Metallic Nanostructures as Nano-sources of Heat. Laser Photonics Rev. 2013, 7, (26) Sambles, J. R. An Electron Microscopy Study of Evaporating Gold Nanoparticles: the Kelvin Equation for Liquid Gold and the Lowering of the Melting Point of Solid Gold Particles. Proc. R. Soc. London, Ser. A 1971, 324, (27) Asoro, M. A.; Kovar, D.; Ferreira, P. J. In situ Transmission Electron Microscopy Observation of Sublimation in Silver Nanoparticles. ACS Nano 2013, 7, (28) Alock, C. B.; Itkin, V. P.; Horrigan, M. K. Vapor Pressure of the Metallic Elements. Can. Metall. Q. 1984, 23, (29) Egry, I.; Lohoefer, G.; Jacobs, G. Surface Tension of Liquid Metals: Results from Measurements on Ground and in Space. Phys. Rev. Lett. 1995, 75, (30) Atkins, P. W. Physical Chemistry, 9th ed.; Oxford University Press: Oxford, U.K., (31) Werner, D.; Ueki, T.; Hashimoto, S. Methodological Improvement in Pulsed Laser-Induced Size Reduction of Aqueous Colloidal Gold Nanoparticles by Applying High Pressure. J. Phys. Chem. C 2012, 116, (32) Shoji, M.; Miyajima, K.; Mafune, F. Ionization of Gold Nanoparticles in Solution by Pulse Laser Excitation as Studied by Mass Spectrometric Detection of Gold Cluster Ions. J. Phys. Chem. C 2008, 112, (33) Liu, Y.-C.; Li, Y.-J.; Huang, C.-C. Information Derived from Cluster Ions from DNA-Modified Gold Nanoparticles under Laser Desorption/Ionization: Analysis of Coverage, Structure, and Single- Nucleotide Polymorphism. Anal. Chem. 2013, 85, (34) Mafune, F.; Kohno, J.; Takeda, Y.; Kondow, T.; Sawabe, H. Formation and Size Control of Silver Nanoparticles by Laser Ablation in Aqueous Solution. J. Phys. Chem. B 2000, 104, (35) Hartland, G. V. Optical Studies of Dynamics in Noble Metal Nanostructures. Chem. Rev. 2011, 111, (36) Yang, W. Improved recursive algorithm for light scattering by a multilayered sphere. Appl. Opt. 2003, 42, (37) Kerker, M. The Scattering of Light and other Electromagnetic Radiation; Academic Press: New York, (38) Scha fer, J.-P. Implementierung und Anwendung Analytischer und Numerischer Verfahren zur Lo sung der Maxwellgleichungen fu r die Untersuchung der Lichtausbreitung in Biologischem Gewebe, Ph.D. Thesis, Ulm University, (39) Grua, P.; Bercegol, H. Laser-Induced Damage in Optical Materials. Proc. SPIE 2000, 4347, (40) Link, S.; El-Sayed, M. A. Size and Temperature Dependence of the Plasmon Absorption of Colloidal Gold Nanoparticles. J. Phys. Chem. B 1999, 103, (41) Hartland, G. V. Coherent Excitation of Vibrational Modes in Metallic Nanoparticles. Annu. Rev. Phys. Chem. 2006, 57, (42) Setoura, K.; Werner, D.; Hashimoto, S. Optical Scattering Spectral Thermometry and Refractometry of a Single Gold Nanoparticle under CW Laser Excitation. J. Phys. Chem. C 2012, 116, (43) Otter, M. Temperaturabha ngigkeit der Optischen Konstanten Massiver Metalle. Z. Phys. 1961, 161,