11246
J. Phys. Chem. C 2007, 111, 11246-11251
Thermionic Emission of Electrons from Gold Nanoparticles by Nanosecond Pulse-Laser Excitation of Interband Kunihiro Yamada, Ken Miyajima, and Fumitaka Mafune´ * Department of Basic Science, Graduate School of Arts and Sciences, The UniVersity of Tokyo, Komaba, Meguro-ku, Tokyo 153-8902, Japan ReceiVed: April 21, 2007; In Final Form: May 18, 2007
Gold nanoparticles exhibit optical absorption bands assignable to a broad interband in the UV region and a narrow intraband (surface plasmon band) in the 520 nm wavelength region. The gold nanoparticles were multiply ionized in an aqueous solution of sodium dodecyl sulfate when excited by a tightly focused nanosecond pulsed laser at 532 nm resonant to the intraband or at 355 nm resonant to the interband. Although the absorbance at 355 nm is almost comparable with the absorbance at 532 nm, gold nanoparticles with higher charge states were formed when the interband at 355 nm was excited. Our experimental findings and the theoretical prediction by Grua et al. (Phys. ReV. B 2003, 68, 035424) lead to the conclusion that electrons are predominantly released from the gold nanoparticles through thermionic emission by the excitation of the interband, whereas excitation of the surface plasmon band hardly contributes to the ionization processes.
1. Introduction The optical response of gold nanoparticles is totally different from that of nonmetallic molecules.1-3 Because they possess metallic properties more or less depending on the size,4 the excited gold nanoparticles are readily relaxed to the ground state due to the characteristic electron-electron and electron-phonon interaction.5 This particular optical behavior puts the particles themselves under the extreme physical and chemical conditions, when they are irradiated with the intense laser pulse in solution.6-10 For instance, gold nanoparticles are temporally heated to several thousand kelvin.6 Right after this, the nanoparticles are pressurized by the solvent molecules in close vicinity, because the solvent molecules are also heated through the heat transfer from the nanoparticles.7 In addition, the gold nanoparticles release some conduction electrons to be multiply ionized.8-10 These phenomena have been observed so far in the several experiments. Ultrafast spectroscopy shows that gold nanoparticles are heated by laser excitation: The excited gold nanoparticles are relaxed to the electronic ground state within subpicoseconds. Right after the relaxation, hot electrons are formed around the Fermi level at first, which are cooled by the electron-phonon interaction, increasing the lattice temperature of the nanoparticles.1-3 Koda and his co-workers observed the blackbody radiation from the gold nanoparticles, and they concluded that the gold nanoparticles are heated to their melting and boiling points due to the multiphoton absorption of the laser photons.6 More recently, Plech et al. observed the structural changes of both gold nanoparticles and water molecules in the vicinity of the particles by time-resolved X-ray scattering, concluding that particles undergo a melting transition. They also found that the heated water molecules in close vicinity are pressurized.7 On the other hand, Kamat and his co-workers observed solvated electrons right after the gold nanoparticles are excited by the laser pulse.8,9 We measured the number density of * Corresponding author. E-mail:
[email protected].
solvated electrons in water by the transient spectroscopy, and we found that gold nanoparticles are multiply ionized.10 Although the heating mechanism has been elucidated rather well, the ionization mechanism is not well established. In the present study, we studied the ionization mechanism by comparing the ionization efficiency at the selected two excitation wavelengths. Obviously, there are two absorption bands in gold nanoparticles which show a totally different nature.11-14 Intraband absorption originates from the optical response of conduction electrons in the particles and, hence, is sensitively affected by the electron distribution around the Fermi level. In contrast, the interband originates from the d band, which relates closely to the low-lying atomic orbital. We measured the ionization efficiency of gold nanoparticles when either the interband or the intraband of them is excited and elucidated the ionization mechanism of gold nanoparticles by the nanosecond laser excitation. 2. Experimental Section Details of the experimental setup have already been described elsewhere,10,15-17 so that only the brief explanation relevant to this study is given here. Gold nanoparticles were prepared by laser ablation of a gold metal plate. The fundamental of a Nd: YAG laser with a pulse energy of 100 mJ was focused onto the metal plate immersed in 10 mL of a 3 × 10-4 M sodium dodecyl sulfate (SDS) aqueous solution, by a lens having a focal length of 250 mm. The size and the shape of the gold nanoparticles were observed by the transmission electron microscopy (TEM). The average size of them was 10.4 ( 4.0 nm. The concentration of the gold atoms dispersed in the solution as gold nanoparticles was 2.1 mM after 36 000 laser shots. Hereafter, when the concentration of gold nanoparticles is given in this paper, it refers to the concentration of gold atoms dispersed as gold nanoparticles. As described laser, gold nanoparticles in the solution are sizereduced or aggregated by the irradiation of the focused laser pulse onto them. Taking advantage of these changes, we
10.1021/jp0730747 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/10/2007
Electron Thermionic Emission of Gold Nanoparticles prepared small nanoparticles with the desired diameter by the laser irradiation:17 We adjusted the pulse energy of the laser and the concentration of SDS in the aqueous solution so as to determine the particle size. The focal volume of the laser beam for the size reduction was so small that more than 12 000 laser shots are required to induce the structural changes of all the nanoparticles in the solution. We confirmed the size of the particles by TEM, after all the nanoparticles in the SDS solution have been size-reduced. The standard deviation of the diameter of the nanoparticles was typically (2 nm. In the nanosecond transient absorption measurements, 5 × 10-5 to 3 × 10-4 M solutions of gold nanoparticles in a 2 × 10-2 M SDS solution, were used. The solution in an optical SiO2 cell was irradiated with either the third harmonic (355 nm) and the second harmonic (532 nm) of a Nd:YAG pulse laser (pulse energy, 50-70 mJ‚pulse-1; pulse width, 10 ns; repetition rate, 10 Hz). The laser pulse was focused by the lens to the solution through the opening of the optical cell. The fluence of the pump laser pulse was set to be 132 MW‚cm-2 at the surface of the aqueous solution. The transient absorption spectrum of the solution was obtained 100 ns after the laser irradiation by using a Xe lamp as a probe light, which was introduced into the optical cell perpendicular to the laser pulse. The probe light whose beam waist was 1 mm in diameter was aligned to overlap with the focused Nd:YAG pulse laser. The broad-band probe light was dispersed by a spectrometer (Hamamatsu C5094), and the light intensities at each wavelength were measured by a MOS linear image sensor equipped with an image intensifier having a 15 ps). These separate experiments indicate that the absorption in the 500550 nm region is depleted temporally when the gold nanoparticles are excited by the 532 nm laser, whereas the absorption in the UV region is enhanced when the interband is excited. The optical bleaching implies that the gold nanoparticles are not ready to absorb a photon within the period after the photoexcitation. Obviously, this effect decelerates the photoexcitation-relaxation cycles. We simulated the photoexcitationrelaxation cycles, taking into consideration that the optical absorption cross-sections at 355 and 532 nm change with time: The absorption cross-section at 355 nm was set to decrease exponentially from 1.8 × 10-17 to 1.5 × 10-17 m2 with the lifetime of 2 ps,3 whereas the absorption cross-section at 532 nm was set to increase exponentially from 0.5 × 10-17 to 1.7 × 10-17 m2 with the lifetime of 2 ps because of the bleach of the intraband.1,2 In addition, we assumed that the lifetime of the electronic excited state is as long as 100 fs and the photon density of the laser pulse is uniform during the 10 ns laser pulse. The results are illustrated in Figure 5, which shows the time of photoexcitation as a bar. The bars are dense by the excitation at 355 nm, indicating that the nanoparticles are able to absorb a photon as soon as the nanoparticles are relaxed to the electronic ground state. We calculated the periods of each photoexcitation-relaxation cycle of gold nanoparticles excited by the 532 or 355 nm laser pulse, which correspond to the time intervals between the adjacent bars in Figure 5. It is shown in Figure 6a,b that the periods of the cycle are distributed in 0-0.5 ps for the 355 nm excitation and in 0-1 ps for the 532 nm excitation. The difference indicates again that the photoexcitation-relaxation cycle induced by the 532 nm laser excitation turns more slowly.
11250 J. Phys. Chem. C, Vol. 111, No. 30, 2007
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Figure 7. TEM images of gold nanoparticles produced after the pulse laser irradiation at (a) 355 and (b) 532 nm in 3 × 10-2 M SDS aqueous solution. Optical absorption spectra of gold nanoparticles formed by the pulse laser excitation at (c) 355 and (d) 532 nm in 3 × 10-2 M SDS aqueous solution. Figure 6. Distribution of the period of each photoexcitation-relaxation cycle when gold nanoparticles are excited by the 532 and 355 nm laser pulse in panels a and b, respectively. The distributions are calculated for different lifetimes of the bleach recovery in panel a and for different lifetimes of the enhanced transient absorption in panel b. At the 355 nm laser excitation, the distribution does not change with the lifetimes of the transient absorption. The distributions are obtained by calculating 500 000 photoexcitation-relaxation cycles.
We also calculated the period of each cycle for different lifetimes of the bleach recovery at the 532 nm laser excitation (see Figure 6a). The distribution does not change so much with the lifetime of the bleach recovery, although it shifts very slightly to the longer time as the lifetime of the bleach recovery increases from 1 to 2 ps. Especially, when the lifetime is longer than 10 ps, the distribution becomes unchanged, and it is essentially the same as the one obtained, assuming that there is no intraband at 532 nm. These findings lead to the important conclusion that the photoexcitation occurs only through the interband, and the intraband cannot contribute to the photoexcitation, as long as the lifetime of the bleach recovery is longer than several picoseconds:1,2 The recovery of the bleach is slow compared to the fast photoexcitation-relaxation cycle. Size-Reduction of Gold Nanoparticles by Coulomb Explosion. Parts a and b of Figure 7 show TEM images of gold nanoparticles, after they were irradiated with the focused laser pulse at 355 and 532 nm, respectively. Obviously, the diameters of the gold nanoparticles were reduced by the pulse laser irradiation. Average diameters of the gold nanoparticles are measured from the TEM images to be 1.7 ( 0.3 and 3.0 ( 0.8 nm after 355 and 532 nm laser irradiation, respectively. Parts c and d of Figure 7 show optical absorption spectra of the gold nanoparticles, whose TEM images are shown in Figure 7a,b, respectively. In both cases, the heights of the surface plasmon band are significantly lowered after the laser irradiation, when compared with that of the gold nanoparticles as prepared (see Figure 1b). Especially, the surface plasmon peak disappears completely after the 355 nm excitation, leaving with only a broad interband in the visible-UV region (see Figure 7c). The
spectral changes also indicate that the gold nanoparticles are fragmented into smaller ones by the laser excitation. In a previous paper, we concluded that the size reduction is caused by the Coulomb explosion of gold nanoparticles: The gold nanoparticles are multiply ionized by the pulse laser irradiation, which are then fragmented into the small ones, because highly charged gold nanoparticles are electronically unstable.10,25,26 According to the liquid drop model, a highly charged particle becomes unstable when the disruptive Coulomb force exceeds the attractive cohesive force.27-29 The criterion for the Coulomb explosion is expressed quantitatively by fissility defined as X ) Ec/2Es, where Ec and Es are the Coulomb energy and the surface energy of the particle, respectively. Multiply charged nanoparticles are expected to readily dissociate into the small ones when X g 1, both the evaporation and Coulomb explosion competitively occur in the range 0.3 < X < 1, and only evaporation can take place when X < 0.3.17-19 The fissility parameter for a gold nanoparticle is calculated to be
q2 n
X ) 0.9
(2)
where q and n are the charge state and the number of atoms included in the particle. In practice, in the time of a 10 ns laser pulse, multiply charged nanoparticles must dissociate into smaller ones, and the product small particles further absorb photons to reach highly charged states. As we measured the number density of solvated electrons right after the 10 ns laser irradiation, the “charge state” described here refers to the nominal total charge that the nanoparticles would finally have after the 10 ns laser irradiation. Now, we examine the wavelength dependence of the size reduction. As calculated in Results, gold nanoparticles are multiply ionized into Au35000+2550 at 355 nm and Au35000+520 at 532 nm, whose fissilities are calculated to be X ) 167 and X ) 7, respectively. In both cases, the Coulomb force overcomes the attractive cohesive force, causing the nanoparticle to break up into small particles by the Coulomb explosion.
Electron Thermionic Emission of Gold Nanoparticles Upon fragmentation by the Coulomb explosion, it is highly likely that both the atoms and the charges in one nanoparticle are divided into the small particles such that the charge state to size ratio, q/n, of the particles does not change. It should be noted that the fissility decreases as the size decreases, because fissility is given by eq 2. A highly charged gold nanoparticle is expected to be divided, until the fissility of the product small nanoparticles becomes X < 1. According to this model, Au35000+2550 and Au35000+520 should be divided into the smaller particles than Au≈208+15 and Au≈5000+74, respectively. As shown in Figure 7, the average diameters of gold nanoparticles formed by the 355 and 532 nm laser irradiation are 1.7 and 3.0 nm, respectively. A 1.7 nm spherical gold nanoparticle consists of about 150 gold atoms, and Au150 should have the charge state of +11 (Au150+11). The fissility of Au150+11 is calculated to be 0.73, indicating that Au150+11 is stable enough against the disruptive Coulomb force. Similarly, a 3.0 nm spherical gold nanoparticle is Au830+12, whose fissility is 0.16. Thus, the smaller nanoparticles were formed, where the gold nanoparticles with the higher charge states are formed by the laser irradiation (see Figure 7). The liquid drop model not only predicts the instability of the highly charged gold nanoparticles generated by the pulse laser irradiation but also explains the stability of the formed small particles.27-29 These findings lead us to conclude again that the size reduction is caused by the Coulomb explosion of gold nanoparticles. 5. Conclusion We studied the dynamics of gold nanoparticles by the nanosecond transient absorption spectroscopy after they are excited by the focused nanosecond laser pulse. It was found that multiply ionized gold nanoparticles are formed. The ionization efficiency is highest when the interband of the gold nanoparticles is resonantly excited, whereas intraband excitation hardly contributes to the ionization. These experimental findings along with the theoretical calculation by Grua and co-workers lead us to conclude that gold nanoparticles are multiply ionized by the thermionic emission when the interband of them is excited repeatedly through the photoexcitation-relaxation cycles. The intraband does not contribute to the characteristic cycles, because it is subjected to optical bleaching due to the hot electrons produced around the Fermi level. Acknowledgment. This work is financially supported by a Grant-in-Aid for Scientific Research from the Ministry of
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