Thermal dynamics in laser excited metal nanoparticles

Chemical Physics Letters 401 (2005) 565–569 www.elsevier.com/locate/cplett

Thermal dynamics in laser excited metal nanoparticles A. Plech a

a,*

, V. Kotaidis a, M. Lorenc b, M. Wulff

b

Fachbereich Physik der Universita¨t Konstanz, Universita¨tsstr. 10, D-78457 Konstanz, Germany b ESRF, BP 220, F-38043 Grenoble, France Received 20 October 2004; in final form 9 November 2004 Available online 15 December 2004

Abstract The transient structural response of laser excited gold nanoparticle sols has been recorded by pulsed X-ray scattering. Time resolved wide angle and small angle scattering (SAXS) record the changes in structure both of the nanoparticles and the water environment subsequent to femtosecond laser excitation. Within the first nanosecond after the excitation of the nanoparticles the water phase shows a signature of compression, induced by a heat-induced evaporation of the water shell close to the heated nanoparticles. The particles themselves undergo a melting transition and are fragmented to form new clusters in the nanometer range.
2004 Elsevier B.V. All rights reserved.

1. Introduction Metal and semiconductor nanoparticle systems attract presently strong attention due to their nonlinear properties [1–4]. A large number of studies have focussed on the transient optical response following to femtosecond laser excitation. Noble metal nanoparticles have been used to study electron and phonon interactions [2,5–7]. The common feature in these studies has been that mostly reversible optical and structural changes have been studied, while irreversible reactions of structural origin are rarely addressed. Link et al. have used elongated gold nanoparticle sols excited by a high power laser to obtain information about the irreversible shape transformation from the rod shape to spheres, most likely through a melting transition. While the interpretation of most studies does not include the interaction with the medium around the nanoparticle, it is essential to understand mechanical interaction and heat transfer from the particles to the bulk [8–12]. *

Corresponding author. E-mail address: [email protected] (A. Plech).

0009-2614/$ - see front matter
2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.11.072

The melting and ablation mechanism of laser irradiated semiconductor surfaces has been examined by optical/X-ray pump and probe techniques [13,14]. One characteristic feature is the change in melting regime with excitation density. At low power lattice melting occurs via the thermalized phonon motion, whereas at high power an ultrafast lattice disorder is found and called non-thermal melting. This event is accompanied by shock waves emitted into the bulk and even ablation of material from the surface. In this Letter, we present evidence that the ultrafast excitation of 35 nm gold nanoparticles suspended in water drives a phase transition of the close water shell, which compresses the bulk liquid. The expanded zone around the particles collapses again within nanoseconds. At the same time the particles are fragmented and the fragments cluster on the microsecond time scale to form ˚ . The study has been new particles with sizes around 9 A performed with methods of ultrafast X-ray scattering [15,16]. We use the signals from the water solvent (wide-angle diffuse scattering), the particle lattice (powder scattering) and the shape scattering from nanoparticles (SAXS) to determine the structure of the particles as well as the local environment.

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2. Materials and methods S(Q) [e.u. / H2O]

1.0 flux [arb. u.]

(a)

40

0.5

20

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5

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25

Energy [keV] 0 2.0

∆S(Q) [e.u. / H2O]

Nanoparticle sols have been synthesized by the well known citrate technique [12,17]. By using a relatively high concentration of citrate as reductant (0.03 wt%) of a 0.8 mM gold hydrochlorate solution, final sizes of 35 nm diameter for the colloids were obtained. The solution displays an absorbance of 0.07 at 400 nm (excitation wavelength) and a 300 lm cross-section. The time resolution for the nanoparticle and solvent structural changes was achieved at beamline ID09B at ESRF by using a femtosecond laser synchronized to the X-ray pulse structure emitted from the synchrotron [15,18]. Briefly, a Ti:Sa femtosecond oscillator (diode pumped 150 fs, Coherent) is phase locked to the bunch clock of the synchrotron accelerating cavities to pulse synchronously to the electron bunch circulation in the ring at 88 MHz. The weak 800 nm pulses emitted by the oscillator are amplified in a regenerative Ti:Sa amplifier (Spectra Physics) to 1 mJ at a repetition rate of 986.3 Hz. The delay between the laser pump and the X-ray probe pulse is controlled electronically and can be tuned in steps of 5 ps with a jitter below 10 ps. The laser light is frequency doubled in a BBO crystal and focussed onto a thin walled borosilicate capillary (Hilgenberg) of 300 lm diameter. The liquid containing the nanoparticles is pumped through the capillary to ensure a fresh portion of the sample at each shot. We used a power of 180 mW with a spot diameter of 0.33 mm at the sample position. The scattering is recorded by a CCD camera (Mar Research) with 133 mm diameter and 64 lm resolution. The range in momentum transfer is limited between 0.13 ˚
1 (detector edge) at the chosen (beam stop size) and 8 A distances. The technique of recording weak intensity differences on a large background induced by laser excitation has been described elsewhere [16,18,19]. The essential point is repetitive exposures with the laser excitation alternating before and after the X-ray probe. The scattering intensity and hence the signal to noise ratio has been maximized by using the full emission spectrum of the in-vacuum undulator. The X-ray spectrum was optimized for smallest bandwidth (3%) at a fundamental of 18.2 keV at a slightly opened magnetic gap (see inset in Fig. 1). By accumulating scattering from about 105 X-ray pulses a relative accuracy of 5 · 10
5 relative signal change can be achieved. The curves in Fig. 1 are obtained by taking the difference in scattering collected before (reference) and after the laser impact. The data is corrected for geometrical distortions and inelastic Compton scattering and can be normalized to absolute scattering units (Thomson scattering of one electron) by scaling the data to the gas scattering of water molecules in the limit of large momentum transfer. Scattering from the different levels of structural organization within the suspension is described within the Born approximation, which leads to the well known

60

2 µs

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5 ns

1.0

1ns 0.5

250 ps 0.0

50ps

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1

2

3

4

5

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-1

Q [Å ] Fig. 1. (a) Scattering S(Q) function for water together with the undulator spectrum (inset). (b) Differences in scattering due to the laser excitation of the nanoparticle sol at different time delays between laser and X-ray pulses. The curves are offset by 0.5 e.u. The positions of the fcc reflections are indicated by the vertical lines.

SAXS formula, liquid (or diffuse scattering) and the occurrence of Bragg refraction from the long range order within the crystal structure (Debye–Scherrer rings) [20]. An important consequence of the SAXS distribution is the approximation Q Æ Rg < 1 (Q = 4p/k Æ sin(2h/2) being the scattering vector and 2h marks the scattering angle), which leads to the Guinier formula !
Q2  R2g 2 IðQÞ ¼ N  ne  exp ð1Þ 3 with the radius of gyration RG (being smaller than the actual radius for a sphere by Rg = R/1.29). The intensity I(Q) scales with the number of scatterers N and the electrons n2e within one scatterer. The liquid scattering is generally described by the form factors of the individual molecules or atoms within the ensemble and the modification of single molecule scattering from the interference between adjacent molecules. This so-called structure factor can be described by Z 1 sin Qr ð2Þ SðQÞ ¼ 1 þ 4pq1 dr r2 ðgðrÞ
1Þ  Qr 0 with the average electron density q1 and the site–site pair correlation function g(r) as function of r (real space distance between scattering centers). As the normalization of the data is done by scaling to the large Q limit in order to isolate weak differences in signal due to photo-excitation, the result is corrected for density changes. The information content is then ascribed to

A. Plech et al. / Chemical Physics Letters 401 (2005) 565–569

the change in molecular pair distribution only. When the average distances of proximate molecules change (i.e., due to expansion or compression of the liquid), correlation peaks will shift and the difference scattering will display positive and negative contributions around the initial scattering peaks [21,22].

3. Results and discussions The scattering yield from the sample is dominated by the liquid scattering of the water solvent (and capillary scattering). The corrected elastic scattering [20] is presented in Fig. 1a showing the characteristic double peak from the molecular nearest neighbor correlation in the liquid (basically the O–O correlation for X-rays [23]). In this presentation the SAXS from the particles as well as the powder scattering from their fcc lattice are almost invisible. Taking the difference DS(Q) from the laser excited sample at a fixed time delay and the reference results in the curves shown in Fig. 1b. There are broad features located around the liquid scattering maxima which show both negative and positive contributions depending on Q. At distinct Q values sharp negative peaks appear, which indicates the reduction of scattering of the powder peaks from the gold fcc lattice as indexed. At the laser power used in the present study the particles completely melt and even fragment [12]. This effect consequently reduces the Bragg scattering throughout the complete time series. The width of these peaks is larger than those from the Scherrer equation as the X-ray spectrum is broader and asymmetrically k shaped [19] (compare inset in Fig. 1a). There is another regime of ˚
1) with strong scattering at low Q values (below 0.3 A laser induced changes. This SAXS region reflects the change in size or shape of larger objects, i.e., the initial nanoparticles or newly formed aggregates. The general trend in DS(Q) as function of time delay can be described as follows. The broad features in DS(Q) related to the liquid scattering are already present at short delays and develop in amplitude up to 1 ns. The shape is essentially constant (even though minor changes can occur). After 1–2 ns this signal vanished quickly. The powder scattering contribution is always negative, proving that the long range order in the particles is lost and never fully restored. This is in agreement with the particles undergoing a melting transition and being destroyed at these high laser powers. The applied power density in the present study is 12 times higher than the limit for purely reversible excitation of the particles in the solid state [12]. The melting transition is already reached at 1/5 of the present power level. We therefore conclude that the lattice temperature of the particle immediately (<50 ps) after laser excitation can attain several thousands of degrees, compared to the

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bulk melting point of gold of 1337 K. The SAXS scattering generally increases with delay, which will be further discussed below. How can one explain the scattering differences from the liquid structure of water on the picosecond time scale? Clearly the high particle temperature heats the surrounding water shell. Heat transfer models [9,24,12] that are verified in the low excitation regime show that heat is transferred within 100–300 ps into the nearby water shell, which can heat up in a region of 10–20 nm. This will lead to a strong pressure increase in this volume element as the liquid can react by expansion only on the time scale of the speed of sound in the material. Calculating the pressure increase within the picosecond heated volume one gets DP(T) = aP/vT Æ DT with the isobaric thermal expansion coefficient aP = 1/V Æ (dV/ dT)|P of water and the isothermal compression coefficient vT =
1/V(dV/dP)|T and the temperature change of 1000 K (estimated from the calculations at lower excitation). The resulting pressure can reach values as high as 5 · 104 atm. However under these extreme conditions, heat transfer and the material coefficients may change drastically and that requires a more detailed modeling. For typical liquids the contribution dS(Q)/dP|V is known to be small as compared to contributions which include a change in volume. The reason is that to first order there is no change in mean molecular distances with increased pressure at fixed volume. The overpressure confined in the nanoscale shell around the nanoparticles will cause the liquid to expand with time. This expansion and pressure release could either proceed in a monotonic manner as typical for thermal grating experiments [25] or induce a nonuniform density and pressure profile radially from the particle nucleus. We compared the laser excitation data with the static pressure derivative dS(P) = dS(Q)/dP|T and temperature derivative dS(T) = dS(Q)/dT|P, which are simply measured by changing the hydrostatic pressure of a sample of pure water by fractions of an atmosphere, respectively, the temperature at fixed (ambient) pressure and collecting the difference scattering to high precision. These two static derivatives include a change in volume of the liquid, either through hydrostatic compression or through thermal expansion. The thermal derivative dS(T) can be normalized to electron units and temperature change, as it is linear within a wide range of temperatures far away from the anomalous temperature at 4 C and reflects the corresponding volume change. The dominance of the volume change on the derivatives is demonstrated in Fig. 2 by displaying dS(P) and
dS(T) scaled to the same level. The curves are basically identical. Generally the scattering is the average in DS(Q, r) over the complete illuminated volume. It is known that for the present conditions it takes some 80 ns for the entire laser illuminated cylinder to expand so that on the

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dS/dP|T

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Amplitude [a. u.]

dS/dT [e.u./K]

dS/dT|P 0.03

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∆ S(Q) [e.u. per H2O]

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WAXS signal SAXS signal parabolic volume change

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Fig. 3. Evolution of the amplitude of the difference scattering in the wide angle scattering (j) and in the SAXS region (s) as a function of delay between laser and X-ray pulses. The solid line is a simulation with a parabolic change in volume.

-0.2 -0.4 -0.6 1

2

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-1

Q [Å ] Fig. 2. (a) Comparison of the static derivatives dS(P) and
dS(T) as measured for pure water. dS(T) is scaled to 1 K temperature change and the amplitude of dS(P) is scaled to match dS(T). (b) The difference scattering at 850 ps as in Fig. 1 together with the experimental scattering difference dS(P) above. The photo-excitation curve has been normalized to absolute units and the static pressure differential has been scaled.

picosecond to 1 ns time scale the average density must be constant [22] and there should be no photo-excitation signal. However this is no longer true, when a bubble of gas or supercritical water is formed around the heated nanoparticles. This bubble will show a completely different scattering pattern than a simple peak shift. The bulk water by contrast has to be compressed locally to allow for the expansion of the bubble. The result is a scattering signature from compressed liquid water. This is evident from Fig. 2 which compares the photo-excitation signal to the static derivative dS(P), which closely match. It is important to note that a monotonic radial pressure decay without a phase change near the particles would cause scattering from expanded as well as compressed regions of liquid water, which would almost cancel out due their inverse sign. The signal strength can be compared to the derivatives to deduce a maximum volume change of the compression area of 3.7 · 10
3 or a mean pressure therein of 80 bars (using bulk water data at 25 C). This allows to estimate the bubble diameter to be 250 nm assumed that the density within is close that of the gas phase. The behaviour of the signal strength with time is depicted in Fig. 3, which shows a steady increase up to 1 ns, then a fast drop until the signal vanishes after tens of nanoseconds. This can be understood by assuming that the bubble starts with a certain expansion velocity, which

is reduced until it finally stops to expand and collapses again. A simple parabolic volume change already describes the shape, further refinement of the velocities will be restricted to future studies. A recent study has shown the occurrence of a phase transition of supercritical ethanol close to a nanosecond laser heated silicon surface, which shows essentially the same steps of gas nucleation, growth and collapse although being quantitatively in a different regime [26]. Finally the surplus in intensity at low scattering vector can be related to the formation of small metal aggregates, when ablated material from the initial particles fuses. The Guinier representation of difference data is shown in Fig. 4. The linear slope of the curves makes the derivation of a radius of gyration of newly formed species plausible. This radius of gyration increases with time delay as indicated by the inset in Fig. 4. On the microsecond time ˚ particles are formed which contain about 230 scale 9 A atoms. This reaction is presumably limited by diffusional recombination of atoms and smaller clusters.

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Q [Å ] Fig. 4. Guinier plot of the scattering intensity at low angles featuring the scattering from the particle shape. The inset is a calculation of the radius of gyration derived from the slope in the Guinier representation.

A. Plech et al. / Chemical Physics Letters 401 (2005) 565–569

The increase in SAXS scattering intensity can be modeled by the increase in scattering power of these aggregates, as it scales with the square of the number of electrons in one object (compare Fig. 3). No signal from the size reduction of the parent particles is observed. The critical scattering vector scales with the inverse of the particle diameter and should therefore ˚
1. This region is masked by the show up below 0.05 A beam stop, but can be made accessible in future experiments in a dedicated small angle scattering setup. The signal amplitude with delay is shown in Fig. 3. The SAXS signal, although already present on the sub-nanosecond time scale, increases after about 10 ns. It is interesting to note that a significant increase in scattering amplitude occurs after the bubble collapse. This may be related to the forces exerted on the particles by the collapsing interface and clearly deserves further studies.

4. Conclusion and outlook It has been shown that pulsed X-ray scattering can resolve the reaction of the embedding medium induced by the structural changes in laser excited nanoparticles. At high laser powers gold nanoparticles are excited to lattice energies equivalent to several 1000 K. They undergo a melting transition and fragmentation. The surrounding water is strongly heated and expands to form a vapor layer in the vicinity of the particles. The expansion compresses the water bulk, which in turn gives rise to important scattering changes. After few nanoseconds these bubbles collapse again. It should be emphasized that extreme thermodynamic conditions are present at short times on the nanoscale making this effect a fascinating object for strong nonequilibrium studies. Furthermore the extreme conditions should have a strong impact on the structural dynamics of the nanoparticles themselves, influencing the mode of phase transition and fragmentation [27,28].

Acknowledgements We thank Q. Kong and W. Reichenbach for the assistance on the beamline and the ESRF for support. We have also enjoyed stimulating discussions with R. Vuilleumier, S. Bratos, G. von Plessen and P. Leiderer. This project is funded by the Deutsche Forschungsgemeinschaft and Zentrum fu¨r den wissenschaftlichen Nachwuchs Konstanz.

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References [1] U. Kreibig, M. Vollmer, Optical Properties of Metal Clusters, Springer, Berlin, 1995. [2] S. Link, M.A. El-Sayed, Int. Rev. Phys. Chem. 19 (2000) 409, and references therein. [3] M. Kaempfe, T. Rainer, K.J. Berg, G. Seifert, H. Graener, Appl. Phys. Lett. 74 (1999) 1200. [4] J.H. Hodak, A. Henglein, G.V. Hartland, J. Chem. Phys. 111 (1999) 8613. [5] J.-Y. Bigot, J.-C. Merle, O. Cregut, A. Daunois, Phys. Rev. Lett. 75 (1995) 4702. [6] M. Perner, P. Bost, U. Lemmer, G. von Plessen, J. Feldmann, U. Becker, M. Mennig, M. Schmitt, H. Schmidt, Phys. Rev. Lett. 78 (1997) 2192. [7] J. Hodak, I. Martini, G.V. Hartland, Chem. Phys. Lett. 284 (1998) 135. [8] N. del Fatti, C. Voisin, F. Chevy, F. Valle´e, C. Flytzanis, J. Chem. Phys. 110 (1999) 11484. [9] M. Hu, G.V. Hartland, J. Phys. Chem. B 106 (2002) 7029. [10] M. Hu, X. Wang, G.V. Hartland, V. Salgueirino-Maceira, L.M. Liz-Marzan, Chem. Phys. Lett. 372 (2003) 767. [11] A. Plech, M. Wulff, S. Kuerbitz, K.-J. Berg, G. Berg, H. Graener, S. Gre`sillon, M. Kaempfe, J. Feldmann, G. von Plessen, Europhys. Lett. 61 (2003) 762. [12] A. Plech, V. Kotaidis, S. Gre´sillon, C. Dahmen, G. von Plessen, Phys. Rev. B 70 (2004) 1195423. [13] K. Sokolowski-Tinten, Ch. Blome, C. Dietrich, A. Tarasevitch, D. von der Linde, M. Horn von Hoegen, A. Cavalleri, J.A. Squier, M. Kammler, Phys. Rev. Lett. 87 (2001) 225701. [14] A. Cavalleri et al., J. Appl. Phys. 85 (1999) 3301. [15] F. Schotte, S. Techert, P.A. Anfinrud, V. Srajer, K. Moffat, M. Wulff, in: D. Mills (Ed.), Third-Generation Hard X-ray Synchrotron Radiation Sources, Wiley, New York, 2002. [16] A. Plech, M. Wulff, S. Bratos, F. Mirloup, R. Vuilleumier, F. Schotte, P.A. Anfinrud, Phys. Rev. Lett. 92 (2004) 125505. [17] J. Turkevich, P.C. Stevenson, J. Hillier, Discuss. Faraday Soc. 11 (1951) 55; B.V. Enu¨stu¨n, J. Turkevich, J. Am. Chem. Soc. 85 (1963) 3317. [18] M. Wulff, A. Plech, L. Eybert, R. Randler, F. Schotte, P. Anfinrud, Faraday Discuss. 122 (2003) 13. [19] A. Plech, R. Randler, A. Geis, M. Wulff, J. Synchrotron Radiat. 9 (2002) 287. [20] B.E. Warren, X-ray Diffraction reprint, Dover Publications, New York, 1990. [21] F. Mirloup, R. Vuilleumier, S. Bratos, M. Wulff, A. Plech, in: M.M. Martin (Ed.), Femtochemistry and Femtobiology: Ultrafast Events in Molecular Science, Elsevier Science Ltd., Amsterdam, 2004. [22] S. Bratos, F. Mirloup, R. Vuilleumier, M. Wulff, A. Plech, Chem. Phys. 304 (2004) 245. [23] A.K. Soper, F. Bruni, M.A. Ricci, J. Chem. Phys. 106 (1997) 247. [24] O.M. Wilson, X. Hu, D.G. Cahill, P.V. Braun, Phys. Rev. B 66 (2002) 224301. [25] P.R. Longaker, M.M. Litvak, J. Appl. Phys. 40 (1969) 4033. [26] F. Lang, P. Leiderer, S. Georgiou, Appl. Phys. Lett. 85 (2000) 2759. [27] G.V. Hartland, M. Hu, J.E. Sader, J. Phys. Chem. B 107 (2003) 7472. [28] J.P. Abid, A.W. Wark, P.F. Brevet, H.H. Girault, Chem. Commun. (2002) 792.