Laser-induced modification of metal nanoparticles formed by laser ablation technique in liquids

Transcription

1 Applied Surface Science 247 (2005) Laser-induced modification of metal nanoparticles formed by laser ablation technique in liquids N.V. Tarasenko *, A.V. Butsen, E.A. Nevar Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 70 Scaryna Avenue, Minsk, Belarus Available online 19 February 2005 Abstract The effects of laser irradiation of silver colloids prepared by laser ablation technique in acetone have been studied. The laser irradiation was performed using laser radiation at different wavelengths (532, 266, 400 and 800 nm). Additional irradiation of colloids resulted in the changes of particles morphology, which were monitored by absorption spectroscopy and transmission electron microscopy methods. The experimental conditions favoring a dimension reduction of the initial particles and a formation of spherical size-controlled nanoparticles were found as well as irradiation conditions aiding the fabrication of nanowires. It was found that both the mean size of the nanoparticles and their stability could be controlled by changing the laser ablation and post-irradiation regimes. # 2005 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; Colloids; Laser-induced modification 1. Introduction Metal nanoparticles has become important for wide range of applications such as surface enhanced Raman spectral probing [1], markers for biosensors and catalytical systems with optimized selectivity and efficiency [2]. The properties of nanoparticles depend on their size and shape and alter considerably from those of the corresponding bulk material of macroscopic dimensions. In this connection particular attention is paid to the preparation methods that allow * Corresponding author. Tel.: ; fax: address: [email protected] (N.V. Tarasenko). synthesis of particles with the nearly uniform size and shape. Recently laser-induced effects in metal nanoparticles have been shown to be attractive for solving this problem [3 8]. These studies have demonstrated that laser irradiation of metal colloids can cause fragmentation [3,4] or enlargement [5,6] of the nanoparticles through the photoinduced heating and melting as well as transformation of nanorods to nanospheres [7], or spherical particles to nanowires [8]. To gain a better control of morphology of nanostructures and to fabricate of nanostructures with desired characteristics in laser ablation processes, further investigation of the mechanisms of interaction between the particles and the laser light is still required. Because of the selectivity of the light absorption by particles due to /$ see front matter # 2005 Elsevier B.V. All rights reserved. doi: /j.apsusc

2 N.V. Tarasenko et al. / Applied Surface Science 247 (2005) their surface plasmon resonance [9], the efficiency of laser excitation is expected to be dependent on the laser wavelength. In this paper we have studied the effect of laser irradiation of silver colloids prepared by laser ablation technique in liquids using laser radiation at different wavelengths (532, 266, 400 and 800 nm). The effect of irradiation time, laser fluence for different laser wavelengths were examined, a comparative analysis of colloids prepared under different experimental conditions was performed in order to optimize the irradiation conditions. The experiments are aimed to elucidate the mechanism of the laser-induced modification of nanoparticles, which is essential for finding the optimal conditions for the control of the process. Laser-induced heating, melting and evaporation, laser-induced charging of particles with the following disintegration as well as a cluster growth and a particle fusion that can contribute to the observed morphology modification of particles after the laser irradiation are discussed. 2. Experimental The silver colloids were prepared by a laser ablation of silver plate in acetone or in distilled water. The pulsed Nd:YAG laser (LOTIS TII, LS2134) operating at the fundamental (1064 nm, 50 mj, 10 ns) or doubled (532 nm, 50 mj, 10 ns) wavelengths was used for ablation. The laser beams were focused on the surface of the silver plate providing the power density on the target surface in the range of to W/cm 2 both at the wavelength 1064 and 532 nm. It should be noted that silver particles prepared by laser ablation in acetone were stable and free from any precipitates for at least several months (without adding any surfactants or other additives). As for colloids prepared in water, they were less stable and after aging for several days they precipitated. Therefore, the experiments were mostly conducted using colloids prepared by laser ablation in acetone. After the laser ablation, the silver plate was removed from the solution and prepared colloids were irradiated for 5 min with unfocused 532 and 266 nm beams of the same Nd:YAG laser (LOTIS TII, LS2134) or with 800 and 400 nm pulses of a Ti sapphire laser (LOTIS TII, LT2211), pumped by the second harmonic of the Nd:YAG laser. The maximal laser fluences were 0.5, 0.1, 0.1 and 0.6 J/cm 2 for the 532, 266, 400 and 800 nm, respectively. The effect of laser irradiation was investigated by measuring optical absorption spectra and TEM images of the colloids with a spectrometer Cary 500 scan and the transmitting electron microscope (JEM-100). 3. Results and discussion The effect of laser irradiation on colloidal solution examined by measuring absorption spectra is shown in Fig. 1. The spectra exhibited absorption bands with a maximum around 400 nm, which is typical for the excitation at the plasmon resonance in silver nanoparticles. Fig. 1 shows spectra for silver colloidal nanoparticles in acetone that have been irradiated for 5 min with different intensity 532 nm laser beams. As it can be seen from Fig. 1 the initial spectrum (curve 1) is changed by laser irradiation. The bandwidth of the plasmon resonance is significantly reduced. The narrowing of the plasmon bandwidth is accompanied by an increase in the absorption magnitude. These spectrum features become stronger with increasing laser fluence from 0.04 to 0.4 J/cm 2. An observed narrowing of the Ag plasmon resonance band can be attributed to the redistribution of the particle sizes (narrowing of the particle size dispersion), its increase Fig. 1. Optical absorption spectra of silver colloidal solution before (1) and after laser irradiation of 532 nm pulsed laser for 5 min at laser fluences 150 mj/cm 2 (2), and 300 mj/cm 2 (3), respectively. The inset shows the difference absorption spectra of the laserirradiated solution and the nonirradiated sample.

3 420 N.V. Tarasenko et al. / Applied Surface Science 247 (2005) in intensity can be assigned to the increase of the particle concentration in the solution. To determine the spectral regions of significant changes after laser irradiation the difference spectra (D D 0 ) of the silver nanoparticles were plotted in the inset in Fig. 1, where D and D 0 represent the absorbance at a given wavelength after and before laser irradiation, respectively. It is evident from the inset in Fig. 1 that in the blue region of the spectrum (around 400 nm) the absorbance increases and in the vicinity of 500 nm decreases after laser irradiation. These spectral changes were found to be increased with increasing laser fluence. Since the plasmon frequency of each single particle is determined by its dimension and shape, the optical absorption profiles of the whole distributions are inhomogeneously broadened. Therefore, irradiation of colloids with laser pulses of a definite photon energy yields resonant plasmon excitation in particles with specific size and shape. By changing the excitation wavelength it is possible to selectively excite particles within a range of sizes and/or shapes. The excitation of the larger particles is expected mainly under laser irradiation at longer wavelengths (532 and 800 nm). With laser excitation at these wavelengths the absorbance decreasing with a maximum in the nm region was observed (Fig. 2). This probably shows that the particles which contribute to the surface plasmon band at 400 nm are unaffected by this longwavelengths excitation. A recorded growth of absorption in the 400 nm region can be attributed to Fig. 2. Optical absorption spectra of silver nanoparticles before (1) and after laser irradiation for 5 min at different wavelengths: 266 nm (2), 400 nm (3), and 800 nm (4). The inset shows the spectra around 400 nm with the higher resolution. the increase of a particle concentration in result of a fragmentation of larger clusters that selectively absorb the incident laser radiation. When the particles were subjected to 400 nm laser excitation the position of the plasmon peak shifted to the blue region and the bandwidth was reduced (Fig. 2). These changes of the absorption spectrum suggest that the mean particle dimension and the size distribution are reduced by laser irradiation. Different changes of the colloid spectrum were observed after laser irradiation at 266 nm wavelength, probably because of the strong absorbance of acetone at this wavelength and a possible photodecomposition of acetone molecules. The red shift of the plasmon peak with broadening of the absorption spectrum to the longer wavelengths indicated the tendency to the formation of larger particles as well as their aggregation. The laser-induced morphological changes of silver nanoparticles were further examined by transmission electron microscopy. Before irradiation (Fig. 3a), colloids prepared by 1064 nm laser ablation (at the power density of W/cm 2 ) consisted of nearly spherical silver particles with diameters between 15 and 50 nm. They exhibited a slight trend to an agglomeration with time. Silver nanoparticles produced by 532 nm laser ablation at the same laser power density were, as a rule, agglomerates, which consisted of disc-like particles larger than 50 nm in diameter (Fig. 4a). Irradiation of the prepared colloids with the 532 nm laser beam at the fluences higher than 100 mj/cm 2 caused changes both a size and a shape of particles. The most of initial particles produced by 1064 nm laser ablation reduced their sizes to the less than 15 nm and larger irradiation fluences led to smaller particles. In addition to the fragmented particles a small fraction of larger particles was observed. They were nearly spherical with diameter of around nm. The fraction of larger particles was about 10%. The fragmented small particles appear to have undergone melting upon laser irradiation and subsequent coalescence of melted particles seemed to yield larger particles. Laser irradiation of the colloids shown in the Fig. 4a caused a transformation of the initial nanodiscs into nanowires. The nanowires had an average size of 100 nm and a length of about 600 nm. Stacks of these

4 N.V. Tarasenko et al. / Applied Surface Science 247 (2005) Fig. 3. TEM images of silver nanoparticles produced by the 1064 nm laser ablation at the power density of W/cm 2 : (a) before and (b) after laser irradiation at 532 nm (400 mj/cm 2 ) for 5 min. nanowires assembled on a carbon film-coated copper grid are seen from Fig. 4b. Consider the main mechanisms of an interaction of a laser radiation with silver nanoparticles. Under laser irradiation parent silver nanoparticles can be heated through the absorption of photons during laser pulse up to boiling point, the heated nanoparticles can be fragmented onto smaller ones with releasing atoms Fig. 4. TEM micrographs of the silver nanoparticles produced by 532 nm laser ablation at the power density of W/cm 2 : (a) before and (b) after laser irradiation with 532 nm laser beam (360 mj/pulse) for 5 min.

5 422 N.V. Tarasenko et al. / Applied Surface Science 247 (2005) and clusters. The silver atoms rapidly aggregate onto small clusters in a nanosecond time scale [10]. These clusters do not exhibit optical absorption in the range of the surface plasmon resonance, although these small clusters show a broad absorption in UV range [11]. The silver clusters begin to contribute to the optical absorption as they grow into nanosized particles by aggregation and attachment to the nanoparticles, followed by coalescence. The possibility of particle heating with consequent melting and evaporation in our experiment was considered by estimating a temperature of particles, which can be reached because of the absorption of laser light. The temperature of the silver particles was estimated on the basis of a balance between the absorbed laser energy and heat loses during the laser pulse using the bulk physical constants. The calculations showed that temperature of the nanoparticles exceeds the silver melting point at the laser fluences higher than 50 mj/ cm 2 and reaches the boiling point at approximately 100 mj/cm 2. Thus, the calculated heat required to melt and evaporate the particles roughly agreed with the value of the experimental threshold of laser fluence for particle modifications. Another possibility for a modification of silver nanoparticles after irradiation is charging of parent particles when they are subjected to laser-pulse excitation and breaking up after accumulating the sufficient charge through the photoelectron emission [12]. Because the work function of silver is about 4.3 ev the photoemission of electrons is a monophotonic process only for 266 nm laser excitation. Thus, this mechanism cannot be obeyed as a significant one in the observed laser-induced fragmentation, although minor contribution of multiphoton photoeffect cannot be completely excluded. 4. Conclusion So, the laser-induced modification of morphological characteristics of colloidal nanoparticles was observed to be initiated by light at four wavelengths between 266 and 800 nm. The difference in the observed spectra after laser irradiation at the indicated wavelengths mainly arises from the fact that differentsized particles are being excited at different laser excitation wavelengths. The observed changes in the absorption spectra caused by laser irradiation appear to correspond to the decrease in the size of the particles in the solution. Experimental parameters such as laser fluence, time of irradiation, wavelength of the incident laser beam were found to influence the efficiency of the fragmentation process. It should be noted that the TEM examination shows the result of laser irradiation to be more complicated than just a fragmentation of initial particles into smaller ones due to the evaporation. Besides fragmented particles, nanosize wire structures were found to form in the solution probably via fusion of photothermally melted nanoparticles. To gain better insight into mechanism of the observed laser-induced modification further investigation of the importance of several processes, accompanying the interaction of nanoparticles with laser radiation is required. It is most likely that the combination of several factors leads to the observed morphological changes of the silver nanoparticles. References [1] J. Neddersen, G. Chumanov, T. Cotton, Appl. Spectrosc. 47 (1993) [2] M.S. Sibbald, G. Chumanov, T.M. Cotton, J. Phys. Chem. 45 (1998) 721. [3] A. Takami, H. Kurita, S. Koda, J. Phys. Chem. B 103 (1999) [4] F. Mafune, J. Kohno, Y. Takeda, T. Kondow, J. Phys. Chem. B 105 (2001) [5] N. Chandrakharan, P.V. Kamat, J. Hu, G. Jones, J. Phys. Chem. B 104 (2000) [6] M.S. Yeh, Y.S. Yang, Y.P. Lee, H.F. Lee, Y.H. Yeh, C.S. Yeh, J. Phys. Chem. B 103 (1999) [7] S. Link, C. Burda, B. Nikoobakht, M.A. El-Sayed, J. Phys. Chem. B 104 (2000) [8] T. Tsuji, N. Watanabe, M. Tsuji, Appl. Surf. Sci. 211 (2003) 189. [9] U. Kreibig, M. Vollmer, Optical Properties of Metal Clusters, Springer, Berlin, [10] A. Dawson, P.V. Kamat, J. Phys. Chem. B 105 (2001) 960. [11] E. Janata, A. Henglein, B.G. Ershov, J. Phys. Chem. 98 (1994) [12] P.V. Kamat, M. Flumiani, G.V. Hartland, J. Phys. Chem. B 102 (1998) 3123.