Pulsed laser deposition of metal films and nanoparticles in vacuum using subnanosecond laser pulses R. A. Ganeev, U. Chakravarty, P. A. Naik, H. Srivastava, C. Mukherjee, M. K. Tiwari, R. V. Nandedkar, and P. D. Gupta A study of silver, chromium, stainless-steel, and indium thin films prepared by subnanosecond laser deposition in vacuum is reported. We compare the laser ablation in vacuum at the weak- and tightfocusing conditions of a Ti:sapphire laser beam and analyze the nanoparticles synthesized in the latter case using absorption spectroscopy, x-ray fluorescence, atomic force microscopy, and scanning electron microscopy. Our results show that the nanoparticle formation can be accomplished using long laser pulses under tight-focusing conditions. © 2007 Optical Society of America OCIS codes: 240.0310, 310.6870. 1. Introduction Nanostructures of different materials are of special interest owing to their potential applications in optoelectronics and nonlinear optics. The structural, optical, and nonlinear optical parameters of nanoparticles are known to differ from those of the bulk materials due to the quantum confinement effect. Silver1,2 (Ag), copper2– 4 (Cu), and gold2,5 are among the most useful metals suited for nanoparticle preparation for optoelectronics and nonlinear optics. Further search of prospective materials in nanoparticle form, their preparation, and application are of considerable importance today. Past studies on nanoparticles prepared using metal vapor deposition,6 reduction of some salts by alkalides,7 and the solution dispersion method8 have revealed many interesting structural and optical properties of these materials. The laser ablation of metals in vacuums and liquids is among the perspective techniques, which can also be successfully applied for the preparation of nanoparticle-containing media. Recently, the application of laser ablation in The authors are with the Raja Ramanna Centre for Advanced Technology, Indore 452013, M.P., India. R. A. Ganeev (r_ganeev@ issp.u-tokyo.ac.jp) is also with the Akadempribor Scientific Association, Academy of Sciences of Uzbekistan, Akademgorodok, Tashkent 700125, Uzbekistan. Received 23 August 2006; revised 2 October 2006; accepted 23 October 2006; posted 30 October 2006 (Doc. ID 74285); published 20 February 2007. 0003-6935/07/081205-06$15.00/0 © 2007 Optical Society of America liquids for the preparation of semiconductor9,10 and metal11 colloids has been demonstrated. It is known that metal ablation in air is significantly less efficient than that in vacuum due to redeposition of the ablated material. The ablation rates in vacuum can be calculated using a thermal model, which also allows estimating the ablation rates for other metals from basic optical and thermal properties.12 A comparison of the morphology of the deposition sites after nanosecond and picosecond ablation shows unequivocally the advantages of short-pulse ablation for the preparation of nanoparticles.13 To prove the generality of the vacuum deposition method and its potential use for preparing nanoparticles, we considered various metals and analyzed this process at different focusing conditions of the laser radiation. The most interesting and new features of laser ablation and nanoparticle formation during irradiation of the solid targets have been recently observed in the case of short laser pulses (100 fs to 1 ps). In this paper, we study the formation of nanoparticles of Ag, chromium, stainless steel, and indium (In) in vacuum, using much longer (subnanosecond) laser pulses. The structural properties of the nanoparticles deposited on different substrates are reported. The effect of focusing conditions of the laser (tight or weak focusing) on the films prepared by the laser ablation of bulk targets in the vacuum is analyzed. Our results show that the nanoparticles can be formed using long laser pulses in tight-focusing conditions. 2. Experimental Details The ablation of various materials was carried out in vacuum using uncompressed pulses (of 300 ps dura10 March 2007 兾 Vol. 46, No. 8 兾 APPLIED OPTICS 1205 tion) from a chirped-pulse amplification-based Ti:sapphire laser system (Thales Lasers). The samples were placed inside a vacuum chamber. Uncompressed pulses from the Ti:sapphire laser (␭ ⫽ 793 nm, ␶ ⫽ 300 ps, E ⫽ 30 mJ, 10 Hz pulse repetition rate) were focused on a bulk target at two regimes of focusing. In the first case (referred to as tight focusing), the intensity of laser radiation was in the range of 2 ⫻ 1012 W cm⫺2, and in the second case (referred to as weak focusing), the intensity was considerably lower 共4 ⫻ 1010 W cm⫺2兲. Ag, In, stainless steel, and chromium were used as targets. Float glass, silicon wafer, and various metal strips (Ag, Cu, and aluminum) were used as the substrates, and were placed at a distance of 50 mm from the targets. The deposition was carried out in oil-free vacuum 共⬇1 ⫻ 10⫺4 mbars兲 at room temperature. The structure of the deposited films of ablated metals was analyzed using different techniques. For this purpose, the nanoparticle-containing films were deposited on different substrates. The nature of the nanoparticles is governed by the thermodynamic conditions at the target surface. The presence of nanoparticles was inferred by analyzing the spatial characteristics of the deposited material and the spectral absorption of the deposited material. The absorption spectra of the deposited films were analyzed using a fiber-optic spectrograph (USB2000). The analysis of the sizes of deposited nanoparticles was carried out using total reflection x-ray fluorescence (TXRF). Details of the TXRF are described in Ref. 14. The structural properties of the deposited films were analyzed using a scanning electron microscope [(SEM), Philips XL30CP], an atomic force microscope [(AFM), SOLVER PRO, NT-MDT], and a transmission electron microscope [(TEM), Philips CM200]. 3. Results and Discussion The absorption spectra of the materials deposited on transparent substrates (float glass) were used to determine the presence or absence of nanoparticles. The presence of nanoparticles was inferred from the appearance of strong absorption bands associated with surface plasmon resonance (SPR). Figure 1 presents the absorption spectra of Ag films deposited on float glass substrates. Earlier work11 has shown that the SPR of spherical silver nanoparticles induces a strong absorption in the range from 410 to 480 nm, depending on the preparation technique. Here, we observed a variation of the position of the absorption of Ag deposition, which depended on the conditions of excitation and evaporation of bulk targets by the 300 ps pulses, which interacted with the surface at different tightfocusing conditions. However, in all these cases, the peaks of SPR were centered in the range from 440 to 490 nm (Fig. 1, curves 1–3). In the case of the deposition of Ag film at the weak-focusing conditions, no absorption peaks were observed in this region, indicating the absence of nanoparticles. Figure 1, curve 4 shows analogous measurements made on a sample of Ag nanoparticles embedded in 1206 APPLIED OPTICS 兾 Vol. 46, No. 8 兾 10 March 2007 Fig. 1. Absorption spectra (curves 1–3) of the Ag films deposited at different tight-focusing conditions and the absorption spectrum of Ag nanoparticles (curve 4) implanted inside the silica glass plate by ion bombardment. silica glasses using the ion bombardment, reported by Ganeev et al.15 In this work, the thickness of the implanted layer was 60 nm, and the size of Ag nanoparticles was reported to vary from 4 to 8 nm. It is observed that the absorption curve of this sample is quite similar to those of Ag deposited on the glass surfaces here (Fig. 1, curves 1–3). The only difference is that the position of the peak of SPR 共415 nm兲 was on the shorter wavelength side (Fig. 1, curve 4). Much attention has been devoted during the past few years to precisely determining the spatial arrangement in 2D and 3D structures of metals. However, the ordering and use of the nanomaterials necessitate synthesis of monodispersed individual nanoparticles, for which no general method is presently available. We describe, later in this paper, the analysis of synthesized nanoparticles by laser ablation of bulk targets at two different conditions of ablation. The structure of the ablated Ag was analyzed by studying the films deposited on silicon substrates. One of the aims of this study was to investigate whether the plumes contain nanoclusters. The presence of the latter could be responsible for the enhancement of nonlinear optical characteristics. In particular, high-order harmonic conversion efficiency may be affected due to the quantum confinement effect during the propagation of a femtosecond laser pulse through a nanoparticle-containing plasma. Harmonic generation using single atoms and multiparticle aggregates has been reported in Ref. 16 for argon atoms and clusters. It was demonstrated that a medium containing intermediate-sized clusters of a few thousand atoms of an inert gas is much better at generating the higher harmonics than a medium of isolated gas atoms of the same density.16 The reported enhancement factor for the third through ninth harmonics was approximately ⫻5. Also, the dependence of the efficiency of harmonic generation on the intensity of the laser radiation was much more pronounced for clusters than for isolated atoms. The Fig. 2. Recorded x-ray fluorescence profile for an In deposition prepared by ablation at weak-focusing conditions. The profile shows that the In is deposited in the form of a continuous monoatomic film instead of nanoparticles. The solid curve shows a fitted profile. The critical angle of In is ⬃0.36° for 8.50 keV x-ray energy. highest harmonic number for clusters was reported in Ref. 16, which was higher than that for the isolated atoms. Total reflection x-ray fluorescence measurements were performed using an in-house-developed TXRF (Ref. 17) for the analysis of the structural properties of the deposited material. The angular dependence of the fluorescence intensity in the total reflection region18,19 can be successfully used to identify the presence of nanoparticles on a flat surface. The structure of the nanoparticles prepared by the laser ablation process was analyzed using TXRF, which identified the presence of nanoparticles. This was done for the deposition in tight-focusing conditions of the laser. In the case of weak focusing, it showed a thick filmlike deposition of metal, without any inclusion of nanoparticles (see the TXRF image for the case of the ablation of indium at the weak-focusing conditions, Fig. 2). It can be seen from this figure that the fluorescence intensity of In-L␣ decreased abruptly below the critical angle for In (⬃0.36° at 8.5 keV). For incident angles larger than the critical angle, In-L␣ fluorescence intensity increased monotonously. It showed behavior like that for a thick film of atomic In deposited on the substrate. The fluorescence measurements were carried out using a Peltier-cooled solid-state detector (EurisusMesures EPXR 10-300), a spectroscopy amplifier AMP 6300, and a multichannel pulse height analyzer card installed in a personal computer. The solid-state detector had an energy resolution of 250 eV at 5.9 keV. A well-collimated primary beam, from a line focus Cu x-ray tube, was used as an excitation source. Figure 3 shows the x-ray fluorescence trace recorded in the case of Ag nanoparticles deposited on a glass substrate at tight-focusing conditions. It can be seen from this figure that the angle-dependent fluores- Fig. 3. Recorded x-ray fluorescence profile of Ag nanoparticles prepared by the laser ablation in vacuum and deposited on a float glass substrate. The dots show experimental data, while the solid curve shows a fitted profile. cence profile of the Ag film shows the presence of nanostructure on the flat surface, as well as indicates a monoatomic layer. The average size of the Ag nanoparticles was determined by fitting the recorded fluorescence profile using the CATGIXRF program.20 The solid curve presents the best theoretical fit, from which the average size of the nanoparticles was estimated to be 60 nm, while the thickness of the layer of monoatomic Ag particles was estimated to be 30 nm. The TEM measurements also confirmed the presence of Ag nanoparticles in these deposited films. Our SEM studies of the structural properties of the deposited films showed that, in tight-focusing conditions, these films contain a lot of nanoparticles with variable sizes. In weak-focusing conditions, the concentration of nanoparticles was considerably smaller compared to the tight-focusing condition. Figure 4(a) shows the SEM images of the deposited chromium nanoparticles on the surface of a silicon wafer. In the case of weak focusing, the deposited film was almost homogeneous with a few nanoparticles appearing in the SEM images [see Fig. 4(a) showing the deposition of chromium], while in tight-focusing conditions, plenty of nanoparticles ranging from 30 to 100 nm appeared in the SEM images [see Fig. 4(b) showing the deposition of stainless steel]. The average size of these nanostructures was estimated to be 60 nm. An enlarged SEM image of the Ag nanoparticles prepared in tight-focusing conditions is presented in Fig. 4(c). The average size of these spherical clusters was also measured to be 60 nm. The same behavior was observed in the case of other targets. These studies showed that the material of the target does not play much of a significant role in the formation of nanoparticles in the case of laser ablation using 300 ps laser pulses in tight-focusing conditions. Hence, further study on the effects of the substrate material on 10 March 2007 兾 Vol. 46, No. 8 兾 APPLIED OPTICS 1207 Fig. 5. SEM images of the Ag nanoparticles deposited on a Cu substrate at (a) weak-focusing conditions and (b) tight-focusing conditions. Fig. 4. SEM images of the chromium, stainless steel, and Ag nanoparticles deposited on silicon wafer as substrate. These images were obtained at (a) weak-focusing (chromium deposition), (b) tight-focusing (stainless-steel deposition), and (c) tight-focusing (Ag deposition) conditions. (Note the different magnifications are used). The average size of the spherical clusters in all three cases was measured to be 60 nm. nanoparticle deposition was carried out using Ag targets. Figure 5 shows the SEM images of Ag deposited on a Cu substrate. One can see a considerable difference in the concentrations of nanoparticles in the cases of weak and tight focusing. It may be pointed out that there is special interest in Ag nanoparticles due to their potential applications. In general, highly dispersed metals have a much higher surface area for a given volume and hence they can be useful for efficient catalytic conversion. Ag nanoparticles are widely used for surface-enhanced Raman scattering. Ag nanoparticles have an advantage over the other metal nanoparticles (e.g., gold and Cu) from the point of view of the position of the SPR energy of Ag, which is far from the interband transition energy. This enables one to investigate the optical and nonlinear optical effects in the Ag nanoparticles by focusing on the surface plasmon contribution. Further studies on the characteristics of nanosized structures of the deposited materials were carried out using atomic force microscopy. AFM measurements were carried out in noncontact mode under an ambi1208 APPLIED OPTICS 兾 Vol. 46, No. 8 兾 10 March 2007 ent environment. Silicon cantilever tips of resonant frequency ⬎180 kHz and spring constant 5.5 N兾m were employed. Figure 6(a) shows the AFM image of the Ag nanoparticles deposited on the surface of a copper strip. The average size of Ag nanoparticles was 65 nm. In contrast to this, the AFM images obtained from the deposited films prepared at weakfocusing conditions showed a considerably smaller number of nanoparticles. Figure 6(b) shows an AFM picture of Ag deposition prepared at these conditions. The image indicates the presence of very few nanoparticles. The same difference in AFM pictures was observed in the case of In deposition under the two focusing conditions. To characterize the ablation process, the temporal and spectral characteristics of the light emitted by the plume were also studied. The oscilloscope traces showed a considerable increase in the duration of the plasma emission in the case of tight focusing that could be expected considering the excitation conditions. A combined analysis of the spectra and oscilloscope traces in the cases of two different regimes of the excitation of surface plasma revealed that the structureless continuum appearing in the spectrum in the case of tight focusing is due to the emission from hot nanoparticles produced during laser ablation. Such hot nanoparticles behave like black-body radiators emitting for a longer time, until they get cooled down. It is well accepted that when a solid target is ablated by laser radiation, the ablating material is in the form of atoms, ions (and electrons), and clusters. These atoms and clusters tend to aggregate during the laser pulse or soon afterward leading to the formation of larger clusters. The reported results (see, for example, Ref. 21) also indicate that the ablation processes in the picosecond and femtosecond time scales are very dif- intensities 共ⱖ1014 W cm⫺2兲, the system can never reach the metastable region, resulting in an almost atomized plume. Pulsed laser deposition using short pulses has gained some interest because of a number of advantages over other processes, such as the possibility of producing materials with a complex stoichiometry and a narrowed distribution of nanoparticle sizes with reduced porosity. Typically, laser deposition is carried out in an ambient gas, which quenches the ablated plume, thus controlling the mean particle size.26 However, some previously reported studies,13,27 as well as the present study, suggest that nanoparticles are generated as a result of some relaxation processes of the extreme material state reached by the irradiated target surface. This stands in stark contrast to the formation of nanoparticles during nanosecond laser ablation in a background gas, where vapor condensation is considered to be an important mechanism. 4. Conclusions A study of the metal deposition by laser ablation in vacuum has been performed using subnanosecond laser pulses under different focusing conditions. It is shown that nanoparticles can be formed using long laser pulses under tight-focusing conditions. The average size of the nanoparticles was ⬃60 nm for the various targets (Ag, In, chromium, and stainless steel) used in the study. Fig. 6. AFM image of (a) the Ag nanoparticles deposited on a Cu strip in tight-focusing conditions and (b) the Ag nanoparticles deposited on an aluminium strip in weak-focusing conditions. Very few nanoparticles are seen in the case of (b) weak-focusing conditions compared to the case of (a) tight-focusing conditions. ferent compared with the nanosecond one. In addition to the early experimental observations, several theoretical studies have suggested that rapid expansion and cooling of the solid-density matter heated by short laser pulses may result in nanoparticle synthesis via different mechanisms. Heterogeneous decomposition, liquid phase ejection and fragmentation, homogeneous nucleation and decomposition, and photomechanical ejection are among those processes that can lead to nanoparticle production.22–24 Short pulses, contrary to the nanosecond pulses, do not interact with the ejected material, thus avoiding complicated secondary laser interactions. Further, short pulses heat a solid to a higher temperature and pressure than do longer pulses of comparable fluence since the energy is delivered before significant thermal conduction can take place. The model developed in Ref. 25 for aluminum predicts that, for short laser pulses at intensities in the range from 1012 to 1013 W cm⫺2, the adiabatic cooling drives the system into a metastable region of its phase diagram, resulting in the production of a relatively large fraction of nanoparticles. At larger R. A. 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