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Comparison of nanosecond and femtosecond pulsed laser deposition of silver nanoparticle films
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 265301 (http://iopscience.iop.org/0957-4484/25/26/265301) View the table of contents for this issue, or go to the journal homepage for more
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Nanotechnology Nanotechnology 25 (2014) 265301 (10pp)
doi:10.1088/0957-4484/25/26/265301
Comparison of nanosecond and femtosecond pulsed laser deposition of silver nanoparticle films I Mirza, G O’Connell, J J Wang and J G Lunney School of Physics and CRANN, Trinity College Dublin, Dublin 2, Ireland E-mail: [email protected] Received 29 September 2013, revised 30 April 2014 Accepted for publication 8 May 2014 Published 11 June 2014 Abstract
Nanoparticle (NP) films of silver were prepared using nanosecond (ns) and femtosecond (fs) pulsed laser deposition (PLD) in vacuum. The flux and energy distribution of the ions in the plasma part of the ablation plume were measured using a Langmuir ion probe. The deposition energy efficiencies of ns and fs silver PLD were also compared. For equivalent thickness up to ∼3 nm the NPs made by ns-PLD are well separated and roughly circular, but for higher thicknesses the NPs begin to coalesce. For equivalent thickness up to 7 nm the fs films are comprised of well separated NPs, though the mean NP size and the surface coverage increase with equivalent thickness. The mean Feret diameter for both ns- and fs-PLD films increases with increasing equivalent solid-density thickness. The surface plasmon resonance peak was observed to red shift for both ns- and fs-PLD films as the equivalent solid-density thickness was increased from 1 nm to 7 nm. Keywords: pulsed laser deposition, nanoparticle Ag films, surface plasmon resonance, laser ablation (Some figures may appear in colour only in the online journal) 1. Introduction
This ablated material expands rapidly away from the target and can be captured on a solid substrate. The process can be performed in vacuum or in a background gas environment. In ns-PLD of NPs in vacuum it seems that the NP growth takes place on the substrate surface by surface diffusion of material which is condensed from the vapour, or plasma, phase [10–12]. Dolbec et al [10] have studied ns-PLD of Pt on highly-oriented pyrolytic graphite and have shown that the mean NP size increases as the areal density, or equivalent solid-density thickness (areal density divided by solid density), is increased. Afonso et al [11] have prepared nanocomposite films of Cu and alumina in vacuum and background gas environments. D’Andrea et al [5] and Smyth et al [13] have investigated the formation and deposition of Ag NPs using ns laser ablation in background gas and vacuum respectively, and demonstrated the utility of the films produced for surface enhanced Raman spectroscopy (SERS). In femtosecond (fs) PLD (fs-PLD) of metals the ablation mechanism is very different from the ns case. In the first
Nanostructured materials and surfaces show interesting optical, electronic, magnetic and catalytic properties which arise when the dimensions of matter are reduced to the nanoscale (1–100 nm) [1–3]. Nanoparticle (NP) films of noble metals are of particular interest since they display a surface plasmon resonance (SPR) in the visible region, which is used to enhance the sensitivity of fluorescence [4] and Raman spectroscopy [5]. The catalytic properties of metal NP films are of interest for the growth of carbon nanotubes [6] and nanorods of various materials [7]. There is a wide range of physical and chemical techniques used for NP film deposition [8, 9]. Among the physical vapour methods, pulsed laser deposition (PLD) is a simple technique that can, in principle, be applied to all solid materials. In conventional PLD a nanosecond (ns) pulsed laser is focused to sufficiently high fluence on the target surface to evaporate a small amount of material from the target surface. 0957-4484/14/265301+10$33.00
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Figure 1. Ion time-of-flight signals for: (a) ns-ablation at 5.8 cm, and (c) fs-laser ablation at 4.7 cm. The corresponding ion energy
distributions are given in (b) and (d); the dotted lines indicate average ion energies.
Table 1. Deposition rate, ion fluence and deposition energy efficiency for ns- and fs-PLD of silver at 0.8 J cm−2.
ns-PLD −2
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Deposition rate (atoms cm ) Ion fluence (ions cm−2) Deposition efficiency (atoms cm−2 mJ−1)
9.9 × 10 (at 8 cm) 6.0 × 1011 (at 8 cm) 3.3 × 1010 (at 8 cm)
fs-PLD 2.4 × 1012 (at 6 cm) 2.6 × 1010 (at 6 cm) 3.4 × 1012 (at 8 cm)
emission was observed at laser fluence higher than ∼8 J cm−2. Chakravarty et al [17] have compared the formation of Ag and Cu NP films on a Si substrate using ps- and fs-PLD in vacuum. They observed that NPs with smaller mean diameter could be generated using fs-PLD as compared to ps-PLD. In another experiment, De Bonis et al [18] used fs-PLD to fabricate Ag NP films on solid substrates for SERS. Using transmission electron microscopy (TEM), they found that the ablation behaviour and NP deposits were quite similar to that found in ns-PLD, indicating that the secondary NP plume, normally observed in fs ablation of metals, may not play an important role in fs-PLD of Ag NP films. In this paper, we present the results of a systematic comparison of ns- and fs-PLD of Ag NP films at 0.8 J cm−2. We have measured the ionization fractions and deposition efficiencies of ns- and fs-ablation. While the ns ablation plume is nearly fully ionized, in fs ablation the ion fraction in the ablation is only ∼1%. We have observed how the NP
stage, the laser energy is absorbed by the free electrons near the metal surface. The electron system thermalises on a timescale of ≈ few tens of fs. On a longer timescale, of 10–20 ps [14], the lattice is heated by electron–phonon relaxation. At the end of this stage a thin layer near the metal surface may be heated to temperatures above the critical temperature but still have a density close to solid density. Using fast photography and optical spectroscopy, it has been shown that the dispersal of this superheated material results in two distinct ablation plumes; the first is composed of partially ionized vapour moving at ∼106 cm s−1, and the second is a plume of hot NPs moving about 100 times slower. These general features are also found in molecular dynamics simulations, where the NPs are seen to be formed by fragmentation and phase explosion [15]. Liu et al [16] have reported that fsPLD of Ni in vacuum leads to polycrystalline NPs with an average diameter less than 10 nm and the NP size is nearly independent of laser fluence. However, large droplet-like NP 2
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hole into the target. The ablated material was deposited on 5 mm × 5 mm × 1 mm thick quartz substrates and carboncoated TEM grids. The target-to-substrate distance was 8 cm for ns-PLD and 6 cm for fs-PLD. A planar Langmuir ion probe of area 0.09 cm2 and biased at −30 V was positioned on the target normal and facing the ablation spot to record the ion time-of-flight signal, from which the ion energy distribution and ion fluence were derived [20, 21]. An ion probe is a sensitive device for gauging the reproducibility of the ablation process. The target-to-probe distance was 5.8 cm for ns ablation and 4.7 cm for fs ablation. A quartz crystal monitor (QCM) was used to measure the equivalent solid density thickness of the deposit, which is obtained by dividing the areal density of the film by the density of solid Ag. The targetto-QCM distance was 6.5 cm for ns-PLD and 4.6 cm for fsPLD. Since the ablation plume expansion is inertial beyond a few mm from the target surface [22], ion probe and deposition measurements made at different distances can be scaled to a fixed position using a 1/(distance)2 scaling. The morphology of deposited films was measured using scanning transmission electron microscopy (STEM) operating at 25 kV. Grazingangle x-ray reflectometry was used to measure the overall thickness and density of the NP films. The optical absorption of the NP films was measured in the 300–800 nm range using a Cary-50 UV-visible spectrophotometer.
3. Results and discussion Figure 2. XRR curves for Ag film deposited on quartz substrate
3.1. Laser ablation plume analysis
morphology and the optical absorption depend on the equivalent solid-density thickness of the deposit. Similar to the observation of De Bonis et al [18], the mean NP size in fsPLD films was observed to increase with equivalent soliddensity thickness.
Figure 1 shows the ion probe signals for (a) ns- and (c) fslaser ablation, where time is measured from the arrival of the laser pulse on the target. Since the plasma plume is mainly accelerated close to the target surface only for a very brief period of time (10–100 ns), the ion velocity can be approximated as the target-probe distance d divided by the ion timeof-flight t. Thus the ion energy distribution dN/dE is given by [12, 23]:
using (a) ns-PLD and (b) fs-PLD. The dotted curve is the best fit to experimental data.
I ( t) t 3 dN = , dE Amed 2
2. Experimental methods
(1)
where I(t) is the time-dependent ion current, m is the ion mass, e is electronic charge and A is the probe area. The energy distributions corresponding to ns- and fs-ablation are shown in figures 1(b) and (d) respectively. It can be seen that the energy distribution extends up to ∼100 eV, with average ion energy of 31 eV for ns-ablation and 38 eV for fs-ablation. Thus it can be seen that PLD is an energetic deposition process whereby the more energetic ions may cause some self-sputtering of deposited films [24, 25]. The deposition rates per laser pulse, measured using a QCM, were 1.7 × 10−4 nm and 4.2 × 10−4 nm for ns- and fs-PLD respectively, which correspond to 9.9 × 1011 and 2.4 × 1012 atoms cm−2, respectively. By integrating the ion signals and scaling for substrate position, the ion fluences per laser pulse were found to be 6.6 × 1011 ions cm−2 for ns-PLD where the substrate was at
The NP films were prepared by laser ablation of a fully-dense Ag target (99.99% purity) using a 248 nm, 25 ns KrF excimer laser and a 800 nm, 130 fs Ti:sapphire laser, both operating at 10 Hz in high vacuum (2 × 10−5 mbar). Typically, the energy of the ns laser was ∼30 mJ per pulse and was imaged to a 2.5 mm × 1.5 mm spot on the target with nearly uniform fluence of ∼0.8 J cm−2. For fs-ablation the laser pulse energy was ∼0.4 mJ per pulse and the beam was focussed with a 20 cm focal length plano-convex lens. The fluence distribution on the target was found by measuring the energy dependence of the ablation craters on Si, using the method described by Liu [19]. The fluence distribution could be approximated as an elliptical Gaussian beam with 1/e2 radii of 145 μm and 225 μm and a peak fluence of ∼0.8 J cm−2. The target was continuously rotated in order to avoid drilling a 3
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Figure 3. STEM images (a)–(d) and corresponding Feret diameter distributions (e)–(h) of 1, 3, 5 and 7 nm equivalent thickness Ag films
prepared using ns-PLD in vacuum. The dotted curve is a log-normal fit, and σ is the standard deviation of the logarithm of the diameter.
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Figure 4. Equivalent thickness variation of (a) mean Feret diameter and (b) areal density and surface coverage of Ag NPs prepared by
ns-PLD.
8 cm, and 2.6 × 1010 ions cm−2 for fs-PLD where the substrate was at 6 cm. Comparison of the ion and the atom fluences shows that the ns-ablation plume is nearly fully ionized, while in the fs case the ion fluence is about 1% of the net deposition. Therefore it can be seen that the ablation plume in fs-PLD contains a small fraction of ions compared to neutral atoms and NPs [26]. It is of interest to compare the deposition energy efficiency of ns- and fs-PLD in terms of atoms deposited per unit area per pulse per mJ of laser energy. For ns-PLD the deposition efficiency at 8 cm was 3.3 × 1010 atoms cm−2 mJ−1. Scaling the fs-PLD rate measured at 6 cm to find the value that would be obtained at 8 cm, gives a deposition energy efficiency value of 3.4 × 1012 atoms cm−2 mJ−1 for fs-PLD. Thus it can be seen that fs-PLD of silver NPs is approximately 100 times more energy efficient than ns-PLD. This observation is consistent with the recent report by Toftmann et al [27] where it was shown that the fs ablation of Ag at 2 J cm−2 is 25 times more energy efficient than ns ablation. There are several factors which contribute to the striking difference in the energy efficiencies of ns- and fs-ablation. In ns-ablation evaporation commences during the laser pulse and a significant part of the laser energy is absorbed in the vapour above the target surface leading to ionization and the relatively high ion production efficiency. In fs-ablation there is no laser interaction with ablated material. Most of the material is removed by nanofragmentation of superheated material [28], which is a relatively energy-efficient process. Depending on the laser spot size, there may also be some difference in the extent to which the ablation plume is forward-directed in the two ablation regimes. The deposition rates, ion fluences and deposition energy efficiency values for ns- and fs-PLD are summarized in table 1.
for a ns-PLD film deposited over 66 min at 10 Hz where the laser fluence was ∼0.8 J cm−2 and the target-substrate distance was 8 cm. The best fit was obtained for film thickness = 9.8 nm, density = 7.0 g cm−3. The fs-PLD film used for XRR was deposited for 27.5 min at 10 Hz with similar laser fluence and a substrate-target distance of 6 cm. The XRR curve (figure 2(b)) was fitted using thickness = 10.0 nm, density = 7.8 g cm−3. It can be seen that the average density is about 66% for the ns-PLD film, and 74% for the fs-PLD film, of the bulk value (10.5 g cm−3 for Ag), indicating the porous nature of the deposited films [30]. Thus, the equivalent soliddensity thickness was ∼6.6 nm for ns-PLD film and ∼7.4 nm for fs-PLD film. There was quite good agreement regarding the amount of deposited material as measured by the QCM and by XRR. For the film produced by fs-PLD, the QCM gave the equivalent solid-density thickness as 7 nm, while fitting the XRR yielded a value of 7.4 nm. 3.3. Microscopy
Figures 3 (a)–(d) show STEM images of ns-PLD Ag NP films deposited on carbon-coated TEM grids, where the equivalent solid-density thicknesses are 1, 3, 5 and 7 nm. The NP micrographs were analysed to obtain the Feret diameter distributions, which are shown in figures 3 (e)–(h). The Feret diameter is the longest distance between any two points on the boundary of the selected NP. These distributions were fitted with a log-normal distribution function (equation (2)) [31] to find the mean NP diameter. 2
ln( x / μ) A f ( x) = e− 2σ 2 , xσ 2π
(2)
where f(x) is the frequency of NPs with Feret diameter x, A is a normalization constant, μ is the geometrical mean diameter, and σ is the standard deviation of the logarithm of the diameter. In the 1 nm film (figures 3(a), (e)), the NPs are wellisolated and circular in appearance with an average size of 5.4 nm. In the 3 nm film (figures 3(b), (f)) the average size has increased to 13.2 nm, and some of the particles are elongated due to the onset of coalescence. At 5 nm the coalescence is well developed, the NPs are quite elongated and the average
3.2. X-ray reflectivity (XRR) analysis of NP films
The thickness and porosity of ns- and fs-PLD Ag NP films were determined by fitting the measured XRR using the LEPTOS software [29]. The samples used for XRR were deposited on fused quartz and the reflectivity was measured for 2θ in the range 0°–6°. Figure 2(a) shows the XRR curve 5
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Figure 5. STEM images (a)–(d) and corresponding size distributions (e)–(h) of 1, 3, 5 and 7 nm equivalent thickness Ag films prepared using
fs-PLD in vacuum. The dotted curve is a log-normal fit, and σ is the standard deviation of the logarithm of the diameter.
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Figure 6. Equivalent thickness variation of (a) mean Feret diameter and (b) areal density and surface coverage of Ag NPs prepared by
fs-PLD.
Figure 7. Mean maximum and minimum NP Feret diameters versus equivalent thickness for (a) ns-PLD and (b) fs-PLD films.
Feret diameter is 32 nm. At 7 nm (figures 3(d), (h)) the elongation of the NPs is further developed and it appears that a percolated nanostructure is formed. Figure 4(a) shows how the mean Feret diameter increases with the equivalent thickness of the deposition and figure 4(b) shows the variation of the areal density of NPs and the surface coverage. Lattice images acquired in TEM analysis showed that some NPs are single crystal, while most are multiply-twinned. It seems that ns-PLD of metal NPs is rather similar to thermal evaporation and sputtering, where the NPs grow by surface diffusion of adatoms [32, 33]. For metal deposition on non-wetting substrates, such as quartz or glass, the early stage of thin film growth proceeds though several distinct regimes. In the initial stage, the film consists of compact and isolated nanoscale islands; as more material is deposited the islands grow, coalesce, become elongated and eventually percolate. It seems that the critical thickness for percolation is 5–7 nm. This is similar to the percolation thickness found in filtered cathodic vacuum arc (FCVA) deposition [34] and distinctly less than the value of 10–14 nm for deposition by magnetron sputtering. It is not surprising that the percolation thickness is similar for ns-PLD and FCVA deposition, since they are both energetic deposition processes; the ion energy extends up to
about 90 eV in the ns-PLD deposition reported here and was 105 eV for the FCVA deposition in [34]. Figures 5(a)–(d) show STEM images for 1, 3, 5 and 7 nm equivalent thickness Ag thin films deposited using fs-PLD; the corresponding Feret diameter distributions are shown in figures 5(e)–(h). As for ns-PLD, NP size increases with equivalent thickness, though for the same equivalent thickness the mean NP size in fs-PLD films is substantially smaller. Figure 6(a) shows that the mean Feret diameter increases from 3 nm to about 11 nm as the equivalent thickness increases from 1 nm to 7 nm. Figure 6(b) shows the thickness dependence of the NP areal density and that the surface coverage depends on thickness. The steady decrease of the areal density of NPs with thickness shows that particle coalescence, and thus material diffusion, is a feature of fsPLD of Ag, and is very different when compared to fs-PLD of Ni [35], where the NPs formed in the ablation process accumulate as a loosely packed film without coalescing. We also observe a small number of mesoscale particles having diameters greater than ∼15 nm (e.g. figure 5(b)), which were not included in the size distribution histogram. The mesoscale particles have also been observed before in fs-PLD of Ni NP 7
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1 nm to 5 nm, and then decreases to a value of 5 nm in the 7 nm film. The interparticle gap is important in applications such as SERS, since it is in the narrow gaps that the electric field of the excitation laser is most amplified and where the SERS signal mainly originates [36]. The physical process giving rise to the unexpected morphology of the fs-PLD NP films is not clear at this stage, but the observations indicate that new NPs arriving in the ablation plume have sufficient surface mobility to reach NPs already formed. This surface mobility may be related to the temperature of the NPs in the ablation plume which are known to have temperatures in the range 1000–3000 K [37]. 3.4. Optical absorption spectra
Figure 8 shows the thickness variation of the optical absorbance spectra for Ag NP films prepared on fused quartz substrates placed beside the TEM grids. The SPR peak is clearly visible in both the ns- and fs-PLD films. The magnitude of the absorbance increases, and the peak absorbance shifts to longer wavelength, as the equivalent solid-density thickness is increased, which is similar to previous reports of ns-PLD of Ag NP films [12]. The red shift and the broadening of the SPR peak with increasing film thickness is widely discussed in the literature [38, 39]. It seems that both dipole–dipole interaction between neighbouring NPs, and electron scattering at the NP surface, can contribute to this behaviour. The electromagnetic interaction between NPs scales as
Figure 8. Optical absorption spectra of Ag NP films with equivalent
2L + 1
( r R)
, where r is radius of the particle, R is the separation between the particles and L is the multipole order of the interaction; L = 1 for dipole, L = 2 for quadrupole, etc [40, 41]. For the fs-PLD films the value of r/R increases from 0.2 in the 1 nm equivalent thickness film to 0.35 in the 7 nm film; thus the observed red-shift of the SPR is consistent with increasing dipole–dipole interaction between NPs as the equivalent thickness is increased. It can also be noted that the SPR absorption band is narrower for fs-PLD films compared to nsPLD films, which can be related to the narrower size distributions observed in fs-PLD films (see figures 3 and 5).
thickness in the range 1–7 nm prepared by (a) ns-PLD and (b) fs-PLD.
films [16], and it was found that the areal density of mesoscale particles increases with increasing laser fluence. Visual inspection of the STEM images in figure 3 shows that the onset of coalescence in ns-PLD causes the NPs to become somewhat elongated, while in fs-PLD (figure 5) the NPs remain rather circular throughout the range of equivalent thicknesses explored. This difference in behaviour is revealed in a more quantitative way by analysing the STEM images to find the mean values of the maximum and minimum Feret diameters. The variation of these mean values with equivalent thickness is plotted in figure 7. In ns-PLD the mean maximum and minimum Feret diameters begin to diverge at ∼3 nm, and differ by a factor of about 2 at 7 nm. For the fs-PLD films the maximum and minimum diameters are comparable from 1 to 7 nm equivalent thickness, indicating the near circularity of the NPs. If we idealize the geometry of the NP films as uniformlysized NPs of radius r, sitting on a hexagonal lattice with separation R, then the areal density of NPs is 1.15/R2, the surface coverage is 3.6 r2/R2 and the interparticle gap is R−2r. The value of the NP diameter, 2r can be derived from the measured values of the areal density and surface coverage, and quite good agreement is obtained with the measured mean Feret diameter. It is then found that the interparticle gap size is lower for the fs-PLD films. In the fs-PLD films it increases from 5 nm to 7 nm as the thickness is increased from
4. Conclusion In conclusion, we have used ns- and fs-PLD to make Ag NP films on fused quartz with equivalent solid-density thicknesses of 1–7 nm. For equivalent thickness up to ∼3 nm the NPs made by ns-PLD are well separated and roughly circular, but for higher thicknesses the NPs begin to coalesce, and by 7 nm appear to have percolated. Over the whole thickness range explored, the fs-PLD films are comprised of well separated NPs, though the mean NP size and the surface coverage increase with equivalent thickness. The physical reasons underlying the difference in NP growth in the two cases are not clear at this stage, but are most likely related to the fact that the ns plume is nearly fully ionized plasma while in fs-PLD most of the material is ablated as NPs and only a small fraction is plasma. Further investigation is required to 8
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fully understand the NP formation for these two PLD techniques, and it seems that molecular dynamics simulation may be a fruitful approach to take. The SPR band is somewhat narrower for the fs-PLD films, which may be related to the narrower NP size distribution.
[15] Amoruso S, Bruzzese R, Wang X, Nedialkov N N and Atanasov P A 2007 Femtosecond laser ablation of nickel in vacuum J. Phys. D: Appl. Phys. 40 331 [16] Liu B, Hu Z, Che Y, Chen Y and Pan X 2007 Nanoparticle generation in ultrafast pulsed laser ablation of nickel Appl. Phys. Lett. 90 044103 [17] Chakravarty U, Naik P A, Mukherjee C, Kumbhare S R and Gupta P D 2010 Formation of metal nanoparticles of various sizes in plasma plumes produced by Ti:sapphire laser pulses J. Appl. Phys. 108 053107 [18] De Bonis A, Galasso A, Ibris N, Sansone M, Santagata A and Teghil R 2012 Ultra-short pulsed laser deposition of thin silver films for surface enhanced Raman scattering Surf. Coat. Technol. 207 279–85 [19] Liu J M 1982 Simple technique for measurements of pulsed Gaussian-beam spot sizes Opt. Lett. 7 196–8 [20] Doggett B and Lunney J G 2009 Langmuir probe characterization of laser ablation plasmas J. Appl. Phys. 105 033306 [21] Toftmann B, Schou J, Hansen T N and Lunney J G 2000 Angular distribution of electron temperature and density in a laser-ablation plume Phys. Rev. Lett. 84 3998 [22] Doggett B and Lunney J G 2011 Expansion dynamics of laser produced plasma J. Appl. Phys. 109 093304-10 [23] Franghiadakis Y, Fotakis C and Tzanetakis P 1999 Energy distribution of ions produced by excimer-laser ablation of solid and molten targets Appl. Phys. A: Mater. Sci. Process. 68 391–7 [24] Fahler S, Sturm K and Krebs H U 1999 Resputtering during the growth of pulsed-laser-deposited metallic films in vacuum and in an ambient gas Appl. Phys. Lett. 75 3766–8 [25] Jordan R, Cole D, Lunney J G, Mackay K and Givord D 1995 Pulsed laser ablation of copper Appl. Surf. Sci. 86 24–8 [26] Donnelly T, Lunney J G, Amoruso S, Bruzzese R, Wang X and Ni X 2010 Dynamics of the plumes produced by ultrafast laser ablation of metals J. Appl. Phys. 108 043309-13 [27] Toftmann B, Doggett B, Budtz-Jørgensen C, Schou J and Lunney J G 2013 Femtosecond ultraviolet laser ablation of silver and comparison with nanosecond ablation J. Appl. Phys. 113 083304-11 [28] Perez D and Lewis J L 2002 Ablation of solids under femtosecond laser pulses Phys. Rev. Lett. 89 255504 [29] www.bruker-axs.com/stress.html [30] Banerjee S, Mukherjee S and Kundu S 2001 Structural study and fabrication of nano-pattern on ultra thin film of Ag grown by magnetron sputtering J. Phys. D: Appl. Phys. 34 L87 [31] Limpert E, Stahel W A and Abbt M 2001 Log-normal distributions across the sciences: keys and clues BioScience 51 341–52 [32] Yu X, Duxbury P M, Jeffers G and Dubson M A 1991 Coalescence and percolation in thin metal films Phys. Rev. B 44 13163–6 [33] Ruffino F and Grimaldi M G 2010 Island-to-percolation transition during the room-temperature growth of sputtered nanoscale Pd films on hexagonal SiC J. Appl. Phys. 107 074301-6 [34] Eungsun B, Thomas W H O and Andre A 2003 Coalescence of nanometer silver islands on oxides grown by filtered cathodic arc deposition Appl. Phys. Lett. 82 1634–6 [35] Amoruso S, Ausanio G, Bruzzese R, Lanotte L, Scardi P, Vitiello M and Wang X 2006 Synthesis of nanocrystal films via femtosecond laser ablation in vacuum J. Phys.: Condens. Matter 18 L49 [36] Pavaskar P, Hsu I K, Theiss J, Hsuan Hung W and Cronin S B 2013 A microscopic study of strongly plasmonic Au and Ag island thin films J. Appl. Phys. 113 034302-6
Acknowledgements This work was supported by Science Foundation Ireland under grant 09/RFP/PHY2422. I Mirza was supported by a postgraduate studentship funded by Trinity College Dublin and the Higher Educational Authority in Ireland. The STEM analysis was done in the Advanced Microscopy Laboratory at Trinity College Dublin.
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Comparison of nanosecond and femtosecond pulsed laser deposition of silver nanoparticle films
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 265301 (http://iopscience.iop.org/0957-4484/25/26/265301) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 137.207.120.173 This content was downloaded on 16/07/2014 at 12:02
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 25 (2014) 265301 (10pp)
doi:10.1088/0957-4484/25/26/265301
Comparison of nanosecond and femtosecond pulsed laser deposition of silver nanoparticle films I Mirza, G O’Connell, J J Wang and J G Lunney School of Physics and CRANN, Trinity College Dublin, Dublin 2, Ireland E-mail: [email protected] Received 29 September 2013, revised 30 April 2014 Accepted for publication 8 May 2014 Published 11 June 2014 Abstract
Nanoparticle (NP) films of silver were prepared using nanosecond (ns) and femtosecond (fs) pulsed laser deposition (PLD) in vacuum. The flux and energy distribution of the ions in the plasma part of the ablation plume were measured using a Langmuir ion probe. The deposition energy efficiencies of ns and fs silver PLD were also compared. For equivalent thickness up to ∼3 nm the NPs made by ns-PLD are well separated and roughly circular, but for higher thicknesses the NPs begin to coalesce. For equivalent thickness up to 7 nm the fs films are comprised of well separated NPs, though the mean NP size and the surface coverage increase with equivalent thickness. The mean Feret diameter for both ns- and fs-PLD films increases with increasing equivalent solid-density thickness. The surface plasmon resonance peak was observed to red shift for both ns- and fs-PLD films as the equivalent solid-density thickness was increased from 1 nm to 7 nm. Keywords: pulsed laser deposition, nanoparticle Ag films, surface plasmon resonance, laser ablation (Some figures may appear in colour only in the online journal) 1. Introduction
This ablated material expands rapidly away from the target and can be captured on a solid substrate. The process can be performed in vacuum or in a background gas environment. In ns-PLD of NPs in vacuum it seems that the NP growth takes place on the substrate surface by surface diffusion of material which is condensed from the vapour, or plasma, phase [10–12]. Dolbec et al [10] have studied ns-PLD of Pt on highly-oriented pyrolytic graphite and have shown that the mean NP size increases as the areal density, or equivalent solid-density thickness (areal density divided by solid density), is increased. Afonso et al [11] have prepared nanocomposite films of Cu and alumina in vacuum and background gas environments. D’Andrea et al [5] and Smyth et al [13] have investigated the formation and deposition of Ag NPs using ns laser ablation in background gas and vacuum respectively, and demonstrated the utility of the films produced for surface enhanced Raman spectroscopy (SERS). In femtosecond (fs) PLD (fs-PLD) of metals the ablation mechanism is very different from the ns case. In the first
Nanostructured materials and surfaces show interesting optical, electronic, magnetic and catalytic properties which arise when the dimensions of matter are reduced to the nanoscale (1–100 nm) [1–3]. Nanoparticle (NP) films of noble metals are of particular interest since they display a surface plasmon resonance (SPR) in the visible region, which is used to enhance the sensitivity of fluorescence [4] and Raman spectroscopy [5]. The catalytic properties of metal NP films are of interest for the growth of carbon nanotubes [6] and nanorods of various materials [7]. There is a wide range of physical and chemical techniques used for NP film deposition [8, 9]. Among the physical vapour methods, pulsed laser deposition (PLD) is a simple technique that can, in principle, be applied to all solid materials. In conventional PLD a nanosecond (ns) pulsed laser is focused to sufficiently high fluence on the target surface to evaporate a small amount of material from the target surface. 0957-4484/14/265301+10$33.00
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Figure 1. Ion time-of-flight signals for: (a) ns-ablation at 5.8 cm, and (c) fs-laser ablation at 4.7 cm. The corresponding ion energy
distributions are given in (b) and (d); the dotted lines indicate average ion energies.
Table 1. Deposition rate, ion fluence and deposition energy efficiency for ns- and fs-PLD of silver at 0.8 J cm−2.
ns-PLD −2
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Deposition rate (atoms cm ) Ion fluence (ions cm−2) Deposition efficiency (atoms cm−2 mJ−1)
9.9 × 10 (at 8 cm) 6.0 × 1011 (at 8 cm) 3.3 × 1010 (at 8 cm)
fs-PLD 2.4 × 1012 (at 6 cm) 2.6 × 1010 (at 6 cm) 3.4 × 1012 (at 8 cm)
emission was observed at laser fluence higher than ∼8 J cm−2. Chakravarty et al [17] have compared the formation of Ag and Cu NP films on a Si substrate using ps- and fs-PLD in vacuum. They observed that NPs with smaller mean diameter could be generated using fs-PLD as compared to ps-PLD. In another experiment, De Bonis et al [18] used fs-PLD to fabricate Ag NP films on solid substrates for SERS. Using transmission electron microscopy (TEM), they found that the ablation behaviour and NP deposits were quite similar to that found in ns-PLD, indicating that the secondary NP plume, normally observed in fs ablation of metals, may not play an important role in fs-PLD of Ag NP films. In this paper, we present the results of a systematic comparison of ns- and fs-PLD of Ag NP films at 0.8 J cm−2. We have measured the ionization fractions and deposition efficiencies of ns- and fs-ablation. While the ns ablation plume is nearly fully ionized, in fs ablation the ion fraction in the ablation is only ∼1%. We have observed how the NP
stage, the laser energy is absorbed by the free electrons near the metal surface. The electron system thermalises on a timescale of ≈ few tens of fs. On a longer timescale, of 10–20 ps [14], the lattice is heated by electron–phonon relaxation. At the end of this stage a thin layer near the metal surface may be heated to temperatures above the critical temperature but still have a density close to solid density. Using fast photography and optical spectroscopy, it has been shown that the dispersal of this superheated material results in two distinct ablation plumes; the first is composed of partially ionized vapour moving at ∼106 cm s−1, and the second is a plume of hot NPs moving about 100 times slower. These general features are also found in molecular dynamics simulations, where the NPs are seen to be formed by fragmentation and phase explosion [15]. Liu et al [16] have reported that fsPLD of Ni in vacuum leads to polycrystalline NPs with an average diameter less than 10 nm and the NP size is nearly independent of laser fluence. However, large droplet-like NP 2
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hole into the target. The ablated material was deposited on 5 mm × 5 mm × 1 mm thick quartz substrates and carboncoated TEM grids. The target-to-substrate distance was 8 cm for ns-PLD and 6 cm for fs-PLD. A planar Langmuir ion probe of area 0.09 cm2 and biased at −30 V was positioned on the target normal and facing the ablation spot to record the ion time-of-flight signal, from which the ion energy distribution and ion fluence were derived [20, 21]. An ion probe is a sensitive device for gauging the reproducibility of the ablation process. The target-to-probe distance was 5.8 cm for ns ablation and 4.7 cm for fs ablation. A quartz crystal monitor (QCM) was used to measure the equivalent solid density thickness of the deposit, which is obtained by dividing the areal density of the film by the density of solid Ag. The targetto-QCM distance was 6.5 cm for ns-PLD and 4.6 cm for fsPLD. Since the ablation plume expansion is inertial beyond a few mm from the target surface [22], ion probe and deposition measurements made at different distances can be scaled to a fixed position using a 1/(distance)2 scaling. The morphology of deposited films was measured using scanning transmission electron microscopy (STEM) operating at 25 kV. Grazingangle x-ray reflectometry was used to measure the overall thickness and density of the NP films. The optical absorption of the NP films was measured in the 300–800 nm range using a Cary-50 UV-visible spectrophotometer.
3. Results and discussion Figure 2. XRR curves for Ag film deposited on quartz substrate
3.1. Laser ablation plume analysis
morphology and the optical absorption depend on the equivalent solid-density thickness of the deposit. Similar to the observation of De Bonis et al [18], the mean NP size in fsPLD films was observed to increase with equivalent soliddensity thickness.
Figure 1 shows the ion probe signals for (a) ns- and (c) fslaser ablation, where time is measured from the arrival of the laser pulse on the target. Since the plasma plume is mainly accelerated close to the target surface only for a very brief period of time (10–100 ns), the ion velocity can be approximated as the target-probe distance d divided by the ion timeof-flight t. Thus the ion energy distribution dN/dE is given by [12, 23]:
using (a) ns-PLD and (b) fs-PLD. The dotted curve is the best fit to experimental data.
I ( t) t 3 dN = , dE Amed 2
2. Experimental methods
(1)
where I(t) is the time-dependent ion current, m is the ion mass, e is electronic charge and A is the probe area. The energy distributions corresponding to ns- and fs-ablation are shown in figures 1(b) and (d) respectively. It can be seen that the energy distribution extends up to ∼100 eV, with average ion energy of 31 eV for ns-ablation and 38 eV for fs-ablation. Thus it can be seen that PLD is an energetic deposition process whereby the more energetic ions may cause some self-sputtering of deposited films [24, 25]. The deposition rates per laser pulse, measured using a QCM, were 1.7 × 10−4 nm and 4.2 × 10−4 nm for ns- and fs-PLD respectively, which correspond to 9.9 × 1011 and 2.4 × 1012 atoms cm−2, respectively. By integrating the ion signals and scaling for substrate position, the ion fluences per laser pulse were found to be 6.6 × 1011 ions cm−2 for ns-PLD where the substrate was at
The NP films were prepared by laser ablation of a fully-dense Ag target (99.99% purity) using a 248 nm, 25 ns KrF excimer laser and a 800 nm, 130 fs Ti:sapphire laser, both operating at 10 Hz in high vacuum (2 × 10−5 mbar). Typically, the energy of the ns laser was ∼30 mJ per pulse and was imaged to a 2.5 mm × 1.5 mm spot on the target with nearly uniform fluence of ∼0.8 J cm−2. For fs-ablation the laser pulse energy was ∼0.4 mJ per pulse and the beam was focussed with a 20 cm focal length plano-convex lens. The fluence distribution on the target was found by measuring the energy dependence of the ablation craters on Si, using the method described by Liu [19]. The fluence distribution could be approximated as an elliptical Gaussian beam with 1/e2 radii of 145 μm and 225 μm and a peak fluence of ∼0.8 J cm−2. The target was continuously rotated in order to avoid drilling a 3
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Figure 3. STEM images (a)–(d) and corresponding Feret diameter distributions (e)–(h) of 1, 3, 5 and 7 nm equivalent thickness Ag films
prepared using ns-PLD in vacuum. The dotted curve is a log-normal fit, and σ is the standard deviation of the logarithm of the diameter.
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Figure 4. Equivalent thickness variation of (a) mean Feret diameter and (b) areal density and surface coverage of Ag NPs prepared by
ns-PLD.
8 cm, and 2.6 × 1010 ions cm−2 for fs-PLD where the substrate was at 6 cm. Comparison of the ion and the atom fluences shows that the ns-ablation plume is nearly fully ionized, while in the fs case the ion fluence is about 1% of the net deposition. Therefore it can be seen that the ablation plume in fs-PLD contains a small fraction of ions compared to neutral atoms and NPs [26]. It is of interest to compare the deposition energy efficiency of ns- and fs-PLD in terms of atoms deposited per unit area per pulse per mJ of laser energy. For ns-PLD the deposition efficiency at 8 cm was 3.3 × 1010 atoms cm−2 mJ−1. Scaling the fs-PLD rate measured at 6 cm to find the value that would be obtained at 8 cm, gives a deposition energy efficiency value of 3.4 × 1012 atoms cm−2 mJ−1 for fs-PLD. Thus it can be seen that fs-PLD of silver NPs is approximately 100 times more energy efficient than ns-PLD. This observation is consistent with the recent report by Toftmann et al [27] where it was shown that the fs ablation of Ag at 2 J cm−2 is 25 times more energy efficient than ns ablation. There are several factors which contribute to the striking difference in the energy efficiencies of ns- and fs-ablation. In ns-ablation evaporation commences during the laser pulse and a significant part of the laser energy is absorbed in the vapour above the target surface leading to ionization and the relatively high ion production efficiency. In fs-ablation there is no laser interaction with ablated material. Most of the material is removed by nanofragmentation of superheated material [28], which is a relatively energy-efficient process. Depending on the laser spot size, there may also be some difference in the extent to which the ablation plume is forward-directed in the two ablation regimes. The deposition rates, ion fluences and deposition energy efficiency values for ns- and fs-PLD are summarized in table 1.
for a ns-PLD film deposited over 66 min at 10 Hz where the laser fluence was ∼0.8 J cm−2 and the target-substrate distance was 8 cm. The best fit was obtained for film thickness = 9.8 nm, density = 7.0 g cm−3. The fs-PLD film used for XRR was deposited for 27.5 min at 10 Hz with similar laser fluence and a substrate-target distance of 6 cm. The XRR curve (figure 2(b)) was fitted using thickness = 10.0 nm, density = 7.8 g cm−3. It can be seen that the average density is about 66% for the ns-PLD film, and 74% for the fs-PLD film, of the bulk value (10.5 g cm−3 for Ag), indicating the porous nature of the deposited films [30]. Thus, the equivalent soliddensity thickness was ∼6.6 nm for ns-PLD film and ∼7.4 nm for fs-PLD film. There was quite good agreement regarding the amount of deposited material as measured by the QCM and by XRR. For the film produced by fs-PLD, the QCM gave the equivalent solid-density thickness as 7 nm, while fitting the XRR yielded a value of 7.4 nm. 3.3. Microscopy
Figures 3 (a)–(d) show STEM images of ns-PLD Ag NP films deposited on carbon-coated TEM grids, where the equivalent solid-density thicknesses are 1, 3, 5 and 7 nm. The NP micrographs were analysed to obtain the Feret diameter distributions, which are shown in figures 3 (e)–(h). The Feret diameter is the longest distance between any two points on the boundary of the selected NP. These distributions were fitted with a log-normal distribution function (equation (2)) [31] to find the mean NP diameter. 2
ln( x / μ) A f ( x) = e− 2σ 2 , xσ 2π
(2)
where f(x) is the frequency of NPs with Feret diameter x, A is a normalization constant, μ is the geometrical mean diameter, and σ is the standard deviation of the logarithm of the diameter. In the 1 nm film (figures 3(a), (e)), the NPs are wellisolated and circular in appearance with an average size of 5.4 nm. In the 3 nm film (figures 3(b), (f)) the average size has increased to 13.2 nm, and some of the particles are elongated due to the onset of coalescence. At 5 nm the coalescence is well developed, the NPs are quite elongated and the average
3.2. X-ray reflectivity (XRR) analysis of NP films
The thickness and porosity of ns- and fs-PLD Ag NP films were determined by fitting the measured XRR using the LEPTOS software [29]. The samples used for XRR were deposited on fused quartz and the reflectivity was measured for 2θ in the range 0°–6°. Figure 2(a) shows the XRR curve 5
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Figure 5. STEM images (a)–(d) and corresponding size distributions (e)–(h) of 1, 3, 5 and 7 nm equivalent thickness Ag films prepared using
fs-PLD in vacuum. The dotted curve is a log-normal fit, and σ is the standard deviation of the logarithm of the diameter.
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Figure 6. Equivalent thickness variation of (a) mean Feret diameter and (b) areal density and surface coverage of Ag NPs prepared by
fs-PLD.
Figure 7. Mean maximum and minimum NP Feret diameters versus equivalent thickness for (a) ns-PLD and (b) fs-PLD films.
Feret diameter is 32 nm. At 7 nm (figures 3(d), (h)) the elongation of the NPs is further developed and it appears that a percolated nanostructure is formed. Figure 4(a) shows how the mean Feret diameter increases with the equivalent thickness of the deposition and figure 4(b) shows the variation of the areal density of NPs and the surface coverage. Lattice images acquired in TEM analysis showed that some NPs are single crystal, while most are multiply-twinned. It seems that ns-PLD of metal NPs is rather similar to thermal evaporation and sputtering, where the NPs grow by surface diffusion of adatoms [32, 33]. For metal deposition on non-wetting substrates, such as quartz or glass, the early stage of thin film growth proceeds though several distinct regimes. In the initial stage, the film consists of compact and isolated nanoscale islands; as more material is deposited the islands grow, coalesce, become elongated and eventually percolate. It seems that the critical thickness for percolation is 5–7 nm. This is similar to the percolation thickness found in filtered cathodic vacuum arc (FCVA) deposition [34] and distinctly less than the value of 10–14 nm for deposition by magnetron sputtering. It is not surprising that the percolation thickness is similar for ns-PLD and FCVA deposition, since they are both energetic deposition processes; the ion energy extends up to
about 90 eV in the ns-PLD deposition reported here and was 105 eV for the FCVA deposition in [34]. Figures 5(a)–(d) show STEM images for 1, 3, 5 and 7 nm equivalent thickness Ag thin films deposited using fs-PLD; the corresponding Feret diameter distributions are shown in figures 5(e)–(h). As for ns-PLD, NP size increases with equivalent thickness, though for the same equivalent thickness the mean NP size in fs-PLD films is substantially smaller. Figure 6(a) shows that the mean Feret diameter increases from 3 nm to about 11 nm as the equivalent thickness increases from 1 nm to 7 nm. Figure 6(b) shows the thickness dependence of the NP areal density and that the surface coverage depends on thickness. The steady decrease of the areal density of NPs with thickness shows that particle coalescence, and thus material diffusion, is a feature of fsPLD of Ag, and is very different when compared to fs-PLD of Ni [35], where the NPs formed in the ablation process accumulate as a loosely packed film without coalescing. We also observe a small number of mesoscale particles having diameters greater than ∼15 nm (e.g. figure 5(b)), which were not included in the size distribution histogram. The mesoscale particles have also been observed before in fs-PLD of Ni NP 7
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1 nm to 5 nm, and then decreases to a value of 5 nm in the 7 nm film. The interparticle gap is important in applications such as SERS, since it is in the narrow gaps that the electric field of the excitation laser is most amplified and where the SERS signal mainly originates [36]. The physical process giving rise to the unexpected morphology of the fs-PLD NP films is not clear at this stage, but the observations indicate that new NPs arriving in the ablation plume have sufficient surface mobility to reach NPs already formed. This surface mobility may be related to the temperature of the NPs in the ablation plume which are known to have temperatures in the range 1000–3000 K [37]. 3.4. Optical absorption spectra
Figure 8 shows the thickness variation of the optical absorbance spectra for Ag NP films prepared on fused quartz substrates placed beside the TEM grids. The SPR peak is clearly visible in both the ns- and fs-PLD films. The magnitude of the absorbance increases, and the peak absorbance shifts to longer wavelength, as the equivalent solid-density thickness is increased, which is similar to previous reports of ns-PLD of Ag NP films [12]. The red shift and the broadening of the SPR peak with increasing film thickness is widely discussed in the literature [38, 39]. It seems that both dipole–dipole interaction between neighbouring NPs, and electron scattering at the NP surface, can contribute to this behaviour. The electromagnetic interaction between NPs scales as
Figure 8. Optical absorption spectra of Ag NP films with equivalent
2L + 1
( r R)
, where r is radius of the particle, R is the separation between the particles and L is the multipole order of the interaction; L = 1 for dipole, L = 2 for quadrupole, etc [40, 41]. For the fs-PLD films the value of r/R increases from 0.2 in the 1 nm equivalent thickness film to 0.35 in the 7 nm film; thus the observed red-shift of the SPR is consistent with increasing dipole–dipole interaction between NPs as the equivalent thickness is increased. It can also be noted that the SPR absorption band is narrower for fs-PLD films compared to nsPLD films, which can be related to the narrower size distributions observed in fs-PLD films (see figures 3 and 5).
thickness in the range 1–7 nm prepared by (a) ns-PLD and (b) fs-PLD.
films [16], and it was found that the areal density of mesoscale particles increases with increasing laser fluence. Visual inspection of the STEM images in figure 3 shows that the onset of coalescence in ns-PLD causes the NPs to become somewhat elongated, while in fs-PLD (figure 5) the NPs remain rather circular throughout the range of equivalent thicknesses explored. This difference in behaviour is revealed in a more quantitative way by analysing the STEM images to find the mean values of the maximum and minimum Feret diameters. The variation of these mean values with equivalent thickness is plotted in figure 7. In ns-PLD the mean maximum and minimum Feret diameters begin to diverge at ∼3 nm, and differ by a factor of about 2 at 7 nm. For the fs-PLD films the maximum and minimum diameters are comparable from 1 to 7 nm equivalent thickness, indicating the near circularity of the NPs. If we idealize the geometry of the NP films as uniformlysized NPs of radius r, sitting on a hexagonal lattice with separation R, then the areal density of NPs is 1.15/R2, the surface coverage is 3.6 r2/R2 and the interparticle gap is R−2r. The value of the NP diameter, 2r can be derived from the measured values of the areal density and surface coverage, and quite good agreement is obtained with the measured mean Feret diameter. It is then found that the interparticle gap size is lower for the fs-PLD films. In the fs-PLD films it increases from 5 nm to 7 nm as the thickness is increased from
4. Conclusion In conclusion, we have used ns- and fs-PLD to make Ag NP films on fused quartz with equivalent solid-density thicknesses of 1–7 nm. For equivalent thickness up to ∼3 nm the NPs made by ns-PLD are well separated and roughly circular, but for higher thicknesses the NPs begin to coalesce, and by 7 nm appear to have percolated. Over the whole thickness range explored, the fs-PLD films are comprised of well separated NPs, though the mean NP size and the surface coverage increase with equivalent thickness. The physical reasons underlying the difference in NP growth in the two cases are not clear at this stage, but are most likely related to the fact that the ns plume is nearly fully ionized plasma while in fs-PLD most of the material is ablated as NPs and only a small fraction is plasma. Further investigation is required to 8
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fully understand the NP formation for these two PLD techniques, and it seems that molecular dynamics simulation may be a fruitful approach to take. The SPR band is somewhat narrower for the fs-PLD films, which may be related to the narrower NP size distribution.
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Acknowledgements This work was supported by Science Foundation Ireland under grant 09/RFP/PHY2422. I Mirza was supported by a postgraduate studentship funded by Trinity College Dublin and the Higher Educational Authority in Ireland. The STEM analysis was done in the Advanced Microscopy Laboratory at Trinity College Dublin.
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