Available online at www.sciencedirect.com Applied Surface Science 254 (2007) 1303–1306 www.elsevier.com/locate/apsusc Pulsed laser deposition of nanoparticle films of Au T. Donnelly *, S. Krishnamurthy, K. Carney, N. McEvoy, J.G. Lunney School of Physics, Trinity College Dublin, Dublin 2, Ireland Available online 16 September 2007 Abstract Nanosecond pulsed laser deposition (PLD) has been used to grow nanoparticle films of Au on Si and sapphire substrates. The equivalent solid density thickness was measured with a quartz crystal monitor and the ion flux was measured with a time-of-flight Langmuir probe. The ion signal yields the ion energy distribution. The angular distribution of deposited material and the ablated mass per pulse were also measured. These values are incorporated into an isentropic plasma expansion model for a better description of the expansion of the ablated material. Atomic force microscopy and UV/vis optical spectroscopy were used to characterise the films. Atomic force microscopy shows that in the equivalent thickness range 0.5–5 nm the deposited material is nanostructured and the surface coverage increases with increasing equivalent thickness. The optical absorption spectra show the expected surface plasmon resonance, which shifts to longer wavelengths and increases in magnitude as the equivalent thickness is increased. # 2007 Elsevier B.V. All rights reserved. Keywords: Gold nanoparticles; Pulsed laser deposition; Surface plasmon resonance; Ablation plume 1. Introduction Nanostructured materials and surfaces have generated great interest in recent years, much of which is due to the unusual optical, electronic, magnetic and catalytic properties that are characteristic of matter when its dimensions are reduced to the nanoscale [1]. In particular, metal nanoparticles (NPs) have been the subject of much research, ranging from the exploitation of the optical properties of noble metal NPs in areas such as sensing, plasmonics and surface enhanced spectroscopies [2] to magnetic NPs and their proposed use for ultrahigh density information storage. The catalytic properties of NP metal films have also been investigated for the growth of nanorods and nanotubes. Many of the standard thin film deposition techniques can be readily adapted for the production of nanostructured materials. Molecular beam epitaxy, thermal evaporation, sputtering and pulsed laser deposition (PLD) have all been used for nanosynthesis and it has been shown that PLD is a relatively simple and effective nanofabrication technique. For nanostructure growth with nanosecond PLD there is much evidence to show that NP growth takes place on the substrate by surface diffusion of the deposited material. In their * Corresponding author. Tel.: +353 1 8962157; fax: +353 1 6711759. E-mail address: tdonnell@tcd.ie (T. Donnelly). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.09.033 study of the growth of Pt on HOPG using nanosecond PLD Dolbec et al. [3] have shown that a power law describes the dependence of the particle diameter on the equivalent solid density thickness. They also demonstrate that changing the plume kinetic energy results in nanostructures of different shape. Afonso et al. [4,5] prepared nanocomposite films of Cu and amorphous alumina in vacuum and different background gases. By studying the morphology dependence on equivalent thickness they concluded that nucleation and growth of the metal NPs takes place on the substrate rather than in the gas phase. Gonzalo et al. [6] looked at the different competing processes during PLD of Au/alumina nanocomposite films in vacuum and concluded that the high kinetic energies involved give rise to a regulation of the NP size through self-sputtering of the deposited material. They also show the presence of a second layer of nanoparticles embedded in the amorphous host that is formed from ion implantation. Irissou et al. [7] studied the effect of Au kinetic energy on the growth of Au thin films and nanocrystals. They report that the kinetic energy determines the crystalline quality of the Au films. For high kinetic energies the films are highly oriented and for low energies the films are nanocrystalline. This study was performed in various background gases and the energies involved were typically <10 eV, which is considerably less than PLD in vacuum. Some researchers have also compared growth of PLD nanoparticles with other methods. Sasaki et al. [8], Warrender and Aziz [9] 1304 T. Donnelly et al. / Applied Surface Science 254 (2007) 1303–1306 and recently Shin and Aziz [10] have compared nanostructure growth by PLD with other deposition techniques. These studies point to the fact that the high atom and ion kinetic energies involved in the PLD process have an influence on the equivalent thickness at which the film reaches the percolation threshold. Other researchers have concentrated on studying the novel properties of NP films produced by PLD. Ag NPs have been deposited and their optical and electronic properties have been studied [11,12]. Magnetic NPs of Fe [13] and Co [14] have been grown for magnetic application and in Ref. [13] it was shown that the PLD grown films have a larger magnetic moment as compared with similar films as grown by thermal evaporation. Fe NPs deposited by PLD have also been used as catalysts for the growth of carbon nanotubes on various substrates [15]. For all these applications it is important to have a good understanding of NP formation by PLD. In this paper, we present results of an investigation of controlled growth of Au nanostructured films on Si and sapphire by PLD. We firstly aim to obtain a well-characterised deposition plasma and secondly to investigate the use of this plasma for the deposition of NP films. To characterise the ablation plasma used in our film deposition we have performed a mass loss experiment to find the total number of particles ablated by the laser pulse. We have used a time resolving Langmuir ion probe [16] to measure both the shape of the plume and the ion kinetic energy distribution of the ablation plasma. A quartz crystal deposition monitor measures the total deposition particle (atom + ion) flux and allows control of the equivalent thickness during film deposition. The deposition plume characteristics can be understood using the isentropic model of plasma expansion developed by Anisimov et al. [17]. We have used atomic force microscopy to investigate how changing the equivalent thickness of the film affects the surface morphology. UV/vis absorption spectra show the expected surface plasmon resonance; the resonance wavelength depends primarily on the equivalent thickness. 2. Experimental setup A Nd:YAG laser operating at 1064 nm, 20 Hz and pulse length of 6 ns was used to ablate a rotating metal Au target (99.99%) in a vacuum chamber at a pressure of 5  105 mbar. The laser spot size on target was 1.5  104 cm2 giving an average fluence of 1 J cm2. Ablated material was deposited on sapphire and native oxide Si substrates placed 9 cm directly in front of the ablation target. A planar Langmuir ion probe of area 5 mm2, biased at 30 V and placed 8 cm from the ablation target, was used to record the ion time-of-flight, from which ion energy distribution is derived. The probe was rotated about the ablation spot to measure the angular variation of the ion flux. The probe also ensured reproducibility of the PLD process. A quartz crystal monitor was used to measure the equivalent thickness of the deposited material. The mass loss due to ablation was measured by weighing the target before and after laser irradiation for 40 min at 20 Hz. The surface morphology of the deposits were characterised using a Digital Instruments multimode scanning probe microscope with a Nanoscope III controller operating in tapping mode AFM. Optical absorption was measured using a dual beam UV/vis spectrophotometer. 3. Results and discussion The ion current measured using the Langmuir probe placed 8 cm from the target (1 cm directly in front of the substrate) is presented in Fig. 1. From the ion time-of-flight the ion energy distribution can be found as described in our previous work [12]. This ion energy distribution is presented in the inset of Fig. 1. The average energy is 110 eV, implying that the deposition is an energetic process in which self-sputtering of the deposited material will take place [18]. Using the quartz crystal monitor the deposition rate was measured to be 1.9  106 g cm2 per laser shot which corresponds to 4.7  1012 atoms cm2 per shot. The integrated ion signal at 8 cm gives an ion dose of 4.8  1012 ions cm2 per shot. Using a 1/r2 scaling, to account for the difference in position Fig. 1. Ion time-of-flight and energy distribution for 1064 nm ablation of Au in vacuum at 1 J cm2. The average energy is 110 eV. Fig. 2. Angular distribution of ion yield fitted using Eq. (1) and k = 2.3. T. Donnelly et al. / Applied Surface Science 254 (2007) 1303–1306 1305 Fig. 3. AFM images for films of equivalent thickness (a) 0.5 nm, (b) 2.5 nm and (c) 4 nm Au on Si. of the probe and deposition monitor, yields a value of 3.8  1012 ions cm2 per shot at the position of the quartz crystal monitor and substrate. The mass lost from the target was measured to be 60 ng per shot. This gives a value of 1.8  1014 for the total number of evaporated particles which corresponds to an ablation depth of 2 nm per pulse. The angular distribution of the integrated ion flux measured in the vertical plane is presented in Fig. 2. The forward peaked nature of the ablation plume can clearly be seen in this figure. The angular distribution was compared with the isentropic gas dynamic expansion model of Anisimov by fitting the data to the following equation [19]: FðuÞ ¼ Fð0Þ  1 þ tan2 u 1 þ k2 tan2 u 3=2 atoms (1.8  1014), but the reason for this discrepancy is not clear at this stage. For the synthesis of NP films the deposition monitor was used to calculate the average deposition per shot (given above) and the number of shots required was adjusted to give a desired (1) F(u) is the ion yield on a hemispherical surface as a function of angle u measured from the target surface normal and k is the aspect ratio of the plasma plume. Fitting the data using this equation gives a value of k = 2.3. Using this k value and the value for integrated ion flux normal to the target the Anisimov model can be used to find the total number of ions in the plume. Thus, we estimate the total number of ions in the plume to be 4.6  1014. This is somewhat larger than the number of ablated Fig. 4. Optical absorption of 0.5 nm, 2.5 nm, 3 nm and 4 nm Au on Al2O3. 1306 T. Donnelly et al. / Applied Surface Science 254 (2007) 1303–1306 equivalent thickness, which is the thickness obtained by dividing the areal density of the film by the density of solid Au. Fig. 3 shows AFM images of 0.5, 2.5 and 4 nm thickness Au deposited on Si. From the images we can see that in the case of 0.5 nm equivalent thickness the Au nanoparticles have a height of 3 nm and an apparent diameter of 66 nm. However, it seems that apparent diameter is determined by the lateral spatial resolution of the AFM. It can be seen that the coverage of the surface increases with increasing equivalent thickness. Fig. 4 shows the absorbance spectra of films prepared under the same conditions but on sapphire and included is the absorption of a 3 nm film. It can be seen that the absorption increases with increasing equivalent thickness as expected. The surface plasmon resonance (SPR) peak shifts to longer wavelengths with increasing thickness which is consistent with our previous work on Ag nanostructured films [12]. The SPR peak shifts from 550 nm for the 0.5 nm film to about 770 nm for the 3 nm film. 4. Conclusion In conclusion we have used PLD to deposit nanostructured films of Au on Si and sapphire in vacuum. We have used a time resolving Langmuir probe to characterise our deposition plasma and have investigated how the film morphology and optical absorption depends on the amount of material deposited. From our analysis of the ablation plasma we have found the average plume energy is 110 eV which is sufficient to cause self-sputtering of the deposited material. Thin films of Au have been deposited and below 5 nm equivalent thickness AFM confirms the formation of NPs. As the equivalent thickness is varied from 0.5 to 4 nm the NP size and surface coverage increases. With increasing deposition the NPs coalesce until a nearly continuous film exists for 4 nm equivalent thickness. The optical measurements show the expected SPR feature which increases in magnitude and shifts to longer wavelengths as the equivalent thickness is increased. Acknowledgment This work was supported by the EU Specific Targeted Research Project DESYGN-IT (No. NMP4-CT-2004-505626). References [1] F. Rosei, J. Phys.: Condens Matter. 16 (2004) S1373–S1436. [2] E. Hutter, J.H. Fendler, Adv. Mater. 16 (2004) 19. [3] R. Dolbec, E. Irissou, M. Chaker, D. Guay, F. Rosei, M.A. El Khakani, Phys. Rev. B 70 (2004) 201406. [4] C.N. Afonso, R. Serna, J.M. Ballesteros, A.K. Petford-Long, D.C. Doole, Appl. Surf. Sci. 127–129 (1998) 339–343. [5] C.N. Afonso, J. Gonzalo, R. Serna, J.C.G. de Sande, C. Ricolleau, C. Grigis, M. Gandais, D.E. Hole, P.D. Townsend, Appl. Phys. A 69 (1999) S201–S207. [6] J. Gonzalo, A. Perea, D. Babonneau, C.N. Afonso, N. Beer, J.P. Barnes, A.K. Petford-Long, D.E. Hole, P.D. Townsend, Phys. Rev. B 71 (2005) 125420. [7] E. Irissou, B. Le Drogoff, M. Chaker, D. Guay, J. Appl. Phys. 94 (8) (2003) 4796–4802. [8] T. Sasaki, N. Koshizaki, K.M. Beck, Appl. Phys. A 69 (1999) S771– S774. [9] J.M. Warrender, M.J. Aziz, Phys. Rev. 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