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Growth process of nanostructured silver films pulsed laser ablated in high-pressure inert gas
Applied Surface Science 255 (2009) 9676–9679
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Growth process of nanostructured silver films pulsed laser ablated in high-pressure inert gas E. Fazio a, F. Neri a, P.M. Ossi b,*, N. Santo c, S. Trusso d a
Dipartimento di Fisica della Materia e Ingegneria Elettronica, Universita` degli Studi di Messina, Salita Sperone 31, 98166 Messina, Italy Dipartimento di Energia & Centre for NanoEngineered MAterials and Surfaces – NEMAS, Politecnico di Milano, Via Ponzio 34-3, 20133 Milano, Italy c Centro Interdipartimentale Microscopia Avanzata, Universita` degli Studi di Milano, Via Celoria 26, 20133 Milano, Italy d Istituto per i Processi Chimico-Fisici del CNR, S.ta Sperone, C.da Papardo, Faro Superiore, 98158 Messina, Italy b
A R T I C L E I N F O
A B S T R A C T
Article history: Available online 21 April 2009
The growth process of silver thin films deposited by pulsed laser ablation in a controlled inert gas atmosphere was investigated. A pure silver target was ablated in Ar atmosphere, at pressures ranging between 10 and 100 Pa, higher than usually adopted for thin film deposition, at different numbers of laser shots. All of the other experimental conditions such as the laser (KrF, wavelength 248 nm), the fluence of 2.0 J cm 2, the target to substrate distance of 35 mm, and the temperature (295 K) of the substrates were kept fixed. The morphological properties of the films were investigated by transmission and scanning electron microscopies (TEM, SEM). Film formation results from coalescence on the substrate of near-spherical silver clusters landing as isolated particles with size in the few nanometers range. From a visual inspection of TEM pictures of the films deposited under different conditions, wellseparated stages of film growth are identified. ß 2009 Elsevier B.V. All rights reserved.
PACS: 81.16.Mk (methods of nanofabrication and processing: laser-assisted deposition) 68.37.Lp (transmission electron microscopy (TEM)) 78.67.Bf (optical properties: nanocrystals and nanoparticles) Keywords: Silver nanoparticles Laser ablation Thin film morphology
1. Introduction In recent years nanostructured materials, in particular noble metal nanoparticles (NPs), have been the object of intense experimental and theoretical research, owing to the strong sensitivity on size of their physical–chemical properties, which find several technological applications [1]. Pulsed laser ablation (PLA) was successfully used to produce materials in NP form under different deposition conditions [2]. Nevertheless an accurate control of several experimental parameters is required to tailor both NP size and the surface morphology of thin films made of NPs, and hence their structural, optical and electronic properties. The ability of an atomic species arriving at the substrate surface to arrange itself so as to result in a specific structure strongly depends on its kinetic energy. Deposition processes characterized by energetic species usually lead to compact and smooth surfaces, while thin film formation by species with low kinetic energy proceeds through cluster coalescence on the substrate surface [3]. PLA in vacuum is a deposition technique in which the produced
* Corresponding author. E-mail address: [email protected] (P.M. Ossi). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.04.050
particles can have very high kinetic energy. Performing the process in presence of inert or reactive gases the energy of the species in the laser generated plasma can be tuned by collision processes between the expanding plasma and the background gas. Nevertheless the plasma–gas interaction is a very complex process, so that the loss and spreading of kinetic energy of plasma species is just one of the outcomes. Chemical reactions, collisions inside the plasma and within the contact layer between the plasma and the gas, collisionally generated excitation processes, shock wave formation and propagation are usually observed when plasma expansion takes place in an ambient gas. As a consequence, film growth is related in a complex way to the background gas nature and pressure. In this work we investigated the processes of silver NP formation and thin film growth by performing the depositions in presence of an inert gas atmosphere (Ar). The evolution of thin film morphology was studied on two sets of samples; a first group of films was grown at fixed number of laser pulses, changing argon pressure, a second set was grown at a fixed gas pressure, while varying the laser pulse number. This way the temporal evolution of the film growth process could be observed and different stages of thin film formation were identified by SEM and TEM imaging. Optical properties of the films are also presented and discussed.
E. Fazio et al. / Applied Surface Science 255 (2009) 9676–9679
2. Experiment Film depositions were performed in a high vacuum chamber with a residual pressure lower than 1.0 10 4 Pa. The beam from a KrF excimer laser (wavelength 248 nm, pulse duration 25 ns, repetition rate 10 Hz) was focused onto the surface of a silver target using a quartz lens. Targets were mounted on a rotating holder and positioned 35 mm far from the substrates. Films were grown onto (1 0 0)Si, Corning 5049 glass and Cu TEM grids covered by a thin formvar/carbon copper grid film. The laser energy density was kept fixed at 2.0 J cm 2. A set of samples was grown in a controlled atmosphere at the Ar pressures (pAr) of 10, 40, 70 and 100 Pa while keeping the laser shot number fixed at 10,000. A second set was grown at the fixed pAr of 70 Pa, changing the laser shot number between 500 and 10,000. The argon gas flux needed to maintain the desired background gas pressure was fed into the chamber using a mass flow controller. SEM images were acquired with a Zeiss Supra 40 field ion SEM, whereas TEM observations were performed with a Zeiss Leo 912AB microscope working at 80 kV. All pictures were acquired with a charged coupled device (CCD) Esi Vision ProScan camera, with a resolution of 1024 1024 pixels. Uv–vis spectra of the deposited films were acquired by means of a Perkin Elmer Lambda 900 spectrophotometer. 3. Results and discussion In Fig. 1a and b are shown SEM images of the sample surfaces deposited in Ar at the pressure of 10 and 100 Pa respectively, with a fixed laser shot number of 10,000. In both cases the deposition process was ended before the coverage of the substrates was completed. Qualitative differences between the surface morphol-
9677
ogies of the two films are clearly visible. The surface of the sample grown at the lower Ar pressure consists of silver islands with smooth rounded edges, with typical size in the range of few tens of nanometers. The elongated shape of most of such islands indicates that they result from coalescence of considerably smaller nearly spherical particles, some of which can be still observed as isolated NPs on the film surface. A similar morphology was observed for the sample grown at 40 Pa. When looking at the surface of the sample grown at the pressure of 100 Pa (Fig. 1b) the surface morphology appears quite different. The surface is mostly covered by a random distribution of isolated sphere-like particles; there is no evidence for a coalescence process. The sample grown at 70 Pa shows a similar morphology. In Fig. 1c and d the TEM images of the same samples are shown. The pictures were taken from samples deposited on amorphous-C supported Cu grids. In Fig. 1c two strongly different morphologies can be observed. In the center of the picture there is a low-density near-circular area; such a feature is surrounded by a rather uniform network of connected islands with inter-island channels considerably filled in. This morphology corresponds to the top-view offered by the corresponding SEM picture (Fig. 1a). The low-density central area in Fig. 1c is likely to be due to a micrometric droplet directly ejected from the target. Such a droplet masked the surface for a while during deposition, reducing the flux of atoms or clusters coming from the target, then it detached from the substrate, allowing film growth to start again. Although particulate is a major drawback of PLD [4], in this case the accidental occurrence of a droplet and its subsequent detachment from the substrate allow for a better understanding of film formation mechanism. Indeed, inside the low-density central area the coalescence process of nearly spherical NPs into larger, elliptical islands is clearly observable. The TEM image of the
Fig. 1. Surface morphology (SEM) of the films deposited (a) at pAr = 10 Pa and (b) at pAr = 100 Pa. In (c) and (d) the corresponding TEM pictures are shown.
E. Fazio et al. / Applied Surface Science 255 (2009) 9676–9679
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Fig. 2. Surface morphology (SEM) of films deposited at pAr = 70 Pa with (a) 500 shots, (b) 1000 shots, (c) 5000 shots and (d) 10,000 shots, respectively.
sample grown at 100 Pa is shown in Fig. 1d. Here a micrometric spherical (dark) particle lies onto the sample surface. The surface is characterized by the predominant presence of small NPs with spherical shape. The shadowing effect of the big particle can be observed as a crescent area on its left side where the particle number density is lower than on the remaining surface. No NP coalescence is observed, in agreement with the corresponding SEM picture (Fig. 1b). The second set of samples was prepared keeping the Ar pressure fixed at 70 Pa and changing the laser pulse number between 500 and 10,000. The SEM images of the corresponding sample surfaces are shown in Fig. 2a–d. The scenario differs from the one discussed above. All of the sample surfaces show evidence for the presence of NPs whose number density increases as a function of the laser shot number. The surface of the sample obtained with 500 laser shots consists of isolated spherical NPs (Fig. 2a); with increasing number of laser pulses more and more NPs crowded onto the substrate surfaces. No coalescence is observed; the latter appears to be typical of film growth at high laser shot number, performed at low buffer gas pressures (see Table 1). The growth paths above described refer to the assembling onto a substrate of clusters synthesized in gas phase during the propagation of the plasma plume from the target to the substrate. A specific study on plasma dynamics, moving from plasma diagnostics [5,6], was performed recently on silver ablated under the same conditions adopted in this work [7]; the average sizes of deposited nano-clusters, deduced from TEM pictures of samples grown at the fixed number of laser shots of 10,000, were in agreement with those predicted by a model for cluster synthesis in
the expanding plume [8,9]. NP sizes were evaluated from the lowdensity areas like those in Figs. 1c and d [8]; for NPs deposited at 10, 40, 70 and 100 Pa the average sizes were 0.7, 1.7, 2.8 and 4.1 nm, respectively. It is worth noticing that within the relatively narrow ranges of Ar pressure and laser shot number investigated well-differentiated morphologies of the deposited films were observed. In particular, at pAr = 40 Pa and 10,000 laser shots the film consists of clustered NPs, while slightly increasing the pressure up to pAr = 70 Pa with the same laser shot number a population of isolated NPs results. At fixed pAr (70 Pa) the NP number density onto the substrate surface can be finely tuned. This is of major relevance to obtain nanostructured metal films owing to their peculiar optical properties. In noble metals a surface plasmon resonance (SPR), resulting from the coherent oscillation of surface electrons excited by an electromagnetic field, can be observed. Bulk silver shows a PR in the near ultraviolet region of the spectrum [10,11]. In the case of isolated particles with size of few nanometers a SPR peak is observed near 400 nm [12]. The position and shape of the SPR peak critically depend on size, shape and spatial distribution of the NPs. By chemical methods cubic, triangular, rod and wire shaped NPs were produced, all of which with different optical properties [13– 16]. Thus a fine control of the morphological properties of a nanostructured thin film results in turn in the control of its optical properties. In Fig. 3 the Uv–vis absorption spectra of the samples deposited at different Ar pressures are reported. Each spectrum is normalized to its own maximum value. Both the position (vp) and the full width at half maximum (FWHM) of the SPR peak are reported in Table 1. It can be observed that keeping fixed the
Table 1 Position (vp) and FWHM of the plasmon resonance peak for films deposited under different experimental conditions. The observed surface morphologies are also reported. Fixed laser shot number
Fixed background Ar pressure
pAr (Pa)
Laser shots
vp (nm)
FWHM (nm)
Film morphology
pAr (Pa)
Laser shots
vp (nm)
FWHM (nm)
Film morphology
10 40 70 100
10,000
632 560 544 440
>630 336 304 150
Percolated Clustered NPs NPs NPs
70
500 1,000 5,000 10,000
434 438 478 544
120 120 160 304
NPs NPs NPs NPs
E. Fazio et al. / Applied Surface Science 255 (2009) 9676–9679
9679
Fig. 3. Optical absorbance of the films deposited at different Ar pressures with fixed laser shot number of 10,000. All spectra are normalized to their own maximum.
Fig. 4. Optical absorbance of films deposited in Ar at pAr = 70 Pa with different numbers of laser shots. All spectra are normalized to their own maximum.
number of laser shots at 10,000 and decreasing pAr from 100 Pa down to 10 Pa the plasmon position red-shifts from 440 to 643 nm while its width progressively increases from 150 nm up to more than 650 nm. Such trends indicate that the films no more consist of isolated spherical NPs, as confirmed by SEM and TEM pictures. A different behaviour is observed for the plasmon position of the samples deposited at fixed pAr = 70 Pa, as shown in Fig. 4. vp redshifts from 438 to 544 nm when the number of laser shots increases from 500 to 10,000 (see Table 1). A broadening of the absorption band is evident only for the sample grown at 10,000 pulses, revealing that in this film the clustering process among particles affected the optical properties of the film, although the effect of the process it is not yet clearly visible from the SEM pictures. In the case of silver deposition, with the adopted parameters, playing with ambient gas (Ar) pressure at fixed number of laser pulses (10,000) a qualitative change in the strategy of selfassembling of NPs on the substrate occurs and involves coalescence. When Ar pressure is kept fixed and the number of laser pulses is changed a control of the spatial density of isolated spherical NPs deposited on the substrate is possible. The observed vp red-shifts are due to an increase of NP size and/or to clustering process among NPs, depending on the experimental growth parameters adopted. The measure of the SPR of the films and its trend indicates that it is possible to finely tailor the nanostructure of this family of films and concurrently their optical properties.
process is associated to the deposition of isolated NPs onto the substrate. Subsequent film formation proceeds via coalescence of the deposited NPs into larger islands, until a completely percolated structure results. The optical properties of the deposited films strongly depend on film morphology. The shifts in position and width of the surface plasmon resonance of silver NPs were studied as functions of the adopted deposition conditions. The optical properties of nanostructured silver thin films can be controlled by performing the ablation process at different Ar pressures and/or selecting the number of laser shots. Such a feature is promising for the preparation of active substrates for surface enhanced Raman scattering.
4. Conclusions The growth process of nanostructured silver thin films was investigated as a function of the ambient gas pressure and laser pulse number. SEM and TEM imaging allowed identifying different temporal stages of film formation. A first in flight cluster growth
References [1] M.F. Jarrold, in: H. Haberland (Ed.), Clusters of Atoms and Molecules, Springer, Berlin, 1994, p. 315. [2] L. Chen, in: D.B. Chrisey, G.K. Hubler (Eds.), Pulsed Laser Deposition of Thin Films, Wiley, New York, 1994, p. 167. [3] R. Messier, A.P. Giri, R. Roy, J. Vac. Sci. Technol. A2 (1984) 500. [4] D. Ba¨uerle, Laser Processing and Chemistry, 3rd ed., Springer, Berlin, 2000, p. 470. [5] S. Trusso, E. Barletta, F. Barreca, F. Neri, Appl. Phys. A 79 (2004) 1997. [6] S. Trusso, E. Barletta, F. Barreca, E. Fazio, F. Neri, Laser Particle Beams 23 (2005) 149. [7] B. Fazio, S. Trusso, E. Fazio, F. Neri, P.M. Ossi, N. Santo, Radiat. Effects Defects Solids 163 (2008) 673. [8] E. Fazio, F. Neri, P.M. Ossi, N. Santo, S. Trusso, Laser Particle Beams. 27 (2009) 281. [9] A. Bailini, P.M. Ossi, Europhys. Lett. 79 (2007) 35002. [10] P. Taneja, P. Ayyub, R. Chandra, Phys. Rev. B 65 (2002) 245412. [11] S. Lau Truong, G. Levi, F. Bozon-Verduraz, A.V. Petrovskaya, A.V. Simakin, G.A. Shafeev, Appl. Phys. A 89 (2007) 373. [12] P. Mulvaney, Langmuir 12 (1996) 788. [13] Y. Sun, Y. Xia, Science 298 (2002) 2176. [14] R. Jin, Y.W. Cao, C.A. Mirkin, K.L. Kelly, G.C. Shatz, J.G. Zheng, Science 294 (2001) 1901. [15] C. Ni, P.A. Hassan, E.W. Kaler, Langmuir 21 (2001) 3334. [16] J.-Q. Hu, Q. Chen, Z.-X. Xie, G.-B. Han, R.-H. Wang, B. Ren, Y. Zhang, Z.-L. Yang, Z.-Q. Tian, Adv. Funct. Mater. 14 (2004) 183.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Growth process of nanostructured silver films pulsed laser ablated in high-pressure inert gas E. Fazio a, F. Neri a, P.M. Ossi b,*, N. Santo c, S. Trusso d a
Dipartimento di Fisica della Materia e Ingegneria Elettronica, Universita` degli Studi di Messina, Salita Sperone 31, 98166 Messina, Italy Dipartimento di Energia & Centre for NanoEngineered MAterials and Surfaces – NEMAS, Politecnico di Milano, Via Ponzio 34-3, 20133 Milano, Italy c Centro Interdipartimentale Microscopia Avanzata, Universita` degli Studi di Milano, Via Celoria 26, 20133 Milano, Italy d Istituto per i Processi Chimico-Fisici del CNR, S.ta Sperone, C.da Papardo, Faro Superiore, 98158 Messina, Italy b
A R T I C L E I N F O
A B S T R A C T
Article history: Available online 21 April 2009
The growth process of silver thin films deposited by pulsed laser ablation in a controlled inert gas atmosphere was investigated. A pure silver target was ablated in Ar atmosphere, at pressures ranging between 10 and 100 Pa, higher than usually adopted for thin film deposition, at different numbers of laser shots. All of the other experimental conditions such as the laser (KrF, wavelength 248 nm), the fluence of 2.0 J cm 2, the target to substrate distance of 35 mm, and the temperature (295 K) of the substrates were kept fixed. The morphological properties of the films were investigated by transmission and scanning electron microscopies (TEM, SEM). Film formation results from coalescence on the substrate of near-spherical silver clusters landing as isolated particles with size in the few nanometers range. From a visual inspection of TEM pictures of the films deposited under different conditions, wellseparated stages of film growth are identified. ß 2009 Elsevier B.V. All rights reserved.
PACS: 81.16.Mk (methods of nanofabrication and processing: laser-assisted deposition) 68.37.Lp (transmission electron microscopy (TEM)) 78.67.Bf (optical properties: nanocrystals and nanoparticles) Keywords: Silver nanoparticles Laser ablation Thin film morphology
1. Introduction In recent years nanostructured materials, in particular noble metal nanoparticles (NPs), have been the object of intense experimental and theoretical research, owing to the strong sensitivity on size of their physical–chemical properties, which find several technological applications [1]. Pulsed laser ablation (PLA) was successfully used to produce materials in NP form under different deposition conditions [2]. Nevertheless an accurate control of several experimental parameters is required to tailor both NP size and the surface morphology of thin films made of NPs, and hence their structural, optical and electronic properties. The ability of an atomic species arriving at the substrate surface to arrange itself so as to result in a specific structure strongly depends on its kinetic energy. Deposition processes characterized by energetic species usually lead to compact and smooth surfaces, while thin film formation by species with low kinetic energy proceeds through cluster coalescence on the substrate surface [3]. PLA in vacuum is a deposition technique in which the produced
* Corresponding author. E-mail address: [email protected] (P.M. Ossi). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.04.050
particles can have very high kinetic energy. Performing the process in presence of inert or reactive gases the energy of the species in the laser generated plasma can be tuned by collision processes between the expanding plasma and the background gas. Nevertheless the plasma–gas interaction is a very complex process, so that the loss and spreading of kinetic energy of plasma species is just one of the outcomes. Chemical reactions, collisions inside the plasma and within the contact layer between the plasma and the gas, collisionally generated excitation processes, shock wave formation and propagation are usually observed when plasma expansion takes place in an ambient gas. As a consequence, film growth is related in a complex way to the background gas nature and pressure. In this work we investigated the processes of silver NP formation and thin film growth by performing the depositions in presence of an inert gas atmosphere (Ar). The evolution of thin film morphology was studied on two sets of samples; a first group of films was grown at fixed number of laser pulses, changing argon pressure, a second set was grown at a fixed gas pressure, while varying the laser pulse number. This way the temporal evolution of the film growth process could be observed and different stages of thin film formation were identified by SEM and TEM imaging. Optical properties of the films are also presented and discussed.
E. Fazio et al. / Applied Surface Science 255 (2009) 9676–9679
2. Experiment Film depositions were performed in a high vacuum chamber with a residual pressure lower than 1.0 10 4 Pa. The beam from a KrF excimer laser (wavelength 248 nm, pulse duration 25 ns, repetition rate 10 Hz) was focused onto the surface of a silver target using a quartz lens. Targets were mounted on a rotating holder and positioned 35 mm far from the substrates. Films were grown onto (1 0 0)Si, Corning 5049 glass and Cu TEM grids covered by a thin formvar/carbon copper grid film. The laser energy density was kept fixed at 2.0 J cm 2. A set of samples was grown in a controlled atmosphere at the Ar pressures (pAr) of 10, 40, 70 and 100 Pa while keeping the laser shot number fixed at 10,000. A second set was grown at the fixed pAr of 70 Pa, changing the laser shot number between 500 and 10,000. The argon gas flux needed to maintain the desired background gas pressure was fed into the chamber using a mass flow controller. SEM images were acquired with a Zeiss Supra 40 field ion SEM, whereas TEM observations were performed with a Zeiss Leo 912AB microscope working at 80 kV. All pictures were acquired with a charged coupled device (CCD) Esi Vision ProScan camera, with a resolution of 1024 1024 pixels. Uv–vis spectra of the deposited films were acquired by means of a Perkin Elmer Lambda 900 spectrophotometer. 3. Results and discussion In Fig. 1a and b are shown SEM images of the sample surfaces deposited in Ar at the pressure of 10 and 100 Pa respectively, with a fixed laser shot number of 10,000. In both cases the deposition process was ended before the coverage of the substrates was completed. Qualitative differences between the surface morphol-
9677
ogies of the two films are clearly visible. The surface of the sample grown at the lower Ar pressure consists of silver islands with smooth rounded edges, with typical size in the range of few tens of nanometers. The elongated shape of most of such islands indicates that they result from coalescence of considerably smaller nearly spherical particles, some of which can be still observed as isolated NPs on the film surface. A similar morphology was observed for the sample grown at 40 Pa. When looking at the surface of the sample grown at the pressure of 100 Pa (Fig. 1b) the surface morphology appears quite different. The surface is mostly covered by a random distribution of isolated sphere-like particles; there is no evidence for a coalescence process. The sample grown at 70 Pa shows a similar morphology. In Fig. 1c and d the TEM images of the same samples are shown. The pictures were taken from samples deposited on amorphous-C supported Cu grids. In Fig. 1c two strongly different morphologies can be observed. In the center of the picture there is a low-density near-circular area; such a feature is surrounded by a rather uniform network of connected islands with inter-island channels considerably filled in. This morphology corresponds to the top-view offered by the corresponding SEM picture (Fig. 1a). The low-density central area in Fig. 1c is likely to be due to a micrometric droplet directly ejected from the target. Such a droplet masked the surface for a while during deposition, reducing the flux of atoms or clusters coming from the target, then it detached from the substrate, allowing film growth to start again. Although particulate is a major drawback of PLD [4], in this case the accidental occurrence of a droplet and its subsequent detachment from the substrate allow for a better understanding of film formation mechanism. Indeed, inside the low-density central area the coalescence process of nearly spherical NPs into larger, elliptical islands is clearly observable. The TEM image of the
Fig. 1. Surface morphology (SEM) of the films deposited (a) at pAr = 10 Pa and (b) at pAr = 100 Pa. In (c) and (d) the corresponding TEM pictures are shown.
E. Fazio et al. / Applied Surface Science 255 (2009) 9676–9679
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Fig. 2. Surface morphology (SEM) of films deposited at pAr = 70 Pa with (a) 500 shots, (b) 1000 shots, (c) 5000 shots and (d) 10,000 shots, respectively.
sample grown at 100 Pa is shown in Fig. 1d. Here a micrometric spherical (dark) particle lies onto the sample surface. The surface is characterized by the predominant presence of small NPs with spherical shape. The shadowing effect of the big particle can be observed as a crescent area on its left side where the particle number density is lower than on the remaining surface. No NP coalescence is observed, in agreement with the corresponding SEM picture (Fig. 1b). The second set of samples was prepared keeping the Ar pressure fixed at 70 Pa and changing the laser pulse number between 500 and 10,000. The SEM images of the corresponding sample surfaces are shown in Fig. 2a–d. The scenario differs from the one discussed above. All of the sample surfaces show evidence for the presence of NPs whose number density increases as a function of the laser shot number. The surface of the sample obtained with 500 laser shots consists of isolated spherical NPs (Fig. 2a); with increasing number of laser pulses more and more NPs crowded onto the substrate surfaces. No coalescence is observed; the latter appears to be typical of film growth at high laser shot number, performed at low buffer gas pressures (see Table 1). The growth paths above described refer to the assembling onto a substrate of clusters synthesized in gas phase during the propagation of the plasma plume from the target to the substrate. A specific study on plasma dynamics, moving from plasma diagnostics [5,6], was performed recently on silver ablated under the same conditions adopted in this work [7]; the average sizes of deposited nano-clusters, deduced from TEM pictures of samples grown at the fixed number of laser shots of 10,000, were in agreement with those predicted by a model for cluster synthesis in
the expanding plume [8,9]. NP sizes were evaluated from the lowdensity areas like those in Figs. 1c and d [8]; for NPs deposited at 10, 40, 70 and 100 Pa the average sizes were 0.7, 1.7, 2.8 and 4.1 nm, respectively. It is worth noticing that within the relatively narrow ranges of Ar pressure and laser shot number investigated well-differentiated morphologies of the deposited films were observed. In particular, at pAr = 40 Pa and 10,000 laser shots the film consists of clustered NPs, while slightly increasing the pressure up to pAr = 70 Pa with the same laser shot number a population of isolated NPs results. At fixed pAr (70 Pa) the NP number density onto the substrate surface can be finely tuned. This is of major relevance to obtain nanostructured metal films owing to their peculiar optical properties. In noble metals a surface plasmon resonance (SPR), resulting from the coherent oscillation of surface electrons excited by an electromagnetic field, can be observed. Bulk silver shows a PR in the near ultraviolet region of the spectrum [10,11]. In the case of isolated particles with size of few nanometers a SPR peak is observed near 400 nm [12]. The position and shape of the SPR peak critically depend on size, shape and spatial distribution of the NPs. By chemical methods cubic, triangular, rod and wire shaped NPs were produced, all of which with different optical properties [13– 16]. Thus a fine control of the morphological properties of a nanostructured thin film results in turn in the control of its optical properties. In Fig. 3 the Uv–vis absorption spectra of the samples deposited at different Ar pressures are reported. Each spectrum is normalized to its own maximum value. Both the position (vp) and the full width at half maximum (FWHM) of the SPR peak are reported in Table 1. It can be observed that keeping fixed the
Table 1 Position (vp) and FWHM of the plasmon resonance peak for films deposited under different experimental conditions. The observed surface morphologies are also reported. Fixed laser shot number
Fixed background Ar pressure
pAr (Pa)
Laser shots
vp (nm)
FWHM (nm)
Film morphology
pAr (Pa)
Laser shots
vp (nm)
FWHM (nm)
Film morphology
10 40 70 100
10,000
632 560 544 440
>630 336 304 150
Percolated Clustered NPs NPs NPs
70
500 1,000 5,000 10,000
434 438 478 544
120 120 160 304
NPs NPs NPs NPs
E. Fazio et al. / Applied Surface Science 255 (2009) 9676–9679
9679
Fig. 3. Optical absorbance of the films deposited at different Ar pressures with fixed laser shot number of 10,000. All spectra are normalized to their own maximum.
Fig. 4. Optical absorbance of films deposited in Ar at pAr = 70 Pa with different numbers of laser shots. All spectra are normalized to their own maximum.
number of laser shots at 10,000 and decreasing pAr from 100 Pa down to 10 Pa the plasmon position red-shifts from 440 to 643 nm while its width progressively increases from 150 nm up to more than 650 nm. Such trends indicate that the films no more consist of isolated spherical NPs, as confirmed by SEM and TEM pictures. A different behaviour is observed for the plasmon position of the samples deposited at fixed pAr = 70 Pa, as shown in Fig. 4. vp redshifts from 438 to 544 nm when the number of laser shots increases from 500 to 10,000 (see Table 1). A broadening of the absorption band is evident only for the sample grown at 10,000 pulses, revealing that in this film the clustering process among particles affected the optical properties of the film, although the effect of the process it is not yet clearly visible from the SEM pictures. In the case of silver deposition, with the adopted parameters, playing with ambient gas (Ar) pressure at fixed number of laser pulses (10,000) a qualitative change in the strategy of selfassembling of NPs on the substrate occurs and involves coalescence. When Ar pressure is kept fixed and the number of laser pulses is changed a control of the spatial density of isolated spherical NPs deposited on the substrate is possible. The observed vp red-shifts are due to an increase of NP size and/or to clustering process among NPs, depending on the experimental growth parameters adopted. The measure of the SPR of the films and its trend indicates that it is possible to finely tailor the nanostructure of this family of films and concurrently their optical properties.
process is associated to the deposition of isolated NPs onto the substrate. Subsequent film formation proceeds via coalescence of the deposited NPs into larger islands, until a completely percolated structure results. The optical properties of the deposited films strongly depend on film morphology. The shifts in position and width of the surface plasmon resonance of silver NPs were studied as functions of the adopted deposition conditions. The optical properties of nanostructured silver thin films can be controlled by performing the ablation process at different Ar pressures and/or selecting the number of laser shots. Such a feature is promising for the preparation of active substrates for surface enhanced Raman scattering.
4. Conclusions The growth process of nanostructured silver thin films was investigated as a function of the ambient gas pressure and laser pulse number. SEM and TEM imaging allowed identifying different temporal stages of film formation. A first in flight cluster growth
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