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Selective gold nanoparticles formation by pulsed laser interference
Applied Surface Science 258 (2012) 9223–9227
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Selective gold nanoparticles formation by pulsed laser interference R.J. Peláez a,∗ , G. Baraldi a , C.N. Afonso a , S. Riedel b , J. Boneberg b , P. Leiderer b a b
Laser Processing Group, Instituto de Optica, CSIC, Serrano 121, 28006 Madrid, Spain Faculty of Physics, University of Konstanz, Universitätsstrasse 10, 78457 Konstanz, Germany
a r t i c l e
i n f o
Article history: Available online 17 August 2011 Keywords: Laser Patterning Interference Metal Films Nanoparticles Melting
a b s t r a c t Discontinuous Au films are prepared on glass substrates by pulsed laser deposition with two different metal coverages that lead to a film being formed by irregular coalesced nanoparticles (NPs) and to another film close to the percolation limit. The films are exposed to three interfering beams at different intensities produced by the fourth harmonic of a Nd:YAG laser (266 nm, 10 ns). Scanning electron microscopy and extinction spectra are used respectively to study the structural and optical properties before and after the laser structuring. Round metal NPs appear in the laser transformed areas due to melting followed by rapid solidification that is reflected in the extinction spectra by the appearance of a surface plasmon resonance around 530–540 nm. The areas with NPs are surrounded by non-transformed areas forming a periodic pattern that evolves from a 2D array to parallel lines when local laser intensity increases to cover the whole sample at high intensity. The accumulation of several pulses at low fluence can also transform the metal film almost completely by creating alternating areas having different NP dimensions. The accumulation of metal in some areas of the pattern is consistent with mass transport towards the lower temperature regions. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Spatially arranged micro- and nano-objects are attracting a lot of interest as platforms for several applications in physics, chemistry or biology [1,2]. In order to produce such ordered nano-structures, surface structuring is the most extensively used approach that includes various chemical processes or more complex lithographic techniques. Laser based techniques offer a fast and simple alternative for surface structuring and periodic patterns can easily be produced by using amplitude or phase masks [3]. More recently, several advanced laser methods for precision engineering including laser interferometer approaches have been reviewed [2]. Direct patterning by laser interference using 2–4 beams has been reported that has the additional advantages to produce patterns at a small period (a few hundred nm), under a single exposure over a large (>mm2 ) area in a single step process [4–7]. Nanostructured metals are the subject of strong research activity mostly stimulated by the possibility of generating large electromagnetic field enhancement via surface plasmon resonance (SPR), and Ag or Au are particularly interesting since their SPR is in the visible spectrum. There are several works on laser irradiation of Au films [8] or nanoparticles (NPs) [9] that typically report the break up of the film or the conversion of irregularly shaped
∗ Corresponding author. E-mail address: [email protected] (R.J. Peláez). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.08.048
NPs into spherical NPs with reduced dimension dispersion. Exposure of Au films to the interfering laser beams has produced either microbumps or de-wetting [4] at the hotter regions of the pattern caused by laser induced melting of the metal and mass transport to colder regions is typically observed [4,10]. The thickness of the starting film has been pointed out as an essential parameter controlling the final transformation [4,8] and most reported work has been performed on films thicker than the percolation limit. The aim of this work is to provide a deeper insight on the transformation induced by pulsed laser interference in metal films as well as the relation between structural features to the optical properties. The focus will be on discontinuous gold films, i.e. thickness smaller than the percolation limit, rather than to continuous films as in most earlier works. 2. Experimental Au films have been produced by PLD in vacuum (≈3–7 × 10−6 mbar) by focusing an ArF laser beam ( = 193 nm, = 20 ns full width half maximum-FWHM) on Au targets at an angle of 45◦ with respect to its normal. The laser repetition rate has been set to 5 Hz and the fluence at the target surface ≈2.7 J cm−2 . The substrate was held at room temperature, placed 38 mm away from the target and rotated along an axis parallel to the plasma expansion axis and shifted a few mm. This rotation leads to samples with homogeneous effective thickness or gold coverage in an area >100 mm2 . Deposition was performed on glass substrates
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Fig. 1. SEM images of (a–c) the 2500 and (d–f) 3500 samples: (a and d) as-grown samples; (b and e) patterns produced with one laser exposure; and (c and f) magnifications of (b and e) showing the central area exposed to the lowest intensity surrounded by other two areas exposed to the highest intensity. The latter is marked in (e) by arrows. Images in the same row have the same magnification.
and a 10 nm thick a-Al2 O3 buffer layer has in situ been produced at 20 Hz before the laser ablation of the gold target since the growth of Au NPs on this surface by PLD as well as the evolution of NPs upon irradiation with nanosecond laser pulses has been reported elsewhere [9]. The number of pulses used to ablate the gold target has been 2500 and 3500 in order to change the gold coverage and this number will be used from now on to refer to the samples. Laser pulses from the fourth harmonics of a Nd:YAG laser ( = 266 nm, FWHM 10 ns) have been used for the laser interference experiments. The laser beam has an almost Gaussian intensity distribution in space and time. It is first split into three beams that are recombined on the sample at an angle of incidence ≈12.5◦ . The laser fluence before the beam splitting was 23 mJ cm−2 and the intensity ratio of the three interfering beams was 1.000:0.422:0.279. The morphology of the Au films has been characterized by scanning electron microscopy (SEM) in a Zeiss CrossBeam 1540XB microscope. The optical extinction spectra of both as-grown and patterned areas have been calculated as ln(1/T), where T is the transmittance measured at 0◦ of incidence angle in the range of 400–800 nm.
3. Results and discussion Fig. 1a and d shows images of the two as-grown samples studied in this work, where it is seen that metal is discontinuous in both samples, the metal coverage in the 2500 sample (68%) being lower than that in the 3500 sample (78%) as expected. While clearly isolated and irregular NPs can be identified in the 2500 sample, metal is close to percolation in the 3500 sample. Fig. 1b and e shows the areas of the 2500 and 3500 samples patterned with a single
exposure to the interfering laser beams. The pattern in the former sample is formed by parallel lines having a period of ≈0.65 m in which areas with different morphologies can be seen. The comparison of the central area of the magnification included in Fig. 1c to the as-grown film in Fig. 1a shows that they are identical and thus the former areas will be referred to from now on as nontransformed areas. The two lateral areas in Fig. 1c surrounding the non-transformed areas show different features, i.e. they are formed by round and well isolated NPs. The formation of NPs evidences that the laser exposure has induced the melting of the initial film. Due to the large surface energy difference between the substrate and the metal, the liquid metal dewets and tends to form spheres. Similar comments can be applied to the 3500 sample although the pattern in Fig. 1e is much more difficult to distinguish since all metal has been transformed into NPs. The same periodicity can be identified in Fig. 1e than Fig. 1b through the approximately vertical lines of NPs of larger diameter that have been marked by the arrows in Fig. 1e. The results clearly evidence that the laser fluence used in the experiments with respect to laser fluence threshold for transformation is higher in the 3500 sample than in the 2500 sample. This result is most likely related to a higher absorption of the 3500 sample at 266 nm that is consistent with the decrease of threshold fluence reported for several metals (included Au) as thickness increases [8]. Since we are using three interfering beams, the fact that the pattern is organised in 1D rather than in 2D features is somehow intriguing. In order to analyze this result, we have taken advantages of the Gaussian envelope of the laser beam profile that actually allows studying the dependence of the patterning process as a function of beam intensity by moving away from the centre of the patterned area towards the edge. This was done for the 3500
R.J. Peláez et al. / Applied Surface Science 258 (2012) 9223–9227
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Fig. 3. Calculated intensity distributions for a 3 beam interference and having intensity ratios of 1.000:0.422:0.279.
Fig. 2. SEM images of the 3500 sample taken at the central region (a), intermediate region (b), and near to the border (c) of the patterned region. The profile in (b) is the metal distribution in the transformed areas calculated by integrating the volume of the NPs that are assumed to be spheres along 30 nm broad vertical stripes. All images have the same magnification.
sample and the results are shown for the central region in Fig. 2a, an intermediate region in Fig. 2b and a region near the edge in Fig. 2c as intensity decreases. It is seen how the pattern evolves from an almost completely transformed film (Fig. 2a), to an area showing parallel fringes in which transformed and non-transformed areas alternate (Fig. 2b) and to isolated transformed areas organised in 2D (Fig. 2c). The fact that the 2D pattern is only observed for the laser intensities close to the threshold is thus a clear consequence of the different energies of the three interfering beams that makes the effect of the two more intense ones dominant at high energies. This is more clearly understood in Fig. 3 that shows the intensity distribution at the sample surface calculated using electrical field vectors and our experimental conditions. The comparison of Figs. 2c and 3 shows that the match between the experimentally obtained pattern and the one obtained by the calculation is excellent. As discussed for the case of the 2500 sample, the transformed areas for the intermediate intensities in the 3500 sample (Fig. 2b) are also formed by isolated NPs. There is something that can also
be identified in Fig. 1c but it becomes now clearer in Fig. 2b, i.e. the NPs become bigger at the interface of the transformed and non-transformed areas where there is even accumulation of metal. This is more obvious in Fig. 2c since the laser intensity becomes very close to the threshold for transformation. It leads to small transformed areas, some of which have its centre almost empty. Assuming the NPs are spherical, we have calculated the distribution of metal in the transformed areas of image in Fig. 2b by integrating the volume of the NPs along 30 nm broad vertical stripes and the result is the profile included. It is indeed observed that there is an accumulation of mass at the interface of transformed and non-transformed areas and only 30% of the metal is located in the transformed area. This was earlier experimentally observed [4] and theoretically studied [10] for thicker films and related to the mass transport of the liquid from hot to cold regions driven by the strong temperature gradients along the sample surface produced by the interfering beams. In addition, the inhomogeneous melting of metal at the interface can promote further mass movement. Upon melting, the metal starts to contract and at the interface, the liquid and solid make contact causing the former to move towards the latter [11]. Nanoparticles thus become bigger and tend to agglomerate at the regions where the laser intensity is lower. For the thicker sample (Fig. 2b), they even tend to form a rim leaving the area exposed to the highest intensity almost empty and approach to what was earlier reported for a 18 nm thick Au film [4]. The shape of the NPs produced in the transformed areas (excluding the interface) is very similar to what has earlier been reported for both continuous [8] and discontinuous [9] gold films upon the laser irradiation with no interfering beams and even using very different wavelengths that range from 248 nm to 650 nm. This result supports the conclusion that once the laser melts the metal, the process is mostly regulated by the non-wettability of the metal to the substrate rather than by the initial structure or thickness of the metal. Instead, the mean size and size distribution of the NPs produced upon laser exposure depends on the initial metal features. We have determined both magnitudes for the two studied samples in patterns with comparable degree of transformation such as those shown in Figs. 1c and 2b. The results show that the mean diameters are 16 nm and 27 nm for the 2500 and 3500 samples respectively, i.e. the diameter increases as initial thickness increases, and the number density is 3.5 times higher in the 2500 samples. These
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Fig. 5. Extinction spectra of as-grown and patterned films after one exposure: 2500 (dashed line) and 3500 (full line) samples.
Fig. 4. Magnified SEM images of regions patterned in the 2500 sample with (a) one and (b) ten exposures. The central area of the images corresponds to an area exposed to the lowest intensity. Both images have the same magnification.
diameters are consistent with the 11–13 nm diameter reported earlier for a thinner film [9]. Fig. 4 compares the areas of the 2500 sample exposed to single (a) and 10 (b) pulses and thus allows us to analyze the effect of accumulating several exposures in the same area. Both images have the non-transformed area at its centre and two transformed areas at either side. Actually, only a small area of irregular width <100 nm remains non-transformed in the sample exposed to 10 pulses (Fig. 4b). The main effect of accumulating pulses is to increase the width of the transformed areas and interestingly, the NPs diameter decreases when moving away from the area exposed to the highest intensity to that exposed to the lowest one. At the interface with the non-transformed area, some big NPs are again observed. The comparison of Figs. 1f and 4b shows that while the use of high fluence transforms the whole sample in an almost homogenous way, i.e. there are little reminds of the original beam intensity distribution, the accumulation of pulses at low fluence can also lead to the whole transformation of the sample having areas with NPs of different dimensions. The first pulse produces fringes in which areas with NPs and as-grown material alternate as in Fig. 4a. The absorption of the next pulse is substantially different from that of the first one not only because the absorption of the NPs is different but also an important part of the intensity is absorbed by the a-Al2 O3 buffer layer. This is consistent with the appearance of irregularly shaped areas with very dark contrast in the region exposed to the highest intensity in Fig. 4b that are thought to be holes in this buffer layer. These holes have only been observed under multipulse irradiation or at very high fluences for which the whole sample is transformed as shown in Fig. 2a. In addition, the melting temperature of metal NPs decreases as their diameter decreases [12] for NPs smaller than ≈35 nm as it is the case (Fig. 4a), making the intensity
required for melting smaller than under the first pulse. Both factors together with the inhomogeneous melting at the interface of transformed and non-transformed regions make the former regions to become broader at the expense of the latter. The NPs in the transformed regions upon repetitive melting tend to grow at the expense of the smaller ones, the lower the quenching rate, the higher the NPs and thus, bigger NPs are formed at the hotter regions and smaller ones at the surrounding intermediate regions, as seen in Fig. 4b. Fig. 5 shows the extinction spectra of the two as-grown films together with those of patterned areas with one exposure to the interfering beams. The spectra of the as-grown is consistent with the existence of isolated NPs in the 2500 sample since it shows a broad maximum around 690 nm associated to the SPR of the coalesced and irregular NPs in the as-grown materials (Fig. 1a). For the case of the 3500 sample, this band is no longer observed consistently with its quasi-percolated character and thus its higher metal coverage or effective thickness. The spectra obtained in the patterned areas were measured in the areas imaged in Fig. 1b and e and both show a narrow band peaking around 530–540 nm with nearly negligible extinction in the near IR part, similarly to what was earlier reported under the homogeneous laser irradiation at several longer wavelengths [9]. This band is clearly associated to the SPR of the round NPs produced upon melting and rapid solidification induced by the laser exposure. The fact that this band is slightly shifted and broader to longer wavelength in the 2500 sample with respect to that of the 3500 sample is most likely related to the fact that the former sample has, in addition to the NPs, irregularly shaped NPs in the non-transformed area whose contribution is stronger in the infra-red region. 4. Conclusions The exposure of discontinuous gold films to the interference laser beams produces patterns in which melting of the metal is produced in the hotter regions and round NPs are formed due to the metal contraction and low wettability of the metal to the substrate. In addition, the accumulation of metal at the interface of transformed and non-transformed areas in the form of big NPs or nearly continuous metal rims is observed, both related to mass transport driven by the temperature gradient induced by laser beam intensity distribution and inhomogeneous melting. The optical response of the patterned areas is dominated by that of the NPs and while their shapes are determined by their nonwettability of metal to the substrate and is not significantly related to the initial sample or irradiation conditions, their sizes depend on the effective thickness. The thicker the sample is the bigger
R.J. Peláez et al. / Applied Surface Science 258 (2012) 9223–9227
the NPs are. In addition, the NPs mean size and size distribution depend on the irradiation conditions as well. Single exposures can produce NPs with little size dispersion even at laser fluences high enough to completely transform the whole sample. Multi-exposure at intermediate fluences can also completely transform the whole sample and produce alternating areas of different NPs dimensions. The observed differences are found consistent with the different absorption and thermodynamic properties of the transformed and non-transformed regions.
References
Acknowledgments
[10]
This work has been partially been supported by the joint DE2009-0004 project within Acciones Integradas Program between Spain and Germany. R.P. and G.B. respectively acknowledge a grant from the JAE-doc and JAE-predoc programs, co-funded by European Social Fund.
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Selective gold nanoparticles formation by pulsed laser interference R.J. Peláez a,∗ , G. Baraldi a , C.N. Afonso a , S. Riedel b , J. Boneberg b , P. Leiderer b a b
Laser Processing Group, Instituto de Optica, CSIC, Serrano 121, 28006 Madrid, Spain Faculty of Physics, University of Konstanz, Universitätsstrasse 10, 78457 Konstanz, Germany
a r t i c l e
i n f o
Article history: Available online 17 August 2011 Keywords: Laser Patterning Interference Metal Films Nanoparticles Melting
a b s t r a c t Discontinuous Au films are prepared on glass substrates by pulsed laser deposition with two different metal coverages that lead to a film being formed by irregular coalesced nanoparticles (NPs) and to another film close to the percolation limit. The films are exposed to three interfering beams at different intensities produced by the fourth harmonic of a Nd:YAG laser (266 nm, 10 ns). Scanning electron microscopy and extinction spectra are used respectively to study the structural and optical properties before and after the laser structuring. Round metal NPs appear in the laser transformed areas due to melting followed by rapid solidification that is reflected in the extinction spectra by the appearance of a surface plasmon resonance around 530–540 nm. The areas with NPs are surrounded by non-transformed areas forming a periodic pattern that evolves from a 2D array to parallel lines when local laser intensity increases to cover the whole sample at high intensity. The accumulation of several pulses at low fluence can also transform the metal film almost completely by creating alternating areas having different NP dimensions. The accumulation of metal in some areas of the pattern is consistent with mass transport towards the lower temperature regions. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Spatially arranged micro- and nano-objects are attracting a lot of interest as platforms for several applications in physics, chemistry or biology [1,2]. In order to produce such ordered nano-structures, surface structuring is the most extensively used approach that includes various chemical processes or more complex lithographic techniques. Laser based techniques offer a fast and simple alternative for surface structuring and periodic patterns can easily be produced by using amplitude or phase masks [3]. More recently, several advanced laser methods for precision engineering including laser interferometer approaches have been reviewed [2]. Direct patterning by laser interference using 2–4 beams has been reported that has the additional advantages to produce patterns at a small period (a few hundred nm), under a single exposure over a large (>mm2 ) area in a single step process [4–7]. Nanostructured metals are the subject of strong research activity mostly stimulated by the possibility of generating large electromagnetic field enhancement via surface plasmon resonance (SPR), and Ag or Au are particularly interesting since their SPR is in the visible spectrum. There are several works on laser irradiation of Au films [8] or nanoparticles (NPs) [9] that typically report the break up of the film or the conversion of irregularly shaped
∗ Corresponding author. E-mail address: [email protected] (R.J. Peláez). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.08.048
NPs into spherical NPs with reduced dimension dispersion. Exposure of Au films to the interfering laser beams has produced either microbumps or de-wetting [4] at the hotter regions of the pattern caused by laser induced melting of the metal and mass transport to colder regions is typically observed [4,10]. The thickness of the starting film has been pointed out as an essential parameter controlling the final transformation [4,8] and most reported work has been performed on films thicker than the percolation limit. The aim of this work is to provide a deeper insight on the transformation induced by pulsed laser interference in metal films as well as the relation between structural features to the optical properties. The focus will be on discontinuous gold films, i.e. thickness smaller than the percolation limit, rather than to continuous films as in most earlier works. 2. Experimental Au films have been produced by PLD in vacuum (≈3–7 × 10−6 mbar) by focusing an ArF laser beam ( = 193 nm, = 20 ns full width half maximum-FWHM) on Au targets at an angle of 45◦ with respect to its normal. The laser repetition rate has been set to 5 Hz and the fluence at the target surface ≈2.7 J cm−2 . The substrate was held at room temperature, placed 38 mm away from the target and rotated along an axis parallel to the plasma expansion axis and shifted a few mm. This rotation leads to samples with homogeneous effective thickness or gold coverage in an area >100 mm2 . Deposition was performed on glass substrates
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R.J. Peláez et al. / Applied Surface Science 258 (2012) 9223–9227
Fig. 1. SEM images of (a–c) the 2500 and (d–f) 3500 samples: (a and d) as-grown samples; (b and e) patterns produced with one laser exposure; and (c and f) magnifications of (b and e) showing the central area exposed to the lowest intensity surrounded by other two areas exposed to the highest intensity. The latter is marked in (e) by arrows. Images in the same row have the same magnification.
and a 10 nm thick a-Al2 O3 buffer layer has in situ been produced at 20 Hz before the laser ablation of the gold target since the growth of Au NPs on this surface by PLD as well as the evolution of NPs upon irradiation with nanosecond laser pulses has been reported elsewhere [9]. The number of pulses used to ablate the gold target has been 2500 and 3500 in order to change the gold coverage and this number will be used from now on to refer to the samples. Laser pulses from the fourth harmonics of a Nd:YAG laser ( = 266 nm, FWHM 10 ns) have been used for the laser interference experiments. The laser beam has an almost Gaussian intensity distribution in space and time. It is first split into three beams that are recombined on the sample at an angle of incidence ≈12.5◦ . The laser fluence before the beam splitting was 23 mJ cm−2 and the intensity ratio of the three interfering beams was 1.000:0.422:0.279. The morphology of the Au films has been characterized by scanning electron microscopy (SEM) in a Zeiss CrossBeam 1540XB microscope. The optical extinction spectra of both as-grown and patterned areas have been calculated as ln(1/T), where T is the transmittance measured at 0◦ of incidence angle in the range of 400–800 nm.
3. Results and discussion Fig. 1a and d shows images of the two as-grown samples studied in this work, where it is seen that metal is discontinuous in both samples, the metal coverage in the 2500 sample (68%) being lower than that in the 3500 sample (78%) as expected. While clearly isolated and irregular NPs can be identified in the 2500 sample, metal is close to percolation in the 3500 sample. Fig. 1b and e shows the areas of the 2500 and 3500 samples patterned with a single
exposure to the interfering laser beams. The pattern in the former sample is formed by parallel lines having a period of ≈0.65 m in which areas with different morphologies can be seen. The comparison of the central area of the magnification included in Fig. 1c to the as-grown film in Fig. 1a shows that they are identical and thus the former areas will be referred to from now on as nontransformed areas. The two lateral areas in Fig. 1c surrounding the non-transformed areas show different features, i.e. they are formed by round and well isolated NPs. The formation of NPs evidences that the laser exposure has induced the melting of the initial film. Due to the large surface energy difference between the substrate and the metal, the liquid metal dewets and tends to form spheres. Similar comments can be applied to the 3500 sample although the pattern in Fig. 1e is much more difficult to distinguish since all metal has been transformed into NPs. The same periodicity can be identified in Fig. 1e than Fig. 1b through the approximately vertical lines of NPs of larger diameter that have been marked by the arrows in Fig. 1e. The results clearly evidence that the laser fluence used in the experiments with respect to laser fluence threshold for transformation is higher in the 3500 sample than in the 2500 sample. This result is most likely related to a higher absorption of the 3500 sample at 266 nm that is consistent with the decrease of threshold fluence reported for several metals (included Au) as thickness increases [8]. Since we are using three interfering beams, the fact that the pattern is organised in 1D rather than in 2D features is somehow intriguing. In order to analyze this result, we have taken advantages of the Gaussian envelope of the laser beam profile that actually allows studying the dependence of the patterning process as a function of beam intensity by moving away from the centre of the patterned area towards the edge. This was done for the 3500
R.J. Peláez et al. / Applied Surface Science 258 (2012) 9223–9227
9225
Fig. 3. Calculated intensity distributions for a 3 beam interference and having intensity ratios of 1.000:0.422:0.279.
Fig. 2. SEM images of the 3500 sample taken at the central region (a), intermediate region (b), and near to the border (c) of the patterned region. The profile in (b) is the metal distribution in the transformed areas calculated by integrating the volume of the NPs that are assumed to be spheres along 30 nm broad vertical stripes. All images have the same magnification.
sample and the results are shown for the central region in Fig. 2a, an intermediate region in Fig. 2b and a region near the edge in Fig. 2c as intensity decreases. It is seen how the pattern evolves from an almost completely transformed film (Fig. 2a), to an area showing parallel fringes in which transformed and non-transformed areas alternate (Fig. 2b) and to isolated transformed areas organised in 2D (Fig. 2c). The fact that the 2D pattern is only observed for the laser intensities close to the threshold is thus a clear consequence of the different energies of the three interfering beams that makes the effect of the two more intense ones dominant at high energies. This is more clearly understood in Fig. 3 that shows the intensity distribution at the sample surface calculated using electrical field vectors and our experimental conditions. The comparison of Figs. 2c and 3 shows that the match between the experimentally obtained pattern and the one obtained by the calculation is excellent. As discussed for the case of the 2500 sample, the transformed areas for the intermediate intensities in the 3500 sample (Fig. 2b) are also formed by isolated NPs. There is something that can also
be identified in Fig. 1c but it becomes now clearer in Fig. 2b, i.e. the NPs become bigger at the interface of the transformed and non-transformed areas where there is even accumulation of metal. This is more obvious in Fig. 2c since the laser intensity becomes very close to the threshold for transformation. It leads to small transformed areas, some of which have its centre almost empty. Assuming the NPs are spherical, we have calculated the distribution of metal in the transformed areas of image in Fig. 2b by integrating the volume of the NPs along 30 nm broad vertical stripes and the result is the profile included. It is indeed observed that there is an accumulation of mass at the interface of transformed and non-transformed areas and only 30% of the metal is located in the transformed area. This was earlier experimentally observed [4] and theoretically studied [10] for thicker films and related to the mass transport of the liquid from hot to cold regions driven by the strong temperature gradients along the sample surface produced by the interfering beams. In addition, the inhomogeneous melting of metal at the interface can promote further mass movement. Upon melting, the metal starts to contract and at the interface, the liquid and solid make contact causing the former to move towards the latter [11]. Nanoparticles thus become bigger and tend to agglomerate at the regions where the laser intensity is lower. For the thicker sample (Fig. 2b), they even tend to form a rim leaving the area exposed to the highest intensity almost empty and approach to what was earlier reported for a 18 nm thick Au film [4]. The shape of the NPs produced in the transformed areas (excluding the interface) is very similar to what has earlier been reported for both continuous [8] and discontinuous [9] gold films upon the laser irradiation with no interfering beams and even using very different wavelengths that range from 248 nm to 650 nm. This result supports the conclusion that once the laser melts the metal, the process is mostly regulated by the non-wettability of the metal to the substrate rather than by the initial structure or thickness of the metal. Instead, the mean size and size distribution of the NPs produced upon laser exposure depends on the initial metal features. We have determined both magnitudes for the two studied samples in patterns with comparable degree of transformation such as those shown in Figs. 1c and 2b. The results show that the mean diameters are 16 nm and 27 nm for the 2500 and 3500 samples respectively, i.e. the diameter increases as initial thickness increases, and the number density is 3.5 times higher in the 2500 samples. These
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R.J. Peláez et al. / Applied Surface Science 258 (2012) 9223–9227
Fig. 5. Extinction spectra of as-grown and patterned films after one exposure: 2500 (dashed line) and 3500 (full line) samples.
Fig. 4. Magnified SEM images of regions patterned in the 2500 sample with (a) one and (b) ten exposures. The central area of the images corresponds to an area exposed to the lowest intensity. Both images have the same magnification.
diameters are consistent with the 11–13 nm diameter reported earlier for a thinner film [9]. Fig. 4 compares the areas of the 2500 sample exposed to single (a) and 10 (b) pulses and thus allows us to analyze the effect of accumulating several exposures in the same area. Both images have the non-transformed area at its centre and two transformed areas at either side. Actually, only a small area of irregular width <100 nm remains non-transformed in the sample exposed to 10 pulses (Fig. 4b). The main effect of accumulating pulses is to increase the width of the transformed areas and interestingly, the NPs diameter decreases when moving away from the area exposed to the highest intensity to that exposed to the lowest one. At the interface with the non-transformed area, some big NPs are again observed. The comparison of Figs. 1f and 4b shows that while the use of high fluence transforms the whole sample in an almost homogenous way, i.e. there are little reminds of the original beam intensity distribution, the accumulation of pulses at low fluence can also lead to the whole transformation of the sample having areas with NPs of different dimensions. The first pulse produces fringes in which areas with NPs and as-grown material alternate as in Fig. 4a. The absorption of the next pulse is substantially different from that of the first one not only because the absorption of the NPs is different but also an important part of the intensity is absorbed by the a-Al2 O3 buffer layer. This is consistent with the appearance of irregularly shaped areas with very dark contrast in the region exposed to the highest intensity in Fig. 4b that are thought to be holes in this buffer layer. These holes have only been observed under multipulse irradiation or at very high fluences for which the whole sample is transformed as shown in Fig. 2a. In addition, the melting temperature of metal NPs decreases as their diameter decreases [12] for NPs smaller than ≈35 nm as it is the case (Fig. 4a), making the intensity
required for melting smaller than under the first pulse. Both factors together with the inhomogeneous melting at the interface of transformed and non-transformed regions make the former regions to become broader at the expense of the latter. The NPs in the transformed regions upon repetitive melting tend to grow at the expense of the smaller ones, the lower the quenching rate, the higher the NPs and thus, bigger NPs are formed at the hotter regions and smaller ones at the surrounding intermediate regions, as seen in Fig. 4b. Fig. 5 shows the extinction spectra of the two as-grown films together with those of patterned areas with one exposure to the interfering beams. The spectra of the as-grown is consistent with the existence of isolated NPs in the 2500 sample since it shows a broad maximum around 690 nm associated to the SPR of the coalesced and irregular NPs in the as-grown materials (Fig. 1a). For the case of the 3500 sample, this band is no longer observed consistently with its quasi-percolated character and thus its higher metal coverage or effective thickness. The spectra obtained in the patterned areas were measured in the areas imaged in Fig. 1b and e and both show a narrow band peaking around 530–540 nm with nearly negligible extinction in the near IR part, similarly to what was earlier reported under the homogeneous laser irradiation at several longer wavelengths [9]. This band is clearly associated to the SPR of the round NPs produced upon melting and rapid solidification induced by the laser exposure. The fact that this band is slightly shifted and broader to longer wavelength in the 2500 sample with respect to that of the 3500 sample is most likely related to the fact that the former sample has, in addition to the NPs, irregularly shaped NPs in the non-transformed area whose contribution is stronger in the infra-red region. 4. Conclusions The exposure of discontinuous gold films to the interference laser beams produces patterns in which melting of the metal is produced in the hotter regions and round NPs are formed due to the metal contraction and low wettability of the metal to the substrate. In addition, the accumulation of metal at the interface of transformed and non-transformed areas in the form of big NPs or nearly continuous metal rims is observed, both related to mass transport driven by the temperature gradient induced by laser beam intensity distribution and inhomogeneous melting. The optical response of the patterned areas is dominated by that of the NPs and while their shapes are determined by their nonwettability of metal to the substrate and is not significantly related to the initial sample or irradiation conditions, their sizes depend on the effective thickness. The thicker the sample is the bigger
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the NPs are. In addition, the NPs mean size and size distribution depend on the irradiation conditions as well. Single exposures can produce NPs with little size dispersion even at laser fluences high enough to completely transform the whole sample. Multi-exposure at intermediate fluences can also completely transform the whole sample and produce alternating areas of different NPs dimensions. The observed differences are found consistent with the different absorption and thermodynamic properties of the transformed and non-transformed regions.
References
Acknowledgments
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This work has been partially been supported by the joint DE2009-0004 project within Acciones Integradas Program between Spain and Germany. R.P. and G.B. respectively acknowledge a grant from the JAE-doc and JAE-predoc programs, co-funded by European Social Fund.
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