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Nanosecond pulsed laser nanostructuring of Au thin films: Comparison between irradiation at low and atmospheric pressure
Accepted Manuscript Title: Nanosecond pulsed laser nanostructuring of Au thin films: comparison between irradiation at low and atmospheric pressure Authors: C. S´anchez-Ak´e, A. Canales-Ramos, T. Garc´ıa-Fern´andez, M. Villagr´an-Muniz PII: DOI: Reference:
S0169-4332(17)30203-9 http://dx.doi.org/doi:10.1016/j.apsusc.2017.01.181 APSUSC 34976
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Received date: Revised date: Accepted date:
20-9-2016 3-1-2017 18-1-2017
Please cite this article as: C.S´anchez-Ak´e, A.Canales-Ramos, T.Garc´ıa-Fern´andez, M.Villagr´an-Muniz, Nanosecond pulsed laser nanostructuring of Au thin films: comparison between irradiation at low and atmospheric pressure, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2017.01.181 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Nanosecond pulsed laser nanostructuring of Au thin films: comparison between irradiation at low and atmospheric pressure
C. Sánchez-Aké1,*, A. Canales-Ramos1, T. García-Fernández2, M. Villagrán-Muniz1
1
Centro de Ciencias Aplicadas y Desarrollo Tecnológico, Universidad Nacional Autónoma de México, Circuito Exterior S/N, C. U., Delegación Coyoacán, C.P. 04510, México D. F. México 2
Universidad Autónoma de la Ciudad de México (UACM), Prolongación San Isidro 151, Col. San Lorenzo Tezonco, México D.F., C.P. 09790, México.
*
Corresponding author: [email protected]
Highlights
Background pressure plays an important role in NPs formation and its characteristics The NPs diameter and their size dispersion are smaller when irradiating in vacuum The plasmon resonance shifts 15 nm to higher frequencies when irradiating in vacuum Film partial ablation cannot be neglected for thickness in the range 40-80 nm In situ optical techniques monitor the timescale of the process and ablation dynamics
Abstract Au thin films with tens of nm in thickness deposited on glass substrates were irradiated with nanosecond UV (355 nm) laser pulses at atmospheric pressure and in vacuum conditions (600 and 10-5 Torr). We studied the effect of the laser fluence (200-400 mJ/cm2), thickness of the starting film ( 40-80 nm) and surrounding pressure on the partial ablation/evaporation of the films and the morphology of the produced nanoparticles (NPs). The dynamics of NPs formation was studied by measuring in real time the transmission of the samples upon continuous-wave laser exposure, and by means of probe beam deflection technique. The ejection of material from the film as a result of the irradiation was confirmed by time-resolved shadowgraphy technique. Experiments show that the NPs diameter and their size distribution are smaller when the irradiation is performed in vacuum regardless the laser fluence and thickness of the started film. It is also shown that the plasmon band shifts to higher frequencies with lower background pressure. The optical measurements show that the films melt and ablate during the laser pulse, but the transmission of the irradiated areas continues changing during tens of microseconds due to ejection of material and solidification of the remaining gold. Our results indicate that partial ablation cannot be neglected in nanostructuration by ns-pulsed irradiation of thin films when their thickness is in the studied range.
Keywords Gold nanoparticles; laser ablation; localized surface plasmon resonance; in situ transmittance; probe beam deflection.
Introduction The optical properties of nobel metal nanoparticles (NPs) and their potential application in sensors and photovoltaic devices [1] have result in a growing interest in developing fast, effective and lowcost methods of synthesis. The optical absorption of the NPs is dominated by the surface plasmon resonance band (SPR) which depends on their size, shape, separation between them and surrounding media [2]. Therefore, controlling the morphology of the NPs is very important for further applications.
Laser irradiation of thin metal films has shown to be an effective method for the synthesis of NPs attached or even embedded in dielectric substrates [3-5]. It has also been applied for producing NPs in periodic arrays by using interference beam patterns [6] and for nanostructuring of dielectric materials by multipulse irradiation [7]. The formation of NPs due to pulsed-laser irradiation has been explained in terms of the melting of the film followed by (i) heterogeneous nucleation from liquid phase on defects, and (ii) rupture of the film due to spinodal dewetting [8]. The influence of experimental parameters on the morphology of the produced nanostructures has been studied over a wide range of laser fluences, film thicknesses and number of pulses [3, 4, 6, 9-11]. Most of the works reported use very thin films, with thickness smaller than 20 nm or even below the percolation limit [12]. In those cases, it has been found that the NPs diameter increases with the thickness of the starting film [11, 13]. Besides, for thicknesses up to 15 nm it has been reported a good agreement between the experimental observed NPs diameter and that calculated theoretically using thermal models based on the dewetting phenomenon [11, 14, 15]. Despite this confirmed dependence between the film thickness and the size of the NPs, it is not clear if this tendency continues for larger values of the thickness of the starting film. On the other hand, most of the theoretical models used to describe laser nanostructuring assume that during the laser irradiation, there are no losses of the metal due to evaporation from the surface of the film and the substrate [11]. However, metal evaporation along with the formation of NPs have been reported repeatedly in experimental works. Ruffino et al. [16] detected by Rutherford backscattering spectrometry (RBS) loss of Au by one-pulse laser irradiation of a 20 nm thin film. Chiu et al. [17] monitored by mass spectrometry the formation of gaseous gold cluster ions during the multi-pulse irradiation of gold films with thickness from 10 to 100 nm. Henley et al. [3] suggested that the implantation of metal NPs into the surface of glass substrates is due to boiling of both the film and a surface layer of the glass. Furthermore, metal evaporation has also been reported by irradiating noble metal islands and NPs previously produced either by direct deposition or by thin films irradiation. As a consequence of the evaporation of the metal, the irradiation of preexisting metal islands or NPs results in a decrease of the size of the starting metal particles [18, 19] and/or in the nanodrilling of the substrate [7, 20, 21]. This is particularly important in multi pulse irradiation of thin films since the first pulses can lead to the formation of NPs and the subsequent pulses to the modification of them. Despite these reports, the range of laser fluences and the thickness of the starting film in which there is partial evaporation of the metal film along with NPs formation has not been identified. In addition, the influence of the evaporated material and its possible back condensation to the substrate on the formation of the NPs is still not fully understand. The aim of this work is to investigate the partial evaporation of the starting metal film and the formation of NPs by irradiating gold films with thickness of 44 – 82 nm. The films were irradiated at atmospheric pressure and in vacuum in order to determine its effect on the morphology and size of the obtained NPs. Additionally, we employed optical in situ techniques to characterize the timescale of melting, evaporation and solidification processes associated to the NPs formation. Experimental details
Au thin films were deposited at room temperature by DC sputtering on corning glass (Soda lime glass 2947) substrates. The thicknesses of the films h measured by Atomic force Microscopy (AFM), were 44 ± 7, 55 ± 9, 60 ± 9 and 82 ± 13 nm. The films were irradiated in vacuum (≈ 3x10-5 Torr) and at atmospheric pressure in Mexico City ( 580 Torr), by a sequence of 5 pulses with duration of 6 ns, of a Nd:YAG laser operating at 355 nm at a 10 Hz repetition rate. For irradiation, the laser beam was directed normal to the surface of the film. The Gaussian beam diameter on the surface was about 2.42 ± 0.05 mm. The values of the laser fluence used for irradiation were 200, 300, and 400 mJ/cm2. These values were obtained by varying the laser energy, but keeping fixed the spot diameter. Both, the as-grown and the irradiated samples were characterized by scanning electron microscopy (SEM) using a FEI helios nanolab 600 microscope. The SEM images were processed by an in-house program based on anisotropic diffusion [22]. Besides, the absorbance of the films before and after irradiation was measured at a normal incidence angle in the range of 350-700 nm with an UV-Vis Cary 5000 dual beam spectrometer. The absorbance was measured using the software ABS function: A = -log(I/I0). Two cw He-Ne lasers ( = 632 nm) were used to study in situ the nanoparticles formation process during 200 µs after the first pulse. One beam, which was directed to the center of the irradiated area at an incident angle of 45°, was used to record in real time the transmission of the sample. Similar optical measurements have been used to study the timescale of the melting and solidification process and NPs formation [6] and the formation of periodic surface structures [23] by irradiation of metallic thin films. The second laser, running parallel to the substrate, was used as a probe beam to detect its deflection due to material emerging from the surface of the sample. The probe beam with 1 mm of diameter was kept as close as possible to the surface without blocking the former. This kind of measurements has been applied to study the ablation of materials in bulk or even in thin films [24]. Both signals were detected by to two fast photodiodes (PD) and recorded by a 500 MHz oscilloscope which was triggered with a third PD placed near the output of the pulsed laser. We employed an interference filter in front of the PD that detects the transmission intensity to suppress any contribution of the laser pulses to the recorded signals. In order to verify the ejection of material from the film and/or from the substrate, we employed laser shadowgraphy. This technique has been used previously to study ablation of thin metal films [25, 26]. As illuminating source we used the emission of a frequency-doubled Nd:YAG laser ( = 532 nm) with pulses of 7 ns. This beam was aligned parallel to the surface of the film and was expanded to illuminate a region of approximately 1 cm in diameter in the vicinity of the irradiated area. Synchronization between irradiation and illumination pulses was performed by means of a pulse delay generator and the images were captured through a charged-coupled device (CCD) working in video mode. Results and discussion Nanoparticles characterization
All films were irradiated with 5 pulses to guarantee the formation of NPs for the three studied fluences. Drastic morphology changes by pulsed laser irradiation as a function of the number of pulses have been reported before in numerous works [5, 10, 11, 21, 27-29]. In our case, single pulse irradiation with the lowest fluence modified the surface morphology of the 60 nm-thick film, producing a discontinuous structure with asymmetric undulations and only few NPs were observed. In contrast, after 5 pulses numerous spherical NPs and no apparent remnant of the starting film were observed in all irradiated area. The size and morphology of the Au NPs produced by laser irradiation of thin films at atmospheric pressure were significantly different from those irradiated in vacuum. Figure 1 shows representative SEM images of a 55 nm-thick film after irradiation with a sequence of 5 laser pulses at fluences of 200, 300 and 400 mJ/cm2, under atmospheric pressure and vacuum conditions. The particles size distribution is shown in each image. These distributions were calculated on a statistic population of at least 800 islands by using SEM images of different regions near the center of the irradiated area. We studied only this region since the irradiation was performed with a Gaussian beam, thus the local fluence varies with the distance from the beam center. As a result, the NPs formation was not uniform in all the irradiated area. This effect was clearly seen by the naked-eye since the irradiation produced a strongly coloration in the center of the spot, but the periphery of the irradiated area looked as clean glass. The coloration is produced by the surface plasmon resonance and indicates the presence of nanoparticles formed by dewetting or vaporization and re-condensation [3]. On the contrary, the apparent absence of gold in the rim of the spot suggests that the energy density was not enough for melting and boiling the film. When the laser fluence is below the melting threshold, but sufficient to induce thermal expansion and stress in both, the film and the substrate, the former can be ablated in solid phase [25]. This take place when the mechanical stress is high enough to tear off part of the film. Because of the dependence of the energy density with the position, SEM and absorbance analyses were performed only in a region near the center of the laser spot. From SEM images obtained after irradiation of films with different thickness (h), we calculated the mean diameter and standard deviation of the nanoparticles, as reported in Table 1. The most important feature is that for all the used thicknesses, smaller NPs are observed in the experiments performed in vacuum in comparison with that produced at atmospheric pressure. In addition, the standard deviation of the size distribution also decreases with the background pressure. A possible reason for this is that a part of the film evaporates leaving a distribution of nanoparticles formed by the dewetting of the remaining material on the substrate. This possible mechanism will be discussed later. One of the main drawbacks of the laser irradiation technique is that the size distribution of the produced NPs is large in comparison with other methods of synthesis. Therefore, our results indicate that this technique can be improved performing the irradiation at low pressure. Furthermore, the morphology of the NPs produced in both conditions is quite different (see Fig. 1), the NPs produced in vacuum are spherical with smooth surface whereas that those produced at atmospheric pressure have multiple small NPs grafted to their surface. This behavior was observed in all thicknesses of the starting film. Other authors have reported the formation of NPs decorated with smaller ones by laser irradiation at atmospheric pressure of thin
films [16] and NPs [28]. Since this kind structure was only observed in the experiments performed at atmospheric pressure, their formation may be due to condensation back of evaporated material because of the ambient gas. From table 1, it is not possible to clearly describe how the fluence, in the range from 200 to 400 mJ/cm2 and the thickness of the starting film affect the size of the obtained NPs. With the aim to compare the effect of the background pressure, we calculated the average diameter over the three fluences, for each value of thickness h. These average values, plotted in Fig. 2(a), show the already mentioned effect of the surrounding media: for all the thicknesses, the NPs produced in vacuum have smaller diameter. The average NPs size seems to be weakly dependent on the starting film thickness except for the thinnest film irradiated at atmospheric pressure. In that case, the diameter of the nanoparticles is approximately 40 % larger than those produced with all other thicknesses. Given that the background pressure affects the size of the NPs, it is expected that it also has an effect on the optical absorbance of the irradiated films. Fig. 2(b) displays the wavelength position of the surface plasmon resonance (max) as a function of the thickness of the starting film. Again, each point is the average value calculated for the three used fluences. Regarding the effect of the film thickness, the wavelength of the plasmon band remains approximately constant throughout h 60 nm and shifts to the blue for the case of the thickest film. Furthermore, there is a clear difference between the films irradiated in vacuum and at atmospheric pressure. The former exhibits values of max shifted to the blue for all used thicknesses and substrates. This was expected, due to the fact that the smaller NPs were produced by irradiation in vacuum. However, there are other morphological aspects that can affect the surface plasmon resonance. For instance, NPs with separations smaller than their diameter can interact resulting in a shift of the plasmon band to the red [12]. In our case, the NPs produced at atmospheric pressure showed to have smaller NPs grafted to their surface (see for example Fig. 1(a)-(c)), thus it may contribute to the red shift. The results showed in Fig.1 and 2 evidence that the background pressure is an important parameter when processing thin films by laser irradiation. The main factors that differentiate the NPs formation in vacuum from that at atmospheric pressure are: (i) heat dissipation from the film to the surrounding media, (ii) boiling temperature of the film and, (iii) dynamics of the evaporated material (expansion and confinement near the film surface). In the case of (i), the heat loss from the Au surface to the ambient can be ignored in comparison to that dissipated within the substrate [28]. However, (ii) plays an important role since it depends on the pressure exerted on the film surface by the surrounding media: the higher the pressure, the higher the vaporization temperature [29]. For the case of Au in bulk, the vaporization temperature drops approximately from 2800° to 1120° C when the pressure decreases from 585 Torr to 7 x 10-5 Torr [30]. Thus, the films irradiated at atmospheric pressure reach a higher temperature than those processed in vacuum. Besides, the increase of the vapor pressure with the background pressure in turn implies that the fluence threshold to vaporization also increases. Since the amount of removed material is linearly proportional to the laser energy density above its threshold [29], it is expected that a
larger fraction of material evaporates at vacuum. Regarding (iii), the background pressure plays a critical role in the dynamics of the ejected material. This material can expand freely in vacuum but at atmospheric pressure it can be confined and possibly condensed back on the surface. This will be discussed in more detail in the last part of this section. To determine if there is loss of gold in the films as a result of the laser irradiation, we analyzed their absorbance UV-Vis spectra. Figure 3(a) depicts the absorbance spectra of a representative 55 nm-thick film irradiated with the three used fluences under both pressure conditions. It can be noted that for all wavelengths and in particular for the UV region (from 350-400 nm), the absorbance of the films processed at atmospheric pressure is larger than that for the samples irradiated in vacuum. Absorbance at wavelengths in the UV could enable an estimation of Au concentration due to that its main contribution comes from interband transitions in metallic gold [31]. In contrast, for wavelengths above 400 nm, the absorbance is increasingly influenced by the localized surface plasmon resonance [32]. Thus, by choosing an adequate wavelength it is possible to estimate the amount of gold regardless of particle size. This method has been applied to determine Au concentration in colloids [31-33] and in thin films [34]. Its main advantages are that the determination of the amount of gold does not depend on the particle size and shape [31] and, that it can be applied for NPs in liquids and supported on transparent substrates. However, both the presence of NPs with diameter < 2.5 nm [35] and organic substances [31, 32] can contribute to the absorbance below 400 nm thus restricting the validity of the measurement. In order to compare the loss of Au as a function of the thickness of the starting film, we measured the absorbance at 350 nm (vertical dotted line in Fig. 3(a)) after and before irradiation. The measured absorbance values after irradiation at atmospheric pressure were 0.29 ± 0.04, 0.26 ± 0.05, 0.26 ± 0.07 and 0.19 ± 0.09 for the starting films with thickness 44, 55, 60 and 82 nm respectively. Whereas in vacuum were 0.15 ± 0.04, 0.11 ± 0.02, 0.14 ± 0.04 and 0.08 ± 0.04 for the same values of the thickness. Fig. 3(b) shows the relative absorbance at 350 nm, defined as the absorbance ratio of the irradiated areas to the as-grown film. Since the value of Abs350 nm is proportional to the amount of gold on the substrate, the relative absorbance gives the proportion of Au after irradiation compared with the Au amount before irradiation (as-grown film). We found that regardless the thickness, the relative absorbance at 350 nm is always smaller in the films irradiated in vacuum, thus revealing that the gold loss is larger in this condition. This is not a surprising result due to the difference in the threshold of energy density for vaporization discussed above. Since this threshold is below for irradiation in vacuum, a large amount of the film is ablated and the nanoparticles are formed by the remaining material on the substrate. Furthermore, the maximum temperature that the films reach under irradiation at atmospheric pressure is higher than in vacuum and consequently, the temperature reached by the surface of the substrate is also higher. This is because the thickness h of our films fulfills -1 < h < LT, as the values of the optical absorption length and the thermal diffusion length are -1 = 18 nm and LT = 1237 nm for gold in bulk, respectively [16]. The fact that the substrate reaches higher temperatures at ambient pressure than in vacuum may favor that a larger fraction of gold remains on the substrate. This is suggested since previous reports have demonstrated that the adhesion of Au thin films increases with the substrate temperature during the deposition and with post annealing at high
temperatures [36]. Moreover, higher temperatures favour the softening or even melting of the glass. It has been reported that the remaining material on the substrate after irradiation of metallic thin films is due to the melting of both, the substrate and the film [37]. Figure 3(b) also reveals that the proportion of the remaining gold after irradiation decreases with the thickness of the starting film regardless the surrounding pressure. As the thickness increases with constant pressure and fluence, there are two parameters in competition that influence the amount of ejected material: the ablation threshold and the temperature that the substrate reaches. By one hand, for thicknesses h < LT, the ablation threshold increases linearly with the thickness [24]. Thus, it would be expected larger amount of removed material for the thinnest film. On the other hand, the temperature reached by the surface of the substrate is inversely proportional to the film thickness since the same deposited energy on the film heats different amounts of gold. Therefore, as mentioned above, the adhesion to the substrate is better for the thinnest film. Our results suggest that under our conditions, the impact of the substrate temperature is more important than the ablation threshold dependence on the thickness. As mentioned above, the NPs size was affected by the surrounding pressure as shown in Fig. 1 and Table 1. In general, both the NPs average diameter and the size distribution were larger when irradiating at atmospheric pressure regardless the thickness of the starting film and the laser fluence. This is likely a consequence of two main causes. First, the amount of evaporated material is smaller in the case of irradiation at high pressure, thus the amount of available remaining Au to form NPs is higher than in vacuum. Several previous works have explained in terms of the dewetting that, for different number of pulses, the mean nanoparticle diameter increases with the thickness of the starting film [9-11, 13, 27]. The second cause is related with the maximum temperature reached during irradiation, which is considerable higher in the case of the experiment at atmospheric pressure [30]. Higher temperatures increase the mobility of the droplets on the substrate, as their surface boils, probably allowing some of the surviving drops to coalesce. The coalescence of the NPs with near neighbors may produce the formation of observed larger Au NPs and the increase of the size distribution since coalescence depends on the distance between neighbor NPs [23]. Some previous reports support this hypothesis; Henley et al [9] attributed the increase of the NPs size when increasing the laser fluence, to the coalescence of droplets when the fluence is above the boiling point. Wang et al. [23] reported that the coalescence of neighbor Au NPs increases their average size as the temperature increases. Dynamics of nanoparticles formation The dynamics of NPs formation was studied by measuring in real time: (i) the transmitted intensity of a continuous beam directed at the irradiated area and, (ii) the deflection of a continuous probe beam oriented parallel and grazing to the area of irradiation. In addition, time-resolved shadowgraphy images of the vicinity of the film were acquired at different time delays from ns to µs. Fig. 4(a)-(b) show the transmission and deflection signals directly obtained from the oscilloscope for both pressure conditions, corresponding to the first pulse irradiation of a 55 nmthick untreated thin film at 300 mJ/cm2. Fig. 4(c)-(f) shows the corresponding shadowgraphy images at different time delays for irradiation performed at atmospheric pressure. The recorded
intensity of the probe beams and the shadowgraph images showed noticeable changes for the first pulse but not for the subsequent pulses, suggesting that the main ablation of the film takes place in the first laser pulse. Lorenz et al. [38] reported similar behavior of the transmission intensity when irradiating Cr films with different number of pulses. Even though the subsequent pulses did not produce noticeable changes in the recorded intensity of the probe beams, those pulses might affect the remnant droplets either boiling or melting them. However further studies are needed to confirm this hypothesis. For both pressure conditions, the transmission intensity showed in Fig. 4(a) starts with a very rapid increase during the laser pulse used for irradiation (approximately 10 ns after its onset). After that, this signal increases monotonically for long time until reaching a maximum value after tens of µs in the case of irradiation in vacuum. In contrast, at atmospheric pressure, the transmission shows two minima (at 500 ns and 15 µs) and after that rises monotonically until reaching a final level at 40 µs. The behavior of this signal integrates the ejection of gold from the surface of the film and solidification of the remaining material on the substrate. We attribute the abrupt rise of the transmission signal at the first ns under both pressure conditions, to the melting and ablation of the film. Previous reports have demonstrated by measuring the reflectivity of metal thin films under irradiation in similar conditions that the films melt during the laser pulse [6, 23, 39]. The final transmitted intensity is consistent with the films becoming more transparent at the probe wavelength (632 nm), as it is evidenced in the absorbance spectra in Fig. 3(a). The ejection of material from the film due to irradiation was studied by means of probe deflection technique. Fig. 4(b) shows the deflection signals corresponding to irradiation in vacuum and at atmospheric pressure. It is well known that light deflection is consequence of a transient gradient of refractive index, which can be caused by heat, shock waves, vapor or plasma plumes [24]. The deflection signal in vacuum shows two minima. Since the film melts/ablates during the laser pulse, we attribute them to two groups of material ejected with travelling velocities of 60 m/s and 10 m/s. Probably, these groups corresponds to ablated Au+, Au, clusters of Au [17] and delaminated gold flakes [25]. As expected, the velocity of both groups increases with the laser fluence. For the first group from 50 m/s at 200 mJ/cm2 to 60 m/s at 400 mJ/cm2, whereas for the second one from 8 to 16 m/s respectively. For the case of irradiation at atmospheric pressure, the changes in the deflection signal can be compared with the shadowgraphy images (4(c)-(f)). This signal starts decreasing at 500 ns which corresponds to the shock wave observed in Fig. 4(c). The formation of this shock wave also affects the transmitted intensity as showed in Fig. 4(a). The minimum at 22 µs is attributed to ejected material that slowly expands as illustrated in Fig. 4(d)-(f). Since this experiment was performed at atmospheric pressure, one part of the ejected material can be confined by the surrounding pressure and another moves away from the substrate. Therefore, the transmission and deflection signals integrate both contributions. The fact that the principal minimum appears approximately 20 µs later in vacuum than at high pressure is assumed due to the difference of the maximum temperature reached in both conditions. Since the evaporation temperature in vacuum is lower, the kinetic energy of the ejected material is also lower than the one obtained at atmospheric pressure. Moreover, the two groups observed in vacuum may be aggregated in one group when irradiating at atmospheric pressure, because lower-energy ejected
particles are contained within the vicinity of the substrate. One part of this confined material probably condenses back into the substrate. This may be the cause of the decoration of the NPs observed in Fig. 1(a)-(c). Conclusions We compare the NPs formation by laser irradiation of thin films in vacuum and at atmospheric pressure. It was found that NPs produced in vacuum have smaller diameter and size dispersion than those produced at atmospheric pressure. Moreover, the morphology of the NPs is also affected by the background pressure. As a consequence, the plasmon band shifts to lower wavelengths by decreasing the pressure. The real time optical measurements indicate that the NPs formation starts with the melting and partial ablation of the film during the laser pulse-film interaction. The absorbance spectra and shadowgraphy images reveal partial ablation of the films. This ablation process depends on the surrounding media because of the vapor pressure. The lower the pressure, the lower evaporation temperature of the gold film. Consequently, a large amount of gold can be ablated due to the ablation threshold fluence and the temperature reached by the glass substrate are also lower. Since the NPs are formed with the remaining material on the substrate, their size and morphology also depend on the surrounding pressure. The changes in optical signals last tens of microseconds due to material ejection and solidification of remaining gold. In vacuum, the ejected material moves away from the substrate; while at atmospheric pressure, low-energy particles can be confined and possible condense back to the substrate, affecting the final NPs morphology. Our experimental results show that, in contrast with previous reports for films thinner than 20 nm, there is partial ablation of the film when its thickness is in the studied range (40-80 nm). Moreover, this partial ablation plays an important role in the formation of NPs and their characteristics. Acknowledgements This work was supported by project PAPIIT-IG100415. The authors thank the financial support from CONACyT by means of the Red de Nanociencias y Nanotecnología and the project CB 176705. The authors gratefully thank Professor J.L. Sánchez-Llamazares for his support and help in this research and A. Esparza for the deposition of the films. We also acknowledge J.G Bañuelos, G. Labrada and A.I. Peña and the Laboratories LUCE-CCADET and LINAN-IPICyT for providing the characterization of the samples.
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Hashimoto, S., et al., Mechanistic Aspect of Surface Modification on Glass Substrates Assisted by Single Shot Pulsed Laser-Induced Fragmentation of Gold Nanoparticles. Journal of Physical Chemistry C, 2011. 115: p. 4986-4993. Lorenz, P., et al., Nanodrilling of fused silica using nanosecond laser radiationP. Applied Surface Science, 2015. 351: p. 935-945. Perona, P. and J. Malik, Scale-space and edge detection using anisotropic diffusion. IEEE Transactions on Pattern Analysis and Machine Intelligence, 1990. 12(7): p. 629-339. Wang, J., et al., Approach and Coalescence of Gold Nanoparticles Driven by Surface Thermodynamic Fluctuations and Atomic Interaction Forces. ACS Nano, 2016. 10(2): p. 2893-2902. Matthias, E., et al., In-situ investigation of laser ablation of thin films Thin solid films, 1995. 254: p. 139-146. Tóth, Z., et al., Pulsed laser ablation mechanisms of thin metal films. Proceedings of the SPIE, 1999. 3822: p. 18-26. Gregorcic, P., et al., Optodynamic energy conversion efficiency during laser ablation on metal surfaces measured by shadow photography. Applied Physics A, 2014. 117: p. 353357. Trice, J., et al., Pulsed-laser-induced dewetting in nanoscopic metal films: Theory and experiments. Physical Review B, 2007. 75: p. 235439. Hashimoto, S., et al., Gold nanoparticle-assisted laser surface modification of borosilicate glass substrates. The Journal of Physical Chemistry C, 2009. 113(48): p. 20640-20647. Chen, K.R., et al., Laser-solid interaction and dynamics of laser-ablated materials. Applied Surface Science, 1996. 96-98: p. 45-49. Bichsel, H. and D. Gray, American Institute of Physics Handbook. McGraw-Hill, New York, 1972. Scarabelli, L., et al., A “Tips and Tricks” Practical Guide to the Synthesis of Gold Nanorods. The Journal of Physical Chemistry Letters, 2015. 6(21): p. 4270-4279. Hendel, T., et al., In Situ Determination of Colloidal Gold Concentrations with UV−Vis Spectroscopy: Limitations and Perspectives. Analytical Chemistry, 2014. 86: p. 1111511124. Mafuné, F., et al., Formation of Gold Nanoparticles by Laser Ablation in Aqueous Solution of Surfactant. journal of physical Chemistry B, 2001. 105: p. 5114-5120. Rao, P. and R. Doremus, Kinetics of growth of nanosized gold clusters in glass. journal of Non-Crystalline Solids, 1996. 203: p. 202-205. Balamurugan, B. and T. Marayuma, Evidence of an enhanced interband absorption in Au nanoparticles: Size-dependent electronic structure and optical properties. applied Physics Letters, 2005. 87: p. 143105-1-3. Yoo, K.S., I.W. Sorensen, and W.S. Glaunsinger, Adhesion, surface morphology, and gas sensing characteristics of thin‐gold‐film chemical sensors. Journal of Vacuum Science & Technology A, 1994. 12: p. 192-198. Lee, S.K., et al., Excimer laser ablation removal of thin chromium films from glass substrates. Surface and Coatings Technology, 1999. 113: p. 63-74. Lorenz, P., et al., Dynamics of the laser-induced nanostructuring of thin metal layers: experiment and theory. Materials Research Express, 2015. 2: p. 026501. Boneberg, J., J. Bischof, and P. Leiderer, Nanosecond time-resolved reflectivity determination of the melting of metals upon pulsed laser annealing. Optics Communications, 2000. 174: p. 145-149.
Figure and table captions Figure 1. SEM images and size distribution of gold NPs produced after the irradiation of 5 laser pulses on a thin film of 55 nm in thickness at atmospheric pressure (a)-(c) and at 10-5 Torr (d)-(f). The fluences used were (a) and (d) 200 mJ/cm2, (b) and (e) 300 mJ/cm2, and (c) and (f) 400 mJ/cm2. Figure 2. (a) Average diameter and (b) SPR wavelength of the NPs produced at atmospheric pressure and at 10-6 Torr as a function of the starting film thickness. The average was calculated over all used fluences. Figure 3. (a) Absorbance of laser exposed areas of a thin film of 55 nm in thickness with different fluences; the films were irradiated at atmospheric pressure and at 10-5 Torr. The spectra correspond to the samples showed in Fig. 1. (b) Absorbance ratio (at 350 nm) of the irradiated areas to the as grown film as a function of the film thickness. Figure 4. On the left: (a) Beam transmission signal in vacuum (top) and at atmospheric pressure (bottom), (b) Probe beam deflection at the same configuration, corresponding to single pulse irradiation at 300 mJ/cm2 of a 55 nm-thick film. On the right, shadowgraph images of an adjacent region to the thin film at different time delays after irradiation at atmospheric pressure: (c) 0.5 µs, (d) 10 µs, (e) 20 µs and (f) 40 µs (the laser pulse comes from left to right).
Figure 1
Figure 2
Figure 3
Figure 4 (a)
(c)
(d)
(e)
(e) (d)
(d) 10 µs
(e) 20 µs
(f) 40 µs
(f)
(b)
(c)
(c) 0.5 µs
(f)
Table captions
Thickness of the starting film (nm)
Table 1. Mean diameter and standard deviation (in parenthesis) of the nanoparticles produced in vacuum and at atmospheric pressure as a function of the laser fluence (F) and the thickness of the starting film (T).
Table 1
Mean diameter D and standard deviation (SD) in nm Pressure vacuum conditions Fluence 200 300 400 (mJ/cm2) 13.4 13.9 11.6 h1=44 nm (10.2) (7.3) (7.1) 17.5 8.0 8.7 h2=55 nm (10.3) (5.1) (4.7) 12.7 14.3 12.8 h3=60 nm (8.9) (8.7) (9.5) 10.2 13.0 12.4 h4=82 nm (6.8) (6.9) (6.5)
Patm 200
300
400
21.7 (13.0) 13.3 (11.3) 17.0 (10.8) 17.5 (9.4)
23.8 (21.0) 20.0 (15.2) 14.3 (14.3) 17.2 (12.5)
20.3 (17.1) 12.9 (9.8) 19.6 (16.6) 15.0 (11.3)
S0169-4332(17)30203-9 http://dx.doi.org/doi:10.1016/j.apsusc.2017.01.181 APSUSC 34976
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APSUSC
Received date: Revised date: Accepted date:
20-9-2016 3-1-2017 18-1-2017
Please cite this article as: C.S´anchez-Ak´e, A.Canales-Ramos, T.Garc´ıa-Fern´andez, M.Villagr´an-Muniz, Nanosecond pulsed laser nanostructuring of Au thin films: comparison between irradiation at low and atmospheric pressure, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2017.01.181 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Nanosecond pulsed laser nanostructuring of Au thin films: comparison between irradiation at low and atmospheric pressure
C. Sánchez-Aké1,*, A. Canales-Ramos1, T. García-Fernández2, M. Villagrán-Muniz1
1
Centro de Ciencias Aplicadas y Desarrollo Tecnológico, Universidad Nacional Autónoma de México, Circuito Exterior S/N, C. U., Delegación Coyoacán, C.P. 04510, México D. F. México 2
Universidad Autónoma de la Ciudad de México (UACM), Prolongación San Isidro 151, Col. San Lorenzo Tezonco, México D.F., C.P. 09790, México.
*
Corresponding author: [email protected]
Highlights
Background pressure plays an important role in NPs formation and its characteristics The NPs diameter and their size dispersion are smaller when irradiating in vacuum The plasmon resonance shifts 15 nm to higher frequencies when irradiating in vacuum Film partial ablation cannot be neglected for thickness in the range 40-80 nm In situ optical techniques monitor the timescale of the process and ablation dynamics
Abstract Au thin films with tens of nm in thickness deposited on glass substrates were irradiated with nanosecond UV (355 nm) laser pulses at atmospheric pressure and in vacuum conditions (600 and 10-5 Torr). We studied the effect of the laser fluence (200-400 mJ/cm2), thickness of the starting film ( 40-80 nm) and surrounding pressure on the partial ablation/evaporation of the films and the morphology of the produced nanoparticles (NPs). The dynamics of NPs formation was studied by measuring in real time the transmission of the samples upon continuous-wave laser exposure, and by means of probe beam deflection technique. The ejection of material from the film as a result of the irradiation was confirmed by time-resolved shadowgraphy technique. Experiments show that the NPs diameter and their size distribution are smaller when the irradiation is performed in vacuum regardless the laser fluence and thickness of the started film. It is also shown that the plasmon band shifts to higher frequencies with lower background pressure. The optical measurements show that the films melt and ablate during the laser pulse, but the transmission of the irradiated areas continues changing during tens of microseconds due to ejection of material and solidification of the remaining gold. Our results indicate that partial ablation cannot be neglected in nanostructuration by ns-pulsed irradiation of thin films when their thickness is in the studied range.
Keywords Gold nanoparticles; laser ablation; localized surface plasmon resonance; in situ transmittance; probe beam deflection.
Introduction The optical properties of nobel metal nanoparticles (NPs) and their potential application in sensors and photovoltaic devices [1] have result in a growing interest in developing fast, effective and lowcost methods of synthesis. The optical absorption of the NPs is dominated by the surface plasmon resonance band (SPR) which depends on their size, shape, separation between them and surrounding media [2]. Therefore, controlling the morphology of the NPs is very important for further applications.
Laser irradiation of thin metal films has shown to be an effective method for the synthesis of NPs attached or even embedded in dielectric substrates [3-5]. It has also been applied for producing NPs in periodic arrays by using interference beam patterns [6] and for nanostructuring of dielectric materials by multipulse irradiation [7]. The formation of NPs due to pulsed-laser irradiation has been explained in terms of the melting of the film followed by (i) heterogeneous nucleation from liquid phase on defects, and (ii) rupture of the film due to spinodal dewetting [8]. The influence of experimental parameters on the morphology of the produced nanostructures has been studied over a wide range of laser fluences, film thicknesses and number of pulses [3, 4, 6, 9-11]. Most of the works reported use very thin films, with thickness smaller than 20 nm or even below the percolation limit [12]. In those cases, it has been found that the NPs diameter increases with the thickness of the starting film [11, 13]. Besides, for thicknesses up to 15 nm it has been reported a good agreement between the experimental observed NPs diameter and that calculated theoretically using thermal models based on the dewetting phenomenon [11, 14, 15]. Despite this confirmed dependence between the film thickness and the size of the NPs, it is not clear if this tendency continues for larger values of the thickness of the starting film. On the other hand, most of the theoretical models used to describe laser nanostructuring assume that during the laser irradiation, there are no losses of the metal due to evaporation from the surface of the film and the substrate [11]. However, metal evaporation along with the formation of NPs have been reported repeatedly in experimental works. Ruffino et al. [16] detected by Rutherford backscattering spectrometry (RBS) loss of Au by one-pulse laser irradiation of a 20 nm thin film. Chiu et al. [17] monitored by mass spectrometry the formation of gaseous gold cluster ions during the multi-pulse irradiation of gold films with thickness from 10 to 100 nm. Henley et al. [3] suggested that the implantation of metal NPs into the surface of glass substrates is due to boiling of both the film and a surface layer of the glass. Furthermore, metal evaporation has also been reported by irradiating noble metal islands and NPs previously produced either by direct deposition or by thin films irradiation. As a consequence of the evaporation of the metal, the irradiation of preexisting metal islands or NPs results in a decrease of the size of the starting metal particles [18, 19] and/or in the nanodrilling of the substrate [7, 20, 21]. This is particularly important in multi pulse irradiation of thin films since the first pulses can lead to the formation of NPs and the subsequent pulses to the modification of them. Despite these reports, the range of laser fluences and the thickness of the starting film in which there is partial evaporation of the metal film along with NPs formation has not been identified. In addition, the influence of the evaporated material and its possible back condensation to the substrate on the formation of the NPs is still not fully understand. The aim of this work is to investigate the partial evaporation of the starting metal film and the formation of NPs by irradiating gold films with thickness of 44 – 82 nm. The films were irradiated at atmospheric pressure and in vacuum in order to determine its effect on the morphology and size of the obtained NPs. Additionally, we employed optical in situ techniques to characterize the timescale of melting, evaporation and solidification processes associated to the NPs formation. Experimental details
Au thin films were deposited at room temperature by DC sputtering on corning glass (Soda lime glass 2947) substrates. The thicknesses of the films h measured by Atomic force Microscopy (AFM), were 44 ± 7, 55 ± 9, 60 ± 9 and 82 ± 13 nm. The films were irradiated in vacuum (≈ 3x10-5 Torr) and at atmospheric pressure in Mexico City ( 580 Torr), by a sequence of 5 pulses with duration of 6 ns, of a Nd:YAG laser operating at 355 nm at a 10 Hz repetition rate. For irradiation, the laser beam was directed normal to the surface of the film. The Gaussian beam diameter on the surface was about 2.42 ± 0.05 mm. The values of the laser fluence used for irradiation were 200, 300, and 400 mJ/cm2. These values were obtained by varying the laser energy, but keeping fixed the spot diameter. Both, the as-grown and the irradiated samples were characterized by scanning electron microscopy (SEM) using a FEI helios nanolab 600 microscope. The SEM images were processed by an in-house program based on anisotropic diffusion [22]. Besides, the absorbance of the films before and after irradiation was measured at a normal incidence angle in the range of 350-700 nm with an UV-Vis Cary 5000 dual beam spectrometer. The absorbance was measured using the software ABS function: A = -log(I/I0). Two cw He-Ne lasers ( = 632 nm) were used to study in situ the nanoparticles formation process during 200 µs after the first pulse. One beam, which was directed to the center of the irradiated area at an incident angle of 45°, was used to record in real time the transmission of the sample. Similar optical measurements have been used to study the timescale of the melting and solidification process and NPs formation [6] and the formation of periodic surface structures [23] by irradiation of metallic thin films. The second laser, running parallel to the substrate, was used as a probe beam to detect its deflection due to material emerging from the surface of the sample. The probe beam with 1 mm of diameter was kept as close as possible to the surface without blocking the former. This kind of measurements has been applied to study the ablation of materials in bulk or even in thin films [24]. Both signals were detected by to two fast photodiodes (PD) and recorded by a 500 MHz oscilloscope which was triggered with a third PD placed near the output of the pulsed laser. We employed an interference filter in front of the PD that detects the transmission intensity to suppress any contribution of the laser pulses to the recorded signals. In order to verify the ejection of material from the film and/or from the substrate, we employed laser shadowgraphy. This technique has been used previously to study ablation of thin metal films [25, 26]. As illuminating source we used the emission of a frequency-doubled Nd:YAG laser ( = 532 nm) with pulses of 7 ns. This beam was aligned parallel to the surface of the film and was expanded to illuminate a region of approximately 1 cm in diameter in the vicinity of the irradiated area. Synchronization between irradiation and illumination pulses was performed by means of a pulse delay generator and the images were captured through a charged-coupled device (CCD) working in video mode. Results and discussion Nanoparticles characterization
All films were irradiated with 5 pulses to guarantee the formation of NPs for the three studied fluences. Drastic morphology changes by pulsed laser irradiation as a function of the number of pulses have been reported before in numerous works [5, 10, 11, 21, 27-29]. In our case, single pulse irradiation with the lowest fluence modified the surface morphology of the 60 nm-thick film, producing a discontinuous structure with asymmetric undulations and only few NPs were observed. In contrast, after 5 pulses numerous spherical NPs and no apparent remnant of the starting film were observed in all irradiated area. The size and morphology of the Au NPs produced by laser irradiation of thin films at atmospheric pressure were significantly different from those irradiated in vacuum. Figure 1 shows representative SEM images of a 55 nm-thick film after irradiation with a sequence of 5 laser pulses at fluences of 200, 300 and 400 mJ/cm2, under atmospheric pressure and vacuum conditions. The particles size distribution is shown in each image. These distributions were calculated on a statistic population of at least 800 islands by using SEM images of different regions near the center of the irradiated area. We studied only this region since the irradiation was performed with a Gaussian beam, thus the local fluence varies with the distance from the beam center. As a result, the NPs formation was not uniform in all the irradiated area. This effect was clearly seen by the naked-eye since the irradiation produced a strongly coloration in the center of the spot, but the periphery of the irradiated area looked as clean glass. The coloration is produced by the surface plasmon resonance and indicates the presence of nanoparticles formed by dewetting or vaporization and re-condensation [3]. On the contrary, the apparent absence of gold in the rim of the spot suggests that the energy density was not enough for melting and boiling the film. When the laser fluence is below the melting threshold, but sufficient to induce thermal expansion and stress in both, the film and the substrate, the former can be ablated in solid phase [25]. This take place when the mechanical stress is high enough to tear off part of the film. Because of the dependence of the energy density with the position, SEM and absorbance analyses were performed only in a region near the center of the laser spot. From SEM images obtained after irradiation of films with different thickness (h), we calculated the mean diameter and standard deviation of the nanoparticles, as reported in Table 1. The most important feature is that for all the used thicknesses, smaller NPs are observed in the experiments performed in vacuum in comparison with that produced at atmospheric pressure. In addition, the standard deviation of the size distribution also decreases with the background pressure. A possible reason for this is that a part of the film evaporates leaving a distribution of nanoparticles formed by the dewetting of the remaining material on the substrate. This possible mechanism will be discussed later. One of the main drawbacks of the laser irradiation technique is that the size distribution of the produced NPs is large in comparison with other methods of synthesis. Therefore, our results indicate that this technique can be improved performing the irradiation at low pressure. Furthermore, the morphology of the NPs produced in both conditions is quite different (see Fig. 1), the NPs produced in vacuum are spherical with smooth surface whereas that those produced at atmospheric pressure have multiple small NPs grafted to their surface. This behavior was observed in all thicknesses of the starting film. Other authors have reported the formation of NPs decorated with smaller ones by laser irradiation at atmospheric pressure of thin
films [16] and NPs [28]. Since this kind structure was only observed in the experiments performed at atmospheric pressure, their formation may be due to condensation back of evaporated material because of the ambient gas. From table 1, it is not possible to clearly describe how the fluence, in the range from 200 to 400 mJ/cm2 and the thickness of the starting film affect the size of the obtained NPs. With the aim to compare the effect of the background pressure, we calculated the average diameter over the three fluences, for each value of thickness h. These average values, plotted in Fig. 2(a), show the already mentioned effect of the surrounding media: for all the thicknesses, the NPs produced in vacuum have smaller diameter. The average NPs size seems to be weakly dependent on the starting film thickness except for the thinnest film irradiated at atmospheric pressure. In that case, the diameter of the nanoparticles is approximately 40 % larger than those produced with all other thicknesses. Given that the background pressure affects the size of the NPs, it is expected that it also has an effect on the optical absorbance of the irradiated films. Fig. 2(b) displays the wavelength position of the surface plasmon resonance (max) as a function of the thickness of the starting film. Again, each point is the average value calculated for the three used fluences. Regarding the effect of the film thickness, the wavelength of the plasmon band remains approximately constant throughout h 60 nm and shifts to the blue for the case of the thickest film. Furthermore, there is a clear difference between the films irradiated in vacuum and at atmospheric pressure. The former exhibits values of max shifted to the blue for all used thicknesses and substrates. This was expected, due to the fact that the smaller NPs were produced by irradiation in vacuum. However, there are other morphological aspects that can affect the surface plasmon resonance. For instance, NPs with separations smaller than their diameter can interact resulting in a shift of the plasmon band to the red [12]. In our case, the NPs produced at atmospheric pressure showed to have smaller NPs grafted to their surface (see for example Fig. 1(a)-(c)), thus it may contribute to the red shift. The results showed in Fig.1 and 2 evidence that the background pressure is an important parameter when processing thin films by laser irradiation. The main factors that differentiate the NPs formation in vacuum from that at atmospheric pressure are: (i) heat dissipation from the film to the surrounding media, (ii) boiling temperature of the film and, (iii) dynamics of the evaporated material (expansion and confinement near the film surface). In the case of (i), the heat loss from the Au surface to the ambient can be ignored in comparison to that dissipated within the substrate [28]. However, (ii) plays an important role since it depends on the pressure exerted on the film surface by the surrounding media: the higher the pressure, the higher the vaporization temperature [29]. For the case of Au in bulk, the vaporization temperature drops approximately from 2800° to 1120° C when the pressure decreases from 585 Torr to 7 x 10-5 Torr [30]. Thus, the films irradiated at atmospheric pressure reach a higher temperature than those processed in vacuum. Besides, the increase of the vapor pressure with the background pressure in turn implies that the fluence threshold to vaporization also increases. Since the amount of removed material is linearly proportional to the laser energy density above its threshold [29], it is expected that a
larger fraction of material evaporates at vacuum. Regarding (iii), the background pressure plays a critical role in the dynamics of the ejected material. This material can expand freely in vacuum but at atmospheric pressure it can be confined and possibly condensed back on the surface. This will be discussed in more detail in the last part of this section. To determine if there is loss of gold in the films as a result of the laser irradiation, we analyzed their absorbance UV-Vis spectra. Figure 3(a) depicts the absorbance spectra of a representative 55 nm-thick film irradiated with the three used fluences under both pressure conditions. It can be noted that for all wavelengths and in particular for the UV region (from 350-400 nm), the absorbance of the films processed at atmospheric pressure is larger than that for the samples irradiated in vacuum. Absorbance at wavelengths in the UV could enable an estimation of Au concentration due to that its main contribution comes from interband transitions in metallic gold [31]. In contrast, for wavelengths above 400 nm, the absorbance is increasingly influenced by the localized surface plasmon resonance [32]. Thus, by choosing an adequate wavelength it is possible to estimate the amount of gold regardless of particle size. This method has been applied to determine Au concentration in colloids [31-33] and in thin films [34]. Its main advantages are that the determination of the amount of gold does not depend on the particle size and shape [31] and, that it can be applied for NPs in liquids and supported on transparent substrates. However, both the presence of NPs with diameter < 2.5 nm [35] and organic substances [31, 32] can contribute to the absorbance below 400 nm thus restricting the validity of the measurement. In order to compare the loss of Au as a function of the thickness of the starting film, we measured the absorbance at 350 nm (vertical dotted line in Fig. 3(a)) after and before irradiation. The measured absorbance values after irradiation at atmospheric pressure were 0.29 ± 0.04, 0.26 ± 0.05, 0.26 ± 0.07 and 0.19 ± 0.09 for the starting films with thickness 44, 55, 60 and 82 nm respectively. Whereas in vacuum were 0.15 ± 0.04, 0.11 ± 0.02, 0.14 ± 0.04 and 0.08 ± 0.04 for the same values of the thickness. Fig. 3(b) shows the relative absorbance at 350 nm, defined as the absorbance ratio of the irradiated areas to the as-grown film. Since the value of Abs350 nm is proportional to the amount of gold on the substrate, the relative absorbance gives the proportion of Au after irradiation compared with the Au amount before irradiation (as-grown film). We found that regardless the thickness, the relative absorbance at 350 nm is always smaller in the films irradiated in vacuum, thus revealing that the gold loss is larger in this condition. This is not a surprising result due to the difference in the threshold of energy density for vaporization discussed above. Since this threshold is below for irradiation in vacuum, a large amount of the film is ablated and the nanoparticles are formed by the remaining material on the substrate. Furthermore, the maximum temperature that the films reach under irradiation at atmospheric pressure is higher than in vacuum and consequently, the temperature reached by the surface of the substrate is also higher. This is because the thickness h of our films fulfills -1 < h < LT, as the values of the optical absorption length and the thermal diffusion length are -1 = 18 nm and LT = 1237 nm for gold in bulk, respectively [16]. The fact that the substrate reaches higher temperatures at ambient pressure than in vacuum may favor that a larger fraction of gold remains on the substrate. This is suggested since previous reports have demonstrated that the adhesion of Au thin films increases with the substrate temperature during the deposition and with post annealing at high
temperatures [36]. Moreover, higher temperatures favour the softening or even melting of the glass. It has been reported that the remaining material on the substrate after irradiation of metallic thin films is due to the melting of both, the substrate and the film [37]. Figure 3(b) also reveals that the proportion of the remaining gold after irradiation decreases with the thickness of the starting film regardless the surrounding pressure. As the thickness increases with constant pressure and fluence, there are two parameters in competition that influence the amount of ejected material: the ablation threshold and the temperature that the substrate reaches. By one hand, for thicknesses h < LT, the ablation threshold increases linearly with the thickness [24]. Thus, it would be expected larger amount of removed material for the thinnest film. On the other hand, the temperature reached by the surface of the substrate is inversely proportional to the film thickness since the same deposited energy on the film heats different amounts of gold. Therefore, as mentioned above, the adhesion to the substrate is better for the thinnest film. Our results suggest that under our conditions, the impact of the substrate temperature is more important than the ablation threshold dependence on the thickness. As mentioned above, the NPs size was affected by the surrounding pressure as shown in Fig. 1 and Table 1. In general, both the NPs average diameter and the size distribution were larger when irradiating at atmospheric pressure regardless the thickness of the starting film and the laser fluence. This is likely a consequence of two main causes. First, the amount of evaporated material is smaller in the case of irradiation at high pressure, thus the amount of available remaining Au to form NPs is higher than in vacuum. Several previous works have explained in terms of the dewetting that, for different number of pulses, the mean nanoparticle diameter increases with the thickness of the starting film [9-11, 13, 27]. The second cause is related with the maximum temperature reached during irradiation, which is considerable higher in the case of the experiment at atmospheric pressure [30]. Higher temperatures increase the mobility of the droplets on the substrate, as their surface boils, probably allowing some of the surviving drops to coalesce. The coalescence of the NPs with near neighbors may produce the formation of observed larger Au NPs and the increase of the size distribution since coalescence depends on the distance between neighbor NPs [23]. Some previous reports support this hypothesis; Henley et al [9] attributed the increase of the NPs size when increasing the laser fluence, to the coalescence of droplets when the fluence is above the boiling point. Wang et al. [23] reported that the coalescence of neighbor Au NPs increases their average size as the temperature increases. Dynamics of nanoparticles formation The dynamics of NPs formation was studied by measuring in real time: (i) the transmitted intensity of a continuous beam directed at the irradiated area and, (ii) the deflection of a continuous probe beam oriented parallel and grazing to the area of irradiation. In addition, time-resolved shadowgraphy images of the vicinity of the film were acquired at different time delays from ns to µs. Fig. 4(a)-(b) show the transmission and deflection signals directly obtained from the oscilloscope for both pressure conditions, corresponding to the first pulse irradiation of a 55 nmthick untreated thin film at 300 mJ/cm2. Fig. 4(c)-(f) shows the corresponding shadowgraphy images at different time delays for irradiation performed at atmospheric pressure. The recorded
intensity of the probe beams and the shadowgraph images showed noticeable changes for the first pulse but not for the subsequent pulses, suggesting that the main ablation of the film takes place in the first laser pulse. Lorenz et al. [38] reported similar behavior of the transmission intensity when irradiating Cr films with different number of pulses. Even though the subsequent pulses did not produce noticeable changes in the recorded intensity of the probe beams, those pulses might affect the remnant droplets either boiling or melting them. However further studies are needed to confirm this hypothesis. For both pressure conditions, the transmission intensity showed in Fig. 4(a) starts with a very rapid increase during the laser pulse used for irradiation (approximately 10 ns after its onset). After that, this signal increases monotonically for long time until reaching a maximum value after tens of µs in the case of irradiation in vacuum. In contrast, at atmospheric pressure, the transmission shows two minima (at 500 ns and 15 µs) and after that rises monotonically until reaching a final level at 40 µs. The behavior of this signal integrates the ejection of gold from the surface of the film and solidification of the remaining material on the substrate. We attribute the abrupt rise of the transmission signal at the first ns under both pressure conditions, to the melting and ablation of the film. Previous reports have demonstrated by measuring the reflectivity of metal thin films under irradiation in similar conditions that the films melt during the laser pulse [6, 23, 39]. The final transmitted intensity is consistent with the films becoming more transparent at the probe wavelength (632 nm), as it is evidenced in the absorbance spectra in Fig. 3(a). The ejection of material from the film due to irradiation was studied by means of probe deflection technique. Fig. 4(b) shows the deflection signals corresponding to irradiation in vacuum and at atmospheric pressure. It is well known that light deflection is consequence of a transient gradient of refractive index, which can be caused by heat, shock waves, vapor or plasma plumes [24]. The deflection signal in vacuum shows two minima. Since the film melts/ablates during the laser pulse, we attribute them to two groups of material ejected with travelling velocities of 60 m/s and 10 m/s. Probably, these groups corresponds to ablated Au+, Au, clusters of Au [17] and delaminated gold flakes [25]. As expected, the velocity of both groups increases with the laser fluence. For the first group from 50 m/s at 200 mJ/cm2 to 60 m/s at 400 mJ/cm2, whereas for the second one from 8 to 16 m/s respectively. For the case of irradiation at atmospheric pressure, the changes in the deflection signal can be compared with the shadowgraphy images (4(c)-(f)). This signal starts decreasing at 500 ns which corresponds to the shock wave observed in Fig. 4(c). The formation of this shock wave also affects the transmitted intensity as showed in Fig. 4(a). The minimum at 22 µs is attributed to ejected material that slowly expands as illustrated in Fig. 4(d)-(f). Since this experiment was performed at atmospheric pressure, one part of the ejected material can be confined by the surrounding pressure and another moves away from the substrate. Therefore, the transmission and deflection signals integrate both contributions. The fact that the principal minimum appears approximately 20 µs later in vacuum than at high pressure is assumed due to the difference of the maximum temperature reached in both conditions. Since the evaporation temperature in vacuum is lower, the kinetic energy of the ejected material is also lower than the one obtained at atmospheric pressure. Moreover, the two groups observed in vacuum may be aggregated in one group when irradiating at atmospheric pressure, because lower-energy ejected
particles are contained within the vicinity of the substrate. One part of this confined material probably condenses back into the substrate. This may be the cause of the decoration of the NPs observed in Fig. 1(a)-(c). Conclusions We compare the NPs formation by laser irradiation of thin films in vacuum and at atmospheric pressure. It was found that NPs produced in vacuum have smaller diameter and size dispersion than those produced at atmospheric pressure. Moreover, the morphology of the NPs is also affected by the background pressure. As a consequence, the plasmon band shifts to lower wavelengths by decreasing the pressure. The real time optical measurements indicate that the NPs formation starts with the melting and partial ablation of the film during the laser pulse-film interaction. The absorbance spectra and shadowgraphy images reveal partial ablation of the films. This ablation process depends on the surrounding media because of the vapor pressure. The lower the pressure, the lower evaporation temperature of the gold film. Consequently, a large amount of gold can be ablated due to the ablation threshold fluence and the temperature reached by the glass substrate are also lower. Since the NPs are formed with the remaining material on the substrate, their size and morphology also depend on the surrounding pressure. The changes in optical signals last tens of microseconds due to material ejection and solidification of remaining gold. In vacuum, the ejected material moves away from the substrate; while at atmospheric pressure, low-energy particles can be confined and possible condense back to the substrate, affecting the final NPs morphology. Our experimental results show that, in contrast with previous reports for films thinner than 20 nm, there is partial ablation of the film when its thickness is in the studied range (40-80 nm). Moreover, this partial ablation plays an important role in the formation of NPs and their characteristics. Acknowledgements This work was supported by project PAPIIT-IG100415. The authors thank the financial support from CONACyT by means of the Red de Nanociencias y Nanotecnología and the project CB 176705. The authors gratefully thank Professor J.L. Sánchez-Llamazares for his support and help in this research and A. Esparza for the deposition of the films. We also acknowledge J.G Bañuelos, G. Labrada and A.I. Peña and the Laboratories LUCE-CCADET and LINAN-IPICyT for providing the characterization of the samples.
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Figure and table captions Figure 1. SEM images and size distribution of gold NPs produced after the irradiation of 5 laser pulses on a thin film of 55 nm in thickness at atmospheric pressure (a)-(c) and at 10-5 Torr (d)-(f). The fluences used were (a) and (d) 200 mJ/cm2, (b) and (e) 300 mJ/cm2, and (c) and (f) 400 mJ/cm2. Figure 2. (a) Average diameter and (b) SPR wavelength of the NPs produced at atmospheric pressure and at 10-6 Torr as a function of the starting film thickness. The average was calculated over all used fluences. Figure 3. (a) Absorbance of laser exposed areas of a thin film of 55 nm in thickness with different fluences; the films were irradiated at atmospheric pressure and at 10-5 Torr. The spectra correspond to the samples showed in Fig. 1. (b) Absorbance ratio (at 350 nm) of the irradiated areas to the as grown film as a function of the film thickness. Figure 4. On the left: (a) Beam transmission signal in vacuum (top) and at atmospheric pressure (bottom), (b) Probe beam deflection at the same configuration, corresponding to single pulse irradiation at 300 mJ/cm2 of a 55 nm-thick film. On the right, shadowgraph images of an adjacent region to the thin film at different time delays after irradiation at atmospheric pressure: (c) 0.5 µs, (d) 10 µs, (e) 20 µs and (f) 40 µs (the laser pulse comes from left to right).
Figure 1
Figure 2
Figure 3
Figure 4 (a)
(c)
(d)
(e)
(e) (d)
(d) 10 µs
(e) 20 µs
(f) 40 µs
(f)
(b)
(c)
(c) 0.5 µs
(f)
Table captions
Thickness of the starting film (nm)
Table 1. Mean diameter and standard deviation (in parenthesis) of the nanoparticles produced in vacuum and at atmospheric pressure as a function of the laser fluence (F) and the thickness of the starting film (T).
Table 1
Mean diameter D and standard deviation (SD) in nm Pressure vacuum conditions Fluence 200 300 400 (mJ/cm2) 13.4 13.9 11.6 h1=44 nm (10.2) (7.3) (7.1) 17.5 8.0 8.7 h2=55 nm (10.3) (5.1) (4.7) 12.7 14.3 12.8 h3=60 nm (8.9) (8.7) (9.5) 10.2 13.0 12.4 h4=82 nm (6.8) (6.9) (6.5)
Patm 200
300
400
21.7 (13.0) 13.3 (11.3) 17.0 (10.8) 17.5 (9.4)
23.8 (21.0) 20.0 (15.2) 14.3 (14.3) 17.2 (12.5)
20.3 (17.1) 12.9 (9.8) 19.6 (16.6) 15.0 (11.3)