Laser treatment of seed-mediated nanostructured silver film morphology

Materials Chemistry and Physics 113 (2009) 187–191

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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Laser treatment of seed-mediated nanostructured silver film morphology a ´ J. Ebothé a,∗ , R. Miedzinski , A.H. Reshak b,c , K. Nouneh d , M. Oyama d , A. Aloufy e , M. El Messiry e a

LMEN, E.A. 3799, UFR Sciences, Université de Reims, B.P. 138, 51685 Reims, France Institute of Physical Biology, South Bohemia University, Nove Hrady 37333, Czech Republic c Institute of System Biology and Ecology, Academy of Sciences, Nove Hrady 37333, Czech Republic d Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8520, Japan e NAMN Centre, Faculty of Engineering, Alexandria University, 21599 Alexandria, Egypt b

a r t i c l e

i n f o

Article history: Received 23 June 2008 Received in revised form 4 July 2008 Accepted 9 July 2008 Keywords: Optical materials Nanoparticles Photoinduced Nonlinear optics

a b s t r a c t An efficient photoinduced method to operate by average sizes and heights of silver nanoparticles deposited on transparent and conductive substrates by seed-mediated growth method is proposed. The study is carried out with indium tin oxide (ITO) and fluorine doped tin oxide (FTO) substrates having 4 and 14 /square sheet resistance, respectively. These two substrates determine different photoinduced aggregation of silver nanoparticles. The substrate resistance appears as the principal origin of the observed photoinduced aggregation features. The nanosecond kinetics of phototransparency engenders temporary oscillations in the range of nanosecond time. Several roles of the local overheating occurring through the electron–phonon interactions are evidenced. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Handling the nanoparticles size of noble metals such as gold and silver by optical treatment recently became an investigation topic of enhanced interest [1]. For that purpose, a shift of the photoinduced spectra towards the material surface plasmon resonance wavelength (SPR), located at 2.1–2.3 eV for the case of Ag nanoparticles (NP), was observed even under low power illumination of about 8.5 mW mm−2 . It was shown that this illumination engenders the decrease of the Ag NP average size. The application of this in the building of sensors based on the SPR wavelength established that the nanoparticle geometrical parameters such as their size, sheet thickness and inter-distance play the main role in the sensor performance [2]. Therefore, their control appears to be an important technological task for the production of sensors operating from the SPR absorption spectra to UV spectra of noble metals. It is clear that technologically nanoclusters with high size monodispersion in the desired range of the sheet thickness are difficult to obtain. One of the research orientation for that could be based on the formation of clusters by an appropriate laser excitation [3,4]. In our recent paper [5], we showed the possibility to form a grating by single pulses of the 10 ns Nd:YAG laser. It should be mentioned that the excitation involved here is away from the near-resonance SPR line. It is likely induced by the second harmonic

∗ Corresponding author. Tel.: +33 3 325 25 65 01; fax: +33 3 325 25 65 00. E-mail address: [email protected] (J. Ebothé). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.07.071

generation occurring at the borders separating the noble metallic nanoparticle and the ITO substrate. It was established that the sheet resistance of the ITO substrate plays the main role in the optically induced gratings formation and the geometrical size of the resulting aggregates. Generally, the photoinduced light forms an additional doubled frequency beam due to the coherent bicolour interaction of the electromagnetic waves with the Ag NP/ITO interfaces. Therefore, the so formed staying wave modifies the main topographical parameters of the nanostructured metallic film. In the present work, we investigate the influence of the substrate resistance on the aggregation process under the influence of an external light. A particular attention is paid to the dependence of the structure on the number of single laser pulses. Additionally, we investigate the changes of the spectral properties near the SPR maxima and the temporary kinetics of the observed photoinduced effects. Sections 2 and 3 are devoted to the sample preparation and the substrate features. The results and discussion on the changes occurred in the surface morphology, optical absorption and temporary kinetics are proposed in Section 4. 2. Sample features The glass substrates coated with fluorine doped tin oxide (FTO) was purchased from FlexiTec, Brazil, whose sheet resistance is 14 /square. The indium tin oxide (ITO) coated glass substrate with sheet resistance of 4 /square is the product of Kuramoto Seisakusho Co., Ltd. Japan. The deposition of silver nanoparticles is

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performed by a seed-mediated growth method as described previously [6,7]. As the actual procedures, briefly, an ITO or FTO substrate was first immersed for 2 h in a seed solution containing 4 nm Ag nanoparticles, which was produced by the NaBH4 reduction of AgNO3 in the presence of trisodium citrate. Then, after washing the substrate by pure water and drying by nitrogen gas, it was immersed in a growth solution, which contained AgNO3 , ascorbic acid, NaOH and cetyltrimethylammonium bromide. 3. Photoinduced experimental method Photoinduced experiment was performed in several steps. At the beginning, the samples were irradiated by a 18 ns 1064 nm Nd:YAG laser with a frequency of about 1 Hz (Fig. 1a). The sample surface morphology obtained is afterwards studied by atomic force microscope (AFM) referring to the non-illuminated surface. Finally, we analyzed the changes of the absorption spectra after appropriate illumination. Each sample was irradiated by 1, 3 and 5 single pulses of the Nd:YAG laser. The power density of the photoinducing laser was deviated not more than 6% from pulse to pulse. That means we had irradiated regions on the sample for a comparison—several irradiated by laser places and non-irradiated one. Each pulse of fundamental photoinducing Nd:YAG laser (FL) was measured (Fig. 1a) by an energy detector (ED) connected to the calibrated energy meter (EM) (Newport Power Meter 2935-C). Simultaneously the pulse duration was measured by highly sensitive, fast-respond AC-coupled photodiode (PD2) connected to a digital oscilloscope (OSC) (Tektronix TDS 2024B) with 2 GS s−1 . Additionally, we used a 2 mW continuous wave (CW) of 685 nm diode probe laser (PL) to obtain the transparency of the irradiated areas before, during and after irradiation. The probe CW laser has illuminated a fastrespond amplified photodiode (PD1) by low power laser beam. Beam surfaces of both laser beams were overlapped on surface about 0.25 cm2 . The photoinduced and photo-probing laser beams were temporary synchronized. This one allows to synchronize the initiation of energy counting (called output energy) and “opening” of oscilloscope which starts to measure the signals from PD1 and PD2 with precision up to 100 ps. Afterwards the signals from oscilloscope and energy meter were sent to PC by USB port.

The response curves (Fig. 1b) from PD1 (Fig. 1a) were described by sum of the two functions: f = fa + fb

(1)

where fa and fb is given by fa = a ln(t) + b fb = A0 e

− t

(1a)

sin(ωT t + ϕ0 )

(1b)

where A0 , amplitude of oscillation; , damping ratio; ωt , natural frequency; ϕ0 , phase of oscillation. Parameters a and b from (1a) were calculated by the logarithmic least squares fittings. Four parameters from (1b) were calculated by fitting theoretical curves generated by computer to experimental curve taken from oscilloscope. As best fitting criterion assumed: S=

m 

(yi − zi )2

(2)

i=1

Besides, we calculated the Pearson product-moment correlation coefficient (3) for the fitting curves: =

N  (yi − y )(zi − z ) i=1

(3)

N y z

The coefficient ranges from −1 to 1. A value of 1 shows that a linear equation describes the relationship perfectly and positively, with all data points lying on the same line and with y increasing with z. A score of −1 shows that all data points lie on a single line but that y increases as z decreases. A value of 0 shows that a linear model is inappropriate—that there is no linear relationship between the variables [8]. After the irradiation of the sample surfaces, we studied the surface morphology by AFM NT-MDT Smena. The force constant of the used tip is located in the interval 0.01–0.08 N m−1 while the resonant frequency at 7–14 kHz. All the AFM measurements were performed in contact mode with a scanning frequency of 1.5 Hz and a picture resolution of 512 × 512 pixels. Nanotec Electronica WSxM software [9] was used to render the AFM picture and obtain the topography analysis. The OceanOptics HR4000 spectrometer was used to obtain transmission spectra. Spectral range of this spectrometer is 200–1100 nm with a resolution of 0.25 nm. 4. Results and discussion The principal results of the surface modification induced by single laser pulses of 18 ns Nd:YAG lasers are presented in Figs. 2 and 3. It can be observed here that if the initial surface morphological feature of the samples are almost similar on each of the investigated substrates before the optical treatment (0 pulse), a real change in the surface morphology occurs after the different number of treated pulses. To study an influence of the surrounding dielectric permittivity the results are presented for the substrates with two different resistivity: 4 /sheet and 14 /sheet. All the measurements were done at the same conditions. Tables 1 and 2 summarize the values Table 1 Measurement of the topography parameters of the Ag nanostructured films

Fig. 1. (a) Experimental setup. PL, probe laser; FL, fundamental laser; PD1, photodiode; PD2, amplified AC-coupled photodiode; ED, energy detector; EM, energy meter; P, polarizer; OSC, oscilloscope; S, sample. (b) Theoretical approximation of curves from the PD1 detector.

Sample

0 pulses

RMS (nm)

Ag:ITO Ag:FTO

9.63 19.86

Average height (nm)

Ag:ITO Ag:FTO

34.3 69.9

1 pulse

3 pulses

5 pulses

36.19 57.51

50.91 66.71

35.80 77.23

115.3 149.8

147.58 193.5

87.4 200.3

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Fig. 2. AFM top view of silver nanoparticles on ITO (4 /sheet) surfaces irradiated by 0 (a), 1 (b), 3 (c) and 5 (c) pulses of the Nd:YAG laser.

Fig. 3. AFM top view of silver nanoparticles on FTO (14 /sheet) surfaces irradiated by 0 (a), 1 (b), 3 (c) and 5 (c) pulses of the Nd:YAG laser.

of the root-mean square surface roughness (RMS) of the nanostructured Ag films and the average height of the Ag nanoparticles after the different number of the pulses. One can see from Table 1 for all the samples that the RMS and the average height of the silver grains are always enhanced after the treatment. Besides, the values of both parameters increase with the enhanced number of pulses at least till 3-pulse treatment. On the other hand, it clearly appears that FTO substrate engenders higher values than ITO that can be estimated as being two times larger. This fact reflects the difference in the conditions evolved during the aggregates formation likely due the specific resistance value of each substrate. It is interesting to notice that after the optical treatment even by 1 pulse we have a drastic increase of the nanoparticle effective grain sizes for both substrates. The principal difference is observed after the 3 laser pulses where we have a continuation of the size increase with FTO while ITO substrate leads to a

drastic decrease of the effective sizes between the 3 and 5 laser pulses. Following the results presented in Figs. 4–6 one can see that the substrate resistance value plays the main role in the results

Table 2 Measurements of the input and output energy densities after the different pulse treatments Sample

Input energy density (mJ/cm2 )

Output energy density: 3 pulses (mJ/cm2 )

5 pulses (mJ/cm2 )

76

76 (1) 148 (2) 248 (3)

80 (1) 156 (2) 252 (3) 260 (4) 292 (5)

216

244 (1) 292 (2) 316 (3)

276 (1) 304 (2) 340 (3) 344 (4) 340 (5)

1 pulse (mJ/cm2 )

Ag:ITO

Ag:FTO

400

400

Fig. 4. Height distribution of examined films after 0, 1, 3 and 5 pulses of Nd:YAG laser: (a) silver nanoparticles on ITO film and (b) silver nanoparticles on FTO film.

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Fig. 5. Output energy density for Ag:ITO and Ag:FTO corresponding to the five pulse treatment. The presented data show only the limiting cases.

Fig. 7. Average profile of Ag:ITO and Ag:FTO taken from Figs. 2a and 3a (0 pulse).

Fig. 6. Average height of silver nanoparticles incorporated on ITO and FTO films after laser treatment.

Fig. 8. Curves recorded by oscilloscope. Solid line, signal from fundamental laser; triangle, response signal from glass; circle line, response signal from Ag:ITO; square lines, response signal from Ag:FTO.

obtained in Table 1. It may be caused by different local heating on interfaces separating the metallic nanoparticles and the substrates. At the same time the different resistivity cause the different surrounding background covering each nanoparticle. Consequently, the number of free carriers interacting with the substrates will be different. The results presented in Figs. 4 and 6 clearly show the increase of the effective sizes and decrease of their monodispersion. The latter may reflect several local overheating which at 18 ns may be substantial. It is crucially that the behaviour of the transmitted intensities seems to be opposite to the features of the sizes. So the effects are not caused by simple light scattering due to the nanoparticles size increase. To understand more deeply the observed photoinduced changes we have done additional investigation of the photoinduced temporary kinetics presented in Figs. 7 and 8. We have studied pump-probe dependencies with 0.1 ns time resolution. From Figs. 7 and 8, one can see that the pump-probe time delaying response is substantially larger for FTO substrate compared to the ITO ones. It may reflect substantially larger electron–phonon anharmonicity for the second case. It is well known that due to the existence of photoinduced anharmonic phonons [10] one can favour additional non-centrosymmetry. The latter will be determined by local thermoheating and photoinduced charge carrier kinetics. After interaction of the fundamental wavelength with the local acentrical clusters one can expect an occurrence of the interference between the fundamental and doubled frequency beams. The latter will determine the observed morphology and the appropriate photokinetics. The observed relaxation have a period of several nanoseconds, which may reflect a competition between the photoexcited car-

riers originated from the SPR and local heating waves occurring after excitations near the SPR. Because we excited by the 1064 nm wavelength we observe an occurrence of the interface doubled frequency 532 nm wavelength near the SPR maxima. This wave will be crucially dependent on the wavelengths of the SPR which are determined by the effective dielectric susceptibility of the substrate. It is crucial that the samples possessing larger resistance have larger response. This may indicate that the local heating manifested through the electron–phonon interactions plays here principal role. Moreover, the vibration damping amplitudes are attenuated in time (Figs. 7 and 8). However, principal role here begin to optically induced acoustical waves which have typical features. So the effects of local overheating may be crucial for the single pulse aggregation in the 1064 nm for Ag NP deposited on the ITO/FTO (Table 3). At the end it would be of interest to compare the changes of the principal SPR bands, which show that principal deviations in the photoinduced spectra occurred in the spectral range 30–450 nm corresponding to the near the SPR spectral region. Principal role for such effects play photoinduced electron–phonon anharmonicities [11–13].

Table 3 Main parameters of silver NPs on the two investigated substrates

Response time Amplitude Frequency and Period time Damping ratio Pearson correlation

FTO

ITO

11.6 ns 1.81 V 45.871 MHz, 21.8 ns 15 010 000 0.9390

8.2 ns 1.51 V 45.871 MHz, 21.8 ns 16 020 000 0.9495

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5. Conclusions

References

During investigations of optical treatment of the Ag NP deposited on the ITO and FTO substrates we have established an increase of the nanoparticles effective grain sizes for both substrates during treatment by single laser. The same tendency is observed for treatment by up to 3 laser pulses. The principal difference is observed after the 3 laser pulses. Particularly, a continuation of the size increase is obtained for FTO while ITO substrate leads to a drastic decrease of the effective sizes between the 3 and 5 laser pulses. Following the results presented in Figs. 4–6, one can see that the resistance of the substrate plays the main role in the observed dependencies. This may be due to the different local heating of interfaces separating the metallic nanoparticles and the substrates. At the same time the resistance difference cause different surrounding backgrounds covering particular nanoparticles. As a consequence, the numbers of free carriers interacting with the substrates will be different.

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