Laser Induced Microrelief Superstructure of Ag/ITO Seed-Mediated Nanocomposites

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Superlattices and Microstructures 46 (2009) 637–644

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Superlattices and Microstructures

journal homepage: www.elsevier.com/locate/superlattices

Laser induced microrelief superstructure of Ag/ITOseed-mediated nanocompositesR. Miedziński a,b, J. Ebothé∗,b, G. Kozlowski c, J. Kasperczyk a, I.V. Kityk d,∗,I. Fuks-Janczarek a, K. Nouneh e, M. Oyama e, M. Matusiewicz d, A.H. Reshak fa Institute of Physics, J. Długosz University of Częstochowa, Al. Armii Krajowej 13/15, Czestochowa, Polandb LMEN, E.A. 3799, UFR Sciences, Université de Reims, B.P. 138, 51685 Reims, Francec Physics Department, Wright State University, 3640 Col. Glenn Hwy., Dayton, OH 45435, USAd Electrical Engineering Department, Czestochowa Technological University, Al. Armii Krajowej 17/19, Czestochowa, Polande Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8520, Japanf Institute of Physical Biology, South Bohemia University, Nove Hrady 37333, Czech Republic

a r t i c l e i n f o

Article history:Received 22 March 2009Received in revised form4 April 2009Accepted 20 April 2009Available online 17 May 2009

Keywords:SuperlatticePhotoinduced phenomenaNanocomposites

a b s t r a c t

The topography of indium tin oxide (ITO) films with incorporatedsilver nanoparticles and irradiated by single pulses of 18 nsNd:YAGlaser has been investigated. The study was carried out with twoAg/ITO films having resistances of 50 �/ITO sheet and 4 �/ITOsheet. The periodic structures in both samples were created afterthe laser treatment. The photo-induced periodic structures have adifferent character while the sheet resistance plays a major rolein the growth process of these structures. The results of opticaland non-linear optical investigations lead us to the conclusion thatthe temporary polarization of samples during laser treatment isresponsible for the shape of periodic structures.

© 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Our preliminary studies [1] have shown the formation of micro-sized grains in Ag/ITO films underthe influence of nanosecondNd–YAG laser pulses. It shows a relatively periodic structurewith a heightmodulation of about 150 nm. A principal role of the ITO sheet resistance has been underlined. Inaddition, the crucial requirements were related to the pumping power density and the number ofpulses used during irradiation.

∗ Corresponding author. Tel.: +48 34 322 18 41.E-mail address: [email protected] (I.V. Kityk).

0749-6036/$ – see front matter© 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.spmi.2009.04.006


638 R. Miedziński et al. / Superlattices and Microstructures 46 (2009) 637–644

Fig. 1. Photo-induced experimental set-up: Nd:YAG fundamental pulsed laser; GaAlAs semiconductor probe laser; P1 and P2polarizers; PD1 and PD2 photo-detectors; ED energy detector; PM power meter; OSC digital storage oscilloscope.

Generally in the nanocrystallites illuminated by nanosecond laser pulses we deal with the twoprincipal effects: the photothermoheating and the interactions with the surface Plasmon resonance(SPR) waves. Superposition of these two effectsmay give some grating relief whichwill determine thebasic principles of the effects.In our present work, we have studied the height profiles of the Ag/ITO films as a function of ITO

sheet resistance and the time kinetics of the photoinduced changes of the films in the vicinity of theSPR.

2. Experiment

Transparent and conductive indium tin oxide (ITO) films deposited on a glass substrate werepurchased from Kuramoto Seisakusho Co., Ltd. Japan. Two types of ITO films characterized by 4 �and 50� sheet resistance were studied. The silver coatings were formed by a seed mediated growthprocess as described in [2]. The two step process consists of a seed solution phase in which the silverseeds were formed and a growth-solution phase in which the seeds evolved into grains.Our previous investigations have shown that the morphology after photoillumination is closely

related to the thermoheating [1]. So the main goal of this study to study the influence of laserillumination on the morphological changes appearing after the illumination.The initial Ag/ITO sample surfaces are modified by nanosecond Nd:YAG laser irradiation of 1, 3

or 5 pulses. The experimental set-up for photo-induced treatment of the sample surface is given inFig. 1. A signal was triggered by a PC interface to the Nd:YAG fundamental laser with a pulse durationof 20 ns at 1064 nm wavelength and energy density of 400 mJ/cm2. Simultaneously, the energydetector (ED) connected to the power meter (PM) is switched off and the digital storage oscilloscope(OSC) begins to measure signals from photo-detectors PD1 and PD2. The surface irradiated by thefundamental laser is probed by a GaAlAs semi-conductor continuous wave (CW) laser (2 mW powerand 685 nm wavelength). The sample is placed at the center where beams from both lasers overlapwithin an area of 0.25 cm2. The signal from the GaAlAs laser is registered by photo-detector PD2.The incident beam splits into two beams when the fundamental laser pulse propagates through thesample. The first beam goes to the energy detector and the second to the photo-detector PD1. All dataare acquired in PC computer and PM indicates the output beam energy in the same time. Based onthese measurements, absorbed and diffracted energies are estimated. PD1 and OSC detect the pulseduration of the fundamental laser and PD2 monitors the reference signal before, during and after thebeam interaction with the films. Each sample has been irradiated with 1, 3 or 5 single pulses of thefundamental Nd:YAG laser. Microstructure and topography measurements of the irradiated surfaceswere registered by a SMENA NT-MDT atomic force microscope (AFM) operating in a contact modewith a scanning frequency of 1.5 Hz. The picture resolution was equal to about 512× 512 pixels andthe tip spring constant was between 0.01 N/m and 0.08 N/m. The AFMmicrographs and topographyanalysis were collected and supplied by Nanotec Electronica WSxM software [3]. The spectroscopyanalysis was performed with an OceanOptics HR4000 high resolution spectrometer. The absorbancespectra were collected in the range between 200 nm and 1100 nm.


R. Miedziński et al. / Superlattices and Microstructures 46 (2009) 637–644 639

a b

Fig. 2. 2D-AFM micrographs of the Ag/ITO surface before irradiation: (a) Ag/ITO(50 �), σ = 2.57 nm, AvH = 13.67 nm;(b) Ag/ITO(4�), σ = 5.31 nm, AvH = 20.46 nm.

3. Results and discussion

3.1. Morphology and topography of Ag/ITO films

Fig. 2(a) and (b) present the 2D-AFM images (2.5 µm × 2.5 µm) of the Ag/ITO samples beforeirradiation. The Ag/ITO filmwith 50� ITO sheet resistance (ITO/50�) exhibits a microstructure withsmall grains (see Fig. 2(a)) while an image in Fig. 2(b) obtained for 4� ITO sheet resistance (ITO/4�)shows aggregated grains. The root-mean-squared surface roughness (σ ) for these samples has beenevaluated as being σ50 � = 2.57 nm and σ4 � = 5.31 nm. Measurements of average height (AvH)result in the following values: AvH50 � = 13.67 nm and AvH4 � = 20.46 nm. It shows that a lowerITO sheet resistance leads to higher topography parameters.The morphology and topography of the samples are drastically changed under the laser pulse

treatment. The surface morphology of the Ag/ITO films after single pulse irradiation is presented inFigs. 3 and 4. The 2D-AFM micrographs in Figs. 3(a) and 4(a) show a periodicity in the morphologyof the films. The corresponding surface profiles in Figs. 3(b) and 4(b) suggest that the period (Λ)in morphology may be affected by the ITO sheet resistance. We have found from Figs. 3 and 4 thatΛ ≈ 1.15 µm for Ag/ITO(50 �) film andΛ ≈ 3.43 µm for Ag/ITO(4 �), respectively. Figs. 3(b) and4(b) show that the average amplitude of the film profile is about 110 nm for Ag/ITO(50�) and about60 nm for Ag/ITO(4�). The root-mean-squared surface roughness for these samples amounts to thevalue of σ50 � = 32.89 nmand σ4 � = 22.75 nmand the average height (AvH) of AvH50 � = 79.84 nmand AvH4 � = 62.64 nm. These results show that topographic parameters of the irradiated films arehigher for 50� ITO sheet resistance than 4� in contrast to the results for the non-irradiated films.An increase in the pulse duration of the irradiation leads to a progressive collapse of the grating’s

regularity. As we can see in Fig. 5, the effect of the three consecutive pulses applied to the samplemodifies its surface morphologically in a different way than for samples not subjected to irradiation.The surface features in Fig. 5(a) obtained for the Ag/ITO(50 �) film exhibits a short-range and rice-like periodic structure. This periodicity is attenuated in the Ag/ITO(4�) film as seen in Fig. 5(b). Thetopography analysis results in surface roughness values of σ50 � = 32.87 nm and σ4 � = 25.42 nmand average heights of AvH50 � = 110.17 nm and AvH4 � = 83.05 nm, respectively. A surfaceexamination of the film irradiated with five consecutive pulses (Fig. 6) clearly shows that the periodicpattern on the film surface completely disappears. The silver coating is almost completely destroyed inthe Ag/ITO(50�) film (see Fig. 6(a)) while Ag is still present in the Ag/ITO(4�) film. A topographicalstudy of these samples leads to the values of σ50 � = 27.45 nm and AvH50 � = 79.08 nm inAg/ITO(50 �) film but these values cannot represent properties of the whole film because of itssurface destruction mentioned above. The silver film morphology in Ag/ITO(4 �) film (see Fig. 6(b))



ba

Fig. 3. 2D-AFMmicrograph and depth profile of the Ag/ITO(50�) after one pulse irradiation by Nd:YAG laser (1064 nm).

a b

Fig. 4. 2D-AFMmicrograph and depth profile of the Ag/ITO(4�) after one pulse irradiation by Nd:YAG laser (1064 nm).

Table 1Topographical parameters of the investigated Ag/ITO films subjected to laser irradiation.

Number of pulses Ag/ITO(50�) Ag/ITO(4�)σ (nm) AvH (nm) σ (nm) AvH (nm)

0 2.57 13.67 5.31 20.461 32.89 79.84 22.75 62.643 32.87 110.17 25.42 83.055 27.45 79.08 26.67 67.07

still remains visible over its surface. The surface roughness σ4 � = 26.67 nm and average heightAvH4 � = 79.08 nm of the Ag/ITO(4 �) film can be more reliable than for Ag/ITO(50 �) becausethe surface destruction is not as apparent there. The topographical parameters for different films aresummarized in Table 1. It shows that σ4 � is almost unchanged after a single pulse treatment whilstthe highest AvH4 � value is obtained after a three pulse treatment. It is necessary to add that thereexists some difference in the values of the photoinduced relief on the borders and in the center ofthe photoinduced beam. This difference does not exceed 13%–15%whichmay indicate the substantialinfluence of photothermal melting of the surfaces.

3.2. Optical studies

The optical studies of the absorption spectra were restricted to the wavelength from 300 nm to700 nm. The registered signals from the PD1 and PD2 photo-detectors allow one to determine theresponse time (τ ) of the examined samples.Fig. 7(a) presents the absorption spectra of silver coating on the Ag/ITO(50 �) film. Generally,

higher absorptions have been observed for samples at the near UV region. It can be seen from



a b

Fig. 5. 2D-AFM micrographs of the Ag/ITO surfaces after three pulse irradiation: (a) Ag/ITO(50 �), σ = 32.87 nm, AvH =110.17 nm; (b) Ag/ITO(4�), σ = 25.42 nm, AvH = 83.05 nm.

a b

Fig. 6. 2D-AFM micrographs of the Ag/ITO surface after five pulse irradiation: (a) Ag/ITO(50 �), σ = 27.45 nm, AvH =79.08 nm; (b) Ag/ITO(4�), σ = 26.67 nm, AvH = 67.07 nm.

Fig. 7(a) that the absorption spectrum of the non-irradiated samples is relatively flat as a functionof wavelength and has a shallow maximum at 493 nm. The absorption spectra of all treated samplesover the investigated wavelength region are significantly higher in comparison to the non-irradiatedsamples.After one pulse irradiation, the absorption in the range of wavelengths from 300 nm to 464 nm is

lower than absorption for the samples irradiated by three and five single pulses. We observe that thespectrum of the three pulse irradiated sample has the highest absorption in the whole investigatedspectral region. The five pulse irradiated sample has a lower absorption than the one pulse irradiatedin the vicinity of 464 nm wavelength region. Moreover, its absorption decreases tremendously in thewavelength range between 600 nm and 700 nm.Fig. 7(b) presents the absorption spectra for the silver coating in the Ag/ITO(4�) film. Herewe also

observe the highest absorption for all samples at the near UV region. The absorption spectrum of thenon-treated sample has the lowest absorption spectrum with two shallow spectral peaks at 465 nmand 355 nm. An interesting fact is that the latter peak is located near the third harmonic frequencyof the applied Nd:YAG laser. An incremental increase of the absorption is observed in the one pulsetreated sample. The highest absorption is seen in the three pulse treated Ag/ITO(4�) film.



Fig. 7. Absorption spectra of (a) Ag/ITO(50�) and (b) Ag/ITO(4�) films before and after laser treatment, respectively.

Fig. 8. Relaxation curves due to the fundamental laser pulse of 400 mJ/cm2 power.

The existence of higher absorption after increasing the number of pulses is caused by additionalthermoheating.As shown in Fig. 1, the sample was also illuminated by a CW laser. This laser signal is registered

by PD2 connected to a digital oscilloscope. The set of two polarizers (P1 and P2) gives rise to thereference signal of 1 V. During the photo-induced process this set-up arrangement can detect: (1) theresponse time of the material (τ ) and (2) the change in the polarization of the probe laser. As wecan observe in Fig. 8, the duration of Nd:YAG lasers detected by PD1 is about 17 ns. The signals fromPD2 detector have a damped sinus-like shape. The calculated response time of the studied samplesare τ1 = 4.6 ns for the Ag/ITO(50 �) film and τ2 = 8.0 ns for the Ag/ITO(4 �) film. This timeresponse τ is caused by electron–phonon interaction andmaterial heating which, in turn, depends onthe material resistance. The change of polarization caused by the optically induced Kerr effect is dueto birefringence modification. This effect will be additionally investigated in the near future.



Fig. 9. Correlation between the output THG and the averaged height of the photoinduced grating.

It is necessary to emphasize that the effect is observed only in the nanostructural effects. So role of thenano-confined states should play here a decisive role. Superposition of the SPRwaveswith the thermalwaves will be interesting near the surface reliefs with periods of about several micrometers. Severalexplanations may be given by taking into account possible nonlinear optical effects [4,5] occurringon the silver nanoparticles/ITO substrate border. Second harmonic generation occurring may interactwith the fundamental wavelength and due to the presence of non-confined states interacting withthe phonon sub-system one can observe an occurrence of a modulated structure. An additionalnumber of pulses leads to local overheating of the periodic structures formed and finally to theirdestruction. Depending on the periodicity and the shape on the resistivity of the substrate indicatesthe principal role of the photoinduced carrier kinetics in the observed effects. However, these carrierswere trapped on the inter-border trapping states, which are sensitive both to the SPR as well as tothe electron–phonon local anharmonicities. Changing the laser pulses and the pulse duration one canobserve changes undergoing from periodicity up to a flower-like structure [6]. It is necessary to addthat similar experiments which were performed on the ITO films of different resistivities did not playany role in the occurrence of the photoinduced relief.In order to check the role of the third harmonic generationwehave done additional experiments on

the third harmonic generation in the investigated lasers.We have found that there exists a correlationbetween the period of the photoinduced structure and the output SHG (see Fig. 9) for 1064 nmfundamental wavelength. So the role of the THG is crucial here.

4. Conclusion

The average heightmeasurements for Ag/ITO filmswith 50� and 4� ITO sheet resistance amountto values of AvH50 � = 13.67 nm and AvH4 � = 20.46 nm, respectively. These results suggest thata lower ITO sheet resistance leads to higher topographical parameters showing a periodical patternon the surface of the films (see, Figs. 3–5). We have found that the periodicity of Λ ≈ 1.15 µm forAg/ITO(50 �) film and Λ ≈ 3.43 µm for Ag/ITO(4 �) film is observed. These results show thattopographical parameters of the irradiated film are higher for Ag/ITO(50�) than Ag/ITO(4�) films incontrast to the non-treated samples. An examination of the five single pulse irradiated sample clearlyshows (Fig. 6) that the periodicmorphological pattern of the film surface disappears. The silver coatingis almost destroyed in the Ag/ITO(50�) film (Fig. 6(a)) while it is still present in the Ag/ITO(4�) film.The signals from the PD2 detector have a damped sinus-like shape. The calculated response time ofthe studied samples are τ1 = 4.6 ns for the Ag/ITO(50 �) film and τ2 = 8.0 ns for the Ag/ITO(4 �)film, respectively. This time response τ is probably caused by the electron–phonon interaction andmaterial heating which depends on the material’s resistance.



References

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Laser Induced Microrelief Superstructure of Ag/ITO Seed-Mediated Nanocomposites

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