Films by Continuous Wave Laser Proce - American Chemical Society

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One-Step Microstructuring of TiO2 and Ag-TiO2 Films by Continuous Wave Laser Processing in the UV and Visible Ranges N. Crespo-Monteiro,† N. Destouches,*,† L. Saviot,‡ S. Reynaud,† T. Epicier,§ E. Gamet,† L. Bois,∥ and A. Boukenter† †

Université de Lyon, F-42023 Saint-Etienne, France; CNRS, UMR 5516, Laboratoire Hubert Curien, 18 rue Pr. Lauras F-42000 Saint-Etienne, France; and Université de Saint-Etienne, Jean-Monnet, F-42000 Saint-Etienne, France ‡ Laboratoire Interdisciplinaire Carnot de Bourgogne, UMR 6303, CNRS-Université de Bourgogne, 9 Av. A. Savary, BP 47 870, F-21078 Dijon Cedex, France § Matériaux, Ingénierie et Sciences (MATEIS), UMR 5510 CNRS, Université de Lyon, INSA-Lyon, 7 avenue Jean Capelle, 69621 Villeurbanne, France ∥ Laboratoire Multimatériaux et Interfaces, Université de Lyon, Université Claude Bernard Lyon 1, Bat Berthollet, 69622 Villeurbanne, France ABSTRACT: The transformation of three kinds of mesoporous films of amorphous TiO2 under exposure to continuous wave (CW) lasers emitting at UV and visible wavelengths is investigated. Silver-free films, films impregnated with a silver salt, and films loaded with silver nanoparticles are considered. The silver containing films had previously been shown to exhibit a photochromic behavior. Intensity thresholds leading to laser-induced crystallization are evaluated using a combination of Raman spectroscopy, atomic force microscopy, and high-resolution electron microscopy. Crystallization in the anatase and rutile phases as well as ablation of the films are shown at 244 nm wavelength for increasing intensity, whatever the nature of the films. However, under visible light exposure, crystallization only occurs in the silver containing films. The oxidized-silver containing films, that are shown to be as transparent as the silver-free films in the visible range, appear to crystallize under visible light. The temperature rise occurring during visible light exposure and due to the plasmon-induced nanoparticle heating is measured. Finally, such a CW laser-induced crystallization is proven to be an accurate technique to produce permanent patterns with submicrometer widths and nanometer depths that can be tuned to within only a few nanometers. The patterns resist to temperature rises of 1000 °C.

1. INTRODUCTION

induced crystallization has also been investigated in such matrixes.10,11 Recently, we have demonstrated that photochromic films of silver containing TiO2 could be used as rewritable or writable data carriers depending on the laser intensity.4 Permanent data are written by crystallizing the TiO2 under visible or UV CW laser. However, no precise study has been done concerning the ability to control the film properties in the crystallized areas, and the influence of silver on the laserinduced crystallization of TiO2 has not been characterized. In this article, we compare the transformation of three kinds of mesoporous films of amorphous TiO2 under exposure to CW lasers emitting at UV and visible wavelengths: silver-free films, films impregnated with a silver salt, and films loaded with silver nanoparticles. Intensity thresholds leading to laserinduced crystallization are evaluated using a combination of

TiO2 has many singular properties that have drawn a lot of interests for technological applications such as photocatalysis, hydrogen and electric energy production, gas sensor, protective coatings, optical coatings, or self-cleaning surfaces.1 It has also been proven to exhibit multicolor photochromism when combined with silver nanoparticles, which leads to other applications like rewritable color copy paper, smart glass, or multiwavelength optical memory.2−4 The development of some of these technologies relies on the ability to micronanostructure the material with fast and accurate techniques. Ultrashort lasers have been used to crystallize or damage titania leading to single-step patterning techniques by direct writing5,6 or laser-beam interference.7 Few studies have also been reported on the use of continuous wave (CW) lasers to induce physicochemical transformations of TiO2. Bulk vitreous TiO2 and TiO2 films, whose absorption was significant in the visible range, have been crystallized or ablated under high intensity exposures.8,9 The role of cobalt cations on the laser© 2012 American Chemical Society

Received: September 27, 2012 Revised: November 21, 2012 Published: November 21, 2012 26857

dx.doi.org/10.1021/jp3096264 | J. Phys. Chem. C 2012, 116, 26857−26864

The Journal of Physical Chemistry C

Article

Raman microspectroscopy (LabRam ARAMIS) was used to characterize the crystal phase of the TiO2 matrix. Spatially resolved measurements were performed with an excitation wavelength of 633 nm and an intensity of 63 kW·cm−2. These conditions did not lead to crystallization or densification during the measurements. The estimation of the temperature rise during crystallization was done by recording simultaneously the Stokes and anti-Stokes Raman spectra down to 10 cm−1. The Raman setup used in this case was a Renishaw inVia with 532 nm excitation and a BragGrate notch filter from OptiGrate. The film mesostructure and the silver nanoparticles were characterized by transmission electron microscopy (TEM, Hitachi H-800 operating at 200 kV) and scanning electron microscopy (SEM, FEI Nova nanoSEM 200). High-resolution transmission electron microscopy (HRTEM, Jeol 2010F TEM, equipped with a Field-Emission Gun) was used to identify small silver nanoparticles in the nonabsorbing silver containing films and TiO2 nanocrystals, especially in the early stages of crystallization when Raman spectroscopy was not sensitive enough to detect the phase changes. Topographic measurements were performed with an atomic force microscope (Agilent Technologies 5500) in acoustic AC mode, and UV−visible absorbance spectra were recorded with a Perkin-Elmer Lambda 900 spectrometer.

Raman spectroscopy, atomic force microscopy, and highresolution electron microscopy. Laser-induced phase changes of the TiO2 matrix are characterized for a large range of intensities at 244 and 488 nm wavelengths and are shown to allow an accurate control of the surface structuring depth depending on the exposure parameters. An interesting result is that crystallization under visible light occurs on the colorless TiO2 films loaded with oxidized silver, whereas they do not show more absorbance at the incident wavelength than the pure TiO2 films that cannot be crystallized. Plasmon-induced nanoparticle heating under visible illumination is proven by measuring the real-time temperature rise deduced from the Stokes/anti-Stokes Raman intensity ratio. Finally, the stability of micropatterns resulting from a local crystallization or ablation of the TiO2 film is demonstrated under high temperature rise.

2. EXPERIMENTAL METHODS Mesoporous titania films were prepared following a previously published procedure.4,12 A solution of P123 ((PEO)20(PPO)70(PEO)20) (1 g), used as structuring agent, in ethanol (12 g) was added to a solution of tetrabutylorthotitanate (TBT) (3.4 g) in concentrated HCl (3.2 g). The films were deposited by dip-coating on cleaned glass slides at a withdrawal rate of 20 mm·min−1 and dried at 150 °C for 1 h before the copolymer was removed using a hot ethanol extraction. Silver-salt containing films were obtained by immersion of the mesoporous titania films during 30 min in an aqueous ammoniacal silver solution obtained by adding a NH3 solution to a 0.5 M silver nitrate solution until a clear solution was observed. They were rinsed with water and dried at room temperature for at least 12 h. Such films that are initially colorless exhibit a photochromic behavior, well explained in refs 2, 4, and 13, which allows to reversibly switch between a colorless and a colored film. This photochromism results from the growth of silver NPs under UV light and from their oxidation under visible light. In order to investigate the influence of the localized surface plasmon resonance (LSPR) of silver NPs on the laser-induced crystallization, we considered both films with silver nanoparticles that exhibit an absorption band in the visible range and films with oxidized silver that do not absorb in the visible range. Silver-nanoparticle containing films were obtained by illuminating the as-impregnated films with a CW doubled Ar laser emitting at 244 nm for 10 min with an intensity of 0.15 W·cm−2. Oxidized-silver containing films were obtained after exposing the as-impregnated films to a CW Ar laser emitting at 488 nm for 2 h with an intensity of 4 W·cm−2. The last exposure was not completely needed since such as-impregnated films look colorless and contain Ag(I) species, i.e., oxidized silver,2,13 but when they are used a few days after the introduction of the silver salt, the beginning of a LSPR band corresponding to the spontaneous formation of few silver NPs may appear. To be sure that no absorption band was detected and to work in the same conditions whatever the sample, the visible exposure was performed systematically before any crystallization study of the oxidized-silver containing films. The samples were illuminated through a circular aperture centered on the expanded beam in order to get a homogeneous intensity on the whole treated surface. Laser-induced TiO2 crystallization was performed at the same wavelengths, 244 and 488 nm, but under a focused beam to reach much greater irradiances.

3. RESULTS AND DISCUSSION 3.1. Film Characteristics before Crystallization. The mesoporous TiO2 films are transparent and colorless and their absorbance spectrum does not show any absorption band in the visible range (green curve in Figure 1a). Their thickness is estimated to 250 ± 50 nm from profilometry measurements (Dektak 3 ST) and SEM pictures on the cross-section. TEM and SEM images of the films (Figure 1b,c, respectively) indicate a disordered structure of mesopores whose size equals 7 ± 2 nm. The oxidized-silver containing films have the same absorbance spectrum as the nonimpregnated ones (red curve in Figure 1a). However, HRTEM micrographs (Figure 1d,e) show the presence of numerous small silver (face-centered cubic structure with a unit cell length of 0.407 nm) nanoparticles of about 1−3 nm in diameter; a few NPs smaller than 4 nm are also imaged as white dots on the film surface by SEM (Figure 1e). Such nanoparticles have a very low absorption cross-section, typically more than 74 times smaller than the one of 9 nm large NPs, according to the Mie theory. It appears here that such a low absorption cannot be detected in such thin films by UV−visible spectroscopy. The films seem then nonabsorbent in the visible range even if they very slightly absorb. The presence of small nanoparticles is also asserted by low-frequency Raman scattering. Raman peaks, which correspond to confined acoustic vibrations in small nanoparticles,14 are observed at about 20 cm−1 (Figure 1h, oxidized Ag before crystallization). These peaks are attributed to the spheroidal S = 2 vibrations of silver nanoparticles because the laser is resonant with their LSPR,15 which is further confirmed by their absence in the silver-free samples. The shape of Raman peaks is a complex function of the size and shape distributions of the nanoparticles and laser wavelength,16 and its detailed study is clearly beyond the scope of the present work, precisely because of the complexity of these size and shape distributions of nanoparticles in our samples. However, using simple assumptions, the mean diameter calculated from the frequency of the peaks is about 2.5 nm assuming free spheres and 3.5 nm 26858



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for Ag spheres in an anatase TiO2 matrix.17 This is consistent with the very small silver nanoparticles actually detected in the oxidized-silver containing films. The films in which silver nanoparticles have been grown photocatalytically show a high absorbance band centered at 485 nm (Figure 1a) that gives them a brown color. This band is characteristic of the presence of silver nanoparticles whose size can be estimated from TEM and SEM characterizations (Figure 1f) to 11 ± 4 nm within the film and ranging from 10 to 70 nm on the top surface (Figure 1g). These NPs mainly grow within the first tens of nanometers under the film surface, as attested by many TEM pictures of film sections (Figure 1f). This local growth results from the high absorption coefficient of TiO2 at 244 nm wavelength and from the fact that Ag NPs essentially grow where the TiO2 is photoexcited that is preferentially near the top surface. Here also, the presence of small NPs is asserted by the presence of a Raman peak at low frequency (Figure 1h, Ag NP before crystallization). This peak is enlarged toward low frequencies compared with the one of oxidized-silver containing films, meaning that larger NPs are present. Note, however, that Ag NPs whose diameter is larger than about 7−8 nm cannot be observed with the used Raman setup because their vibration frequency is less than 10 cm−1. No NP diameter can therefore be estimated from these measurements. The silver-free films have also been characterized by Raman microspectroscopy and, as the silver containing films, they did not exhibit any signal that may attest the presence of crystallized TiO2. XRD patterns recorded on nonimpregnated films confirm that the TiO2 matrix is amorphous. The next two sections will be devoted to the characterization of the laserinduced crystallization under UV and visible light and to the determination of intensity thresholds leading to phase changes in the three kinds of films. 3.2. Laser-Induced Crystallization at 244 nm Wavelength. The 244 nm wavelength of a doubled Ar laser corresponds to a photonic energy of 5.08 eV that is much greater than the band gap of TiO2 (∼3.2 eV). This wavelength is strongly absorbed by titania and can lead to phase changes within the films or even to laser damages. The initial amorphous TiO2 matrix can be crystallized in its anatase or rutile phase provided that the incident intensity is high enough. Such high intensities are easily obtained at the focal plane of microscope objectives and lead to local nanocrystallizations of the film on areas of only a few μm2. The nanocrystallization can be evidenced by Raman microspectroscopy, HRTEM, and, in an indirect manner, atomic force microscopy. HRTEM shows that crystallization leads to grains of nanometer size, but it is obviously not a technique to easily determine the threshold above which phase transformations occur. Raman spectroscopy is a reliable and nondestructive technique to characterize the TiO2 crystal phases, but when carried out on so thin films, it turns to be weakly sensitive and needs a high concentration of nanocrystals to detect the phase changes. Finally, AFM can be a complementary nondestructive technique to gain sensitivity, even if it cannot prove the phase changes by itself. It has been observed that a densification occurs and leads to the formation of a trough on the film surface under the laser beam, largely before any phase change can be detected by Raman spectroscopy. These troughs can be detected by AFM as soon as they become deeper than the film roughness (of the order of one nanometer on a few micrometer squared areas), and it has been checked by HRTEM that nanograins of crystallized TiO2 are actually present in their region. Nano-

Figure 1. (a) Absorbance spectra of a silver-free mesoporous TiO2 film, a film containing oxidized silver, or silver NPs. (b,c) TEM and SEM pictures of silver-free mesoporous TiO2 films. (d,e) Oxidizedsilver containing films: HRTEM micrograph showing a 2 nm wide silver NP (d) and SEM picture (e). (f,g) Cross-section TEM and SEM pictures of silver-NP containing films. (h) Low-frequency Raman spectra of silver containing films before and after crystallization. 26859



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Figure 2. Optical (a−d) and SEM (e−h) pictures of silver-free mesoporous TiO2 films deposited on glass substrates before irradiation (a,e) and after irradiation at 244 nm wavelength with an intensity of 97 (b,f), 129 (c,g), and 242 kW·cm−2 (d,h). The corresponding Raman spectra are given below each SEM picture (i−l).

peak. These values agree with the Raman active modes of anatase TiO2 reported in the literature.18,19 The bands at 144, 197, and 639 cm−1 are assigned to Eg modes, the band at 399 cm−1 to the B1g mode, and the one at 514 cm−1 to a doublet of A1g and B1g modes. The significant thickness reduction of the film is then likely to result not only from the switch of the TiO2 matrix phase from amorphous to anatase but also from a collapse of the film mesostructure. Under an intensity of 129 kW·cm−2, the drawn lines look more heterogeneous in the center, and homogeneous bands appear on the edges of the illuminated area (Figure 2c). SEM characterizations in the center of the line give an average film thickness of about 80 nm and show the presence of many holes in the glass substrate just below the film (Figure 2g). The TiO2 film is sometimes lifted and forms bumps above the holes. The Raman spectrum recorded in this region (Figure 2k) exhibits four peaks at 143 (B1g), 234 (combination of modes), 447 (Eg), and 611 (A1g) cm−1 that can be assigned to Raman active modes of rutile TiO2.18,19 To crystallize into the rutile phase, TiO2 is supposed to reach temperatures greater than 600 °C. However, below 800−1000 °C, both anatase and rutile phases generally coexist.8,20 Here, the Raman spectrum of Figure 2k shows only one phase suggesting that temperatures greater than 800 °C are reached, probably leading to a melting of the glass substrate. Damages occurring in the substrate are responsible for the heterogeneous aspect of the sample under optical microscope. They do not occur when working on silica substrates, as done in section 3.4. In the homogeneous external bands, the film thickness increases slightly up to 100 nm. Raman characterization was not reliable in these bands because

crystallization of TiO2 is therefore considered to be the cause of the observed local densification, and in the following, the intensity threshold leading to TiO2 crystallization will be defined as the lowest intensity required to create a topographic variation on the film surface measurable by AFM. For silver-free mesoporous TiO2 films, the intensity threshold is estimated to 3 ± 1 kW·cm−2. The presence of silver NPs or oxidized silver within the mesoporous films does not significantly affect the intensity threshold. Actually, most absorption of the incident light at 244 nm comes from the titania matrix itself rather than from silver. Above the threshold, different phase transformations can be obtained depending on the incident laser intensity. Crystallization into the anatase and rutile phases as well as ablation of the film have been obtained at increasing intensity. Patterns were drawn dynamically by moving the samples under the focused laser beam at a scanning speed of 2 cm·s−1. They were observed in top view by optical microscopy and on the cross -section by SEM, and they were characterized by Raman microspectroscopy. Only experiments carried out on films without silver are shown below. Before irradiation, the films look homogeneous (Figure 2a) with a thickness of about 250 nm (Figure 2e) and their Raman spectrum (Figure 2i) shows no features, except for the broad peaks due to Raman scattering by the glass substrate. Using an intensity of 97 kW·cm−2, a bright yellow color appears under optical microscope (Figure 2b), the film thickness goes down to 96 nm (Figure 2f), and four dominant peaks at 144, 399, 514, and 639 cm−1 appear on the Raman spectrum (Figure 2j). A shoulder at 197 cm−1 can also be found on the foot of the first 26860



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Figure 3. Topography and phase changes on mesoporous TiO2 films loaded with oxidized silver under exposure at 488 nm wavelength. (a) AFM profiles of troughs created with various exposure times at 10 kW·cm−2. The inset shows a 3D view of the AFM topography of the trough made at 10 kW·cm−2 for 30 s. (b) HRTEM micrograph of an anatase TiO2 nanocrystallite observed at the early stage of densification (the inset diffractogram shows the indexing of the lower part as the [331̅] zone axis of anatase). (c) Same as that in panel a but under a 1 min long exposure. (d) HRTEM image of a TiO2 crystallite appeared after illumination at 125 kW·cm−2 (the inset diffractogram shows the indexing of the [111̅] zone axis of rutile).

The intensity threshold leading to crystallization, as defined previously, has been estimated to be 0.4 ± 0.1 kW·cm−2 for the silver-NP containing films. These films exhibit a large absorbance band in the visible region (Figure 1a) due to the LSPR of silver nanoparticles. When exposed to a visible wavelength, the latter strongly absorbs the incident photons and heats the surrounding TiO2 matrix at high enough temperatures to cause a phase transformation. The oxidizedsilver containing films, which appear as transparent as the silverfree films at 488 nm (see Figure 1a), can also be crystallized under exposure at this wavelength. In this case, the intensity threshold is estimated to br 10 ± 1 kW·cm−2, i.e., 25 times the threshold measured on the silver-NP containing films. This higher intensity threshold means that the temperature rise at a given intensity is lower in the oxidized-silver containing films than in the silver-NP containing films, which will be confirmed below. This is due to the much lower absorption of the films at this wavelength. Above the intensity threshold, AFM characterizations show that the trough depth resulting from the laser-induced crystallization depends on both the intensity and exposure

of the lack of spatial resolution, but in comparison with the results obtained at 97 kW·cm−2, the external bands are likely to be crystallized in the anatase phase. At 242 kW·cm−2, the film seems to be ablated in the central part of the illuminated area since no film is clearly observed on the cross-section (Figure 2h), and no significant peak arises on the Raman spectrum except for the broad band of the glass substrate at about 600 cm−1 (Figure 2l). Optical characterization informs, by comparison with the previous results, that rutile and anatase phases are very likely to be present on the edges of the ablated area, as expected from a temperature profile proportional to the incident intensity and corresponding to a Gaussian shape.21 3.3. Laser-Induced Crystallization at 488 nm Wavelength. A significant difference with the previous section is that TiO2 does not absorb the 488 nm wavelength. Exposures of silver-free mesoporous TiO2 films at incident intensities reaching 3 MW·cm−2 (limit of our experimental setup) do not induce any crystallization nor changes in the topography. However, they gain sensitivity to visible wavelengths after introduction of silver. 26861



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Figure 4. (a) Time variations of the temperature rise within silver containing films under exposure at 532 nm with an incident intensity of 36 kW·cm−2. An exponential fit of each data set is also shown. (b) Stokes and anti-Stokes Raman spectra of Ag-NP containing films after 1 s and 118 s of exposure.

reached on a crystallized area when using a probe beam intensity of 3.6 kW·cm−2 (Raman spectra named ″after cryst.″ in Figure 1h) has been assumed to be the ambient temperature. Actually, this may not be completely true, and the calculated temperature values are probably slightly underestimated, especially for the Ag-NP containing films. It follows that, after 1 s of exposure, the temperature reaches 435 °C in the AgNP containing films and 380 °C in the oxidized-silver containing films. Therefore, the plasmon induced heating in the colorless films loaded with oxidized silver and containing small silver NPs (Figure 1d) is significant, even if the film absorption is too low to be measured by absorption spectroscopy (Figure 1a). As expected, the temperature reached in NP containing films is higher than the one of the colorless films, but not by much. Both estimated temperatures give rise to TiO2 nanocrystallization; according to the very small size of TiO2 crystallites identified in HRTEM (see Figure 3b,d), nanocrystallization may occur from a relatively low temperature rise as was already reported in ref 8. Figure 4a also shows that the film temperature rapidly decreases under exposure, before stabilizing around 200 °C. This decrease can be explained by changes in the NP size during the illumination time. TEM characterizations of films after crystallization show the presence of large spheroidal silver nanoparticles whose size reaches several tens of nanometers; silver NPs seem to grow and coalesce during high intensity visible laser exposure. According to theoretical estimations of the NP heating under laser exposure,23,24 the temperature rise of NPs in steady state is proportional to the ratio of their absorption cross-section to their radius. Using the Mie theory to calculate it, this ratio first increases with the NP radius, then decreases when the radius typically exceeds 10 to 20 nm. The decrease in temperature measured during illumination is therefore likely to result from a significant growth of Ag NPs up to sizes reaching 50 to 150 nm. More detailed investigations of changes in the NP morphology and organization under high intensity CW visible exposures will be presented in another article. It can also be noted that the temperature decrease also goes with a shift of the Raman peaks toward lower frequencies (Figure 4b), which agrees with previously published results on the temperature dependence of the band position for anatase TiO2.8 Changes over time of the low-frequency Raman spectra confirm the disappearance of at least the smallest silver nanoparticles. After 1 s of laser exposure, these low-frequency peaks disappear, and they are not observed anymore even after the laser exposure (Figure 1h, see spectra named “after cryst.”).

time. At intensities close to the threshold, the trough depth increases during about 1 min before reaching a saturation level. Figure 3a shows AFM profiles of troughs formed in an oxidized-silver containing film when using a laser intensity of 10 kW·cm−2. At such low intensity, the trough depth can be tuned down to a few nanometers with a high accuracy by controlling the exposure time to within a few hundreds of milliseconds. From about a 30 s long exposure time, the amount of nanocrystallites in the film is high enough to measure a Raman signal characteristic of an anatase phase in the film. However, some TiO2 anatase nanocrystallites can be found in the films by HRTEM from the early stages of densification (Figure 3b). When using the threshold intensity of 10 kW·cm−2, the trough depth reaches a maximum value of about 100 nm after 1 min. This maximum value increases with the incident intensity up to about 175 nm (Figure 3c), whereas the time needed to reach it rapidly falls down below 1 s (shorter times were not accurately measured). Laser-induced crystallization can also be performed at other visible wavelengths such as 514, 532, 568, 633, or 647 nm, provided that the intensity can be raised above the threshold that depends on the wavelength. Depending on the exposure conditions, anatase (Figure 3b) or rutile (Figure 3d) phases can be obtained in the form of a few nanometer wide crystallites. The mesoporous films loaded with silver NPs exhibit a similar behavior compared to the oxidized-silver containing films, except that the intensities required to form the troughs were lower. The latter films exhibit a high enough absorbance at 488 nm to significantly heat during exposure. The temperature rise occurring during titania crystallization under visible exposure has been estimated from Raman measurements, using the ratio of Stokes to anti-Stokes intensities at 146 and −146 cm−1, and compared for both kinds of films loaded with silver (Figure 4). When a film loaded with reduced or oxidized silver is illuminated at 532 nm wavelength for 100 s with an intensity of 3.6 kW·cm−2, no crystallization occurs (Raman spectra named “before cryst.” in Figure 1h). At 36 kW·cm−2, however, the peaks identifying an anatase phase instantaneously appear on the Raman spectra. This intensity is high enough to crystallize both kinds of films. Raman spectra have been recorded as a function of time with an integration time of 1 s. The ratio of Stokes to anti-Stokes intensities of the peaks at 146 and −146 cm−1 has been used to estimate the temperature of the crystallized nanodomains using the Boltzmann distribution22 (Figure 4a). In order to calibrate the system, the temperature 26862



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resulting from the photochromic behavior4 of Ag-TiO2, can be written on the same films.

3.4. Stability of the Crystallized Patterns under High Temperature Rise. As it has been shown in the previous sections, the CW laser induced crystallization or ablation of TiO2 can lead to the formation of micropatterns with widths down to less than 1 μm and depths accurately controlled to within a few nanometers. The technique can then be considered to engrave, in a one-step process, 2D sets of data using depth as a tunable and encoding parameter. Its use for permanent data storage also requires a high stability under temperature rise. Figure 5 shows a micropattern drawn under

4. CONCLUSIONS Because of the strong absorption of the TiO2 matrix at 244 nm wavelength, the three kinds of films can be crystallized in the anatase or rutile phase or ablated, provided that intensities of a few tens or a few hundreds of kW·cm−2 are involved. They all show a similar intensity threshold for TiO2 nanocrystallization meaning that the presence of silver in the films does not contribute in a significant manner to the temperature rise occurring under UV exposure. Under visible laser exposure, especially at 488 nm wavelength, silver-free TiO2 films do not show any photosensitivity and cannot be crystallized, but they gain sensitivity when they are loaded with silver NPs or oxidized silver. The oxidized-silver containing films do not exhibit more absorption at 488 nm than the silver-free films according to spectroscopic measurements. However, they contain some small silver NPs that are likely to heat the surrounding TiO2 matrix under high enough intensity. Silver NPs act as heat nanosources for laser excitations close to their surface plasmon resonance. High temperature rise can then be reached, as proven by measurements of the relative amplitude of the Stokes and anti-Stokes Raman peaks of TiO2. This leads to crystallization, with an intensity threshold 25 times higher in the oxidized-silver containing films than in the Ag-NP containing films, which show a high absorption in the visible. In all cases, CW laser exposures under a focused beam allow to produce high-resolution patterns with submicrometer widths and an accurately controlled depth that can be tuned from a few nanometers to a few hundreds of nanometers depending on the incident intensity and exposure time. This opens the way to a one-step marking process with CW lasers for the direct writing of micropatterns whose depth can be modulated. The depth modulation can be used for encoding more information on a 2D surface. Another interesting property of these micropatterns is that they appear to be heat-resistant up to 1000 °C.

Figure 5. Resistance to temperature rise of laser inscriptions performed at 488 nm wavelength and 867 kW·cm−2 at a scanning speed of 5 mm·s−1 on a mesoporous TiO2 film loaded with silver NPs.

visible focused laser beam on an Ag-NP containing film deposited on a silica substrate. Laser marking has been performed at 488 nm wavelength and 867 kW·cm−2 at a scanning speed of 5 mm·s−1 in order to locally crystallize the film in the rutile phase. Crystallization in the rutile phase rather than in the anatase phase provides a better resistance to temperature. The latter has been evaluated by checking the readability of the written data under optical microscope after heating the sample at various temperatures for 5 h. Such a film usually starts cracking from a temperature of 900 °C probably due to strong stresses at the film−substrate interface resulting from their different thermal expansion coefficients. However, temperatures up to 1300 °C can be reached without cracking if silver is at least partly removed from the film after crystallization with visible laser light. For that, the film is first bleached for 2 h at 488 nm wavelength (4 W·cm−2) to oxidize silver, then soaked in an aqueous solution of sodium thiosulfate at 0.05 mol·L−1 during 10 min under ultrasound, rinsed with water, and dried. In such conditions, the laser written data remain safe up to 1000 °C, then the contrast becomes low due to crystallization of the whole film in the rutile phase. Using UV laser light to write the data, similar results can also be obtained on silver-free films. UV or visible CW laser induced microstructuring of such titania films is therefore useful for the storage of permanent data resistant to high temperature rise. In other words, with the same lasers and depending on the incident intensity, permanent data, resulting from the laser induced local crystallization of TiO2, or updatable data,

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the French National Research Agency (ANR) for its financial support in the framework of project UPCOLOR no. JCJC 2010 1002 1. We also thank CLYM (www.clym.fr) for access to the Jeol 2010F TEM and the METSA network (www. metsa.fr) for financial support. Y. Lefkir from University Jean Monnet is gratefully acknowledged for TEM measurements.

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REFERENCES

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Films by Continuous Wave Laser Proce - American Chemical Society

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