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Spatial variation of structural order in sputtered WO3 films
Electrochimica Acta 46 (2001) 1937– 1944 www.elsevier.nl/locate/electacta
Spatial variation of structural order in sputtered WO3 films E. Cazzanelli a,*, L. Papalino a, A. Pennisi b, F. Simone b a
Dipartimento di Fisica, Uni6ersita` della Calabria and Unita` INFM, I-87036 Arca6acata di Rende (CS), Italy b Dipartimento di Fisica, Uni6ersita` di Catania and Unita` INFM, Corso Italia 57, I-95129 Catania, Italy Received 23 August 2000; received in revised form 13 November 2000
Abstract In this work the degree of structural order has been analyzed in different regions of WO3 films deposited by r.f. magnetron sputtering on a K-glass with a layer of SnO2:F as electronic conductor. The variability of film crystallization in many regions of the samples has been studied, by using as heating agent the same laser beam of 632.8 nm of the micro-Raman apparatus. Comparison of the spectral shape with those of annealed films and microcrystalline powders allows to estimate the degree of crystal order reached in the various spots, and to evaluate the influence of initial structural configuration. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: WO3 films; Sputtering; Laser-induced crystallization; Crystal order; Raman spectra
1. Introduction The experimental evidence of many studies performed on WO3 by using various techniques [1,2] indicates a wide variation of the structural, optical and electrochemical characteristics of the films by changing the preparation methods and, within any method, by changing the deposition parameters. The resulting films can be basically grouped in two main classes: crystallike films and so called ‘‘amorphous’’ films. The real amorphous nature of the latter can be questioned because the absence of sharp XRD peaks is also consistent with a nanocrystalline configuration, as proposed by Nanba and Yasui [3]. In sputtered films, for instance, electron diffraction measurements [4] confirm such a hypothesis, but also suggest a space variation of the structural order within the same film sample. However, an important effect, observed in many cases [1], must be considered when analyzing the various experimental data on the structural order in WO3 * Corresponding author. Tel.: +39-984-493135; fax: + 39984-493187. E-mail address: [email protected] (E. Cazzanelli).
films, i.e. the perturbative action of the probes (electron, X-ray or laser beams), due to the appreciable amount of energy concentrated on the sample, such that a crystallization process can be induced. In fact, regions having a different degree of crystal order require, in principle, different amounts of energy to reach the most stable crystalline phase; furthermore, the same amount of locally transferred energy makes it possible to reach different metastable configurations. In any case, different final results of a local crystallization process, under the heating effect of some probe, are related to the initial state. To have a better comprehension of such phenomena, we have performed an extensive Raman scattering investigation for a set of film samples all deposited by r.f. magnetron sputtering, very similar to those films previously investigated for their interaction with liquid crystal layers [4 – 6]. By using micro-Raman equipment it was possible to explore small spots on the samples (order of a few micrometers), to change the employed laser power and to collect rapidly enough the spectra during the light exposure. This way we can strictly associate the agent of crystallization (the focused laser beam), with a local measurement of the transformation,
0013-4686/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 1 ) 0 0 3 6 3 - 2
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performed at short intervals (seconds). In summary, we try to measure, via Raman spectroscopy, a local crystallization response to a controlled thermal stimulus. The Raman spectral patterns allow an easy discrimination between various structural phases such as ‘‘amorphous’’, nanocrystalline, microcrystalline and crystalline films [1,7,8]. In fact, most of the crystal phases having a distorted ReO3-type structure show in the higher frequency part of the spectrum two distinct bands at 715 and 810 cm − 1 (RT values), which are quite insensitive to several phase transitions [9,10], as well as to the static disorder induced by mechanical treatments [11]. On the contrary, most of the as-deposited films, obtained by different techniques, show a very broad band having a maximum at about 770–780 cm − 1 and a narrower band at about 950 cm − 1, due to the terminal double bond WO [1,7,8]. It also appears in the bulk phases, but only after prolonged mechanical treatments inducing a strong decrease of the crystal domain size, and its relative intensity is a lot smaller than in the films [11]. Thus a topological difference between crystalline and ‘‘amorphous’’ films seems to be relevant: in the first case a continuous connection of single bonds WO is established, whereas in the latter one the strong contribution of the terminal bonds suggests a non-compact structure, always containing voids at nanometric level [3]. The transformation between these two states can be induced by thermal treatments. Some intermediate steps are known to occur during this process, which includes a dehydration process, followed by a collapse of the open nanocrystalline structures into the more compact bulk-crystal configuration [12]. The local heating of the films analyzed in the present work must be compared with such evolution, obtained in the equilibrium condition. Furthermore, to associate the local crystallization process with the Raman spectral changes, we have tried to fit the spectra of the transformed spots by using the frequency and linewidth data reported for annealed films.
2. Experimental
2.1. Film deposition and handling All the samples analyzed here were prepared using r.f. magnetron sputtering at the Universita` di Catania laboratories and were obtained by fixing the following sputtering parameters: deposition pressure 2–4x10 − 2 Torr, O2/Ar ratio 0.05, r.f. power 250 W. The substrates were K-glasses coated with a layer of SnO2 –F. The time of deposition, correlated with the thickness, ranges from 10 to 40 min.
The lateral dimensions of the samples were 2 × 1 cm2. These films were successively stored in air and measured at different time intervals.
2.2. TEM measurements, XRD, etc. Some TEM measurements were made on similar film samples, deposited with the same sputtering parameters, by collecting lateral sections of the films as well as top views. The X-ray measurement made on this kind of film showed no XRD peaks for the as-deposited films, while typical patterns of a crystalline phase appeared after annealing of such films at 400°C.
2.3. Raman equipment A Raman microprobe ISA-Jobin Yvon Labram equipped with a CCD detector and a He– Ne laser (632.8 nm u), was used. The power of the laser out of the objective, a 100 × Mplan Olympus with NA of 0.90, was about 5 mW, and the focused laser spot had about 10 mm in diameter. By assuming such values of spot diameter and laser power, we have for the unfiltered laser beam an irradiance of about 6 – 7 kW/ cm2 on the spot. By using the 0.3 OD filter, this value decreased by about 53%. Each sample was measured using the method described below: 1. Image capture of the sample using the CCD camera connected with the microscope. 2. 100 s exposure of the sample using a 0.3 OD filter that reduces the laser power by 53%, and continuous recording of the spectral evolution with aperture hole of 400 mm. 3. Image capture after first exposure. 4. 240 s exposure of the sample without any filter and continuous recording of the spectra evolution with aperture hole of 400 mm. 5. Image capture after second exposure. 6. 120 s exposure of the sample without any filter and continuous recording of the spectra evolution with aperture hole of 400 mm. 7. Image capture after third exposure. In this way the transformations of the spectra, under laser exposure, were continuously monitored, following any eventual evolution toward a microcrystalline structure, and the use of the filter made it possible to reveal threshold effects for such transformations. The distance between the spots we have measured was at least 30 mm.
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3. Results and discussion
3.1. WO3 films homogeneity Earlier studies indicate the existence of density fluctuations and coexistence of regions having different structural order [4]. In the films grown by r.f. magnetron sputtering, previously studied with regard to their interactions with liquid crystal layers [4–6], electron diffraction data [4] and TEM images give evidence of different states of aggregation coexisting in the same film. For instance, in Fig. 1 a lateral section of the film is shown, and it is evident that an amorphous or nanocrystalline structure is present above the columnar structures, already observed in other samples [13,14].
3.2. Raman spectra and crystal order in WO3 A remarkable variability of results has been obtained from the Raman measurements also, between the two extreme cases of ‘‘bulk-crystal spectrum’’ and ‘‘amorphous film spectrum’’. The ‘‘amorphous-like’’ spectra are obtained when the sample is not perturbed, i.e. by using a 0.3 OD filter, or
Fig. 1. TEM Image of a lateral section of WO3 sputtered film, showing columnar features and a non-structured layer.
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at the very beginning of the exposure to the laser beam (see in Fig. 2, spectrum A). Only a minor modification of the spectra is observed in many points of the films not undergoing the strong crystallization effect: the 950 cm − 1 band, assigned to the terminal bonds, shifts to higher frequencies (by about 10 cm − 1) and broadens. This behavior is consistent with the evolution previously observed [4] on similar sputtered films annealed at 305°C, which is not high enough to induce crystallization in our samples, but enough to initiate the preliminary dehydration process. The loss of water without crystallization seems to destroy the structural connectivity of the film and increases the static disorder. This is mainly responsible for the observed broadening of the WO band, beside the expected temperature effect. The shift at higher frequencies can be explained by a lower coupling to the other basic units, due to the destruction of the hydrogen bond network. For many other spots, a more remarkable transformation is induced using the unfiltered laser, but the ‘‘final results’’, defined as the states of the irradiated spots at the end of the experimental procedure previously described, show an appreciable variability as can be seen in Fig. 2. Each reported spectrum of Fig. 2 is representative of a class among the 400 spectra we have recorded. For a particular spot the time evolution of the spectral shape has been followed, and the initial, intermediate and final states are shown, corresponding to the bottom, middle and top spectra of Fig. 3, respectively. The final state shown in Fig. 3 is representative of the majority of the analyzed spots. It is characterized by a good amount of connected crystalline material, but the appreciable intensity of the 956 cm − 1 bands indicates a very high surface-to-volume ratio for the crystalline domains. In fact, a first reasonable assumption to explain an incomplete crystallization is that a lower temperature is reached, with respect to the ‘‘critical’’ one for the total crystallization. On the contrary, an almost total crystallization can be deduced from the spectrum collected in another spot of the same sample, after continuous laser irradiation (Fig. 4a), as can be seen by comparison with the spectrum of an annealed sample (at 400°C), reported in Fig. 4b. Furthermore, the frequency and the linewidth values of the crystal-like peaks are roughly consistent with the temperature dependence measured in microcrystalline powders investigated in a previous work [10]; the temperature value deduced from the spectrum of the irradiated spot, by using the data of the powders, is about 400°C or slightly higher, which is sufficient for the total crystallization, as demonstrated by the previous annealing experiments. Similar films used in previous works had crystallization temperatures between 300 and 400°C, as demonstrated by Raman measurement and
Fig. 2. Raman spectra collected in various spots of a WO3 film of estimated thickness about 1 mm (the minimum distance between the spots is 30 mm); spectrum A is obtained by filtered laser or after short exposure; all the other spectra represent the result of the last exposure after the long measurement procedure described in the text (three exposures: 100 s with OD 0.3 filter, and 240 and 120 s without filter).
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Fig. 3. Spectra collected at the same spot after different exposure times, cumulatively: 100 s with filter on (bottom), 240 s without filter (middle), 120 s without filter (top).
XRD collected after annealing treatments: at the lower temperature the films were non-crystallized [4], while good evidence of an ReO3-type crystal phases was obtained after annealing at 400°C (see Fig. 4b). To separate the vibrational contribution of the ‘‘connected’’ crystal structure from that of the nanocrystalline phase, we have tried to fit all the spectra of Fig. 3 by using the peaks characteristic of the crystal phase, represented by lorentzian functions peaking at 715 and 810 cm − 1, with a fixed intensity ratio, as deduced by the spectra of well-crystallized films (annealed at 400°C and checked by XRD). In addition the typical spectral features of the films are inserted: the 950 cm − 1 peaks and a broad contribution centered at about 680 cm − 1, characteristic of the nanocrystalline phase. We found good results from this approach in partially laser crystallized regions, while the spectra generated from not crystallized spots give the best fit by using two different functions, gausssians instead of lorentzians, having the barycentre down-shifted with respect to the crystal peaks and strongly broadened. These modifications in the best fit parameters confirm a strong inhomogeneous disorder and confinement effects in the as-deposited films, in agreement with previous Raman studies [15], but the appreciable down-shift and broadening of the fitting functions suggest a real change of the symmetry and topology of the structure. The results of this fitting procedure are shown in Fig. 5a–c.
It shows not only the decrease of the nanocrystalline phase contribution, but also the transformations of the terminal bonds band, when passing from the ‘‘amorphous film’’ spectrum (Fig. 5c) to the state of maximum crystallization (Fig. 5a). For the bands characteristic of the bulk-crystal state, it is interesting to note the excess broadening. We can assume that a major contribution to the linewidth is due to the very small size of the connected crystalline regions and to the still great static disorder, overcoming the homogeneous temperature dependent term, comparable to that found in microcrystals [10].
3.3. Statistics of Raman measurements After the first evidence of sudden crystallization during the Raman measurements, we adopt for any spot the standard procedure of analysis described above, in Section 2. Spectra typical of the amorphous or nanocrystalline films are always generated when the sample is not perturbed (see Fig. 2, spectrum A). Only in the cases of freshly deposited samples, and in the central zone of the films, does full power laser application also induce this kind of spectra. By using the unfiltered laser, in general, many different spectra are obtained: a statistic approach has been tried, exploring the films from the border toward the
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center. The first crystallized spots were detected after one week, only on the sample edge, while after one month it was possible to have crystallization almost everywhere. In the case of spectra showing an advanced degree of crystallization (see Fig. 2, spectra E and F) it is interesting to describe the kinetics: in the early stages of the films, most of the typical crystal-like spectral features appeared after an exposure time between 30
and 180 s, and the crystallization process appears to be stabilized after another comparable irradiation time. In the samples stored for longer times before the test the crystal-like spectral features appeared suddenly, after an average exposure time of 6 – 10 s (each spectrum required 3 s, which is our time uncertainty) and the spectral evolution did not show further relevant changes, even after hundreds of seconds under laser
Fig. 4. (a) Spectrum from a spot of the film, showing the maximum degree of crystallization, after laser exposure, fitted by two lorentzian functions, representing the two typical crystal peaks, and a Voigt function with gaussian character, to represent the band of the nanocrystalline structure; the fit results are: peak frequency and bandwidth at 676 and 142 cm − 1, 709 and 33 cm − 1, and 805 and 32 cm − 1, respectively. (b) Spectrum previously obtained from another sputtered film after annealing at 400°C in air, by using an oven; it has been fitted by two lorentzian functions only and the fit results are: frequency peak and bandwidth at 716 and 31 cm − 1, and 810 and 25 cm − 1, respectively.
E. Cazzanelli et al. / Electrochimica Acta 46 (2001) 1937–1944
irradiation. All these findings suggest that the aging process in air promotes laser-induced crystallization. By moving the microscope objective up and down, evidence is obtained of a maximum crystallization not on the surface, but some tenths of a micron below, though not extending down to the support glass: at such a low level the spectrum also shows an appreciable contribution of the glass.
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It seems reasonable that columnar structures, growing in many cases below the more disordered surface material, constitutes a better seed for crystallization: when the energy transferred by the probe is close to the critical value to start the transformation (using half laser power we do not observe any crystallized spot), such structures crystallize easily, while the more disordered material deposited above generates the still
Fig. 5. Tentative fits of the WO3 Raman spectra representing different steps of crystallization in the same spot. Fit (a) corresponds to the top spectrum of Fig. 3, fit results are: peaks and bandwidth at 679 and 134 cm − 1, 715 and 85 cm − 1, 809 and 59 cm − 1, and 956 and 80 cm − 1, respectively. Fit (b) corresponds to the middle spectrum, fit results are: peaks and bandwidth at 686 and 151 cm − 1, 715 and 94 cm − 1, 803 and 96 cm − 1, and 956 and 74 cm − 1, respectively. Fit (c) corresponds to the bottom spectrum, fit results are: peaks and bandwidth at 678 and 309 cm − 1, 778 and 173 cm − 1, and 950 and 57 cm − 1, respectively. Note that in the last case it was not possible to have a simple fit by using the lorentzian functions associated to the bulk-crystal Raman peaks.
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Fig. 5. (Continued).
‘‘amorphous-like’’ spectra observed by analyzing the Z dependence in our study.
4. Conclusions In summary, what can be remarked from this initial investigation is the sensitivity of these films to laser beam exposure, modulated by an aging process in room atmosphere. The space variability of the crystallization could be connected, in principle, with heat conductivity fluctuations, allowing different amounts of energy to remain localized in the irradiated spot. On the other hand, the local stress fluctuations, enhanced by the preliminary dehydration process, can be invoked as a modulating agent of the energy barriers, separating the metastable disordered nanocrystalline configurations from the stable bulk-crystal structure. This great horizontal variability response of the films can lead, in principle, to a structural mapping on a mesoscopic scale, but to realize this goal further studies are necessary. For instance, a more detailed investigation of the power thresholds for crystallization has to be performed and a more theoretically founded analysis of the spectral shapes must be developed. The possibility of inducing controlled local laser transformations is also interesting for eventual applications, as well as for spatially resolved structural studies.
References
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K. Bange, Sol Energy Mater. Sol. Cells 58 (1999) 1. C.G. Granqvist, Electrochim. Acta 44 (1999) 3005. T. Nanba, I. Yasui, J. Solid State Chem. 83 (1989) 304. G. Strangi, N. Scaramuzza, C. Versace, E. Cazzanelli, F. Simone, A. Pennisi, R. Bartolino, Ionics 3 – 4 (1999) 275. G. Strangi, D.E. Lucchetta, E. Cazzanelli, N. Scaramuzza, C. Versace, R. Bartolino, Appl. Phys. Lett. 4 (1999) 534. E. Cazzanelli, N. Scaramuzza, G. Strangi, C. Versace, A. Pennisi, F. Simone, Electrochim. Acta. 44 (1999) 3101. J.V. Gabrusenoks, P.D. Cikmach, A.R. Lusis, J.J. Kleperis, G.M. Ramans, Solid State Ionics. 14 (1984) 25. M.F. Daniel, B. Desbat, J.C. Lassegues, R. Garie, J. Solid State Chem. 73 (1988) 127. E. Cazzanelli, G. Mariotto, C. Vinegoni, A. Kuzmin, J. Purans, Ionics 5 – 6 (1999) 335. E. Cazzanelli, C. Vinegoni, G. Mariotto, A. Kuzmin, J. Purans, Solid State Ionics 123 (1999) 67. A. Kuzmin, J. Purans, E. Cazzanelli, C. Vinegoni, G. Mariotto, J. Appl. Phys. Lett. 10 (1998) 5515. A. Agrawal, H. Habibi, Thin Solid Films 169 (1989) 257. E. Masetti, M.L. Grilli, G. Dautzenberg, G. Macrelli, M. Adamik, Sol. Energy Mater. Sol. Cells 3 – 4 (1999) 259. P. Ta¨gtstro¨m, U. Jansson, Thin Solid Films 352 (1999) 107. Y. Shingesato, A Murayama, T. Kamimori, K. Matsuhiro, Appl. Surf. Sci. 33/34 (1987) 804.
Spatial variation of structural order in sputtered WO3 films E. Cazzanelli a,*, L. Papalino a, A. Pennisi b, F. Simone b a
Dipartimento di Fisica, Uni6ersita` della Calabria and Unita` INFM, I-87036 Arca6acata di Rende (CS), Italy b Dipartimento di Fisica, Uni6ersita` di Catania and Unita` INFM, Corso Italia 57, I-95129 Catania, Italy Received 23 August 2000; received in revised form 13 November 2000
Abstract In this work the degree of structural order has been analyzed in different regions of WO3 films deposited by r.f. magnetron sputtering on a K-glass with a layer of SnO2:F as electronic conductor. The variability of film crystallization in many regions of the samples has been studied, by using as heating agent the same laser beam of 632.8 nm of the micro-Raman apparatus. Comparison of the spectral shape with those of annealed films and microcrystalline powders allows to estimate the degree of crystal order reached in the various spots, and to evaluate the influence of initial structural configuration. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: WO3 films; Sputtering; Laser-induced crystallization; Crystal order; Raman spectra
1. Introduction The experimental evidence of many studies performed on WO3 by using various techniques [1,2] indicates a wide variation of the structural, optical and electrochemical characteristics of the films by changing the preparation methods and, within any method, by changing the deposition parameters. The resulting films can be basically grouped in two main classes: crystallike films and so called ‘‘amorphous’’ films. The real amorphous nature of the latter can be questioned because the absence of sharp XRD peaks is also consistent with a nanocrystalline configuration, as proposed by Nanba and Yasui [3]. In sputtered films, for instance, electron diffraction measurements [4] confirm such a hypothesis, but also suggest a space variation of the structural order within the same film sample. However, an important effect, observed in many cases [1], must be considered when analyzing the various experimental data on the structural order in WO3 * Corresponding author. Tel.: +39-984-493135; fax: + 39984-493187. E-mail address: [email protected] (E. Cazzanelli).
films, i.e. the perturbative action of the probes (electron, X-ray or laser beams), due to the appreciable amount of energy concentrated on the sample, such that a crystallization process can be induced. In fact, regions having a different degree of crystal order require, in principle, different amounts of energy to reach the most stable crystalline phase; furthermore, the same amount of locally transferred energy makes it possible to reach different metastable configurations. In any case, different final results of a local crystallization process, under the heating effect of some probe, are related to the initial state. To have a better comprehension of such phenomena, we have performed an extensive Raman scattering investigation for a set of film samples all deposited by r.f. magnetron sputtering, very similar to those films previously investigated for their interaction with liquid crystal layers [4 – 6]. By using micro-Raman equipment it was possible to explore small spots on the samples (order of a few micrometers), to change the employed laser power and to collect rapidly enough the spectra during the light exposure. This way we can strictly associate the agent of crystallization (the focused laser beam), with a local measurement of the transformation,
0013-4686/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 1 ) 0 0 3 6 3 - 2
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performed at short intervals (seconds). In summary, we try to measure, via Raman spectroscopy, a local crystallization response to a controlled thermal stimulus. The Raman spectral patterns allow an easy discrimination between various structural phases such as ‘‘amorphous’’, nanocrystalline, microcrystalline and crystalline films [1,7,8]. In fact, most of the crystal phases having a distorted ReO3-type structure show in the higher frequency part of the spectrum two distinct bands at 715 and 810 cm − 1 (RT values), which are quite insensitive to several phase transitions [9,10], as well as to the static disorder induced by mechanical treatments [11]. On the contrary, most of the as-deposited films, obtained by different techniques, show a very broad band having a maximum at about 770–780 cm − 1 and a narrower band at about 950 cm − 1, due to the terminal double bond WO [1,7,8]. It also appears in the bulk phases, but only after prolonged mechanical treatments inducing a strong decrease of the crystal domain size, and its relative intensity is a lot smaller than in the films [11]. Thus a topological difference between crystalline and ‘‘amorphous’’ films seems to be relevant: in the first case a continuous connection of single bonds WO is established, whereas in the latter one the strong contribution of the terminal bonds suggests a non-compact structure, always containing voids at nanometric level [3]. The transformation between these two states can be induced by thermal treatments. Some intermediate steps are known to occur during this process, which includes a dehydration process, followed by a collapse of the open nanocrystalline structures into the more compact bulk-crystal configuration [12]. The local heating of the films analyzed in the present work must be compared with such evolution, obtained in the equilibrium condition. Furthermore, to associate the local crystallization process with the Raman spectral changes, we have tried to fit the spectra of the transformed spots by using the frequency and linewidth data reported for annealed films.
2. Experimental
2.1. Film deposition and handling All the samples analyzed here were prepared using r.f. magnetron sputtering at the Universita` di Catania laboratories and were obtained by fixing the following sputtering parameters: deposition pressure 2–4x10 − 2 Torr, O2/Ar ratio 0.05, r.f. power 250 W. The substrates were K-glasses coated with a layer of SnO2 –F. The time of deposition, correlated with the thickness, ranges from 10 to 40 min.
The lateral dimensions of the samples were 2 × 1 cm2. These films were successively stored in air and measured at different time intervals.
2.2. TEM measurements, XRD, etc. Some TEM measurements were made on similar film samples, deposited with the same sputtering parameters, by collecting lateral sections of the films as well as top views. The X-ray measurement made on this kind of film showed no XRD peaks for the as-deposited films, while typical patterns of a crystalline phase appeared after annealing of such films at 400°C.
2.3. Raman equipment A Raman microprobe ISA-Jobin Yvon Labram equipped with a CCD detector and a He– Ne laser (632.8 nm u), was used. The power of the laser out of the objective, a 100 × Mplan Olympus with NA of 0.90, was about 5 mW, and the focused laser spot had about 10 mm in diameter. By assuming such values of spot diameter and laser power, we have for the unfiltered laser beam an irradiance of about 6 – 7 kW/ cm2 on the spot. By using the 0.3 OD filter, this value decreased by about 53%. Each sample was measured using the method described below: 1. Image capture of the sample using the CCD camera connected with the microscope. 2. 100 s exposure of the sample using a 0.3 OD filter that reduces the laser power by 53%, and continuous recording of the spectral evolution with aperture hole of 400 mm. 3. Image capture after first exposure. 4. 240 s exposure of the sample without any filter and continuous recording of the spectra evolution with aperture hole of 400 mm. 5. Image capture after second exposure. 6. 120 s exposure of the sample without any filter and continuous recording of the spectra evolution with aperture hole of 400 mm. 7. Image capture after third exposure. In this way the transformations of the spectra, under laser exposure, were continuously monitored, following any eventual evolution toward a microcrystalline structure, and the use of the filter made it possible to reveal threshold effects for such transformations. The distance between the spots we have measured was at least 30 mm.
E. Cazzanelli et al. / Electrochimica Acta 46 (2001) 1937–1944
3. Results and discussion
3.1. WO3 films homogeneity Earlier studies indicate the existence of density fluctuations and coexistence of regions having different structural order [4]. In the films grown by r.f. magnetron sputtering, previously studied with regard to their interactions with liquid crystal layers [4–6], electron diffraction data [4] and TEM images give evidence of different states of aggregation coexisting in the same film. For instance, in Fig. 1 a lateral section of the film is shown, and it is evident that an amorphous or nanocrystalline structure is present above the columnar structures, already observed in other samples [13,14].
3.2. Raman spectra and crystal order in WO3 A remarkable variability of results has been obtained from the Raman measurements also, between the two extreme cases of ‘‘bulk-crystal spectrum’’ and ‘‘amorphous film spectrum’’. The ‘‘amorphous-like’’ spectra are obtained when the sample is not perturbed, i.e. by using a 0.3 OD filter, or
Fig. 1. TEM Image of a lateral section of WO3 sputtered film, showing columnar features and a non-structured layer.
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at the very beginning of the exposure to the laser beam (see in Fig. 2, spectrum A). Only a minor modification of the spectra is observed in many points of the films not undergoing the strong crystallization effect: the 950 cm − 1 band, assigned to the terminal bonds, shifts to higher frequencies (by about 10 cm − 1) and broadens. This behavior is consistent with the evolution previously observed [4] on similar sputtered films annealed at 305°C, which is not high enough to induce crystallization in our samples, but enough to initiate the preliminary dehydration process. The loss of water without crystallization seems to destroy the structural connectivity of the film and increases the static disorder. This is mainly responsible for the observed broadening of the WO band, beside the expected temperature effect. The shift at higher frequencies can be explained by a lower coupling to the other basic units, due to the destruction of the hydrogen bond network. For many other spots, a more remarkable transformation is induced using the unfiltered laser, but the ‘‘final results’’, defined as the states of the irradiated spots at the end of the experimental procedure previously described, show an appreciable variability as can be seen in Fig. 2. Each reported spectrum of Fig. 2 is representative of a class among the 400 spectra we have recorded. For a particular spot the time evolution of the spectral shape has been followed, and the initial, intermediate and final states are shown, corresponding to the bottom, middle and top spectra of Fig. 3, respectively. The final state shown in Fig. 3 is representative of the majority of the analyzed spots. It is characterized by a good amount of connected crystalline material, but the appreciable intensity of the 956 cm − 1 bands indicates a very high surface-to-volume ratio for the crystalline domains. In fact, a first reasonable assumption to explain an incomplete crystallization is that a lower temperature is reached, with respect to the ‘‘critical’’ one for the total crystallization. On the contrary, an almost total crystallization can be deduced from the spectrum collected in another spot of the same sample, after continuous laser irradiation (Fig. 4a), as can be seen by comparison with the spectrum of an annealed sample (at 400°C), reported in Fig. 4b. Furthermore, the frequency and the linewidth values of the crystal-like peaks are roughly consistent with the temperature dependence measured in microcrystalline powders investigated in a previous work [10]; the temperature value deduced from the spectrum of the irradiated spot, by using the data of the powders, is about 400°C or slightly higher, which is sufficient for the total crystallization, as demonstrated by the previous annealing experiments. Similar films used in previous works had crystallization temperatures between 300 and 400°C, as demonstrated by Raman measurement and
Fig. 2. Raman spectra collected in various spots of a WO3 film of estimated thickness about 1 mm (the minimum distance between the spots is 30 mm); spectrum A is obtained by filtered laser or after short exposure; all the other spectra represent the result of the last exposure after the long measurement procedure described in the text (three exposures: 100 s with OD 0.3 filter, and 240 and 120 s without filter).
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Fig. 3. Spectra collected at the same spot after different exposure times, cumulatively: 100 s with filter on (bottom), 240 s without filter (middle), 120 s without filter (top).
XRD collected after annealing treatments: at the lower temperature the films were non-crystallized [4], while good evidence of an ReO3-type crystal phases was obtained after annealing at 400°C (see Fig. 4b). To separate the vibrational contribution of the ‘‘connected’’ crystal structure from that of the nanocrystalline phase, we have tried to fit all the spectra of Fig. 3 by using the peaks characteristic of the crystal phase, represented by lorentzian functions peaking at 715 and 810 cm − 1, with a fixed intensity ratio, as deduced by the spectra of well-crystallized films (annealed at 400°C and checked by XRD). In addition the typical spectral features of the films are inserted: the 950 cm − 1 peaks and a broad contribution centered at about 680 cm − 1, characteristic of the nanocrystalline phase. We found good results from this approach in partially laser crystallized regions, while the spectra generated from not crystallized spots give the best fit by using two different functions, gausssians instead of lorentzians, having the barycentre down-shifted with respect to the crystal peaks and strongly broadened. These modifications in the best fit parameters confirm a strong inhomogeneous disorder and confinement effects in the as-deposited films, in agreement with previous Raman studies [15], but the appreciable down-shift and broadening of the fitting functions suggest a real change of the symmetry and topology of the structure. The results of this fitting procedure are shown in Fig. 5a–c.
It shows not only the decrease of the nanocrystalline phase contribution, but also the transformations of the terminal bonds band, when passing from the ‘‘amorphous film’’ spectrum (Fig. 5c) to the state of maximum crystallization (Fig. 5a). For the bands characteristic of the bulk-crystal state, it is interesting to note the excess broadening. We can assume that a major contribution to the linewidth is due to the very small size of the connected crystalline regions and to the still great static disorder, overcoming the homogeneous temperature dependent term, comparable to that found in microcrystals [10].
3.3. Statistics of Raman measurements After the first evidence of sudden crystallization during the Raman measurements, we adopt for any spot the standard procedure of analysis described above, in Section 2. Spectra typical of the amorphous or nanocrystalline films are always generated when the sample is not perturbed (see Fig. 2, spectrum A). Only in the cases of freshly deposited samples, and in the central zone of the films, does full power laser application also induce this kind of spectra. By using the unfiltered laser, in general, many different spectra are obtained: a statistic approach has been tried, exploring the films from the border toward the
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center. The first crystallized spots were detected after one week, only on the sample edge, while after one month it was possible to have crystallization almost everywhere. In the case of spectra showing an advanced degree of crystallization (see Fig. 2, spectra E and F) it is interesting to describe the kinetics: in the early stages of the films, most of the typical crystal-like spectral features appeared after an exposure time between 30
and 180 s, and the crystallization process appears to be stabilized after another comparable irradiation time. In the samples stored for longer times before the test the crystal-like spectral features appeared suddenly, after an average exposure time of 6 – 10 s (each spectrum required 3 s, which is our time uncertainty) and the spectral evolution did not show further relevant changes, even after hundreds of seconds under laser
Fig. 4. (a) Spectrum from a spot of the film, showing the maximum degree of crystallization, after laser exposure, fitted by two lorentzian functions, representing the two typical crystal peaks, and a Voigt function with gaussian character, to represent the band of the nanocrystalline structure; the fit results are: peak frequency and bandwidth at 676 and 142 cm − 1, 709 and 33 cm − 1, and 805 and 32 cm − 1, respectively. (b) Spectrum previously obtained from another sputtered film after annealing at 400°C in air, by using an oven; it has been fitted by two lorentzian functions only and the fit results are: frequency peak and bandwidth at 716 and 31 cm − 1, and 810 and 25 cm − 1, respectively.
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irradiation. All these findings suggest that the aging process in air promotes laser-induced crystallization. By moving the microscope objective up and down, evidence is obtained of a maximum crystallization not on the surface, but some tenths of a micron below, though not extending down to the support glass: at such a low level the spectrum also shows an appreciable contribution of the glass.
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It seems reasonable that columnar structures, growing in many cases below the more disordered surface material, constitutes a better seed for crystallization: when the energy transferred by the probe is close to the critical value to start the transformation (using half laser power we do not observe any crystallized spot), such structures crystallize easily, while the more disordered material deposited above generates the still
Fig. 5. Tentative fits of the WO3 Raman spectra representing different steps of crystallization in the same spot. Fit (a) corresponds to the top spectrum of Fig. 3, fit results are: peaks and bandwidth at 679 and 134 cm − 1, 715 and 85 cm − 1, 809 and 59 cm − 1, and 956 and 80 cm − 1, respectively. Fit (b) corresponds to the middle spectrum, fit results are: peaks and bandwidth at 686 and 151 cm − 1, 715 and 94 cm − 1, 803 and 96 cm − 1, and 956 and 74 cm − 1, respectively. Fit (c) corresponds to the bottom spectrum, fit results are: peaks and bandwidth at 678 and 309 cm − 1, 778 and 173 cm − 1, and 950 and 57 cm − 1, respectively. Note that in the last case it was not possible to have a simple fit by using the lorentzian functions associated to the bulk-crystal Raman peaks.
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Fig. 5. (Continued).
‘‘amorphous-like’’ spectra observed by analyzing the Z dependence in our study.
4. Conclusions In summary, what can be remarked from this initial investigation is the sensitivity of these films to laser beam exposure, modulated by an aging process in room atmosphere. The space variability of the crystallization could be connected, in principle, with heat conductivity fluctuations, allowing different amounts of energy to remain localized in the irradiated spot. On the other hand, the local stress fluctuations, enhanced by the preliminary dehydration process, can be invoked as a modulating agent of the energy barriers, separating the metastable disordered nanocrystalline configurations from the stable bulk-crystal structure. This great horizontal variability response of the films can lead, in principle, to a structural mapping on a mesoscopic scale, but to realize this goal further studies are necessary. For instance, a more detailed investigation of the power thresholds for crystallization has to be performed and a more theoretically founded analysis of the spectral shapes must be developed. The possibility of inducing controlled local laser transformations is also interesting for eventual applications, as well as for spatially resolved structural studies.
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