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Synthesis of nanocrystalline ceria thin films by low-temperature thermal decomposition of Ce-propionate
Thin Solid Films 520 (2012) 1949–1953
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Synthesis of nanocrystalline ceria thin films by low-temperature thermal decomposition of Ce-propionate P. Roura a,⁎, J. Farjas a, S. Ricart b, M. Aklalouch b, R. Guzman b, J. Arbiol b, c, T. Puig b, A. Calleja b, O. Peña-Rodríguez b, M. Garriga b, X. Obradors b a b c
GRMT, Dept. of Physics, University of Girona, Campus Montilivi, Edif. PII, E17071 Girona, Catalonia, Spain Institut de Ciència de Materials de Barcelona (CSIC), Campus de la UAB, 08193 Bellaterra, Catalonia, Spain Institució Catalana de Recerca i Estudis Avançats (ICREA), Barcelona, Catalonia, Spain
a r t i c l e
i n f o
Article history: Received 5 May 2011 Received in revised form 13 September 2011 Accepted 23 September 2011 Available online 6 October 2011 Keywords: Ceria Cerium propionate Thermal decomposition Pyrolysis Optical absorption Thermogravimetry Crystal microstructure
a b s t r a c t Thin films of Ce-propionate (thickness below 20 nm) have been deposited by spin coating and pyrolysed into ceria at temperatures below 200 °C. After 1 h of thermal treatment, no signature of the vibrational modes of Ce-propionate is detected by infrared spectroscopy, indicating that decomposition has been completed. The resulting ceria films are nanocrystalline as revealed by X-ray diffraction (average grain size of 2–2.5 nm) and confirmed by microscopy. They are transparent in the visible region and show the characteristic band gap absorption below 400 nm. A direct band gap energy of 3.50± 0.05 eV has been deduced irrespective of the pyrolysis temperature (160, 180 and 200 °C). © 2011 Elsevier B.V. All rights reserved.
1. Introduction Ceria thin films deserve great attention due to the outstanding chemical and physical properties of CeO2. Owing to their high ionic conductivity at moderate temperatures and catalytic activity, they can be incorporated in solid-oxide fuel cells [1] and oxygen gas sensors [2]. They also play a role as barriers to diffusion in buffer layers such as those needed in high-temperature superconducting tapes [3]. Their high dielectric constant has led to propose their use in microelectronics where they could be a substitute for SiO2 in metal oxide semiconductor devices and in metal-insulator–metal devices [4,5]. Finally, their high transparency in the visible and infrared regions and their high absorbance in the UV region make them suitable for optical coatings [6,7]. Chemical solution deposition techniques constitute a versatile low-cost route for the synthesis of nanocrystalline oxide thin films. However, the need for a pyrolysis step at T ≥ 400 °C to decompose the molecular precursor into the desired oxide [8] discards their use in substrates such as polymers that cannot stand moderate temperatures. In the particular case of ceria, the decomposition temperature of its precursors could be substantially lowered with respect to the value reported in the literature [9,10]. This is so because, as we have
⁎ Corresponding author. E-mail address: [email protected] (P. Roura). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.09.058
shown in a detailed analysis of the decomposition of Ce-propionate [11], its decomposition in air is triggered by Ce 3+ oxidation to Ce 4+ instead of dissociation of the organic ligands. Consequently, for ceria thin films, the decomposition temperature could reach a minimum value. This finding can be of great interest is view of the present search for precursors that would allow the synthesis of other complex oxide films at low temperature [12]. In this paper, we will show that nanocrystalline ceria thin films can be obtained by pyrolysing Ce-propionate in air, below 200 °C. The films structure has been analysed by infrared (IR) spectroscopy, x-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM) and the optical properties of the films have been measured by optical transmittance in the visible (VIS) and ultraviolet (UV) regions and by spectroscopic ellipsometry. 2. Experimental details Ce-propionate powders were used directly for the thermal analysis experiments or were dissolved in propionic acid for film deposition. A 0.25 M solution was spin-coated on 0.5 mm-thick, 10× 10 mm2 square quartz plates at 6000 rpm for 2 min with 1 s acceleration time. A thickness of CeO2 films around 20 nm is expected according to spin-coated samples on YSZ (100) single crystals. Alternatively, to obtain films of hundreds of nm a more diluted (0.04 M) solution was used to freely
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P. Roura et al. / Thin Solid Films 520 (2012) 1949–1953
spread a microdrop on the surface of a glass disc (10 mm in diameter). The films were heated either in the TG furnace or on a hot plate with static air. Thermogravimetric (TG) analyses were done with a SDTA851 eLF thermobalance from Mettler Toledo. The Ce-propionate powders were placed inside uncovered alumina pans to facilitate gas exchange and the films were measured on their substrate. High purity synthetic air and nitrogen were used at a flow rate of 40 mL/min. IR spectra of the films were measured with an attenuated total reflexion (ATR) accessory mounted on a high-resolution spectrometer FT-IR from Perkin-Elmer with a CdSe ATR crystal. The spectra were recorded in the 4000–700 cm − 1 range. XRD measurements were performed in reflexion mode by a DX Advance powder diffractometer from Bruker AXS with a Cu X-ray source. The crystallinity of the samples was also characterised by HRTEM using a JEOL 2010FEG electron microscope operated at 200 kV (point to point resolution of 0.19 nm). The films were scratched from the substrate, dispersed in n-hexane and then deposited on holey carbon grids. Finally, the topography of the samples was investigated by AFM using an Agilent Technologies 5100 instrument provided with a silicon tip and operating at a frequency of 190 kHz. The scans were performed in the intermittent mode at room temperature. Image analysis was carried out with Mountains Map software from Digital Surf. Spectroscopic ellipsometry was measured at several angles of incidence with a rotating polariser system equipped with a CCD detector (GES5E by Sopralab). Absorbance spectra in the UV–VIS were taken with a Varian Cary 5 E spectrophotometer. 3. Results and discussion 3.1. Thermal analysis of Ce-propionate decomposition The decomposition temperature of Ce-propionate powders reported in the literature is around 300 °C [9,10]. Our own TG results confirm this value (Fig. 1) and make clear that decomposition occurs at a lower temperature when done a) in air instead of nitrogen and b) with a lower initial mass, mi. In a previous study [11], we concluded that the dependence on the mass, in an oxidative atmosphere, is a consequence of the oxygen depletion that occurs near the sample surface due to its consumption by the oxidative decomposition [11]. Consequently, the decomposition temperature will reach a minimum value when the initial mass tends to zero. The inset of Fig. 1 shows that, in this limit, the decomposition begins at 200 °C when heating at 10 K/min in air, i.e. about 100 °C below the value for large masses. Furthermore, owing to the fact that decomposition of Ce-propionate
Fig. 1. TG thermograms of the Ce-propionate decomposition in air and in N2. Note the dependence of the decomposition temperature on the initial mass, mi. Inset: dependence of the decomposition threshold, T0.9 (m/mi = 0.9), on mi for two heating rates.
is a thermal activated process, it occurs at lower temperature for slower heating rates (inset of Fig. 1). The ensemble of these results obtained on Ce-propionate powders indicates that, when dealing with Ce-propionate thin films, decomposition into Ce-oxide films can reach completion during isothermal treatments of reasonable duration in air. On the one hand, the higher surface to volume ratio of thin films compared to powders facilitates the oxygen transport to the sample and, consequently, lowers the decomposition temperature. This prediction is confirmed by the two dashed TG curves plotted in Fig. 2 corresponding to an initial mass of 2 mg. The film decomposes at a temperature 25 °C below that for the powders. On the other hand, isothermal treatments could, in principle, lead to complete decomposition provided that its duration is long enough. To estimate the duration needed for thin films at T b 200 °C, we have analysed the decomposition kinetics with a series of experiments done on thin films at 2, 5 and 10 K/min. The corresponding TG curves have been plotted in Fig. 2. We have used standard techniques [13] to extract the kinetic parameters of the transformation and used them to predict the duration needed to reach m/mi = 0.55. The results of this analysis are 9.5, 17.2 and 35.3 min at 200, 180 and 160 °C, respectively. We conclude that 1 h at a temperature as low as 160 °C should be enough to transform Ce-propionate into ceria thin films. For the rest of the paper, we will report on the results obtained by decomposing thin films at 160, 180 and 200 °C for 1 h. 3.2. Thin film structure First of all, we have verified by infrared spectroscopy that the characteristic bonds of Ce-propionate have disappeared in the treated films. In Fig. 3 we compare the IR-ATR spectra obtained from an asdeposited film and a film treated at 180 °C. The peaks due to the stretching C\H vibrational modes (at 3000–2800 cm − 1) [14], clearly observed in the as-deposited film, are absent in the pyrolysed film. The same evolution occurs for the absorption bands appearing in the 1700–1200 cm − 1 range. In addition to several vibrational modes related to C\H bonds, the stretching modes of the carboxylic group of acid salts are expected to absorb in this region [14]. Finally, the broad band appearing below 1000 cm − 1 in the IR-ATR spectra is due to the glass substrate [15]. The film's crystallinity and the existing phases have been analysed by XRD and TEM. To facilitate these measurements the films have been scratched. However, by doing so, we are unable to analyse any film anisotropy (texture or grain elongation with respect to the plane of the substrate). That said, in contrast with physical methods
Fig. 2. The films decompose quicker (at a lower temperature) than a powdered sample of the same initial mass (dashed curves). The thermograms of films decomposed at 2, 5 and 10 K/min serve to predict the decomposition time at isothermal conditions (see text).
P. Roura et al. / Thin Solid Films 520 (2012) 1949–1953
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In fact, we are surprised by the large crystallinity of these films decomposed at such low temperatures. In our previous work [11], where the decomposition mechanisms of Ce-propionate were analysed, we suggested that crystallisation could be enhanced when the precursor was in the liquid state. However, our present results, obtained below the melting point of Ce-propionate (230 °C [11]), contradict this hypothesis and reveal that crystallisation takes place even in the solid state precursor. Finally, the spin coated films have been observed by AFM. Fig. 6 presents an AFM picture of the surface morphology that has a rounded granular and porous structure. Roughness rms is 3.9 nm while the mean height of such grains lies between 10 and 12 nm and the base widths range from 0.5 to 1.6 μm. Note that the grains observed by the AFM analysis show a surface occupancy of 30%, which is in agreement with the model obtained from the ellipsometry study detailed below. 3.3. Optical characterization Fig. 3. IR spectra measured by ATR on a Ce-propionate film in its as-deposited state and after thermal treating at 180 °C. The characteristic peaks of Ce-propionate disappear after decomposition (below 1250 cm− 1 the signal is due to the quartz substrate).
for depositing ceria thin films [16,17], we do not expect any anisotropy in our films. This is well known from the structure of similar oxide films obtained through chemical solution deposition of metalorganic precursors treated at higher temperatures [18]. The XRD curve of a film treated at 200 °C is plotted in Fig. 4. The peaks correspond to CeO2 crystallised as ceria in the fluorite structure. No secondary peaks from other crystalline phases are detected. From the full-width at half maximum of the ceria peaks, the crystallite size can be estimated through application of the Scherrer formula [19]. We obtain an average size of 2–2.5 nm. This value is clearly smaller than that obtained from powders treated at 400 °C (5.8 nm), which agrees with the general trend observed by other authors [10]. So, we must conclude that the main characteristic of the films decomposed at low temperature resides in their smaller grain size. Observation of the film treated at 180 °C by high-resolution TEM confirms that it is polycrystalline (see Fig. 5a). As revealed by the Fast Fourier Transform (FFT) of this micrograph (Fig. 5b), some diffraction pattern rings could be indexed, indicating this polycrystalline distribution. Crystallites below 4 nm in diameter are clearly observed (Fig. 5c) and the FFT done on one crystallite delivers a diffraction pattern coherent with the crystalline structure of ceria (Fig. 5c).
Fig. 4. XRD curve of a film obtained at 200 °C. It has been fitted to a number of Voigt functions to extract the peak width that corresponds to a grain size of 2–2.5 nm. By comparison, the powder treated at 500 °C leads to coarser grains (sharper XRD peaks).
Optical properties of deposited films have been measured with variable angle ellipsometry. Spectra between 1.3 and 5.0 eV were taken at angles of incidence from 40 to 75° in 5 degree steps. The observed spectra have been fit to a multilayer model [20] to determine film thickness and optical properties. All the measured spectra were fitted simultaneously with the same model. Best fit results were obtained with a four phase model (inset of Fig. 7) consisting of quartz as substrate, a CeO2 layer, a mixture layer of air and CeO2, and air as ambient. The quartz substrate's refractive index was determined with an independent ellipsometric measurement of a bare substrate. The refractive index and extinction coefficient of CeO2 (Fig. 7) were best described with a Tauc–Lorentz model [21], and the mixture layer was described with Bruggeman's effective medium approximation. The fitted model has eight variable parameters: thickness of the two layers [d(CeO2) = 5.3 nm and d(air + CeO2) = 23.9 nm]; air fraction (xair = 0.53) in the mixture layer; and five that define the Tauc-Lorentz dielectric function of CeO2 [ε1(∞) = 3.92, Eg = 2.95 eV, E0 = 3.64 eV, A = 19 eV, C = 1.28 eV]. The total CeO2 effective film thickness is 16.5 nm, consistent with the 20 nm expected from spin coating conditions. The observed CeO2 optical functions (Fig. 7) are smaller than those reported for other CeO2 films [17,22]. This fact, together with the composition and relatively larger thickness of the mixture layer, gives an indication that the ceria film is rather porous, with porosity increasing near the film surface (see Fig. 6). As for the spectral behaviour of the CeO2 dielectric function, Eg and E0 in the Tauc–Lorentz model [21] correspond to the onset of absorption and the strongest interband transition respectively. The obtained values are very similar to those reported in [17] for the indirect band gap of amorphous CeO2 (2.96 eV) and direct transitions of crystalline CeO2 (3.60 eV). The optical absorption of thin films deposited by spin coating has been measured. The spectra obtained after decomposition at 160, 180 and 200 °C are very similar (Fig. 8). We think that the slight difference in the absorption level observed for the 180 °C film is not due to the temperature of decomposition (if this were the case, one would expect a systematic evolution with temperature) but to the impossibility of reproducing the same film thickness. Consequently, we deduce that the 180 °C film is about 15% thinner than the other two films. The spectra of Fig. 8 reveal that these films are highly transparent in the visible region and absorb in the UV region, beginning with a threshold at about 400 nm. After a peak at 300 nm, an additional threshold appears at about 220 nm. All these features are similar to those reported for thin films grown by alternative methods [17,22]. As described in the literature, the threshold at 400 nm is due to the band gap absorption of ceria [22]. For ceria nanocrystalline films, the band gap shifts to lower energy and the absorption coefficient diminishes, reaching a minimum value when films become amorphous [17,22].
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Fig. 5. HRTEM micrograph of a film treated at 180 °C (a) and its fast Fourier transform showing the polycrystalline rings of ceria (b). With higher magnification, it is possible to analyse the FFT of a single nanocrystal, which can be indexed according to the cubic structure of ceria (c).
We have deduced the energy of the direct band gap of our films by replotting the spectra with suitable axes (Fig. 9) and extrapolating to zero the absorption threshold. We have obtained EgD = 3.50 ± 0.05 eV.
Fig. 6. AFM topographic image of a 12 × 12 μm2 film area and extracted linear profile showing the presence of granular structure.
This value agrees with that reported for nanocrystalline films of similar grain size (3.57 ± 0.1 eV for an average grain size of 4 nm [23]). Thicker films of CeO2 (thickness around 200 nm) have been obtained by free spreading of a drop of Ce-propionate solution. In these conditions, the films are thick enough to become opaque below 300 nm (Fig. 8). Unfortunately, they became cracked. The relationship between film thickness and cracking is well known in the field of chemical solution deposition. Internal stresses develop as the result of film shrinking when the film is pyrolysed. Since these stresses are higher for thicker films, there is a critical thickness above which cracks
Fig. 7. Refractive index (n) and extinction coefficient (k) of CeO2 obtained from a fit of measured ellipsometry spectra to the multilayer model shown in the inset. Model details and parameters are given in the text.
P. Roura et al. / Thin Solid Films 520 (2012) 1949–1953
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2x1012
film thickness = 16.5 nm 160°C
(α·E)2 (cm-2·eV2)
200°C 180°C 1x10
12
Eg (direct) = 3.55-3.45 eV 0 3.0
3.5
4.0
4.5
5.0
E (eV) Fig. 8. Optical absorption spectra of thin films obtained by spin coating and pyrolysed at several temperatures (the contribution of the quartz substrate has been subtracted). The much higher absorbance of a thick film obtained by drop free spreading is also shown.
develop. The cracks are at the origin of the reduced transparency of the drop-coated film observed below the absorption threshold (Fig. 8). 4. Conclusions and perspectives Although the decomposition temperature of Ce-propionate reported in the literature is around 300 °C, we have shown that it can be substantially lowered when this precursor is in the form of thin film and is pyrolysed at isothermal conditions in air. Thin films of ceria have been obtained through the decomposition of Ce-propionate at temperatures as low as 160 °C during 1 h. The films are nanocrystalline with grain sizes below 4 nm. The films are transparent in the visible region and show the characteristic absorption threshold at about 400 nm due to the ceria band gap. The absorbance of films of similar thickness is the same irrespectively of the pyrolysis temperature in the 160–200 °C range and the measured direct band gap of 3.5 ± 0.05 eV is consistent with a nanocrystalline grain size distribution. Our results offer a route to obtain ceria thin films on substrates that cannot stand moderate temperatures. Further research is being carried out to optimise the process parameters that would allow the synthesis of thicker ceria films free of cracks. Acknowledgements We acknowledge the financial support from MICINN (MAT200908385, MAT2008-01022, Consolider Project NANOSELECT: CSD200700041), EU project NESPA-RTN and by the Generalitat de Catalunya (Catalan Pla de Recerca 2009SGR-185, 2009-SGR-770 and XaRMAE). A.C. thanks MICINN for the Spanish Ramon y Cajal Contract.
Fig. 9. Plot of the optical absorption curves of Fig. 7 to obtain the value of the direct band gap of the ceria thin films. α is the absorption coefficient (according to the ellipsometric results a film thickness of 16.5 nm has been assumed). Since the 180 °C film is thinner, its absorption coefficient is underestimated in the figure.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
E.P. Murray, T. Tsai, S.A. Barnett, Nature 400 (1999) 649. N. Izu, W. Shin, N. Murayama, Sens. Actuators B 93 (2003) 449. A. Goyal, M. Parans Paranthaman, U. Schoop, MRS Bull. 29 (2004) 552. E.K. Evangelou, S. Rahman, A. Dimoulas, IEEE Trans. Electron Devices 56 (2009) 399. S.M. Yang, C.H. Chien, J.J. Huang, T.F. Lei, M.J. Tsai, L.S. Lee, Appl. Phys. Lett. 91 (2007) 262104. X. Luo, B. Zhu, C. Xia, G.A. Niklasson, C.G. Granqvist, Sol. Energy Mater. Sol. Cells 53 (1998) 341. K.C. Krogman, T. Druffel, M.K. Sunkara, Nanotechnology 16 (2005) S338. R. Schwartz, T. Schneller, R. Waser, C.R. Chim. 7 (2004) 433. M. Kim, T.R. Hinklin, R.M. Laine, Chem. Mater. 20 (2008) 5154. Jian-dong Hu, Yong-xiu Li, Xue-zhen Zhou, Ming-xia Cai, Mater. Lett. 61 (2007) 4989. P. Roura, J. Farjas, J. Camps, S. Ricart, J. Arbiol, T. Puig, X. Obradors, J. Nanopart. Res. 13 (2011) 4085. K.K. Banger, Y. Yamashita, K. Mori, R.L. Peterson, T. Leedham, J. Rickard, H. Sirringhaus, Nat. Mater. 10 (2011) 45. B. Roduit, P. Folly, B. Berger, J. Mathieu, A. Sarbach, H. Andres, M. Ramin, B. Vogelsanger, J. Term. Anal. Calorim. 93 (2008) 153. G. Socrates, Infrared and Raman characteristic group frequencies, 3rd edition, John Wiley & Sons, Chichester, 2001, Chap. 1. D. Das, J. Farjas, P. Roura, G. Viera, E. Bertran, Appl. Phys. Lett. 79 (2001) 3705. C. Mansilla, Solid State Sci. 11 (2009) 1456. S. Guo, H. Arwin, S.N. Jacobsen, K. Järrendahl, U. Helmersson, J. Appl. Phys. 73 (1995) 5369. F. Sandiumenge, A. Cavallaro, J. Gazquez, T. Puig, X. Obradors, J. Arbiol, H.C. Freyhardt, Nanotechnology 16 (2005) 1809. P. Scherrer, Nachr. Wiss. Gott. 26 (1918) 98. R.M.A. Azzam, N.M. Bashara, Ellipsometry and polarized light, North-Holland, Amsterdam, 1987. G.E. Jellison, F.A. Modine, Appl. Phys. Lett. 69 (1996) 371 69, (1996) 2137. P. Patsalas, S. Logothetidis, L. Sygellou, S. Kennou, Phys. Rev. B 68 (2003) 035104. T. Suzuki, I. Kosacki, V. Petrovski, H.U. Anderson, J. Appl. Phys. 91 (2002) 2308.
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Synthesis of nanocrystalline ceria thin films by low-temperature thermal decomposition of Ce-propionate P. Roura a,⁎, J. Farjas a, S. Ricart b, M. Aklalouch b, R. Guzman b, J. Arbiol b, c, T. Puig b, A. Calleja b, O. Peña-Rodríguez b, M. Garriga b, X. Obradors b a b c
GRMT, Dept. of Physics, University of Girona, Campus Montilivi, Edif. PII, E17071 Girona, Catalonia, Spain Institut de Ciència de Materials de Barcelona (CSIC), Campus de la UAB, 08193 Bellaterra, Catalonia, Spain Institució Catalana de Recerca i Estudis Avançats (ICREA), Barcelona, Catalonia, Spain
a r t i c l e
i n f o
Article history: Received 5 May 2011 Received in revised form 13 September 2011 Accepted 23 September 2011 Available online 6 October 2011 Keywords: Ceria Cerium propionate Thermal decomposition Pyrolysis Optical absorption Thermogravimetry Crystal microstructure
a b s t r a c t Thin films of Ce-propionate (thickness below 20 nm) have been deposited by spin coating and pyrolysed into ceria at temperatures below 200 °C. After 1 h of thermal treatment, no signature of the vibrational modes of Ce-propionate is detected by infrared spectroscopy, indicating that decomposition has been completed. The resulting ceria films are nanocrystalline as revealed by X-ray diffraction (average grain size of 2–2.5 nm) and confirmed by microscopy. They are transparent in the visible region and show the characteristic band gap absorption below 400 nm. A direct band gap energy of 3.50± 0.05 eV has been deduced irrespective of the pyrolysis temperature (160, 180 and 200 °C). © 2011 Elsevier B.V. All rights reserved.
1. Introduction Ceria thin films deserve great attention due to the outstanding chemical and physical properties of CeO2. Owing to their high ionic conductivity at moderate temperatures and catalytic activity, they can be incorporated in solid-oxide fuel cells [1] and oxygen gas sensors [2]. They also play a role as barriers to diffusion in buffer layers such as those needed in high-temperature superconducting tapes [3]. Their high dielectric constant has led to propose their use in microelectronics where they could be a substitute for SiO2 in metal oxide semiconductor devices and in metal-insulator–metal devices [4,5]. Finally, their high transparency in the visible and infrared regions and their high absorbance in the UV region make them suitable for optical coatings [6,7]. Chemical solution deposition techniques constitute a versatile low-cost route for the synthesis of nanocrystalline oxide thin films. However, the need for a pyrolysis step at T ≥ 400 °C to decompose the molecular precursor into the desired oxide [8] discards their use in substrates such as polymers that cannot stand moderate temperatures. In the particular case of ceria, the decomposition temperature of its precursors could be substantially lowered with respect to the value reported in the literature [9,10]. This is so because, as we have
⁎ Corresponding author. E-mail address: [email protected] (P. Roura). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.09.058
shown in a detailed analysis of the decomposition of Ce-propionate [11], its decomposition in air is triggered by Ce 3+ oxidation to Ce 4+ instead of dissociation of the organic ligands. Consequently, for ceria thin films, the decomposition temperature could reach a minimum value. This finding can be of great interest is view of the present search for precursors that would allow the synthesis of other complex oxide films at low temperature [12]. In this paper, we will show that nanocrystalline ceria thin films can be obtained by pyrolysing Ce-propionate in air, below 200 °C. The films structure has been analysed by infrared (IR) spectroscopy, x-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM) and the optical properties of the films have been measured by optical transmittance in the visible (VIS) and ultraviolet (UV) regions and by spectroscopic ellipsometry. 2. Experimental details Ce-propionate powders were used directly for the thermal analysis experiments or were dissolved in propionic acid for film deposition. A 0.25 M solution was spin-coated on 0.5 mm-thick, 10× 10 mm2 square quartz plates at 6000 rpm for 2 min with 1 s acceleration time. A thickness of CeO2 films around 20 nm is expected according to spin-coated samples on YSZ (100) single crystals. Alternatively, to obtain films of hundreds of nm a more diluted (0.04 M) solution was used to freely
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P. Roura et al. / Thin Solid Films 520 (2012) 1949–1953
spread a microdrop on the surface of a glass disc (10 mm in diameter). The films were heated either in the TG furnace or on a hot plate with static air. Thermogravimetric (TG) analyses were done with a SDTA851 eLF thermobalance from Mettler Toledo. The Ce-propionate powders were placed inside uncovered alumina pans to facilitate gas exchange and the films were measured on their substrate. High purity synthetic air and nitrogen were used at a flow rate of 40 mL/min. IR spectra of the films were measured with an attenuated total reflexion (ATR) accessory mounted on a high-resolution spectrometer FT-IR from Perkin-Elmer with a CdSe ATR crystal. The spectra were recorded in the 4000–700 cm − 1 range. XRD measurements were performed in reflexion mode by a DX Advance powder diffractometer from Bruker AXS with a Cu X-ray source. The crystallinity of the samples was also characterised by HRTEM using a JEOL 2010FEG electron microscope operated at 200 kV (point to point resolution of 0.19 nm). The films were scratched from the substrate, dispersed in n-hexane and then deposited on holey carbon grids. Finally, the topography of the samples was investigated by AFM using an Agilent Technologies 5100 instrument provided with a silicon tip and operating at a frequency of 190 kHz. The scans were performed in the intermittent mode at room temperature. Image analysis was carried out with Mountains Map software from Digital Surf. Spectroscopic ellipsometry was measured at several angles of incidence with a rotating polariser system equipped with a CCD detector (GES5E by Sopralab). Absorbance spectra in the UV–VIS were taken with a Varian Cary 5 E spectrophotometer. 3. Results and discussion 3.1. Thermal analysis of Ce-propionate decomposition The decomposition temperature of Ce-propionate powders reported in the literature is around 300 °C [9,10]. Our own TG results confirm this value (Fig. 1) and make clear that decomposition occurs at a lower temperature when done a) in air instead of nitrogen and b) with a lower initial mass, mi. In a previous study [11], we concluded that the dependence on the mass, in an oxidative atmosphere, is a consequence of the oxygen depletion that occurs near the sample surface due to its consumption by the oxidative decomposition [11]. Consequently, the decomposition temperature will reach a minimum value when the initial mass tends to zero. The inset of Fig. 1 shows that, in this limit, the decomposition begins at 200 °C when heating at 10 K/min in air, i.e. about 100 °C below the value for large masses. Furthermore, owing to the fact that decomposition of Ce-propionate
Fig. 1. TG thermograms of the Ce-propionate decomposition in air and in N2. Note the dependence of the decomposition temperature on the initial mass, mi. Inset: dependence of the decomposition threshold, T0.9 (m/mi = 0.9), on mi for two heating rates.
is a thermal activated process, it occurs at lower temperature for slower heating rates (inset of Fig. 1). The ensemble of these results obtained on Ce-propionate powders indicates that, when dealing with Ce-propionate thin films, decomposition into Ce-oxide films can reach completion during isothermal treatments of reasonable duration in air. On the one hand, the higher surface to volume ratio of thin films compared to powders facilitates the oxygen transport to the sample and, consequently, lowers the decomposition temperature. This prediction is confirmed by the two dashed TG curves plotted in Fig. 2 corresponding to an initial mass of 2 mg. The film decomposes at a temperature 25 °C below that for the powders. On the other hand, isothermal treatments could, in principle, lead to complete decomposition provided that its duration is long enough. To estimate the duration needed for thin films at T b 200 °C, we have analysed the decomposition kinetics with a series of experiments done on thin films at 2, 5 and 10 K/min. The corresponding TG curves have been plotted in Fig. 2. We have used standard techniques [13] to extract the kinetic parameters of the transformation and used them to predict the duration needed to reach m/mi = 0.55. The results of this analysis are 9.5, 17.2 and 35.3 min at 200, 180 and 160 °C, respectively. We conclude that 1 h at a temperature as low as 160 °C should be enough to transform Ce-propionate into ceria thin films. For the rest of the paper, we will report on the results obtained by decomposing thin films at 160, 180 and 200 °C for 1 h. 3.2. Thin film structure First of all, we have verified by infrared spectroscopy that the characteristic bonds of Ce-propionate have disappeared in the treated films. In Fig. 3 we compare the IR-ATR spectra obtained from an asdeposited film and a film treated at 180 °C. The peaks due to the stretching C\H vibrational modes (at 3000–2800 cm − 1) [14], clearly observed in the as-deposited film, are absent in the pyrolysed film. The same evolution occurs for the absorption bands appearing in the 1700–1200 cm − 1 range. In addition to several vibrational modes related to C\H bonds, the stretching modes of the carboxylic group of acid salts are expected to absorb in this region [14]. Finally, the broad band appearing below 1000 cm − 1 in the IR-ATR spectra is due to the glass substrate [15]. The film's crystallinity and the existing phases have been analysed by XRD and TEM. To facilitate these measurements the films have been scratched. However, by doing so, we are unable to analyse any film anisotropy (texture or grain elongation with respect to the plane of the substrate). That said, in contrast with physical methods
Fig. 2. The films decompose quicker (at a lower temperature) than a powdered sample of the same initial mass (dashed curves). The thermograms of films decomposed at 2, 5 and 10 K/min serve to predict the decomposition time at isothermal conditions (see text).
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In fact, we are surprised by the large crystallinity of these films decomposed at such low temperatures. In our previous work [11], where the decomposition mechanisms of Ce-propionate were analysed, we suggested that crystallisation could be enhanced when the precursor was in the liquid state. However, our present results, obtained below the melting point of Ce-propionate (230 °C [11]), contradict this hypothesis and reveal that crystallisation takes place even in the solid state precursor. Finally, the spin coated films have been observed by AFM. Fig. 6 presents an AFM picture of the surface morphology that has a rounded granular and porous structure. Roughness rms is 3.9 nm while the mean height of such grains lies between 10 and 12 nm and the base widths range from 0.5 to 1.6 μm. Note that the grains observed by the AFM analysis show a surface occupancy of 30%, which is in agreement with the model obtained from the ellipsometry study detailed below. 3.3. Optical characterization Fig. 3. IR spectra measured by ATR on a Ce-propionate film in its as-deposited state and after thermal treating at 180 °C. The characteristic peaks of Ce-propionate disappear after decomposition (below 1250 cm− 1 the signal is due to the quartz substrate).
for depositing ceria thin films [16,17], we do not expect any anisotropy in our films. This is well known from the structure of similar oxide films obtained through chemical solution deposition of metalorganic precursors treated at higher temperatures [18]. The XRD curve of a film treated at 200 °C is plotted in Fig. 4. The peaks correspond to CeO2 crystallised as ceria in the fluorite structure. No secondary peaks from other crystalline phases are detected. From the full-width at half maximum of the ceria peaks, the crystallite size can be estimated through application of the Scherrer formula [19]. We obtain an average size of 2–2.5 nm. This value is clearly smaller than that obtained from powders treated at 400 °C (5.8 nm), which agrees with the general trend observed by other authors [10]. So, we must conclude that the main characteristic of the films decomposed at low temperature resides in their smaller grain size. Observation of the film treated at 180 °C by high-resolution TEM confirms that it is polycrystalline (see Fig. 5a). As revealed by the Fast Fourier Transform (FFT) of this micrograph (Fig. 5b), some diffraction pattern rings could be indexed, indicating this polycrystalline distribution. Crystallites below 4 nm in diameter are clearly observed (Fig. 5c) and the FFT done on one crystallite delivers a diffraction pattern coherent with the crystalline structure of ceria (Fig. 5c).
Fig. 4. XRD curve of a film obtained at 200 °C. It has been fitted to a number of Voigt functions to extract the peak width that corresponds to a grain size of 2–2.5 nm. By comparison, the powder treated at 500 °C leads to coarser grains (sharper XRD peaks).
Optical properties of deposited films have been measured with variable angle ellipsometry. Spectra between 1.3 and 5.0 eV were taken at angles of incidence from 40 to 75° in 5 degree steps. The observed spectra have been fit to a multilayer model [20] to determine film thickness and optical properties. All the measured spectra were fitted simultaneously with the same model. Best fit results were obtained with a four phase model (inset of Fig. 7) consisting of quartz as substrate, a CeO2 layer, a mixture layer of air and CeO2, and air as ambient. The quartz substrate's refractive index was determined with an independent ellipsometric measurement of a bare substrate. The refractive index and extinction coefficient of CeO2 (Fig. 7) were best described with a Tauc–Lorentz model [21], and the mixture layer was described with Bruggeman's effective medium approximation. The fitted model has eight variable parameters: thickness of the two layers [d(CeO2) = 5.3 nm and d(air + CeO2) = 23.9 nm]; air fraction (xair = 0.53) in the mixture layer; and five that define the Tauc-Lorentz dielectric function of CeO2 [ε1(∞) = 3.92, Eg = 2.95 eV, E0 = 3.64 eV, A = 19 eV, C = 1.28 eV]. The total CeO2 effective film thickness is 16.5 nm, consistent with the 20 nm expected from spin coating conditions. The observed CeO2 optical functions (Fig. 7) are smaller than those reported for other CeO2 films [17,22]. This fact, together with the composition and relatively larger thickness of the mixture layer, gives an indication that the ceria film is rather porous, with porosity increasing near the film surface (see Fig. 6). As for the spectral behaviour of the CeO2 dielectric function, Eg and E0 in the Tauc–Lorentz model [21] correspond to the onset of absorption and the strongest interband transition respectively. The obtained values are very similar to those reported in [17] for the indirect band gap of amorphous CeO2 (2.96 eV) and direct transitions of crystalline CeO2 (3.60 eV). The optical absorption of thin films deposited by spin coating has been measured. The spectra obtained after decomposition at 160, 180 and 200 °C are very similar (Fig. 8). We think that the slight difference in the absorption level observed for the 180 °C film is not due to the temperature of decomposition (if this were the case, one would expect a systematic evolution with temperature) but to the impossibility of reproducing the same film thickness. Consequently, we deduce that the 180 °C film is about 15% thinner than the other two films. The spectra of Fig. 8 reveal that these films are highly transparent in the visible region and absorb in the UV region, beginning with a threshold at about 400 nm. After a peak at 300 nm, an additional threshold appears at about 220 nm. All these features are similar to those reported for thin films grown by alternative methods [17,22]. As described in the literature, the threshold at 400 nm is due to the band gap absorption of ceria [22]. For ceria nanocrystalline films, the band gap shifts to lower energy and the absorption coefficient diminishes, reaching a minimum value when films become amorphous [17,22].
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Fig. 5. HRTEM micrograph of a film treated at 180 °C (a) and its fast Fourier transform showing the polycrystalline rings of ceria (b). With higher magnification, it is possible to analyse the FFT of a single nanocrystal, which can be indexed according to the cubic structure of ceria (c).
We have deduced the energy of the direct band gap of our films by replotting the spectra with suitable axes (Fig. 9) and extrapolating to zero the absorption threshold. We have obtained EgD = 3.50 ± 0.05 eV.
Fig. 6. AFM topographic image of a 12 × 12 μm2 film area and extracted linear profile showing the presence of granular structure.
This value agrees with that reported for nanocrystalline films of similar grain size (3.57 ± 0.1 eV for an average grain size of 4 nm [23]). Thicker films of CeO2 (thickness around 200 nm) have been obtained by free spreading of a drop of Ce-propionate solution. In these conditions, the films are thick enough to become opaque below 300 nm (Fig. 8). Unfortunately, they became cracked. The relationship between film thickness and cracking is well known in the field of chemical solution deposition. Internal stresses develop as the result of film shrinking when the film is pyrolysed. Since these stresses are higher for thicker films, there is a critical thickness above which cracks
Fig. 7. Refractive index (n) and extinction coefficient (k) of CeO2 obtained from a fit of measured ellipsometry spectra to the multilayer model shown in the inset. Model details and parameters are given in the text.
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2x1012
film thickness = 16.5 nm 160°C
(α·E)2 (cm-2·eV2)
200°C 180°C 1x10
12
Eg (direct) = 3.55-3.45 eV 0 3.0
3.5
4.0
4.5
5.0
E (eV) Fig. 8. Optical absorption spectra of thin films obtained by spin coating and pyrolysed at several temperatures (the contribution of the quartz substrate has been subtracted). The much higher absorbance of a thick film obtained by drop free spreading is also shown.
develop. The cracks are at the origin of the reduced transparency of the drop-coated film observed below the absorption threshold (Fig. 8). 4. Conclusions and perspectives Although the decomposition temperature of Ce-propionate reported in the literature is around 300 °C, we have shown that it can be substantially lowered when this precursor is in the form of thin film and is pyrolysed at isothermal conditions in air. Thin films of ceria have been obtained through the decomposition of Ce-propionate at temperatures as low as 160 °C during 1 h. The films are nanocrystalline with grain sizes below 4 nm. The films are transparent in the visible region and show the characteristic absorption threshold at about 400 nm due to the ceria band gap. The absorbance of films of similar thickness is the same irrespectively of the pyrolysis temperature in the 160–200 °C range and the measured direct band gap of 3.5 ± 0.05 eV is consistent with a nanocrystalline grain size distribution. Our results offer a route to obtain ceria thin films on substrates that cannot stand moderate temperatures. Further research is being carried out to optimise the process parameters that would allow the synthesis of thicker ceria films free of cracks. Acknowledgements We acknowledge the financial support from MICINN (MAT200908385, MAT2008-01022, Consolider Project NANOSELECT: CSD200700041), EU project NESPA-RTN and by the Generalitat de Catalunya (Catalan Pla de Recerca 2009SGR-185, 2009-SGR-770 and XaRMAE). A.C. thanks MICINN for the Spanish Ramon y Cajal Contract.
Fig. 9. Plot of the optical absorption curves of Fig. 7 to obtain the value of the direct band gap of the ceria thin films. α is the absorption coefficient (according to the ellipsometric results a film thickness of 16.5 nm has been assumed). Since the 180 °C film is thinner, its absorption coefficient is underestimated in the figure.
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