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Optical properties of europium-doped Gd2O3 waveguiding thin films prepared by the sol–gel method
Optical Materials 19 (2002) 161–168 www.elsevier.com/locate/optmat
Optical properties of europium-doped Gd2O3 waveguiding thin films prepared by the sol–gel method A. Garcıa-Murillo a,*, C. Le Luyer a, C. Garapon a, C. Dujardin a, E. Bernstein b, C. Pedrini a, J. Mugnier a a
Laboratoire de Physico-Chimie des Mat eriaux Luminescents, CNRS-UMR 5620, Universit e Claude Bernard Lyon 1, 43 Bd du 11 novembre 1918, 69622 Villeurbanne Cedex, France b D epartement de Physique des Mat eriaux, CNRS UMR 5586, Universit e Claude Bernard Lyon 1, 43 Bd du 11 novembre 1918, 69622 Villeurbanne Cedex, France
Abstract Recently, there has been a growth of interest in the preparation of new phosphor films for high-resolution X-ray imaging systems. Europium activated gadolinium oxide is very interesting because of its scintillation properties especially as a red component. The sol–gel method has been used to synthesize europium-doped gadolinium oxide films. The films present waveguiding properties and this special feature is used to study their microstructure by Raman spectroscopy in waveguiding configuration. Structural results, confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM) observations, show that the crystallization in the cubic phase occurs at 700 °C. Optogeometrical parameters were determined with respect to the annealing temperature. After annealing at 1000 °C, very dense europium-doped gadolinium oxide films are obtained with a thickness of 390 nm and a refractive index of 1.88 at 632.8 nm. Spectroscopic results constituted by emission spectra (UV and X-ray excitation) and decay measurements are presented. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Europium-activated phosphors have been known for a long time [1]. In recent years there has been a growth of interest in preparation of phosphors activated by rare earths or with rare earths as host lattice constituents for X-ray imaging systems. One of the most efficient phosphors found
* Corresponding author. Tel.: +33-472-448-338; fax: +33-472431-130. E-mail address: [email protected] (A. GarcıaMurillo).
was europium-doped dense oxide. For example, europium-doped polycrystalline gadolinium oxide as powder is currently used as a red component because of its high effective atomic number. Moreover the development of X-ray systems with improved performance such as high-resolution images requires low diffusion loss. Thus, very dense phosphor films play an important role in high-resolution display devices such as cathode ray tubes (CRTs) [2] used in medical imaging. Gadolinium oxide waveguiding thin films prepared by the sol–gel process have been recently studied and characterized [3]. The sol–gel method has the advantage to provide negligible diffusion
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loss, high quality and purity. Moreover, the planar waveguide properties of the elaborated layers are a proof of their high optical quality and allow their study in convenient waveguiding configuration. The preliminary results obtained on the synthesis and on the microstructure of undoped gadolinium oxide films were used to elaborate europiumdoped gadolinium oxide films for scintillating applications. The sol–gel processing of europium-doped Gd2 O3 films is presented. Differential thermal analysis (DTA) and Fourier transform infrared (FTIR) spectroscopy are at first conducted on the corresponding sol–gel powder in order to forecast the formation and the crystallization temperature dependence of the material in powder form. These results can be used only as a rough indication when studying sol–gel evolution versus annealing temperature because it is well known that sol–gel powder and thin film behavior can be different [4]. The coating layers are then heat-treated at different temperatures and their structural properties are determined using Raman spectroscopy. The microstructure of the more densified layers heattreated at 1000 °C is confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Refractive index and thickness values were determined by m-lines spectroscopy technique. Emission spectra and emission decays were measured to estimate the scintillating potential of these layers.
2. Preparation of Gd2 O3 -doped Eu (5 mol%) films Europium-doped Gd2 O3 films were prepared using the sol–gel process. The precursor used was gadolinium isopropoxide in toluene–isopropanol (3:2) 99.9% (Johnson Mattey, Alfa products) and europium (III) nitrate pentahydrate, 99.9% (Aldrich). The transparency, homogeneity and stability of the precursor solution are predominant criteria in order to ensure reproducible coatings of high optical quality, especially when multiple coatings are necessary to obtain thick films. Nevertheless, the metal alkoxide is highly reactive towards hydrolysis and condensation. It has to be stabilized in order to slow down the reactions and to prevent
precipitation, to control the growth of oxide particles and to form a stable precursor solution. This can be performed via the chemical modification of gadolinium alkoxide by acetylacetone, 99% (Aldrich), a chelating agent frequently used in sol–gel chemistry [5]. The precursor solution was prepared under controlled atmosphere (argon flux) by mixing the components under strong agitation. The final concentration of gadolinium is 50 g/l. The Eu3þ ions were added in the ratio Eu/Gd ¼ 5 mol%. This solution remains stable after storage under argon for one month, which is interesting, viewed from practical and economical aspects. The europium-doped gadolinium solution was filtered through 0:2 lm and deposited on carefully cleaned pure silica (Herasilâ from Heraeus) substrates in a glove box using the dip-coating technique also under argon controlled atmosphere. Ten and 50 layers were deposited to obtain monomode and multimode transverse electric waveguides, respectively. The layers were heattreated at 350 °C for 30 min between each coating under oxygen flow to promote organic burnout and partial densification of the film. The films were finally annealed 1 h at final required temperatures, ranging from 350 to 1000 °C. Crack-free and transparent layers were obtained and studied at room temperature.
3. Sol–gel powder analyses The DTA and thermo-gravimetry (TG) measurements (Fig. 1) were conducted, using a Netzsch-STA 409C apparatus, on the europiumdoped sol–gel powder obtained by drying the precursor solution at 100 °C. The DTA and TG thermograms were recorded at a scan rate of 5 °C/ min in air flux. This study allowed to forecast the crystallization temperature of the material. The small endothermic peak around 100 °C mainly corresponds to the evaporation of volatile organic solvents. The narrow exothermic peak centered at 460 °C arises from the oxidation of organic compounds. The TG curve continuously decreases up to 500 °C indicating the removal of solvents. The large exothermic peaks around 700 °C are assigned to crystallization of the material.
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a 700–1000 °C heat treatment correspond to cubic Gd2 O3 phase [3].
4. Microstructure of Eu (5 mol%)-doped Gd2 O3 films
Fig. 1. DTA and TG thermograms on the 5 mol% Eu3þ :Gd2 O3 sol–gel powder treated from room temperature to 1000 °C, using a ramp of 5 °C/min.
Room temperature IR spectra (Fig. 2) are obtained using a FTIR 2000 Perkin–Elmer on the sol–gel powder treated for 1 h from 100 °C to 1000 °C, in order to complete DTA and TG experiments. The bands located around 1400 and 3400 cm1 are attributed to the presence of solvents as alcohol and toluene, and are characteristic to the gem-dimethyl structure [–CðCH3 Þ2 –] of the alkoxide. These bands are observed on the powder dried at 110 °C. Some of them are still present at 500 °C, but they totally disappear at 700 °C. This observation confirms the removal of acetyl groups and organic oxidation up to 500 °C. The bands observed from 520 to 370 cm1 which appear after
Fig. 2. Room temperature IR spectra of 5 mol% Eu3þ :Gd2 O3 sol–gel powder treated for 1 h from 100 to 1000 °C.
The determination of Eu (5 mol%)-doped Gd2 O3 film microstructure is essential to understand the optical properties and to explain the fluorescence behavior of the films for scintillation applications. As optical properties are strongly linked with structural characteristics, the structural evolution of Eu-doped films with annealing temperature are investigated using Raman spectroscopy. Waveguide Raman spectroscopy (WRS) is conducted at 647.1 nm on films heat-treated from 400 to 900 °C. Details of the WRS experimental set-up have already been described elsewhere [6]. This highly sensitive and non-destructive technique is extremely powerful especially for sol–gel waveguides analysis. At temperature higher than 900 °C, the film loses its waveguiding properties and WRS measurements cannot be carried out. MicroRaman spectroscopy (DilorXY apparatus) is thus used to analyze the local microstructure of a 50 coatings layer annealed at 1000 °C for 1 h (around 400 nm thick, see Section 5) and to compare with the Raman spectrum of the 5 mol% Eu3þ :doped gadolinium oxide powder which we obtain by solid state reaction method. WRS spectra of Eu-doped Gd2 O3 waveguides treated at 400 and 500 °C could not be obtained due to the luminescence of organic compounds remaining in the layer, which hides the Raman signal. WRS spectra obtained on films annealed from 600 to 900 °C are represented on Fig. 3. The Raman spectrum recorded on sample heat-treated at 600 °C indicates that the material transforms from amorphous phase into crystallized phase at 600 °C. Both phases are detected at this temperature. The Raman spectrum of the amorphous phase exhibits two weak broad bands around 90 and 350 cm1 as already observed on amorphous undoped gadolinium films [3]. The weak band at 360 cm1 is attributed to the main strongest band of the Gd2 O3 cubic phase. The amorphous phase
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totally disappears after 700 °C heat treatment. The Raman bands at 93, 117, 313, 359, 442 and 565 cm1 observed on the Raman spectrum of the 5 mol% Eu3þ :Gd2 O3 powder obtained by the solid state reaction method are assigned to cubic phase of gadolinium oxide [3]. These bands are clearly identified on the Eu-doped Gd2 O3 film treated at 1000 °C (Fig. 3). The phase structure of the 1000 °C heat-treated Eu3þ :Gd2 O3 waveguide is confirmed by X-ray analysis and TEM observations. The complete series of Bragg reflections observed on the corresponding XRD pattern (Fig. 4) correspond to the Gd2 O3 cubic phase. Electron microscopy observation of the Eu3þ :Gd2 O3 layer annealed 1 h at 1000 °C is shown on Fig. 5(a). The crystallized film presents a dense and polycrystalline structure made up by randomly oriented crystallites with grain size varying between 30 and 100 nm. The presence of crystallites as great as 100 nm explains the loss of waveguiding properties of the layer annealed at 1000 °C. The electron diffraction pattern of the surface area of the film is represented on Fig. 5(b). The rings correspond to the lattice spacings of the Gd2 O3 cubic phase (JCPDS 431014).
Fig. 3. Room temperature Raman spectra of europium-doped gadolinium oxide films recorded in a waveguiding configuration for layers heat-treated from 600 to 900 °C. Raman spectra of the non-waveguiding layer annealed at 1000 °C and of the cubic powder obtained by solid state reaction method are obtained using a microRaman set-up.
Fig. 4. XRD spectrum of 5 mol% Eu3þ :Gd2 O3 layers deposited on a SiO2 /Si substrate and heat-treated 1 h at 1000 °C.
Fig. 5. TEM observations (a) and electron diffraction pattern (b) of 5 mol% Eu3þ :Gd2 O3 planar waveguide heat-treated 1 h at 1000 °C.
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Fig. 6. Evolution of thickness at 632.8 nm (a) and refractive index (b) of 5 mol% Eu3þ :Gd2 O3 films heat-treated under different annealing temperatures.
5. Opto-geometrical properties The refractive index and the thickness of europium-doped Gd2 O3 films were determined by mlines spectroscopy conducted at 632.8 nm. This method is described elsewhere [7]. For this purpose we used the multimode layers dip coated 50 times. These layers support two transverse electric (TE) modes and two transverse magnetic (TM) modes after the successive 1 h heat treatments from 350 to 1000 °C. Refractive index and thickness versus annealing temperatures determined for TE modes are shown in Fig. 6. A decrease of the thickness from 550 1 nm (350 °C) to 387 1 nm (1000 °C), illustrates the densification of the film. This densification is correlated with an increase of the refractive index from 1:712 0:001 (amorphous phase heat-treated at 350 °C) to 1:877 0:001 (crystalline phase heat-treated at 1000 °C). The increase of the refractive index is a consequence of the organic compounds removal and structural changes in the europium-doped gadolinium oxide films.
6. Spectroscopic properties of Eu (5 mol%)-doped Gd2 O3 films The fluorescence measurements were conducted on the films in order to study the environment of
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Eu3þ ions in the Gd2 O3 sol–gel host lattice as a function of heat treatments. The properties of Gd2 O3 , which we synthesized as thin film by the sol–gel process, are compared to the already known fluorescence properties of Gd2 O3 powders synthesized by solid state reaction method. The cubic structure obtained for the films contain two inequivalent cation sites of C2 and S6 symmetry [8]. The ratio of C2 to S6 site numbers is 3 to 1 but there is no preferential occupation by the Eu3þ ions of these two sites [9]. The luminescence of Eu3þ in Gd2 O3 is very similar to that of extensively studied phosphor Y2 O3 [9]. For 5 mol% Eu3þ -doped Gd2 O3 it essentially consists of transitions from the 5 D0 to the 7 FJ levels as 5 D1 emission is quenched by cross relaxation [10]. The 7 F0;1 and 5 D0;1 energy levels scheme for Eu3þ in both sites are given in [10,11]. Transitions between any two of the levels are governed by different selection rules for the two sites. For the S6 site, which has an inversion center, only weak magnetic dipole transitions are present. For the C2 , site forced electric dipole transitions of higher intensities are observed in addition to the magnetic dipole transitions [12]. 6.1. Fluorescence after UV excitation The emission spectra and fluorescence decays were recorded at room temperature using excitation at 308 nm by an excimer XeCl laser (Lumonics 527). The fluorescence was analyzed with a 1 m monochromator (Hilger Watts), detected by a AsGa photomultiplier (RCA 31084) and an Ortec photoncounter. Fluorescence decays were registered with a Canberra multichannel analyzer. Films were excited tangentially and the fluorescence was collected perpendicularly to the film surface. 6.1.1. Emission spectra Emission spectra were recorded for 50 layers Gd2 O3 film doped with 5 mol% Eu3þ that was submitted to successive 1 h annealing treatments from 350 to 1000 °C. Excitation at 308 nm takes place into the low energy wing of the charge transfer band peaking at 250 nm and also into the 6 P5=2 multiplet of the
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Fig. 7. Room temperature emission spectra of 5 mol% Eu3þ :Gd2 O3 films heat-treated 1 h at different temperatures.
Gd3þ ions [10]. Gadolinium to europium energy transfer and fast relaxation among Eu3þ energy levels populate the 5 D0 emitting level. The emission spectra are shown in Fig. 7. The emission spectra of Eu3þ -doped films heattreated between 350 and 600 °C, are characterized by five broad bands located between: 570–580, 580–603, 603–640, 640–665 and 680–715 nm, which correspond to unresolved 5 D0 ! 7 FJ transitions (J ¼ 0 to 4), respectively. These broad bands confirm the location of the Eu3þ ions in a disordered gadolinium oxide matrix for low annealing temperatures. The emission spectra of the films heat-treated from 700 to 1000 °C change drastically in comparison with those annealed at lower temperature. After 700 °C heat treatment the five broad bands change to sharp emission lines. The same spectrum was observed for conventional polycrystalline europium-doped Gd2 O3 powders, obtained by solid state reaction method. These emission lines presented in Fig. 8 are characteristic of the crystallized cubic phase which remains until 1000 °C. The line positions are in good agreement with the level energies given in [9,10], according to which we made transition assignments. 6.1.2. Fluorescence decays Decay measurements were recorded at room temperature on Eu3þ -doped Gd2 O3 films heat-
Fig. 8. Room temperature emission spectra of 5 mol% Eu3þ :Gd2 O3 films heat-treated 1 h at 700 °C.
treated from 350 to 1000 °C at the wavelength of the predominant 5 D0 ! 7 F2 ðsite C2 Þ emission after excitation at 308 nm (Fig. 9). The emission decays corresponding to the temperatures of 350 and 600 °C, (Eu3þ is in a disordered Gd2 O3 host lattice) are, almost exponential with a mean time constant of about 1 ms. However, a change in the 5 D0 ! 7 F2 fluorescence decay is clearly observed for 500 °C annealing temperature, for which the non-exponential character is more pronounced. This must be related to Eu3þ environment modification induced by organic compounds burnout detected by ATD and
Fig. 9. Dependence on the heat-treatment of the 5 D0 ! 7 F2 emission decay under excitation at 308 nm in 5 mol% Eu3þ :Gd2 O3 films (curves were arbitrarily translated for figure clearness).
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Table 1 Lifetime values determined by double exponential fit of the experimental luminescence decays (k ¼ 611 nm) obtained for Eu3þ -doped Gd2 O3 films as a function of heat-treatments Annealing temperature (°C) 700 800 900 1000
5
D0 ! 7 F2 lifetime (ms)
C2
S6
1.00 1.01 1.02 1.03
– 3.41 3.46 3.49
IR analyses at this annealing temperature. Modification of the broad emission spectra profiles was also noticed, indicating a crystal field distribution evolution. For films heat-treated from 700 to 1000 °C, which have cubic phase as crystallization, the decays are approximately identical and have become non-exponential. They appear as being constituted by two components and may be fitted satisfactorily by the sum of two exponential functions, the lifetimes of which are collected in Table 1. The fast component of 1 ms gives a dominant contribution to the intensity, the slower component of 3.5 ms lifetime corresponding to an about 100 times weaker intensity. The main 1 ms component is due to the C2 site emission in agreement with the literature data [9]. The weak component may correspond to feeding of the C2 sites from the S6 sites by an energy transfer, which is known as very efficient [9]. This attribution is confirmed by the fluorescence decays of the 5 D0 ! 7 F0 line of the site C2 and 5 D0 ! 7 F1 line of the site S6 (Fig. 10). The decay of the 5 D0 ! 7 F1 emission of the site S6 at 581.6 nm is non-exponential as expected in case of energy transfer for the donor decay, with a predominant long time component having the same time constant of about 3.5 ms. This value is similar to literature values for similar concentrations. The decay of the 5 D0 ! 7 F0 emission of the site C2 at 580.6 nm is dominated by a 1 ms component, as for the 5 D0 ! 7 F2 emission at 611.6 nm, the increase in the 3.5 ms component being due to the overlap with the wing of the 5 D0 ! 7 F1 line of the site S6 at 581.6 nm. Due to the complicated level populating processes, induced by excitation at 308 nm we did not try to study more
Fig. 10. Emission decay curves on 5 mol% Eu3þ :Gd2 O3 films heat-treated at 900 °C under excitation at 308 nm for emission wavelengths of 580.6, 581.6 and 611.6 nm.
precisely neither the S6 –C2 energy transfer nor to explain the rise time observed for some decays. Nevertheless, the measured lifetimes, 1 and 3.5 ms for C2 and S6 sites, respectively, which correspond to most of the intensity, are in agreement with literature data reported for crystals. 6.2. Emission spectrum after X-ray excitation For X-ray imagery applications, the most important feature is the scintillation properties of these films. In other words the conversion of absorbed X-rays into visible radiation.
Fig. 11. Room temperature emission spectrum under X-ray excitation of 5 mol% Eu3þ :Gd2 O3 thin films heat-treated 1 h at 1000 °C.
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To test these properties we have measured the emission of europium-doped Gd2 O3 films under X-ray radiation. The emission spectrum presented in Fig. 11 was recorded using an X-ray source operated at 40 kV. The light emitted was analyzed with a monochromator (Jobin Yvon H20) and detected by a photomultiplier (EMI 9789). The film surface was placed 45° of X-ray radiation and fluorescence collected 45° to the film surface. The emission spectrum recorded at room temperature shows the same band around 611 nm as the emission spectrum after UV excitation, this visible emission band is clearly assigned to the 5 D0 ! 7 F2 transition of europium ion. Unfortunately, we could not resolve the spectrum mainly because of the very thin thickness. However, the spectrum of europium-doped gadolinium oxide confirms their scintillation properties.
7. Conclusions Sol–gel process has been used to synthesize europium-doped gadolinium oxide thin films with high optical quality and waveguiding properties. The stability of the sol has allowed to elaborate multimode thin films after 50 coatings, which were used to study the structure and chemical evolution as a function of heat treatments. The increase of refractive index and the decrease of thickness values are representative of crystallization and densification of the films due to increased heat treatment temperature. Good waveguiding properties enable waveguide Raman spectroscopy measurements which indicate that the films after 700 °C annealing are crystallized in cubic phase. The structure was confirmed by X-ray analysis and TEM observations. The crystallization of the film into cubic phase, remains stable up to 1000 °C. The photoluminescence studies evidence that europium-doped gadolinium oxide thin films matrix have the same sharp emission bands as europium-
doped Gd2 O3 powders produced by solid state reaction method, corresponding to the 5 D0 ! 7 FJ transitions (J ¼ 0 to 4). These characteristics emission bands of cubic phase appear at 700 °C by sol–gel process although with conventional method they become visible only after 900 °C. Fluorescence decay properties are consistent with the presence of two sites known for Eu3þ in cubic Gd2 O3 with 5 D0 lifetime of about 1 ms for the C2 site and about 3 ms for the S6 site. Finally, X-ray-induced emission leads to the same 5 D0 ! 7 F2 red emission lines as observed under UV excitation which shows the potentiality of these films as a scintillating material.
Acknowledgements The authors are grateful to EZUS (Universite Lyon I) for financial support and Conacyt-SFERE Program for A.G.-M. scholarship. The authors would like to thank Delphine Pennaneac’h for DTA-TG measurements.
References [1] A. Bril, W.L. Wanmaker, J. Electrochem. Soc. 111 (1964) 1363. [2] R.P. Rao, Solid State Commun. 99 (1996) 439. [3] A. Garcıa-Murillo, C. Le Luyer, C. Dujardin, C. Pedrini, J. Mugnier, Opt. Mater. 16 (1–2) (2001) 39. [4] M. Bahtat, J. Mugnier, L. Lou, J. Serughetti, SPIE, Sol–gel Opt. II 173 (1992) 1758. [5] D.C. Bradley, R. Bohra, D.P. Gaur, Metal Alkoxides, Academic Press, New York, 1978. [6] C. Urlacher, J. Mugnier, J. Raman Spectrosc. 27 (1996) 785. [7] R. Ulrich, R. Torge, Appl. Opt. 12 (1973) 2901. [8] P.E. Caro, J. Less–Common Met. 16 (1968) 367. [9] D.B.M. Klaasen et al., J. Lumin. 40–41 (1988) 651. [10] M. Buijs, A. Meijerink, G. Blasse, J. Lumin. 37 (1987) 9. [11] C. Linares, J. Phys. 29 (1968) 917. [12] N.C. Chang, J.B. Gruber, J. Chem. Phys. 41 (1964) 3227.
Optical properties of europium-doped Gd2O3 waveguiding thin films prepared by the sol–gel method A. Garcıa-Murillo a,*, C. Le Luyer a, C. Garapon a, C. Dujardin a, E. Bernstein b, C. Pedrini a, J. Mugnier a a
Laboratoire de Physico-Chimie des Mat eriaux Luminescents, CNRS-UMR 5620, Universit e Claude Bernard Lyon 1, 43 Bd du 11 novembre 1918, 69622 Villeurbanne Cedex, France b D epartement de Physique des Mat eriaux, CNRS UMR 5586, Universit e Claude Bernard Lyon 1, 43 Bd du 11 novembre 1918, 69622 Villeurbanne Cedex, France
Abstract Recently, there has been a growth of interest in the preparation of new phosphor films for high-resolution X-ray imaging systems. Europium activated gadolinium oxide is very interesting because of its scintillation properties especially as a red component. The sol–gel method has been used to synthesize europium-doped gadolinium oxide films. The films present waveguiding properties and this special feature is used to study their microstructure by Raman spectroscopy in waveguiding configuration. Structural results, confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM) observations, show that the crystallization in the cubic phase occurs at 700 °C. Optogeometrical parameters were determined with respect to the annealing temperature. After annealing at 1000 °C, very dense europium-doped gadolinium oxide films are obtained with a thickness of 390 nm and a refractive index of 1.88 at 632.8 nm. Spectroscopic results constituted by emission spectra (UV and X-ray excitation) and decay measurements are presented. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Europium-activated phosphors have been known for a long time [1]. In recent years there has been a growth of interest in preparation of phosphors activated by rare earths or with rare earths as host lattice constituents for X-ray imaging systems. One of the most efficient phosphors found
* Corresponding author. Tel.: +33-472-448-338; fax: +33-472431-130. E-mail address: [email protected] (A. GarcıaMurillo).
was europium-doped dense oxide. For example, europium-doped polycrystalline gadolinium oxide as powder is currently used as a red component because of its high effective atomic number. Moreover the development of X-ray systems with improved performance such as high-resolution images requires low diffusion loss. Thus, very dense phosphor films play an important role in high-resolution display devices such as cathode ray tubes (CRTs) [2] used in medical imaging. Gadolinium oxide waveguiding thin films prepared by the sol–gel process have been recently studied and characterized [3]. The sol–gel method has the advantage to provide negligible diffusion
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loss, high quality and purity. Moreover, the planar waveguide properties of the elaborated layers are a proof of their high optical quality and allow their study in convenient waveguiding configuration. The preliminary results obtained on the synthesis and on the microstructure of undoped gadolinium oxide films were used to elaborate europiumdoped gadolinium oxide films for scintillating applications. The sol–gel processing of europium-doped Gd2 O3 films is presented. Differential thermal analysis (DTA) and Fourier transform infrared (FTIR) spectroscopy are at first conducted on the corresponding sol–gel powder in order to forecast the formation and the crystallization temperature dependence of the material in powder form. These results can be used only as a rough indication when studying sol–gel evolution versus annealing temperature because it is well known that sol–gel powder and thin film behavior can be different [4]. The coating layers are then heat-treated at different temperatures and their structural properties are determined using Raman spectroscopy. The microstructure of the more densified layers heattreated at 1000 °C is confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Refractive index and thickness values were determined by m-lines spectroscopy technique. Emission spectra and emission decays were measured to estimate the scintillating potential of these layers.
2. Preparation of Gd2 O3 -doped Eu (5 mol%) films Europium-doped Gd2 O3 films were prepared using the sol–gel process. The precursor used was gadolinium isopropoxide in toluene–isopropanol (3:2) 99.9% (Johnson Mattey, Alfa products) and europium (III) nitrate pentahydrate, 99.9% (Aldrich). The transparency, homogeneity and stability of the precursor solution are predominant criteria in order to ensure reproducible coatings of high optical quality, especially when multiple coatings are necessary to obtain thick films. Nevertheless, the metal alkoxide is highly reactive towards hydrolysis and condensation. It has to be stabilized in order to slow down the reactions and to prevent
precipitation, to control the growth of oxide particles and to form a stable precursor solution. This can be performed via the chemical modification of gadolinium alkoxide by acetylacetone, 99% (Aldrich), a chelating agent frequently used in sol–gel chemistry [5]. The precursor solution was prepared under controlled atmosphere (argon flux) by mixing the components under strong agitation. The final concentration of gadolinium is 50 g/l. The Eu3þ ions were added in the ratio Eu/Gd ¼ 5 mol%. This solution remains stable after storage under argon for one month, which is interesting, viewed from practical and economical aspects. The europium-doped gadolinium solution was filtered through 0:2 lm and deposited on carefully cleaned pure silica (Herasilâ from Heraeus) substrates in a glove box using the dip-coating technique also under argon controlled atmosphere. Ten and 50 layers were deposited to obtain monomode and multimode transverse electric waveguides, respectively. The layers were heattreated at 350 °C for 30 min between each coating under oxygen flow to promote organic burnout and partial densification of the film. The films were finally annealed 1 h at final required temperatures, ranging from 350 to 1000 °C. Crack-free and transparent layers were obtained and studied at room temperature.
3. Sol–gel powder analyses The DTA and thermo-gravimetry (TG) measurements (Fig. 1) were conducted, using a Netzsch-STA 409C apparatus, on the europiumdoped sol–gel powder obtained by drying the precursor solution at 100 °C. The DTA and TG thermograms were recorded at a scan rate of 5 °C/ min in air flux. This study allowed to forecast the crystallization temperature of the material. The small endothermic peak around 100 °C mainly corresponds to the evaporation of volatile organic solvents. The narrow exothermic peak centered at 460 °C arises from the oxidation of organic compounds. The TG curve continuously decreases up to 500 °C indicating the removal of solvents. The large exothermic peaks around 700 °C are assigned to crystallization of the material.
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a 700–1000 °C heat treatment correspond to cubic Gd2 O3 phase [3].
4. Microstructure of Eu (5 mol%)-doped Gd2 O3 films
Fig. 1. DTA and TG thermograms on the 5 mol% Eu3þ :Gd2 O3 sol–gel powder treated from room temperature to 1000 °C, using a ramp of 5 °C/min.
Room temperature IR spectra (Fig. 2) are obtained using a FTIR 2000 Perkin–Elmer on the sol–gel powder treated for 1 h from 100 °C to 1000 °C, in order to complete DTA and TG experiments. The bands located around 1400 and 3400 cm1 are attributed to the presence of solvents as alcohol and toluene, and are characteristic to the gem-dimethyl structure [–CðCH3 Þ2 –] of the alkoxide. These bands are observed on the powder dried at 110 °C. Some of them are still present at 500 °C, but they totally disappear at 700 °C. This observation confirms the removal of acetyl groups and organic oxidation up to 500 °C. The bands observed from 520 to 370 cm1 which appear after
Fig. 2. Room temperature IR spectra of 5 mol% Eu3þ :Gd2 O3 sol–gel powder treated for 1 h from 100 to 1000 °C.
The determination of Eu (5 mol%)-doped Gd2 O3 film microstructure is essential to understand the optical properties and to explain the fluorescence behavior of the films for scintillation applications. As optical properties are strongly linked with structural characteristics, the structural evolution of Eu-doped films with annealing temperature are investigated using Raman spectroscopy. Waveguide Raman spectroscopy (WRS) is conducted at 647.1 nm on films heat-treated from 400 to 900 °C. Details of the WRS experimental set-up have already been described elsewhere [6]. This highly sensitive and non-destructive technique is extremely powerful especially for sol–gel waveguides analysis. At temperature higher than 900 °C, the film loses its waveguiding properties and WRS measurements cannot be carried out. MicroRaman spectroscopy (DilorXY apparatus) is thus used to analyze the local microstructure of a 50 coatings layer annealed at 1000 °C for 1 h (around 400 nm thick, see Section 5) and to compare with the Raman spectrum of the 5 mol% Eu3þ :doped gadolinium oxide powder which we obtain by solid state reaction method. WRS spectra of Eu-doped Gd2 O3 waveguides treated at 400 and 500 °C could not be obtained due to the luminescence of organic compounds remaining in the layer, which hides the Raman signal. WRS spectra obtained on films annealed from 600 to 900 °C are represented on Fig. 3. The Raman spectrum recorded on sample heat-treated at 600 °C indicates that the material transforms from amorphous phase into crystallized phase at 600 °C. Both phases are detected at this temperature. The Raman spectrum of the amorphous phase exhibits two weak broad bands around 90 and 350 cm1 as already observed on amorphous undoped gadolinium films [3]. The weak band at 360 cm1 is attributed to the main strongest band of the Gd2 O3 cubic phase. The amorphous phase
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totally disappears after 700 °C heat treatment. The Raman bands at 93, 117, 313, 359, 442 and 565 cm1 observed on the Raman spectrum of the 5 mol% Eu3þ :Gd2 O3 powder obtained by the solid state reaction method are assigned to cubic phase of gadolinium oxide [3]. These bands are clearly identified on the Eu-doped Gd2 O3 film treated at 1000 °C (Fig. 3). The phase structure of the 1000 °C heat-treated Eu3þ :Gd2 O3 waveguide is confirmed by X-ray analysis and TEM observations. The complete series of Bragg reflections observed on the corresponding XRD pattern (Fig. 4) correspond to the Gd2 O3 cubic phase. Electron microscopy observation of the Eu3þ :Gd2 O3 layer annealed 1 h at 1000 °C is shown on Fig. 5(a). The crystallized film presents a dense and polycrystalline structure made up by randomly oriented crystallites with grain size varying between 30 and 100 nm. The presence of crystallites as great as 100 nm explains the loss of waveguiding properties of the layer annealed at 1000 °C. The electron diffraction pattern of the surface area of the film is represented on Fig. 5(b). The rings correspond to the lattice spacings of the Gd2 O3 cubic phase (JCPDS 431014).
Fig. 3. Room temperature Raman spectra of europium-doped gadolinium oxide films recorded in a waveguiding configuration for layers heat-treated from 600 to 900 °C. Raman spectra of the non-waveguiding layer annealed at 1000 °C and of the cubic powder obtained by solid state reaction method are obtained using a microRaman set-up.
Fig. 4. XRD spectrum of 5 mol% Eu3þ :Gd2 O3 layers deposited on a SiO2 /Si substrate and heat-treated 1 h at 1000 °C.
Fig. 5. TEM observations (a) and electron diffraction pattern (b) of 5 mol% Eu3þ :Gd2 O3 planar waveguide heat-treated 1 h at 1000 °C.
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Fig. 6. Evolution of thickness at 632.8 nm (a) and refractive index (b) of 5 mol% Eu3þ :Gd2 O3 films heat-treated under different annealing temperatures.
5. Opto-geometrical properties The refractive index and the thickness of europium-doped Gd2 O3 films were determined by mlines spectroscopy conducted at 632.8 nm. This method is described elsewhere [7]. For this purpose we used the multimode layers dip coated 50 times. These layers support two transverse electric (TE) modes and two transverse magnetic (TM) modes after the successive 1 h heat treatments from 350 to 1000 °C. Refractive index and thickness versus annealing temperatures determined for TE modes are shown in Fig. 6. A decrease of the thickness from 550 1 nm (350 °C) to 387 1 nm (1000 °C), illustrates the densification of the film. This densification is correlated with an increase of the refractive index from 1:712 0:001 (amorphous phase heat-treated at 350 °C) to 1:877 0:001 (crystalline phase heat-treated at 1000 °C). The increase of the refractive index is a consequence of the organic compounds removal and structural changes in the europium-doped gadolinium oxide films.
6. Spectroscopic properties of Eu (5 mol%)-doped Gd2 O3 films The fluorescence measurements were conducted on the films in order to study the environment of
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Eu3þ ions in the Gd2 O3 sol–gel host lattice as a function of heat treatments. The properties of Gd2 O3 , which we synthesized as thin film by the sol–gel process, are compared to the already known fluorescence properties of Gd2 O3 powders synthesized by solid state reaction method. The cubic structure obtained for the films contain two inequivalent cation sites of C2 and S6 symmetry [8]. The ratio of C2 to S6 site numbers is 3 to 1 but there is no preferential occupation by the Eu3þ ions of these two sites [9]. The luminescence of Eu3þ in Gd2 O3 is very similar to that of extensively studied phosphor Y2 O3 [9]. For 5 mol% Eu3þ -doped Gd2 O3 it essentially consists of transitions from the 5 D0 to the 7 FJ levels as 5 D1 emission is quenched by cross relaxation [10]. The 7 F0;1 and 5 D0;1 energy levels scheme for Eu3þ in both sites are given in [10,11]. Transitions between any two of the levels are governed by different selection rules for the two sites. For the S6 site, which has an inversion center, only weak magnetic dipole transitions are present. For the C2 , site forced electric dipole transitions of higher intensities are observed in addition to the magnetic dipole transitions [12]. 6.1. Fluorescence after UV excitation The emission spectra and fluorescence decays were recorded at room temperature using excitation at 308 nm by an excimer XeCl laser (Lumonics 527). The fluorescence was analyzed with a 1 m monochromator (Hilger Watts), detected by a AsGa photomultiplier (RCA 31084) and an Ortec photoncounter. Fluorescence decays were registered with a Canberra multichannel analyzer. Films were excited tangentially and the fluorescence was collected perpendicularly to the film surface. 6.1.1. Emission spectra Emission spectra were recorded for 50 layers Gd2 O3 film doped with 5 mol% Eu3þ that was submitted to successive 1 h annealing treatments from 350 to 1000 °C. Excitation at 308 nm takes place into the low energy wing of the charge transfer band peaking at 250 nm and also into the 6 P5=2 multiplet of the
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Fig. 7. Room temperature emission spectra of 5 mol% Eu3þ :Gd2 O3 films heat-treated 1 h at different temperatures.
Gd3þ ions [10]. Gadolinium to europium energy transfer and fast relaxation among Eu3þ energy levels populate the 5 D0 emitting level. The emission spectra are shown in Fig. 7. The emission spectra of Eu3þ -doped films heattreated between 350 and 600 °C, are characterized by five broad bands located between: 570–580, 580–603, 603–640, 640–665 and 680–715 nm, which correspond to unresolved 5 D0 ! 7 FJ transitions (J ¼ 0 to 4), respectively. These broad bands confirm the location of the Eu3þ ions in a disordered gadolinium oxide matrix for low annealing temperatures. The emission spectra of the films heat-treated from 700 to 1000 °C change drastically in comparison with those annealed at lower temperature. After 700 °C heat treatment the five broad bands change to sharp emission lines. The same spectrum was observed for conventional polycrystalline europium-doped Gd2 O3 powders, obtained by solid state reaction method. These emission lines presented in Fig. 8 are characteristic of the crystallized cubic phase which remains until 1000 °C. The line positions are in good agreement with the level energies given in [9,10], according to which we made transition assignments. 6.1.2. Fluorescence decays Decay measurements were recorded at room temperature on Eu3þ -doped Gd2 O3 films heat-
Fig. 8. Room temperature emission spectra of 5 mol% Eu3þ :Gd2 O3 films heat-treated 1 h at 700 °C.
treated from 350 to 1000 °C at the wavelength of the predominant 5 D0 ! 7 F2 ðsite C2 Þ emission after excitation at 308 nm (Fig. 9). The emission decays corresponding to the temperatures of 350 and 600 °C, (Eu3þ is in a disordered Gd2 O3 host lattice) are, almost exponential with a mean time constant of about 1 ms. However, a change in the 5 D0 ! 7 F2 fluorescence decay is clearly observed for 500 °C annealing temperature, for which the non-exponential character is more pronounced. This must be related to Eu3þ environment modification induced by organic compounds burnout detected by ATD and
Fig. 9. Dependence on the heat-treatment of the 5 D0 ! 7 F2 emission decay under excitation at 308 nm in 5 mol% Eu3þ :Gd2 O3 films (curves were arbitrarily translated for figure clearness).
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Table 1 Lifetime values determined by double exponential fit of the experimental luminescence decays (k ¼ 611 nm) obtained for Eu3þ -doped Gd2 O3 films as a function of heat-treatments Annealing temperature (°C) 700 800 900 1000
5
D0 ! 7 F2 lifetime (ms)
C2
S6
1.00 1.01 1.02 1.03
– 3.41 3.46 3.49
IR analyses at this annealing temperature. Modification of the broad emission spectra profiles was also noticed, indicating a crystal field distribution evolution. For films heat-treated from 700 to 1000 °C, which have cubic phase as crystallization, the decays are approximately identical and have become non-exponential. They appear as being constituted by two components and may be fitted satisfactorily by the sum of two exponential functions, the lifetimes of which are collected in Table 1. The fast component of 1 ms gives a dominant contribution to the intensity, the slower component of 3.5 ms lifetime corresponding to an about 100 times weaker intensity. The main 1 ms component is due to the C2 site emission in agreement with the literature data [9]. The weak component may correspond to feeding of the C2 sites from the S6 sites by an energy transfer, which is known as very efficient [9]. This attribution is confirmed by the fluorescence decays of the 5 D0 ! 7 F0 line of the site C2 and 5 D0 ! 7 F1 line of the site S6 (Fig. 10). The decay of the 5 D0 ! 7 F1 emission of the site S6 at 581.6 nm is non-exponential as expected in case of energy transfer for the donor decay, with a predominant long time component having the same time constant of about 3.5 ms. This value is similar to literature values for similar concentrations. The decay of the 5 D0 ! 7 F0 emission of the site C2 at 580.6 nm is dominated by a 1 ms component, as for the 5 D0 ! 7 F2 emission at 611.6 nm, the increase in the 3.5 ms component being due to the overlap with the wing of the 5 D0 ! 7 F1 line of the site S6 at 581.6 nm. Due to the complicated level populating processes, induced by excitation at 308 nm we did not try to study more
Fig. 10. Emission decay curves on 5 mol% Eu3þ :Gd2 O3 films heat-treated at 900 °C under excitation at 308 nm for emission wavelengths of 580.6, 581.6 and 611.6 nm.
precisely neither the S6 –C2 energy transfer nor to explain the rise time observed for some decays. Nevertheless, the measured lifetimes, 1 and 3.5 ms for C2 and S6 sites, respectively, which correspond to most of the intensity, are in agreement with literature data reported for crystals. 6.2. Emission spectrum after X-ray excitation For X-ray imagery applications, the most important feature is the scintillation properties of these films. In other words the conversion of absorbed X-rays into visible radiation.
Fig. 11. Room temperature emission spectrum under X-ray excitation of 5 mol% Eu3þ :Gd2 O3 thin films heat-treated 1 h at 1000 °C.
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To test these properties we have measured the emission of europium-doped Gd2 O3 films under X-ray radiation. The emission spectrum presented in Fig. 11 was recorded using an X-ray source operated at 40 kV. The light emitted was analyzed with a monochromator (Jobin Yvon H20) and detected by a photomultiplier (EMI 9789). The film surface was placed 45° of X-ray radiation and fluorescence collected 45° to the film surface. The emission spectrum recorded at room temperature shows the same band around 611 nm as the emission spectrum after UV excitation, this visible emission band is clearly assigned to the 5 D0 ! 7 F2 transition of europium ion. Unfortunately, we could not resolve the spectrum mainly because of the very thin thickness. However, the spectrum of europium-doped gadolinium oxide confirms their scintillation properties.
7. Conclusions Sol–gel process has been used to synthesize europium-doped gadolinium oxide thin films with high optical quality and waveguiding properties. The stability of the sol has allowed to elaborate multimode thin films after 50 coatings, which were used to study the structure and chemical evolution as a function of heat treatments. The increase of refractive index and the decrease of thickness values are representative of crystallization and densification of the films due to increased heat treatment temperature. Good waveguiding properties enable waveguide Raman spectroscopy measurements which indicate that the films after 700 °C annealing are crystallized in cubic phase. The structure was confirmed by X-ray analysis and TEM observations. The crystallization of the film into cubic phase, remains stable up to 1000 °C. The photoluminescence studies evidence that europium-doped gadolinium oxide thin films matrix have the same sharp emission bands as europium-
doped Gd2 O3 powders produced by solid state reaction method, corresponding to the 5 D0 ! 7 FJ transitions (J ¼ 0 to 4). These characteristics emission bands of cubic phase appear at 700 °C by sol–gel process although with conventional method they become visible only after 900 °C. Fluorescence decay properties are consistent with the presence of two sites known for Eu3þ in cubic Gd2 O3 with 5 D0 lifetime of about 1 ms for the C2 site and about 3 ms for the S6 site. Finally, X-ray-induced emission leads to the same 5 D0 ! 7 F2 red emission lines as observed under UV excitation which shows the potentiality of these films as a scintillating material.
Acknowledgements The authors are grateful to EZUS (Universite Lyon I) for financial support and Conacyt-SFERE Program for A.G.-M. scholarship. The authors would like to thank Delphine Pennaneac’h for DTA-TG measurements.
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