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White light generation in Al2O3:Ce3+:Tb3+:Mn2+ films deposited by ultrasonic spray pyrolysis
Thin Solid Films 518 (2010) 5724–5730
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
White light generation in Al2O3:Ce3+:Tb3+:Mn2+ films deposited by ultrasonic spray pyrolysis R. Martínez-Martínez a, E. Álvarez b, A. Speghini c, C. Falcony d,1, U. Caldiño d,⁎ a
Instituto de Física y Matemáticas, Universidad Tecnológica de la Mixteca, Carretera a Acatlima, Km. 2.5, Huajuapan de León, Oaxaca 69000, Mexico Departamento de Física, Universidad de Sonora (UNISON), Boulevard Luis Encinas y Rosales s/n, Hermosillo, Sonora 83000, Mexico DiSTeMeV, Università di Verona, and INSTM, UdR Verona, Via della Pieve 70, I-37029 San Floriano, Verona, Italy d Departamento de Física, Universidad Autónoma Metropolitana-Iztapalapa, PO Box 55-534, México, DF 09340, Mexico b c
a r t i c l e
i n f o
Available online 20 May 2010 Keywords: Aluminium oxide Al2O3 3+ Ce 3+ Tb 2+ Mn Films Spray pyrolysis Photoluminescence Nonradiative energy transfer
a b s t r a c t Aluminium oxide (Al2O3) films doped with CeCl3, TbCl3 and MnCl2 were deposited at 300 °C with the ultrasonic spray pyrolysis technique. The films were analysed using the X-ray diffraction technique and they exhibited a very broad band without any indication of crystallinity, typical of amorphous materials. Sensitization of Tb3+ and Mn2+ ions by Ce3+ ions gives rise to blue, green and red simultaneous emission when the film activated by such ions is excited with UV radiation. The overall efficiency of such energy transfer results to be about 85% upon excitation at 312 nm. Energy transfer from Ce3+ to Tb3+ ions through an electric dipole–quadrupole interaction mechanism appears to be more probable than the electric dipole– dipole one. A strong white light emission for the Al2O3:Ce3+(1.3 at.%):Tb3+(0.2 at.%):Mn2+(0.3 at.%) film under UV excitation is observed. The high efficiency of energy transfer from Ce3+ to Tb3+ and Mn2+ ions, resulting in cold white light emission (x = 0.30 and y = 0.32 chromaticity coordinates) makes the Ce3+, Tb3+ and Mn2+ triply doped Al2O3 film an interesting material for the design of efficient UV pumped phosphors for white light generation. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The preparation of phosphor materials for application in advanced illumination technologies has been the subject of intense research during recent decades. In fact, special attention has been paid to the development of phosphors that can be pumped in the UV and/or blue region (in the 300–400 nm range) because of the necessity of increasing the efficiency in white light emitting solid state devices, which represent an alternate lightning source, as well as a possible next generation of lighting lamps [1]. The Ce3+ ion exhibits broad absorption (excitation) bands in the UV [2], so that it can be pumped by UV light emitting diodes or mercury vapor lamps. White light generation using the simultaneous emission of blue, green and red emitting centers upon UV excitation was attained for the first time in borate based glasses containing Ce3+, Tb3+ and Mn2+ as activators [3]. Aluminium oxide (alumina, Al2O3) is one of the most cost effective and widely used materials in the family of engineering ceramics. The raw materials, from which this high performance technical grade
⁎ Corresponding author. E-mail address: [email protected] (U. Caldiño). 1 On leave from Centro de Investigaciones del IPN, Departamento de Física, 07000 México, DF, Mexico. 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.05.057
ceramic is made, are readily available and reasonably priced, resulting in cheap fabricated alumina shapes. With an excellent combination of properties and an attractive price, it is no surprise that fine grain technical grade alumina has a very wide range of applications. Alumina is characterized by hardness, wear-resistant, excellent dielectric properties, resistance to strong acid and alkali attack, also at elevated temperatures, good thermal conductivity and high stiffness [4]. Moreover, Al2O3 has shown excellent electronic quality, which can be attractive for field effect transistor applications [5]. Because of its wide energy band gap and chemical stability, Al2O3 can be an important component in thin film electroluminescent devices and optical active layers in flat-panel displays [6]. At the present, alumina has turned out to be of great interest for these applications, since it is a good host to incorporate rare earth ions, such as Ce3+ [7,8], Tb3+ [9] and Eu3+ [10], as well as Mn2+ ions [7], which emit through excitation–relaxation processes within their own energy levels. Recently, efficient blue and red emissions were achieved in CeCl3 and MnCl2 doped Al2O3 films deposited by spray pyrolysis (SP) under specific deposition conditions [7]. SP is a low cost deposition technique suitable for large area deposition of thin films [11]. These properties make thin films excellent candidates for applications in advanced illumination technologies. The lighting system technology will experience a very important transformation with the contribution of these thin film materials. From an environmental point of view,
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it is also important to limit significantly the discharge of greenhouse gases reducing the quantity of energy necessary for artificial lighting. Taking into account that both Ce3+ and Tb3+ ions are important activators, so that they can be widely used in phosphors for fluorescent lamps and displays [12], as well as the importance to find efficient luminescent materials for the design of optical devices based on Al2O3, in the present investigation a spectroscopic study of the sensitization of the Tb3+ blue and green luminescence by Ce3+ ions is presented. Moreover, emission of cold white light from Al2O3 films triply doped with Ce3+, Tb3+ and Mn2+ ions has been observed.
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Table 1 Atomic percent content of oxygen, chlorine, aluminium, cerium, terbium and manganese in the undoped and doped films as measured by EDS. Film
Oxygen (at.%)
Chlorine (at.%)
Aluminium (at.%)
Cerium (at.%)
Terbium (at.%)
Manganese (at.%)
AO AOT AOC AOCT AOCTM
62.8 62.5 63.1 69.1 66.0
4.5 4.8 5.4 6.2 5.0
32.7 32.0 29.8 23.5 27.2
– – 1.7 0.9 1.3
– 0.7 – 0.3 0.2
– – – – 0.3
2. Experimental details Al2O3 films doped with Ce3+, Tb3+ and Mn2+ ions were prepared using the ultrasonic SP technique. The spraying solution was a 0.07 M solution of Al2Cl3·6H2O (Aldrich, 99+%) dissolved in de-ionized water (18 MΩcm− 1), in which appropriate amounts of CeCl3, TbCl3 and MnCl2 (Aldrich 99.99+%) were added as doping materials [4], with 5, 1 and 1 at.% concentrations with respect to Al3+ ions, respectively. The spray was produced by means of an ultrasonic generator at a frequency of 0.8 MHz. Filtered air was used as the transport gas, at a flow rate of 10 l min− 1. The solution was sprayed at a rate of 1 ml min− 1. Corning 7059 glass slides were used as substrates. The substrate temperature during the film deposition was 300 °C. The preparation time was 6 min for a film thickness of about 5 μm, as measured with a profilometer, Sloan Dektak IIA. The surface roughness of the films, as measured by the profilometer, turned out to be 0.66 ± 0.03 μm. The crystalline structure of the films was analysed by the X-ray diffraction (XRD) technique using a 1.540 Å (Cu Kα radiation) Siemens D5000 diffractometer, operating at 30 keV with a current between anode and Cu target of 30 mA. The film was mounted on a goniometer and gradually rotated 1°/min while being bombarded with X-rays, producing a diffraction pattern. The chemical composition of the films was measured using energy dispersive spectroscopy (EDS) with a Leica Cambridge Electron Microscope model Stereoscan 440 equipped with a beryllium window X-ray detector, using a Link-Isis software and the ZAF method. The standard for the EDS measurements was the Multielement X-ray Reference Standard (Microspec, Serial 0034, Part. No. 8160-53). This microscope was used to obtain scanning electron microscopy (SEM) images. The electron beam voltage and current were 20 kV and 1000 pA, respectively. Each data point was determined from the average of three measurements on different areas of the film. Photoluminescence measurements were carried out using Perkin– Elmer LS50B and SPEX Fluoro-Max-P spectrophotometers. Emission spectra were recorded upon excitation at 312 nm, which fits to the requirements of an UVP light tube FL15EUV320 or a 310 nm AlGaN/ GaN UV LED chip T039-UVTOP. Lifetime data were obtained using a PTI GL300 and GL302 Dye nitrogen pulsed laser, which produces a pulse of about 600 ps in duration and 0.1 nm bandwidth at 337.1 nm. The resulting transient fluorescence signal was analysed with a Jobin-Yvon monochromator Triax 550 and detected with HORIBA-Jobin Yvon i-Spectrum Two ICCD. All measurements were carried out at room temperature. 3. Results and discussion 3.1. EDS measurements The chemical composition measured from EDS spectra for the films deposited with solution concentrations of 5 at.% of CeCl3, 1 at.% of TbCl3 and 1 at.% of MnCl2 with respect to the Al content are listed in Table 1. The Al2O3 films studied: undoped, Al2O3:TbCl3(1 at.%), Al2O3: CeCl3(5 at.%):TbCl3(1 at.%) and Al2O3:CeCl3(5 at.%): TbCl3(1 at.%):
MnCl2(1 at.%) will be referred to hereafter as AO, AOT, AOCT and AOCTM, respectively. From Table 1, it can be noted that for the undoped sample the aluminium (32.7 at.%), oxygen (62.8 at.%) and chlorine (4.5 at.%) contents are consistent with a stoichiometric Al2O3 film. However, in the Al2O3 films doped with Ce3+, Tb3+ and Mn2+, prepared by adding CeCl3, TbCl3 and MnCl2 to the spraying solution, the aluminium content decreases in correlation with an increase of oxygen and chlorine contents, so that the incorporated chlorine ions could act as charge compensators to preserve the electrical neutrality for the Mn2+ ions substituting Al3+ cations [13]. The at.% content of the Ce3+, Tb3+ and Mn2+ dopants measured by EDS is referenced to the presence of Al, O and Cl, whereas in the case of the spraying solution the at.% concentration of CeCl3, TbCl3 and MnCl2 is referenced only to Al. In consequence, we have found a lower amount of the dopants measured by EDS than that of the dopants mixed in the spraying solution. In Ce3+-doped Al2O3 films deposited by SP the integrated luminescence intensity of Ce3+ as a function of the cerium amount measured by EDS has been previously investigated by Falcony et al. [8]. It was observed that the light emission increases quickly with the amount of Ce3+ going through a maximum, and then it decreases at a Ce3+ concentration above 1.1 at.%. Thus, in the AOCT and AOCTM films (with 5 at.% of CeCl3) we have achieved contents (as measured from EDS) of 0.9 and 1.3 at.% of Ce3+, respectively, which are close to the Ce3+ threshold concentration for maximum intensity of cerium emission. According to this perspective and taking into account that a efficient sensitization of the Mn2+ red emission by Ce3+ ions (close to 100%) was observed in SP deposited Al2O3:CeCl3(5 at.%):MnCl2(1 at.%) films excited at 254 nm [7], then 5 at.% of CeCl3, 1 at.% of TbCl3 and 1 at.% of MnCl2 with respect to the Al content might be appropriate concentrations to achieve a high efficiency of Tb3+ and Mn2+ luminescence sensitized by Ce3+. 3.2. XRD measurements The crystalline structure of the films was analysed by means of XRD. All the films studied exhibited a very broad band without any indication of crystallinity, typical of amorphous materials. Such non-crystalline structure can be observed from Fig. 1, which shows the diffraction pattern of the AOCTM film. In spite of the non-crystalline nature of the studied films, the Ce3+, Tb3+ and Mn2+ ion substitution of Al3+ cations might induce a substantial rearrangement of the local environment of the Al3+ site. This site would rearrange to incorporate the Ce3+ (1.28 Å, in eight-fold coordination), Tb3+ (1.18 Å, in eight-fold coordination) and Mn2+ (0.81 Å, in six-fold coordination) ions, which are much larger than the Al3+ ion (0.68 Å, in six-fold coordination) [14], as it was found for polycrystalline corundum α-Al2O3 activated by Eu3+ and Er3+ ions [15]. Moreover, the charge compensation due to the Mn2+ substitution might be achieved by residual chlorines. In Ce3+ and Mn2+ codoped Al2O3 films, for example, such rearrangement of the nearest environment of the Al3+ ions allows an efficient sensitization of the Mn2+ red luminescence by Ce3+ ions close to 100% upon excitation at 254 nm [7].
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Fig. 1. X-ray diffraction pattern of the AOCTM triply doped film.
3.3. SEM measurements Fig. 2 shows the SEM surface morphology of the investigated samples, from which can be observed rough films with good adherence to the substrate. This fact along with a high chemical stability of the films, allow that they can remain as long as one year in the laboratory atmosphere without visible signs of deterioration. It is conceivable that the growth rate and the surface inhomogeneity of the films would increase on increasing the metal concentrations in the starting solution, as it was found for ZnO thin films prepared by the same technique [16]. The SEM micrographs (Fig. 2) show that the surface morphology is not very homogeneous and well defined in spite of the low solution concentration (0.07 M) of the aluminium precursor used in the spraying solution, probably due to the low temperature (300 °C) employed in the SP process [16]. 3.4. Photoluminescence of Al2O3:Ce3+ and Al2O3:Tb3+ Optical data of the singly doped films of cerium (AOC) and terbium (AOT) are required first to achieve a meaningful interpretation of spectroscopic data in the (AOCT) doubly and (AOCTM) triply doped films. The excitation and emission spectra of the AOC film are shown in Fig. 3. The emission spectrum was recorded with 312 nm excitation. The spectrum consists of a broad band, peaking at 380 nm, which is attributed to the 5d → 4f emission of Ce3+ ions. The excitation spectrum corresponds to the Ce3+ emission monitored at 400 nm. It consists of several bands associated with the 4f → 5d absorption transitions of Ce3+.
Fig. 3. Excitation (dashed line) and emission (solid line) spectra of AOC. The excitation spectrum was monitored at 400 nm. The emission spectrum was recorded with 312 nm excitation.
Decay time measurements performed on the Ce3+ emission in the AOC film were carried out under 337 nm laser pulsed excitation within the Ce3+ 4f → 5d absorption band. The Ce3+ emission decay could be fitted to a simple exponential, with a lifetime value of 25 ± 1 ns. This value was found to be independent of the emission wavelength selected for the decay recording. The excitation and emission spectra recorded for the AOT film are shown in Fig. 4 and inset of Fig. 4, respectively. The excitation spectrum was monitored at 545 nm, into the 5D4 → 7F5 emission transition of Tb3+. It consists of characteristic Tb3+ excitation bands, which correspond to transitions from the 7F6(4f8) ground state to higher energy states of the 4f8 and 4f75d configurations. The two excitation bands associated with the 4f8 → 4f75d electric dipole allowed transitions, located at around 239 nm and 267 nm, appear to be significantly more intense than those associated with the 4f 8 → 4f 8 forbidden transitions centred at 283 nm (7F6 → 5I8, 5F4, 5F5, 5 H4), 303 nm (7F6 → 5H5, 5H6), 319 nm (7F6 → 5H7, 5D1), 339 nm ( 7 F 6 → 5 L 7,8 , 5 G 3 ), 351 nm ( 7 F 6 → 5 L 9 , 5 D 2 , 5 G5 ) and 373 nm (7F6 → 5 L10, 5G6, 5D3). The emission spectrum (inset of Fig. 4) was recorded under excitation into the 4f 8 → 4f75d absorption band at 240 nm. It consists of several bands associated with 4f 8 → 4f 8 transitions from the 5D4 level to the 7F6, 7F5, 7F4 and 7F3 multiplets, which are centred at 488, 545, 585 and 620 nm, respectively. No emissions from the 5D3 level are observed, which suggests that at 1 at.% concentration of TbCl3 (0.7 at.% of Tb3+ measured by EDS) there is a non-radiative relaxation from the 5D3 to the 5D4 level promoted by the excitation from the 7F6 to the 7F0 level through a cross-relaxation process [17]. Thus, the 5D3 to 7FJ transitions are quenched by the energy
Fig. 2. SEM micrograph of surface morphology of: (a) undoped, (b) AOC, (c) AOCT and (d) AOCTM films.
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Fig. 4. Excitation and emission (inset) spectra of AOT. The excitation spectrum was monitored at 545 nm. The emission spectrum was recorded after excitation at 240 nm.
transfer of identical centers, 5D3 + 7F6 → 5D4 + 7F0, so that only 5D4 to 7FJ emissions are observed. 3.5. Sensitization of Tb3+ by Ce3+ ions in the Cerium and Terbium codoped film The emission spectrum of the Al2O3 film doped with Ce3+ (0.9 at.%) and Tb3+ (0.3 at.%) ions (AOCT film) recorded with 312 nm excitation, within the 4f→ 5d absorption band of Ce3+, is shown in Fig. 5. The spectrum consists of the 5d→ 4f UV-blue emission broad band assigned to transitions of Ce3+ ions, and the 5D4 → 7F6,5,4,3 green-red emissions assigned to transitions of Tb3+ ions. The global emission of these emissions was characterized by its chromaticity coordinates in a CIE diagram (Fig. 6), resulting in blue green-white light, with x = 0.25 and y = 0.33, when the film is excited at 312 nm. Fig. 7 portrays the excitation spectra recorded for the AOT and AOCT films. The excitation spectra were monitored at 545 nm, inside the 5 D4 → 7F5 emission of Tb3+. The excitation spectrum displayed by the codoped film exhibits a broad band similar to the 4f → 5d Ce3+ absorption band observed in the AOC excitation spectrum (see Fig. 3),
Fig. 5. Emission spectrum of the AOCT film excited at 312 nm.
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Fig. 6. Chromaticity coordinates characteristic of the emissions observed in the AOCT (solid circle symbol) and AOCTM (circle symbol) films excited at 312 nm.
which evidences a Ce3+→Tb3+ energy transfer. In fact, the 5d–4f electric dipole allowed transitions of Ce3+ are several orders of magnitude stronger than the 4f intraconfigurational transitions of Tb3+, such that Ce3+ ions can strongly absorb UV radiation and transfer a part of its energy to Tb3+ ions. Besides, the Ce3+ emission overlaps the 7 F6 → 5 L7,8,5G3, 7F6 → 5 L9,5D2,5G5, 7F6 → 5 L10,5G6,5D3 and 7F6 → 5D4 absorption (excitation) transitions of Tb3+, as it can be appreciated from the spectra shown in Fig. 8. Lifetime measurements of the 5D4 → 7F6,5 emissions of Tb3+ and Ce3+ emission in the AOCT doubly-doped film were performed monitoring the Ce3+ (420 nm) and Tb3+ (488 and 545 nm) emissions after 337 nm pulsed laser excitation within the Ce3+ 4f → 5d absorption transition. The 5D4 level emission decay was found to consist of a singleexponential decay with a lifetime value of 1.2 ± 0.1 ms. The decay time curve of the Ce3+ emission was analysed to determine the nature of the Ce3+–Tb3+ interaction and the Ce3+ → Tb3+ energy transfer microparameter. According to previous models (Yokota-Tanimoto [18] and
Fig. 7. Excitation spectra monitored at 545 nm for the AOT (dashed curve) and AOCT (solid curve) films.
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be concluded that an electric dipole–quadrupole (d–q) interaction mechanism could be dominant in the Ce3+ → Tb3+ energy transfer process. Moreover, such short range interaction mechanism, occurring in Ce3+-Tb3+ clusters, is induced by the electric dipole allowed 4f → 5d transition of Ce3+ and the 4f → 4f forbidden transitions of Tb3+ given their very short and long decay time, respectively. For such interaction, the time dependence of the Ce3+ emission can be fitted to the relation: t 3=8 KD t ; Iðt Þ = I0 exp o γ8 t τD
ð2Þ
where γ8 is given by: γ8 =
Fig. 8. Overlap region of Tb3+ absorptions (solid line) and Ce3+ emission (dashed line). The Tb3+ absorption spectrum was taken from the AOT excitation spectrum displayed in Fig. 4.
Inokuti-Hirayama [19]), considering a short-time excitation and the presence of energy migration among donors, such decay time curve follows the expression: t 3=S KD t ; Iðt Þ = I0 exp o γS t τD
where I0 is the initial intensity at t = 0, τDo is the decay time of Ce3+ (donor ions) in absence of Tb3+ (activator ions), γS is a measure of the direct Ce3+ → Tb3+ energy transfer, KD represents the migration rate of excitation energy between donor ions, and S is the multipolar interaction parameter. The time dependence of the Ce3+ emission for the AOCT film excited at 337 nm was fitted to Eq. (1) with τDo = 25 ns (measured from the decay time of the Ce3+ emission in the AOC film, see Section 3.4) and assuming dipole–dipole (S = 6), dipole–quadrupole (S = 8) and quadrupole–quadrupole (S = 10) electric couplings. The best agreement between the decay data and the theoretical fit given by Eq. (1) is attained for S = 8, as it can be appreciated from Fig. 9. Therefore, it can
PDA =
i 3ħ4 c4 fq λ2D QA h 4 ∫ FD ðEÞFA ðEÞ = E dE; 4πn4 τoD fd R8DA
ð4Þ
where τDo is the donor intrinsic lifetime in absence of the activator, QA is the oscillator strength of the activator absorption transitions, fq and fd are the oscillator strengths of the activator ion electric quadrupole and dipole transitions, respectively, λD is the wavelength position of the donor emission, and ∫ [FD(E)FA(E)/E4]dE is the spectral overlap integral between the normalised line-shape functions of the donor emission FD(E) and activator absorption FA(E), with E being the average energy of the overlapping transition. The remaining symbols in Eq. (4) have their usual meaning. In the AOCT film the donor-activator average interaction distance (RDA ≈ 12 Å) was estimated from the amounts of Ce3+ (8.7 × 1020 ions/ cm3) and Tb3+ (2.6 × 1020 ions/cm3) measured by EDS assuming a random ion distribution. The optical absorption spectrum of Tb3+ ions in the overlap region with the Ce3+ emission was hardly detectable due to the very weak intensity of the forbidden 4f–4f transitions. Then, in Eq. (4) the QA integrated absorption coefficient of Tb3+ was estimated using the relation derived by Blasse, QA = 4.8 × 10− 20 eV m2 • fd [22]. In the region of Ce3+ emission (340–440 nm) the fd electric dipole oscillator strength of the Tb3+ ion is very low (3× 10− 7 [23]). The overlap integral in Eq. (4) was calculated using the normalised lineshape functions of the cerium emission FD(E) and terbium absorptions FA(E) in the overlap region. Using n=1.64, QA =1.44×10− 26 eV m2, fq/fd ∼ 10− 2–10− 3 [23], RDA ≈ 12 Å and the value estimated for the overlap integral (6.3 × 10− 3 eV− 5) in Eq. (4) is found that dq PDA ≈6×107–6×106 s− 1, within which lies the rate of Ce3+ radiative emission (1/τDo =4×107 s− 1). For comparison the transfer rate for an dd electric dipole–dipole (d–d) interaction mechanism, PDA , was estimated dq from its relation with the PDA rate [24]: dd
PDA =
Fig. 9. Time dependence of the Ce3+ emission measured for the AOCT film. Solid line is the best fit to Eq. (1) for electric dipole–quadrupole (S = 8) interaction.
ð3Þ
being ρA the activator density and CDA the energy transfer microparameter. From the fitting of the decay curve in Fig. 9 to Eq. (2) is obtained that γ8 = 45.7 s− 3/8 and KD = 320 s− 1. Thus, from Eq. (3) CDA resulted to be about 8.2 × 10− 53 cm8/s using the Tb3+ concentration measured by EDS (ρA ≈ 2.6 × 1020 ions/cm3). This CDA value is higher than that (3.8 × 10− 54 cm8/s) reported for a HfO2:CeCl3(3 at.%):TbCl3(6 at.%) film [20], which involves that the ability of the Ce3+–Tb3+ system for energy transfer in aluminium oxide might be even better than in hafnium oxide. dq The transfer rate PDA for an electric d–q interaction mechanism is given by [21]: dq
ð1Þ
4π 3=8 ð1:43ÞρA CDA ; 3
R2DA fd dq PDA : λ2D fq
ð5Þ
dd rate for an interaction distance of 12 Å turned out to be The PDA 4 −1 6 × 10 s , which is quite smaller than the rate of Ce3+ radiative emission. Therefore, the Ce3+ → Tb3+ energy transfer through an
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electric d–q interaction mechanism appears to be more probable than the electric dipole–dipole one.
3.6. Sensitization of Tb3+ and Mn2+ by Ce3+ ions in the Cerium, Terbium and Manganese triply doped film In addition to Tb3+ ions, Mn2+ ions can also be sensitized by Ce3+ ions, since there exists an overlapping between the cerium emission and 6A1(S) → 4 T1(G), 4 T2(G), 4A1(G), 4E(G), 4 T2(D), 4E(D) and 4 T1(P) absorptions (excitations) of manganese [7]. Fig. 10 displays the emission spectrum of the Al2O3 film triply doped with Ce3+ (1.3 at.%), Tb3+ (0.2 at.%) and Mn2+ (0.3 at.%) ions (AOCTM film) recorded with 312 nm excitation within the Ce3+ 4f → 5d absorption transition. The spectrum exhibits, in addition to the UV-blue broad band associated with the Ce3 + 5d → 4f emission, four narrow green-red bands due to the 5D4 → 7F6,5,4,3 emissions of Tb3+ and a red broad band due to the 4 T1(G) → 6A1(S) emission of Mn2+. The combination of these emissions results in white light, with x = 0.30 and y = 0.32 chromaticity coordinates, as it can be appreciated from the CIE diagram portrayed in Fig. 6. Such chromaticity coordinates correspond to cold white light with a colour temperature of about 7320 K, and its deviation from the planckian black-body radiator locus is small (0.005). Cold white light have potential applications in public lighting. The emission spectrum recorded for the AOC film (with 1.7 at.% of Ce3+) measured in the same experimental conditions, is also displayed in Fig. 10 for comparison. It is evident that the addition of Tb3+ and Mn2+ in the Ce3+ doped film causes a strong decrease of the cerium overall emission. This fact provides evidence that energy transfer from Ce3+ to Tb3+ and Mn2+ ions takes place by means of a non-radiative process. Fig. 11 shows the excitation spectra of the AOCTM film monitored at (a) 645 nm (inside the 4 T1(G) → 6A1(S) emission of Mn2+) and (b) 545 nm (inside the 5D4 → 7F5 emission of Tb3+). In addition to the (6A1(S) → 4A1(G), 4E(G), 6A1(S) → 4 T2(G) and 6A1(S) → 4 T1(G)) Mn2+ and (7F6 → 5 L10,5G6,5D3) Tb3+ absorption (excitation) transitions, the 4f→ 5d Ce3+ absorption (excitation) band (see Fig. 3) is also observed, which evidences an energy transfer from Ce3+ to Tb3+ and Mn2+ ions. Decay time measurements of the Ce3+ emission in the AOCTM film were performed monitoring the emission at 420 nm after 337 nm laser pulsed excitation. The Ce3+ emission decay is simple exponen-
Fig. 10. Emission spectra of the AOCTM (solid line) and AOC (dashed line) films excited at 312 nm. Note that the AOCTM emission spectrum is multiplied by 5.
Fig. 11. Excitation spectra of the AOCTM film monitored at (a) 645 nm and (b) 545 nm.
tial, with a lifetime value of 20 ± 1 ns. The increase in the Ce3+ emission decay rate on codoping with terbium and manganese suggests a non-radiative energy transfer from Ce3+ to Tb3+ and Mn2+ ions. In order to evaluate whether Ce3+ ion acts as a good donor to Tb3+ and Mn2+ ions the η efficiency of energy transfer from Ce3+ to Tb3+ and Mn2+ ions was measured from the intensities of the donor emission (Ce3+) in the presence (ID) and absence (IoD) of the activators (Tb3+ and Mn2+). Using the cerium emission spectra portrayed in Fig. 10 η( = 1 − ID/IoD [25]) resulted in being about 85% when the film is excited at 312 nm. The high efficiency of Ce3+ → Tb3+ and Ce3+ → Mn2+ energy transfer, along with the possibility of cold white light generation, makes the Ce3+, Tb3+ and Mn2+ triply doped Al2O3 films an interesting material for the design of efficient UV pumped phosphors for cold white light emission. Lifetime measurements of the 5D4 level of Tb3+ and 4 T1(G) level of Mn2+ were carried out with Ce3+ excitation at 337 nm. The 5 D4 → 7F6,5 emission decay (monitored at 488 and 545 nm) was found to consist of a single-exponential decay with a lifetime value of 0.9 ± 0.1 ms, which is comparable to that measured for the same Tb3+ emissions in the AOCT film. The Mn2+ emission decay (monitored at 645 nm) was found to consist of a single-exponential decay with a lifetime value of 1.9 ± 0.2 ms, which is comparable to that (1.7 ± 0.3 ms) previously measured for the Mn2+ emission in a Al2O3:Ce3+(2.7 at.%):Mn2+(0.2 at.%) film excited through Ce3+ ions at 320 nm [7]. It is worth noting that upon excitation at 337 nm, the donor emission is partially reabsorbed by the same Ce3+ ions, due to an overlap between the Ce3+ emission and absorption profiles. In fact, from Fig. 3 it is evident that the high energy side of the emission (in the 330–380 nm region) overlaps with the low energy side of the absorption profile. The photons involved in this auto-absorption process could be partly dissipated, as a result of the energy migration between Ce3+ ions, as it was estimated for
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the AOCT film (KD = 320 s− 1), and subsequent transfer to traps [26] can lead to a luminescence quenching. The long lifetime measured for the Mn2+ and Tb3+ emissions is in accordance with the forbidden nature of the manganese 3d → 3d and terbium 4f → 4f transitions. Thus, in the AOCTM amorphous film is expected that the efficient energy transfer from Ce3+ to Mn2+ and Tb3+ ions could occur through electric d–q interaction mechanisms in Ce3+–Mn2+ and Ce3+–Tb3+ clusters, as it was demonstrated for the Ce3+ → Mn2+ and Ce3+ → Tb3+ energy transfers observed in Al2O3:Ce3+(2.7 at.%):Mn2+(0.2 at.%) [7] and AOCT amorphous films, respectively. 4. Conclusion Films of aluminium oxide activated with only Ce3+, with Ce3+ and Tb , or with Ce3+, Tb3+ and Mn2+, were prepared using the ultrasonic spray pyrolysis technique. The crystalline structure of the samples was monitored by the XRD technique. All the films under investigation exhibited a very broad band without any indication of crystallinity, typical of amorphous materials. Sensitization of Tb3+ and Mn2+ by Ce3+ allows a simultaneous emission of these ions under UV excitation (312 nm). The Ce3+ → Tb3+ energy transfer through an electric dipole– quadrupole interaction mechanism appears to be more probable than the electric dipole–dipole one. The possibility of white light emission by the Al2O3:Ce3+(1.3 at.%):Tb3+(0.2 at.%):Mn2+(0.3 at.%) film is demonstrated. The overall efficiency of energy transfer from Ce3+ to Tb3+ and Mn2+ ions measured from the Ce3+ emission intensities in the absence and presence of the activators (Tb3+ and Mn2+) resulted to be about 85% upon excitation at 312 nm. This high efficiency of energy transfer, resulting in cold white light emission (x = 0.30 and y = 0.32 chromaticity coordinates and 7320 K colour temperature), makes the Ce3+, Tb3+ and Mn2+ triply doped film an interesting material for the design of efficient UV pumped phosphors for generation of cold white light. 3+
Acknowledgments This work was supported by the CONACyT under project contract 78802-2F and the ICIT-DF. We would like to thank CBI chemical
laboratory of the Universidad Autónoma Metropolitana-Iztapalapa for sharing its equipment. The authors thank Z. Rivera, M. Guerrero and A.B. Soto for their technical assistance.
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
White light generation in Al2O3:Ce3+:Tb3+:Mn2+ films deposited by ultrasonic spray pyrolysis R. Martínez-Martínez a, E. Álvarez b, A. Speghini c, C. Falcony d,1, U. Caldiño d,⁎ a
Instituto de Física y Matemáticas, Universidad Tecnológica de la Mixteca, Carretera a Acatlima, Km. 2.5, Huajuapan de León, Oaxaca 69000, Mexico Departamento de Física, Universidad de Sonora (UNISON), Boulevard Luis Encinas y Rosales s/n, Hermosillo, Sonora 83000, Mexico DiSTeMeV, Università di Verona, and INSTM, UdR Verona, Via della Pieve 70, I-37029 San Floriano, Verona, Italy d Departamento de Física, Universidad Autónoma Metropolitana-Iztapalapa, PO Box 55-534, México, DF 09340, Mexico b c
a r t i c l e
i n f o
Available online 20 May 2010 Keywords: Aluminium oxide Al2O3 3+ Ce 3+ Tb 2+ Mn Films Spray pyrolysis Photoluminescence Nonradiative energy transfer
a b s t r a c t Aluminium oxide (Al2O3) films doped with CeCl3, TbCl3 and MnCl2 were deposited at 300 °C with the ultrasonic spray pyrolysis technique. The films were analysed using the X-ray diffraction technique and they exhibited a very broad band without any indication of crystallinity, typical of amorphous materials. Sensitization of Tb3+ and Mn2+ ions by Ce3+ ions gives rise to blue, green and red simultaneous emission when the film activated by such ions is excited with UV radiation. The overall efficiency of such energy transfer results to be about 85% upon excitation at 312 nm. Energy transfer from Ce3+ to Tb3+ ions through an electric dipole–quadrupole interaction mechanism appears to be more probable than the electric dipole– dipole one. A strong white light emission for the Al2O3:Ce3+(1.3 at.%):Tb3+(0.2 at.%):Mn2+(0.3 at.%) film under UV excitation is observed. The high efficiency of energy transfer from Ce3+ to Tb3+ and Mn2+ ions, resulting in cold white light emission (x = 0.30 and y = 0.32 chromaticity coordinates) makes the Ce3+, Tb3+ and Mn2+ triply doped Al2O3 film an interesting material for the design of efficient UV pumped phosphors for white light generation. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The preparation of phosphor materials for application in advanced illumination technologies has been the subject of intense research during recent decades. In fact, special attention has been paid to the development of phosphors that can be pumped in the UV and/or blue region (in the 300–400 nm range) because of the necessity of increasing the efficiency in white light emitting solid state devices, which represent an alternate lightning source, as well as a possible next generation of lighting lamps [1]. The Ce3+ ion exhibits broad absorption (excitation) bands in the UV [2], so that it can be pumped by UV light emitting diodes or mercury vapor lamps. White light generation using the simultaneous emission of blue, green and red emitting centers upon UV excitation was attained for the first time in borate based glasses containing Ce3+, Tb3+ and Mn2+ as activators [3]. Aluminium oxide (alumina, Al2O3) is one of the most cost effective and widely used materials in the family of engineering ceramics. The raw materials, from which this high performance technical grade
⁎ Corresponding author. E-mail address: [email protected] (U. Caldiño). 1 On leave from Centro de Investigaciones del IPN, Departamento de Física, 07000 México, DF, Mexico. 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.05.057
ceramic is made, are readily available and reasonably priced, resulting in cheap fabricated alumina shapes. With an excellent combination of properties and an attractive price, it is no surprise that fine grain technical grade alumina has a very wide range of applications. Alumina is characterized by hardness, wear-resistant, excellent dielectric properties, resistance to strong acid and alkali attack, also at elevated temperatures, good thermal conductivity and high stiffness [4]. Moreover, Al2O3 has shown excellent electronic quality, which can be attractive for field effect transistor applications [5]. Because of its wide energy band gap and chemical stability, Al2O3 can be an important component in thin film electroluminescent devices and optical active layers in flat-panel displays [6]. At the present, alumina has turned out to be of great interest for these applications, since it is a good host to incorporate rare earth ions, such as Ce3+ [7,8], Tb3+ [9] and Eu3+ [10], as well as Mn2+ ions [7], which emit through excitation–relaxation processes within their own energy levels. Recently, efficient blue and red emissions were achieved in CeCl3 and MnCl2 doped Al2O3 films deposited by spray pyrolysis (SP) under specific deposition conditions [7]. SP is a low cost deposition technique suitable for large area deposition of thin films [11]. These properties make thin films excellent candidates for applications in advanced illumination technologies. The lighting system technology will experience a very important transformation with the contribution of these thin film materials. From an environmental point of view,
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it is also important to limit significantly the discharge of greenhouse gases reducing the quantity of energy necessary for artificial lighting. Taking into account that both Ce3+ and Tb3+ ions are important activators, so that they can be widely used in phosphors for fluorescent lamps and displays [12], as well as the importance to find efficient luminescent materials for the design of optical devices based on Al2O3, in the present investigation a spectroscopic study of the sensitization of the Tb3+ blue and green luminescence by Ce3+ ions is presented. Moreover, emission of cold white light from Al2O3 films triply doped with Ce3+, Tb3+ and Mn2+ ions has been observed.
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Table 1 Atomic percent content of oxygen, chlorine, aluminium, cerium, terbium and manganese in the undoped and doped films as measured by EDS. Film
Oxygen (at.%)
Chlorine (at.%)
Aluminium (at.%)
Cerium (at.%)
Terbium (at.%)
Manganese (at.%)
AO AOT AOC AOCT AOCTM
62.8 62.5 63.1 69.1 66.0
4.5 4.8 5.4 6.2 5.0
32.7 32.0 29.8 23.5 27.2
– – 1.7 0.9 1.3
– 0.7 – 0.3 0.2
– – – – 0.3
2. Experimental details Al2O3 films doped with Ce3+, Tb3+ and Mn2+ ions were prepared using the ultrasonic SP technique. The spraying solution was a 0.07 M solution of Al2Cl3·6H2O (Aldrich, 99+%) dissolved in de-ionized water (18 MΩcm− 1), in which appropriate amounts of CeCl3, TbCl3 and MnCl2 (Aldrich 99.99+%) were added as doping materials [4], with 5, 1 and 1 at.% concentrations with respect to Al3+ ions, respectively. The spray was produced by means of an ultrasonic generator at a frequency of 0.8 MHz. Filtered air was used as the transport gas, at a flow rate of 10 l min− 1. The solution was sprayed at a rate of 1 ml min− 1. Corning 7059 glass slides were used as substrates. The substrate temperature during the film deposition was 300 °C. The preparation time was 6 min for a film thickness of about 5 μm, as measured with a profilometer, Sloan Dektak IIA. The surface roughness of the films, as measured by the profilometer, turned out to be 0.66 ± 0.03 μm. The crystalline structure of the films was analysed by the X-ray diffraction (XRD) technique using a 1.540 Å (Cu Kα radiation) Siemens D5000 diffractometer, operating at 30 keV with a current between anode and Cu target of 30 mA. The film was mounted on a goniometer and gradually rotated 1°/min while being bombarded with X-rays, producing a diffraction pattern. The chemical composition of the films was measured using energy dispersive spectroscopy (EDS) with a Leica Cambridge Electron Microscope model Stereoscan 440 equipped with a beryllium window X-ray detector, using a Link-Isis software and the ZAF method. The standard for the EDS measurements was the Multielement X-ray Reference Standard (Microspec, Serial 0034, Part. No. 8160-53). This microscope was used to obtain scanning electron microscopy (SEM) images. The electron beam voltage and current were 20 kV and 1000 pA, respectively. Each data point was determined from the average of three measurements on different areas of the film. Photoluminescence measurements were carried out using Perkin– Elmer LS50B and SPEX Fluoro-Max-P spectrophotometers. Emission spectra were recorded upon excitation at 312 nm, which fits to the requirements of an UVP light tube FL15EUV320 or a 310 nm AlGaN/ GaN UV LED chip T039-UVTOP. Lifetime data were obtained using a PTI GL300 and GL302 Dye nitrogen pulsed laser, which produces a pulse of about 600 ps in duration and 0.1 nm bandwidth at 337.1 nm. The resulting transient fluorescence signal was analysed with a Jobin-Yvon monochromator Triax 550 and detected with HORIBA-Jobin Yvon i-Spectrum Two ICCD. All measurements were carried out at room temperature. 3. Results and discussion 3.1. EDS measurements The chemical composition measured from EDS spectra for the films deposited with solution concentrations of 5 at.% of CeCl3, 1 at.% of TbCl3 and 1 at.% of MnCl2 with respect to the Al content are listed in Table 1. The Al2O3 films studied: undoped, Al2O3:TbCl3(1 at.%), Al2O3: CeCl3(5 at.%):TbCl3(1 at.%) and Al2O3:CeCl3(5 at.%): TbCl3(1 at.%):
MnCl2(1 at.%) will be referred to hereafter as AO, AOT, AOCT and AOCTM, respectively. From Table 1, it can be noted that for the undoped sample the aluminium (32.7 at.%), oxygen (62.8 at.%) and chlorine (4.5 at.%) contents are consistent with a stoichiometric Al2O3 film. However, in the Al2O3 films doped with Ce3+, Tb3+ and Mn2+, prepared by adding CeCl3, TbCl3 and MnCl2 to the spraying solution, the aluminium content decreases in correlation with an increase of oxygen and chlorine contents, so that the incorporated chlorine ions could act as charge compensators to preserve the electrical neutrality for the Mn2+ ions substituting Al3+ cations [13]. The at.% content of the Ce3+, Tb3+ and Mn2+ dopants measured by EDS is referenced to the presence of Al, O and Cl, whereas in the case of the spraying solution the at.% concentration of CeCl3, TbCl3 and MnCl2 is referenced only to Al. In consequence, we have found a lower amount of the dopants measured by EDS than that of the dopants mixed in the spraying solution. In Ce3+-doped Al2O3 films deposited by SP the integrated luminescence intensity of Ce3+ as a function of the cerium amount measured by EDS has been previously investigated by Falcony et al. [8]. It was observed that the light emission increases quickly with the amount of Ce3+ going through a maximum, and then it decreases at a Ce3+ concentration above 1.1 at.%. Thus, in the AOCT and AOCTM films (with 5 at.% of CeCl3) we have achieved contents (as measured from EDS) of 0.9 and 1.3 at.% of Ce3+, respectively, which are close to the Ce3+ threshold concentration for maximum intensity of cerium emission. According to this perspective and taking into account that a efficient sensitization of the Mn2+ red emission by Ce3+ ions (close to 100%) was observed in SP deposited Al2O3:CeCl3(5 at.%):MnCl2(1 at.%) films excited at 254 nm [7], then 5 at.% of CeCl3, 1 at.% of TbCl3 and 1 at.% of MnCl2 with respect to the Al content might be appropriate concentrations to achieve a high efficiency of Tb3+ and Mn2+ luminescence sensitized by Ce3+. 3.2. XRD measurements The crystalline structure of the films was analysed by means of XRD. All the films studied exhibited a very broad band without any indication of crystallinity, typical of amorphous materials. Such non-crystalline structure can be observed from Fig. 1, which shows the diffraction pattern of the AOCTM film. In spite of the non-crystalline nature of the studied films, the Ce3+, Tb3+ and Mn2+ ion substitution of Al3+ cations might induce a substantial rearrangement of the local environment of the Al3+ site. This site would rearrange to incorporate the Ce3+ (1.28 Å, in eight-fold coordination), Tb3+ (1.18 Å, in eight-fold coordination) and Mn2+ (0.81 Å, in six-fold coordination) ions, which are much larger than the Al3+ ion (0.68 Å, in six-fold coordination) [14], as it was found for polycrystalline corundum α-Al2O3 activated by Eu3+ and Er3+ ions [15]. Moreover, the charge compensation due to the Mn2+ substitution might be achieved by residual chlorines. In Ce3+ and Mn2+ codoped Al2O3 films, for example, such rearrangement of the nearest environment of the Al3+ ions allows an efficient sensitization of the Mn2+ red luminescence by Ce3+ ions close to 100% upon excitation at 254 nm [7].
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Fig. 1. X-ray diffraction pattern of the AOCTM triply doped film.
3.3. SEM measurements Fig. 2 shows the SEM surface morphology of the investigated samples, from which can be observed rough films with good adherence to the substrate. This fact along with a high chemical stability of the films, allow that they can remain as long as one year in the laboratory atmosphere without visible signs of deterioration. It is conceivable that the growth rate and the surface inhomogeneity of the films would increase on increasing the metal concentrations in the starting solution, as it was found for ZnO thin films prepared by the same technique [16]. The SEM micrographs (Fig. 2) show that the surface morphology is not very homogeneous and well defined in spite of the low solution concentration (0.07 M) of the aluminium precursor used in the spraying solution, probably due to the low temperature (300 °C) employed in the SP process [16]. 3.4. Photoluminescence of Al2O3:Ce3+ and Al2O3:Tb3+ Optical data of the singly doped films of cerium (AOC) and terbium (AOT) are required first to achieve a meaningful interpretation of spectroscopic data in the (AOCT) doubly and (AOCTM) triply doped films. The excitation and emission spectra of the AOC film are shown in Fig. 3. The emission spectrum was recorded with 312 nm excitation. The spectrum consists of a broad band, peaking at 380 nm, which is attributed to the 5d → 4f emission of Ce3+ ions. The excitation spectrum corresponds to the Ce3+ emission monitored at 400 nm. It consists of several bands associated with the 4f → 5d absorption transitions of Ce3+.
Fig. 3. Excitation (dashed line) and emission (solid line) spectra of AOC. The excitation spectrum was monitored at 400 nm. The emission spectrum was recorded with 312 nm excitation.
Decay time measurements performed on the Ce3+ emission in the AOC film were carried out under 337 nm laser pulsed excitation within the Ce3+ 4f → 5d absorption band. The Ce3+ emission decay could be fitted to a simple exponential, with a lifetime value of 25 ± 1 ns. This value was found to be independent of the emission wavelength selected for the decay recording. The excitation and emission spectra recorded for the AOT film are shown in Fig. 4 and inset of Fig. 4, respectively. The excitation spectrum was monitored at 545 nm, into the 5D4 → 7F5 emission transition of Tb3+. It consists of characteristic Tb3+ excitation bands, which correspond to transitions from the 7F6(4f8) ground state to higher energy states of the 4f8 and 4f75d configurations. The two excitation bands associated with the 4f8 → 4f75d electric dipole allowed transitions, located at around 239 nm and 267 nm, appear to be significantly more intense than those associated with the 4f 8 → 4f 8 forbidden transitions centred at 283 nm (7F6 → 5I8, 5F4, 5F5, 5 H4), 303 nm (7F6 → 5H5, 5H6), 319 nm (7F6 → 5H7, 5D1), 339 nm ( 7 F 6 → 5 L 7,8 , 5 G 3 ), 351 nm ( 7 F 6 → 5 L 9 , 5 D 2 , 5 G5 ) and 373 nm (7F6 → 5 L10, 5G6, 5D3). The emission spectrum (inset of Fig. 4) was recorded under excitation into the 4f 8 → 4f75d absorption band at 240 nm. It consists of several bands associated with 4f 8 → 4f 8 transitions from the 5D4 level to the 7F6, 7F5, 7F4 and 7F3 multiplets, which are centred at 488, 545, 585 and 620 nm, respectively. No emissions from the 5D3 level are observed, which suggests that at 1 at.% concentration of TbCl3 (0.7 at.% of Tb3+ measured by EDS) there is a non-radiative relaxation from the 5D3 to the 5D4 level promoted by the excitation from the 7F6 to the 7F0 level through a cross-relaxation process [17]. Thus, the 5D3 to 7FJ transitions are quenched by the energy
Fig. 2. SEM micrograph of surface morphology of: (a) undoped, (b) AOC, (c) AOCT and (d) AOCTM films.
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Fig. 4. Excitation and emission (inset) spectra of AOT. The excitation spectrum was monitored at 545 nm. The emission spectrum was recorded after excitation at 240 nm.
transfer of identical centers, 5D3 + 7F6 → 5D4 + 7F0, so that only 5D4 to 7FJ emissions are observed. 3.5. Sensitization of Tb3+ by Ce3+ ions in the Cerium and Terbium codoped film The emission spectrum of the Al2O3 film doped with Ce3+ (0.9 at.%) and Tb3+ (0.3 at.%) ions (AOCT film) recorded with 312 nm excitation, within the 4f→ 5d absorption band of Ce3+, is shown in Fig. 5. The spectrum consists of the 5d→ 4f UV-blue emission broad band assigned to transitions of Ce3+ ions, and the 5D4 → 7F6,5,4,3 green-red emissions assigned to transitions of Tb3+ ions. The global emission of these emissions was characterized by its chromaticity coordinates in a CIE diagram (Fig. 6), resulting in blue green-white light, with x = 0.25 and y = 0.33, when the film is excited at 312 nm. Fig. 7 portrays the excitation spectra recorded for the AOT and AOCT films. The excitation spectra were monitored at 545 nm, inside the 5 D4 → 7F5 emission of Tb3+. The excitation spectrum displayed by the codoped film exhibits a broad band similar to the 4f → 5d Ce3+ absorption band observed in the AOC excitation spectrum (see Fig. 3),
Fig. 5. Emission spectrum of the AOCT film excited at 312 nm.
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Fig. 6. Chromaticity coordinates characteristic of the emissions observed in the AOCT (solid circle symbol) and AOCTM (circle symbol) films excited at 312 nm.
which evidences a Ce3+→Tb3+ energy transfer. In fact, the 5d–4f electric dipole allowed transitions of Ce3+ are several orders of magnitude stronger than the 4f intraconfigurational transitions of Tb3+, such that Ce3+ ions can strongly absorb UV radiation and transfer a part of its energy to Tb3+ ions. Besides, the Ce3+ emission overlaps the 7 F6 → 5 L7,8,5G3, 7F6 → 5 L9,5D2,5G5, 7F6 → 5 L10,5G6,5D3 and 7F6 → 5D4 absorption (excitation) transitions of Tb3+, as it can be appreciated from the spectra shown in Fig. 8. Lifetime measurements of the 5D4 → 7F6,5 emissions of Tb3+ and Ce3+ emission in the AOCT doubly-doped film were performed monitoring the Ce3+ (420 nm) and Tb3+ (488 and 545 nm) emissions after 337 nm pulsed laser excitation within the Ce3+ 4f → 5d absorption transition. The 5D4 level emission decay was found to consist of a singleexponential decay with a lifetime value of 1.2 ± 0.1 ms. The decay time curve of the Ce3+ emission was analysed to determine the nature of the Ce3+–Tb3+ interaction and the Ce3+ → Tb3+ energy transfer microparameter. According to previous models (Yokota-Tanimoto [18] and
Fig. 7. Excitation spectra monitored at 545 nm for the AOT (dashed curve) and AOCT (solid curve) films.
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be concluded that an electric dipole–quadrupole (d–q) interaction mechanism could be dominant in the Ce3+ → Tb3+ energy transfer process. Moreover, such short range interaction mechanism, occurring in Ce3+-Tb3+ clusters, is induced by the electric dipole allowed 4f → 5d transition of Ce3+ and the 4f → 4f forbidden transitions of Tb3+ given their very short and long decay time, respectively. For such interaction, the time dependence of the Ce3+ emission can be fitted to the relation: t 3=8 KD t ; Iðt Þ = I0 exp o γ8 t τD
ð2Þ
where γ8 is given by: γ8 =
Fig. 8. Overlap region of Tb3+ absorptions (solid line) and Ce3+ emission (dashed line). The Tb3+ absorption spectrum was taken from the AOT excitation spectrum displayed in Fig. 4.
Inokuti-Hirayama [19]), considering a short-time excitation and the presence of energy migration among donors, such decay time curve follows the expression: t 3=S KD t ; Iðt Þ = I0 exp o γS t τD
where I0 is the initial intensity at t = 0, τDo is the decay time of Ce3+ (donor ions) in absence of Tb3+ (activator ions), γS is a measure of the direct Ce3+ → Tb3+ energy transfer, KD represents the migration rate of excitation energy between donor ions, and S is the multipolar interaction parameter. The time dependence of the Ce3+ emission for the AOCT film excited at 337 nm was fitted to Eq. (1) with τDo = 25 ns (measured from the decay time of the Ce3+ emission in the AOC film, see Section 3.4) and assuming dipole–dipole (S = 6), dipole–quadrupole (S = 8) and quadrupole–quadrupole (S = 10) electric couplings. The best agreement between the decay data and the theoretical fit given by Eq. (1) is attained for S = 8, as it can be appreciated from Fig. 9. Therefore, it can
PDA =
i 3ħ4 c4 fq λ2D QA h 4 ∫ FD ðEÞFA ðEÞ = E dE; 4πn4 τoD fd R8DA
ð4Þ
where τDo is the donor intrinsic lifetime in absence of the activator, QA is the oscillator strength of the activator absorption transitions, fq and fd are the oscillator strengths of the activator ion electric quadrupole and dipole transitions, respectively, λD is the wavelength position of the donor emission, and ∫ [FD(E)FA(E)/E4]dE is the spectral overlap integral between the normalised line-shape functions of the donor emission FD(E) and activator absorption FA(E), with E being the average energy of the overlapping transition. The remaining symbols in Eq. (4) have their usual meaning. In the AOCT film the donor-activator average interaction distance (RDA ≈ 12 Å) was estimated from the amounts of Ce3+ (8.7 × 1020 ions/ cm3) and Tb3+ (2.6 × 1020 ions/cm3) measured by EDS assuming a random ion distribution. The optical absorption spectrum of Tb3+ ions in the overlap region with the Ce3+ emission was hardly detectable due to the very weak intensity of the forbidden 4f–4f transitions. Then, in Eq. (4) the QA integrated absorption coefficient of Tb3+ was estimated using the relation derived by Blasse, QA = 4.8 × 10− 20 eV m2 • fd [22]. In the region of Ce3+ emission (340–440 nm) the fd electric dipole oscillator strength of the Tb3+ ion is very low (3× 10− 7 [23]). The overlap integral in Eq. (4) was calculated using the normalised lineshape functions of the cerium emission FD(E) and terbium absorptions FA(E) in the overlap region. Using n=1.64, QA =1.44×10− 26 eV m2, fq/fd ∼ 10− 2–10− 3 [23], RDA ≈ 12 Å and the value estimated for the overlap integral (6.3 × 10− 3 eV− 5) in Eq. (4) is found that dq PDA ≈6×107–6×106 s− 1, within which lies the rate of Ce3+ radiative emission (1/τDo =4×107 s− 1). For comparison the transfer rate for an dd electric dipole–dipole (d–d) interaction mechanism, PDA , was estimated dq from its relation with the PDA rate [24]: dd
PDA =
Fig. 9. Time dependence of the Ce3+ emission measured for the AOCT film. Solid line is the best fit to Eq. (1) for electric dipole–quadrupole (S = 8) interaction.
ð3Þ
being ρA the activator density and CDA the energy transfer microparameter. From the fitting of the decay curve in Fig. 9 to Eq. (2) is obtained that γ8 = 45.7 s− 3/8 and KD = 320 s− 1. Thus, from Eq. (3) CDA resulted to be about 8.2 × 10− 53 cm8/s using the Tb3+ concentration measured by EDS (ρA ≈ 2.6 × 1020 ions/cm3). This CDA value is higher than that (3.8 × 10− 54 cm8/s) reported for a HfO2:CeCl3(3 at.%):TbCl3(6 at.%) film [20], which involves that the ability of the Ce3+–Tb3+ system for energy transfer in aluminium oxide might be even better than in hafnium oxide. dq The transfer rate PDA for an electric d–q interaction mechanism is given by [21]: dq
ð1Þ
4π 3=8 ð1:43ÞρA CDA ; 3
R2DA fd dq PDA : λ2D fq
ð5Þ
dd rate for an interaction distance of 12 Å turned out to be The PDA 4 −1 6 × 10 s , which is quite smaller than the rate of Ce3+ radiative emission. Therefore, the Ce3+ → Tb3+ energy transfer through an
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electric d–q interaction mechanism appears to be more probable than the electric dipole–dipole one.
3.6. Sensitization of Tb3+ and Mn2+ by Ce3+ ions in the Cerium, Terbium and Manganese triply doped film In addition to Tb3+ ions, Mn2+ ions can also be sensitized by Ce3+ ions, since there exists an overlapping between the cerium emission and 6A1(S) → 4 T1(G), 4 T2(G), 4A1(G), 4E(G), 4 T2(D), 4E(D) and 4 T1(P) absorptions (excitations) of manganese [7]. Fig. 10 displays the emission spectrum of the Al2O3 film triply doped with Ce3+ (1.3 at.%), Tb3+ (0.2 at.%) and Mn2+ (0.3 at.%) ions (AOCTM film) recorded with 312 nm excitation within the Ce3+ 4f → 5d absorption transition. The spectrum exhibits, in addition to the UV-blue broad band associated with the Ce3 + 5d → 4f emission, four narrow green-red bands due to the 5D4 → 7F6,5,4,3 emissions of Tb3+ and a red broad band due to the 4 T1(G) → 6A1(S) emission of Mn2+. The combination of these emissions results in white light, with x = 0.30 and y = 0.32 chromaticity coordinates, as it can be appreciated from the CIE diagram portrayed in Fig. 6. Such chromaticity coordinates correspond to cold white light with a colour temperature of about 7320 K, and its deviation from the planckian black-body radiator locus is small (0.005). Cold white light have potential applications in public lighting. The emission spectrum recorded for the AOC film (with 1.7 at.% of Ce3+) measured in the same experimental conditions, is also displayed in Fig. 10 for comparison. It is evident that the addition of Tb3+ and Mn2+ in the Ce3+ doped film causes a strong decrease of the cerium overall emission. This fact provides evidence that energy transfer from Ce3+ to Tb3+ and Mn2+ ions takes place by means of a non-radiative process. Fig. 11 shows the excitation spectra of the AOCTM film monitored at (a) 645 nm (inside the 4 T1(G) → 6A1(S) emission of Mn2+) and (b) 545 nm (inside the 5D4 → 7F5 emission of Tb3+). In addition to the (6A1(S) → 4A1(G), 4E(G), 6A1(S) → 4 T2(G) and 6A1(S) → 4 T1(G)) Mn2+ and (7F6 → 5 L10,5G6,5D3) Tb3+ absorption (excitation) transitions, the 4f→ 5d Ce3+ absorption (excitation) band (see Fig. 3) is also observed, which evidences an energy transfer from Ce3+ to Tb3+ and Mn2+ ions. Decay time measurements of the Ce3+ emission in the AOCTM film were performed monitoring the emission at 420 nm after 337 nm laser pulsed excitation. The Ce3+ emission decay is simple exponen-
Fig. 10. Emission spectra of the AOCTM (solid line) and AOC (dashed line) films excited at 312 nm. Note that the AOCTM emission spectrum is multiplied by 5.
Fig. 11. Excitation spectra of the AOCTM film monitored at (a) 645 nm and (b) 545 nm.
tial, with a lifetime value of 20 ± 1 ns. The increase in the Ce3+ emission decay rate on codoping with terbium and manganese suggests a non-radiative energy transfer from Ce3+ to Tb3+ and Mn2+ ions. In order to evaluate whether Ce3+ ion acts as a good donor to Tb3+ and Mn2+ ions the η efficiency of energy transfer from Ce3+ to Tb3+ and Mn2+ ions was measured from the intensities of the donor emission (Ce3+) in the presence (ID) and absence (IoD) of the activators (Tb3+ and Mn2+). Using the cerium emission spectra portrayed in Fig. 10 η( = 1 − ID/IoD [25]) resulted in being about 85% when the film is excited at 312 nm. The high efficiency of Ce3+ → Tb3+ and Ce3+ → Mn2+ energy transfer, along with the possibility of cold white light generation, makes the Ce3+, Tb3+ and Mn2+ triply doped Al2O3 films an interesting material for the design of efficient UV pumped phosphors for cold white light emission. Lifetime measurements of the 5D4 level of Tb3+ and 4 T1(G) level of Mn2+ were carried out with Ce3+ excitation at 337 nm. The 5 D4 → 7F6,5 emission decay (monitored at 488 and 545 nm) was found to consist of a single-exponential decay with a lifetime value of 0.9 ± 0.1 ms, which is comparable to that measured for the same Tb3+ emissions in the AOCT film. The Mn2+ emission decay (monitored at 645 nm) was found to consist of a single-exponential decay with a lifetime value of 1.9 ± 0.2 ms, which is comparable to that (1.7 ± 0.3 ms) previously measured for the Mn2+ emission in a Al2O3:Ce3+(2.7 at.%):Mn2+(0.2 at.%) film excited through Ce3+ ions at 320 nm [7]. It is worth noting that upon excitation at 337 nm, the donor emission is partially reabsorbed by the same Ce3+ ions, due to an overlap between the Ce3+ emission and absorption profiles. In fact, from Fig. 3 it is evident that the high energy side of the emission (in the 330–380 nm region) overlaps with the low energy side of the absorption profile. The photons involved in this auto-absorption process could be partly dissipated, as a result of the energy migration between Ce3+ ions, as it was estimated for
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the AOCT film (KD = 320 s− 1), and subsequent transfer to traps [26] can lead to a luminescence quenching. The long lifetime measured for the Mn2+ and Tb3+ emissions is in accordance with the forbidden nature of the manganese 3d → 3d and terbium 4f → 4f transitions. Thus, in the AOCTM amorphous film is expected that the efficient energy transfer from Ce3+ to Mn2+ and Tb3+ ions could occur through electric d–q interaction mechanisms in Ce3+–Mn2+ and Ce3+–Tb3+ clusters, as it was demonstrated for the Ce3+ → Mn2+ and Ce3+ → Tb3+ energy transfers observed in Al2O3:Ce3+(2.7 at.%):Mn2+(0.2 at.%) [7] and AOCT amorphous films, respectively. 4. Conclusion Films of aluminium oxide activated with only Ce3+, with Ce3+ and Tb , or with Ce3+, Tb3+ and Mn2+, were prepared using the ultrasonic spray pyrolysis technique. The crystalline structure of the samples was monitored by the XRD technique. All the films under investigation exhibited a very broad band without any indication of crystallinity, typical of amorphous materials. Sensitization of Tb3+ and Mn2+ by Ce3+ allows a simultaneous emission of these ions under UV excitation (312 nm). The Ce3+ → Tb3+ energy transfer through an electric dipole– quadrupole interaction mechanism appears to be more probable than the electric dipole–dipole one. The possibility of white light emission by the Al2O3:Ce3+(1.3 at.%):Tb3+(0.2 at.%):Mn2+(0.3 at.%) film is demonstrated. The overall efficiency of energy transfer from Ce3+ to Tb3+ and Mn2+ ions measured from the Ce3+ emission intensities in the absence and presence of the activators (Tb3+ and Mn2+) resulted to be about 85% upon excitation at 312 nm. This high efficiency of energy transfer, resulting in cold white light emission (x = 0.30 and y = 0.32 chromaticity coordinates and 7320 K colour temperature), makes the Ce3+, Tb3+ and Mn2+ triply doped film an interesting material for the design of efficient UV pumped phosphors for generation of cold white light. 3+
Acknowledgments This work was supported by the CONACyT under project contract 78802-2F and the ICIT-DF. We would like to thank CBI chemical
laboratory of the Universidad Autónoma Metropolitana-Iztapalapa for sharing its equipment. The authors thank Z. Rivera, M. Guerrero and A.B. Soto for their technical assistance.
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