- Email: [email protected]
Er3+ doped Y2O3
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 216–221
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Change in structural morphology on addition of ZnO and its effect on fluorescence of Yb3+/Er3+ doped Y2O3 R.V. Yadav, R.K. Verma, G. Kaur, S.B. Rai ⇑ Laser and Spectroscopy Laboratory, Department of Physics, Banaras Hindu University, Varanasi 221 005, India
h i g h l i g h t s 3+
g r a p h i c a l a b s t r a c t
3+
" Yb /Er :Y2O3 phosphor and its
" "
"
"
composite with ZnO are synthesized by combustion method. On addition of ZnO, a small shifting in XRD pattern of Y2O3 is observed. Strong green with blue and red emissions through UC is seen on NIR excitation. On addition of ZnO, the intensity of these emissions is enhanced several times. Dual mode property is also observed on excitation with 532 nm excitation.
a r t i c l e
i n f o
Article history: Received 19 August 2012 Received in revised form 8 October 2012 Accepted 10 October 2012 Available online 16 November 2012 Keywords: Upconversion Luminescence Composite Phosphor
a b s t r a c t Yb3+/Er3+ codoped Y2O3 phosphor and its composite with ZnO have been synthesized by combustion method. Morphology of the materials has been investigated using X-ray diffraction pattern (XRD) and scanning electron microscopy (SEM) techniques. XRD confirms the constituents as Y2O3 and ZnO, with average crystallite size of 112 nm. On addition of ZnO, a small shifting in XRD pattern of Y2O3 is observed. SEM pattern suggests that the average particle size lies in micro-range (0.5 lm). A dumble like structure is observed for hybrid material on annealing at 1473 K. A strong green (525, 546 nm) with weak blue (411 nm) and red (657 nm) emissions through upconversion has been observed from the phosphor on excitation with 976 nm diode laser. The observed emissions involve 2H9/2 ? 4I15/2, 2H11/2 ? 4I15/2, 4 S3/2 ? 4I15/2 and 4F9/2 ? 4I15/2 electronic transitions, respectively. The upconversion process has been confirmed by power dependence measurements and its slope value was found to be 1.85, 1.72 for green and red emissions, respectively. On addition of ZnO, the intensity of these emissions is enhanced several times. The reason behind the enhancement is discussed with the help of the emitting level lifetime. An interesting dual mode property (upconversion and downconversion) to the same material has been observed on excitation with 532 nm laser source. Ó 2012 Elsevier B.V. All rights reserved.
Introduction There is an augmented interest in rare earth ions doped phosphor materials due to their several novel optical properties and applications in various fields [1–5]. Upconversion (UC) has currently become an important tool for several photonic devices as ⇑ Corresponding author. Tel.: +91 542 230 7308; fax: +91 542 236 9889. E-mail address: [email protected] (S.B. Rai). 1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2012.10.054
well as for fiber applications [6–8]. UC describes a nonlinear optical process in which low energy photons are used to generate high-energy photons. Usually infrared (IR) radiation is used to generate visible and ultraviolet (UV) light and in some cases visible to visible or IR to IR upconversion has also been reported [9,10]. Down conversion on the other hand has its own importance and it can generate visible light on visible/UV excitations and has a number of applications such as in display devices, sensors, energy harvesting, bio-imaging, and security purposes [11–13].
R.V. Yadav et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 216–221
The emission efficiency of rare earth depends upon various parameters of the host matrix, nano particles shape and size, synthesis technique, excitation mechanisms, etc. Y2O3 is an inorganic phosphor and well known luminescent host because its phonon frequency is comparatively low. Y2O3 has another advantage that its band gap is around 6 eV and thus it is insulating in nature [14,15]. The large band gap also supports the feature of strong radiative transitions because of reduced loses and weak scattering. It has been observed that addition of another organic/inorganic host improves the optical property of the material [16,17]. Moreover, the so called composite material further has wide applications. In the present work, we have examined a rare earth doped composite phosphor containing yttrium oxide and zinc oxide and studied the structural and optical properties in detail. Yb3+ is one of the most studied lanthanide ion [18] especially as it has a simple electron level scheme. The two lowest states of it are 2F7/2 (ground state) and 2F5/2 about 10,000 cm1 above 2F7/2 level. It is a long lived state and can decay both either through radiative emission or through excitation energy transfer to another suitable atom or ion. An interesting feature of Yb3+ ion is that though no band is usually seen at 488 nm in its absorption spectrum, an emission at this wavelength is seen with appreciable intensity. This has been explained as due to cooperative emission [19,20]. Though, the process responsible for this emission is not very well understood it has been invoked in many upconversion studies [21]. Such codoped materials have applications in optical switching through optical bistability, planar laser, etc. [22,23]. In Yb3+/Er3+ codoped Y2O3, Yb3+ transfers energy to Er3+ ions which gives, intense upconversion emissions at different wavelengths. Several techniques have been used in preparation of various kinds of luminescent phosphor powders [24–26], among which the combustion is one of the most important as there are numerous advantages with this method. First, the starting ingredients are liquid and each component can be accurately controlled and uniformly mixed with the help of magnetic stirrer. Second, the nano-scale materials can be prepared in very short time using this method. The third advantage is that it converts the whole material into gel at 333 K and due to organic fuel burn at 773 K. The fourth advantage is that prepared phosphor most of the time is crystalline in nature and average particle size is in micro-range which is more suitable for optical properties [27]. In this article, we report the results of our study on the optical properties of Yb3+/Er3+ codoped Y2O3 phosphor on excitation with 976 and 532 nm laser source. The effect of calcinations of the phosphor material has been analyzed. Effect of ZnO on the structural as well as on luminescence properties in its composite form has also been investigated in detail.
217
the as-synthesized composite was divided into three parts which were separately annealed at three different temperatures viz. 1073 K, 1273 K and 1473 K. The annealing time was three hours for each. The development of crystallinity and the existence of different phases in the three annealed composites were checked by powder X-ray diffraction (XRD) studies with Cu Ka as the source of radiation (1.5406 Å). The luminescence emissions from Er3+ on excitation with 976 and 532 nm laser light were recorded with a fluorescence spectrometer [iHR-320 Horiba jobin, Yvon]. The luminescence spectra of samples were also recorded at low temperatures. The life time of state, of Er3+ has also been measured. Results and discussion Structural analysis To study the structural morphology, X-ray diffraction (XRD) and scanning electron microscopy techniques were used. The XRD patterns were recorded for 2h = 10–80° at a scanning speed of 3°/min (see Fig. 1). The material show a single phase Y2O3 and the planes h k l are (2 1 1), (2 2 2), (4 0 0), (4 1 1), (3 3 2), (1 3 4), (4 4 0), (4 4 3), etc. The crystallite size was calculated using scherrer formula as:
t¼
0:9 k b cos h
where t is the crystallite size for the h k l planes, k is the wavelength of the incident X-ray [Cu Ka (1.54056 Å)], b is the full width at half maximum (FWHM) and h is the diffraction angle for h k l planes. When the sample is heat treated at higher temperatures, the FWHM of the peak decreases (see Supplementary Fig. 1). This decrease in peak width is a direct consequence of increase in particle size. Interestingly, the XRD peaks are shifted towards the higher angles in presence of ZnO. This shift in peak position may be due to several reasons such as increase in strain in the material and due to this interface dislocation. The second valuable reason may be the change in dhkl value on addition of ZnO which is directly related
Experimental Several phosphor samples were prepared through the combustion method with following compositions of reagent grade ingredients
Y2 O3 þ xYb2 O3 þ yEr2 O3 Y2 O3 þ 20ZnO þ xYb2 O3 þ yEr2 O3 where x = 2.5, 3.0, 3.5; y = 0.5, 0.75, 1.0, 1.25 mol%. To prepare the composite phosphor, we used the stoichiometric ratio of these reagents and dissolved them in a small amount of de-ionized water. Urea was then added to this solution and the mixture was stirred at 333 K in a beaker till it converts into gel. The gel was kept in a closed furnace maintained at a fixed temperature (973 K), self ignition was seen to occur after few minutes and the composite phosphor was formed. In order to grow crystals,
Fig. 1. XRD pattern of Y2O3 in (a) as-synthesized sample, (b) sample heat treated at 1473 K.
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R.V. Yadav et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 216–221
Fig. 2. (a and b) SEM morphology of Y2O3 phosphor and it shows phome like structure, (c and d) SEM morphology of ZnO:Y2O3 composites which shows nearly spherical shape particles.
to the lattice parameters. The phase of the material does not change on addition of ZnO while its d value may change. This reveals that the crystallite size decreases on addition of ZnO. The third reason may be the overlapping of the peaks of ZnO to Y2O3 phase. In presence of ZnO the FWHM of the peaks slightly increases which supports our results. The shift in peak corresponding to the plane (2 2 2) is 0.083°. The important thing is that in presence of ZnO the particles show a reduced mean morphology change but the phase is the same. The SEM image of the phosphor sample in case of as-synthesized and the sample annealed at 1473 K was recorded to investigate the surface morphology of the phosphor material. The phosphor shows the presence of agglomerated particles with irregular shape and sizes and also sheet like structure. However, the particles are not densely packed and it seems there exist some irregular voids/gaps among them. The porous structure may result due to liberation of different gases during the combustion process (see Fig. 2a). On annealing the sample the particle size slightly increases. This increase in particle size is due to agglomeration of the particles and it shows pebble like dense structure. The SEM picture of the material annealed at 1473 K is shown in Fig. 2b. However, on addition of ZnO, the morphology of the sample is drastically changed and particles turned into a mixture of two phases one like powder and the other like sphere (see Fig. 2c). The mixed sample on annealing at 1473 K turned into dumbled shape as shown in Fig. 2d. The inset of the figure shows one dumble structure which reveals that the spheres of dumble shape joined by some powder
sample and these dumble and powder may be due to ZnO and Y2O3 phases, respectively. The spherical shape is very important for fluorescence because in these cases probably the scattering is reduced due to which the efficiency of emission of rare earth ions are increased.
Upconversion emission with Infrared excitation (kexc = 976 nm) The upconversion spectra of Yb3+/Er3+ doped Y2O3 has been recorded using 976 nm diode laser and the fluorescence emitted is shown in Fig. 3. In the spectrum three bands viz. one in blue1 region, second in green region and third in red region are seen. The green bands are strongest among these emission bands. Red emission is slightly less intense but the intensity of the blue emission is very weak compared to green band. The peaks centralized at 411, 525, 546 and 657 nm are ascribed to arise due to 2H9/2 ? 4I15/2, 4S3/2/ 2 H11/2 ? 4I15/2 and 4F9/2 ? 4I15/2, electronic transitions respectively. When the phosphor is excited by 976 nm diode laser, both Er3+ and Yb3+ ions are excited with a different rate due to a large difference in their absorption cross section for infrared radiation (Yb3+ has many times larger absorption cross-section particularly for 976 nm radiation in comparison to Er3+ ion). The Er3+ ions absorb 976 nm photons and excited to the level 4I11/2 (4I15/2 ? 4I11/2) through ground state 1 For interpretation of color in Fig. 3, the reader is referred to the web version of this article.
R.V. Yadav et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 216–221
Fig. 3. Upconversion spectrum of Yb3+/Er3+ doped phosphor (kexc = 976 nm) and the effect of calcination.
absorption (GSA). The ions subsequently in this state reabsorb 976 nm photons and promoted to higher lying 4F7/2 state through excited state absorption process (ESA). The Er3+ ions in this state decay non-radiatively to lower excited states 2H11/2 and 4S3/2. The excited ions from these states decay radiatively to ground state via 2H11/2 ? 4 I15/2 and 4S3/2 ? 4I15/2 transitions and non-radiatively to 4F9/2 state followed by radiative emission corresponding 4F9/2 ? 4I15/2 transition. The blue emission does not appear in this case. Even these emissions also are weak in intensity. On the other hand, Yb3+ ions also absorb 976 nm radiation and get excited to its only low lying excited state 2F5/2 (2F7/2 ? 2F5/2). The excited Yb3+ ions transfer their excitation energy to Er3+ ions through energy transfer (ET) process (2F5/2 to 4 I11/2) and increase the Er3+ in 4I11/2 level. In addition to this, at higher concentrations two Yb3+ ions make a pair and give a cooperative upconversion emission at 488 nm. The cooperative blue emission energy matches with the energy of 4F7/2 level of Er3+ ion and promote Er3+ ions to 4F7/2 level. The ions from this state transfer energy
Fig. 4. Energy level diagram of Yb3+/Er3+ and mechanism of energy transfer on excitation with 976 nm diode laser and 532 nm from Verdi-V 5 laser.
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radiatively to ground state emitting blue band. Some of the Er3+ ions in 4F7/2 level at the same time relax non-radiatively to populate 4S3/2 and 2H11/2 levels. These results are very strong upconversion emission from these levels. The combined partial energy level diagram with mechanism of upconversion is shown in Fig. 4. The UC mechanism on excitation with 976 nm radiation has been further verified through power dependence. The UC emission was plotted in natural log i.e. ln I (intensity of UC emission) versus ln P (applied laser input power). The number of photons involved in the emission process could be calculated with the slope of the curve. Interestingly the slope for green and red bands are 1.85 and 1.72 respectively, which indicates that two photons are involved in the emission. When the sample is heat treated at higher temperatures intensity of all the three bands observed increases. This is due to removal of quenching centers (unwanted impurity materials such as OH, NO3,. . . etc.) at higher temperatures which are still present with the phosphor material. All the bands observed are splitted in number of Stark components clearly indicating that Y2O3 is a highly crystalline host. This is also confirmed by X-ray diffraction pattern. The crystalline nature of the material favors the fluorescence efficiency. At higher temperatures, the crystallinity of the sample increases from nanosize to microsize. The microsize crystallinity favors the fluorescence intensity. Giri et al. have reported that in Y2O3 host, the fluorescence intensity increases rapidly up to 1673 K [27]. For higher temperatures the particle size reaches to critical size (a size at which fluorescence efficiency is maximum), above which the scattering becomes a dominant factor which quenches the fluorescence intensity. On addition of ZnO, the material is converted into composite form which has been verified by XRD and SEM techniques as we have discussed above. We found that the composite material is comparatively better host not only for the luminescence point of view but from stability point of view also. Verma et al. [16] in a recent work have reported that stability of composite ZnO:Ca12Al33O15 is larger than the Ca12Al33O15 phosphor. As discussed earlier, on mixing of ZnO with Y2O3 the structure changes and a dumble shape morphology is obtained. There are two possibilities that the rare earth ions either sit at the site of ZnO or at the site of Y2O3 or on both. Though both the phases are in nanoform but ZnO is in spherical shape while Y2O3 is in random shape. The spherical shape has a tendency to increase surface to volume ratio in nanoregime and in case of rare earth to sit at this site has a possibility to enhance the emission efficiency. In presence of ZnO the emission efficiency of Er3+ ion is increased appreciably (see Fig. 5). However, this enhancement is not similar for all the bands. In the
Fig. 5. Upconversion emission in absence and presence of ZnO on excitation with 976 nm diode laser.
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Fig. 7. Up and down conversion emission on excitation with 532 nm laser.
Fig. 6. Decay pattern of 4S3/2 ? 4I15/2 transition on 976 nm excitation.
case of Y2O3 host the emission is greenish i.e. red emission is relatively weak. However in presence of ZnO with Y2O3 the glow becomes yellowish clearly indicating a large increase in red emission. The yellowish color reveals that green and red color mix nearly in equal proportions as is seen in Fig. 5. The color coordinates of Y2O3 phosphor and Y2O3 + ZnO composites are (0.34, 0.65) and (0.38, 0.54), respectively which are represented by CIE diagram (see Supplementary Fig. 2). We measured the lifetime of 4 S3/2 level in presence and absence of ZnO. The decay curves in two situations are shown in Fig 6. It is found that in presence of ZnO the lifetime of 4S3/2 level is increased from 478 ls to 634 ls. There are two reasons behind this enhancement. The first reason is that in presence of ZnO the crystallinity of the material increases. This increase in crystallinity enhance the life time of 4S3/2/2H11/2 levels of Er3+. The second reason is the change in morphology of the host (structural change) which is also of equal importance. The presence of ZnO in the composite causes environment effect (this is due to weak interaction of ZnO with Y2O3) which may increase the radiative relaxation of ions. Actually ZnO and Y2O3 are weakly bonded to each other in dumble formation. This weak bonding reduces the freedom of Y2O3 molecules. Wang et al. [6] have reported that the fluorescence of lanthanides depends upon the crystallite size and in case of microcrystallites it is better than the nano-crystallites. Moreover, ZnO annealed at higher temperatures always enhance red emission as is it in present case also.
are shown in Fig. 7. A band appears in red at 657 nm in case of down conversion splitted into several components due to 4F9/2 ? 4 I15/2 electronic transitions [we do not see green emission clearly due to superposition of laser wavelength]. Similarly we observe a band in blue in case of the upconversion splitted into several components at 411 nm due to 2H9/2 ? 4I15/2 transition. Here again green emission is not recorded since the pump laser line match to the transition of the Er3+ ions. The emission intensity increases on heat treatment due to decrease in unwanted impurity as reported earlier (see Fig 8) [16]. The slope of ln I–ln P plots for the blue emission shows a clear quadratic dependence with value to be 1.75 (not shown here). The mechanism and energy levels involved in this case is shown in Fig. 4. First the Er3+ ions are promoted to 2H11/2 level on absorption of 532 nm photons. These ions relax non-radiatively to 4S3/2 level a part of which populates 4F9/2 level also. Since 4S3/2 level is
Stokes and anti-Stokes emission on excitation of 532 nm laser radiation The upconversion and down conversion spectra of Er3+ doped sample have been recorded using 532 nm excitation source and
Fig. 8. Effect of annealing in case of downconversion emission (kexc = 532 nm).
R.V. Yadav et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 216–221
metastable, ions in this level again absorb 532 nm photons and promoted to some higher lying levels. A multi phonon relaxation from there populates 4G11/2 level. The ions from this level emit photons of energy which lie in blue region. Most probable phonon assisted ESA are follows as: 4
I15=2 þ hm!2 H11=2 þ phonons!4 S3=2 !4 I15=2 þ Green emission
4
I15=2 þ hm!2 H11=2 þ hm ! Higher levels!4 G11=2 !4 I15=2 þ Blue photon
Interestingly, on addition of ZnO the emission intensity increases in this case also. This is probably due to energy transfer from ZnO to Er3+. Since ZnO has a direct band gap and in case of UV excitation it emits emission near 380 nm along with two defect emission in green and red regions [28]. On excitation with 532 nm it may be that it populates green defect band and subsequently red band through non-radiative channel. It is therefore expected that an energy transfer from defect level to Er3+ ion occurs which results an enhancement in emission intensity. Conclusions Upconversion properties of Er3+/Yb3+ doped combustion synthesized material were investigated. Yb3+ works as sensitizer for Er3+ ions due to strong absorption cross section for infra red radiations. Effect of annealing on the intensity of emission bands was investigated. The effect of addition of ZnO on the emissions were also investigated and correlated to the structural morphology in the composite phosphor material (a dumble like structure has been observed which may be due to agglomeration of ZnO and Y2O3). Lifetime measurements carried out to explain the enhancement in emission intensity. In case of direct excitation of Er3+ ions using 532 nm laser source, up as well as down conversion emissions were observed. A possibility of energy transfer from ZnO to Er3+ ions has been discussed. Acknowledgments R.K. Verma would like to thank CSIR for a senior research fellowship. Grants from UGC, Government of India, New Delhi are also acknowledged.
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Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2012.10.054. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
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Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Change in structural morphology on addition of ZnO and its effect on fluorescence of Yb3+/Er3+ doped Y2O3 R.V. Yadav, R.K. Verma, G. Kaur, S.B. Rai ⇑ Laser and Spectroscopy Laboratory, Department of Physics, Banaras Hindu University, Varanasi 221 005, India
h i g h l i g h t s 3+
g r a p h i c a l a b s t r a c t
3+
" Yb /Er :Y2O3 phosphor and its
" "
"
"
composite with ZnO are synthesized by combustion method. On addition of ZnO, a small shifting in XRD pattern of Y2O3 is observed. Strong green with blue and red emissions through UC is seen on NIR excitation. On addition of ZnO, the intensity of these emissions is enhanced several times. Dual mode property is also observed on excitation with 532 nm excitation.
a r t i c l e
i n f o
Article history: Received 19 August 2012 Received in revised form 8 October 2012 Accepted 10 October 2012 Available online 16 November 2012 Keywords: Upconversion Luminescence Composite Phosphor
a b s t r a c t Yb3+/Er3+ codoped Y2O3 phosphor and its composite with ZnO have been synthesized by combustion method. Morphology of the materials has been investigated using X-ray diffraction pattern (XRD) and scanning electron microscopy (SEM) techniques. XRD confirms the constituents as Y2O3 and ZnO, with average crystallite size of 112 nm. On addition of ZnO, a small shifting in XRD pattern of Y2O3 is observed. SEM pattern suggests that the average particle size lies in micro-range (0.5 lm). A dumble like structure is observed for hybrid material on annealing at 1473 K. A strong green (525, 546 nm) with weak blue (411 nm) and red (657 nm) emissions through upconversion has been observed from the phosphor on excitation with 976 nm diode laser. The observed emissions involve 2H9/2 ? 4I15/2, 2H11/2 ? 4I15/2, 4 S3/2 ? 4I15/2 and 4F9/2 ? 4I15/2 electronic transitions, respectively. The upconversion process has been confirmed by power dependence measurements and its slope value was found to be 1.85, 1.72 for green and red emissions, respectively. On addition of ZnO, the intensity of these emissions is enhanced several times. The reason behind the enhancement is discussed with the help of the emitting level lifetime. An interesting dual mode property (upconversion and downconversion) to the same material has been observed on excitation with 532 nm laser source. Ó 2012 Elsevier B.V. All rights reserved.
Introduction There is an augmented interest in rare earth ions doped phosphor materials due to their several novel optical properties and applications in various fields [1–5]. Upconversion (UC) has currently become an important tool for several photonic devices as ⇑ Corresponding author. Tel.: +91 542 230 7308; fax: +91 542 236 9889. E-mail address: [email protected] (S.B. Rai). 1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2012.10.054
well as for fiber applications [6–8]. UC describes a nonlinear optical process in which low energy photons are used to generate high-energy photons. Usually infrared (IR) radiation is used to generate visible and ultraviolet (UV) light and in some cases visible to visible or IR to IR upconversion has also been reported [9,10]. Down conversion on the other hand has its own importance and it can generate visible light on visible/UV excitations and has a number of applications such as in display devices, sensors, energy harvesting, bio-imaging, and security purposes [11–13].
R.V. Yadav et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 216–221
The emission efficiency of rare earth depends upon various parameters of the host matrix, nano particles shape and size, synthesis technique, excitation mechanisms, etc. Y2O3 is an inorganic phosphor and well known luminescent host because its phonon frequency is comparatively low. Y2O3 has another advantage that its band gap is around 6 eV and thus it is insulating in nature [14,15]. The large band gap also supports the feature of strong radiative transitions because of reduced loses and weak scattering. It has been observed that addition of another organic/inorganic host improves the optical property of the material [16,17]. Moreover, the so called composite material further has wide applications. In the present work, we have examined a rare earth doped composite phosphor containing yttrium oxide and zinc oxide and studied the structural and optical properties in detail. Yb3+ is one of the most studied lanthanide ion [18] especially as it has a simple electron level scheme. The two lowest states of it are 2F7/2 (ground state) and 2F5/2 about 10,000 cm1 above 2F7/2 level. It is a long lived state and can decay both either through radiative emission or through excitation energy transfer to another suitable atom or ion. An interesting feature of Yb3+ ion is that though no band is usually seen at 488 nm in its absorption spectrum, an emission at this wavelength is seen with appreciable intensity. This has been explained as due to cooperative emission [19,20]. Though, the process responsible for this emission is not very well understood it has been invoked in many upconversion studies [21]. Such codoped materials have applications in optical switching through optical bistability, planar laser, etc. [22,23]. In Yb3+/Er3+ codoped Y2O3, Yb3+ transfers energy to Er3+ ions which gives, intense upconversion emissions at different wavelengths. Several techniques have been used in preparation of various kinds of luminescent phosphor powders [24–26], among which the combustion is one of the most important as there are numerous advantages with this method. First, the starting ingredients are liquid and each component can be accurately controlled and uniformly mixed with the help of magnetic stirrer. Second, the nano-scale materials can be prepared in very short time using this method. The third advantage is that it converts the whole material into gel at 333 K and due to organic fuel burn at 773 K. The fourth advantage is that prepared phosphor most of the time is crystalline in nature and average particle size is in micro-range which is more suitable for optical properties [27]. In this article, we report the results of our study on the optical properties of Yb3+/Er3+ codoped Y2O3 phosphor on excitation with 976 and 532 nm laser source. The effect of calcinations of the phosphor material has been analyzed. Effect of ZnO on the structural as well as on luminescence properties in its composite form has also been investigated in detail.
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the as-synthesized composite was divided into three parts which were separately annealed at three different temperatures viz. 1073 K, 1273 K and 1473 K. The annealing time was three hours for each. The development of crystallinity and the existence of different phases in the three annealed composites were checked by powder X-ray diffraction (XRD) studies with Cu Ka as the source of radiation (1.5406 Å). The luminescence emissions from Er3+ on excitation with 976 and 532 nm laser light were recorded with a fluorescence spectrometer [iHR-320 Horiba jobin, Yvon]. The luminescence spectra of samples were also recorded at low temperatures. The life time of state, of Er3+ has also been measured. Results and discussion Structural analysis To study the structural morphology, X-ray diffraction (XRD) and scanning electron microscopy techniques were used. The XRD patterns were recorded for 2h = 10–80° at a scanning speed of 3°/min (see Fig. 1). The material show a single phase Y2O3 and the planes h k l are (2 1 1), (2 2 2), (4 0 0), (4 1 1), (3 3 2), (1 3 4), (4 4 0), (4 4 3), etc. The crystallite size was calculated using scherrer formula as:
t¼
0:9 k b cos h
where t is the crystallite size for the h k l planes, k is the wavelength of the incident X-ray [Cu Ka (1.54056 Å)], b is the full width at half maximum (FWHM) and h is the diffraction angle for h k l planes. When the sample is heat treated at higher temperatures, the FWHM of the peak decreases (see Supplementary Fig. 1). This decrease in peak width is a direct consequence of increase in particle size. Interestingly, the XRD peaks are shifted towards the higher angles in presence of ZnO. This shift in peak position may be due to several reasons such as increase in strain in the material and due to this interface dislocation. The second valuable reason may be the change in dhkl value on addition of ZnO which is directly related
Experimental Several phosphor samples were prepared through the combustion method with following compositions of reagent grade ingredients
Y2 O3 þ xYb2 O3 þ yEr2 O3 Y2 O3 þ 20ZnO þ xYb2 O3 þ yEr2 O3 where x = 2.5, 3.0, 3.5; y = 0.5, 0.75, 1.0, 1.25 mol%. To prepare the composite phosphor, we used the stoichiometric ratio of these reagents and dissolved them in a small amount of de-ionized water. Urea was then added to this solution and the mixture was stirred at 333 K in a beaker till it converts into gel. The gel was kept in a closed furnace maintained at a fixed temperature (973 K), self ignition was seen to occur after few minutes and the composite phosphor was formed. In order to grow crystals,
Fig. 1. XRD pattern of Y2O3 in (a) as-synthesized sample, (b) sample heat treated at 1473 K.
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Fig. 2. (a and b) SEM morphology of Y2O3 phosphor and it shows phome like structure, (c and d) SEM morphology of ZnO:Y2O3 composites which shows nearly spherical shape particles.
to the lattice parameters. The phase of the material does not change on addition of ZnO while its d value may change. This reveals that the crystallite size decreases on addition of ZnO. The third reason may be the overlapping of the peaks of ZnO to Y2O3 phase. In presence of ZnO the FWHM of the peaks slightly increases which supports our results. The shift in peak corresponding to the plane (2 2 2) is 0.083°. The important thing is that in presence of ZnO the particles show a reduced mean morphology change but the phase is the same. The SEM image of the phosphor sample in case of as-synthesized and the sample annealed at 1473 K was recorded to investigate the surface morphology of the phosphor material. The phosphor shows the presence of agglomerated particles with irregular shape and sizes and also sheet like structure. However, the particles are not densely packed and it seems there exist some irregular voids/gaps among them. The porous structure may result due to liberation of different gases during the combustion process (see Fig. 2a). On annealing the sample the particle size slightly increases. This increase in particle size is due to agglomeration of the particles and it shows pebble like dense structure. The SEM picture of the material annealed at 1473 K is shown in Fig. 2b. However, on addition of ZnO, the morphology of the sample is drastically changed and particles turned into a mixture of two phases one like powder and the other like sphere (see Fig. 2c). The mixed sample on annealing at 1473 K turned into dumbled shape as shown in Fig. 2d. The inset of the figure shows one dumble structure which reveals that the spheres of dumble shape joined by some powder
sample and these dumble and powder may be due to ZnO and Y2O3 phases, respectively. The spherical shape is very important for fluorescence because in these cases probably the scattering is reduced due to which the efficiency of emission of rare earth ions are increased.
Upconversion emission with Infrared excitation (kexc = 976 nm) The upconversion spectra of Yb3+/Er3+ doped Y2O3 has been recorded using 976 nm diode laser and the fluorescence emitted is shown in Fig. 3. In the spectrum three bands viz. one in blue1 region, second in green region and third in red region are seen. The green bands are strongest among these emission bands. Red emission is slightly less intense but the intensity of the blue emission is very weak compared to green band. The peaks centralized at 411, 525, 546 and 657 nm are ascribed to arise due to 2H9/2 ? 4I15/2, 4S3/2/ 2 H11/2 ? 4I15/2 and 4F9/2 ? 4I15/2, electronic transitions respectively. When the phosphor is excited by 976 nm diode laser, both Er3+ and Yb3+ ions are excited with a different rate due to a large difference in their absorption cross section for infrared radiation (Yb3+ has many times larger absorption cross-section particularly for 976 nm radiation in comparison to Er3+ ion). The Er3+ ions absorb 976 nm photons and excited to the level 4I11/2 (4I15/2 ? 4I11/2) through ground state 1 For interpretation of color in Fig. 3, the reader is referred to the web version of this article.
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Fig. 3. Upconversion spectrum of Yb3+/Er3+ doped phosphor (kexc = 976 nm) and the effect of calcination.
absorption (GSA). The ions subsequently in this state reabsorb 976 nm photons and promoted to higher lying 4F7/2 state through excited state absorption process (ESA). The Er3+ ions in this state decay non-radiatively to lower excited states 2H11/2 and 4S3/2. The excited ions from these states decay radiatively to ground state via 2H11/2 ? 4 I15/2 and 4S3/2 ? 4I15/2 transitions and non-radiatively to 4F9/2 state followed by radiative emission corresponding 4F9/2 ? 4I15/2 transition. The blue emission does not appear in this case. Even these emissions also are weak in intensity. On the other hand, Yb3+ ions also absorb 976 nm radiation and get excited to its only low lying excited state 2F5/2 (2F7/2 ? 2F5/2). The excited Yb3+ ions transfer their excitation energy to Er3+ ions through energy transfer (ET) process (2F5/2 to 4 I11/2) and increase the Er3+ in 4I11/2 level. In addition to this, at higher concentrations two Yb3+ ions make a pair and give a cooperative upconversion emission at 488 nm. The cooperative blue emission energy matches with the energy of 4F7/2 level of Er3+ ion and promote Er3+ ions to 4F7/2 level. The ions from this state transfer energy
Fig. 4. Energy level diagram of Yb3+/Er3+ and mechanism of energy transfer on excitation with 976 nm diode laser and 532 nm from Verdi-V 5 laser.
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radiatively to ground state emitting blue band. Some of the Er3+ ions in 4F7/2 level at the same time relax non-radiatively to populate 4S3/2 and 2H11/2 levels. These results are very strong upconversion emission from these levels. The combined partial energy level diagram with mechanism of upconversion is shown in Fig. 4. The UC mechanism on excitation with 976 nm radiation has been further verified through power dependence. The UC emission was plotted in natural log i.e. ln I (intensity of UC emission) versus ln P (applied laser input power). The number of photons involved in the emission process could be calculated with the slope of the curve. Interestingly the slope for green and red bands are 1.85 and 1.72 respectively, which indicates that two photons are involved in the emission. When the sample is heat treated at higher temperatures intensity of all the three bands observed increases. This is due to removal of quenching centers (unwanted impurity materials such as OH, NO3,. . . etc.) at higher temperatures which are still present with the phosphor material. All the bands observed are splitted in number of Stark components clearly indicating that Y2O3 is a highly crystalline host. This is also confirmed by X-ray diffraction pattern. The crystalline nature of the material favors the fluorescence efficiency. At higher temperatures, the crystallinity of the sample increases from nanosize to microsize. The microsize crystallinity favors the fluorescence intensity. Giri et al. have reported that in Y2O3 host, the fluorescence intensity increases rapidly up to 1673 K [27]. For higher temperatures the particle size reaches to critical size (a size at which fluorescence efficiency is maximum), above which the scattering becomes a dominant factor which quenches the fluorescence intensity. On addition of ZnO, the material is converted into composite form which has been verified by XRD and SEM techniques as we have discussed above. We found that the composite material is comparatively better host not only for the luminescence point of view but from stability point of view also. Verma et al. [16] in a recent work have reported that stability of composite ZnO:Ca12Al33O15 is larger than the Ca12Al33O15 phosphor. As discussed earlier, on mixing of ZnO with Y2O3 the structure changes and a dumble shape morphology is obtained. There are two possibilities that the rare earth ions either sit at the site of ZnO or at the site of Y2O3 or on both. Though both the phases are in nanoform but ZnO is in spherical shape while Y2O3 is in random shape. The spherical shape has a tendency to increase surface to volume ratio in nanoregime and in case of rare earth to sit at this site has a possibility to enhance the emission efficiency. In presence of ZnO the emission efficiency of Er3+ ion is increased appreciably (see Fig. 5). However, this enhancement is not similar for all the bands. In the
Fig. 5. Upconversion emission in absence and presence of ZnO on excitation with 976 nm diode laser.
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Fig. 7. Up and down conversion emission on excitation with 532 nm laser.
Fig. 6. Decay pattern of 4S3/2 ? 4I15/2 transition on 976 nm excitation.
case of Y2O3 host the emission is greenish i.e. red emission is relatively weak. However in presence of ZnO with Y2O3 the glow becomes yellowish clearly indicating a large increase in red emission. The yellowish color reveals that green and red color mix nearly in equal proportions as is seen in Fig. 5. The color coordinates of Y2O3 phosphor and Y2O3 + ZnO composites are (0.34, 0.65) and (0.38, 0.54), respectively which are represented by CIE diagram (see Supplementary Fig. 2). We measured the lifetime of 4 S3/2 level in presence and absence of ZnO. The decay curves in two situations are shown in Fig 6. It is found that in presence of ZnO the lifetime of 4S3/2 level is increased from 478 ls to 634 ls. There are two reasons behind this enhancement. The first reason is that in presence of ZnO the crystallinity of the material increases. This increase in crystallinity enhance the life time of 4S3/2/2H11/2 levels of Er3+. The second reason is the change in morphology of the host (structural change) which is also of equal importance. The presence of ZnO in the composite causes environment effect (this is due to weak interaction of ZnO with Y2O3) which may increase the radiative relaxation of ions. Actually ZnO and Y2O3 are weakly bonded to each other in dumble formation. This weak bonding reduces the freedom of Y2O3 molecules. Wang et al. [6] have reported that the fluorescence of lanthanides depends upon the crystallite size and in case of microcrystallites it is better than the nano-crystallites. Moreover, ZnO annealed at higher temperatures always enhance red emission as is it in present case also.
are shown in Fig. 7. A band appears in red at 657 nm in case of down conversion splitted into several components due to 4F9/2 ? 4 I15/2 electronic transitions [we do not see green emission clearly due to superposition of laser wavelength]. Similarly we observe a band in blue in case of the upconversion splitted into several components at 411 nm due to 2H9/2 ? 4I15/2 transition. Here again green emission is not recorded since the pump laser line match to the transition of the Er3+ ions. The emission intensity increases on heat treatment due to decrease in unwanted impurity as reported earlier (see Fig 8) [16]. The slope of ln I–ln P plots for the blue emission shows a clear quadratic dependence with value to be 1.75 (not shown here). The mechanism and energy levels involved in this case is shown in Fig. 4. First the Er3+ ions are promoted to 2H11/2 level on absorption of 532 nm photons. These ions relax non-radiatively to 4S3/2 level a part of which populates 4F9/2 level also. Since 4S3/2 level is
Stokes and anti-Stokes emission on excitation of 532 nm laser radiation The upconversion and down conversion spectra of Er3+ doped sample have been recorded using 532 nm excitation source and
Fig. 8. Effect of annealing in case of downconversion emission (kexc = 532 nm).
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metastable, ions in this level again absorb 532 nm photons and promoted to some higher lying levels. A multi phonon relaxation from there populates 4G11/2 level. The ions from this level emit photons of energy which lie in blue region. Most probable phonon assisted ESA are follows as: 4
I15=2 þ hm!2 H11=2 þ phonons!4 S3=2 !4 I15=2 þ Green emission
4
I15=2 þ hm!2 H11=2 þ hm ! Higher levels!4 G11=2 !4 I15=2 þ Blue photon
Interestingly, on addition of ZnO the emission intensity increases in this case also. This is probably due to energy transfer from ZnO to Er3+. Since ZnO has a direct band gap and in case of UV excitation it emits emission near 380 nm along with two defect emission in green and red regions [28]. On excitation with 532 nm it may be that it populates green defect band and subsequently red band through non-radiative channel. It is therefore expected that an energy transfer from defect level to Er3+ ion occurs which results an enhancement in emission intensity. Conclusions Upconversion properties of Er3+/Yb3+ doped combustion synthesized material were investigated. Yb3+ works as sensitizer for Er3+ ions due to strong absorption cross section for infra red radiations. Effect of annealing on the intensity of emission bands was investigated. The effect of addition of ZnO on the emissions were also investigated and correlated to the structural morphology in the composite phosphor material (a dumble like structure has been observed which may be due to agglomeration of ZnO and Y2O3). Lifetime measurements carried out to explain the enhancement in emission intensity. In case of direct excitation of Er3+ ions using 532 nm laser source, up as well as down conversion emissions were observed. A possibility of energy transfer from ZnO to Er3+ ions has been discussed. Acknowledgments R.K. Verma would like to thank CSIR for a senior research fellowship. Grants from UGC, Government of India, New Delhi are also acknowledged.
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Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2012.10.054. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
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