Optical Materials 35 (2013) 1487–1492
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Laser induced thermal effect on upconversion luminescence and temperature-dependent upconversion mechanism in Ho3+/Yb3+-codoped Gd2(WO4)3 phosphor Wei Liu a, Jiashi Sun a,⇑, Xiangping Li a, Jinsu Zhang a, Yue Tian a, Shaobo Fu a, Hua Zhong a, Tianhong Liu a,b, Lihong Cheng a, Haiyang Zhong a, Haiping Xia c, Bin Dong d, Ruinian Hua d, Xiangqing Zhang a, Baojiu Chen a,⇑ a
Department of Physics, Dalian Maritime University, Dalian 116026, PR China School of Computer Science and Information Technology, Liaoning Normal University, Dalian 116024, PR China c Key laboratory of Photo-electronic Materials, Ningbo University, Ningbo 315211, PR China d College of Life Science, School of Physics and Material Engineering, Dalian Nationalities University, Dalian 116600, PR China b
a r t i c l e
i n f o
Article history: Received 8 November 2012 Received in revised form 12 March 2013 Accepted 13 March 2013 Available online 6 April 2013 Keywords: Gd2(WO4)3:Ho3+/Yb3+ Thermal effect Time-scanning Thermal quenching Upconversion luminescence
a b s t r a c t Sub-micro sized Gd2(WO4)3:Yb3+/Ho3+ phosphor was synthesized via a co-precipitation reaction. The crystal structure and morphology of the phosphor were characterized by XRD and SEM. The time scanning of green and red upconversion emissions displayed that the upconversion luminescent intensities were dependent on the irradiation time and the excitation powder, which was resulted from the thermal effect induced by LD (laser diode) irradiation. The upconversion luminescence of Gd2(WO4)3:Yb3+/Ho3+ phosphor at different sample temperatures was studied. It was found that at room temperature the red and green upconversion emissions were 1.5- and 2-photon processes, respectively. With increasing sample temperature the 2-photon process for the red upconversion emission and 3-photon process for green upconversion emission occurred. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The upconversion luminescence (UCL) is a light emitting process in which the long wavelength photons are converted into short wavelength photons, which is obviously an anti-Stokes process, and also known as frequency upconversion process. The UCL phenomenon in rare earth (RE) was ﬁrst discovered by Auzel [1,2]. During the past several decades, many efforts have been devoting to the investigations on UCL in RE3+ doped materials, which were driven by both the scientiﬁc interest and potential applications in short wavelength lasers, 3-dimensional displays and biomedicine [3–6]. The UCL mechanisms of RE3+ [6,7] and progresses in their corresponding applications [5,8,9] were reviewed in detail. Most of the recent study interest relevant to RE3+ doped upconversion was focused on the synthesis techniques, ﬂuorescence concentration quenching, particle surface and size effects and the upconversion luminescence material applications in lasers and biomedicine [10–19]. Nevertheless, less attention has been paid to the thermal inﬂuence of laser irradiation on the upconver-
⇑ Corresponding authors. Tel./fax: +86 411 84728909. E-mail address: [email protected]
(B. Chen). 0925-3467/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2013.03.008
sion process and the UCL mechanism of RE3+ doped materials at high temperature. It is well known that the nonradiative transition rate between contiguous two levels of RE3+ is strongly dependent on the temperature [20,21], meanwhile the energy transfer rate can also be affected by the temperature , and thus the upconversion behavior in RE3+ doped materials should be dependent on the sample temperature. Romerro et al. have ﬁrstly found that infrared laser irradiation induced the thermalisation of excited state of Nd3+, and new emission could be observed . Örücü et al. have conducted a thorough investigation on the spectroscopic properties of Cr3+ at various temperatures . In this work, we attempt to study on the inﬂuence of laser irradiation on the upconversion luminescence and temperature effect on the UCL mechanism. Yb3+ is usually introduced as sensitizer to enhance UCL efﬁciency through cascade energy transfer from Yb3+ to the upconversion emitting centers such as Er3+ (green and red emissions), Tm3+ (blue emission), Pr3+ (red emission) and Ho3+ (green emission), since Yb3+ possesses simple energy level structure and strong absorption at around 980 nm which matches the output of the commercial high power laser diodes. In this study, we choose Gd2(WO4)3 as host for accommodating the doping centers Ho3+ and Yb3+ since it contains Gd3+ which features similar chemical
W. Liu et al. / Optical Materials 35 (2013) 1487–1492
properties with other RE3+, thus enabling reliable doping of Ho3+ and Yb3+ in it. Meanwhile, Gd2(WO4)3 is chemically stable and environment-friendly, and can be easily prepared via solid state reaction or co-precipitation reaction. Based on above mentioned issue, in this work the Gd2(WO4)3:Ho3+/Yb3+ phosphor was prepared via a co-precipitation reaction, and its crystal structure and morphology were characterized by means of X-ray diffraction (XRD) and ﬁeld emission scanning electron microscope (FE-SEM). The effects of laser irradiation and temperature on the UCL were studied. It was found that the laser heating could caused the change of UCL intensity with increasing irradiation time, and the mechanisms of the Gd2(WO4)3:Ho3+/Yb3+ phosphor are different at various temperatures. 2. Experimental Gd2(WO4)3 phosphor doped with 5 mol.% Ho3+ and 10 mol.% Yb3+ was prepared via a co-precipitation process. In the preparation the gadolinium, holmium and ytterbium chlorides with crystal waters (GdCl36H2O, HoCl36H2O and YbCl36H2O) were produced via a recrystallization technique by using their corresponding spectroscopically pure RE oxides as starting materials. The detailed synthesis procedure can be found in our previous work [25,26]. The obtained RE chlorides and analytically pure Na2WO4 were used as raw materials in the synthesis of target phosphor. First, certain amount of GdCl36H2O, HoCl36H2O and YbCl36H2O were weighed according to the designed composition and dissolved in 10 ml deionized water in the same beaker to form solution I. Appropriate amount of Na2WO42H2O was dissolved in 30 ml deionized water in another beaker to form solution II. After that, solution I was poured slowly into solution II drop by drop under magnetic drastic stir at room temperature. White suspension occurred during the pouring process. After the reaction was completed (about 30 min), the emulsion mixture was centrifuged at 7000 rpm for 20 min, and then the obtained precipitation was washed by deionized water twice and by absolute ethanol once. The precipitation was dried at 100 °C for 5 h, and was calcined consequently at 900 °C for 4 h to achieve ﬁnal product Gd2(WO4)3:Ho3+/Yb3+ phosphor. It should be mentioned that the pH value of Na2WO4 had to be adjusted to be lower than 10 by using NH3H2O. If the pH value is higher than 10, the ﬁnal products other than Gd2(WO4)3 will be achieved [25,26]. The XRD data taken on a SHIMADZU XRD (X-ray Diffraction) 6000 X-ray diffractometer with Cu Ka1 radiation (k = 1.5406 Å) were used to identify the crystal structure of the studied sample. In the measurement of XRD, the scanning region of 2h angle is from 10° to 60°, and the scanning step size is 0.02°. The morphology of phosphor particles was characterized by using a HITACHI S-4800 FE-SEM. The upconversion emission spectra and time scanning upconversion emission spectra were recorded on a Hitachi F-4600 ﬂuorescence spectrometer. A power adjustable laser diode with maximum two watts output and peak wavelength of 980 nm was used as excitation source for the measurements of upconversion emission spectra. The excitation power density was estimated to be around 35 W/cm2 when the working current of laser diode is 1000 mA. The area of the laser beam spot was unchanged if without speciﬁc statement. The temperatures of the studied sample were controlled by a sample temperature controlling system DMU-TC 450 produced in our lab. 3. Results and discussion 3.1. Crystal structure and morphology characterization Fig. 1a shows the XRD pattern (top part) for the prepared Gd2(WO4)3:5 mol.% Ho3+/10 mol.% Yb3+ together with the pattern
(bottom part) plotted by using the data reported in JCPDS card No. 23-1076 for Gd2(WO4)3 powder. It can be seen that most of the diffraction peak positions are in good agreements with those appearing in the JCPDS card, the extra diffraction peaks at 30° and 50° marked by asterisks in the ﬁgure can be ascribed to diffraction from (2 2 0) and (4 2 2) planes of residual Na2WO4. This fact means that Gd2(WO4)3:Ho3+/Yb3+ can be obtained from the co-precipitation route, and that the introduction of Ho3+ and Yb3+ did not cause obvious change in the Gd2(WO4)3 crystal structure. Fig. 1b displays the SEM image of the studied phosphor. From this image it can be seen that the phosphor is composed of many particles, and some of the particles are agglomerated together. One hundred dispersed particles were counted in to estimate the average particle size via statistic analysis route. Fig. 1c depicts the size distribution of the particles, and it can be found that the particle sizes distributed in a region of 0.2–0.4 lm. The average particle size is calculated to be 0.3 lm with an uncertainty of 0.06 lm. 3.2. Laser induced thermal effect on upconversion luminescence The upconversion emission spectra were measured when the sample was excited by a 980 nm LD working at varied currents from 0.3 to 2.0 A. In the full process of the spectral measurements, the sample was still irradiated by the 980 nm laser, and the area of the laser beam spot was kept as a constant. Two sets of upconversion emission spectra were obtained for two cases in which the time intervals between two spectral measurements were set to be 20 and 120 s, respectively. As a representative, Fig. 2 shows the upconversion emission spectra excited by 980 nm LD working at varied currents in the case of SCTI (spectrum collection time interval) of 20 s. It can be seen that each spectrum contains three emission bands peaking at around 543, 658 and 755.4 nm corresponding, respectively, to 5F4,5S2?5I8, 5F5?5I8 and 5F4,5S2?5I7 transitions [27,28]. Meanwhile, their intensities increase with the increase of LD working currents. In order to study the upconversion mechanism, the dependence of integrated upconversion emission intensity on LD working current was derived by numeric analyses on the upconversion emission spectra for each case of different SCTI. Insets (a) and (b) of Fig. 2 display, respectively, the dependence of green and red upconversion emission intensities on the working currents in both the cases of 20 and 120 s of SCTIs. It is seen that the dependence of green and red upconversion emission intensities on the LD follow different variation trends in the two cases of different SCTIs. When the SCTI is longer, the upconversion emission intensity displays saturation-like behavior. It is well known that for an n-photon process, the upconversion emission intensity Iup is proportional to Pn, here P is excitation power of LD. It is also a commonly accepted fact that the output power P of a LD depends linearly on its working current i, thus this relation can be expressed as P = P0(i i0), here P0 is a constant, and i0 is current threshold of LD. Therefore, the relation between upconversion emission intensity Iup and the LD working current i can be easily derived as 
Iup ¼ I0 ði i0 Þn
Eq. (1) was ﬁt to the experimental data in the insets (a) and (b) of Fig. 2, and the n values were derived to be 2.18 (SCTI = 20 s) and 2.79 (SCTI = 120 s) for the green upconversion emission, 1.77 (SCTI = 20 s) and 2.26 (SCTI = 120 s) for the red upconversion emission. It should be mentioned that in the ﬁtting processes the data at saturated stage were excluded. The ﬁtting results indicate that the green upconversion emission could be 2- or 3-photon process, and the red upconversion emission is approximately 2-photon process, which are very bewildering. In fact the UCL mechanism
W. Liu et al. / Optical Materials 35 (2013) 1487–1492
Fig. 1. (a) XRD pattern for the studied sample Gd2(WO4)3:Er3+/Yb3+ (top part) and together with the pattern plotted by using the data reported in JCPDS No. 23–1076. (b) SEM image of the studied sample. (c) Particle size distribution.
200 20 s 120 s 5
(a) green emission n = 2.79 0.4
LD current increases
n = 2.18
20 s 120 s
n = 2.26
(b) red emission
n = 1.77
Current (A) 5
To clarify the effect of laser-induced heat on the UCL, the time scanning upconversion emission spectra for the varied excitation powers (namely, different LD working currents) were measured by monitoring green (543 nm) and red (658 nm) emissions. Fig. 3a and b shows the time scanning upconversion emission spectra for the green and red emissions of the studied sample,
Wavelength (nm) Intensity (a.u.)
2000 Fig. 2. Upconversion emission spectra under 980 nm LD working at different currents, the time interval of spectral measurement is 20 s. Insert shows the dependence of green (a) and red (b) integrated UC emission intensities on the LD working current. Open squares and solid triangles indicate the data collected for 20 and 120 s time interval, respectively.
1.1A 0.9A 0.7A 0.5A
6000 1.7A 1.5A
should not depend on the SCTI since the experimental conditions are not changed in addition to the time interval between two times spectral measurements. Taking the experimental conditions and the ﬁtting results into account, it may be concluded that the impenetrable ﬁtting results probably is relative to the sample temperature, since the sample was continually irradiated by LD, which may cause an increase of sample temperature due to the fact that the upconversion process in Ho3+/Yb3+ system accompanies nonradiative transitions [30–32].
4000 1.3A 1.1A
0.9A 0.7A 0.5A
Time (s) Fig. 3. Time scanning of green and red upconversion emission Gd2(WO4)3:Er3+/Yb3+ phosphor measured at different working currents of 980 nm LD.
W. Liu et al. / Optical Materials 35 (2013) 1487–1492
Temperature increases 1000 800 600
Wavelength (nm) 3+
3.3. Temperature dependence of upconversion mechanism In Section 3.1, it has been seen that the UCL of Gd2(WO4)3:Ho3+/Yb3+ phosphor can be affected by the sample temperature. However, the UCL mechanism is not clear. In order to comprehensively understand the UCL mechanism, the upconversion emission spectra under varied excitation powers were measured at different temperatures. It is worthwhile to mention that the measurement of each upconversion emission spectrum was done as the sample was just irradiated by the laser, and the wavelength scanning speed was set to be 2400 nm/min. Meanwhile, during the idle time of spectral measurements the laser irradiation was stopped for enough time in the interest of avoiding the laser induced thermal effect. For the sake of exploring the UCL mechanism of the sample at different temperatures, the integrated UCL intensity dependences on LD working current for the green and red emissions at different temperatures were derived and are shown in Fig. 5a. Eq. (1) was
Red (a) RT
Red o 200 C
Red o 300 C
Green o 200 C
Green o 300 C
400 0 293 K
Above results tell us that to identify the multiphoton process through the intensity dependence relationship, namely Eq. (1), the laser induced thermal effect should be considered. In doing so the heat generation caused by laser irradiation should be somehow restricted in case incorrect conclusion is deduced.
respectively. It should be mentioned that each measurement was started when the sample was cooled for enough time to let it be at room temperature. It can be seen that when the excitation power is lower (for example, the LD working currents are lower than 0.9 A) the green UCL intensity almost not changes with irradiation time of LD, but when the excitation power is higher, the UCL intensity increases ﬁrstly and then decreases with irradiation time. It should be noted that the time for achieving maximum UCL intensity is different for different excitation power, and that the excitation power higher, the time shorter. Moreover, the higher the excitation power, the more intense the maximum UCL intensity. All these results can be explained well by the LD induced thermal effect. When the excitation power is lower, the heat energy generated by LD irradiation in the sample is less, and the less heat energy could be quickly consumed by the environment (surrounding the laser beam spot), thus the sample temperature is almost not changed, and the UCL intensity is almost unchanged with irradiation time; when the excitation is higher, the heat energy cannot be completely consumed, thus the sample temperature would be increased. In general, the temperature elevation will lead to the thermal quenching of luminescence centers , nevertheless it should be noted that several non-resonant energy transfers are involved into the upconversion process of Ho3+/Yb3+ codoped system, and the increased temperature in a certain degree may be beneﬁcial to the energy transfers , thus the upconversion emission intensity may increase. However, with the continuous increase of the sample temperature (in the case of higher excitation, the LD irradiation time increases) the non-radiative relaxation rates for the emitting levels and the metastable levels will increase, which will directly cause the decease of the UCL intensity and depress the increased part in virtue of energy transfers. Additionally, it is indubitable that the excitation power is higher, the time of reaching the sample temperature of maximum upconversion emission intensity is shorter, which are in good agreements with the fact as observed in Fig. 2a. The similar results for the red upconversion emission were also observed as shown in Fig. 3b. To further validate above conclusion the upconversion emission spectra at different sample temperatures were measured under excitation of 980 nm LD working at 1.3 A and are shown in Fig. 4. It can be seen that the luminescence intensities of both the green and red emissions increase ﬁrst and then decrease. This variation trend indicates that the analyses on the time scanning data are reliable. We also measured the time scanning upconversion emission spectra when the excitation power was kept as a constant, but the laser beam areas were different. The observed proﬁles are similar to those as shown in Fig. 3. However, the luminescence intensity in the case of large laser beam area gets its maximum faster than that in the case of small laser beam area. This fact indicates that sample temperature change is also dependent on the laser power density.
Fig. 4. Upconversion emission spectra of Gd2(WO4)3:Er /Yb phosphor measured at different sample temperatures under 980 nm LD working at 1.3 A.
Temperature (K) Fig. 5. (a) Dependence of green and red integrated upconversion emission intensities on working current of 980 nm LD measured at different temperature. (b) Dependence of photon number for the upconversion process on the sample temperature.
W. Liu et al. / Optical Materials 35 (2013) 1487–1492
used to ﬁt the data in Fig. 5a, and the n values derived from the ﬁtting processes were displayed in the ﬁgure. Fig. 5b shows the dependence of photon number for the upconversion process on the temperature. It can be seen from Fig. 5b that the n values for both the green and red UCL increase with the increase of the sample temperature, which indicates that with increasing the sample temperature the number of excitation photon needed for emitting one green or red emission photon increases. This fact means that UCL mechanisms at different temperatures are different. The red emission changes from 1.5-photon process at room temperature to 2-photon process at 300 °C, the green emission changes from 2-photon process at room temperature to 2.5-photon process at 300 °C. The 2.5-photon process means that the phosphor may absorb 5 photons to generate 2 photons of green, or the 2-photon and 3-photon processes are existent at the same time. Based on above results the possible UCL mechanisms for Gd2(WO4)3: Ho3+/Yb3+ phosphor at different temperatures can be drown in Fig. 5, where (I–IV) regions display 2- and 1.5-photon processes for red emissions, and 2- and 3-photon processes for green emissions. As have addressed in the introduction section, the Ho3+ cannot be directly excited by 980 nm light, thus Ho3+ ions at ground state can accept the energy from Yb3+ to get into both the 5 I5 and 5I6 levels via phonon assisted energy transfer processes. It should be noted that the energy transfer from 2F5/2(Yb3+) to 5I5 (Ho3+) is easier than that to 5I6 (Ho3+) due to small energy mismatch. The Ho3+ at 5I5 level can be de-excited down to 4I7 level via a cross relaxation 5I5 + 5I8 ? 5I7 + 5I7, as a consequence one 980 nm photon makes two Ho3+ ions populated at 5I7 level. Furthermore, these two Ho3+ ions individually accept the energy from two Yb3+ ions to enter their 5F5 level, and then generate red emission of two photons. Therefore, the 1.5-photon process for red emission is dominant at room temperature (see part II in Fig. 6). When the temperature increases to 200 °C, the non-radiation transition rates of 5I5 and 5I6 levels increase, thus 5I7 begins to be populated and then it accepts the energy from Yb3+ again to get into 5 F5, ﬁnally it can be depopulated via a radiation transition to releases a photon of red emission. This process is a 2-photon process which is co-existent with the 1.5-photon process, therefore at 200 °C the n value derived from the ﬁtting by using Eq. (1) falls into the region of 1.5–2.0 (see part I in Fig. 6). When the sample temperature reaches 300 °C, the non-radiative transitions of 5I5 and 5 I6 levels are dominant since the energy distances from 5I5 to 5I6 and from 5I6 to 5I7 are smaller than that from 5I7 to 5I8, thus the lifetimes for 5I5 and 5I6 is smaller than 5I7 level. Therefore, the 2-photon process is the main process for the red upconversion emission, therefore, n value is close to 2 at 300 °C (see part I in Fig. 6). The
Based on study of UCL in Gd2(WO4)3:Ho3+,Yb3+ phosphor, it was found that the laser induced thermal effect should be considered, and the intensity dependence relationship for analyzing the UCL mechanism should be carefully used. The UCL process might be dependent on the sample temperature. At lower temperature, to generate one UC emitting photon needs less excitation photons, but at higher temperature more excitation photons may need to generate one UC emitting photon. Acknowledgements This work was partially supported by NSFC (National Natural Science Foundation of China, Grant Nos. 51272109, 51002041, 11104024, 11274057, 11104023 and 61078061), Fundamental Research Funds for the Central Universities (Grant No. 2012TD017), Natural Science Foundation of Liaoning Province (Grant Nos. 20111031 and 20111032), the Natural Science Foundation of Zhejiang Province (Grant No. R4100364), and the State Key Development Program for Basic Research of China (973 Program, Grant No. 2012CB626801). References          
   
2-photon process of green UCL has been widely observed [33– 35], which usually explained as that the Ho3+ at 5I6 level continues to accept the energy from Yb3+ and then reaches the green emission levels 5S2/5F4, ﬁnally generates green emission (see part III of Fig. 6). At higher temperature 5F5 level is mainly populated by 2-photon process, and its nonradiative transition rate is higher, therefore the lower levels 5I4, 5I5 and 5I6 can be populated via the nonradiative transition originating from 5F5 level. The Ho3+ ions at 5I4, 5I5 and 5I6 can further accept the energy from Yb3+ and get into green emitting levels 5S2/5F4, thus 3-photon green ﬂuorescence occurs (see part IV in Fig. 6).
  
Energy (cm )
Fig. 6. Scheme for energy level of Er3+ & Yb3+ and illustration of upconversion process: zones (I) and (II) are for the 2-photon and 1.5-photon red upconversion emissions, zones (III) and (IV) for 2-photon and 3-photon green upconversion emissions.
       
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