Sensors and Actuators B 190 (2014) 512–515
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Er3+ -Yb3+ and Eu3+ -Er3+ -Yb3+ codoped Y2 O3 phosphors as optical heater Riya Dey, Anurag Pandey, Vineet Kumar Rai ∗ Laser and Spectroscopy Laboratory, Department of Applied Physics, Indian School of Mines, Dhanbad 826004, Jharkhand, India
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Article history: Received 21 May 2013 Received in revised form 4 September 2013 Accepted 5 September 2013 Available online xxx Keywords: Upconversion emission Optical thermometry Fluorescence intensity ratio Nanoheater
a b s t r a c t The internal optical heating in the cubic phase Er3+ -Yb3+ and Eu3+ -Er3+ -Yb3+ codoped Y2 O3 phosphors synthesized by low temperature, urea assisted combustion technique due to near infrared (NIR) diode laser excitation have been performed by monitoring the upconversion luminescence bands corresponding to the 2 H11/2 → 4 I15/2 and 4 S3/2 → 4 I15/2 transitions of Er3+ ions. The results indicate that the developed phosphor materials are of signiﬁcant interest for making optical heater. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The upconversion (UC) emissions from rare earth ions doped glasses, phosphors and ceramics have been a subject of particular interest now a day because of its versatility in broad ﬁeld of applications such as magnetic resonance imaging (MRI), display devices, temperature sensors, solar cell, thermal imaging [1–5]. Among these applications rare earth doped materials based temperature sensors are attracting much attention to the researchers. For the optical temperature sensing purposes the change of ﬂuorescence intensity ratio (FIR) as a function of external temperature in different rare earth doped/codoped systems have been studied [6–8]. The change of FIR with temperature between two close lying (i.e. thermally coupled) levels of same rare earth ion has been studied mostly for the investigation of temperature sensing behavior of the rare earth doped materials [3,6–11]. The temperature dependent FIR study has also been implemented by considering the Stark sublevels in single ion and considering two close lying levels of different ions [12,13]. The variation of FIR with pump excitation power generates some heating in the material. The internal heating in the material induced by the laser excitation has been reported rarely [14–17]. Tikhomirov et al. have studied the laser induced optical heating for Er3+ -Yb3+ codoped ﬂuoride nanoparticles by monitoring the 2 H11/2 → 4 I15/2 and 4 S3/2 → 4 I15/2 transitions of Er3+ ions , whereas Singh et al. have studied the laser induced optical heating
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in Er3+ -Yb3+ codoped Gd2 O3 phosphor . Hayakawa et al. have observed the optical heating in the Er3+ doped TeO2 -ZnO-Nb2 O5 glass . Verma et al. have also studied the laser induced optical heating in Ho3+ -Yb3+ codoped Ca12 Al14 O33 phosphor where they have monitored the FIR of green emissions coming from the Ho3+ ion . No one has reported the optical heating behavior by using the green emissions from Er3+ ions in the Er3+ -Yb3+ and Eu3+ -Er3+ Yb3+ codoped Y2 O3 phosphors upon NIR excitation till now to the best of our knowledge. Here we report the optical heating behavior of the Er3+ -Yb3+ and Eu3+ -Er3+ -Yb3+ codoped Y2 O3 phosphor powders upon NIR diode laser excitation at 980 nm by using FIR as a function of input excitation power density. The internal heating of the developed phosphors have been computed employing temperature dependent FIR study of the synthesized phosphors and concluded the applicability of the developed materials for photonic applications on the basis of observed results. 2. Experimental The Er3+ -Yb3+ codoped Y2 O3 phosphor has been synthesized by using solution combustion process. The composition used for the synthesis was taken according to the given formula 97.7mol%Y 2 O3 +0.3mol%Er2 O3 +3.0mol%Yb2 O3 The detail of the synthesis used for the preparation of optimized Eu3+ -Er3+ -Yb3+ codoped Y2 O3 phosphor using solution combustion process has been reported in our previous work . The concentrations of Er2 O3 , Eu2 O3 and Yb2 O3 have been optimized to
R. Dey et al. / Sensors and Actuators B 190 (2014) 512–515
UC emission intensity (arbitrary unit)
Er -Yb :Y2O3
Eu - Er -Yb :Y2O3 1.2
(a) 0.6 0
wavelength (nm) 3+
Fig. 2. The plot of the FIR (I523 /I551 ) as a function of input pump power density for (a) Er3+ -Yb3+ (b) Eu3+ -Er3+ -Yb3+ codoped Y2 O3 phosphor powders. 3+
Fig. 1. The UC emission spectra of Er -Yb and Eu -Er -Yb phosphors on increasing excitation power densities.
copdoped Y2 O3
0.3 mol%, 1.5 mol% and 3.0 mol% respectively . Finally the phosphor samples have been heat treated at 800 ◦ C for 2 h and used for further characterization purposes. For the UC emission measurement by using the excitation at 980 nm from a CW diode laser with pump power density 18.51 W/cm2 at different temperature, the excitation source is focused on the sample kept inside a homemade heat chamber and a thermocouple is placed very near (∼3 mm apart) to the focal spot (diameter ∼1.40 mm) in order to avoid the temperature gradient effect . For the study of FIR variation with laser power density, the upconversion (UC) spectra of the phosphors have been recorded at different excitation power densities at room temperature. 3. Results and discussion The green UC emission spectra of optimized cubic phase Er3+ and Eu3+ -Er3+ -Yb3+ codoped Y2 O3 phosphors conﬁrmed by XRD  have been recorded in the 510–580 nm range upon 980 nm diode laser excitation on varying the pump power density (as shown in Fig. 1). The UC emission bands are observed around 523 nm and 551 nm corresponding to the 2 H11/2 → 4 I15/2 and 4 S3/2 → 4 I15/2 transitions of Er3+ ion respectively. From ﬁgure it is seen that the trend of variation in intensity of two bands are not the same on increasing the excitation power density in both case. This dissimilar behavior of emission intensities of two bands with excitation power density forms a variation in their intensity ratio (I523 /I551 ). We know that the variation in FIR of two close lying levels of rare earth (RE) ions is due to change in their populations [10,17,19]. As the energy difference between the 2 H11/2 and 4 S3/2 states is ∼770 cm−1 and follows Boltzmann’s distribution . So, the FIR variation observed corresponding to the 2 H11/2 → 4 I15/2 and 4S 4 3+ 3+ and Eu3+ -Er3+ -Yb3+ codoped 3/2 → I15/2 transitions of Er -Yb Y2 O3 phosphors due to change in laser power density generates an idea of optical heating. For experimental veriﬁcation of the concept of optical heating induced by laser power density, the FIR technique for the same UC emission bands have been used and obtained the factors which affect the change in the intensity ratio. The FIR as a function of laser excitation power density for Er3+ -Yb3+ and Eu3+ -Er3+ -Yb3+ codoped Y2 O3 phosphors have been shown in Fig. 2. Fig. 3 shows the variation in UC emission band on increasing the temperature of the samples. It is observed that on increasing the external temperature the band positions do not change, only the intensity of the UC emission bands change. This change in intensity Yb3+
Power density (W/cm )
of 2 H11/2 → 4 I15/2 and 4 S3/2 → 4 I15/2 transitions cause an increase in the FIR (I523 /I551 ) with temperature according to the formula FIR =
I523 = C exp I551
where, I523 and I551 are the integrated intensities associated with the 2 H11/2 → 4 I15/2 and 4 S3/2 → 4 I15/2 transitions respectively. C is proportionality constant. E is the energy difference between the two thermally coupled levels 4 S3/2 and 2 H11/2 , k (=0.695 K−1 cm−1 ) is the Boltzmann’s constant. As the ‘C’ value varies for different samples, so to obtain the proportionality constant Eq. (1) has been modiﬁed as ln
= ln C −
where all the terms have their usual meanings as mentioned above. A plot of logarithmic of FIR as the function of inverse absolute temperature of Er3+ -Yb3+ codoped Y2 O3 phosphor is shown in Fig. 4. The linear ﬁtting of the experimental data using Eq. (2) gives a slope that corresponds to E/k value. In case of Er3+ -Yb3+ codoped Y2 O3 phosphor, the observed energy difference (E) between the two levels and constant ‘C’ are found to be 474.41 cm−1 and 6.18, respectively as shown in Fig. 4. The temperature dependence FIR study of Eu3+ -Er3+ -Yb3+ codoped Y2 O3 phosphors have been performed and
UC emission intensity (arbitrary unit)
4 4 S3/2 → I15/2
Eu - Er -Yb :Y2O3 2 4 H11/2 → I15/2
Er - Yb :Y2O3
Wavelength (nm) Fig. 3. The UC emission spectra of Er3+ -Yb3+ and Eu3+ -Er3+ -Yb3+ codoped Y2 O3 phosphors at two extreme temperatures.
R. Dey et al. / Sensors and Actuators B 190 (2014) 512–515 Table 1 Values of temperature at different pump power density of Er3+ -Yb3+ and Eu3+ -Er3+ Yb3+ codoped Y2 O3 phosphor corresponding to different FIR values.
Y2O3: Er - Yb
Pump power density (W/cm2 )
ln(I523 / I551)
0.6 Slope=682.61 K
7.01 13.18 18.51 23.63 29.42 34.87 40.13 45.06 50.45 55.84
ΔE = 474.41 cm
Fig. 4. The monolog plot of the FIR (I523 /I551 ) as a function of inverse absolute temperature for Er3+ -Yb3+ codoped Y2 O3 phosphor powders.
the observed energy difference (E) between 4 S3/2 and 2 H11/2 levels and constant ‘C’ were observed to be 738.54 cm−1 and 19.29, respectively . Using the FIR observed at different excitation power densities, we have calculated the temperature (by using Eq. (1)) of both the samples caused by laser heating effect by applying the experimental values of E/k and C obtained from temperature dependence FIR study. Fig. 5 depicts the variation of samples temperature gain with varying excitation power density and the corresponding values are listed in Table 1. It is seen that the temperature is increasing with the increase in pump power density. In case of Er3+ -Yb3+ codoped Y2 O3 phosphor the sample temperature varies from 317 to 347 K with the variation in pump power density 7.01–55.84 W/cm2 whereas with the same power density variation in Eu3+ -Er3+ -Yb3+ codoped Y2 O3 phosphor the temperature of the sample varies within 301–403 K range. The temperature variation in Er3+ -Yb3+ codoped Y2 O3 phosphor is less compared to that of the Eu3+ -Er3+ -Yb3+ codoped Y2 O3 phosphor. This is because of the less variation in spectral properties of the Er3+ -Yb3+ codoped Y2 O3 phosphor powder with laser power density. So, we conclude that optical heat generated by laser power density is much more in the Eu3+ -Er3+ -Yb3+ codoped Y2 O3
Er -Yb :Y2O3 3+
0.715 0.736 0.748 0.765 0.783 0.805 0.824 0.835 0.855 0.865
316.5 320.8 323.2 326.8 330.4 334.8 338.8 340.9 345.0 347.1
0.566 0.635 0.703 0.799 0.887 0.988 1.071 1.151 1.243 1.381
301.0 311.0 320.8 333.6 345.0 357.6 367.6 376.9 387.5 402.9
Eu - Er -Yb :Y2O3
phosphor rather than the Er3+ -Yb3+ codoped Y2 O3 phosphor. The heat generation inside the samples are due to the nonradiative relaxation involved and the crystalline nature of the synthesized materials. The crystalline materials show the efﬁciency of heat generation under optical excitation. The energy gained by the crystalline particles due to laser power density is transferred into heat via nonradiative channels. The other reason for heat production may be the quantum conﬁnement of phonons which results the enhancement of the electron–phonon interaction and ﬁnally the heating of the sample  which is beyond the scope of present investigation.The result shows that the laser excitation induced heat generation have made these phosphor powders useful in local hyperthermia based cancer treatment as the temperature produced are within the required range for hyperthermia based treatment . 4. Conclusion The cubic phase Er3+ -Yb3+ and Eu3+ -Er3+ -Yb3+ codoped Y2 O3 phosphors have been successfully synthesized by cost effective solution combustion technique. The input excitation power density induced FIR change and eventually the optical heating of the phosphors have been studied by varying the input excitation power density of the 980 nm NIR diode laser. By using the experimental values of E/K and C from the graphs the temperature produced inside the phosphors have been calculated corresponding to different pump power density. The internal heat produced from the samples is within the range required for hyperthermia treatment too. So, it may be used in cancer treatment as well as the temperature sensing probe (optical heater). Acknowledgements
Authors are grateful to the University Grants Commission (UGC), New Delhi, India for ﬁnancial assistance. Also, one of the authors Riya Dey acknowledges Indian School of Mines, Dhanbad for providing the fellowship.
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Power density (W/cm ) Fig. 5. The temperature variation of Er3+ -Yb3+ and Eu3+ -Er3+ -Yb3+ codoped Y2 O3 phosphor powders as a function of pump power density.
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Biographies Vineet Kumar Rai, is currently working as Associate Professor in Department of Applied physics, ISM, Dhanbad, India. Presently, he is working in the area of atomic, molecular & laser physics, sensors, nanophotonics, etc. He has published more than 68 articles in reputed international journals, 3 book chapters, and has contributed to a number of national/international conferences/seminars.
Riya Dey, Junior Research Fellow Department of Applied Physics ISM, Dhanbad-826004, Jharkhand, India, Riya Dey is working as a Ph.D. scholar in the Department of Applied Physics at Indian School of Mines, Dhanbad, in the area of “Optical properties of some rare earth doped phosphors and their applications”.
Anurag Pandey is a Ph.D. student in Department of Applied physics, ISM, Dhanbad, India and currently working on “Lanthanides doped oxide materials”.