BaZnLa2O5:Ho3+-Yb3+ phosphor for display and security ink ...

A. K. Soni and V. K. Rai

Vol. 31, No. 9 / September 2014 / J. Opt. Soc. Am. B

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BaZnLa2O5:Ho3+-Yb3+ phosphor for display and security ink application Abhishek Kumar Soni and Vineet Kumar Rai* Laser and Spectroscopy Laboratory, Department of Applied Physics, Indian School of Mines, Dhanbad 826004, Jharkhand, India *Corresponding author: [email protected] Received June 26, 2014; accepted July 22, 2014; posted July 31, 2014 (Doc. ID 214831); published August 26, 2014 The formation of single-phase, morphological information and the impurities present in the developed phosphor has been identified by using x-ray diffraction, scanning electron microscope, and Fourier transform infrared analysis. Nontunable intense green upconversion emission from single-phase tetragonal Ho3 -Yb3 -codoped BaZnLa2 O5 phosphor upon 980 nm excitation has been reported. The effect of codoping with Yb3 ions on the upconversion emission bands in BaZnLa2 O5 :Ho3 phosphor has been discussed, and the possible upconversion mechanisms involved have been explained. The experimental results confirm the suitability of the developed phosphor in display devices and security applications. © 2014 Optical Society of America OCIS codes: (140.2020) Diode lasers; (140.3380) Laser materials. http://dx.doi.org/10.1364/JOSAB.31.002201

1. INTRODUCTION The multicolor emissions observed from luminescent materials have attracted a lot of research attention due to their several advantageous applications in displays, lighting, sensing, security, and biomedical applications [1–12]. Rare-earth (RE)-doped luminescent materials are of particular interest for getting upconversion emission because of the abundant energy levels present in RE ions. The near-infrared (NIR) excitation used to excite the materials is very important for biological studies as well as in clinical applications because of its several benefits, such as small light scattering, being less damaging to cells, and deep penetration capability [13]. Halidebased solid hosts are good upconversion hosts due to their relatively smaller phonon energy compared to other hosts, but due to their hygroscopic and toxic behavior they are not suitable for making luminescent materials; therefore hosts with nontoxic, nonhygroscopic behavior and low phonon frequency are mostly preferred, as they are able to produce efficient luminescence by reducing the energy loss caused by nonradiative relaxations. Luminescent materials, doped with a single RE element, are able to generate luminescence but of poor intensity. As the absorption cross section of RE ions introduced as dopants is smaller due to its 4f-intra configurational transitions, researchers are trying for various approaches that can enrich the luminescence behavior. In this regard, the concept of codoping with other REs has been found to be a promising approach in recent years. Therefore, the effect of codoping with ions showing efficient energy transfer to emitting ions that can effectively enhance the upconversion emission intensity by changing the environment of probe ions has been reported [14,15]. Green-light-emitting phosphors are essential components for tricolor emitting materials for materialization of whitelight-emitting diodes. Holmium-doped phosphors are generally used among the RE ions for green emission, and are 0740-3224/14/092201-07$15.00/0

suitable components for white-light generation. Different combinations of RE elements have been selected by different researchers for generating white light from the phosphor materials [16,17]. The quality of white-light emission depends not only on the dopants but also on the choice of host, which plays a crucial role [18]. In most of the studies reported by different researchers the excitation energy was observed to be distributed among all the bands, thereby reducing the efficiency of the materials to be used for making the single wavelengthbased optical devices. The search for materials that may pinpoint the excitation energy on a single band is an essential demand of modern technology. Today, researchers are much more interested in exploring the new inorganic complex luminescent materials owing to their alluring luminescent behavior [19,20]. To the best of our knowledge, BaZnLa2 O5 as a host material has not been used for preparing upconversion luminescent materials. Moreover, green light emission from Ho3 ∕Yb3 -doped/codoped BaZnLa2 O5 phosphors has not been reported so far. Several synthesis techniques, viz. the combustion technique, spray pyrolysis, the coprecipitation technique, solid-state reaction, hydrothermal synthesis, and the sol-gel route, have been used for the synthesis of phosphor materials [5,21–25]. Among these synthesis techniques, the coprecipitation technique strengthens the upconversion luminescence by minimizing the accumulation of structures [26]. This article reports the upconversion emission in the BaZnLa2 O5 :Ho3 -Yb3 phosphor synthesized by the chemical coprecipitation method upon inexpensive 980 nm diode laser excitation. The structural and surface morphological investigations have been carried out by x-ray diffraction (XRD) and scanning electron microscope (SEM) analysis, respectively. The mechanism involved for the upconversion emission has also been identified by decay curve analysis. The applicability of the present material for display devices and security © 2014 Optical Society of America

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J. Opt. Soc. Am. B / Vol. 31, No. 9 / September 2014


purposes has been demonstrated by varying the pump power and phosphor as security ink, respectively.

3. RESULTS AND DISCUSSION

A. Materials and Method The BaZnLa2 O5 :Ho3 and BaZnLa2 O5 :Ho3 -Yb3 phosphors have been synthesized by using the chemical coprecipitation method. BaCO3 , ZnO, La2 O3 , Ho2 O3 , and Yb2 O3 with 99.9% purity were taken as the starting raw materials. The compositions of the precursor chemicals were taken according to the following equations:

A. XRD Analysis The power XRD pattern of Ho3 -Yb3 -codoped BaZnLa2 O5 phosphor is shown in Fig. 1. This shows the tetragonal symmetry of BaZnLa2 O5 with lattice parameter a 6.9091Å, c 11.5907Å, which suitably matched with the JCPDS file No. 80-1882 [27]. There appear 19 peaks in the XRD pattern. The most intense peak has been observed at 2θ 30.76° due to the reflection from the (202) plane. The crystallite size of the crystallites present in the phosphor can easily be deduced with the help of XRD peak broadening measurement by employing the following Debye Scherrer’s formula [28]:

100-pBaZnLa2 O5 pHo2 O3 ;

t 0.89λ∕βf cos θ;

2. EXPERIMENT

(1)

where p 0.4, 0.5, 0.6, 0.7, and 0.8 wt. %, and 100-p-qBaZnLa2 O5 pHo2 O3 qYb2 O3 ;

(2)

where p 0.6 wt: % and q 0.5, 0.7, 1.0, and 3.0 wt. %. The chemical reactions occurring to form the host material (BaZnLa2 O3 ) during the synthesis process can be described as BaCO3 2HNO3 → BaNO3 2 H2 O CO2 ↑; 2La2 O3 8HNO3 → 4LaNO3 2 4H2 O O2 ↑; ZnO 2HNO3 → ZnNO3 2 H2 O; 2BaNO3 2 4LaNO3 2 2ZnNO3 2 → 2BaZnLa2 O3 8N2 ↑ 21O2 ↑: The stoichiometric amount of precursor materials has been reacted with HNO3 acid and heated at 80°C to form their transparent nitrates. The prepared nitrates of starting materials were mixed together. The precipitate was obtained with the addition of strong base (NH4 OH) into the nitrate mixture and then after filtering with filter paper followed by successive washing with ethanol and distilled water. After that the precipitate was dried at room temperature and then at ∼500°C around 2 to 5 min. The prepared as-synthesized phosphors were crushed into fine powder form and further annealed at ∼800°C for 3 h to obtain the good crystallite formation and impurity reduction. The annealed samples were used further for structural and optical characterizations. B. Characterizations The XRD pattern of the prepared phosphor has been recorded by a BRUKER D8 focus x-ray diffractometer in the 25° ≤ 2θ° ≤ 90° range. The surface morphology has been carried out by using SEM analysis. The impurities in the form of different functional groups have been detected with the help of Fourier transform infrared (FTIR) spectrum recorded in the 400–4000 cm−1 range. The UC emission spectra have been recorded with a Princeton (Acton SP-2300) monochromator attached with a photomultiplier tube (PMT) upon 980 nm continuous-wave (CW) laser excitation. The lifetime measurements have been performed with the help of a fast-responding digital storage oscilloscope. All the measurements have been carried out at room temperature.

(3)

where “t” is the average crystallite size, λ is the x-ray wavelength, and βf and θ are the full width at half-maximum (FWHM) and Bragg’s angle of a measured peak, respectively. The calculated crystallite size corresponding to the reflections from different planes has been given in Table 1. From Table 1, it is marked that the present phosphor exhibits nanocrystallites, ranging from 10 to 20 nm. The effective ionic radii mismatch between the dopant ions (Ho3 89.4 pm, Yb3 85.8 pm) and the sites (La3 117.2 pm) may result in a small stress and hence strain inside the crystal structure. The Williamson–Hall (W-H) relation is used to determine the crystallite size as well as the lattice strain present in the sample [29,30]. The W-H plot for BaZnLa2 O5 :Ho3 -Yb3 phosphor is presented in Fig. 2, which obeys the relation βf cos θ 4ε sin θ 0.89λ∕t;

(4)

where “ε” is the strain present in the sample and other terms have their usual meanings. By deducing the slope value and the intercept of the W-H plot, one can directly estimate the strain and crystallite size, respectively. The calculated values of strain and crystallite size are found to be ∼3.3 × 10−4 and ∼20.4 nm, respectively. This small strain has negligible effect on the crystal structure, whereas the average crystallite size is in close agreement with that calculated by using Scherrer’s formula.

Fig. 1. X-ray diffraction pattern of the BaZnLa2 O5 :Ho3 -Yb3 phosphor annealed at 800°C with JCPDS file No. 80-1882.



Table 1. Crystallite Size Corresponding to the Diffraction from Different Planes with Their FWHM Values (h K l) (211) (202) (204) (006) (400) (330) (433)

2θ (degree)

cos θ

βf (FWHM) in radians

t (nm)

26.82 30.76 40.29 46.92 52.89 56.16 72.91

0.972 0.964 0.938 0.917 0.895 0.882 0.804

0.0073 0.0085 0.0080 0.0073 0.0081 0.0154 0.0088

19.33 16.73 18.27 20.48 18.93 10.09 19.38

B. FTIR and SEM Analysis In order to investigate the presence of vibrational bands corresponding to different functional groups, the FTIR spectrum of BaZnLa2 O5 :Ho3 -Yb3 phosphor has been recorded (Fig. 3). From Fig. 3, we have observed different peaks associated with the different functional groups. The intense band peaking at 436 cm−1 can be described as the cut-off phonon frequency of the developed phosphor. The assignments of the different

Fig. 2. Williamson–Hall plot of BaZnLa2 O5 :Ho3 -Yb3 phosphor.

Fig. 3. FTIR spectrum of BaZnLa2 O5 :Ho3 -Yb3 phosphor annealed at 800°C.

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Table 2. Assignments of FTIR Peaks for BaZnLa2 O5 :Ho3 -Yb3 Phosphor Peak Position (cm−1 )

Mode of Vibration of Different Functional Groups

436.14 645.85, 901.19 1385.94 1654.87 2374.16 3608.06

Stretching vibration of metal–oxygen bond In-plane and out-plane bending CO−2 3 Asymmetric stretching C–O band O–H bending vibration CO−2 3 stretching vibration O–H stretch of crystalline water molecule

functional groups present with their corresponding energies are given in Table 2. Figure 4 shows the SEM micrograph of BaZnLa2 O5 :Ho3 Yb3 phosphor. From the SEM image, it is clearly visible that the prepared phosphor comprises a large number of small, agglomerated particles with particle size in the micrometer range. C. Upconversion Emission Study To observe the maximum UC emission intensity the concentration of dopants (Ho3 , Yb3 ) has been optimized. For this the UC emission spectra of Ho3 ∕Ho3 -Yb3 -doped/codoped BaZnLa2 O5 phosphors upon varying the concentration of dopants have been recorded (Fig. 5). From Fig. 5, it is clearly observed that 0.6 wt. % of Ho3 and 0.7 wt. % Yb3 ions concentration is the optimum concentration. Beyond this concentration a reduction in UC emission intensity has been detected due to the effect of concentration quenching [5,31–33]. The optimized UC emission spectra of 0.6 wt. % Ho3 and 0.6 wt:% Ho3 0.7 wt: % Yb3 -doped/codoped BaZnLa2 O5 phosphors recorded at identical conditions by using 980 nm diode laser excitation are shown in Fig. 6. In the case of singly Ho3 -doped phosphor, three UC emission peaks are observed around 546, 661, and 757 nm due to the 5 F4 , 5 S2 → 5 I8 , 5 F → 5 I , and 5 F , 5 S → 5 I transitions, respectively (in Fig. 6 5 8 4 2 7 the spectrum profile of singly Ho3 -doped phosphor is multiplied by a factor of 20), whereas in the Ho3 -Yb3 -codoped phosphor, one extra peak around 491 nm is observed due to the 5 F3 → 5 I8 transition of the Ho3 ion [34,35]. Upon codoping with the Yb3 ions a significant enhancement in all the UC emission bands has been detected. This enhancement

Fig. 4. SEM image of BaZnLa2 O5 :Ho3 -Yb3 phosphor annealed at 800°C.

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Also, the UC emission spectra of the BaZnLa2 O5 :Ho3 -Yb3 phosphor upon increasing the pump power have been recorded, and significant enhancements in all UC emission bands have been observed. Now to assess the number of NIR pump photons involved in the UC process, the logarithmic of UC emission intensity versus pump power has been plotted (Fig. 7) according to the following relation: Iuc ∝ Ps ;

(5)

in UC emission intensity and origination of band around 491 nm is caused by the energy transfer from Yb3 to Ho3 ions. By comparing the UC emission spectra of Ho3 and Ho3 -Yb3 -doped/codoped phosphors, a remarkable enhancement around ∼80 times has been detected for the green UC emission band in the Ho3 -Yb3 -codoped phosphor compared to the Ho3 -doped phosphor. In addition to the significant enhancement we have also observed the broadening of UC emission bands in the spectral profile of codoped phosphor due to the noticeable amount of disorder around the lanthanide ions. Thus dopants ions essentially create an inhomogeneous local field around the Ho3 ions, this result broadening in the UC bands [36]. The intensity of other UC emission bands has also been enhanced with Yb3 ion codoping. Although the present phosphor material is capable of emitting blue, green, and red UC emission bands simultaneously, the large intensity of the green UC emission band has suppressed the visibility of others. This shows that the energy of photons involved in the UC process is localized and utilized greatly to emit radiation in the green region. The intense green color light emitted from the sample has been confirmed from its photograph given in the inset of Fig. 6.

where “Iuc ” is the UC emission intensity, “P” is the pump power, and “s” is the number of pump photons required to populate the emitting levels responsible for UC emissions [37]. The slopes observed from the plotted curves are found to be around two in all cases, which confirms the occurrence of a two-photon absorption process for all UC emissions. The reduction in the slope value is basically due to the involvement of various nonradiative relaxation channels. The proposed upconversion mechanisms could be explained by the energy level diagram of Ho3 and Yb3 ions (Fig. 8). From the energy level diagram, it is observed that there is a small mismatch (∼1500 cm−1 ) in the energy of the 5 I6 level of Ho3 ions and 980 nm photons, whereas the energy of the 2 F5∕2 excited level of the Yb3 ion is exactly matching with the energy of the excitation beam. Therefore, Yb3 ions sufficiently absorb the pump photon of 980 nm and take part actively in the UC process. In the case of singly Ho3 -doped BaZnLa2 O5 phosphor, the triply ionized ground state holmium ion populates the 5 I6 level by absorbing the 980 nm pump photon through the ground state absorption (GSA) process assisted by nonradiative relaxation. After reabsorbing the second NIR photon of the same energy, the Ho3 ions in the 5 I6 state are pumped to the 5 F4 , 5 S2 level through the excited state absorption (ESA) process. Finally, the 5 F4 , 5 S2 level depopulates either by radiative transition to the 5 I8 level resulting in green emission around 546 nm or by the phonon assisted transitions to the 5 F4 ∕5 S2 and 5 F5 levels. The red and NIR emissions around 661 and 757 nm are due to the 5 F5 → 5 I8 and 5 F4 , 5 S2 → 5 I7 transitions, respectively.

Fig. 6. Upconversion emission spectra of BaZnLa2 O5 :Ho3 and BaZnLa2 O5 :Ho3 -Yb3 phosphors exciting by 980 nm diode laser.

Fig. 7. Logarithmic dependence of UC emission intensity as a function of pump power of BaZnLa2 O5 :Ho3 -Yb3 phosphor.

Fig. 5. Variation of upconversion emission intensity as a function of dopant concentration.



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Fig. 8. Schematic energy level diagram of Ho3 -Yb3 -codoped BaZnLa2 O5 phosphor.

In Ho3 -Yb3 -codoped phosphor the energy transfer from the Yb3 ion to the Ho3 ion becomes prominent. The only possibility of populating the 5 F3 level of Ho3 may be cooperative energy transfer (CET) from the Yb3 to the Ho3 ion followed by the two NIR photon absorption process. In the CET process a pair of the Yb3 ions utilize their energy cooperatively and one of them gets excited to a virtual level (V) having energy twice that of the 2 F5∕2 level. The ground state Ho3 ion after receiving the energy corresponding to this virtual level is promoted to the 5 F3 level of Ho3 ions. The radiative relaxation from the 5 F3 level to the ground level 5 I emits a blue photon corresponding to the 5 F → 5 I tran8 3 8 sition, and a part of the population relaxes nonradiatively to the 5 F4 , 5 S2 state. Since the intensity corresponding to the 5 F3 → 5 I8 transition is very weak and sharp compared to the other transitions, therefore it appears that the 5 F3 → 5 I8 transition is due to the CET from Yb3 to Ho3 ; but with less probability, the enhancement observed in the UC emission bands lying in the green, red, and NIR regions is due to the ET-1 and ET-2 processes. As the intensity corresponding to the 5 F4 , 5 S2 → 5 I8 transition is extremely large compared to the other transitions, the ET-2 process is dominant compared to the other processes involved in UC emission. The purity of color of the light emitted from the BaZnLa2 O5 :Ho3 -Yb3 phosphor can be estimated by calculating the color coordinates (Fig. 9). The calculated color coordinates at different pump powers show no change in color coordinates (X 0.29, Y 0.70) and are found in the green region. This confirms a nontunable pure green color light emitted from the BaZnLa2 O5 :Ho3 -Yb3 phosphor, which is suitable for making display devices [38–40]. D. Lifetime Study The luminescence decay curves of prepared phosphors for green UC emission around 546 nm corresponding to the 5 F4 , 5 S2 → 5 I8 transition have been shown in Fig. 10. The decay times are found to be 332.5 1.9 × 10−6 s and 180.2 0.9 × 10−6 s for the BaZnLa2 O5 :Ho3 and BaZnLa2 O5 :Ho3 Yb3 phosphors, respectively. Thus the decay time of the 5 F , 5 S level decreases upon codoping with the Yb3 ions 4 2 in the BaZnLa2 O5 :Ho3 phosphor. This refers to an increase in the radiative transition probability of the 5 F4 , 5 S2 emitting state in the BaZnLa2 O5 :Ho3 -Yb3 phosphor [41–44]. This result supports the UC emission intensity corresponding to the 5 F4 , 5 S2 → 5 I8 transition being enhanced significantly upon codoping with Yb3 ions in BaZnLa2 O5 :Ho3 phosphor.

Fig. 9. CIE color coordinates at different pump powers for BaZnLa2 O5 :Ho3 -Yb3 phosphor.

E. BaZnLa2 O5 :Ho3 -Yb3 Phosphor as Security Ink The use of codoped phosphor material in the development of security ink has been demonstrated. For this, the codoped phosphor (0.5 g) was disseminated in acetone (5.0 ml), and using this dispersed medium, the letter “LASER” on a plane paper has been written, which is not visible under normal light illumination [Fig. 11(a)]. Upon the excitation with 980 nm radiation, the written letters become visible in green color [Fig. 11(b)], whereas the black background indicates the photograph being taken in the dark condition. As in the case of codoped material, the emission intensity is enhanced ∼80 times, which is more efficient than the result reported in other

Fig. 10. Decay curve corresponding to the 5 F4 , 5 S2 → 5 I8 transition of Ho3 and Ho3 -Yb3 -doped/codoped BaZnLa2 O5 phosphors.

Fig. 11. Optical photograph of the written matter (a) under normal light illumination and (b) under 980 nm excitation.

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work [45–50]. Thus, based on the above demonstration, it can be proposed that the prepared phosphor can be efficiently used as security ink.

4. CONCLUSION The BaZnLa2 O5 :Ho3 -Yb3 phosphor synthesized by the coprecipitation method has been characterized by XRD, SEM, and FTIR analysis. Intense green UC emission along with other UC emissions has been observed upon excitation at 980 nm. The effect of Yb3 ion codoping enriched the UC emission intensities significantly due to the energy transfer from Yb3 to Ho3 ions. The green light emitted from the sample does not show variation with pump power. Based upon the experimental results, the prepared material may be suitable in security ink, fabricating green LEDs, and display devices.

ACKNOWLEDGMENTS The authors are grateful to DST and UGC, New Delhi, India, for providing financial help. Mr. Abhishek Kumar Soni is also very thankful to the Indian School of Mines, Dhanbad, India, for providing financial support in the form of a fellowship.

REFERENCES 1. 2.

3.

4. 5. 6. 7. 8. 9. 10. 11.

12. 13. 14.

K. V. R. Murthy, “Nano phosphors for light emitting diodes (LEDs) synthesis and characterization,” Rec. Res. Sci. Technol. 4, 8–13 (2012). Y. C. Chen and T. M. Chen, “Improvement of conversion efficiency of silicon solar cells using up-conversion molybdate La2Mo2O9:Yb, R (R = Er, Ho) phosphors,” J. Rare Earth 29, 723–726 (2011). A. A. D. Adikaari, I. Etchart, P.-H. Guering, M. Berard, S. R. P. Sliva, A. K. Cheetham, and R. J. Curry, “Near infrared upconversion in organic photovoltaic devices using an efficient Yb3+:Ho3+ co-doped Ln2BaZnO5 (Ln = Y, Gd) phosphor,” J. Appl. Phys. 111, 094502 (2012). P. Sharma, S. Brown, G. Walter, S. Santra, and B. Moudgil, “Nanoparticles for bioimaging,” Adv. Colloid Interface Sci. 123–126, 471–485 (2006). A. Pandey and V. K. Rai, “Improved luminescence and temperature sensing performance of Ho3+-Yb3+-Zn2+:Y2O3 phosphor,” Dalton Trans. 4, 11005–11011 (2013). B. S. Richards, “Enhancing the performance of silicon solar cells via the application of passive luminescence conversion layers,” Sol. Energy Mater. Sol. Cells 90, 2329–2337 (2006). R. L. Toquin and A. K. Cheetham, “Red-emitting cerium-based phosphor materials for solid-state lighting application,” Chem. Phys. Lett. 423, 352–356 (2006). D. S. Zang, J. H. Song, D. H. Park, Y. C. Kim, and D. H. Yoon, “New fast-decaying green and red phosphors for 3D application of plasma display panels,” J. Lumin. 129, 1088–1093 (2009). Y. S. Fran and T. Y. Tseng, “Preparation of aluminum film on phosphor screen for field emission display,” Mater. Chem. Phys. 61, 166–168 (1999). A. Gnach and A. Bednarkiewicz, “Lanthanide-doped upconverting nanoparticles: merits and challenges,” Nano Today 7(6), 532–563 (2012). J. Shen, L. Zhao, and G. Han, “Lanthanide-doped upconverting luminescent nanoparticle platforms for optical imaging-guided drug delivery and therapy,” Adv. Drug Delivery Rev. 65, 744–755 (2013). K. Byrappa, M. K. Devaraju, J. R. Paramesh, B. Basavalingu, and K. Soga, “Hydrothermal synthesis and characterization of LaPO4 for bio-imaging phosphors,” J. Mater. Sci. 43, 2229–2233 (2008). D. K. Chatterjee, A. J. Rufaihah, and Y. Zhang, “Upconversion fluorescence imaging of cells and small animals using lanthanide doped nanocrystals,” Biomaterials 29, 937–943 (2008). V. K. Rai, A. Panday, and R. Dey, “Photoluminescence study of Y2O3:Er3+-Eu3+-Yb3+ phosphor for lighting and sensing applications,” J. Appl. Phys. 113, 083104 (2013).

A. K. Soni and V. K. Rai 15. G. S. Maciel, R. B. Guimaraes, P. G. Barreto, I. C. S. Carvalho, and N. Rakov, “The influence of Yb3+ doping on the upconversion luminescence of Pr3+ in aluminum oxide based powders prepared by combustion synthesis,” Opt. Mater. 31, 1735– 1740 (2009). 16. Z. Xia, W. Zhou, H. Du, and J. Sun, “Synthesis and spectral analysis of Yb3+/Tm3+/Ho3+-doped Na0.5Gd0.5WO4 phosphor to achieve white upconversion luminescence,” Mater. Res. Bull. 45, 1199–1202 (2010). 17. Y. Xu, Y. Wang, L. Shi, L. Xing, and X. Tan, “Bright white upconversion luminescence in Ho3+/Yb3+/Tm3+ triple doped CaWO4 polycrystals,” Opt. Laser Technol. 54, 50–52 (2013). 18. V. R. Bandi, B. K. Grandhe, K. Jang, H.-S. Lee, D. S. Shin, S.-S. Yi, and J.-H. Jeong, “Citric based sol–gel synthesis and luminescence characteristics of CaLa2ZnO5:Eu3+phosphors for blue LED excited white LEDs,” J. Alloys Compd. 512, 264–269 (2012). 19. J. Yang, J. Ding, S. Xiao, Y. Liang, and J. Chai, “Luminescence properties of Yb3+/Ho3+ co-doped Sr3YAl2O7.5 and Sr3LuAl2O7.5 powders,” J. Alloys Compd. 577, 86–89 (2013). 20. Y. S. Chang, “Blue emitting phosphors of BaLa2ZnO5 activated by bismuth ions,” J. Electrochem. Soc. 158, J115–J119 (2011). 21. B. Dong, T. Yang, and M. K. Lei, “Optical high temperature sensor based on green up-conversion emissions in Er3+ doped Al2O3,” Sens. Actuators B 123, 667–670 (2007). 22. Y. He, M. Zhao, Y. Song, G. Zhao, and X. Ai, “Effect of Bi3+ on fluorescence properties of YPO4:Dy3+ phosphors synthesized by a modified chemical co-precipitation method,” J. Lumin. 131, 1144–1148 (2011). 23. F. He, P. Yang, N. Niu, W. Wang, S. Gai, D. Wang, and J. Lin, “Hydrothermal synthesis and luminescent properties of YVO4: Ln3+ (Ln = Eu, Dy, and Sm) microspheres,” J. Colloid Interface Sci. 343, 71–78 (2010). 24. Z. Mu, Y. Hu, L. Chen, X. Wang, G. Ju, Z. Yang, and Y. Jin, “A single-phase, color-tunable, broadband-excited white lightemitting phosphor Y2WO6:Sm3+,” J. Lumin. 146, 33–36 (2014). 25. Y. C. Kang, H. S. Roh, and S. B. Park, “Preparation of Y2O3:Eu phosphor particles of filled morphology at high precursor concentrations by spray pyrolysis,” Adv. Mater. 12, 451–453 (2000). 26. W. L. Feng, Y. Jin, Y. Wu, D. F. Li, and A. K. Cai, “Co-precipitation synthesis and photoluminescence properties of Ba1-xMoO4:xEu3+ red phosphors,” J. Lumin. 134, 614–617 (2013). 27. H. Muller-Buschbaum and S. Mohr, “Zur Kenntnis von BaZnTb2O5 und BaZnLa2O5,” J. Less-Common Met. 170, 127–133 (1991). 28. F. Lei and B. Yan, “Hydrothermal synthesis and luminescence of CaMO4:RE3+ (M = W, Mo; RE = Eu, Tb) submicro-phosphors,” J. Solid State Chem. 181, 855–862 (2008). 29. K. Venkateswarlu, A. C. Bose, and N. Rameshbabu, “X-ray peak broadening studies of nanocrystalline hydroxyapatite by Williamson–Hall analysis,” Physica B 405, 4256–4261 (2010). 30. A. K. Zak, W. H. A. Majid, M. E. Abrishami, and R. Yousef, “X-ray analysis of ZnO nanoparticles by Williamson-Hall and sizes train plot methods,” Solid State Sci. 13, 251–256 (2011). 31. G. Tripathi, V. K. Rai, and S. B. Rai, “Spectroscopy and upconversion of Dy3+ doped in sodium zinc phosphate glass,” Spectrochim. Acta A 62, 1120–1124 (2005). 32. G. Tripathi, V. K. Rai, and S. B. Rai, “Upconversion and temperature sensing behavior of Er3+ doped Bi2O3–Li2O-BaO-PbO tertiary glass,” Opt. Mater. 30, 201–206 (2007). 33. L. Li, H. Lin, X. Zhao, Y. Wang, X. Zhou, C. Ma, and X. Wei, “Effect of Yb3+ concentration on upconversion luminescence in Yb3+, Tm3+ co-doped Lu2O3 nanophosphors,” J. Alloys Compd. 586, 555–560 (2014). 34. G. H. Dieke, “Review of the empirical data,” in Spectra and Energy Levels of Rare Earth Ions in Crystals (Interscience, 1968), Chap. 13, pp. 280–286. 35. N. Niu, P. Yang, Y. Liu, C. Li, D. Wang, S. Gai, and F. He, “Controllable synthesis and up-conversion properties of tetragonal BaYF5:Yb/Ln (Ln = Er, Tm, and Ho) nanocrystals,” J. Colloid Interface Sci. 362, 389–396 (2011). 36. T. Passuello, F. Piccinelli, M. Pedroni, M. Bettinelli, F. Mangiarini, R. Naccache, F. Vetrone, J. A. Capobianco, and A. Speghini, “White light upconversion of nanocrystalline Er/Tm/ Yb doped tetragonal Gd4O3F6,” Opt. Mater. 33, 643–646 (2011).

A. K. Soni and V. K. Rai 37. Z. Yang, Z. Zhao, B. Liu, and Y. Wang, “Preparation and upconversion luminescence of b-SiAlON:Ln3+ (Ln = Yb/Ho, Yb/Er) microparticles,” Mater. Lett. 93, 32–35 (2013). 38. S. Choi, S. W. Tae, J.-H. Seo, and H.-K. Jung, “Preparation of blue-emitting CaMgSi2O6:Eu2+ phosphors in reverse micellar system and their application to transparent emissive display devices,” J. Solid State Chem. 184, 1540–1544 (2011). 39. A. Pandey, V. K. Rai, R. Dey, and K. Kumar, “Enriched green upconversion emission in combustion synthesized Y2O3:Ho3+ Yb3+ phosphor,” Mater. Chem. Phys. 139, 483–488 (2013). 40. C. R. Ronda, “Recent achievements in research on phosphor for lamps and displays,” J. Lumin. 49, 72–74 (1997). 41. D. K. Mohanty and V. K. Rai, “Photoluminescence studies of Pr3+ doped lead germanate glass,” J. Fluoresc. 21, 1455–1460 (2011). 42. A. A. Kaminskii, A. O. Ivanov, S. E. Sarkisov, I. V. Mochalov, V. A. Fedorov, and L. Li, “Comprehensive investigations of the spectral and lasing characteristics of the LuAlO3 crystal doped with Nd3+,” Sov. Phys. JETP 44, 516–524 (1976). 43. B. J. Chen, G. C. Righini, M. Bettinelli, and A. Speghini, “A comparison between different methods of calculating the radiative lifetime of the 4I13/2 level of Er3+ in various glasses,” J. Non-Cryst. Solids 322, 319–323 (2003).


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44. K. Zou, H. Guo, M. Lu, W. Li, C. Hou, W. Wei, J. He, B. Peng, and B. Xiangli, “Broad-spectrum and long-lifetime emissions of Nd3+ ions in lead fluorosilicate glass,” Opt. Express 17, 10001–10009 (2009). 45. T. A. A. de Assumpção, L. R. P. Kassab, A. S. L. Gomes, C. B. de Araújo, and N. U. Wetter, “Influence of the heat treatment on the nucleation of silver nanoparticles in Tm3+ doped PbO-GeO2 glasses,”Appl. Phys. B 103, 165–169 (2011). 46. K. Mishra, N. K. Giri, and S. B. Rai, “Preparation and characterization of upconversion luminescent Tm3+/Yb3+ co-doped Y2O3 nanophosphor,” Appl. Phys. B 103, 863–875 (2011). 47. D. K. Mohanty, V. K. Rai, and Y. Dwivedi, “Yb3+ sensitized Tm3+ upconversion in tellurite lead oxide glass,” Spectrochim. Acta Part A 89, 264–267 (2012). 48. C. Zhao, X. Kong, X. Liu, L. Tu, F. Wu, Y. Zhang, K. Liu, Q. Zeng, and H. Zhang, “Li+ ion doping: an approach for improving the crystallinity and upconversion emissions of NaYF4:Yb3+, Tm3+ nanoparticles,” Nanoscale 5, 8084–8089 (2013). 49. V. Singh, V. K. Rai, I. Ledoux-Rak, L. Badie, and H. Y. Kwak, “Visible upconversion and infrared luminescence investigations of Al2O3 powders doped with Er3+, Yb3+ and Zn2+ ions,” Appl. Phys. B 97, 805–809 (2009). 50. R. Dey and V. K. Rai, “Yb3+ sensitized Er3+ doped La2O3 phosphor in temperature sensors and display devices,” Dalton Trans. 43, 111–118 (2014).