Yb3+ co-doped Y2O3 phosphor

Chemical Physics Letters 599 (2014) 122–126

Contents lists available at ScienceDirect

Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett

Observation of multi-mode: Upconversion, downshifting and quantum-cutting emission in Tm3+/Yb3+ co-doped Y2O3 phosphor Ranvijay Yadav a, S.K. Singh b,⇑, R.K. Verma a, S.B. Rai a a b

Department of Physics, Banaras Hindu University, Varanasi 221005, India Department of Physics, Indian Institute of Technology (Banaras Hindu University), Varanasi 221005, India

a r t i c l e

i n f o

Article history: Received 15 February 2014 In final form 8 March 2014 Available online 15 March 2014

a b s t r a c t Micro-crystalline Y2O3 phosphor co-doped with Yb3+/Tm3+ has been synthesized and characterized. The phosphor material gives efficient multimodal emission via downshifting (DS), upconversion (UC), and downconversion (DC)/quantum cutting (QC) luminescence processes. Cross relaxation and co-operative energy transfer (CET) have been ascribed as the possible mechanism for QC; as result of which a UV/blue photon absorbed by Tm3+ splits into two near infrared photons (wavelength range 950–1050 nm) emitted by Yb3+. The Yb3+ concentration dependent ET efficiency and QC efficiency has also been evaluated. Such multi-mode emitting phosphors could have potential applications in increasing the conversion efficiency of solar cells via spectral modification. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Rare-earth (RE) doped phosphors have made unique impact in the area of luminescence in recent decades. There is strong quest worldwide to search for new applications of RE based materials [1–3]. In this context, observation of multi-mode emission (upconversion (UC), downconversion (DC)/quantum cutting (QC) and downshifting (DS)) of RE ion in such materials is a hot topic of research. RE ions have rich energy-level structure that allows the observation of many different types of emission processes e.g. photoluminescence (PL)/DS, DC/QC, and UC [4]. However, observation of all the above processes in a single host matrix is somewhat typical and challenging. DS is normal PL process that involves transformation of one absorbed high-energy photon into one low-energy photon and so its conversion efficiency does not exceed 100% [5]. UC and QC process are relatively new process in RE ion doped materials and are being explored widely in different exotic materials to go for novel applications [6]. QC is a process which can transform the energy of one absorbed photon into two (or more) emitted low energy photons, with quantum efficiency more than 100%. Initially, it was realized in single lanthanide ions, particularly in Pr3+ and Gd3+ [7]. Later on, the same phenomenon was observed more effectively through energy transfer process between different sets of RE ions. The most effective energy transfer was observed for Gd3+ (of the host matrix) to different trivalent RE ions such as Eu3+, Tb3+, Er3+, etc. [7–9]. New ⇑ Corresponding author. Fax: +91 542 2369889. E-mail addresses: [email protected], [email protected] (S.K. Singh). http://dx.doi.org/10.1016/j.cplett.2014.03.025 0009-2614/Ó 2014 Elsevier B.V. All rights reserved.

QC materials are in great demand recently due to their novel applications in emerging fields like energy harvesting, along with the well explored applications in the plasma display panels, mercury-free fluorescent tubes, etc. [9]. In the UC process, two (or more) photons (mostly infrared photons) are converted into one photon (in visible/UV) which is opposite to the QC process [10]. This process needs low phonon frequency host (to avoid the non-radiative losses), and RE ions featuring ladder-like energy levels, which could facilitate photon absorption and subsequent energy transfer (ET) steps. In this context, Er3+, Tm3+, and Ho3+ ions are generally chosen as activators to give rise to efficient visible emissions under low pump power densities. In addition to this, the Yb3+ ion is usually co-doped as an excellent sensitizer due to its large absorption cross-section in the 900–1100 nm region, corresponding to its 2F5/2 ? 2F7/2 transition [11]. UC based nano-materials is recently a new hope for bio-imaging/bio-tagging process [12–14]. In light of these requirements for both the processes, it is quite imperative to search a combination of suitable host material and doping ions, which can efficiently coordinate both UC and QC along with the normal PL emission. Until now, there are very rare reports which cover all the luminescence mechanism in a single host matrix. In the present work, yttria matrix (Y2O3) has been selected as the host matrix because of its excellent physical properties, such as higher melting point (2400 °C), higher thermal conductivity, wide transparency range (0.2–8 lm), high refractive index (1.8) and band gap of 5.6 eV [15]. In addition to this, Y2O3 is an ideal matrix for UC process with a phonon frequency 400 cm1 [16]. RE ion Tm3+ has been selected as activator because it is very

123

R. Yadav et al. / Chemical Physics Letters 599 (2014) 122–126 0

efficient for visible emission through UC, and at the same time NIR QC is also demonstrated in Tm3+/Yb3+ co-doped fluoride materials [17,18]. So, this combination of host and activator could be an ideal choice to realize the multi-mode emission. 2. Experimental 2.1. Materials and synthesis Analytical reagent (AR) grade yttrium oxide (Y2O3, 99.99%, Himedia), thulium oxide (Tm2O3, 99.9%, Alfa Aesar), ytterbium oxide (Yb2O3, 99.99%, Alfa Aesar), nitric acid (99.9%, Merck,) and urea (99%, Fisher Scientific) were used for synthesis. The phosphor was prepared using solution combustion technique [19]. Following series of phosphors were synthesized to optimize the RE concentration.

Y2 O3 þ xYb2 O3 þ yTm2 O3 where, x = 0, 3, 5, 15 mol%; and y = 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 mol%. Metal nitrates were prepared by dissolving the oxides in nitric acid. The mixture of nitrates and urea was stirred in a beaker to get a homogeneous transparent solution. The solution is then heated at 60 °C to evaporate water and thus a transparent gel is obtained. The gel was then transferred into platinum crucible and allowed for auto ignition inside a close furnace maintained at 600 °C. The obtained foam like product was then grinded and further post annealed at higher temperature to improve the crystallinity of the as-synthesized phosphor. 2.2. Instrumentation Phase identification was carried out using 45 kW Cu rotating anode based high resolution X’PertPRO PANalytical X-ray powder diffractometer (XRD) fitted with a graphite monochromator in the diffracted beam. Data were obtained from 2h = 10° to 80° at a scanning speed of 3°/minute. Micro-structural characterizations were carried out using a scanning electron microscope (SEM: QUANTA 200). Transmission electron microscopy (TEM, TECNAI20G2, 200 kV) was used for the particle size analysis. For UC measurement, the 976 nm radiation from a diode laser (2 W, continuous mode, power tunable) was used to excite the samples. The emission was dispersed and detected using a monochoromator (iHR320, Horiba Jobin Yvon) equipped with PMT (photomultiplier tube) detector (1424 M). Photoluminescence excitation (PLE) and emission (PL) measurements were performed using a Fluorolog-3 spectrofluorometer (Model: FL3-11, Horiba Jobin Yvon) equipped with 450 W xenon flash lamp. The lifetime measurements were performed by using phosphorescence mode using pulsed xenon lamp (25 W). The PMT used in the spectrofluorometer has its higher detection limit at 850 nm. Therefore, the 266 nm excitation wavelength of a Nd:YAG laser, and CCD (charged coupled device) detector (Ocean Optics, QE 65 000) was used for QC measurement. 3. Results and discussion 3.1. Structural analysis Figure 1a shows XRD pattern of as-prepared Tm3+, Yb3+ codoped Y2O3 phosphor. All the observed diffraction peaks have been indexed well by cubic Y2O3 phase with lattice parameter a = 10.60 Å and space group Ia3 (2 0 6) (JCPDS card No. 25-1200). No impurity peak has been observed. The average crystallite size (D) was calculated by Scherer equation:

D¼

k  0:89 b  Cosh

where, k is the wavelength of incident X-ray [CuKa (1.54056 A Å)], b is the FWHM (full width at half maxima) and h is the diffraction angle for (h k l) plane. For the crystallite size calculation, three most intense peaks were selected. The FWHM of these peaks were taken by their Lorentzian peak fitting. In addition to this, prior to the calculation of particle size, instrumental correction in the measured FWHM (which is estimated by recording the XRD pattern for standard silica sample) was made to get the correct FWHM. The average crystallite size thus estimated for the as-synthesized sample was above 100 nm. To verify this and to get exact picture of the particle size and its distribution in the phosphor material, TEM image of the material was recorded. Figure 1b shows the TEM image of the phosphor sample, while picture in Figure 1c is the selected area electron diffraction (SAED) pattern. Bright spots in the SAED pattern confirm the well crystalline nature of the material. Pattern is indexed which substantiate the presence of cubic Y2O3 phase of the material. The image in (b) clearly depicts that most of the particle are of sub-micron size (100–200 nm). Particles do not posses any well defined shape; however, most of the particles are tending towards spherical shape. Particles are attached to one another, i.e. agglomerated, and due to this some of the particles seem still bigger than the value reported. This agglomerated nature of the particles makes the surface of the phosphor particles like a sheet/heap, which is clearly visible in SEM image in Figure 1d. 3.2. Multi-mode luminescence process 3.2.1. UC emission in Tm3+/Yb3+:Y2O3 phosphor On excitation with 976 nm, the Tm3+/Yb3+:Y2O3 phosphor emits in strong blue color. The UC spectrum is shown in Figure 2a. The emission peaks are observed at 362, 488, 655, 796 and 815 nm corresponding to the 1D2 ? 3H6 1G4 ? 3H6, 1G4 ? 3F4, 1G4 ? 3H5, and 3 H4 ? 3H6 electronic transitions of Tm3+ ion, respectively [20,21]. Well known concentration quenching effect is observed at very high concentration of Yb (15 mol%) and UC emission is drastically reduced. The log–log plot for power dependence is shown in Figure 2b while the UC mechanism involved in the process is shown in the partial energy level diagram Figure 3. The upconversion emission of Tm3+ is observed by the involvement of several processes viz. excited state absorption (ESA), energy transfer (ET), cross relaxation (CR), etc. Yb3+ ions in its ground state (2F7/2) absorbs the incident IR photons and are promoted to excited state, i.e. 2F5/2 level. The absorption coefficient of Yb3+ (for 976 nm radiation) is far much better than any rare-earth ions, so, incident energy is absorbed efficiently by Yb3+ ions. The excited Yb3+ ions transfer their excitation energy to Tm3+ ions via two possible channels. In the first mechanism, Yb3+ ions is excited to 2F5/2 energy states, and then transfers its energy non-resonantly (phonon assisted energy transfer) to Tm3+ ions, in ground state, and thus excites them to the 3H5 state., via the channel 2F5/2 (Yb3+), 3H6 (Tm3+) ? 2F7/2 (Yb3+), 3H5 (Tm3+). Since, 3H5 level is known to have a short lifetime [22], therefore, the Tm3+ ions from this level relax nonradiatively to the lower lying metastable state 3F4, where they again get sufficient time to re-absorb a second incident 976 nm photon (excited state absorption, ESA) and get promoted to 3F2 level. The population from 3F2 state relaxes non-radiatively to 3H4 level via involving the intermediate 3F3 level. Finally, Tm3+ ion in 3H4 state absorbs a third 976 nm photon and promoted to the 1G4 level. The 1G4 level gives transition to different lower lying states from where the different radiative transitions are observed. This process is called sequential sensitization. The second mechanism called cooperative sensitization. For the co-operative process, the two excited Yb3+ ions, in the 2F5/2 excited state, interact to each other through a dipole–dipole interaction (most effective) and form a coupled cluster. The coupled cluster state of Yb3+ ions,

124

R. Yadav et al. / Chemical Physics Letters 599 (2014) 122–126

Figure 1. (a) X-ray diffraction patterns, (b) transmission electron microscopic (TEM) image, (c) selected area electron diffraction (SAED) pattern, and (d) scanning electron microscopic image of Tm3+/Yb3+ co-doped Y2O3 phosphor.

essentially a virtual level in nature, transfer energy to a Tm3+ ion in the ground state, 3H6 state, and excite it to 1G4 level, i.e. 2(2F5/ 2 3+ ? (3H6 ? 1G4) Tm3+ [21]. 2 ? F7/2) Yb To verify the above proposed discussion for the mechanism involved in UC process, we measured the intensity of different emission line as a function of incident laser power. The lnI versus lnP curves for 1G4 ? 3H6 and 1G4 ? 3F4 transitions show a slope of 2.24, and 1.37, respectively which is in accordance to the discussion made for UC mechanism. Remarkably, the slope of the curve goes down and shows saturation like behavior at higher pump powers. The decrease in the slope value and the saturation behavior has already been well documented by using energy level diagrams and rate equation [23]. The color coordinate observed by CIE chromaticity diagram for this phosphor is (0.16, 0.20), which confirms a blue color perception of this phosphor. 3.2.2. Photoluminescence and quantum-cutting emission in Tm3+/ Yb3+:Y2O3 phosphor Photoluminescence excitation (PLE) spectra (monitored at 452 nm) show an intense band at 361 nm, corresponding to the transition 3H6 ? 1D2 of Tm3+ ions, shown in Figure 4a. The other peaks are relatively weak. The excitation spectra recorded with an increase in the concentration of Yb3+ shows a decrease in the intensity of different excitation peaks. Figure 4b shows the PL spectra of Y2O3:Tm3+ (0.3 mol%), Yb3+ (0–15 mol%) under 361 nm excitation. Emission peaks are observed at 405, 452, 491, 592 and 651 nm due to transitions 1I6 ? 3H5, 1D2 ? 3F4, 1G4 ? 3H6 and 1G4 ? 3F4 of Tm3+, respectively. The blue peak at 452 nm appears as the most intense peak, contrary to the case of UC, where the intense peak appears at 488 nm. Further, it is observed that, similar to the

excitation spectra, emission spectra also shows a decrease in emission intensity of different peaks as doping concentration is increased from 0 mol% to 15 mol%. It is expected that this decrease in emission intensity is due to an energy transfer from Tm3+ ions to Yb3+ ion (possibility of near infrared QC emission through Yb3+ ions). As the PMT attached with the fluorometer used in PL measurement has its detection limit to monitor the emission peak due to Yb3+ ion, it was not possible to record it. Therefore, the emission due to Yb3+ ion (2F5/2 ? 2F7/2) was monitored by using CCD detector. The 266 nm line of Nd:YAG laser was used as excitation source. Inset to Figure 4b shows an efficient emission peak in 950–1050 nm region, which shows an increase in intensity as the concentration of Yb3+ increases. A decrease in intensity at 15 mol% Yb3+ concentrations is due to self-trapping effect as well as excitation energy migration among adjacent Yb3+ ions followed by nonradiative trapping by defects at high Yb3+ concentration. Thus, with increasing concentration of Yb3+, a decrease in the intensity of the peaks of Tm3+ ion and, contrary to this, an increase in the intensity of the emission peak of Yb3+ clearly supports for an energy transfer from Tm3+ to Yb3+, which is possible via a QC process. The possible mechanism for the observation of QC can be explained as follows. In the presence of Yb3+, the charge transfer state (CTS) arises, which lie around 200–300 nm (though it also depends upon host matrix). The 266 nm excitation populates the CTS and latter on the population is transferred nonradiatively to high lying excited states of other co-dopant Tm3+ ion. The co-dopant Tm3+ is populated directly also. The population is then transferred to the 3 P0 level and relaxes further to the lower level 1G4. The lifetime of this level is relatively long and behaves as a metastable state. The radiative transitions in blue are observed from this level. The

R. Yadav et al. / Chemical Physics Letters 599 (2014) 122–126

Figure 2. (a) Room temperature upconversion (UC) spectrum of the Tm3+/Yb3+ codoped Y2O3 phosphor on 976 nm excitation (b) lnI (intensity of UC emission) versus lnP (applied laser input power) plot. The slope of these curves (n) gives the number of photons involved in the particular UC process of different bands.

125

Figure 4. (a) Photoluminescence excitation spectra by monitoring Tm3+: 1D2 ? 3F4 emission, (b) photoluminescence spectra under 361 nm excitation (3H6 ? 1D2). Inset to the figure shows the quantum cutting emission of Tm3+-Yb3+.

transitions become available and QC emission is observed (Figure 3). The other possible way is the cross relaxation. The relaxation 1G4 ? 3H5 matches well to 2F5/2 ? 2F7/2 transitions, and via this channel QC is also observed. Similar observation has also been reported in other works; summarized in review by Liu et al. and references there in [5]. To realize the effect of energy transfer

Figure 3. Schematic energy level diagram showing the mechanism involved in the upconversion emission (left side) and quantum-cutting process (right side). ESAexcites state absorption, GSA-Ground state absorption, CET-co-operative energy transfer, ET-energy transfer, CTS-charge transfer state.

population in 1G4 level may undergo QC emission through two possible ways. The cooperative level due to two Yb3+ ions matches well (resonant) to the energy of 1G4 state. Thus, population in 1 G4 is relaxed and two photons matching with the 2F5/2 ? 2F7/2

Figure 5. Decay curves of Tm3+ ion for 1G4 ? 3H6 transition in Yb3+/Tm3+ co-doped Y2O3 phosphors under 361 nm excitation wavelength.

126

R. Yadav et al. / Chemical Physics Letters 599 (2014) 122–126

Table 1 Variation in the lifetime (1G4 ? 3H6), energy transfer efficiency and quantum cutting efficiency of Tm3+, Yb3+ co-doped Y2O3 phosphor with a variation in Yb3+ concentration. Yb3+ concentration (mol%)

Life time (s) (ls)

Energy transfer efficiency (gET%)

Quantum efficiency (gQE%)

0 3 5 15

166 106 92 30

0 36 44 81

0 136 144 181

and also to calculate the energy transfer efficiency/efficiency of QC, decay time analysis has been carried out. Figure 5 shows the decay curves (kexc = 361 nm) of the transition 1G4 ? 3H6 with a variation in the concentration of Yb3+. The decay curves are well fitted by single exponential equation:

t I ¼ I0 expð Þ

s

where, I and I0 are the intensity at time t and at 0 s, respectively, and s is the lifetime. The obtained lifetime is given in Table 1, which depicts a decrease in the value of decay time with increasing concentration of Yb3+ in the sample. According to the mentioned mechanism, the QC emission is mainly followed by the occurrence of cross-relaxation and CET channels. It is well known that CET process is not so efficient at lower concentration, due to the intrinsic properties of Yb3+ ion. But at a relatively high concentration, CET process could become efficient, and the energy transfer efficiency of Tm3+ to Yb3+ (gET) can be calculated by equation

gET ¼ 1 

sx s0

where, sx and s0 is the fluorescence lifetime of Tm3+/Yb3+ and singly Tm3+ doped phosphors, respectively. The value of gET estimated for different concentration of Yb3+ using the above relation is given in Table 1. It again shows that ET efficiency increases with increasing Yb3+ concentration, and reaches a maximum of 81% for 15 mol% of Yb3+ concentration. The internal quantum efficiency (QE), which represents the better QE for the phosphor samples, can be calculated by

gQE ¼ gTm ð1  gET Þ þ 2gET The first term in the relation belongs to the visible photons emitted by Tm3+ ions and the second term refers to the NIR photons emitted by Yb3+. Here, gTm represent the quantum efficiency of the emission of Tm3+ ions. Assuming that there is no nonradiative transition (i.e. gTm = 1), the highest possible QE is 181% for

Y2O3:0.3% Tm3+, 15% Yb3+. The calculated value of the internal QE increased notably with increment of the Yb3+ concentration (see Table 1). 4. Conclusions Micro-crystalline Y2O3 phosphor co-doped with Tm3+/Yb3+ ion has been synthesized via solution combustion technique. The phosphor material gives efficient multimodal emission via upconversion (UC), downconversion (DC)/quantum cutting (QC), and downshifting (DS) luminescence process. The energy transfer from Tm3+ to Yb3+ has been verified with efficiency as high as 81%, and corresponding quantum efficiency of quantum cutting are calculated to be 181%. Multi-modal emission behaviour of the phosphor materials brings out its potential application in energy harvesting. Acknowledgements S.K. Singh thankfully acknowledges Department of Science and Technology, New Delhi, India for financial assistance under INSPIRE Faculty Award [IFA12-PH-21]. Authors would like to acknowledge DST, New Delhi for financial assistance. References [1] D. J. Naczynski, M. C. Tan, M. Zevon, B. Wall, J. Kohl, A. Kulesa, S. Chen, C. M. Roth, R. E. Riman, P. V. Moghe, Nature Commun. doi:10.1038/ncomms3199. . [2] Z. Li, Chem. Commun. 49 (2013) 7129. [3] Y. Zhang, F. Zheng, T. Yang, W. Zhou, Y. Liu, N. Man, L. Zhang, N. Jin, Q. Dou, Y. Zhang, Z. Li, L.P. Wen, Nat. Mater. 11 (2012) 817. [4] G. Ajithkumar et al., J. Mater. Chem. B 1 (2013) 1561. [5] X. Huang, S. Han, W. Huang, X. Liu, Chem. Soc. Rev. 42 (2013) 173. [6] W. Zheng, H. Zhu, R. Li, D. Tu, Y. Liu, W. Luo, X. Chen, Phys. Chem. Chem. Phys. 14 (2012) 6974. [7] F. Xiong, Y. Ling, Y. Chen, Z. Luo, Chem. Phys. Lett. 429 (2006) 410. [8] R.T. Wegh, E.V.D. van Loef, A. Meijerink, J. Lumin. 90 (2000) 111. [9] B. Han, H.B. Liang, Y. Huang, Y. Tao, Q. Su, J. Phys. Chem. C 114 (2010) 6770. [10] F. Auzel, Chem. Rev. 104 (2004) 139. [11] S.K. Singh, A.K. Singh, S.B. Rai, Nanotechnology 22 (2011) 275703. [12] X. Li, D. Zhao, F. Zhan, Theranostics 3 (2013) 292. [13] A. Yin, Y. Zhang, L. Sun, C. Yan, Nanoscale 2 (2010) 953. [14] J.G. Bunzli, S.V. Eliseevacd, Chem. Sci. 4 (2013) 1939. [15] J. Zhang, S. Wang, T. Rong, L. Chen, J. Am. Ceram. Soc. 87 (2004) 1072. [16] L.A. Riseberg, in: B.D. Bartolo (Ed.), The relevance of non radiative transitions to solid state lasers, Plenum Press, NewYork, 1980, p. 369. [17] K. Deng, T. Gong, L. Hu, X. Wei, Y. Chen, M. Yin, Opt. Express 19 (2011) 1749. [18] V.D. Rodriguez, J. Mendez-Ramos, V.K. Tikhomirov, Opt. Mater. 34 (2011) 179. [19] S.K. Singh, K. Kumar, S.B. Rai, Appl. Phys. B 94 (2009) 165. [20] A. Patra, S. Saha, M.A.R.C. Alancar, N. Racov, G.S. Maciel, Chem. Phys. Lett. 407 (2005) 477. [21] N.K. Giri, S.K. Singh, D.K. Rai, S.B. Rai, Appl. Phys. B 99 (2010) 271. [22] Z.X. Cheng, X.J. Yi, J.R. Han, H.C. Chen, X.L. Wang, H.K. Liu, S.X. Dou, F. Song, H.C. Guo, Cryst. Res. Technol. 37 (2002) 1318. [23] M. Pollnau, D.R. Gamelin, S.R. Luthi, H.U. Gudel, M.P. Hehlen, Phys. Rev. B 61 (2000) 3337.