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Intense white light emission in Tm3+/Er3+/Yb3+ co-doped Y2O3–ZnO nano-composite

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2013 J. Phys. D: Appl. Phys. 46 275101 (http://iopscience.iop.org/0022-3727/46/27/275101) View the table of contents for this issue, or go to the journal homepage for more

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JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 46 (2013) 275101 (8pp)

doi:10.1088/0022-3727/46/27/275101

Intense white light emission in Tm3+/Er3+/Yb3+ co-doped Y2O3–ZnO nano-composite R S Yadav, R K Verma and S B Rai Laser and Spectroscopy Laboratory, Department of Physics, Banaras Hindu University, Varanasi 221 005, India E-mail: [email protected] (S B Rai)

Received 16 January 2013, in final form 10 April 2013 Published 18 June 2013 Online at stacks.iop.org/JPhysD/46/275101 Abstract The Tm3+ /Er3+ /Yb3+ co-doped Y2 O3 –ZnO nano-composite is synthesized using the solution combustion technique. The structural morphology is monitored using x-ray diffraction, transmission electron microscopy and scanning electron microscopy. The Yb3+ /Tm3+ co-doped nano-phosphor emits intense blue as well as weak red emissions, while Yb3+ /Er3+ co-doped nano-phosphor emits strong green along with red emissions on excitation with 976 nm laser. Joining these together (i.e. Tm3+ /Er3+ /Yb3+ co-doped phosphor) give very strong white light, which is further verified by CIE coordinates (0.32, 0.36). The addition of ZnO with Y2 O3 phosphor gives further enhancement in the intensity of white light. The possible reason for this enhancement is the removal of optical quenching sites. (Some figures may appear in colour only in the online journal)

white light emission in Ln : Y2 O3 nano-crystalline phosphor (Ln : Tm\Er\Yb). The multicolour tunability depends on doping concentrations, annealing temperature and laser power [10]. They have also discussed that the emission intensity increases on annealing due to removal of quenching sites. Song et al have also investigated the enhanced white light emission in Er/Tm/Yb/Li co-doped Y2 O3 nanocrystals and discussed the effect of modifier on the emission intensity [11]. In spite of these, the investigated materials show a common problem of lower emission efficiency. In order to enhance the emission efficiency, the research interests are being emerged in the new class of materials, namely, composites. Actually, composites are a category of hybrid materials having two or more distinct phases and each of these phases have their own optical characteristics [12–14]. The Tm3+ ions exhibit strong blue as well as weak red emissions, whereas Er3+ ions are well known for green and red emissions on 976 nm laser excitation. Since the absorption cross sections of these activators are relatively small, the overall efficiencies are poor. However, the Yb3+ ion which has 7 times larger absorption cross section for infrared wavelength 976 nm and can easily excite these ions through energy transfer. Thus, if Yb3+ is also taken together the overall emission efficiencies of Tm3+ and Er3+ ions can be enhanced by several

1. Introduction There are several ways to generate white light. For example, by emission of a broad band continuum in the visible region, by mixing of primary colours red, green and blue (RGB) light and by mixing light of opposite colours, etc. Among these the most common one is by mixing of RGB. One way to achieve RGB radiations is through the frequency upconversion process. Generally, near infrared radiation sources are used for this purpose [1–5]. Thus, in a recent report, Li et al have reported the high colour purity phosphor LaAlGe2 O7 doped with Tm3+ /Er3+ and obtained vivid blue and green emissions from these ions [6]. Qin et al have observed the emission of white light through upconversion in Tm/Er/Yb tri-doped CaF2 phosphor and explained the factors affecting the white light intensity such as host materials, concentration of rare earth ions, annealing temperature and excitation power density [7]. Zhou et al obtained the white light emission in Tm/Er/Yb tridoped Y2 O3 transparent ceramic and reported the tuning of emission from multicolour to white light [8]. Lin et al have studied the white light emission in the La2 O3 : Yb3+ /Er3+ /Tm3+ system and demonstrated the morphological control and luminescence by adjusting the concentration of rare earth ions [9]. Recently, Giri et al have reported tunable colour to 0022-3727/13/275101+08$33.00

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J. Phys. D: Appl. Phys. 46 (2013) 275101

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times [15–23]. Thus, an appropriate combination of these ions can produce efficient white light. The host material also plays a crucial role in the enhancement of the luminescence. Y2 O3 is a host with low phonon frequency and results in a low non-radiative relaxation and good quantum yield of dopant. Another advantage with Y2 O3 host is that it has high chemical stability, thermal capacity, long durability, etc [24, 25]. The presence of ZnO with Y2 O3 further reduces the non-radiative relaxation and enhances the crystallinity of the material [26, 27]. ZnO is a direct band gap semiconductor and is well known for fluorescence from the band edge along with different defect level emissions. These emissions can directly or indirectly enhance the emissive properties of the localized dopant [28]. In this work, we have synthesized for the first time Tm3+ /Er3+ /Yb3+ co-doped Y2 O3 : ZnO nano-composite through the solution combustion method using urea as organic fuel/reducing agent. The structural characterizations have been made using different techniques (x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive analysis of x-rays (EDAX), etc). The photoluminescence properties of the synthesized samples have been monitored using the 976 nm laser radiation. The phosphor emits intense white light. The presence of ZnO further enhances the intensity of the white light more than two times.

2.2. Characterization XRD patterns of as-synthesized and annealed samples were recorded using Cu, Kα radiation (λ = 0.154 06 nm) from a RINT/DMAX 2200 H/PC (Rigaku, Japan) machine with 5◦ min−1 scan speed at room temperature. Data from International Centre for Diffraction (ICDD) were used to identify the crystallite phase of the as-synthesized and annealed samples. Surface morphology of the samples was studied with scanning electron microscopy (SEM) using JEOL-TM Model JSM 5410 system operated at 15 kV. The transmission electron microscopy (TEM) was used to record the micrograph of the samples using a Technai 20G2 , Philips unit. The EDAX measurement was carried out to verify the presence of different elements and avoid impurities in the synthesized sample. The photoluminescence spectra of different samples were monitored using 976 nm radiation from a diode laser and iHR320, Horiba Jobin Yvon, spectrometer attached with CCD and Ocean Optics QE 65000.

3. Results and discussion 3.1. Structural characterizations 3.1.1. X-ray diffraction. The XRD patterns of as-synthesized and annealed at 1200 ◦ C/5 h Tm3+ /Er3+ /Yb3+ co-doped Y2 O3 samples are recorded in the range 20◦ –80◦ to distinguish the phase and crystallite size and they are shown in figure 1(a). As is evident from the figure the as-synthesized sample is also crystalline. The crystallinity in the samples increases on annealing at higher temperatures as is also evident from figure 1(a). The diffraction patterns match well with the ICDD JCPDS file no 43-1036. The phase of the as-synthesized and annealed samples is cubic with space group la3 (2 0 6) and the cell parameters a = b = c = 10.60 Å with α = β = γ . The average crystallite size (th k l ) for (2 2 2), (4 0 0) and (4 4 0) lattice planes are calculated using the Debye Scherrer equation

2. Experimental 2.1. Synthesis of the sample Following compositions are used for the synthesis of the nanophosphor and its nano-composites: (100 − x − y − z)Y2 O3 + xTm2 O3 + yYb2 O3 + zEr 2 O3 , (1) where x = 0.5, y = 3.0 and z = 0.3 mol%, 0.5 mol%, 0.7 mol% and 0.9 mol%, respectively. (100 − x − y − z − p)Y2 O3 + xTm2 O3 + yYb2 O3 + zEr 2 O3 +pZnO, (2)

th k l =

where x = 0.5, y = 3.0 and z = 0.7 and p = 10 mol%, 20 mol%, 30 mol%, respectively. The stoichiometric ratios of Tm2 O3 , Yb2 O3 and Er2 O3 (99.99% pure) are used for synthesizing the phosphor. These materials were first dissolved in 5 ml of nitric acid and then diluted with de-ionized water and mixed with vigorous stirring. Urea was then added to the solution as organic fuel/reducing agent. The final solution was stirred at a constant temperature of 60 ◦ C for 4 h and a gel was obtained. The gel was taken in a platinum crucible and placed in a closed furnace maintained at 500 ◦ C. The auto-ignition took place within few minutes. Finally, the composition in the form of white powder was obtained. For preparation of composites ZnO was added in the composition mentioned above and the same process was repeated. The final product thus obtained was grinded in an agate mortar to convert it into fine powder. To achieve the crystallinity the powder samples were annealed in batches at 800 ◦ C and 1200 ◦ C for 5 h separately [29].

kλ , βCosθ

where λ is the wavelength of the x-ray radiation, β is the FWHM of the diffraction peak, θ is the angle of diffraction and k is a constant equal to 0.90. The average crystallite size is found to be ∼25 and 40 nm for the as-synthesized and annealed nano-phosphor samples, respectively. The inset in figure 1(a) shows the variation in the full-width at halfmaximum (FWHM) of the peak due to lattice plane (2 2 2) in the two cases which is also an indication of increase in crystallinity of the material. The FWHM of the peak is much larger in the as-synthesized sample than in the annealed one. The reduction in FWHM confirms the grain growth. The XRD patterns of Tm3+ /Er3+ /Yb3+ co-doped Y2 O3 – ZnO nano-composite sample annealed at 1200 ◦ C/5 h recorded in the same range under the same condition is shown in figure 1(b). The XRD patterns of ZnO match well with the JCPDS file no 36-1451 with the tetragonal phase and the cell parameters are a = 3.249, b, c = 5.206 Å with α = β = γ . The XRD patterns due to Y2 O3 are shown with (+) sign while that of ZnO with (*) sign. Figure 1(c) shows the XRD 2

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Figure 2. Scanning electron micrographs of annealed (at 1200 ◦ C/5 h) Tm3+ /Er3+ /Yb3+ co-doped Y2 O3 and Tm3+ /Er3+ /Yb3+ co-doped Y2 O3 –ZnO samples (a), (b). Transmission electron micrograph (c) and the EDAX pattern (d) of annealed (at 1200 ◦ C/5 h) Tm3+ /Er3+ /Yb3+ co-doped Y2 O3 –ZnO sample.

host [30]. The reduction in FWHM (FWHMY2 O3 = 0.21◦ and FWHMY2 O3 +ZnO = 0.17◦ ) further results an enhancement in crystallite size from 40 to 48 nm [26, 27]. 3.1.2. SEM, TEM and EDAX measurements. The morphology of the annealed Tm3+ /Er3+ /Yb3+ co-doped samples at 1200 ◦ C/5 h is recorded using SEM and TEM. The scanning electron micrographs of the annealed Tm3+ /Er3+ /Yb3+ co-doped Y2 O3 and Tm3+ /Er3+ /Yb3+ co-doped Y2 O3 –ZnO samples at 1200 ◦ C/5 h are shown in figures 2(a) and (b). The morphology of the sample appears fluffy, porous and agglomerated in both the cases. The transmission electron micrograph of the annealed Tm3+ /Er3+ /Yb3+ co-doped Y2 O3 –ZnO sample is given in figure 2(c). This figure clearly shows that the particles are linked to each other inside the material. The particle size in the annealed sample is coarser (in 80–150 nm range) which is consistent with the nano-scaled particles and supports the agglomeration. The EDAX measurement shown in figure 2(d) is carried out to verify the presence of desired elements (no impurity) in the composite. The EDAX pattern indicates the presence of Y, Zn, O, Tm, Er and Yb elements in the annealed ZnO composite sample. The presence of Cu lines in the EDAX spectrum is originated from the copper grid mesh used in the measurement.

Figure 1. (a) XRD patterns of as-synthesized and annealed (at 1200 ◦ C/5 h) Tm3+ /Er3+ /Yb3+ co-doped Y2 O3 nano-phosphor sample. (b) XRD patterns of annealed (at 1200 ◦ C/5 h) Tm3+ /Er3+ /Yb3+ co-doped Y2 O3 –ZnO nano-composite sample. (c) Variation of XRD patterns in the (2 2 2) lattice plane of annealed (at 1200 ◦ C/5 h) Tm3+ /Er3+ /Yb3+ co-doped Y2 O3 and its ZnO nano-composite sample.

pattern of the lattice plane (2 2 2) in the range 28◦ –30◦ . This clearly reveals the variation in FWHM between pure Y2 O3 and its ZnO nano-composite sample. The FWHM of the nanocomposite is smaller than that of pure Y2 O3 and is slightly shifted towards higher angle side also. The shifting in peak is due to strain in lattice on addition of ZnO in the pure Y2 O3

3.2. Optical characterization 3.2.1. Tm3+ /Yb3+ co-doped Y2 O3 nano-phosphor. The upconversion emission spectra of the as-synthesized Tm3+ /Yb3+ co-doped Y2 O3 nano-phosphor sample was first monitored 3

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Figure 4. Schematic Energy level diagram of Er3+ /Tm3+ /Yb3+ co-doped Y2 O3 –ZnO nano-composite under 976 nm laser excitation.

the ions in 3 H4 state are promoted to 1 G4 state through ESA. Simultaneously, the Yb3+ ions from its cooperative state also transfer energy to Tm3+ ions in ground state and promote them to 1 G4 excited state. The transitions from 1 G4 state to lower states emit radiations at 491, 658 and 798 nm [32]. Thus, the 658 and 798 nm transitions are ascribed as excited to excited transitions, namely, 1 G4 → 3 F4 and 1 G4 → 3 H5 , respectively. Some of the ions in 1 G4 state are excited further through ESA to 1 D2 and 3 P2 states and then relax to 1 I6 state. The ions in 1 I6 state emit radiations at 301 and 394 nm through the 1 I6 → 3 H6 and 1 I6 → 3 H5 transitions, respectively. The bands observed at 368 nm, 466 nm and 777 nm are ascribed to arise due to 1 D2 → 3 H6 , 1 D2 → 3 F4 and 1 D2 → 3 F3 excited to excited transitions, respectively [18]. Most of these transitions do not appear in the absence of sensitizer Yb3+ . The Yb3+ ions enhance the emission intensity upto orders to make them to be seen [31, 34]. The insets in figure 3 show the UV and NIR upconversion emission spectra. The energy level diagram and possible electronic transitions in Tm3+ /Yb3+ are shown in figure 4. The possible absorption, energy transfer and upconversion mechanism can be easily understood with the help of the energy level diagram. The power dependence measurements are carried out to verify the upconversion mechanisms involved in the emission process. A logarithmic plot of pump power versus emission intensity for (0.5 mol%) Tm3+ : (3.0 mol%) Yb3+ co-doped Y2 O3 nano-phosphor sample for 491 nm transition is shown in figure 5 and the value of n is found to be 2.68 (∼3). This suggests that three photons are involved to populate the 1 G4 state of the Tm3+ ion [17]. The CIE colour coordinates are calculated for this sample. The colour coordinates are found to be (0.85, 0.19) which comes in the blue region. The synthesized sample results strong blue light perception to the naked eye when illuminated with 976 nm radiation.

Figure 3. Emission spectra of Tm3+ /Yb3+ co-doped Y2 O3 samples of as-synthesized and annealed at different temperatures for 5 h under 976 nm laser excitation.

and optimized exciting with 976 nm radiation. The emission intensity was optimum for (0.5 mol%) Tm3+ : (3.0 mol%) Yb3+ ion concentration. The as-synthesized sample was annealed at different temperatures for 5 h and the upconversion emissions thus obtained are shown in figure 3. Large number of emission bands are seen extending from the ultraviolet (UV) to near infrared (NIR) regions at wavelengths 301, 368, 394, 466, 491, 658, 679, 777, 798 and 817 nm and they are assigned to arise due to 1 I6 → 3 H6 , 1 D2 → 3 H6 , 1 I6 → 3 H 5 , 1 D 2 → 3 F 4 , 1 G 4 → 3 H 6 , 1 G 4 → 3 F 4 , 3 F 2 → 3 H 6 , 1 D2 → 3 F3 , 1 G4 → 3 H5 and 3 H4 → 3 H6 electronic transitions, respectively [15–18, 31–33]. All these bands are seen in both the cases i.e. in the annealed samples as well as the as synthesized one although the intensity of the peaks of annealed samples is much larger than in the case of the as-synthesized sample. The most intense emission is observed in the blue region centred at 491 nm due to the 1 G4 → 3 H6 transition. A weak emission in the red region centred at 658 nm appears due to the 1 G4 → 3 F4 transition. Another intense emission is observed at 817 nm due to the 3 H4 → 3 H6 transition in the NIR region [31]. Tm3+ ions absorb 976 nm radiation very weakly and give poor emission. However, in the presence of sensitizer Yb3+ , the emission becomes very strong. Actually the Yb3+ ions are pumped by NIR 976 nm laser from its ground state (2 F7/2 ) to excited state (2 F5/2 ) and transfer their excitation energy to the Tm3+ ions in the ground state (3 H6 ) due to which they are promoted to 3 H5 state. The Tm3+ ions are further excited to the higher excited states by different excitation processes such as through excited state absorption (ESA), excited to excited state energy transfer (EEET), energy transfer upconversion (ETU), etc. Some of the excited Tm3+ ions relax from 3 H5 to 3 F4 state also non-radiatively and transfer some of the Tm3+ in 3 F4 state. These ions are then promoted to 3 F2 state through ESA. Some of these ions in this state relax nonradiatively and populate 3 H4 state. The transitions from 3 F2 and 3 H4 states to ground state are responsible for the radiative transitions at 679 nm and 817 nm, respectively. Some of

3.2.2. Er3+ /Yb3+ co-doped Y2 O3 nano-phosphor. The upconversion emission spectra of the as-synthesized Er3+ /Yb3+ co-doped Y2 O3 nano-phosphor sample were also optimized for maximum photoluminescence under 976 nm excitation and the composition found was (0.7 mol%) Er3+ : (3 mol%) Yb3+ co-doped Y2 O3 . We also monitored the spectra of 4

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green and red bands are increased. This increase in emission intensity is a direct consequence of removal of quenching sites from the as-synthesized sample. The mechanism of emission in these cases is very similar to the previous case (i.e. Tm3+ /Yb3+ ). As mentioned earlier the absorption cross section of Yb3+ ion is high enough for 976 nm laser compared with Er3+ ion. When Er3+ and Yb3+ both are present together in the phosphor the excited Yb3+ ions transfer their energy to Er3+ ions in the ground state as well as in the excited state making the two photon absorption possible. The excited Yb3+ ions also emit strong broad radiation at 976 nm and cooperative upconversion emission at 488 nm. The Er3+ ions at the same time absorb 976 nm radiation and populate the 4 I11/2 state. Moreover, the excited Yb3+ ions in 2 F5/2 state also transfer their energy to Er3+ ions and promote them to the excited state (4 I11/2 ). This process is very efficient and populates 4 I11/2 state very heavily. The de-excitation of some of the ions from 4 I11/2 level populates 4 I13/2 level. The 4 I13/2 as well as 4 I11/2 levels of Er3+ are long lived levels and the ions present in these levels reabsorb 976 nm radiation and promoted to 4 S3/2 , 2 H11/2 and 4 F9/2 levels. The 4 S3/2 and 2 H11/2 levels of Er3+ are populated by energy transfer from cooperatively excited Yb3+ ions. Similarly 4 F9/2 level is populated further by relaxation of excited ions from 4 S3/2 and 2 H11/2 levels. Some of the ions present in 4 S3/2 and 2 H11/2 states reabsorb incident 976 nm radiation and populate higher lying states. Transitions from these higher lying states to ground state emit UV, blue and bluegreen radiations [22, 23]. Similarly, transitions from 4 S3/2 , 2 H11/2 states to ground state result intense green emission and the transition from 4 F9/2 to ground state gives red emission. The energy levels of Er3+ /Yb3+ and observed transitions are shown in figure 4. Wang et al have studied the visible upconversion emission of Er3+ doped ZnO nanocrystals and observed the effect of annealing on the emission intensity. The samples heat treated at higher temperatures result intense emission due to the removal of the optical quenching sites such as OH and CO from the as-synthesized samples [35, 36]. The intensity of the green band is larger than the red band and results a strong orange perception with the naked eye. The CIE diagram is plotted and the colour coordinates is found to be (0.34, 0.65) which results the perception of orange colour. A log–log plot for 566 and 663 nm transitions of Er3+ /Yb3+ co-doped Y2 O3 nano-phosphor sample annealed at 1200 ◦ C is depicted in figure 7. The value of slope (n) is found to be 1.97 and 1.91 (∼2) for 566 and 663 nm, respectively. This clearly suggests that two photons are involved in these transitions. The deviation from the integral value is due to involvement of non-radiative relaxation inside the host material.

Figure 5. Pump power versus the emitted intensity dual logarithmic plot for blue emission of Tm3+ /Yb3+ co-doped Y2 O3 annealed sample (at 1200 ◦ C/5 h) under 976 nm laser excitation.

Figure 6. Emission spectra of as-synthesized and annealed Er3+ /Yb3+ co-doped Y2 O3 samples at different temperatures for 5 h on excitation with 976 nm laser.

the optimized samples annealed at different temperatures for 5 h. The spectra thus obtained are shown in figure 6. Two intense emission bands one in the green and the other in the red region are observed along with large number of other bands. The emission peaks are observed at 364 nm, 393 nm, 411 nm, 478 nm, 491 nm, 524 nm, 566 nm, 663 nm and 855 nm and these peaks are assigned as 2 H9/2 → 4 I15/2 , 4 G11/2 → 4 I15/2 , 2 P3/2 → 4 I13/2 , 4 F3/2 → 4 I15/2 , 4 F5/2 → 4 I15/2 , 2 H11/2 → 4 I15/2 , 4 S3/2 → 4 I15/2 , 4 F9/2 → 4 I15/2 , 4 I9/2 → 4 I15/2 transitions, respectively [19–23, 32]. The transition 2 P3/2 → 4 I13/2 at 411 nm is an excited to excited state transition and appears with relatively weak intensity. The emission intensity of the green band is greater than the red band and they are assigned to arise due to the (4 S3/2 , 2 H11/2 ) → 4 I15/2 and 4 F9/2 → 4 I15/2 transitions, respectively. The insets in figure 6 show the emission bands in the violet and NIR regions. As is evident from figure, on annealing the sample at higher temperatures [35, 36] the intensities of the

3.2.3. Tm3+ /Er3+ /Yb3+ co-doped Y2 O3 nano-phosphor. The upconversion emission spectra of the as-synthesized and annealed at different temperatures for 5 h Tm3+ /Er3+ /Yb3+ co-doped Y2 O3 nano-phosphor samples have been monitored under 976 nm laser excitation and are shown in figure 8. The molar concentrations of Tm3+ : Er3+ : Yb3+ are the optimized value of these ions. The peaks observed in the spectra match well with peaks present in the individual samples. The insets in figure 8 show the emissions in UV and NIR regions. The 5

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Figure 7. Pump power versus the emitted intensity dual logarithmic plot for green and red emissions of Er3+ /Yb3+ co-doped Y2 O3 annealed sample (at 1200 ◦ C/5 h) under 976 nm laser excitation.

Figure 9. Effect of Er3+ concentration on the emission intensity of white light annealed at 1200 ◦ C for 5 h under 976 nm laser excitation.

Figure 8. Emission spectra of as-synthesized and annealed Tm3+ /Er3+ /Yb3+ co-doped Y2 O3 nano-phosphor at different temperatures for 5 h under 976 nm laser excitation. Figure 10. Effect of ZnO concentration on the emission intensity of white light annealed at 1200 ◦ C for 5 h under 976 nm laser excitation.

emission spectra recorded in the range 280–875 nm and it is shown in three parts for clarity. Further, since the tri-doped nano-phosphor sample emits primary colours red, green and blue, their concentrations may be varied so that they can be used for white light generation. The concentrations of the Yb3+ as well as Tm3+ ions are fixed at 3 mol% and 0.5 mol%, respectively, whereas that of the Er3+ ions are made to vary as 0.3, 0.5, 0.7 and 0.9 mol% so that the light emitted is white light [7–11]. It is found that at 0.7 mol% Er3+ ion concentration, white light emission takes place. The CIE coordinates corresponding to this combination is found to be (0.32, 0.36) which is close to the standard value (0.33, 0.33) for white light [11]. As the concentration of the Er3+ ion is increased, the colour of the emission of tri-doped nano-phosphor sample changes from blue to bluish green, whitish green and finally white light. Thus, it gives a possibility to get colour tunability leading to white light. If the concentrations of Er3+ ions are increased further the emission intensity of the white light decreases due

to concentration quenching. The effect of concentration on the emission intensity of white light is depicted in figure 9. 3.2.4. ZnO nano-composite and its effect on the emission of white light. The emission intensity of white light increases further on addition of ZnO in the optimized combination. We analysed the emission efficiency of white light by adding up the different concentrations, namely, 10, 20 and 30 mol% of ZnO in it. It is found that the emission intensity of white light improves with the increase of ZnO in the host. It is optimum for 20 mol% of ZnO. The effect of ZnO concentrations on the emission intensity is shown in figure 10. There may be several reasons for this enhancement. One of the reasons is that ZnO forms composite with Y2 O3 and as a result the phonon frequency of the material is reduced. This decrease in phonon frequency reduces the electron phonon 6

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Figure 11. Emission spectra of as-synthesized and annealed Tm3+ /Er3+ /Yb3+ co-doped Y2 O3 –ZnO nano-composite at different temperatures for 5 h under 976 nm laser excitation.

Figure 12. CIE diagram of Tm3+ /Er3+ /Yb3+ co-doped Y2 O3 –ZnO nano-composite sample annealed at 1200 ◦ C/5 h and its image under 976 nm laser excitation.

coupling and thereby decreases the non-radiative relaxations. The second and important reason may be the removal of remaining quenching sites in the presence of ZnO [39–41]. Actually in the presence of ZnO, the space vacant in the lattice is covered by the ZnO which replaces impurities such as OH and CO present in the host. The third possibility may be an increase in the crystallinity of the host material in the presence of ZnO [26, 27]. Wang et al have reported that the emissive efficiency of rare earth ions is increased in larger crystallite medium [37]. Jung et al have also reported that the increasing the crystallite size of the phosphor essentially improves the fluorescence intensity of the material [38]. The effect of the annealing has also been investigated in the case of composite host. When the composite sample is annealed at higher temperature; the emission intensity of white light increases. The effect of the presence of ZnO on white light emission as a function of temperature is shown in figure 11. As has been reported the annealed samples show an increase in crystallite size. The nanoparticles with larger diameter can absorb the incident light more efficiently than those with smaller diameter [42–44]. Thus, the enhancement in the emission intensity of white light is a combined effect of concentration of rare earth ions, input pump power, annealing temperature and crystallinity of the material. The colour coordinates for white light emission is found to be (0.32, 0.36) which is very close to the standard white light colour coordinates (0.33, 0.33) [11]. The CIE diagram of the Tm3+ /Er3+ /Yb3+ co-doped Y2 O3 –ZnO nano-composite sample is shown in figure 12.

co-doped Y2 O3 nano-phosphor sample gives intense blue emission whereas Er3+ /Yb3+ co-doped Y2 O3 nano-phosphor sample results strong green and slightly weak red emissions. The tri-doped Tm3+ /Er3+ /Yb3+ co-doped Y2 O3 nano-phosphor sample gives intense white light emission with (0.5 mol%) Tm3+ , (0.7 mol%) Er3+ and (3.0 mol%) Yb3+ concentrations. Addition of ZnO in the Tm3+ /Er3+ /Yb3+ co-doped Y2 O3 nano-phosphor sample enhances the efficiency of white light emission further more than two times. The character of the emitted light is verified by CIE coordinates (0.32, 0.36) which is very close to the standard CIE coordinates for white light (0.33, 0.33).

Acknowledgments The authors are grateful to Professor O N Srivastava, Department of Physics, for providing facilities for XRD, SEM, TEM and EDAX. We are also grateful to UGC and DST, New Delhi, for financial assistance.

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4. Conclusions The tri-doped Tm3+ /Er3+ /Yb3+ : Y2 O3 –ZnO nano-composite is synthesized by the solution combustion method. The nanocomposite emits intense white light on excitation with 976 nm laser. The XRD and TEM measurements reveal the strong crystallinity of the nano-composite sample. The Tm3+ /Yb3+ 7

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