Author’s Accepted Manuscript Chromaticity modulation of upconversion luminescence in CaSnO3:Yb3+,Er3+,Li+ phosphors through Yb3+ concentration, pumping power and temperature Tao Pang, Wenhui Lu, Wujian Shen www.elsevier.com/locate/physb
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S0921-4526(16)30376-3 http://dx.doi.org/10.1016/j.physb.2016.08.036 PHYSB309605
To appear in: Physica B: Physics of Condensed Matter Received date: 10 June 2016 Revised date: 19 August 2016 Accepted date: 22 August 2016 Cite this article as: Tao Pang, Wenhui Lu and Wujian Shen, Chromaticity modulation of upconversion luminescence in CaSnO3:Yb3+,Er3+,Li+ phosphors through Yb3+ concentration, pumping power and temperature, Physica B: Physics of Condensed Matter, http://dx.doi.org/10.1016/j.physb.2016.08.036 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
pumping power and temperature Tao Pang*, Wenhui Lu, Wujian Shen College of science, Huzhou University, Zhejiang Huzhou 313000, China *
Corresponding author. Tel.: +86-572-2321593. E-mail address: [email protected]
Abtract CaSnO3:Yb3+,Er3+,Li+ upconverting phosphors are synthesized by a simple solid state method. The doping concentration of Li+ ions is optimized by comparatively studying upconversion luminescence of samples with various Li+ content. The effects of Yb3+ concentration, pumping power and temperature on chromaticity of upconversion luminescence in CaSnO3:Yb3+,Er3+,Li+ phosphors are investigated, the related mechanisms are discussed and the abilities to modulate chromoaticity are evaluated. Keywords: Upconversion; CaSnO3; Rare earth; Li+ ions; Chromaticity Introduction Over the past few decades, considerable effort has been devoted to the study of upconversion luminescence of lanthanide-ions doped optical materials, due to the potential application in solid state lighting, full color display, bio-label and so on[1-4]. Many trivalent lanthanide ions have been used as luminescence centers for upconversion materials, but studies of the luminescence properties in Er3+ doped materials are of greater interest. This is because (1) Er3+ can be efficiently excited in the case of Yb3+/Er3+ co-doping; (2) Er3+ has two thermally coupled levels 2H11/2 and 4S3/2; (3) the transition from 4F9/2 to 4I15/2 produces the red emission around 650 nm that is one of three primary colors. Compared to majority oxide materials, CaSnO3 with relatively low phonon energy (699 cm-1) enables high luminescence efficiency by hindering the nonradiative losses. Recently, luminescence of CaSnO3 activated by lanthanide ions has attracted much attention[9-15], but a little work has been performed on the upconversion luminescence. In this paper, CaSnO3 is selected as the host materials. Meanwhile, to further enhance the
luminescence efficiency, a certain amount of Li+ ions are incorporated into the CaSnO3 lattice. Compare to other methods such as core-shell structure[16-18] and surface plasmon resonance[19-21], introduction of Li+ ions is more suitable for the solid state method. More importantly, the solubility limit of trivalent lanthanide ions in CaSnO3 can be improved by using charge compensation of Li+ that replaces Sn4+ in CaSnO3. Finally, chromaticity modulation of upconversion luminescence in CaSnO3:Yb3+,Er3+,Li+ phosphors through Yb3+ concentration, pumping power and temperature are investigated in detail. 1 Experimental 1.1 Preparation of CaSnO3:Yb3+,Er3+,Li+ phosphors Solid state method is used to synthesize the CaSnO3:Yb3+,Er3+,Li+ phosphors. The starting materials including CaCO3 (AR), SnO2 (CP),YbNO3·6H2O (4N) and ErNO3·6H2O (4N) are mixed in stoichiometric ratio. To improve the upconversion efficiency, x wt% (x = 0, 5, 10, 15,20) Li2CO3 (AR) is incorporated. After throughout grinding in agate mortar, the mixture are put into corundum crucible and then calcined in a silicon-molybdenum stove at 1300 °C for 3 h. Finally, the white products are obtained. 1.2 characterization X-ray diffraction (XRD) analysis was performed at 40 kV and 30 mA using a shimadzu-6000 X-ray diffraction generator with Cu Kα (λ=1.5406Å) radiation. The upconversion spectra were recorded by Hitachi F-4600 fluorescence spectrophotometer equipped with a power tunable 980 nm continuous wave LD. A home-made sample temperature control setup was used, and the temperature controlling and measuring uncertainty is less than 0.5°C. 2 Results and discussion 2.1 Optimization of Li+ content in CaSnO3:Yb3+,Er3+ phosphors Fig .1 shows the XRD patterns of CaSnO3:8%Yb3+,1%Er3+,x wt%Li+ (x = 0, 5, 10, 15, 20) and the standard data for orthorhombic CaSnO3. It is seen that when the value of x is 0, the orthorhombic CaSnO3 is obtained but followed by a small quantity of cubic Yb2O3 (JCPDS No. 76-0161). The measured diffraction peaks slightly shift towards larger angles compared to the standard data indicates that most of Yb3+ ions and all Er3+ ions have been doped into the lattice of CaSnO3. Moreover, because the radii of Yb3+ (r = 0.868 Å with CN = 6) and Er3+ (r = 0.89 Å with CN = 6) is less than that of Ca2+ (r = 1.00 Å with CN = 6) but larger than that of Sn4+ (r = 0.69 Å
with CN = 6), it can be deduced that the doping ions occupy the site of Ca2+[8-15], It is noteworthy that the diffraction peaks corresponding to Yb2O3 disappear after doping Li+ ions. The enhanced dissolving capacity of Yb3+ ions in CaSnO3 can be attributed to the charge compensation of Li+ (r = 0.76 Å with CN = 6) that replaces the Sn4+. However, an excess doping of Li+ ions causes that a certain amount of Sn3O4 (JCPDS No. 20-1293) appear and the crystallinity of CaSnO3 is destroyed.
Fig. 1 XRD patterns of CaSnO3:Yb3+,Er3+,x wt%Li+ (x = 0, 5, 10, 15, 20) and standard data JCPDS No.77-1797
Through a comparative research of upconversion luminescence in CaSnO3:8%Yb3+,1%Er3+,x wt%Li+ (x = 0, 5, 10, 15, 20) under 980 nm excitation (Fig. 2), some interesting phenomena are observed as follows: (1) With increasing Li+ content, the red emission firstly increases and then decreases. When introducing 10 wt% Li+, about 50% intensity enhancement is induced compared to the sample without Li+. The increase process can be attributed to the increased doping level of Yb3+ and the change of crystal field around Er3+, while the decrease process can be explained by the destroyed crystallinity of CaSnO3. (2) With the increase of Li+ content, the intensities of purple
and green emissions increase and decrease, alternately. And when the Li+ concentration exceeds 15 wt%, the peak wavelength of purple emission shifts towards shorter wavelengths and red emission presents the Stark splitting. These may be related to the impurity phases, because the 4f-4f transitions of rare-earth-ions are strongly dependent on the strength and symmetry of crystal field around activators. For the real applications, upconversion emission brightness and upconversion quantum yield (UCQY) is very important. However, it is difficult to measure these two parameters, especially the UCQY. As well known, Y2O3:Yb3+,Er3+ and Y2O2S:Yb3+,Er3+ are the excellent upconverting materials. For the purpose of comparison, Y2O3:10%Yb3+,1%Er3+ is synthesized by using a solid state method and Y2O2S:Yb3+,Er3+ is purchased from Dalian Luming light Co., Ltd. In the same measurement condition, the integrate intensity ratio of red emission is IY2O2S : I optimized sample : IY2O3 = 8.78 : 1.94 : 1.
Fig. 2 Upconversion spectra of CaSnO3:8%Yb3+,1%Er3+,x wt%Li+ (x = 0, 5, 10, 15, 20) under 980 nm excitation with pumping power density of 6.91 W/cm2
2.2 Effect of Yb3+ concentration on chromaticity and Yb3+ concentration dependent
upconversion mechanism From the structure analysis in Fig. 1 and upconversion luminescence properties shown in Fig. 2, the Li+ doping concentration is fixed at 10 wt% in each sample discussed below. Fig.3 shows the upconversion spectra of CaSnO3:x%Yb3+,1%Er3+,10wt%Li+ (x = 0, 4, 8, 12) under 980 nm excitation. It is noted that all the upconversion emissions present a same change regulation, viz. the relative intensity firstly increase and then decrease with the increase of Yb3+ concentration. But the red emission relative to the others is more sensitive for the change of Yb3+ concentration. This means that the shades of upconversion luminescence can be designed by adjusting the Yb3+ concentration[24,25]. The chromaticity coordinate is calculated from the upconversion spectra. As shown in Fig.4a, a larger shift of color point occurs with the change of Yb3+ concentration. According to E
( x1 x2 )2 ( y1 y2 )2
( E is the color aberration， x and y are
chromaticity coordinates), when the Yb3+ concentration changes from 0% to 12%, the color aberration is about 0.1976.
Fig. 3 Upconversion spectra of CaSnO3:x%Yb3+,1%Er3+,10wt%Li+ (x = 0, 4, 8, 12) under 980 nm excitation with pumping power density of 8.32 W/cm2, and the color-matching function of x(λ), y(λ) and z(λ)
Fig. 4 Chromatic diagram and color images of upconversion luminescence for various Yb3+ concentration (a), pumping power (b) and temperature (c)
To understand the chromaticity-tunable upconversion mechanism, the power dependence of upconversion emission for the samples with various Yb3+ concentrations is measured. It is seen from Fig. 5 that both the green and red emissions belong to the two-photon absorption, while the purple emission that only exists in Yb3+/Er3+ codoped samples belongs to the three-photon absorption. In the Er3+ doped sample, the combined action of ground state absorption and excited state absorption is responsible for the population of 4F7/2 level, and then the 2H11/2/4S3/2 and 4F9/2 levels are populated by multi-phonon relaxation from the 4F7/2 level. Since the energy gap (3136 cm-1) between 4S3/2 and 4F9/2 is about 4.5 times of the phonon (699 cm-1) of CaSnO3, the multi-phonon relaxation probability from 4S3/2 to 4F9/2 is every low, resulting in the dominant green emission. With the addition of Yb3+, the energy transfer from Yb3+ to Er3+ dominates the population of 2H11/2/4S3/2 and 4F9/2 levels. Due to high absorption cross-section around 980 nm and resonant energy transfer between Yb3+ and Er3+, the green and red emissions are enhanced and the weak purple emission is produced in the Yb3+/Er3+ codoped samples. It is noteworthy that the emission intensity ratio of Red/Green is strongly increased after doping Yb3+ ions. Comparing
with the 0%Yb3+ sample, the integrate intensity ratio of Red/Green for the 4%Yb3+, 8%Yb3+, and 12%Yb3+ sample are enhanced by 41, 75 and 128 times, respectively. In this paper, the doping ions tend to cluster together in matrix because of the mismatch in radius and charge . Thus the dominated red emission may be related to the back energy transfer from Yb3+ to Er3+ although the Yb3+ content is low. In this case, the more the addition of Yb3+ ions, the more the efficiency of energy transfer from Er3+ to Yb3+, which results in a gradual increase of the Red/Green emission ratio.
Fig. 5 Power dependence of purple (a), green (b) and red (c) emissions for the sample with various Yb3+ concentration (Pumping power density: 8.32 ~ 13.98 W/cm2)
2.3 Effect of pumping power and temperature on chromaticity of upconversion luminescence Fig. 5 shows the power dependence of upconversion emissions for the samples with 4%, 8% and 12% Yb3+. It can be seen that the slops of purple, green and red upconvesion emissions are distinct in each sample. According to the function f ( P) AP
( f ( P) is the intensity of
upconversion emission, P is pumping power, n is slop of double logarithmic curve, A is a constant), the relative intensity ratio of Purple/Green/Red will vary with pumping power. The shift of color point shown in Fig. 4b proves the chromaticity can be adjusted by changing the pumping power. It is interesting that the abilities to modulate chromaticity is different for the three samples. Bringing the spectral power distribution and three color-matching functions in Fig. 2 into the calculation formula of tristimulus values X, Y and Z, it can be found that the contribution of purple to X+Y+Z is much less than those of the green and red emissions. In order to simplify the problem, the contribution of the weak purple emission to chromaticity is ignored. In this case, since the difference between ngreen and nred is lager in 8%Yb3+ and 12%Yb3+ samples than 4%Yb3+ sample, the enhancement extent of intensity ratio of Green/Red is larger in both of the former, resulting in the larger color aberrations after changing pumping power.
Fig. 6 upconversion spectra of CaSnO3:8%Yb3+,1%Er3+,10 wt%Li+ sample under different 2
temperature (pumping power density: 9.74 W/cm )
As well known, most of laser pumping energy is converted to thermal energy during laser irradiation. Therefore, the temperature of sample will be raised under 980 nm excitation, and the upconversion luminescence properties will be affected by the raised temperature. Fig. 6 shows the upconversion spectra of CaSnO3:8%Yb3+,1%Er3+,10wt%Li+ sample under different temperature. When the temperature increases from 300K to 500K, the integrate intensity ratio of Red/Green reduces to 4.53 from 7.40. This indicates that the red emission relative to the green emission is more sensitive for the temperature. To understand the temperature dependence of CaSnO3:Yb3+,Er3+,Li+ phosphors, the plot of Ln( I / I 0 1) vs. 1 / kT is displayed in Fig. 7. Where I (T ) and I 0 is the intensity at a given testing temperature and the initial intensity, respectively; and k is Boltzmann’s constant (8.62X10-5 eV / K ). It is seen that the experimental data corresponding to green emission misfits with the theory, which may be related to the thermal population from 4S3/2 to 2H11/2. Due to the difference between green emission and red emission in temperature sensitivity, the chromaticity can also be adjusted by controlling the
temperature of sample. However, the ability of pumping power to adjust chromaticity of upconversion luminescence is limited (Fig. 4c). When the temperature of sample increases from 300 K to 400 K, the color aberration is only 0.0469.
Fig. 7 Plot of Ln( I / I 0 1) vs. 10000 / T for the green and red emissions
Conclusion Through the comparative study of upconversion spectra and XRD analasis of samples with various Li+ content, the orthorhombic CaSnO3:Yb3+,Er3+ doped with 10wt% Li+ upconverting phosphors are obtained. The effects of Yb3+ concentration, pumping power and temperature on chromaticity of upconversion luminescence in CaSnO3:Yb3+,Er3+,Li+ phosphors are investigated, respectively. Compared to the latter two, the former is proved to be the most efficient for the Chromaticity modulation of upconversion luminescence. The ability of Yb3+ concentration to modulate chromaticity of upconversion luminescence in CaSnO3:Yb3+,Er3+,Li+ phosphors is related to the Yb3+ concentration dependent upconversion mechanism.
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