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Influence of Nd3+ concentration on up-conversion fluorescence colour in YVO4 co-doped with Ho3+, Yb3+ and Nd3+ ions
Materials Letters 88 (2012) 86–88
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Influence of Nd3 þ concentration on up-conversion fluorescence colour in YVO4 co-doped with Ho3 þ , Yb3 þ and Nd3 þ ions Marcin Sobczyk n Faculty of Chemistry, University of Wroc!aw, ul. F. Joliot-Curie 14, 50-383 Wroc!aw, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 June 2012 Accepted 7 August 2012 Available online 17 August 2012
Influence of Nd3 þ concentration on up-conversion fluorescence colour in YVO4 triple doped with Ho3 þ , Yb3 þ and Nd3 þ ions excited by 980 nm laser diode was investigated. A serials of NdxHo0.02 Yb0.05Y0.93 xVO4 (where x ¼ 0, 0.25, 0.5, 0.75 and 1 at%) crystalline materials were synthesised by a solid state reaction. Excitation in the 2F5/2 bands of Yb3 þ produced a strong red and a weaker green emissions at 300 K for Ho3 þ ,Yb3 þ :YVO4 whereas for Ho3 þ ,Yb3 þ ,Nd3 þ :YVO4 a strong green emission was observed. Concentration dependence studies indicated that Nd3 þ concentrations had significant influences on up-conversion mechanisms and emission colour. & 2012 Elsevier B.V. All rights reserved.
Keywords: Up-conversion fluorescence Nd3 þ /Ho3 þ /Yb3 þ co-doped YVO4 Vanadates Emission colour
1. Introduction Over the past years trivalent lanthanide ions—doped yttrium tetraoxovanadates(V) (Ln3 þ :YVO4) have been attracting intensive research attention for their excellent performance in infrared emission and infrared-to-visible up-conversion [1–7]. It is well know that tetraoxovanadates(V) exhibit a strong absorption in the UV-VUV spectral range due to the CT:O2 -V5 þ transitions [8–10]. Consequently, Ln3 þ ions-doped YVO4 shows a strong emission under UV excitation due to the efficient energy transfer from the CT state to the 4fN electronic states of Ln3 þ ions. Eu3 þ :YVO4 is known as a typical commercial red-emitting phosphor used in cathode ray tubes [8]. Ho3 þ ion is a good candidate for up-conversion processes because it has many long-lived intermediate metastable levels (5I7, 5I6), from which excited state absorption can take place. Yb3 þ ion has been used as a very efficient sensitizer due to its large absorption cross section at about 980 nm, and it can efficiently transfer to the excitation energy to Ho3 þ ion [4–7]. Unlike other Yb,Ho-doped materials (for example [11]) Yb3 þ ,Ho3 þ :YVO4 exhibit at 300 K intense red and week green up-converted emissions originating from 5F5 and 5 S2 levels, respectively. The Ho,Yb:YVO4 shows reddish orange fluorescence which changes into yellowish green on cooling down the sample to 4.2 K This change is reversible on warming up [6]. In this letter the effect of Nd3 þ concentration on the infraredto-visible conversion of radiation in YVO4 co-doped with Ho3 þ , Yb3 þ and Nd3 þ ions has been studied. The up-conversion fluorescence has been examined in NdxHo0.02Yb0.05Y0.93 xVO4 (where
n
Tel.: þ48 71 375 7333x7321; fax: þ 48 71 3282348. E-mail address: [email protected]
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.08.026
x¼0, 0.25, 0.50, 0.75 and 1 at%) microcrystals pumping with a 980 nm diode laser at 300 K. The dependence of fluorescence intensity on pump power as well as of the decay time of the 5I7 level of Ho3 þ on Nd3 þ concentration have been measured in order to understand the mechanism of the up-conversion luminescence. More importantly, the UC fluorescence colours can be modulated by adjusting the Nd3 þ ion concentrations.
2. Experimental The microcrystalline samples of Ho3 þ , Yb3 þ and Nd3 þ -doped YVO4 were synthesized using a high temperature solid-state reaction technique. The high purity (99.999%) starting materials were Ho2O3, Y2O3, Eu2O3 and NH4VO3. The doping concentrations of Ho3 þ and Yb3 þ were fixed at 2 and 5 at%, respectively, and Nd3 þ were 0.00, 0.25, 0.50, 0.75 and 1.00 at% with respect to Y3 þ ions. Stoichiometric amounts of reagents were thoroughly mixed and ground using an agate mortar. The reaction mixture was taken in a platinum–rhodium crucible and slowly heated in an electrical furnace at 350 1C for 10 h to facilitate the evolution of NH3 and H2O from NH4VO3 to obtain the highly reactive vanadium pentaoxide (V2O5). In the next step, the mixture was reground and fired at 1100 1C for 72 h, at a heating rate of 5 1C/min, to obtain slightly yellow powder. The X-ray powder diffractograms of the products were recorded on a D8 ADVANCE X-ray diffractometer using Ni-filtered copper radiation ˚ The analysis was performed in the 2y ¼10–601 (l ¼1.5418 A). range and with 0.0161 step. The X-ray examination of obtained sample powders revealed a single phase (see Fig. 1a) [12]. The diffraction lines in XRD patterns are very sharp and narrow, revealing that the synthesized compounds showed high
M. Sobczyk / Materials Letters 88 (2012) 86–88
87
basis of our previous works [6,9]. The strongest absorption bands are due to the 2F7/2-2F5/2 transition of Yb3 þ ion and are located in the diode-pumping region at about 9400–11,500 cm 1 (1064–870 nm). Fig. 2 shows the normalized up-conversion fluorescence spectra of NdxHo0.02Yb0.05Y0.93 xVO4 (where x¼0 and 1 at%) with a pump excitation at 980 nm (P¼0.25 W). The fluorescence bands centred at 649.0 and 543.3 nm correspond to 5F5-5I8 and 5 S2-5I8 transitions of Ho3 þ ion, respectively. It is important to point out that the fluorescence is bright enough to be observed by the naked eye at excitation power as low as 50 mW. The integrated emission ratio, defined as the emission intensity from 5 S2 level to the emission intensity from 5F5 level of Ho3 þ ion, without the Nd3 þ ions is 0.13 and grows to 0.87 for sample with 1 at% of Nd3 þ . This indicates that the red emission from 5F5 state of Ho3 þ is widely quenched by the presence of Nd3 þ . Consequently, mix of these bands yields reddish orange, yellowish orange, yellow, yellow green and yellowish green colour for sample with 0, 0.25, 0.5, 0.75 and 1.0 at% of Nd3 þ ions, respectively. The relationship between green and red emission intensity as the function of Nd3 þ concentration is plotted in inset of Fig. 2. The chromaticity coordinates of the up-conversion emission for Ho,Yb,Nd3 þ :YVO4 recorded at different Nd3 þ concentration are listed in Table 1. The up-conversion fluorescence intensity (IUC) depends on the incident pump power (P) according to the following relation IUC pP n
Fig. 1. (a) Theoretical [11] and experimental XRD patterns of YVO4, (b) absorption spectra of Ho0.02Yb0.05Y0.93VO4 and Ho0.02Yb0.05Nd0.0075Y0.9225VO4.
crystallinity. The powder patterns of the samples were indexed on the tetragonal cell, I41amd space group, Z ¼4, with the lattice ˚ V¼318.7 A˚ 3. Reflection spectra parameters: a ¼b¼7.12, c ¼6.29 A, were recorded on a Cary 5E NIR-Vis-UV spectrophotometer in the 4000–50,000 cm 1 range with the resolution of 0.1 nm. The spectra were automatically converted into absorption units by the instrument software. Infrared light with a wavelength of 980 nm emitted by a diode laser with highest available power of 2 W has been used to excite up-converted (UC) emission. The UC spectra were measured with a High-Resolution spectrometer OceanOptics (model HR-4000). Luminescence decay curves of luminescence from 5I7 excited level of Ho3 þ were recorded with a SPM-2 monochromator and a Tektronix model TDS 3052 digital oscilloscope following the selective excitation by a Continuum Surelite I optical parametric oscillator (OPO) pumped by the third harmonic of a Nd:YAG laser and detected by a Hamamatsu R955 photomultiplier with S-20 spectral response.
ð1Þ
where n is the number of photons involved in the UC process. A plot of logðIUC Þ versus logðP Þ yields a straight line with slope n. The pump power dependence of the UC emission intensity upon the pomp power was analyzed. The fitted n values are 1.75 70.02 and 1.65 70.02 for the 5S2-5I8 and 5F5-5I8 transitions of Ho3 þ ion, respectively for YVO4 doubly doped with Ho3 þ and Yb3 þ ions. The results indicate that a two-photon UC process propels to the 5S2-5I8 and 5F5-5I8 transitions. One note that for the Ho,Yb,Nd3 þ :YVO4 samples the slopes of 5F5-5I8 transition are independent on Nd3 þ concentration but for the 5S2-5I8 transition the slopes slightly increases with increasing the Nd3 þ contents.
3. Results and discussion The room temperature absorption spectra of Ho0.02Yb0.05 Y0.93VO4 and Ho0.02Yb0.05Nd0.0075Y0.9225VO4 are shown in Fig. 1b. The absorption bands are attributed to 4fN-4fN of Nd3 þ , Ho3 þ and Yb3 þ from the ground states 4I9/2, 5I8 and 2F7/2, respectively. The absorption bands could be assigned on the
Fig. 2. UC emission spectra of Ho0.02Yb0.05Y0.93VO4 (red) and Ho0.02Yb0.05Nd0.01Y0.92VO4 (green) (lexc ¼ 980, T¼ 300 K). Inset shows the relationship between green and red integrated emission intensity as the function of Nd3 þ concentration. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
88
M. Sobczyk / Materials Letters 88 (2012) 86–88
Table 1 Influence of Nd3 þ concentration on green to red integrated intensity ratio of upconversion (UC) fluorescence, chromaticity coordinates (CIE 1931) and energy transfer (ET) efficiency between 5I7 level of Ho3 þ ion and 4I15/2 level of Nd3 þ ion in Ho3 þ , Yb3 þ , Nd3 þ -triple doped YVO4, excited by 980 nm. Nd3 þ concentration (at%)
Green-to-red intensity ratio
Chromaticity coordinates
x
y
0.00
0.13
0.615
0.375
0.25
0.33
0.521
0.474
0.50 0.75 1.00
0.51 0.68 0.87
0.483 0.461 0.445
0.511 0.533 0.511
Colour of UC fluorescence
Reddish orange Yellowish orange Yellow Yellow green Yellowish green
ET efficiency (%)
– 66.0 69.6 80.9 83.3
Nd3 þ ions plays the role of donors and acceptors, respectively and ion–ion interaction quenches the 5I7 emission through the energy transfer to the 4I15/2 level of the Nd3 þ . The ET efficiency (Z) can be expressed as
Z ¼ 1
tHo tHo 0
ð2Þ
5 3þ where tHo and tHo 0 are the decay times of the I7 level of the Ho ion with and without Nd3 þ ions, respectively. The Z values are listen in Table 1. It is clearly seen that the ET efficiency is very high and increased from 66 to 83% with increasing Nd3 þ concentration from 0.25 to 1 at%, respectively. Consequently, the red up-conversion emission intensity decreased with increasing Nd3 þ concentration. Fig. 3 pictures the energy level diagrams of the Nd3 þ , Ho3 þ and Yb3 þ ions as well as the probable UC mechanisms accounting for the red and green emissions under 980 nm excitation. One note that the UC fluorescence intensity of the 5S2-5I8 transition was enhanced largely with an increasing Nd3 þ content, which is mainly due to energy transfer from the 4F3/2 metastable state of Nd3 þ to the 5S2 level of Ho3 þ . This mechanism will be studied in detail and described in the next publication.
4. Conclusions
Fig. 3. Energy scheme of Nd3 þ , Ho3 þ and Yb3 þ levels considering the ESA and ETU processes for the UC emissions under 980 nm excitation and fluorescence decay curves of the 5I7 level of Ho3 þ in Ho0.02Yb0.05Y0.93VO4 and Ho0.02Yb0.05 Nd0.01Y0.92VO4 samples.
When the samples are excited with 980 nm laser, the Yb3 þ ions can absorb the radiation efficiently and transfer their excitation energies to 5I6 state of Ho3 þ ions. This is a phonon assisted energy transfer process because of energy mismatch between the 2 F5/2 level of donor (Yb3 þ ) and 5I6 level of acceptor (Ho3 þ ). In the next step the 5S2 level can be populated from 5I6 state by absorption of second excitation phonon. The emission decay times of the 5I6 state is relatively short [7]. Consequently, the green emission from 5S2 state is weak. The 5I7 level of the Ho3 þ ion can be populated via non-radiative energy transfer from the 5 I6 state. In an excited state absorption (ESA) process the 5F5 level can be populated by efficient absorption of second excitation phonon from 5I7 metastable level. Consequently, intense red upconverted emission originating from the 5F5 to the 5I8 ground state of the Ho3 þ ions is observed in the fluorescence spectrum of the Ho3 þ ,Yb3 þ -doped YVO4. The fluorescence decay profiles for the 5I7 level of the Ho3 þ ion manifold in semilog scale are shown in Fig. 3. The decay kinetics of the emitting level are nearly exponential. The lifetime of 5I7 level for the sample with 2 at% of Ho3 þ without the Nd3 þ ions was amounted to 1548 ms and that decreased to 259 ms for sample containing with 1 at% of the Nd3 þ ions. The difference in these lifetimes is relatively large. It indicates that the Ho3 þ and
This letter presents the optical characterization of Ho3 þ , Yb3 þ and Nd3 þ -triple doped in YVO4—promising and efficient phosphors. Excitation in the 2F5/2 bands of Yb3 þ ion produced strong UC emission in the red and green region from 5F5 and 5S2 levels, respectively of the Ho3 þ ion at 300 K. It has been demonstrated that the UC fluorescence colours may be modulated by adjusting the concentration of Nd3 þ ions. The chromaticity coordinated changed from reddish orange to yellowish green when increasing the Nd3 þ concentration from 0 to 1.0 at%, respectively. The ET efficiency between Ho3 þ and Nd3 þ ions is very high.
Acknowledgements The author wish to thank Dr. Rados"aw Lisiecki of the Institute of Low Temperature and Structure Research in Wroc"aw for measurements of the decay curves. References [1] Sliney Jr JG, Leung KM, Birnbaum M, Tucker AW. J Appl Phys 1979;50:3778–9. [2] Go"a˛b S, Ryba-Romanowski W, Dominiak-Dzik G, Łukasiewicz T, S´wirkowicz M. J Alloys Compd 2001;323–324:288–91. [3] Tsuboi T. Phys Rev B 2000;62:4200–3. [4] Ryba-Romanowski W, Solarz P, Dominiak-Dzik G, Lisiecki R, Łukasiewicz T. Laser Phys 2004;14:250–7. [5] Ryba-Romanowski W, Go"a˛b S, Dominiak-Dzik G, Solarz P, Łukasiewicz T. Appl Phys Lett 2001;79:3026–8. [6] Lisiecki R, Dominiak-Dzik G, Ryba-Romanowski W, Łukasiewicz T. J Appl Phys 2004;96:6323–30. [7] Lisiecki R, Ryba-Romanowski W, Łukasiewicz T, Mond M, Petermann K. Laser Phys 2005;15:306–12. [8] Blasse G. Struct Bond 1980:42. [9] Sobczyk M. J Lumin 2009;129:430–3. [10] Sobczyk M, Lisiecki R, Solarz P, Ryba-Romanowski W. J Lumin 2010;130:567–75. [11] Luo XX, Cao WH. Mater Lett 2007;61:3696–700. [12] Milligan WO, Vernon LW. J Phys Chem 1952;56:145–8.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Influence of Nd3 þ concentration on up-conversion fluorescence colour in YVO4 co-doped with Ho3 þ , Yb3 þ and Nd3 þ ions Marcin Sobczyk n Faculty of Chemistry, University of Wroc!aw, ul. F. Joliot-Curie 14, 50-383 Wroc!aw, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 June 2012 Accepted 7 August 2012 Available online 17 August 2012
Influence of Nd3 þ concentration on up-conversion fluorescence colour in YVO4 triple doped with Ho3 þ , Yb3 þ and Nd3 þ ions excited by 980 nm laser diode was investigated. A serials of NdxHo0.02 Yb0.05Y0.93 xVO4 (where x ¼ 0, 0.25, 0.5, 0.75 and 1 at%) crystalline materials were synthesised by a solid state reaction. Excitation in the 2F5/2 bands of Yb3 þ produced a strong red and a weaker green emissions at 300 K for Ho3 þ ,Yb3 þ :YVO4 whereas for Ho3 þ ,Yb3 þ ,Nd3 þ :YVO4 a strong green emission was observed. Concentration dependence studies indicated that Nd3 þ concentrations had significant influences on up-conversion mechanisms and emission colour. & 2012 Elsevier B.V. All rights reserved.
Keywords: Up-conversion fluorescence Nd3 þ /Ho3 þ /Yb3 þ co-doped YVO4 Vanadates Emission colour
1. Introduction Over the past years trivalent lanthanide ions—doped yttrium tetraoxovanadates(V) (Ln3 þ :YVO4) have been attracting intensive research attention for their excellent performance in infrared emission and infrared-to-visible up-conversion [1–7]. It is well know that tetraoxovanadates(V) exhibit a strong absorption in the UV-VUV spectral range due to the CT:O2 -V5 þ transitions [8–10]. Consequently, Ln3 þ ions-doped YVO4 shows a strong emission under UV excitation due to the efficient energy transfer from the CT state to the 4fN electronic states of Ln3 þ ions. Eu3 þ :YVO4 is known as a typical commercial red-emitting phosphor used in cathode ray tubes [8]. Ho3 þ ion is a good candidate for up-conversion processes because it has many long-lived intermediate metastable levels (5I7, 5I6), from which excited state absorption can take place. Yb3 þ ion has been used as a very efficient sensitizer due to its large absorption cross section at about 980 nm, and it can efficiently transfer to the excitation energy to Ho3 þ ion [4–7]. Unlike other Yb,Ho-doped materials (for example [11]) Yb3 þ ,Ho3 þ :YVO4 exhibit at 300 K intense red and week green up-converted emissions originating from 5F5 and 5 S2 levels, respectively. The Ho,Yb:YVO4 shows reddish orange fluorescence which changes into yellowish green on cooling down the sample to 4.2 K This change is reversible on warming up [6]. In this letter the effect of Nd3 þ concentration on the infraredto-visible conversion of radiation in YVO4 co-doped with Ho3 þ , Yb3 þ and Nd3 þ ions has been studied. The up-conversion fluorescence has been examined in NdxHo0.02Yb0.05Y0.93 xVO4 (where
n
Tel.: þ48 71 375 7333x7321; fax: þ 48 71 3282348. E-mail address: [email protected]
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.08.026
x¼0, 0.25, 0.50, 0.75 and 1 at%) microcrystals pumping with a 980 nm diode laser at 300 K. The dependence of fluorescence intensity on pump power as well as of the decay time of the 5I7 level of Ho3 þ on Nd3 þ concentration have been measured in order to understand the mechanism of the up-conversion luminescence. More importantly, the UC fluorescence colours can be modulated by adjusting the Nd3 þ ion concentrations.
2. Experimental The microcrystalline samples of Ho3 þ , Yb3 þ and Nd3 þ -doped YVO4 were synthesized using a high temperature solid-state reaction technique. The high purity (99.999%) starting materials were Ho2O3, Y2O3, Eu2O3 and NH4VO3. The doping concentrations of Ho3 þ and Yb3 þ were fixed at 2 and 5 at%, respectively, and Nd3 þ were 0.00, 0.25, 0.50, 0.75 and 1.00 at% with respect to Y3 þ ions. Stoichiometric amounts of reagents were thoroughly mixed and ground using an agate mortar. The reaction mixture was taken in a platinum–rhodium crucible and slowly heated in an electrical furnace at 350 1C for 10 h to facilitate the evolution of NH3 and H2O from NH4VO3 to obtain the highly reactive vanadium pentaoxide (V2O5). In the next step, the mixture was reground and fired at 1100 1C for 72 h, at a heating rate of 5 1C/min, to obtain slightly yellow powder. The X-ray powder diffractograms of the products were recorded on a D8 ADVANCE X-ray diffractometer using Ni-filtered copper radiation ˚ The analysis was performed in the 2y ¼10–601 (l ¼1.5418 A). range and with 0.0161 step. The X-ray examination of obtained sample powders revealed a single phase (see Fig. 1a) [12]. The diffraction lines in XRD patterns are very sharp and narrow, revealing that the synthesized compounds showed high
M. Sobczyk / Materials Letters 88 (2012) 86–88
87
basis of our previous works [6,9]. The strongest absorption bands are due to the 2F7/2-2F5/2 transition of Yb3 þ ion and are located in the diode-pumping region at about 9400–11,500 cm 1 (1064–870 nm). Fig. 2 shows the normalized up-conversion fluorescence spectra of NdxHo0.02Yb0.05Y0.93 xVO4 (where x¼0 and 1 at%) with a pump excitation at 980 nm (P¼0.25 W). The fluorescence bands centred at 649.0 and 543.3 nm correspond to 5F5-5I8 and 5 S2-5I8 transitions of Ho3 þ ion, respectively. It is important to point out that the fluorescence is bright enough to be observed by the naked eye at excitation power as low as 50 mW. The integrated emission ratio, defined as the emission intensity from 5 S2 level to the emission intensity from 5F5 level of Ho3 þ ion, without the Nd3 þ ions is 0.13 and grows to 0.87 for sample with 1 at% of Nd3 þ . This indicates that the red emission from 5F5 state of Ho3 þ is widely quenched by the presence of Nd3 þ . Consequently, mix of these bands yields reddish orange, yellowish orange, yellow, yellow green and yellowish green colour for sample with 0, 0.25, 0.5, 0.75 and 1.0 at% of Nd3 þ ions, respectively. The relationship between green and red emission intensity as the function of Nd3 þ concentration is plotted in inset of Fig. 2. The chromaticity coordinates of the up-conversion emission for Ho,Yb,Nd3 þ :YVO4 recorded at different Nd3 þ concentration are listed in Table 1. The up-conversion fluorescence intensity (IUC) depends on the incident pump power (P) according to the following relation IUC pP n
Fig. 1. (a) Theoretical [11] and experimental XRD patterns of YVO4, (b) absorption spectra of Ho0.02Yb0.05Y0.93VO4 and Ho0.02Yb0.05Nd0.0075Y0.9225VO4.
crystallinity. The powder patterns of the samples were indexed on the tetragonal cell, I41amd space group, Z ¼4, with the lattice ˚ V¼318.7 A˚ 3. Reflection spectra parameters: a ¼b¼7.12, c ¼6.29 A, were recorded on a Cary 5E NIR-Vis-UV spectrophotometer in the 4000–50,000 cm 1 range with the resolution of 0.1 nm. The spectra were automatically converted into absorption units by the instrument software. Infrared light with a wavelength of 980 nm emitted by a diode laser with highest available power of 2 W has been used to excite up-converted (UC) emission. The UC spectra were measured with a High-Resolution spectrometer OceanOptics (model HR-4000). Luminescence decay curves of luminescence from 5I7 excited level of Ho3 þ were recorded with a SPM-2 monochromator and a Tektronix model TDS 3052 digital oscilloscope following the selective excitation by a Continuum Surelite I optical parametric oscillator (OPO) pumped by the third harmonic of a Nd:YAG laser and detected by a Hamamatsu R955 photomultiplier with S-20 spectral response.
ð1Þ
where n is the number of photons involved in the UC process. A plot of logðIUC Þ versus logðP Þ yields a straight line with slope n. The pump power dependence of the UC emission intensity upon the pomp power was analyzed. The fitted n values are 1.75 70.02 and 1.65 70.02 for the 5S2-5I8 and 5F5-5I8 transitions of Ho3 þ ion, respectively for YVO4 doubly doped with Ho3 þ and Yb3 þ ions. The results indicate that a two-photon UC process propels to the 5S2-5I8 and 5F5-5I8 transitions. One note that for the Ho,Yb,Nd3 þ :YVO4 samples the slopes of 5F5-5I8 transition are independent on Nd3 þ concentration but for the 5S2-5I8 transition the slopes slightly increases with increasing the Nd3 þ contents.
3. Results and discussion The room temperature absorption spectra of Ho0.02Yb0.05 Y0.93VO4 and Ho0.02Yb0.05Nd0.0075Y0.9225VO4 are shown in Fig. 1b. The absorption bands are attributed to 4fN-4fN of Nd3 þ , Ho3 þ and Yb3 þ from the ground states 4I9/2, 5I8 and 2F7/2, respectively. The absorption bands could be assigned on the
Fig. 2. UC emission spectra of Ho0.02Yb0.05Y0.93VO4 (red) and Ho0.02Yb0.05Nd0.01Y0.92VO4 (green) (lexc ¼ 980, T¼ 300 K). Inset shows the relationship between green and red integrated emission intensity as the function of Nd3 þ concentration. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
88
M. Sobczyk / Materials Letters 88 (2012) 86–88
Table 1 Influence of Nd3 þ concentration on green to red integrated intensity ratio of upconversion (UC) fluorescence, chromaticity coordinates (CIE 1931) and energy transfer (ET) efficiency between 5I7 level of Ho3 þ ion and 4I15/2 level of Nd3 þ ion in Ho3 þ , Yb3 þ , Nd3 þ -triple doped YVO4, excited by 980 nm. Nd3 þ concentration (at%)
Green-to-red intensity ratio
Chromaticity coordinates
x
y
0.00
0.13
0.615
0.375
0.25
0.33
0.521
0.474
0.50 0.75 1.00
0.51 0.68 0.87
0.483 0.461 0.445
0.511 0.533 0.511
Colour of UC fluorescence
Reddish orange Yellowish orange Yellow Yellow green Yellowish green
ET efficiency (%)
– 66.0 69.6 80.9 83.3
Nd3 þ ions plays the role of donors and acceptors, respectively and ion–ion interaction quenches the 5I7 emission through the energy transfer to the 4I15/2 level of the Nd3 þ . The ET efficiency (Z) can be expressed as
Z ¼ 1
tHo tHo 0
ð2Þ
5 3þ where tHo and tHo 0 are the decay times of the I7 level of the Ho ion with and without Nd3 þ ions, respectively. The Z values are listen in Table 1. It is clearly seen that the ET efficiency is very high and increased from 66 to 83% with increasing Nd3 þ concentration from 0.25 to 1 at%, respectively. Consequently, the red up-conversion emission intensity decreased with increasing Nd3 þ concentration. Fig. 3 pictures the energy level diagrams of the Nd3 þ , Ho3 þ and Yb3 þ ions as well as the probable UC mechanisms accounting for the red and green emissions under 980 nm excitation. One note that the UC fluorescence intensity of the 5S2-5I8 transition was enhanced largely with an increasing Nd3 þ content, which is mainly due to energy transfer from the 4F3/2 metastable state of Nd3 þ to the 5S2 level of Ho3 þ . This mechanism will be studied in detail and described in the next publication.
4. Conclusions
Fig. 3. Energy scheme of Nd3 þ , Ho3 þ and Yb3 þ levels considering the ESA and ETU processes for the UC emissions under 980 nm excitation and fluorescence decay curves of the 5I7 level of Ho3 þ in Ho0.02Yb0.05Y0.93VO4 and Ho0.02Yb0.05 Nd0.01Y0.92VO4 samples.
When the samples are excited with 980 nm laser, the Yb3 þ ions can absorb the radiation efficiently and transfer their excitation energies to 5I6 state of Ho3 þ ions. This is a phonon assisted energy transfer process because of energy mismatch between the 2 F5/2 level of donor (Yb3 þ ) and 5I6 level of acceptor (Ho3 þ ). In the next step the 5S2 level can be populated from 5I6 state by absorption of second excitation phonon. The emission decay times of the 5I6 state is relatively short [7]. Consequently, the green emission from 5S2 state is weak. The 5I7 level of the Ho3 þ ion can be populated via non-radiative energy transfer from the 5 I6 state. In an excited state absorption (ESA) process the 5F5 level can be populated by efficient absorption of second excitation phonon from 5I7 metastable level. Consequently, intense red upconverted emission originating from the 5F5 to the 5I8 ground state of the Ho3 þ ions is observed in the fluorescence spectrum of the Ho3 þ ,Yb3 þ -doped YVO4. The fluorescence decay profiles for the 5I7 level of the Ho3 þ ion manifold in semilog scale are shown in Fig. 3. The decay kinetics of the emitting level are nearly exponential. The lifetime of 5I7 level for the sample with 2 at% of Ho3 þ without the Nd3 þ ions was amounted to 1548 ms and that decreased to 259 ms for sample containing with 1 at% of the Nd3 þ ions. The difference in these lifetimes is relatively large. It indicates that the Ho3 þ and
This letter presents the optical characterization of Ho3 þ , Yb3 þ and Nd3 þ -triple doped in YVO4—promising and efficient phosphors. Excitation in the 2F5/2 bands of Yb3 þ ion produced strong UC emission in the red and green region from 5F5 and 5S2 levels, respectively of the Ho3 þ ion at 300 K. It has been demonstrated that the UC fluorescence colours may be modulated by adjusting the concentration of Nd3 þ ions. The chromaticity coordinated changed from reddish orange to yellowish green when increasing the Nd3 þ concentration from 0 to 1.0 at%, respectively. The ET efficiency between Ho3 þ and Nd3 þ ions is very high.
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