Journal of Luminescence 179 (2016) 107–113
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Temperature influence on NaLaF4:Er3 þ green luminescence Jurgis Grube 1 Institute of Solid State Physics, University of Latvia, 8 Kengaraga str., Riga, LV-1063, Latvia
art ic l e i nf o
a b s t r a c t
Article history: Received 29 January 2016 Received in revised form 16 June 2016 Accepted 19 June 2016 Available online 23 June 2016
Er3 þ doped NaLaF4 is a promising material for up-conversion luminescence applications due to low phonon energy and multisite nature of the crystalline lattice. In this work, luminescence processes in NaLaF4:Er3 þ materials have been studied at different temperatures. Spectra and decay kinetics of the green luminescence were measured under excitation to 4F7/2 state. Analysis of the green luminescence excitation spectra, the luminescence spectra and the luminescence decay kinetics at different temperatures reveals that the observed single green luminescence spectra at room temperature are related to overlapping of the green luminescence excitation bands from erbium ions located at non-equivalent lattice sites. In addition, spectroscopic data obtained from the green and the infrared luminescence spectra at different temperatures suggest that both 4S3/2 and 2H11/2 states are involved in energy transfer between erbium ions, leading to concentration quenching of the green luminescence. & 2016 Elsevier B.V. All rights reserved.
Keywords: Luminescence NaLaF4 Er3 þ Multisite Temperature dependence
1. Introduction For many years, considerable attention was been paid to studies of up-conversion luminescence - processes involving transformation of lower energy photons (with longer wavelength) into photons with higher energy (with shorter wavelength) by gradually absorbing and accumulating energy of excitation source photons [1]. Up-conversion luminescence can be used for several practical applications, e.g., in solid-state lasers [2], temperature sensors [3], white light simulation devices [4], in solar cells to improve their efficiency [5], in biologic markers [6], photodynamic therapy [7] and in optical fibres for amplification of the signal [8]. Compared to traditional luminescence, up-conversion luminescence is a more complex process. Therefore requirements for materials which would show efficient up-conversion luminescence are higher. At present, hexagonal NaYF4 doped with different rareearth elements is considered to be one of the most promising materials for up-conversion luminescence applications [9–12]. NaLaF4 – a compound isostructural with hexagonal NaYF4 (space group P6) – is another promising material for efficient upconversion luminescence [13–16]. Slightly lower phonon effective energy in NaLaF4 [17], compared to NaYF4 [12], most probably indicates that nonradiative transitions in NaLaF4 could be supressed more effectively than in NaYF4, therefore efficiency of upconversion luminescence in rare-earth doped NaLaF4 is expected to be higher than in NaYF4. E-mail address: jurgis.grube@cfi.lu.lv Tel.: þ371 67 187 471; fax: þ 371 67 132 778.
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http://dx.doi.org/10.1016/j.jlumin.2016.06.046 0022-2313/& 2016 Elsevier B.V. All rights reserved.
Up to now, only a few studies associated with synthesis and spectroscopic characterisation of rare-earth doped NaLaF4 have been performed [17–22]. In Er3 þ doped NaLaF4, intensive green luminescence has been observed. Maximal intensity of green luminescence was observed for Er3 þ doping level of 2 mol%. Above this critical concentration, the green luminescence band is significantly influenced by concentration quenching effect related to cross-relaxation and energy migration processes [23,24]. Site-selective spectroscopy study of NaLaF4:Er3 þ performed at low temperatures revealed at least three different green luminescence spectra, which originate from Er3 þ occupying three nonequivalent NaLaF4 crystalline lattice sites [24]. However, at room temperature, only a single green luminescence spectrum is observed regardless of excitation wavelength [23]. Therefore, in this work, site-selective spectroscopy is used to study temperature dependences of green luminescence spectra and luminescence kinetics in NaLaF4:Er3 þ . Analysis of the obtained experimental results revealed influence of temperature on green luminescence spectra and allows one to better understand mechanisms responsible for green luminescence concentration quenching.
2. Experimental setup The synthesis procedure of NaLaF4 doped with Er3 þ at different concentration was the same as in reference [24]. NaLaF4 samples doped with Er3 þ (2 and 0.1 mol%) were synthesised from a high purity (4N) precursors (LaF3, NaF and ErF3) with a ratio (44-x) LaF3:56NaF:xErF3, x¼2; 0.1 mol%. The mixture of precursors was annealed in He/F2 (90%/10% ratio) gas flow at 150 °C for 1 h and
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afterwards – in argon atmosphere at 750 °C for 1 h. The obtained polycrystalline powder was grinded and annealed following the latter procedure. Afterwards, the synthesised polycrystalline material was washed with water to remove traces of residual NaF and dried at 100 °C for 1 h in He/F2 gas flow. White polycrystalline powder of NaLaF4:Er3 þ was obtained in the result of this synthesis method. In order to excite luminescence, tunable (210–2300 nm) pulsed NT342/3UV EKSPLA laser (pulse length 5 ns, repetition frequency 10 Hz) has been used. Laser linewidth is 4.3 cm 1. Laser wavelength can be varied with a step of 0.1 nm. iCCD ANDOR camera iSTAR DH734_18mm connected to ANDOR SR-303i-B monochromator/spectrometer was used to detect luminescence. ANDOR iDus 1.7 μm InGaAs CCD (DU490A-1.7) camera connected to the same spectrometer was used to detect infrared spectra. Luminescence decay kinetics were obtained as a series of delayed luminescence spectra recorded after excitation laser pulse using iCCD ANDOR camera iSTAR DH734_18 mm. For low temperature measurements, the sample was placed into closed cycle He cryostat DE202N, Advanced Research Systems. LakeShore 325 temperature controller (accuracy 71 K) was used to control the temperature of the sample.
3. Results Upon 980 nm excitation of NaLaF4:Er3 þ (2 mol%) at room temperature, characteristic Er3 þ luminescence bands are observed in the green and the red spectral regions with a dominant green luminescence band at 540 nm (Fig. 1). Further analysis will be focused on the green luminescence band, as intensity of this band is more than ten times higher, comparing to intensity of the red luminescence band. Inset in Fig. 1 illustrates energy level diagram of Er3 þ with possible up-conversion mechanisms: excited state absorption (ESA) and energy transfer up-conversion (ETU). At room temperature regardless of excitation wavelength, no explicit dependence on the excitation wavelength can be observed for the green luminescence band in the infrared spectral region at 950–1000 nm for up-conversion processes or at direct excitation in the visible spectral region, for example, 470–495 nm (4I15/ 4 2- F7/2). On the contrary, at low temperatures (15 K), the green luminescence spectra vary upon different excitation wavelengths (Fig. 2 15 K). Our previous work [24] has shown that Er3 þ can occupy three non-equivalent NaLaF4 crystalline lattice sites and as
Fig. 1. Up-conversion luminescence spectrum of NaLaF4:Er3 þ (2 mol%) measured at room temperature upon 980 nm. Inset: Er3 þ energy level diagram with possible up-conversion mechanisms.
a result three distinct green luminescence spectra can be observed. Excitation at 482.8 nm corresponds to Er(1) excitation, excitation at 484.8 nm corresponds to Er(2) excitation and excitation at 458.2 nm – to Er(3) excitation. In order to analyse and describe the observed differences in the green luminescence spectra, spectral measurements were performed at different sample temperatures. In temperature range from 15–100 K, Er3 þ green luminescence spectra retain their distinct form, typical for Er(1), Er(2) and Er(3), when excited at appropriate wavelength. Above 150 K, Er3 þ green luminescence spectra become indistinguishable regardless of excitation wavelength. The observed persistency of green luminescence spectra could be attributed to overlapping of excitation spectra. Indeed, it can be seen in the green luminescence excitation spectra, shown in Fig. 3, that the excitation spectra for the green luminescence of Er(1), Er(2) and Er(3) maintain their distinct form at temperatures lower than 150 K. As temperature increases above 150 K, overlapping of the Er(1), Er(2) and Er(3) green luminescence excitation bands occur. As the result, all Er3 þ located at different sites are excited simultaneously and single green luminescence spectrum – combination of the distinct Er(1), Er(2) and Er(3) spectra – is observed. Assumption about overlapping of the green luminescence excitation bands is confirmed by measurements of the green luminescence kinetics (Fig. 4). At low temperatures (15 K) green luminescence kinetics at various luminescence wavelengths are not identical. This is because mainly erbium ions occupying one crystalline lattice site are excited after excitation with a specific wavelength, while luminescence is observed also from erbium ions occupying other lattice sites. This is possible due to energy transfer between Er3 þ occupying different crystalline lattice sites [24]. For example, the excitation of Er(2) leads to a simple decay profile for the green luminescence of Er(2), while the luminescence of Er(1) has a well-pronounced rising part in the beginning of the kinetics curve, indicating energy transfer from Er(2) to erbium ions located at Er(1). As the sample temperature increases, the rising part in the green luminescence kinetics of Er(1) becomes less pronounced. The rising part in Er(1) green luminescence kinetics is not observed above 150 K. In addition, the Er(1) and the Er(2) green luminescence kinetics become similar, which signifies that all erbium ions are excited at the same moment during the excitation time (in our case 5 ns) due to overlapping of Er3 þ green luminescence excitation bands above 150 K, and the pronounced energy transfer part in luminescence decay kinetics is not observed. The previously mentioned variations in luminescence kinetics also apply in case when mainly Er(1) or Er(3) erbium ions are excited and luminescence is observed from erbium ions occupying other NaLaF4 crystalline lattice sites. Not only the rising part becomes less pronounced in the green luminescence kinetics, as the sample temperature increases, but also the kinetics of the green luminescence become shorter, as it is shown in Fig. 4. When excitation and luminescence occur in the same type of erbium ions, for example, Er(2)-Er(2), shortening of the green luminescence kinetics occurs in a discontinuous manner (Fig. 5a). In temperature range from 15 K to 150 K, shape of the luminescence kinetics curves remains the same, but, starting from 150 K and above, shortening of the green luminescence kinetics occurs. Inset in Fig. 5. a shows effective transition probability of the green luminescence kinetics obtained using the following R 1 R [25]. Effective transition equation: wef f ¼ τeff1 ¼ t IðtÞ= IðtÞ probability weff was used instead of radiative transition probability obtained from a single exponential fit, because, as the sample temperature increases above 150 K, the green luminescence kinetics become non-exponential. Also from Fig. 5. a, we can see
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Fig. 2. Green luminescence spectra of NaLaF4:Er3 þ (2 mol%) measured at different temperatures under excitation of Er3 þ occupying different sites in NaLaF4 crystalline lattice (Er(1), Er(2) and Er(3)). Arrows indicate characteristic luminescence wavelengths for Er(1) – 544.4 nm, Er(2) – 545.5 nm and Er(3) – 539.9 nm.
Fig. 3. Green luminescence excitation spectra of Er3 þ (2 mol%), which occupies different NaLaF4 crystalline lattice sites, recorded at different temperatures. Excitation spectra are normalised.
that, above 150 K, the green luminescence kinetics become faster and, when reaching 300 K, weff value increases almost twice indicating that depopulation of 4S3/2 state occurs faster. This means that, without radiative transitions, additional mechanisms appear, which reduces 4S3/2 state population. In the previous report of our group on Er3 þ concentration impact on luminescence processes in NaLaF4 [23], we have proposed that the decrease of the effective lifetime (increase of the effective transition probability weff) and the observed nonexponential nature of the green luminescence (540 nm) kinetics at room temperature are associated with cross-relaxation: (4S3/2; 4 I15/2)-(4I13/2; 4I9/2) and (4S3/2; 4I15/2)-(4I9/2; 4I13/2). The final states in both cross-relaxation processes are 4I13/2 and 4I9/2. After reaching 4I9/2 state, Er3 þ relaxes to 4I11/2 state in a nonradiative way. Therefore an increase of the infrared luminescence intensity at 980 nm (4I11/2-4I15/2) was observed with a simultaneous decrease of green luminescence intensity. In order to check whether the mentioned cross-relaxation processes are also responsible for the variations of the green luminescence kinetics at different temperatures, infrared luminescence spectra were measured for 980 nm luminescence band at different temperatures under the 484.8 nm excitation (4I15/2-4F7/2) (Fig. 6). The relative intensity of 980 nm luminescence band did not change in the
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Fig. 4. Decay kinetics of the green luminescence band of NaLaF4:Er3 þ (2 mol%) at 544.4 nm (Er(1)) and 545.5 nm (Er(2)) under 484.8 nm excitation (Er(2)) at different temperatures.
Fig. 5. Green luminescence kinetics of NaLaF4:Er3 þ (2 mol%) at a) 545.5 nm and b) 544.5 nm under excitation at 484.8 nm (Er(2)) at different temperatures. Inset in a) shows 545.5 nm luminescence kinetics effective transition probability vs sample temperature.
temperature interval 15–100 K. As the temperature rises above 150 K, the intensity of 980 nm luminescence band increases (Fig. 6 inset). Simultaneous increase of 980 nm luminescence band intensity and
shortening of the green luminescence kinetics (Fig. 5a inset) upon increasing of the sample temperature allows us to assume that these simultaneous changes are caused by cross-relaxation processes. The
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Fig. 6. Luminescence spectra of NaLaF4:Er3 þ in infrared spectral region excited at 484.8 nm, measured at different temperatures. Inset shows 980 nm luminescence band intensity vs sample temperature.
Fig. 7. Green luminescence spectra (525 nm) of NaLaF4:Er3 þ (2 mol%) excited at 484.8 nm (Er(2)) at different temperatures. Inset shows 525 nm luminescence band intensity vs sample temperature.
question is – why the cross-relaxation efficiency increases considerably at temperatures above 150 K? Additional analysis of temperature dependence of luminescence spectra indicates that the band at 525 nm, caused by the transition from 2H11/2 state (2H11/2-4I15/2), could be involved in the crossrelaxation processes. (Fig. 7). At low temperatures (15–100 K), luminescence from 2H11/2 state is not observed. Starting from 150 K, the 525 nm luminescence band appears and its intensity increases upon increasing of the sample temperature. This fact implies growth of 2H11/2 state population which is in accordance with Boltzmann's law, which follows from thermal population distribution between states 2H11/2 and 4S3/2 having energy gap up to 0.09 eV. Correlation of growth of 525 nm (Fig. 7) and 980 nm (Fig. 6) luminescence band intensities with shortening of green luminescence kinetics (Fig. 5a) suggests participation of 2H11/2 state in cross-relaxation processes at temperatures above 150 K (Fig. 8). In addition cross-relaxation processes (2H11/2; 4I15/2)-(4I13/2; 4I9/2) and (2H11/2; 4I15/2)-(4I9/2; 4I13/2) are more preferable than (4S3/2; 4I15/2)(4I13/2; 4I9/2) and (4S3/2; 4I15/2)-(4I9/2; 4I13/2), as energy mismatch is smaller for the transition from 2H11/2 state than from 4S3/2 state.
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Fig. 8. Scheme of the processes of cross-relaxation in Er3 þ involving excited 2H11/2 state: thick arrows – radiation transitions (luminescence), dashed arrows – processes of energy transfer, dotted arrows – nonradiative transition.
In order to verify assumption about 2H11/2 involvement in cross-relaxation processes, spectroscopic measurements were performed for NaLaF4 with a smaller Er3 þ concentration (0.1 mol%) when probability of cross-relaxation processes are negligible. At room temperature, green luminescence bands at 525 nm and 540 nm are observed just like for the NaLaF4:Er3 þ (2 mol%). In temperature interval 15–300 K, no significant changes could be observed in the green luminescence decay kinetics (Fig. 9a), compared to NaLaF4 with 2 mol% Er3 þ (Fig. 5). This indicates that probability of energy transfer and cross-relaxation processes which could reduce number of excited ions in 4S3/2 and 2 H11/2 states is small. In addition, from the infrared luminescence spectra (Fig. 9b) it is seen that intensity of luminescence band centred at 980 nm, intensity of which is linked with probability of the mentioned cross-relaxation processes, does not increase significantly as the sample temperature is increased from 15 K to 300 K, indicating that number of excited Er3 þ in 4I11/2 state does not change significantly. Therefore correlation of growth of 525 nm (Fig. 7) and 980 nm (Fig. 6) luminescence band intensities with shortening of green luminescence kinetics (Fig. 5a) is connected with increased cross-relaxation efficiency at higher Er3 þ concentration in NaLaF4. As it is shown in Fig. 5a, shortening of the green luminescence kinetics in discontinuous manner is related to cross-relaxation processes when excitation and luminescence occur from erbium ions occupying the same crystalline lattice site, for example, Er (2)-Er(2). In case where energy transfer occurs between erbium ions (Er(2)-Er(1)), shortening of the green luminescence kinetics occurs in a different way (Fig. 5b) than in Er(2)-Er(2) case. This is because the green luminescence kinetics consist of several processes. As temperature increases in a low temperature interval (15 K–100 K), the rising part (which prolongs the kinetics) in Er (1) green luminescence kinetics becomes less pronounced due to overlapping of the green luminescence excitation bands. At higher temperatures (150 K–300 K), 2H11/2 state becomes populated and the cross-relaxation processes is responsible for shortening of the green luminescence kinetics, as it is in case when excitation and luminescence occur in the same type of erbium ions, for example, Er(2)-Er(2). One of the difficulties with 980 nm up-conversion excitation is heating up. When temperature is increased, green luminescence intensity decreases. More Er3 þ are thermally excited from 4S3/2 into 2H11/2 state at higher temperatures, therefore cross-relaxation may occur more frequently.
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Fig. 9. a) Green luminescence (at 544.4 nm) kinetics of NaLaF4:Er3 þ (0.1 mol%) under excitation at 484.8 nm (Er(2)) at different temperatures. b) Luminescence spectra of NaLaF4:Er3 þ in infrared spectral region excited at 484.8 nm at different temperatures.
As it is mentioned above in the manuscript, decrease of the green luminescence intensity is connected with reduced population of 4S3/2 state. This does not depend on whether direct or upconversion excitation occurs. As one of the final state for crossrelaxation is 4I11/2, the excitation energy is not “lost irreversibly”. 4 I11/2 state is involved to excite the green up-conversion luminescence via ETU. Therefore green up-conversion luminescence could be excited via ETU if there is another Er3 þ excited in 4I11/2 state in an appropriate distance.
4. Conclusions Site selective spectroscopy reveals that the green luminescence excitation bands for Er3 þ located at different NaLaF4 crystalline lattice sites overlap upon increasing of temperature and it becomes impossible to excite Er3 þ , located at different NaLaF4 crystalline lattice sites, separately. As a result:
it becomes impossible to observe energy transfer between different localised Er ions upon increasing temperature;
a single green luminescence spectrum is observed at room temperature (300 K) independently from excitation wavelength. Above 150 K, shortening of the green luminescence kinetics is caused by strongly increased cross-relaxation processes (2H11/2; 4 I15/2)-(4I13/2; 4I9/2) and (2H11/2; 4I15/2)-(4I9/2; 4I13/2) due to increasing of thermal population of 2H11/2 state.
Acknowledgement Financial support provided by Scientific Research Project for Students and Young Researchers Nr. SJZ2015/23 realized at the Institute of Solid State Physics, University of Latvia is greatly acknowledged.
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