Research article Received: 27 September 2013,
Revised: 26 February 2014,
Accepted: 6 May 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/bio.2712
Luminescence and spectroscopic studies of halosulfate phosphors: a review S. C. Gedam,a* P. S. Thakreb and S. J. Dhoblec ABSTRACT: This review discusses the photoluminescence (PL) characteristics of halosulfate phosphors developed by us. Halosulfate phosphors KCaSO4Cl:X,Y (X = Eu or Ce; Y = Dy or Mn) and Na6(SO4)2FCl (doped with Dy, Ce or Eu) were prepared using a solid-state diffusion method. The mechanism of energy transfer from Eu2+→Dy3+, Ce3+→Dy3+ and Ce3+→Mn2+ has also been studied. Dy3+ emission in the host at 475 and 570 nm is observed due to 4F9/2→6H15/2 and 4F9/2→6H13/2 transition, whereas the PL emission spectra of Na6(SO4)2FCl:Ce phosphor shows Ce3+ emission at 322 nm due to 5d→4f transition of the Ce3+ ion. The main property of KCaSO4Cl is its very high sensitivity, particularly when doped by Dy, Mn or Pb activators. This review also discusses the PL characteristics of some new phosphors such as LiMgSO4F, Na6Pb4(SO4)6Cl2, Na21Mg(SO4)10Cl3 and Na15(SO4)5F4Cl. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: photoluminescence; spectroscopy; halosulfate; rare earths; phosphors
Introduction To date, several sulfate-based phosphors have been studied. This review discusses the photoluminescence (PL) properties of halosulfate phosphors. Sulfates are known to be good optical materials, and the sulfate phosphor CaSO4:Dy is an efficient phosphor used in thermoluminescence dosimetry for ionizing radiation (1). Alkaline earth sulfates activated with rare earth (RE) ions are known phosphors used in thermoluminescence dosimetry, imaging plates and thin-film electroluminescence displays (2–5). Alkaline earth sulfates activated with Eu3+ and Sm3+ ions are promising candidates for optical information storage (6). Gong et al. (7–9) showed the influence of γ-ray irradiation on the crystal structure and PL of alkaline earth sulfates nanocrystalline-activated with Eu3+ and Sm3+ ions. The authors state that PL quenching in alkaline earth sulfate nanocrystalline materials is due to dipole–quadrupole interaction. An unwanted feature of energy transfer is the reduction in emission. Indeed, there are many more examples of energy transfer resulting in a reduction in the desired emission than there are of energy transfer in which sensitization is achieved. The concentration quenching of RE emission most often takes place via energy transfer. In addition to f–d allowed transitions and charge transfer (CT) bands, strong excitation can often be achieved by energy transfer. A RE ion or other species may absorb energy and transfer it to another RE ion, which may then lose the energy radiatively. When energy transfer results in an increase in RE emission it is termed sensitization. The RE ion from which the emission results is called the activator and the one that absorbs the energy is the sensitizer. As far as spectroscopic studies of RE and inner transition metal ions are concerned, the Ce3+ ground state is split into two levels, 2 F5/2 and 2F7/2, which are the only levels possible for the 4f configuration. f→f transitions in Ce3+ are in the infrared (IR) region. At room temperature, they occur as unresolved bands with a maximum at ~ 2200–2300 cm-1 and a half-width of 250–300 cm-1. At low temperatures, the band splits into somlines, which are due to f→f transitions and electrovibronic transitions.
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Dy3+ emission mainly falls within two lines in the visible region, arising from 4F9/2→6H15/2 (470–500 nm) and 4F9/2 → 6H13/2 (570 nm) transitions. The relative intensities of the two bands depend on the local symmetry. When the ratio of blue to green emission is optimal, white emission can be obtained using Dy3+. This property has generated some interest in Dy3+ luminescence. UV cannot efficiently excite Dy3+ because its CT state and the 5d levels are situated above 50,000 cm-1. The luminescence of Eu2+ ions in different hosts has recently attracted much attention due to its peculiar properties. Its excitation and emission spectra are usually broadband due to transition between the 4f7 (8S7/2) ground state and the crystal field components of the 4f6 5d excited state configuration. Eu2+ emission results from two types of transitions. The most common is that due to 4f6 5d→4f7 (8S7/2). Because the position of the band corresponding to the 4f6 5d configuration is strongly influenced by the host, the emission can be anywhere from 365 nm (e.g. in BaSO4) to 650 nm (e.g. in CaS). In case of the Tb3+ ion, absorption is usually due to the allowed f–d transition. From the excited state of the 4f7 5d1 configuration, the electron loses energy to the lattice and comes to 5Dj. 5D3→7Fj emission is in the UV and blue regions, whereas 5 D4→7Fj emission is predominantly green. At lower concentrations, blue emission is observed, but at higher concentrations, there is energy transfer between Tb3+ ions, e.g. cross-relaxation: Tb3þ 5 D3 þ Tb3þ 7 Fj →Tb3þ 5 D4 þ Tb3þ 7 F0 ; * Correspondence to: S. C. Gedam, Department of Physics, K. Z. S. Science College, Kalmeshwar, Nagpur - 441501, India. Tel: +91 9373286707. E-mail: [email protected] a
Department of Physics, K. Z. S. Science College, Kalmeshwar, Nagpur, 441501, India
b
Hutatma Rashtriya College of Science, Ashti, Wardha - 442202, India
c
Department of Physics, R.T.M. Nagpur University, Nagpur-440033, India
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S. C. Gedam et al. owing to which the blue emission is quenched, increasing the green emission at the same time. Transition metal ions have an incompletely filled d-shell, i.e. their electron configuration is dn (0 < n < 10). The luminescence properties of divalent manganese (Mn2+) have been studied intensively and it is used in many luminescent materials. All optical absorption transitions are parity and spin forbidden. In general, Mn2+-activated phosphors are divided into two classes: those with green emission and those with orange-to-red emission. In octahedral surroundings with a large crystal field, the emission is usually red; in tetrahedral surroundings with a much smaller crystal field, the emission is usually green. KCaSO4Cl can be easily prepared using a solid-state diffusion method. A large number of sulfates with well-characterized structures are known, and it was decided to study the emission of various REs such as Ce3+, Dy3+, Eu2+, Tb3+, as well as a transition metal like Mn2+, in the presented hosts. Typical Ce3+ emission bands were observed at 321 and 332 nm. Co-doping with Dy3+ or Mn2+ leads to very efficient whitish to yellow emission at 475 and 572 nm due to Dy3+. Efficient energy transfer in Ce3+→Mn2+ leads to efficient Mn2+ emission at 515 nm. This corresponds to the blue–green part of the visible spectrum. The emission spectra of Eu2+ ions contain two overlapping broad peaks at around 400 and 450 nm. Efficient energy transfer results in Eu2+→Dy3+ emission at around 490 and 575 nm. Studies on KCaSO4Cl and Na6(SO4)2FCl have published presented previously (10,11). Along with this new halosulfate, phosphors such as LiMgSO4F, Na6Pb4(SO4)6Cl2, Na21Mg(SO4)10Cl3 and Na15(SO4)5F4Cl are introduced in this review.
Experimental The compounds KCaSO4Cl, Na6(SO4)2FCl and Na6Pb4(SO4)6Cl2 were prepared using the solid-state diffusion method, as detailed elsewhere (10,11). In the solid-state diffusion method, to prepare KCaSO4Cl:Eu,Dy; KCaSO4Cl:Ce,Dy and KCaSO4Cl:Ce, Mn, the constituents KCl, CaSO4 and the sulfate salts of cerium, europium and dysprosium were taken in a stoichiometric ratio and crushed in a crucible for 1 h. This material was then heated at 800ºC for 8 h, to give KCaSO4Cl:Eu,Dy in a powder form. The samples were then slowly cooled at room temperature. The resultant polycrystalline mass was crushed to fine particles in a crucible, and this powder was used in the subsequent study. A similar method was adopted for KCaSO4Cl:Ce,Dy and KCaSO4Cl: Ce,Mn. For Na6(SO4)2FCl, the constituents Na2SO4, NaCl, NaF and the abovementioned dopants were used in preparation of the sample. LiMgSO4F, Na21Mg(SO4)10Cl3 and Na15(SO4)5F4Cl were prepared using a wet chemical method. For LiMgSO4F:Ce3+, MgSO4 and LiF of analar grade were taken in a stoichiometric ratio and dissolved separately in double-distilled de-ionized water, resulting in a solution of LiMgSO4F. A water-soluble sulfate salt of cerium was then added to the solution to obtain LiMgSO4F:Ce. It was confirmed that no undissolved constituents had been left behind, and all the salts had completely dissolved in water and thus reacted. LiMgSO4F (pure) and LiMgSO4F:Ce in its powder form were obtained by evaporation at 80ºC for 8 h. The dried samples were then slowly cooled at room temperature. The resultant polycrystalline mass was crushed to fine particles in a crucible. The powder was used in the subsequent study. Formation of the compound was confirmed by examining the X-ray diffraction (XRD) pattern. The PL emission spectra of
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the samples were recorded using a fluorescence spectrometer (Hitachi F-4000). The same amount of sample was used in each case. Emission and excitation spectra were recorded using a spectral slit width of 1.5 nm. Similarly, the remaining samples were prepared according to the following reactions and were prepared using a wet chemical method. The constituents used were: CaSO4 þ KCl→KCaSO4 Cl
(1)
2Na2 SO4 þ NaCl þ NaF→Na6 ðSO4 Þ2 FCl
(2)
MgSO4 þ LiF→LiMgSO4 F
(3)
2Na2 SO4 þ 4PbSO4 þ 2NaCl→Na6 Pb4 ðSO4 Þ6 Cl2
(4)
9Na2 SO4 þ 3 NaCl þ MgSO4 →Na21 MgðSO4 Þ10 Cl3
(5)
5Na2 SO4 þ 4NaF þ NaCl→Na15 ðSO4 Þ5 F4 Cl
(6)
Results and discussion Photoluminescence of Ce3+ in KCaSO4Cl The results for KCaSO4Cl have been presented previously (10). Ce3+, which is in the 4f1 configuration, shows efficient luminescence owing to 4f–5d transitions. The luminescent colors or wavelengths of these ions change from the near UV to the red region, depending on the nature of the host lattices (12). The 4f–5d transition of Ce3+ ions in solids is a parity-allowed electric dipole transition (e–d), has large oscillator strength and produces efficient broadband luminescence. It has a larger Stokes shift than the other RE ions because of the extended radial wave functions of the 5d state (13). Figure 1 shows the XRD pattern of KCaSO4Cl, which did not indicate the presence of CaSO4 or KCl, or other likely phases such as CaCl2 or K2SO4; this is indirect evidence for the formation of the desired compound. These results indicate that the final product was formed in an homogeneous form. All Ce3+-doped KCaSO4Cl phosphors were fine powders. Figure 2 shows the PL spectra of Ce3+ in KCaSO4Cl host halosulfates. In Fig. 2, some selected emission spectra for various Ce3+ contents are shown. From these data and the PL spectra, it can be seen that the Ce3+ content affects not only the peak
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Figure 1. XRD pattern of KCaSO4Cl.
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A review on photoluminescence in halosulfate phosphors
Figure 2. Excitation and emission spectra of KCaSO4Cl:Ce (a) 5 mol%, (b) 2 mol% and (c) 1 mol%.
height, but also the peak profile. Obviously, with an increase in Ce3+ content, the emission intensity increases, as does the emission peak position. When the Ce3+ content varies from 1 to 5 mol%, the peak wavelength position varies from 321 to 332 nm. Excitation spectra are shown for all samples (λex = 254 nm) monitored at 321 nm emission. Curves a, b and c in Fig. 2 are the emission spectra for 5, 2 and 1 mol% Ce3+, and show the broadband nature of the emission. The well-known UV emission of Ce3+ ions in these phosphors is at around 321–332 nm. This band is due to the allowed transition from 5d to 4f of Ce3+ ions, giving maximum intensity for an excitation wavelength of 254 nm. This feature can be explained as follows. The excitation energy matches the energy separation between the ground state and lowest state of the 5d level of the ion. This gives a maximum population for the lowest 5d level, which favors maximum emission intensity. Another characteristic feature of the emission band is the absence of the expected doublet arising from the transition from 5d→2F5/2 and 2F7/2 levels due to spin orbit splitting of the 4f1 ground state of Ce3+ ions.
solid-state lasers. Here, we used KCaSO4Cl halosulfate as the host for the luminescence and energy transfer process in Ce–Dy ions. The emission and excitation spectra of the co-doped Ce–Dy ions in the KCaSO4Cl halosulfate host are shown in Fig. 3, which also shows the excitation spectrum recorded and plotted at 474 nm emission. The excitation spectra (monitored at 474 nm emission) show a maximum at 266 nm and a faint shoulder at ~ 254 nm. Selecting 254 nm as the excitation wavelength, we recorded the emission spectra for Ce5mol% and Dy0.1mol% contents in this sample. The emission spectra show two prominent peaks at 475 and 572 nm with very sharp and high intensity. In these samples, singly doped Dy3+ does not give PL on excitation at 254 nm. Because Ce3+ and Dy3+ ions were co-doped in the system, the Ce3+ emission almost disappeared with correspondingly intense emission lines of Dy3+ observed at 474 and 572 nm. Dy3+ emission consists of the transitions 4F9/2→6H15/2 at 472 nm and 4F9/2→6H13/2 at 572 nm. These results indicate that very efficient energy transfer from Ce3+ to Dy3+ takes place in KCaSO4Cl halosulfate lattices. This is due to energy transfer from Ce3+ to Dy3+ ions. Figure 4 shows the mechanism of Ce3+→Dy3+ energy transfer in KCaSO4Cl halosulfate material.
Ce3+→Mn2+ energy transfer in KCaSO4Cl Transition metal ions have been widely used in luminescent materials, e.g. Mn2+, a transition metal center, has been doped
Ce3+→Dy3+ energy transfer in KCaSO4Cl Energy transfer between pairs of RE ions at dilution levels below the self-quenching limit is known to take place via multipolar, e.g. dipole–dipole interactions or dipole–quadrupole, interactions (14–16). The Ce3+ ion can be used as a sensitizer as well as an activator, depending on the splitting of the 5d excited levels by the crystal field symmetry. Much work has been carried out on the energy transfer from Ce3+ to different activator ions in different host lattices (17,18). The emission for Dy3+ originates from the 4 F9/2 level and transitions to 6H15/2 (~475 nm) and 6H13/2 (~570 nm) dominate. The latter has ΔJ = 2 and is hypersensitive. The emission is whitish to yellow in host lattices where hypersensitivity is pronounced. It is possible to obtain near white emission with Dy3+-activated phosphors because of the location of strong emissions in the regions 470–500 and 560–600 nm, together with some weak emissions at longer wavelengths (19–21). However, for lamp applications, Dy3+ cannot be excited by low- or highpressure mercury discharges that emit at wavelengths > 250 nm. This is because the CT level and also because the lowest 5d level of Dy3+ is situated at energies of 250,000 cm-1. Energy transfer from donor to acceptor plays an important role in luminescence and
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Figure 3. PL emission and excitation spectra of KCaSO4Cl:Ce5 mol%, Dy0.1 mol%.
3+
Figure 4. Ce →Dy material.
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3+
energy transfer mechanism in KCaSO4Cl halosulfate
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S. C. Gedam et al. into more than 500 inorganic hosts (22,23) for luminescence within an emission range from 490 to 750 nm. It is possible that in a halosulfate host co-doped with Ce3+→Mn2+, some form of energy transfer takes place, with the Ce3+ ions acting as the sensitizer and the Mn2+ ions acting as the activator. KCaSO4Cl: Mn2+ does not show any Mn2+ emission by UV excitation. Bhapat (24) commented that energy transfer from Ce3+ to Mn2+ should not result in sensitization because the PL used by him has the same efficiency at emission wavelengths of Ce3+ (306 and 324 nm) and Mn2+ (~500 nm). In our experiment, strong Mn2+ emission was observed at 557 nm due to the transition of 4 T1→6A1 of the Mn2+ ion in KCaSO4Cl materials with Ce3+ as the sensitizer ion (λex = 263 nm) The excitation and emission spectra of the KCaSO4Cl:Ce,Mn and the Ce3+→Mn2+ energy transfer mechanism are shown in Fig. 5. The excitation spectrum (monitored at 516 nm emission) shows maximum intensity at 267 nm, and a faint shoulder is observed at ~ 254 nm. Selecting 267 nm as the excitation wavelength, the emission spectra were recorded. Figure 5 shows the emission spectra for Ce5mol% and Mn0.1mol%. In these emission spectra, there are two peaks, the first, having low intensity, is at 334 nm and the second, with very sharp and high intensity, is at 516 nm. The 334 nm peak shows the emission position of Ce3+ and the 516 nm peak shows the emission of the Mn2+ ion. The 516 nm peak can be assigned to the Mn2+ emission from the 4T1 state. These results can be interpreted in terms of absorption by Ce3+ at 254 nm Ce3+→Mn2+ energy transfer, followed by efficient energy migration through the halosulfate lattice. In our experiment, strong Mn2+ emission is observed at 516 nm due to transition 4T1–6A1 of the Mn2+ ion in the KCaSO4Cl lattice on sensitization by the Ce3+ ion. The Mn2+ emission may be due to energy transfer from the Ce3+ ion to the Mn2+ ion. Mn2+ has a 3d5 configuration and from the Tanabe–Sugano diagram, it follows that the ground level is 6A1. Emission arises from the 4T1 (4G) level, which shifts to lower energies for higher crystal field strengths (25).
by the host, the emission can be anywhere from 365 nm (e.g. in BaSO4) to 650 nm (e.g. in CaS). Blasse (36) listed the Eu2+-doped compounds, showing that the emission color of Eu2+ can vary over a broad range from ultraviolet to red. Curve a of Fig. 6 is the PL excitation spectrum and curve b is emission spectrum of KCaSO4Cl:Eu for an excitation wavelength of 325 nm. It is known that Eu2+ emission is in the form of a band, the position of which differs from host to host. In our case, the emission spectra contain two overlapping broad peaks around 400 and 450 nm. We speculate that both peaks correspond to Eu2+ emission arising from transitions from the eg to t2g levels of the 4f6 5d configuration to the 8S7/2 levels of the 4f6 5d configuration, but with Eu2+ occupying two different lattice sites (Fig. 6c). Eu2+substituting K+ ions in the host lattice may lead to emission around 450 nm, while substituting Ca2+ may give emission around 400 nm. Because Eu2+ shows emission in a CaSO4 lattice around 385 nm, it is possible that Eu ions may also enter the lattice in their trivalent form.
Eu2+→Dy3+ energy transfer in KCaSO4Cl Figure 7 shows excitation spectra of KCaSO4Cl:Eu,Dy for an emission wavelength of 575 nm. Figure 7 shows the PL emission spectra of KCaSO4Cl:Eu,Dy for 325 nm excitation. Three peaks are obtained: one at around 440 nm, and the other two at around 490 and 575 nm. The first peak is characteristic of the Eu2+ emission arising from transitions of the 4f 65d configuration to the 8S7/2 level of the 4f7 configuration, and the other two peaks can be assigned to Dy3+ emission due to the transitions
Photoluminescence of Eu2+ in KCaSO4Cl The luminescence of Eu2+ ions in different hosts has recently attracted much attention due to its peculiar properties. Efficient Eu2+ emission has been obtained in many compounds (26–35) and many such phosphors have found applications. UV-emitting phosphors are useful in erythemal and photocopying lamps. Eu2+ emission results from two types of transitions. The most common is that due to 4f6 5d→4f7 (8S7/2). Because the position of the band corresponding to the 4f6 5d configuration is strongly influenced
Figure 5. PL excitation and emission spectra of KCaSO4Cl:Ce5 mol% Mn0.1 mol% and 3+ 2+ Ce →Mn energy transfer mechanism in KCaSO4Cl.
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Figure 6. (a) PL excitation spectra (for wavelength 325 nm) and (b) PL emission 6 8 spectra of KCaSO4Cl:Eu. (c) 4f 5d to S7/2 transitions.
Figure 7. Photoluminescence excitation and emission spectra of KCaSO4Cl:Eu, Dy for an excitation wavelength of 325 nm.
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A review on photoluminescence in halosulfate phosphors F9/2→6H15/2 and 4F9/2→6H13/2, respectively. It can be seen in Fig. 7 that the excitation spectrum of the Dy3+ emission contains not only a band corresponding to excitation of the Dy3+ ion itself, but also a one that corresponds to excitation of the Eu2+ ion, giving evidence for the occurrence of energy transfer from Eu2+ to Dy3+. Figure 8 shows the Eu2+→Dy3+ energy transfer mechanism in a KCaSO4Cl host. The doped amount of Dy3+ is too low in the matrix materials. However, if the doped amount of Dy3+ is too large, it may result in concentration quenching and lower the luminescent effect. The peak positions in the emission spectra depend strongly on the nature of the Eu2+ surroundings. Host lattices with Eu2+ ions show more than one emission band. 4
Photoluminescence of Ce3+ in Na6(SO4)2FCl The XRD pattern of Na6(SO4)2FCl agrees well with JCPDS No. 741186. The Ce3+ ground state is split (2F5/2, 2F7/2) and these are the only levels possible for the 4f configuration. f→f transitions in Ce3+ are within the IR region. At room temperature, they occur as unresolved bands with a maximum at ~ 2200–2300 cm-1 and a half-width of 250–300 cm-1. The excited state, above the 2F7/2 level, belongs to the 5d configuration in the form of broad bands. Most commonly observed emission is characteristic of the 5d→4f transition. Both absorption and emission usually have a broadband character, showing the splitting characteristic of the 2 Fj states. Ce usually shows a high quenching temperature in silicates, borates and phosphates. Because Ce3+ has strong absorption in many hosts and emission matching the 4f levels of other RE impurities, it can be used as a sensitizer for other REs (37). Figure 9 shows the PL excitation spectra of Na6(SO4)2FCl:Ce3+ phosphor, which show a broad band at 265 nm with a prominent peak (λem = 322 nm). The PL emission spectra of the Ce3+ ion in a Na6(SO4)2FCl host (for 0.1, 0.2 and 0.5 mol%) are also given in Fig. 9. The emission peak wavelength and its relative intensity are shown for different contents of Ce3+ ion. From these PL spectra, it can be seen that the Ce3+ content affects peak height. Obviously, with an increase in Ce3+ content, the emission intensity increases. This indicates that the Na6(SO4)2FCl lattice is more suitable for higher concentrations of Ce3+ ions. The PL emission spectra of Na6(SO4)2FCl:Ce phosphors show Ce3+ emission at 322 nm due to 5d→4f transition of the Ce3+ ion. The observed variation in PL emission intensity may be due to cross-relaxation between Ce3+ ions at high concentrations of Ce3+.
Figure 9. PL excitation and emission spectra of Na6(SO4)2FCl:Ce (a) 0.5, (b) 0.2 and 3+ (c) 0.1 mol% and energy level diagram showing 5d to 4f transition in Ce ion.
The 5d→4f transition of the Ce3+ ion in Na6(SO4)2FCl is also shown in Fig. 9.
Photoluminescence of Dy3+ in Na6(SO4)2FCl Dy3+ emission falls in two lines in the visible region, arising from F9/2→6H15/2 (475–500 nm) and 4F9/2→6H13/2 (570 nm) transitions. When the ratio of blue to green emission is optimal, white emission can be obtained using Dy3+. This property has generated some interest in Dy3+ luminescence. The emission and excitation spectra of the Dy-doped ions in the Na6(SO4)2FCl host are shown in Fig. 10. In Fig. 10, the excitation spectrum is recorded and plotted, as monitored at 475 nm emission. At this wavelength, emission spectra are observed for different concentrations (0.1 and 0.5 mol%). An intense broad excitation band with a maximum at 270 nm is observed. Selecting 270 nm as the excitation wavelength, we recorded the emission spectra for the same samples. Strong PL emission of Dy3+ ions is observed at 475 and 570 nm in Na6(SO4)2FCl:Dy phosphor. Dy3+ emissions at 475 and 570 nm are due to 4F9/2→6H15/2 and 4F9/2→6H13/2 transitions, respectively (at an excitation wavelength of 270 nm) (Fig. 10). The emission in Dy3+ comes via a non-radiative transition to the 4F9/2 level, followed by radiative transitions to the 6H15/2 and 6H13/2 levels, as shown on the right-hand side of Fig. 10. The excitation spectra 4
3+
2+
Figure 8. Eu →Dy
Luminescence 2014
3+
radiative energy transfer mechanism in KCaSO4Cl host.
Figure 10. (A) PL excitation spectra of Na6(SO4)2FCl:Dy . (B) Emission spectra for 3+ Na6(SO4)2FCl:Dy (higher intensity, 0.5 mol% and lower intensity 0.1 mol%). (C) 3+ Energy level showing that an emission in Dy comes via a non-radiative transition 4 6 6 to the F9/2 level followed by radiative transitions to H15/2 and H13/2 levels.
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S. C. Gedam et al. of Na6(SO4)2FCl:Dy show a broad band in the UV region of the spectrum containing excitation at 270 nm. Photoluminescence of Eu2+ in Na6(SO4)2FCl
Figure 12. Schematic diagram of the induced luminescence from Na6(SO4)2FCl 2+ 3+ resulting from the radiative energy transfer from Eu to Eu ions.
200 180 160 Intensity (arb.unit)
Figure 11(A) shows excitation spectra peaking at 260 and 325 nm. The emission spectra of Na6(SO4)2FCl:Eu2+ (0.1, 0.2, 0.5 mol%) phosphors are shown in Fig. 11(B). As can be seen, the emission has a prominent peak at around 440 nm, which can be assigned to Eu2+ emission arising from transitions of the 4f6 5d configuration to the 8S7/2 level of the 4f7 configuration. The results show that all the spectra are broadband and the main emission peaks of these phosphors are at ~ 410–440 nm. Recently, Yu et al. (38) studied Eu and found excellent luminescence. There is no special Eu3+ emission in the spectra, which indicates that Eu3+ ions are reduced to Eu2+ completely in the Na6(SO4)2FCl:Eu2+ phosphor. However, for 0.5 mol% Eu, two more peaks are observed at 580 and 615 nm, which show the Eu3+ emission. This shows that, particularly in this lattice for 0.5 mol% Eu, Eu2+ as well as Eu3+ emissions are possible, as shown in the Fig. 11; Eu2+ emission has a very broad and intense nature and Eu3+ emission has a very low intensity with line emission. For 0.1 mol% Eu, a broad band is split into two peaks at 410 and 480 nm. The splitting may be due to the lower Eu concentration. The induced luminescence of Eu3+ via the radiative energy transfer consists of six processes, as shown in Fig. 12. The population at the 5D3 level of Eu3+ increases via radiative energy transfer from Eu2+ to Eu3+. Finally, the population arrives at the emitting 5D0 level via non-radiative transitions from the upper levels. The trivalent europium ion is very useful for studying the nature of the metal coordination in various systems, owing to its non-degenerate emitting 5D0 state. With increasing concentrations of Eu2+ ions, the intensity of the peaks increases with a maximum observed for Eu0.5mol%.
140 120 100 80 60 40 20 0 200
250 300 wavelength (nm)
350
Photoluminescence of Ce3+ in LiMgSO4F Figure 13 shows the PL excitation spectra of LiMgSO4F:Ce3+ phosphor (λem = 335 nm); a broad band is observed at around 235 nm. Figure 14 shows the PL emission spectra of Ce3+ ions in LiMgSO4F phosphors at different concentration under
2+
Figure 11. (A) Excitation spectra of Na6(SO4)2FCl:Eu showing two peaks at 2+ 260 and 325 nm. (B) Emission spectra for Na6(SO4)2FCl:Eu : (a) 0.5 mol%, (b) 0.2 mol%, (c) 0.1 mol.
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Figure 13. Excitation spectra of LiMgSO4F:Ce for 5 mol%.
3+
Figure 14. PL emission spectra of LiMgSO4F:Ce ions (λex = 232 nm, λem = 335 nm): (a) 5 mol%, (b) 2 mol%, (c) 1 mol% and (d) 0.5 mol% (higher peak, 5 mol%; lower peak, 0.5 mol%).
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A review on photoluminescence in halosulfate phosphors excitation at 235 nm. Peaks are observed at 335 nm, and are assigned to the 5d→4f transition of Ce3+ ions. With increasing concentrations of Ce3+ ions, the peak intensity increases with the maximum intensity being observed for 5 mol% Ce3+ ions. This indicates that the LiMgSO4F lattice is more suitable for higher concentrations of Ce3+ ions. The PL emission spectra show very strong Ce3+ emission at 335 nm for 5 mol% Ce3+ due to the 5d→4f transition of Ce3+ ions. The observed variation in the PL emission intensity may be due to cross-relaxation between Ce3+ ions at high concentrations of Ce3+. The PL emission peak intensity of Ce3+ is not same at different concentrations. The peak intensity curves are shifted towards shorter wavelengths. LiMgSO4F:Ce5mol% phosphors may be useful candidates for scintillation applications. LiMgSO4F as a new luminescent host material, Dy3+, Mn2+, Eu2+ emission and the energy transfer from Ce3+→Dy3+ and Ce3+→Mn2+ ions are taken into consideration for further study. Figure 15 shows a schematic energy level diagram of Ce3+ in LiMgSO4F.
Ce →Dy 3+
3+
energy transfer in LiMgSO4F
3+
Ce ions can be used as a sensitizer as well as an activator, depending on the splitting of the 5d excited levels by crystal field symmetry. Much work has been done on the energy transfer from Ce3+ to different activator ions in different host lattices. Figure 16 shows that LiMgSO4F:Dy does not show Dy3+ emission under UV (340 nm) excitation. A strong PL emission of Dy3+ ions was observed at 482 and 571 nm in LiMgSO4F:Ce,Dy phosphor due to the presence of Ce3+ ions as a sensitizer. The observed Dy3+ emission at 482 and 571 nm was due to 4F9/2→6H15/2 and 4F9/2→6H13/2 transition of the Dy3+ ion, respectively. The transfer of energy from Ce3+ to Dy3+ ions in a LiMgSO4F lattice returns Ce3+ to the ground state and Dy3+ to the excited state. The emission in Dy3+ comes via a non-radiative transition to the 4F9/2 level, followed by radiative transitions to the 6H15/2 and 6H13/2 levels. The excitation spectra of LiMgSO4F:Ce,Dy show a broad band in the UV region of the spectrum, containing excitation of Ce3+ (340 nm); emission of Ce3+ at 340 nm occurs due to energy transfer from Ce3+ to Dy3+ ions.
Figure 15. Schematic energy level diagram of Ce λem = 335 nm).
Luminescence 2014
3+
Figure 16. PL excitation and emission spectra of LiMgSO4F:Ce5 (λex = 340 nm).
%
Dy0.1%
Photoluminescence of Ce3+ in Na6Pb4(SO4)6Cl2 Figure 17 shows the XRD pattern of Na6Pb4(SO4)6Cl2 material, which matched the standard JCPDF data No. 27-1416. The XRD pattern did not indicate the presence of constituents such as Na2SO4, PbSO4 or NaCl and other likely phases, which is direct evidence for formation of the desired compound. These results indicate that the final product was formed in an homogeneous form. Figure 18(A) shows PL excitation spectra for Na6Pb4(SO4) 3+ 6Cl2:Ce , a broad band is observed at around 260 nm (λ em = 340 nm). Figure 18(B) shows the PL spectra of Ce3+ ions in Na6Pb4 (SO4)6Cl2 phosphor at different concentrations under excitation at 260 nm. Peaks are observed at 340 nm for all concentrations, and are assigned to the 5d→4f transition of Ce3+ ions. As the concentration of Ce3+ ions increases, the peak intensity of 340 nm increases and the maximum intensity is observed for 5 mol% Ce3+ (Fig. 18C). This indicates that the Na6Pb4(SO4)6Cl2 lattice is more suitable for higher concentrations of Ce3+. The PL emission spectra of Na6Pb4(SO4)6Cl2:Ce phosphor show Ce3+ emission at 340 nm due to 5d→4f transition of the Ce3+ ion. The observed variation in PL emission intensity may be due to cross-relaxation between Ce3+ ions at high concentrations of Ce3+. Figure 18(D) shows energy level diagram of Ce3+ in Na6Pb4(SO4)6Cl2.
in LiMgSO4F (λex = 232 nm, Figure 17. XRD pattern of Na6Pb4(SO4)6Cl2.
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S. C. Gedam et al.
Figure 18. (A) Excitation spectra of Na6Pb4(SO4)6Cl2:Ce5 mol% (λex = 260 nm). (B) PL emission spectra of Na6Pb4(SO4)6Cl2:Ce: (a) 5 mol%, (b) 2 mol%, (c) 1 mol% (λem = 340 nm). 3+ (C) Increase in intensity as the concentration increases. (D) Energy level diagram of Ce in Na6Pb4(SO4)6Cl2.
Ce3+→Dy3+ energy transfer in Na6Pb4(SO4)6Cl2 Na6Pb4(SO4)6Cl2:Dy did not show any Dy3+ emission under UV (260 nm) excitation. A strong PL emission of Dy3+ ions was observed at 475 and 575 nm in Na6Pb4(SO4)6Cl2:Ce,Dy phosphor due to the presence of Ce3+ as a sensitizer. Dy3+ emission at 475 and 575 nm is due to 4F9/2→6H15/2 and 4F9/2→6H13/2 transition of the Dy3+ ion, respectively (under an excitation wavelength of 260 nm). From Fig. 19, it is clear that Dy3+ emission is observed at higher concentrations (0.5 mol%). Only some results are plotted. The transfer of energy from Ce3+ to Dy3+ ions in a Na6Pb4(SO4) 3+ to the ground state and Dy3+ to the 6Cl2 lattice brings Ce excited state. The emission in Dy3+ comes via non-radiative transition to the 4 F9/2 level, followed by radiative transitions to the 6H15/2 and 6H13/2 levels, as shown in Fig. 3. The emission spectra of the Na6Pb4(SO4)6Cl2 host overlap with the excitation spectrum of Dy and this provides the fundamental condition for energy transfer from Ce3+ to Dy3+ ions in Na6Pb4(SO4)6Cl2. Figure 20 shows an energy transfer process accomplished via re-absorption transfer. Similar results for the PL enhancement of Dy3+ emission in CaSO4 lattice were reported in 1999 by Lakshmanan.
Figure 19. (A) Excitation spectra and (B) PL emission spectra of Na6Pb4(SO4)6Cl2: (a) Ce5%, Dy0.5% ; (b) Ce5%, Dy0.1%
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3+
Figure 20. Ce →Dy
3+
energy transfer in Na6Pb4(SO4)6Cl2.
Ce3+→Tb3+ energy transfer in Na6Pb4(SO4)6Cl2 The Ce3+ to Tb3+ energy transfer process in different host matrices is well known. Broadband emitters are often used to sensitize the luminescence of RE ions. The overlap of the normalized Ce3+ emission and Tb3+ excitation is found much more easily. The energy transfers from Ce3+ to Tb3+ might be very efficient. This could improve the excitation efficiency and brightness. Figure 21 (A) shows the PL excitation spectra of Na6Pb4(SO4)6Cl2:Ce5%, Tb0.1% observed at 340 nm and Fig. 21(B) shows the PL emission spectra of Na6Pb4(SO4)6Cl2:Ce5mol%,Tb0.1mol% observed at 490 and 550 nm, with a small peak at 580 nm. From this result, three emission transitions can be observed: 5D4→7F6 (490 nm), 5 D4→7F5 (550 nm) and 5D4→7F4 (580 nm). The 5D4→7F5 transition may be responsible for the observation of green color as a single peak (39). The Ce emission moves towards the lower energy side due to the new host ligands. Thus, the energy transfer rate should be much higher. Thus, Na6Pb4(SO4)6Cl2:Ce,Tb is an interesting blue and green phosphor and may have application in display devices.
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Luminescence 2014
A review on photoluminescence in halosulfate phosphors
Figure 21. (A) PL excitation of Na6Pb4(SO4)6Cl2Ce5%, Tb0.1%. (B) PL emission spectra of Na6Pb4(SO4)6Cl2Ce5%, Tb0.1% for the emission of 550 nm.
Figure 22 is a schematic energy level diagram showing Ce3+→Tb3+ energy transfer in Na6Pb4(SO4)6Cl2. In this host, even at the lowest concentrations, only green emission is observed and the blue emission is quenched by Tb3+ ions. Hence, the quantum efficiency of the luminescence may be high when the f–d absorption is at longer wavelengths. Efficient energy transfer from the broad (i.e. Ce3+) to the narrow line emitter (i.e. Tb3+) is possible only between nearest neighbors in the crystal lattice and with optimal spectral overlap.
Photoluminescence of Ce3+ in Na21Mg(SO4)10Cl3 Figure 23 shows the PL excitation spectra of Na21Mg(SO4)10Cl3: Ce3+ phosphor (λem = 341 nm), broadband is observed at around 247 nm. Figure 24 shows the PL emission spectra of Ce3+ ions in Na21Mg(SO4)10Cl3 phosphor at different concentrations under excitation at 247 nm. A peak is observed at 341 nm, which is assigned to the 5d→4f transition of Ce3+ ions. With increasing concentrations of Ce3+ ions, the peak intensity of 341 nm increases, with the maximum intensity observed for 5 mol% Ce3+. This indicates that the Na21Mg(SO4)10Cl3 lattice is more suitable for higher concentrations of Ce3+.The PL emission spectra of Na21Mg(SO4)10Cl3:Ce3+ (5 mol%) phosphors show very strong
3+
Figure 22. Ce → Tb
Luminescence 2014
3+
energy transfer in Na6Pb4(SO4)6Cl2.
Figure 23. Excitation spectra of Na21Mg(SO4)10Cl3:Ce (λex = 247 nm) for λem = 341 nm.
Ce3+ emission at 341 nm due to 5d→4f transition of the Ce3+ ions. The observed variation in PL emission intensity may be due to cross-relaxation between Ce3+ ions in cases of high concentrations of Ce3+. The strong Ce3+ emission in the Na21Mg(SO4)10Cl3 lattice may be useful as a scintillator. The PL emission peak intensity of Ce3+ ion in Na21Mg(SO4)10Cl3 lattice is not of the same
Figure 24. PL emission spectra of Na21Mg(SO4)10Cl3:Ce: (a) 5 mol%, (b) 2 mol%, (c) 1 mol% (excitation at 247 nm).
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S. C. Gedam et al. intense. Individual emissions are not observed for Dy3+, Mn2+, Eu2+, whereas with Ce3+ as a co-activator ion, Dy3+and Mn2+ emission may be possible and this should be taken into consideration for further study. Further investigations into the energy transfer from Ce3+→Dy3+, Ce3+→Mn2+ and other possible mechanisms are ongoing. The emission intensity of Dy3+ and Mn2+ individual ions is not sufficient for luminescence applications.
300
Intensity (arb units)
250 200 150 100
Conclusions
50 0 200
250
300
350
Wavelength (nm) Figure 25. Excitation spectra of Na15(SO4)5F4Cl:Ce1 mol% (λex = 247 nm).
Figure 26. PL emission spectra Na15(SO4)5F4Cl:Ce: (a) 5 mol%, (b) 2 mol%, and (c) 1 mol% (excitation at 247 nm).
intensity for different concentration of Ce3+ ions. The PL emission peak intensity is dependent on the concentration of the Ce3+ ion. The intensity of the Ce3+ ion in the above phosphor shows that the maximum Ce3+ emission is observed in the Ce (5 mol%) phosphor. Na21Mg(SO4)10Cl3:Ce (5 mol%) phosphors may be useful candidates for scintillation applications. Photoluminescence of Ce3+ in Na15(SO4)5F4Cl Figure 25 shows the PL excitation spectra of Na15(SO4)5F4Cl:Ce3+ phosphor. A broad band is observed at 247 nm with a prominent shoulder around 230, 260 and 290 nm (λem = 341 nm). Figure 26 shows the PL emission spectra of Ce3+ ions in Na15(SO4)5F4Cl phosphor at different concentrations under excitation at 247 nm. Peaks are observed at 341 nm for concentrations of 5, 2 and 1 mol%, which are assigned to the 5d→4f transition of Ce3+ ions. On increasing the doping concentration of Ce3+ ions, the intensity of the 341 nm peak is found to increase and maximum intensity is observed for 5 mol% Ce3+. This indicates that the Na15(SO4)5F4Cl lattice is more suitable for higher concentrations of Ce3+ ions. The observed variation in the PL emission intensity may be due to cross-relaxation between Ce3+ ions at high concentrations of Ce3+. All the phosphors may be useful in scintillation applications. Among these halosulfate phosphors, Na15(SO4)5F4Cl:Ce is more
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Ce3+ emission in these phosphors is observed around 321–332 nm, due to the allowed 5d→4f transition for an excitation wavelength of 254 nm. Very efficient energy transfer from Ce3+ to Dy3+ takes place in KCaSO4Cl halosulfate. The emission spectra show two prominent peaks at 475 and 572 nm, with very sharp and high intensity. Dy3+ emission consists of transitions 4F9/2→6H15/2 at 472 nm and 4F9/2→6H13/2 at 572 nm. In our sample, strong Mn2+ emission is observed at 516 nm because of transition of 4T1–6A1 of the Mn2+ ion in the KCaSO4Cl lattice on sensitization by Ce3+. Efficient energy transfer from Ce3+ → Mn2+ leading to efficient Mn2+ emission at 515 nm was observed in this host. This corresponds to the blue–green region of the visible spectrum. In KCaSO4Cl:Eu, the observed emission spectra contain two overlapping broad peaks around 400 and 450 nm for an excitation wavelength of 325 nm, due to transitions from the eg to t2g levels of the 4f6 5d configuration to the 8S7/2 levels of the 4f6 5d configuration. Eu2+-substituting K+ ions in the host lattice may lead to emission around 450 nm, while substituting Ca2+ may give emission around 400 nm. A PL emission spectrum of KCaSO4Cl:Eu,Dy is observed for 325 nm excitation. Three peaks are obtained, one at around 440 nm and two at around 490 and 575 nm. The first peak is characteristic of the Eu2+ emission arising from transitions of the 4f 65d configuration to the 8S7/2 level of the 4f7 configuration and the other two peaks can be assigned to the Dy3+ emission due to the transitions 4F9/2→6H15/2 and 4F9/2→6H13/2, respectively. Acknowledgements SCG thanks THE University Grant Commission (UGC), New Delhi and SJD thanks the BRNS, Government of India, for their financial support.
References 1. Nambi KSV. Science Today (India). Progress of thermoluminescence research. 1981; Dec. 12. 2. Yamashita N, Yamamoto I, Ninagawa K, Wada T, Yamashita Y, Nakao Y. Investigation of TLD Phosphors by Optical Excitation - Luminescence 2+ of Eu Centers in MgSO4, CaSO4, SrSO4, BaSO4. Jpn J Appl Phys Part 1 1985;24:1174–8. 3. Morishita T, Ichii A, Matsui M, Tomomura S, Konishi T. Blue Electroluminescence in SrSO4:Ce-Based Thin-Films. Jpn J Appl Phys Part 1 1998;37:3992–6. 4. Konrad A, Fries T, Gahn A, Kummer F, Herr U, Tideck R, et al. Chemical vapor synthesis and luminescence properties of nanocrystalline cubic Y2O3:Eu. Appl Phys Lett 1999;86:3129–31. 5. Justel T, Nikol H, Ronda C. New developments in the field of luminescent materials for lighting and displays. Angew Chem Int Ed Engl 1998;37:3084–7. 6. Winnacker A, Shellby RM, Macfarlane RM. Photon-gated hole burning: a new mechanism using two-step Photoionization. Opt Lett 1985;10:350–2. 7. Gong X, Wu PF, Chan WK, Chen WJ. Effect of γ-ray irradiation on structures and luminescent properties of nanocrystalline MSO4: 3+ (M = Ca, Sr, Ba; x = 0.001 0.005). J Phys Chem Solid xEu 2000;61:115–21.
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A review on photoluminescence in halosulfate phosphors 8. Gong X, Liu L, Chan WK, Chen WJ. Structures and fluorescence of 3+ nanocrystallines MSO4:xSm (M = Ca, Sr, Ba; x = 0.001, 0.005) with γ-ray irradiation. Opt Mater 2000;15:143–8. 9. Gong X, Liu L, Chen W. Preparation and photoluminescence of nano3+ crystalline MSO4: xTb (M=Ca, Sr, and Ba; x = 0.001 0.005). J Appl Phys 2000;88:4389–93. 10. Thakre PS, Gedam SC, Dhoble SJ, Atram RG. Luminescence of KCaSO4Cl:X, Y (X = Eu or Ce; Y = Dy or Mn) halosulfate material. J Lumin 2011;131:1612–6. 11. Thakre PS, Gedam SC, Dhoble SJ, Atram RG. Luminescence investigations on sulfate apatite Na6(SO4)2FCl:RE (RE : Dy, Ce or Eu) phosphors. J Lumin 2011;131:2683–9. 12. Ajith Kumar G, Biju PR, Jose G, Unnikrishnan NV. Static energy 2+ 3+ transfer for Mn : Pr system in Phosphate glasses. Mater Chem Phys 1999;60:247–5. 13. Bartolo B, editor. Advances in nonradiative processes in solids. New York: Plenum Press, 1991:282–9. 3+ 3+ 14. Joshi BC, Pande UC. Interaction between Tb and Er ions in phosphate glass. J Phys Chem Solid 1989;50:599–601. 15. Agrawal AK, Lohant NC, Pant TC, Pant KC. Buildup of the acceptor 3+ 3+ emission as a result of energy transfer from Tb to Sm in barium borate glass. J Solid State Chem 1984;54:219–25. 16. Sobha KC, Rao KJ. Luminescence of, and energy transfer between 3+ 3+ Dy and Tb in NASICON-type phosphate glasses. J Phys Chem Solid 1996;57:1263–7. 17. Meijerink A, Nuyten J, Blasse G. Luminescence and energy migration 7 6 in (Sr,Eu)B4O7, a system with a 4f -4f 5d crossover in the excited state. J Lumin 1989;44:19–22. 18. Kiliaan HS, Kothe JK, Blasse G. Energy Transfer in the Luminescent System Na(Y,Gd)F4:Ce,Tb. J Elecrochem Soc 1987;134:2359–64. 19. Verstegen JM, Sommerdijk JL, Erriet JG. Cerium and terbium luminescence in LaMgAl11O19. J Lumin 1973;6:425–31. 20. Srivastava AM, Sobieraj MT, Ruan SK, Banks E. Sensitization of the 3+ 3+ 3+ 3+ Gd lattice by Pr in GdBO3 and energy transfer to Ln (Ln = 3+ 3+ 3+ Dy , Sm , and Tb ). Mat Res Bull 1986;21:1455–63. 21. Dieke GH, Singh S. Absorption, fluorescence and energy levels of the dysprosium ion. J Opt Soc Am 1956;46:495–9. 3+ 22. Sommerdijk JL, Bril A. Efficiency of Dy Activated Phosphors. J Electrochem Soc 1975;122:952–4. 23. Blasse G, Bril A. The absorption and emission spectra of some important activators. Phillips Tech Rev 1970;31:304–12.
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24. Bapat VN. Energy transfer phenomenon in TL of calcium sulfate doped with lanthanide rare earth and manganese [dissertation]. Bombay University, 1977. 25. Sun Y, Jiang W, Gao Y, Ding F, Xu Y, Bian Z, et al. Iridium Complexes: Synthesis, Luminescent Properties and Their Application in Cell Imaging. J Inorg Chem 2010;49:3252–60. 2+ 26. Barry TL. Fluorescence of Eu Activated Phases in Binary Alkaline Earth Orthosilicate Systems. J Electrochem Soc 1968;115:1181–90. 2+ 27. Blasse G. Brill A. Fluorescence of Eu activated alkaline earth aluminates. Philip Res Rep 1968;23:201–6. 2+ 28. Blasse G, Brill A, de Vries J. Fluorescence of Eu Activated Barium Octaborate. J Electrochem Soc 1968;115:977–8. 29. Blasse G, Brill A, de Vries J. Luminescence of alkaline-earth boratephosphates activated with divalent Europium. J Inorg Nucl Chem 1969;31:568–70. 2+ 30. Hoffman MV. Eu Activation in Some Alkaline Earth Strontium Phosphate Compounds. J Electrochem Soc 1968;115:560–3. 31. Palilla FC, Levine AK, Tomkus MR. Fluorescent Properties of Alkaline Earth Aluminates of the Type MAl2O4 Activated by Divalent Europium. J Electrochem Soc 1968;115(6):642–4. 32. Palilla FC, O’Reilly BC. Alkaline Earth Halophosphate Phosphors Activated by Divalent Europium. J Electrochem Soc 1968;115(10): 1076–81. 2+ 33. Wachtel A. Eu Activated Aluminosilicate Phosphors. J Electrochem Soc 1969;116(1):61–4. 34. Wanmaker WL, ter Vrugt JW. Lanthanide activated tube lighting and cathode ray tube phosphors. Philip Res Rept 1967;22:355–9. 2+ 35. Wanmaker WL, ter Vrugt JW. The quenching of 5d–4f emission in Eu . Philip Res Rept 1968;23:362–8. 36. Blasse G. Luminescence of Inorganic Solids. US: Springer, 1978; 457–94. 3+ 37. Buchanan RA, Rast HE, Caspers HH. Infrared Absorption of Ce in LaF3 and of CeF3. J Chem Phys 1966;44:4063–5. 38. Yu H, Deng D, Li Y, Xu S, Li Y, Yu C, et al. Electronic structure and photoluminescence properties of Yellow-emitting Ca10Na(PO4)7: 2+ Eu phosphor for white light-emitting diodes. J Lumin 2013;143:132–6. 39. Lakshmanan AR. Photoluminescence and thermoluminescence processes in rare earth doped CaSO4 phosphors. Prog Mater Sci 1999;44:1–187. 3+ 40. Lee S, Park S. Preparation and luminescent properties of Tb and 3+ 3+ Tb -Ce doped Ba9Y2Si6O24 phosphors. J Lumin 2013;143:215–8.
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Revised: 26 February 2014,
Accepted: 6 May 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/bio.2712
Luminescence and spectroscopic studies of halosulfate phosphors: a review S. C. Gedam,a* P. S. Thakreb and S. J. Dhoblec ABSTRACT: This review discusses the photoluminescence (PL) characteristics of halosulfate phosphors developed by us. Halosulfate phosphors KCaSO4Cl:X,Y (X = Eu or Ce; Y = Dy or Mn) and Na6(SO4)2FCl (doped with Dy, Ce or Eu) were prepared using a solid-state diffusion method. The mechanism of energy transfer from Eu2+→Dy3+, Ce3+→Dy3+ and Ce3+→Mn2+ has also been studied. Dy3+ emission in the host at 475 and 570 nm is observed due to 4F9/2→6H15/2 and 4F9/2→6H13/2 transition, whereas the PL emission spectra of Na6(SO4)2FCl:Ce phosphor shows Ce3+ emission at 322 nm due to 5d→4f transition of the Ce3+ ion. The main property of KCaSO4Cl is its very high sensitivity, particularly when doped by Dy, Mn or Pb activators. This review also discusses the PL characteristics of some new phosphors such as LiMgSO4F, Na6Pb4(SO4)6Cl2, Na21Mg(SO4)10Cl3 and Na15(SO4)5F4Cl. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: photoluminescence; spectroscopy; halosulfate; rare earths; phosphors
Introduction To date, several sulfate-based phosphors have been studied. This review discusses the photoluminescence (PL) properties of halosulfate phosphors. Sulfates are known to be good optical materials, and the sulfate phosphor CaSO4:Dy is an efficient phosphor used in thermoluminescence dosimetry for ionizing radiation (1). Alkaline earth sulfates activated with rare earth (RE) ions are known phosphors used in thermoluminescence dosimetry, imaging plates and thin-film electroluminescence displays (2–5). Alkaline earth sulfates activated with Eu3+ and Sm3+ ions are promising candidates for optical information storage (6). Gong et al. (7–9) showed the influence of γ-ray irradiation on the crystal structure and PL of alkaline earth sulfates nanocrystalline-activated with Eu3+ and Sm3+ ions. The authors state that PL quenching in alkaline earth sulfate nanocrystalline materials is due to dipole–quadrupole interaction. An unwanted feature of energy transfer is the reduction in emission. Indeed, there are many more examples of energy transfer resulting in a reduction in the desired emission than there are of energy transfer in which sensitization is achieved. The concentration quenching of RE emission most often takes place via energy transfer. In addition to f–d allowed transitions and charge transfer (CT) bands, strong excitation can often be achieved by energy transfer. A RE ion or other species may absorb energy and transfer it to another RE ion, which may then lose the energy radiatively. When energy transfer results in an increase in RE emission it is termed sensitization. The RE ion from which the emission results is called the activator and the one that absorbs the energy is the sensitizer. As far as spectroscopic studies of RE and inner transition metal ions are concerned, the Ce3+ ground state is split into two levels, 2 F5/2 and 2F7/2, which are the only levels possible for the 4f configuration. f→f transitions in Ce3+ are in the infrared (IR) region. At room temperature, they occur as unresolved bands with a maximum at ~ 2200–2300 cm-1 and a half-width of 250–300 cm-1. At low temperatures, the band splits into somlines, which are due to f→f transitions and electrovibronic transitions.
Luminescence 2014
Dy3+ emission mainly falls within two lines in the visible region, arising from 4F9/2→6H15/2 (470–500 nm) and 4F9/2 → 6H13/2 (570 nm) transitions. The relative intensities of the two bands depend on the local symmetry. When the ratio of blue to green emission is optimal, white emission can be obtained using Dy3+. This property has generated some interest in Dy3+ luminescence. UV cannot efficiently excite Dy3+ because its CT state and the 5d levels are situated above 50,000 cm-1. The luminescence of Eu2+ ions in different hosts has recently attracted much attention due to its peculiar properties. Its excitation and emission spectra are usually broadband due to transition between the 4f7 (8S7/2) ground state and the crystal field components of the 4f6 5d excited state configuration. Eu2+ emission results from two types of transitions. The most common is that due to 4f6 5d→4f7 (8S7/2). Because the position of the band corresponding to the 4f6 5d configuration is strongly influenced by the host, the emission can be anywhere from 365 nm (e.g. in BaSO4) to 650 nm (e.g. in CaS). In case of the Tb3+ ion, absorption is usually due to the allowed f–d transition. From the excited state of the 4f7 5d1 configuration, the electron loses energy to the lattice and comes to 5Dj. 5D3→7Fj emission is in the UV and blue regions, whereas 5 D4→7Fj emission is predominantly green. At lower concentrations, blue emission is observed, but at higher concentrations, there is energy transfer between Tb3+ ions, e.g. cross-relaxation: Tb3þ 5 D3 þ Tb3þ 7 Fj →Tb3þ 5 D4 þ Tb3þ 7 F0 ; * Correspondence to: S. C. Gedam, Department of Physics, K. Z. S. Science College, Kalmeshwar, Nagpur - 441501, India. Tel: +91 9373286707. E-mail: [email protected] a
Department of Physics, K. Z. S. Science College, Kalmeshwar, Nagpur, 441501, India
b
Hutatma Rashtriya College of Science, Ashti, Wardha - 442202, India
c
Department of Physics, R.T.M. Nagpur University, Nagpur-440033, India
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S. C. Gedam et al. owing to which the blue emission is quenched, increasing the green emission at the same time. Transition metal ions have an incompletely filled d-shell, i.e. their electron configuration is dn (0 < n < 10). The luminescence properties of divalent manganese (Mn2+) have been studied intensively and it is used in many luminescent materials. All optical absorption transitions are parity and spin forbidden. In general, Mn2+-activated phosphors are divided into two classes: those with green emission and those with orange-to-red emission. In octahedral surroundings with a large crystal field, the emission is usually red; in tetrahedral surroundings with a much smaller crystal field, the emission is usually green. KCaSO4Cl can be easily prepared using a solid-state diffusion method. A large number of sulfates with well-characterized structures are known, and it was decided to study the emission of various REs such as Ce3+, Dy3+, Eu2+, Tb3+, as well as a transition metal like Mn2+, in the presented hosts. Typical Ce3+ emission bands were observed at 321 and 332 nm. Co-doping with Dy3+ or Mn2+ leads to very efficient whitish to yellow emission at 475 and 572 nm due to Dy3+. Efficient energy transfer in Ce3+→Mn2+ leads to efficient Mn2+ emission at 515 nm. This corresponds to the blue–green part of the visible spectrum. The emission spectra of Eu2+ ions contain two overlapping broad peaks at around 400 and 450 nm. Efficient energy transfer results in Eu2+→Dy3+ emission at around 490 and 575 nm. Studies on KCaSO4Cl and Na6(SO4)2FCl have published presented previously (10,11). Along with this new halosulfate, phosphors such as LiMgSO4F, Na6Pb4(SO4)6Cl2, Na21Mg(SO4)10Cl3 and Na15(SO4)5F4Cl are introduced in this review.
Experimental The compounds KCaSO4Cl, Na6(SO4)2FCl and Na6Pb4(SO4)6Cl2 were prepared using the solid-state diffusion method, as detailed elsewhere (10,11). In the solid-state diffusion method, to prepare KCaSO4Cl:Eu,Dy; KCaSO4Cl:Ce,Dy and KCaSO4Cl:Ce, Mn, the constituents KCl, CaSO4 and the sulfate salts of cerium, europium and dysprosium were taken in a stoichiometric ratio and crushed in a crucible for 1 h. This material was then heated at 800ºC for 8 h, to give KCaSO4Cl:Eu,Dy in a powder form. The samples were then slowly cooled at room temperature. The resultant polycrystalline mass was crushed to fine particles in a crucible, and this powder was used in the subsequent study. A similar method was adopted for KCaSO4Cl:Ce,Dy and KCaSO4Cl: Ce,Mn. For Na6(SO4)2FCl, the constituents Na2SO4, NaCl, NaF and the abovementioned dopants were used in preparation of the sample. LiMgSO4F, Na21Mg(SO4)10Cl3 and Na15(SO4)5F4Cl were prepared using a wet chemical method. For LiMgSO4F:Ce3+, MgSO4 and LiF of analar grade were taken in a stoichiometric ratio and dissolved separately in double-distilled de-ionized water, resulting in a solution of LiMgSO4F. A water-soluble sulfate salt of cerium was then added to the solution to obtain LiMgSO4F:Ce. It was confirmed that no undissolved constituents had been left behind, and all the salts had completely dissolved in water and thus reacted. LiMgSO4F (pure) and LiMgSO4F:Ce in its powder form were obtained by evaporation at 80ºC for 8 h. The dried samples were then slowly cooled at room temperature. The resultant polycrystalline mass was crushed to fine particles in a crucible. The powder was used in the subsequent study. Formation of the compound was confirmed by examining the X-ray diffraction (XRD) pattern. The PL emission spectra of
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the samples were recorded using a fluorescence spectrometer (Hitachi F-4000). The same amount of sample was used in each case. Emission and excitation spectra were recorded using a spectral slit width of 1.5 nm. Similarly, the remaining samples were prepared according to the following reactions and were prepared using a wet chemical method. The constituents used were: CaSO4 þ KCl→KCaSO4 Cl
(1)
2Na2 SO4 þ NaCl þ NaF→Na6 ðSO4 Þ2 FCl
(2)
MgSO4 þ LiF→LiMgSO4 F
(3)
2Na2 SO4 þ 4PbSO4 þ 2NaCl→Na6 Pb4 ðSO4 Þ6 Cl2
(4)
9Na2 SO4 þ 3 NaCl þ MgSO4 →Na21 MgðSO4 Þ10 Cl3
(5)
5Na2 SO4 þ 4NaF þ NaCl→Na15 ðSO4 Þ5 F4 Cl
(6)
Results and discussion Photoluminescence of Ce3+ in KCaSO4Cl The results for KCaSO4Cl have been presented previously (10). Ce3+, which is in the 4f1 configuration, shows efficient luminescence owing to 4f–5d transitions. The luminescent colors or wavelengths of these ions change from the near UV to the red region, depending on the nature of the host lattices (12). The 4f–5d transition of Ce3+ ions in solids is a parity-allowed electric dipole transition (e–d), has large oscillator strength and produces efficient broadband luminescence. It has a larger Stokes shift than the other RE ions because of the extended radial wave functions of the 5d state (13). Figure 1 shows the XRD pattern of KCaSO4Cl, which did not indicate the presence of CaSO4 or KCl, or other likely phases such as CaCl2 or K2SO4; this is indirect evidence for the formation of the desired compound. These results indicate that the final product was formed in an homogeneous form. All Ce3+-doped KCaSO4Cl phosphors were fine powders. Figure 2 shows the PL spectra of Ce3+ in KCaSO4Cl host halosulfates. In Fig. 2, some selected emission spectra for various Ce3+ contents are shown. From these data and the PL spectra, it can be seen that the Ce3+ content affects not only the peak
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Figure 1. XRD pattern of KCaSO4Cl.
Luminescence 2014
A review on photoluminescence in halosulfate phosphors
Figure 2. Excitation and emission spectra of KCaSO4Cl:Ce (a) 5 mol%, (b) 2 mol% and (c) 1 mol%.
height, but also the peak profile. Obviously, with an increase in Ce3+ content, the emission intensity increases, as does the emission peak position. When the Ce3+ content varies from 1 to 5 mol%, the peak wavelength position varies from 321 to 332 nm. Excitation spectra are shown for all samples (λex = 254 nm) monitored at 321 nm emission. Curves a, b and c in Fig. 2 are the emission spectra for 5, 2 and 1 mol% Ce3+, and show the broadband nature of the emission. The well-known UV emission of Ce3+ ions in these phosphors is at around 321–332 nm. This band is due to the allowed transition from 5d to 4f of Ce3+ ions, giving maximum intensity for an excitation wavelength of 254 nm. This feature can be explained as follows. The excitation energy matches the energy separation between the ground state and lowest state of the 5d level of the ion. This gives a maximum population for the lowest 5d level, which favors maximum emission intensity. Another characteristic feature of the emission band is the absence of the expected doublet arising from the transition from 5d→2F5/2 and 2F7/2 levels due to spin orbit splitting of the 4f1 ground state of Ce3+ ions.
solid-state lasers. Here, we used KCaSO4Cl halosulfate as the host for the luminescence and energy transfer process in Ce–Dy ions. The emission and excitation spectra of the co-doped Ce–Dy ions in the KCaSO4Cl halosulfate host are shown in Fig. 3, which also shows the excitation spectrum recorded and plotted at 474 nm emission. The excitation spectra (monitored at 474 nm emission) show a maximum at 266 nm and a faint shoulder at ~ 254 nm. Selecting 254 nm as the excitation wavelength, we recorded the emission spectra for Ce5mol% and Dy0.1mol% contents in this sample. The emission spectra show two prominent peaks at 475 and 572 nm with very sharp and high intensity. In these samples, singly doped Dy3+ does not give PL on excitation at 254 nm. Because Ce3+ and Dy3+ ions were co-doped in the system, the Ce3+ emission almost disappeared with correspondingly intense emission lines of Dy3+ observed at 474 and 572 nm. Dy3+ emission consists of the transitions 4F9/2→6H15/2 at 472 nm and 4F9/2→6H13/2 at 572 nm. These results indicate that very efficient energy transfer from Ce3+ to Dy3+ takes place in KCaSO4Cl halosulfate lattices. This is due to energy transfer from Ce3+ to Dy3+ ions. Figure 4 shows the mechanism of Ce3+→Dy3+ energy transfer in KCaSO4Cl halosulfate material.
Ce3+→Mn2+ energy transfer in KCaSO4Cl Transition metal ions have been widely used in luminescent materials, e.g. Mn2+, a transition metal center, has been doped
Ce3+→Dy3+ energy transfer in KCaSO4Cl Energy transfer between pairs of RE ions at dilution levels below the self-quenching limit is known to take place via multipolar, e.g. dipole–dipole interactions or dipole–quadrupole, interactions (14–16). The Ce3+ ion can be used as a sensitizer as well as an activator, depending on the splitting of the 5d excited levels by the crystal field symmetry. Much work has been carried out on the energy transfer from Ce3+ to different activator ions in different host lattices (17,18). The emission for Dy3+ originates from the 4 F9/2 level and transitions to 6H15/2 (~475 nm) and 6H13/2 (~570 nm) dominate. The latter has ΔJ = 2 and is hypersensitive. The emission is whitish to yellow in host lattices where hypersensitivity is pronounced. It is possible to obtain near white emission with Dy3+-activated phosphors because of the location of strong emissions in the regions 470–500 and 560–600 nm, together with some weak emissions at longer wavelengths (19–21). However, for lamp applications, Dy3+ cannot be excited by low- or highpressure mercury discharges that emit at wavelengths > 250 nm. This is because the CT level and also because the lowest 5d level of Dy3+ is situated at energies of 250,000 cm-1. Energy transfer from donor to acceptor plays an important role in luminescence and
Luminescence 2014
Figure 3. PL emission and excitation spectra of KCaSO4Cl:Ce5 mol%, Dy0.1 mol%.
3+
Figure 4. Ce →Dy material.
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3+
energy transfer mechanism in KCaSO4Cl halosulfate
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S. C. Gedam et al. into more than 500 inorganic hosts (22,23) for luminescence within an emission range from 490 to 750 nm. It is possible that in a halosulfate host co-doped with Ce3+→Mn2+, some form of energy transfer takes place, with the Ce3+ ions acting as the sensitizer and the Mn2+ ions acting as the activator. KCaSO4Cl: Mn2+ does not show any Mn2+ emission by UV excitation. Bhapat (24) commented that energy transfer from Ce3+ to Mn2+ should not result in sensitization because the PL used by him has the same efficiency at emission wavelengths of Ce3+ (306 and 324 nm) and Mn2+ (~500 nm). In our experiment, strong Mn2+ emission was observed at 557 nm due to the transition of 4 T1→6A1 of the Mn2+ ion in KCaSO4Cl materials with Ce3+ as the sensitizer ion (λex = 263 nm) The excitation and emission spectra of the KCaSO4Cl:Ce,Mn and the Ce3+→Mn2+ energy transfer mechanism are shown in Fig. 5. The excitation spectrum (monitored at 516 nm emission) shows maximum intensity at 267 nm, and a faint shoulder is observed at ~ 254 nm. Selecting 267 nm as the excitation wavelength, the emission spectra were recorded. Figure 5 shows the emission spectra for Ce5mol% and Mn0.1mol%. In these emission spectra, there are two peaks, the first, having low intensity, is at 334 nm and the second, with very sharp and high intensity, is at 516 nm. The 334 nm peak shows the emission position of Ce3+ and the 516 nm peak shows the emission of the Mn2+ ion. The 516 nm peak can be assigned to the Mn2+ emission from the 4T1 state. These results can be interpreted in terms of absorption by Ce3+ at 254 nm Ce3+→Mn2+ energy transfer, followed by efficient energy migration through the halosulfate lattice. In our experiment, strong Mn2+ emission is observed at 516 nm due to transition 4T1–6A1 of the Mn2+ ion in the KCaSO4Cl lattice on sensitization by the Ce3+ ion. The Mn2+ emission may be due to energy transfer from the Ce3+ ion to the Mn2+ ion. Mn2+ has a 3d5 configuration and from the Tanabe–Sugano diagram, it follows that the ground level is 6A1. Emission arises from the 4T1 (4G) level, which shifts to lower energies for higher crystal field strengths (25).
by the host, the emission can be anywhere from 365 nm (e.g. in BaSO4) to 650 nm (e.g. in CaS). Blasse (36) listed the Eu2+-doped compounds, showing that the emission color of Eu2+ can vary over a broad range from ultraviolet to red. Curve a of Fig. 6 is the PL excitation spectrum and curve b is emission spectrum of KCaSO4Cl:Eu for an excitation wavelength of 325 nm. It is known that Eu2+ emission is in the form of a band, the position of which differs from host to host. In our case, the emission spectra contain two overlapping broad peaks around 400 and 450 nm. We speculate that both peaks correspond to Eu2+ emission arising from transitions from the eg to t2g levels of the 4f6 5d configuration to the 8S7/2 levels of the 4f6 5d configuration, but with Eu2+ occupying two different lattice sites (Fig. 6c). Eu2+substituting K+ ions in the host lattice may lead to emission around 450 nm, while substituting Ca2+ may give emission around 400 nm. Because Eu2+ shows emission in a CaSO4 lattice around 385 nm, it is possible that Eu ions may also enter the lattice in their trivalent form.
Eu2+→Dy3+ energy transfer in KCaSO4Cl Figure 7 shows excitation spectra of KCaSO4Cl:Eu,Dy for an emission wavelength of 575 nm. Figure 7 shows the PL emission spectra of KCaSO4Cl:Eu,Dy for 325 nm excitation. Three peaks are obtained: one at around 440 nm, and the other two at around 490 and 575 nm. The first peak is characteristic of the Eu2+ emission arising from transitions of the 4f 65d configuration to the 8S7/2 level of the 4f7 configuration, and the other two peaks can be assigned to Dy3+ emission due to the transitions
Photoluminescence of Eu2+ in KCaSO4Cl The luminescence of Eu2+ ions in different hosts has recently attracted much attention due to its peculiar properties. Efficient Eu2+ emission has been obtained in many compounds (26–35) and many such phosphors have found applications. UV-emitting phosphors are useful in erythemal and photocopying lamps. Eu2+ emission results from two types of transitions. The most common is that due to 4f6 5d→4f7 (8S7/2). Because the position of the band corresponding to the 4f6 5d configuration is strongly influenced
Figure 5. PL excitation and emission spectra of KCaSO4Cl:Ce5 mol% Mn0.1 mol% and 3+ 2+ Ce →Mn energy transfer mechanism in KCaSO4Cl.
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Figure 6. (a) PL excitation spectra (for wavelength 325 nm) and (b) PL emission 6 8 spectra of KCaSO4Cl:Eu. (c) 4f 5d to S7/2 transitions.
Figure 7. Photoluminescence excitation and emission spectra of KCaSO4Cl:Eu, Dy for an excitation wavelength of 325 nm.
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Luminescence 2014
A review on photoluminescence in halosulfate phosphors F9/2→6H15/2 and 4F9/2→6H13/2, respectively. It can be seen in Fig. 7 that the excitation spectrum of the Dy3+ emission contains not only a band corresponding to excitation of the Dy3+ ion itself, but also a one that corresponds to excitation of the Eu2+ ion, giving evidence for the occurrence of energy transfer from Eu2+ to Dy3+. Figure 8 shows the Eu2+→Dy3+ energy transfer mechanism in a KCaSO4Cl host. The doped amount of Dy3+ is too low in the matrix materials. However, if the doped amount of Dy3+ is too large, it may result in concentration quenching and lower the luminescent effect. The peak positions in the emission spectra depend strongly on the nature of the Eu2+ surroundings. Host lattices with Eu2+ ions show more than one emission band. 4
Photoluminescence of Ce3+ in Na6(SO4)2FCl The XRD pattern of Na6(SO4)2FCl agrees well with JCPDS No. 741186. The Ce3+ ground state is split (2F5/2, 2F7/2) and these are the only levels possible for the 4f configuration. f→f transitions in Ce3+ are within the IR region. At room temperature, they occur as unresolved bands with a maximum at ~ 2200–2300 cm-1 and a half-width of 250–300 cm-1. The excited state, above the 2F7/2 level, belongs to the 5d configuration in the form of broad bands. Most commonly observed emission is characteristic of the 5d→4f transition. Both absorption and emission usually have a broadband character, showing the splitting characteristic of the 2 Fj states. Ce usually shows a high quenching temperature in silicates, borates and phosphates. Because Ce3+ has strong absorption in many hosts and emission matching the 4f levels of other RE impurities, it can be used as a sensitizer for other REs (37). Figure 9 shows the PL excitation spectra of Na6(SO4)2FCl:Ce3+ phosphor, which show a broad band at 265 nm with a prominent peak (λem = 322 nm). The PL emission spectra of the Ce3+ ion in a Na6(SO4)2FCl host (for 0.1, 0.2 and 0.5 mol%) are also given in Fig. 9. The emission peak wavelength and its relative intensity are shown for different contents of Ce3+ ion. From these PL spectra, it can be seen that the Ce3+ content affects peak height. Obviously, with an increase in Ce3+ content, the emission intensity increases. This indicates that the Na6(SO4)2FCl lattice is more suitable for higher concentrations of Ce3+ ions. The PL emission spectra of Na6(SO4)2FCl:Ce phosphors show Ce3+ emission at 322 nm due to 5d→4f transition of the Ce3+ ion. The observed variation in PL emission intensity may be due to cross-relaxation between Ce3+ ions at high concentrations of Ce3+.
Figure 9. PL excitation and emission spectra of Na6(SO4)2FCl:Ce (a) 0.5, (b) 0.2 and 3+ (c) 0.1 mol% and energy level diagram showing 5d to 4f transition in Ce ion.
The 5d→4f transition of the Ce3+ ion in Na6(SO4)2FCl is also shown in Fig. 9.
Photoluminescence of Dy3+ in Na6(SO4)2FCl Dy3+ emission falls in two lines in the visible region, arising from F9/2→6H15/2 (475–500 nm) and 4F9/2→6H13/2 (570 nm) transitions. When the ratio of blue to green emission is optimal, white emission can be obtained using Dy3+. This property has generated some interest in Dy3+ luminescence. The emission and excitation spectra of the Dy-doped ions in the Na6(SO4)2FCl host are shown in Fig. 10. In Fig. 10, the excitation spectrum is recorded and plotted, as monitored at 475 nm emission. At this wavelength, emission spectra are observed for different concentrations (0.1 and 0.5 mol%). An intense broad excitation band with a maximum at 270 nm is observed. Selecting 270 nm as the excitation wavelength, we recorded the emission spectra for the same samples. Strong PL emission of Dy3+ ions is observed at 475 and 570 nm in Na6(SO4)2FCl:Dy phosphor. Dy3+ emissions at 475 and 570 nm are due to 4F9/2→6H15/2 and 4F9/2→6H13/2 transitions, respectively (at an excitation wavelength of 270 nm) (Fig. 10). The emission in Dy3+ comes via a non-radiative transition to the 4F9/2 level, followed by radiative transitions to the 6H15/2 and 6H13/2 levels, as shown on the right-hand side of Fig. 10. The excitation spectra 4
3+
2+
Figure 8. Eu →Dy
Luminescence 2014
3+
radiative energy transfer mechanism in KCaSO4Cl host.
Figure 10. (A) PL excitation spectra of Na6(SO4)2FCl:Dy . (B) Emission spectra for 3+ Na6(SO4)2FCl:Dy (higher intensity, 0.5 mol% and lower intensity 0.1 mol%). (C) 3+ Energy level showing that an emission in Dy comes via a non-radiative transition 4 6 6 to the F9/2 level followed by radiative transitions to H15/2 and H13/2 levels.
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S. C. Gedam et al. of Na6(SO4)2FCl:Dy show a broad band in the UV region of the spectrum containing excitation at 270 nm. Photoluminescence of Eu2+ in Na6(SO4)2FCl
Figure 12. Schematic diagram of the induced luminescence from Na6(SO4)2FCl 2+ 3+ resulting from the radiative energy transfer from Eu to Eu ions.
200 180 160 Intensity (arb.unit)
Figure 11(A) shows excitation spectra peaking at 260 and 325 nm. The emission spectra of Na6(SO4)2FCl:Eu2+ (0.1, 0.2, 0.5 mol%) phosphors are shown in Fig. 11(B). As can be seen, the emission has a prominent peak at around 440 nm, which can be assigned to Eu2+ emission arising from transitions of the 4f6 5d configuration to the 8S7/2 level of the 4f7 configuration. The results show that all the spectra are broadband and the main emission peaks of these phosphors are at ~ 410–440 nm. Recently, Yu et al. (38) studied Eu and found excellent luminescence. There is no special Eu3+ emission in the spectra, which indicates that Eu3+ ions are reduced to Eu2+ completely in the Na6(SO4)2FCl:Eu2+ phosphor. However, for 0.5 mol% Eu, two more peaks are observed at 580 and 615 nm, which show the Eu3+ emission. This shows that, particularly in this lattice for 0.5 mol% Eu, Eu2+ as well as Eu3+ emissions are possible, as shown in the Fig. 11; Eu2+ emission has a very broad and intense nature and Eu3+ emission has a very low intensity with line emission. For 0.1 mol% Eu, a broad band is split into two peaks at 410 and 480 nm. The splitting may be due to the lower Eu concentration. The induced luminescence of Eu3+ via the radiative energy transfer consists of six processes, as shown in Fig. 12. The population at the 5D3 level of Eu3+ increases via radiative energy transfer from Eu2+ to Eu3+. Finally, the population arrives at the emitting 5D0 level via non-radiative transitions from the upper levels. The trivalent europium ion is very useful for studying the nature of the metal coordination in various systems, owing to its non-degenerate emitting 5D0 state. With increasing concentrations of Eu2+ ions, the intensity of the peaks increases with a maximum observed for Eu0.5mol%.
140 120 100 80 60 40 20 0 200
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Photoluminescence of Ce3+ in LiMgSO4F Figure 13 shows the PL excitation spectra of LiMgSO4F:Ce3+ phosphor (λem = 335 nm); a broad band is observed at around 235 nm. Figure 14 shows the PL emission spectra of Ce3+ ions in LiMgSO4F phosphors at different concentration under
2+
Figure 11. (A) Excitation spectra of Na6(SO4)2FCl:Eu showing two peaks at 2+ 260 and 325 nm. (B) Emission spectra for Na6(SO4)2FCl:Eu : (a) 0.5 mol%, (b) 0.2 mol%, (c) 0.1 mol.
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Figure 13. Excitation spectra of LiMgSO4F:Ce for 5 mol%.
3+
Figure 14. PL emission spectra of LiMgSO4F:Ce ions (λex = 232 nm, λem = 335 nm): (a) 5 mol%, (b) 2 mol%, (c) 1 mol% and (d) 0.5 mol% (higher peak, 5 mol%; lower peak, 0.5 mol%).
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Luminescence 2014
A review on photoluminescence in halosulfate phosphors excitation at 235 nm. Peaks are observed at 335 nm, and are assigned to the 5d→4f transition of Ce3+ ions. With increasing concentrations of Ce3+ ions, the peak intensity increases with the maximum intensity being observed for 5 mol% Ce3+ ions. This indicates that the LiMgSO4F lattice is more suitable for higher concentrations of Ce3+ ions. The PL emission spectra show very strong Ce3+ emission at 335 nm for 5 mol% Ce3+ due to the 5d→4f transition of Ce3+ ions. The observed variation in the PL emission intensity may be due to cross-relaxation between Ce3+ ions at high concentrations of Ce3+. The PL emission peak intensity of Ce3+ is not same at different concentrations. The peak intensity curves are shifted towards shorter wavelengths. LiMgSO4F:Ce5mol% phosphors may be useful candidates for scintillation applications. LiMgSO4F as a new luminescent host material, Dy3+, Mn2+, Eu2+ emission and the energy transfer from Ce3+→Dy3+ and Ce3+→Mn2+ ions are taken into consideration for further study. Figure 15 shows a schematic energy level diagram of Ce3+ in LiMgSO4F.
Ce →Dy 3+
3+
energy transfer in LiMgSO4F
3+
Ce ions can be used as a sensitizer as well as an activator, depending on the splitting of the 5d excited levels by crystal field symmetry. Much work has been done on the energy transfer from Ce3+ to different activator ions in different host lattices. Figure 16 shows that LiMgSO4F:Dy does not show Dy3+ emission under UV (340 nm) excitation. A strong PL emission of Dy3+ ions was observed at 482 and 571 nm in LiMgSO4F:Ce,Dy phosphor due to the presence of Ce3+ ions as a sensitizer. The observed Dy3+ emission at 482 and 571 nm was due to 4F9/2→6H15/2 and 4F9/2→6H13/2 transition of the Dy3+ ion, respectively. The transfer of energy from Ce3+ to Dy3+ ions in a LiMgSO4F lattice returns Ce3+ to the ground state and Dy3+ to the excited state. The emission in Dy3+ comes via a non-radiative transition to the 4F9/2 level, followed by radiative transitions to the 6H15/2 and 6H13/2 levels. The excitation spectra of LiMgSO4F:Ce,Dy show a broad band in the UV region of the spectrum, containing excitation of Ce3+ (340 nm); emission of Ce3+ at 340 nm occurs due to energy transfer from Ce3+ to Dy3+ ions.
Figure 15. Schematic energy level diagram of Ce λem = 335 nm).
Luminescence 2014
3+
Figure 16. PL excitation and emission spectra of LiMgSO4F:Ce5 (λex = 340 nm).
%
Dy0.1%
Photoluminescence of Ce3+ in Na6Pb4(SO4)6Cl2 Figure 17 shows the XRD pattern of Na6Pb4(SO4)6Cl2 material, which matched the standard JCPDF data No. 27-1416. The XRD pattern did not indicate the presence of constituents such as Na2SO4, PbSO4 or NaCl and other likely phases, which is direct evidence for formation of the desired compound. These results indicate that the final product was formed in an homogeneous form. Figure 18(A) shows PL excitation spectra for Na6Pb4(SO4) 3+ 6Cl2:Ce , a broad band is observed at around 260 nm (λ em = 340 nm). Figure 18(B) shows the PL spectra of Ce3+ ions in Na6Pb4 (SO4)6Cl2 phosphor at different concentrations under excitation at 260 nm. Peaks are observed at 340 nm for all concentrations, and are assigned to the 5d→4f transition of Ce3+ ions. As the concentration of Ce3+ ions increases, the peak intensity of 340 nm increases and the maximum intensity is observed for 5 mol% Ce3+ (Fig. 18C). This indicates that the Na6Pb4(SO4)6Cl2 lattice is more suitable for higher concentrations of Ce3+. The PL emission spectra of Na6Pb4(SO4)6Cl2:Ce phosphor show Ce3+ emission at 340 nm due to 5d→4f transition of the Ce3+ ion. The observed variation in PL emission intensity may be due to cross-relaxation between Ce3+ ions at high concentrations of Ce3+. Figure 18(D) shows energy level diagram of Ce3+ in Na6Pb4(SO4)6Cl2.
in LiMgSO4F (λex = 232 nm, Figure 17. XRD pattern of Na6Pb4(SO4)6Cl2.
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S. C. Gedam et al.
Figure 18. (A) Excitation spectra of Na6Pb4(SO4)6Cl2:Ce5 mol% (λex = 260 nm). (B) PL emission spectra of Na6Pb4(SO4)6Cl2:Ce: (a) 5 mol%, (b) 2 mol%, (c) 1 mol% (λem = 340 nm). 3+ (C) Increase in intensity as the concentration increases. (D) Energy level diagram of Ce in Na6Pb4(SO4)6Cl2.
Ce3+→Dy3+ energy transfer in Na6Pb4(SO4)6Cl2 Na6Pb4(SO4)6Cl2:Dy did not show any Dy3+ emission under UV (260 nm) excitation. A strong PL emission of Dy3+ ions was observed at 475 and 575 nm in Na6Pb4(SO4)6Cl2:Ce,Dy phosphor due to the presence of Ce3+ as a sensitizer. Dy3+ emission at 475 and 575 nm is due to 4F9/2→6H15/2 and 4F9/2→6H13/2 transition of the Dy3+ ion, respectively (under an excitation wavelength of 260 nm). From Fig. 19, it is clear that Dy3+ emission is observed at higher concentrations (0.5 mol%). Only some results are plotted. The transfer of energy from Ce3+ to Dy3+ ions in a Na6Pb4(SO4) 3+ to the ground state and Dy3+ to the 6Cl2 lattice brings Ce excited state. The emission in Dy3+ comes via non-radiative transition to the 4 F9/2 level, followed by radiative transitions to the 6H15/2 and 6H13/2 levels, as shown in Fig. 3. The emission spectra of the Na6Pb4(SO4)6Cl2 host overlap with the excitation spectrum of Dy and this provides the fundamental condition for energy transfer from Ce3+ to Dy3+ ions in Na6Pb4(SO4)6Cl2. Figure 20 shows an energy transfer process accomplished via re-absorption transfer. Similar results for the PL enhancement of Dy3+ emission in CaSO4 lattice were reported in 1999 by Lakshmanan.
Figure 19. (A) Excitation spectra and (B) PL emission spectra of Na6Pb4(SO4)6Cl2: (a) Ce5%, Dy0.5% ; (b) Ce5%, Dy0.1%
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3+
Figure 20. Ce →Dy
3+
energy transfer in Na6Pb4(SO4)6Cl2.
Ce3+→Tb3+ energy transfer in Na6Pb4(SO4)6Cl2 The Ce3+ to Tb3+ energy transfer process in different host matrices is well known. Broadband emitters are often used to sensitize the luminescence of RE ions. The overlap of the normalized Ce3+ emission and Tb3+ excitation is found much more easily. The energy transfers from Ce3+ to Tb3+ might be very efficient. This could improve the excitation efficiency and brightness. Figure 21 (A) shows the PL excitation spectra of Na6Pb4(SO4)6Cl2:Ce5%, Tb0.1% observed at 340 nm and Fig. 21(B) shows the PL emission spectra of Na6Pb4(SO4)6Cl2:Ce5mol%,Tb0.1mol% observed at 490 and 550 nm, with a small peak at 580 nm. From this result, three emission transitions can be observed: 5D4→7F6 (490 nm), 5 D4→7F5 (550 nm) and 5D4→7F4 (580 nm). The 5D4→7F5 transition may be responsible for the observation of green color as a single peak (39). The Ce emission moves towards the lower energy side due to the new host ligands. Thus, the energy transfer rate should be much higher. Thus, Na6Pb4(SO4)6Cl2:Ce,Tb is an interesting blue and green phosphor and may have application in display devices.
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Luminescence 2014
A review on photoluminescence in halosulfate phosphors
Figure 21. (A) PL excitation of Na6Pb4(SO4)6Cl2Ce5%, Tb0.1%. (B) PL emission spectra of Na6Pb4(SO4)6Cl2Ce5%, Tb0.1% for the emission of 550 nm.
Figure 22 is a schematic energy level diagram showing Ce3+→Tb3+ energy transfer in Na6Pb4(SO4)6Cl2. In this host, even at the lowest concentrations, only green emission is observed and the blue emission is quenched by Tb3+ ions. Hence, the quantum efficiency of the luminescence may be high when the f–d absorption is at longer wavelengths. Efficient energy transfer from the broad (i.e. Ce3+) to the narrow line emitter (i.e. Tb3+) is possible only between nearest neighbors in the crystal lattice and with optimal spectral overlap.
Photoluminescence of Ce3+ in Na21Mg(SO4)10Cl3 Figure 23 shows the PL excitation spectra of Na21Mg(SO4)10Cl3: Ce3+ phosphor (λem = 341 nm), broadband is observed at around 247 nm. Figure 24 shows the PL emission spectra of Ce3+ ions in Na21Mg(SO4)10Cl3 phosphor at different concentrations under excitation at 247 nm. A peak is observed at 341 nm, which is assigned to the 5d→4f transition of Ce3+ ions. With increasing concentrations of Ce3+ ions, the peak intensity of 341 nm increases, with the maximum intensity observed for 5 mol% Ce3+. This indicates that the Na21Mg(SO4)10Cl3 lattice is more suitable for higher concentrations of Ce3+.The PL emission spectra of Na21Mg(SO4)10Cl3:Ce3+ (5 mol%) phosphors show very strong
3+
Figure 22. Ce → Tb
Luminescence 2014
3+
energy transfer in Na6Pb4(SO4)6Cl2.
Figure 23. Excitation spectra of Na21Mg(SO4)10Cl3:Ce (λex = 247 nm) for λem = 341 nm.
Ce3+ emission at 341 nm due to 5d→4f transition of the Ce3+ ions. The observed variation in PL emission intensity may be due to cross-relaxation between Ce3+ ions in cases of high concentrations of Ce3+. The strong Ce3+ emission in the Na21Mg(SO4)10Cl3 lattice may be useful as a scintillator. The PL emission peak intensity of Ce3+ ion in Na21Mg(SO4)10Cl3 lattice is not of the same
Figure 24. PL emission spectra of Na21Mg(SO4)10Cl3:Ce: (a) 5 mol%, (b) 2 mol%, (c) 1 mol% (excitation at 247 nm).
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S. C. Gedam et al. intense. Individual emissions are not observed for Dy3+, Mn2+, Eu2+, whereas with Ce3+ as a co-activator ion, Dy3+and Mn2+ emission may be possible and this should be taken into consideration for further study. Further investigations into the energy transfer from Ce3+→Dy3+, Ce3+→Mn2+ and other possible mechanisms are ongoing. The emission intensity of Dy3+ and Mn2+ individual ions is not sufficient for luminescence applications.
300
Intensity (arb units)
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Conclusions
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Wavelength (nm) Figure 25. Excitation spectra of Na15(SO4)5F4Cl:Ce1 mol% (λex = 247 nm).
Figure 26. PL emission spectra Na15(SO4)5F4Cl:Ce: (a) 5 mol%, (b) 2 mol%, and (c) 1 mol% (excitation at 247 nm).
intensity for different concentration of Ce3+ ions. The PL emission peak intensity is dependent on the concentration of the Ce3+ ion. The intensity of the Ce3+ ion in the above phosphor shows that the maximum Ce3+ emission is observed in the Ce (5 mol%) phosphor. Na21Mg(SO4)10Cl3:Ce (5 mol%) phosphors may be useful candidates for scintillation applications. Photoluminescence of Ce3+ in Na15(SO4)5F4Cl Figure 25 shows the PL excitation spectra of Na15(SO4)5F4Cl:Ce3+ phosphor. A broad band is observed at 247 nm with a prominent shoulder around 230, 260 and 290 nm (λem = 341 nm). Figure 26 shows the PL emission spectra of Ce3+ ions in Na15(SO4)5F4Cl phosphor at different concentrations under excitation at 247 nm. Peaks are observed at 341 nm for concentrations of 5, 2 and 1 mol%, which are assigned to the 5d→4f transition of Ce3+ ions. On increasing the doping concentration of Ce3+ ions, the intensity of the 341 nm peak is found to increase and maximum intensity is observed for 5 mol% Ce3+. This indicates that the Na15(SO4)5F4Cl lattice is more suitable for higher concentrations of Ce3+ ions. The observed variation in the PL emission intensity may be due to cross-relaxation between Ce3+ ions at high concentrations of Ce3+. All the phosphors may be useful in scintillation applications. Among these halosulfate phosphors, Na15(SO4)5F4Cl:Ce is more
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Ce3+ emission in these phosphors is observed around 321–332 nm, due to the allowed 5d→4f transition for an excitation wavelength of 254 nm. Very efficient energy transfer from Ce3+ to Dy3+ takes place in KCaSO4Cl halosulfate. The emission spectra show two prominent peaks at 475 and 572 nm, with very sharp and high intensity. Dy3+ emission consists of transitions 4F9/2→6H15/2 at 472 nm and 4F9/2→6H13/2 at 572 nm. In our sample, strong Mn2+ emission is observed at 516 nm because of transition of 4T1–6A1 of the Mn2+ ion in the KCaSO4Cl lattice on sensitization by Ce3+. Efficient energy transfer from Ce3+ → Mn2+ leading to efficient Mn2+ emission at 515 nm was observed in this host. This corresponds to the blue–green region of the visible spectrum. In KCaSO4Cl:Eu, the observed emission spectra contain two overlapping broad peaks around 400 and 450 nm for an excitation wavelength of 325 nm, due to transitions from the eg to t2g levels of the 4f6 5d configuration to the 8S7/2 levels of the 4f6 5d configuration. Eu2+-substituting K+ ions in the host lattice may lead to emission around 450 nm, while substituting Ca2+ may give emission around 400 nm. A PL emission spectrum of KCaSO4Cl:Eu,Dy is observed for 325 nm excitation. Three peaks are obtained, one at around 440 nm and two at around 490 and 575 nm. The first peak is characteristic of the Eu2+ emission arising from transitions of the 4f 65d configuration to the 8S7/2 level of the 4f7 configuration and the other two peaks can be assigned to the Dy3+ emission due to the transitions 4F9/2→6H15/2 and 4F9/2→6H13/2, respectively. Acknowledgements SCG thanks THE University Grant Commission (UGC), New Delhi and SJD thanks the BRNS, Government of India, for their financial support.
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