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Although the emission of lanthanide (Ln) ions doped cubic sesquioxides has been subject of extensive studies, there are fundamental issues still to be elucidated. Specifically, compared to the well-understood emission of Ln at C2 sites, the emission at the inversion S6 (C3i) sites, representing ¼ of the total sites, has been identified only for Pr, Nd, Eu, Sm and Yb. Here, we present a first report and improved identification of the emission, excitation and decay properties of Tb, Dy and Pr at S6 sites in Y2O3 by use of time-gated luminescence spectroscopy. The green emission of Tb at S6 sites is characterized by an intensity ratio relative to the cyan band at 490 nm of 10 compared to 2 measured for Tb at C2 sites. Dy at S6 sites displays a relatively intense, near-infrared emission at 765 nm which is red-shifted by ca. 200 nm relative to yellow peaked emission of Dy at C2 sites. The emission lifetimes of 9.4 and 4.8 ms, associated with Tb and Dy at S6 sites, exceed by a factor of 5 and 10 those of C2 counterparts. It is also found that Dy may be regarded as a sensitive probe for the inversion symmetry, comparable with the more recognized Eu. The participation of Ln at S6 sites to the up-conversion emission is revealed for the first time in Sm doped Y2O3 and Eu /Tb, Yb co-doped Y2O3 and explained in terms of successive ground state and excited state absorptions and cooperative energy transfer, respectively.
© 2016 Optical Society of America
1. Introduction
Over the past decades, lanthanide (Ln) doped cubic sesquioxides, such as Y2O3, have been extensively investigated especially for phosphor related applications, such as temperature sensors, light displays, laser materials, Si- compatible photonic devices, luminescent bio labels and nanoscale scintillators [1–7].
Y2O3 belongs to the cubic space group, Th7, with a unit cell formed by two types of cationic sites, assumed to be randomly distributed on two non-equivalent sites, 24d of inversion-less C2 symmetry and 8b of inversion C3i or S6 symmetry sites, respectively [8,9]. Upon doping into Y2O3, the emission of C2 Ln benefits both from the more numerous C2 relative to S6 centers and the relative more intense, characteristic forced electric dipole (ED) emission transitions. In comparison, the emission of S6 Ln is longer-lived and of much weaker intensity, as it contains only the magnetic dipole (MD) transitions (ΔJ = 0, ± 1 in the intermediate coupling) allowed by the selection rules of centrosymmetric sites. From an experimental point of view, this means that in the absence of a simultaneous spectral and temporal selectivity, it may be challenging to discriminate the weaker emission of S6 Ln from the, typically order(s) of magnitude more intense emission associated with C2 Ln. The S6 centers are not isolated from the C2 ones, since the two types of centers are interconnected via a relative efficient energy transfer. It is demonstrated in the literature that, generally the excitation energy flows from the higher energy located metastable level of S6 Ln which act as donor to lower energy of C2 Ln which acts as acceptor, though an inverse transfer from C2 to S6 centers is also reported in Eu doped Y2O3 and isostructural Lu2O3 [10, 11]. More recently, it has been a growing interest on how the MD transitions of Ln doped Y2O3 can be used as atomic scale probes of optical magnetic fields or to directly modulate light emission from integrated sources faster than their radiative lifetime [12,13].
The dopant site distribution, site characteristic emission properties and inter-site interactions represent key elements in understanding and optimizing the emission of lanthanide based optical materials, such as Ln doped Y2O3. It is therefore intriguing that, to date, the emission of S6 Ln dopants in cubic sesquioxides has been identified for a few Ln, such as Pr [14], Nd [15], Eu [10,16], Sm [17] and Yb [18].
Here, we present a first report and improved identification of the emission properties of Tb, Dy and Pr at S6 sites in Y2O3 by use of time-gated luminescence spectroscopy. The emission properties are analyzed in terms of individual emission and excitation spectra as well as emission decay. Furthermore, the participation of S6 Ln to up-conversion emission is revealed for the first time in Sm doped Y2O3 and Eu/ Tb, Yb co-doped Y2O3. Energy transfer between S6 and C2 Ln as well as the mechanisms of up-conversion are also discussed.
2. Experimental
2.1. Materials
(Yb) Ln (co)-doped Y2O3 (Ln = Pr, Sm, Eu, Tb and Dy) were synthetized by using the citrate complexation method already reported [19] and measured at a calcination temperature of 800 °C. Ln concentration was fixed at 0.1% (Pr, Sm), 1% (Dy, Eu, Tb) and 2% (Yb). Their structures were investigated by X-ray diffraction (XRD), Diffuse Reflectance (DR-UV/Vis/NIR) spectroscopy and Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS).
2.2. Characterization
Powder X-ray diffraction (XRD) patterns were recorded on a Schimadzu XRD - 7000 diffractometer using Cu Kα radiation (λ = 1.5418 Å, 40 kV, 40 mA) at a scanning speed of 0.10 degrees min−1 in the 15 ÷ 90 degrees 2Θ range. Fourier transform infrared (FTIR) spectra were measured with a Thermo Electron Nicolet 4700 FTIR spectrometer with a Smart Accessory for diffuse reflectance measurements. The IR spectra were scanned in the region of 4000–400 cm−1 at the resolution of 4 cm−1. The final spectra resulted from accumulation of 200 scans. Diffuse reflectance optical (DR-UV–Vis) spectra were recorded at room temperature on a Analytik Jena Specord 250 spectrophotometer with an integrating sphere for reflectance measurements and MgO as the reflectance standard. DR-UV–Vis spectra of the photocatalysts were recorded in reflectance units and were transformed in Kubelka–Munk remission function F(R).
2.3. Photoluminescence measurements
The photoluminescence measurements were carried out at room temperature and T = 80 K (by use of exchange gas cryostat) using a Fluoromax 4 spectrofluorometer (Horiba) operated in both the fluorescence and the phosphorescence mode. The repetition rate of the xenon flash lamp was 25 Hz, the integration window varied between 0.1 and 0.5 s, the delay after flash varied between 0.03 and 25 ms, and up to 30 flashes were accumulated per data point. For excitation spectra, the slits were varied from 0.1 nm to 1 nm in excitation and from 1 to 29 nm in emission. The emission decays were measured by using the “decay by delay” feature of the phosphorescence mode. Time resolved (gated) emission spectra (TRES) were recorded at room temperature and T = 80 K (by use of exchange gas cryostat) using a wavelength tunable NT340 Series EKSPLA OPO (Optical Parametric Oscillator) for samples excitation at 210 ÷ 1000 nm operated at 10 Hz as excitation light source. The tunable wavelength laser has a narrow linewidth (< 5 cm−1, which makes the laser a high selective excitation source) with scanning step and output energy depending on the spectral region. As detection system an intensified CCD (iCCD) camera (Andor Technology) coupled to a spectrograph (Shamrock 303i, Andor) was used. The TRES were collected using the box car technique. The gain of the micro-chanel plate (MCP gain) was set to 100. The emission was detected in the spectral range of 450 nm < λem < 800 nm with a spectral resolution from 0.05 to 0.88 nm and the input slit of the spectrograph was set to 10 µm. The temperature of the iCCD was lowered at −20 °C for a better signal to noise ratio (S/N). For all measurements done by iCCD, a cut off filter of 450 nm was used to protect the detector from the excitation light. The emission decays were analyzed by fitting with a multiexponential function f(t) using the commercial software (OriginPro 8): f(t) = ∑ Ai exp(− t/τi) + B, where Ai is the decay amplitude, B is a constant (the baseline offset) and τi is the time constant of the decay i.” More details of used luminescence set-up and method can be found elsewhere [20].
3. Results and discussion
3.1. Emission of S6 Ln (Ln = Pr, Tb and Dy) under direct excitation
Figure 1 illustrates the cubic structure of Y2O3 with C2 and S6 symmetry sites colored with dark blue and green color. Both centers have six - fold coordination of O2- ions: the C2 site originates from an eight - fold cubic structure with two oxygen vacancies on a face diagonal, with three groups of Y - O2- distances of 2.26, 2.28 and 2.35 Å. For the inversion S6 site, the two vacancies lies on a body diagonal, so the six Y - O2- bonds a have similar length of 2.25 Å [8].
Figure 2 illustrates a selection of representative XRD patterns corresponding to the investigated samples. All patterns could be indexed to cubic Y2O3 (JPCDS cards #01-0831) with estimated sizes varying between 25 and 30 nm and no parasite phases could be detected.
The emission and excitation spectra and emission decays corresponding to Ln at S6 sites (Ln = Pr, Tb and Dy) are gathered in Figs. 3-5 with the MD transitions highlighted with green line. For comparison only, the spectra and decays of Ln at C2 sites were also included (black lines) whose shapes agree well to those reported in literature. The emission of Pr represented in Fig. 3(a) corresponds mainly to 1D2 - 3H4 (ΔJ = 2) that is expected to display only a weak magnetic component [21]. The excitation proceeds predominantly via the intense, spin allowed f-d transitions of Pr [14] with maxima around 285 and 324 nm, assigned to C2 and S6 centers, respectively. It is also noticeable that, compared to previous reports [14] no f-f absorption transitions can be detected for S6 Pr, that is, the emission at the inversion site is exclusively sensitized by Pr f-d absorption bands. The emission spectrum of S6 Pr (excited at 324 nm and measured after a delay of 1 ms to exclude further interference from C2 Pr) is unexpectedly broader with a more complex Stark splitting pattern than that of C2 Pr. Such emission shape may be caused by a combination of factors, such as (weak) interference from C2 emission, vibronic lines [16] or slighter lower symmetry then S6 due to Y/Pr ionic radii mismatch (0.9 nm/0.99 nm) [22]. However, strong evidence that the green line emission is likely to be connected with S6 Pr is given by the value of its average lifetime which surpasses by an order of magnitude that of C2 Pr (1.8 compared to 0.18 ms, respectively) which is consistent with a MD nature of the associated emission (Fig. 3(b)).
Fig. 3 Excitation and emission spectra (a) and emission decays (b) corresponding to C2 (black) and S6 (green) Pr doped Y2O3 measured at 80 K. Emission and excitation wavelengths and the delays after the laser pulse, δt, are indicated on the Figure. Also included is a diagram with the MD transitions highlighted. Black and green dotted vertical lines in Fig. 3(a) localize the emission and excitation wavelengths used in experiments.
Fig. 4 Excitation and emission spectra (a) and emission decays (b) corresponding to C2 (black) and S6 (green) Tb doped Y2O3 measured at 80 K. Emission and excitation wavelengths and the delays after the laser pulse, δt, are indicated on the Figure. Also included is a diagram with the MD transitions highlighted. Black and green dotted vertical lines in Fig. 4(a) localize the emission and excitation wavelengths used in experiments.
Fig. 5 Excitation and emission spectra (a) and emission decays (b) corresponding to C2 (black) and S6 (green) Dy doped Y2O3 measured at 80 K. Emission and excitation wavelengths and the delays after the laser pulse, δt, are indicated on the Figure. Also included is a diagram with the MD transitions highlighted. Superimposed on the emission spectrum of S6 Dy in Fig. 5(a) is the fingerprint emission spectrum (blue lines) of Dy at Oh site of CeO2 (see also text). Black and green dotted vertical lines in Fig. 5(a) localize the emission and excitation wavelengths used in experiments.
Figure 4 gathers the excitation and emission spectra and emission decays related to Tb at C2 and S6 sites.
For both Tb centers, the emission is dominated by 5D4 - 7F5 green emission at 545 nm that is known to have strongly mixed ED and MD nature [23]. As expected, the 5D4 - 7F5 emission is relatively much more intense than 5D4 - 7F6 emission for S6 Tb than C2 Tb as for inversion symmetry the ED transitions are forbidden. We can define an asymmetry ratio for Tb as the ratio of ED and MD transition intensities, R (Tb) = I(5D4 - 7F6)/I(5D4 - 7F5). The estimated ratio values are 0.1 and 0.5 for S6 Tb and C2 Tb, respectively. We recall that, as it was firstly established for the Eu lanthanide, a closer to zero value of its asymmetry ratio (defined as R (Eu) = I(5D0 - 7F2)/I(5D0 - 7F1)) signifies a more symmetrical environment around the metal. The ratio values for Tb confirm the distinct oxygen environments or local symmetries around the two centers. The excitation spectra corresponding to both C2 and S6 Tb are dominated by the strong, spin allowed f-d absorption transitions of Tb centered at 275 nm and at 310 nm, respectively. The emission decay of S6 Tb is remarkable long, with an estimated lifetime around 9.4 ms which is 4-5 fold greater than the value for C2 Tb (1.9 ms). The value is slightly longer than that measured for S6 Sm (8.4 ms, measured for diluted concentration of 0.065% in Sm doped Y2O3 ceramics [17]) and comes close to that of Tb doped CaF2 (12 ms) which is one of the greatest value reported so far for Tb doped nanoparticles [24]. It should be noted the presence of the 310 nm based shoulder in the excitation spectrum of C2 Tb that may result from the insufficient spectral separation of the Tb centers but also can signal the occurrence of the energy transfer from S6 Tb to C2 Tb.
Figure 5 gathers the excitation/emission spectra and emission decays related to Dy at C2 and S6 sites of Y2O3.
The emission of C2 Dy follows the typical shape of Dy emission as found in a wide range of hosts, that is, dominated by the 4F9/2 - 6H13/2 yellow transition at 570 nm. Recently, by use of energy-momentum spectroscopy, the electric and magnetic dipole emission rates of near-infrared transitions in Dy doped Y2O3 were quantified [25]. It has been found that the overlapping 4F9/2 - 6F11/2 and 4F9/2 - 6H9/2 transitions of Dy at C2 site display the greatest MD emission. Here, we find that the fingerprint emission of S6 Dy is striking different to that of C2 Dy with a relatively intense NIR emission corresponding to the MD 4F9/2 - 6H9/2/6F11/2 transition at 765 nm. The excitation spectra of C2 and S6 Dy show a strongly similar structure, except for the region around 450 nm where the MD absorption assigned to 4H15/2 - 4I15/2 transition is located [21]. The average emission lifetime of S6 Dy is estimated to be around 4.8 ms, which is one order of magnitude greater than the value of C2 Dy (0.43 ms), representing also one of the greatest values reported so far in the literature for Dy doped nanoparticles. We should note that Dy is used as a structural probe in a wide range of hosts with the emission intensity ratio defined as I(4F9/2 - 6H13/2)/I(4F9/2 - 6H15/2) employed as an indicator for the local symmetry [26]. In light of the present results we suggest that a more appropriate asymmetry ratio should be defined by the intensity ratio, R(Dy) = I (ED) (4F9/2 - 6H13/2)/I (MD) (4F9/2 - 6H9/2/6F11/2).
The R(Dy) values of ~0.5 for S6 Dy and ~6.5 for C2 Dy witness for the strongly distinct oxygen environment around Dy at the two sites. Finally, it is interesting to compare the Dy emission at S6 sites in Y2O3 and cubic, Oh, sites in CeO2 [27]. Although in both Y2O3 and CeO2 Dy resides at the center of a cube, Dy presents a 6-fold coordination determined by two missing oxygens lying on a body diagonal in Y2O3 (Fig. 1) whilst in CeO2 Dy has 8-fold coordination due to oxygen presence in all eight corners of the cube. The different local crystal field determined by coordination and presumed Dy - oxygen bond lengths (2.25 Å in Y2O3 [8] and 2.343 Å in CeO2 [28], with both values representing the Y - O and Ce - O bond lengths, respectively) lead to the predominance of single MD emission line at 765 nm in Y2O3 whilst two comparable emission lines of MD nature at 678.5 nm (corresponding to 4F9/2 - 6H11/2) and 766 nm are observed in CeO2 (see the blue line spectrum represented in Fig. 5(a)). To our knowledge, this is a first demonstration that Dy can be employed as a powerful luminescent probe for the inversion symmetry, being comparable with the more acknowledged Eu lanthanide.
3.2. The emission of S6 Ln (Ln = Sm, Eu and Tb) under up-conversion excitation
Over the past decades, along with down-conversion emission properties, the up-conversion (UPC) emission of Ln - doped Y2O3 has also been extensively investigated, being exclusively assigned to the more emissive C2 Ln. Here, we present the first evidence of UPC emission assignable to S6 Ln in single doped Sm and Yb, Eu/Tb – co-doped Y2O3 by use of time-gated emission up-conversion spectroscopy [29,30]. Figure 6 illustrates the UPC emission in Sm - Y2O3 excited by a pulsed laser at 952 nm (corresponding to 6H5/2 - 6F11/2 transition, see also the diagram). Increasing the delay after the laser pulse, the UPC emission of Sm gradually changes from the well- known C2 type emission [31] to one dominated by MD 4G5/2 - 6H5/2 and mixed MD + ED 4G5/2 - 6H7/2 transitions at 563 and 607.5 nm. Such emission shape is readily identifiable as the emission of S6 Sm [17], superimposed on a small contribution of C2 Sm emission. Since the C2 Sm emission was still observed at delays longer than 10 ms, an inter-centre energy transfer flowing from the longer lived S6 Sm to the shorter lived C2 Sm, previously evidenced with direct/down-conversion studies [17] cannot be discarded. As for the UPC mechanism, an energy transfer between the two neighbor Sm ions was considered responsible for the C2 UPC emission in (3%) Sm - Y2O3 [31]. Considering the low concentration used here of only 0.1% Sm, a ground state absorption followed by excited state absorption (see also the diagram in Fig. 6) is more likely to explain the mechanism of UPC emission for both the C2 and S6 centers.
Fig. 6 Up-conversion emission of C2/S6 Sm doped Y2O3. A diagram that illustrates the main energy levels involved in up-conversion process is also included. For comparison purpose, with red and green lines are drawn the emission spectra related to C2 and S6 centres of Sm separated under down-conversion / direct excitation.
Figure 7 gathers the UPC of Eu(1%), Yb (2%) and Tb (1%), Yb (2%) co-doped in Y2O3 excited by use of a pulsed laser into the maximum of Yb absorption at 976 nm. The previously reported up-conversion emission of similar systems was assigned to C2 centers only and explained in terms of a cooperative UPC process [32,33].
Fig. 7 Up-conversion emission of C2/S6 Eu, Yb (a) and Tb, Yb co-doped Y2O3(b). A diagram that illustrates the main energy levels involved in up-conversion process is also included. For comparison purpose, with red and green lines are drawn the emission spectra related to C2 and S6 centres of Eu and Tb separated under down-conversion / direct excitation.
In this mechanism, pairs of Yb are first excited from 2F7/2 to 2F5/2 upon 976 pulsed excitation that simultaneously transfer the excitation energy and excite one nearby Eu/Tb ion from their ground-state levels to 5D4 (Tb) or 5D2 (Eu). The diagram included in Fig. 7 illustrates the energy levels of Yb, Eu and Tb involved in the UPC. The UPC emission of S6 Eu and Tb could be discriminated against the major C2 up-conversion emission only by use of long delay of 5 ms and 8 ms, respectively.
As illustrated in Fig. 7(a) the delayed UPC emission is characterized by two strong MD 5D0 - 7F1 transition at 582.7 nm and 592.7 nm along with some weak vibronic line around 596.2 nm assigned in literature to S6 Eu [10,16]. Similarly to the case of Sm discussed above, a non-negligible emission of C2 center at 611 nm corresponding to 5D0 - 7F2 was found to accompany the emission characteristic of S6 Eu which could not be diminished even at delays above 50 ms. In fact, a non-zero contribution of C2 Eu is always observed in the literature even under selective down-conversion excitation of S6 Eu. Our time-gated up-conversion results confirm thus the occurrence of S6 to C2 energy transfer, previously observed for Eu (and Sm) doped Y2O3 under down-conversion excitation [10,11,17]. Finally, in Fig. 7(b), the delayed UPC emission of Tb obtained also with excitation into Yb absorption at 976 nm agrees well in shape, with the emission of S6 Tb illustrated above in Fig. 4(a). Note that, some emission interference from an Er impurity [32] originating from 2H11/2/4S3/2 - 4I15/2 at 550 nm to UPC emission of S6 Tb was safely removed at the used delay of 8 ms. Differences in the emission shapes of S6 Tb measured in down- conversion (Fig. 4(a)) and up-conversion excitation (Fig. 7(b)) are mostly due to higher temperature (300 K) used in UPC measurements though the occurrence of a S6 to C2 energy transfer cannot yet be discarded. Further concentration dependent emission measurements with Tb - Y2O3 are needed to decide between the two possibilities.
4. Conclusions
In summary, we present a first report and improved identification of the emission, excitation and decay properties of Tb, Dy and Pr at the inversion symmetry, S6 sites in Y2O3 by use of time-gated luminescence spectroscopy. A first observation of up-conversion emission in Sm doped Y2O3 and Eu /Tb, Yb –co- doped Y2O3 assignable to Sm and Eu/Tb at S6 sites is also presented. Our work represents an important advance in magnetic dipole luminescence of lanthanide doped Y2O3 that may be exploited further to optimize the applications of this important class of phosphor materials.
Acknowledgments
DA and CT acknowledge the Romanian National Authority for Scientific Research (CNCS-UEFISCDI) (project number PN-II-IDPCE-2011-3-0534) for the financial support. BC and MF acknowledge the Romanian National Authority for scientific Research, CNDI–UEFISCDI, for financial support through the project PCCA-II-56/2014.
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