Low temperature micro Raman and laser induced upconversion and downconversion spectra of europium doped silver tungstate Ag_2−3xEu_xWO₄ nanorods

Figure 1 presents the X-ray powder diffraction (XRD) patterns of α-(Ag_2−3xEu_x)WO₄ samples. All of the peaks observed in the patterns can be indexed to the pure orthorhombic phase of α-Ag₂WO₄ and are in good accordance with PDF no 34-0061. These results indicate that Eu³⁺ cations have been introduced efficiently into the α-Ag₂WO₄ lattice until 4 mol% Eu³⁺. For 8 mol% Eu³⁺ it is possible to observe small peaks related to Silver Europium Tungsten Oxide (PDF no 60-0818) at 27.80°, 28.5° and 29.8° indicated with “*” in Fig. 1. This is evidence of saturation of the α-Ag₂WO₄ orthorhombic structure by the Eu³⁺ cations.

The substitution of monovalent Ag⁺ cations by trivalent Eu³⁺ cations will cause strain in the lattice. This is because for every Eu³⁺ cation occupying an Ag⁺ cation site two more Ag⁺ cations have been displaced from the lattice leaving two cation vacancies, \({\text{Ag}}^{ + }_{\text{v}}\) this maintains charge compensation in the lattice but both the vacancies and the 3⁺ charged cations cause strain as can be seen from the fact that as the proportion of the Eu³⁺ cations is increased the Eu³⁺ cations are rejected from occupying the vacant sites. This means that in the α-(Ag_2−3xEu_x)WO₄ structure x cannot be tolerated much above 0.04 Mol%. So, in this lattice charge compensation can only be tolerated to a small extent. In fact at 0.04mol% Eu³⁺ 0.12mol% of Ag+ cations have been removed (that is around one in eight of the Ag⁺ cations present. Though the ionic radius of Ag⁺ is 1.22 Å, and that of Eu³⁺ is 1.087 Å the latter will tend to distort the crystal site normally occupied by the former because of its greater charge [42‐44]. The synthetic conditions would not favour Eu²⁺ formation (reducing conditions would be required), if Eu²⁺ was present it would be observed as a broad peak in the photoluminescence spectrum.

Structural refinement data of the α-(Ag_2−3xEu_x)WO₄ (x = 0.0–0.08) samples confirm the orthorhombic structure with a Pn2n symmetry space group. The experimental lattice parameters and unit cell volumes calculated using the Rietveld refinement method are shown in Table 1. This table shows low deviations of statistical parameters (weighted profile R-factor (R_wp), expected R-factor (R_exp), residual of least-squares refinement (R_p) and reflection intensity-based R-factors (R_Bragg) which suggests that the refinement results are quite reliable and with good numerical results. These results were obtained by keeping the occupancy of W⁶⁺ fixed to 1 while Eu³⁺ cations were set to share occupancies in the Ag⁺ sites.

The size distribution and morphology of the crystals are important parameters in both luminous flux and their ordering on a surface (for device application). For many applications it would be ideal to have uniform sub-micron phosphor particles without any reduction in the luminous efficacy (compared to larger micron sized particles.

In Fig. 2a–d FESEM micrographs of the samples are presented. It is possible to see clear boundaries between the crystals indicating that there is no strong adhesion among the crystals. Moreover, there are in all cases many crystals of similar size and shape showing no evidence of aggregation, nor formation of larger structures. Most of the crystals are smooth surfaced nanorods of around 100 nm in width and 1–2 µm in length. The nanorod morphology should facilitate preferential alignment, dense packing and directional emission characteristics.

The Raman spectra of the phosphors may be useful for obtaining information about parameters which affect the luminescence efficiency. These parameters include crystallographic phases, lattice phonon energies, and the site symmetries of activators [45]. As α-Ag₂WO₄ belongs to the C _2v ¹⁰ point group and has two molecular formulas per unit cell (Z = 2). Group theory predicts that there are 21 different Raman modes [46], of these 14 modes have previously been identified [21].

Figures 3 and 4 present the room temperature Raman spectra of the α-(Ag_2−3xEu_x)WO₄ (x = 0–0.08 mol) samples excited at 532 and 633 nm, respectively. These spectra are very similar to those reported and assigned for the α-Ag_2−3xEu_xWO₄ (x = 0.0–0.01 mol) phosphor samples [21]. The differences in the Raman spectra presented in Figs. 3 and 4 and those presented in reference 21 are due to the presence of some luminescence (emission bands from the larger concentrations of Eu³⁺ present in the samples studied in this work, we will present evidence for this in later Figures and discuss this more below. The Raman modes and their respectively assignments are shown in the Figures. The Raman modes below 251 cm⁻¹ are related to the translational lattice vibrations of Ag⁺ and W⁶⁺, mainly contributed by the motion of heavy Ag⁺ ions. The out of plane bending modes of WOOW are observed at 306 cm⁻¹. The modes at 340 and 367 cm⁻¹ are due to the bending vibrations of O–W–O, WOOW, and W–O–W. The modes at 489 and 510 cm⁻¹ are related to the out of plane wagging of WOOW. The mode at 589 cm⁻¹ is the bending vibrations of W–O–W and WOOW. The mode at 666 cm⁻¹ is due to the stretching modes of WOOW. The modes at 749 and 779 cm⁻¹ result from the symmetric stretching modes of WOOW and W–O. The mode at 805 cm⁻¹ arises from the asymmetric stretching modes of W–O–W and W–O and the mode observed at 883 cm⁻¹ is from the symmetric stretching of W–O [47].

The main features observed are for samples with x > 2 mol% (λ_exc = 532 nm), in which the mode A_1g at 883 cm⁻¹ appears to shifts to higher wavenumber because of the this is because of an underlying emission band that gets stronger and causes the apparent movement of the Raman band as the Eu²⁺ concentration increases in the α-Ag₂WO₄ lattice. These underlying emission bands also affect the spectra collected with the 633 nm laser line (Fig. 4), saturation also causes broadening of the bands as observed on the spectra excited at 633 nm. However in the latter case these emission bands are less intense and so only a broadening is observed that becomes a shoulder at the higher Eu³⁺ concentrations.

In Figs. 5 and 6 the room temperature Anti-Stokes and Stokes luminescence and Raman spectra of the α-(Ag_2−3xEu_x)WO₄ (x = 0–0.08 mol%) samples excited at 532 and 633 nm are presented. In the case of the spectra excited at 633 nm (Fig. 6) the emission bands at higher energy are observed due to a combination of the energy of the laser line plus the thermal energy of the electrons in the ⁷F_j states [48]. For the 532 nm excited spectra the higher energy emission bands may arise from a similar process or alternatively (but less likely) they may be excited by a two photon upconversion process. The strongest Raman modes typical of α-Ag₂WO₄ are seen on both anti-Stokes and Stokes sides of the spectra. The characteristic emission bands seen in Figs. 5 and 6 at around 591, 616, 652 and 702 nm are due to the ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂, ⁵D₀ → ⁷F₃ and ⁵D₀ → ⁷F₄ transitions, respectively [48].

The temperature dependence of the Raman spectra of α-(Ag_1.97Eu_0.01)WO₄ excited at 633 nm is presented in Fig. 7. These spectra show that as the temperature decreases, improvement of the Raman modes definition occurs due to less lattice vibrations (thermal energy) at lower temperatures depopulating the luminescence spectra [48]. Thus the broad emission bands seen below 550 cm⁻¹ in the 20 and 10 °C spectra in Fig. 5 are not present in the −50 °C spectrum.

The temperature dependence of the Stokes and anti-Stokes emission spectra of α-(Ag_1.95Eu_0.01)WO₄ excited at 633 nm is presented in Fig. 8. These spectra indicate that the intensities of all of the anti-Stokes and Stokes luminescence lines manifest temperature-dependent behavior (losing intensity as the temperature decreases) in line with the fact that less energy is available to thermally populate the ⁷F₀, ⁷F₁, ⁷F₂, ⁷F₃ and ⁷F₄ states of Eu³⁺ at the lower temperatures [48]. It is apparent that as the temperature decreases to −190 °C the only remaining bands on the low energy side in Fig. 6 are the Raman bands. It is apparent from Fig. 8 that the emission bands are no longer present at −100 °C.