Concentration and Cyrstalline Phase Effects on the Spectroscopic Properties of Sol ‐ Gel Synthesized Er 3 + : Y 2 Si 2 O 7

Nanosized yttrium disilicate powders activated with trivalent erbium ions were produced by Sol-Gel known as wet chemical process. The structure and morphology of the synthesized powders were characterized by using X-ray diffraction spectroscopy (XRD), Transmission electron microscopy (TEM) and Photoluminescence spectroscopy. The XRD analysis revealed that the formation of triclinic  a-Y 2 Si 2 O 7 and monoclinic  b-Y 2 Si 2 O 7 polymorphs were obtained at 1050  and 1450 , respectively. The photoluminescence properties were also investigated in terms of sintering temperature and doping effect on different polymorphs.

Yttrium disilicate (Y2Si2O7) is one of the binary disilicates with high thermal and chemical stability (Melting point : 1775 ), low dielectric constant, low linear coefficient of thermal expansion and low thermal conductivity. It shows y, α, β, γ, and δ phases because of complex polymorphism resulting in comparative broad emission [11][12]. It is very difficult to obtain a pure phase of yttrium disilicate because of its intricate polymorphous structure.
In our former studies, we have discussed the structural and spectroscopic properties of Y2Si2O7 : Nd and Y2Si2O7 : Yb phosphors [26,27] and the upconversion properties of Y2Si2O7 : Yb, Er nanophosphors obtained by sol-gel technique [28].
In the present study structural characterization and the spectroscopic properties of Er 3+ doped nanocrystalline yttrium disilicate samples fabricated using sol-gel method are reported depend upon cyrstalline phase properties. Due to the efficient luminescence properties at the telecommunication wavelength of 1.54 μm, Erbium doped materials have received considerable interest in optical amplifiers and silicon photonics [15,29].

Material and Method
Er 3+ : Y2Si2O7 nanopowders were synthesized in the phase diagram of SiO2-Y2O3 binary system using the sol-gel method to obtain two different phases. TEOS (Si (OC2H5)4) with 99.9% purity, yttrium nitrate (Y (NO3) ⋅6H2O), and erbium nitrate (Er (NO3) ⋅5H2O) salts with 99.9% purity were used to produce the nanopowders. Detail explanation of the fabrication process could be reached in our recent study [28]. The fabricated samples were then annealed at 1050 o C and 1450 o C for 12 hours to produce nanocrystalline Y2Si2O7 powders. Three different samples were synthesized with 0.5, 1.0, 1.5 mol % ratios for Er 3+ ions; they were labeled as YSE1, YSE2, YSE3, respectively.
The structural properties of two powders were conducted using X-ray diffraction spectroscopy (Model of Rigaku-XRD 2200 D/MAX) with the Cu-Kα source operated with the wavelength at λ = 1.5418 Å.
Slit systems, step-size (0.02°), source voltage (40 kV), and current (30 mA) were kept constant during the scans that were conducted in θ-2θ coupled mode. The morphological properties of the powders were also investigated by TEM with model number JEOL JEM-2100.
The photoluminescence (PL) spectrums of all powders were conducted using a diode laser (Laser Drive Inc.-LDI-820) with a wavelength of 800 nm. The emissions resulting from the dopant ions of the powders were transmitted to a Monochromotor (McPherson Inc. Model 2051) and measured by an InGaAs semiconductor detector (Princeton Inc. Model ID-441-C) with a preamplifier (Stanford Res. Model SR560). A short wavelength pass filter (800 nm) was placed infront of the monochromator to eliminate the scattering lights in the measurements.
The decay measurements of the dopant ions in the powders under pulsed excitation were taken by using the titanium sapphire laser (Model of Schwarz Electro-Optics Inc. Titan-P) and a digital oscilloscope (Model of Tektronix-TDS3052B). All spectral output of the nanopowders was recorded at room temperature.

X-ray diffraction analysis
The phase evolution of the phosphors heat-treated at different temperatures was determined using X-ray diffraction analysis. The XRD patterns of the Er 3+ doped Y2Si2O7 (YSE1, YSE2 and YSE3) samples calcined at two different temperatures are shown in Figure 1. It can be seen in Figure 1a, 1b that -Y2Si2O7 (JCPDS file no. 38-0223) is the dominant (main) phase for the Er 3+ doped powders calcined at 1050 . Annealing the samples to 1450 resulted in the growth of the -Y2Si2O7 crystalline phase which is well consistent with the JCPDS file no. 21-1454. We observed also the presence of the SiO2 (JCPDS file no. 39-1425) peak located at 2 22° (indicated with an asterix * in Figure 1b). Similar results have been recently reported by Becerro et al and Diaz et al. [30,31]. The structure of the other phases and the intensity of the peaks do not change as a function of Er 3+ doping concentration, indicating that the doping ions do not contribute to the formation of new phases.
The polymorphs of yttrium disilicates transform from one phase into another with increasing temperature [32]. However, phase transition temperatures vary considerably in the studies of different researchers [17,18,33,34]. This demonstrates that the yttrium disilicate structure is strongly dependent on the synthesis methods, annealing temperatures and pressure.

Transmission electron microscopy results
Morphological properties and the particle size distribution of calcined powders were identified with transmission electron microscope. Figure 2 illustrates the TEM images of 1.0% Er 3+ doped yttrium disilicate samples (YSE2) annealed at 1050 and 1450 . The yttrium disilicate powders have regular crystalline form and the particle size of crystallites increase with the increasing annealing temperatures. The average size of nano particles estimated from TEM images is about ~100 nm for -Y2Si2O7, and 0.5 µm for β-Y2Si2O7.  Figure 3 shows the spectral output of photoluminescence measurement of Er 3+ :Y2Si2O7 nanopowders in the 1425-1675 nm wavelength range at room temperature when excited with diode laser operating at 800 nm. The spectral intensity of the lines and band broadening of the spectral output decreased with the increase of Er 3+ mole concentration (Fig. 3a, 3b). The sample doped with 1.0 mole % Er 3+ ions portrayed higher luminescence intensity than the other concentrations of Er 3+ ions for each phase. The change in intensity with Er 3+ concentration could be on account of the interaction among the Er 3+ ions causing the energy transfer mechanisms.

Photoluminescence (PL) properties
Each spectrum contains an emission band with the strongest emission peak at about 1550 nm which correspond to the transition from the 4 I13/2 to the ground level ( 4 I15/2), within the 4f shell of the Er 3+ ions. The J-manifolds of Er 3+ ions are split due to the low symmetry (C1 or C2) of the Y2Si2O7 cation sites [25]. We observed that the spectral form and the count of the Stark components showed similar behavior for each Er 3+ ions transitions in the powders at each phase. This phenomenon can be ascribed to the crystal field effect of ligand ions surrounding the Er 3+ ions due to the same crystalline phase.  Figure 3c presents the comparison of emission bands for α and β-Er 3+ :Y2Si2O7 1% phases. The spectral output of Er 3+ emission shows strong dependency of the crystalline phase.
All these results can be attributed to the varied numbers of Y 3+ (Er 3+ ) sites in the structures. In βphase has one Y 3+ site with C2 symmetry and in αphase there are four different Y 3+ sites with C1 symmetry. Er 3+ ions can substitute in four different crystallographic sites of Y 3+ ions in α-phase and emission from these four sites can occur in the widening of the emission band. Thus, the spectral output of the emission indicates a more significant difference of the silicate crystalline phase for Er 3+ ions [25]. The decay curves of the 4 I13/2 level have been measured for all samples excitation wavelength of 798 nm at room temperature. Figure 4 represents the decay plots in semi-log scale for three powders. The decay time of the 4 I13/2 emission decreases with increasing Er 3+ mole concentration in the crystalline powders (Fig. 4a, 4b). The comparison of the PL decay curves of YSE2 sample in the three crystalline phases has been shown in Fig. 4c. All these decay curves of and β-phases have similar lifetime values.

Conclusion
-Y2Si2O7 and -Y2Si2O7 phosphors were successfully synthesized by sol-gel route using yttrium nitrate, erbium nitrate salts, TEOS (Si (OC2H5)4) and hydrochloric acid as a catalyst for the hydrolysis of TEOS as the starting materials. The structure and the phase purity of the powders are determined by XRD and TEM analysis. and β-phases of crystalline Er 3+ :Y2Si2O7 phosphors were obtained via changing thermal treatment process.
The influences of cyristalline phase on the spectral profile of Er 3+ :Y2Si2O7 phosphors were studied in detail. Although doping of rare earth ion amounts do not contribute to the phase evolution of the powders, it is observed that the spectroscopic properties depend strongly on phase properties of Y2Si2O7. We also observed that Y2Si2O7 : Er 3+ 1.0 mol % showed higher PL intensity for each phase than the other concentrations of Er 3+ ions.