Synthesis and Characterization of a Novel Sm+3 Activated NaCdVO4 Phosphors for Red Emitting Material

Sm 3+ activated NaCdVO 4 phosphors were prepared by the simple solid-state reaction method. X-ray diffraction, dispersive energy (EDS), scanning electron microscope (SEM), infrared as well as photoluminescence (PL) techniques were used to characterize obtained samples. Irregular and non-uniform structures were observed by SEM. EDS spectra confirmed the presence of Na, Cd, V, O and Sm elements in each sample. Uuder 405nm excitation, the NaCd 1-x VO 4 : xSm (x=0.01, 0.03 and 0.05) exhibits a bright red emission consisting mainly of four wavelength peaks at 556, 593, 650 and 700 nm. The highest emission intensity was found with a composition of x=0.05. The analysis of PL spectra suggest that studied samples can be used as a red emitting phosphors candidates for fabrication of white LEDs. The CIE chromaticity coordinates of prepared samples were close to the blue-emitting phosphors for NaCdVO 4 and red-emitting ones for NaCd 0.99 Sm 0.01 VO 4 , NaCd 0.97 Sm 0.03 VO 4 , and NaCd 0.95 Sm 0.05 VO 4 . The band gap energies of phosphors were calculated from reflectance data using K-M function.


Introduction
In recent years, phosphors-transformed LEDs present the most promising solid-state light source compared to conventional incandescent and fluorescent lamps due to their long lifetime, high luminescence efficiency as well as environmental friendly characteristics [1][2][3][4][5]. Vanadate compounds containing [VO 4 ] 3present a significant family of luminophores. They show wide and intense charge transfer absorption bands in the UV region and visible emission bands in the 400-700nm region due to the O 2--V 5+ charge transfer transition [6][7][8][9][10]. Moreover, the energy transfers from [VO 4 ] 3groups to the dopants Sm 3+ ion through O 2--V 5+ charge transfer [11]. Therefore, intense luminescence characteristics are expected in rare earth ion activated vanadates. Sm 3+ as an important member of RE ions is an excellent activator to generate red-orange or red emissions due to intrinsic 4f-4f transitions [12][13][14].]. Nevertheless, much effort still should be dedicated to RE ion activated vanadates to further improve their luminescence properties.
In present work, we chose as a luminescent host, the NaCdVO 4 which has a space group of Cmcm and a lattice parameter varying with concentration Sm 3+ . Sm 3+ -activated NaCdVO 4 phosphors were successfully prepared by conventional solid-solid technique.
The structure is determined by X-ray (XRD) and confirmed by FTIR and EDX. The morphology is by SEM. Photoluminescence (PL) allow us to found the optimal concentration of dopant corresponding to the maximum of emission in visible region. The package CIE give us the chromaticity coordinates and diffusive reflectance is used in order to obtain the energy gap.

Experimental method
The powdered samples with general chemical formula NaCd (1-3/2x) Sm x VO 4 (x=0.01, 0.03, 0.05) were successfully synthesized via a typical solid state reaction method at high temperature of 873K according the following chemical path: The precursor materials are NaVO 3 , CdO, and Sm 2 O 3 of high purity. These materials are mixed in stoichiometric amounts in the first step. The obtained powders are pressed into pellets.
The pellets are annealed at a temperature of 873K for 12h and cooled down to the room temperature.

Samples characterization:
X-ray powder diffraction was used in order to check of the phases purity of prepared compounds using X'Pert PROMPD diffactometer equipped with Cu-K radiation (λ Cu =1.5046 Å). The intensity data was recorded by continuous scan in 2/ mode from 10 to 60° with a step (2) = 0.017°.
The elemental composition of samples was carried out by analyzing the energy dispersive with an X-ray detector attached to the Zeiss-SEM instrument and the average size of grains was estimated.
The Infrared spectra was recorded at room temperature with a Perkin Elmer spectrophotometer in the wave number range 50-1100cm -1 . The PL spectra were measured by Leica spectrophotometer equipped with a Xe lamp as excitation source.
UV-Vis spectra of investigated compounds were carried out at room temperature using UV-3101PC scanning spectrophotometer (Integrated Sphere) in the range of wavelength (200-800) nm.

Results and discussions a. X-ray analysis
The XRD patterns of NaCdVO 4 doped Sm 3+ ions are shown in Figure1. The obtained patterns were in close agreement with the orthorhombic structure belonging to space group Cmcm. XRD profiles of all specimens match well with our previous work [15] It was observed that Sm 3+ doping NaCdVO 4 did not influence the main structure of the host because no significant shift of observed peaks in series of doped samples since the doping with Sm 3+ which it's ionic radii r(Sm 3+ , CN=6)=1.09 Å [16] into the Cd 2+ ( [r(Cd 3+ , CN=6)=0.95 Å] site is assumed owing to the small radius percentage difference Dr of both ions. In fact, the percentage difference in ionic radii between substituted and doped ions should be less than 30%. the equation below can calculate the difference between Cd 2+ and the doped rare ions (Sm 3+ ) in radius percentage where Dr is the radius percentage difference; CN is the coordination number; Rm(CN) is the radius of the host cation; and Rd(CN) is the radius of the doped ion. The calculated value of Dr is about 14%.
The unit cell parameter for each sample has been refined by the least square method from powder data and summarized in the table 1.

b. EDS and SE
In order to check of the elemental composition, a chemical analysis was performed on the different samples. Figure2 (a, b and [17][18][19].
The analysis of the IR spectra shows the formation of a unique phase with a good agreement with XRD results.

d. Luminescence properties
The PL emission spectra of NaCdVO 4 :Sm 3+ phosphor obtained under 405nm excitation is done in Figure 5. The emission band observed around 474nm is due to the dominating charge transfer of [VO 4 ] 3group [20]. It is also observed that each emission spectrum of different concentration exhibits four emission bands in the visible region around 556nm, 593nm, 650nm and another small peak around 700nm which are ascribed to the electronic transitions of Sm 3+ from the excited state 4 G 5/2 to the lower energy levels 6 H 5/2 , 6 H 7/2, 6 H 9/2 , and 6 H 11/2 respectively [21].
The green 4 G 5/2 → 6 H 5/2 (556nm) allowed transition is purely magnetic dipole (MD) and its intensity is influenced by the environment around the Sm 3+ ions in the host lattices, while 4 G 5/2 → 6 H 9/2 (640nm) allowed transition is purely electric dipole (ED). The strongest peak at 593 nm ( 4 G 5/2 → 6 H 7/2 ) is symmetry sensitive transition implicating that Sm +3 ions occupies simultaneously symmetry sites of the NaCdVO 4 host lattice. In addition, the shape and intensity of this peak indicate that Sm 3+ is an excellent candidate for NaCdVO 4 doping under 405nm excitation wavelength.
In order to better explain the luminescence mechanism of Sm 3+ in the NaCdVO 4 host, a simplify energy level diagram based on selection rules is displayed in Figure 6. The Sm 3+ ions are firstly excited under 405nm after that due to the fast transition the 4 G 5/2 level is populated. Then, the emissions of Sm 3+ ion will be achieved because of the following radiative transitions, 4 G 5/2 → 6 H 5/2 at 546nm, 4 G 5/2 → 6 H 7/2 at 593nm, 4 G 5/2 → 6 H 9/2 at 640nm and 4 G 5/2 → 6 H 9/2 at 700nm.
The luminescence emission intensity of Rare earth ion-activated phosphors can be in general affected by the dopant concentration. As described in figure 5, all the samples exhibit the same characteristics of Sm 3+ emissions as the PL profiles except the intensity.
In fact, the emission intensity increased with the increase in concentration and reached a quenching concentration at x=0.05. This concentration is mainly due, according to the Van Uitert, to two different types of mechanism, namely exchange and electric multipolemultipole interactions [22]. The interaction mechanism can be identified through analyzing the critical distance between samarium and samarium ions.
The critical distance defined by using the following formula of Blasse's [23]: Where R c is the critical distance of Sm concentration, V is the volume of the unit cell, Z is and X c is the number of cations.
For NaCd 0.95 Sm 0.05 VO 4 , V=374.174Å 3 , X c =0.05, and Z=4. The R c was determined to be 15.28 Å which is larger than 5Å. Thus, the concentration quenching mechanism for Sm 3+ in the NaCdVO 4 :xSm 3+ is dominated by multipole-multipole interaction.
On the basis of Dexter's energy transfer expression for multipole interaction, the relation between emission intensity and activator concentration can be achieved, as defined below [24,25]: Ln(x) (Figure 7), the Q value was found to be 6.54. This result suggest that the energy transfer mechanism for the concentration quenching in NaCdVO 4 :Sm 3+ was dipole-dipole interaction.
The CIE chromaticity coordinates are important for the performance of phosphors and have been calculated to estimate the luminous color of the phosphor material. CIE is the standard reference for defining colors and is obtained by considering the sensitivity of the human eye to different colors. In the order to determine the band gap, diffuse reflectance spectra were converted using the Kubelka-Munk function expressed by [26]: Where F(R) is the Kubelka-Munk function and t (mm) is the thickness of pellets.
The energy band gap E g of material is related to the absorption coefficient α by the following Tauc equation as [26]: Where m is equals to 2 for an indirect transition and 1/2 for a direct transition. A Tauc plots were plotted between (h) 2 , (h) 1/2 and h for each sample. The shapes of these curves favor the direct transition for all compositions. The value of direct band gap Eg was obtained from the extrapolating of the linear fitted region at (h)=0 in the plot of (h) 2 versus h as shown in Figure 9. The band gap energy showing an increase with of the increase of doping concentration from 3.57eV for NaCd 0.99 Sm 0.01 VO 4 to 3.69eV for NaCd 0.97 Sm 0.03 VO 4 then decreasing to 3.5eV for NaCd 0.95 Sm 0.05 VO 4 .

Conclusions
Sm 3+ doped NaCdVO 4 phosphors doped with varying concentration were prepared by a simple solid-state reaction technique. The samples were found to crystallize in the orthorhombic system with Cmcm space group. The EDS spectra confirmed the presence of Sm 3+ in all compositions. The average particle sizes were estimated in the region of 1-8mµ.
Upon 405nm excitation, the characteristic Sm 3+ -activated NaCdVO 4 were observed and the Intensity showed a maximum of emission when Sm 3+ ion content was above 5%. It was found that the concentration quenching mechanism occurred as a result of dipoledipole interaction according to Dexter's theory. The CIE chromaticity coordinates of the obtained phosphors suggest that can be used as red-emitting phosphor for white LEDs applications. Figure 1: XRD patterns of NaCdVO4:xSm phosphors   Size distribution histograms of NaCdVO4:xSm phosphors Figure 4 IR spectra of NaCdVO4:xSm phosphors Room-temperature PL emission spectra of the as a function of wavelength under 405nm excitation Re ectance spectra of NaCdVO4:xSm phosphors Figure 10 Plots of (αhv)1/2 and (αhv )2 versus (hv ) of NaCdVO4:xSm phosphors