3.1 Phase structure
XRD patterns of (BaBi0.01)TiO3 and (Ba1-xBi0.01)TiO3 doped Dy ions are calcined at 1200°C (Fig.1). The phase pure and rare earth composition of the as-synthesized (Ba1-xBi0.01)TiO3:Dy3+(x=0,0.01,0.03,0.05 and 0.07) phosphors producing peaks are indicated in the XRD pattern. The sharp and intense diffraction peaks formed seem to be in good agreement with the patterns confirming the sample materials consign well with the pure Barium Titanate phase. These phases are consistent with the standard JCPDS file no. 89-2475. The synthesized phosphors of the cubic phase perovskite structure with lattice parameters belong to a=b=c=4.017 and Pm mm space group. The BaTiO3 perovskite structure is designed into a framework of corner-linked TiO6 octahedra with centrally occupied Ba2+ ions [12, 13]. These phases are indexed with miller indices (hkl) and compared with the standard JCPDS data as depicted in Fig.1. The maximum intensity is recorded for (110) plane that is in close agreement with JCPDS: 89-2475. The radii of Dy3+ ions are similar to those of Ba2+ ions present in the host, thus enhancing it to occupy the position of Ba2+ ions present in the host. No other crystalline phase has been detected, indicating that all the prepared samples are in single phase state and there appears to be no impact of activator (Dy3+) and co-activator (Bi2+) in the phase of BaTiO3.This signifies that the pure and rare earth doped (Ba1-xBix0.01)TiO3 composite are fruitfully prepared by this simple solid state method and the Dy3+ ions have been efficiently occupied in the (Ba1-xBi0.01)TiO3 host by substitution process.
3.2 Absorption spectrum
The optical absorption spectrum of (Ba1-xBi0.01)TiO3 doped Dy3+ phosphors is recorded for the wavelength range 200-700 nm and represented in Fig. 2. The (Ba1-xBi0.01)TiO3:xDy3+ shows thick absorption band in UV–Vis region due to host absorption and also accounts for the charge transfer band between Dy3+ ions and O2—ions . The charge transfer phenomenon is effected as the consequence of O- 2p electron transition to 4f orbital of Dy3+ ion the empty shell . This can be achieved on exciting the electrons from the ground level to the excited level. An absorption shoulder is presented in the UV–Vis region and its location displays tiny changes due to their variation in the doping concentration. This indicates the Dy3+ ions can be incorporated into the host lattice without change in the structure of the synthesized compounds.
3.3 Morphology structure of (Ba1-xBi0.01)TiO3:0.03Dy3+ phosphors
SEM micrographs of (Ba1-xBi0.01)TiO3:Dy3+ phosphors shown in Fig.3. Display the SEM photographs at three different resolutions. The obtained micrographs show agglomerated and irregular morphology for x=0.03. The elemental analysis on the synthesized products is performed using energy dispersive X-ray analysis (EDX) technique. EDX mapping for the elemental distribution of Ba, Ti, Dy, Bi and O has revealed that Bi2O3 and Dy2O3 are highly dispersed in the BaTiO3 matrix.
3.4 Photoluminescence analysis
Photoluminescence spectra of (Ba1-xBi0.01)TiO3:Dy3+(x=0.01, 0.03, 0.05 and 0.07) phosphors at room temperature is shown in Fig. 4. The photoluminescence spectra of (Ba1-xBi0.01) TiO3:xDy3+ excited under the 387 nm wavelength. On increasing the Dy3+ content up to x=0.05, the emission intensity is enhanced to the maximum value that diminishes on any further increase in the concentration of Dy3+ions. This indicates the quenching effect concentration of Dy3+ ions as well as its dependence on the concentration of rare earth element, which further reduces the emission intensity. Such an experience may be attributed to one' of the Quantum Mechanic rules that the location of rare earth ion at a high centosymmetric rate forbids the f-f levels of electric dipole transition [16,17]. In the photoluminescence spectra, broad peak can be seen in the yellow light emission region centred at 572 nm corresponding to the 4F9/2 to 6H13/2 transition of Dy3+ ions [18, 19]. Such a forced electric dipole transition is allowed only at low symmetries without any inversion centres in the host crystal structure, suggestive of dominant occupation of Dy3+ at Ba2+ sites. Thus, the optimal doping concentration of Dy3+ in the (Ba1-xBi0.01)TiO3 host is facilitated to gain the strongest PL emission intensity, which is obtained at x=0.05.
3.5 Energy Level diagram
The energy level diagram of Dy3+ ions depicted in Fig. 5 explains luminescence mechanism of synthesized phosphors. This phosphor is special of its kind as the yellow emission obtained is of high intensity with appropriate doping concentration of Dy3+ ions. It is as well conjectured that yellow emissions is suitable for preparing white luminescence with blue LED chip. The emission of white light can be achieved from three different processes: selection of a suitable host composition, changing the concentration of Dy3+ ions and, co-doping with appropriate sensitizer . The present work attempts to produce high intensity yellow emission suitable for preparing efficient white LED with blue LEDs chip. The phosphor materials get excited from the ground state (6H13/2) to excited state of (4F7/2) . After excitation, the incited electrons relax to the lower energy level by non-radiative (NR) transition lifted from the 4F7/2 state to 4F9/2. In non-radiative transition process, the energy is carried in the form of vibrations or motion of the electron without the luminescence process . On reaching 4F9/2 state the transfer to the ground state occurs by releasing the characteristic emission spectra from the excited state of 4F9/2 to ground state of 6H13/2 .The radiative mechanism is held responsible for the high intensity yellow spectra of Dy3+ions.
3.6 Decay time study
In order to confirm the ET between the Dy3+ ions, the luminescence decay profiles are assessed (Fig. 6) The decay time is recorded by exciting the sample material with 387 nm wavelength xenon flash lamp. The monitored emission at 572 nm (4F9/2 to 6 H13/2) reveals that the decay curves seem suitable for bi-exponential fitting :
I(t) refers to Infer luminescence intensity, A1 and A2 indicate the amplitudes and τ1 & τ2 signify the decay times of the prepared sample. The Decay time of the synthesized phosphors can be evaluated from the following equation:
The decay time range of the (Ba1-xBi0.01)TiO3:Dy3+ phosphors is 0.04, 0.05, 0.02 and 0.03 ms under 387 nm and 572 nm wavelength for different doping concentrations of Dy ions in the host.
3.7 Photometric analysis:
The stalwart in illumination, Commission Internationale de l’Eclairage (CIE) has chalked out certain chromaticity colour coordinate parameters and colour correlated temperature (CCT) to study the features of the prepared phosphor compound. The chromaticity colour illustration of Dy3+doped (Ba1-xBi0.01)TiO3 phosphor material with varying concentration is explained in Fig.7. The CIE chromaticity coordinates of the synthesized phosphor calculated from the emission spectrum lie in the yellow region. Fig. 7 illustrates the chromaticity image of (Ba1-xBi0.01)TiO3 doped Dy3+ for various doping concentrations. It is clearly noticed that by adjusting the concentration of Dy3+ ions, the photoluminescence colour is altered in the yellow region. It produces the CIE co-ordinates at x=0.487, y=0.511 & x=0.488, y=0.522 & x=0.477, y=0.510 & x=0.475, y=0.508 resulting in the concentrations, 0.01,0.03,0.05 and 0.07 respectively. These colour co-ordinates imply the presence of luminescence in the yellow region and its suitability for generating efficient WLEDs with blue LED chip.
Besides, the studied sample is subjected to CCT investigations and the results are evaluated from the renowned McCamy's relation from the given formula :
n is equated for x-0.332/y-0.186 and x,y stands for colour coordinates of the present sample. The correlated colour temperatures (3081K, 3068K, 3134K and 3146 K) are recorded from the chromaticity colour coordinates by altering the concentration of Dy3+ ions. The above referred CCT, 3134 K reads the maximum concentration of Dy3+ generating efficacious yellow emission best suited for LED applications. Also, the CCT lower than 4,000 K is noteworthy for fashionable indoor lighting purposes .