*3.2.1. Enhancement of luminescence intensity by the persuade of alkali metal ions in Ca0.5R1-x(MoO4)2:xLn3+,M+*

In the phosphor material, by introducing the alkali metal chlorides, nitrates or fluorides, which substantially increase the luminescence intensity owing to the charge compensation effect between unequal ions [18]. On our previous work in Ca0.5R1*-x*(MoO4)2:*x*Ln3+ (R = Y, La) phosphor, the doping of alkali ions appreciably improves the emission properties by charge compensation using solid-state reaction method [15, 19].

Influence of Alkali Metal Ions on Luminescence Behaviour of Ca0.5R1-*x*(MoO4)2:*x*Ln3+ (R = Y, La)... http://dx.doi.org/10.5772/65006 39

**Figure 4.** The AFM images of Ca0.5La(MoO4) 2 :Eu3+ thin film (a) 3D surface topography, (b) 2D surface scan image, (c) line profile for vertical cross-section in 5 × 5 µm2 scan area and (d) histogram analysis.

**Figure 3.** The AFM images of Ca0.5Y(MoO4)2:Eu3+ thin film (a) 3D surface topography, (b) 2D surface scan image, (c) line

In the powder XRD pattern, all the peaks are indexed perfectly which indicates a pure tetragonal phase having scheelite crystal structure and the planes (1 0 1), (1 1 2), (0 0 4), (2 0 0), (2 0 4), (2 2 0), (1 1 6) and (1 3 2) are in well accordance with the JCPDS card no. 82-2369 of NaY(MoO4)2. No deleterious phases are found. The peak shift is not noticed with respect to

doping. An intense peak with plane (1 1 2) is found at 28.95° [2, 15, 17].

**3.2. Photoluminescence properties of laser-ablated thin phosphor films:**

38 Applications of Laser Ablation - Thin Film Deposition, Nanomaterial Synthesis and Surface Modification

*3.2.1. Enhancement of luminescence intensity by the persuade of alkali metal ions in*

compensation using solid-state reaction method [15, 19].

scan area and (d) histogram analysis.

 **(R = Y, La; Ln = Eu, Tb and Dy; M = Li, K and Na)**

In the phosphor material, by introducing the alkali metal chlorides, nitrates or fluorides, which substantially increase the luminescence intensity owing to the charge compensation effect between unequal ions [18]. On our previous work in Ca0.5R1*-x*(MoO4)2:*x*Ln3+ (R = Y, La) phosphor, the doping of alkali ions appreciably improves the emission properties by charge

profile for vertical cross-section in 3 × 3 µm2

**Ca0.5R1-***x***(MoO4)2:***x***Ln3+,M+**

*Ca0.5R1-x(MoO4)2:xLn3+,M+*

**Figure 5.** XRD patterns of (a) Ca0.5Y(MoO4)2:Ln3+,Na+ , (b) Ca0.5La(MoO4)2:Ln3+,Na+ (Ln = Eu, Tb and Dy).

In our system, Eu3+, Tb3+, Dy3+ and M+ co-doped in Ca0.5R(MoO4)2 (R = Y, La) matrix would induce a distortion in lattice, and consequently, the lattice symmetry is desperately lowered [20]. The co-doped Eu3+, Tb3+, Dy3+ and M+ at the Ca2+ sites in prepared thin film samples would play a role of dominance with enhanced luminescence intensity [21]. This is due to altering the symmetry and their surroundings in the locality of rare earth ions by adding the charge compensators of alkali metal ions [22]. **Figures 6** and **7** show the PL emission spectra of Ca0.5Y1-*x*(MoO4)2:*x*Ln3+,M+ (Ln = Eu, Tb and Dy; M = Li, K and Na) and Ca0.5La1-*x*(MoO4)2:*x*Ln3+, M+ (Ln = Eu, Tb and Dy; M = Li, K and Na) thin film phosphors. The luminescence emission intensity is deliberately increased for Na+ ion co-doped Ca0.5R1-*x*(MoO4)2:*x*Ln3+ (R = Y, La; Ln =  Eu, Tb and Dy). This could be owing to the charge compensation effect, and the proposed mechanism is Ca2+ → 2M+ (M = Li+ , K+ , Na+ ) [21, 22]. Furthermore, the ionic radius of Na+ (0.97  Å) is closer to the Ca2+ (1.12 Å) which is somewhat better than that of K+ (1.33 Å) and Li+ (0.59  Å) [3, 15]. Hence, there is an efficient replacement of Ca2+ ions by alkali metal ions. This forms the basis for the increased luminescence intensity, and obviously, Na+ is having the best charge compensation effect. The electronic configurations and its transitions are explained in the subsequent sections.

**Figure 6.** PL emission spectrum of the thin film phosphor Ca0.5Y(MoO4)2:Eu3+ co-doped with various alkali metal ions [Li+ (Green), K+ (Red) and Na+ (Blue)].

Influence of Alkali Metal Ions on Luminescence Behaviour of Ca0.5R1-*x*(MoO4)2:*x*Ln3+ (R = Y, La)... http://dx.doi.org/10.5772/65006 41

**Figure 7.** PL emission spectrum of the thin film phosphor Ca0.5La(MoO4)2:Eu3+ co-doped with various alkali metal ions [Li+ (Green), K+ (Red) and Na+ (Blue)].

### *3.2.2. Photoluminescence excitation studies*

In our system, Eu3+, Tb3+, Dy3+ and M+

[20]. The co-doped Eu3+, Tb3+, Dy3+ and M+

intensity is deliberately increased for Na+

(M = Li+

, K+ , Na+

Å) is closer to the Ca2+ (1.12 Å) which is somewhat better than that of K+

the basis for the increased luminescence intensity, and obviously, Na+

Ca0.5Y1-*x*(MoO4)2:*x*Ln3+,M+

mechanism is Ca2+ → 2M+

subsequent sections.

[Li+

(Green), K+

(Red) and Na+

(Blue)].

co-doped in Ca0.5R(MoO4)2 (R = Y, La) matrix would

at the Ca2+ sites in prepared thin film samples would

ion co-doped Ca0.5R1-*x*(MoO4)2:*x*Ln3+ (R = Y, La; Ln = 

(0.97 

(0.59 

(1.33 Å) and Li+

is having the best charge

) [21, 22]. Furthermore, the ionic radius of Na+

(Ln = Eu, Tb and Dy; M = Li, K and Na) and Ca0.5La1-*x*(MoO4)2:*x*Ln3+,

induce a distortion in lattice, and consequently, the lattice symmetry is desperately lowered

40 Applications of Laser Ablation - Thin Film Deposition, Nanomaterial Synthesis and Surface Modification

play a role of dominance with enhanced luminescence intensity [21]. This is due to altering the symmetry and their surroundings in the locality of rare earth ions by adding the charge compensators of alkali metal ions [22]. **Figures 6** and **7** show the PL emission spectra of

M+ (Ln = Eu, Tb and Dy; M = Li, K and Na) thin film phosphors. The luminescence emission

Eu, Tb and Dy). This could be owing to the charge compensation effect, and the proposed

Å) [3, 15]. Hence, there is an efficient replacement of Ca2+ ions by alkali metal ions. This forms

compensation effect. The electronic configurations and its transitions are explained in the

**Figure 6.** PL emission spectrum of the thin film phosphor Ca0.5Y(MoO4)2:Eu3+ co-doped with various alkali metal ions

#### *3.2.2.1. Ca0.5Y1-x(MoO4)2:xLn3+,Na+ (Ln = Eu, Tb and Dy)*

**Figure 8a** shows the room temperature PL excitation spectra of Ca0.5Y1-*x*(MoO4)2:*x*Ln3+ (Ln = Eu, Tb and Dy) thin film phosphors. The PL excitation spectrum of Ca0.5Y1-*x*(MoO4)2:*x*Eu3+,Na+ is having a wavelength range of 225–575 nm. It is showing two regions with a broad band and intense sharp peaks. The broad band is located from 225 to 350 nm with a centre at ∼306 nm attributed to the O2- to Eu3+ charge transfer band (CTB) and also designated as ligand-to-Eu3+ metal charge transfer transitions (LMCT) [23, 24]. Above 350 nm, intense sharp peaks are found at 362 nm ( 7 F0 → <sup>5</sup> D4), 382 nm ( 7 F0 → <sup>5</sup> L7), 395 nm ( 7 F0 → <sup>5</sup> L6), 416 nm ( 7 F0 → <sup>5</sup> D3), 465 nm ( 7 F0 → <sup>5</sup> D2) and 536 nm ( 7 F0 → <sup>5</sup> D1). Among which, the strong and intense peak is found at 395  nm. The characteristic configurations were attributed to the transition from the 7 F0 ground state of Eu3+ to the upper excited states (5 D1,2,3,4 and L6,7). This strongest peak in UV region is more suitable for exciting Eu3+ ions.

**Figure 8b** depicts the room temperature excitation spectrum of Ca0.5Y1-*x*(MoO4)2:*x*Tb3+,Na+ with a wavelength range of 270–390 nm in the UV region. It consists of two regions. One is from 270 to 360 nm, a highly intense and wide band designated as charge transfer band (CTB) having centred at ∼295 nm is ascribed to the charge transfer of molybdate host lattice [17]. The other is due to f-f transitions of Tb3+ and its peak is at 376 nm (7 F6 → <sup>5</sup> G6) which is less intense than CTB. The energy transfer is being occurred from 4f8 to 4f7 5d configuration of Tb3+ ions [17].

The excitation spectrum for the Ca0.5Y1-*x*(MoO4)2:*x*Dy3+,Na+ thin film phosphor is shown in **Figure 8c**. The wavelength of the excitation spectrum ranges from 240 to 480 nm. The strong broad band is ranging from 240 to 340 nm bears a centre at ∼270 nm. Above 340 nm, the numerous intense f-f transitions of Dy3+ ions are found. The f-f transitions are located at 353 nm (6 H15/2 → <sup>6</sup> P7/2), 367 nm (6 H15/2 → <sup>6</sup> P5/2), 388 nm (6 H15/2 → <sup>4</sup> I3/2), 425 nm (6 H15/2 → <sup>4</sup> G11/2), 453 nm (6 H15/2 → <sup>4</sup> I15/2) and 475 nm (6 H15/2 → <sup>4</sup> F9/2) [25]. The highly intense peak is found at 353 nm which is the best candidate for exciting Dy3+ ions.

**Figure 8.** PL excitation spectrum (a, b, and c) for the thin film phosphors Ca0.5Y(MoO4)2:Ln3+ (Ln = Eu, Tb and Dy).

#### *3.2.2.2. Ca0.5La1-x(MoO4)2:xLn3+,Na+ (Ln = Eu, Tb and Dy)*

The room temperature PL excitation spectra of Ca0.5La1-*x*(MoO4)2:*x*Ln3+ (Ln = Eu, Tb and Dy) thin phosphor films are illustrated in **Figure 9a**. The PL excitation spectrum of Ca0.5Y1 *x*(MoO4)2:*x*Eu3+,Na+ comprises of wavelength range of 200–550 nm. It consists of two regions with a wide band and highly intense sharp peaks. The wide band is found from 220 to 350 nm with a centre at ∼278 nm ascribed to the O2- to Eu3+ ligand-to-Eu3+ metal charge transfer transitions (LMCT) [19]. Above 350 nm, intense sharp peaks are found at 360 nm (7 F0 → <sup>5</sup> D4), 382 nm (7 F0 → <sup>5</sup> L7), 394 nm (7 F0 → <sup>5</sup> L6), 415 nm (7 F0 → <sup>5</sup> D3), 464 nm (7 F0 → <sup>5</sup> D2) and 535 nm (7 F0 → <sup>5</sup> D1). In that, the strongest and highly intense peak is found at 394 nm. This strongest peak in UV region is good for exciting Eu3+ ions.

Influence of Alkali Metal Ions on Luminescence Behaviour of Ca0.5R1-*x*(MoO4)2:*x*Ln3+ (R = Y, La)... http://dx.doi.org/10.5772/65006 43

**Figure 9.** PL excitation spectrum (a, b, and c) for the thin film phosphors Ca0.5La(MoO4)2:Ln3+ (Ln = Eu, Tb and Dy).

**Figure 9b** shows that the photoluminescence excitation spectrum of Ca0.5La1-*x*(MoO4)2:*x*Tb3+,Na+ possess a wavelength range of 220–420 nm in the UV region. Among the two regions, the broad region is from 220 to 340 nm, ascribed to charge transfer band (CTB) which is centred at ∼278  nm. The other sharp peaks are due to f-f transitions of Tb3+ with peaks at 369 nm (7 F6 → <sup>5</sup> G5) and 378 nm (7 F6 → <sup>5</sup> G6) is having lesser intensity than charge transfer band (CTB) [19].

The room temperature excitation spectrum for the Ca0.5La1-*x*(MoO4)2:*x*Dy3+,Na+ thin film phosphor is depicted in **Figure 9c**. The range of the excitation spectrum is from 240 to 440 nm. The broad band ranges from 240 to 340 nm bears a centre at ∼271 nm. After 340 nm, a number of highly intense f-f transitions of Dy3+ ions are located. The f-f transitions are found at 352 nm ( 6 H15/2 → <sup>6</sup> P7/2), 367 nm (6 H15/2 → <sup>6</sup> P5/2), 388 nm (6 H15/2 → <sup>4</sup> I3/2) and 428 nm (6 H15/2 → <sup>4</sup> G11/2) [19]. The most intense peak that is situated at ∼352 nm is fit for exciting Dy3+ ions.

### *3.2.3. Photoluminescence emission studies*

The excitation spectrum for the Ca0.5Y1-*x*(MoO4)2:*x*Dy3+,Na+

H15/2 → <sup>6</sup>

H15/2 → <sup>4</sup>

42 Applications of Laser Ablation - Thin Film Deposition, Nanomaterial Synthesis and Surface Modification

P7/2), 367 nm (6

*3.2.2.2. Ca0.5La1-x(MoO4)2:xLn3+,Na+*

*x*(MoO4)2:*x*Eu3+,Na+

F0 → <sup>5</sup>

L7), 394 nm (7

in UV region is good for exciting Eu3+ ions.

F0 → <sup>5</sup>

382 nm (7

→ <sup>5</sup>

I15/2) and 475 nm (6

which is the best candidate for exciting Dy3+ ions.

nm (6

nm (6

H15/2 → <sup>6</sup>

H15/2 → <sup>4</sup>

**Figure 8c**. The wavelength of the excitation spectrum ranges from 240 to 480 nm. The strong broad band is ranging from 240 to 340 nm bears a centre at ∼270 nm. Above 340 nm, the numerous intense f-f transitions of Dy3+ ions are found. The f-f transitions are located at 353

H15/2 → <sup>4</sup>

I3/2), 425 nm (6

F9/2) [25]. The highly intense peak is found at 353 nm

P5/2), 388 nm (6

**Figure 8.** PL excitation spectrum (a, b, and c) for the thin film phosphors Ca0.5Y(MoO4)2:Ln3+ (Ln = Eu, Tb and Dy).

 *(Ln = Eu, Tb and Dy)*

transitions (LMCT) [19]. Above 350 nm, intense sharp peaks are found at 360 nm (7

L6), 415 nm (7

The room temperature PL excitation spectra of Ca0.5La1-*x*(MoO4)2:*x*Ln3+ (Ln = Eu, Tb and Dy) thin phosphor films are illustrated in **Figure 9a**. The PL excitation spectrum of Ca0.5Y1-

with a wide band and highly intense sharp peaks. The wide band is found from 220 to 350 nm with a centre at ∼278 nm ascribed to the O2- to Eu3+ ligand-to-Eu3+ metal charge transfer

F0 → <sup>5</sup>

D1). In that, the strongest and highly intense peak is found at 394 nm. This strongest peak

comprises of wavelength range of 200–550 nm. It consists of two regions

D3), 464 nm (7

F0 → <sup>5</sup>

thin film phosphor is shown in

H15/2 → <sup>4</sup>

G11/2), 453

F0 → <sup>5</sup>

D2) and 535 nm (7

D4),

F0

#### *3.2.3.1. Ca0.5Y1-x(MoO4)2:xLn3+,Na+ (Ln = Eu, Tb and Dy)*

The room temperature PL emission spectra for Ca0.5Y1-*x*(MoO4)2:*x*Ln3+,Na+ (Ln = Eu, Tb and Dy) thin phosphor films are shown in **Figure 10a**. The emission spectra monitored at 395 nm UV excitation for Ca0.5Y1-*x*(MoO4)2:*x*Eu3+,Na+ illustrates a number of intra-configurational 4f-4f transitions arising from Eu3+ <sup>5</sup> D0 excited state to the 7 FJ (J = 1, 2, 3 and 4) ground states [26–28]. Upon other excitations and also with LMCT, there is no significant change in emission spectra. The strong and most intense emission peak is found at 616 nm upon 395 nm UV excitation is ascribed to the 5 D0 → <sup>7</sup> F2 electric-dipole transition depicts hypersensitive red emission which is parity forbidden (ΔJ = 2) [26]. Also, it shows two sub-peaks arises due to Stark energy splitting, that is (2J + 1) Stark components of J-degeneracy splitting [27]. The predominant electric-dipole transition signifies that Eu3+ ions are found at sites without inversion symmetry. The other transitions found at 587 nm (5 D0 → <sup>7</sup> F1) show orange emission owing to magneticdipole transition. The other relatively weaker transitions are located at 655 nm (5 D0 → <sup>7</sup> F3) and 702 nm (5 D0 → <sup>7</sup> F4) [28]. The red emission to orange emission (R/O) ratio for the thin film phosphor is 5.2536. From these findings, it is evident that Ca0.5Y1-*x*(MoO4)2:*x*Eu3+,Na+ possess scheelite tetragonal structure having C3v site symmetry could be used for display applications.

The PL emission spectrum for Ca0.5Y1-*x*(MoO4)2:*x*Tb3+,Na+ upon ∼295 nm UV excitation is shown in **Figure 10b**, comprises four PL emission bands having peaks at 489 nm (5 D4 → <sup>7</sup> F6), 545 nm (5 D4 → <sup>7</sup> F5), 587 nm (5 D4 → <sup>7</sup> F4) and 621 nm (5 D4 → <sup>7</sup> F3). In these emission peaks, the highly remarkable green colour is located at 545 nm related to the predominant transition 5 D4 → <sup>7</sup> F5 [27]. It is due to the energy transfer from the host which is populating only 5 D4 level. The dominant emission peak which is having two sub peaks is due to the Stark energy splitting and forms the suitable candidate for display applications.

**Figure 10.** PL emission spectra (a, b, and c) of the thin film phosphors Ca0.5Y(MoO4)2:Ln3+,M+ (Ln = Eu, Tb and Dy; M =  Na).

The room temperature PL emission spectrum (**Figure 10c**) for the thin film phosphor Ca0.5Y1-*x*(MoO4)2:*x*Dy3+,Na+ excited upon 353 nm excitation wavelength. The emission spectrum consists of two major peaks with respective peaks at 485 nm ascribed to magnetic dipole transition of (4 F9/2 → <sup>6</sup> H15/2) and 576 nm related to forced electric dipole transition of 4 F9/2 → <sup>6</sup> H13/2. The magnetic dipole transition is lesser sensitive to the coordination environment [27, 28]. The forced electric dipole transition is appeared only in the case of Dy3+ ions which are found at the local sites without inversion centre symmetry [28]. The respective blue emission having a centre at 485 nm is relatively lower intense than predominant yellow emission. The yellow-to-blue line ratio is 6.8026 which signifies that forced electric dipole transition is in dominance thereby indicating that the Dy3+ ions would found at the local sites with non-inversion centre symmetry in the Ca0.5Y1-*x*(MoO4)2 host.

#### *3.2.3.2. Ca0.5La1-x(MoO4)2:xLn3+,Na+ (Ln = Eu, Tb and Dy)*

The strong and most intense emission peak is found at 616 nm upon 395 nm UV excitation is

is parity forbidden (ΔJ = 2) [26]. Also, it shows two sub-peaks arises due to Stark energy splitting, that is (2J + 1) Stark components of J-degeneracy splitting [27]. The predominant electric-dipole transition signifies that Eu3+ ions are found at sites without inversion symmetry.

phosphor is 5.2536. From these findings, it is evident that Ca0.5Y1-*x*(MoO4)2:*x*Eu3+,Na+ possess scheelite tetragonal structure having C3v site symmetry could be used for display applications.

The PL emission spectrum for Ca0.5Y1-*x*(MoO4)2:*x*Tb3+,Na+ upon ∼295 nm UV excitation is

F4) and 621 nm (5

highly remarkable green colour is located at 545 nm related to the predominant transition 5

dominant emission peak which is having two sub peaks is due to the Stark energy splitting

shown in **Figure 10b**, comprises four PL emission bands having peaks at 489 nm (5

F5 [27]. It is due to the energy transfer from the host which is populating only 5

D4 → <sup>7</sup>

**Figure 10.** PL emission spectra (a, b, and c) of the thin film phosphors Ca0.5Y(MoO4)2:Ln3+,M+

and forms the suitable candidate for display applications.

D0 → <sup>7</sup>

dipole transition. The other relatively weaker transitions are located at 655 nm (5

44 Applications of Laser Ablation - Thin Film Deposition, Nanomaterial Synthesis and Surface Modification

F2 electric-dipole transition depicts hypersensitive red emission which

F4) [28]. The red emission to orange emission (R/O) ratio for the thin film

D4 → <sup>7</sup>

F1) show orange emission owing to magnetic-

D0 → <sup>7</sup>

F3). In these emission peaks, the

F3) and

D4 → <sup>7</sup>

D4 level. The

(Ln = Eu, Tb and Dy; M = 

F6),

D4

ascribed to the 5

702 nm (5

545 nm (5

→ <sup>7</sup>

Na).

D0 → <sup>7</sup>

D4 → <sup>7</sup>

D0 → <sup>7</sup>

The other transitions found at 587 nm (5

F5), 587 nm (5

The room temperature PL excitation spectrum of Ca0.5La1-*x*(MoO4)2:*x*Eu3+ thin phosphor films monitored at 394 nm excitation wavelength is shown in **Figure 11a**, consists of numerous intraconfigurational 4f-4f transitions. As the Eu3+ concentration in the Ca0.5La(MoO4)2 host lattices increases, the photoluminescence emission of the host is suppressed due to the overcoming of Eu3+ ions. The intensities of the different <sup>5</sup> D0 → <sup>7</sup> FJ transitions might depend on the local symmetry of crystal field of Eu3+ ions [19]. The highly intense and strong emission peak is located at 615 nm upon 394 nm UV excitation corresponds to the 5 D0 → <sup>7</sup> F2 electric-dipole transition showing hypersensitive red emission with parity forbidden (ΔJ = 2). The split-up in the peaks is due to the Stark energy splitting, which is having (2J + 1) Stark components of Jdegeneracy splitting [28]. The predominant electric-dipole transition implies that Eu3+ ions would be situated at sites with non-inversion symmetry [15]. The transitions located at 586 nm show emission in the orange region which is associated with magnetic-dipole transition (5 D0 → <sup>7</sup> F1) and is not affected by the chemical surroundings of Eu3+. The remaining transitions at 654 nm (5 D0 → <sup>7</sup> F3) and 701 nm (5 D0 → <sup>7</sup> F4) are the weakest ones. The red-to-orange emission (R/O) ratio of the phosphor is 5.5311 which suggests the sites symmetry of the respective Eu3+ ions. Based on these observations, it is suggested that Ca0.5La1-*x*(MoO4)2:*x*Eu3+,Na+ might be a suitable phosphor candidate for solid-state lighting applications.

The PL emission spectrum for Ca0.5La1-*x*(MoO4)2:*x*Tb3+,Na+ recorded at ∼278 nm UV excitation is shown in **Figure 11b** which consists of four PL emission peaks at 489 nm (5 D4 → <sup>7</sup> F6), 545 nm (5 D4 → <sup>7</sup> F5), 585 nm (5 D4 → <sup>7</sup> F4) and 621 nm (5 D4 → <sup>7</sup> F3). Among these emission peaks, the sensitive green colour is located at 545 nm associated to the predominant transition 5 D4 → <sup>7</sup> F5 [19]. It is based on the energy transfer from the host populates only 5 D4 level. The dominant emission peak possesses two sub peaks which correspond to the Stark energy splitting.

The room temperature PL emission spectrum (**Figure 11c**) for the thin phosphor film Ca0.5La1 *x*(MoO4)2:*x*Dy3+,Na+ excited with 352 nm excitation wavelength. The emission spectrum comprises of two major peaks with peak positions at 477 nm attributed to magnetic dipole transition of (4 F9/2 → <sup>6</sup> H15/2) and 570 nm corresponds to forced electric dipole transition of 4 F9/2 → <sup>6</sup> H13/2 [25]. The magnetic dipole transition is least sensitive to the coordination environment. The forced electric dipole transition would be found only in the case of Dy3+ ions situated at the local sites without inversion symmetry [19]. The emission in the blue region having a centre at 477 nm is having relatively least intense than predominant yellow emission. The yellow-toblue line ratio is 6.0076 which implies that the forced electric dipole transition is most dominant hence indicating that the Dy3+ ions are situated at the local sites with non-inversion symmetry in the Ca0.5La1-*x*(MoO4)2 host.

**Figure 11.** PL emission spectra (a, b, and c) of the thin film phosphors Ca0.5La(MoO4)2:Ln3+,M+ (Ln = Eu, Tb and Dy; M =  Na).

### **3.3. Photometric characterization and decay-time analysis**

**Figures 12a**, **b** and **13a**, **b** show the decay time profile and Commission Internationale de I'Eclairage (CIE) colour chromaticity coordinates of the Ca0.5Y1-*x*(MoO4)2:*x*Ln3+,Na+ (Ln = Eu, Tb and Dy) and Ca0.5La1-*x*(MoO4)2:*x*Ln3+,Na+ (Ln = Eu, Tb and Dy) phosphors. The CIE colour chromaticity coordinate of these phosphors was estimated and is given in **Table 1**. Furthermore, to know about the characteristic emission of these phosphors, the value of colour purity was derived by the equation [24]

$$\text{Colorparity} = \frac{\sqrt{\left(x - x\_l\right)^2 + \left(y - y\_l\right)^2}}{\sqrt{\left(x\_d - x\_l\right)^2 + \left(y\_d - y\_l\right)^2}} 100 \tag{1}$$

where (*x*, *y*) is denoted as CIE chromaticity coordinate of the synthesized sample, (*xi* , *yi* ) is the CIE white illumination, and (xd, *yd*) is the CIE chromaticity coordinate of the dominant wavelength. Thus, the colour purities of the Ca0.5Y1-*x*(MoO4)2:*x*Eu3+,Na+ , Ca0.5Y1-*x*(MoO4)2:*x*Tb3+,Na+ and Ca0.5Y1-*x*(MoO4)2:*x*Dy3+,Na+ phosphors are 90.0, 87.5 and 81.3%, respectively. Similarly, the colour purities of the Ca0.5La1-*x*(MoO4)2:*x*Eu3+,Na+ Ca0.5La1-*x*(MoO4)2:*x*Tb3+,Na+ and Ca0.5La1 *<sup>x</sup>*(MoO4)2:*x*Dy3+,Na+ phosphors are 95.0, 91.9 and 81.7%. From the results, it is suggested that these phosphors with remarkable CIE chromaticity coordinate with high colour purities might be suitable for applications in display devices as the best red, green and yellow emitting phosphors.

the local sites without inversion symmetry [19]. The emission in the blue region having a centre at 477 nm is having relatively least intense than predominant yellow emission. The yellow-toblue line ratio is 6.0076 which implies that the forced electric dipole transition is most dominant hence indicating that the Dy3+ ions are situated at the local sites with non-inversion symmetry

46 Applications of Laser Ablation - Thin Film Deposition, Nanomaterial Synthesis and Surface Modification

**Figure 11.** PL emission spectra (a, b, and c) of the thin film phosphors Ca0.5La(MoO4)2:Ln3+,M+

**Figures 12a**, **b** and **13a**, **b** show the decay time profile and Commission Internationale de I'Eclairage (CIE) colour chromaticity coordinates of the Ca0.5Y1-*x*(MoO4)2:*x*Ln3+,Na+ (Ln = Eu, Tb and Dy) and Ca0.5La1-*x*(MoO4)2:*x*Ln3+,Na+ (Ln = Eu, Tb and Dy) phosphors. The CIE colour chromaticity coordinate of these phosphors was estimated and is given in **Table 1**. Furthermore, to know about the characteristic emission of these phosphors, the value of colour purity

( ) ( )

*xx yy*

2 2 Colourpurity <sup>100</sup> *i i*


2 2

( ) ( )


*xx yy*

*di di*

**3.3. Photometric characterization and decay-time analysis**

was derived by the equation [24]

(Ln = Eu, Tb and Dy; M = 

(1)

in the Ca0.5La1-*x*(MoO4)2 host.

Na).

**Figure 12.** Luminescence decay profiles (a and b) for the thin film phosphors Ca0.5R1-*x*(MoO4)2:*x*Ln3+ (R3+ = Y, La),Na+ (Ln = Eu, Tb and Dy).

**Figure 13.** CIE diagram (a, b) for the thin film phosphors Ca0.5Y(MoO4)2:Ln3+,Na+ (Ln = (A) Eu, (B) Tb and (C) Dy).

The representative PL decay curves for luminescence emission for the phosphors Ca0.5Y1 *x*(MoO4)2:*x*Ln3+,Na+ (Ln = Eu, Tb and Dy) and Ca0.5La1-*x*(MoO4)2:*x*Ln3+,Na+ (Ln = Eu, Tb and Dy) are shown in **Figure 12a**, **b**. This can be fitted well into a single exponential function [15, 27] as

$$I = I\_0 \exp\left(\frac{-t}{\tau}\right) \tag{2}$$

where *I*0 is the luminescence intensity at times *t* = 0 and *τ* is its associated luminescence lifetime. The photometric quantities and luminescence decay time values are given in **Table 1**.


**Table 1.** Photometric parameters, color purity and luminescence decay time for the phosphors Ca0.5R1-*x*(MoO4)2:*x*Ln3+, Na+ (R = Y, La; Ln = Eu, Tb and Dy).

### **3.4. Photoluminescence emission studies from nano-architectures**

The thin phosphor films grown from nano-powder are being synthesized by the hydrothermal method, and the synthesis procedure is described previously by our group [17]. The luminescence emission intensity is being enhanced by co-doping of alkali metal ions. Furthermore, for the co-doping of alkali precursors, instead of using alkali chloride, alkali carbonates were taken and converted them into alkali nitrates. These alkali nitrates were co-doped with the existing precursors following the hydrothermal method nano-powders were synthesized and thin films were deposited from these powders [17]. The room temperature PL emission spectrum for Ca0.5R1-*x*(MoO4)2:*x*Eu3+,Na+ (R = Y, La) as the representative thin phosphor films are shown in **Figure 14**. The emission spectra monitored at 395 nm UV excitation for both the phosphors shows a number of intra-configurational ff transitions. The strong and most intense emission peak is found at 616 nm for Ca0.5Y1 *x*(MoO4)2:*x*Eu3+,Na+ and 615 nm for Ca0.5La1-*x*(MoO4)2:*x*Eu3+,Na+ is attributed to the 5 D0 → <sup>7</sup> F2 electric-dipole transition possess hypersensitive red emission [15, 28, 29]. The Stark energy splitting is mildly shown for both the phosphors. It is noticed that the splitting of the electric dipole transition is uniform and homogeneous between the two thin phosphor films. The intensity of the spectral peaks for the nano-thin phosphor film is nearly close to that of those from the bulk thin phosphor film, as peak intensity is related to reduced particle size and improved homogeneity [17, 29]. The dominant electric-dipole transition suggests that Eu3+ ions are found at sites with non-inversion symmetry [30]. The other transitions found at 587 nm ( 5 D0 → <sup>7</sup> F1) for Ca0.5Y1-*x*(MoO4)2:*x*Eu3+,Na+ and 586 nm ( 5 D0 → 7 F1) for Ca0.5La1-*x*(MoO4)2:*x*Eu3+,Na+ show orange emission due to magnetic-dipole transition. The other relatively weaker transitions are found at 654 nm (5 D0 → <sup>7</sup> F3) and 702 nm (5 D0 → <sup>7</sup> F4) for both the nano-thin phosphor films. The photometric parameters for both the phosphors are under further investigation. From these results, it is indicated that Ca0.5R1-*x*(MoO4)2:*x*Eu3+,Na+ (R = Y, La) phosphors are best candidates for display applications.

**Figure 14.** PL emission spectrum of the thin film phosphors Ca0.5Y(MoO4)2:Eu3+,Na+ and Ca0.5La(MoO4)2:Eu3+,Na+ prepared from nano-phosphors.
