**6. Photoluminescence using 355 nm wavelength**

**Figure 12.** Photoluminescence spectra of Tb3+ ions (A), Tb3+ ions with Bi nanoparticles (B) and the (Tb(Sal)<sup>3</sup>

(reproduced from Kaur et al. [24]).

added to TbCl<sup>3</sup>

140 Acrylic Polymers in Healthcare

and the (Tb(Sal)<sup>3</sup>

the <sup>5</sup> D4 →<sup>7</sup> F5

(Tb(Sal)<sup>3</sup>

photon excites the 5

Tb3+ ions emanating from 5

complex with Bi NPs (C) in PVA in the range of 375–700 nm using 266 nm radiation exciting the SPR band of Bi NPs

The excitation spectrum of Tb3+ ion in PVA sample also shows a few weak bands at 341, 352, 358, 369, 377, and 488 nm wavelengths corresponding to absorption of Tb3+ ions. When Bi NPs were

and laser-ablated Bi NPs in PVA depicts an extensive excitation band between 275 and 375 nm that may be attributed to n→π\* transition of the salicylate ligands. There seems to be appreciable enhancement in the intensity of the bands corresponding to the Tb3+ ion emission. This vividly signifies effectual sensitization of Tb3+ ions by the ligands pointing to a competent antenna effect [20].

The photoluminescence spectra depicts the emission bands of Tb3+ ions, Tb3+ ions with Bi NPs

The spectra of Tb3+ ions exhibit characteristic emission peaks at 487, 544, 583, and 618 nm for

was enhanced on incorporating Bi NPs, but the effect is more prominent in the case of the

The mechanism for augmentation of the emission intensity of the observed transitions may be elucidated with the help of a partial energy-level diagram showing different routes of excitation of Tb3+ ions, and the respective emissions are shown in **Figure 13**. Primarily, the 266 nm

> D3 and <sup>5</sup> D4

(Phen)) complex in PVA as in this case, the emission emanating from the 5

F6 →<sup>5</sup> H7

broad plasmonic band for Bi NPs at 285 nm. The excitation spectrum (Tb(Sal)<sup>3</sup>

with the SPR band of NPs using 266 nm radiation and is shown in **Figure 12**.

**5. Photoluminescence using 266 nm wavelength**

D4 →<sup>7</sup> FJ

level through the <sup>7</sup>

FJ

appear, which is an additional interesting feature.

H7

these level to lower lying levels (<sup>7</sup>

excited Tb3+ ions relax nonradiatively down to 5

, the excitation intensity of the bands got improved. There also appears a weak

(Phen)) complex with Bi NPs in PVA in the range of 375–700 nm on excitation

transition (544 nm) is the most intense one. The emission intensity of Tb3+ bands

(J = 6, 5, 4, 3) transitions, respectively, and among them,

; J = 1–6). This excitation radiation, i.e., 266 nm, moreover,

absorption transition of Tb3+ ions. Then the

levels to yield the emissions from

(Phen))

(Phen)) complex

D3

level also

The photoluminescence spectra of Tb3+ ion, Tb3+ ion with Bi NPs and (Tb(Sal)<sup>3</sup> (Phen)) complex with Bi NPs in PVA in the range between 375 and 700 nm using 355 nm excitation radiation, namely, nonresonant excitation is shown in **Figure 14**. The photoluminescence emission spectra is similar to the previous one, but the photoluminescence emission intensity for (Tb(Sal)<sup>3</sup> (Phen)) complex with Bi NPs is enhanced to a large extent in the present case. This improvement in the photoluminescence emission intensity can be understood by the following mechanism.

This nonresonant 355 nm excitation radiation excites equally the Tb3+ ion in addition to the Sal ligand to their excited states. It should be mentioned here that the Bi NPs do not absorb this wavelength. This incident excitation energy is directly absorbed by the 5 L9 level of Tb3+ ion. It

**Figure 14.** Photoluminescence spectra of Tb3+ ions (A), Tb3+ ions with Bi NPs (B), and (Tb(Sal)<sup>3</sup> (Phen)) complex with Bi NPs (C) in PVA in the range of 375–700 nm on excitation with 355 nm radiation (reproduced from Kaur et al. [24]).

then relaxes nonradiatively and populates the emitting 5 D4 level. Along with this, the optical energy absorbed by the Sal ligand is also transferred to the resonating Tb3+ ions populating the 5 D4 level via intersystem crossing and the consequent energy transfer process that is the reason for enhancing the emission intensity. Also, the Bi NPs form a local plasmonic field around the (Tb(Sal)<sup>3</sup> (Phen)) complex, and the high-field gradients of NPs increase the lifetime of the emitting 5 D4 level of Tb3+ ions [42]. The coupling between the radiative transitions, and the field effect is the fundamental basis for the enhancement in intensity as shown in the inset of **Figure 14**. The increase in the lifetime of the 5 D4 level of Tb3+ ion is clearly observed in the decay curves for the <sup>5</sup> D4 →<sup>7</sup> F5 transition of Tb3+ ions in the presence and absence of Bi NPs (as seen in **Figure 15**). Herein, the point to mention is that different transitions of Tb3+ ion respond differently to the

**Figure 15.** The decay curves for the 5 D4 →<sup>7</sup> F5 transition at 544 nm of Tb3+ ions with and without NPs and the (Tb(Sal)<sup>3</sup> (Phen)) complex with and without Bi NPs in PVA using 355 nm radiation (reproduced from Kaur et al. [24].

local field gradients of Bi NPs. The reason behind this may be that magnetically allowed dipole transitions differ in interaction with the surface plasmon field of NPs than the electrically allowed dipole transitions. Nevertheless, the photoluminescence emission intensity of Tb3+ ion on excitation with 355 nm nonresonant radiation is larger to a great extent than that of 266 nm resonant excitation. It undoubtedly reveals that the transfer of energy to Tb3+ ions through the salicylic acid ligand is more proficient as compared with other channels of energy transfer.
