**5. Photoluminescence using 266 nm wavelength**

The photoluminescence spectra depicts the emission bands of Tb3+ ions, Tb3+ ions with Bi NPs and the (Tb(Sal)<sup>3</sup> (Phen)) complex with Bi NPs in PVA in the range of 375–700 nm on excitation with the SPR band of NPs using 266 nm radiation and is shown in **Figure 12**.

The spectra of Tb3+ ions exhibit characteristic emission peaks at 487, 544, 583, and 618 nm for Tb3+ ions emanating from 5 D4 →<sup>7</sup> FJ (J = 6, 5, 4, 3) transitions, respectively, and among them, the <sup>5</sup> D4 →<sup>7</sup> F5 transition (544 nm) is the most intense one. The emission intensity of Tb3+ bands was enhanced on incorporating Bi NPs, but the effect is more prominent in the case of the (Tb(Sal)<sup>3</sup> (Phen)) complex in PVA as in this case, the emission emanating from the 5 D3 level also appear, which is an additional interesting feature.

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 photon excites the 5 H7 level through the <sup>7</sup> F6 →<sup>5</sup> H7 absorption transition of Tb3+ ions. Then the excited Tb3+ ions relax nonradiatively down to 5 D3 and <sup>5</sup> D4 levels to yield the emissions from these level to lower lying levels (<sup>7</sup> FJ ; J = 1–6). This excitation radiation, i.e., 266 nm, moreover,

**Figure 12.** Photoluminescence spectra of Tb3+ ions (A), Tb3+ ions with Bi nanoparticles (B) and the (Tb(Sal)<sup>3</sup> (Phen)) 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 (reproduced from Kaur et al. [24]).

Spectroscopic Investigations on Polyvinyl Alcohol Film with Complex of Terbium Ions along... http://dx.doi.org/10.5772/intechopen.70274 141

**Figure 13.** Partial energy-level diagram showing different routes of excitation of Tb3+ ions and the respective emissions exciting the SPR band of Bi NPs and the Sal ligand, respectively, on excitation with 266 and 355 nm radiations for improved green emission. The inset to the figure depicts Tb3+ ions, Bi NPs, and (Tb(Sal)<sup>3</sup> (Phen)) complex in PVA host presenting the plasmonic and the field effect to boost the emission of Tb3+ ion. SPR refers to surface plasmon resonance, LFE stands for local field effect, and ET represents energy transfer (reproduced from Kaur et al. [24]).

excites the surface plasmon band of Bi NPs. This engrossed excitation energy from the surface plasmon band of Bi NPs is transferred to Tb3+ ions. It further improves the build up of population of higher 5 D3 and <sup>5</sup> D4 levels of Tb3+ ion. This is cause for enhancement in the photoluminescence emission intensity. The great absorption cross-section of the bismuth plasmon band grounds for an amplified excitation of 5 D4 level of Tb3+ ion by means of energy transfer from the excited surface plasmon resonance band of Bi NPs. It is noteworthy to mention that the ligand Sal does not absorb at 266 nm radiation as no energy level of Sal exists at this energy. It merely encapsulates the Tb3+ ion to cut it off from the host vibrations and intensifies the emission from the complexed Tb3+ ion, thus resulting in the emergence of emission from the 5 D3 level.
