Dazzling green emission from graphene oxide nanosheet-embedded co-doped Ce³⁺ and Tb³⁺:PVA polymer nanocomposites for photonic applications

Abstract

Novel dazzling green emission has been obtained from graphene oxide nanosheet-embedded Ce³⁺ and Tb³⁺:PVA polymer nanocomposites under UV excitation. We have successfully synthesized Tb³⁺:PVA, Ce³⁺ + Tb³⁺:PVA and Ce³⁺ + Tb³⁺ + graphene oxide nanosheet (GO NS):PVA polymer films by the solution casting method. For these polymer films, their XRD and FTIR spectral profiles have been analyzed for structure details and ion–polymer interaction mechanism. Tb³⁺ doped at different concentrations in the PVA polymer films displayed a green emission at 546 nm (⁵D₄ → ⁷F₅) and under 370 nm (⁷F₆ → ⁵L₁₀) excitation. Upon co-doping with Ce³⁺ to the Tb³⁺:PVA polymer film, it exhibits enriched green emission compared to the single Tb³⁺:PVA under the same excitation due to energy transfer from Ce³⁺ to Tb³⁺. Surprisingly a common excitation band has been found at 370 nm for Tb³⁺, Ce³⁺ and the GO NS. After good dispersion of the GO NS in the co-doped Ce³⁺ + Tb³⁺:PVA polymer films, the photoluminescence properties are remarkably enhanced and a prominent green emission is observed at 370 nm. The green emission of Tb³⁺ is significantly enhanced though an efficient energy transfer process from Ce³⁺ to Tb³⁺ and the GO NS to Tb³⁺. A possible energy transfer mechanism is clearly demonstrated by several fluorescence methods and lifetime decay dynamics. From these results, these GO NS-embedded Ce³⁺ + Tb³⁺:PVA polymer films could be suggested as promising candidates for green luminescent materials for photonic devices.

---

1. Introduction

A great effort has been devoted to modulate the properties of polymer-based photonic devices for their use in novel photonic systems. Rare earth metal-doped polymer-based devices have several advantages over inorganic devices in the device design and fabrication such as being light weight, having design flexibility, their shape, and ease of fabrication.¹ In recent years, special attention has been focused on the luminescent rare earth ion-doped complexes for their superior properties such as narrow emission bands for high color purity, large Stokes shifts, long lifetime, high quantum efficiency and good processing ability. The luminescent rare earth complexes have widely been considered as light-converting molecular devices (LCMDs) and these could open up a new class of materials in the field of photonics.² Rare earth ion-incorporated polymer composites are expected to be versatile integrated functional devices, such as waveguide amplifiers, optical sensors, electroluminescent displays, light-emitting diodes, polymer fiber lasers and compact lasers that constitute a major part of the ever-growing field of photonics.³ Polyvinyl alcohol (PVA) polymer films have been identified as a potential host matrix for rare earth ions, nanoparticles and dyes which can be used for a wide range of applications in holography, image storage, laser applications, display applications, optical sensors and photovoltaic cells.⁴ The PVA polymer material is identified as a promising material which readily accepts rare earth ions for their use in several applications. Co-doped polymer systems have shown a significant sensitivity enhancement over a single-doped polymer matrix. The fluorescence efficiency of rare earth complexes, which play a pivotal role in the field of photonic devices, may be improved by a variety of methods. Several methods have been used to enhance the luminescence efficiency of rare earth ions by increasing the dopant concentration. But it does not prosper at a higher concentration due to aggregation of the ions taking place and the aggregation acts as quenching centers.⁵ To overcome this concentration quenching effect over luminescence efficiency, the addition of a secondary suitable rare earth ion to the polymer complex is being employed. The fluorescence efficiency of the primary rare earth ion is significantly enhanced with the addition of the secondary suitable rare earth ion by an energy transfer process in the polymer system. In this co-doped system, the primary rare earth ion acts as an activator and the secondary rare earth ion acts as a sensitizer. The energy transfer from the sensitizer to the activator may occur, in principle, in whole or partly non-radiative and/or radiative processes. Such energy transfer processes have been attracting significant attention for their practical utility in optical devices.⁶ In order to increase the emission performance of activator ions, we generally increase the concentration of sensitizer ions in the polymer matrix. However, the activator emission intensities might be decreased at a higher concentration of sensitizer ions in the co-doped system. However, this could not be occurred due to quenching centers; it needs to observed as ionic aggregation. These aggregation portions act as quenching centers. In our present work, Tb³⁺ acts as an active rare earth ion and it is activated by Ce³⁺ ions initially. The sensitizing ability will be saturated at a particular optimized concentration of the sensitizer, and it would be declared based on the emission performances of the activator in the co-doped system. For a further increase in the luminescence efficiency of the activator, we can choose another option which is the addition of a tertiary rare earth ion or nanofiller to the co-doped polymer system. The role of a nanofiller in the co-doped polymer films is crucial as it serves to enhance the luminescence efficiency.⁷ Homogeneous dispersion of the nanofiller in the polymer matrix could become a new field of research for several advanced applications. Several research efforts have been made towards the improvement of the luminescence efficiency while retaining the mechanical properties and stability towards metallic anodes by the incorporation of nano-size fillers. The graphene family group has attracted attention as a promising candidate to create new polymer nanocomposites for improving polymer properties due to their excellent characteristics such as an extremely high aspect ratio, high conductivity, unique graphitized planar structure and low manufacturing costs.⁸ Incorporation of graphene oxide in to the polymer matrix enhances the optical and electrical properties of the polymer complex. Among the various graphene-based materials, graphene oxide has potential applications for further functionalization due to its unique chemical structure. Graphene oxide (GO) consists of two-dimensional sheets of covalently-bonded carbon atoms bearing various oxygen functional groups on the basal planes and edges. Graphene oxide substituted with oxygen-containing functional groups forms individually dispersed single layers when assisted by sonication in water. Modified graphene oxide functional groups enhance the polymer compatibility, reinforcing graphene oxide in polymer composites. The meritorious results of the graphene oxide-based polymer nanocomposites have been reported in previous work.⁹ Based on these properties, we have introduced graphene oxide nanostructures into the polymer matrix. Moreover, an interesting strategy exists here as the common excitation for Ce³⁺ and graphene oxide nanostructures is found to be 370 nm. By using this excitation wavelength, we can excite the Tb³⁺ ions for enhancement of photoluminescence efficiency of green emission. In our present study, graphene oxide nanosheets (GO NSs) are synthesized by the sol–gel method. The prepared graphene oxide nanosheets are dispersed in co-doped Ce³⁺ + Tb³⁺:PVA blended polymer films at different concentrations to obtain further enhancement of the photoluminescence efficiency of Tb³⁺ by an energy transfer process. The energy transfer phenomenon has been demonstrated by the lifetime decay dynamics.

2. Experimental studies

2.1 Synthesis of graphene oxide nanosheets (GO NSs)

Initially graphite oxide was prepared using the Hummers method.¹⁰ Graphite powder was purchased from Sigma-Aldrich, USA. 3.0 g of graphite powder was added into 150 mL of concentrated H₂SO₄ at 0 °C in an ice bath and stirred for 1 h. Subsequently, 12.5 g of KMnO₄ and NaNO₃ were infused in the mixture at a temperature of <10 °C for 4 h and stirred for 1 h at 35 °C. 100 mL of triple-distilled water and 40 mL of 30% H₂O₂ for dilution were poured sequentially into the mixture and stirred for 15 h. The precipitate was washed thoroughly with 1 L of 5% HCl and 4 L of H₂O. The product (graphite oxide) was dried at 60 °C for 24 h. 0.5 g of the graphite oxide thus prepared was placed into 300 mL of ethylene glycol and stirred for 1 h, followed by ultrasonication for 12 h. As a reducing agent, 0.5 mL of hydrazine hydrate was added drop-wise into the mixture and stirred for 30 min. The final mixture was treated thermally at 180 °C for 16 h in an autoclave. The resulting precipitate was washed 5 times with H₂O and 3 times with ethanol, and later dried at 60 °C.¹¹ The final product grapheme oxide nanosheets (GO NSs) were taken for measurement.

2.2 Preparation of GO NS-embedded Ce³⁺ + Tb³⁺:PVA polymer nanocomposite films

Polymer films were prepared using the solution casting method. The chemicals used were polyvinyl alcohol (PVA) (MW = 186 × 10³) from Sigma-Aldrich Company. Here, we have used the precursor polymer material which has a high molecular weight for better properties, especially in view of its good thermal stability and durability. Films (thickness ∼ 100 μm) of PVA polymers doped with Tb(NO₃)₃·5H₂O and Ce(NO₃)₃·6H₂O were taken in an appropriate weight percentage ratio using triple-distilled water as the solvent. PVA and these rare earth nitrates were dissolved in triple-distilled water along with the synthesized GO NSs in different concentrations (0.005, 0.01, 0.015, 0.02 and 0.025 wt%) and stirred at room temperature (∼30 °C) for 10–12 h to get a homogeneous mixture. In order to make a homogeneous mixture of the polymer nanocomposites, a high intensity ultrasonic processor (750 W, Probe type sonicator-Vibra Cell-VC 750, Cole-Parmer, USA) was used. Its working conditions were at 20 kHz with a 6.5 mm microtip, applying an amplitude of 30% of the maximum power supplied by the instrument corresponding to 20 W and pulsed cycles of 5 s on and 5 s off for 15 min. Due to applying a low power in the order of 20 W, the polymer chains could not be affected. The solution was cast onto polypropylene dishes and those solutions were allowed to evaporate slowly at room temperature. The final product was dried upon warming to remove all traces of the solvent. The dried nanosheet-embedded rare earth ion co-doped polymer nanocomposite films were collected from the polypropylene dishes and stored in a dry vacuum box.

2.3 Characterization

XRD spectra of the rare earth metal-doped PVA polymer with and without GO NSs were recorded with SEIFERT 303 TT X-ray diffraction (XRD) with Cu Kα radiation (1.5405 Å), and it was operated at 40 kV voltage and 50 mA anode current at an angle ranging from 10° to 90°, with a scan speed of 10° min⁻¹. Phillips TECHNAI FE 12 transmission electron microscopy (TEM) was used for particle/sheet size and shape confirmation. FTIR spectra of the RE³⁺:PVA films and RE³⁺ + GO NS:PVA films were carried out on an EO-SXB IR spectrometer in the range of 530–​4000 cm⁻¹. The absorption spectra of the RE³⁺:PVA films and RE³⁺ + GO NS:PVA films were measured on a Scinco absorption spectrophotometer in the range of 215–650 nm. The TG measurements for the pure PVA, RE³⁺:PVA and RE³⁺ + GO NS:PVA polymer films were carried out using thermogravimetric analysis and differential scanning calorimetry (model: SDTQ600TA Instrument, specimens were scanned in nitrogen atmosphere from 30–60° at a heating rate of 10 °C min⁻¹) from room temperature to 700 °C. The photoluminescence (excitation and emission) spectra of the RE³⁺:PVA film and RE³⁺ + GO NS:PVA films were recorded on a Scinco FluoroMate FS-2 visible fluorescence spectrometer with a Xe arc lamp of 150 W power as an excitation source for studying the state emission spectra.

3. Results and discussion

3.1 XRD analysis

The XRD pattern of the synthesized pure GO NS is shown in Fig. 1(f). A broad diffraction peak at a 2θ value of 24.1° corresponding to the (002) plane of GO NS is observed.¹² This means that the conjugated GO is formed successfully in the form of thin layered nanosheets which consist of an inter-planar distance of 0.31 nm. X-ray diffraction investigations are carried out to infer the microstructural changes that take place as well as to characterize the molecular structure of the various polymer composites. The diffraction patterns of pure PVA, singly rare earth ion-doped PVA, and GO NS-embedded dual rare earth ion (Ce³⁺, Tb³⁺)-doped PVA polymer nanocomposite films are presented in Fig. 1(a–e). The pure PVA polymer film exhibits an intense peak at 19.3°, a weak shoulder at 22.2° and a low intense shallow peak at 40.3°. These two peaks are characteristic of PVA, representing reflections from the (101), (200) and (111) planes of the monoclinic unit cell respectively (JCPDS: 53-1587).^13,14 Upon addition of the external dopant ions (Ce³⁺, Tb³⁺) into the PVA polymer matrix, the characteristic XRD peak intensity is reduced and becomes broadened. This variation in intensity suggests that strong interactions between the polymer chains and the dopant ions may exist and that there is disruption of the semi-crystalline phase of the PVA bonding scheme.¹⁵

Fig. 1 XRD profiles of pure (a) PVA, (b) Ce³⁺:PVA, (c) Tb³⁺:PVA, (d) Tb³⁺ + Ce³⁺:PVA, (e) Tb³⁺ + Ce³⁺ + GO NS (0.015 wt%):PVA polymer films and (f) GO NS.

3.2 TEM analysis

The TEM image of the pure GO NS after synthesis is shown in Fig. 2(a) and the GO NSs dispersed in the co-doped Ce³⁺ + Tb³⁺:PVA polymer nanocomposites in different concentrations are shown in Fig. 2(b–f). Conjugated GO NSs are prepared and it is confirmed through TEM analysis also as shown in Fig. 2(a). After proper functionalization of the GO NSs, the GO NSs are added to the co-doped PVA polymer matrix in different concentrations. At higher concentration of the GO NSs, agglomeration of the NSs has been noticed in the co-doped PVA polymer matrix as shown in Fig. 2(f).

Fig. 2 TEM images of (a) the synthesized pure GO NS, and (b) 0.005, (c) 0.01, (d) 0.015, (e) 0.02, and (f) 0.025 wt% dispersed GO NS Tb³⁺ (0.15 wt%) + Ce³⁺ (0.1 wt%):PVA polymer nanocomposite films.

3.3 FTIR analysis

Fourier transform infrared (FTIR) spectroscopy is a versatile tool to confirm the chemical structure of the composite films, existence of the possible interactions between host and dopant ions, and formation of the complex. FTIR spectra of the pure PVA, PVA:Ce³⁺, PVA:Tb³⁺, PVA:Ce³⁺ + Tb³⁺, and GO NS-embedded PVA:Ce³⁺ + Tb³⁺ polymer matrix are studied in the range 4000–530 cm⁻¹ as shown in Fig. 3. A broad peak centered at 3287 cm⁻¹ is observed. This peak is attributed to symmetric stretching vibrations of O–H from the intramolecular and intermolecular hydrogen bonds in PVA. As a rule of thumb, the stretching vibrational peak –OH is sensitive to hydrogen bonding. The bands observed at 2847 cm⁻¹ and 2917 cm⁻¹ are assigned to the symmetric and anti-symmetric stretching vibrational modes of C–H alkyl groups.¹⁶ The acetyl C=C group is observed at 1663 cm⁻¹ in the PVA polymer film. The bands at 1584 cm⁻¹ and 1429 cm⁻¹ are observed and assigned to the aromatic C=C stretching group and CH₂ bending vibrations respectively.¹⁷ The CH₂ wagging vibrational mode is noticed at 1357 cm⁻¹. The hydroxyl C–O stretching band has been observed at 1092 cm⁻¹. The combination of the C–H wagging vibration and C–O stretching of acetyl groups is observed at 1256 cm⁻¹. The vibrational modes of C–C stretching and CH₂ stretching are observed at 947 cm⁻¹ and 846 cm⁻¹.¹⁸ The band assignments of the FTIR profiles of PVA and the other synthesized polymer composite materials are presented in Table 1. When compared to pure PVA, the –OH stretching peak position is shifted to a higher wavenumber in the doped PVA polymer system and the C=O stretching peak (1662 cm⁻¹) is enhanced in the rare earth metal-doped PVA polymer matrix, suggesting the presence of hydrogen bonding interactions between the hydroxyl groups on the PVA molecular chains.

Fig. 3 FTIR profiles of (a) PVA, (b) Ce³⁺:PVA, (c) Tb³⁺:PVA, (d) Tb³⁺ + Ce³⁺:PVA, and (e) Tb³⁺ + Ce³⁺ + GO NS (0.015 wt%):PVA polymer films.

Table 1 FTIR band assignments of the (a) PVA, (b) PVA:Ce³⁺ (0.1), (c) PVA:Tb³⁺ (0.15), (d) PVA:Ce³⁺ (0.1) + Tb³⁺ (0.15), and (e) PVA:Ce³⁺ (0.1) + Tb³⁺ (0.15) + GO NS (0.015) nanoparticles

Assignment of the bands	Wavenumber (cm^−1)					Ref.
	a	b	c	d	e
OH stretching vibrations	3287	3289	3296	3308	3326	16 and 17
CH symmetric stretching	2847	2849	2854	2856	2857	16 and 17
CH anti-symmetric stretching	2917	2919	2919	2925	2932	16 and 17
Acetyl C=C group	1663	1665	1672	1684	1689	17
Aromatic C=C stretching group	1584	1592	1601	1607	1610	17
CH2 bending vibrations	1429	1432	1432	1432	1432	17
CH2 wagging vibration	1357	1360	1362	1364	1367	17
C–H wagging mode, C–O stretching of acetyl groups	1256	1257	1257	1258	1258	17
C–C stretching vibrational mode	947	948	948	948	948	18
CH2 stretching vibration	846	848	848	848	850	18

---

Upon addition of the rare earth ions (Ce³⁺ and Tb³⁺) with and without GO nanosheets, we detected some structural modifications. These modifications can be identified by doping with rare earth ions along with the GO nanosheets. The investigation of the FTIR spectral peaks and intensity parameters upon doping gives information regarding the formation of a polymer complex. A band at 1741 cm⁻¹ is observed and it is assigned to the stretching vibration of the carbonyl groups (C=O). The spectral band intensity of the C=O stretching vibrational band at 1741 cm⁻¹ decreased with addition of the dopant salt. This could suggest an increase in hydrogen bonding between the polymer and dopant ions. The band at 2847 cm⁻¹ related to C–H symmetric vibrations is found to shift to the higher wavenumber side (nearly 2857 cm⁻¹) upon addition of the rare earth ions and GO NSs. The oxygen-containing functional groups on the GO NS can generate possible interactions with the PVA matrix mainly by hydrogen bonding. The exfoliated GO NS exhibits characteristic peaks at 3444 cm⁻¹ and 1662 cm⁻¹ corresponding to hydroxyl and benzene carboxyl groups respectively. These bands are overlapped with the PVA polymer characteristic bands of OH stretching and acetyl C–C groups at 3687–3250 cm⁻¹ and 1663 cm⁻¹, respectively. The absorption of the hydrogen ions from the free OH groups plays a pivotal role in the formation of complexes. Moreover, it is clearly observed that the OH stretching vibrational band at 3668–3000 cm⁻¹ exhibits a decreased pattern with incorporation of dopant ions and GO NSs, and also it is shifted to the higher wavenumber side. This quite encouraging result suggests the formation of non-covalent bonds between Ce³⁺, Tb³⁺ and OH groups of the PVA skeleton.^17,19 The polymer is probably connected with the GO NS through a weak physical force rather than strong chemical bonding in the GO NS-impregnated PVA:Ce³⁺ + Tb³⁺ polymer nanocomposite. The transmittance decreases with the addition of dopant ions and GO NS; this might be due to the presence of GO NSs within the polymer.¹⁹ The reduced intensity pattern of FTIR could affirmatively support the complex formation between rare earth ions and the PVA polymer matrix. These results are in good agreement with the XRD results also.

3.4 Thermal analysis

The influence of the incorporation of rare earth ions (Ce³⁺, Tb³⁺) and GO NSs on the thermal properties of the PVA polymer matrix was examined by TG/DTA analysis. Thermograms were recorded for (a) pure PVA, (b) Ce³⁺:PVA, (c) Tb³⁺:PVA, (d) Tb³⁺ + Ce³⁺:PVA, and (e) Tb³⁺ + Ce³⁺ + GO NS (0.015%):PVA polymer films in the temperature range from 25–700 °C, and are shown in Fig. 4. The weight losses observed for each step along with the corresponding temperature for all the samples are given in Table 2. The decomposition of pure PVA occurs in three steps. The initial weight loss (around 7%) at 95 °C may be due to degradation of large polymer chains into small fragments through primary decomposition and the removal of absorbed water because PVA is a hydrophilic polymer.²⁰ The second decomposition has been found to occur at 289 °C with a weight loss of 68% which is due to decomposition of acetate and the side chain of PVA. The third decomposition occurs at 420 °C with a weight loss of 17%. This third degradation of the polymer is due to oxidation combustion of the PVA polymer main chain. The complete decomposition of the pure PVA polymer has been found at 465 °C.²¹ In the case of Ce³⁺ and Tb³⁺-doped and co-doped PVA polymer films with and without GO NSs, the first weight loss has been noticed as the same as that of pure PVA at 93 °C, 89 °C, 92 °C and 97 °C with weight losses of 7%, 7%, 6% and 5% for (b) Ce³⁺:PVA, (c) Tb³⁺:PVA, (d) Tb³⁺ + Ce³⁺:PVA, and (e) Tb³⁺ + Ce³⁺ + GO NS (0.015%):PVA polymer films respectively. This could be due to splitting or volatilization of small molecules or evaporation of residual absorbed water in the polymer composites. This might resulting from micro-Brownian motion of the main chain back bone of the PVA polymer matrix. A small percentage of weight loss has occurred in doped samples at 172 °C, 178 °C, 176 °C and 174 °C with a weight loss of 6%, 4%, 4% and 6% respectively. This lower value of weight loss enables one to suggest that the phase transition observed indicates the existence of a physical transition upon addition of dopant ions. The third degradation temperature point has been observed in all respects of the doped samples at 205 °C, 229 °C, 225 °C and 218 °C with a weight loss of 42%, 45%, 46% and 32% for (b) Ce³⁺:PVA, (c) Tb³⁺:PVA, (d) Tb³⁺ + Ce³⁺:PVA, and (e) Tb³⁺ + Ce³⁺ + GO NS (0.015%):PVA polymer films respectively. This thermal degradation occurred due to the heating arrangement of the polyene structure to the polyaromatic form. The decomposition of polyenes results in formation of macro-radicals which further decompose to form cis and trans derivatives. Furthermore, it can form polyconjugated aromatic structures as a result of intramolecular cyclization and condensation reactions according to the Diels–Alder mechanism. The final step of the degradation has been observed in all doped samples above 440 °C and it is due to thermo-oxidation of the carbonized residue.²² All the thermal degradations have been substantiated by the endothermic and exothermic peaks of the DTA profiles of all the polymer samples. The glass transition and melting temperatures have been determined from the DTA profiles of the entire polymer composite and these values are reported in Table 2. A slight variation in the T_g values of the rare earth metal-doped PVA polymer samples has been observed whereas a noteworthy variation is observed in the GO NS-embedded dual rare earth ion-doped PVA polymer nanocomposite. This variation in thermal properties is mainly due to the changes in polymer interaction with respect to hydrogen bonding and covalent bonding with dopant ions. Among all these, polymer interactions between polymer molecules and dopant salts or nanofillers are most important because so many other parameters such as crystallinity, molecular packing and the degree of dispersion of the nanosheets are also dependant on the interactions.²³ The thermal stability and melting temperatures of the rare earth ion-doped PVA polymer composites and polymer nanocomposites are found to decrease when compared to pure PVA. This might be due to a reduction in the number of intra- and inter-hydrogen bonds of PVA after doping or dispersion of nanosheets which is presumably due to the decrease in the degree of crystallinity as proven by XRD.²⁴

Fig. 4 TG/DTA profiles of (a) PVA, (b) Ce³⁺:PVA, (c) Tb³⁺:PVA, (d) Tb³⁺ + Ce³⁺:PVA, (e) Tb³⁺ + Ce³⁺ + GO NS (0.015 wt%):PVA nanocomposite films.

Table 2 TG/DTA results: (a) PVA, (b) PVA:Ce³⁺(0.1), (c) PVA:Tb³⁺ (0.15), (d) PVA:Ce³⁺ (0.1) + Tb³⁺ (0.15), and (e) PVA:Ce³⁺ (0.1) + Tb³⁺ (0.15) + GO NS (0.015%) polymer films

Sample	TG (degradation temperature with weight loss) (°C) (%) ± 2				DTA
	I	II	III	IV	Tg (°C)	Tm (°C)
a	95 (7%)	—	289 (68%)	420 (17%)	113	339
b	93 (7%)	172 (6%)	205 (42%)	442 (10%)	115	218
c	89 (7%)	178 (4%)	229 (45%)	432 (10%)	111	264
d	92 (6%)	176 (4%)	225 (46%)	436 (11%)	109	265
e	97 (5%)	174 (6%)	218 (32%)	441 (10%)	131	236

---

3.5 Optical absorption studies

The optical absorption spectral features of the singly rare earth ion (Ce³⁺, Tb³⁺)-doped and co-doped PVA polymer films along with GO NSs are shown in Fig. 5. The optical absorption spectral study is one of the most important tools to understand the band structure and electronic properties of the pure and doped polymer composites. The observed absorption band centered at 245 nm of the pure PVA polymer is assigned to the π → π* electronic transition of the PVA molecule. Another hump is also observed at 285 nm which is assigned to the n–π* transition.²⁵ The rare earth ions are characterized by partially filled 4f shells, which are shielded by the 5s² and 5p⁶ electrons. For Tb³⁺ ions, all the possible electronic transitions in the absorption spectrum start from the ground state of ⁷F₆ to various excited states. A strong absorption band observed at 282 nm for the Tb³⁺-doped PVA polymer composite is assigned to the electronic transition ⁷F₆ → ⁵I₈.²⁶ In the case of the Ce³⁺:PVA polymer film, the optical absorption band observed at 274 nm is attributed to the electric dipole transitions from the ²F_5/2 ground state to 5d levels.²⁷ In the co-doped (Ce³⁺ + Tb³⁺) PVA sample, an additional absorption band appears at 324 nm which could be related to the f–d forbidden high-spin transition from the ⁵F₁ level to the ⁵d₁ (HS) state.²⁸ Two absorption bands are observed at 274 nm (Ce³⁺) and 324 nm (Tb³⁺) from the dual Ce³⁺ + Tb³⁺-doped PVA polymer film, which suggests that both ions are present in the PVA polymer films. A broad absorption band observed at 274 nm in the GO NS-embedded co-doped Ce³⁺ + Tb³⁺:PVA polymer film indicates the presence of GO NSs in the co-doped PVA polymer nanocomposite.²⁹

Fig. 5 Absorption spectrum of (a) pure PVA, (b) Ce³⁺:PVA, (c) Tb³⁺:PVA, (d) Tb³⁺ + Ce³⁺:PVA, and (e) Tb³⁺ + Ce³⁺ + GO NS (0.015 wt%):PVA polymer films.

3.6 Photoluminescence analysis

The excitation and emission spectral profile of the Ce³⁺:PVA polymer film is shown in Fig. 6. We observed a broad excitation band centered at 370 nm. This excitation band is due to the absorption of the incident radiation by Ce³⁺ ions. This leads to the excitation of electrons from the 4f¹ ground state (²F_5/2, ²F_7/2) to the excited 5d¹ level (²D). From the emission spectrum, the major emission spectral peak centered at 422 nm is attributed to typical double-band emission of Ce³⁺ 5d–4f.³⁰ The excitation spectrum of the Tb³⁺:PVA polymer composite film is shown in Fig. 7. The excitation spectrum exhibits several excitation bands at 327 nm, 340 nm, 353 nm, 370 nm, 379 nm, and 489 nm and these bands are assigned to the corresponding electronic transitions of ⁷F₆ → ⁵D₁, ⁷F₆ → ⁵L₆, ⁷F₆ → ⁵D₂, ⁷F₆ → ⁵L₁₀, ⁷F₆ → ⁵G₆, and ⁷F₆ → ⁵D₄ respectively.³¹ Among all the excitation bands, 370 nm (⁷F₆ → ⁵L₁₀) is found to be prominent. Using this excitation wavelength, the emission spectra of the Tb³⁺:PVA polymer composite films at different concentrations of Tb³⁺ ions are studied and the spectra are shown in Fig. 8. The emission spectra exhibit four emission bands centered at 491 nm (⁵D₄ → ⁷F₆), 546 nm (⁵D₄ → ⁷F₅), 586 nm (⁵D₄ → ⁷F₄) and 621 nm (⁵D₄ → ⁷F₃), and similar band positions are reported for the Tb³⁺:PVA polymer composite films by Kesavulu et al.³² A well-defined prominent green emission band has been observed form the Tb³⁺:PVA polymer composite films at 546 nm (⁵D₄ → ⁷F₅). Among all the concentrations of the Tb³⁺ ion-doped PVA polymer composite films, the 0.15 wt% concentration Tb³⁺:PVA polymer exhibits a prominent emission intensity. Hence, 0.15 wt% concentration has been found to be the optimized concentration for the Tb³⁺ ion in the PVA polymer composite films.

Fig. 6 Excitation and emission spectra of the Ce³⁺ (0.1 wt%):PVA polymer film.

Fig. 7 Excitation spectrum of the Tb³⁺ (0.15 wt%):PVA polymer film.

Fig. 8 Emission spectra of Tb³⁺ (0.025, 0.05, 0.075, 0.1, 0.15 and 0.2 wt%):PVA polymer film.

In order to enhance the photoluminescence performance of the Tb³⁺ ions in the PVA polymer composites, we introduced another appropriate dopant ion such as Ce³⁺ into the Tb³⁺:PVA polymer composite film. From the overlapped spectral profile of the excitation spectra of the Tb³⁺ ions and emission spectrum of the Ce³⁺ ion-doped PVA polymer composite films, there are well overlapped spectral features which are shown in Fig. 9. Therefore, it is expected that efficient energy transfer could occur between the Ce³⁺ and Tb³⁺ ions in the PVA polymer composite films.³³ Dramatic emission enhancement pertaining to Tb³⁺ ions in the Tb³⁺ (0.15 wt%):PVA polymer films has been observed while co-doping with Ce³⁺ ions. The emission spectra for the addition of Ce³⁺ ions in different concentrations under the common excitation of 370 nm are shown in Fig. 10. Due to co-doping with Ce³⁺ ions, the green emission of Tb³⁺ has remarkably been enhanced in the PVA polymer composites. At 0.1 wt% concentration of the Ce³⁺ ions in the co-doped Ce³⁺ (0.1 wt%) + Tb³⁺ (0.15%):PVA polymer film, a prominent green emission is exhibited compared to the other samples. Hence, it is found that the optimized sensitizing concentration of the Ce³⁺ ions is 0.1 wt%. The emission intensities of Tb³⁺ decreased after 0.1 wt% concentration of the Ce³⁺ sensitization in the co-doped PVA system. This could be due to a concentration quenching effect.³⁴ Here, Ce³⁺ ions act as a sensitizer and Tb³⁺ ions act as an activator. The possible emission photons of the Ce³⁺ ions are successfully absorbed by Tb³⁺ in the PVA polymer composites. This analysis unambiguously states that efficient energy transfer takes place from Ce³⁺ to Tb³⁺ ions.³⁵ A schematic diagram of the energy transfer process of the co-doped Ce³⁺ + Tb³⁺:PVA polymer composites is shown in Fig. 11. Initially Ce³⁺ ions absorb UV light of 370 nm; electrons are pumped to the 5d excited state and then relax to the lowest component of the 5d state non-radiatively. Finally they decay to ²F_5/2 and ²F_7/2 states through a radiative process which emits photons (422 nm). The value of the energy level of the excited 5d state of Ce³⁺ is very close to ⁵D₃ and the other energy levels of Tb³⁺ ions. This favourable situation highly supports the energy transfer from Ce³⁺ to Tb³⁺ ions and it promotes the energy from the ⁷F₆ ground state to ⁵D₃ and other levels. The excited Tb³⁺ ions relax to the ⁵D₄ levels non-radiatively and give the strong green emission of Tb³⁺ at 546 nm (⁵D₄ → ⁷F₅).³³

Fig. 9 Overlapped emission spectrum of Ce³⁺ and excitation spectrum of Tb³⁺:PVA polymer films.

Fig. 10 Emission spectra of co-doped Tb³⁺ (0.15 wt%) + Ce³⁺ (0.025, 0.05, 0.1, 0.15 and 0.2 wt%):PVA polymer films.

Fig. 11 Partial energy level scheme diagram of energy transfer from Ce³⁺ to Tb³⁺ and GO NSs to Tb³⁺ in PVA polymer films.

The photoluminescence excitation spectrum of the pure GO NS is shown in Fig. 12(a). Three major excitation bands at 320, 344 and 370 nm are observed in the excitation spectrum of the pure GO NS. Among all the excitation bands, the band at 370 nm is found to be prominent. The electronic transition of the excitation band at 370 nm can be considered as a transition from the σ orbital (HOMO−1) to π* (LUMO) or from the π orbital (HOMO) to σ* (LUMO+1). Dramatically, this 370 nm band is also found to be a common excitation for Ce³⁺ and Tb³⁺ ions as discussed earlier. Using 370 nm as the major excitation wavelength, the emission spectrum of the pure GO NSs is obtained and is shown in Fig. 12(b). Three emission bands are observed in the emission spectrum of the GO NS at 413 nm, 467 nm and 510 nm. A prominent emission band is observed at 413 nm under excitation of 370 nm. This might be due to optical selection of quantum sizes and defects in the GO NS.³⁶

Fig. 12 (a) Excitation and (b) emission spectra of synthesized graphene oxide nanosheets.

3.6.1 Energy transfer phenomena from Ce³⁺ to Tb³⁺ ions and GO NSs to Tb³⁺ in the co-doped PVA system. For further improvement of the photoluminescence properties of the co-doped (Ce³⁺ + Tb³⁺):PVA polymer films, several approaches have been made for obtaining prominent photoluminescence properties. In the second part of the energy transfer, graphene oxide nanosheets have been used to incorporate into the Ce³⁺ + Tb³⁺: PVA polymer films. Actually, two kinds of electronic transitions in the Tb³⁺ ions, such as 4fⁿ → 4fⁿ and 4fⁿ → 4fⁿ⁻¹ 5d, contribute bands in the visible and near-UV regions. The 4fⁿ → 4fⁿ transitions are forbidden and result in weak bands due to perturbations; this partly changes the character of the 4f sub-orbital. Moreover, 4f–5d transitions are allowed and are most important for the energy transfer process, because these transitions are easily perturbed by traditional interactions such as a crystal field, electrostatic interactions and spin–orbit interactions. In a previous discussion, we observed the enhanced Tb³⁺ emission intensities due to the addition of Ce³⁺ ions by energy transfer from Ce³⁺ to Tb³⁺. There are no literature reports on the energy transfer from GO NSs to rare earth ions so far; we made an attempt for the first time with GO NSs for enhancement of the photoluminescence efficiency of the Tb³⁺ ions. In the present work, we observed energy transfer from GO NSs to terbium ions due to localized interactions. The photoluminescence spectra of the Ce³⁺ + Tb³⁺:PVA polymer films under a common excitation of 370 nm in the presence and absence of GO NSs are shown in Fig. 13. Surprisingly, we observed a remarkable enhancement in the Tb³⁺ emission intensity in the presence of GO NSs. A photograph of the (0.015 wt%) GO NS-embedded co-doped Ce³⁺ + Tb³⁺:PVA polymer nanocomposite under the UV excitation source is shown in the inset of Fig. 13. The two most possible mechanisms that might be responsible for this enhancement are either that there is energy transfer from the GO NS to Tb³⁺ ions, increasing the population of excited ions, or there might be an increase in the lifetime of the emitting level of ⁵D₄ of the terbium ions due to interaction with the GO NS. In the sample of the Ce³⁺ + Tb³⁺:PVA polymer films containing GO NSs, excitation with 370 nm excites the GO plasmon band as well as the band due to terbium ions. The absorption cross-section for the GO plasmon band is very large and many GO NSs are excited from the lower to higher energy states. Furthermore, a near proximity of the excitation energies of the plasmon level and the ⁵D₄ level of the terbium ions makes energy transfer from the GO NS to Tb³⁺ ions more likely. The result of the ⁵D₄ level would naturally enhance all the ⁵D₄ → ⁷F_J (J = 0–6) transitions with increasing the Tb³⁺ ions, which lie between 480 nm and 700 nm.³⁷ The relative increase in intensities of the different bands will depend on the intrinsic probabilities for the individual transitions. However, the largest increase in the intensity takes place for the transition ⁵D₄ → ⁷F₅ which is a magnetic dipole allowed hypersensitive transition. This hypersensitive nature is responsible for the large enhancement in intensity as seen in the presence of GO NSs. The energy transfer probability is proportional to the superposition integral of the two spectral shapes, namely donor (GO NS) emission and acceptor (Tb³⁺ ions) excitation.³⁸ A well overlapped spectral region has been observed between the emission of the GO NS and Tb³⁺ excitation as shown in Fig. 14(a). Moreover, no overlapped region has been noticed between Ce³⁺ emission and GO NS excitation which suggests that the energy transfer might not take place from Ce³⁺ to GO NS as shown in Fig. 14(b) (inset). From the above discussion, we suggest unambiguously that the possible energy transfer takes place from Ce³⁺ to Tb³⁺ and the GO NS to Tb³⁺ under the excitation of 370 nm.

Fig. 13 Emission spectra of the co-doped GO NS (0.005, 0.01, 0.015, 0.02 and 0.025%)-embedded Tb³⁺ (0.15 wt%) + Ce³⁺ (0.1 wt%):PVA polymer nanocomposite films.

Fig. 14 Overlapped emission spectra of the synthesized GO nanosheets and excitation spectrum of the Tb³⁺:PVA polymer film.

Fig. 15 exhibits the CIE chromaticity diagram regarding Tb³⁺ (0.15 wt%):PVA, co-doped Tb³⁺ (0.15 wt%) + Ce³⁺ (0.1 wt%):PVA, and GO (0.015 wt%)-embedded co-doped Tb³⁺ (0.15 wt%) + Ce³⁺ (0.1 wt%):PVA polymer nanocomposites. Low violet-green emission has been noticed in the Tb³⁺ (0.15 wt%):PVA and co-doped Tb³⁺ (0.15 wt%) + Ce³⁺ (0.1 wt%):PVA polymer samples. Upon addition of the Ce³⁺ ions to the Tb³⁺:PVA polymer matrix, the color index has been increased compared to singly-doped Tb³⁺:PVA polymer complex. The x and y values of the CIE chromaticity coordinates for (i) Tb³⁺ (0.15 wt%):PVA, (ii) co-doped Tb³⁺ (0.15 wt%) + Ce³⁺ (0.1 wt%):PVA and (iii) GO NS (0.015 wt%)-embedded co-doped Tb³⁺ (0.15 wt%) + Ce³⁺ (0.1 wt%):PVA polymer nanocomposites are presented in Table 3. Upon addition of the GO NS at 0.15 wt% concentration, the chromaticity coordinates could be tuned to the green position under 370 nm excitation. It is also an external evidence for the possible energy transfer process between the GO NSs and Tb³⁺ ions.

Fig. 15 CIE chromaticity diagram for Tb³⁺ (0.15 wt%):PVA, co-doped Tb³⁺ (0.15 wt%) + Ce³⁺ (0.1 wt%):PVA, and GO (0.015 wt%)-embedded co-doped Tb³⁺ (0.15 wt%) + Ce³⁺ (0.1 wt%):PVA polymer nanocomposites.

Table 3 CIE chromaticity coordinates of PVA:Tb³⁺ (0.15), PVA:Ce³⁺ (0.1) + Tb³⁺ (0.15) and PVA:Ce³⁺(0.1) + Tb³⁺ (0.15) + GO NS (0.015%) polymer composites under the common excitation of 370 nm

Sl. No	Sample composition (wt%)	CIE coordinates (x, y)
1	PVA:Tb^3+ (0.15)	(0.1924, 0.2842)
2	PVA:Ce^3+ (0.1) + Tb^3+ (0.15)	(0.1954, 0.3162)
3	PVA:Ce^3+ (0.1) + Tb^3+ (0.15) + GO NS (0.015)	(0.2241, 0.4124)

---

3.7 Decay analysis

In order to further understand the energy transfer phenomenon, the lifetime decay dynamics of Ce³⁺ and Tb³⁺ in the singly-doped and co-doped polymer systems have been systematically measured and extensively studied with the depicted logarithmic intensity as shown in Fig. 16. The lifetime of the blue emission (422 nm) under the excitation of 370 nm pertaining to the Ce³⁺ singly-doped PVA polymer film is calculated and it is found to be 1.89 μs. This decay curve is well fitted in a single exponential function as shown in Fig. 16(a). The green emission (546 nm) lifetime of the Tb³⁺ ion in the singly-doped PVA polymer film with an excitation at 370 nm is calculated and it is found to be 410 μs. It can be seen that the decay curve of the singly-doped Tb³⁺:PVA polymer film is well fitted in the single exponential function as shown in Fig. 16(d). From Fig. 16, we can calculate the lifetime and investigate the luminescence dynamics of the polymers. It is found that the curves are well fitted to follow a single exponential decay. The decay lifetime is obtained by fitting the decay curve to the equation as:

I_t = I₀ exp(−t/τ) (1)

where I_t and I₀ are the intensities at time t and 0, and τ is defined as the luminescence lifetime. In the co-doped system, the decay curves exhibit a non-exponential nature due to the different sites of the Tb³⁺ ions. Hence, the decay of the luminescence has been fitted to the expression given by:

I(t) = A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂) (2)

where I(t) is the emission intensity, A₁ and A₂ are constants, and τ₁ and τ₂ are the short and long lifetimes for the exponential components respectively. The average lifetimes (τ_avg) of the blue and green emissions of the Ce³⁺ and Tb³⁺ ions have been determined by the formula:

Fig. 16 Decay curves of Ce³⁺ and Tb³⁺ emissions in the singly- and co-doped PVA polymer matrix with and without GO NSs.

However, the decay curves of the Ce³⁺ and Tb³⁺ singly-doped PVA polymer films have slightly deviated from the single exponential nature compared with the decay curves of the co-doped PVA polymer system. This could be due to modification of the fluorescence dynamics by the co-doped ions. The lifetime of the excited state of the Ce³⁺ ions in the co-doped Ce³⁺ + Tb³⁺:PVA polymer film has been evaluated and it is found to be 1.15 μs. The decrease in the lifetime of the Ce³⁺ ion within the co-doped system indicates that possible energy transfer occurs between the Ce³⁺ and Tb³⁺ ions.³⁹ It has also been noticed that the decay profile of the co-doped system exhibits a non-exponential nature. This indicates the interaction between the acceptor and donor ions. In the co-doped system, the concentration of the availability of the donor ions (Ce³⁺) per acceptor ion (Tb³⁺) will be more, therefore the average distance between the acceptor–donor ions will be decreased and electronic multipole–multipole interactions start and the energy migration effect take place, and as a result of this, the emission decay profiles exhibit a non-exponential nature. Therefore the above evidences confirm that a possible energy transfer process take place from Ce³⁺ to Tb³⁺ in the PVA polymer film.⁴⁰ According to Dexter theory, the probability of energy transfer via multipolar interaction can be expressed by the following equation:

where P represents the energy transfer probability, τ_D is the decay time of the donor emission, Q_A is the total absorption cross-section of the acceptor, R is the distance between the donor and the acceptor, and b and c are parameters that depend on the type of the energy transfer.⁴¹

The calculated lifetime values corresponding to Ce³⁺ and Tb³⁺ in the singly-doped and co-doped PVA polymer matrix are presented in Table 4. From the calculated lifetime values shown in Table 4, the decay time of the cerium emission was observed to shorten in the co-doped system when compared to the singly-doped polymer matrix. According to eqn (1), the energy transfer probability is inversely proportional to the decay time of τ_D, and the shortened decay time of the Ce³⁺ emission confirms the presence of an energy transfer pathway from Ce³⁺ to Tb³⁺ ions.⁴¹

Table 4 Calculated lifetime values

Polymer composite	Excitation wavelength (nm)	Emission wavelength (nm)	Lifetime (μs)
PEO + PVP:0.1Ce^3+	370	422	1.89
PEO + PVP:1.5Tb^3+	370	546	410
PEO + PVP:0.1Ce^3+ + 1.5Tb^3+	370	422 (Ce^3+)	1.15
PEO + PVP:0.1Ce^3+ + 1.5Tb^3+	370	546 (Tb^3+)	540
PEO + PVP:0.1Ce^3+ + 1.5Tb^3+ + 0.015GO NS	370	546 (Tb^3+)	590

---

In order to understand the energy transfer process from Ce³⁺ to Tb³⁺ ions, the energy transfer efficiency (η_ET) from the sensitizer Ce³⁺ ions to activator Tb³⁺ ions was estimated using the following equation:

η_ET = 1 − τ_d/τ_d0 (5)

where τ_d0 is the intrinsic lifetime of the sensitizer (Ce³⁺) and τ_d is the lifetime of the sensitizer in the presence of the activator (Tb³⁺). The energy transfer efficiency has been calculated as 39.1%.^42,43

While co-doped with Ce³⁺ along with Tb³⁺, the Tb³⁺ emission peak (546 nm) intensity has been remarkably enhanced and consequently its lifetime also increased. The co-doped system Tb³⁺ lifetime has been found to be 540 μs. Upon adding GO NSs (4 wt%) to the co-doped Ce³⁺ + Tb³⁺:PVA polymer system, the lifetime of Tb³⁺ has significantly been increased. This is due to the fact that the coupling of the plasmonic field of the GO NSs to dual rare earth ions affects the lifetime of the radiative level of RE ions as well as the energy transfer. With the GO NS presence along with both Ce³⁺ and Tb³⁺, the Tb³⁺ green emission lifetime is found to be 590 μs. The enhancement of the lifetime upon adding Ce³⁺ and GO NSs indicates the occurrence of energy transfer from these two Ce³⁺ and GO NSs to Tb³⁺ in the sample studied. The non-exponential nature of the co-doped system Tb³⁺ ion emission lifetime decay curve indicates an effective interaction between the donor and acceptor ions.⁴⁴ The energy transfer mechanism is clearly demonstrated by the energy level scheme diagram as shown in Fig. 11.

4. Conclusions

In summary, it is concluded that we have successfully synthesized a transparent and semi-transparent pure PVA polymer film and also with dopant ions such as Ce³⁺ and Tb³⁺ separately and also together, with and without GO nanosheets using the solution casting method. The semi-crystalline nature of the polymer films has been confirmed based on the XRD spectral profiles. Dispersion studies of the polymer nanocomposites of the GO nanosheet-embedded co-doped PVA polymer complex have been investigated by TEM analysis. FTIR analysis confirms the complex formation of the polymers due to the presence of these dual rare earth ions in the polymer films along with GO nanosheets. Both absorption and photoluminescence spectra have been measured to study their optical properties. The Tb³⁺ ion-doped PVA polymer films exhibit green emission under a UV source. The photoluminescence efficiency of the Tb³⁺ ions is enhanced significantly due to co-doping with Ce³⁺ ions through an energy transfer process. In the GO nanosheet-embedded Ce³⁺ + Tb³⁺:PVA polymer system, energy transfer between the GO nanosheets and Ce³⁺ ions also takes place in the host polymer matrix. Due to the sensitizing effect of the GO nanosheets and Ce³⁺ ions, it could enhance the emission performance of the Tb³⁺ ions in two ways simultaneously within the PVA polymer matrix, demonstrated in the partial energy level scheme diagram. The energy transfer phenomenon has been substantiated by the overlapped spectral profiles, photoluminescence analysis and CIE chromaticity diagram. These results suggest that GO NS-embedded co-doped Ce³⁺ + Tb³⁺:PVA polymer films are promising materials for green luminescence photonic devices.

Acknowledgements

This research work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of the Science ICT & Future Planning (NRF-2015R1A1A3A04001268).

References

1. K. Yamashita, N. Takeuchi, H. Taniguchi, S. Yuyama, K. Oe, N. Mibuka, A. Suzuki and H. Mataki, J. Lumin., 2009, 129, 526–530.
2. K. Binnemans, Chem. Rev., 2009, 109, 4283–4374.
3. Q. D. Ling, D. J. Liaw, C. Zhu, D. S. H. Chan, E. T. Kang and K. G. Neoh, Prog. Polym. Sci., 2008, 33, 917–978.
4. B. Karthikeyan, Phys. B: Condens. Matter., 2005, 364, 328–332.
5. R. S. Yadav, R. K. Verma, A. Bahadur and S. B. Rai, Spectrochim. Acta, Part A, 2015, 137, 357.
6. C. Parthasaradhi Reddy, V. Naresh, R. Ramaraghavulu, B. H. Rudramadevi, K. T. Ramakrishna Reddy and S. Buddhudu, Spectrochim. Acta, Part A, 2015, 144(5), 68–75.
7. K. Naveen Kumar, B. Chandra Babu and S. Buddhudu, J. Lumin., 2015, 161, 456–464.
8. A. Geim and K. Novoselov, Nat. Mater., 2007, 6, 183–191.
9. D. Chen, H. Zhu and T. Liu, ACS Appl. Mater. Interfaces, 2010, 2, 3702–3708.
10. J. Chen, B. Yao, C. Li and G. Shi, Carbon, 2013, 64, 225–229.
11. B. S. Kwak, H. Lee and M. Kang, Chem. Eng. J., 2014, 255, 613–622.
12. S. Park, J. An, J. R. Potts, A. Velamakanni, S. Murali and R. S. Ruoff, Carbon, 2011, 49, 3019–3023.
13. C.-M. Tang, Y.-H. Tianand and S.-H. Hsu, Materials, 2015, 8, 4895–4911.
14. H. J. Salavagione, G. Martınez and M. A. Gomez, J. Mater. Chem., 2009, 19, 5027–5032.
15. E. Sheha, M. Nasr and M. K. El-Mansy, Phys. Scr., 2013, 88, 035701–035709.
16. X. Qi, X. Yao, S. Deng, T. Zhou and Q. Fu, J. Mater. Chem. A, 2014, 2, 2240–2249.
17. T. A. Hamdalla and T. A. Hanafy, Optik, 2016, 127, 878–882.
18. A.-M. Grigoriu, C. Luca, E. Horoba and S. Dunca, Rev. Chim., 2013, 64, 606–611.
19. M. Z. Kassaee, M. Mohammadkhani, A. Akhavan and R. Mohammadi, Struct. Chem., 2011, 22, 11.
20. K. Zhou, S. Jiang, C. Bao, L. Song, B. Wang, G. Tang, Y. Hu and Z. Gui, RSC Adv., 2012, 2, 11695–11703.
21. S. Ramesh and A. K. Arof, J. Power Sources, 2001, 99, 41–47.
22. A. Y. Shaulov, S. M. Lomakin, T. S. Zarkhina, A. D. Rakhimkulov, N. G. Shilkina, Y. B. Muravlev and A. A. Berlin, Dokl. Phys. Chem., 2005, 403, 154.
23. S. K. Sharma, J. Prakash and P. K. Pujari, Phys. Chem. Chem. Phys., 2015, 17, 29201–29209.
24. Y. Pavani, M. Ravi, S. Bhavani, A. K. Sharma and V. V. R. Narasimha Rao, Polym. Eng. Sci., 2012, 52, 1685–1692.
25. Y. GagandeepKaur, S. Dwivediand and B. Rai, J. Fluoresc., 2011, 21, 423–432.
26. M. Sekita, S. Miyazawa, H. Sekiwa and Y. Sato, Appl. Phys. Lett., 1994, 65, 2380.
27. I. Kebaili and M. Dammak, Journal of Theoretical and Applied Physics, 2012, 6(21), 1–8.
28. A. Podhorodecki, N. V. Gaponenko, M. Bański, T. Kim and J. Misiewicz, ECS Trans., 2010, 28(3), 81.
29. P. Huang, C. Xu, J. Lin, C. Wang, X. Wang, C. Zhang, X. Zhou, S. Guo and D. Cui, Theranostics, 2011, 1, 240–250.
30. C. Feldmann, T. Justel, C. R. Ronda and P. J. Schmidt, Adv. Funct. Mater., 2003, 13, 511–517.
31. J. F. M. dos Santos, I. A. A. Terra, N. G. C. Astrath, F. B. Guimaraes, M. L. Baesso, L. O. A. Nunes and T. Catunda, J. Appl. Phys., 2015, 117, 053102.
32. C. R. Kesavulu, A. C. Almeida Silva, M. R. Dousti, N. O. Dantas, A. S. S. de Camargo and T. Catunda, J. Lumin., 2015, 165, 77–84.
33. H. Guo, H. Zhang, J. J. Li and F. Li, Opt. Express, 2010, 18(26), 27257–27262.
34. X. Chen and Z. Xia, J. Solid State Light., 2014, 4, 1–10.
35. M. Xu, L. Wang, D. Jia and H. Zhao, Phys. Chem. Chem. Phys., 2015, 17, 28802–28808.
36. H.-X. Zhao, Yu-C. Wang, L.-Y. Zhang and M. Wang, New J. Chem., 2015, 39, 98–101.
37. R. K. Verma, K. Kumar and S. B. Rai, Solid State Commun., 2010, 150, 1947–1950.
38. M. Mattarelli, M. Montagna, L. Zampedri, A. Chiasera, M. Ferrari, G. C. Righini, L. M. Fortes, M. C. Goncalves, L. F. Santos and R. M. Almeida, Europhys. Lett., 2005, 71, 394.
39. J. Olesiak-Banska, M. Nyk, D. Kaczmarek, K. Matczyszyn, K. Pawlik and M. Samoc, Opt. Mater., 2011, 33, 1419.
40. Yu-C. Li, Y.-H. Chang, Yu-F. Lin, Y.-S. Chang and Yi-J. Lin, J. Alloys Compd., 2007, 439, 367.
41. S. Yeon Kim, Y.-H. Won and H. S. Jang, Sci. Rep., 2015, 5, 7866.
42. Z. Zhu, Y. Zhang, Y. Qiao, H. Liu and D. Liu, J. Lumin., 2013, 134, 724–728.
43. A. Bahadur, Y. Dwivedi and S. B. Rai, Spectrochim. Acta, Part A, 2014, 118, 177–181.
44. B. Han, H. Liang, Y. Huang, Y. Tao and Q. Su, Appl. Phys. B, 2011, 104, 241–246.

---