Photoluminescence behavior of rare earth doped self-activated phosphors (i.e. niobate and vanadate) and their applications

Abstract

In the present study, the photoluminescence behaviors of rare earth doped self-activated phosphors are discussed briefly. Different techniques were used to develop these phosphor samples. We prepared pure and rare earth doped phosphor samples to look for their various applications. The structural confirmations and surface morphologies were performed using X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements, respectively. The upconversion (UC) phenomenon was investigated in Tm³⁺/Yb³⁺ and Ho³⁺/Yb³⁺ co-doped niobate and vanadate based phosphors, which gave intense blue/NIR and green/red emissions with a 980 nm diode laser as an excitation source. Pure niobate and vanadate phosphor materials are self-activated hosts which give broad blue emission under UV excitation. Upon UV excitation, intense broad blue emission along with sharp emissions due to Tm³⁺ and Ho³⁺ ions are observed via energy transfer between niobate/vanadate and rare earth ions. These self-activated hosts show prominent downshifting (DS) behavior. Broad band quantum cutting (QC) was observed in these self-activated hosts, in which a blue emitting photon is converted into two NIR photons by co-doping Yb³⁺ ions in it. The multimodal (upconversion, downshifting and quantum cutting) behaviors of these phosphors make them very promising in various applications, such as spectral converters to enhance the efficiency of a c-Si solar cell, security ink and color tunable materials.

---

A. Dwivedi

Dr Abhishek Dwivedi is currently working as Assistant Professor at Sunbeam Women's College, Varuna Varanasi, India. He completed his PhD at Banaras Hindu University, Varanasi, India. After this, he joined IIT BHU as a Postdoctoral Fellow in 2018. He got a Kothari Postdoctoral Fellowship from UGC, India, in 2019 and joined the University of Allahabad in 2020. His areas of interest are the optical behaviour of rare earth doped materials and their several applications as spectral converters, color tunable materials, temperature sensors, security ink applications, etc. He has published more than 30 research articles in various international journals.

A. Roy

Mr Abhishek Roy is a research scholar in the Department of Physics at Banaras Hindu University, Varanasi, India. He is doing his PhD under the supervision of S. B. Rai at Banaras Hindu University. His current research interests include multifunctional materials doped with rare-earth ions, e.g., ceramics, phosphors, etc., and their applications in security ink and anticounterfeiting applications. He has authored/co-authored around 10 articles in peer-reviewed international journals.

S. B. Rai

Dr S. B. Rai has been a professor in the Department of Physics at Banaras Hindu University, Varanasi, India, since 1994. He gained his PhD (Physics) in 1974 from Banaras Hindu University and visited Germany as an Alexander von Humboldt Fellow in 1982–84 and several times later on. His current research interests include multifunctional materials doped with rare-earth ions, e.g. ceramics, phosphors, quantum dots, hybrid nanostructures, etc., and their applications in various emerging fields. He has edited many book chapters and authored/co-authored around 500 articles in peer-reviewed national and international journals.

---

1. Introduction

Phosphors are materials that possess the phenomenon of luminescence. These materials have various applications in the fields of physics, chemistry, biology and material sciences. Self-activated hosts give their own intense emission in the visible region. Thus, these self-activated phosphors play a very crucial role for the excitation of different activators doped in them. Phosphors doped with rare earth ions have various applications, such as lasers, LEDs, sensors, display devices, bio-imaging, security ink, and color tunable materials.^1–12 In this work, two types of hosts, ANbO₄ and AVO₄ (where A = Y, Gd and La), are investigated. These host materials are chemically, thermally, and mechanically stable and give intense broad blue emission on UV excitation. Most of the rare earth ions absorb radiation in the blue region and therefore their photoluminescence intensity on doping in these host materials is enhanced. This makes them useful for various applications. This blue emission is also helpful in white light generation as well as in achieving a color tunable radiation source. The downshifting (DS) behavior of the rare earth ions is enhanced significantly due to the self-activation behavior of the host.

ANbO₄ (A = Gd, Y, La) with fergusonite structure was described by Blasse^13,14 as early as 1976. The niobate group [(NbO₄)³⁻] in a matrix acts as a fluorescence center and gives blue emission under UV excitation. YNbO₄ and LaNbO₄ show efficient blue emission, whereas GdNbO₄ shows relatively weak blue emission on UV excitation. Since very little information is available about the luminescence properties of GdNbO₄ based phosphors, it is interesting to study in detail this material and its properties. In GdNbO₄, the niobium atom is attached tetrahedrally to the oxygen atom in a highly distorted site.^15–18 AVO₄ (A = Y and Gd) is a wide band gap host material since vanadate is a self-activated host which gives broad blue emission on UV excitation due to the presence of the (VO₄)³⁻ group.^19–22

Different rare earth ions (Tm³⁺, Ho³⁺, Yb³⁺) were used as activators doped into these self activated hosts. These activators have different spectral luminescence behavior in different regions. In this study, rare earth ion doped ANbO₄ and AVO₄ phosphors are discussed in terms of their multimodal properties and applications. For the multimodal behavior, we focused particularly on the upconversion (UC), downshifting (DS) and quantum cutting (QC) processes and looked for appropriate applications. For this purpose, Ho³⁺ and Tm³⁺ ions were used due to their ladder-like energy levels and intense optical properties.

2. Experimental methods

2.1. Materials used

All the starting materials used for the synthesis of the rare earth doped ANbO₄, AVO₄ and ATaO₄ (where A = Y, Gd and La) phosphors were in oxide form. ABO₄ phosphor materials can be synthesized by several techniques, such as the sol–gel method, solid state reaction method, combustion method, etc. Here, we used the solid state reaction method as it is easy.

2.2. Synthesis technique

All the phosphor samples were synthesized via the solid state reaction technique. Here, all the raw materials in oxide form were mixed homogeneously in an agate mortar for one hour using acetone (about 50 ml) as the mixing media. The raw mixtures were then calcined separately in an electric furnace at optimized higher temperatures (1400–1600 K) for 4–5 hours. The dry powders were then cooled to room temperature and finally ground into powder form using an agate mortar. These synthesized powder samples were used for the structural and optical characterizations.

3. Structural behavior

3.1. ANbO₄ (A = Y, Gd and La)

ANbO₄ (A = Y, Gd and La) compounds exhibit a fergusonite crystal structure and are thermally and chemically very stable. Lanthanide-activated YNbO₄ crystallizes in a monoclinic phase with a space group of C2/c and lattice parameters of a = 7.6176(1) Å, b = 10.9454(2) Å, c = 5.2967(1) Å and β = 138.4308(8),^23,24 as shown in Fig. 1(a). Lanthanide-activated GdNbO₄ possesses an I2 space group in the monoclinic cell with refined lattice parameters of a = 5.3674(3) Å, b = 11.0772(6) Å, c = 5.1039(3) Å, and β = 94.554(4) (Fig. 1(b)).²⁵ Similarly, lanthanide-activated LaNbO₄ also crystallizes with monoclinic structure but with space group I2/a and lattice parameters a = 5.2297(1) Å, b = 10.8270(1) Å, c = 5.0416(1) Å, and β = 94.4029(6).²⁶ In the monoclinic crystal structure, each A³⁺ ion coordinates with eight oxygen ions to form a dodecahedron and remains located at C₂ symmetry sites. The Nb⁵⁺ ions remain coordinated with four oxygen ions to form a tetrahedral structure.

Fig. 1 (i) X-ray diffraction patterns of (a) YNbO₄, (b) GdNbO₄ and (c) LaNbO₄ phosphors. (ii) SEM images of (a) YNbO₄, (b) GdNbO₄ and (c) LaNbO₄ phosphors. (Reproduced with permission from ref. 30 with copyright 2016, Institute of Physics).

The SEM images show that the surface particles are well separated from each other and are of nearly spherical shape. Particles are similar in shape and size in both the YNbO₄ and GdNbO₄ phosphors. However, in the case of LaNbO₄, the particles are agglomerated with each other and smaller in size compared to other two phosphors.

3.2. AVO₄ (A = Y and Gd)

Fig. 2 shows the XRD spectra of the YVO₄ and GdVO₄ phosphors. The XRD pattern of YVO₄ shows a tetragonal phase and matches well with JCPDS file no. 72-0274 with cell parameters of a = 7.100 Å, c = 6.270 Å and α = β = γ which confirm the pure phase formation of the prepared sample. The XRD peaks of the GdVO₄ phosphor sample match well with JCPDS file no. 17-0260, confirming the tetragonal phase formation with cell parameters of a = 2.721 nm and c = 6.346 nm. The sharp and intense peaks of the XRD patterns show the pure crystalline nature of the synthesized samples.

Fig. 2 X-ray diffraction patterns of YVO₄ and GdVO₄ phosphors.

4. Optical behavior

4.1. Upconversion (UC) behavior

4.1.1. ANbO₄:X³⁺,Yb³⁺ (where A = Y/Gd/La and X = Tm/Ho) phosphors. Fig. 3 shows the UC emission behavior of the rare earth (Ho³⁺/Yb³⁺ and Tm³⁺/Yb³⁺) doped YNbO₄, GdNbO₄ and LaNbO₄ phosphors in the visible region on excitation with a 980 nm diode laser.^25,28–30 Intense green and moderately weak red emissions are observed in the GdNbO₄:Ho³⁺,Yb³⁺ phosphor, as shown in Fig. 3(a), in which optimization was performed to get intense UC emission by varying the concentrations of Ho³⁺ and Yb³⁺ ions, shown in the inset of the figure. It was also observed that the incorporation of Yb³⁺ enhances the overall UC emission by several times (I and II spectra in the figure). Fig. 3(b) shows the power dependent study of UC emission, particularly in the green and red emissions, to understand the involvement of the number of photons by a log–log plot of green/red emissions with pump power. This shows that two photons are involved in the green and red emissions, as obtained from the slope. This slope decreases at higher pump powers due to the heating effect and the dominance of non-radiative relaxation. Similarly, Fig. 3(c) and (d) show the UC emission of the Tm³⁺/Yb³⁺ codoped YNbO₄ and LaNbO₄ phosphors in which strong blue and NIR emissions were observed at 476 (¹G₄ → ³H₆) and 803 (³H₄ → ³H₆) nm, respectively. From Fig. 3(c), it can also be observed that the overall UC emission enhances significantly on the incorporation of Yb³⁺ ion, which suggests that Yb³⁺ also acts as a sensitizer ion.

Fig. 3 (a) UC emission spectra of GdNbO₄:Ho³⁺,Yb³⁺ in the range 400–800 nm (I) without Yb³⁺ and (II) with Yb³⁺; the optimized green and red UC emission intensity with concentration of Ho³⁺ ion is shown in inset. (b) Ln I versus ln P plot of green and red emissions. (c) UC emission spectra of YNbO₄:Tm³⁺ and YNbO₄:Tm³⁺,Yb³⁺ phosphors in the range 350–850 nm; inset shows the optimized UC intensity with concentration of Yb³⁺ ion. (d) UC emission spectrum of LaNbO₄:Tm³⁺,Yb³⁺ phosphor in the range 350–850 nm. ((a) and (b) are reproduced with permission from ref. 16 with copyright 2014, Royal Society of Chemistry).

4.1.2. AVO₄:X³⁺,Yb³⁺ (where A = Y/Gd and X: Ho/Tm) phosphors. Fig. 4(a) shows the UC emission spectrum of YVO₄:Ho³⁺,Yb³⁺ phosphor in the range of 350–800 nm on excitation at 980 nm. In this case, weak green (540 nm) and intense red (663 nm) UC emissions are observed due to the ⁵F₄/⁵S₂ → ⁵I₈ and ⁵F₅ → ⁵I₈ transitions, respectively.^31,32 However, in the case of Ho³⁺/Yb³⁺ co-doped phosphors, green UC emission is dominant compared to red UC emission. Thus, this host is suitable for UC emission in the red region on excitation with a 980 nm diode laser.

Fig. 4 (a) UC emission spectrum of YVO₄:Ho³⁺,Yb³⁺ phosphor in the range 350–800 nm. (b) UC emission spectra of Tm³⁺/Yb³⁺ co-doped GdVO₄ and YVO₄ phosphors in the range from 350–850 nm. ((b) is reproduced with permission from ref. 32 with copyright 2020, Elsevier).

Fig. 4(b) shows the UC emission spectra of the GdVO₄:Tm³⁺,Yb³⁺ and YVO₄:Tm³⁺,Yb³⁺ phosphors in the range of 350–850 nm on excitation with a 980 nm diode laser. Intense sharp blue (475 nm) and intense broad NIR (803 nm) UC emission lines are observed due to the ¹G₄ → ³H₆ and ³H₄ → ³H₆ transitions, respectively. From the figure, it is observed that the UC emission of GdVO₄:Tm³⁺,Yb³⁺ is more intense than that of the YVO₄:Tm³⁺,Yb³⁺ phosphor due to their surface morphologies.³² In this host, the emission intensities of both the blue and NIR band are nearly equal, which is not observed in the niobate host due to the involvement of a different mechanism in the UC emission.

4.1.3. Mechanism for the UC emission. Fig. 5(a) shows the energy level diagrams of the Ho³⁺/Yb³⁺ and Tm³⁺/Yb³⁺ ions to discover the mechanism behind the UC emission on excitation with a 980 nm diode laser. In these systems, the Yb³⁺ ion acts as a sensitizer and the Ho³⁺/Tm³⁺ ions as an activator. When the Ho³⁺/Yb³⁺ co-doped phosphor is irradiated by the 980 nm diode laser, the Yb³⁺ ion absorbs the incident radiation strongly due to it matching an energy level in Yb³⁺ and a higher cross-section of absorption.^33,34 The de-excitation energy of the Yb³⁺ ion is transferred to the Ho³⁺ ion. Though the Ho³⁺ ion does not have a matching energy level, this is possible via phonon assistance. This energy transfer process is termed phonon-assisted energy transfer. Thus, the Ho³⁺ ion in the ground state (⁵I₈) is promoted to ⁵I₆ via phonon-assisted energy transfer. The Ho³⁺ ions in the ⁵I₆ level absorb the next photons and are promoted to the ⁵F₄/⁵S₂ level via the excited state absorption/energy transfer upconversion (ESA/ETU) process. The various transitions in the green and red regions are as follows.

hν (980 nm) + Yb³⁺ (²F_7/2) → Yb³⁺ (²F_5/2)

²F_5/2 (Yb³⁺) + ⁵I₈ (Ho³⁺) → ²F_7/2 (Yb³⁺) + ⁵I₆ (Ho³⁺)

²F_5/2 (Yb³⁺) + ⁵I₆ (Ho³⁺) → ²F_7/2 (Yb³⁺) + ⁵F₄/⁵S₂ (Ho³⁺)

⁵F₄/⁵S₂ (Ho³⁺) → hν (green) + ⁵I₈ (Ho³⁺)

Fig. 5 Energy level diagram of (a) Ho³⁺/Yb³⁺ions (reproduced with permission from ref. 19 with copyright 2019, American Chemical Society). (b) Tm³⁺/Yb³⁺ ions doped in ANbO₄/AVO₄ phosphor to reveal the UC emission mechanism (reproduced with permission from ref. 32 with copyright 2020, Elsevier).

This channel dominates in Ho³⁺/Yb³⁺ co-doped ANbO₄ phosphors, giving intense green UC emission as compared to the red UC emission. Part of the energy in the ⁵F₄/⁵S₂ level relaxes non-radiatively to the close-lying ⁵F₅ level and emits a red photon.

Another channel is also present in which UC emission from another excited level is possible. There are large numbers of quenching centers present in the system. One is non-radiative relaxation from ⁵I₆ to ⁵I₇ via collisions. The ions in the ⁵I₇ level absorb other photons and are promoted to the ⁵F₅ level which is partly responsible for the red emission. The Ho³⁺ ion in the ⁵F₅ level may be promoted via an ETU/ESA process. After absorbing a third photon, the Ho³⁺ ion is promoted to the ⁵G₄ level. The relaxation from this high-lying level to the ⁵F₄/⁵S₂ and ⁵F₅ levels gives emissions in the green and red regions. The mechanism can be represented as follows.

²F_7/2 (Yb³⁺) + hν → ²F_5/2 (Yb³⁺)

²F_5/2 (Yb³⁺) + ⁵I₈ (Ho³⁺) → ²F_7/2 (Yb³⁺) + ⁵I₆ (Ho³⁺)

⁵I₆ (Ho³⁺) → ⁵I₇ (Ho³⁺) [via relaxation]

²F_5/2 (Yb³⁺) + ⁵I₇ (Ho³⁺) → ²F_7/2 (Yb³⁺) + ⁵F₅ (Ho³⁺)

²F_5/2 (Yb³⁺) + ⁵F₅ (Ho³⁺) → ²F_7/2 (Yb³⁺) + ⁵G₄ (Ho³⁺)

The Ho³⁺/Yb³⁺ co-doped AVO₄ phosphor follows this mechanism to build up population in the higher levels. Therefore, the green emission is very weak compared to the red emission in the AVO₄:Ho³⁺,Yb³⁺ phosphor.

Another channel for UC emission is cooperative energy transfer (CET) between two Yb³⁺ ions and one Ho³⁺ ion. In this process, two Yb³⁺ ions interact such that their deexcitation energy is transferred to the Ho³⁺ ion to simultaneously excite it to the ⁵F₄/⁵S₂ level. The relaxation from this level to lower levels gives transitions in various regions.

2 ²F_5/2 (Yb³⁺) + ⁵I₈ (Ho³⁺) → 2 ²F_7/2 (Yb³⁺) + ⁵F₂ (Ho³⁺)

This channel is not as effective as the other two channels.

The Tm³⁺ ion doped alone in the phosphors shows very weak UC emission at only a high pump power density of the 980 nm diode laser through the ESA process. The efficiency of the UC emission can be enhanced by several times through adding Yb³⁺ ion as a sensitizer. As mentioned in the earlier section, the absorption cross-section of Yb³⁺ ion for 980 nm is very large compared to those of other rare earth ions. Yb³⁺ ion efficiently absorbs NIR photons (980 nm radiation) in its ground state (²F_7/2) and is promoted to the excited state (²F_5/2), as shown in Fig. 5(b). The de-excitation energy of the excited Yb³⁺ ion is transferred to the Tm³⁺ ion by two different possible paths. In the first process, the Yb³⁺ ion transfers its excitation energy non-resonantly (phonon assisted ET) to the Tm³⁺ ion, which is in its ground state (³H₆), and excites it to the higher state (³H₅). Since the lifetime of the ³H₅ state is very short, the excited Tm³⁺ ions relax non-radiatively to the lower lying metastable ³F₄ state where they reabsorb a second 980 nm photon (ESA) and are promoted to the ³F₂ state. From there, the ions relax to the ³H₄ state via the intermediate state ³F₃. Finally, the Tm³⁺ ion in the ³H₄ state absorbs a third 980 nm photon and is promoted to the ¹G₄ state. The Tm³⁺ ion in the ¹G₄ level gives different transitions to low lying states, emitting in different wavelength regions. This can be represented by the multistep ET from Yb³⁺ to Tm³⁺ as follows.

hν (980 nm) + Yb³⁺ (²F_7/2) → Yb³⁺ (²F_5/2)

²F_5/2 (Yb³⁺) + ³H₆ (Tm³⁺) → ³H₅ (Tm³⁺) + ²F_7/2 (Yb³⁺)

²F_5/2 (Yb³⁺) + ³F₄ (Tm³⁺) → ³F₂ (Tm³⁺) + ²F_7/2 (Yb³⁺)

²F_5/2 (Yb³⁺) + ³F₃ (Tm³⁺) → ¹G₄ (Tm³⁺) + ²F_7/2 (Yb³⁺)

This process is called sequential sensitization. In the second mechanism, the sensitization process is known as cooperative sensitization, as discussed earlier. In this process, two excited Yb³⁺ ions in the excited state ²F_5/2 after the absorption of a 980 nm photon interact with each other through dipole–dipole interaction, which is most effective. Due to this interaction, two emission photons from the excited Yb³⁺ are coupled to each other and this energy directly excites the Tm³⁺ ion to the excited level ¹G₄. From there, further emissions are possible due to different transitions to lower lying levels, i.e. the cooperative sensitization process.

2 ²F_5/2 (Yb³⁺) + ³H₆ (Tm³⁺) → ¹G₄ (Tm³⁺) + 2 ²F_7/2 (Yb³⁺)

4.2. Downshifting behavior

4.2.1. ANbO₄:Tm³⁺,Yb³⁺ (where A = Y, Gd and La). Fig. 6 shows the photoluminescence excitation (PLEx) and photoluminescence emission (PLEm) spectra of the ANbO₄:Tm³⁺,Yb³⁺ (where A = Y, Gd and La) phosphors. We already discussed the self-activation behavior of niobate-based phosphors in which broad blue emission was observed on UV excitation. The PLEx spectra were recorded by monitoring emission at 456 nm due to the ¹D₂ → ³F₄ transition in ANbO₄:Tm³⁺,Yb³⁺ (ANOTmYb, where A = Y, Gd and La) phosphors in the range of 230–400 nm, as shown in Fig. 6(a). In the spectra, a broad band is observed at 264 nm due to the (NbO₄)³⁻ group along with a sharp band at 362 nm due to the ³H₆ → ¹D₂ transition of the Tm³⁺ ion. From the excitation spectra, it was confirmed that there is energy transfer from the (NbO₄)³⁻ group to the Tm³⁺ ion. The excitation band at 264 nm is more prominent in YNOTmYb than in the others, while the 362 nm peak is more prominent in GVOTmYb. Fig. 6(b) shows the PLEm spectra of the ANbO₄:Tm³⁺,Yb³⁺ (ANOTmYb, where A = Y, Gd and La) phosphors on excitation at 264 nm in which a broad blue emission is observed at 415 nm due to the (NbO₄)³⁻ group along with a sharp emission at 456 nm due to the ¹D₂ → ³F₄ transition of the Tm³⁺ ion. The emission is more intense in YNOTmYb due to the strong excitation band of the (NbO₄)³⁻ group which is responsible for the strong energy transfer between the host and the Tm³⁺ ion.

Fig. 6 (a) Photoluminescence excitation and (b) photoluminescence emission spectra of ANbO₄:Tm³⁺,Yb³⁺ (where A = Y, Gd and La) phosphors.

4.2.2. YVO₄:Ho³⁺ phpsphor. Fig. 7 shows the PLEx and PLEm spectra of the YVO₄:Ho³⁺ phosphor. In this phosphor, a broad excitation band is observed at 327 nm due to the (VO₄)³⁻ group, along with a sharp f–f transition due to the Ho³⁺ ion (Fig. 7(a)). This excitation spectrum was recorded by monitoring emission at 539 nm due to the ⁵F₄/⁵S₂ → ⁵I₈ transition of the Ho³⁺ ion. The photoluminescence emission spectrum was monitored in the range of 350–850 nm on excitation at 327 nm in the YVO₄:Ho³⁺ phosphor, as shown in Fig. 7(b). A strong emission band is observed at 440 nm due to the (VO₄)³⁻ group along with sharp emissions at 539 and 659 nm due to the ⁵F₄/⁵S₂ → ⁵I₈ and ⁵F₅ → ⁵I₈ transitions of the Ho³⁺ ion. There is a strong energy transfer from the (VO₄)³⁻ group to the Ho³⁺ ion. Therefore, YVO₄ is a self-activated host which gives strong broad blue emission on UV excitation. This broad blue emission along with the green and red emissions gives white light or even tunable light.

Fig. 7 (a) Photoluminescence excitation and (b) photoluminescence emission spectra of YVO₄:Ho³⁺ phosphor.

4.2.3. Mechanism of downshifting emission. The mechanism behind the downshifting emission in GdNbO₄ is explained by the possible energy level diagram shown in Fig. 8(a). With λ_ex = 265 nm, the CTB of the NbO₄ group is directly excited. This CTB excitation gives two results: first, radiative transition at 442 nm due to ³T₁ → ¹A₁ of the NbO₄ group and second, some excitation energy transfer to the Tm³⁺ ion and the Gd³⁺ ion. Thus, the Tm³⁺ and Gd³⁺ ions are excited due to ET from the NbO₄ group and the characteristic emission peaks from the Tm³⁺ ion are observed. Due to transfer of part of the energy from the (NbO₄)³⁻ group to the Gd³⁺ ion, the emission intensity of the (NbO₄)³⁻ group is not more prominent compared to other two.

Fig. 8 Energy level diagrams of (a) GdNbO₄:Tm³⁺ (reproduced with permission from ref. 25 with copyright 2015, Institute of Physics) and (b) YVO₄:Ho³⁺ phosphors (reproduced with permission from ref. 19 with copyright 2019, American Chemical Society).

Similarly, the mechanism behind this DS behavior in YVO₄:Ho³⁺ is well explained by the energy level diagram shown in Fig. 8(b). The (VO₄)³⁻ group is excited at 327 nm. The emission from the (VO₄)³⁻ group is observed in a broad blue region at 440 nm. There is resonant energy transfer from the (VO₄)³⁻ group to the ⁵G₄ level of the Ho³⁺ ion. The Ho³⁺ ions relax from that level to lower levels and give various emissions in different regions.

4.3. Quantum cutting (QC) behavior

In general, QC can be understood as a cascade emission of photons from a single rare earth ion or via ET through different sets of rare earth ions. NIR QC has been observed in various systems co-doped with Ln³⁺–Yb³⁺ (Ln = Tm, Pr, Er, Ho, Nd, Tb and Dy)^35–41 and also in systems with single rare earth doping (e.g. Ho³⁺, Tm³⁺ or Er³⁺).^42–44 In QC, there are two types of energy transfer (ET): (1) first order ET and (2) second order ET (cooperative sensitization process). In the first case, there is spectral overlap between the donor emission and the acceptor absorption. If there is no spectral overlap between the donor emission and the acceptor absorption, then second order ET becomes the dominant process, also called cooperative sensitization. In the second case, the sum of the energy of the absorption (excitation) of two acceptors must equal the energy of the donor emissions for the resonant condition.⁶

Host matrices play a crucial role in the sensitization process. GdNbO₄ is known as a self activated compound which has a charge transfer band (CTB) in the UV region and gives blue emission upon UV excitation.^25,30 In GdNbO₄, the niobate group [(NbO₄)³⁻] acts as a fluorescence center in the matrix and gives blue emission under UV excitation. In the previous section, it was observed that there is a broad CTB at 266 nm and also a broad blue emission at 442 nm due to the (NbO₄)³⁻ group since the resonance condition of CET from (NbO₄)³⁻ to Yb³⁺ ion is well satisfied. Therefore, CET takes place and NIR emission (≈900–1040 nm) due to the Yb³⁺ ion was observed (Fig. 9(a)). This phenomenon of NIR QC by CET from the host to the acceptor ion has not been much explored. The possible mechanism of NIR QC can be explained in terms of the possible energy level diagram in Fig. 10. Upon 266 nm Nd:YAG laser excitation, the ¹T₂ level of the (NbO₄)³⁻ group is populated. This population relaxes non-radiatively to the ³T₁ level from which the blue emission (at 442 nm) is obtained. The emission from ³T₁ to ¹A₁ of the (NbO₄)³⁻ group (≈442 nm) excites two Yb³⁺ ions in the excited level ²F_5/2. Then, the further transition from ²F_5/2 to ²F_7/2 gives NIR emission (≈900–1040 nm), as shown in Fig. 9(a).

Fig. 9 Photoluminescence emission spectra of (a) GdNbO₄ and Gd_0.9Yb_0.1NbO₄ and (b) Gd_0.99Tm_0.01NbO₄ and Gd_0.89Tm_0.01Yb_0.1NbO₄ phosphors on excitation with 266 nm. (Reproduced with permission from ref. 25 with copyright 2015, Institute of Physics).

Fig. 10 Energy level diagram to explain the mechanism of quantum cutting (QC) behavior. (Reproduced with permission from ref. 25 with copyright 2015, Institute of Physics).

Fig. 9(b) shows the emission spectra for the Gd_0.9Tm_0.01NbO₄ and Gd_0.8Tm_0.01Yb_0.1NbO₄ phosphors using λ_ex = 266 nm. It can be observed that both spectra are similar, except the emission in the range of 900–1040 nm in the case of the Gd_0.8Tm_0.01Yb_0.1NbO₄ phosphor. The possible mechanism for the observed NIR emission can be explained as follows: with 266 nm excitation, the Tm³⁺ ion is promoted to the high lying level ³P₂ as shown in Fig. 10. From there, the low lying level ¹D₂ is populated via non-radiative relaxation, then further to lower level ¹G₄. The lifetime of this level is relatively high and it behaves as a metastable state. From this level, blue emission is observed at 477 nm. There may be two possibilities of QC emission in Yb³⁺ ions from the ¹G₄ level: the first is cooperative energy transfer from the ¹G₄ level of the Tm³⁺ ion to two Yb³⁺ ions (since the energy of the ¹G₄ level is just double of the energy level of two Yb³⁺ ions) and the second is a cross relaxation process between ¹G₄ → ³H₅ transition (Tm³⁺ ion) and ²F_5/2 → ²F_7/2 transition (Yb³⁺ ion).

5. Applications

5.1. As a spectral converter

The multimodal (UC, DS and QC) behavior of Tm³⁺/Yb³⁺ or Ho³⁺/Yb³⁺ co-doped ANbO₄/AVO₄ phosphors suggests that such materials can be used as a spectral converter. In this, UV/visible light is converted to visible/NIR light via a DS process and NIR light is converted to visible light via a UC process. Along with these two processes, one very peculiar phenomenon is also observed in which one high energy photon (in the UV region) is converted to two or more low energy photons (in the visible or NIR region) via a QC process. These types of materials are very useful for the efficiency enhancement of c-Si solar cells due to the spectral conversion behavior.²⁷ Fig. 11 shows a schematic diagram to explain these spectral conversion processes.

Fig. 11 Schematic diagram to explain the spectral conversion process.

Energy losses due to thermalization of high energy photons higher than the band gap and to transmission of low energy photons less than the band gap can be minimized by downconverting (QC) high energy photons to low energy photons and by upconverting low energy photons from the solar spectrum to high energy photons, respectively. Trupke et al.^45,46 discussed the efficiency of a solar cell equipped with a downconverting layer located on the front surface of a single junction solar cell and an upconverting layer on the rear side as shown in Fig. 12. A perfect reflector is located at the rear of the upconverter.

Fig. 12 Schematic diagram of (a) a UC layer and (b) a DS or DC layer of phosphor material on a solar cell. (Reproduced with permission from ref. 25 with copyright 2015, Institute of Physics).

5.2. Security ink application

Rare earth doped self activated phosphors have potential application as security ink. As seen above, phosphor materials show very intense emission upon UV and NIR excitation. A security coding using this material can be coated on any document; it is not visible in normal light, but the coded security surface becomes visible when this document is exposed to UV or NIR radiation. The security ink applications of phosphor materials have been used for anti-counterfeiting applications. This technique makes items easier to authenticate from fake brands and documents. The optical emission properties of the phosphor are invariant when it is dispersed in acetone or ethyl alcohol. For the development of this type of security ink, we dispersed an appropriate amount of phosphor within a particular liquid medium. The main challenges in this field are high stability, good dispersion of the luminescent nanophosphor, the sticky nature of the ink medium on surfaces and the low cost and availability of the ink medium (Fig. 13).

Fig. 13 The paper with BHU written on it (a) in normal light illumination, (b) showing blue emission on excitation with 365 nm radiation and (c) showing green emission on excitation with 980 nm radiation. (reproduced with permission from ref. 5 with copyright 2020, Elsevier).

5.3. Color tunability with variation of parameters (concentration, external power and external temperature)

Rare earth doped self activated phosphors show color tunable behaviors when varying the concentration of activators/sensitizers, pump power, and external temperature. These color tunable materials are currently used in various applications such as anti-counterfeiting, sensing, security, etc. In Fig. 14, the color tunability is shown in the UC emission process in Ho³⁺/Tm³⁺/Yb³⁺ doped in these hosts by plotting the CIE diagram in which the UC emission intensity can be tuned from light green to light blue by varying the concentration of Yb³⁺ ion. The color tunability is observed when varying the pump power of the diode laser in which the laser induced optical heating effect becomes dominant with the pump power and causes different relaxation processes. Due to these relaxation processes, there is color tunability with pump power. This color tunability was further investigated by varying the external temperature around the activator ion.

Fig. 14 Color tunability of Ho³⁺/Tm³⁺/Yb³⁺ co-doped phosphors in the UC emission process when varying the concentration, pump power and external temperature.

6. Conclusions

Self activated hosts play a very crucial role in the significant enhancement and variation of optical properties of the rare earth ions doped in these host materials. The ANbO₄ (where A = Y, Gd and La) phosphor shows self activation behavior in which broad blue emission is observed from the host on UV excitation. Similarly, the AVO₄ (where A = Y and Gd) phosphor also shows self activation behavior when monitoring the photoluminescence excitation and emission spectra. Tm³⁺/Yb³⁺ and Ho³⁺/Yb³⁺ co-doped niobate and vanadate phosphors show intense blue, NIR, green and red emissions via a UC emission process. DS behavior shows the self activation of ANbO₄ and AVO₄ phosphors and also shows the energy transfer process between the host and the activator ions (i.e., Tm³⁺ and Ho³⁺ ions). The QC process is also observed in these self-activated phosphors and is a new and interesting phenomenon. Broad band QC is observed in ANbO₄/AVO₄ phosphors due to the self-activation behavior of the host. The multimodal behavior in such materials makes them useful for spectral conversion processes which have been frequently used for the enhancement of the efficiency of c-Si solar cells. The color tunable behaviors of these materials make them very useful in various display devices. Currently, luminescence tags are most the popular security elements for authenticating various items.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this manuscript.

Acknowledgements

One of the authors, Mr Abhishek Roy thanks UGC for providing BHU Research fellowship as financial support, respectively.

References

1. S. V. Eliseeva and J. C. G. Bünzli, Rare earths: jewels for functional materials of the future, New J. Chem., 2011, 35(6), 1165–1176.
2. Y. Chen, M. Gong, G. Wang and Q. Su, High efficient and low color-temperature white light-emitting diodes with Tb₃Al₅O₁₂:Ce³⁺ phosphor, Appl. Phys. Lett., 2007, 91(7), 071117.
3. M. Leskelä, Rare earths in electroluminescent and field emission display phosphors, J. Alloys Compd., 1998, 275, 702–708.
4. A. Escudero, A. I. Becerro, C. Carrillo-Carrión, N. O. Nunez, M. V. Zyuzin, M. Laguna, D. González-Mancebo, M. Ocaña and W. J. Parak, Rare earth based nanostructured materials: synthesis, functionalization, properties and bioimaging and biosensing applications, Nanophotonics, 2017, 6(5), 881–921.
5. A. Roy, A. Dwivedi, H. Mishra, D. Kumar and S. B. Rai, Dual mode (upconversion and downshifting) behavior of Ho³⁺/Yb³⁺/Bi³⁺co-doped YTaO₄ phosphor and its application as a security ink, J. Alloys Compd., 2020, 821, 153202.
6. M. Mondal, V. K. Rai, C. Srivastava, S. Sarkar and R. Akash, Enhanced frequency upconversion in Ho³⁺/Yb³⁺/Li⁺: YMoO₄ nanophosphors for photonic and security ink applications, J. Appl. Phys., 2016, 120(23), 233101.
7. Z. Xia, W. Zhou, H. Du and J. Sun, Synthesis and spectral analysis of Yb³⁺/Tm³⁺/Ho^3+- doped Na_0.5Gd_0.5WO₄ phosphor to achieve white upconversion luminescence, Mater. Res. Bull., 2010, 45(9), 1199–1202.
8. M. Wang, M. Li, A. Yu, J. Wu and C. Mao, Rare earth fluorescent nanomaterials for enhanced development of latent fingerprints, ACS Appl. Mater. Interfaces, 2015, 7(51), 28110–28115.
9. J. Ballato, J. S. Lewis and P. Holloway, Display applications of rare-earth-doped materials, MRS Bull., 1999, 24(9), 51–56.
10. K. Soga, K. Tokuzen, K. Tsuji, T. Yamano and H. Hyodo, Kishimoto,NIR Bioimaging: Development of Liposome Encapsulated, Rare Earth Doped Y₂O₃ Nanoparticles as Fluorescent Probes, Eur. J. Inorg. Chem., 2010, 18, 2673–2677.
11. W. Yuhua, Z. Ge, X. Shuangyu, W. Qian, L. Yanyan, W. Quansheng, W. Chuang, W. Xicheng, D. Xin and G. Wanying, Recent development in rare earth doped phosphors for white light emitting diodes, J. Rare Earths, 2015, 33(1), 1–12.
12. J. Qiao, G. Zhou, Y. Zhou, Q. Zhang and Z. Xia, Divalent europium-doped near-infrared-emitting phosphor for light-emitting diodes, Nat. Commun., 2019, 10(1), 1–7.
13. G. Blasse and A. Bril, Luminescence phenomena in compounds with fergusonite structure, J. Lumin., 1970, 3(2), 109–131.
14. A. H. Buth and G. Blasse, Luminescence and energy transfer in yttrium niobate (YNbO4), Phys. Status Solidi A, 1981, 64(2), 669–676.
15. I. Nowak and M. Ziolek, Niobium compounds: preparation, characterization, and application in heterogeneous catalysis, Chem. Rev., 1999, 99(12), 3603–3624.
16. A. Dwivedi, A. K. Singh and S. B. Rai, Down-shifting and upconversion photoluminescence in Ho^3+/Yb³⁺codoped GdNbO₄: effect of the Bi³⁺ ion and the magnetic field, Dalton Trans., 2014, 43(42), 15906–15914.
17. J. M. Jehng and I. E. Wachs, Structural chemistry and Raman spectra of niobium oxides, Chem. Mater., 1991, 3(1), 100–107.
18. D. J. Jovanović, Ž. Antić, R. M. Krsmanović, M. Mitrić, V. Đorđević, B. Bártová and M. D. Dramićanin, Annealing effects on the microstructure and photoluminescence of Eu³⁺-doped GdVO₄ powders, Opt. Mater., 2013, 35(10), 1797–1804.
19. A. Dwivedi, E. Rai, D. Kumar and S. B. Rai, Effect of synthesis techniques on the optical properties of Ho³⁺/Yb³⁺ co-doped YVO₄ phosphor: a comparative study, ACS Omega, 2019, 4(4), 6903–6913.
20. W. Piotrowski, L. Dalipi, K. Elzbieciak-Piecka, A. Bednarkiewicz, B. Fond and L. Marciniak, Self Referenced Temperature Imaging with Dual Light Emitting Diode Excitation and Single Band Emission of AVO4:Eu³⁺(A=Y,La,Lu,Gd) Nanophosphors, Adv. Photonics Res., 2021, 2100139.
21. H. Zhou, N. Guo, X. Lü, Y. Ding, L. Wang, R. Ouyang and B. Shao, Ratiometric and colorimetric fluorescence temperature sensing properties of trivalent europium or samarium doped self-activated vanadate dual emitting phosphors, J. Lumin., 2020, 217, 116758.
22. P. Botella, F. Enrichi, A. Vomiero, J. E. Muñoz-Santiuste, A. B. Garg, A. Arvind, F. J. Manjón, A. Segura and D. Errandonea, Investigation on the luminescence properties of InMO₄ (M= V⁵⁺, Nb⁵⁺, Ta⁵⁺) crystals doped with Tb³⁺ or Yb³⁺ rare earth ions, ACS Omega, 2020, 5(5), 2148–2158.
23. A. Dwivedi, A. K. Singh and S. K. Singh, Wide-bandgap lanthanide niobates: optical properties and applications, Mater. Res. Bull., 2020, 131, 110960.
24. A. Dwivedi, K. Mishra and S. B. Rai, Role of Gd³⁺ ion on downshifting and upconversion emission properties of Pr³⁺, Yb³⁺ co-doped YNbO₄ phosphor and sensitization effect of Bi³⁺ ion, J. Appl. Phys., 2016, 120(4), 043102.
25. A. Dwivedi, K. Mishra and S. B. Rai, Multi-modal luminescence properties of RE3+ (Tm3+, Yb3+) and Bi3+ activated GdNbO4 phosphors—upconversion, downshifting and quantum cutting for spectral conversion, J. Phys. D: Appl. Phys., 2015, 48, 435103.
26. K. Li, Y. Zhang, X. Li, M. Shang, H. Lian and J. Lin, Host-sensitized luminescence in LaNbO₄:Ln³⁺(Ln³⁺= Eu³⁺/Tb³⁺/Dy³⁺) with different emission colors, Phys. Chem. Chem. Phys., 2015, 17(6), 4283–4292.
27. A. Dwivedi, K. Mishra and S. B. Rai, Investigation of Upconversion, downshifting and quantum–cutting behavior of Eu³⁺, Yb³⁺, Bi³⁺ co-doped LaNbO₄ phosphor as a spectral conversion material, Methods Appl. Fluoresc., 2018, 6(3), 035001.
28. C. Niu, L. Li, X. Li, Y. Lv and X. Lang, Upconversion photoluminescence properties of Ho³⁺/Yb³⁺ co-doped YNbO₄ powder, Opt. Mater., 2018, 75, 68–73.
29. X. Li, Z. Guan, Y. Duan, R. Shen, Y. Tian, X. Wang, S. Xu, H. Yu, X. Zhang and B. Chen, Bright green up-conversion luminescence of LaNbO4: Nd3+/Yb3+/Ho3+ phosphors under 808 nm and 980 nm excitations and the effects of dopant concentration, J. Lumin., 2022, 241, 118524.
30. A. Dwivedi, K. Mishra and S. B. Rai, Tm³⁺, Yb³⁺ activated ANbO₄ (A= Y, Gd, La) phosphors: a comparative study of optical properties (downshifting and upconversion emission) and laser induced heating effect, J. Phys. D: Appl. Phys., 2016, 50(4), 045602.
31. K. Dharendra, A. Roy, A. Dwivedi, A. K. Rai, S. B. Rai and D. Kumar, Two steps enhancement of dual mode (UC and DS) behaviour of Ho³⁺/Yb³⁺ and Tm³⁺/Yb³⁺ co-doped GdVO₄ phosphors: improvement in spectral and color purity, Mater. Res. Bull., 2021, 139, 111280.
32. A. Dwivedi, D. Kumar, S. B. Rai and A. K. Rai, Effect of host on the radiative (upconversion emission) as well as non-radiative relaxation (laser induced optical heating) in Tm3+/Yb3+ co-doped phosphors, J. Lumin., 2020, 226, 117421.
33. J. Li, J. Zhang, Z. Hao, L. Chen, X. Zhang and Y. Luo, Intense Upconversion Luminescence of CaSc₂O₄:Ho³⁺/Yb³⁺ from Large Absorption Cross Section and Energy-Transfer Rate of Yb³⁺, ChemPhysChem, 2015, 16(7), 1366–1369.
34. J. Sun, Y. Lan, Z. Xia and H. Du, Sol–gel synthesis and green upconversion luminescence in BaGd₂(MoO₄)₄:Yb³⁺,Er³⁺ phosphors, Opt. Mater., 2011, 33(3), 576–581.
35. Q. Zhang, B. Zhu, Y. Zhuang, G. Chen, X. Liu, G. Zhang, J. Qiu and D. Chen, Quantum cutting in Tm³⁺/Yb³⁺codoped lanthanum aluminum germanate glasses, J. Am. Ceram. Soc., 2010, 93(3), 654–657.
36. J. Hu, H. Xia, H. Hu, Y. Zhang, H. Jiang and B. Chen, Synthesis and efficient near-infrared quantum cutting of Pr³⁺/Yb³⁺ co-doped LiYF₄ single crystals, J. Appl. Phys., 2012, 112(7), 073518.
37. B. Fan, C. Chlique, O. Merdrignac-Conanec, X. Zhang and X. Fan, Near-infrared quantum cutting material Er³⁺/Yb³⁺ doped La₂O₂S with an external quantum yield higher than 100, J. Phys. Chem. C, 2012, 116(21), 11652–11657.
38. D. C. Yu, X. Y. Huang, S. Ye and Q. Y. Zhang, Efficient first-order resonant near-infrared quantum cutting in β-NaYF₄:Ho³⁺,Yb³⁺, J. Alloys Compd., 2011, 509(41), 9919–9923.
39. H. K. Dan, T. D. Tap, H. X. Vinh, H. T. Nguyen-Truong, J. Qiu, D. Zhou and N. M. Ty, Effects of La³⁺ on the enhancement NIR quantum cutting and UC emissions in Nd³⁺–Yb³⁺ co-doped transparent silicate glass-ceramics for solar cells, Opt. Mater., 2019, 95, 109229.
40. Q. Y. Zhang, C. H. Yang, Z. H. Jiang and X. H. Ji, Concentration-dependent near-infrared quantum cutting in GdBO₃:Tb³⁺,Yb³⁺ nanophosphors, Appl. Phys. Lett., 2007, 90(6), 061914.
41. Y. Yang, L. Liu, S. Cai, F. Jiao, C. Mi, X. Su, J. Zhang, F. Yu, X. Li and Z. Li, Up-conversion luminescence and near-infrared quantum cutting in Dy³⁺, Yb³⁺ co-doped BaGd₂ZnO₅ nanocrystal, J. Lumin., 2014, 146, 284–287.
42. D. C. Yu, X. Y. Huang, S. Ye, M. Y. Peng and Q. Y. Zhang, Efficient near-infrared quantum splitting in YVO₄:Ho³⁺ for photovoltaics, Sol. Energy Mater. Sol. Cells, 2012, 101, 303–307.
43. Y. Z. Wang, D. C. Yu, H. H. Lin, S. Ye, M. Y. Peng and Q. Y. Zhang, Broadband three-photon near-infrared quantum cutting in Tm³⁺ singly doped YVO₄, J. Appl. Phys., 2013, 114(20), 203510.
44. X. Huang, D. Chen, L. Lin, Z. Wang, Z. Feng and Z. Zheng, Infrared quantum cutting in Er³⁺: NaYF₄ nanostructured glass ceramics for solar cells, Optik, 2014, 125(1), 565–568.
45. T. Trupke, M. A. Green and P. Würfel, Improving solar cell efficiencies by up-conversion of sub-band-gap light, J. Appl. Phys., 2002, 92(7), 4117–4122.
46. T. Trupke, M. A. Green and P. Würfel, Improving solar cell efficiencies by down-conversion of high-energy photons, J. Appl. Phys., 2002, 92(3), 1668–1674.

---