White light from dispersible lanthanide-doped LaVO₄ core–shell nanoparticles

Abstract

White light has been generated from dispersible lanthanide ion-doped LaVO₄ core–shell nanoparticles by using a single UV excitation source. The core–shell nanoparticles are excited with single wavelength 280 nm UV light, in which the energy transfer from [VO₄]³⁻ groups to Ln³⁺ ions occurs to produce bright white light. The absolute quantum yields of LaVO₄:Tm³⁺/Tb³⁺/Eu³⁺ (Type 1) and LaVO₄:Tm³⁺/Dy³⁺ (Type 2) core–shell nanoparticles have been estimated as 5% and 7% respectively. The control core nanoparticles doped with all the lanthanide ions did not produce white light which proves the need of a core–shell structure. The core–shell structure of LaVO₄ nanoparticles has been investigated by TEM, EDX, XRF, and PL spectroscopy techniques.

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Introduction

Nanocrystalline phosphors attract a great deal of interest for the generation of cheap and efficient white light for applications such as display devices, general lighting, and active materials in lasers.^1–3 The devices that have been developed to produce white light include incandescent lamps, fluorescent light tubes, compact fluorescent lamps, etc. and the latest additions to the field are organic LED's and inorganic LED's which exploit electricity for light generation.^4–11 In these devices, white light has been generated either by mixing three different monochromatic sources (RGB) or conversion of UV/blue light by using phosphors.¹² These devices suffer either from single or multiple issues such as long term stability of emitters, high cost, and low efficiency. For the three converter (RGB) system, the manufacture cost is high and the blue emission intensity is poor because of strong reasbsorption of blue light by the red and green-emitting ions.^13,14 Therefore, to circumvent these disadvantages there is a need to produce a single phase material that would be more stable, efficient, and durable as well as cost effective. To this end, lanthanide ion-doped nanomaterials are gaining popularity due to their increased photostability, sharp emission lines, large stokes shift, and longer lifetime.^15–17 They can produce light by three photophysical processes: 1) up-conversion^18–20 (conversion of two or more lower energy photons into one higher energy photon), 2) conventional ‘stokes’ shift fluorescence,²¹ (conversion of one higher energy photon into one lower energy photon), and, 3) quantum cutting^22,23 (conversion of one higher energy photon into two or more lower energy photons). There are reports available on the generation of white light from 1) up-conversion nanoparticles,²⁴ 2) multielement (Ln³⁺)-doped core nanomaterials,²⁵ 3) lanthanide-doped bulk materials,^26,27 and 4) lanthanide-complexes.²⁸ Few reports are available on the generation of white light from lanthanide-doped nanomaterials through up-conversion,^29–31 however, the efficiency is very low (<0.5%).^29,32 Though multielement-doped nanomaterials produce white light, their efficiency can be limited by the internal energy transfer between them. Additionally, processing will be a challenging task for lanthanide-doped bulk materials in fabrication of devices. It is well known that lanthanide complexes have inferior optical properties compared to lanthanide-doped inorganic matrices. Furthermore, there are a number of reports available on the generation of white, red, green and blue light by conventional ‘stokes’ shift fluorescence because this process is more efficient than up-conversion.^22,33 However, they have several limitations such as, inefficient direct excitation of lanthanide ions, internal energy transfer due to multiple dopant in a core matrix, poor dispersibility in organic solvents, use of multiple phosphors, multiple excitation sources, etc.³⁴ To our knowledge, nobody has so far reported the generation of white light through ‘stokes’ shift fluorescence from a single type dispersible lanthanide-doped core nanoparticle by using a single excitation source (e.g. UV light).

In this article, we report the generation of white light from dispersible lanthanide ion-doped LaVO₄ core–shell nanoparticles via a ‘stokes’ shift fluorescence process by using a single excitation source (UV light). We note that direct excitation of lanthanide ions is an inefficient process due to the lesser extinction coefficient (1–10 M⁻¹ cm⁻¹) compared to energy transfer from a host material (LnVO₄, LnPO₄, TiO₂, etc.) or other ions^13,15 (Ce³⁺ and Yb³⁺).^35–40 The vanadate charge transfer transition has several orders of magnitude of extinction coefficient and a broad absorption range (260–340 nm) compared to lanthanide ions.³⁶ Additionally, thermally activated energy migration from [VO₄]³⁻ to lanthanide ions is very efficient and it has been universally used as a sensitizer for most of the lanthanide ions.⁴¹

Experimental

Materials

All the lanthanide salts and sodium orthovanadate were obtained from Sigma–Aldrich. Ethanol was obtained from Merck's chemicals and dichloromethane, oleic acid, and triethylamine were obtained from Qualigens India Ltd. All the chemicals and salts were used without any further purification.

Preparation of oleic acid stabilized lanthanide-doped LaVO₄ core–shell nanoparticles

Oleic acid (3 ml) was dissolved in 70 ml of distilled water–ethanol (1 : 1) mixture. The pH of the solution was adjusted to 6 by adding a few drops of triethylamine (30% w/v) and the solution was heated to 75 °C. The total stoichiometric amount (6 mmol for type 1 and 4 mmol for type 2 core–shell particles) of aqueous solution of sodium orthovanadate (3 mL) was added dropwise to the above solution, followed by the dropwise addition of 2 ml aqueous solution of La(NO₃)₃ and Tm(NO₃) salts (total 1.33 mmol). The mixture was stirred for 10 min to prepare the core nanoparticles followed by the dropwise addition of 2 ml aqueous solution of La(NO₃)₃ salts (total 1.33 mmol) to prepare the inert shell. The mixture was again stirred for 10 min and subsequent shells were prepared by adding Ln(NO₃)₃ salt after every 10 min to prepare the shells. After the addition of all the lanthanide salt portions, the mixture was allowed to stir for 2 h and the precipitate was washed with water and ethanol to remove the excess oleic acid. The type 1 core–shell particles consist of Tm³⁺ ions (20%) in the LaVO₄ core, Tb³⁺ (10%) and Eu³⁺ (0.5%) ions in the second and fourth LaVO₄ shell respectively. The first, third and fifth shells are undoped LaVO₄ shell. In type 2 core-shell particles, Tm³⁺ (15%) and Dy³⁺ (0.5%) ions are doped in the LaVO₄ core and second LaVO₄ shell, respectively, whereas the first and third shells are undoped LaVO₄ shell.

Characterizations

The photoluminescence spectra and decay curves were recorded using an Edinburgh instruments FLSP 920 fluorescence system equipped with a red-sensitive Peltier element cooled Hamamatsu R928-P PMT. Emission and excitation spectra were measured by exciting the samples with a 450 W steady state Xe lamp. Decay curves were measured with a Nd:YAG laser, attached to an optical parametric oscillator (OPO) with an optical range from 210–2400 nm. Absolute quantum yields were measured with an integrating sphere coated with BaSO₄. The TEM images were obtained from FEI Technai G² U-Twin (200 KeV) instrument. ¹H NMR and FTIR spectra were measured by using a Bruker AC 300 and Bruker Alfa respectively. XRD analyses were done using a Siemens D5000 Bragg-Brentano θ–2θ diffractometer. The nanoparticles (20–25 mg) were smeared onto a zero-diffraction quartz plate using ethanol. Step-scan X-ray powder-diffraction data were collected over the 2θ range 3–100° with Cu-Kα (40 kV, 40 mA) radiation on a Siemens D5000 Bragg-Brentano θ–2θ diffractometer equipped with a diffracted-beam graphite monochromator crystal, 2 mm (1°) divergence and anti-scatter slits, 0.6 mm receiving slit, and incident beam Soller slit. The scanning step size was 0.04° 2θ with a counting time of 1.5 s step⁻¹. X-ray fluorescence spectroscopy data were obtained from a ZSX primus series, Rigaku corporation spectrometer. EDAX analysis was done with a Carl Zeiss NTS GmbH-SUPRA, 40 VP FESEM.

Results and discussion

The white light has been generated from two types of core–shell nanoparticles: The type 1 core–shell nanoparticle is doped with Tm³⁺ (blue)/Tb³⁺ (green)/Eu³⁺ (red) ions and type 2 is doped with Tm³⁺ (blue)/Dy³⁺ (yellow) ions in different LaVO₄ shells, separated by an inert (un-doped) LaVO₄ layer (Fig. 1a). The core–shell nanoparticles are excited with single wavelength 280 nm UV light, in which the energy transfer from [VO₄]³⁻ groups to Ln³⁺ ions is exploited to produce bright white light.⁴¹ Moreover, these core–shell nanoparticles can be dispersible in organic solvents and polymers, which can easily be processed via spin-coating for flat panel displays and LEDs applications. The CIE co-ordinates of the light emitted from the nanoparticles can also be changed by adjusting the concentration and ratio of lanthanide ions doped in different shells. Furthermore, these particles can be excited with a broad range of UV wavelength (260–340 nm) which is advantageous to general lighting applications.

Fig. 1 a) Schematic representation of the generation of white light from LaVO₄:Tm³⁺/Tb³⁺/Eu³⁺ (Type 1) and LaVO₄:Tm³⁺/Dy³⁺ (Type 2) core–shell nanoparticles and b) digital photographs of blue, green, red, yellow and white light emissions from colloidal dispersions of LaVO₄:Tm³⁺, LaVO₄:Tb³⁺, LaVO₄:Eu³⁺, LaVO₄:Dy³⁺ core nanoparticles and LaVO₄:Tm³⁺/Tb³⁺/Eu³⁺ (Type 1) and LaVO₄:Tm³⁺/Dy³⁺ (Type 2) core–shell nanoparticles in dichloromethane solvent (1 mg mL⁻¹). The samples were excited with a 280 nm Xe lamp light.

The LaVO₄ core–shell particles stabilized with oleic acid were prepared by the co-precipitation method. Fig. 1a shows the schematic representation of type 1 (LaVO₄:Tm³⁺/Tb³⁺/Eu³⁺) and type 2 (LaVO₄:Tm³⁺/Dy³⁺) LaVO₄ core–shell nanoparticles. In type 1, Tm³⁺ ions (20%) are doped in the LaVO₄ core, Tb³⁺ (10%) and Eu³⁺ (0.5%) ions are doped in the second and fourth LaVO₄ shell respectively. The first, third and fifth shells are inert shell (un-doped), which separate the lanthanide-ions from each other to prevent quenching due to internal energy transfer. The outer inert shell also prevents quenching from environmental effects such as solvents and OH groups. In type 2 Tm³⁺ (15%) and Dy³⁺ (0.5%) ions are doped in the LaVO₄ core and second LaVO₄ shell, respectively, whereas the first and third shells are inert shells. Fig. 1b shows the digital photographs of bright white light emissions from LaVO₄:Tm³⁺/Tb³⁺/Eu³⁺ (Type 1), and LaVO₄:Tm³⁺/Dy³⁺ (Type 2) core–shell nanoparticles dispersed in dichloromethane by excitation with 280 nm light. Additionally, bright blue, green, red, and yellow emissions have been observed by excitation with 280 nm light from LaVO₄:Tm³⁺, LaVO₄:Tb³⁺, LaVO₄:Eu³⁺, LaVO₄:Dy³⁺ core nanoparticles, respectively. The digital photographs clearly suggest that the bright white light from both the types of core–shell particles can easily be seen by naked eye.

Fig. 2a demonstrates the emission spectrum of LaVO₄:Tm³⁺/Tb³⁺/Eu³⁺ (Type 1) core–shell nanoparticles dispersed in dichloromethane (λ_ex = 280 nm). It is evident from the emission spectrum that the peaks corresponding to Tm³⁺, Tb³⁺, and Eu³⁺ emissions can easily be seen. The major emission peak at 475 nm is from ¹G₄ to ³H₆ level of Tm³⁺ ions, and the one at 545 nm is from ⁵D₄ to ⁷F₅ level of Tb³⁺ ions. The peaks at 591, 612, 650, and 695 nm are from ⁵D₀ to ⁷F₁, ⁷F₂, ⁷F₃, and ⁷F₄ levels of Eu³⁺ ions, respectively. The emission peak from Tb³⁺ ions (490 nm, ⁵D₄ to ⁷F₅) is overlapped with the 475 nm emission of Tm³⁺ ions. The emission peaks at 575 (⁵D₄ to ⁷F₄) and 625 nm (⁵D₄ to ⁷F₃) from Tb³⁺ ions also overlapped with Eu³⁺ ion emissions. The emission spectrum from the type 2 core–shell nanoparticles is shown in Fig. 2b. The emission peaks at 475 nm and 575 nm are from Tm³⁺ and Dy³⁺ (⁴F_9/2 to ⁶H_13/2) ions, respectively. The CIE co-ordinates of the emitted light from type 1 and 2 nanoparticles were found to be (0.35, 0.34) and (0.32, 0.34) respectively, compared to (0.33, 0.33) for pure white light (Fig. 2d). Excitation with a range of UV wavelengths (260–330 nm) has also produced bright white light and CIE colour co-ordinates were within the white light region (data not given). In order to prove the non-existence of internal energy transfer between lanthanide ions, both the core–shell type nanoparticles were excited (direct excitation of lanthanide ions) with 410 nm for Eu³⁺ ions (⁵D₃), 488 nm for Tb³⁺ ions (⁵D₄), 475 nm for Tm³⁺ ions (¹G₄), and 430 nm for Dy³⁺ ions (⁴G_11/2) light and the emissions corresponding to the direct excitation of lanthanide ions were only observed. This clearly suggests that there was no internal energy transfer between lanthanide-ions suggesting the formation of a core–shell structure with inert LaVO₄ shells in between the emissive layers. Our control experiment (doping all the lanthanide ions in the LaVO₄ core nanoparticles) did not emit white light (Fig. 3). The emission spectrum (Fig. 3a) shows the presence of Tm³⁺, Tb³⁺, and Eu³⁺ ions emissions, however, the Eu³⁺ emission dominates compared to Tm³⁺ and Tb³⁺ emissions. Similarly, in Fig. 3b, the Dy³⁺ emission dominates compared to Tm³⁺ emission. We attribute this to the efficient energy transfer from [VO₄]³⁻ group to Eu³⁺ and Dy³⁺ ion compared to Tm³⁺ and Tb³⁺ ion. This clearly proves the need for a core–shell structure with the inert shell in between the emissive layers.

Fig. 2 Emission spectra a) LaVO₄:Tm³⁺/Tb³⁺/Eu³⁺ (Type 1), b) LaVO₄:Tm³⁺/Dy³⁺ (Type 2) core–shell nanoparticles (λ_ex = 280 nm), c) excitation spectra of Tm³⁺, Tb³⁺, Eu³⁺, and Dy³⁺-doped LaVO₄ nanoparticles, and d) CIE colour co-ordinates of the white light generated from type 1 and type 2 nanoparticles.

Fig. 3 Emission spectra a) LaVO₄:Tm³⁺/Tb³⁺/Eu³⁺ core nanoparticles (control experiment for type 1, λ_ex = 280 nm) and b) LaVO₄:Tm³⁺/Dy³⁺ core nanoparticles (control experiment for type 2, λ_ex = 280 nm).

Fig. 2c shows the excitation spectra of Tm³⁺, Tb³⁺, Dy³⁺, and Eu³⁺ ions doped in LaVO₄ core nanoparticles. The broad band from 260 to 340 nm clearly proves that the emission from all the Ln³⁺ ions is mainly due to energy transfer from [VO₄]³⁻ group, by a charge-transfer transition in the V–O bond. We note that the excitation peaks of Eu³⁺, Tb³⁺, Tm³⁺, and Dy³⁺ ions have not been observed due to a dilution effect and efficient energy transfer. This also proves that the lanthanide ions can be excited through energy transfer by using a single UV excitation source with a broad range of wavelength. We have also measured the absolute quantum yield of generated white light as 5 ± 1% and 7 ± 1% for type 1 and type 2 nanoparticles, respectively by using an integrating sphere. The quantum yield of the generated white light is on the lower side compared to bulk material which may be due to the size effect and fluorescence quenching by water molecules involved in the synthesis procedure. We also note that the quantum yield of LaVO₄:Eu³⁺ (5%) core–shell nanoparticles is estimated to be ~75%. It is well known that Tm³⁺ ion is a weak emitter and its quantum yield is very low (~0.008%). Since, Eu³⁺ (red) and Dy³⁺ (yellow) ions are strong emitters, we have reduced their doping concentration to ~0.5% which also limits our white light quantum yield. Furthermore, the average lifetimes of Tm³⁺, Tb³⁺, Eu³⁺, and Dy³⁺ ions are 0.45 ± 0.05, 1.4 ± 0.1, 1.6 ± 0.12, and 1.1 ± 0.15 ms respectively. (Fig. S1, ESI†) The X-ray powder diffraction pattern (Fig. 4d) of LaVO₄ was refined in space group D¹⁹_4h with the Rietveld refinement method and it shows that LaVO₄ core–shell nanoparticles are in the highly crystalline zircon type phase in contrast to the bulk monazite phase. This matches with the previous reports.¹⁸ Furthermore, FTIR (Fig. S2, ESI†) and ¹H NMR (Fig. S3, ESI†) spectra prove the formation of LaVO₄ nanoparticles with oleic acid as the stabilizing ligand. Broad bands at 789 and 3000 cm⁻¹ in FTIR show the characteristic peak of the [VO₄]³⁻ and the CH stretching vibrations of oleic acid respectively (Fig. S2, ESI†). A broad band above 3000 cm⁻¹ suggests that the surfaces of LaVO₄ nanoparticles are terminated with V–OH bonds. Additionally, a broad NMR peak at 5.2 ppm, attributed to the CH=CH protons clearly suggests the presence of oleic acid (Fig. S3, ESI†).

Fig. 4 a) TEM image of LaVO₄:Eu³⁺ core nanoparticles, b) TEM image of LaVO₄:Tm³⁺/Tb³⁺/Eu³⁺ core–shell nanoparticles, c) TEM image of LaVO₄:Tm³⁺/Dy³⁺ core–shell nanoparticles (particles are identified with white dotted lines for clarity), and d) XRD pattern of LaVO₄:Tm³⁺/Tb³⁺/Eu³⁺ core–shell nanoparticles.

In order to prove the core–shell structure, LaVO₄:Eu³⁺ core, type 1, and type 2 LaVO₄ core–shell particles were investigated by point energy dispersive X-ray spectroscopy (EDX), X-ray fluorescence spectroscopy (XRF), fluorescence spectroscopy along with transmission electron microscopy. The TEM images clearly show that the average size of the LaVO₄:Eu³⁺ core (Fig. 4a), type 1 core–shell (Fig. 4b) and type 2 core–shell (Fig. 4c) nanoparticles are 13 ± 2, 19 ± 2, and 23 ± 2 nm, respectively. The TEM images suggest that there is a clear shift in particle size distribution for core–shell particles (average shell thickness of 1 nm per each shell) compared to core particles which clearly suggests the formation of core–shell structure. We note that since the core and shells are made of the same host materials TEM images will not show any contrast between the core and shell structures. In order to further support the formation of the core–shell structure, EDX (Fig. S4 and S5, ESI†) and XRF (Fig. S6 and S7, ESI†) studies have been performed at various points and clearly show the presence of all the lanthanide ions along with carbon and oxygen (due to the presence of oleic acid) in the same composition in various places. This further supports the formation of the core–shell structure. Additionally, increase in the lifetime of Eu³⁺ ions in the LaVO₄ core–shell particles (τ_av = 1.6 ± 0.12 ms) compared to core particles (τ_av = 0.98 ± 0.09 ms) suggests that the europium ions are in the core–shell structure and are protected from the external quenching events. This matches with the previous reports and further supports the formation of a core–shell structure. From the above observations, we can eliminate the doubt of formation of separate LaVO₄ nanoparticles doped with different lanthanide ions. Moreover, the direct excitation of lanthanide ions (λ_ex = 395 nm for Eu³⁺, λ_ex = 488 nm for Tb³⁺, λ_ex = 475 nm for Tm³⁺ nm for type 1 core–shell particles, and λ_ex = 355 nm for Tm³⁺ nm for type 2 core–shell particles, and λ_ex = 428 nm for Dy³⁺ nm) gives only the corresponding lanthanide ion emissions. This clearly proves that there is no internal energy transfer between the lanthanide ions, which also suggests that the shells could be homogeneous.

Conclusions

In conclusion, bright white light has been generated from LaVO₄:Tm³⁺/Tb³⁺/Eu³⁺ (Type 1) and LaVO₄:Tm³⁺/Dy³⁺ (Type 2) core–shell nanoparticles by excitation with a single 280 nm light. The absolute quantum yields of the white light have been found to be 5% and 7% for type 1 and type 2 nanoparticles respectively. We have also shown that emission from all the Ln³⁺ ions is mainly due to energy transfer from VO₄ group and the inert LaVO₄ shell between the emissive layers prevents the quenching due to internal energy transfer between lanthanide-ions.

Acknowledgements

DST and DST nanomission, India supports are gratefully acknowledged.

References

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Footnote

† Electronic Supplementary Information (ESI) available: Decay curve, NMR spectra, and FTIR spectra of LaVO₄ samples. See DOI: 10.1039/c2ra21586a

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