Formation of green-emitting LaPO₄:Ce,Tb nanophosphor layer and its application to highly transparent plasma displays

Abstract

Uniformly sized LaPO₄:Ce,Tb nanophosphors were produced by using lanthanide nitrates and phosphoric acid along with citric acid and polyvinylpyrrolidone (PVP). The most efficient nanophosphors were obtained by varying their post-annealing temperature and chemical composition. The optimized nanophosphors were subsequently dispersed in 2-methoxyethanol and a highly transparent green-emitting nanophosphor layer was formed on a glass substrate via spin-coating of the dispersion. The spin-coating was repeated up to 7 times to increase the thickness of the nanophosphor layer. Despite multiple coatings, all nanophosphor layers maintained an excellent visible transparency. Ultimately, using the nanophosphor layer deposited on glass substrate, mini-sized transparent plasma display panels (PDPs) were fabricated and their luminance characteristics were described.

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Introduction

Lanthanide orthophosphates (LnPO₄) are useful host lattices for doping rare earth ions to generate various emission colors with high quantum efficiencies. Among them, LaPO₄ codoped with Ce³⁺ and Tb³⁺ ions (LaPO₄:Ce,Tb), which has been commercially applied as a green emitter in fluorescent lamps, cathode ray tubes, and plasma display panels (PDPs),¹ not only possesses excellent chemical and thermal stabilities, but also is a highly efficient phosphor with a quantum yield of >70%.^2–4 Strong green emission of LaPO₄:Ce,Tb phosphor results from the excitation of the 4f → 5d transition of Ce³⁺ ions, followed by the highly efficient energy transfer from Ce³⁺ to neighboring Tb³⁺ ions.

During the last decade, luminescent oxide-based nanocrystals have progressed in synthetic technology and application. Rare earth-doped oxide nanocrystals with a high quantum yield have been considered to be useful as active components in a wide range of applications including displays, lighting, lasers, biological labels, and so on.^2,5–7 A variety of strategies for synthesizing aggregated or non-aggregated LaPO₄:Ce,Tb nanoparticles have been designed including ionic liquid,^2,8,9 hydrothermal,¹ organic liquid phase,^4,10 reverse micelle,¹¹ and aqueous precipitation chemistries.^7,12

Flexibility and transparency are among important attributes which are needed in advanced display devices. Transparent displays have been demonstrated mainly in the field of organic light-emitting devices.^13–15 If the formation of a transparent phosphor layer is possible in PDPs, transparent PDPs may be realized. The transparency of conventional PDPs is limited mainly by the use of micron-sized phosphors that exhibit substantial light scattering. However, by introducing nanophosphors for the formation of scattering-free/-less luminescent layers, transparent PDPs would be feasible. In very recent years, Feldmann et al. reported the microwave-assisted ionic liquid synthesis of LaPO₄:Ce,Tb nanophosphors and formation of a transparent luminescent layer via ink-jet printing for fabrication of a transparent dielectric barrier discharge lamp.⁹

In this paper, we report green-emitting LaPO₄:Ce,Tb nanophosphors with an average size of ∼30 nm that were synthesized by a modified citric acid-assisted sol–gel route. After post-annealing at 1000 °C, the nanophosphors were dispersed in 2-methoxyethanol. Using this dispersion, a highly transparent nanophosphor layer was formed on a glass substrate by multiple spin-coatings. Using the above transparent luminescent layer-deposited glass as the rear plate and combining this rear plate with the front plate of conventional alternating current (ac)-PDPs, a mini-sized transparent plasma display panel was demonstrated for the first time.

Experimental

Synthesis of LaPO₄:Ce,Tb nanophosphors

LaPO₄:Ce,Tb nanophosphors were synthesized by a modified citric acid-assisted sol–gel method, where La(III) nitrate (La(NO₃)₃·6H₂O, 99.99%), Ce(III) nitrate (Ce(NO₃)₃·6H₂O, 99.999%), Tb(III) nitrate (Tb(NO₃)₃·5H₂O, 99.9%), phosphoric acid (H₃PO₄, ≥98%), citric acid, and polyvinylpyrrolidone (PVP, M_w = 10 000) were used as-received. In a typical synthesis of La_0.4PO₄:Ce_0.4,Tb_0.2 nanophosphors, 3.87 mmol of La nitrate, 3.87 mmol of Ce nitrate, 1.93 mmol of Tb nitrate, 10.63 mmol of phosphoric acid, 10 g of citric acid, and 1 g of PVP were dissolved in 50 mL of water and stirred for 12 h at room temperature. This mixture was aged at 80 °C for 2 h and subsequently heated to 150 °C for 12 h. The resulting solidified product was ground and annealed at 800, 1000 or 1200 °C for 4 h in an air atmosphere.

Transparent ac-PDP fabrication

For preparation of a dispersion solution containing LaPO₄:Ce,Tb nanophosphors, 0.1 g of as-annealed nanophosphors were agitated with a standard ultrasonicator (power: 280 W) in 5 mL of 2-methoxyethanol for 2 h. A transparent nanophosphor layer was deposited on 6 cm × 6 cm sized, 5 mm thick soda lime glass substrate by spin coating (3000 rpm, 60 s). After coating, the nanophosphor layer with an area of 3.5 cm × 3.5 cm was dried at 150 °C for 10 min on a hot plate. To increase the layer thickness, the above coating process was repeated up to 7 times. A transparent PDP was fabricated using mini-sized test panels of ac-PDP. The front plate of the test panels has the same structure as that in conventional PDPs. In brief, the front plate consisted of glass substrate, electrodes, dielectric layer and MgO thin film. Electrodes in the front plate consisted of two parts: transparent indium tin oxide (ITO) electrode and silver bus electrode. Because of the high resistance of the ITO electrode, the silver bus electrode is typically required for driving of a large-sized PDP panel. For sustaining a PDP discharge and protecting electrodes from plasma discharge, a 30 μm thick dielectric layer was formed by screen-printing and subsequent sintering at 580 °C. Finally, a ∼500 nm thick MgO thin film that serves as a dielectric protective layer and secondary electron emitter was deposited by an e-beam evaporator. Then the front plate and nanophosphor-coated rear plate were combined with a gap of 100 μm and filled with Ne–20% Xe discharge gas mixture of 400 Torr. To perform discharge tests of the fabricated panels, a conventional pulsed ac power supply was used with a frequency of 30 kHz and a voltage of 280 V to initiate glow discharge of mini-sized test panels.

Characterization

The powder X-ray diffraction (XRD) pattern of annealed nanophosphors was obtained with a Philips APD 3720 diffractometer with a Cu Kα radiation source. A field emission scanning electron microscope (FESEM, Hitachi S-4300) operated at 10 kV and a transmission electron microscope (TEM, JEOL 200 CX) operated at 200 kV were used to obtain information on the size and morphology of the nanophosphors. Photoluminescence (PL) excitation and emission spectra of nanophosphors were recorded by a spectrofluorometer (Jobin Yvon Inc., Fluorolog 3) equipped with a 450 W Xe lamp with a spectral resolution of 1 nm. Using a D₂ lamp and a charge coupled device detector (PTE-VUVD2L-2000, PSI), the emission spectra of LaPO₄:Ce,Tb nanophosphor and a commercial phosphor (Kasei Optonix) were compared with an excitation wavelength of 147 nm. The VUV excitation spectra were also acquired by observing the whole visible emission with a photomultiplier tube detector and normalized with the signal of sodium salicylate powder. UV-visible spectroscopy (Shimadzu, UV-2450) was used to measure transmittance of deposited nanophosphor layers. Luminance of fabricated transparent mini-sized test panels was measured with a spectroradiometer (CS-1000, Minolta).

Results and discussion

A typical XRD pattern of 1000 °C-annealed La_0.4PO₄:Ce_0.4,Tb_0.2 nanophosphor is shown in Fig. 1(a) and it is well indexed to the monazite LaPO₄ phase with a high temperature polymorphic form of the monoclinic structure (JCPDS no. 32-0493).^5,16,17 No additional peaks indicative of other phases were detected, implying that a relatively high doping with Ce³⁺ and Tb³⁺ ions did not lead to the modification of the LaPO₄ host crystal structure and formation of secondary phases. The Debye-Scherrer approximation using a shape factor of 0.9 was employed to calculate a mean particle size of 27 nm, using the highest peak of (012) reflection at 2θ = 31.2°. These nanophosphors are morphologically somewhat elongated into a rod-like shape as shown in the SEM and TEM (inset) images of Fig. 1(b). Several morphologies of lanthanide phosphate-based nanomaterials could be prepared in the spherical,^2,6–8,10 rod-like,^1,11 fibrous,¹⁸ and ellipsoidal⁴ forms, resulting from their own synthetic pathways that may cause preferential growth along a certain axis in the unit cell. The widths of our rod-like La_0.4PO₄:Ce_0.4,Tb_0.2 nanophosphors from both images were averaged to be ∼30 nm, which is close to the size from the above XRD broadening.

Fig. 1 (a) XRD pattern of 1000 °C-annealed La_0.4PO₄:Ce_0.4,Tb_0.2 nanophosphors, showing a monoclinic monazite LaPO₄ phase (JCPDS no: 32-0493). (b) SEM and TEM (inset) images of the nanophosphors.

Using an excitation wavelength of 276 nm, PL emission spectra of La_0.4PO₄:Ce_0.4,Tb_0.2 nanophosphors annealed at different temperatures of 800–1200 °C were recorded as shown in Fig. 2(a). Characteristic Tb³⁺-related green emission resulted from f–f transitions between the excited ⁵D₄ state and the ⁷F_J (J = 0–6) ground states of Tb³⁺ ion.^1,2,4,6 While Tb³⁺-related excitation is not efficient due to its spin- and parity-forbidden transitions, Ce³⁺ ion with optically allowed d–f transitions serves as an excellent sensitizer for pumping Tb³⁺ emission. Therefore, UV excitation of Ce³⁺ ion gives rise to strong Tb³⁺ emission peaks via an efficient energy transfer from Ce³⁺ to the high excitation levels of Tb³⁺ (4f⁸), followed by cross relaxation to the ⁵D₄ level of Tb³⁺. Upon increasing the annealing temperature from 800 to 1000 °C, the size of the nanophosphors increased (which was confirmed by SEM measurement and XRD broadening) and their crystallinity would be improved as well. These synergetic effects might result in a substantial increase in Tb³⁺ emission intensity from 1000 °C-annealed nanophosphors, as shown in Fig. 2(a). But a higher annealing temperature of 1200 °C did not further enhance luminescence efficiency. The particle size of 1200 °C-annealed La_0.4PO₄:Ce_0.4,Tb_0.2 nanophosphors was larger than that of 1000 °C-annealed ones due to inter-particle sintering. Thus, considering the luminescence efficiency and particle size, the annealing temperature was fixed to be 1000 °C for all subsequent samples.

Fig. 2 Comparison of PL emission spectra of (a) La_0.4PO₄:Ce_0.4,Tb_0.2 nanophosphors annealed at different temperatures of 800–1200 °C, (b) La_xPO₄:(Ce,Tb)_1−x nanophosphors with x = 0.3–0.5 and a fixed Ce/Tb ratio of 2, and (c) La_0.4PO₄:(Ce,Tb)_0.6 nanophosphors with varied Ce/Tb ratios of 1–4. The samples in (b,c) were annealed at 1000 °C and emission spectra of all samples were collected with an excitation wavelength of 276 nm.

To find the chemical composition of LaPO₄:Ce,Tb nanophosphor with a maximum luminescence efficiency, the ratios of La, Ce, and Tb were tuned. The molar fraction (x) of La³⁺ in La_xPO₄:(Ce,Tb)_1−x was varied from 0.3 to 0.5, keeping an approximate Ce/Tb ratio of 2. As shown in Fig. 2(b), the highest luminescence intensity was observed for the composition of La_0.4PO₄:Ce_0.4,Tb_0.2. To further optimize the nanophosphor composition showing an efficient energy transfer between Ce³⁺ and Tb³⁺, the molar ratio of Ce/Tb was changed from 1 to 4 in La_0.4PO₄:(Ce,Tb)_0.6. The Tb³⁺ emission increased rapidly with an increasing Ce/Tb ratio from 1 (i.e., Ce_0.3,Tb_0.3) to 2 (i.e., Ce_0.4,Tb_0.2) due to a larger amount of Ce³⁺ excitation energy transferred to Tb³⁺ ion. However, with a further increasing Ce/Tb ratio, the luminescence was quenched, indicating that in the case of excessive Ce³⁺ ions the excitation energy would be consumed nonradiatively through the energy migration between Ce³⁺ ions, thus resulting in a low luminescent efficiency.

The PL properties of our optimized LaPO₄:Ce,Tb nanophosphor, i.e., 1000 °C-annealed La_0.4PO₄:Ce_0.4,Tb_0.2, were compared with a commercially available bulk phosphor as shown in Fig. 3. The vacuum ultraviolet (VUV) excitation spectrum of the nanophosphor was similar to that of the bulk reference. A predominant broad band peaking around 150–160 nm is ascribed to the absorption of PO₄³⁻ and a shoulder peak at ∼179 nm could be assigned to the absorption of Ce³⁺ ion.¹⁹ A series of excitation bands in the range of 190–310 nm correspond to the transitions from the ground state ²F_5/2 of Ce³⁺ ion to the spin–orbit components (²D_5/2 and ²D_3/2) of the excited 5d states that are strongly split by the crystal field.^4,5,19 It is noted that the VUV region that Xe gas-based plasma generates is well overlapped with the VUV excitation band of LaPO₄:Ce,Tb nanophosphor. Thus, our LaPO₄:Ce,Tb nanophosphor could be applied to transparent ac plasma displays operated under the discharge of Xe gas. As shown in Fig. 3(b), the PL emission intensities of 1000 °C-annealed La_0.4PO₄:Ce_0.4,Tb_0.2 nanophosphor and commercial LaPO₄:Ce,Tb bulk reference (with sizes of 1.1–2.9 μm) were compared with an excitation wavelength of 157 nm. Considering that the quantum yield of the bulk phosphor is 0.7–0.9, depending on the excitation wavelength used,^2–4 that of La_0.4PO₄:Ce_0.4,Tb_0.2 nanophosphor may be roughly surmised to be 0.35–0.45. The luminescence of doped nanomaterials is limited mainly by a high probability of surface state-related nonradiative relaxation and/or luminescence quenching of near surface dopants, usually exhibiting a lower quantum yield compared to the bulk material.⁶

Fig. 3 Comparison of (a) VUV excitation and (b) emission spectra of 1000 °C-annealed La_0.4PO₄:Ce_0.4,Tb_0.2 nanophosphor and commercial LaPO₄:Ce,Tb bulk phosphor. The excitation and emission spectra were collected with a detection wavelength of 544 nm and an excitation wavelength of 157 nm, respectively.

La_0.4PO₄:Ce_0.4,Tb_0.2 nanophosphors have been dispersed in various solvents including alcoholic media, and it turned out that 2-methoxyethanol is a suitable medium for dispersing them through a standard sonication. The dispersion of nanophosphors in 2-methoxyethanol was not highly transparent, but quite stable without a noticeable sedimentation for weeks. This simple dispersion protocol without introducing any organic stabilizing component (for example, amine-based agents) would be beneficial from a processing point of view, because an additional heat treatment to eliminate the organic molecules remaining inside the layer deposited is not necessary. The concentration of the nanophosphors in the dispersion was about 0.122 mol/L. The nanophosphor dispersion was spin-deposited on a glass substrate to form a transparent nanophosphor layer. To increase the layer thickness the spin-coating was repeated up to 7 times. A 5 times-spin-deposited nanophosphor layer on glass is shown in Fig. 4(a), where a square-shaped nanophosphor layer could hardly be seen by a naked eye under room light, indicative of the excellent visible transparency. Under a 254 nm hand-held UV light, the green-emitting transparent nanophosphor layer came into view as shown in Fig. 4(b). Fig. 4(c) presents the comparison of the optical transmittances of nanophosphor layers spin-deposited repeatedly up to 7 times along with that of a bare glass substrate. The Rayleigh scattering intensity of particles is proportional to the particle diameter to the sixth power.²⁰ Accordingly, the nano-sizing of a phosphor is one of most effective ways to reduce light scattering loss, thereby maximizing the transmittance. Upon 7 times-spin-coating, the transmittance of the nanophosphor layer formed was measured to be ∼97.6% at 500 nm, indicating that its scattering loss was negligible. In contrast, the conventional phosphor layer consists of bulk phosphors, whose size is typically 5 to 8 μm in diameter. A significant visible light scattering by those micron-sized phosphors is unavoidable, resulting in a very limited light transmittance. The 45°-tilted surface SEM images of representative 1 time- and 5 times-spin-deposited nanophosphor layers are shown in Fig. 5. With only 1 time-coating, the nanophosphor layer was formed in a discontinuous fashion (Fig. 5(a)). However, a repeated coating would fill the uncoated region continually, and eventually 5 times-coating generated an almost continuous nanophosphor layer with constituent particles compactly interconnected (Fig. 5(b)).

Fig. 4 Photographs of transparent La_0.4PO₄:Ce_0.4,Tb_0.2 nanophosphor layer formed by 5 times-spin coating under (a) room light and (b) hand-held UV (254 nm) lamp. (c) Variations of UV-visible transmittance of the nanophosphor layers with coating time.

Fig. 5 45°-tilted surface SEM images of (a) 1 time- and (b) 5 times-coated transparent La_0.4PO₄:Ce_0.4,Tb_0.2 nanophosphor layers formed on glass substrates.

As depicted in the Experimental section, a mini-sized transparent PDP was fabricated with the front plate used in conventional PDPs. A schematic diagram of the front plate is shown in Fig. 6(a). The transmittance of the front plate was ∼47% in the visible range of 420–700 nm. The rear plate (nanophosphor layer/glass) was sealed with the front plate with a gap of 100 μm, and the transmittance of the test panel was ∼42% in the same visible range. Compared with the front plate, additional reduction of transmittance of the test panel is mainly caused by the rear glass substrate rather than the nanophosphor layer, because the transmittance of the bare glass used only reached about 90% in the visible region as shown in Fig. 4(c). The photograph of a completed test panel is shown in Fig. 6(b), demonstrating a decent transparency.

Fig. 6 (a) Schematic diagram showing a mini-sized transparent PDP consisting of the rear plate (nanophosphor layer on glass) and the front plate used in conventional PDPs. (b) Photograph of a fabricated test panel with a transmittance of ∼42% in the visible range.

Upon gas discharge, excited Xe species generate two main VUV lines of 147 (resonance radiation) and 173 nm (molecular radiation), followed by the absorption of these VUV photons by LaPO₄:Ce,Tb nanophosphors, resulting in green emission. Fig. 7(a) presents the luminance variations of transparent panels versus coating time of nanophosphor layer. The luminance increased rapidly to ∼25 cd/m² for a 3 time-coated panel and then increased gradually to ∼30 cd/m² for a 7 time-coated one. Nonlinear increase of luminance with coating time seems to be related with the thickness of nanophosphor layer. Since the dispersion solution used for coating was low in viscosity, the thickness of nanophosphor layer would not be linearly proportional to coating time, which is also evident from transmittance data in Fig. 4(c). The test panel without a nanophosphor layer under gas discharge (280 V of pulsed ac voltage) exhibits orange-reddish emission due to the radiative relaxation of excited Ne species as shown in Fig. 7(b). Under the same gas discharge conditions, the stable green emission of the transparent panels with 1- and 5-times coatings of nanophosphor layer are representatively shown in Fig. 7(c and d). The Ne gas-related orange-reddish emission was completely screened by a strong green emission from LaPO₄:Ce,Tb nanophosphors.

Fig. 7 (a) Variations of luminance of transparent test panels versus coating time of nanophosphor layer. Photographs showing (b) Ne gas-related orange-reddish luminescence from a test panel without a nanophosphor layer, and green luminescence from ones with (c) 1 time-coated nanophosphor layer and (d) 5 times-coated nanophosphor layer.

Conclusions

Green-emitting LaPO₄:Ce,Tb nanophosphors with an average size of ∼30 nm were synthesized by a modified citric acid-assisted sol–gel route. Considering the size and luminescence efficiency, the post-annealing temperature and chemical composition of the nanophosphors were tuned to be 1000 °C and La_0.4PO₄:Ce_0.4,Tb_0.2, respectively. The emission of the optimized nanophosphors was compared with that of a commercial phosphor under VUV excitation, and their quantum yield was approximately estimated to be ∼50% of that of the bulk counterpart. A transparent nanophosphor layer was formed by multiple spin-coatings of a 2-methoxyethanol-based dispersion. Even a 7 times-coated nanophosphor layer exhibited an excellent transmittance of ∼97.6% at 500 nm, indicating that its light scattering loss was negligible. A mini-sized transparent PDP was constructed using the front plate of a conventional PDP. Upon gas discharge, the stable green emission of the transparent panels was observed. The luminance of the panels increased with the thickness of the nanophosphor layer on the rear plate and ∼30 cd/m² could be achieved from the panel with a 7-times-coated rear plate.

Acknowledgements

This work was supported by Seoul Research and Business Development Program (10555).

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