Article pubs.acs.org/JPCC
Synthesis and Luminescent Properties of GdNbO4:RE3+ (RE = Tm, Dy) Nanocrystalline Phosphors via the Sol−Gel Process Ying Lü,† Xinghua Tang,† Liushui Yan,† Kexin Li,† Xiaoming Liu,*,† Mengmeng Shang,‡ Chunxia Li,‡ and Jun Lin*,‡ †
School of Environment and Chemical Engineering, Nanchang Hangkong University, Nanchang 330063, P. R. China State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China
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ABSTRACT: Nanocrystalline GdNbO4:Tm3+ and GdNbO4:Dy3+ phosphors were prepared through a Pechini-type sol−gel process. X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, photoluminescence, and cathodoluminescence (CL) spectra were utilized to characterize the synthesized phosphors. XRD reveals that the samples begin to crystallize at 900 °C and the pure GdNbO4 phase can be obtained at 1000 °C. FE-SEM images indicate that the GdNbO4:Tm3+ and GdNbO4:Dy3+ samples consist of fine and spherical grains with a size around 30−50 nm. Under the excitation of UV light (264 nm) and low-voltage electron beams (1−3 kV), the GdNbO4:Tm3+ and GdNbO4:Dy3+ phosphors showed the characteristic emissions of Tm3+ (1D2 → 3H6, 3H5, 3H4, and 1G4 → 3H6 transitions) and Dy3+ (4F9/2 → 6H15/2 and 4F9/2 → 6 H13/2 transitions), respectively. The blue CL of the GdNbO4:Tm3+ phosphor has higher color purity and comparable intensity to the Y2SiO5:Ce3+ commercial product. A singlecomposition white light emitting in response to near UV and low-voltage electron beam excitation has been realized in GdNbO4:Dy3+ phosphor. The obtained GdNbO4:Tm3+ and GdNbO4:Dy3+ phosphors have potential applications in the areas of near UV white-light-emitting diodes and field emission display devices.
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INTRODUCTION Phosphors have been widely used in almost any device involving the artificial production of light, such as light-emitting diodes (LEDs), cathode ray tubes (CRTs), field emission displays (FEDs), vacuum fluorescent displays (VFDs), plasma display panels (PDPs), liquid crystal displays (LEDs), and Xray imaging scintillators.1−8 Oxide phosphors have gained much interest due to their better thermal and chemical stability, and they are environmentally friendly compared with sulfides, which are currently used in the areas of LEDs, flat-panel displays (FPDs), FEDs, and VFDs.9−15 Therefore, more attention has been paid to the improvement of original oxide phosphors and developing new oxide phosphor materials with respect to the wide possible applications.16−21 Ultraviolet light-emitting diodes (UVLEDs) based on wide band gap III-nitride compound semiconductors have attracted much attention of researchers because of their potential applications for solid-state white lighting. The quest for new UVLED converted phosphors has triggered active research efforts in the investigation of white-emitting materials using UVLEDs (300−410 nm) instead of the blue irradiation (460 nm) from GaInN chips as the excitation source.22,23 This has generated a great interest in developing new white-emitting phosphors with high luminous efficiency, small size, controllable morphology, and environmentally friendly characteristics for UVLED via chemical approaches.24−27 Rare earth ion doped crystallites have been playing an important role in modern lighting and display fields owing to © 2013 American Chemical Society
their excellent luminescent properties. Since the 4f electrons of rare earth ions are shielded by the outer 5s and 5p electrons, the intra-4f emission spectra of rare earth ions are characterized by narrow lines with high color purity.28,29 Thulium-doped phosphors have attracted substantial attention in recent years because Tm3+ ions can provide blue luminescence with appropriate lifetimes, excellent color rendering properties, and potential application in LEDs, CRTs, FEDs, VFDs, plasma display panels (PDPs), and electroluminescence devices, such as Y2O3:Tm3+, La2O3:Tm3+, LaAlO3:Tm3+, Y3Al5O12:Tm3+, SrHfO3:Tm3+, Y3GaO6:Tm3+, LaPO4:Tm3+, LaAlGe2O7:Tm3+, LaGaO3:Tm3+, etc.30−38 The luminescence of trivalent dysprosium Dy3+ mainly consists of narrow lines in the blue (470−500 nm, 4F9/2−6H15/2) and yellow orange (570−600 nm, 4 F9/2−6H13/2) regions. The latter belongs to the hypersensitive transition (ΔL = 2, ΔJ = 2), which is strongly influenced by the environment.39 At a suitable yellow-to-blue intensity ratio, Dy3+ will emit white light.40,41 However, unlike the most frequently used Eu3+ and Tb3+ (in oxide hosts), which have allowed a charge-transfer absorption band (CTB) or 4f8−4f75d absorption band in the UV region, respectively, the excitation spectrum of Dy3+ consists of only narrow f−f transition lines from 300 to 500 nm (both a CTB and a 4f9−4f85d band of Dy3+ are located below 200 nm).42 As a result, the Received: August 29, 2013 Revised: September 22, 2013 Published: September 24, 2013 21972
dx.doi.org/10.1021/jp4086415 | J. Phys. Chem. C 2013, 117, 21972−21980
The Journal of Physical Chemistry C
Article
formed. After being dried in an oven at 110 °C for 10 h, the gels were ground and prefired at 450 °C for 4 h in air. Then, the samples were fully ground and fired at 1000 °C for 3 h to produce the final samples. X-ray diffraction (XRD) measurements were carried out on a Rigaku-Dmax 2500 diffractometer using Cu Kα radiation (λ = 0.15405 nm). The morphologies of the samples were inspected using a field emission scanning electron microscope (FE-SEM, JSF-6700) and a JEOL 2010 transmission electron microscope (TEM) equipped with a field emission gun at a voltage of 200 kV for FE-SEM and TEM images, respectively. The PL measurements were performed on a Hitachi F-7000 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. Diffuse reflectance spectra were obtained using a JASCO V-560 UV−visible diffuse reflectance spectrometer equipped with an integration sphere using Spectralon as a reference. The CL measurements were carried out in an ultrahigh-vacuum chamber (
