Improvement of light emission of Mn-doped Zn 2 SiO 4 phosphors with sodiumby Yi Hu, Jen-Pu Yang, Jiun-Shing Liu

Luminescence

About

Year
2012
DOI
10.1002/bio.1370
Subject
Chemistry (miscellaneous) / Biophysics

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Zn2SiO4:Mn phosphor. It has been confirmed that the sol–gel predetermined amounts of zinc nitrate, manganese acetate and sodium excitation source and the excitation wavelength was kept at 270nm.

Research article

Accepted: 10 October 2011, Published online in Wiley Online Libraryprocess has more advantages in lowering the firing temperature, distributing the activator ions homogeneously and improving the emission efficiency for the powder phosphors (12,14). In this study, the effect of sodium carbonate (Na2CO3) addition on the luminescence properties was studied. Na2CO3 is usually used as the flux in the solid-state reactions to improve the luminescence properties. However, the effect of flux on synthesizing the phosphors through the sol–gel method has not been reported. The flux is defined as an inert high-temperature

Results and discussion

Figure 1 shows the XRD patterns for samples with different sodium contents. All the diffraction peaks belong to crystalline willemite Zn2SiO4 (JCPDS No. 37–1485). No other phase was detected from the XRD patterns of the samples. However, theUp to now, commercial Zn2SiO4:Mn particles have been mainly prepared by solid-state reactions, with or without flux.

In solid-state reactions it is difficult to prepare phase-pure willemite, due to lower chemical homogeneity and the difference in volatilities of raw materials. To overcome the disadvantage of the solid-state reaction method, various processes have been introduced for preparing Zn2SiO4:Mn phosphor. These include polymer pyrolysis (10), the sol–gel process (11,12) and the hydrothermal method (13).

In this study, the sol–gel method was adopted to prepare the and heated at 70 C until it became a clear gel. The gels were dried and heat treated at 1000 C for 3 h in air at a heating rate of 10 C/min.

After that the samples were cooled and ground into powders.

The crystallinity of the samples was analysed by X-ray diffraction (XRD) using Cu–Ka radiation (Siemens D5000 X-ray diffractometer). Electron paramagnetic resonance (EPR) measurements on the powders were performed at 300 K, using a Bruker EMX-10 EPR spectrometer operating in the X-band (9.633GHz). 1,1′-diphenyl-2-picrylhydrazyl (DPPH) was used for the calibration of the klystron frequency. Photoluminescent excitation and emission spectra were measured at room temperature, using a Jasco

FP-6300 spectrophotometer. A Xe lamp of 150W power was used as theImprovement of light e

Zn2SiO4 phosphors wit

Yi Hu*, Jen-Pu Yang and Jiun-Shing

ABSTRACT: Mn-doped willemite (Zn2SiO4:Mn) green phosph addition of sodium, as in the composition Zn(1.92 – X) NaXMn results revealed that sodium ion is incorporated into the lat pairs. The maximum emission intensity of the sample under u of x=~0.03. The green emission at about 525nm is assigned highest intensity of the green emission for x=~0.03 is we

Copyright © 2012 John Wiley & Sons, Ltd.

Keywords: Zn2SiO4:Mn phosphors; sodium; sol–gel; Mn–Mn pair

Introduction

Manganese-doped Zn2SiO4 has been used as a green phosphor in fluorescent lamps, cathode ray tubes and flat panel displays because of its chemical stability and semi-conduction properties (1–4). The photoluminescence (PL) process of Zn2SiO4:Mn phosphors has been characterized by the transition of 3 d5 electrons in the manganese ion acting as an activating centre in the willemite structure (5,6). The transition from the lowest excited state to the ground state, i.e. 4 T1( 4 G)! 6A1(6S) transition of

Mn2+ under the excitation of UV, is directly responsible for the green light emission. Intensive research has been carried out to improve their physical, chemical and photoluminescent properties through new synthetic processes and optimization of the host and activator species (4,7–9).

Received: 20 April 2011, Revised: 23 August 2011, (wileyonlinelibrary.com) DOI 10.1002/bio.1370solvent to accelerate particle growth. Since the flux materials usually have low melting points, they behave like a liquid phase to facilitate particle growth. In this study, it was verified that the brightness of this phosphor was improved through the addition

Luminescence 2012 Copyright © 2012 Johncarbonate were all dissolved in nitric acid 0.01mol/L HNO3 to form a metal nitrate aqueous solution. For the sol–gel procedure, a volume of the above solution was added to a solution of TEOS dissolved in ethanol, and then stirred at room temperature. The solution was then coveredwere synthesized by sol–gel technology. The effect of the

SiO4, on the emission behavior was studied. FT–IR and EPR e and results in the formation of isolated Mn2+ and Mn–Mn aviolet (UV) excitation occurred at the sodium concentration

Mn2+–Mn2+ pair centres on nearest neighbour Zn sites. The lose to the highest concentration of the Mn2+–Mn2+ pair. of sodium carbonate. The role of sodium in the phosphor was studied and the mechanism of the enhanced green emission is the focus of this paper.

Experimental

The starting materials used for the preparation of Zn2SiO4:Mn powders were Zn(NO3)2 (99.5%), Mn(OOCCH3)24H2O (99.5%), Na2CO3 (99.5%) and tetraethoxysilane Si(OC2H5)4 (TEOS; 98%). The composition of the sample in this study was labelled as Zn(1.92 – X)NaX Mn0.08SiO4. First,ission of Mn-doped sodium u* Correspondence to: Y. Hu, Department of Materials Engineering, Tatung

University, Taipei, Taiwan, Republic of China. E-mail: huyi@ttu.edu.tw

Department of Materials Engineering, Tatung University, Taipei, Taiwan,

Republic of China

Wiley & Sons, Ltd.

SiO4); 460, 396 and 380 cm –1 (asymmetrical deformation of SiO4); 578 cm–1 (totally symmetrical stretching of ZnO4); and 616 cm –1 (asymmetrical stretching ZnO4) (15–17). In addition, the group with peaks at about 1200 and 1055 cm–1 are assigned to the absorption of Si–O–Si asymmetrical stretching vibrations (18) and the absorption at 470 cm–1 corresponds to ZnO stretching (19).

It was found that both the absorption peaks at about 1200 cm–1 (silicate stretching) and at 470 cm–1 (ZnO stretching) shift to a lower wave number after sodium addition. In addition, the intensity of the absorption at 616 cm–1 decreases with increasing sodium content. This indicates that the addition of sodium would cause a lowering of the molecular vibration force constant and therefore shifting of the peak associated with the asymmetrical stretching of the Si–O–Si bond to lower wave number (22). In addition, it has been reported that sodium ions can be incorporated into the SiO4 tetrahedral structure and act as a perturbation of silicate stretching vibrations (19).