SYNTHESIS, CHARACTERIZATION AND PHOTOLUMINESCENCE STUDIES OF Sr2CeO4: EU NANO PHOSPHOR
Paleena Thulimilli1, Dr.K.V.R.Murthy2
1Department of Physics, St.Pious X Degree & PG College for Women, Hyderabad, Telengana
2DisplayMaterials Laboratory, Applied Physics Department, Faculty of Technology
Engineering, M.S University of Baroda, Baroda–390001, India
Abstract: Nanotechnology is a common word these days, but many of us don’t realize the amazing impact it has on our daily lives. It is also a rapidly expanding field. Scientists and engineers are having great success making materials at the Nano scale to take advantage of enhanced properties in display applications.For the last one and half decade the nanotechnology, with size limitation of less than 100nm, has been moving at a pace and gaining momentum, research in this field is becoming more and more active. In this regard the phosphor research has also awakened to the challenge and new and better materials with the size limitations are being pursued rigorously. The current world market place for phosphors is about $450m annually with an expectation to grow in the 8-10% range. And, they have to compete with and against several display technologies and applications such as liquid crystal, plasma, LEDs, OLEDs, automotive and field emission, which in turn require a growing range of phosphor types to satisfy new and existing uses. Up till now the commercial Nano phosphors are synthesized using solid state reaction which requires the raw materials to be ground using an high speed grinders/ball mills and fired at 1200-1600°C or more depending on the synthesis temperature for many hours with two or more intermediate grindings. By considering all the applications into account in this paper the detailed description of synthesis of the Sr2CeO4 Nano phosphor by various techniques and their luminescent properties are discussed
Keywords: Sr2CeO4, solid state reaction technique, sol-gel technique, Photoluminescence, XRD; SEM
Nano crystals and the size dependent properties of the Phosphors
The optical properties of the solids have been an area of interest for the past few centuries, but for the past few decades they have been an area of high interest for those working with Nano crystals. The optical properties can give an insight to the mechanism and the nature of the material formed. These properties have led to the discovery of various inorganic hosts having different technological applications. The concentration of these have mainly been on the metallic Nano crystals 1 and semiconductors (mainly the Doped Nano crystals (DNC)), which shows the quantum confined effect.
In the last two decades considerable interest has been directed towards the synthesis and characterization of economical and efficient rare earth doped phosphors using different hosts has been developed for various applications. The red 5D0 ? 7F2 and orange emission 5D0?7F1 of Eu3+ and green emission of Tb3+ 5D4? 7F5, is usually used in these phosphors. These phosphors find applications in fluorescent lamps, color TV screen, cathode ray tubes, Plasma Display Panels, discharge lamps etc. These phosphors are not commercially synthesized in India, and are imported 2, 3. Also no systematic efforts were made towards studying the use of indigenous chemicals for the synthesis of these phosphors. 4,5.6.
Solid State Reaction: The starting materials taken for solid state reactions were SrCO3, CeO2 purchased from S.D. fine chemicals (Boisar). The stoichiometric ratio of Sr:Ce was taken as 2:1. The samples were first grinded using agate mortar and pestle and then kept in a furnace. The temperature of the furnace was set at 1200°C from the room temperature. The heating rate of the furnace was fixed at 6.67°C/minute. The samples were kept in the furnace at 1200°C for 10 to 30 hours with two or three intermediate grindings. After the heating was done, the sample was allowed to cool by switching it off. The samples were again grinded using agate mortar and pestle. The resulting powder was white in colour.
Sol-gel technique: The starting materials taken were Sr(NO3)3, Ce(NO3)3.6H2O, Citric Acid, Ethylene Glycol and Liquid ammonia (NH3) purchased from S.D. fine chemicals (Boisar, Mumbai, India). The stoichiometric ratio of Sr: Ce were taken in 2:1. The nitrates were first dissolved in around 20ml of double distilled water and kept for stirring on a magnetic plate at room temperature until it becomes transparent in color and all the salt have mixed. Gradually the temperature of the plate was increased to 40-50°C. After a certain time of continuous stirring, citric acid was added to the transparent solution. The pH of the resulting solution was maintained 6-7 by adding droplets of ammonia. The temperature of the solution was raised to 60-80°C. The solution at this stage becomes milky in colour due to the precipitation taking place in the solution due to citric acid acting as a chilating agent. After few hours of stirring Ethylene glycol was added to the solution and stirring was maintained. The solution now changes its colour and becomes yellow, becoming more viscous and the formation of gel takes place. The starting materials used are nitrates in aqueous media, which form stable gels through gelation with citric acid, followed by cross-linking after polycondensation of ethylene glycol at increased temperature. The synthesis of Sr2CeO4 by sol-gel has been shown in figure-3. This mechanism ensures that the gel formation takes place. The gel is then kept in an oven at 110°C for 4 hours to remove water content if any and for drying. The dried gel was then made into 4 parts. Each part was given a different heat treatment.
Part- 1 A – Kept for heating at 400 C for 2 hrs.
Part- 2 B – Kept for heating at 800 C for 4 hrs.
Part- 3 C – Kept for heating at 1000 C for 4 hrs.
Part- 4 D – Kept for heating at 1200 C for 2 hrs.
X-ray Diffraction studies: by using X-ray powder diffraction using RIGAKU D’MAX III Diffractometer having Cu K? radiation (?=1.54nm) Phase identification of the powders was carried out. The scan range was kept from 50 to 800 was the scan range at the scan speed of 0.050 per second.
Scanning Electron microscope (SEM): The SEM images of the samples were taken using JEOL make JSM-5610 LV for studying the morphology of the compound.
Photoluminescence measurements: The photoluminescence (Emission and Excitation spectra) were recorded at room temperature using spectrofluorophotometer RF-5301 PC of SHIMADZU. Xenon lamp is used as source. For the emission and excitation the slit width was kept at 1.5nm for all the measurements. To remove the second order peak of the excitation light in the PL measurements a filter was used. The sensitivity of the instrument was set as high unless stated otherwise.
Results & discussion:
1. X-ray diffraction studies: The crystallinity of the compound as revealed by the XRD pattern, increased on raising the calcining temperature. From the analysis of the XRD pattern, it was understood that the introduction of activator Eu3+ did not influence the crystal structure of the phosphor matrix. The calculated average crystal size of the sample calculated by measuring the full width half maxima was found to be of about 40 nm for europium 1 mole percentage doped sample.
2. Scanning Electron microscope (SEM) studies The Scanning Electron Microscopy studies have been done on the particles prepared by both the solid state reaction as well as Sol-gel technique. All the SEM micrographs have been presented in the Figure-1From the SEM micrographs we can see that the morphology of the samples prepared by the sol-gel is better. The shape and the size of the sol-gel prepared are round and they appear to be less agglomerated when compared with the solid state one. The solid state synthesized sample has been sintered and appear heavily agglomerated, the morphology is not uniform and they are tightly aggregated to one another to form large secondary particles. They also do not have narrow size distribution and appear very hard. The sol-gel synthesized samples are spheroidal in shape and even at high temperature of annealing they appear less agglomerated. The size of the sol-gel synthesized is very small compared to solid state and they appear very soft with respect to solid state ones. The micrographs of the sample prepared from the europium doped Sr2CeO4 and one plain are shown in figure-1 for comparison.
Figure 1: All Europium doped Sr2CeO4 micrographs and one plain shown for comparison
Photoluminescence Emission Spectra of Europium doped Sr2CeO4:
The emission spectra of the europium (0.5mol %) doped in Sr by sol-gel has been shown in figure-2. The peak at 595nm is highest in intensity for the 5D0?7F1 transitions and even higher than that at 467nm (5D2?7F0) but as the percentage of europium doping is increased the intensity of this decreases. The stark splitting for the 5D0?7F1 would be three (2J+1) theoretically and experimentally all the three are observed with the intensity of the 595nm being highest, increasing the concentration decreases it with subsequent increase of the 586nm intensity. This is unique observation for the 0.5mol% doped and not for all. The stark splitting for 5D0?7F2 would be five theoretically and four lines are found experimentally.
Figure-2 The Photoluminescence emission spectra of the sol-gel synthesized Sr2CeO4 with europium (0.5mol%) doping excited with 254nm wavelength.
The emission spectra measured for different excitation wavelengths, 254nm, 280nm and 467nm respectively is shown in the figure-3. It is observed that the emission is highest for the 280nm wavelength and the emission pattern is same for excitation wavelengths except for 467nm. The emission observed from 467nm excitation wavelength shows minor changes than the other two. As the emission coming from 467nm is not governed by charge transfer band hence we can see the small shift of the 585nm peak towards higher wavelength region, the stark splitting of the 5D0?7F1 transitions is also not clear. These results prove that the different excitation wavelength can change the composition of different sites spectrum components.
Figure-3 The Photoluminescence emission spectra of the sol-gel synthesized Sr2CeO4 with europium (0.5 mol%) doping excited with 254nm wavelength.
Conclusions: The main conclusions that can be drawn by studying the effect of europium doping on the luminescence properties of the Sr2CeO4 are as follows:
? Europium doped Sr2CeO4 phosphor was synthesized successfully using sol-gel technique.
? Comparison with solid state reaction revealed a marked difference in the emission characteristics from 580nm-630nm for the 0.5mol% doped europium sample. This may be due to the Nano crystal size (~55nm) of the phosphor formed with sol-gel.
? Greater splitting of the 5D0?7F1, 5D0?7F2 when compared with solid state reaction and few additional lines were seen at 595nm and 611nm for the sol-gel prepared sample.
? Excellent tunablity of phosphor observed when doped with various concentrations of Europium.
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