Charge-Transfer Associated Photoluminescence Of Rare-Earths Doped Oxide Phosphors

Abstract

Luminescent materials can be found in a broad range of everyday applications. While in the seventies and eighties, the field of luminescent materials seemed to be fairly well covered, research in nineties has been revitalized both in industry and academia. Improvements over the last three decades have led to phosphor materials that operate close to their physical limits. It cannot be expected that properties such as quantum yield and spectral energy distribution will be significantly improved or that distinctly better materials will be found in the near future. Recently, there is a considerable research activity in the field of luminescent materials for lighting and displays to improve the chemical stability and to adopt the materials to the production technology. Ongoing miniaturization, lifetime improvement and spectral stability of fluorescent lamps on the one hand and brightness and contrast improvement in imaging systems on the other hand demand luminescent materials with very high stability that is invariable to operating conditions. All of the today's efficient lighting sources are based on either direct or indirect light emission from plasma discharges. During the pioneering stage, fluorescent lamp industries predominantly used mixtures of two photo luminescent materials: (Zn,Be)2SiO4.'Mn2+ having two emission maxima at 520 and 600 nm and MgW04 with 480 nm emission. The emission from these two phosphors covers the major portion of the visible spectrum. However, the compound (Zn,Be)2Si04 is hazardous to health because of its beryllium content. In 1942, Jenkins showed that Ca5(PO4)3(F,Cl):Sb,Mn was a very efficient emitter. The halophosphates emit both in the blue (Sb3+) as well as in the orange (Mn2+) spectral region, thus in addition yield white light. By carefully adjusting the ratio of Sb3+ and Mn2+ ion concentrations, a white light emitting phosphor was obtained with color temperatures ranging between 6500 and 2700K. However, the drawback of the halophosphate lamps is that it is impossible to have simultaneously high brightness and high color rendering; if the brightness is high (efficacy -80 lm W"1), the color rendering index (CRI) is of the order of 60, the CRI value can be improved up to 90, but then brightness decreases (-50 lm W"1). In 1974, another important breakthrough came in the form of compact fluorescent lamp, based on the trichromatic phosphor blend which resulted color rending values of 80-85 (color 80 lamps) at high efficiencies of 100 lm W"1. The fluorescent lamps with very high color rendering and efficiency can be obtained if three narrow band emitters with emission maxima at 450, 540 and 610 nm are employed. A typical trichromatic lamp phosphor blend comprises of (i) Sr5(PO4)3Cl:Eu2\ BaMgAl1()O,7:Eu2' as blue component, (ii) Ce0.67Tbo.33MgAl,,0,9, LaPO4,Le3\Tb3+ as green component and (iii)Y2C>3:Eiru as the red component. The color 80 lamps employ line emitters that generated light in discrete wavelength intervals. Colored objects that absorb outside these spectral regions appear with a slightly different body color when illuminated with these lamps rather than with a black body radiator such as the light bulb. For these purposes, color 90 or Deluxe lamps have been developed. The emission maximum of the blue phosphor can be shifted towards longer Wavelength by substituting BaMgAli0Oi7:Eu2+ with Sr4Ali4025:Eu2+. The red and green line emitters can be substituted by broad band emitters covering the whole spectral range. For this concept, (Ce,Gd,Tb)MgB5Oi0:Mn has been developed as a red emitter in which energy transfer from Ce3+ via Gd3+ to Mn2+ gives rise to an additional broad band at 630 nm. On the other hand, (Ba,Sr,Ca)2Si04:Eu has been developed as an alternative green-band emitter in which depending on the exact composition, the phosphor emits between 550 and 580 nm with a high quantum yield. Unfortunately, the host lattice is not stable in water, which prevents its deposition on the lamp bulb from aqueous suspensions and for environmental reasons more and more lamps producers use water as the suspending solvent in production instead of butyl acetate. Therefore, it is necessary to develop a new full color emitting phosphors, which has both thermal and chemical stability for application in luminescent lighting.

The classical cathode ray tube (CRT) invented as the brown tube more than 100 years ago has developed into a remarkably mature product considering the complexity of its manufacturing process. Cathode rays are a beam of fast electrons, the accelerating voltage in a television picture tube is high (>10 kV). Basic requirements of display phosphors are stability (2000 hr operation) and emission color purity according to the standards set by the European Broadcasting Union (EBU). The blue and green phosphors are still the very cheep ZnS based materials, essentially the same ever since color-TV was introduced in fifties. On the other hand, (Zn,Cd)S, Ag+,C1" was originally used as the red phosphor however, the broad emission centered at 650 nm due to intrinsic donor-acceptor transition leads to rather low lumen equivalent as large fraction of the emission integral lies outside the eye sensitivity curve. For this and the environmental reasons, it has been replaced by the much more expensive Y2O2S:Eu with main emission lines at 612 and 628 nm. Recently, the big electronic companies are trying to enforce flat panel displays e.g. PDPs (plasma display panels) and FEDs (field emission displays). This is because of the fact that when compared to the CRT screen pigments, FED phosphors are required to operate at lower voltages and higher current densities. Although the voltages used in FEDs are only 0.1 to < 2 kV, the high-energy surface excitation on the phosphor particles causes degradation of sulfides, leaving the oxide hosts as the only favorable choice. The phosphor blends used are mixtures of SrTiO3:Pr3+ (red), Y2Si05:Tb (green) and Y2Si05:Ce (blue). However, the white light generation efficiency is very low (-5 lm W"1) and required improvement of phosphor efficacy because of its distinct advantages such as a very wide range of operational temperatures, stability under rugged conditions and wide viewing angle of emission. Similarly, in PDPs blue emitting BaMgAlioOniEu, green emitting Z^SiO^Mn and red emitting (Y,Gd)BO3:Eu are mostly used which shows a screen efficiency of about 1.5 lm W"1, just only half that of a CRT used in today's TV sets. However, the advantages of PDPs over CRTs are that it is not sensitive towards the display manufacturing process, which includes high temperature annealing up to about 600°C and it is stable under the harsh conditions of a Ne/Xe plasma used in PDPs (ion bombardment, VUV radiation). This puts pressure on the development of phosphor for maximum brightness and high stability to replace completely the classical CRTs.

On the other hand, the invention of the blue-light emitting diode (LED) based on GaN can be regarded as a triumph of materials chemistry. In principle, it is possible to vary the emission wavelength of blue GaN-based LEDs between 370 nm (band-gap of pure GaN) and 470 nm by increasing the indium (In) content in InGaN devices. Assuming a conversion from the incident light by a phosphor material emitting at 555 nm, InGaN is coated with (Yi.xGdx)3(Ali-yGay)5Oi2:Ce (YAG:Ce) which has broad yellow band varying between 510 and 580 nm. This allows the adjustment of white color temperature from 8000 down to 3000 K. Recently, S^SiCU and S^SiOs have attracted current interest due to their potential applications in developing white light-emitting-diodes (LEDs) because GaN (400 nm chip)-coated with Sr2Si04:Eu2+ or Sr3SiC>5;Eu2+ exhibits better luminous efficiency than that of the industrially available product such as InGaN (460 nm chip)-coated with YAG:Ce. However, the major drawback of this combination is the strongly decreasing overall efficiency upon lowering the color temperature. This can be solved by using a phosphor material that has sufficient absorption at the emission wavelength of the blue diode, the quantum yield should be high under UV/Vis excitation and the FWHM of the emission band should be as small as possible in order to achieve high luminous output. The search for stable inorganic rare-earths phosphors with high absoiption in the near UV/blue spectral region is therefore an attractive research work.

Since luminescence materials are a key component for lighting and display concept, research in the field of rare-earths doped oxide phosphors is carried out. Although state-of-the-art materials fulfill most requirements, improvements are still necessary to further boost the efficiency of the phosphor materials. Since it is not expected that materials will be found that perform better than the already established phosphor, the present work concentrates on the improvements of the phosphor by modifying the chemical and niicrostructurai features as well as the crystal structure.

Chapter I gives a brief introduction to luminescence in solids, physical aspects and applications. Chapter II describes the synthesis and various experimental techniques employed in the investigation. Chapter III deals with photoluminescence and energy transfer involving charge transfer states in Sr2-xLnxCe04+x/2 (Ln = Eu and Sm) leading to an efficient full color emitting phosphor for luminescent lighting. Chapter IV and V describe charge transfer transition involving interface states associated with transitional nanophaseprecipitates leading to photoluminescence enhancement of SrTiO3:Pr3+,Al3+ and SrAli2Oi9:Pr3+,Ti4\ The light induced charge transfer leading to changing oxidation state of Eu in Sr2Si04 involving transient crystal structure results an efficient material for optical storage is presented in Chapter VI.Photoluminescence due to efficient energy transfer from Ce3+ to Tb3+ and Mn2t in SnAlioSi02o leading to an efficient phosphor for FEDs is presented in Chapter VII. Chapter VIII describes charge transfer transition involving trap states leading to long phosphorescence in SrAl2-xBxO4 (0<x<0.2) and Sr4Al14.xBxO25 (0.1<x<0.4) co-doped with Eu2+ and Dy3+. Chapter IX presents the role of particle size on the charge transfer associated luminescence of GdVO4:Ln3+ (Ln = Eu and Sm). A summary of the important findings and the conclusions arrived on the basis of results from these investigations are presented at the end of the thesis.