Long Afterglow Phosphors

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Long After-glow Phosphors

• Prof. U.V. Varadaraju
• Materials Science Research Centre
• Indian Institute of Technology Madras
• Chennai 600 036.

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Long after-glow phosphors

• A long after-glow material
• ? absorbs energy (e.g., UV) excitation
• ? stores the absorbed energy retention
• ? releases the energy in the visible region
  emission
• ? shows persistent glow even after removal of
  the
• excitation source after-glow

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Long after-glow phosphor
Emission center
Trapping center
Emission center necessary to produce
luminescence colours Eg. Transition metal ions
(Mn, Cu), RE ions (Eu, Ce) Trapping center
necessary to store absorbed energy Eg. Defects
in the host, cations (Cu, Mn, Ag, Al) anions
(Cl,O), RE ions (Dy, Nd) Oxides emission
center-Eu, Ce, Mn trapping center-Dy, Nd,
defects
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Long after-glow phosphor
Emission center
Trapping center
Emission center necessary to produce
luminescence colours Eg. Transition metal ions
(Mn, Cu), RE ions (Eu, Ce) Trapping center
necessary to store absorbed energy Eg. Defects
in the host, cations (Cu, Mn, Ag, Al) anions
(Cl,O), RE ions (Dy, Nd) Oxides emission
center-Eu, Ce, Mn trapping center-Dy, Nd,
defects
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Oxide phosphors - Mile Stones

• SrAl2O4 (1996)
• ZnGa2O4 (1997)
• Sr2MgSi2O7 (2000)
• Ca3MgSi2O8 (2001)

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SrAl2O4
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• SrAl2O4Eu2
• ? Green phosphor (520 nm)
• ? Studied for lamp and CRT application
• ? Nonstoichiometric (excess Al2O3) SrAl2O4Eu2
  is
• brighter than the stoichiometric one
• ? After-glows for few seconds
• ? Trapping mechanism involving hole conduction
• ? Photoconductivity measurements holes are
  major

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Structure of SrAl2O4

• ? hcp of O2-
• ? Corner sharing AlO4 tetrahedra
• ? Each O2- shared with two Al3
• ? Each tetrahedron has a net negative charge of
  unity
• Charge balance by Sr2 cations occupying
  interstitial sites within tetrahedral framework
• Sr2 - O2- 9 CN 2 sites

a
b
c
SrAl2O4 Tridymite structure
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Sr 2
Sr 1
Von A.R. Schulze, Müller-Buschbaum, Z. anorg.
Allg. Chem. 475, 205-210 (1981)
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SrAl2O4 Eu,Dy(Wonder material)

• Co-activator for hole trapping
• Sr vacancy ? Dy3
• SrAl2O4 Eu2,Dy3 - new long after-glow phosphor
• Brightness after-glow 10 times more than
  conventional ZnS phosphors

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Solid State Method or Ceramic Route for Solid
Powders

• Reactants-Sr, Al, RE oxides/carbonates
• Mixing Stoichiometric amounts of powders
• Heating at high temperatures (1300-1500ºC)
• Diffusion controlled reaction through product
  layer
• Uniform mixing is difficult especially at very
  low dopant level
• Second phase formation/presence of unreacted
  reactants
• Long heating schedules required

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Self-propagating High-temperature Synthesis (SHS)
or Combustion Synthesis

• Fuel Oxidizer ? Exothermic reaction (?H)
• Reactants ?H Chemical reaction
• Metal salts (Oxidizers) dissolved in solvent-
• homogeneous molecular level mixing
• Sr2 RE3
• Al3
• Fuel addition
• Evaporation of solvent ignition
• Fluffy solid fine powder precursor

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Exc. Em. Spectra of SrAl2O4 Eu,Dy
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Decay curve
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Hole trapped transported detrapped mechanism
Conduction Band
Eu (5d)
6.52eV
2.38eV
3.44eV
Dy
Eu (4f)
0.65eV
0.06eV
Valence Band
T. Matsuzawa et al., J. Electrochem. Soc., 143(8)
2670 (1996)
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Conduction Band
Eu
RE
Valence Band
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Energy level scheme of the Eu2 ions involved in
the UV-excited and persistent luminescence
processes in MAl2O4Eu2, RE3
2.38 eV
3.38 eV
Jorma Holsa, Hogne Jungner, Mika Lastusaari and
Janne Niittykoski, J.Alloys Compds., 323-324 326
(2001)
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• ? Co-activator influence on optical properties
  
• of phosphors involving trapping mechanism
• ? Optimum trap depth of co-activator
• activation energy ET of a hole from a trap
  level
• to the valence band of the host lattice
• Lower ET fast decay
• Higher ET no phosphorescence

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• Co-activators in SrAl2O4Eu2
• Co-doping other RE3 ions
• ? Nd, Dy, Ho Er 1eV
• ? Ce, Pr, Gd Tb 1 3eV too deep
• ? Sm, Eu, Tm Yb 0eV shallow
• SrAl2O4 Eu2 - Dy3 (0.65eV) - excellent
  long
• after-glow green
• CaAl2O4 Eu2, Nd3 - violet
• E. Nakazawa and T. Mochida, J. Luminescence,
  72-74 236 (1997)

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Emission wavelengths of MAl2O4 Eu2,Dy3 (M
Ca, Sr, Ba)

• Curve a - CaAl2O4 Eu2,Dy3 - 445 nm
• Curve b - BaAl2O4 Eu2,Dy3 - 496 nm
• Curve c - SrAl2O4 Eu2,Dy3 - 518 nm
• Yuanhua Lin , Zhongtai Zhang, Zilong Tang,
  Junying Zhang,
• Zishan Zheng, Xiao Lu, Mat. Chem.
  Phys., 70 156 (2001)

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4f65d
Yellow-Green
Blue-Green
UV
4f7
Ca
Ba
Sr
?
Schematic energy level diagram of Eu2 ion vs.
the crystal field ? in MAl2O4 (MCa,SrBa)
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Thermoluminescence glow curves of MAl2O4 Eu,Dy
(M Ca,SrBa) phosphors
Peak of the TL glow curve
Ca 124ºC Sr 77ºC Ba 173ºC
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Photoluminescence Studies on Eu2 activated
Li2SrSiO4 a Potential Orange Yellow Phosphor
for Solid State Lighting
Combustion synthesis

• Hexagonal unit cell with space group P3121

Fig. Powder XRD patterns of Li2SrSiO4 0.005 Eu2
synthesized by solid state reaction and
combustion methods

• Refinement based on
• Li2EuSiO4
• Profile Factors
• Rp 4.74, Rwp 6.12,
• Rexp 3.56, ?2 2.95
• Refined Lattice Parameters
• a 5.023(5) Å, c 12.457(1)

Fig. Rietveld refinement plot of X-ray
diffraction data for Li2SrSiO4
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PL intensity solid state Vs combustion
Fig. Photoluminescence emission spectra of
Li2SrSiO4 0.005Eu2 synthesized by Solid State
Reaction and Combustion methods. The spectra of
CS samples post annealed at various temperatures
are shown.

• Sintering improves emission intensity
• Combustion sample intensity higher than SSR
  sample
• high degree of activator homogeneity.

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Photoluminescence of Phosphor coated on LED Chip

• InGaN chip Supported on SiC substrate
• InGaN (420 nm) phosphor epoxy resin
• CIE, shows better values than commercial
• YAGCe
• CIE Coordinates
• Li2Sr0.995Eu0.005SiO4 (0.3346,0.3401)
• YAGCe
  (0.3069,0.3592)
• Luminous efficiency
• Li2Sr0.995Eu0.005SiO4 35 lm/W
• YAGCe 40 lm/W

Fig. Luminescence spectra of white-LED fabricated
using InGaN (?emi 420 nm) based Li2SrSiO4
0.005Eu2, In inset InGaN (?emi 460 nm) based
Li2SrSiO4 0.005Eu2
A InGaN (?em 420 nm), B InGaN (?em 420
nm) Li2SrSiO4Eu2
M. Pardha Saradhi and U.V. Varadaraju, Chem.
Mater., 18 (22), (2006) 5267
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Multiple energy transfer and fabrication of UV
pumped white LEDs in Sr2SiO4 Ce3,Eu2,by Dy3
Combustion synthesis
Structure type - ß-K2SO4 Orthorhombic unit
cell with space group Pmnb
Combustion synthesized sample shows Higher PL
intensity higher degree of homogeneity of
activators
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Chromaticity Coordinates
Tristimulous Response of the Human Eye
X Red response Y Green response Z Blue
response
Chromaticity Coordinates (x,y,z) x X /
XYZ y Y/ XYZ z Z/ XYZ x y z
1. LED wavelength (nm) CIE 360 nm
(0.2745, 0.2673) 370 nm
(0.2571, 0.2494) 380 nm
(0.2874, 0.2986)
M. Pardha Saradhi and U.V. Varadaraju, (to be
communicated)
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Fabricated White Light Emitting Diodes
UV pumped LEDs Sr2SiO4 Ce3,Dy3,Eu2
InGaN (?em 420 nm) Li2SrSiO4 Eu2
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Combustion synthesis of phosphor nano particles
Eu3 doped Y2O3
a
b

• Particle homogeneity and
• luminescence intensity
• significantly improved by
• combustion method

Fig. (a) SEM image of the Eu doped Y2O3
synthesized using oxygen rich combustion
method,(b) nanoparticles Bottom Fluorescence of
particles under UV excitation
Ce3, Tb3 doped Y2SiO5
Plate like morphology with high amount of
agglomeration and porosity due to combustion
synthesis Porosity due to production of super
heated gases (N2,CO2) escaping form the reaction
J.A. Gonzalez-ortega, E.M. Tejeda, N. Perea, G.A.
Hirata, E.J. Bosze and J. Mckittrick, Opt.
Mater., 27 (2005) 1221
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Solid state reaction or Ceramic method
Diffusion controlled reaction Ficks Ist
law J -D (dc/dx) Repeated grinding,
heating to high temp.. Required
E.g. Commercial blue phosphor BaMgAl10O17 Eu2
BaCO3 Al2O3 MgCO3 Eu2O3 ? 1500 ?
C/5h with a small quantity of
crystallizer BaF2
Wet chemical method..,
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Combustion Synthesis

• Wet chemical synthesis does not required
  further calcinations
• Highly exothermic - flame temperature gt 1600 ?
  C
• Lower sintering temperature
• Chemical homogeneity Nano particle size
• High porosity - due to gases produced during
  the combustion of fuel
• Oxidizer and fuels are required - ratio O/f 1
• Metal nitrates are oxidizers organic compounds
  are fuels
• 
  (urea, glycine, hydrazine, carbohydrazide)
• Based on propellant chemistry - formal
  valencies or elements assigned
• as C
  - 4, H - 1, V - 5, B - 3, N 0, O -2

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E.g. Synthesis of Al2O3
Al2O3
Table. Oxidizing and reducing valencies of metal
nitrates and fuels
S. Ekambaram, K.C Patil and M. Maaza, J. Alloys
Comp., 393 (2005) 81 - 92