Optical Properties of Nanocrystalline Y 2 O 3 :Eu 3+ Powder Phosphor Anushree Roy Indian Institute of Technology Kharagpur, India E-mail : anushree@phy.iitkgp.ernet.in Ms. Sudeshna Ray Prof. P. Pramanik (pramanik@chem.iitkgp.ernet.in)
What is a Phosphor? The substance that emits visible light after being energized Energy (eV) Energy (eV) Energy (eV) 2. Electrons lose its 3. Electrons lose energy 1. Electrons absorb energy to neighbouring and make a transition to energy and go to atoms and make a transition ground state band excited state band to impurity state band.
Application of Phosphor [Application] [Phosphor] [Excitation] Cathode ray Ultarviolet ray X-ray • TV screen • Computer monitor • Fluorescent lamp • Electroluminescence lamp • Plasma display
Advantage of Powder Phosphors • No area limit : can cover a large area of the emissive display • Efficiency is higher : light loss through internal reflection etc. is minimized • Possibility of having all colours in one plane : required in colour TV display • Improved crystallization and uniform distribution of dopant in the host
Why Nanomaterials? Particle size shape crystallite boundary play an important role to control phosphor properties. Possibility of higher efficiency and narrower emission band in nanocrystalline phosphors
Nanocrystalline Y 2 O 3 :Eu 3+ Phosphor Fundamental and technological importance Application • Lighting industry • Thermal lensing • Permanent laser based devices
Sample preparation by soft chemical method Y 2 O 3 Eu 2 O 3 pH-3 Concentrated nitric acid Y(NO 3 ) 3 Eu(NO 3 ) 3 180 0 -200 0 C TEA (chelating agent) Fluffy mass Calcined at 900 0 C Calcined at 1200 o C Nano sample Bulk sample
25 Morphology 20 15 Frequency 10 5 0 8 10 12 14 16 18 20 22 24 26 Diameter of the particle (nm) Assuming log-normal size distribution 2 1 ln( d / d ) P ( d ) exp d 2 2 0.29 nm d : av. Size of the particles : related to size distribution • Estimated values for nanosample : d =15nm = 0.14 • Lattice spacing same as expected in bulk 1nm
X-ray diffraction pattern : information about formation of the material impurity content (222) 14000 NC sample Bulk sample Intensity (440) 6000 4000 (400) 2000 (134) (332) (125) (411) 0 25 30 35 40 45 50 2 (Degree) • Identical diffraction patterns for nano and bulk samples Cubic (corresponds to space group Ia3) • No distortion in nanocrystalline sample
14000 12000 10000 Intensity 8000 6000 4000 2000 0 28.6 28.8 29.0 29.2 29.4 2 (Degree) We use Sherrer formula FWHM of the line 0 . 94 : wavelength of copper K line d cos : diffraction angle to estimate the average particle size using broadening of x-ray line (we also take into account the instrumental broadening)
Structure of Y 2 O 3 Yttrium Oxygen Vacancy diagonal S 6 (25%) C 2 (75%) (w/ inversion symmetry) (w/o inversion symmetry) N. C. Chang and J.B. Gruber, J. Chem Phys. 41, 3227 (1964). J. Silver et al, R. J. Phys Chem B, 106, 7200 (2001).
Luminescence spectrum 1.2 1.0 Emission intensity 5 D 0 - 7 F 4 Emission intensity 1.0 0.9 5 D 0 - 7 F 0 * 0.8 0.8 5 D 0 - 7 F 3 5 D 1 - 7 F 1 0.6 0.7 5 D 2 - 7 F 3 5 D 0 - 7 F 1 5 D 0 - 7 F 6 0.4 0.6 x9 5 D 1 - 7 F 2 x4.5 5 D 0 - 7 F 5 0.2 0.5 x3 5 D 1 - 7 F 0 0.0 0.4 500 520 540 560 580 600 620 640 650 675 700 725 750 775 800 Wavelength (nm) Wavelength (nm) D 0 – 7 F 4,6 : Electric dipole in character ( J=0, ±1 with J=0 ↔ J'=0) : satisfying magnetic 5 D 0 – 7 F 3,5 :First order magnetic dipole dipole selection rules (S 6 site) transition in the even parity crystal 5 D 0 - 7 F 0 : mixing of odd parity J=1 field and the electric dipole transitions states into the 7 F 0 and 5 D 0 states. to second order
Energy transfer between S 6 to C 2 site 11 10 5 D 0 – 7 F 1 (587 nm) 9 I D 0 -F 0 /I D 0 -F 1 originates from S 6 site 8 5 D 0 – 7 F 0 (581 nm) 7 originates from C 2 site 6 1 2 3 4 5 6 7 3+ At wt. % of Eu • Ratio of emission intensity of S 6 site to the emission intensity of C 2 site decreases with increase in Europium concentration in the sample • Effect of fluorescence quenching after 4 at wt% of Eu + doping
Strongest line at 611 nm due to 5 D 0 – 7 F 2 transition 3 12 x10 Theory of Judd and Ofelt : Intensity (arb. unit) 5 D 0 – 7 F 2 transition becomes 10 electric dipole type, due to 8 an admixture of opposite 6 parity 4 f n-1 5 d states by an odd parity crystal field 4 component 2 B. R. Judd, Phys. Rev. 127, 750 (1962). 605 610 615 620 Wavelength (nm) 6 G. S. Ofelt, J. Chem. Phys. 37, 511 3 x10 (1962). 5 Emission intensity of 4 • Increase in emission intensity of D 0- F 2 D 0 -F 2 transition 3 with increase in Europium concentration 2 • Effect of fluorescence quenching after 1 4 at wt% of Eu 3+ doping 0 1 2 3 4 5 6 7 3+ At. wt. % of Eu
16% Increase in luminescence yield for nanocrystalline Y 2 O 3 :Eu 3+ 12 3 x10 Intensity (arb. unit) 10 8 6 4 2 606 608 610 612 614 616 618 Wavelength (nm) Possible cause : Dramatic shortening of lifetime of the luminescence states i.e. slower and less efficient surface recombination process.
24 3 x10 5 D 2 20 5 D 1 -1 ) 5 D 0 Energy (cm 16 12 8 7 F 6 4 7 F 5 7 F 4 7 F 3 7 F 2 0 7 F 0 / 7 F 1 J=1 J=-1 J=0 A schematic diagram of the energy level of Eu 3+ in nanocrystalline Y 2 O 3 :Eu 3+
Crystal field effect Crystal field Hamiltonian w/o taking into account mixing of different J- state H B C ( i ), CF l , m l , m l , m i Crystal field parameter m B ( 1 ) B l =2,4,6, … and l , m l , m m =0,±2, ±4 … ± l. 1 / 2 4 C ( i ) Y ( , ) and l , m lm i i 2 l 1 • One needs an extra crystal field term to include J-mixing • Intensity of 5 D 0 - 7 F 0 line is proportional to 2 5 D 0 - 7 F 2 line is proportional to G. Nishimura, M. Tanaka, A. Kurita and T.Kushida, J. Lumin, 48 & 49, 473 (1991).
Crystal field effect in nanocrystalline Y 2 O 3 :Eu 3+ 1400 1300 I 4 -1 ) 0 0 (cm 1200 I 75 0 2 20 1100 1000 : crystal field parameter 20 : Energy separation between 900 1 2 3 4 5 6 7 7 F 2 and 7 F 0 levels (900 cm -1 ) 3+ At wt % of Eu Value of of for nanocrystalline Y 2 O 3 with 4 at wt % Eu 3+ : 1077 cm -1 for bulk Y 2 O 3 :Eu 3+ : 995 cm -1
Effect of energy transfer in co-doped sample Nanocrystalline Y 2 O 3 :Eu 3+ (4 At wt%) 6000 Nanocrystalline Y 2 O 3 :Sm 3+ ( 0.25 At wt %) Emission intensity Nanocrystalline Y 2 O 3 :Eu 3+ :Sm 3+ 5000 Sm 3+ : as Donor 4000 Eu 3+ : as Acceptor 3000 2000 1000 605 610 615 620 Wavelength (nm)
Electronic energy levels for Eu 3+ and Sm 3+
Singly doped sample 6 3 2.3 x10 3 5 x10 2.2 Emission intensity Emission intensity 4 2.1 2.0 3 1.9 2 1.8 1.7 1 D 0- F 2 transition of Eu 3+ G 5/2 - H 7/2 transition of Sm 3+ 1.6 at 611 nm at 609 nm 0 1.5 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 8 3+ 3+ At. wt. % of Eu At wt% of Sm Only donor Only acceptor
Increase in D 0- F 2 transition of 4 At wt %Eu 3+ in co-doped sample 8 7 Emission intensity 4 At wt %Eu 3+ 6 5 4 3 2 0 1 2 3 4 5 6 7 8 3+ At wt % of Sm
Energy transfer efficiency with Sm 3+ concentration Energy transfer efficiency Energy transfer efficiency 0.4 F 0.3 1 DA 0.2 F D 0.1 (neglecting donor-donor transfer) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 F DA : Fluorescence inten. of donor (Sm 3+ ) 3+ At wt% of Sm in presence of acceptor (Eu 3+ ) F D : Fluorescence inten. of donor (Sm 3+ ) • Increase in energy transfer efficiency with Sm 3+ concentration for 4 at wt% of Eu 3+
Summary • Optical properties of nanocrystalline red-emitting phosphor, Europium doped Yttria (Y 2 O 3 :Eu 3+ ) has been investigated. • Intensity of the strongest emission line at 611 nm of this material doped with 4 at. wt.% of Europium is found to be 16% more than that for the corresponding bulk system. • Narrow electronic emission spectrum suggests a crystalline surrounding in this nanomaterial. • Crystal field parameter and equilibrium thermal energy of the nanocrystalline system has been estimated. • Increase in luminescence efficiency in the nanocrystalline samples in presence of donor has been investigated.
Work in Progress • Variation of optical properties of the phosphor with the size of the nanoparticles. • Energy transfer process in more detail by lifetime measurements. • More accurate estimation of energy transfer efficiency using Dexter- Förster Theory.
Property Remark 15 nm Size of the particle ±5 nm Size distribution Cubic (corresponds to Phase space group Ia3) Intensity of the strongest 16% more compared to emission line at 611 nm that of bulk system 1077 cm -1 Linear crystal field parameter 57 cm -1 Equilibrium energy
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