Optical Properties of Nanocrystalline Y 2 O 3 :Eu 3+ Powder Phosphor - - PowerPoint PPT Presentation

Optical Properties of Nanocrystalline Y2O3:Eu3+ Powder Phosphor

• Ms. Sudeshna Ray
• Prof. P. Pramanik

(pramanik@chem.iitkgp.ernet.in)

Anushree Roy

Indian Institute of Technology Kharagpur, India E-mail : anushree@phy.iitkgp.ernet.in

What is a Phosphor? The substance that emits visible light after being energized

• 1. Electrons absorb

energy and go to excited state band

• 2. Electrons lose its

energy to neighbouring atoms and make a transition to impurity state band.

• 3. Electrons lose energy

and make a transition to ground state band

Energy (eV) Energy (eV) Energy (eV)

Application of Phosphor

• TV screen
• Computer monitor
• Fluorescent lamp
• Electroluminescence lamp
• Plasma display

Cathode ray Ultarviolet ray X-ray

[Excitation] [Phosphor] [Application]

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 Y2O3:Eu3+ Phosphor

Fundamental and technological importance

Application

• Lighting industry
• Thermal lensing
• Permanent laser based devices

Fluffy mass

Sample preparation by soft chemical method

Eu2O3 Y2O3 Concentrated nitric acid pH-3 Y(NO3)3 Eu(NO3)3 TEA (chelating agent) 1800-2000C Calcined at 9000C Nano sample Calcined at 1200oC Bulk sample

Morphology

8 10 12 14 16 18 20 22 24 26 5 10 15 20 25

Frequency Diameter of the particle (nm)

2

2 ) / ln( exp 2 1 ) (            d d d d P Assuming log-normal size distribution 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

0.29 nm

X-ray diffraction pattern :

information about formation of the material impurity content

25 30 35 40 45 50 2000 4000 6000 14000

(125) (134) (332) (411) (440) (400) (222)

Intensity 2 (Degree)

• Identical diffraction patterns for nano and bulk samples Cubic

(corresponds to space group Ia3)

• No distortion in nanocrystalline sample

NC sample Bulk sample

28.6 28.8 29.0 29.2 29.4 2000 4000 6000 8000 10000 12000 14000

Intensity 2 (Degree)

We use Sherrer formula to estimate the average particle size using broadening of x-ray line (we also take into account the instrumental broadening)

  cos 94 . d  

FWHM of the line  : wavelength of copper K line  : diffraction angle

S6 (25%) (w/ inversion symmetry) C2 (75%) (w/o inversion symmetry) Yttrium Oxygen Vacancy diagonal

Structure of Y2O3

• 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).

500 520 540 560 580 600 620 640 0.0 0.2 0.4 0.6 0.8 1.0 1.2

x3

x9

Emission intensity Wavelength (nm)

650 675 700 725 750 775 800 0.4 0.5 0.6 0.7 0.8 0.9 1.0

x4.5

Emission intensity Wavelength (nm)

5D0-7F0 : mixing of odd parity J=1

states into the 7F0 and 5D0 states.

5D0-7F0

D0 –7F4,6 : Electric dipole in character

5D0 –7F3,5 :First order magnetic dipole

transition in the even parity crystal field and the electric dipole transitions to second order

5D0-7F3 5D0-7F4 5D0-7F5 5D0-7F6

*

Luminescence spectrum

(J=0, ±1 with J=0↔J'=0) : satisfying magnetic dipole selection rules (S6 site)

5D2-7F3 5D1-7F1 5D1-7F2 5D1-7F0 5D0-7F1

Energy transfer between S6 to C2 site

5D0 –7F1 (587 nm)

• riginates from S6 site

5D0 –7F0 (581 nm)

• riginates from C2site
• Ratio of emission intensity of S6 site to the emission intensity of C2 site

decreases with increase in Europium concentration in the sample

• Effect of fluorescence quenching after 4 at wt% of Eu+ doping

1 2 3 4 5 6 7 6 7 8 9 10 11

ID0-F0/ID0-F1 At wt. % of Eu

3+

605 610 615 620

2 4 6 8 10 12

x10

3

Intensity (arb. unit) Wavelength (nm)

Theory of Judd and Ofelt :

5D0 –7F2 transition becomes

electric dipole type, due to an admixture of opposite parity 4f n-15d states by an

• dd parity crystal field

component

• B. R. Judd, Phys. Rev. 127, 750 (1962).
• G. S. Ofelt, J. Chem. Phys. 37, 511

(1962).

Strongest line at 611 nm due to 5D0 –7F2 transition

1 2 3 4 5 6 7 1 2 3 4 5 6 x10

3

Emission intensity of D0-F2 transition

• At. wt. % of Eu

3+

• Increase in emission intensity of D0-F2

with increase in Europium concentration

• Effect of fluorescence quenching after

4 at wt% of Eu3+ doping

16% Increase in luminescence yield for nanocrystalline Y2O3:Eu 3+

Possible cause : Dramatic shortening of lifetime of the luminescence states i.e. slower and less efficient surface recombination process.

606 608 610 612 614 616 618 2 4 6 8 10 12 x10

3

Intensity (arb. unit) Wavelength (nm)

4 8 12 16 20 24

J=-1 J=1 J=0

7F6 5D2 5D1 5D0 7F4 7F2

x10

3

7F5 7F3 7F0/ 7F1

Energy (cm

• 1)

A schematic diagram of the energy level of Eu3+ in nanocrystalline Y2O3:Eu3+

Crystal field Hamiltonian w/o taking into account mixing of different J- state

), (

, , ,

i C B H

i m l m l m l CF

 





Crystal field effect

m l m m l

B B

 

 

, ,

) 1 (

Crystal field parameter

) , ( 1 2 4 ) (

2 / 1 , i i lm m l

Y l i C           

and

• One needs an extra crystal field term to include J-mixing
• Intensity of 5D0-7F0 line is proportional to 2

5D0-7F2 line is proportional to 

l=2,4,6,… and m=0,±2, ±4…±l.

• G. Nishimura, M. Tanaka, A. Kurita and T.Kushida, J. Lumin, 48 & 49, 473 (1991).

1 2 3 4 5 6 7

900 1000 1100 1200 1300 1400

 (cm

• 1)

At wt % of Eu

3+

Crystal field effect in nanocrystalline Y2O3:Eu 3+

20 2

75 4  

 

 I I

: crystal field parameter 20 : Energy separation between

7F2 and 7F0 levels (900 cm-1)

Value of of for nanocrystalline Y2O3 with 4 at wt % Eu 3+ : 1077 cm-1 for bulk Y2O3:Eu 3+ : 995 cm-1

Effect of energy transfer in co-doped sample

605 610 615 620 1000 2000 3000 4000 5000 6000

Emission intensity Wavelength (nm)

Nanocrystalline Y2O3:Eu 3+ (4 At wt%) Nanocrystalline Y2O3:Sm3+ (0.25 At wt %) Nanocrystalline Y2O3:Eu3+ :Sm 3+ Sm3+ : as Donor Eu3+ : as Acceptor

Electronic energy levels for Eu3+ and Sm3+

1 2 3 4 5 6 7 1 2 3 4 5 6 x10

3

Emission intensity

• At. wt. % of Eu

3+

1 2 3 4 5 6 7 8 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 x10

3

Emission intensity At wt% of Sm

3+

D0-F2 transition of Eu 3+ at 611 nm G5/2- H7/2 transition of Sm3+ at 609 nm

Singly doped sample Only acceptor Only donor

1 2 3 4 5 6 7 8 2 3 4 5 6 7 8

Emission intensity At wt % of Sm

3+

Increase in D0-F2 transition of 4 At wt %Eu 3+ in co-doped sample 4 At wt %Eu 3+

Energy transfer efficiency with Sm 3+ concentration

0.5 1.0 1.5 2.0 2.5 3.0 3.5 0.0 0.1 0.2 0.3 0.4

Energy transfer efficiency At wt% of Sm

3+

Energy transfer efficiency

D DA

F F  1

(neglecting donor-donor transfer) FDA : Fluorescence inten. of donor (Sm3+) in presence of acceptor (Eu3+) FD : Fluorescence inten. of donor (Sm3+)

• Increase in energy transfer efficiency with Sm3+ concentration for 4 at wt%
• f Eu3+

Summary

• Optical properties of nanocrystalline red-emitting phosphor,

Europium doped Yttria (Y2O3:Eu3+) 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 Size of the particle Size distribution 15 nm ±5 nm Phase Cubic (corresponds to space group Ia3) Intensity of the strongest emission line at 611 nm 16% more compared to that of bulk system Linear crystal field parameter 1077 cm-1 Equilibrium energy 57 cm-1

Estimation of equilibrium energy in nanocrystalline Y2O3:Eu 3+ using Raman measurements

• 400
• 380
• 360

360 380 400 1200 1300 1400 1500 1600 Anti-Stokes Stokes

Intensity Raman shift (cm

• 1)

       kT I I

AS s

  exp

Intensity ratio of Stokes and Antistokes Raman scattering

• Equilibrium energies of

nanocrystalline sample : 57 cm-1 bulk sample : 53 cm-1