Phonon side band and colorimetric analysis of rare earths...

Chapter 4

Phonon side band and colorimetric analysis

of rare earths/nanocrystallites doped

titania-zirconia hybrids

The titania-zirconia matrices codoped with europium ions and

CdS/CdSe nanocrystallites were prepared through the sol-gel route.

The phonon side band analysis of the excitation spectrum provides

information about the coupling between vibrational modes and

electronic transitions.The vibrational state energy and the local site

symmetry in sol-gel systems around Eu3+ ions are investigated in

detail. The emission spectrum intensities of electric dipole transitions

with respect to magnetic dipole transition give a measure of the

distortion from the inversion symmetry around the Eu3+ site. The

vibrational state bands along with Raman measurements give

information on the structural units in the vicinity of the dopant ion.

A correlation between the Raman spectral data and phonon side

band analysis gives an average structure of the bulk glass and a near

estimate of the maximum vibrational state energy. In order to obtain

white light emission, the terbium ions were also codoped with the

above samples. The incorporation of rare earths/nanocrystallites

doped titania-zirconia matrix provides desired emission colors for

lighting and other optical applications.

Chapter 4

122

4.1Introduction

The inherent favourable characteristics of both titania and

zirconia can be fully explored by using them in combination with

each other through sol-gel processing which possess many unique

advantages[1]. Therefore, among various metal oxides, the titania-

zirconia combination has attracted much attention in recent years [2,

3]. Mixing two dissimilar oxides can cause the formation of new

stable compounds, which can lead to totally different physical,

chemical properties and catalytic behaviour. Zirconium titanate

based ceramic materials have many attractive properties like high

dielectric constant, resistivity, permittivity at microwave frequencies

and excellent temperature stability for microwave properties.

Lanthanide ions in general have a fairly unique property of sharp

spectral lines (4f–4f) in the solid phase [4] and the nonradiative

relaxation rates depend on the highest energy of phonons available

in the matrix. The optical absorption or excitation spectra of glasses

containing rare earth ions often show weak vibronic features on the

low energy side of the spectrum representing phonons created by the

lattice vibrational modes [5-7]. The extent of electron phonon

coupling is used in the determination of the intensity of these

vibronic transitons. These are of special interest as diagnostic probes

for the rare earth ligand interaction and crystal field dynamics [8].

Europium is an excellent indicator of the site symmetry and chemical

bonding in glasses since Eu3+ ions incorporated in low-symmetry

Phonon side band and colorimetric analysis of rare earths/nanocrystallites....

123

sites exhibit enhanced f–f transition probabilities [9].Because of its

simple structure it is widely used as a probe to investigate local

structure. In the excitation spectrum of Eu3+, the phonon side bands

are associated with the 7F0→5D2 transition. Their analysis has been

used for the understanding of phonon energy and local site

symmetry in sol-gel systems.

In recent years, there has been large interest in cheap, efficient

generation of (white) light sources for a variety of purposes, such as

displays, liquid crystal displays, etc. [10]. Lanthanide ions have been

used as basic constituents of luminescent materials, such as

illumination lamps, cathode ray tube displays and optical imaging of

cells due to their colorimetric purity [11, 12]. In the past, materials for

practical uses were limited to inorganic solids, whereas lanthanide

complexes were always excluded just because of their limited

thermal stability. The new applications that have been investigated

in recent years make such metal complexes an attractive research

area. The common solution is to link lanthanide complexes to

inorganic parts via covalent bonds to generate hybrid materials [13,

14].The sol–gel process has been successfully used in the past few

years for the production of novel phosphors and one of the most

important aspects of the realization of the white LED lighting. In

particular, single phosphors that can emit blue, green and red are

drawing attention as potential white light sources since they offer

Chapter 4

124

higher luminous efficiencies and lower manufacturing costs that

require multiple phosphors to achieve the same blend of colors.

In this work europium ions and CdS/CdSe nanocrystallites

are doped in titania-zirconia matrices.The optical properties mainly

the phonon side band analysis was done to understand the

vibrational state energy and the local site symmetry in sol-gel

systems around Eu3+ ions.The vibrational state bands along with

Raman measurements give information on the structural units in the

vicinity of the dopant ion. An attempt has been made to understand

the coupling between the electronic transition of the dopant ion and

the lattice vibrations. A correlation between the Raman spectral data

and phonon side band analysis gives an average structure of the bulk

glass and a near estimate of the maximum vibrational state

energy.RGB emission was observed from titania-zirconia matrix

codoped with CdS/CdSe nanocrystallites and Tb3+/Eu3+ rare earth

ions. The color coordinates corresponding to the prominent

emissions are determined using a fibre optic spectrometer.

4.2Experimental

TiO2-ZrO2 gels codoped with Tb3+/Eu3+ rare earth ions and

CdS/CdSe nanocrystallites were prepared through sol-gel route with

titanium iso-propoxide and zirconium IV propoxide as precursors in

the presence of ethanol. The dopants were added in the form of

europium nitrate, terbium nitrate, cadmium acetate, thiourea and


125

selenic acid. Cadmium acetate and thiourea were used as cadmium

and sulphur sources respectively. Similarly Cadmium acetate and

selenic acid were used as cadmium and selenium sources

respectively. A measured volume of 1M HNO3 was added as

catalyst.

The samples were prepared with the following compositions

Sample A TiO2-ZrO2 -75:25 + Eu3+( 3%)

Sample B TiO2-ZrO2 -75:25 + Eu3+( 3%)+ CdS (3%)

Sample C TiO2-ZrO2 -75:25 + Eu3+( 3%)+ CdSe (3%)

Sample D TiO2-ZrO2 -75:25 + Eu3+( 3%)+Tb3+( 3%)+ CdS (3%)

Sample E TiO2-ZrO2 -75:25 + Eu3+( 3%)+Tb3+( 3%)+ CdSe (3%)

In all the samples dopant concentration was maintained at 3%. The

resulting mixture was stirred continuously using a magnetic stirrer

for about an hour at room temperature till it formed a uniform clear

solution. The mixture (sol) is poured into polypropylene containers,

which is sealed and kept to form stiff gel for one month. The samples

were clear, transparent and colorless. The CdS doped sample was

heated at 600C for two days. The CdSe doped sample was annealed

at 5000C for two hours. The excitation and emission spectra were

taken using spectrophotofluorimeter (Shimadzu-RFPC 5301). The

colorimetric analysis was done using a fibre optic spectrometer.

Chapter 4

126

4.3 Theoretical considerations

4.3.1 Phonon side band analysis

The phonon sideband spectra provide useful experimental

data to analyse and study the variations of the nonradiative decay

rate of rare earth ions doped in different glassy matrices. The

generation of one phonon process is rapid for a small energy gap

while a multiphonon relaxation process is expected for a larger

energy gap. The total decay rate of an excited state is given by

W=Wr +Wnr =1/τr +1/ τnr

Wnr =Wmp +WET +WCR +WOH

Wmp, WET, WCR, and WOH are the rates of multiphonon decay, energy

transfer, cross relaxation and decay due to water contained in the

glasses respectively. The radiative decay rate is influenced by

variations of the local crystal field symmetry at the rare earth site.

The host matrix into which the ion is placed determines these

variations. In addition to changes in field symmetry, the local

vibrational density of states (D0S) of the host also provides a

mechanism for depopulation of the excited state energy. Electron-

phonon coupling allows an excited rare earth ion to decay

nonradiatively via the production of lattice vibrations. WET

represents an additional nonradiative loss mechanism involving the

transfer of excited state energy between rare earth ions that

terminates when the energy counters a defect or trap. The WOH

(4.1)

(4.2)


127

component for the nonradiative decay can be neglected for water free

glasses. Determination of the intrinsic radiative decay rate Wr was

accomplished through Judd-Ofelt analysis of the absorption spectra

[15, 16]. Using the values of Wr, from the Judd-Ofelt analysis together

with the experimentally observed excited state decay rate W, the

nonradiative contribution Wnr, can be determined. The nonradiative

decay rate was estimated as

Wnr =1/τ-Arad (4.3)

where τis the measured lifetime and Arad transition probability

obtained from Judd-Ofelt analysis. The probability for nonradiative

process, which arises outof the production of lattice vibrations

(optical phonons), is determined by the strength of the electron-

phonon coupling.

According to Miyakawa and Dexter, [17] the nonradiative decay rate

due to the multiphonon relaxation process is given by

Wmp=Woe-αΔE (4.4)

α=ħω-1[ln[p/g(n+1)]-1] (4.5)

p=ΔE/ ħω (4.6)

where ΔE is the energy gap, ħω is the phonon energy, p is the

phonon number, g is the electron phonon coupling strength, Wo is

the experimental parameter corresponding to the decay rate at zero

energy gap and zero phonon emission and n is the Planck

Chapter 4

128

distribution function on the population of phonon as a function of

temperature and ħω, being expressed by [18]

n=(e ħω/kT-1)-1 (4.7)

From these equations, it is quite evident that the larger the phonon

energy and electron-phonon coupling strength, the larger the

multiphonon decay rate. If Wo is constant, the relative nonradiative

decay due to the multiphonon relaxation process can be estimated

from the phonon energy, electron-phonon coupling strength and

energy gap. The electron phonon coupling strength (g) and phonon

energy (ħω) can be determined from the phonon side band spectra.

Using the values obtained for g and ħω, the host dependant

parameter α and the relative nonradiative decay rate Wmp/W0, can be

calculated.

.

4.3.2 Colorimetry

Chromaticity coordinates are the ratio of the intensities of the

three primary lights that appear identical to a series of

monochromatic lights of equal energy traversing the visible

spectrum. A diagram in which any one of the three-chromaticity

coordinates is plotted against any other is called a chromaticity

diagram [19–21]. Chromaticity coordinates are derived from the color

matching functions. The CIE standard color primaries model the

response of the three photoreceptors in the human eye and are

referred to as ‘standard CIE observer’. The finalized response curves


129

existing in the literature are known as the ‘Tristimulus Response

Curves’.These are defined in terms of the color matching functions

yx, and z or the tristimulus response functions. The energy of any

spectral curve is defined as a summation of intensities times

wavelengths. By multiplying the spectral curve energy by the

overlap of each tristimulus response curve we get the ‘Tristimulus

values’.

Tristimulus values are obtained from the following integrals

(4.8)

∫=780

380

)()()( λλλλ dTySKY (4.9)

∫=780

380

)()()( λλλλ dTzSKZ (4.10)

∫= 780

380)()(

100

λλλ dySK (4.11)

is a normalizing factor, where S(λ) is relative spectral power

distribution of the illuminant, T(λ) is spectral transmittance of the

color object. A color is said to be achromatic if X=Y=Z. These

tristimulus values XYZ are useful for defining a color, but for the

visualization the CIE 1931(x,y)- Chromaticity Diagram is used where

x, y and z are chromaticity coordinates calculated from the

tristimulus values.

∫=780

380

)()()( λλλλ dTxSKX

Chapter 4

130

ZYX

Zz

ZYX

Yy

ZYX

Xx

++=

++=

++= ,, (4.12)

Since chromaticity parameters are defined as x + y + z = 1, it is

sufficient to give only two of the three coordinates—commonly x and

y [22, 23]. Since monochromatic radiation is a boundary of color-

mixing, the Chromaticity Coordinate Diagram can be constructed

with x and y, the advantage of which is that we have a set of

normalized values which can be used to compare colors having

different intensity values. We can find any color or hue, in terms of

its x and y coordinates. These diagrams are based on the 1931 2-

degree CIE xyz color matching functions that remain international

standards in both colorimetry and photometry. International

Telecommunication Union uses 1931 CIE color matching functions in

their recommendations for worldwide unified colorimetry (ITU-R

BT.709-4, ITU-R BT.1361). Most color monitors comply with this

standard. This makes it possible to display 1931 CIE diagrams

correctly on different color monitors. CIE also defined a standard set

of real primaries of wavelengths Red = 700, Green = 546 and Blue =

435 nm. They have defined the chromaticity coordinates for this in

the CIE space as R (0.73467, 0.26533, 0.0), G (0.27376, 0.71741, 0.00883)

and B (0.16658, 0.00886, 0.82456) respectively.


131

4.4Results and discussion

4.4.1 Phonon side band analysis

The excitation spectra of Eu3+/CdS/CdSedoped titania-zirconia

matrices were recorded in the wavelength range 350-600 nm at an

emission wavelength of 614 nm [Figure 4.1].

350 400 450 500 550 600

0

50

100

150

200

250

300

350

400

Inte

nsi

ty(a

rb.u

nit

)

Wavelength(nm)

A-Eu3+(3%)

B-Eu3+(3%)+CdS(3%)

C- Eu3+(3%)+CdSe(3%)

A

B

C

Figure 4.1: Excitation spectra of (A) Eu3+ (B) Eu3+/CdS

and (C) Eu3+/CdSe doped TiO2-ZrO2matrices

In the spectrum the main peak is observed at 464 nm, corresponding

to the7F0→5D2transition. The phonon side bands at the high energy

side are associated with the 7F0→5D2 transition and are shown in

Figure 4.2. The phonon side bands with four fold magnification are

shown in the inset. In the three samples the pure electronic transition

(PET) or zero vibrational state line is peaked at 21553 cm-1. In the case

of the matrix doped only with Eu3+, the phonon side band occurs at

energy shifts of 910 and 1160 cm-1 from the zero vibrational state line.

Chapter 4

132

In the case of CdS/Eu3+ doped samples,they occur at energy shifts of

690 and 940 cm-1. Similarly in CdSe/Eu3+ doped samples, the phonon

side bands occur at energy shifts of 590 and 840 cm-1. The phonon

side band energy values obtained from this study supports the

assignment of the vibrational state bands to Zr-O-Ti vibrations. The

vibrational state band peaks,which correspond to the highest

vibrational state energy available in nanocrystallites doped samples,

are found to be shifted to lower values.

21000 21500 22000 22500 23000 23500

0

50

100

150

200

250

300

350

400

450

Inte

nsi

ty (

arb.

un

it)

Energy Shift (cm-1)

A-EuB-Eu+CdSC-Eu+CdSe

A

B

C

Figure 4.2: Phonon side bands associated with the 7F0→5D2 transition of

Eu3+ ion for Eu3+, Eu3+/CdS and Eu3+/CdSe doped TiO2-ZrO2matrices

The energy differences between these phonon side bands and

the zero phonon line gives a direct measure of the local vibrational

state energies associated with the rare earth ion. The phonon side

bands provide useful information on the electron vibrational state

22200 22400 22600 22800

8

10

12

14

16

18

Inte

nsi

ty(a

rb.u

nit

)

Energy Shift(cm-1)

A

B

C


133

coupling in the rare earth doped glasses. Phonon side bands arise

from the simultaneous excitation of the electronic transitions of Eu3+

and vibrational modes around the dopant ions [24]. The partial

energy level diagram of Eu3+ with the phonon side band process is

schematically shown in Figure 4.3.

Figure 4.3: Partial energy level diagram of Eu3+ ion

showing Phonon side bands

The presence of nanosized semiconductors may produce

shifting of side bands, which is attributed to some differences in the

Chapter 4

134

vibrational state energy in the local structure around the rare earth

ions in the two cases. The vibrational state energy obtained from the

vibrational state band measurements clearly gives the local

vibrational environment around the rare earth ion. This corresponds

to the strongest among several local variations that contributes to

multi vibrational state relaxation. The parameters of multi

vibrational state relaxation in the samples have been calculated and

are summarized in Table 4.1.

Table 4.1:Vibrational state maxima (ħω),electron–vibrational state

coupling strength(g) and non-radiative decay rates for 5D1,5D2 and 5D3

transitions of three samples

Glass g

ħω

(cm-1)

7F0→5D1 7F0→5D2 7F0→5D3

α

(10-3

cm

)

Wmp/

W0

(10-5)

α

(10-3

cm

)

Wmp/W0(

10-5)

α

(10-3

cm

)

Wmp

/W0

(10-5)

Eu3+

0.007

3 910

5.0

2 15.30

5.4

1 0.134

5.5

3 0.019

0.006

5

116

0

3.8

0 129.4

4.1

4 3.200

4.2

4 0.698

CdS/Eu3

+

0.009

5 690

6.5

8 0.098

7.0

9 0.002

7.2

5 0.0002

0.007

8 940

4.7

4 24.98

5.1

2 0.280

5.2

4 0.043

CdSe/E

u3+

0.007

5 590

7.7

5 0.130

8.3

5 0.00009

8.5

5

0.0000

04

0.007

0 840

5.3

0 8.600

5.7

8 0.053

5.9

0 0.007


135

The values of pare determined using the known values of the energy

gap ΔE for 5D1,5D2 and 5D3levels of the Eu3+ ion. The phonon side

band (PSB) area divided by the zero phonon line (ZPL) area gives the

values of g. The values of n are calculated using equation (4.7).Using

the values of n, p, and g we have calculated the values of Wmp /W0

and α by using equation (4.4) and (4.5). The low energy and high

energy shifts are termed as PSB I and PSB II for the respective cases

[Table 4.I]. In all the samples the value of g is higher for PSB 1 than

PSB 2.Also the CdS/CdSe doped samples have higher g value than

Eu alone doped sample. The higher value of g is an indication of

increasing covalency and a shortening of Eu-O bond, because of the

strong covalent bonding of rare earth ions with the local site and

higher nonradiative decay [25].

The emission spectra of Eu3+/CdS/CdSe doped titania-zirconia

matrices at an exciting wavelength 393 nm are shown in Figure 4.4.

Chapter 4

136

450 500 550 600 650 700

0

100

200

300

400

In

ten

sity

(arb

.un

it)

Wavelength(nm)

A-Eu3+(3%)

B- Eu3+(3%)+CdS(3%)

C- Eu3+(3%)+CdSe(3%)

A

B

C

Figure 4.4: Emission spectra of (A) Eu3+ (B) Eu3+/CdS and (C) Eu3+/CdSe

doped TiO2-ZrO2matricesfor an excitation wavelength of 393 nm.

The spectra consist of 5D0→7FJ (J=0, 1, 2, 3, 4) transitions. The intense

peaks are observed at 590 nm and 612 nm corresponding to the

5D0→7F1 and5D0→7F2 transitions. The emission around 590 nm is

allowed by magnetic dipole considerations. So it is relatively

indifferent to the local symmetry. The emission around 612 nm is

allowed by electric dipole considerations and is subjected to local

symmetry. A considerable enhancement in the emission intensity

occurs corresponding to the 5D0→7F2 transition for CdS/Eu3+ and

CdSe/Eu3+ doped titania-zirconia matrices. The structural features

play a critical role on the fluorescence enhancement because the

complex dielectric function of the composite medium depends

directly on the structural features of the particles involved. In Eu3+


137

doped samples the 5D0→7F2 transition is hypersensitive to the

chemical bond formed between Eu3+ and its surrounding ligands. A

larger transition probability of the 5D0→7F2 hypersensitive transition

may correspond to an increase of covalent bonding. This bonding is

induced by the presence of CdS and CdSe in the nearest neighbour

coordination of the Eu3+ which changes the geometrical atomic

arrangement as well as the bond strength. The incorporation of CdS

and CdSe to the first coordination shell of Eu3+ provides a relative

softening of the crystal field strength. However, it also contributes to

distort the anion symmetry around the rare earth and therefore

promotes an enhancement of the transition rates. The intensities of

electric dipole transitions with respect to magnetic dipole transition

give a measure of the distortion from the inversion symmetry at the

Eu3+ site. This asymmetric ratio is given as

∫∫

→

→=γ

γ

dI

dIAS

10

20

where I0→j denotes the intensity of 5D0→7Fj transition. This

ratio is influenced by the site symmetry, electronegativity and

covalency of the ligand atoms.The ratio of emission intensity of 612

nm (red) and 590 nm(orange) gives a value of 1.142 for Eu3+ alone

doped sample. For Eu3+/CdS and Eu3+/CdSe doped samples the ratio

is evaluated to be 2.39 and 2.67.Improved efficient luminescence was

obtained forEu3+/CdS andEu3+/CdSe codoped titania-zirconia matrix

due to the reduction of concentration quenching (more dispersion of

(4.13)

Chapter 4

138

Eu3+ ions) compared to Eu3+ single doped samples. This shows that

the presence of nanocrystallites is responsible for an increase of both

the covalency and the polarization of the local vicinities of the Eu3+

cations. A higher ratio corresponds to a more distorted or

asymmetric local cation environment and an increase in covalency of

europium. The dominance of 5D0→7F2 transition indicates a lack of an

inversion centre for the local symmetry of the Eu3+ ion in the matrix.

Luminescence channels of Eu3+ ions are shown in Figure 4.5.

Figure 4.5: Luminescence channels of Eu3+ ions


139

4.4.2 Raman measurements

The Raman spectra of Eu3+/CdS/CdSe doped titania-zirconia

matrices are shown in Figure 4.6.The bands at 150,393,515 and 638

cm-1 are observed in the spectrum. The bands 150 cm-1 and 515 cm-1

indicate the presence of hydrous zirconium titanate, which shows the

bonding Zr-O-Ti. The band at 393 cm-1 corresponds to crystalline

anatase titania. The band at 638 cm-1 corresponds to monoclinic

zirconia[26].Only zirconia-titania bands are observed in the 200-900

cm-1region of the Raman spectra. Additional intensive part of spectra

was detected at wave-numbers higher than 1000 cm-1.

200 400 600 800 1000 1200 14000

500

1000

1500

2000

Inte

nsi

ty (

arb

.un

it)

Raman Shift (cm-1)

A-Eu B-Eu+CdS C-Eu+CdSe

C

A

B

Figure4.6: Raman spectra of Eu3+/CdS/CdSe

doped TiO2-ZrO2matrices

In CdS doped sample the peak is obtained at 301 cm–1

corresponding to longitudinal optical phonon (1LO) associated with

the CdS nanocrystals. The 1 LO phonon frequency for a single crystal

Chapter 4

140

of CdS was reported as 305 cm–1[27].This frequency shifts of the 1 LO

Raman peak in CdS nanoparticles is mainly ascribed to the grain size

effect (grain size =7.8 nm [reported and explained in chapter 2]) [28,

29]. In CdSe nanocrystals doped sample the LO phonon frequency is

at 205 cm-1andin bulk CdSe crystals it is at 212 cm–1 [30]. Thus, we see

a decrease in the frequencies of LO phonons due to their spatial

confinement (grain size = 10.8 nm [reported and explained in chapter

3]).The aforesaid shift in the LO frequencies is ascribed to the atomic

arrangement on and near the grain boundaries. This arrangement is

far from equilibrium and hence the Raman signals from these layers

may be more similar to those from amorphous structures than those

from crystalline ones [31, 32].

Depending on the local bonding environment of the rare

earth ion within the glass matrix, some of these vibration modes may

or may not be coupled to electronic excited states. The vibrational

state side band lines for the three samples can be accounted for by

considering the appearance of the various vibrational modes

detected in the Raman spectra. The vibrational state side band lines

and Raman data show a close relationship between the vibrational

state side band maxima and the vibrational modes. It is summarized

in Table 4.2. From the table it is quite clear that there exist a strong

coupling between vibrational modes and electronic transitions.


141

Table 4.2: Relationship between vibrational state side band maxima and

vibrational frequencies

Glass ħω (cm-1) Vibrational frequency (cm-1)

1 2 Total

Eu3+ 910 393 515 908

1160 638 515 1153

CdS/Eu3+ 690 393 301 694

940 638 301 939

CdSe/Eu3+ 590 393 205 598

840 638 205 843

4.4.3Colorimetric analysis

White light emission is achieved from titania-zirconia matrix

codoped with CdS/CdSe nanocrystals and rare earths like europium

and terbium. The Eu3+ usually serves as the red emitter, titania-

zirconia matrix as the blue emitter and Tb3+ as both the blue and

green emitters. Terbium ions excited at a wavelength of 350 nm is

shown in Figure 4.7. It is well known thatTb3+sensitizes Eu3+ [33] as

the Eu3+ emits strongly when excited at wavelengths at which Tb3+

absorbs. The luminescence of Tb3+ under UV excitation is mainly

from the 5D3 and 5D4 levels [34, 35]. The emission due to the 5D4→7F5

transition is usually so strong that almost all phosphors activated

with Tb3+ show green luminescence. The terbium ions show emission

at 490, 540, 585 and 620 nm wave-lengths. In Eu3+ ions the

fluorescence is mainly due to the purely electronic 5D0→7FJ (J=0, 1, 2, 3,

4) transitions at 578,589,613,650 and 699 nm respectively.

Chapter 4

142

Figure 4.7: Energy level diagram showing the excitation scheme for

obtaining white light with a single UV excitation

The emission spectrum of Eu3+ and Tb3+ ions together with

CdS nanocrystallites doped in titania-zirconia matrix excited at 360

nm is shown in Figure 4.8.The spectrum shows that the Tb3+ emission

is not fully quenched as the energy transfer from CdS

nanocrystallites to Tb3+ is also operative [36]. This can allow for

multiple rare-earth ions along with CdS nanocrystals to be placed

into the titania-zirconia matrix. This inhibits undesirable quenching

while at the same time allowing ions to partially transfer energies

such that sensitization of an acceptor ion can occur. The fluorescence

3+


143

spectrum of Eu3+ and Tb3+ ions together with CdSe nanocrystallites

doped in titania-zirconia matrix excited at 360 nm is shown in Figure

4.9.

450 500 550 600 6500

50

100

150

200

250

Inte

nsi

ty (

arb

.un

it)

Wavelength (nm)

Sample D

Figure 4.8: Fluorescence spectrum of CdS/ Eu3+/ Tb3+

doped TiO2-ZrO2matrix at an excitation wavelength of 360 nm

450 500 550 600 6500

50

100

150

200

250

Inte

nsi

ty (a

rb.u

nit)

Wavelength (nm)

Sample E

Figure 4.9: Fluorescence spectrum of CdSe/ Eu3+/ Tb3+ doped TiO2-

ZrO2matrix at an excitation wavelength of 360 nm

Chapter 4

144

The chromaticity (x,y) coordinates in the Commission

Internationale d Eclairage (CIE) diagram were calculated. The

coordinates were calculated to be (0.294, 0.333), (0.291,0.33) for the

CdS and CdSe doped sample. The calculated colour coordinates fall

within the white region of the 1931 CIE diagram. The CIE

chromaticity diagram with the colour coordinates are shown in

Figure 4.10.

Figure 4.10: The CIE Chromaticity for CdS/CdSe/ Eu3+/ Tb3+

doped TiO2-ZrO2matrices


145

4.5Conclusions

The phonon side bands provide information about the

coupling between vibrational modes and electronic transitions. The

phonon side bands measurements also give the values of vibrational

state maxima and electron vibrational state coupling strength. The

emission spectrum intensities of electric dipole transitions with

respect to magnetic dipole transitions give a measure of the

distortion from the inversion symmetry at the Eu3+ site. The

vibrational state side band lines and Raman data show a close

relationship between the vibrational state side band maxima and the

vibrational modes. White light emission was achieved from titania-

zirconia matrix codoped with CdS/CdSe nanocrystals and rare earths

like europium and terbium. The presence of nanocrystallites

increased the intensity of rare earth ions. The emission spectrum for

the sample has been converted to the CIE 1931 colour coordinate

system. The CIE coordinates of D sample and E sample are close to

the standard equal energy white light illuminate(X=0.333,

Y=0.333).Thus the incorporation of rare earths/nanocrystallites doped

titania-zirconia matrix provide desired emission colours for lighting

and other optical applications.

Chapter 4

146

References

[1] M.F. Garcia, A.M. Arias, J.C. Hanson, J.A. Rodriguez, Chem.

Rev.104(2004)4063.

[2] W. Lin, L. Lin, Y.X. Zhu, J. Mol.Catal. A Chem.226 (2005) 263.

[3] G. Colon, M.C. Hidalgo,J.A. Navio, Appl. Catal.A 231 (2002)

185.

[4] S. Todoroki, K. Hira, N. Soga J. Non-Cryst. Solids 143(1992)

46.

[5] M. Peng, J. Qiu, D. Chen, X. Meng, I. Yang, X. Jiang, C. Zhu,

Opt. Lett.29 (2004)1998.

[6] M. Nogami, S. Ito, Phys. Rev.B 61 (2000) 14295.

[7] H. Toratani,T. Izumitani,H. Kuroda, J. Non-Cryst.Solids

52(1982) 303.

[8] G.F. Imbuschin,In: Bartolo BD (ed) Advances in Nonradiative

Processes in solids,Plenum Press,New York(1991).

[9] C. Brecher, L. Riseberg, Phys. Rev. B 13 (1976) 81.

[10] C. Adachi, M.A. Baldo, M.E. Thompson, S.R. Forrest, J. Appl.

Phys.90 (2001) 5048.

[11] Y. Yang, X. Liu, A. Nakamura, K. Binnemans, J. Liu, J.Phys.

Chem. B 112 (2008) 5291.

[12] Y.G. Galyametdinov, A.A. Knyazev, V.I. Dzhabarov, T.

Cardinaels, K. Driesen, C. G. walrand, K. Binnemans, Adv.

Mater.20 (2008) 252.


147

[13] L.D. Carlos, R.A.S. Ferreira, V. de, Z. Bermudez, S.J.L. Ribeiro,

Adv.Mater.21(2009) 509.

[14] H.F. Lu, B. Yan, J.L. Liu, Inorg.Chem.48 (2009) 3966.

[15] B.R. Judd, Phys. Rev.127(1962) 750.

[16] G.S. Ofelt,J.Chem. Phys.37(1962) 511.

[17] T. Miyakawa,D.L. Dexter, Phys. Rev. B.1(1970) 2961.

[18] C. Kittel,Introduction to Solid State Physics. Wiley,New York

(1992).

[19] D.L. MacAdam,Color Measurements, 2nd edn. Springer,

Berlin,Heidelberg(1985).

[20] G. Wyszecki, W.S. Stiles,Color Science, Concepts and

Methods, 2nd edn. WileyNew York(1982).

[21] H. Arens,Color Measurement, Leipziger Druckhaus,Leipzig

(1967).

[22] V.T. Bich, N.T. Binh, P.H. Duong,T.K. Anh, Commun. In

Phys.8 (1998) 145.

[23] R.C. Ropp,Luminescence and the solid state. Elsevier(1991).

[24] M. Wachtler, A. Speghini, S. Pigorini, R. Rolli,M.

Bettinelli,J.Non-Cryst. Solids217(1997) 111.

[25] S. Tanabe,K. Hirao,N. Soga, J. Non-Cryst. Solids142(1992) 148.

[26] M. Andrianainarivelo, J.P Robert, Corriu, D. Leclercq, H.M.P.

Andre Vioux J.Mater. Chem.7(1997) 279.

[27] B. Tell, T.C. Damen,S.P.S. Porto, Phys. Rev.144(1966) 771.

[28] D.R. Chuu, C.M. Dai,Phys. Rev. B45 (1992) 11805.

Chapter 4

148

[29] R.P. Rajeev,M. Abdul Khadar,Bull. Mater. Sci. 31(2008) 511.

[30] O. Madelung, M. Schultz, H. Weiss,Landoldt–Börnstein New

Series: Numerical Data and Functional Relationships in

Science and Technology. Springer,Verlag, Berlin (1982).

[31] S. Bobovich Ya, J. Appl. Spectrosc. 49(1988) 863.

[32] V.S. Vinogradov, G. Karczewski, I.V. Kucherenko, N.N.

Melnik,P. Fernandez, Physics of the Solid State50(2008) 164.

[33] T. Ishizaka, R. Nozaki, Y. Kurokawa, J.Phys. Chem.Solids

63(2002) 613.

[34] L.G.V. Uitert,L.F. Johnson, J. Chem. Phys.44(1966) 3514.

[35] E. Nakazawa,S. Shionoya, J. Chem. Phys.47(1967) 3211.

[36] R. Reisfeld, M. Gaft, T. Saridarov, G. Panczer, M.

Zelner,Mater. Lett. 45(2000) 154.

Phonon side band and colorimetric analysis of rare earths...

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