CHAPTER 2

Dzflerent samples of calcium sulphide (W: CaS(Ce) , CaS(Sm) and CaS(Sm. Ce)

phosphors have been prepared The absorption, emission and excitation spectra

of the powder phosphors were obtained at room temperature. The absorption

spectra, luminescence bands obtained in excitation and emission spectra are

discussed in detail. The spechai characteristics of the impurities, cerium and

samarium trivalent ions incorporated in the host CaS lattice are also discussed A

brief analysis of enera trun~fer processes taking place between the impuriry

ions, and the host and impurig ions are incorporated The doubly doped CaS

phosphors show similar emission spectru to that of Sm doped samples for lower

wavelength excitations. The emission spectra for Ce doped and co-doped samples

excited ar 460nm showed the green emission of ce3' around 503nm and a

shoulder at j60nm. The (ransitions are attributed to ' ~ 5 1 2 and 'F,,,' state

respectively The phosphorescence decay of the samples has also been studied

systematicaliy The trap depth has been evaluated by analyzing the decay curves.

The observed decay could be explained satisfactorily by assuming a

superposition scheme.

Chapter 2 38

2.1. Introduction

Solid materials in powder form that give luminescence when suitably excited are

called phosphors. From 1950 onwards the extensive study of luminescence

characteristics of sulphide phosphors have been started. Effect of impurities on

the energy levels of sulphide phosphors has become an active field of

investigation. Alkaline earth sulphide phosphors activated with specific metallic

impurities and rare earth ions are of considerable practical importance. Many

investigators have reported luminescence studies of alkaline earth sulphides

activated with one or more rare earth ions and transition metal ions [l-61. Most of

the rare earth ions exhibit good fluorescence when they are incorporated in solid

matrix [7]. The optical properties of these ions depend on the matrix

environment. sm3+ ions give very strong fluorescence in orange red region in a

variety of crystals, glasses and phosphors [8-101. ce3+ ions act as the best co-

activator because of its characteristic UV and visible emission [ I 11. The activated

phosphors are widely used in cathode ray tube, IR sensors and

thermoluminescence (TL) materials in dosimetry. Doubly doped phosphors act as

IR stimulable materials [I 1-13]. Photostimulated luminescence (PSL) in Eu, Ce

and Sm co-doped sulphides (CaS) has been studied by Kravets 1141 in order to

develop a novel erasable and rewritable optical memory using the

photoluminescence method

The dopant may act as trapping/recombination/luminescent centers in the host

material. Optically stimulated luminescent studies of doubly doped phosphors

were reported in which energy transfer takes place between sm3'and ce3+. Also

reported that thermoluminescence and photoluminescence emission are

characteristics of sm3+ ions irrespective of second dopant [12]. The unexcited

cerium and samarium ions occur in the phosphor as ce3+ and sm3+, which is the

usual valency state of the rare earths. The fundamental state of ce3+ is a doublet 2 FSR and 2 ~ 7 ~ separated by 0.2eV and the excited state is most probably ' ~ 3 1 2 and

2 ~ 5 , 2 , split by the electrical field in the crystal. Gupta [I51 studied the

phosphorescence mechanism in CaS:Er and CaS:Cu phosphors and observed that

Photoluminescence Studies of CaS:(Sm) and CaS:(Sm,Ce) Phosphors 39

inactivated phosphor did not show any luminescence while the luminescence is

observed when the base is activated with either Er or Cu or other activators. The

phosphorescence mechanism has been studied with reference to the

phosphorescence decay characteristics and the spectral distribution of

luminescence emission by many workers on CaS phosphors and other alkaline

earth sulphides (AES) [3,6,15-191

Studies of phosphorescence decay are to yield valuable information regarding the

distribution of trap levels. The results obtained from thermo luminescence studies

combined with phosphorescence decay measurements give insight into the type

of kinetics involved in phosphorescence. The theory suggested by Randall and

Wilkiris [20,21] has been verified for a variety of phosphors. Alkaline earth

sulphide phosphors have gained significance since the discovery of their utility

for the sensitized luminescence and infrared stimulation. Room Temperature

Phosphorescence (RTP) is now a well-established detection technique widely

used in many laboratories for organic trance analysis. The exceptional analytical

features of the RTP phenomenon, the attainable sensitivity, selectivity and the

low cost of the necessary instrumentation point to phosphorimetry as a highly

attractive and promising detection mode for optical sensors development. In fact,

scientific research during the last few years on the use of RTP as a powerful

detection technique to develop new optical sensors and sensing systems for a

wide variety of analyses and applications has been very active [22]. Jin et al

reported the development of a room temperature phosphorescence (RTP)

optosensor for pH monitoring in aqueous media based on the effect of the pH on

the energy transfer from a phosphor molecule (acting as a donor) to an adequate

mixture of pH-sensitive dyes (acceptors) [23]. Qiu et al observed long-lasting

phosphorescence in oxygen-deficient Ge-doped silica glasses at room

temperature due to irradiation of focused 120 fs laser pulses at 800 nrn induced

long-lasting phosphorescence. Based on the time dependence of the intensity of

the phosphorescence, the long lasting phosphorescence in these glasses is

considered to be due to the thermally activated electron-hole recombination at

shallow traps [24].

Chapter 2 40

The aim of the present study is (i) to elucidate the luminescence mechanisms in

alkaline earth sulphides activated with ce3+, sm3+ and (sm3': ce3+) (ii) to study

various energy transfer mechanisms taking place in the doped phosphors (iii) to

investigate the pronounced green emission observed for cerium doped samples

(iv) to study the excitation dependence of emission spectra of the rare earth doped

calcium sulphide phosphors by observing the excitation and emission spectra and

(v) to understand the type of kinetics involved in phosphorescence decay and to

obtain information about the distribution of trapping levels in the phosphors.

2.2. Theoretical considerations

In many crystalline phosphors the luminescent emission originates in impurity

systems called activators .In sulphide phosphors, however these properties seem

to be associated more with lattice itself than activators. The impurities can be

introduced in two ways (i) They may be impurity atoms occurring in relatively

small concentration in the host material (ii) They may be stochiometric excess of

one of the constituents of the host material, which is called self-activation The

incorporation of an activator in crystalline solid gives rise to certain localized

energy levels in the forbidden band. Depending upon the energy levels involved

we can distinguish characteristic and non-characteristic luminescence. For

characteristic luminescence the energy levels involved are those of the activator

atoms or modified perhaps by the host lattice. Here an activator atom absorbs the

incident quantum of energy by the transition of one of these electrons from one

quantum state to another. When the excited atom returns to the ground state, it

loses a part of energy due to lattice interaction and hence emits a photon of less

energy. In non-characteristic luminescence a charge transfer through the lattice is

taking place. This also involves the energy levels of the host lattice modified due

to activator atoms 1251.

The co-activator is an additional impurity, which is necessary for luminescence in

sulphide phosphors. But it does not have the pronounced effect on emission

spectrum that the activator has. Usually co-activators are identified as donors and

the activators as acceptors. Adding suitable flux materials such as sodium

Photoluminescence Studies of CuS:(Sm) and CuS:(Sm,Ce) Pho~phors 4 1

thiosulphate compensates the lack of positive charges created due to the addition

of monovalent or trivalent impurity ions. The addition of flux only serves to alter

relative importance of different groups of traps and not their mean depth or

additional trapping levels. The flux facilitates the solution and distribution of the

activators in the host crystal on firing. It probably acts to provide a charge

compensating coactivator, although the atoms of the flux do not always go into

the lattice. If they do the flux may also fmi sh trapping centers [26].

Luminescence caused by intentionally incorporated impurities is classified as

extrinsic luminescence [25]. In ionic crystals and semiconductors they are of two

types: unlocalized and localized. In the unlocalized type the electrons and holes

of the host lattice participate in the luminescence process while in the case of

localized type the excitation and emission are confined in a localized

luminescence center. Localized-type centers are classified into (a) allowed

transition type and (b) forbidden transition type in regard to electric dipole

transitions. The f e d transition in ce3' and Eu2+ are examples for allowed

transition type (oscillator strength z 10-~-10"). The ce3+ ion with the

4f1 configuration shows efficient luminescence owing to 4E t t 5 d transition, and

the luminescence colours or wavelengths change widely from near ultraviolet to

red regions depending on the nature of the host lattice. [11,25]

The f t, f transitions in ~r'.'. ~ d ~ ' , sm3', Eu3+ and other trivalent rare earth ions

are examples for forbidden type transitions (oscillator strength ; lo4 - 10.') 2 6 Trivalent rare earth ions of ce3'-yb3' have the electron configuration 4f"5s 5p

(n = 1 to13) .The 4f shell is located inside the 5s5p shell so the influence of the

crystal field on the 4f energy levels is weak. As a result the 4f levels in solids are

not very different from those of fiee ions and further do not change much when

the host lattice is changed. So the absorption and emission spectra of trivalent

earth ions due to 4 f e 4f transitions composed of a number of sharp lines. The

energy levels of ~ 2 ' and sm3' [2] rare earth ions incorporated with calcium

sulphide phosphors prepared for our studies are shown in Figure 2.1.

" I;) i 0 (3)

Chapter 2 42

Figure 2.1.a

20

Energy level diagram of sm3' Figure 2.1.b Energy level diagram ion below 20 x10 3crn" of ce3+ in CaS (number of stoke's components in brackets)

(3$Gli?

I ................................................................................................

Phosphorescence decay

The hyperbolic decay can be represented by the equation of the type

I = ......... 2. 1 where I is the intensity at time t, 10 is the intensity at the beginning of decay and b

is the decay constant Hyperbolic decay can be explained by the monomolecular

superposition theory. According to this theory the hyperbolic decay is the result

of superposition of exponentials corresponding different traps. The trap depths

corresponding to different exponentials were calculated from the slopes of the

straight lines on the semi log plot using Randall and Wilkin's equation for the

phosphorescence decay

I = 10 e-P' ......... 2. 2

where p = ~ e - ~ ~ k ~ ......... 2. 3

is the probability of an electron escaping from a trap, E is the trap depth, k is the

Boltzmann constant and S is the escape frequency factor, which is usually

Photoluminescence Studies of CaS:(Sm) and CaS:(Sm, Ce) Phosphors 43

obtained from thennoluminescence studies. The trap depth E is given by the

formula

E = kT [ log, S- log, (log 1011-log t )] ......... 2. 4

or E=kTlnS/a )

where a is the slope of each linear portion of semi log plot.

2.3. Experimental Procedure

The phosphors were prepared by firing the mixture of pure CaS04 as host lattice

and sodium thiosulphate as flux. Analytical grade carbon powder is used as

reducing agent. Initially the weighed quantities of starting materials and dopant in

proper proportion are mixed well with distilled water.

Table 2.1.Concentrations (wt%) of samarium and cerium in the prepared samples

of calcium sulphide phosphors

Sample code Concentration (wt %)

Samarium ix) 1 t Cerium (v)

CaS --

CaSSm,,

CaSSrnXl

CaSSrn,,

CasSm,~

CaS Ce,,

CaSCe,>

CaSCe,,

CaSCeV4

CaSSm,Ce,,

CaSSm,Ce,,

CaSSrn,Ce,3 - CaSSm,Cey4

It was dried at 80UC and powdered with mortar and pestle. The charge was fired

in the central zone of a high temperature furnace at 1000°C for one hour. After

firing it was suddenly quenched to the room temperature. The phosphors thus

0

0.00 1

0.002

0.005

0.009

0

0

0

0

0.00 1

0.00 1

0.00 1

0.00 1

0

0 -

0

0

0

0.0009 -

0.0018

0.003

0.004

0.0002 J

0.0005

0.001

0.002

Chapter 2 44

obtained were finely powdered and used for the studies. A number of CaS

phosphors doped with cerium and samarium ions were prepared as shown in

Table 2.1

The crystalline nature of the prepared samples was confirmed by XRD.

Figure 2.2 is a representative XRD of CaS (Smx,Cey2). The absorption spectra of

Figure 2.2 XRD of sample CaSSm,Ceyz

all the powdered samples were recorded with a UVPC Shimadzu Photometer at

room temperature. The excitation spectra of all the samples were monitored for

three different wavelengths 480nm, 503nm and 569nm. The emission spectra

were also observed for four different wavelengths 388nm, 259nn1, 306nm and

460nm. All the spectral distributions were recorded with the same instrument

(RFPC5301 Shimadzu spectrophotofluorimeter) having specific experimental

configuration at room temperature.

To study the phosphorescence decay, the samples were excited to a saturation

using 259nm line of Xenon lamp. The phosphorescence emission was monitored

for a wavelength 569 nm that of samarium. The decay intensity was measured by

using RFPC 5301 Shimadzu spectrofluorimetr in Time course mode, with certain

modification of the set up of the apparatus, at room temperature.

Photoluminescencr Studies of CuS:(Sm) and CuS:(Sm, Ce) Phosphors 45

2.4 Results and Discussion

2.4.1.Absorption spectra

Figure 2.3 shows the absorption spectra of pure CaS, representative samples of

Ce doped, Sm doped and Ce, Sm co-doped phosphors. Since the concentration of

om

a. CaS b. CaSSm,, c. CaSCe,,

0 . 4 ~ ~ d. CaSSm,Ce,, - ?

0 C

0 . . _ , . --.. . . \ -._

d ---.,>- / /- - - - - - - -4

0.2 . - - - - . - . - .. . - - - - - - - - - - - - .. ,.---------- C

a

Figure 2.3 Absorption spectra of representative samples

rare earth ions cerium and samarium are too small the respective absorption

bands are weak to identify. However a broad band around 300nm has been

observed which can be attributed to the characteristic absorption of the host

calcium sulphide

2.4.2. Excitation spectra:

Figures 2.4, 2.5 and 2.6 show the excitation spectra of representative samples,

which were obtained for emission wavelengths 480nm, 503nm and 569nm

respectively. h,, = 480nm corresponds to that of calcium sulphide host,

A,,= 569nm corresponds to sm3+ and h,, =503nm that of ce3'.

Figure 2.4 Excitation spectra of different samples (L, = 480 nm)

46 Chapter 2

Wavelength (nm)

Figure 2.5 Excitation spectra of different samples (L, =503 nm)

600

- 2 400 - 0 .- rn

s *

a - CasCe,~ b - CaS

2. :: c - CaSSm,, : :;*; 1 i 0 , . d - caSSm,Ce,~ ,. ,11' :

d: .

c -

350 400 450 250 300 Wavelength (~11)

Photoluminescence Studies ofCuS:(Sm) and CuS:(Sm,Ce) Phosphors 47

Wavelength (nm)

Figure 2.6. Excitation spectra of different samples (Lm=569 nm)

The excitation spectra of each of the samples observed at (L, = 480nm) were

found that all samples showed three broad bands around 259nm, 306nm and

388nm. These are the fundamental absorption of host lattice. The cerium doped

samples showed a fourth peak with high intensity at 460nm. When the excitation

spectrum was monitored for 503nm that of Ce3' in the present sample, cerium

doped sample showed two broad peaks at 259nrn and 460nm When observed for

kc,= 569nm. samarium doped samples showed excitation band at 259nm,

broadened with a shoulder around 306nm.The third peak observed at 388nm for

other excitation spectra disappears. The intensity of 259nm band increases with

samarium concentration (Figure 2.7).

The Sm, Ce co-doped samples showed double peaks one at 259nm similar to that

of samarium doped and the other at 460nm similar to that of cerium doped

samples. The characteristic cerium excitation band at 460nm consists of certain

fine structure lines along the band. The broad band at 259nm and shoulder at

306nm can be assigned to the excitation process of electronic 4f5 + 4P5d

Chapter 2 48

transition in samarium ion because the intensity of the band increases with

concentration of samarium ion in the phosphor [2]. The broadband and shoulder

can be assigned to 4r4(t2g) and 'r4(t2,) band originating from 4fs+ 4P5d

transition. The broad peak at 460nm observed for all cerium doped samples

irrespective of emission wavelength is due to the crystal field splitting of 5d ('D)

state [I 11.

In Ce doped samples it may be assumed that ce3+ ion occupies a distorted cube

so that the 5d levels split into a lower doublet and a higher triplet by the cubic

component of crystal field. This may be the reason for the fine structure

components observed along the broad band around 460nm. The excitation spectra

Figure 2.7. Excitation spectra of CaS(Sm) samples forhe,=569nm

rmrrm-

m.mo

of all samples containing cerium ion showed the fine structure lines at 448nm,

451nm, 456nm, 461nm and 465nm. These values along with the emission lines

observed at 503nm with a shoulder at 560 nm that helps to propose an energy

level diagram for ce3+ ions in alkaline earth sulphide phosphors.

a. CaSSm,, b. CaSSm,, c. CaSSm,,

- . . ; .. d. CaSSm,&

,, ' . d

m -

-.-. -- - - - - - + ---+

0.m 230 0 330.0 400.0 5m.O

Wavelength (nm)

Photoluminescence Studies of CuS:fSm) and CaS:(Sm, Ce) Phosphors 49

2.4.3.Emission spectra:

The samples were excited for the four prominent wavelengths (388,259,306 and

460mn) and the emission spectra were recorded. The Figures 2.8, 2.9, 2.10, 2.1 1

and 2.12 represent emission spectra of the representative samples.

CaS and CaS(Ce) phosphors:

The emission spectrum of pure calcium sulphide host (CaS) sample showed a

broad band around 483nm, except for 460mn excitation. For cerium doped

samples the broadband emission shifts towards longer wavelength side to a peak

wavelength 503nm with a shoulder around 560nm in the order of excitation

wavelength 388nm, 306nm. 259nm and 460nm Also found that the cerium

emission intensity increases with concentration, gives maximum for 0.001 wt%

of Ce and then decreases. The theory of luminescence emission assumes that the

activator is an important part of luminescence center. The emission can be

attributed to the interaction between the host crystal lattice and emission center.

Inside the host crystal the activator is surrounded by other ions and hence it is

Figure 2.8. Emission spectra of pure CaS and CaSCey3 samples excited at 259nm

Chapter 2 50

subjected to the electrical field of these ions, which perturb the ground and the

excited state of activator and bring them nearer. It reduces the energies of

transition.

a - CaS b - CaSSm,, c - CaSSm,Ce,,

Wavelength (nm) Figure 2.9 Emission spectra of different samples h,=388 nm

The emission band around 483nm observed in pure and doped samples indicate

that the emission is due to the native defect in host crystal such as cation or anion

vacancies. In the samples studied the self-emission wavelengths can be attributed

to sulphur vacancies that may be created during the process of the incorporation

of the activator and depends on the base material used.

CaS (Sm) phosphors:

The photoluminescence spectra of CaS(Sm,) samples excited for 259nm and

306nm consists of three groups of emission bands located around 569nm, 605nm

and 648nm with fine structure components at 559,565,569,576,600,605,642,648

and 655nm in addition to the host emission broad band at 483nm (Figure 2.10

and 2.1 1). The fine structure spectra of samarium in CaS host can be explained as

follows. The 4P electrons of a sm3' ion substituting a cation in a crystal are free

from the electron phonon interaction, as they are electrically shielded by the 2 6 5s 5p electron clouds. Each 4P energy levels, which is mainly determined by the

Photoluminescence Studies of CaS:(Sm) and CaS:(Sm,Ce) Pho~phors 51

electrostatic and spin orbit interactions, slightly splits to Stark components caused

by crystal field around the sm3+ ion.

The transitions within the 4f configuration, therefore, are observed as line

spectra with fine structure. The photoluminescence spectrum of a sm3' ion in a

crystal is expected to be composed of 12 groups of emission lines, which are due

to the electronic transitions 4 ~ 5 1 2 . 6~~ (J=5/2,7/2,9/2,11/2,13/2,15/2) and

6 ~ J (J=1/2,3/2,5/2,7/2,9/2,11/2). The three groups of lines observed in our study

500

Wavelength (nm)

Figure 2.10. Emission spectra of different samples h,,=259 nm

can be identified with the transition from 4 ~ 5 n -t 6~~ (J=5/2,7/2,9/2) states in

sm3' ion respectively 121.

The fine structure component observed associates with each group are the spin

multiplets 'HJ in alkaline earth sulphides. The intensity variation of sm3'

emission lines with variation of excitation wavelength at same temperature

suggests that ' G S , ~ state may not be the initial state of the lines belonging to the

group. Also the unexpected enhancement in sm3+ emission intensity of doped

Chapter 2 52

samples for excitation wavelengths at 259nm may be due to the transfer of energy

from the host to the sm3' ion [27].

This may be the reason for the decrease in intensity of host emission for the

aforesaid cases. In the samples studied as samarium concentration increases the

Wavelength (nm)

Figure 2.11. Emission spectra of CaS, CaSSmx3 and CaSSmXCey3

emission intensity of prominent lines of samarium increases for both excitation

wavelength, reaches maximum for CaSSme and then decreases. The normalized

intensity values are tabulated in Table 2.2. The variation of intensity of 569nrn

emission of sm3+ with concentration is shown in Figure 2.12 a.

Samples CaSm* and CaSSmx4 showed almost same intensity, indicating that

there is a concentration quenching due to non-radiative transitions with in the

ions. However the energy difference between the components and groups of sm3+

emission in CaS host remains the same as those values reported in other host

materials [2,28].

Photoluminescence Studies of CaS:(Sm) and CaS:(Sm,Ce) Phosphors 53

Table 2.2. Normalised Intensity values of sm3' emission of the samples for

excitation at 259nm and 306 nm

So the energy parameters for sm3+ ions in CaS phosphors have the same value to

that in other host materials. Dela RosaCruz et a1 [29] studied the evidence of non-

radiative energy transfer from the host to the active ions in monoclinic ~r02:sm"

prepared by the sol-gel process and annealed at 1000°C . Under excitation at

320nm the ~ rn~ ' -do~ed monoclinic zirconium oxide shows strong emission at the

green (569nm) and red (607, 613 and 618nm) bands, corresponding to the

samarium transitions, whereas the undoped sample only shows a broadband

emission centred at 495nm. The main mechanism that allows the samarium

emission under UV excitation appears to be non-radiative energy transfer from

the ZrOz host to the sm3' ions. In CaS Sm, phosphors also the similar behavior is

observed.

Sample code

J.

Excitation wavelength h,, =259nm

Peak intensity of emission bands

Excitation wavelength A,, =306m

Peak intensity of emission bands

Chapter 2 54

In the study of characteristic emission of sm3' in Zn phosphors fired in reductive

atmosphere at 1050°C, Tang et a1 found that Sm doping favors the formation of

Concentration (wt Yo)

Figure 2.12 a Variation of intensity of emission of 569nm line for L, = 259 nm and 306 nm with concentration of sm3'

Concentration (wt%)

Figure 2.12 b Variation of intensity of emission of 569nm line for &, = 259 nm and 306 nrn with concentration of ce3'

Photoluminescence Studies of CuS:(Sm) and CuS:(Sm, Ce) Phosphors 55

hexagonal phase in host lattice of ZnS:Sm. Also it was noted that the increase in

hexagonal phase content will boost the overall photoluminescence emission

intensity and so self-activated luminescence intensity increases with the increase

in the amount of Sm doping [30].

CaS(Sm,Ce) phosphors:

For 259nm and 306nrn excitation, the emission spectra showed only the

emissions of the host and samarium. It is because the excitation wavelengths are

in the absorption region of CaS and CaS (Sm) but far away from the absorption

of cerium ions in the present samples. In ce3+ codoped samples as cerium

concentration increases for constant samarium concentration the intensity of

samarium emission decreases as shown in Table 2.2. Variation of intensity of

569nm line of sm3+ with increasing concentration of cerium is shown in

Figure 2.12 b. For lower excitation wavelengths the ce3'emission will not occur.

For an excitation wavelength 460nm, which is in the absorption region of ce3+

and far away from that of CaS and CaS(Sm), ce3+ ions only may be excited and

hence the luminescence of ce3+ ion is observed. The fluorescence emission of

ce3' ion originates from a transition from one or more of the 5d levels to 2 ~ 5 / 2 or

2 ~ 7 1 2 ground state. The emission spectra (Figure 2.13) for he, = 460 nm of all ce3+

doped samples shows the peak at 503nm and a shoulder at 560nm, which are

separated by = 2000cm-I. As reported by Blase et a1 [ l 11 the doublet ground state

of ce3+ in alkaline earth sulphides is separated by = 2000cm-'. The f + d

transitions in cerium are allowed-transition type and such spectra are always

broad ball shaped bands [25] Hence the doublet emission observed in the present

Ce doped samples is due to that of ce3' ions. The position of the lowest 5d levels

of ce3+ ions lowers with increasing nephelauxetic effect and crystal field and

hence in sulphide lattice the emission lies in the visible region. The Ce emission

intensity increases with concentration reaching a maximum after which it showed

a slow quenching effect. Optimum cerium concentration was about 0.001wt%.

Figure 2.14 represents the variation of emission intensity and wavelength for

same sample CaSm,lC:e,3 for different excitation wavelengths. It leads to the idea

Chapter 2 56

of energy transfer between the host and the dopant 13 1) and between the dopants

for different excitation wavelengths.

a - CaSSm.ce,~ b - CaSSmXCe,2 c - CaSSmxCe,~

f - CaSSm,,

Wavelength (nm)

Figure 2.13. Emission spectra of different samples Lx=460 nm

a - he, = 388 b-A,,=306nm c - h., = 259 nm d-hC,=460nm

Wavelength (nm)

Figure 2.14. Emission spectra of samples CaSSmxCe9 excited for different wavelengths

Photoluminescence Studies ofCaS:(Sm) and CaS:(Sm, Ce) Phosphors 57

2.4.4.Phosphorescence decay characteristics

Analysis of decay studies are done based on Randall-Wilkins's theory. The

probability of escaping from the trap is given by p=~e""T. The above equation

2O.M 40.00 Time (sac)

Figure 2.15 Decay of phosphorescence intensity (I vs t ) for samples

CaSSm,l and CaSSmVCe,~

assumes that retrapping is negligible and radiative transition predominates after

the release of electrons. To study the mode of decay, the afterglow intensity as a

function of time was recorded for all the phosphors as described in the

experimental procedure. Since the phosphorescence decay was monitored for

569nm, which is the emission wavelength of samarium, only the samples

containing samarium showed the decay curves The intensity I verses time t plot

showed a hyperbolic rather than an exponential nature. Figure 2.15 shows the

decay characteristics of representative samples CaSSm,+ool and

CaSSrnxqoolCe\=o 001. Ln I verses r plots are shown in Figure 2.16.

They are not straight lines as expected, indicating that the decay is not

exponential. However Ln I versus Ln t plots are found to be almost linear which

points to the hyperbolic type decay. Typical Ln I vs Ln t plot of CaSSm,, and

CaSSm,Ce,l are shown in Figure 2.17. The value of decay constant b can be

Chapter 2 58

calculated from the slope of the Ln I - Ln t plot. The trap depth E was calculated

with reference to equation (2.4). The values of E and b are tabulated in Tables 2.3

and 2.4 respectively. The value of b obtained is of the order of 1, indicates that

the kinetics involved in the processes is first order. of straight lines and trap depth

are calculated. Using the peeling off method described in chapter 1, the Ln I

versus time curves are split into minimum number of straight lines and trap depth

are calculated. Since three trap depths are involved in the phosphorescence

process of these samples the decay rate depends on the population of electrons in

the traps. The relative population of trapping levels N, at t=O can be obtained by

the extrapolation of Ln I vs time curve using the relati~nNn(t),~ = I,(t) ,o .rn

E -2 w - C -

1 .m

0.000 0.00 20.00 40.00 60.00

Time (sac)

Figure 2.16 Decay of phosphorescence intensity (Ln I versus t ) for samples CaSSm,l and CaSSm,Cey~

where r,= l/p, , is the lifetime of electrons trapped in a trap of width En The ratio

Nn(t)-o/ Nn(t),+ will be the relative population at t = 0 with respect to that at a

later time t = Ssec. and can be calculated from peeling of component of semi log

plot. The corresponding ratios are summarized in Table.2.3. From the Table 2.4

for b and r it is clear that deeper traps are having higher lifetime. The hyperbolic

decay is supposed to result from the superposition of intensities, each of which is

Photoluminescence Studies of CaS:(Sm) and Cas:(Sm, Ce) p h ~ ~ ~ h ~ ~ ~ 59

L n ( t ) Figure 2.17. Ln I vs Ln t Plot of sample CaSSm,CeYl & CaSSm,,l

varying with time. when different trap depths are involved in contributing

phosphorescence [ 151.

The samples containing cerium along with samarium takes longer time to decay.

This shows that the presence of cerium in the codoped samples changes the

trapping levels. The phosphorescence in inorganic phosphors is due to absorption

Chapter 2 60

of electrons below conduction band. The thermal energy at room temperature is

sufficient to empty the shallow traps. The values of trap depth, also known as

activation energy, calculated from thermo luminescence glow c w e s (Chapter 5)

by UV excitation are in fairly good agreement with values obtained fiom

phosphorescence studies. The phosphorescence and thermoluminescence studies

reveal that there is only one group of traps having depth in the region of 0.5 to

0.7eV, which can be associated with host lattice defects. The conclusions made

by earlier investigators are that there is only one group of traps in CaS phosphors,

which is associated with host lattice defects, and the activators have only little

effect on the trapping level. However the present observation of phosphorescence

decay suggests that the co-activators has modified the distribution of trapping

levels.

Table 2.3 Trap depth values and population ratios of for each group of traps

corresponding to three exponentials of CaS:Sm, and CaS:Sm,Ce,

phosphors

CaSSmxCey2

CaSSmxCey3

CaSSm,Cey4

0.596

0.602

0.600

0.636

0.642

0.623

1.570

2.660

2.500

0.646

0.667

0.638

1.090

1.330

1.330

- 1.058

1.110

I .250

Photoluminescence Studies of CaS:(Sm) and CaS:(Sm, Ce) Phosphors 6 1

Table 2.4 Decay constant values (b) and life time (7,) for each group of traps

corresponding to three exponentials of CaS phosphor samples

2.5 Conclusions

The undoped and doped CaS phosphors show a broad emission around 483nm,

which is the characteristic emission of self activated CaS phosphors. Exciting the

phosphor at 259 nrn can enhance the sm3' characteristic emission intensity in

CaS host and this may be due to the sharing of energy of host lattice by sm3+

ions. The excitation of the cerium-doped samples at 460nm enhances the green

emission of ~ e " ions as reported in alkaline earth sulphide hosts. The critical

concentration of cerium and samarium for maximum green emission is 0.001 wt

% of the host. The excitation dependence of emission spectra of the rare earth

doped calcium sulphide phosphors leads to the idea of energy transfer between

the host and the dopant and between the dopants. The decay characteristic studies

of CaS phosphors containing trivalent samarium ions reveals the effect of

activators in the host lattice in the distribution of electron traps. It is also

confirmed that the presence of codopant in the phosphors modifies the

distribution of trapping levels.

Chapter 2 62

References

1. Xiao1in.S.. Guangyan H. Xinyong D, Xiao Dong, Guilan 2, Tang ~uoqing , Chen Wenju, J. Phys. Chem. solids 62 (2000) 807

2. YamashitaN and Asano S; J. Phys. Soc. Jpn. 56 (1987) 352

3. Gupta C S, Ind. J. Pure. Appl. Phys. 36(1998) 765 4. YamashitaN; Jpn. J. Appl. Phys.30 (1991) 33 5. Sharma D and Arnar Singh, Ind. J. Pure. Appl. Phys. 9 (1971) 810

6. Gupta C S; Ind. J. Pure. Appl. Phys. 38 (2000) 821 7. Kumar G A; J. Phys. Chem. Solids 62 (2001) 1327 8. Souza Filho A G, Mendes Filho J, Melo F.E.A, Custodio M C C, Lebullenger

R Hemandes A C; J. Phys. Chem. solids 61 (2000) 1535 9. Aruna V,. HussainN. S and. Buddhudu S; Mat. Res. Bull. 33(1998) 149

10. Curie D, "Lwninescence in crystals" (Butler and Tanner Ltd. London) (1963) 221 11. Blasse G and Bril A; J. Chem. Phys. 47(1967) 5139 12. ~ K M a d . l u r V 4 J F R h o d e s a n d R J M ; J . ~ l . P ~ . 6 4 ( 1 9 8 8 ) 1 3 6 3 13. Bapat M N, Sivaraman S; Ind. J. Pure Appl. Phys. 23 (1985) 535 14. Kravets V G,Opt. Mater. 16 (2001) 369 15. Gupta C S, Ind. J. Pure Appl. Phys., 37 (1999) 906 16. Lawangar R D, Narlikar A V Ind. J. Pure Appl. Phys. 7 (1969) 163 17. Lawangar R D, Narlikar A V Ind. J. Pure. Appl. Phys. 10 (1971) 6171 18. Ghosh P K and Jain K L Ind. J. Pure Appl. Phys. 12 (1974)188 19. Jain K L andRanade J D Ind. J. Pure. Appl. Phys. 11 (1973) 602 20. Randall J T and Wilkins MH F, Proc. Roy. Soc. A 184(1945) 366 21. Randall J T and Wilkins MH F, Proc. Roy. Soc. A 184(1945) 390 22. Costa Fernandez J M, SanzMedel A. Quimica Analitica 19 (2000) 189

23. Jin WJ CostaFernandez JM, SanzMedel A .Analytica Chimica Acta 431( 2001)l 24. Qiu JR Gaeta, AL, Hirao K Chem. Phys. Lett. 333 ( 2001) 236 25. Vij D R ,Luminescence ofsolids, Plenum press , New York (1998) 26. Marton L(ed) Methods of experimental Physics. Solid state of physics Vo1.6

Academic Press New York (1959) 27. Dexter D L, J. Chem. Phys., 21 (1953) 836 28. Reisfeld R Greenberg E and.Biron E ,J. Solid State Chemistry 9(1974)224 29. Dela RosaCruz E, DiazTorres LA, Salas P, Castano VM, Hernandez JM,

J.Physics D - Appl. Physics 34 (2001) 83

30. Tang T P, Yang M R, Chen K S, Ceramics International, 26 (2000) 153 31. Forster T, Ann. Phys., 2 (1948) 55