Indian 10urnal of Engineering & Materials Sciences Vol. 10, August 2003, pp. 329-334

Preparation and characterization of silver doped (ZnCd)S mixed mechanoluminophors

Sanjay Tiwari·, Shikha Tiwarib & B P Chandrac

' Department of Physics, Goverment Science College, Raipur 492 010, India bState Forensic Science Laboratory, Raipur 492 001 , India

CDepartment of Physics, R.S. Uni versity, Raipur 492001 , India

Received 29 August 2002; accepted 8 May 2003

Mechanoluminescence (ML) in sil ver doped (ZnCd)S phosphors is reported. Considering the basic processes involved in (ML), a theoretical approach is made to understand the mechanical characteri stics of ML. The expressions derived are able to expl ain the dependence of ML intensity on several parameters like temperature, time, area of newly created surfaces and surface charge density. Spectroscopic studies of ML are made. The wavelength corresponding to the peak of both the ML and photoluminescence (PL) spectra shi ft towards longer wavelength with increasing CdS contents. The mechanism of ML is di scussed and it is concluded that the impul sive deformation of these phosphors may be due to piezo-electrification of newly created surfaces . The similarity of ML spectra with e lectro luminescence (EL) and photoluminescence (PL) spectra suggests that although there is a difference in the process of excitation of electrons whereas the re laxation with the emi ssion of photons involves the same process.

Mechanoluminescence (ML) is emission that occurs as a result of mechanical stress excitation of solids. The ML links mechanical, spectroscopic, electrical, structural and other properties of solids. A large number of organic and inorganic crystals and amorphous sol ids exhibit the phenomenon of ML I

-7

•

ML can be class ified into three types on the bas is of deformation needed for producing ML: fracto­induced ML, pl astico-induced ML and elastico­induced ML. The ML occurring during fracture of solids may be called fracto-induced ML, the ML occurring during plastic deformation of solids may be plastico- induced ML and the ML occurring during elastic deformation of solids may be called elastico­induced ML. Such cl ass ification is essential, since the mechanisms of ML production during elastic and plastic deformation and fracture of solids are di fferent.

Generally, all the piezoelectric crystals and some non-pi ezoelectric crystals exhibit frac to-induced ML. The plastico-i nduced ML has been observed in the crystals of coloured alkali halides8

-lo as well as non­

coloured alkali halides 13·14

, II-VI compounds l-3

,

metals II, alkali ne earth ox ides 12 and in certain variety of rubbers l 5

. The elastico-induced ML is exhibited by the single crystals of X or y-irradiated alkali halides8

-lo

.

Experimental Procedure The preparation of phosphors was calTied out

fo llowing the conventional technique reported earlier l6

. The (ZnCd)S:Ag,CI phosphors (hexagonal) and (ZnCd)S:Ag, CI phosphors (cubic) were prepared by firing for one hour in nitrogen atmosphere at llOO°C and 900°C respectively. The acti vator concentration was varied from 0 to 10,000 ppm and concentration of CdS was varied from 0 to 40 mol%.

The schematic diagram of the device used fo r measuring the ML activi ty is shown in Fig.I. For measuring the ML acti vity, 5 mg phosphor was placed on a transparent Lucite plate below the guiding cylinder. An RCA 931 photomultiplier tube was used for monitoring the luminescence fro m below the Lucite plate. The phosphor was covered with a thin aluminum fo il and fixed with an adhesive tape. This arrangement was made to eliminate the error in the measurement of ML intensity due to scattering of crys tallite fragment during the impact of load onto the crystal. The ML was excited impulsively by droppi ng a hollow cylinder of 800 g (2 cm di a) from different heights through a guiding hollow cylinder. The output of the PMT was connected to Scientific HM 307 oscilloscope having P7 phosphorescent screen capable of sustaining a trace in dark for more than a mi nute. The ML versus time curve was determined by

330 INDIAN J. ENG. MATER. SCI. , AUGUST 2003

Fig. I- Schematic diagram of the experimental arrangement used for measuring the time dependence of ML in phosphors (I) stand, (2) pulley, (3) metallic wire, (4) load; (5) guiding cylinder, (6) aluminum foil , (7) sample, (8) transparent lucite plate, (9) wooden block, (I 0) photomultiplier tube, and (II) iron base mounted on a table

measuring the area below this curve. The error in the measurement of ML intensity was found to be ± 6%. No correction in ML intensity was made for spectral response of the photomultiplier tube.

For measuring the effect of temperature on the ML of phosphors, the phosphors were placed onto a Lucite plate, which was heated by two heating filaments. The ML measurements were carried out when the device had attained a steady state temperature.

The ML spectra were recorded with the help of series of optical filters. The electroluminescence (EL) and photoluminescence (PL) spectra were recorded using grating monochromater as described earlier7

.

Results The time dependence of the ML of (ZnCd)S:Ag, CI

(1000 ppm Ag, 20% CdS) phosphors for different impact velocities of a load of 800 g is shown in Fig. 2. The ML intensity increases with time, attains a maximum value and then decreases with time. Fig. 3 shows the total ML intensity h versus time curve. It

4,-------------------------,

0.8 1.0

Time,s

Fig. 2-The dependence of ML intensity of (ZnCd )S :Au,Cl phosphors for different impact velocities

2 'c ::l

.0

-"= £ C 'iii c

'" E ...J :::;: 0; (5 f-

8,-----------------------------,

I.

'" 20% CdS (H«) • 0% CdS (Hex) to. 20% CdS (Cub)

xl.

Impact velocity, cmls

x2

300

Fig. 3-Dependence of total intensi ty of (ZnCd)S:Au,CI phosphors on the impact velocity

initially increases with the impact velocity Vo and then attains a saturated value of the impact velocity. Fig. 4 shows that the plot of log h versus 1000/Va, is a straight line with negative slope, fo llowing the relation.

TIW ARI el al.: SILVER DOPED (ZnCd)S MIXED MECHANOLUMINOPHORS 331

o 20% CdS (Hex) • 0% CdS (Hex) l!. 20% CdS (Cub)

10

Fig. 4-Plot of logarithm of total ML intensity IT versus 1000/vo for (ZnCd)S:Au,C1

1.00

0.75

~ 1 z

0.50

c

~ ...j ::;:

0.2 5

450 100 110 600

Wavelenglh.nm

Fig. 5- ML spectra of (ZnCd)S:A u,CI phosphors for different CdS concentration. [( I) 0% CdS (Hex), (2) 10% CdS (Hex), (3) 20% CdS (Hex), (4) 30% CdS (Hex); (5) 40% CdS (Hex) and (6) 20% CdS (Cub)]

where 1\ and Vc are constants . Fig. 5 shows the ML spectra of cubic and

hexagonal (ZnCd)S:Ag, CI phosphors, respectively for 1000 ppm activator concentration and for differen t contents of CdS in the phosphors. It is seen from these figures that the peaks of the ML spectra shift towards longer wavelength values with increasing CdS contents. Fig. 6 shows the EL spectra of (ZnCd)S :Ag, CI phosphors. It is observed that the peaks of the EL spectra shift towards higher wavelength values with increasing CdS contents in the phosphors.

Figure 7 shows the PL spectra of si lver doped (ZnCd)S mixed phosphors. It seen that the peak of PL spectra shifts with increasi ng CdS percentage. Fig. 8 shows the effect of temperature on the total ML intensi ty h. It is observed that h decreases with temperature and finally di sappears beyond a particular

-0 0.71

.~ ~ o Z

~ 0.50 1l ~ CD

-' w 0.21

2/

41 0

/ 5 .'

500

Wavelength, nm

\ 6

510 600

Fig. 6- EL spectra of (ZnCd)S:Au ,CI phosphors for different CdS concentration. [( I) 0% CdS (Hex), (2) 10% CdS (Hex ), (3) 20% CdS (Hex), (4) 30% CdS (Hex) ; (5) 40% CdS (Hex) and (6) 20% CdS (Cub)]

0.71

0.21

Wavelength, nm

\

'· 6 '<"

Fig. 7- PL spectra of (ZnCd)S:A u,C1 phosphors for different CdS concentration. [(I) 0% CdS (Hex), (2) 10% CdS (Hex), (3) 20% CdS (Hex), (4) 30% CdS (Hex); (5) 40% CdS (Hex) and (6) 20% CdS (Cub)]

temperature Tc which is much less than the melting point of the phosphor. The decrease of the ML intensity with temperature shown in Fig. 9 follows the relation :

where f\ is constant, n the slope of log IT versus (1-TlTc) plot and its value lies between 0.90 and 1.10 for (ZnCd)S phosphors, Tc is the temperature at which ML disappears . The value of Tc for 20% of CdS content in (ZnCd)S :Ag,Cl is 130°C and 140°C in cubic and hexagonal phosphors, respectively and 136°C for hexagonal phosphors without CdS content.

Discussion The ML in silver doped (ZnCd)S phosphors may be

discussed with respect to three main features: (a)

332 INDIAN J. ENG. MATER. SCI., AUGUST 2003

9 0% CdS (Hex) • 20% CdS (Hel( t 20% CdS (Cub)

" " , " '-

'-"- '--............... ...... ' "- " ,

.......... ..... ", °3~0-----5LO-----7~0----~90----~11-0 ----~1~30~--~150

Temperature7C

Fig. 8- Effect of temperature on total ML intensity IT of (ZnCd)S :Au,CI phosphors

i 20\1, CdS (lie , > ;;, 0% CdS (Hex) , 20% CdS (Cuh)

°1LOO~------~------~4r---~---L--10~1----~

I-Trrc

Fig. 9-Plot of logari thm of IT versus ( 1-TITc) for hexagonal phosphors

mechanical characteristics of ML, (b) effect of temperature on the ML and (c) mechanism of ML. Mechanical characteristics of ML

It is known that II-VI compounds ex hibit ML during their elastic, plastic and fracture deformations 17.18 . Since the ML was excited impulsively, the fracture of phosphor crystallites was the major factor responsible for the ML emission and the ML intensity is found to be directly proportional to the total area of newly created surfaces in the crystals 19.20.

Consider the crystal having length L, breadth Wand thickness H. If the crystals are cleaved aiong the plane parallel to its breadth side, the rate of the creation of new surfaces is given by 2 Wv, where v is the velocity

of the separation of cleavage plane or, in other words , the velocity of crack propagation . If y is the charge density of the newly created surface of the crystal, the rate of generation of surface charge is given by:

... (l)

It is to be noted that the charged surfaces may be created due to the piezoelectrification, movement of charged di slocations and baro-diffusion of charged defects21

. When the charged surfaces are created, the surface charge density may get relaxed, firstly, by the charge caniers produced due to the breakdown of the crystal, and secondly, by electrons and ions produced due to the breakdown of intervening gases. If al and a 2 are the rate constants of the relaxation of charges on the newly created surfaces by the crystal and gaseous breakdown processes, respectively, then we may write the following rate equation:

dQ or -= gs - a 3 Q

dt ... (2)

where a 3 = (al + a2) and Q is the surface charge at any time t.

As Q = ° at t=O, the solution of Eq. (2) gives

.. . (3)

The ML intensity I will be directly proportional to the rate of discharge (-dQldt). If T] I is the luminescence efficiency related to the movement of carriers produced by the dielectric breakdown of crystais and T] 2 is the luminescence efficiency related to the movement of electrons and ions produced by the dielectric breakdown of intervening gases, then the ML intensity may be expressed as:

... (4)

From Eqs (3) and (4), we get

... (5)

Kinetics of ML For a 3t < I, Eq. (5) may be written as:

TIW ARI et aL.: SILVER DOPED (ZnCd)S MIXED MECHANOLUMINOPHORS 333

1= (ll! (XI + Tl 2 (X2) gst or I = 2(TlI (XI + Tl 2 (X2)Y W vt ... (6)

The above equation shows that when a crystal is cleaved, initially the ML intensity should rise linearly with time t.

After the completion of cleavage of the crystal at t=tm, v becomes zero and hence gs= 0, at t =tm thus, from Eq. (2), we have:

dQ = _ (X3 Q dt

.. . (7)

Taking Q =Qo at t = tni and integrating Eq. (7) , we get:

. . . (8)

As tm is very short, negligible discharge occurs within the time duration, zero to tm and therefore we may take Qo= 2WHy, i.e., the total charge of the newly created surfaces.

From Eqs (4) and (8), we get

I = [TIl (XI + Tl2(X2 ] Qo exp [-(X3 (t - tm)] ... (9)

Eq. (9) shows the exponential decay of the ML intensity after cleavage of the crystals .

Estimation of tl/l For the thickness H of the crystal, the time tm taken

for cleaving the crystal is given by:

tm = Hlv .. . (10)

Thus, the ML may provide a new tool for determining the velocity of crack propagation in crystals.

Estimation of 11/1 From Eqs (6) and (10), the value of the peak ML

intensity at t = tm is given by:

.. . (11)

where area of newly created surfaces A = WH The above equation shows that 1m should increase

linearly with the area of newly created surfaces A and the surface charge density y.

Estimation of Ir The total ML intensity h, i.e. , the integrated area

below the ML intensity versus time curve may be given by:

h= f Idt ... (12)

It is found experimentally from the ML intensity versus time curve that the integrated light emitted during the rising portion of the ML intensity with time is negligible as compared to the integrated light emitted during the decaying portion of the ML intensity with time. Thus from Eqs (9) and (12), we may express the total ML intensity as:

~

h = [TlI(X1 + Tl 2(X2] Qo f exp [-(X3(t- te)] dt

(13)

The above equation shows that the total ML intensity should be directly proportional to the area of newly created surfaces A. It is also evident that h should be higher for the crystals having higher values of the surface charge density y.

Spectroscopy of ML

For (X2 = 0, there will be no gaseous discharge ML and the ML emission will primarily be due to the bulk ML. In this case (X3 = (XI and Eqs (11) and (13) may be written as :

(14)

(15)

For the crystals, where (XI is comparable with (X2,

the ML emission should consist of both the solid state ML and gaseous discharge ML and in this case Eqs (11) and (13) should be obeyed.

Temperature dependence of ML Eqs (11) and (13) shows that the temperature

dependence of both 1m and h should be primarily due to the effect of temperature on TIl. Tl 2 and 12

.23

. Since all three decreases with increasing temperature of the crystals, 1m and h should decrease with increasing temperature of the crystals. Beyond a particular temperature, y may decrease to such a value where the breakdown of gases and solids may not be possible and thereby the ML may not appear beyond a particular temperature of solids.

Mechanism of mechanoIuminescence The crystals of ZnS and CdS have non­

centrosymmetric crystal structure; hence ML excitation in these crystals or phosphors may also be

334 INDIAN I. ENG. MATER. SCI., AUGUST 2003

+ + + + +~------------~

Fig. IO-Electric field produced during the movement of a crack in a piezoelectric crystal

due to the piezoelectrification. It is well-known that with few exceptions, generally all the piezoelectric crystals exhibit ML and crystals not exhibiting ML are non-piezoelectric7

.24

. This result indicates the piezoelectric origin of ML.

According to Langevin model25, when stress is

applied to piezoelectric crystal , one of its surfaces gets positively charged and the opposite surface negatively charged (Fig. 10). Owing to the movement of a crack in the crystal new surface are created. The newly created surfaces nearer to the positively charged surface of the crystal get negatively charged and those nearer to the negatively charged surface of the crystal get positively charged. Thus, an intense electric field may be produced between the newly created surfaces of the crystal.

The intense piezoelectic field near the tip of the mobile crack may produce electrons and holes due to the dielectric breakdown of solids and, in tum, the recombination of electrons with holes may give rise to luminescence, the powder phosphor of (ZnCd)S are crystallites of micron size where the fracture create charged surface which in tum may give rise to intense electric field. It can be concluded that ML excitation during the impulsive deformation of (ZnCd)S :Ag, CI mixed phosphors may be due to the piezoelectrification of the newiy created surface.

The similarity of ML spectra with EL and PL spectra suggests that although there is difference in the process of excitation, relaxation with photon emission involves the same optical transition centres as in other types of luminescence.

Conclusion In the present investigation, a theoretical approach

is made to understand the mechanical characteristics

of ML and it is concluded that the impulsive deformation of these phosphors may due to piezo­electrification of newly created surfaces. The similarity of ML spectra with EL and PL spectra suggests that although there is a difference in the process of excitation of electrons whereas the relaxation with the emission of photons involves the same process. The present study may be helpful in preparation of intense mechanoluminophors for several possible applications

Acknowledgement One of the authors (ST) is indebted to M.P. Council

of Science and Technology, Bhopal, for providing financial assistance.

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