Electro-Optical Studies of KBr and KCl Crystals

Transcription

1 155 Chapter V Electro-Optical Studies of KBr and KCl Crystals V.1 Introduction The subject of crystal growth has advanced greatly in the last few decades due to the practical applications of the crystals. The most common methods of crystal growth are solution growth [1] and melt growth [2]. In practice, all materials can be grown in single crystal from the melt; provided they melt congruently, they do not decompose before melting and they do not undergo a phase transition between the melting point and the room temperature. Among the normal freezing methods, Bridgeman technique is one of the oldest methods for growing crystals. This technique produces nucleation on a single solid-liquid interface by carrying out the crystallization in a temperature gradient. Many of the technically important crystals are obtained by this method. The principle is that a melt of the correct composition of the substance is slowly cooled from above the equilibrium melting point to produce the desired crystal. Melt crystallization is often considered to be commercially attractive, since it offers the potential for low energy separation compared to distillation, because latent heats of fusion are generally much lower than latent heats of vaporization [3]. Due to their simple cubic structure, the alkali halides have played a very important role in the development of Solid State Physics. Europium doped alkali halide single crystal has been the subject of very intensive investigation due to its

2 156 applications in digital medical radiography, optical memories and environmental dosimetry etc [4]. The present chapter proposes the electro-optical studies of the undoped and Mn doped KBr and KCl crystals prepared by melt growth. The lattice of KBr/KCl is face centered cubic with lattice parameters a = b = c = 6.65Å for KBr (JCPDS CAS : ) and a = b = c = Å for KCl [5]. The basis consists of one K atom and one Br/Cl atom separated by one half the body diagonal of a unit cube. There are four units of KBr/KCl in each unit cube. The inter ionic separation of pure KBr crystal is 3.3 Å and that of pure KCl crystal is Å [5]. V.2 Results and Discussion V.2.1 Structural Properties Structures of the KBr and KCl crystals in the present study is analyzed by taking XRD using Cu Kα (λ =.15456nm) radiation and are compared with JCPDS data card. The XRD patterns of KBr and KCl crystals (precursor, undoped and Mn doped) with d and (hkl) values are shown in Fig. 5.1 and Fig.5.2 respectively. The XRD pattern reveals a crystalline nature for the crystals. The orientation of higher intensity peak is found to be along (2) and (22) planes at 2θ =27.73 o and 38.6 o for KBr crystal and (2) and (22) planes at 2θ = o and 4.82 o for KCl crystal. The presence of other orientations such as (111), (311), (222), (4), (42) and (422) is also detected at 2θ = o, o, o, o, o and o respectively for KBr. Similarly orientations such as (222), (4), (42) and (422) is also detected at 2θ = 5.45 o, 59 o, 66.81oand o respectively for KCl crystals with lower intensities.

3 157 4 wt % 3 wt % 2 wt % 1 wt % L intensity (A.U) d = 3.81 (2) d = (22) d = 2.33 d = d = (111) (311) (222) (4) d = (42) d = (422) d = crystal undoped precursor θ o Fig.5.1. XRD pattern of precursor, undoped and Mn doped KBr crystal. Extra peaks corresponding to the dopant or their compounds are not detected, for any crystals but the intensity of the prominent peaks in the host sample is decreased in the crystals on Mn doping. The intensities of the peaks are further decreased on increasing the concentration (in wt %) of the dopant. It is due to the decrease in the host atomic density in these planes. Increase in dopant concentration leads to the movement of Mn 2+ ions to the interstitial sites and also increases the state of amorphous nature and disorders.

4 158 4 wt % 3 wt % 2 wt % 1 wt % L intensity (A.U) (2) d = d = (22) d = (222) d = (4) d = 1.47 (42) (422) crystal undoped precursor Fig.5.2. XRD pattern of precursor, undoped and Mn doped KCl crystal. 2θ o V.2.2 Diffused Reflectance Spectroscopy Band gap (E g ) of melt grown KBr and KCl crystals is measured from Diffused Reflectance Spectroscopy as described in [6]. Diffused Reflectance Spectrogram for these crystals with percentage of reflectivity R versus wavelength λ is incorporated in Fig. 5. 3(a) and Fig. 5.4 (a). E g is found by extrapolating the straight line graph of {(k/s)hυ} 2 versus hυ (Fig. 5.3 (b) and Fig. 5.4(b)) at k =. Where k is the absorption coefficient and s is the scattering coefficient and E g is found to be 5.5 ev for KBr and 4.94 ev for KCl crystal. Wide band gap compounds are especially promising for light emitting devices in the short wavelength region of visible light.

5 (a) 2 % R [(k/s) hν] λ (nm) 5 (b) hν (ev) Fig.5.3. (a). Reflectivity R versus wavelength λ, (b). {(k/s)hυ} 2 versus hυ of KBr crystal (a) 2 [(k/s) hν] % R λ (nm) 5 (b) hν (ev) Fig.5.4. (a). Reflectivity R versus wavelength λ, (b). {(k/s)hυ} 2 versus hυ of KCl crystal.

6 16 V.2.3 Electro-Optical Studies V Photoconductivity Studies The basic principle of photoconductivity (PC) is the production of free charge carriers in a material by optical excitation. Saturated photocurrent is reached after some time as the PC cell is exposed to excitation source. The samples are annealed to various temperatures such as 5 o C, 1 o C, 15 o C, 2 o C, and 25 o C. A plot of the time dependence of Photocurrent (primary photocurrent) of KBr and KCl samples at 1 o C annealing temperature is shown in Fig.5.5 (Saturated value of photocurrent of the crystal is measured for each case separately). Saturated photocurrent is different at different annealing temperature and it increases as annealing temperature increases and reaches the maximum for the samples annealed at 1 o C (Fig.5.6). Annealing increases the crystallanity of the KBr and KCl crystals, which produces an increment in photocurrent [7]. 14 Photo Current (µa) KBr KCl Time (mins.) Fig.5.5. Variation of photocurrent versus time of KBr and KCl crystals annealed at 1 o C.

7 161 Saturated photocurrent decreases as the annealing temperature is increased from 1 o C to 25 o C. The observed decrease in photocurrent when the sample is annealed above 1 o C can be explained on the basis of defects in the material [8] as explained in section IV It is found that saturated value of photocurrent increases with increasing intensity of excitation (Fig.5.7) and also with increase in applied voltage (Fig. 5.8). More and more charge carriers reach at the respective electrodes and the photocurrent increases with the increase in intensity of light and applied voltage. The non-linearity in Fig.5.7 and Fig.5.8 for these crystals represents the dependence of saturated value of photocurrent (secondary photocurrent) on the intensity of excitation and applied voltage [8] Photo Current (µa) KBr KCl Temperature ( o C) Fig.5.6. Variation of saturated photocurrent of KBr and KCl crystals with annealing temperature.

8 Photo Current (µa) KBr KCl Intensity (mw/cm 2 ) Fig.5.7. Variation of saturated photocurrent of KBr and KCl crystals at different intensities of light Photo Current (µa) KBr KCl V (volts) Fig.5.8. Variation of saturated photo current of KBr and KCl crystals with applied voltage.

9 Photo Current (µa) KBr KCl Concentration (Arb. Units) Fig.5.9. Variation of saturated photocurrent of KBr and KCl crystals at different concentrations of the dopant. Fig. 5.9 shows the variation of the saturated value of photocurrent with concentration of the dopant Mn for KBr and KCl crystals. As the concentration increases, the charge carriers also increase and it is observed that photocurrent increases and reaches the maximum at 2 wt % concentration of Mn for both the samples. At this concentration, the charge concentration seems to be optimum for better PC [8]. PC is found to be decreased on further increase in the concentration of Mn. The increase in concentration of Mn causes increases the charge carrier concentration and photocurrent. But, when it goes beyond an optimum value the carrier collision probability increases which results in reduction of the photocurrent.

10 164 V Photovoltaic Studies Photovoltaic (PV) effect is involved, when absorption of radiation by a material causes the formation of a p.d. between the two portions of the material. Fig.5.1 shows the variation of photovoltage of KBr and KCl crystals annealed at 1 o C respectively with respect to time as the PV cell is exposed to light source. As annealing temperature increases, it is observed that the saturated photovoltage increases and becomes the maximum at 1 o C for the crystals (Fig.5.11). (Saturated value of photovoltage of crystals is measured for each case separately). When the annealing temperature is increased further, saturated photovoltage decreases which is due to the similar cause as given in section IV The maximum value of the photovoltage also increases with increasing intensity of excitation (Fig.5.12). With the increase in intensity, more and more charge carriers are separated at the respective electrodes and the photovoltage is increased [8] KBr KCl Photo Voltage (mv) Time(mins.) Fig.5.1. Variation of photovoltage of KBr and KCl crystals annealed at 5 o C with respect to time.

11 Photo Voltage (mv) KBr KCl Temperature ( o C) Fig Variation of saturated photovoltage of KBr and KCl crystals with temperature Photo Voltage (mv) KBr KCl Intensity (mw/cm 2 ) Fig Variation of saturated photovoltage of KBr and KCl crystals at different intensities of light.

12 Photo Voltage (mv) KBr KCl Concentration (wt %) Fig Variation of saturated photovoltage of KBr and KCl crystals at different concentrations of the dopant. Fig depicts the variation of the saturated photovoltage of KBr and KCl crystals with concentration of the dopant Mn and it is observed that PV is the maximum at 2 wt % concentration of Mn. At this concentration, the charge separation may be more in number compared to other cases for better PV [8]. Photo-electronic properties of the prepared crystals are very much influenced by the presence of defects in the original crystal lattice [8]. PV increases on increasing the concentration of Mn and reached the maximum at 2 wt % concentration and decreases on increasing the dopant concentration. Decrease of PV on increasing dopant concentration, may be attributed to increase in amorphous phase and concentration quenching, consequent to doping. V Electroluminescence Studies Electroluminescence (EL) is the phenomenon in which electrical energy is converted directly into electromagnetic energy in the visible

13 167 region of the spectrum. In this process heating does not play an essential part and an electroluminescent device is not designed to operate at an incandescent temperature. The observation of EL was first reported by Round [9], but the phenomenon was originally discovered by Lossew [1-12]. This effect in inorganic phosphors was first observed by Destriau [13-15]. A great deal of progress has been made recently in improving the performances of various classes of EL devices. The light emitted from the AC/DC EL cell, on application of suitable voltage, was detected by the PMT and the corresponding photocurrent is measured with the help of a digital nanoammeter. V AC Electroluminescence The mechanism of AC EL is as explained in section IV The integrated light intensity is accurately given by the expression [16-19], B= A exp (-C/V 1/2 ) (5.1) -.2 Log (B) (Arb. Units) /V 1/2 (volts -1 ) Fig Plot of Log (B) Vs. V -1/2 of KBr crystal. where A and C are constants independent of the voltage. This expresses that the mechanism of excitation is an acceleration- collision one. Log (B)

14 168 (Brightness) versus V -1/2 (applied AC voltage) graph of KBr and KCl electroluminors is given in Fig and Fig respectively. Linearity of these plots holds the above relation and proves the mechanism of excitation is acceleration- collision one. The EL phenomenon is very much influenced by presence of defects in the original lattice [8] Log (B) (Arb. Units) /V 1/2 (volts -1 ) Fig Plot of Log (B) Vs. V -1/2 of KCl crystal. AC EL Brightness as a function of applied a.c voltage for the two electroluminors at different concentrations (up to 4 wt %) of the dopant (Mn) is given in Fig and Fig.5.17 respectively. The EL intensity is almost same as that of the undoped sample below 1 wt % concentration of the dopant. Hence the corresponding curves are not included in the graph. EL intensity increases with increasing applied voltage. EL intensity increases as the concentration of the dopant increases and becomes the maximum at 1wt % for KBr and KCl crystals. EL intensity then decreases

15 169 EL Intensity (Arb.units) wt% 2wt% undoped 3wt% 4wt% AC Voltage Fig AC EL of KBr crystal at different concentrations of the dopant (Mn). 4 1wt% EL Intensity (Arb.units) wt% 3wt% undoped 4wt% AC Voltage Fig AC EL of KCl crystal at different concentrations of the dopant (Mn). on increasing the concentration above 1wt %. This decrease in intensity is due to concentration quenching. AC EL intensity of the electroluminors at different annealing temperatures is given in Fig and Fig.5.19 respectively. EL intensity increases with increasing annealing temperature from room temperature (crystallanity increases) and reaches the

16 17 EL Intensity (Arb.units) C 1 C 15 C 2 C 25 C Room Temperature AC Voltage Fig AC EL of KBr crystal at different annealing temperature. EL Intensity (Arb.units) C 1 C 15 C Room Temperature 2 C 25 C AC Voltage Fig AC EL of KCl crystal at different annealing temperature.

17 171 Fig.5.2. AC EL emissionphotograph of KBr crystal. Fig AC EL emissionphotograph of KCl crystal. maximum at 5 C for KBr and KCl crystals. EL intensity then decreases on increasing annealing temperature due to the increase in the amorphous phase and disorders. The AC EL emission of both the electroluminors is bluish. AC EL emission photographs of the electroluminors taken with a digital camera are given in the figures (Fig. 5.2 for KBr and Fig.5.21 for KCl crystals). V DC Electroluminescence DC EL powder panels have become reasonably successful as displays. An efficient DC powder EL device was first reported by A. Vecht [2]. Two essential features of any DC EL panel are that the phosphor particles are in contact with each other and with the electrodes. The mechanism of DC EL is as explained in section IV

18 172 4 EL Intensity (Arb.units) wt% 2wt% undoped 3wt% 4wt% DC Voltage Fig DC EL of KBr crystal at different concentrations of the dopant (Mn). 4 1wt% EL Intensity (Arb.units) wt% 3wt% undoped 4wt% DC Voltage Fig DC EL of KCl crystal at different concentrations of the dopant (Mn).

19 173 EL Intensity (Arb.units) C 1 C 15 C 2 C 25 C Room Temperature DC Voltage Fig DC EL of KBr crystal at different annealing temperature. DC EL Brightness as a function of applied DC voltage of KBr and KCl electroluminor at different concentrations of the dopant (Mn) is given in Fig and Fig.5.23 respectively. The EL intensity is almost same as that of the undoped sample below 1 wt % concentration of the dopant. Hence the corresponding curves are not included in the graph. EL intensity increases with increasing Mn concentration and reaches the maximum at 1 wt % then decreases on increasing concentration due to concentration quenching. DC EL intensities of KBr and KCl electroluminor at different annealing temperatures are given in Fig and Fig.5.25 respectively. EL intensity increases with increasing annealing temperature (crystallanity increases) and reaches the maximum at 5 C then decreases on increasing annealing temperature due to the increase in the amorphous phase and disorders. The DC EL emission of KBr and KCl electruluminor is also bluish.

20 C EL Intensity (Arb.units) C 15 C 2 C 25 C Room Temperature DC Voltage Fig DC EL of KCl crystal at different annealing temperature. DC EL emission photographs of the two electroluminors taken with a digital camera are given in the following figures (Fig for KBr and Fig for KCl crystals). Fig DC EL emission photograph of KBr crystal. Fig DC EL emission photograph of KCl crystal.

21 175 V Photoluminescence Studies Photoluminescence (PL) is the process in which absorption of UV/optical photons is followed by electronic transitions, associated with the emission of photons. PL spectra are recorded by using a Flourimeter. Excitation and emission spectra were taken by changing the excitation wavelength (λ ex ) under a fixed emission wavelength (λ em ) and vice versa. The highest resolution used were.1nm for excitation and.3nm for the emission. When excited with λ ex = 31 nm, the KBr crystal phosphors show a broad 18 PL Intensity (Arb.units) excitation emission undoped 1 wt% 2 wt% 3 wt% 4 wt% λ (nm) Fig PL emission at λ em = 357 nm with λ exc = 31nm spectra of KBr crystal at different concentrations of the dopant (Mn). emission band which is observed at λ em = 357 nm (Fig. 5.28). The broad peak observed around 357 nm corresponds to the 1 S 3 P 2 transition (4p 4 4p 4 ) of Br [21]. The normal luminescent bands are attributed to interaction between emission centers and the host crystal lattice [22]. The luminescent emission is usually originated due to presence of some defects in the host lattice, which produces certain impurity site or centers

22 176 during the preparation [8]. Shoulder at 334 nm is originated because of the presence of some defects in the host lattice. When excited with λ ex = 274 nm, the KCl crystal phosphors show a broad emission band which is observed at λ em = 34 nm (Fig. 5.29).. The broad peak observed around 34 nm corresponds to the 5 D 1 S transition (3s 2 3p 3 (4S o ) 4d 3s 2 3p 3 (2D o ) 3d) of Cl [21]. 18 PL Intensity (Arb.units) excitation emission undoped 1 wt% 2 wt% 3 wt% 4 wt% λ (nm) Fig PL emission at λ em = 34 nm with λ exc = 274 nm spectra of KCl crystal at different concentrations of the dopant (Mn). PL emission spectra of the photoluminors doped with Mn are given in Fig and Fig Normally the luminescent emission depends upon the nature of the activator and its concentration in the host lattice. The peak position is not affected because the energy levels of these additives (Mn 2+ ions) lie at the same level as that of the host materials KBr and KCl. The emission band corresponds to Mn 2+ ion found in [23] and [24] is

23 177 merged in the broad peak of KBr and KCl. The merged transitions of Mn 2+ ion are at 339nm, corresponding to the band assignment 6 A 1 (S) 4 T 1 (P), 363nm corresponding to the band assignment ( 6 A 1 (S) 4 E(D) and 376nm corresponding to the band assignment 6 A 1 (S) 4 T 2 (D). PL intensity of the peak decreases with increasing Mn concentration for both the crystals due to concentration quenching [25]. At higher concentration, the activator atoms destroy the matrix, which results in quenching of emission [26]. PL emission may be delayed due to the presence of traps [27]. 2 PL Intensity (Arb.units) C Room Temp 1 C 15 C 2 C λ (nm) Fig.5.3. PL emission spectra of KBr crystal at different annealing temperatures.

24 PL Intensity (Arb.units) C Room Temp 1 C 15 C 2 C λ (nm) Fig PL emission spectra of KCl crystal at different annealing temperatures. PL emission spectra of both the photoluminors at different annealing temperatures are given in Fig. 5.3 and Fig PL intensity increases with increasing annealing temperature and reaches the maximum at 5 C then decreases on increasing annealing temperature. Shoulder at 334 nm of KBr crystal produced from the defects is disappeared due to increase in crystallanity of the material on annealing. Increase in PL intensity with increase in annealing temperature up to 5 o C can also be explained on the basis of impurity effect in the material. Photosensitivity will be increased if imperfections capture more minority carriers than majority carriers. Imperfections acting as efficient recombination centers decrease the photosensitivity. The decrease in PL intensity on increasing the temperature beyond 5 o C is due to the increase in the amorphous nature and disorders. This can also happen due to the

25 179 recombination of electrons, which are thermally freed from traps with photo excited holes held at centers as in quenching effects reported [7]. V.3 Conclusion Conditions for the growth of crystals of KBr and KCl crystals by melt growth using a cost effective mini crystal growth setup have been optimized and their crystalline nature has been confirmed by carrying out X-ray diffraction. On Mn doping, no extra peaks corresponding to them or their compounds were detected but the intensity of the prominent peaks was decreased due to the decrease in the atomic density in these planes which leads to the movement of Mn 2+ ions to the interstitial sites and also increases the amorphous phase and disorders. Band gap (E g ) is found to be 5.5 ev for KBr and 4.94 ev for KCl crystals. PC effects of the crystals were studied and found that both the materials are more photosensitive at 1 o C. It is found that the maximum value of photocurrent increases with increasing intensity of excitation and also with increase in applied voltage. Mn doping makes the photosensitive crystals more photosensitive. As the concentration of Mn increases, it is observed that PC increases and reaches the maximum at 2 wt % concentration of Mn. KBr is found to be more photoconducting than KCl. PV effects of the crystals were studied and found that both the materials show greater photovoltage at an annealing temperature of 1 o C. Maximum value of the photovoltage also increases with increasing intensity of excitation. PV increases on increasing the concentration of Mn in the doped crystals and becomes the maximum at 2 wt% concentration and decreases on increasing the dopant concentration.

26 18 Electroluminescence Brightness increases with the applied electric field. The brightness of a powder EL cell increases non-linearly as excitation voltage is increased. The EL phenomenon is very much influenced by presence of defects in the crystal lattice. AC/DC EL Brightness as a function of applied AC/DC voltage of both crystals at different concentrations of the dopant (Mn) shows that EL intensity increases with increasing Mn concentration (in wt %) and reaches the maximum at 1 wt % then decreases on increasing concentration due to concentration quenching. AC/DC EL Brightness as a function of applied AC/DC voltage of both electroluminors at different annealing temperatures shows that EL intensity increases with increasing annealing temperature and reaches the maximum at an annealing temperature of 5 C then decreases on increasing annealing temperature. The AC/DC EL emissions of KBr and KCl electruluminors are bluish in colour. The AC/DC EL brightness intensity of plane KBr reveals that it is a better electroluminor than plane KCl crystal. But on doping and annealing the AC/DC EL brightness intensity of KCl is more than that of KBr crystal. When excited with λ ex = 31 nm, the KBr crystal phosphors show a broad emission band which is observed at λ em = 357 nm. The broad peak observed around 357 nm corresponds to the 1 S 3 P 2 transition of Br. Shoulder at 334 nm is originated because of the presence of some defects in the host lattice. When excited with λ ex = 274 nm, the KCl crystal phosphors show a broad emission band which is observed at λ em = 34 nm. The broad peak observed around 34 nm corresponds to the 5 D 1 S transition of Cl.

27 181 The peak positions of both crystals are not much affected on Mn doping because the energy levels of these additives (Mn 2+ ions) lie at the same level as that of the host materials KBr and KCl. The emission band corresponds to Mn 2+ ion is merged in the broad peak of KBr and KCl. The merged transitions of Mn 2+ ion are at 339nm, corresponding to the band assignment 6 A 1 (S) 4 T 1 (P), 363nm corresponding to the band assignment ( 6 A 1 (S) 4 E(D) and 376nm corresponding to the band assignment 6 A 1 (S) 4 T 2 (D). PL intensity of the peak decreases with increasing Mn concentration for both the crystals due to concentration quenching. At higher concentration, the activator atoms destroy the matrix, which results in quenching of emission. The emission peak intensity reveals that the prepared materials could be used as a scintillator phosphor. PL intensity increases as annealing temperature increased from room temperature and reaches the maximum at 5 C then decreases on increasing annealing temperature. Shoulder at 334 nm (originated out of defects) of KBr crystal is disappeared due to increase in crystallanity of the material on annealing. The PL intensity of KBr peak reveals that it is a better photoluminor than KCl.

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