95 CHAPTER 5 STUDIES ON GROWTH, CHARACTERIZATION AND IRRADIATION EFFECTS OF 4, 4’- DIMETHYLBENZOPHENONE SINGLE CRYSTALS 5.1 INTRODUCTION Currently, second-order nonlinear optical (NLO) properties of molecular materials are widely investigated for their potential applications in the newly emerging optoelectronic and optical signal processing. Optical nonlinearity of the crystals with O-H bond has been extensively studied (Xu and Xue 2008, Xue and Zhang 1999, Xue and Zhang 1995). The advantages offered by organic over inorganic systems include high electronic second order susceptibility (χ (2) ) through high molecular Hyperpolarizability (β), fast response time, facile modification through standard synthetic methods, and relative ease of device processing. The large nonlinearities of certain organic compounds appear to arise from extended π-conjugated systems, as well as the presence of asymmetrical charge transfer processes. Charge transfer originates from the electron donating and electron accepting properties of aromatic ring substituent. Other important requirements for efficient second harmonic generation (SHG) are: (1) a lack of center of symmetry for the molecular charge transfer; (2) a significant change in dipole moment upon excitation from the electronic ground state to some excited states; (3) small to moderate excitation energies of the corresponding excited states and (4) Hammett constants of the substituents (Oudar 1977, Katz et al 1987).
Transcript
95
CHAPTER 5
STUDIES ON GROWTH, CHARACTERIZATION AND
IRRADIATION EFFECTS OF 4, 4’-
DIMETHYLBENZOPHENONE SINGLE CRYSTALS
5.1 INTRODUCTION
Currently, second-order nonlinear optical (NLO) properties of
molecular materials are widely investigated for their potential applications in
the newly emerging optoelectronic and optical signal processing. Optical
nonlinearity of the crystals with O-H bond has been extensively studied (Xu
and Xue 2008, Xue and Zhang 1999, Xue and Zhang 1995). The advantages
offered by organic over inorganic systems include high electronic second
order susceptibility (χ(2)) through high molecular Hyperpolarizability (β), fast
response time, facile modification through standard synthetic methods, and
relative ease of device processing. The large nonlinearities of certain organic
compounds appear to arise from extended π-conjugated systems, as well as
the presence of asymmetrical charge transfer processes. Charge transfer
originates from the electron donating and electron accepting properties of
aromatic ring substituent. Other important requirements for efficient second
harmonic generation (SHG) are: (1) a lack of center of symmetry for the
molecular charge transfer; (2) a significant change in dipole moment upon
excitation from the electronic ground state to some excited states; (3) small to
moderate excitation energies of the corresponding excited states and (4)
Hammett constants of the substituents (Oudar 1977, Katz et al 1987).
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4, 4’-Dimethylbenzophenone (DMBP) is a novel organic nonlinear optical crystal, which crystallizes in the orthorhombic structure with space group P212121 (Kojic-Prodic et al 1990). In this chapter, the growth of DMBP and characterization studies are discussed. The effect of irradiation of 50 MeV Li3+ on the dielectric and optical behavior and changes in the DMBP crystal due to the irradiation were studied.
5.2 SOLUBILITY OF 4, 4’-DIMETHYLBENZOPHENONE AND CRYSTAL GROWTH OF DMBP
DMBP is purified by recrystallization, several times in xylene. For crystal growth of organic materials, purification of starting material has been found to be an important step. DMBP is soluble in acetone and xylene. Selection of suitable solvents is very definitive for the growth of good quality single crystals (Sherwood 1998). Xylene is a good solvent for growth of DMBP. The solubility of DMBP in xylene was assessed as a function of temperature in the range 20-35oC. The saturated solution was allowed to reach the equilibrium in about one day at a chosen temperature and then the solubility was gravimetrically analyzed. The same process was repeated for different temperatures and the solubility curve was obtained. The DMBP exhibits good solubility and a positive solubility-temperature gradient in xylene. The knowledge of metastable zone width (MZW) is very important in terms of designing crystallization processes and obtaining desired crystal sizes, shapes, and purities. The different experimental solutions were prepared at desired saturation temperatures. Then the solution was heated 4 oC above the saturation temperature and kept there for 30 min. The solution was cooled at 4 oC/hour until nucleation occurred. The difference between saturation and nucleation temperature was taken as MZW at arbitrary conditions. Figure 5.1 shows the solubility curve and supersaturation curve of DMBP. The growth of DMBP crystals can easily lead to spurious nucleation, the perfection of seeds is very important for growing good crystals. The seeds must be without any macroscopic defects such as flaws and inclusions because it may cause
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spurious crystallization in solution. The DMBP crystals were grown from saturated solution at 35 oC by slow evaporation technique. The crystal of size 41128 mm3 was harvested after 20 days. The photograph of the as-grown crystal of DMBP is shown in Figure 5.2.
2 .5
3 .0
3 .5
4 .0
4 .5
1 0 1 5 20 25 30 35
T em p eratu re (oC )
Con
cent
ratio
n(g/
10m
l)
So lub ility cu rv e Su p ersatu ratio n cu rv e
Figure 5.1 Solubility curve and supersaturation curve of DMBP
Figure 5.2 Grown single crystal of DMBP
5.3 SINGLE CRYSTAL X-RAY DIFFRACTION STUDIES
The grown crystals were subjected to X-ray diffraction studies using Bruker AXS (kappa Apex II) single crystal X-ray difffractometer, using
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MoKα (λ=0.7107 Å). Cell parameters were obtained from least-squares
refinement of the setting angles of 25 reflections. The lattice parameters of DMBP are a = 7.954(3) Å, b = 12.167(4) Å, c = 12.265(3) Å, V = 1187.76 Å 3
in close agreement with reported values (Kojic-Prodic et al 1990). The crystal system is orthorhombic system and space group P212121. ORTEP diagram of DMBP is shown in Figure 5.3. Figure 5.4 shows the packing diagram of DMBP. The disorder of the methyl groups is observed in the Figure 5.3.
Figure 5.3 Structure of DMBP showing 50% probability ellipsoids and
the labeling scheme using ORTEP-3
Figure 5.4 Packing diagram of DMBP
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5.4 HIGH RESOLUTION X-RAY DIFFRACTION (HRXRD)
ANALYSIS
To reveal the crystalline perfection of the grown crystals, a multicrystal X-ray diffractometer has been used to record high-resolution diffraction curves (DCs) (Lal and Bhagavannarayana 1989). The specimen can be rotated about a vertical axis, which is perpendicular to the plane of diffraction, with minimum angular interval of 0.2 arc sec. The diffracted intensity is measured by using a scintillation counter which is mounted on the radial arm of the turn table. The diffraction curves were recorded by changing the glancing angle (angle between the incident X-ray beam and the surface of
the specimen) around the Bragg diffraction peak position B starting from a
suitable arbitrary glancing angle (denoted as zero). The detector was kept at
the same angular position 2B with wide opening for its slit, the so-called
scan. Before recording the diffraction curve, the specimen surface was prepared by lapping and polishing and then the surface was chemically etched by a non-preferential chemical etchant (water and acetone in 1:2 ratio). This process also ensures to get rid of the non-crystallized solute atoms from the surface (Bhagavannarayana et al 2005).
Figure 5.5 shows the high-resolution diffraction curve (DC) recorded for a typical DMBP single crystal grown by slow evaporation solution growth technique (SEST) using (011) diffracting planes in symmetrical Bragg geometry by employing the multicrystal X-ray
diffractometer with MoK1 radiation. As in the Figure 5.5, the DC contains a
single peak and shows that this specimen is free from structural grain boundaries. However, the full width at half maximum (FWHM) of this curve which is 52 arc sec is much more than that expected from the plane wave theory of dynamical X-ray diffraction (Betterman and Cole 1964). It is interesting to see the asymmetry of the DC with respect to the peak position
(denoted by the dotted line). For a particular angular deviation () of glancing
angle with respect to the peak position, the scattered intensity is much more in
100
-200 -100 0 100 2000
100
200
300
400
500
600
700
52"
4-4 dimethyl(011) PlanesMoK1(+,-,-,+)
Diff
ract
ed X
-ray
inte
nsity
[c/s
]
Glancing angle [arc s]
the positive direction in comparison to that of the negative direction. This feature clearly indicates that the crystal contains predominantly interstitial type of defects rather than vacancy defects. This can be well understood by the fact that due to interstitial defects which may be due to self interstitials, impurity atoms including the solvent atoms or molecules in the crystalline matrix, the lattice around these defects undergo compressive stress and the lattice parameter d (interplanar spacing) decreases and leads to give more scattered (also known as diffuse X-ray scattering) intensity at slightly higher Bragg angles (θB) as d and sin θB are inversely proportional to each other in the Bragg equation (2d sin θB = nλ; n and λ being the order of reflection and wavelength respectively which are fixed). However, these point defects with much lesser density as in the present case (if the concentration is high, the FWHM would be much higher and often lead to structural grain boundaries) hardly give any effect in the performance of the devices based on such crystals.
Figure 5.5 Diffraction curve recorded for DMBP single crystal for
(011) diffracting planes by employing the multicrystal X-ray
diffractometer with MoK1 radiation
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5.5 NLO PROPERTY
Powder second-harmonic generation efficiency was measured following the Kurtz and Perry powder method (Kurtz and Perry 1968). In this measurement, a fundamental wavelength emitted from a Q-switched Nd: YAG laser (1064 nm, 8 ns, 10 Hz, 1.5 mJ/pulse) was used. The power of the incident beam was measured using a power meter. The transmitted fundamental wave was passed over a monochromator (Czemy Turner monochromator), which separates 532 nm (second harmonic signal) from 1064 nm, and absorbed by a CuSO4 solution, which removes the 1064 nm light, and passed through BG-34 filter to remove the residual 1064 nm light and an interference filter with bandwidth of 4 nm and central wavelength of 532 nm. The green light was detected by a photomultiplier tube (Hamamatsu). KDP (Potassium dihydrogen phosphate) crystal was powdered to the identical size and was used as reference material in the SHG measurement. A quantitative measurement of the SHG conversion efficiency of DMBP crystal was determined by the modified version of powder technique developed by Kurtz and Perry. The SHG signal energy outputs are 55 mV and 66 mV for KDP and DMBP sample respectively. The SHG relative efficiency of DMBP crystal was found to be 1.2 times higher than that of KDP.
5.6 LASER DAMAGE MEASUREMENTS
The details of experimental setup have already been discussed in section 3.6. Single shot and multiple shot (30 pulses) surface laser damage thresholds are determined to be 64.07 GW/cm2 and 27.46 GW/cm2 respectively at 532 nm laser radiation. The Figure 5.6 shows the optical micrograph of the single shot damage profile of 532 nm laser radiation of DMBP.
The damage pattern of DMBP (Figure 5.6) shows circular blobs
surrounding the core of the damage. Such circular blobs are generally seen in
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crystals where the damage is mainly due to thermal effects resulting in
melting and solidification or decomposition of the material (Glass and
Guenther 1973). Recent investigations into laser damage in various optical
materials by nano second pulses have shown that the temperature reached at
the damage site could be as high as 12000 K (Carr et al 2004). Since DMBP
decomposes at around 98 oC it is most likely that in the present case damage
occurs due to decomposition of the crystal. Hence, in DMBP we can expect
the damage to be of thermal origin. However, one cannot rule out other
mechanisms being operative simultaneously, as the damage mechanism is
quite complex and depends on the nature of the material and various
experimental parameters.
Figure 5.6 Laser damage profile of DMBP
5.7 FTIR SPECTRAL ANALYSIS OF DMBP
The FTIR spectra of DMBP crystals were recorded in the range
4000-450 cm-1 employing a Perkin Elmer Fourier transform infrared
spectrometer by the KBr pellet method to study the functional groups in the
sample. The FTIR spectral analysis of DMBP crystals is shown in Figure 5.7.
The C=O stretching overtone is observed at 3434 cm-1. The peak at 2920 cm-1
could be attributed to methyl C-H stretching. The peak at 1646 cm-1 is due to
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carbonyl (C=O) stretching. Skeletal vibrations of aromatic rings are observed
at 1605 cm-1 and 1407 cm-1. The CH3 deformation occurred at 1378 cm-1. The
peaks at 1311, 1276, 1209, 1175, 1146, 1116, and 1018 cm-1 are all due to in-
plane bending modes of aromatic C-H bonds. The peaks at 953, 925, 844,
820, 785, 751, 680, 578, and 466 cm-1 are all due to out-of-plane bending
modes. The assignment of obtained frequencies is in conformity with
characteristic transmission bands of DMBP samples.
Linear optical properties of the crystals were studied using a Perkin
Elmer Lambda 35 UV-Vis Spectrometer in the region 200-1100 nm. The UV-
Vis-NIR spectrum is shown in Figure 5.8. It is evident that the transmission of
DMBP crystal has a wide frequency range; its transparency power is above
40% in this range. Also, DMBP crystal has a UV cut off at 400 nm, which is
sufficient for SHG laser radiation of 1064 nm or other application in the blue
region.
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2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0
0
1 0
2 0
3 0
4 0
5 0
Tran
smitt
ance
(%)
w av e len g th (n m )
Figure 5.8 Transmittance spectrum of DMBP crystal
5.9 THERMAL ANALYSIS
The thermal stability was identified by thermo gravimetric (TG) and differential thermal analyses (DTA). The thermal analysis was carried out using Perkin Elmer Diamond TG/DTA in the temperature range 40-420 oC at a heating rate of 10 oC/min in nitrogen atmosphere. Figure 5.9 illustrates the TG and DTA curves for the grown DMBP samples. From the DTA curve it is seen that the material is stable and there is no phase transition upto 98 oC. Since there is no endo- or exothermic transitions below 98 oC, the material is proved to be stable in this region. It is seen from the TGA curve that decomposition starts at 142 oC and the material is fully decomposed at 257 oC. The sharpness of the endothermic peak shows good degree of crystallinity of the grown sample. The specific heat (Cp) at 35 oC is 2.44 Jg-1K-1. From earlier investigations on NLO crystals, it is known that crystals having higher values of specific heat exhibit high values of laser damage threshold. The observed high damage threshold may be related to the specific heat of DMBP which is reasonably high. Figure 5.10 shows the heating and cooling curve of the DMBP crystal carried out by DSC. The sharp peak in the heating DSC curve corresponds to the melting temperature of DMBP. The endothermic peak is asymmetric. The growth of DMBP crystal from melt by Bridgman method was not successful.
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0 50 100 150 200 250 300 350 400 450-20
0
20
40
60
80
100
TGA DTA
Temperature (oC)
Wei
ght l
oss (
%)
-40
-20
0
20
40
60
80
Endo
dow
n M
icro
volts
98oC
Figure 5.9 TG/DTA curves of DMBP
20 40 60 80 100 120 140 160-15
-12
-9
-6
-3
0
3
Hea
t flo
w (m
W/m
g)
Temperature (oC)
Heating Cooling
99oC
47.5oC
Figure 5.10 DSC curve for DMBP
5.10 DIELECTRIC MEASUREMENT
The capacitance (Ccrys) and dielectric loss (tanδ) were measured using the conventional parallel plate capacitor method with frequency range (20 Hz to 1 MHz) using Agilent 4284A LCR meter at various temperatures
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ranging from 313 K to 353 K. Dielectric properties are correlated with the electro-optic property of the crystals (Boomadevi and Dhanasekaran 2004). The magnitude of dielectric constant depends on the degree of polarization charge displacement in the crystals. The dielectric constant of materials is due to the contribution of electronic, ionic, dipolar and space charge polarizations which depend on the frequencies (Dharmaprakash and Mohan Rao 1989). At low frequencies, all these polarizations are active. The space charge polarization is generally active at lower frequencies. The frequency dependence of the dielectric constant at different temperatures is shown in Figure 5.11. The dielectric constant decreases with increasing frequency and becomes almost saturated beyond 10 KHz for all temperatures. The variation of dielectric loss with frequency is shown in Figure 5.12. It is observed that the dielectric loss decreases with increasing frequency.
The low value of dielectric loss indicates that the grown crystals of
DMBP are of reasonably good quality. The dielectric constant and dielectric
loss studies of DMBP establish the normal behavior.
0 1 2 3 4 5 61
2
3
4
5
6
7
8
Die
lect
ric c
onsta
nt
lo g frequ en cy
353 K 343 K 333 K 323 K 313 K
Figure 5.11 Plot of log frequency versus dielectric constant
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0 1 2 3 4 5 6
0 .0
0 .1
0 .2
0 .3
0 .4
Die
lect
ric lo
ss
lo g fre q u e n cy
3 5 3 K 3 4 3 K 3 3 3 K 3 2 3 K 3 1 3 K
Figure 5.12 Plot of log frequency versus dielectric loss
5.11 MECHANICAL PROPERTIES
The selected smooth surfaces of DMBP crystals were used for micro hardness measurements at room temperature, using a Vickers micro hardness tester (MITUTOYO) attached to an incident-light microscope. Loads ranging from 10 to 50 g were used for indentation, keeping the indentor at right angles to the crystal plane for 10 s in all cases. The hardness measurements were made on the (010) plane of DMBP crystal. Figure 5.13 shows the variation of Hv as a function of applied load ranging from 10 to 50 g on (010) plane for the DMBP crystal. It is very clear from the figure that Hv decreases with the increase of load. The phenomenon of dependence of microhardness of a solid on the applied load, at low level of testing load is known as indentation size effect (ISE).
5.12 IRRADIATION STUDIES
Good quality single crystals were subjected to irradiation using 15UD Pelletron accelerator developed at Inter University Accelerator Centre (Formerly Nuclear Science Centre), New Delhi (Figure 5.14). DMBP single
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crystals were irradiated at room temperature (RT) and at liquid nitrogen temperature (LNT) with 50 MeV Li3+ ions at fluences 1×1012 and 1×1013 ions/cm2. The ion beams were made to be incident on the sample
mounted inside a 1.5 m diameter high vacuum chamber (pressure 5 × 10-8 Pa).
The ion beam was magnetically scanned on 10 mm × 10 mm large area on the sample surface for uniform irradiation. The ion beam fluence was measured by integrating the ion charge on the sample ladder, which was insulated from the chamber.
10 20 30 40 5014
15
16
17
18
19
Hv in
kg\
mm
2
Load P in gm
Figure 5.13 Plot of Vickers hardness number vs load
Figure 5.14 Schematic of Materials science beam line at Inter University
Accelerator Centre, New Delhi
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5.12.1 Dielectric Measurements
The dielectric measurements carried out in the frequency range
20–106 Hz and temperature range 313–353 K, were recorded with the help of
an Agilent 4284A LCR meter. The dielectric constant was calculated by using
the relation
0
rCt
A
(5.1)
where is the permitivitty of free space, t is the thickness of the sample, C is
the capacitance and A is the area of cross section.
The dielectric constant was found to increase, after all fluences of
Li3+ ion irradiation. The large value of dielectric constant at low frequency is
due to the presence of space charge polarization (Ishwar Bhat et al 2002).
Relationship between the dielectric constant and corresponding defect
concentration of lithium niobate crystals is studied (Xue and Kitamura 2002).
The decrease in the values of dielectric constant with the frequency takes
place when the jumping frequency of electric charge carriers cannot follow
the alternation of the ac electric field applied beyond a certain critical
frequency (Ponpandian et al 2002). The dielectric constant was found to
increase, after Li3+ ion irradiation of fluence of 1×1013 ions/cm2. The drastic
increase in dielectric constant due to ion irradiation may be correlated to the
defects created along the ion tracks. Incident heavy ions get embedded in the
crystal, they lose energy by both the inelastic collisions dominant near the
surface and the elastic collisions, which dominate near the low end of the
range of implanted ions. The increase in dielectric constants for Li3+ ion
irradiated samples may be attributed to the disordering of the crystal lattice by
the ion beam (Wooster 1953). The increase in dielectric constant due to ion
irradiation may be correlated to the defects created along the ion tracks and
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the structural modifications induced in the surrounding regions (Ishwar Bhat
et al 2002). As we increase the fluence to 1×1013 ions/ cm2, more ions are
activated with the lattice disorderliness causing more activation of interaction
between the ions. This increases the capacitance, hence the dielectric constant
increases. The increase in dielectric constant (εr) is more for room temperature
irradiated sample compared to liquid nitrogen temperature irradiated sample
because, irradiation at liquid nitrogen temperature causes less lattice
disorderliness compared to irradiation at room temperature. Figure 5.15
shows the variations of dielectric constant with respect to temperature for
1 KHz frequency and the temperature range of 313-353 K. The dielectric loss
may be due to the perturbation of the phonon system by an electric field. The
energy transferred to the phonons dissipates in the form of heat. The dielectric
loss is observed to be very high at low frequencies. Also the dielectric loss
increases with increase of fluence. Figure 5.16 shows the variations of
dielectric loss with respect to temperature for 1 KHz frequency and the
temperature range of 313-353 K. Due to the similarity of the trend at all
frequency range considered; the trend at the 1 KHz is only presented.
31 0 3 20 3 3 0 34 0 35 0 3 602.8
3 .0
3 .2
3 .4
3 .6
3 .8
4 .0
4 .2
4 .4
D ie le c tr ic c o n s ta n t a t 1 K H z U n ir ra d ia te d sa m p le 1 x 1 0 1 2io n s/cm 2(R T ) 1 x 1 0 1 3io n s/cm 2(R T ) 1 x 1 0 1 2io n s/cm 2(L N T ) 1 x 1 0 1 3io n s/cm 2(L N T )
Die
lect
ric c
onst
ant
T e m p e ra tu re (K ) Figure 5.15 Plot of dielectric constant versus temperature of DMBP
crystals at 1 KHz frequency
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31 0 320 3 30 34 0 350 3600 .080
0 .085
0 .090
0 .095
0 .100
0 .105
0 .110
0 .115
0 .120
0 .125
D ielec tr ic lo ss a t 1 K H z U n irra d ia ted sa m p le L i1 x 1 0 13 io n s/cm 2(R T ) L i1 x 1 0 13 io n s/cm 2(L N T ) L i1 x 1 0 12 io n s/cm 2(R T ) L i1 x 1 0 12 io n s/cm 2(L N T )
Die
lect
ric lo
ss
T em p eratu re (K )
Figure 5.16 Plot of dielectric loss versus temperature of DMBP crystals
at 1 KHz frequency
5.12.2 Optical Behaviour of DMBP Before and After Irradiation
Absorption of the unirradiated and irradiated DMBP crystals was studied using a Perkin Elmer Lambda 35 UV-Vis Spectrometer in the region 300-1100 nm. The results of optical absorption studies are given in Figure 5.17. The electronic structure of DMBP crystals can be visualized from the study of UV–Visible spectra. The unirradiated DMBP crystal shows its characteristic peak at around 385 nm, the characteristic peak at 394 nm is observed for irradiated DMBP crystals at fluence of 1 × 1013 ions/cm2 at Liquid nitrogen temperature. The characteristic peaks at 392 nm and 381 nm are observed for irradiated DMBP crystals at fluences of 1 × 1012 ions/cm2 at LNT and at RT respectively. The characteristic peak at 382 nm is observed for irradiated DMBP crystals at fluence of 1 × 1013 ions/cm2 at room temperature. With increase in irradiation fluence delivered to the DMBP crystals higher concentration of defects is formed. The increase in absorption may be due to capture of excited electrons by existing ion vacancies and the formation of additional defect centers. Absorption of DMBP crystal decreases
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at the fluence of 1 × 1013 ions/cm2 irradiated at room temperature and Liquid nitrogen temperature, this is due to the fall in the free radical production, and this is due to the thermal effect caused on irradiation (Rotblat and Simmons 1963), but no additional absorption peaks were found. This may be due to the fact that the energy that ions absorbed from swift heavy ions is not enough to move from lattice to substitution positions. In other words it is difficult to form ion vacancies. The change in the absorption may also be attributed to the creation of some intermediate energy levels due to structural rearrangements.
From the wavelength corresponding to the bandgap of the material in the absorbance curve, a sudden rise in the absorbance is expected after irradiation (Desai and Rao 1983). Working on this hypothesis, energy gap of the as grown DMBP and irradiated DMBP crystals was determined from the absorption spectra. According to the Tauc relation, the absorption co-efficient α is given by
(αhν)n =A(hν − Eg) (5.2)
where Eg is the energy gap, A is a constant and varies for different transitions, hν is the energy of photon and n is an index equal to 1/2, which enumerates that the absorption edge in this crystal is due to the indirect allowed transition.
The graph is plotted between (αhν)1/2 and hν (Figure 5.18) and extrapolation of the linear part gives the value of band gap of the sample. The value of bandgap of unirradiated DMBP crystal is 3 eV. The values of bandgap determined are 2.2 and 2 eV for irradiated DMBP of 1×1012 and 1×1013 ions/cm2, respectively at liquid nitrogen temperature. The values of bandgap determined are 2.6 and 2.8 eV for irradiated DMBP of 1×1012 and 1×1013 ions/cm2, respectively at room temperature. The decrease in bandgap energy upon irradiation may be attributed to the creation of some intermediate energy levels due to structural rearrangements (Virk et al 2001). Hence with relatively swift heavy ion fluences, it is possible to create electrically
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transparent windows along with the optical activity of laser structure without any damage to the active region.
U n irra d ia te d s a m p le ir r 1 x 1 0 13 R T ir r 1 x 1 0 13 L N T ir r 1 x 1 0 12 R T ir r 1 x 1 0 12 L N T
Abs
orba
nce
(arb
uni
ts)
W a v e le n g th (n m )
Figure 5.17 UV-Visible spectrum of unirradiated and irradiated DMBP
crystals
1 .0 1 .5 2 .0 2 .5 3 .0 3 .5
1 .0
1 .5
2 .0
2 .5
3 .0
3 .5
(h
)1/2
P h o to n E n e rg y (eV )
u n ir ra d ia ted sa m p le ir r 1 x 1 0 1 2io n s /cm 2-L N T irr 1 x 1 0 1 2io n s /cm 2-R T irr 1 x 1 0 1 3io n s /cm 2-L N T ir r 1 x 1 0 1 3io n s /cm 2-R T
Figure 5.18 (αhν)1/2 versus photon energy
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5.12.3 Photoluminescence Studies
The photoluminescence (PL) spectrum was measured using a JY Fluorolog-3-11 consisting of a two-stage monochromator, a photomultiplier tube (PMT) with a lock-in amplifier for PL detection, and a Xenon Lamp operating at 390 nm for excitation in all the measurements. The Photoluminescence spectrum is shown in Figure 5.19. The luminescent emission of the DMBP crystal is determined at room temperature. For the unirradiated DMBP crystal, the intensity pattern displays remarkable high intensity band around blue region at 439 nm. The peak intensity has decreased when compared to that of unirradiated sample. Luminescence has been observed during irradiation at various fluences and the spectra do not show any shift in the band position. This observation suggests the existence of states, which remain unaffected by irradiation. More studies are needed in this direction to understand the mechanism.
The PL intensity is sensitive to the damage created by swift heavy ions. Initially, a strong PL intensity indicates dominant radiative transitions. Hence a slight increase in intensity occurs with the increase of fluence from 1012 to 1013 ions/cm2 at LNT. As the concentration of the colour centers increases, the rate of radiative transitions also increases, resulting in an increase in the luminescence intensity. At higher fluences the sample becomes rich with defects, which does affect the radiative transitions. Thus, due to the excessive defects, the radiative transition rate decreases, resulting in a decrease of the integrated PL intensity from the sample with increase of fluence from 1012 to 1013 ions/cm2 (Skuratov et al 2002). The decrease in intensity may be attributed to lattice deformation produced due to displacement of cations or due to modification of ligand field in irradiated region.
