The research paper published by IJSER journal is about Dielectric Properties of L-Glycine Formate Single Crystal 1
ISSN 2229-5518
S.Suresh and P.Mani
Abstract— Single crystal of L-Glycine Formate (LGF) was grown by slow evaporation method. Dielectric constant and dielectric loss have been obtained as a function of frequenc y between 50 Hz -5 MHz and temperature range between 35ºC-75ºC. The dependence of tan δ, ε ״ and ζac on temperature and frequency of the applied field (50 Hz–5 MHz) is established D.C. conductivity has been deduced from the A.C. conductivity data and activation energy is calculated.
Key words: Single crystal, dielectric loss, dielectric constant, AC and DC conductivity.
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he fast development in the field of optoelectronics has stimulated the search for novel non linear optical crystals for efficient signal processing, optical storage, optical
communication, photonics, electro-optic modulation, optical parametric amplifiers, optical image processing etc., New non linear optical frequency conversion materials can have a sig- nificant impact on laser technology, optical communication etc., A molecular crystal suitable for use in a number of opti- cally non linear devices required atleast four basic qualities, such as high molecular first order polarizabiity suitable crys- tallographic structure, sufficient crystal quality and high opti- cal damage threshold [1,2]. The search for suitable materials displaying excellent second- order nonlinear optical (SONLO) properties is the focus of much current research activity due to their potential applications in optoelectronics, telecommunica- tion and optical storage devices. Materials with large second- order optical nonlinearities, short transparency cut-off wave- lengths and stable physicochemical performances are needed in order to realize many of these applications [3]. From Single crystal X-ray diffraction studies were carried out. The L- glycine formate crystal held its monoclinic structure with lat- tice parameters a = 5.07 Å, b = 11.93 Å, c = 5.42 Å, V = 945 Å3, β= 110.64°.
Single crystal of L-glycine formate was grown by slow evapo- ration technique of an equimolar solution of glycine and for- mic acid are taken and dissolved in double distilled water and stirred well for about six hours. Then it was filtered and
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Department of Physics, Loyola College, Chennai-600 034, India.
E-mail: sureshsagadevan@yahoo.co.in
Department of Physics, Hindustan Institute of Technology, Padur, India.
allowed to crystallize by slow evaporation method. Within 20 days, crystals with good transparency were obtained. Figure 1 shows the photograph of the LGF single crystal.
Fig.1.Photograph of as-grown LGF single crystal
Dielectric properties are correlated with electro-optic property of the crystals: particularly when they are non con- ducting materials. [4]. Microelectronics industry needs low dielectric constant (εr) materials as an interlayer dielectric [5] LGF crystal was subjected to dielectric studies using a HIOKI model 3532-50 LCR HITESTER with a conventional two ter- minal sample holder. The sample was electroded on either side with air-dying silver paste so that it behaves like parallel capacitor. The studies were carried from 35ºC-75ºC for fre- quency varying from 50 Hz to 5 MHz. Figure 2 shows the variations of dielectric constant with log frequency. The dielec- tric constant is calculated using the formula
E-mail: mani_hce@yahoo.co.in
'
Ct
0 A
(1)
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The research paper published by IJSER journal is about Dielectric Properties of L-Glycine Formate Single Crystal 2
ISSN 2229-5518
Where C is capacitance (F), t is the thickness (m), A the area of sample, εo is the absolute permittivity in the free space having a value of 8.854 x 10-12 Fm-1. Fig. 2 shows the variation of di- electric constant of LGF crystal as a function of frequency at different temperatures. It is seen from the plot that the sample has high dielectric constant in the low frequency region. The very high value of dielectric constant at lower frequencies may be due to the space charge polarization. From Fig.3, it is ob- served that the dielectric loss decreases with increase in fre- quency at different temperatures.
Fig.2. Dielectric constant with log frequency
Fig. 3. Dielectric loss with log frequency
The AC conductivity study of the LGF crystal was carried out at 1 kHz. In the high temperature (intrinsic) region, the effect of impurity on electrical conduction has not made any appreciable change whereas in the low temperature (ex- trinsic) region, the presence of impurity in the crystal has an impact and particularly increases its conductivity. The electric- al conduction in dielectrics is mainly a defect controlled process in the low temperature region. The presence of impur- ities and vacancies predominantly determine this region. The energy needed to form the defect is much larger than the energy needed for its drift. It is found from the Arrhenius plots (Figs. 4 and 5) for the LGF crystal that the conductivity increases with temperature. Accordingly, the value of activa- tion energy is estimated from the slope of the plot and its value is found to be 0.070 eV.
Fig.4.Variation of AC conductivity with 1000/T
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Fig.5. Plot of ln (ζac) T versus 1000/T
The research paper published by IJSER journal is about Dielectric Properties of L-Glycine Formate Single Crystal 3
ISSN 2229-5518
The DC conductivity of LGF crystal was carried out. The expe- riment was carried out using the conventional two-probe technique at different temperature ranging from 35ºC-75ºC. Well-sized crystal of LGF was used for conductivity study. The crystal was perfectly cut in to rectangular slaps and then po- lished using silicon carbide paper. The DC electrical conduc- tivity (σdc) of the crystal was calculated using the
relation dcT d / RA , where R is the measured resistance,
d is the thickness of the sample and A is the area of face in con-
tact with the electrode. The sdc values were fitted into the eq- uation dcT 0 exp( E / KT ) . The conductivities of LGF are shown in Figs. 6 and 7. Fig. 6 represents the temperature
dependence of conductivity of the sample is found to increase with increase in temperature for LGF crystal. Electrical con- ductivity depends on thermal treatment of crystal. Thus the conductivity at low temperatures depends on the cooling speed from melting point temperature to room temperature. Thus, for slow cooing there making of the lattice can occur by the migration of interstitials to vacancies, recombination of Schottky defects or migration of vacancies to the surface or along dislocation channels. On quenching or rapid cooling, a fraction of the vacancies freeze and the pre-exponential term includes a contribution from those frozen vacancies [6]. The DC activation energy of the LGF crystal is found to be
0.055 eV.
Fig.6.Variation of DC conductivity with 1000/T
Fig. 7. Plot of ln (ζdc) T versus 1000/T
Field dependence of dark and photocurrent of LGF are shown in Figure 8. The photocurrent is found to be less than the dark current at every applied electric field. This phe- nomenon is known as negative photoconductivity. It is inter- esting to note that LGF crystals exhibit negative photoconduc- tivity nature.
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The research paper published by IJSER journal is about Dielectric Properties of L-Glycine Formate Single Crystal 4
ISSN 2229-5518
Fig. 8. Field dependent photoconductivity of LGF single crystal
The dielectric constants (ε’ and ε’’), dielectric loss (tan δ) and conductivity (σac) of LGF crystals are strongly de- pendent on temperature and frequency of the applied ac field, the variation depends on the ranges of temperature and frequency. The dielectric constant and dielectric loss decreases with increasing frequency and higher values of dielectric constant occurs at higher temperature. The rate of variation of imaginary dielectric constant (ε") with temperature is strongly dependent on temperature and frequency of the applied field. The activation energy is de- termined from the plots for AC/DC conductivity. It is con- cluded from the photoconductivity studies that LGF has negative photoconducting nature. From all those analysis, it can be concluded that L-glycine formate is not only a potential Non linear Optical material but also a promising low εr value dielectric material, expected to be useful in the microelectronics industry. The encouraging dielectric properties of the crystal indicate the suitability of this crystal for photonics device fabrication.
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[6] I. Bunget, M. Popescu, in: Physics of Solid Dielectrics, Elsevier, New
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