Optical and Electrical Behavior of Gel Grown Strontium Incorporated Nickel Cadmium Oxalate Trihydrate Crystals for Opto-Electronic Applications

Crystal growth is the field of growing new variety of crystals for scientific and commercial purposes. Crystal growth in gels is an efficient and inexpensive technique to grow water insoluble single crystals at ambient temperature [1-4]. The chemically inert gel permits the reagents to diffuse at a desirable controlled rate. In recent years crystal growth in silica gel medium has attracted the attention of many researchers [5]. The growth of various mixed crystals like rare earth mixed crystals; transition metal mixed crystals has become an interesting field in the modern years, particularly after the discovery of ceramic superconductors [6]. Due to their extended and endless periodicity of atoms or molecules throughout the specimen, single crystalline solids are considered to be compelling class of materials [7]. For the elemental research in the field of organic electronics, the organic single crystals and organo-metallic compounds are considered to be influential materials because of the grain boundary vacancy, presence of minimum amount of impurities, unique structure, excessively good surface crystallinity and excellent interface quality [8]. Microelectronics and optoelectronics are regarded as the fields in which single crystals are stipulated and they are used as structural, high temperature materials [9]. The present paper aims to carry out the growth and characterization of strontium incorporated nickel cadmium oxalate (SNCO) single crystals in silica gel at ambient temperature. Generally, most of the oxalate crystals exhibit water insolubility, corrosion resistance, insulating behaviour, possesses high leakage resistance and good thermal stability [10]. In view of this, the grown crystals were subjected for various characterization techniques.

EXPERIMENTAL:

MATERIALS AND METHODS

The crystallization of SNCO was done using single diffusion gel technique. Chemicals used for growing SNCO crystals are Sodium Meta Silicate (Na₂SiO₃.5H₂O), Oxalic acid (C₂H₂O₄.2H₂O), Nickel chloride (NiCl₂.6H₂O), Cadmium chloride (CdCl₂.2.5H₂O) and Strontium chloride (SrCl₂.9H₂O) of AR grade. The silica gel was set by acidification of sodium metasilicate (SMS) by oxalic acid so that the pH of the solution was brought to the desired value.

General procedure:

After setting the gel, strontium chloride, nickel chloride and cadmium chloride solution was poured over the surface of the gel with the ratio (0.2ml : 2ml : 2ml) along the sides of the test tube in such a way that the gel should not be disturbed. The gel is a semisolid, and hence a semisolid-liquid interface was setup and Ni²⁺, Cd²⁺ and Sr²⁺ cations from the supernatant solutions are diffused into the gel in an accurately composed manner through the fine pores in the gel. Within three weeks crystal attain maximum size and no further growth is observed [11-12]. The experiment was repeated by modifying the growth parameters such as concentrations of supernatant solutions, pH, specific gravity and density of the gel medium. Between 1.030 and 1.060 g.cm^-3, the specific gravity of the gel was varied. 1.036 to 1.046 g.cm^-3 is considered to be the gel density at which the crystals of good quality with maximum size are obtained [13-14]. The gel density above 1.050 g.cm^-3, there is non-uniformity in the surface of the grown crystals. The values of pH of the gel are set at 4, 5, 6, 7 and 8. There is reduction in the size of the crystal as the pH increases above 5 and good quality crystals are obtained when the pH of the gel is below 5. The molarity of oxalic acid was varied from 0.1 M–0.7 M and for the supernatant solutions like nickel chloride, cadmium chloride and strontium chloride, the variation range is between 0.2 M-1 M to recognize changes in the crystallization [15-16]. At descended concentration of the reactants the crystal size was diminished and larger sized crystals are obtained at higher concentration of the reactants. The optimum conditions to obtain good quality and larger sized crystals are summarized in Table 1. The growth setup for SNCO crystals and fully-grown crystals are shown in Figure 1.

Table 1: Growth Profile of SNCO crystal.

Figure 1: Synthesis and extracted crystals.

Detection Method

The Powder X-Ray Diffraction (PXRD) pattern of the grown crystal was conducted by Rigaku MiniFlex600 X-ray diffractometer with wavelength of X-ray 0.15406 nm (CuKα) at a scan speed of 5? per minute. Fourier Transform Infrared (FTIR) spectrum of the grown crystal was recorded using IR Prestige-21 SHIMADZU FTIR spectrophotometer in the region 400 - 4000 cm₋₁. Field Emission Scanning Electron Microscope- Energy Dispersive X-ray (FESEM-EDX) spectrum of SNCO crystal was analyzed using CARL ZEISS FESEM attached with the EDS system (Oxford Instruments). Thermal analysis was carried out using the DSC-TGA TA (SDT-Q600) system in the nitrogen gas atmosphere. UV-Visible-NIR absorption spectrum was recorded in the UV-Vis-NIR spectrophotometer (UV-1800 SHIMADZU) with a scanning speed of 480 nm.min-1 with the wavelength range 190 to 1100 nm. The electrical and dielectric behavior was studied using Agilent 4294A precision impedance analyzer and Keithley source meter studies of the crystal.

RESULTS AND DISCUSSION:

Morphology and composition analysis:

Figure 3: EDX Spectrum of SNCO Crystal.

Table 2: Cationic distribution of the grown Crystal.

Fourier Transform Infrared Spectroscopy (FTIR):

Figure 4: FTIR Spectrum of SNCO Crystal.

Fourier transform infrared spectrum of SNCO crystal is shown in Figure 4. The spectra show broad absorption peaks at 3441.14 cm^-1 and 3179.52 cm^-1 due to O-H stretching of water [18]. The very strong bands at 1596.31 cm^-1 were attributed to the C-O and C-C stretch of the carbonyl group and the peaks at 1316.27 cm^-1 were assigned to C=O symmetric modes [19]. Due to the incorporation of metal and alkaline earth metal ions of cadmium, nickel and strontium, there exists many vibrational bands below 800 cm^-1 which represent metal-oxygen bonds (M-O) [20-21]. Hence, the infrared spectral studies reveal the existence of water of crystallization and oxalate group in the crystal geometry.

X-Ray Diffraction studies:

The Powder X-Ray Diffraction (PXRD) spectra of SNCO crystal are shown in Figure 5. The high crystalline nature of the grown crystal is indicated by the occurrences of well-defined peaks at specific Bragg angles 2θ [22]. From the spectrum, d- values for different h k l were computed using Powder X software and obtained (h k l) values are also indicated in the figure [23]. There is no any ICDD files found for this crystal. The cell parameters have been obtained from UNITCELL software and tabulated in Table 3. The grown crystals belong to triclinic (α ≠ β ≠ γ ≠ 90º) crystal system with P_-1 space group. The average crystallite size (Cs) was determined by the Scherer equation [24],

Cs = K λ  β cosθ

Where λ is the wavelength of the X-ray radiation, K is the Scherer constant and β is the

Figure 5: X-Ray Diffractogram- Showing high crystallinity.

FWHM (full width half maximum) of the reflection peak that has the same maximum intensity in the diffraction pattern. The number of molecules in the unit cell for SNCO crystal, Z=2, hence the density of the crystal is calculated using the relation [27]:

ρx=M.ZN.V

Where M is the molecular weight of the crystal, V is the volume of the unit cell and N is the Avogadro number.

Table 3: Powder X-ray diffraction Profile.

Thermal Analysis:

In Figure 6 the thermogravimetric analysis (TGA) plot shows two major decomposition stages. The initial change in weight is due to the evaporation of water, which starts at 40? C and ends at 200? C, which results in the formation of anhydrous SNCO crystal from SNCO trihydrate crystal, resulting in the observed weight loss 20.626% (Calculated weight loss 22.179%).  The next represent the decomposition of anhydrous strontium doped nickel cadmium oxalate crystal into strontium doped nickel cadmium oxide in the temperature range of 260? C and 400? C with observed weight loss 27.490% (Calculated weight loss 29.556%), which shows the release of carbon monoxide (CO) and carbon dioxide (CO₂) molecules as gases. Differential thermal analysis (DTA) and differential scanning calorimetry (DSC) curves are shown in Figure 7. The figure shows an endothermic peak at 118.09?C is due to the decomposition of SNCO trihydrate into anhydrous SNCO crystal by the removal of three water molecules. The exothermic peak at 349.92? C results in the formation of strontium doped nickel cadmium oxide by the release of carbon monoxide and carbon dioxide molecules. Using the atomic weight percentage from EDX spectra and the weight loss percentage from thermogravimetric analysis, the molecular weight of the crystal is calculated. The calculated weight loss, observed weight loss, number of decomposed molecules and the decomposition temperature of the sample are tabulated in Table 4. The proposed chemical formula for the SNCO crystal is obtained from EDX, FTIR and TG data as Sr_0.0127 [Ni_0.1953: Cd_0.7919 C₂O₄]. 3H₂O with molecular weight 243.669 g.mol^-1.

Figure 7: DTA/DSC Profile

Table 4: Thermal studies- molecules decomposed list

UV-Visible Spectral Studies:

UV-visible absorbance spectrum of SNCO crystal is active in the visible and ultra-violet region having the lowest cut off wavelength, λ=229.02 nm is depicted in Figure 8. The fundamental absorption edge and the band corresponding to the intrinsic absorption process lie in the ultraviolet region. The absorption band around 235 nm seen in the spectrum is attributed to the optical absorption due to the excitons. Excitons greatly affect the shape of absorption spectra near the fundamental edge. Transmittance spectrum of SNCO is also shown in the figure 8 which show a wide transparency in the visible region ³⁷. This is naturally so, because the energy gap seperating the valence band from the conduction band is quite large.  The energy bandgap is determined using the Tauc’s plot is shown in Figure 9. The extrapolation of the linear part of the graph gives the optical bandgap energy, E_g =5.420 eV. This value agrees well with the theoretically calculated value of E_g =5.416 eV, using the equation [29]:

Eg=1240λeV

If the energy gap of a material is greater than 0 eV, then the refractive index (n) of the material is calculated using the expression [30]:

Eg.en=36.

Figure 8: Absorbance and Transmittance curve of SNCO Crystals.

Figure 9: Tauc’s plot of SNCO Crystals.

Reflectance (R) of the crystal is calculated by using the expression,

R= (n-1)2(n+1)2

From the optical constants, electrical susceptibility (χ_e) could be calculated using the following relation:

εr=1+χe=n2

Therefore,

χe=εr-1=n2-1

The real part of the dielectric constant (ε_r) and the imaginary part of dielectric constant (ε_i) could be calculated from the following relations,

ε=εr-iεi

Where, εr=n2-k2

k=λα4π

The calculated band gap energy, high value of the refractive index, low value of reflectance, electrical susceptibility, real and imaginary parts of the dielectric constant are summarized in Table 5.

Table 5: Calculated optical parameters.

DC Conductivity Study:

Pellets of 1mm thickness and 13mm diameter were prepared using hydraulic press. The electrical conductivity of the SNCO crystal was measured using two-probe method by maintaining constant temperature using PID controlled oven (PID-TZ).  The pellets were placed between the silver plates to make good contact between the probes ⁴⁶. A high voltage power supply and Pico ammeter (Keithley source meter) was used to measure voltage and current values. The experiment was carried out by varying voltage (10-200 V) at constant temperature. The variation of current with voltage is shown in Figure 10. The current (μA) increases linearly with the increase of applied voltage. The DC conductivity is calculated using the relation:

  σDC=I.tV.A

Where I is the current passing through the pellet of thickness t=1 mm and area A=0.25 cm². For V=100 V the crystal exhibit σ = 0.77×10^-6 S.m^-1, and leakage resistance R_L= 51.89 MΩ.

Figure 10: DC Conductivity plot.

AC Conductivity Study:

The dielectric studies exhibit the opto-electric properties of SNCO crystals. The silver plates were attached to the surface of pelletized SNCO crystal for electrical contact, hence the setup acted as parallel plate capacitor. The experiment was performed at 30º C in the frequency range 40 Hz - 1 MHz. Conductivity (σ), dielectric loss (tanδ) and capacitance (C_P) were noted. The dielectric constant (ε_r) was calculated using the equation:

  εr=CdεoA

Where ε_o is the free space dielectric constant, thickness d=1mm and area of the sample A.

Figure 11: Variation of Dielectric Constant of SNCO Crystals.

Dielectric constant of strontium doped nickel cadmium oxalate crystals reduces deliberately with increasing frequency which is evident from the Figure 11. At diminished frequency the higher value of dielectric constant will be accredited to the lower electrostatic binding strength which appears due to the space charge polarization close to the grain boundary interface of the crystal lattice.

Figure 12: Variation of Dielectric loss of SNCO Crystals.

There is local movement of electrons in the guidance of electric field as a result of electronic transfer of the number of ions in crystals and this result in polarization. The variation in the dielectric loss with frequency is depicted in Figure 12. It is noticed initially that as the frequency increases dielectric loss is almost constant and then start to decrease and at the end almost remains constant. The lower value of dielectric loss and dielectric constant at higher range of frequencies confirms the optical quality of the grown crystals with lesser number of defects. The variation of AC conductivity of the grown crystals with frequency is depicted in Figure 13. The figure shows the increasing nature of AC conductivity with the frequency of the AC signal. The conductivity of SNCO crystals increase with an increase in frequency (10^-5 to 10^-4 S.m^-1). The conductivity curve is almost constant till 50 kHz. Above 50 kHz the conductivity increases and attain maximum at 1 MHz. This increase in conductivity is associated with the diminishing capacitive reactance of the sample holder. The sample holder acts as a parallel plate capacitor, whose impedance decreases with frequency, which in turn increases the AC conductivity.

Figure 13: Variation of AC Conductivity of SNCO Crystals.

Photoconductivity Study:

The SNCO crystal sample was coated with silver to make good electrical contact and then exposed to the light from the halogen lamp of 100Watts, to study the photoconductivity. The applied voltage and illumination of light to the crystal sample produces photocurrent which varies in the order of 10^-8 to 10^-6A. There is a linear increase in dark current (I_d) and photo current (I_ph) of the sample corresponding with the applied electric field and it is noticed that the dark current is found to be less compared to the photo current, which is indisputable from the Figure 14.  Hence we can conclude that the crystals expose a nature of positive photoconductivity as result of the propagation of mobile charge carriers by the photon absorption. The photosensitivity (P_s) of SNCO crystal was determined using the equation:

Ps=IphId

And it is found to be 1.18.

Figure 14: Variation of Photocurrent of SNCO Crystals.