Remarkable effect of l-Ascorbic acid on crystal morphology, structural, crystalline perfection, optical, photoluminescence and dielectric properties of Zinc(tris) thiourea sulphate (ZTS) single crystals

2.1 Synthesis and crystal growth

For the current purpose we have purchased the high grade zinc sulphate heptahydrate [ZnSO₄·7H₂O], thiourea [CS(NH₂)₂] from Sigma Aldrich and l-ascorbic acid [C₆H₈O₆] (LAA) from Merck company. The calculated amounts of zinc sulphate heptahydrate and thiourea in 1:3 stoichiometric ratio were taken and dissolved in double distilled water in separate beakers, after getting the transparent solutions they were mixed to form Zinc(tris) thiourea sulphate (ZTS) and subsequently different concentrations (1%, 5% and 10%) of LAA was added to the solutions. All the prepared solutions were continuously stirred at 500 rpm for more than 24 h at 45 °C using a well temperature controlled magnetic stirrer (Isotemp, Fishers Scientific). The above temperature was maintained during the whole synthesis process for proper chemical reaction to achieve a uniform mixture of ZTS and LAA added ZTS. The solvent was evaporated at 50 °C to get the crude solid salts of ZTS and LAAZTS.

To grow the single crystals the synthesized materials were dissolved in double distilled water at 40 °C without further purification. The continuous stirring was done to get the highly transparent solutions and the temperature was reduced to 28 °C. All the prepared solutions were filtered using good quality Whatman filter paper in well cleaned three different beakers and covered with a perforated lid. These beakers containing the solution were housed in a stable constant temperature bath at the same temperature. After a span of 15 day white transparent of pure [see Fig. 1(a)] and after about 50 days light and dark yellow color single crystals of ZTS in presence of different concentrations of LAA were grown as publicized in Fig. 1(b–d). The size of the grown crystals at higher concentration of LAA (i.e. 10%) is found to be ∼23 × 12 × 10 mm³. It can be clearly seen in figure that LAA has remarkable effect on the morphology of ZTS crystals. The addition of LAA in ZTS completely modify the morphology of pure ZTS (see inset in Fig. 1).

Close

Fig. 1 As grown single crystal of (a) pure, (b) 1% LAA, (c) 5% LAA and (d) 10% LAA added ZTS.

Export to PPT

2.2 Characterization methods

The grown crystals were powdered and in parallel cuts and polished for different aimed characterizations. For structural studies the powdered specimens were studied by a Shimadzu X-600 Japan powder X-ray diffractometer (PXRD) comprise CuK_α radiation and operated at 40 kV, 30 mA by keeping the conditions same for all the specimens during the measurement. The crystalline perfection was assessed by High Resolution X-ray Diffraction (HRXRD) (PANalytical X’Pert PRO MRD System) using CuK_α1 radiation (λ = 1.5,40,598 Å). Optical properties were studied in solid as well as in solution using diffused reflectance and UV–Vis-NIR spectroscopy measurements by a Shimadzu (UV-3600) and JASCO (V-570) UV–Vis-NIR spectrophotometers respectively. Lumina fluorescence spectrophotometer (Thermo Fisher Scientific) was used to record the Photoluminescence emission spectra at three different excitation wavelengths in the wavelength range of 300–750 nm at 293 K by keeping all the parameters same. The impedance measurement was carried out on a KEITHLEY 4200-SCS system at 293 K in frequency range of 3 kHz–10 MHz. For mechanical testings a Shimadzu mechanical tester coupled with microscope was used.

3.1 Structural studies

Fig. 2(a) shows the measured X-ray diffraction patterns of ZTS and LAAZTS crystals. The intensity of peaks [mainly (0 0 2), (1 1 1), (2 0 0), (2 1 1) and (1 2 3) planes] is found to be enhanced due to LAA addition in ZTS crystals which confirms the enrichment of its crystallinity. The X-ray diffraction results confirms the single phase even at higher concentration of LAA, however there is great effect on crystal morphology (see Fig. 1). The XRD pattern for pure LAA was also recorded and compared with ZTS and LAAZTS crystals in higher angular range from 30 to 50° as shown in Fig. 2(b). It can be seen in figure (b) that there are some extra peaks in LAAZTS crystals which are belongs to LAA and confirm the presence of LAA in ZTS matrix and also shift in peaks position can be seen. From Fig. 1 it can be observed that the pure ZTS crystal contains all its faces like: (1 0 0), (0 0 1), (1 0 1), (0 1 1), (0 1 0) (Ushasree et al., 1999b) [see Fig. 1(a)], most of these planes are found to be disappear and growth is taking place along (1 0 0) plane in presence of 1% LAA. However, at higher concentrations of LAA these planes are found to be completely modified and the morphology of ZTS becomes a prism and the possible planes of growth are may be (1 0 1) and (0 0 1). Lattice parameters refinement was done using POWDERX software by taking the recorded diffraction data as input and tabulated in Table 1 and also the variation has been shown in figure (c) and (d). From figure (d) it can be observe that the unit cell volume is increasing with increasing the concentration of LAA in ZTS. All the crystal systems are of orthorhombic structure with space group Pca2₁ and point group mm2 [JCPDS#76-0778] (Andreetti et al., 1968, Ushasree et al., 1999b, Ushasree et al., 1999a, Moitra & Kar, 2007), which confirms the single phase of ZTS even at higher concentration of LAA. The crystallite size (L), lattice strain (ε) and dislocation density (δ) parameters were also determined (as these parameters are directly related to full width half maxima of the diffractions peaks and so on crystallinity) using the well- known relations (Scherrer, 1918): $D=\frac{k\lambda }{\beta \cos \theta }$ , $\epsilon =\frac{\beta \cos \theta }{4}$ and $\delta =\frac{1}{D^2}$ , where D, k (= 0.94), λ, θ and β are having their standard meanings. The calculated average values (corresponding to the main diffractions peaks which are indexed in the pattern) of these parameters are listed in Table 1. From the data one can conclude that the value of D is increasing with increasing the LAA concentrations and the values of ε and δ are found to be decreased which gives a clear support to enhancement of crystallinity of the crystals.

Close

Fig. 2 (a, b) X-ray diffraction patterns, (c) plots of variation of a and b and (d) c and V for pure and LAAZTS crystals.

Export to PPT

3.2 Crystalline perfection analysis by HRXRD

The HRXRD diffraction curve for (2 0 0) diffraction planes of pure and 1, 5 and 10 mol% l-Ascorbic acid (LAA) added ZTS crystals are shown in Fig. 3(a–d). The HRXRXD rocking curve of pure ZTS shows sharp single peak and possess very low value of full width at half maxima (FWHM) of the order of 8 arc sec the same has been recently reported (Kushwaha et al., 2011). The rocking curve of 1 mol% LAA added ZTS crystal is much broader compared to that of pure ZTS crystal having FWHM ∼ 175 arc sec. The presence of LAA has yielded significant increase in the FWHM value from 8 to 175 arc sec with clear cut asymmetry in −ve side also shows a small peak that indicate the presence of grain boundaries. The 5 mol.% presence of LAA into ZTS has drastically increased the FWHM value to ∼443 arc sec and also changed the asymmetry nature of the rocking curve from −ve side to +ve side. Though this curve looks a bit symmetrical in nature compared to 1 mol% but angular scattering is much larger that has yielded higher FWHM value. Surprisingly, at higher concentration (i.e. 10 mol%) of LAA has given a very symmetric rocking curve with quite low value of FWMH i.e. ∼20 arc sec that shows up to this level of LAA presence into ZTS crystal can yield good quality single crystals without any grain boundaries or defects.

Close

Fig. 3 HRXRD curves for (a) ZTS, (b) 1% LAAZTS, (c) 5% LAAZTS and (d) 10% LAAZTS crystals.

Export to PPT

3.3 Optical activity studies

3.3.1

3.3.1 UV–Vis-NIR and energy gap studies

To know the appropriateness of the grown single crystals in optical device applications the UV–Vis-NIR absorbance spectra was measured by preparing their solutions in double distilled water and shown in Fig. 4(a). From figure we can conclude that the grown crystals are having very low absorbance that results in high optical transparency. However, the absorbance or transparency was found to be reduced with increasing impurity concentrations but still have higher % of transmittance, viz. ∼80% at 10% LAA concentration [see Fig. 4(b)]. These characteristics makes it appropriate in optical windows for second harmonic generation and laser radiation applications (Rao, 1974, Shkir and Abbas, 2014, Shakir et al., 2010c). It can be seen in figure that the cut-off wavelength shifting towards the higher wavelength in presence of LAA and also shift in wavelength will change the band gap of ZTS. Hence, we have determined the band gap of LAAZTS crystals and compared with pure ZTS using Tauc's plot acquire using Tauc's relation [see Fig. 4(c)]. The value of energy gap (E_g) was acquire from the Tauc's plot, ${(\alpha h\nu )}^2$ vs. $h\nu$ (where $h\nu$ and $\alpha$ respectively measure the photon energy and absorption coefficient) through the point of joint [where ${(\alpha h\nu )}^2=0$ ] of the extrapolated straight portion with the abscissa axis. To determine the values of $\alpha$ , Beer–Lambert relation was used i.e. $\alpha =2.303A/d$ , where A is absorbance and d is the path length of cuvette (10 mm). The E_g values are found to be between 4.2 and 4.4 eV for pure as well as LAAZTS crystals. A new figure with more clarity has been plotted as shown in Fig. 4(d). However, these values are found to be less than the previous reported value of pure ZTS by us which was calculated from diffused reflectance data in solid samples (Shkir, 2016, Shkir et al., 2016). The color of the grown crystals in presence of LAA is found to be changed [see Fig. 1] that results in reduction of E_g value. The calculated value of E_g shows that the grown crystals belongs to high band gap materials category and can be applied in opto-electronic devices (Shakir et al., 2009a, Shkir et al., 2012, Kirschman, 1999, Casady and Johnson, 1996, Chow and Tyagi, 1994).

Close

Fig. 4 (a) UV–vis-NIR spectra, (b) transmittance, (c) Energy vs. ( α h υ ) 2 plot and (d) close view of Energy vs. ( α h υ ) 2 plot of ZTS and LAAZTS crystals.

Export to PPT

3.3.2

3.3.2 Photoluminescence (PL) analysis

ZTS and LAAZTS crystals were subjected to PL measurement at 295 K and the recorded emission spectra has been presented in Fig. 5(a–c). The emission spectra was measured at three different excitation wavelengths, ${\lambda }_{\mathit{exc}}$  = 310, 358 and 385 nm for all crystals. In figure (a), the grown pure and LAAZTS crystals are showing a UV-A emission band at ∼346 nm, however a new blue emission band was observed ∼460 nm in LAAZTS crystals grown in presence of 1%, 5% and 10% LAA when excited at 310 nm. From figure (b) (recorded at excitation wavelength 358 nm) it can be seen that the pure ZTS crystal possess a violet-blue emission band at ∼408 nm, which becomes broader and broader with increasing the LAA concentrations in ZTS, however a new emission band was also observed at ∼460 nm in LAAZTS crystals. From figure (c) which is recorded at excitation wavelength 385 nm shows one violet emission band at ∼441 nm in pure, 1% LAAZTS and shifted to higher wavelength to ∼444 nm and ∼449 nm in 5% and 10% LAAZTS crystals along with this band one more blue emission band was observed at ∼480 nm, 473 nm and 470 nm in 1%, 5% and 10% LAAZTS crystals, respectively.

Close

Fig. 5 PL emission spectra for PZTS and LAAZTS single crystals excited at (a) 310 nm, (b) 358 nm and (c) 385 nm.

Export to PPT

The PL emission band observed at ∼441, 444 and 449 nm in pure ZTS, 1%, 5% and 10% LAAZTS, respectively are may be ascribed to improved S²⁻ vacancies (Bol et al., 2002) or zinc interstitials (Pal et al., 2014) in the grown crystals which result in increase of vacancy type of defects (Shkir, 2016). The emission band is found to be more sharp for ZTS crystals when grown in presence of LAA and can be used as blue light emitting diodes (BLEDs). However, the highest PL intensity was observed for 10% LAAZTS crystal, when excited at 358 nm wavelength and will be more efficient for the fabrication of BLEDs.

3.4 Dielectric studies

Dielectric properties of any optical materials plays a vital role to define its application in various solid state devices as it mainly concerns with storage and dissipation of electric and magnetic energy in any materials. It also helps to explain various phenomenon in electro-optics, solid-state physics and also in biophysics. Hence, the dielectric studies were carried out on LAAZTS crystals. The capacitance (C), $\tan \delta$ and impedance (Z) was measured in the frequency range from 3 kHz to 10 MHz. Using these parameters the dielectric constant ( ${\epsilon }_1$ ) and loss $({\epsilon }_2)$ values were computed through following relations (Kaygili et al., 2013, Kaygili et al., 2015): ${∊}^{\prime }=\frac{\mathit{Cd}}{{\epsilon }_o{A}^{\prime }}$ , and ${∊}^{″}=\tan \delta \times {\epsilon }_1$ , where ${\epsilon }_{\text{o}}$ is known permittivity of free space ( ${\epsilon }_{\text{o}}$  = 8.854 × 10⁻¹² F m⁻¹), d is thicknesses, A is area of sample. The calculated values of dielectric constant ( ${∊}^{\prime }$ ) as a function of frequency for all crystals are shown in Fig. 6(a) and shows that ${∊}^{\prime }$ is depend on frequency and have stable value in the tested frequency range with minute variation. However, the value of ${∊}^{\prime }$ is found to be increased after 2 MHz. The average value of ${∊}^{\prime }$ is found to be enriched for LAAZTS crystals from 13 to 14 and higher value is achieved for 10% LAAZTS crystals which is higher than pure and other LAAZTS as well as reported pure and doped ZTS crystals (Bhandari et al., 2014). The possible reason of enhanced dielectric constant is may be due to high dielectric polarization and lowering of defects by added impurity in ZTS. The reduction of defects clearly indicates that the grown crystals are of good crystalline perfection and can be used in electro-optic device applications. The defect concentrations can be discussed on the basis of dielectric loss, ${∊}^{″}$ and hence it was calculated and shown in Fig. 6(b). From figure it can be noticed that the grown crystals of ZTS in presence if LAA are showing low loss values compared to ZTS and hence the number of defects are reduced by LAA impurity which was also confirmed by HRXRD studies. However, the behavior of ${∊}^{″}$ is quite similar to ${∊}^{\prime }$ . Moreover, the total alternating current (ac) electrical conductivity $({\sigma }_{\mathit{tot}.\mathit{ac}})$ was also evaluated by equation (Kaygili et al., 2013, Kaygili et al., 2015): ${\sigma }_{\mathit{tot}.\mathit{ac}}=\frac{d}{\mathit{ZA}}$ and the plot of variation of ${\sigma }_{\mathit{tot}.\mathit{ac}}$ with frequency is shown in Fig. 6(c). The value of ${\sigma }_{\mathit{tot}.\mathit{ac}}$ is found to be increased with increasing the frequency as well as impurity concentration and obey the universal frequency power law. Such features in ${\sigma }_{\mathit{tot}.\mathit{ac}}$ is may be due to hopping mechanism, that happens in the existence of field applied to the crystal. For understanding and determination of conductivity mechanism in the grown crystals the Jonscher relation is used (Jonscher, 1977): ${\sigma }_{\mathit{tot}.\mathit{ac}}={\sigma }_{\mathit{dc}}+B{\omega }^s$ , here, ${\sigma }_{\text{dc}}$ , B, $\omega$ and s are direct current conductivity, constant, angular frequency and frequency exponent, respectively. The value of s is determined from slope of the linear part of the curve between $\mathit{ln}{\sigma }_{\mathit{ac}}\mathit{vs}.\mathit{ln}\omega$ [Fig. 6(c)] and obtained to be 1.028, 1.027 and 1.024, with standard error 0.170, 0.173 and 0.169 for respective 1%, 5% and 10 wt% LAAZTS crystals. Hence, the s values are ∼≤1. The value of s shows that the grown crystals encompasses hoping mechanism of conduction that involves a translational motion with sudden carrier hopping within the prepared material (Shkir and AlFaify, 2017, Shkir et al., 2017). The enhanced dielectric constant, low loss value and high electrical properties makes the LAAZTS crystals more favorable in optoelectronic device applications compared to pure ZTS crystals.

Close

Fig. 6 Plots of variation of (a) dielectric constant, (b) loss and (c) total ac electrical conductivity with frequency.

Export to PPT