Pure and dyed ZTS crystals were synthesized by mixing zinc sulfate heptahydrate [ ] and thiourea [CS(NH₂)₂] in 1:3 stoichiometric ratio in two separate glass pots following a previous procedure.^[38] The pure sample was untreated, while in the dyed sample, 0.1 wt% uranine [C₂₀H₁₀Na₂O₅] ( g of uranine dye in 1 g of ZTS) was added and continuously stirred at ∼320 K. Both samples were then dissolved in deionized water and recrystallized at room temperature to achieve a high-purity material for the optimized growth of single crystals. The filtrates of the prepared solutions were transferred into their corresponding beakers, roofed with a perforated sheet, and immersed in a steady-temperature bath at (27±1 °C. Single crystals of UZTS (∼26 mm×15 mm×10 mm) were harvested from the parent solution after 30 days as shown in Fig. 1(a). Furthermore, the dyed ZTS crystals were grown mainly along the [100] direction, consistent with that reported for pure ZTS crystals.^[38] The images of the sliced and polished crystal specimens from the bulk crystal are depicted in Figs. 1(b) and 1(c), and were used for further analyses. Observably, the grown crystals were highly transparent, and thus, suitable for optoelectronic applications.

Fig. 1.

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	Fig. 1. (a) As-grown and ((b), (c)) sliced and polished single crystals of UZTS.

For crystal structure and lattice parameters analyses, the grown crystals were subjected to PXRD measurement using a Shimadzu X-600 x-ray diffractometer with CuK_α radiation (40 kV, 30 mA, λ = 0.1543 nm) at a scan rate of 2°/min over an angular range of 5°–70°. Furthermore, energy-dispersive x-ray spectroscopy (EDXS) analysis with scanning electron microscopy (SEM) was performed using a JSM 6360 LA (JEOL, Japan) to confirm the presence of the dopant in the ZTS crystals. Vibrational studies were conducted with FT-IR and FT-Raman spectroscopies using THERMO SCIENTIFIC, DXR DT-IR, and FT-RAMAN spectrophotometer (Waltham, Massachusetts, USA). For optical analysis, a Shimadzu UV-vis-NIR spectrophotometer model (UV-3600) (Kyoto, Kyoto Prefecture, Japan) was employed to record the diffuse reflectance (DR) within 190–1000 nm. The room-temperature PL spectrum was recorded using a Lumina Fluorescence Spectrometer (Thermo Fisher Scientific, USA) at different excitation wavelengths. Thermal analysis was accomplished by differential scanning calorimetry (DSC) 60 (C30455001206SA) at a heating rate of 15 °C/min over the temperature range of 30–300 °C. The prepared samples were sputter-coated on both sides for dielectric measurement with a KEITHLEY 4200-SCS system (Keithley (a Tektronix company, USA). The dielectric constant, dielectric loss, and alternate current (AC) conductivity were obtained over a high-frequency range of 1 kHz to 10 MHz.

Figure 2 shows the recorded PXRD pattern with hkl indexing for the pure and dyed ZTS crystalline powder specimens. The peak intensities of the dyed ZTS crystals were enriched, which signifies the enhancement in the quality of the crystal. For further structural analysis, the recorded diffraction data were outsourced from POWDERX software (http://www.ccp14.ac.uk/tutorial/powderx/). The crystal system was orthorhombic, belonging to the space group, with unit cell dimensions of a = 11.120 Å, b = 7.770 Å, c = 15.490 Å, and V = 1338.37314 Å; and a = 11.121 Å, b = 7.771 Å, c = 15.493 Å, and V = 1338.789 ų for the pure and dyed ZTS crystals, respectively, which agrees with earlier reported data.^[14] The c value and, effectively, the volume, increased due to the presence of the dye in the ZTS crystal lattice. Upon close inspection of the diffraction pattern, shifts in the peak positions were observed, as depicted in Fig. 2(b). Evidently, the diffraction peaks of the dyed specimen shifted toward the lower angle, which signifies the interaction of uranine dye with the ZTS crystalline matrix. The presence of the uranine dye phase was confirmed by the appearance of low-intensity XRD peaks 33.48° ( , 36.12° ( ), 37.82° (423), and 40.36°(225) (JCDPS card No. #51-2376)^[39] in the spectrum of the dyed crystals (Fig. 2(c)). This was further supported by the EDX spectrum as shown in Fig. 2(d). Moreover, evidence of sodium ions in the ZTS matrix from its interaction with uranine dye was observed. Thus, we successfully prepared uranine-dyed ZTS crystals.

Fig. 2.

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	Fig. 2. (a) Indexed x-ray diffraction patterns, (b) magnified view of high-intensity peaks, (c) magnified view of dye peaks, and (d) EDX spectrum of UZTS crystals.

For the vibrational studies, the FT-IR and FT-Raman spectra of the grown crystals were recorded over the wavenumber ranges of 4000 to 400 cm⁻¹ and 1800 to 50 cm⁻¹ (main), respectively, as shown in Figs. 3(a) and 3(b). Symmetric and asymmetric NH stretching vibrations of several N–H–O bonds in ZTS were observed in the higher wavenumber region (i.e., 3000–3500 cm⁻¹). All IR and Raman vibration modes, and their corresponding assignments are presented in Table 1. Clearly, almost all the peak positions are slightly shifted, due to the interaction of the dye with the ZTS matrix. Furthermore, the intensities of the Raman peaks were observably enhanced due to doping. Therefore, improvement in the luminescence properties of the ZTS crystals is expected.

Fig. 3.

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	Fig. 3. (a) FT-IR and (b) FT-Raman spectra for pure and dyed ZTS crystals.

Table 1.

Table 1.

Table 1. Calculated mechanical parameters for pure and dyed ZTS crystals. . Crystal Meyerʼs law (Meyers No.) Hays–Kendall approach PSR model W/g A1 a b /kg/mm−2 ZTS 122 2.45 –10.82 0.073 135.3 –9.32 0.095 176.1 UZTS 192 2.63 –15.63 0.1152 213.6 –20.26 0.114 211.4

Table 1. Calculated mechanical parameters for pure and dyed ZTS crystals. .

Figures 3(a) and 3(b) reveal a broad envelope between 1700 and 3000 cm⁻¹ due to the symmetric and asymmetric modes of NH₂ in zinc-coordinated thiourea molecules. The strong bands at 1633 cm⁻¹ in the pure crystal spectrum and at 1632 cm⁻¹ in the UZTS crystal spectrum both correspond to the plane bending vibration of NH₂. The stretching vibration modes of N–C–N bonds were observed at 1517 and 1540 cm⁻¹, and at 1515 and 1539 cm⁻¹ in pure and UZTS crystal spectra, respectively. The vibration bands observed at 1442 and 1403 cm⁻¹, and at 1440 and 1404 cm⁻¹ in the IR spectra of ZTS and UZTS correspond to the asymmetric stretching of CN and stretching of CS bonds, respectively. The coordination of zinc with thiourea through sulfur was confirmed by the presence of C = S vibration bands at 1089 cm⁻¹ in the spectra of both crystals.

The vibration bands at 1411 and 730 cm⁻¹ attributed to the C=S stretching vibration of thiourea were shifted toward lower wavelengths of 1410 and 716 cm⁻¹ in the ZTS spectrum, and 1403 and 710 cm⁻¹ in the UZTS spectrum, respectively, indicating the interaction of the dye with the pure material. The respective stretching vibrations of SO₄ and NH₂ at 1112 and 1093 cm⁻¹ in the ZTS spectrum, were slightly shifted to 1107 and 1086 cm⁻¹ in the UZTS spectrum, respectively. Similarly, vibration bands assigned to S=C–N and SO₄ vibrations at 1035 and 957 cm⁻¹, respectively, in the ZTS spectrum, were observed at 1024 and 947 cm⁻¹ in the UZTS spectrum, suggesting the interaction of sulfur with the dye. The symmetric stretching and rocking vibrations of C–N at ∼716, 478, 607, and 432 cm⁻¹ in the spectrum of pure crystals were shifted to 710, 481, 609, and 426 cm⁻¹ in that of the doped samples, respectively. The vibrational band assigned to Zn–S at 271 cm⁻¹ in the ZTS spectrum was observed at 263 cm⁻¹ in the UZTS spectrum.^[36] Furthermore, the vibrational peaks at 241 and 182 cm⁻¹ in the ZTS spectrum and at 233 and 173 cm⁻¹ in the UZTS spectrum were attributed to lattice vibrations. These vibrational modes concur with previous reports on pure and dyed ZTS.^[20,40–42] The position of the vibration bands and their intensities were shifted and enhanced, respectively, because of the interaction of the dye with the ZTS crystal.

The PL study involves the evaluation of the trajectory of electron transition due to photo-excitation in the given energy states of the material, effectively probing the associated recombination rate and electronic purity of the material.^[48–50] Figures 5(a), 5(b), and 5(c) show the recorded PL spectra of the grown crystals at room temperature under different excitation wavelengths. The emission spectra of the dyed crystals were recorded under excitation wavelengths of ∼310, 358, and 385 nm. Accordingly, the spectra of the pure ZTS crystal possess only one emission band at ∼348, 409, and 442 nm, under each respective excitation wavelength, along with a few low-intensity bands between 360 and 470 nm. The spectrum of the dyed single crystal exhibits an intense UV emission band at 346 nm and low-intensity violet-blue/blue bands at ∼420 and 445 nm when excited at 310 nm. Correspondingly, these bands were observed at ∼408 (intense), 448, and 459 nm, and 440 (intense), 460 nm under excitation wavelengths of ∼358 and 385 nm, respectively. Furthermore, a strong green emission band at ∼511, 508, and 512 nm appeared under each respective excitation wavelength. This new emission band might have resulted from the presence of uranine dye in the ZTS matrix. An emission band at ∼528 nm was observed in our previous report on ZTS crystals with titan yellow dye.^[37] However, it may have shifted toward a lower wavelength due to the presence of uranine dye. The appearance of luminescence bands at 408 and 416 nm could be attributed to the presence of intrinsic defects.^[52,53] The luminescence band at was induced by increased vacancies^[54] or zinc interstitials.^[55] This band was previously observed at 434 nm in ZTS and 433 nm in TYZTS.^[37] Moreover, previous reports on pure and dyed ZTS crystals documented luminescence bands at 348 and 416 nm (λ_exc = 310 nm), 423 nm (λ_exc = 358 nm), and 406 nm (λ_exc = 249 nm) with a broad emission band between 370–500 nm.^[20,53,56] The current PL study demonstrates the tremendous potential of UZTS crystals for applications in biochemical/biomedical systems for the detection of compounds.^[57]

Fig. 5.

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	Fig. 5. PL emission spectra of UZTS crystals at excitation wavelengths of (a) 320 nm, (b) 358 nm, and (c) 385 nm.

Figures 6(a) and 6(b) show the recorded DSC curves for non-dyed and dyed ZTS crystals, respectively. DSC is an excellent tool for thermal study and provides accurate information on the thermal stability of any material, suggesting its application in nonlinear devices. Here, the influence of the dye on the thermal behavior of ZTS crystal was studied. Figure 6 shows that both pure and dyed ZTS crystals possess good thermal stability. Furthermore, upon melting, both crystals did not exhibit any weight loss from water evaporation, as water was the solvent for crystal growth. The performance of the prepared samples was described by the sharpness of the DSC peaks. The endothermic peaks of the ZTS and UZTS crystals were observed at 241 and 243 °C, respectively. Both crystals were stable up to these temperatures, which correspond to their melting point. Evidently, the melting point of ZTS improved by due to uranine dye doping. The melting points of pure and dyed ZTS crystals were higher and comparable with that of organic, semi-organic, inorganic, and metal-organic materials.^[58–63]

Fig. 6.

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	Fig. 6. DSC curves for (a) pure and (b) dyed ZTS crystals.

Dielectric materials are widely used in optoelectronic devices, because the dielectric constant is directly related to the refractive index and poling voltage. A low dielectric constant reduces the discrepancy in refractive index and facilitates low poling voltage. For dielectric and electrical conductivity analyses, we measured the capacitance (C), loss tangent ( ), and impedance (Z) of the crystals. The dielectric constant ( ) was calculated from the equation: , where t is the thickness of the sample, ε₀ is vacuum permittivity, and A is the area of electrodes. Using the values of , the dielectric loss ( ) was evaluated with the equation: . Figure 7(a) plots the variations of and with frequency. It also reveals that the value of the dielectric constant is higher in the low-frequency region and significantly decreases with increasing frequency but stabilizes at a certain point. The high dielectric constant at low frequency is attributed to electronic, ionic, dipolar, and space-charge polarizations,^[64,65] while the low and stable value at higher frequency is a consequence of the minimal contribution from the above mentioned polarizations. The low dielectric constant of the dyed crystal validates its applications in photonics, second-harmonic generation (SHG), and NLO devices.^[66] The value of for the ZTS crystals was enhanced with uranine dye, and was higher than the previously reported values of pure and dyed ZTS crystals.^[20,51] Based on Fig. 7(a), the dielectric loss also follows a similar trend as it decreases with frequency. Consequently, a very low value was attained, which indicates that the grown crystals contain small amounts of defects. Therefore, they exhibit good optical quality and are desirable in nonlinear optical applications.^[67] The total AC conductivity (σ_ac.tot.) was evaluated through the equation: . Its variation with frequency is illustrated in Fig. 7(b). Observably, the value of σ_ac.tot. increases with frequency and follows a universal frequency power law. The conductivity mechanism in the grown crystals was analyzed using the Jonscher Law:^[68] , where σ_dc, B, and ω represent the direct current conductivity, a constant, and angular frequency, respectively, and s is the exponent of frequency. The value of s was determined from the slope of vs. (Fig. 7(b)) to be 1.0227 with a standard error of 0.1044. Its value less than unity (∼0.9183) was associated with the lack of measurable direct current conductivity. Hence, the hopping mechanism in the crystals involved translational motion with sudden hopping. These findings show that the values of , , and σ_ac.tot. of ZTS can be tuned with the addition of uranine.

Fig. 7.

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	Fig. 7. Plots of (a) and versus and (b) versus .

The hardness of the single crystal is affected by the Debyeʼs temperature, lattice energy, heat of formation, and interatomic bonding, which are also important in studying various strength parameters.^[69,70] The indentation technique is a simple but unique method to examine different properties, e.g., normal indentation size effect (NISE), reverse indentation size effect (RISE), yield point of fracture, and brittleness.^[71] Therefore, to study the effect of load on the (100) plane of pure and UZTS crystals, Vickerʼs indentation test was performed over the load range of 10–100 g. To calculate the different strength parameters of the crystals, 26 mm×15 mm×10 mm samples were prepared, load tested, and evaluated using the following relation:

Fig. 8.

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	Fig. 8. Plots of (a) HV versus P, (b) ln P versus lnd, (c) P versus , and (d) P/d versus d.

3.7.1. Meyerʼs law

Meyerʼs law describes RISE in crystals by the following relation:^[73]

where P is the applied load, d is the diagonal length of the indentation mark, a is a constant for any given material, and n is the Meyer index or work hardening coefficient. Here, the n values were calculated from the slope of lnP vs. ln d plots (Fig. 8(b)). The corresponding values for pure and UZTS crystals are given in Table 1. Consequently, samples with higher values of n, i.e., , exhibit RISE. This trend was intensively discussed in previous studies.^[72,74] However, Meyerʼs law is suitable only for setups with lower loads. To compensate for this, we implemented Hays–Kendallʼs law.

3.7.2. Hays–Kendallʼs law

According to Hays–Kendallʼs law, the relation between the applied load, P, and the indentation diagonal, d, is given by^[64]

where W is the minimum load required for plastic deformation, and A₁ is a load-independent coefficient.^[75,76] The plots of P versus (Fig. 8(c)) provide the values of W and A₁, which are listed in Table 1. Using the values of A₁, the corrected hardness, , is calculated using the equation . With this approach, several reports suggest that the positive values of w imply NISE, while negative values signify RISE.^[72] Here, pure and UZTS crystals both yielded negative values, indicating the existence of RISE. Furthermore, to understand the RISE at load-independent and dependent regions, the proportional specimen resistance (PSR) model was adopted.

3.7.3. Proportional specimen resistance (PSR) model

According to the PSR model,^[74,76] the load, P, and indentation diagonal, d, are related by

where a is the frictional resistance developed at the indenter-specimen interface, and b is the load-independent constant. These values were obtained from the P/d versus d (Fig. 8(d)) plots. In literature, materials with undergo NISE, while those with experience RISE. Presently, both samples have , which indicates the presence of RISE. Moreover, the load-independent hardness was also calculated using the equation, ,^[74] the results for which are presented in Table 1.