In recent times, much attention has been focused on tin sulfide (SnS), with an extensive range of applications such as in near-infrared detectors, electrochemical capacitors 1, holographic recording, photovoltaic cells, and lithium-ion batteries, 2-8. SnS is a semiconductor belongs to the IV-VI group with the layered orthorhombic crystal structure, which it has a long b-axis with lattice constants of a = 0.4321 nm, b = 1.11923 nm, and c = 0.39838 nm 9. According to Fig.
1, SnS consists of two weakly van der Waals force bonded layers, in which atoms are tightly bonded with covalence bond. SnS has a variety of energy band gap depending on the preparation method, which it has been reported as 1.3–2.3 eV for direct band gap and 1.
0–1.2 eV for indirect band gap 10. Because of the unique features of SnS such as high absorption coefficient (>104 cm?1) 11, suitable carrier concentration 9, plentiful in nature, non-toxicity, and cost efficiency, it was a promising candidate for use in absorber layers in thin film solar cell applications. Various methods have been used to prepare SnS such as spray pyrolytic-deposition 12-14, molecular beam epitaxy (MBE) 15, hydrothermal method 6, 7, 16, chemical bath deposition 17-20, electron beam evaporation 21, 22, SILAR method 23, 24,and electrodeposition technique 11. Among these methods, the electrochemical technique is a good method due to simplicity, cost-efficiency, and the facility of controlling its parameters with high accuracy. To estimate the crystallite size of material the Scherrer’s method has been applied. Nevertheless, two important factors including inhomogeneous strain and instrumental effects have not taken into account for acquiring crystallite size. Therefore, the Williamson-Hall (W-H)- and the size-strain plot (SSP) methods are an average method to have a much realistic estimation of the crystallite size and lattice strain 26.
As we know, the deviation from perfect crystallinity creates a broadening of the diffraction peaks. From peak width analysis, it can be obtained the crystallite size and lattice strain. The particle size is almost bigger than crystallite size due to the aggregation of crystallite structures 27. In order to a real crystal deviate from a perfect crystal, the lattice strain has been created. The sources of lattice strain are the distribution of lattice constants arising from crystal imperfections, such as lattice dislocation, and the grain boundary triple junction, contact, or sinter stresses, stacking faults, coherency stresses etc. Some structural parameters such as peak width, the intensity of the peak and the shift in peak position are affected by crystallite size and lattice strain. The peak width and the lattice strain varies as 1/cos? and tan?, respectively 28.
In order to obtain the crystallite size and lattice strain as a function of 2?, two methods named Williamson-Hall (W-H)- and the size-strain plot (SSP) methods can be applied. In this work, six samples (containing undoped SnS and In-doped SnS) have been synthesized by electrochemical deposition from an aqueous solution. With the use of XRD data, the crystallite size, lattice strain, and other related parameters have been achieved by applying following methods. Three models of the W-H method containing: (i) uniform deformation model (UDM), (ii) uniform strain deformation model (UDSM), (iii) uniform deformation energy density model (UDEDM), and the size-strain plot (SSP) method. The crystallite size values acquired from Scherrer’s-, W–H, and SSP methods confirmed by TEM image. There is no report on W–H method, and SSP analysis of nanostructured In-doped SnS thin films. A three-electrode electrochemical cell was applied to deposit Nanostructured In-doped SnS thin films on fluorine-doped tin oxide (FTO) coated glass substrate. The effective dimension of FTO substrates (used as working electrode) was considered as 1 cm × 1 cm.
The anode and the reference electrode were a platinum sheet and a saturated calomel electrode (SCE), respectively. The electrolyte was 2 mM SnCl2 and 16 mM Na2S2O3, and the In-dopant was supplied by a 1mM InCl3 aqueous solution. The pH of the electrolyte was 3.8, which is reduced to 2.1 by adding diluted H2SO4.
The FTO and platinum sheet was cleaned in an ultrasonic bath and then rinsed with ethanol/acetone and distilled water. All deposition parameters except the In-dopant concentration were kept constant during electrochemical process. The bath temperature and deposition time considered as 60 ? and 30 minutes, respectively. The deposition potential was tuned at -1 V for all samples by a computer-controlled electrochemical analyzer (potentiostat, Autolab, A3ut71167, Netherlands). At the end of the electrodeposition process, the substrates were taken out from the bath. Then they washed with distilled water and lastly dried with an air jet.
The formation of SnS on FTO substrates is occurred according to the following reaction, According to the above reactions, the Na2S2O3 is unstable in acidic media. Therefore, it is easy to separate the sulfur, and consequently, the Sn2+ and S reduced at the cathode (substrate). In this research, we performed our analyses on six samples with different In-dopant concentrations.
The undoped SnS named as In 0, and the In-doped SnS thin films named as In 1-In 5. Using EDX analysis, the atomic percentage of In-dopant in In 1, In 2, In 3, In 4, and In 5 samples obtained 1.30, 2.13, 2.59, 2.90, and 3.58 %, respectively. To examine the phases of the deposited thin films, a Philips X’Pert-MPD X-ray diffraction diffractometer (XRD) system with Cu-K? radiation has been employed.
Elemental analysis was performed by a TE-SCAN field emission scanning electron microscope (FESEM) with an energy dispersive X-ray analyzer (EDX) attachment was used. The surface topography of the deposited samples checked by atomic force microscopy (AFM- ARA AFM). A PHILIPS CM120 TEM was used to study the shape and size of SnS particles. Varian-Cary Eclipse room temperature photoluminescence (PL) was used to analyze the optical properties of nanostructured SnS thin films.X-ray diffraction (XRD) test is a robust nondestructive method that used for characterizing the structural phases of various materials. It offers information on crystal structure, phase analysis, preferred crystal orientation (texture), and other structural parameters, such as average grain size, crystallinity, lattice strain, and crystal defects. X-ray diffraction pattern is the fingerprint of the periodic atomic arrangements in a given material.
Therefore, XRD is a unique method in determination of crystallinity of a compound. Fig. 2a depicted the XRD patterns of undoped- and In-doped SnS thin films. All the films showed polycrystalline nature with the orthorhombic crystal structure of preferred orientation. The observed peak position values compared with the standard JCPDS files and the Miller indices of SnS compound referring to JCPDS 039-0354. As it was evident in this figure, the preferred orientations of In 0, In 1, In 2, and In 3 samples were (021) and (111). Whereas, those were (101) and (040) for In 4 and In 5 samples.
Therefore, it is interesting that the increase in In-dopant concentration leads to change in preferential orientation of as-deposited In-doped SnS thin films. Also, no trace of In, In2O3, and In2S3 or other impurities cannot be detected in all samples. As it is observed in XRD patterns, with an increase in In-dopant concentration, the peaks will become less intense and broader, which indicating a decrease in crystallinity of samples. Hence, it shows a significant increase in crystalline defects and mismatching due to In-doping. In order to better investigate the effect of In-doping on the structural properties of SnS, the plot of I-2? for (111) plane diffraction peak (Fig. 2b) of all samples has been drawn. Due to the difference in the effective ionic radii between Sn2+ (0.93 A) and In3+ (0.
80 A), a shift of (111) peak position to higher 2? has been observed. The lattice parameters of undoped- and In-doped SnS thin films can be obtained from the following relation 30, where (hkl) is the lattice plane index for the planes with higher intensity, i.e. (040), (021), (111) planes, and the dhkl is inter-planar distance. The calculated lattice parameters and other structural parameters of undoped- and In-doped SnS samples listed in Table 1. It is clear that the substitution of In3+ for Sn2+ in the SnS lattice leads to a decrease in the unit cell volume. The reason for this phenomenon is the smaller effective ionic radii of In3+ compared with Sn2+, which it caused a decrease in the dhkl and consequently unit cell volume. Fig.
3 shows the variation of lattice parameters of undoped SnS after In-doping. As it was apparent in this figure, due to the effective ionic radii of In3+ is smaller than Sn2+, an increase in In-dopant concentration in the SnS lattice leads to decrease in lattice parameters (a, b, and c). This occurrence clearly indicates that the In-dopant is substitutionally doped into SnS lattice.
Some structural properties of undoped- and In-doped SnS thin films originated from XRD patterns.Using atomic force microscopy (AFM) scanned over an area of 6µm × 6µm, the topographical examinations of In 0, In 1, and In 2 samples was done. Fig. 4 shows AFM images of deposited films. Therefore, the variation in the morphology of In-doped SnS nanostructures with an increase in In-dopant concentration showed that the In had been doped successfully in SnS lattice.
In this section, we use different methods to calculate crystallite size and lattice strain. These methods are Scherrer’s method, W-H method (including UDM, UDSM, and UDEDM), and SSP method. Using XRD patterns, the crystallite size (D) is estimated from Scherrer’s equation 31, 32, where D is the crystallite size, K is a shape-dependent constant equal 0.94, ? is the X-ray wavelength of Cu-K? radiation (0.154056 nm), ?hkl is the peak width at half maximum intensity (FWHM), and ?B is the Bragg’s angle.
The width of the Bragg’s angle is formed by the combination of instrumental- and sample dependent effects. The instrumental effect is evaluated from the line broadening of a reference sample such as silicon. Therefore, considering the instrumental effect, ?hkl can be obtained as follows 31, The crystallite size (D) was evaluated from the slope of cos? versus 1/?hkl plot using Eq. 5.
Consequently, the value of k?/slope shows the crystallite size value. Fig. 5 depicts Scherrer’s plots of undoped- and nanostructured In-doped SnS thin films.
It is clear that the crystallite size D of SnS is decreased after In-doping. The decrease in crystallite size of undoped SnS thin films after In-doping can be due to the difference in effective ionic radii of Sn2+ and In3+. Therefore, the crystalline quality of undoped SnS has been decreased after In-doping, which it could be attributed to the created lattice mismatch.