AbstractAn electrochemical route has beenemployed to prepare undoped- and In-doped SnS thin films. Six samples includingundoped- and In-doped SnS thin films were depositedon the fluorine-doped tin oxide (FTO)substrate. An aqueous solution containing 2mM SnCl2 and 16 mM Na2S2O3 are used as the mainelectrolyte. Different In-doped SnS samples wereprepared by adding a differentamount of 1 mM InCl3 solution into the main electrolyte. The applied potential (E), time ofdeposition (t), pH and bath temperature (T) were kept at -1 V, 30 minutes, 2.1 and 60 ?, respectively.
For all samples, except the In-dopant concentration, all the deposition parametersare the same. After preparation, X-ray diffraction (XRD), field emission scanning electron microscopy(FESEM) with an energy dispersive X-ray analyzer (EDX) attachment, atomic forcemicroscopy (AFM), and transmission electron microscopy (TEM) were used todetermine structural properties of as-deposited films. XRD pattern showed thatthe synthesized undoped- and In-doped SnS thin films were crystallized in the orthorhombic structure. The shape of SnScrystals was spherical in the TEM image. X-ray peak broadening studies was done by applying Scherrer’s method, Williamson-Hall(W-H) models (assuming uniform deformation model (UDM), uniform straindeformation model (UDSM), and uniform deformation energy density model (UDEDM)),and size-strain plot (SSP) method. The crystallite size and lattice strain havebeen estimated using these methods. There was a good agreement in the particlesize obtained from W-H- and SSPmethods with TEM image.
Keywords:X-ray analysis, SnS nanostructures, In-doping, Williamson-Hall,size-strain plot. 1. IntroductionIn recent times, muchattention has been focused on tinsulfide (SnS), with an extensive range of applications such as in near-infrareddetectors, electrochemical capacitors 1, holographic recording, photovoltaic cells, and lithium-ion batteries, 2-8.
SnSis a semiconductor belongs to the IV-VIgroup with the layered orthorhombiccrystal structure, which it has a long b-axis with lattice constants of a = 0.4321 nm, b = 1.11923 nm, and c = 0.39838nm 9. According to Fig. 1, SnS consists of two weakly van der Waalsforce bonded layers, in which atoms aretightly bonded with covalence bond. SnS has a varietyof energy band gap depending on the preparation method, which it has been reported as 1.3–2.
3 eV for direct band gapand 1.0–1.2 eV for indirect band gap 10. Because of the unique features of SnS such as high absorptioncoefficient (>104 cm?1) 11, suitable carrier concentration 9, plentiful in nature, non-toxicity, and cost efficiency, it was apromising candidate for use in absorber layers inthin film solar cell applications. Various methods have been used toprepare SnS such as spray pyrolytic-deposition 12-14,molecular beam epitaxy (MBE) 15, hydrothermal method 6, 7, 16, chemicalbath deposition 17-20, electronbeam evaporation 21, 22, SILARmethod 23, 24,and electrodeposition technique 11. Among these methods, the electrochemical technique is a good method dueto simplicity, cost-efficiency, and the facility of controlling its parameters withhigh accuracy. To estimate thecrystallite size of material the Scherrer’smethod has been applied.
Nevertheless,two important factors including inhomogeneous strain and instrumental effects have not taken into account for acquiringcrystallite size. Therefore, the Williamson-Hall (W-H)- and the size-strainplot (SSP) methods are an average methodto have a much realistic estimation ofthe crystallite size and lattice strain 25. As we know, the deviation from perfect crystallinity creates abroadening of the diffraction peaks. From peak width analysis, it can be obtained the crystallite size and latticestrain.
The particle size is almost bigger than crystallite size due to theaggregation of crystallite structures 26. In order to a real crystaldeviate from a perfect crystal, the lattice strain has been created. The sources of lattice strain are the distribution oflattice constants arising from crystal imperfections, such as latticedislocation, and the grain boundary triple junction, contact, or sinterstresses, stacking faults, coherency stresses etc.
Some structural parameters such as peak width, the intensity of the peak and theshift in peak position are affected by crystallite size and lattice strain. Thepeak width and the lattice strain varies as 1/cos? and tan?, respectively 27. In order to obtain thecrystallite size and lattice strain as a function of 2?, two methodsnamed Williamson-Hall (W-H)- and the size-strain plot (SSP) methods can be applied. In this work, six samples (containingundoped SnS and In-doped SnS) have beensynthesized by electrochemical depositionfrom an aqueous solution. With the use of XRDdata, the crystallite size, lattice strain, and other related parameters have beenachieved by applying following methods. Three models of the W-H methodcontaining: (i) uniform deformation model (UDM), (ii) uniform straindeformation model (UDSM), (iii) uniform deformation energy density model(UDEDM), and the size-strain plot (SSP) method. The crystallite sizevalues 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-dopedSnS thin films. 2. Experimental2.1. Materials and processing A three-electrode electrochemical cell was applied todeposit Nanostructured In-doped SnS thin films on fluorine-doped tin oxide (FTO)coated glass substrate. The effectivedimension of FTO substrates (used as working electrode) was considered as 1 cm × 1 cm. The anode and the referenceelectrode were a platinum sheet and a saturated calomel electrode (SCE),respectively. The electrolyte was 2 mM SnCl2 and 16mM 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 anddistilled water. All deposition parameters except the In-dopant concentrationwere kept constant during electrochemical process. The bath temperature anddeposition time considered as 60 ? and 30minutes, respectively. The deposition potential wastuned at -1 V for all samples by a computer-controlled electrochemicalanalyzer (potentiostat, Autolab, A3ut71167, Netherlands). At the end of theelectrodeposition process, the substrates were taken out from the bath.
Thenthey washed with distilled water and lastly dried with an air jet. The formation of SnS on FTO substratesis occurred according to the following reaction, Accordingto the above reactions, the Na2S2O3 isunstable in acidic media. Therefore, it is easy to separate the sulfur, and consequently, the Sn2+ and S reduced at the cathode (substrate). In thisresearch, we performed our analyses on six samples with different In-dopantconcentrations. The undoped SnS named asIn 0, and the In-doped SnS thin films named as In 1-In 5. Using EDX analysis, theatomic percentage of In-dopant in In 1, In 2, In 3, In 4, and In 5 samples obtained1.30, 2.
13, 2.59, 2.90, and 3.58 %, respectively. 2.
2.Characterization of the films To examine the phases of the deposited thin films, a PhilipsX’Pert-MPD X-ray diffractiondiffractometer (XRD) system with Cu-K? radiation has been employed. Elementalanalysis was performed by a TE-SCAN fieldemission scanning electron microscope (FESEM) with an energy dispersiveX-ray analyzer(EDX) attachment was used. The surfacetopography of the deposited samples checkedby atomic force microscopy (AFM- ARA AFM). A PHILIPS CM120 TEM was used to study the shape and size of SnSparticles. Varian-Cary Eclipse room temperature photoluminescence (PL) was used toanalyze the optical properties of nanostructured SnS thin films. 3.
Results anddiscussion3.1. XRD analysisX-ray diffraction (XRD) test is a robust nondestructive method that usedfor characterizing the structural phasesof various materials. Itoffers information on crystal structure, phase analysis, preferred crystalorientation (texture), and other structural parameters, such as average grainsize, crystallinity, lattice strain, and crystal defects. X-raydiffraction pattern is the fingerprint of the periodic atomic arrangements in agiven material. Therefore,XRD is a unique method in determination of crystallinity of a compound. Fig.
2adepicted the XRD patterns of undoped- and In-doped SnS thin films. All thefilms showed polycrystalline nature with the orthorhombiccrystal structure of preferredorientation. The observed peak position values comparedwith the standard JCPDS files and the Miller indices of SnS compound referringto JCPDS 039-0354. As it was evident in thisfigure, the preferred orientations of In0, 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-dopantconcentration leads to change in preferential orientation of as-depositedIn-doped SnS thin films. Also, no traceof In, In2O3, and In2S3or other impurities cannot be detected inall samples.
As it is observed in XRDpatterns, with an increase in In-dopantconcentration, 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 toIn-doping. In order tobetter investigate the effect ofIn-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 theeffective 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 latticeparameters of undoped- and In-doped SnS thin films can be obtained from the following relation 28, where(hkl) isthe lattice plane index for the planes with higher intensity, i.e. (040),(021), (111) planes, and the dhkl is inter-planar distance. Thecalculated lattice parameters and other structural parameters of undoped- and In-dopedSnS samples listed in Table 1. It is clear thatthe substitution of In3+ for Sn2+ in the SnS latticeleads to a decrease in the unit cell volume. The reason for this phenomenon isthe 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. Asit was apparent in this figure, due tothe effective ionic radii of In3+is smaller than Sn2+, an increase in In-dopant concentration in theSnS lattice leads to decrease in lattice parameters (a, b, and c). Thisoccurrence clearly indicates that theIn-dopant is substitutionally doped into SnS lattice.
Using atomicforce microscopy (AFM) scanned over an area of 6µm × 6µm, the topographicalexaminations of In 0, In 1, and In 2 samples was done. Fig. 4 shows AFM imagesof deposited films. Therefore, the variation in the morphology of In-doped SnS nanostructureswith an increase in In-dopant concentration showed that the In had been doped successfully in SnS lattice.
3.2.Crystallite size and strainIn thissection, we use different methods to calculate crystallite size and latticestrain. These methods are Scherrer’smethod, W-H method (including UDM, UDSM, and UDEDM), and SSP method.
3.2.1.Scherrer’s methodUsing XRDpatterns, the crystallite size (D) isestimated from Scherrer’s equation 29, 30, where Dis the crystallite size, K is a shape-dependent constant equal 0.
94, ?is the X-ray wavelength of Cu-K? radiation (0.154056 nm), ?hklis the peak width at half maximum intensity (FWHM), and ?B isthe Bragg’s angle. The width of the Bragg’s angle is formed by the combinationof instrumental- and sample dependent effects. The instrumental effect is evaluated from the line broadening of areference sample such as silicon. Therefore, considering the instrumentaleffect, ?hkl can be obtained as follows 29, Scherrer’sequation can be rearranged by applyingthe corrected ?hkl as follows, The crystallitesize (D) was evaluated from the slope ofcos? versus 1/?hkl plot using Eq. 5. Consequently, thevalue of k?/slope shows the crystallite size value. Fig.
5 depictsScherrer’s plots of undoped- and nanostructured In-doped SnS thin films. It isclear that the crystallite size D of SnS isdecreased after In-doping. The decrease in crystallite size of undopedSnS thin films after In-doping can be due to the difference in effective ionicradii of Sn2+ and In3+. Therefore, the crystallinequality of undoped SnS has been decreasedafter In-doping, which it could be attributedto the created lattice mismatch. 3.
2.2.W-H methodsThe Williamson-Hall(W-H) and size-strain plot (SSP) are two methods to estimate the crystallitesize and lattice strain. In this paper,three models of a W-H method includingUDM, UDSM, and UDEDM have been used to estimate the structural parameters. To examine the uniform strain in the crystallinelattices, a simplified method named UDM model isproposed. In this method, the W-H formula relies on the size broadeningeffect and strain broadening effect. Size broadening effect is described with Debye-Scherer(?DS) formula as in Eq. 3, while the strain broadening termoriginated from imperfection and/ordistortion in crystal lattices (Eq.
6).In the aboveequation, ? is a maximum tensilestrain, or maximum compressive strain andC is a constant that assumed as 4. The above relation originated from the differentialof the Bragg equation (n?=2dsin?B) concerning the d-spacing and thediffraction angle. According to the lattice strain is considered as ?d/d=2?,the strain-inducing term is obtained as follows 31, In order to ?hkl=??+?DS,the Eq. 8 obtained as follows 32, By rearrangingthe above equation, the W-H relation (Eq. 9) is obtained 29. Accordingto Eq.
9, the crystallite size and lattice strain can be achieved. The slope and the y-intercept (at ?hkl.cos?= 0) values of the ?hkl.cos? vs.
4sin? plot show strain (?) andk?/D, respectively. The W-Hdiagrams of un- and In-doped SnS thin films shown in Fig. 6. As shown in thisfigure, due to the difference in the effective ionic radii of Sn2+and In3+, by increasing the In-dopant concentration in the SnSlattice, the crystallite size and lattice strain of In-doped SnS samplesdecreased and increased, respectively. To intrinsic anisotropic nature of elastic constant of materials, themicrostrain is not uniform in all crystallographic direction.
By consideringthe anisotropic nature of Young’s modulus, the UDSM and UDEDM methods have beenused to measure structural characterizations of crystalline lattices. In theUDSM model, it is assumed that thelattice stress (?) is uniform in all crystallographic directions.Therefore, the anisotropic nature of elastic modulus of materials isresponsible for the anisotropic nature of micro strain (?hkl)and energy density (u) 33. As we know, the Hooke’s law is valid in the elastic deformationzone. According to the Hooke’s law, the stress and strain have linear variations to each other.Therefore, the stress is obtained by using? = ? Ehkl formula, which ?, ?,and Ehkl are lattice stress, lattice strain, and elastic modulusin the vertical direction to the crystalline planes (hkl) crystalline lattices, respectively. Therefore, the Eq.
9 modified by putting the value of ?=?/Ehklas follows, According tothe above equation, the UDSM plot has been drawnby considering ?cos? as the y-axisand 4sin?/Ehkl as the x-axis.Therefore, the crystallite size and lattice stress are estimated from the y-interceptand the slope of this plot, respectively. Young’smodulus in the orthorhombic structures obtained as follows, 34,where liis the unit vector for a particular (hkl) plane and s11,s12, s13, s22, s23,s33, s44, s55, and s66are the elastic compliance of SnS with values of 11.92, -2.
93, -4.32, 10.07,-8.2, 19.
08, 19.46, 35.8, and 35.27 (TPa)-1, respectively.The obtained results showed that anincrease in In-doping concentration in SnS lattice results in a decrease and anincrease in crystallite size and lattice stress, respectively. Therefore, itcan be said that the micro tensile stress in the In-doped SnS thin films may be dueto the formation of grain boundaries 32.
The other formof W-H method is UDEDM. In this method, the energy density (u) isassumed uniform in all crystallographic directions, while the deformationstress (?) is presumed anisotropic 33. According to the Hooke’s law in the elastic deformation zone, theenergy density (u) defines as u=?2Ehkl/2.Therefore, the UDEDM formula is obtainedby rearranging the Eq. 9, In order to estimate the crystallite size andenergy density, the UDEDM graphs have beenplotted. The UDEDM curves are drawnwith ?cos? against 4sin?/(Yhkl/2)1/2.It is obvious in the Eq. 12 that thecrystallite size (D) and energy density (u) are estimated by the y-interceptand slope of the fit, respectively.
Based on Eq. 12, the crystallite size andenergy density are calculated using the below equation. According to UDEDM model, the crystallite sizeand energy density of undoped SnS aredecreased and increased, respectively, after In-doping. As previouslydescribed, introduction an ion (In3+) with different effective ionicradii compared with Sn2+ in SnS lattice leads to create mismatch andimperfection in SnS lattice. Therefore, it increased the energy density inIn-doped SnS crystalline lattice. According to Hooke’s law, an increase inenergy density leads to an increase in lattice stress and lattice strain. The calculatedcrystallite size values from W-H methods including UDM, UDSM, and UDEDM are ingood agreement with each other.
These are becausethe presence of strain in various models of W-H analysis has a minimal effect on the average crystallitesize of SnS thin films. Also, the valueof average crystallite size of un- and In-doped SnS samples estimated from Scherrer’smethod and W-H analysis shows a variation, which this is due to (i) strainbroadening effect and (ii) the difference in averaging the particle sizedistribution 26. Similar results were observedin 29. 3.2.3.
SSPmethodSize strainplot (SSP) technique is another suitable method to investigate the crystallitesize and lattice strain. It is consideredfor the isotropic nature of the crystal structure which gives less weight todata from reflections at high angles, where the accuracy is usually lower 35. In this method, the Lorentzian function and the Gaussian functiondescribe the crystallite size- and strain profile, respectively 25. According to the SSP method, thefollowing relation is employed to describe the relation between lattice strainand crystallite size 36, where A is a constant which equals ¾ for sphericalparticles. By plotting (?hkl.cos?.dhkl)2versus d2hkl.?hkl.
cos? as shown in Fig. 9 (SSP plot), thecrystallite size and lattice strain of undoped- and In-doped SnS thin films canbe achieved. According to Fig. 9 and Eq. 13,the crystallite size and the lattice strain can beobtained from the slope and y-intercept (in which (?hkl.
cos?.dhkl)2= 0), respectively. The obtained results from Scherrer’smethod, W-H method, and SSP methodare summarized in Table 2. In addition, the obtained values of crystallitesize and lattice strain for undoped- and In-doped SnS thin films using Scherrer’s-, W-H- and SSP methods are compared in Fig. 10. As can be seen in Fig. 10, with the increase ofIn-dopant concentration in the SnS lattice, the crystallite size decreased and subsequently, the lattice strain increased. Itis due to the fact that with introducing theindium ions into SnS lattice, the lattice has beenaccompanied by mismatches related to different effective ionic radii ofIn3+ and Sn2+ ions.
The results showed that the crystallitesize obtained from Scherrer method is less than that of W-H and SSP method thatit can occur due to the effect of strainvalue and shows that the role of strain is important 37. Consequently, it can be saidthat there is good accordance between structuralparameters obtained from UDEDM, UDM, UDSM models, SSP method, and the resultsof the Scherrer’s formula and TEM image. 3.2.3. TEM methodIn order to discover the reality of obtained data from XRD analysis, TEManalysis was applied.
TEM is an excellent analysis to examine the size andthe shape of deposited SnS. The TEM image for undoped SnS thin films is shown in Fig. 11. It is clear in TEM imagethat the average particle size is in good agreement with the averagecrystallite size estimated from W-H and SSP methods. 3.3. PLPhotoluminescence is light emissionfrom any form of matter after the absorption of photons. PL is anon-destructive test for examination the crystalline quality of materials.
Theroom temperature PL spectra of undoped- and In-doped SnS thin films are shown in Fig. 10. The photo-excitationwavelength was 350 nm. As it is evidentin this figure, two emission peaks containing a blue emission peak at 482 nmand a green emission peak at 559 nm observedfor all samples. Thus, the In-doped SnS thin films can be used as blue and/or greenlight emitters or other devices owing to these emission bands. Based on ourinvestigations, these peaks are assigned to a high density of sulfur and tinvacancies and various kinds of defects such as interstitials, stacking faults, etc.
38, 39.Liu et al. observed two peaks at 365 nm and 464 nm for SnO2nanoparticles that the peak at 464 nm is related to oxygen vacancies 40. The observed blue emission peak is similar to that reported in 38, 41, and the green emission peak is similar to that reported in 42. Due to PL detection limitations, the band-to-band emission (bandgap energy) of undoped- and In-doped SnS samples is not seen. As it is obvious inFig.
10, with an increase in In-dopant concentration in SnS lattice, ablue- or red shift has been seen comparedto undoped SnS film. In addition,compared to the undoped SnS thin film, the PL intensity of In-doped SnS samplesdecreased that it showed the crystalline quality of In-doped SnS is decreased in comparison with undoped SnSfilm. This observance is in good accordance with XRD patterns.
4. ConclusionSix deposited samples containing undoped SnS and In-doped SnS thin filmshave been prepared using an electrodeposition method on the FTO substrates. Theresults of XRD patterns clearly showedthat all of the deposited thin films were orthorhombic polycrystalline. In thisresearch, the line broadening investigations on un- and In-doped SnS thin filmshave been investigated. Therefore, the Scherrer’s method, modified forms of W-H (UDM) method,and the SSP method have been used toanalyze the line broadening of undoped- and In-doped SnS (with differentconcentration of In-dopant) samples. The results obtained by these methods showedthat an increase in In-dopant concentration in SnS lattice leads to a decreasein the crystallite size and an increase in the lattice strain. There were happened due to the variation in theeffective ionic radii of In3+ and Sn2+ ions.
Therefore, substitutionof In3+ for Sn2+ in the SnS lattice leads to createsmismatches in the SnS crystal lattice. This lattice mismatch is responsible forthe reduction of the crystalline quality and the increase in lattice strain. In addition, the result of TEM image confirms our obtained results.