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UV irradiated wet chemical deposition and characterization of nanostructured tin sulfide thin films

  • A. J. Ragina
  • K. V. Murali
  • K. C. Preetha
  • K. Deepa
  • T. L. Remadevi
Article

Abstract

Tin sulfide (Sn2S3) films were synthesized both by chemical bath deposition method without (CBD) and with simultaneous irradiation of UV light (UV–CBD). The influence of UV illumination on the synthesis of Sn2S3 thin films were investigated through the structural, compositional, morphological, optical and electrical studies. CBD films were Sn2S3 having orthorhombic structure with some impurities while that of UV–CBD films exhibit excellent crystallinity of pure single phase Sn2S3 having the same structure. Morphology of CBD films was spherical grains of different sizes while that of UV–CBD films was uniformly distributed thin long nanoworms. Optical properties of the films were different and the optical band gap for CBD and UV–CBD films was 1.20 and 1.57 eV respectively. Refractive index of the films lies in 1.84–2.02 in the 700–1,500 nm wavelength range. Electrical resistivity of the CBD and UV–CBD films was 104 and 103 Ωcm for respectively. UV irradiation during the synthesis of tin sulfide films had highly enhanced the properties demanded by various optoelectronic applications. UV–CBD is a novel, simple and cost effective approach, having the potential to stimulate new research in the study of tin sulfide thin films and other metal chalcogenides.

Keywords

Full Width Half Maximum Chemical Bath Deposition Thioacetamide SnS2 Metal Chalcogenide 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

1 Introduction

Tin sulfide belongs to groups IV–VI of compounds formed with tin as the cation and sulfur as the anion. The constituent elements are nontoxic and abundant in nature leading to the development of devices that are environmentally safe and public acceptance [1]. Tin sulfide is particularly important because of its interesting optical, transport properties and the consequent potential technological applications. Owing to the versatile coordinating ability of tin and sulfur, tin sulfide shows a variety of phases such as SnS, Sn2S3, Sn3S4, Sn4S5, and SnS2. The narrow band gap and the interesting structural property of Sn2S3 make it a potential candidate as a solar absorber in thin film solar cells and in applications like near-infrared detector. Thin films of tin sulfide have shown conversion efficiency in photovoltaic devices similar to those found in silicon films [2]. Among the different phases, Sn2S3 crystallizes with an orthorhombic crystal structure. The optical band gap of Sn2S3 films varies from 0.95 to 2.2 eV [3, 4]. Sn2S3 is a suitable material for preparing near-lattice-matched hetero-junctions used in the detection and generation of infrared radiation [5].

Thin films of tin sulfide have been deposited using different techniques such as vacuum evaporation [6], spray pyrolysis [3, 7, 8], electro-deposition [9, 10], SILAR method, [11] atmospheric pressure CVD [12, 13] and chemical bath deposition [1, 14, 15]. Every technique of thin film deposition has its own merits and demerits but the one, which is economically feasible for large area deposition and capable of producing uniform and well adherent thin films with single phase and equal distribution of grain size, is of interest to researchers. Among these methods, chemical bath deposition (CBD) which is also known as solution growth deposition has emerged as the feasible method to grow high quality layers over large areas of metal chalcogenide thin films [14, 15, 16, 17].

The tin sulfide films in the present study were synthesized both by CBD without and with simultaneous irradiation of ultra-violet (UV) light from photochemical deposition (PCD) apparatus. PCD set-up consists of a high-pressure mercury lamp with a provision for UV light illumination. In PCD, the reaction is activated by the illumination of UV light and the film is deposited on the immersed substrate [18]. Synthesis of patch free, uniform and crystalline tin sulfide film preparation using CBD technique alone is still a challenging task. Therefore, the CBD reaction is activated by the illumination of UV light and the film is deposited on the immersed substrate. This technique of UV assisted chemical reaction is efficient to break molecular bonds, generate new species and induce its rapid deposition in the illuminated area [19].

The main aim of this study was to investigate the influence of UV illumination on tin sulfide thin films through observing the changes in structural, morphological and optical properties of tin sulfide thin films. The results were discussed and compared to that of the samples prepared from CBD alone. To the best of our knowledge, such an investigation on the effect of UV-irradiation on the tin sulfide thin film preparation by CBD is rare in the literature. The introduced UV–CBD method could remove the difficulty in realizing high quality tin sulfide thin films at room temperature in a single step. This approach of clubbing wet chemical synthesis and UV irradiation is a cost effective, simple and novel idea for synthesizing highly reproducible tin sulfide films.

2 Materials and methods

All chemical reagents were AR grade (from Merck) and used as received. Tin sulfide thin films were deposited both by typical CBD and by UV light irradiated CBD (UV–CBD). The typical CBD method has been extensively used to synthesize tin sulfide films [14, 15, 20, 21]. UV–CBD method is the fabrication of thin films on glass substrates by chemical bath deposition with simultaneous UV light illumination using a PCD set-up. The used PCD set-up consists of a high-pressure UV light (180 W) for illumination. The glass container (5 cm diameter) containing the growth solution was kept in a thin walled glass water bath, which was stirred constantly during the deposition. The temperature of the chemical solution was thus maintained at room temperature during the illumination. UV light source was arranged 8 cm away from the substrate to fall light normally on the substrate after passing through the growth solution. The schematic representation of the experimental set-up of UV–CBD is shown in Fig. 1.
Fig. 1

Schematic model of experimental set up. a CBD, b UV–CBD

The growth solution was prepared by dissolving 1.4 g of tin chloride (SnCl2·2H2O) in 10 ml acetone, followed by the sequential addition of 30 ml triethanolamine (3.7 M) (TEA), 10 ml thioacetamide (1 M) and 50 ml ammonia (4 M) solution to complete a volume of 100 ml. Desired concentrations of TEA, thioacetamide and ammonia were initially prepared by using the respective chemical and distilled water. The reaction mixture was magnetically stirred. Substrates were pre-cleaned using detergent solution, chromic acid and distilled water sequentially before the deposition of the films. CBD thin films were prepared by keeping the substrates inclined to the vertical in half of the reaction mixture taken in a beaker kept at room temperature for 24 h.

UV–CBD films were deposited by a two-step procedure using the other half of the reaction mixture taken in a glass container. The pre-cleaned glass substrate was immersed vertically in the solution as shown in Fig. 1. At first, this solution was irradiated for 3 h using UV light from the PCD set-up. The irradiation time was optimized to 3 h after a series of experimentation. UV irradiation for less than 3 h duration created non-uniform coating while more than 3 h generated patches in the film. Secondly, after the irradiation, the substrate was left in the beaker and was kept at room temperature for further 24 h. Both the films were dried by blowing hot air before characterization. Film from the CBD bath was named as CBD-SNS and that from the UV–CBD bath was named as UV–CBD–SNS.

The as-prepared films were characterized by X-ray diffraction (XRD) recorded by Bruker AXS-8 advance X-ray diffractometer using X-ray source Cu-Kα wavelength 1.5406 Å (operated at 40 kV and 35 mA) and scanning electron microscope (SEM) photographs taken by JEOL Model JSM6490 operated at voltage of 20 kV for their surface morphology. Chemical elemental stoichiometry was also examined from energy dispersive X-ray analysis (EDAX) linked with the SEM unit operated at voltage of 20 kV. Thickness of the films was determined by gravimetric method using an analytical balance of readability 0.1 mg. The optical properties of films were investigated by using Hitachi-U-3410 UV–Vis-NIR spectrophotometer in the 250–2,000 nm wavelength range. Keithley source measure unit (Model SMU Keithley 2400) was used for electrical studies.

3 Results and discussion

3.1 Deposition mechanism

In this work, SnCl2·2H2O was dissolved in acetone instead of water. Water was not used as the solvent to dissolve SnCl2·2H2O in order to prevent the formation of the insoluble basic salt Sn (OH) Cl in the solution [17, 22, 23]. SnCl2·2H2O dissolved in acetone forms a transparent solution. TEA was added in the solution to develop tin complexes. Like in aqueous solution, tin (II) exists as [Sn(TEA)k]2+ complex ions with TEA. Aqueous ammonia precipitates white Sn (OH)2 with tin(II). However, the precipitate of tin (II) hydroxide dissolves in excess aqueous ammonia. So the use of adequate amount of ammonia prevents the formation of Sn(OH)2 in the solution which might have reduced the concentration of Sn2+ ions in solution. Moreover, ammonia used in the bath prevents the formation of powdery film in chemical bath deposition [24]. Thioacetamide is an organosulfur compound. This white crystalline solid is nicely soluble in water and serves as a source of sulfide ions. Therefore, treatment of tin cations to a solution of thioacetamide generates tin sulfide.

UV light is used to initiate or accelerate the chemical reaction by providing an alternative pathway or mechanism with a lower activation energy. Interference of molecular pathways is the key to govern the chemical reactions by using photons with adequate energy [25, 26, 27, 28]. However, it does not change the conditions of equilibrium of the reaction but plays a major role in controlling the outcomes of chemical reactions. By delivering a range of energies, photons can induce vibrational or rotational motion in a molecule, which in turn affects the way it interacts with other photons and with other chemical constituents in the solution. It is also important to note that the photon is not used in the reaction but rather supplies a part of energy. The dissociation of tin complex ion into tin ion may be a slow process at room temperature. However, on interaction with photons from UV light, rate of dissociation of tin complexes and the generation of S2− ions increase and there will be sufficient amount of Sn2+ ions and S2− ions available in the solution, which promotes deposition of film on the substrate.

The overall chemical reaction processes [14, 29] may be described as follows
$$ \left[ {{\text{Sn }}\left( {\text{TEA}} \right){\text{ k}}} \right]^{2+} \to {\text{ Sn}}^{2+} + {\text{k }}({\text{TEA}}) $$
(1)
$$ {\text{CH}}_{ 3} {\hbox{-}} {\text{CS}} {\hbox{-}} {\text{NH}}_{ 2} + {\text{ H}}_{ 2} {\text{O}} \to {\text{ CH}}_{ 3} {\hbox{-}} {\text{C}} {\hbox{-}} {\text{NH}}^{ + } + {\text{ H}}_{ 3} {\text{O}}^{ + } + {\text{S}}^{ 2- } $$
(2)
$$ 2 {\text{Sn}}^{2+} + {\text{ 2S}}^{ 2- } \to {\text{Sn}}_{ 2} {\text{S}}_{ 2} $$
(3)
$$ {\text{Sn}}_{ 2} {\text{S}}_{ 2}^{{}} + {\text{ 2S}}^{{}} \to {\text{ 2SnS}}_{ 2} $$
(4)
$$ 2 {\text{SnS}}_{ 2} \to {\text{Sn}}_{ 2} {\text{S}}_{ 3} + {\text{ S}} $$
(5)

3.2 Structural characterization

Tin sulfide thin films were characterized by X-ray diffraction analysis. Figure 2 shows the XRD patterns of the tin sulfide thin films synthesized by CBD technique. The peak observed at 31.82° was found to be the strongest for films prepared by CBD in this work. The corresponding ‘d’ value was found to be 2.809 Å which is identical to the (211) orientation of Sn2S3 phase with orthorhombic structure. Small intensity peaks corresponding to reflections from the planes (220) and (111) planes are also observed in the XRD patterns of the CBD films. The X-ray diffraction pattern for films synthesized by UV–CBD is shown in Fig. 3. Similar to the CBD film, the XRD pattern contains the only one peak at 2θ = 31.91° having the d-value of 2.802 Å, corresponding to the orientation of (211) phase of Sn2S3 with orthorhombic structure. Peak position and corresponding ‘d’ values of both films were compared with the standard JCPCDS Card No. 72-0031, which confirmed the formation of Sn2S3 films from both the baths. The observed ‘d’ and ‘2θ’ values of both films corresponding to the high intensity reflection from the plane (211) along with the standard values were given in Table 1.
Fig. 2

XRD patterns of the tin sulfide thin films synthesized by CBD

Fig. 3

XRD patterns of the tin sulfide thin films synthesized by UV–CBD

Table 1

Structural parameters and grain size of Sn2S3 films

Sample

Observed, d

Standard, d

Observed, 2θ

Standard, 2θ

Plane

Structure

Grain size (nm)

CBD–SNS

2.8099

2.8036

31.820

31.894

211

Orthorhombic

12

UV–CBD–SNS

2.8020

2.8036

31.912

31.894

211

Orthorhombic

10

The XRD patterns of CBD and UV–CBD films have considerable differences and exposed important facts. Structure of the CBD and UV–CBD films in this work is similar to that of the Sn2S3 films reported by Khadraoui et al. [5] using the spray pyrolysis method. However, the CBD films have a low degree of crystallinity as indicated by the low intensity peaks in Fig. 2. CBD films exhibit crystallite orientations along the planes (211), (220) and (111) with preferential orientation along the (211) plane. The overlapped peaks in the XRD patterns may be due to the occurrence of the small crystallites consist in the film. On the other hand, UV–CBD films exhibit excellent crystallinity with a single high intensity peak showing crystallite orientation along the (211) plane only. UV–CBD films have grain size slightly less than that obtained in the case of CBD films as depicted in Table 1. X-ray diffraction studies confirmed that both films were comprised of nanostructured grains.

The mean crystallite sizes of tin sulfide films were determined using the full width half maximum (FWHM) of the (211) peak using the Debye–Scherrer’s equation,
$$ D = \frac{A\lambda }{\beta \cos \theta } $$
(6)
where ‘A’ is the shape factor which is 0.94, ‘λ’ is the wavelength of X-rays which is 1.5406 nm for CuKα, ‘β’ is the full width half maximum of diffraction peak measured in radians and ‘θ’ is the Bragg’s angle. The grain size of the films is depicted in Table 1.
The line shifting of observed XRD patterns from that of the standard patterns indicate the strain developed in the films during synthesis. Micro-strain in the films in the direction normal to the diffracting plane was computed by using Eq. (7) [30].
$$ \mu = \frac{{d_{o} - d_{s} }}{{d_{s} }} $$
(7)
where ‘d S ’ and ‘d O ’ denotes inter planar spacing of the standard and the observed value of unstrained and strained samples respectively. Microstrain in the (211) plane oriented nano-crystals of the films is shown in Fig. 4. Even though strain developed is small in both cases it can be noted that strain is positive in the case of CBD films whereas it is negative and negligible for UV–CBD films.
Fig. 4

Microstrain in CBD and UV–CBD tin sulfide films

3.3 Composition analysis

The elemental composition of the Sn2S3 films prepared by both methods was evaluated. Sn/S ratio of CBD films exhibited sulfur deficiency and the evaluated value is 0.81. However, films prepared by UV–CBD were nearly stoichiometric having a Sn/S ratio of 0.61. The deficiency of sulfur in the CBD films grown at room temperature may be due to the lack of sulfur atoms released in the solution at room temperature to react with tin. This excess of tin present in the film layers might react with atmospheric oxygen to form tin-oxy-sulfides [31]. This was creating an amorphous nature to the CBD films as evident from the XRD analysis discussed in Sect. 3.2, while when UV irradiation was carried out to prepare the films, near stoichiometry (with some excess sulfur) was achieved together with crystallinity without the presence of tin-oxy-sulfides.

3.4 Film thickness

Thickness of the films was determined by gravimetric method. The film thickness (t) was evaluated using Eq. (8).
$$ t = \frac{m}{dA} $$
(8)
where ‘m’ is the mass of the film deposited on a known area ‘A’ and ‘d’ being the bulk density of the material tin sulfide. The bulk density of the material was taken as 5.22 × 103 kg/m3. The thickness of the films synthesized by CBD and UV–CBD obtained was 680 and 730 nm respectively.

3.5 Film morphology

Figures 5a, b and 6a, b show the SEM images of tin sulfide thin films synthesized by CBD and UV–CBD techniques respectively. The SEM images show that the films synthesized by CBD have spherical grains of different sizes. It seems that each grain is comprised of small particles. This gives the films a powdery appearance. The small and big grains are evenly distributed on the substrate surface.
Fig. 5

SEM images of CBD tin sulfide films

Fig. 6

SEM images of UV–CBD tin sulfide films

SEM image of UV–CBD films exhibit uniformly distributed thin but lengthy nanoworms. These worms are closely packed and randomly ordered giving good continuity to the film surface. The average thickness of a worm is approximately 65 nm. Moreover, the powdery appearance as observed in CBD films was not present in UV–CBD films. This type of morphology can provide greater surface area for reaction, when used for any surface related application like gas sensors. The observed closed packed morphology helped in decreasing the resistivity of UV–CBD films by one order than that of the CBD films which is discussed in Sect. 3.7. Hence, UV irradiation during the synthesis of tin sulfide films has tremendous effect on the evolution of surface morphology and the consequent transport properties.

3.6 Optical properties

The CBD–SNS and UV–CBD–SNS films appeared brown in color. Optical properties of the tin sulfide films prepared by UV–CBD method has significant differences with the films synthesized by CBD technique. Figure 7 shows the absorption spectra of tin sulfide thin film synthesized using CBD and UV–CBD techniques. Both the films show very high absorption in the UV region. In the visible and near-infrared region, absorption of UV–CBD films is comparatively lower than that of the CBD films. Even though both the films were synthesized using the same chemical constituents and the films obtained were of the same phase of tin sulfide Sn2S3, a relative blue shifting can be observed in the case of UV–CBD films. This can be attributed to the effect of UV radiation during the synthesis.
Fig. 7

Absorption spectra of CBD and UV–CBD tin sulfide films

Figure 8 shows the optical transmission spectra of CBD and UV–CBD films. Likewise, in absorption spectra, transmission spectra of both the films show considerable differences. Transmission is low in CBD films and is below 60 % in the entire visible-NIR region. For UV–CBD films, it is continuously increasing from 500 nm and it is above 60 % beyond 700 nm wavelength in the entire region. This smooth increase is due to the high crystalline nature of the films. In the 900–1,200 nm region, transmission of the films is nearly 100 %. It is evident from the spectra that the transmission of the tin sulfide thin films strongly depends on the mode of preparation.
Fig. 8

Transmission spectra of CBD and UV–CBD tin sulfide films

The optical band gap energy E g , was evaluated using Eq. (9) [32, 33] for the allowed direct transition in a crystal.
$$ \alpha = \frac{{A\left( {h\nu - E_{g} } \right)}}{h\nu }^{\frac{1}{2}} $$
(9)
where ‘α’ is the absorption coefficient, is the photon energy and ‘A’ is a constant depending on the material properties. The equation gives the band gap ‘E g ,’ when linear portion of (αhν)2 against the photon energy () plot is extrapolated at α = 0.
Figure 9 shows a plot of (αhν)2 versus hν curves of films prepared by CBD and UV–CBD which is linear at the absorption edge, confirming the direct band gap material. The optical band gap energy ‘E g ’ determined was 1.20 eV for films prepared from CBD technique. This value agrees with the reported values for orthorhombic Sn2S3 thin film [8]. The UV–CBD tin sulfide films possess an optical band gap of 1.57 eV, which is slightly greater than the reported values. This broadening of band gap may be due to the quantum size effect induced by very small grains in the films [34, 35, 36] and also due to the presence of some excess sulfur in the films as discussed in Sect. 3.3 [37].
Fig. 9

Plot of (αhν)2 versus hν of tin sulfide films

Figure 10 shows the optical reflectance spectra of CBD and UV–CBD films. Films show significant reflectance only beyond 700 nm wavelength. For CBD–SNS films, reflectance gradually increased to 20 % and beyond 800 nm it stands steady. But for UV–CBD–SNS films, reflectance is decreasing from 20 % value and reaches nearly to zero at 1,000 nm and very gradually increases to 5 % and seems to be steady beyond 1,400 nm wavelength.
Fig. 10

Reflectance spectra of CBD and UV–CBD tin sulfide films

Refractive index, n, of the films was determined from the extinction coefficient ‘k’ and the reflectance ‘R’ using Eqs. (10) and (11) [38].
$$ k = \frac{\alpha \lambda }{4\pi } $$
(10)
$$ n = \frac{{(1 + R) + \sqrt {4R - (1 - R^{2} )k^{2} } }}{{(1 - R)}} $$
(11)
The plot of refractive index as a function of wavelength of the CBD and UV–CBD films is shown in Fig. 11. The refractive index spectra of the films have a different nature. The values of ‘n’ obtained for the films lie in the range 1.84–2.02 in the 700–1,500 nm wavelength range. The values of refractive index obtained in this work agree with the reported values for the Sn2S3 films [5]. Refractive index of CBD–SNS films show an increase from 1.85 to 2.02 with increase in wavelength and become steady in the entire wavelength region. Whereas UV–CBD–SNS films show a decrease from 2.02 to 1.84 and then increases with increase in wavelength and appears steady beyond 1,400 nm wavelength. Hence, it is clear that the UV–CBD method used gives different results compared to the CBD method.
Fig. 11

Variation of refractive index of CBD and UV–CBD tin sulfide films

3.7 Electrical properties

The electrical behavior of films was examined through the electrical resistivity using source measuring unit and carrier type by hot probe method. Measurements were carried out by dc two-point probe method using silver electrode contacts. Hot probe studies indicated p-type conductivity of films and the electrical resistivity of the films were 104 and 103 Ωcm for the CBD and UV–CBD films respectively. The high resistivity of Sn2S3 phase films were reported earlier [39]. The relative low resistivity of UV–CBD films achieved was as a matter of UV-irradiation. This fact is well supported by the compact and uniform surface morphology of these films discussed in Sect. 3.5.

4 Conclusion

Nanostructured Sn2S3 thin films have been synthesized on glass substrates by CBD method without and with simultaneous irradiation of UV light. The UV–CBD technique used was simple and economic and it requires less monitoring. Influence of UV irradiation during the synthesis on the structure, morphology, optical and the electrical properties of the films was demonstrated. The XRD studies confirmed the formation of crystalline, pure, and single-phase Sn2S3 films having orthorhombic structure when the wet chemical synthesis was coupled with UV irradiation. The SEM studies revealed the formation of films with different morphology. UV–CBD films have low absorption and high transmission compared to the CBD films. Optical band gap for CBD and UV–CBD films was 1.20 and 1.57 eV respectively. Refractive index of the films lies in 1.84–2.02 in the 700–1,500 nm wavelength range. Electrical resistivity of the UV–CBD films was one order less than that of the CBD films. UV irradiation during the synthesis had highly enhanced the properties that demanded by various optoelectronic applications and gas sensors. This work provides a novel approach to stimulate new research in the study of metal chalcogenide films.

Notes

Acknowledgments

The authors would like to express their sincere thanks to the authorities of Nehru Arts & Science College, Kanhangad, Kerala for offering facilities and STIC, Cochin for their technical support.

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Copyright information

© Springer Science+Business Media, LLC 2012

Authors and Affiliations

  • A. J. Ragina
    • 1
    • 2
  • K. V. Murali
    • 1
    • 2
  • K. C. Preetha
    • 1
    • 3
  • K. Deepa
    • 1
    • 4
  • T. L. Remadevi
    • 1
    • 4
  1. 1.Department of Physics, School of Pure and Applied PhysicsKannur UniversityKannurIndia
  2. 2.Department of PhysicsNehru Arts and Science CollegeKanhangadIndia
  3. 3.Department of PhysicsSree Narayana CollegeKannurIndia
  4. 4.Department of PhysicsPazhassi Raja N.S.S. CollegeMattannurIndia

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