Introduction

Tin sulfides (SnS2, Sn2S3, and SnS) are members of the IV–VI family of semiconductors that have shown promise in photovoltaic and optoelectronic applications [1–4]. Tin(II) sulfide (SnS) in particular has been seen as a potential candidate as an absorber layer in photovoltaic cells due to its 1.4 eV direct band gap that can harvest the visible and near-IR regions of the EM spectrum, the lower costs and toxicity of the constituent elements as compared to other potential materials (e.g., PbS and CdS), and the simplicity of the binary system compared to multicomponent materials such as copper zinc tin sulfide (CZTS) and copper indium gallium sulfide (CIGS) [5–8].

Single-source precursors (SSPs) are compounds that are designed to decompose to materials of specific compositions, by containing the desired elements. In many cases, the uses of SSPs have granted control of both its physical and optical properties that dual-source precursors cannot [9–11]. In the last 20 years [12–18], many SSPs comprised metal (N,N-dialkyldithiocarbamates) [M(S2CNR2)n] have been used to synthesize metal sulfide nanocrystals. More recently, complexes containing (O-alkyl)xanthate (−S2COAk) ligands have been viewed as a potentially useful class of SSPs for the production of metal sulfide nanomaterials. The decomposition of metallo-organic xanthates is known to take place via the relatively low-temperature and clean Chugaev elimination reaction [19]. The use of the xanthate ligand in SSPs has permitted the formation of many metal sulfides, including, but not limited to, MoS2 [20], CdS [21], NiS, PdS [22], and CZTS [23], at lower temperatures than those needed by their respective (N,N-dialklydithiocarbamato-) analogs. Recently, we have reported the preparations of PbS/polymer composites from both lead(II)xanthate and lead(II)dithiocarbamate complexes by a melt process [18, 24], finding that the decomposition of Pb(S2COnBu)2 in a polymer matrix produced pure cubic PbS nanocrystals at 150 °C; significantly lower temperatures than 275 °C are needed to decompose Pb(S2CNnBu2)2. As a result, the xanthate-containing SSP can be used in a wider temperature window, giving greater control over nanocrystal size, shape variation, and orientation preference of the PbS crystals. Among the other known methods to SnS nanomaterials [18, 25], explorations of the syntheses of orthorhombic SnS nanoparticles [26–28] and films [13, 29, 30], using Sn-SSPs such as [SnII(S2CNR2)2] and [R′2SnIV(S2CNR2)2], have been reported. To date, however, no studies on the uses of tin(O-alkylxanthate) complexes have been documented.

In this report, we investigate the use of the SSP [Sn(S2COEt)2] as a coating material for the production of herzenbergite SnS films on glass. We focus on both the annealing temperature and the role of the xanthate ligand during the decomposition process for the potential in controlling the structural and optoelectronic properties of the SnS films produced.

Experimental

Materials and methods

Potassium ethyl xanthate, chloroform and tetrahydrofuran were purchased from Sigma-Aldrich. Tin(II) chloride was purchased from Alfa Aesar. All chemicals were used as received. Elemental (EA) and thermogravimetric (TGA) analyses were carried out by the Microelemental Analysis service at University of Manchester. EA was performed using a Flash 2000 Thermo Scientific elemental analyzer and TGA data obtained with Mettler Toledo TGA/DSC1 stare system between the ranges of 30–600 °C at a heating rate of 10 °C min−1 under nitrogen flow. Scanning electron microscopy (SEM) analysis was performed using a Philips XL30 FEG microscope, with energy-dispersive X-ray spectroscopy (EDX) data obtained using a DX4 instrument. Thin-film X-ray diffraction (XRD) analyses were carried out using an X-Pert diffractometer with a Cu-K α1 source (λ = 1.54059 Å), the samples were scanned between 20° and 75°, the applied voltage was 40 kV, and the current was 30 mA. Raman spectra were measured using a Renishaw 1000 Micro-Raman System equipped with a 514 nm laser. UV–Vis measurements were made using a Shimadzu UV-1800 spectrophotometer.

Synthesis of tin(II)(O-ethylxanthate)

[Sn(S2COEt)2] was prepared by a procedure that was modified for that described in literature [19, 31]. An aqueous solution of potassium ethylxanthate (10.0 g, 12.5 mmol) was added to a stirred solution of tin(II) chloride (5.9 g, 6.2 mmol) in distilled water (100 ml) and stirred for a further 30 min. The yellow precipitate produced was filtered by vacuum filtration, washed three times with water, and finally dried in a vacuum oven at room temperature for 2 h. Yield = 7.2 g (67 %). Melting point = 44–53 °C. Anal. Calcd for [Sn(S2COEt)2]: C, 19.98; H, 2.79; S, 35.45; Sn, 32.91 Found: C, 19.67; H, 2.74; S, 35.45; Sn, 32.17. FTIR data (cm−1); 2986.8 (w), 2930.7 (w), 1457 (w), 1355 (w), 1195.6 (s), 1108.1(s), 1020.6 (s), 852.0(w), 801.3(w), 563.4 (w).

Preparation SnS thin films by spin coating and heating

Glass slides were cut to 20 mm × 15 mm, cleaned by sonication in acetone (twice) and water, and allowed to dry. Three cycles of coating was performed; in each cycle, 300 μL of a 3 M [Sn(S2COEt)2] solution in THF was coated onto the glass slide by spin coating at 700 rpm for 60 s and allowed to dry. The resulting films were loaded into a glass tube for decomposition in a dry nitrogen environment. The tube was then heated in the furnace to the desired temperature (150–400 °C) at a rate of ∼3 °C min−1 and held at that temperature for 60 min; after this time had elapsed, the furnace was turned off and the tube allowed to cool to room temperature.

Results and discussion

The tin xanthate precursor [Sn(S2COEt)2] was synthesized by the literature procedure [19, 31] and elemental analyses confirming its purity. The yellow powder is readily soluble in THF and many other common organic solvents. It was found that storage at −20 °C was necessary to limit decomposition. The thermal decomposition of [Sn(S2COEt)2] was studied using thermogravimetric analysis (TGA). The thermogram showed rapid single decomposition step between 80 and 130 °C (Fig. S1). The final weight of the residue (44.1 %) is close to the predicted value for residual SnS (41.8 %). The precursor is predicted to break down via the Chugaev elimination mechanism [19], as shown in Scheme S1. The IR spectrum of [Sn(S2COEt)2] shows bands corresponding to ν(C–O) (1196 and 1224 cm−1) and ν(C–S) (1021 and 1108 cm−1). A peak at 563 cm−1 is also observed consistent with a ν(Sn–S) mode (Fig. S2).

SnS films were prepared by coating glass slides with the [Sn(S2COEt)2] precursor, followed by heating step in an N2 environment, at 150, 200, 250, 300 and 400 °C for 1 h. The resulting films were gray and uniform at all the heating temperatures (Fig. S3); the films were found to be between 2.2 and 2.9 μm thick. Both the morphology and composition of the SnS films produced showed a dependence on the heating temperature. The films contained nearly spherical structures with some flakes (Fig. 1a, b, c, d). Elemental maps for the film produced at all temperatures (elemental maps for 300 °C shown in Fig. 1e, f; maps for the other films in Fig. S4) demonstrate the uniform distribution of these elements among the films. The Sn/S ratio within the films steadily decreased when higher reaction temperatures were used (Table 1; Fig. 2): Sn/S ratios of 1.31 and 1.32 (Sn1.31S and Sn1.32S) were seen in the films produced at 150 and 200 °C (sulfur deficient). Increasing the heating temperature also increased the sulfur content in the film, reactions performed at 250 and 300 °C produced SnS films with reduced sulfur deficiency (Sn/S ratios of 1.15:1 (Sn1.15S) and 1.11:1 (Sn1.11S), respectively), and a stoichiometric SnS film was obtained upon heating at 400 °C (Sn/S ratio 1.03:1; (Sn1.03S)). Such control of the stoichiometry in SnS nanomaterials has previously been observed [32, 33].

Figure 1
figure 1

SEM images of SnS films grown on glass substrates from [Sn(S2COEt)2] at a 200 °C, b 250 °C, c 300 °C, and d 400 °C (scale bars represent 1 μm); e and f the elemental maps of the SnS film produced at 300 °C (scale bars represent 10 μm)

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Table 1 Thicknesses, compositions, and unit cell parameters of the SnS films produced by the melt method
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Figure 2
figure 2

The Sn/S ratio by EDX for samples heated for 60 min at temperatures between 150 and 400 °C

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The p-XRD patterns of the SnS thin films produced at temperatures between 200 and 400 °C gave peaks that can be indexed to herzenbergite-SnS with the expected orthorhombic crystal structure (matches ICCD pattern No. 00-039-0354; see Fig. 3a); no other peaks are observed that correspond to other tin oxide or sulfide species. The calculated unit cell parameters of the herzenbergite SnS films (shown in Table 1) match the expected (Pbnm) space group, with lattice parameters that closely match with those reported in literature [34, 35]. The films heated at 400 °C have strong, well-defined diffraction peaks. At lower temperatures, however, the films were found to exhibit broader peaks, possibly due to the increasing sulfur deficiency within the crystalline film. The temperature during [Sn(S2COEt)2] decomposition seems crucial; at lower temperatures, the rate at which the precursor decomposes will be slowed considerably, with the resulting intermediate species exposed to temperatures that may promote evaporation. In contrast, the Sn1.31S film produced at 150 °C was found to consist of a mixture of orthorhombic and cubic phases, with the latter phase matching well with a rock-salt SnS phase (space group Fm-3 m; ICCD pattern No. 04-004-8426) recently discussed by first-principle calculations [36, 37]. In addition, Raman spectroscopy of all of the SnS films (Fig. 3b) revealed Raman bands at 94, 160, 188, and 218 cm−1, in good agreement with the herzenbergite SnS phase reported previously [38–40].

Figure 3
figure 3

a p-XRD patterns of SnS films grown on glass substrate at different temperatures, accompanied by reference patterns of herzenbergite SnS (ICCD pattern No. 00-039-0354) and rock-salt SnS (ICCD pattern No. 04-004-8426). b Raman spectra for the SnS films grown on glass substrates from [Sn(S2COEt)2] at 150, 200, 250, 300, and 400 °C

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The optical band gaps (Eg) of the SnS films (produced at temperatures between 200 and 400 °C) were determined from optical absorption measurements by the Tauc method (Fig. 4a) [41, 42]. All of the films analyzed have high absorption coefficients (α > 104 cm−1 above the fundamental absorption). The band gaps were evaluated by extending linear part of the plots of (αhν)2 versus hν [43]. The band gap of the Sn1.32S films formed at 200 °C is 1.88 eV. Increased decomposition temperatures gave lower band gaps; 250 (Sn1.15S), 300 (Sn1.11S), and 400 °C (Sn1.03S) gave band gaps at 1.75, 1.49, and 1.26 eV, respectively. It is clear that the control of the stoichiometry that we have achieved in the syntheses allows for tuning of optical band gaps. The band gaps of SnS films previously reported [44–48] show similar change with sulfur deficiency (Fig. 4b). However, care needs to be taken when comparing the results obtained from literature to our dataset, as the films produced by each citation varies from both our work and each other. These experimental variations will introduce variations in the macrostructures, elemental stoichiometries, film thicknesses, and concentrations of Sn x S y -based impurities contained within the SnS films documented. The physical properties of the materials and the potential of quantum confined materials may also need to be considered.

Figure 4
figure 4

a UV–Vis and Tauc plots (inset) for the SnS films grown on glass substrates from [Sn(S2COEt)2] at 200, 250, 300, and 400 °C. b Graph showing the relationship between the Sn/S ratio of SnS1−x materials with its measured band gap. Data in black represents the findings in this report, whereas data in red were obtained from literature

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Conclusions

A simple process has been described for the growth of SnS films. Heating of substrates spin-coated with tin(II)O-ethylxanthate at different temperatures between 150 and 400 °C produced SnS films mainly in orthorhombic phase with good crystallinity. Analyses of the films reveal them to be sulfur deficient with Sn/S ratio controlled by selecting the heating temperatures. In addition, a correlation was found between the optical band gap of the SnS films (as determined by UV–Vis spectroscopy) and the stoichiometry. The measured optical band gaps were lowered from 1.88 to 1.26 eV. We believe that the process described in this report could be used in the production of SnS films with tuneable band gaps for solar cell applications.