Synthesis of SnS nanocrystals by the solvothermal decomposition of a single source precursor

SnS nanocrystals (NCs) were synthesized from bis(diethyldithiocarbamato) tin(II) in oleylamine at elevated temperature. High-resolution transmission electron microscopy (HRTEM) investigation and X-ray diffraction (XRD) analysis showed that the synthesized SnS particles are monocrystalline with an orthorhombic structure. The shape and size tunability of SnS NCs can be achieved by controlling the reaction temperature and time, and the nature of the stabilizing ligands. The comparison between experimental optical band gap values shows evidence of quantum confinement of SnS NCs. Prepared SnS NCs display strong absorption in the visible and near-infrared (NIR) spectral regions making them promising candidates for solar cell energy conversion.


Introduction
Among the extensively studied IV-VI semiconductor materials, SnS has attracted particular attention as a low-toxicity [1,2] photoconductor for the fabrication of photoelectric energy conversion and near-infrared (NIR) detector materials. Semiconductor SnS has an optical band gap value of 1.1 eV [3], a large optical absorption coefficient of >10 4 cm -1 [4][5][6] and a high photoelectric conversion efficiency (up to 25%) [7,8].
Conventional SnS synthetic techniques have been applied most often for the fabrication of bulk SnS films [4][5][6][8][9][10][11][12][13][14]. There have been a number of reports of syntheses of nanocrystalline SnS. SnS NCs have been prepared by the reaction of powdered tin with elemental sulfur in a parafilm oil [15] or diglyme [16], and by the solvothermal route using thiourea, thiocyanate, elemental sulfur as sulfur precursors and tin(II) chloride as the tin precursor [17][18][19][20][21]. A more versatile approach to the controlled colloidal synthesis of semiconductor NCs from single source precursors was recently developed for a range of II-VI and IV-VI semiconductor materials [22][23][24], demonstrating an efficient route to high quality, crystalline nanoparticles. A typical synthetic procedure involves the solvothermal decomposition of preformed single source precursors (metal alkyl xanthates, thiocarbamates and thiocarbonates) in a mixture of coordinating solvents at relatively low temperatures [24]. This particular method has great potential for the production of highquality SnS NCs with predetermined functionalities.
In this paper, for the first time, we describe a synthetic method for a preparation of SnS NCs from a single source precursor. The synthesis of SnS NCs from bis(diethyldithiocarbamato) tin(II) (Sn(Et 2 Dtc) 2 ) in oleylamine does not require the use of hazardous materials such as phosphines and volatile organometallic compounds. The crystalline SnS NCs prepared using this new procedure display strong optical absorption in the visible and NIR spectral regions making them very attractive for spectroscopic investigations and for incorporation into optical devices.

Synthesis
All synthetic steps were conducted inside a nitrogenfilled, dry glove box. Bis(diethyldithiocarbamato) tin(II) was synthesized using a procedure similar to that used elsewhere [13,14]. Typically, stock solutions of 0.379 g of SnCl 2 and 0.45 g of diethylammonium diethyldithiocarbamate were prepared in 6 ml of anhydrous methanol and purged with argon. With continued stirring, a solution of SnCl 2 was added dropwise to a solution of diethylammonium diethyldithiocarbamate under a stream of argon. White crystals of Sn(Et 2 Dtc) 2 were precipitated, isolated by centrifugation and washed twice with methanol. Resulted Sn(Et 2 Dtc) 2 crystals were dried under vacuum (0.32 g, 40%).
For the synthesis of SnS NCs, the mixture of 0.16 g Sn(Et 2 Dtc) 2 , 1 ml oleic acid and 6 ml of oleylamine contained in a 50 ml three neck flask (1) was degassed and purged by argon. The solution was heated at 45°C under an argon flow for about 10 min until Sn(Et 2 Dtc) 2 was completely dissolved. This mixture was injected under vigorous stirring and an argon flow into another flask (2) containing a hot (170°C or 205°C) solution of 5 ml of degassed oleylamine and 0.2 g of tetradecylphosphonic acid (TDPA). After the temperature decreased to about 150°C, resulting from the injection of the precursor, the solution was held for 30 s finally being removed from the reaction vessel with a glass syringe. To increase SnS particles size the reaction was allowed to continue for 3 h at 170°C. Then the solution was cooled down to room temperature. The resulting dark brown solution containing the SnS nanoparticles was purified to remove unreacted precursors. Purification process included repeated precipitation and dissolution of SnS NCs. In order to precipitate SnS NCs, an appropriate portion of anhydrous acetone was added to the product solution. After acetone treatment, a flocculate is obtained due to insolubility of SnS NCs in the short chain ketone and then separated by centrifugation. The retrieved flocculate precipitate containing the desired SnS NCs was redissolved in chloroform. The above purification steps were repeated twice. Finally, the purified SnS NCs were redispersed in chloroform.

Characterization
Powder XRD measurements were made using a Scintag X1 powder diffractometer. The samples for XRD analysis were prepared by dropping the solution of NCs onto a silicon substrate. HRTEM analysis was done using a Philips CM20 TEM operating at 200 kV. The samples for TEM investigation were prepared by dropping a solution of washed SnS NCs onto carbon coated copper grids. UV-VIS-NIR absorption spectra were measured at room temperature with a Cary 5000 UV-VIS-NIR spectrometer (Varian).

Results and discussion
Bis(diethyldithiocarbamato) tin(II) (Fig. 1) is a more desirable precursor for the synthesis of high quality semiconductor SnS NCs due to its low cost and low toxicity. As indicated by the thermoanalytical data [13,14], the substantially complete thermal decomposition of Sn(Et 2 Dtc) 2 with bulk SnS formation occurs at high temperatures (210-360°C) in a nitrogen atmosphere. Contrary to the thermal decomposition procedure, the solvothermal route gives an additional degree of control over the material particle size and size distribution [21,25]. SnS NCs have been synthesized by the solvothermal decomposition of single source precursor in a coordinating solvent at elevated temperature. Low-cost and controllable synthetic procedure is highly reproducible with repeated preparations of different batches of samples. This procedure is similar to others published by O'Brian et al. [22,23] for the synthesis of high quality semiconductor NCs (CdS, ZnS), where less toxic single-molecule organic complexes of heavy metals with dithiocarbamates and non-phosphine containing solvents are used. As it was indicated by Efrima et al. [24], Lewis base alkylamine solvents promote the decomposition reaction of metal alkyl xanthates, thiocarbamates and thiocarbonates at relatively low temperatures. Indeed, the heating of the reaction mixture without amines at elevated temperature did not result in SnS formation. By contrast, using hexadecylamine or oleylamine as a reaction solvent promotes Sn(Et 2 Dtc) 2 decomposition at temperature as low as 85°C. Alkylamines also act as a stabilizing agent for the formed particles permitting control of their size. In the work presented here, the SnS NCs are formed from Sn(Et 2 Dtc) 2 in an oleylamine/oleic acid mixture. The presence of oleic acid in the reaction mixture serves as a ligand and also plays a vital role in the formation of nanoscale tin sulfide by controlling the reactivity of precursors [26,27]. XRD analysis verified the formation of highly crystalline SnS NCs (Fig. 2). The reflections were indexed and assigned to SnS of orthorhombic structure with the lattice parameters a = 0.4328 nm, b = 0.1119 nm, and c = 0.3978 nm (JCPDS 39-354, Herzenbergite). Some small additional peaks from trace impurities were observed as well. The broadening of the XRD peaks is naturally associated with the formation of NCs. Representative TEM micrographs of SnS NCs, synthesized from the single source precursor at various conditions are shown in Fig. 3. The SnS NCs synthesized by this solvothermal procedure were polydisperse in size. A hierarchy of coagulated NCs could be explained by insufficient surface passivation, leading to aggregate formation [28]. The dimensions of SnS NCs were in a range of 5-200 nm depending on synthetic conditions. Fast nucleation and growth leaded to the formation small (5-10 nm) NCs (Fig. 3a, c), whereas prolonged heating caused big NCs to be grown (Fig. 3b). As may be seen clearly in the TEM images, the shape of the SnS NCs seems to be dependent on the nature of the stabilizing agents used in addition to the thermal conditions of the preparation. Isotropic and anisotropic growth of SnS NCs is achieved by use different capping molecules. As indicated in Fig. 3 (a-c), the chemical nature of stabilizing agents can significantly affect the surface energy of the different facets of growing SnS NCs, leading to the formation of rode-like (Fig. 3a), polygonal (Fig. 3b) or spherical morphologies (Fig. 3c, d) of semiconductor nanomaterials [28,29]. Addition of TDPA to the reaction mixture as a cosurfactant terminates anisotropic growth of SnS inducing the formation of spherical NCs (Fig. 3c, d). The morphology of nonspherical NCs depends more on the surface energies of the specific crystalline faces, whereas spherical morphology corresponds to the lowest surface energy for small NCs, which have large atomic surface/volume ratio [28]. Additionally, the existence of lattice planes on HRTEM images of these particles stretching through entire NCs (Fig. 3d) confirms the high crystallinity of the samples, even though the size distribution is broad. The representative optical absorption spectrum of sub-10 nm SnS NCs synthesized in oleylamine/oleic acid mixture at 170°C is shown in Fig. 4(a). The absorption coefficient for SnS nanoparticles a, was calculated from the average absorption index (A) as a ¼ 4pA=k [4]. The spectral behavior of the absorption coefficient as a function of energy, hv, is shown in Fig. 4(b). SnS NCs have high absorption coefficient >10 5 cm -1 in the wavelength range from 400 nm to 800 nm.
To determine the energy band gap, E g , and the type of optical transition responsible for this intense optical absorption, the absorption spectrum was analyzed using the equation for the near-edge absorption (Eq. 1) [30].
In Eq. 1, k and n are constants and E g is the band gap energy of the bulk semiconductor. The n value is 4 for indirect-gap materials. Values of the optical band gap for the samples were obtained by the extrapolation of the linear region of the plot of (ahv) 1/2 against photon energy (hv) as shown in Fig. 4(c). Clearly, the absorption corresponds to an indirect allowed transition with an energy gap of 1.6 eV for the nanocrystalline particles, higher than the literature value (1.1 eV) for bulk films of SnS [30,31]. Calculated the same way band gap value of sub-200 nm SnS particles synthesized by prolonged heating of Sn(Et 2 Dtc) 2 precursor is 1.06 eV which close to reported one for bulk SnS. Since this approach to band gap calculation is not particularly accurate for polydisperse solutions of nanoparticles, these reported bandgap values should be taken as approximate. The increased values of band gap for SnS NCs compared with the bulk material can be explained by quantum confinement of the carriers in semiconductor NCs [32]. When the size of the particles decreases, then quantum confinement leads to a size dependent enlargement of the band gap resulting in a blue shift in the absorbance onset [33], as observed in this work.
In conclusion, for the first time, SnS NCs on a sub-10 nm scale were synthesized from bis(diethyldithiocarbamato) tin(II) in oleylamine at elevated temperature. The shape and size tunability of SnS NCs can be achieved by controlling the reaction temperature and time, and the nature of stabilizing ligands. HRTEM investigation and XRD analysis showed that the synthesized SnS particles are monocrystalline with an orthorhombic structure. The synthesized,