## 3 Results  
### Synthesis and Characterization of CaS nanoclusters  
The molecular level heating associated with the use of microwave radiation (MW) offers the unique advantage to stop the decomposition of DMSO and turn \"on\" and \"off\" the supply of sulfide necessary for metal-sulfide synthesis [27]. The approach offers the unique advantage to stop the supply of sulfide ions at different stages of the chemical processes allowing to halt the nucleation and growth processes and control cluster or particle size. In the case of CaS, the process can be described according to the following process:  
\[\mathrm{nCa^{2+\frac{DMS}{MW}}\left(CaS\right)_{n}} \tag{1}\]  
with \(n=1\) (monomers), 2, (dimers) and so on, until nanostructures are formed. Exposure of diluted solutions of Ca(CH\({}_{3}\)CO\({}_{2}\))\({}_{2}\) in DMSO to microwave radiation for short periods of time results in the formation of CaS monomers and clusters, as revealed by the lack of absorption bands due to phonon-coupled electronic transitions from the valence to the conduction band. Solutions exposed to microwave heating with initial Ca(CH\({}_{3}\)CO\({}_{2}\))\({}_{2}\) toDMSO ratios larger than 10\({}^{-3}\) result in the formation of CaS nanostructures, as evidenced by the well-defined structure that results from the phonon-coupled valence to conduction band electronic transitions [29; 30]. Representative UV visible absorption spectra of the dispersions employed in the experiments described here are displayed in Figure 1. The absorption spectrum of the dispersion with a 2.0 \(\times\) 10\({}^{-4}\) Ca\({}^{2+}\)-to-DMSO ratio has a well-defined band below 300 nm and well-defined features at 320, 387, and 415 nm. The absorption spectrum of CaS obtained from the dispersion with a 6.0 \(\times\) 10\({}^{-4}\) Ca(CH\({}_{3}\)CO\({}_{2}\))\({}_{2}\)-to-DMSO mole ratio, on the other hand, has a well-defined band that extends to 600 nm. These observations are consistent with the absorption of light by CaS reported in the literature. The optical absorption of CaS monomers in the gas phase has transitions at 300, 500, and 1200 nm, while only transitions in the UV are predicted in dispersions of the monomers in DMSO [29; 31; 32]. Fluorescence measurements, on the other hand, of these dispersions reveal an emission band centered at 497 nm upon excitation with 405 nm [28; 29; 33; 34]. The emission spectra, illustrated in Figure 1b, are identical to the (a) one obtained when the solutions of Na\({}_{2}\)S and Ca(CH\({}_{3}\)CO\({}_{2}\))\({}_{2}\) in DMSO are mixed and (b) dispersions obtained by dissolving solid CaS in DMSO. We conclude that the microwave-mediated decomposition of DMSO in the presence of Ca(CH\({}_{3}\)CO\({}_{2}\))\({}_{2}\) results in the formation of CaS. STM images of dry deposits of these dispersions, as well as particle size distributions obtained from light scattering and imaging measurements, are summarized in Figure 2. The average particle sizes obtained from light scattering measurements of CaS dispersions are summarized in Table 1. The light scattering measurements indicate that CaS nanostructures prepared with the initial Ca(CH\({}_{3}\)CO\({}_{2}\))\({}_{2}\)/DMSO 4 \(\times\) 10\({}^{-4}\) ratio have a hydrodynamic diameter that is below the detection limit of 0.8 nm of the instrument employed for the measurement [8]. This result is consistent with the STM image presented in Figure 2a, which shows discrete roughness features smaller than 1 nm. The particle size distribution obtained from the STM and light scattering measurements for a dispersion with a Ca(CH\({}_{3}\)CO\({}_{2}\))\({}_{2}\)/DMSO ratio of 6 \(\times\) 10\({}^{-4}\) are summarized in Figure 2c,d. The STM measurements reveal an average particle diameter of (3.1 \(\pm\) 0.3) nm, while a hydrodynamic diameter of (1.1 \(\pm\) 0.2) nm is determined for dispersions with initial Ca(CH\({}_{3}\)CO\({}_{2}\))\({}_{2}\)/DMSO ratios of 6 \(\times\) 10\({}^{-4}\).