Quantum Confinement Effects in Calcium Sulfide: The Role of Indirect Transitions in the Red Shift of the Band Edge in Semiconductor Nanoparticles

Abstract

Calcium sulfide (CaS) nanoparticles are cadmium free fluorescent nanostructures with potential applications in nanomedicine and photovoltaic cells. We report on the synthesis and optical properties of CaS nanoparticles prepared by the reaction of Ca(CH3CO2)2 and DMSO in a microwave. The absorption spectra of CaS prepared from this method consists of a well-defined peak in the UV and a long wavelength tail that extends above 700 nm. Emission bands centered at around 500 nm with a long wavelength tail that extends above 600 nm are observed upon excitation at 405 nm. STM measurements reveal the formation of CaS nanoparticles with an average diameter of (3.2 ± 0.7) nm. The direct and indirect band gaps are estimated to be (0.403 ± 0.003) eV and (4.135 ± 0.006) eV, respectively. Theoretical calculations on small CaS clusters are used to establish the physical properties of calcium sulfide nanoclusters, including the optical absorption spectra. Unique to CaS nanostructures is the absorption of light at wavelengths longer that in the bulk material instead of the blue shift associated with quantum confinement effects in semiconductors. Indeed, the strong absorption bands in the visible region of the spectra of the CaS nanostructures do not have a counterpart in the gas or solid phases. The optical absorption spectra are proposed to have a significant contribution from indirect transitions which are discussed in terms of the dispersion of the phonon frequency.

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“…Exposure of diluted solutions of 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 to DMSO 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 ].…”

“…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 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 in DMSO are mixed and (b) dispersions obtained by dissolving solid CaS in DMSO.…”

“…The microwave synthesis was performed by exposing this solution to microwave radiation in cycles of 5-s “on” and 15-s “off”. This process was repeated for up to 15 intervals of exposure to microwave radiation in the sample [ 28 , 29 ]. The resulting solution was filtered several times using a P5 Fisher filter paper prior to the measurements reported here.…”