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

Published online by Cambridge University Press:  23 September 2014

Daniel Rivera-Vázquez ,
Yohaselly Santiago-Rodríguez ,
Miguel A. González  and
Miguel E. Castro-Rosario
Daniel Rivera-Vázquez
Affiliation:
Chemical Imaging Center, Department of Chemistry, The University of Puerto Rico at Mayaguez, Mayaguez, Puerto Rico 00680
Yohaselly Santiago-Rodríguez
Affiliation:
Department of Chemical Engineering, The University of Puerto Rico at Mayaguez, Mayaguez, Puerto Rico, 00680
Miguel A. González
Affiliation:
Chemical Imaging Center, Department of Chemistry, The University of Puerto Rico at Mayaguez, Mayaguez, Puerto Rico 00680
Miguel E. Castro-Rosario*
Affiliation:
Chemical Imaging Center, Department of Chemistry, The University of Puerto Rico at Mayaguez, Mayaguez, Puerto Rico 00680
*
*Corresponding author: miguel.castro2@upr.edu
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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.

Keywords

nanoscalenanostructureCa
Type
Articles
Information
MRS Online Proceedings Library (OPL) , Volume 1694: Symposium EE – Advances in Inorganic Semiconductor Nanoparticles and Their Applications , 2014 , mrss14-1694-ee10-09
Copyright
Copyright © Materials Research Society 2014 

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