CdS/ZnS-doped silico-phosphate films prepared by sol-gel synthesis
Introduction
In recent years, II-VI binary semiconducting compounds, belonging to cadmium chalcogenide family have been intensively investigated mainly due to their quantum confinement effect with potential use in optoelectronics devices [1], [2], [3]. CdS quantum dots (QDs) have many applications, which, sometimes, require to embed the nanocrystals into a solid matrix host, allowing optical information or signals to be transmitted with high propagation efficiency [4]. The host material plays the important role of providing a stable matrix, preventing the agglomeration of the dopant nanoparticles and, in the context of optical properties, it should be transparent in the region of interest.
Several approaches have been developed for producing bulk or thin films materials doped with semiconductor QDs: melt glasses, [5], [6] polymers [7], [8] and sol-gel glasses [9].
Optical and structural characterization of CdS QDs for solar concentrators was reported in [10] and luminescent down-shifting layer for crystalline Si solar cells was presented in [11].
The sol-gel method, as compared to the classical solid-state synthetic routes offers advantages, such as: the mild synthetic conditions, versatility, high homogeneity of dopants and purity of the final material, which are fundamental for the development of new advanced materials [12].
The paper presents the preparation as well as optical and structural characterization of luminescent silico-phosphate films doped with CdS/ZnS quantum dots QDs.
Section snippets
Experimental
Silico-phosphate films doped with CdS/ZnS quantum dots (QDs) have been deposited on silicon substrate, by sol-gel method, spin coating technique. The following regents were used as precursors: tetraethyl ortho-silicate (TEOS), phosphoric acid (H3PO4) and ethanol (EtOH), with the molar ratio 1.5/0.82/1.04. CdS/ZnS (QDs) have been previously prepared by chemical synthesis of CdS core nanoparticles followed by growth of ZnS shell [13]. The final solution was prepared by using 0.5 ml of precursor
Optical absorption and X-ray diffraction analysis
The ellipsometry modeling of the absorption spectra of CdS/ZnS-doped sol-gel films used the Bruggeman Effective Medium Approximation (BEMA) [14], considering that the precursor solution for each deposited layer has the following molar composition: 31.26% SiO2, 19.38% P2O5 and 49.35% CdS/ZnS (QDs). The model takes into consideration nine similar layers deposited successively on silicon substrate, the last upper layer being in contact with atmosphere (Fig. 1). The interface between those nine
Conclusions
Optical absorption of the CdS-doped films deposited on silicon substrate was found to be decreasing over 300 nm. This aspect was attributed to CdS semiconductor dopant particles having about 2.75 nm size.
Luminescence spectrum of CdS-doped sol-gel films, provided by 340 nm excitation showed a broad band in the visible range, specific to the electrons in the conduction band, excitonic states and trap states.
FTIR and Raman spectra revealed specific molecular vibrations of the silico-phosphate matrix.
Acknowledgements
The author Dr. Cristina Bartha (NIMP) gratefully acknowledges the Romanian Research Ministry (“Core Program 2016-2017” projects no. P2”. The other authors are grateful to Executive Unity for Financing of Higher Education, Research and Innovation, Romania, for the financial support in the frame of projects: 51/2011-Ideas Program and 17/2012 – Partnership Program as well as to Core Program, under the support of ANCS, project OPTRONICA IV no. PN 16.400102. The SEM analyses on samples were possible
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