Manganous ion dictated morphology change and ferromagnetism in CdS nanocrystals

Abstract

We report a simple chemical route for converting the dimorphic CdS nanocrystals into pure hexagonal structure by loading ~5 % Mn²⁺ into CdS in presence of a surfactant rather than under harsh conditions reported earlier. Differential scanning calorimetric measurement was used to quantify the percentages of cubic and hexagonal components at lower manganese ion loading, which correlated well with the Short and Steward procedure using X-ray diffraction analysis. Annealing of all lower (<5 %) manganese ion-loaded samples transforms the cubic component into its hexagonal form. Systematic variation of manganese ion concentration in the range of 0–5 % helps tune in a new combination of photoluminescence bands, composed of a band edge, surface and manganese d–d emissions. The manganese ions located inside the CdS matrix studied by electron paramagnetic resonance (EPR) reveal progressively changing presence of (i) distorted tetrahedral nature of isolated Mn²⁺ and (ii) cluster formation through dipolar and/or exchange coupling arising from Mn²⁺–Mn²⁺ interaction. There is a neat correlation between PL and EPR properties on the one hand and between EPR and magnetism on the other. Most important part of this investigation lies in manganese coupled ferromagnetism (FM) as compared to our earlier reported anti-FM-coupled superparamagnetism, the difference originating from the use of different ligands. Pre-annealed (as synthesized) and post-annealed CdS/Mn²⁺ nanocrystals exhibit FM of manganese origin, the latter exhibiting increased FM due to annealing promoted migration of manganese ions to form more clusters, as confirmed by magnetization experiments. Electron microscopy reveals the formation of nanorods and nanoparticles.