Luminescent and ferromagnetic CdS:Mn²⁺/C core-shell nanocrystals

The Mn-doped CdS nanocrystals encapsulated by carbon (Cd _1-xMn_xS/C) were synthesized by a one-step, kinetically controlled, solid-state reaction under autogenic pressure at elevated temperatures. The ∼50 nm wurtzite Cd_1-xMn_xS core was encapsulated by a 5-11 nm disordered carbon shell, and with the increase in Mn concentration, a gradual change from isotropic nanocrystals to one-dimensional nanorods was observed. Electron paramagnetic resonance studies showed that Mn²⁺ could be efficiently doped into the CdS lattice up to a Mn:Cd atomic ratio of 0.012. The 0.9-1.8 atomic % manganese-doped CdS samples were found to be ferromagnetic at room temperature, and the magnetic moment did not saturate even at 2 K, likely due to the coexistence of superparamagnetic fractions and antiferromagnetic coupling between the Mn²⁺ spins. The lowest-doped samples (Mn:Cd = 0.009 and 0.012) display the highest magnetic moments (4.43 ± 0.04 and 4.52 ± 0.04 μ_B/Mn), respectively. The more concentrated samples exhibit weaker magnetic moments (2.85 ± 0.03 μ_B/Mn for Mn:Cd = 0.018) as a result of antiferromagnetic coupling between Mn²⁺ second neighbors. Cathodoluminescence spectroscopy experiments were performed from 50 to 300 K to assess the temperature dependence of emissions related to the CdS near band edge, the Mn intra d-shell⁴T₁ → ⁶A ₁ transition, defectrelated surface state transitions, and the effect of surface passivation with carbon. The temperature-dependent spectral line shape variations, the emission intensities, and energies of the various components were examined for each Mn doping density to evaluate the incorporation of Mn²⁺ into the host CdS nanocrystal lattice.