Comparison of the magnetic and optical properties of wide-gap (II,Mn)VI nanostructures confined in mesoporous silica

Arrays of highly ordered Cd1-xMnxSe, Cd1-xMnxS, and Zn1-xMnxS nanoparticles with x ranging from 0.01 to 0.3 and with lateral dimensions of 3, 6, and 9 nm were synthesised within mesoporous SiO2 host structures of the MCM-41 and SBA15 type. The hexagonal symmetry of these arrays (space group p6m) and the high degree of order were confirmed by X-ray diffraction and transmission electron microscope studies. Physisorption measurements show the progressive filling of the pores of the SiO2 host structures, while photoluminescence excitation (PLE) and electron paramagnetic resonance (EPR) studies confirm the good crystalline quality of the incorporated (II,Mn)VI guest species. The effects of the reduction of the lateral dimensions on the electronic properties of the diluted magnetic semiconductor were studied by PLE spectroscopy. Due to the quantum confinement of the excitons in the nanostructures an increase of the direct band gap with decreasing particle size is observed. The magnitude of the confinement effects increases with increasing excitonic Bohr radius of the (II,Mn)VI, i.e. it is largest for (Cd,Mn)Se and smallest for (Zn,Mn)S. Moreover, an increased band-gap bowing with Mn content is observed for the Cd compounds and can be correlated with the modification of the magnetic properties. Surprisingly, no band-gap bowing with x is observed for the Zn1-xMnxS nanostructures. Analysis of the EPR linewidth and EPR intensity show that macroscopic magnetic properties such as the Curie-Weiss temperature are affected by the reduction of the lateral dimensions. The microscopic coupling between the Mn ions (e.g. the exchange constants J(nn) and J(nnn)) is not altered to a first approximation. (c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005.