Magnetic Quantum Dots:  Synthesis, Spectroscopy, and Magnetism of Co²⁺- and Ni²⁺-Doped ZnO Nanocrystals

Abstract

We report a method for the preparation of colloidal ZnO-diluted magnetic semiconductor quantum dots (DMS-QDs) by alkaline-activated hydrolysis and condensation of zinc acetate solutions in dimethyl sulfoxide (DMSO). Mechanistic studies reveal that Co(2+) and Ni(2+) dopants inhibit nucleation and growth of ZnO nanocrystals. In particular, dopants are quantitatively excluded from the critical nuclei but are incorporated nearly isotropically during subsequent growth of the nanocrystals. The smaller nanocrystal diameters that result upon doping are explained by the Gibbs-Thompson relationship between lattice strain and crystal solubility. We describe methods for cleaning the nanocrystal surfaces of exposed dopants and for redispersion of the final DMS-QDs. Homogeneous substitutional doping is verified by high-resolution low-temperature electronic absorption and magnetic circular dichroism (MCD) spectroscopies. A "giant Zeeman effect" is observed in the band gap transition of Co(2+):ZnO DMS-QDs. MCD and Zeeman spectroscopies are used to quantify the magnitude of the p-d exchange interaction (N(0)beta) that gives rise to this effect. N(0)beta values of -2.3 +/- 0.3 eV (-18 500 cm(-1)) for Co(2+):ZnO and -4.5 +/- 0.6 eV (-36 300 cm(-1)) for Ni(2+):ZnO have been determined. Ligand-to-metal charge-transfer transitions are observed in the MCD spectra of both Co(2+):ZnO and Ni(2+):ZnO DMS-QDs and are analyzed in the context of an optical electronegativity model. The importance of these charge-transfer states in determining N(0)beta is discussed. Ferromagnetism with T(C) > 350 K is observed in aggregated nanocrystals of Co(2+):ZnO that unambiguously demonstrates the existence of intrinsic high-T(C) ferromagnetism in this class of DMSs.

Magnetic Properties of ZnO Nanoparticles

“…ZnO NPs were prepared by sol-gel following the method described in [7,18,19] and subsequently capped with three different organic molecules:…”

Strong Room-Temperature Ferromagnetism in Co2+-Doped TiO2 Made from Colloidal Nanocrystals

“…We have previously observed that high-T C ferromagnetism can be induced in paramagnetic ZnO DMS NCs by room-temperature aggregation in air. 14,30 The change from paramagnetic nanocrystals to ferromagnetic nanocrystalline aggregates was proposed to arise from the increased domain volumes and the introduction of donor-type defects upon aggregation. The data in Figure 6A suggest that a similar phenomenon may also occur for TiO 2 DMS NCs, although the magnitude of the effect in this case is too small to be analyzed quantitatively.…”

Synthesis, Structural and Optical Properties of Mo-Doped ZnO Thin Films

“…For example, studies have been reported on Ti--doped [6], V -doped [7], Cr-doped [8], Mn-doped [9], Fe--doped [10], Co-doped [11], Ni-doped [12], and Cu-doped [13] ZnO.…”