Synthesis of Colloidal Mn²⁺:ZnO Quantum Dots and High-T_C Ferromagnetic Nanocrystalline Thin Films

Abstract

We report the synthesis of colloidal Mn(2+)-doped ZnO (Mn(2+):ZnO) quantum dots and the preparation of room-temperature ferromagnetic nanocrystalline thin films. Mn(2+):ZnO nanocrystals were prepared by a hydrolysis and condensation reaction in DMSO under atmospheric conditions. Synthesis was monitored by electronic absorption and electron paramagnetic resonance (EPR) spectroscopies. Zn(OAc)(2) was found to strongly inhibit oxidation of Mn(2+) by O(2), allowing the synthesis of Mn(2+):ZnO to be performed aerobically. Mn(2+) ions were removed from the surfaces of as-prepared nanocrystals using dodecylamine to yield high-quality internally doped Mn(2+):ZnO colloids of nearly spherical shape and uniform diameter (6.1 +/- 0.7 nm). Simulations of the highly resolved X- and Q-band nanocrystal EPR spectra, combined with quantitative analysis of magnetic susceptibilities, confirmed that the manganese is substitutionally incorporated into the ZnO nanocrystals as Mn(2+) with very homogeneous speciation, differing from bulk Mn(2+):ZnO only in the magnitude of D-strain. Robust ferromagnetism was observed in spin-coated thin films of the nanocrystals, with 300 K saturation moments as large as 1.35 micro(B)/Mn(2+) and T(C) > 350 K. A distinct ferromagnetic resonance signal was observed in the EPR spectra of the ferromagnetic films. The occurrence of ferromagnetism in Mn(2+):ZnO and its dependence on synthetic variables are discussed in the context of these and previous theoretical and experimental results.

Ferromagnetism in Fe-doped ZnO nanocrystals: Experiment and theory

“…This effect is greater at lower temperatures. 60 The inset of Fig. 8 depicts a measure of the total number of spins participating in producing signals A and B.…”

Chemical Manipulation of High-TCFerromagnetism in ZnO Diluted Magnetic Semiconductors

“…We recently reported strong high-T C ferromagnetism in 0.20% Mn 2 :ZnO prepared by a direct chemical synthesis route in which Mn 2 :ZnO colloids were used as precursors for spin coating nanocrystalline films [15]. To explain the strong ferromagnetism [M S (300 K) 1:35 B =Mn 2 ], we hypothesized that calcination of the surface-capping amines left behind nitrogen that could serve as a p-type defect [15].…”

“…To explain the strong ferromagnetism [M S (300 K) 1:35 B =Mn 2 ], we hypothesized that calcination of the surface-capping amines left behind nitrogen that could serve as a p-type defect [15]. To test this hypothesis, we have now performed the split-batch experiment described below.…”

“…To test this hypothesis, we have now performed the split-batch experiment described below. High-quality Mn 2 :ZnO nanocrystals (6 1 nm diameter) were prepared as described previously [15]. After growth in dimethylsulfoxide (DMSO), the nanocrystals were precipitated with ethyl acetate and washed with heptane.…”

Nonvolatile bistability effect based on the electrically controlled phase transition in scaled magnetic semiconductor nanostructures

“…Note for instance two recent reports of the DMS with T c ≥ 300 K. 5,7 Since the search for an ideal DMS is only at the beginning stage, we assume a hypothetical material with the characteristics similar to GaMnAs except T 0 c in Eq. (9), which is treated as a variable in a wide temperature range.…”