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

Authors

Nick S. Norberg, University of Washington
Kevin R. Kittilstved, University of Washington
James E. Amonette, Pacific Northwest National Laboratory
Ravi K. Kukkadapu, Pacific Northwest National LaboratoryFollow
Dana A. Schwartz, University of WashingtonFollow
Daniel R. Gamelin, University of WashingtonFollow

Date of this Version

2004

Comments

J. AM. CHEM. SOC. 2004, 126, 9387-9398

Abstract

We report the synthesis of colloidal Mn²⁺-doped ZnO (Mn²⁺:ZnO) quantum dots and the preparation of room-temperature ferromagnetic nanocrystalline thin films. Mn²⁺: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 Mn²⁺ by O₂, allowing the synthesis of Mn2+:ZnO to be performed aerobically. Mn2+ ions were removed from the surfaces of as-prepared nanocrystals using dodecylamine to yield high-quality internally doped Mn2+: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 Mn²⁺ with very homogeneous speciation, differing from bulk Mn²⁺:ZnO only in the magnitude of D-strain. Robust ferromagnetism was observed in spincoated thin films of the nanocrystals, with 300 K saturation moments as large as 1.35 µB/Mn²⁺ 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 Mn²⁺:ZnO and its dependence on synthetic variables are discussed in the context of these and previous theoretical and experimental results.