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Article

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

Supporting Info

Nick S. Norberg,^† Kevin R. Kittilstved,^† James E. Amonette,^‡ Ravi K. Kukkadapu,^‡ Dana A. Schwartz,^† and Daniel R. Gamelin*^†

Contribution from the Department of Chemistry, University of Washington, Seattle, Washington 98195-1700, and Pacific Northwest National Laboratory, Richland, Washington 99352

J. Am. Chem. Soc., 2004, 126 (30), pp 9387–9398

DOI: 10.1021/ja048427j

Publication Date (Web): July 14, 2004

†

University of Washington.

‡

Pacific Northwest National Laboratory.

In papers with more than one author, the asterisk indicates the name of the author to whom inquiries about the paper should be addressed.

Gamelin@chem.washington.edu

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 spin-coated thin films of the nanocrystals, with 300 K saturation moments as large as 1.35 μ_B/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.