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

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• 作者：Nick S. Norberg ; Kevin R. Kittilstved ; James E. Amonette ; Ravi K. Kukkadapu ; Dana A. Schwartz ; and Daniel R. Gamelin
• 刊名：Journal of the American Chemical Society
• 出版年：2004
• 出版时间：August 4, 2004
• 年：2004
• 卷：126
• 期：30
• 页码：9387 - 9398
• 全文大小：272K
• 年卷期：v.126,no.30(August 4, 2004)
• ISSN：1520-5126

文摘

We report the synthesis of colloidal Mn²⁺-doped ZnO (Mn²⁺:ZnO) quantum dots and thepreparation of room-temperature ferromagnetic nanocrystalline thin films. Mn²⁺:ZnO nanocrystals wereprepared by a hydrolysis and condensation reaction in DMSO under atmospheric conditions. Synthesiswas 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 beperformed aerobically. Mn²⁺ ions were removed from the surfaces of as-prepared nanocrystals usingdodecylamine to yield high-quality internally doped Mn²⁺:ZnO colloids of nearly spherical shape and uniformdiameter (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 issubstitutionally incorporated into the ZnO nanocrystals as Mn²⁺ with very homogeneous speciation, differingfrom 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 r.gif">_B/Mn²⁺ and T_C >350 K. A distinct ferromagnetic resonance signal was observed in the EPR spectra of the ferromagneticfilms. The occurrence of ferromagnetism in Mn²⁺:ZnO and its dependence on synthetic variables arediscussed in the context of these and previous theoretical and experimental results.