  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 Bohr magneton/Mn2+ and TC > 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.
