Charge-controlled magnetism in colloidal doped semiconductor nanocrystals.

Electrical control over the magnetic states of doped semiconductor nanostructures could enable new spin-based information processing technologies. To this end, extensive research has recently been devoted to examination of carrier-mediated magnetic ordering effects in substrate-supported quantum dots at cryogenic temperatures, with carriers introduced transiently by photon absorption. The relatively weak interactions found between dopants and charge carriers have suggested that gated magnetism in quantum dots will be limited to cryogenic temperatures. Here, we report the observation of a large, reversible, room-temperature magnetic response to charge state in free-standing colloidal ZnO nanocrystals doped with Mn(2+) ions. Injected electrons activate new ferromagnetic Mn(2+)-Mn(2+) interactions that are strong enough to overcome antiferromagnetic coupling between nearest-neighbour dopants, making the full magnetic moments of all dopants observable. Analysis shows that this large effect occurs in spite of small pairwise electron-Mn(2+) exchange energies, because of competing electron-mediated ferromagnetic interactions involving distant Mn(2+) ions in the same nanocrystal.

Room-temperature electron spin dynamics in free-standing ZnO quantum dots.

Conduction band electrons in colloidal ZnO quantum dots have been prepared photochemically and examined by electron paramagnetic resonance spectroscopy. Nanocrystals of 4.6 nm diameter containing single S-shell conduction band electrons have g(*)=1.962 and a room-temperature ensemble spin-dephasing time of T(2)(*)=25 ns, as determined from linewidth analysis. Increasing the electron population leads to increased g(*) and decreased T(2)(*), both associated with formation of P-shell configurations. A clear relationship between T(2)(*) and hyperfine coupling with 67Zn(I=5/2) is observed.

Electronic structure origins of polarity-dependent high-TC ferromagnetism in oxide-diluted magnetic semiconductors.

Future spintronics technologies based on diluted magnetic semiconductors (DMSs) will rely heavily on a sound understanding of the microscopic origins of ferromagnetism in such materials. Discoveries of room-temperature ferromagnetism in wide-bandgap DMSs hold great promise, but this ferromagnetism remains poorly understood. Here we demonstrate a close link between the electronic structures and polarity-dependent high-TC ferromagnetism of TM(2+):ZnO DMSs, where TM(2+) denotes 3d transition metal ions. Trends in ferromagnetism across the 3d series of TM(2+):ZnO DMSs predicted from the energies of donor- and acceptor-type excited states reproduce experimental trends well. These results provide a unified basis for understanding both n- and p-type ferromagnetic oxide DMSs.

Stable photogenerated carriers in magnetic semiconductor nanocrystals.

We report the preparation and investigation of charged colloidal Co2+:ZnO and Mn2+:ZnO nanocrystals. Although both charged and magnetically doped colloidal semiconductor nanocrystals have been reported previously, colloidal charged and magnetically doped semiconductor nanocrystals as described herein have not. Conduction band electrons were introduced into colloidal ZnO diluted magnetic semiconductor (DMS) nanocrystals photochemically, and the resulting TM2+-e-CB interactions were observed by electron paramagnetic resonance spectroscopy (TM2+ = Co2+ or Mn2+). This new motif of colloidal charged magnetic semiconductor nanocrystals reveals attractive new opportunities for studying spin effects in DMS nanostructures relevant to proposed spintronics technologies.

Spectroscopy of photovoltaic and photoconductive nanocrystalline Co2+-doped ZnO electrodes.

Photocurrent and photoconductivity measurements have been used in combination with absorption and magnetic circular dichroism (MCD) spectroscopic measurements to elucidate the mechanism of photoinduced carrier generation in nanocrystalline Co(2+):ZnO electrodes. These experiments allowed direct observation of two broad Co(2+) charge transfer (CT) bands extending throughout the visible energy range. The lower energy CT transition is assigned as a Co(2+) --> conduction band excitation (ML(CB)CT). Sensitization of this ML(CB)CT level by (4)A(2) --> (4)T(1)(P) ligand-field excitation is concluded to be responsible for the distinctive structured photocurrent action spectrum of these electrodes at ca. 14 000 cm(-1). The higher energy CT transition is assigned as a valence band --> Co(2+) excitation (L(VB)MCT) and is found to have an internal quantum efficiency for charge separation that is approximately four times larger than that of the ML(CB)CT excitation. The different internal quantum efficiencies for the two CT excitations are related to differences in excited-state wave functions arising from configuration interaction with the 1S excitonic levels of ZnO. Whereas the ML(CB)CT excited state is best described as a localized Co(3+) + e(-)(CB) configuration, the L(VB)MCT excited state (Co(+) + h(+)(VB)) has a 4-fold greater admixture of delocalized excitonic (Co(2+) + h(+)(VB) + e(-)(CB)) character in its wave function, a conclusion supported by quantitative analysis of the CT absorption intensities. Practical factors controlling the overall photovoltaic efficiencies of the photoelectrochemical cells, including electrode conductivity and porosity, were also examined.