Kevin R. Kittilstved

Ferromagnetism in oxide semiconductors

Over the past five years, considerable work has been carried out in the exploration of candidate diluted oxide magnetic semiconductors with high Curie temperatures. Fueled by early experimental results and theoretical predictions, claims of ferromagnetism at and above room temperature in doped oxides have abounded. In general, neither the true nature of these materials nor the physical causes of the magnetism have been adequately determined. It is now apparent that these dilute magnetic systems are deceptively complex. We consider two well-characterizedn-type magnetically doped oxide semiconductors and explore the relationship between donor electrons and ferromagnetism.

Above-room-temperature ferromagnetic Ni2+-doped ZnO thin films prepared from colloidal diluted magnetic semiconductor quantum dots

We report the preparation of spin-coated nickel-doped zinc oxide nanocrystalline thin films using high-quality colloidal diluted magnetic semiconductor (DMS) quantum dots as solution precursors. These films show robust ferromagnetism with Curie temperatures above 350K and 300K saturation moments up to 0.1Bohr magnetons per nickel. These results demonstrate a step toward the use of colloidal zero-dimensional DMS nanocrystals as building blocks for the bottom-up construction of more complex ferromagnetic semiconductor nanostructures.

Near-Infrared→Visible Light Upconversion in a Molecular Trinuclear d-f-d Complex

Giving the green light: The connection of two CrIII sensitizers around a central ErIII acceptor in a self-assembled cation provides high local metal concentrations that favor efficient nonlinear energy transfer upconversion luminescence (see picture). Upon selective low-energy near-infrared irradiation of CrIII-centered transitions, 1 displays an unprecedented molecular two-photon upconverted green ErIII-centered emission. Copyright

Optimizing Millisecond Time Scale Near-Infrared Emission in Polynuclear Chrome(III)–Lanthanide(III) Complexes

This work illustrates a simple approach for optimizing long-lived near-infrared lanthanide-centered luminescence using trivalent chromium chromophores as sensitizers. Reactions of the segmental ligand L2 with stoichiometric amounts of M(CF(3)SO(3))(2) (M = Cr, Zn) and Ln(CF(3)SO(3))(3) (Ln = Nd, Er, Yb) under aerobic conditions quantitatively yield the D(3)-symmetrical trinuclear [MLnM(L2)(3)](CF(3)SO(3))(n) complexes (M = Zn, n = 7; M = Cr, n = 9), in which the central lanthanide activator is sandwiched between the two transition metal cations. Visible or NIR irradiation of the peripheral Cr(III) chromophores in [CrLnCr(L2)(3)](9+) induces rate-limiting intramolecular intermetallic Cr→Ln energy transfer processes (Ln = Nd, Er, Yb), which eventually produces lanthanide-centered near-infrared (NIR) or IR emission with apparent lifetimes within the millisecond range. As compared to the parent dinuclear complexes [CrLn(L1)(3)](6+), the connection of a second strong-field [CrN(6)] sensitizer in [CrLnCr(L2)(3)](9+) significantly enhances the emission intensity without perturbing the kinetic regime. This work opens novel exciting photophysical perspectives via the buildup of non-negligible population densities for the long-lived doubly excited state [Cr*LnCr*(L2)(3)](9+) under reasonable pumping powers.

Tuning the Potentials of “Extra” Electrons in Colloidal n-Type ZnO Nanocrystals via Mg2+ Substitution

Colloidal reduced ZnO nanocrystals are potent reductants for one-electron or multielectron redox chemistry, with reduction potentials tunable via the quantum confinement effect. Other methods for tuning the redox potentials of these unusual reagents are desired. Here, we describe synthesis and characterization of a series of colloidal Zn(1-x)Mg(x)O and Zn(0.98-x)Mg(x)Mn(0.02)O nanocrystals in which Mg(2+) substitution is used to tune the nanocrystal reduction potential. The effect of Mg(2+) doping on the band-edge potentials of ZnO was investigated using electronic absorption, photoluminescence, and magnetic circular dichroism spectroscopies. Mg(2+) incorporation widens the ZnO gap by raising the conduction-band potential and lowering the valence-band potential at a ratio of 0.68:0.32. Mg(2+) substitution is far more effective than Zn(2+) removal in raising the conduction-band potential and allows better reductants to be prepared from Zn(1-x)Mg(x)O nanocrystals than can be achieved via quantum confinement of ZnO nanocrystals. The increased conduction-band potentials of Zn(1-x)Mg(x)O nanocrystals compared to ZnO nanocrystals are confirmed by demonstration of spontaneous electron transfer from n-type Zn(1-x)Mg(x)O nanocrystals to smaller (more strongly quantum confined) ZnO nanocrystals.

Magnetic circular dichroism of ferromagnetic Co2+-doped ZnO

Cobalt-doped ZnO (Co2+:ZnO) films were studied by magnetic circular dichroism (MCD) spectroscopy. A broad 300K ferromagnetic MCD signal was observed between 1.4 and 4.0eV after exposure of paramagnetic Co2+:ZnO films to zinc metal vapor, attributed to low-energy photoionization transitions originating from a spin-split donor impurity band in ferromagnetic n-type Co2+:ZnO.

Manipulating polar ferromagnetism in transition-metal-doped ZnO: Why manganese is different from cobalt (invited)

High-temperature magnetic ordering in Mn2+- and Co2+-doped ZnO diluted magnetic semiconductors has been predicted theoretically and confirmed experimentally to have different charge-carrier requirements. This paper summarizes some of these experimental and theoretical results and relates the different carrier polarity requirements for 300K ferromagnetism in Mn2+:ZnO and Co2+:ZnO to differences in the charge-transfer electronic structures of these two materials.

Speciation of Cr(III) in intermediate phases during the sol–gel processing of Cr-doped SrTiO3 powders

The local environment of Cr3+ during the sol–gel synthesis of 1% and 5% Cr3+-doped SrTiO3 (Cr3+:SrTiO3) bulk powders was systematically studied by structural characterization techniques and dopant-specific spectroscopies. After calcination at 800 °C, the precursors were annealed between 850 °C and 1050 °C for up to 6 h. We observe the formation of numerous phases in addition to the final product of SrTiO3. One of these is a metastable Ruddelsden–Popper phase (Sr2TiO4) that forms when the precursor is annealed at 1050 °C for less than 2 h. Electron paramagnetic resonance (EPR) spectroscopy reveals a new signal that correlates with appearance of the Sr2TiO4 phase and is consistent with Cr3+ substitution into the axially-compressed Ti4+ site in Sr2TiO4. The best agreement between experiment and the simulated EPR spectra of Cr3+:Sr2TiO4 is when |D| = 0.0207 cm−1, g∥ = 1.9803 and g⊥ = 1.9793. The majority of the sample is converted to Cr3+:SrTiO3 with increasing annealing times at 1050 °C as detected by low-temperature emission and EPR spectroscopies, and powder X-ray diffraction.

Ground-State Electronic Structure of Vanadium(III) Trisoxalate in Hydrated Compounds

The ground-state electronic structures of K3V(ox)3.3H2O, Na3V(ox)3.5H2O, and NaMgAl1-xVx(ox)3.9H2O (0 < x <or= 1, ox = C2O42-) have been studied by Fourier-transform electronic absorption and inelastic neutron scattering spectroscopies. High-resolution absorption spectra of the 3Gamma(t2g2) --> 1Gamma(t2g2) spin-forbidden electronic origins and inelastic neutron scattering measurements of the pseudo-octahedral [V(ox)3]3- complex anion below 30 K exhibit both axial and rhombic components to the zero-field-splittings (ZFSs). Analysis of the ground-state ZFS using the conventional S = 1 spin Hamiltonian reveals that the axial ZFS component changes sign from positive values for K3V(ox)3.3H2O (D approximately +5.3 cm-1) and Na3V(ox)3.5H2O (D approximately +7.2 cm-1) to negative values for NaMgAl1-xVx(ox)3.9H2O (D approximately -9.8 cm-1 for x = 0.013, and D approximately -12.7 cm-1 for x = 1) with an additional rhombic component, |E|, that varies between approximately 0.8 and approximately 2 cm-1. On the basis of existing crystallographic data, this phenomenon can be identified as due to variations in the axial and rhombic ligand fields resulting from outer-sphere H-bonding between crystalline water molecules and the oxalate ligands. Spectroscopic evidence of a crystallographic phase change is also observed for K3V(ox)3.3Y2O (Y = H or D) with three distinct lattice sites below 30 K, each with a unique ground-state electronic structure.

Control over Fe3+ speciation in colloidal ZnO nanocrystals

The incorporation of potentially redox active dopant ions holds much promise for applications in catalysis and energy. Here we report the room-temperature synthesis of colloidal Fe-doped ZnO nanocrystals. By combining detailed dopant-specific spectroscopy with known single crystal data we are able to elucidate the locations of paramagnetic Fe3+ ions in the colloidal ZnO nanocrystals. Electron paramagnetic resonance (EPR) spectra of 0.15–2.0% Fe-doped ZnO nanocrystals are consistent with the Fe dopants occupying both pseudo-tetrahedral (substitutional at the Zn-site) and pseudo-octahedral (surface and interstitial) coordination environments. The evolution of the spectra as a function of ZnO growth time allow us to provide additional mechanistic insight into the formation of doped colloidal ZnO nanocrystals using a simple room temperature synthetic method. We also demonstrate control over the speciation of the Fe dopants in colloidal ZnO nanocrystals by changing the growth and/or surface-ligand treatment times.