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

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.Comment: To be published in the Journal of the American Chemical Society Web on July 14, 2004 (http://dx.doi.org/10.1021/ja048427j

Chemical Stabilization of Perovskite Solar Cells with Functional Fulleropyrrolidines.

While perovskite solar cells have invigorated the photovoltaic research community due to their excellent power conversion efficiencies (PCEs), these devices notably suffer from poor stability. To address this crucial issue, a solution-processable organic chemical inhibition layer (OCIL) was integrated into perovskite solar cells, resulting in improved device stability and a maximum PCE of 16.3%. Photoenhanced self-doping of the fulleropyrrolidine mixture in the interlayers afforded devices that were advantageously insensitive to OCIL thickness, ranging from 4 to 190 nm. X-ray photoelectron spectroscopy (XPS) indicated that the fulleropyrrolidine mixture improved device stability by stabilizing the metal electrode and trapping ionic defects (i.e., I-) that originate from the perovskite active layer. Moreover, degraded devices were rejuvenated by repeatedly peeling away and replacing the OCIL/Ag electrode, and this repeel and replace process resulted in further improvement to device stability with minimal variation of device efficiency

Persistent radical anion polymers based on naphthalenediimide and a vinylene spacer

Persistent n-doped conjugated polymers were achieved by doping the electron accepting PDNDIV and PFNDIVpolymers with ionic (TBACN) or neutral (TDAE) dopants. The great electron affinities, as indicated by the low LUMO levels of PDNDIV (−4.09 eV) and PFNDIV (−4.27 eV), facilitated the chemical reduction from either TBACN or TDAE. The low-lying LUMOs of the neutral polymers PDNDIV and PFNDIV were achieved by incorporation of vinylene spacers between the electron poor NDI units to increase the conjugation length without the use of an electron donor, and this was lowered further by an electron-withdrawing fluorinated N-substituent on the NDI moiety. The polymer radical anions were found to persist for several days under ambient conditions by EPR spectroscopy. A distinguishing and noteworthy feature of these polymers is that they can be consecutively reduced by up to four electrons in acetonitrile. Conductivity measurements demonstrate the prospective impact of PDNDIV and PFNDIV for organic electronics

Reversible Control of the Mn Oxidation State in SrTiO3 Bulk Powders

We demonstrate a low-temperature reduction method for exhibiting fine control over the oxidation state of substitutional Mn ions in strontium titanate (SrTiO3) bulk powder. We employ NaBH4 as the chemical reductant that causes significant changes in the oxidation state and oxygen vacancy complexation with Mn2+ dopants at temperatures <350°C where lattice reduction is negligible. At higher reduction temperatures, we also observe the formation of Ti3+ in the lattice by diffuse-reflectance and low-temperature electron paramagnetic resonance (EPR) spectroscopy. In addition to Mn2+, Mn4+, and the Mn2+ complex with an oxygen vacancy, we also observe a sharp resonance in the EPR spectrum of heavily reduced Mn-doped SrTiO3. This sharp signal is tentatively assigned to surface superoxide ion that is formed by the surface electron transfer reaction between Ti3+ and O2. The ability to control the relative amounts of various paramagnetic defects in SrTiO3 provides many possibilities to study in a model system the impact of tunable dopant-defect interactions for spin-based electronic applications or visible-light photocatalysis

Manipulating Polar Ferromagnetism in Transition Metal Doped ZnO: Why Manganese is Different from Cobalt

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 300 K ferromagnetism in Mn2+:ZnO and Co2+:ZnO to differences in the charge-transfer electronic structures of these two materials

Electronic Structure and Photophysics of Pseudo-Octahedral Vanadium(III) Oxo Complexes

The electronic structure and the photophysical properties of the vanadium(III)ion in pseudo-octahedral oxygen coordination is reviewed. V3+ has received much interest from spectroscopists in recent years due to the advancement of state-of-the-art experimental techniques such as inelastic neutron scattering and high-field electron paramagnetic resonance spectroscopy that directly interrogate its large ground state zero-field splittings (ZFSs) and to rational parameterization of the ligand fieldp arameters using the angular overlap model. However, for V3+ these ZFSs can be large enough to also be probed directly by high-resolution electronic absorption spectroscopy of intra-configurational (t22g → t22g) spin-forbidden transitions in the near-IR and visible regions. The luminescent properties of V3+ with hexa-oxo and tris-bidentate di-oxo-coordination are quite disappointing compared to its neighbor in the periodic table, Cr3+, in similar environments. The efficient non-radiative pathways in these compounds are reviewed and compared to recent work on V3+ doped into NaMgAl(ox)3⋅9H2O. The poor luminescence quantum efficiencies of V3+ oxo complexes is explained by strong coupling of multi-phonon processes with a dynamic Jahn-Teller distortion originating from the 3E trigonal component of the 3T1g ground state

Cation Exchange in Small ZnS and CdS Molecular Analogues

The simplest means of altering the chemistry and electronic structure of any material, from molecular clusters to single crystals, is by the introduction of chemical impurities. We present a systematic study of the cation exchange reaction involving Co²⁺ ions with metal benzenethiolate clusters, [M₄(SPh)₁₀]^2– (M = Zn, Cd), yielding diluted magnetic clusters having the general formula [(M_1–<i>x</i>Co_<i>x</i>)₄(SPh)₁₀]^2–. This method allows high concentrations of doping at the molecular level without forming concentrated magnetic clusters such as [Co₄(SPh)₁₀]^2–. Changes in the electronic structure of the molecular species containing on average <1 Co²⁺ per cluster were observed and characterized by a variety of analytical (high-resolution electrospray mass spectrometry) and spectroscopic techniques (electronic absorption including stopped-flow kinetics, luminescence, and paramagnetic ¹H NMR). The mass spectrometry results strongly suggest that the cation exchange reaction with Co²⁺ is thermodynamically favored for the [Zn₄(SPh)₁₀]^2– cluster compared to the [Cd₄(SPh)₁₀]^2– clusters at room temperature. The rate of the cation exchange is orders of magnitude faster for the [Cd₄(SPh)₁₀]^2– cluster than for [Zn₄(SPh)₁₀]^2– and is governed by ligand interconversion processes. This simple room temperature cation exchange into molecular clusters is a model reaction that provides important structural information regarding the effect of Co²⁺ doping on the cluster stability

Above-Room-Temperature Ferromagnetic Ni2+: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 350 K and 300 K saturation moments up to 0.1 Bohr 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