The solid-state synthesis and controllable speciation of Cr dopants in the layered perovskite Sr2TiO4 is reported. We employed a chemical reduction procedure with NaBH4 at relatively mild temperatures (<450 °C) to impart sensitive control over the relative concentration of Cr3+ dopants, the charge-state of oxygen-vacancy defects, and presence of Ti3+ defects in highly reduced Cr-doped Sr2TiO4. The electron paramagnetic resonance (EPR) spectra of the reduced powder samples reveal a 12-fold increase in the Cr3+ concentration within the axially compressed Ti4+-site of the Sr2TiO4 host. The increase in Cr3+ content is achieved through the reduction of higher-valence Cr ions that are either EPR silent or diamagnetic. The spin-Hamiltonian parameters for Cr3+ substituted at the B-site of Sr2TiO4 were refined to D = −201 × 10–4 cm–1, g⊥ = 1.980, and g∥ = 1.978. In addition, the Cr3+ ion exhibits a temperature-dependent axial component to the zero-field splitting of the 4A2 ground term that is accounted for by ligand field theory and an isotropic contraction of the Sr2TiO4 lattice with decreasing temperature. The observed changes to the electronic structure upon reduction are quantitatively reversible upon reoxidation of the sample under aerobic annealing at the same temperature and duration as the reduction conditions. This temperature dependence of the Cr3+ content in the Cr-doped Sr2TiO4 powders is discussed and contrasted to our recent study on Cr-doped SrTiO3.

The effects of photodoping on the electronic structure of Fe3+-doped ZnO colloidal nanocrystals are presented. We observe disappearance of the spectroscopic signatures attributed to both substitutional Fe3+ and interstitial Fe3+ in the ZnO host as a function of photodoping time, which precede the appearance of the well-known localized surface plasmon resonance from conduction band electrons in ZnO nanocrystals. These results suggest that the oxidation state of Fe3+ defects can be reversibly switched in ZnO nanocrystals.

The effect of chemical reduction by NaBH4 on the electronic structure of Cr-doped SrTiO3−δ bulk powders prepared by a solid-state reaction was systematically studied as a function of reduction temperature. Electron paramagnetic resonance (EPR) and diffuse reflectance spectroscopies (DRS) were utilized to monitor changes in the electronic structures of both intrinsic defects (oxygen vacancies and/or Ti3+) and extrinsic dopants (Cr3+) at different reduction temperatures. We identify the existence of two temperature regimes where changes occur within 30 min. The first temperature regime occurs between 300–375 °C and results in (1) reduction of oxygen-related surface defects, and (2) an increase in the concentration of Cr3+ by over an order of magnitude, suggesting that EPR-silent Cr4+ or Cr6+ is being reduced to Cr3+ by NaBH4. The second temperature regime occurs between 375–430 °C where we observe clear evidence of Ti3+ formation by EPR spectroscopy that indicates chemical reduction of the SrTiO3 lattice. In addition, the oxygen-related surface defects observed in regime 1 are not formed in regime 2, but instead lattice oxygen vacancies (VO) are observed by EPR. The changes to the Cr-doped SrTiO3 electronic structure after chemical reduction in regime 1 are quantitatively reversible after aerobic annealing at 400 °C for 30 min. The internal oxygen vacancies formed during the higher temperature reductions in regime 2 require increased temperatures of at least 600 °C to be fully reoxidized in 30 min. The effect of these different oxygen-related defects on the EPR spectrum of substitutional Cr3+ dopants is discussed. These results allow us to independently tune the dopant and host electronic structures of a technologically-relevant multifunctional material by a simple ex situ chemical perturbation.

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.

We report on the metal ion exchange between Co2+ and CdS-based molecular clusters. These studies demonstrate that exchange into the smaller [Cd4(SPh)10]2− clusters is facile compared to the larger [Cd10S4(SPh)16]4− and [Cd17S4(SPh)28]2− clusters. This trend correlates with the rate of benzenethiolate interconversion and μ-S2− and μ-SPh− content among the clusters.

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 [(M1–xCox)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.

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.

We report on the metal ion exchange between Co2+ and CdS-based molecular clusters. These studies demonstrate that exchange into the smaller [Cd4(SPh)10]2− clusters is facile compared to the larger [Cd10S4(SPh)16]4− and [Cd17S4(SPh)28]2− clusters. This trend correlates with the rate of benzenethiolate interconversion and μ-S2− and μ-SPh− content among the clusters.

The effects of photodoping on the electronic structure of Fe3+-doped ZnO colloidal nanocrystals are presented. We observe disappearance of the spectroscopic signatures attributed to both substitutional Fe3+ and interstitial Fe3+ in the ZnO host as a function of photodoping time, which precede the appearance of the well-known localized surface plasmon resonance from conduction band electrons in ZnO nanocrystals. These results suggest that the oxidation state of Fe3+ defects can be reversibly switched in ZnO nanocrystals.

The effect of chemical reduction by NaBH4 on the electronic structure of Cr-doped SrTiO3−δ bulk powders prepared by a solid-state reaction was systematically studied as a function of reduction temperature. Electron paramagnetic resonance (EPR) and diffuse reflectance spectroscopies (DRS) were utilized to monitor changes in the electronic structures of both intrinsic defects (oxygen vacancies and/or Ti3+) and extrinsic dopants (Cr3+) at different reduction temperatures. We identify the existence of two temperature regimes where changes occur within 30 min. The first temperature regime occurs between 300–375 °C and results in (1) reduction of oxygen-related surface defects, and (2) an increase in the concentration of Cr3+ by over an order of magnitude, suggesting that EPR-silent Cr4+ or Cr6+ is being reduced to Cr3+ by NaBH4. The second temperature regime occurs between 375–430 °C where we observe clear evidence of Ti3+ formation by EPR spectroscopy that indicates chemical reduction of the SrTiO3 lattice. In addition, the oxygen-related surface defects observed in regime 1 are not formed in regime 2, but instead lattice oxygen vacancies (VO) are observed by EPR. The changes to the Cr-doped SrTiO3 electronic structure after chemical reduction in regime 1 are quantitatively reversible after aerobic annealing at 400 °C for 30 min. The internal oxygen vacancies formed during the higher temperature reductions in regime 2 require increased temperatures of at least 600 °C to be fully reoxidized in 30 min. The effect of these different oxygen-related defects on the EPR spectrum of substitutional Cr3+ dopants is discussed. These results allow us to independently tune the dopant and host electronic structures of a technologically-relevant multifunctional material by a simple ex situ chemical perturbation.

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.

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.