Concurrent La and A-site Vacancy Doping Modulates the Thermoelectric Response of SrTiO3. Experimental and Computational Evidence

To help understand the factors controlling the performance of one of the most promising n-type oxide thermoelectric SrTiO3, we need to explore structural control at the atomic level. In Sr1–xLa2x/3TiO3 ceramics (0.0 ≤ x ≤ 0.9), we determined that the thermal conductivity can be reduced and controlled through an interplay of La-substitution and A-site vacancies and the formation of a layered structure. The decrease in thermal conductivity with La and A-site vacancy substitution dominates the trend in the overall thermoelectric response. The maximum dimensionless figure of merit is 0.27 at 1070 K for composition x = 0.50 where half of the A-sites are occupied with La and vacancies. Atomic resolution Z-contrast imaging and atomic scale chemical analysis show that as the La content increases, A-site vacancies initially distribute randomly (x < 0.3), then cluster (x ≈ 0.5), and finally form layers (x = 0.9). The layering is accompanied by a structural phase transformation from cubic to orthorhombic and the formation of 90° rotational twins and antiphase boundaries, leading to the formation of localized supercells. The distribution of La and A-site vacancies contributes to a nonuniform distribution of atomic scale features. This combination induces temperature stable behavior in the material and reduces thermal conductivity, an important route to enhancement of the thermoelectric performance. A computational study confirmed that the thermal conductivity of SrTiO3 is lowered by the introduction of La and A-site vacancies as shown by the experiments. The modeling supports that a critical mass of A-site vacancies is needed to reduce thermal conductivity and that the arrangement of La, Sr, and A-site vacancies has a significant impact on thermal conductivity only at high La concentration

Nanoscale silver oxide: A rewritable optical recording medium

Measurements are presented that demonstrate the possibility of rewritable optical recording on nanoscale silver oxide films. Three different sputtering techniques are studied, seeking an optimum and requisite film structure. Estimates of the energy requirements of the recording process are made for comparison with conventional thermal-based recording mechanisms. The recording process appears purely photonic and does not require optical heating to bring about the reversible physical changes that constitute the presence or absence of data. Although more complex than conventional optical recording techniques in that it requires the use of different wavelengths for the write and readout processes, and the presence or absence of data is signified by the emission or lack of radiation at yet a third wavelength, the advantages of its nonthermal nature should outweigh this additional complexity

Ab initio calculations of the electronic structure and magnetism of iron porphyrin-type molecules: a benchmarking study

The iron porphyrin molecule is one of the most important biomolecules. In spite of its importance to life science, on a microscopic scale its electronic properties are not yet well-understood. In order to achieve such understanding we have performed an ab initio computational study of various molecular models for the iron porphyrin molecule. Our ab initio electronic structure calculations are based on the density functional theory (DFT) and have been conducted using both the Generalised Gradient Approximation (GGA) and the GGA+U approach, in which an additional Hubbard-U term is added for the treatment of on-site electron-electron correlations. In our investigations we have, first, optimised the molecular structures by computing the minimal-energy atomic distances, and second, benchmarked our computational approach by comparison to existing calculated results obtained by quantum-chemical methods. We have considered several models of ligated porphyrin (Cl and NH3 ligated), as well as charged and non-charged molecules. In this way, the changes in the electronic, structural, and magnetic properties of the iron atom have been investigated as a function of the oxidation state and local environment of the iron atom. Our results for some of the model molecules reproduce the earlier quantum-chemical calculations done by Johansson and Sundholm [J. Chem. Phys. 120 (2003) 3229]. We find that the GGA+U approach provides a better description of the molecular electronic properties, which indicates that electron correlation effects on the iron are important and play an essential role, particularly for the spin moment on the iron atom. Also, we proceed beyond the relatively small molecular models to a larger, more realistic porphyrin molecule, for which we also find that the GGA+U results are in better agreement with experiments

GGA+U modeling of structural, electronic, and magnetic properties of iron porphyrin-type molecules

AB An ab initio computational study of various iron porphyrin-type molecules has been performed. Our ab initio calculations are based on the density functional theory (DFT) and have been conducted using the generalized gradient approximation (GGA, with PW91 & PBE versions) as well as GGA + U approach, in which an additional Hubbard-U term is added for the treatment of strong on-site 3d electron-electron interactions on Fe. We have, first, optimized the atomic distances for the porphyrin models by minimizing the total energy. Second, we benchmarked our computational approach by comparison to existing calculated results for relatively small porphyrin models obtained by the Becke-Lee-Yang-Parr (BLYP) exchange-correlation functional. We have considered several models of ligated porphyrins (Cl and NH3 ligated), as well as charged and neutral molecules, to study properties of the molecules as a function of oxidation state and local chemical environment of the Fe atom. We find that the GGA + U (with U approximate to 4 eV) approach provides a better description of the molecular electronic properties for five coordinated (Fe-III) systems than the standard GGA approach, which indicates that Coulombic electron interaction effects on the Fe are important and play an essential role, particularly for the spin moment on the molecule. Also, we proceed to a larger, more realistic Fe-porphyrin model (FeP), for which we also investigate the performance of the GGA and GGA + U functionals. The character of the electronic states involved in the chemical bonding has been analyzed with the aid of energy resolved charge densities

Halide Ligated Iron Porphines: A DFT+Uand UB3LYP Study

We apply the density functional theory + U (DFT+U) and unrestricted hybrid functional DFT-UB3LYP methods to study the electronic structure and magnetic properties of two prototypical iron porphines: iron(III) porphine chloride (FePCl) and difluoro iron(III−IV) porphine. Plain DFT within the generalized gradient approximation (GGA) implementation fails in describing the correct high-spin ground state of these porphine molecules, whereas DFT+U and UB3LYP provide an improved description. For a range of U values (4−8 eV), we compare the results of the DFT+U approach to those obtained previously with the hybrid functional (B3LYP) and with the CASPT2 approach. The DFT+U and UB3LYP methods successfully predict the molecular high spin (S = 5/2) ground state of FePCl, and also provide the nontrivial S = 3 high spin ground state for FePF2. For the latter six-coordinated Fe porphine, our DFT+U calculations show that the S = 2, S = 5/2, and S = 3 states are energetically very close together (differences of 30 meV). Nonetheless, S = 3 is obtained as the ground state of the whole molecule, in accordance with the spin expected from the electron count. Our DFT+U calculations show furthermore that the Fe 3d occupancy is similar for FePF2 and FePCl, i.e., DFT+U does not support Fe(IV) for FePF2, but rather an Fe(III) porphyrin π-cation radical species, with an Fe high spin SFe = 5/2, and an additional S = 1/2 stemming from spin density distributed over the porphine ring. This observation is also supported by our UB3LYP calculations

Nature of the magnetic interaction between Fe-porphyrin molecules and ferromagnetic surfaces

We have investigated computationally the magnetic spin state of free metalloporphyrins and how magnetic ordering in metalloporphyrins can be induced through contact with the metallic surface and what the origin of the exchange interaction is. To this end, we performed density functional theory (DFT) and DFT + U studies for a series of isolated, ligated as well as unligated Fe-porphyrin (FeP) molecules as well as various FeP molecules on surfaces. Our calculations for isolated FePs clearly demonstrate that the usual DFT-based exchange-correlation functionals (such as the generalized gradient approximation) cannot predict the experimental high-spin ground state of these molecules. Instead, one has to resort to DFT + U calculations with a Coulomb U of about 4 eV on the Fe atoms, to obtain the correct single-molecule spin state. The magnetic interaction between FeP and a Co surface has been studied computationally with the DFT and DFT + U approaches. Our total energy DFT and DFT + U calculations predict an optimal Fe - substrate distance of 3.5 Å and a ferromagnetic exchange coupling of FeP to the substrate, in accordance with recent experiments. For Fe-porphyrin chloride (FePCl), on the other hand, an antiferromagnetic coupling is computed to be more favorable. Our study demonstrates that due to an indirect exchange interaction, which is mediated through the four nitrogen atoms, ferromagnetic ordering on the FeP is stabilized. © 2009 Elsevier Ltd. All rights reserved

Manipulation of spin state of iron porphyrin by chemisorption on magnetic substrates

One of the key factors behind the rapid evolution of molecular spintronics is the efficient realization of spin manipulation of organic molecules with a magnetic center. The spin state of such molecules may depend crucially on the interaction with the substrate on which they are adsorbed. In this paper we demonstrate, using ab initio density functional calculations, that the stabilization of a high spin state of an iron porphyrin (FeP) molecule can be achieved via chemisorption on magnetic substrates of different species and orientations, viz., Co(001), Ni(001), Ni(110), and Ni(111). The signature of chemisorption of FeP on magnetic substrates is evident from broad features in N K x-ray absorption (XA) and Fe L2,3 x-ray magnetic circular dichroism (XMCD) measurements. Our theoretical calculations show that the strong covalent interaction with the substrate increases Fe-N bond lengths in FeP and hence a switching to a high spin state (S=2) from an intermediate spin state (S=1) is achieved. Due to chemisorption, ferromagnetic exchange interaction is established through a direct exchange between Fe and substrate magnetic atoms as well as through an indirect exchange via the N atoms in FeP. The mechanism of exchange interaction is further analyzed by considering structural models constructed from ab initio calculations. Also, it is found that the exchange interaction between Fe in FeP and a Ni substrate is almost 4 times smaller than with a Co substrate. Finally, we illustrate the possibility of detecting a change in the molecular spin state by XMCD, Raman spectroscopy, and spin-polarized scanning tunneling microscopy