No abstract

Transition metal oxide heterostructures are interesting due to the distinctly different properties that can arise from their interfaces, such as superconductivity, high catalytic activity, and magnetism. Oxygen point defects can play an important role at these interfaces in inducing potentially novel properties. The design of oxide heterostructures in which the oxygen defects are manipulated to attain specific functionalities requires the ability to resolve the state and concentration of local oxygen defects across buried interfaces. In this work, we utilized a novel combination of hard X-ray photoelectron spectroscopy (HAXPES) and high resolution X-ray diffraction (HRXRD) to probe the local oxygen defect distribution across the buried interfaces of oxide heterolayers. This approach provides a nondestructive way to qualitatively probe locally the oxygen defects in transition metal oxide heterostructures. We studied two trilayer structures as model systems: the La 0.8 Sr 0.2 CoO 3−δ /(La 0.5 Sr 0.5 ) 2 CoO 4−δ / La 0.8 Sr 0.2 CoO 3−δ (LSC 113 /LSC 214 ) and the La 0.8 Sr 0.2 CoO 3−δ /La 2 NiO 4+δ /La 0.8 Sr 0.2 CoO 3−δ (LSC 113 /LNO 214 ) on SrTiO 3 (001) single crystal substrates. We found that the oxygen defect chemistry of these transition metal oxides was strongly impacted by the presence of interfaces and the properties of the adjacent phases. Under reducing conditions, the LSC 113 in the LSC 113 /LNO 214 trilayer had less oxygen vacancies than the LSC 113 in the LSC 113 /LSC 214 trilayer and the LSC 113 single phase film. On the other hand, LSC 214 and LNO 214 were more reduced in the two trilayer structures when in contact with the LSC 113 layer compared to their single phase counterparts. The results point out a potential way to modify the local oxygen defect states at oxide heterointerfaces.

Understanding the stability and reactivity of iron sulfide phases is key to developing predictive capabilities for assessing their corrosion and catalytic activity. The differences between the free surface and the bulk interior of such phases are of particular importance in this context. Here, we employ density functional theory to investigate the formation energetics and electronic structure of intrinsic Fe and S vacancies in bulk pyrite (FeS(2)) and on the pyrite (100) surface. The formation energies of intrinsic bulk vacancies of all charge states are found to be high, ranging from 1.7 to 3.7 eV. While the formation energies of surface vacancies are lower, varying from 1.4 to 2.1 eV for S vacancies and from 0.3 to 1.7 eV for Fe vacancies, they are too large to result in significant sub-stoichiometry in bulk pyrite at moderate temperatures. On the basis of charged defect formation energies and defect equilibria calculations, intrinsic charge carriers are expected to outnumber point defects by several orders of magnitude, and therefore, pure pyrite is not expected to demonstrate p-type or n-type conductivity. The presence of surface states is observed to cause a reduction in the band gap at the (100) surface, which was captured computationally and experimentally using tunneling spectroscopy measurements in this work. The vacancy-induced defect states behave as acceptor-like or donor-like defect states within the bulk band gap. The findings on the stoichiometry and the electronic structure of active sites on the (100) surface have important implications for the reactivity of pyrite.

No abstract

Pyrite (FeS2), being a promising material for future solar technologies, has so far exhibited in experiments an open-circuit voltage (OCV) of around 0.2 V, which is much lower than the frequently quoted 'accepted' value for the fundamental bandgap of ∼0.95 eV. Absorption experiments show large subgap absorption, commonly attributed to defects or structural disorder. However, computations using density functional theory with a semi-local functional predict that the bottom of the conduction band consists of a very low intensity sulfur p-band that may be easily overlooked in experiments because of the high intensity onset that appears 0.5 eV higher in energy. The intensity of absorption into the sulfur p-band is found to be of the same magnitude as contributions from defects and disorder. Our findings suggest the need to re-examine the value of the fundamental bandgap of pyrite presently in use in the literature. If the contribution from the p-band has so far been overlooked, the substantially lowered bandgap would partly explain the discrepancy with the OCV. Furthermore, we show that more states appear on the surface within the low energy sulfur p-band, which suggests a mechanism of thermalization into those states that would further prevent extracting electrons at higher energy levels through the surface. Finally, we speculate on whether misidentified states at the conduction band onset may be present in other materials.