Comparison of the electronic structure of LnBaCo(2)O(5+delta) (Ln = Gd, Dy; Ln-112) and LnBaCo(4)O(7) (Ln = Yb; Ln-114) single-crystal surfaces using resonant photoemission

A comparison of the electronic structure of LnBaCo2O5+( (Ln = Gd, Dy; Ln-112) and LnBaCo4O7+δ (Ln = Yb; Ln-114) single-crystal surfaces has been made using synchrotron photoemission spectroscopy. Resonant photoemission is used to identify the atomic parentage of the valence band states of Ln-114. The states close to the Fermi energy are found to be of mixed Co 3d/O 2p character. Comparison of the photoemission results for the two systems allows unambiguous identification of the spectral signal due to low spin octahedral Co3+ in Ln-112. High resolution valence band spectra taken as a function of temperature reveal the presence of the metal–insulator (MI) transition in Ln-112 in the 300–400 K temperature range. The gradual changes in the spectral profile of the low energy states with temperature rule out a sudden ‘high spin–low spin’ switch as the mechanism of the MI transition. They are instead consistent with a gradually shifting equilibrium between three states—low spin, intermediate spin and high spin

Chemically-specific time-resolved surface photovoltage spectroscopy: Carrier dynamics at the interface of quantum dots attached to a metal oxide

All rights reserved.We describe a new experimental pump-probe methodology where a 2D delay-line detector enables fast (ns) monitoring of a narrow XPS spectrum in combination with a continuous pump laser. This has been developed at the TEMPO beamline at Synchrotron SOLEIL to enable the study of systems with intrinsically slow electron dynamics, and to complement faster measurements that use a fs laser as the pump. We demonstrate its use in a time-resolved study of the surface photovoltage of the m-plane ZnO (101¯0) surface which shows persistent photoconductivity, requiring monitoring periods on ms timescales and longer. We make measurements from this surface in the presence and absence of chemically-linked quantum dots (QDs), using type I PbS and type II CdSe/ZnSe (core/shell) QDs as examples. We monitor signals from both the ZnO substrate and the bound QDs during photoexcitation, yielding evidence for charge injection from the QDs into the ZnO. The chemical specificity of the technique allows us to observe differences in the extent to which the QD systems are influenced by the field of the surface depletion layer at the ZnO surface, which we attribute to differences in the band structure at the interface

Dynamics in next-generation solar cells: time-resolved surface photovoltage measurements of quantum dots chemically linked to ZnO (101[combining macron]0)

The charge dynamics at the surface of the transparent conducting oxide and photoanode material ZnO are investigated in the presence and absence of light-harvesting colloidal quantum dots (QDs). The time-resolved change in surface potential upon photoexcitation has been measured in the m-plane ZnO (101[combining macron]0) using a laser pump-synchrotron X-ray probe methodology. By varying the oxygen annealing conditions, and hence the oxygen vacancy concentration of the sample, we find that dark carrier lifetimes at the ZnO surface vary from hundreds of μs to ms timescales, i.e. a persistent photoconductivity (PPC) is observed. The highly-controlled nature of our experiments under ultra-high vacuum (UHV), and the use of band-gap and sub-band-gap photoexcitation, allow us to demonstrate that defect states ca. 340 meV above the valence band edge are directly associated with the PPC, and that the PPC mediated by these defects dominates over the oxygen photodesorption mechanism. These observations are consistent with the hypothesis that ionized oxygen vacancy states are responsible for the PPC in ZnO. The effect of chemically linking two colloidal QD systems (type I PbS and type II CdS-ZnSe) to the surface has also been investigated. Upon deposition of the QDs onto the surface, the dark carrier lifetime and the surface photovoltage are reduced, suggesting a direct injection of charge carriers into the ZnO conduction band. The results are discussed in the context of the development of next-generation solar cells

Electronic and Optical Properties of Dye-Sensitized TiO2 Interfaces

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