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Abstract: Hybridization of electronic states and orbital symmetry in transition metal oxides are generally considered key ingredients in the description of both their electronic and magnetic properties. In the prototypical case of La 0.65 Sr 0.35 MnO 3 (LSMO), a landmark system for spintronics applications, a description based solely on Mn 3 d and O 2 p electronic states is reductive. We thus analyzed elemental and orbital distributions in the LSMO valence band through a comparison between density functional theory calculations and experimental photoelectron spectra in a photon energy range from soft to hard x rays. We reveal a number of hidden contributions, arising specifically from La 5 p , Mn 4 s , and O 2 s orbitals, considered negligible in previous analyses; our results demonstrate that all these contributions are significant for a correct description of the valence band of LSMO and of transition metal oxides in general.

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Abstract: One of the most important functionalities of the atomically thin insulator hexagonal boron nitride (hBN) is its ability to chemically and electronically decouple functional materials from highly reactive surfaces. It is therefore of utmost importance to uncover its structural properties on surfaces on an atomic and mesoscopic length scale. In this paper, we quantify the relative coverages of structurally different domains of a hBN layer on the Ni(111) surface using low-energy electron microscopy and the normal incidence x-ray standing wave technique. We find that hBN nucleates on defect sites of the Ni(111) surface and predominantly grows in two epitaxial domains that are rotated by 60 ∘ with respect to each other. The two domains reveal identical adsorption heights, indicating a similar chemical interaction strength with the Ni(111) surface. The different azimuthal orientations of these domains originate from different adsorption sites of N and B. We demonstrate that the majority ( ≈ 70 % ) of hBN domains exhibit a ( N , B ) = ( top , fcc ) adsorption site configuration while the minority ( ≈ 30 % ) show a ( N , B ) = ( top , hcp ) configuration. Our study hence underlines the crucial role of the atomic adsorption configuration in the mesoscopic domain structures of in situ fabricated two-dimensional materials on highly reactive surfaces.

Open Access

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Abstract: Germanium sulfide and germanium selenide bulk crystals were prepared using a melt growth technique. X-ray photoemission spectroscopy (XPS) was used to determine ionisation potentials of 5.74 and 5.48 eV for GeS and GeSe respectively. These values were used with the previously-measured band gaps to establish the natural band alignments with potential window layers for solar cells and to identify CdS and TiO2 as sensible choices. The ionisation potential of GeS is found to be smaller than in comparable materials. Using XPS and hard x-ray photoemission (HAXPES) measurements in conjunction with density-functional theory calculations, we demonstrate that stereochemically active Ge 4s lone pairs are present at the valence-band maxima. Our work thus provides direct evidence for active lone pairs in GeS and GeSe, with important implications for the applications of these and related materials, such as Ge-based perovskites.

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Abstract: In this paper, we report insights into the local atomic and electronic structure of NiCo 2 O 4 epitaxial thin films and its correlation with electrical, optical, and magnetic properties. We grew structurally well-defined NiCo 2 O 4 epitaxial thin films with controlled properties on Mg Al 2 O 4 ( 001 ) substrates using pulsed laser deposition. Films grown at low temperatures ( < 400 ∘ C ) exhibit a ferrimagnetic and metallic behavior, while those grown at high temperatures are nonmagnetic semiconductors. The electronic structure and cation local atomic coordination of the respective films were investigated using a combination of resonant photoemission spectroscopy, x-ray absorption spectroscopy, and ab initio calculations. Our results unambiguously reveal that the Ni 3 + valence state promoted at low growth temperature introduces delocalized Ni 3 d -derived states at the Fermi level ( E F ), responsible for the metallic state in NiCo 2 O 4 , while the Co 3 d -related state is more localized at higher binding energy. In the semiconducting films, the valence state of Ni is lowered and ∼ + 2 . Further structural and defect chemistry studies indicate that the formation of oxygen vacancies and secondary CoO phases at high growth temperature are responsible for the Ni 2 + valence state in NiCo 2 O 4 . The Ni 3 d -related state becomes localized away from E F , opening a band gap for a semiconducting state. The band gap of the semiconducting NiCo 2 O 4 is estimated to be < 0.8 eV , which is much smaller than the quoted values in the literature ranging from 1.1 to 2.58 eV. Despite the small band gap, its optical transition is d − d dipole forbidden, and therefore, the semiconducting NiCo 2 O 4 still shows reasonable transparency in the infrared-visible light region. The present insights into the role of Ni 3 + in determining the electronic structure and defect chemistry of NiCo 2 O 4 provide important guidance for use of NiCo 2 O 4 in electrocatalysis and opto-electronics.

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Abstract: SnS and SnS2 are earth abundant layered semiconductors that owing to their optoelectronic properties have been proposed as materials for different photovoltaic, photosensing and photocatalytic applications. The intrinsic efficiency of these materials for such applications is driven by their charge transfer dynamics, which in turn depend on their electronic structure and the interaction of the molecular orbitals involved in the charge transfer process. In this publication, we provide a step-by-step description of the use of the core hole clock method to obtain orbital dependent charge transfer times for SnS2 and SnS down to the attosecond time scale. We use both S 1s and Sn 3d core holes, with natural core hole lifetimes of 1.3 fs and 300 as, as time references to obtain a complete picture of electron delocalisation times across the conduction band of both materials. Ultrafast electron delocalisation times, lower than 5 femtoseconds and as low as tens of attoseconds, are measured for electrons excited to the unoccupied molecular orbitals of both materials. SnS delocalisation times are found to be shorter than those for SnS2 for all probed molecular orbitals, with delocalisation times being more than an order of magnitude shorter for higher lying molecular orbitals. DFT calculations show that charge transfer dynamics are strongly influenced by the interlayer interaction within these materials. The slower delocalisation of electrons in SnS2 can be linked to the restricted out of plane spatial dispersion of the molecular orbitals in the conduction band and consequent limitation of the electron delocalisation pathways. The results presented in this paper highlight the high potential of combining the core hole clock method with high-level DFT calculations to study orbital dependent charge transfer in semiconductor materials for optoelectronic applications down to the attosecond scale.