The carrier-type of the emerging photovoltaic Sb2Se3 was evaluated for both thin films and bulk crystals via a range of complementary techniques. X-ray photoelectron spectroscopy (XPS), hot probe, Hall effect, and surface photovoltage spectroscopy showed films and crystals synthesized from the Sb2Se3 granulate material to be n-type with chlorine identified as an unintentional n-type dopant via secondary ion mass spectrometry analysis. The validity of chlorine as a dopant was confirmed by the synthesis of intrinsic crystals from metallic precursors and subsequent deliberate n-type doping by the addition of MgCl2. Chlorine was also shown to be a substitutional n-type shallow dopant by density functional theory calculations. TiO2/Sb2Se3 n–n isotype heterojunction solar cells with 7.3% efficiency are subsequently demonstrated, with band alignment analyzed via XPS.

Atomically thin two-dimensional (2D) materials exhibit superlative properties dictated by their intralayer atomic structure, which is typically derived from a limited number of thermodynamically stable bulk layered crystals (e.g., graphene from graphite). The growth of entirely synthetic 2D crystals, those with no corresponding bulk allotrope, would circumvent this dependence upon bulk thermodynamics and substantially expand the phase space available for structure-property engineering of 2D materials. However, it remains unclear if synthetic 2D materials can exist as structurally and chemically distinct layers anchored by van der Waals (vdW) forces, as opposed to strongly bound adlayers. Here, we show that atomically thin sheets of boron (i.e., borophene) grown on the Ag(111) surface exhibit a vdW-like structure without a corresponding bulk allotrope. Using X-ray standing wave-excited X-ray photoelectron spectroscopy, the positions of boron in multiple chemical states are resolved with sub-angström spatial resolution, revealing that the borophene forms a single planar layer that is 2.4 Å above the unreconstructed Ag surface. Moreover, our results reveal that multiple borophene phases exhibit these characteristics, denoting a unique form of polymorphism consistent with recent predictions. This observation of synthetic borophene as chemically discrete from the growth substrate suggests that it is possible to engineer a much wider variety of 2D materials than those accessible through bulk layered crystal structures.

We study 1s and 2p hard x-ray photoemission spectra (XPS) in a series of late transition metal oxides: Fe2O3 (3d 5 ), FeTiO3 (3d 6 ), CoO (3d 7 ) and NiO (3d 8 ). The experimental spectra are analyzed with two theoretical approaches: MO6 cluster model and local density approximation (LDA) + dynamical mean-field theory (DMFT). Owing to the absence of the core-valence multiplets and spin-orbit coupling, 1s XPS is found to be a sensitive probe of chemical bonding and nonlocal charge-transfer screening, providing complementary information to 2p XPS. The 1s XPS spectra are used to assess the accuracy of the ab-initio LDA+DMFT approach, developed recently to study the material-specific charge-transfer effects in core-level XPS.

Sensitivity to the "bulk" oxygen core orbital makes hard X-ray photoelectron spectroscopy (HAXPES) an appealing technique for studying oxygen redox candidates. Various studies have reported an additional O 1s peak (530−531 eV) at high voltages, which has been considered a direct signature of the bulk oxygen redox process. Here, we find the emergence of a 530.4 eV O 1s HAXPES peak for three model cathodesLi 2 MnO 3 , Li-rich NMC, and NMC 442that shows no clear link to oxygen redox. Instead, the 530.4 eV peak for these three systems is attributed to transition metal reduction and electrolyte decomposition in the near-surface region. Claims of oxygen redox relying on photoelectron spectroscopy must explicitly account for the surface sensitivity of this technique and the extent of the cathode degradation layer.

This contribution reports the first X-ray standing wave structural characterization of self-assembled NLO-active chromophoric multilayers (SAMs). These siloxane-based self-assembled stilbazolium multilayers are intrinsically acentric and exhibit very large second-order nonlinear optical responses. The locations of bromide ions within the SAMs were precisely determined using X-ray standing waves generated by total external reflection from mirror surfaces as well as by Bragg diffraction from layered synthetic microstructures. The large coherent fraction (i.e., small Gaussian distribution width) of the Br- ions provides direct evidence for the high structural regularity of these self-assembled multilayers along the surface normal direction. These results are supported by atomic force microscopic (AFM) and X-ray photoelectron spectroscopic (XPS) studies which probe the structural regularity and chemical composition of SAMs, respectively. The anion surface coverage has also been measured in this study (2.5(5) × 1014 Br-/cm2) and is in excellent agreement with the cation surface coverage measured by second harmonic generation (2(1) × 1014 molecules/cm2). These results clearly demonstrate the utility of X-ray standing wave analyses as a quantitative microstructural probe for self-assembled mono- and multilayers, especially for SAMs with incommensurate structures.