X-ray photoelectron spectroscopy studies of non-stoichiometric superconducting NbB2+x

TL;DR

This study uses X-ray photoelectron spectroscopy to analyze how varying boron content in NbB2+x affects surface chemistry, electronic structure, and superconducting properties, revealing chemical shifts and electronic modifications.

Contribution

It provides detailed surface and electronic structure analysis of non-stoichiometric NbB2+x, linking compositional changes to electronic and superconducting properties with experimental and theoretical insights.

Findings

Nb and B oxides are present on the surface and decrease after etching.

Core level binding energies increase with boron content, indicating chemical shifts.

Electronic structure changes correlate with modifications in superconducting properties.

Abstract

Polycrystalline samples of NbB2+x with nominal composition (B/Nb) = 2.0, 2.1, 2.2, 2.3, 2.4 and 2.5 were studied by X-ray photoelectron spectroscopy (XPS). The spectra revealed Nb and B oxides on the surface of the samples, mainly B2O3 and Nb2O5. After Ar ion etching the intensity of Nb and B oxides decreased. The Nb 3d5/2 and B 1s core levels associated with the chemical states (B/Nb) were identified and they do not change with etching time. The Binding Energy of the Nb 3d5/2 and B 1s core levels increase as boron content increases, suggesting a positive chemical shift in the core levels. On the other hand, analysis of Valence Band spectra showed that the contribution of the Nb 4d states slightly decreased while the contribution of the B 2p(pi) states increased as the boron content increased. As a consequence, the electronic and superconducting properties were substantially modified, in good agreement with band-structure calculations.