Near-Atomic-Scale Compositional Complexity in a 2D Transition Metal Oxide

Abstract

2D materials hold transformative promise for next-generation nanoelectronics. However, successfully integrating these materials from laboratory-scale discoveries into real-world devices depends on precisely controlling their properties, which are fundamentally determined by their composition. Detailed characterisation using atom probe tomography of 2D Ti0.87O2, a candidate high-$κ$ dielectric, reveals deviations from its commonly assumed stoichiometry. Compositional analysis and comparison with the bulk K0.8[Ti1.73Li0.27]O4 precursor evidences an oxygen deficit indicative of oxygen vacancy formation in the 2D material, as well as the retention of low concentrations of alkali metals that were presumed to be removed during synthesis. Such deviations from stoichiometry indicate a reconstruction mechanism that mitigates the effect of the characteristic, negatively charged vacancies on the titanium sublattice, thereby influencing the local electronic structure and, consequently, functional properties. These findings underscore the importance of a detailed compositional analysis in both understanding and optimizing the extraordinary functional properties of 2D materials, opening pathways to tailored functionalities in next-generation nanoelectronics.

Figures (6)

• Figure 1: Synthesis of the potassium lithium titanate precursor and derived 2D transition metal oxide. (a) Synthesised K_0.8[Ti_1.73Li_0.27]O4 powder. (b) Exfoliated 2D Ti_0.87O2 nanosheets. (c) Top-view electron micrograph of the 2D material film, with inset optical photograph highlighting its optical transparency. All scanning electron microscopy images were acquired using the secondary electron imaging mode. (d) X-ray diffraction data for the lepidocrocite-type K_0.8[Ti_1.73Li_0.27]O4 powder and the 2D Ti_0.87O2 film.
• Figure 2: High-resolution X-ray photoelectron Ti $2p$, O $1s$, and K $2p$ core level spectra for the 2D Ti_0.87O2 film.
• Figure 3: Influence of the electrostatic field conditions on the measured oxygen to titanium ratio in bulk K_0.8[Ti_1.73Li_0.27]O4 and 2D Ti_0.87O2, as inferred from the TiO++ to TiO+ charge state ratio. The fitted line for the bulk K_0.8[Ti_1.73Li_0.27]O4 data points is intended solely as a guide to illustrate the calibration curve.
• Figure 4: APT analysis of the 2D Ti_0.87O2 material coated in situ with palladium. (a) Reconstructed 3D atom map. (b) 1D compositional profile ($\varnothing$10 x 40) across the region of interest as indicated in (a). Errors are estimated according to counting statistics.
• Figure 5: First nearest-neighbour analysis for lithium and potassium in 2D Ti_0.87O2, calculated within a sub-volume containing more than 80Ti + O. Sample width ion-pair 0.01nm.