Experimental and Theoretical Studies of the Surface Oxidation Process of Rare‐Earth Tritellurides

Abstract

Recent studies have established Van der Waals (vdW) layered and 2D rareearth tritellurides (RTe 3 ) as superconductors and near room-temperature charge density wave (CDW) materials. Their environmental stability raises natural concern owing to aging/stability effects observed in other tellurium-based layered crystals. Here, the results establish the stability and environmental aging characteristics of these RTe 3 systems involving a variety of metals such as La, Nd, Sm, Gd, Dy, and Ho. The atomic force microscopy (AFM) and scanning electron microscopy (SEM) results show that all the RTe 3 sheets oxidize to form thin TeO x layers that are primarily confined to the surface, edges, and grain boundaries. Time-resolved in situ Raman spectroscopy measurements are used to understand the kinetics of the oxidization process for different lanthanide metal cations and establish their relative stability/resilience to oxidization. Overall results indicate that the vdW layers show higher air stability as the 4f electron number decreases going from Ho to La, resulting in the most stable LaTe 3 compared to the least stable HoTe 3 . Comprehensive quantum mechanical simulations reveal that environmental degradation originates from a strong oxidizing reaction with O 2 molecules, while humidity (H 2 O) plays a negligible role unless Te vacancies are present. Moreover, the simulations explain the effects of 4f electrons on the work function and Te vacancies formation, which directly impact the aging characteristics of RTe 3 layers. Interestingly, optical and electrical measurements show that the CDW response is still observed in aged RTe 3 layers owing to the presence of underlying pristine/ nonoxidized RTe 3 layers, except CDW transition temperatures increase due to the thickness effect. Overall results offer the first in-depth environmental aging studies on these materials, which can be applied to engineer and design their chemical stability, surface properties, and overall CDW characteristics.

Solution Synthesis of NdTe3 Magnetic Nanosheets

“…This cannot be explained by an impurity in the synthesis, and we suggest that this is may be attributed to the reduced thickness of unoxidized NdTe 3 . This effect has been observed in the increase in T CDW with aging as observed for RTe 3 …”

“…This effect has been observed in the increase in T CDW with aging as observed for RTe 3 . 47 The M vs H data for bulk NdTe 3 indicates that the material does not saturate and there is a metamagnetic transition around 5 T. 24 We found that the magnetization vs field measurements of the two materials differ: thick NdTe 3 nanosheets show a near linear moment as a function of applied field (typical of an antiferromagnet), while the thin NdTe 3 thin nanosheets exhibit a soft 's' curve. While this shape could indicate ferromagnetic coupling, there is no evidence of hysteresis or an increased saturation magnetization making this conclusion unlikely.…”

“…Based on EDS and XPS analysis of the sputtered NdTe 3 nanosheets, there was no evidence of Li from the Li(HMDS) used in the synthesis of the thicker NdTe 3 nanosheets. The lanthanide tritellurides exhibit air sensitivity, and the first evidence of oxidation is TeO 2 , which may be observed in the Raman spectroscopy 25 , 47 and here evidenced in the XPS as Te(IV) ( SI-2 ). It was possible to remove this surface oxide by sputtering with argon ions, simplifying spectra for analysis of the tellurium oxidation state ( Figure 5 ).…”