Mechanistic Studies of Gas Reactions with Multicomponent Solids: What Can We Learn By Combining NAP XPS and Atomic Resolution STEM/EDX?

Abstract

Rapid development of experimental techniques has enabled real time studies of solid−gas reactions at the level reaching the atomic scale. In the present paper, we focus on a combination of atomic resolution STEM/EDX, which visualizes the reaction zone, and near ambient pressure (NAP) XPS, which collects information for a surface layer of variable thickness under reaction conditions. We compare the behavior of two affined topological insulators, Bi 2 Te 3 and Sb 2 Te 3 . We used a simple reaction with molecular oxygen occurring at 298 K, which is of practical importance to avoid material degradation. Despite certain limitations, a combination of in situ XPS and ex situ cross-sectional STEM/EDX allowed us to obtain a self-consistent picture of the solid−gas reaction mechanism for oxidation of Sb 2 Te 3 and Bi 2 Te 3 crystals, which includes component redistribution between the oxide and the subsurface layer and Te segregation with formation of a thin ordered layer at the interface. The process is multistep in case of both compounds. At the very beginning of the oxidation process the reactivity is determined by the energy benefit of the corresponding element−oxygen bond formation. Further in the oxidation process, the behavior of these two compounds becomes similar and features component redistribution between the oxide and the subsurface layer.

Atomic and Electronic Structure of a Multidomain GeTe Crystal

“…%, giving a hole concentration from 10 19 to 10 21 cm –3 for Ge- and Te-boundaries, respectively. − Our samples are Te-enriched and, therefore, the bulk density of vacancies is sufficient to form BB interfaces with a mean spacing of ∼10 nm. The BB interface seems to be similar to the vdW gap in layered Bi 2 Te 3 . , The decisive role of vacancies in the formation of such interfaces is supported by the fact that the domain structure with a planar inversion domain is favored for Te-rich GeTe. This agrees with Sist et al ., who speculated the possibility of vacancy accumulation at domain boundaries.…”

“…The BB interface seems to be similar to the vdW gap in layered Bi 2 Te 3 . 36,37 The decisive role of vacancies in the formation of such interfaces is supported by the fact that the domain structure with a planar inversion domain is favored for Te-rich GeTe. This agrees with Sist et al, 35 who speculated the possibility of vacancy accumulation at domain boundaries.…”

Nanoscale phase separation in the oxide layer at GeTe (111) surfaces

“…It was established that an X-ray beam and/or an emitting electron beam can both modify the reaction rate and alter the reaction pathway. 24 Our recent observations made for compound semiconductors such as Bi 2 Te 3 , Sb 2 Te 3 , and some mixed crystals in their reaction with oxygen, 24,25,44 have revealed essential enhancement of the oxidation rate during the in situ experiments, while the underlying chemical mechanism typically remains intact. The possible influence of radiation can be investigated by comparing the data obtained in situ and ex situ.…”

“…40 Generally, the mechanism of GeTe oxidation is very similar to that of Sb 2 Te 3 , which has recently been reported in ref. 24 including the formation of Ge or Sb oxide at the very surface with elemental tellurium formed underneath. The main reason for this is the higher energy benefit of Ge-O (−560 kJ mol −1 ) 45 or Sb-O bond formation compared to that of Te-O (−323 kJ mol −1 ).…”

“…Nowadays, the state-of-the-art methods to probe the reactivity and solid-gas reaction kinetics are near ambient pressure (NAP) X-ray photoelectron spectroscopy (XPS), which collects information for a surface layer of variable thickness under the reaction conditions, 21,22 and atomic resolution scanning transmission electron microscopy (STEM) combined with energy-dispersive X-ray spectroscopy (EDX), which visualizes the reaction zone. 23 Despite certain limitations, their combination allowed obtaining a self-consistent picture of the solid-gas reaction mechanism, for instance, for the oxidation of Sb 2 Te 3 and Bi 2 Te 3 crystals 24 and the mixed crystals of (Bi,Sb) 2 Te 3 . 25 The present paper is devoted to advanced studies of the growth kinetics and structure of the oxide layer formed at the GeTe surface, as well as to the mechanism of its formation.…”