Kinetics of the Topochemical Transformation of (PbSe)_m(TiSe₂)_n(SnSe₂)_m(TiSe₂)_n to (Pb_0.5Sn_0.5Se)_m(TiSe₂)_n

Abstract

Solid-state reaction kinetics on atomic length scales have not been heavily investigated due to the long times, high reaction temperatures, and small reaction volumes at interfaces in solid-state reactions. All of these conditions present significant analytical challenges in following reaction pathways. Herein we use in situ and ex situ X-ray diffraction, in situ X-ray reflectivity, high-angle annular dark field scanning transmission electron microscopy, and energy-dispersive X-ray spectroscopy to investigate the mechanistic pathways for the formation of a layered (Pb 0.5 Sn 0.5 Se) 1+δ (TiSe 2 ) m heterostructure, where m is the varying number of TiSe 2 layers in the repeating structure. Thin film precursors were vapor deposited as elemental-modulated layers into an artificial superlattice with Pb and Sn in independent layers, creating a repeating unit with twice the size of the final structure. At low temperatures, the precursor undergoes only a crystallization event to form an intermediate (SnSe 2 ) 1+γ (TiSe 2 ) m (PbSe) 1+δ (TiSe 2 ) m superstructure. At higher temperatures, this superstructure transforms into a (Pb 0.5 Sn 0.5 Se) 1+δ (TiSe 2 ) m alloyed structure. The rate of decay of superlattice reflections of the (SnSe 2 ) 1+γ (TiSe 2 ) m (PbSe) 1+δ (TiSe 2 ) m superstructure was used as the indicator of the progress of the reaction. We show that increasing the number of TiSe 2 layers does not decrease the rate at which the SnSe 2 and PbSe layers alloy, suggesting that at these temperatures it is reduction of the SnSe 2 to SnSe and Se that is rate limiting in the formation of the alloy and not the associated diffusion of Sn and Pb through the TiSe 2 layers.

Synthesis of Metastable Inorganic Solids with Extended Structures

“…[81] For example, if one compares the relative rates of mixing of the elemental layers two precursors, a|x|a|x|a|x|b|x versus 3a|3x|b|x, the a-x layer with a single bilayer that is three times thicker, [3a|3x], will mix 9 times slower relative to the layer with three layers that are 1/3 of the thickness if Fick's laws for diffusion hold. [97] This difference in thickness can be used to control which layer nucleates and grows first. The layer sequence in a precursor can also have dramatic effects on the sequence of compounds that form, particularly if there are large differences in the interdiffusion rates of the elements through similar matrices.…”

Atomic Resolution Analysis of Defect Structures in Multi-layer Chalcogenide Films

“…Motivated by these possibilities, the ability to grow hierarchical chalcogenide structures with precisely tailored atomic-scale composition has advanced rapidly in recent years. For instance, the modulated elemental reactants approach [1] has enabled rigorous control of 2D layer sequencing, producing a wide variety of novel metal chalcogenide layer configurations (e.g., references [2][3][4]). Although the layer-sequencing can now be controlled with great precision, a key remaining challenge is to control the order and defect configurations within the layers themselves.…”