Sutherland Duncan R, Merrill Devin R, Ditto Jeffrey, Moore Daniel B, Medlin Douglas, Johnson David C
Materials Science Institute and Department of Chemistry and Biochemistry , University of Oregon , Eugene , Oregon 97403 , United States.
Sandia National Laboratories , Livermore , California 94551 , United States.
J Am Chem Soc. 2019 Jan 16;141(2):922-927. doi: 10.1021/jacs.8b10681. Epub 2018 Dec 28.
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 (PbSnSe)(TiSe) heterostructure, where m is the varying number of TiSe 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)(TiSe) (PbSe)(TiSe) superstructure. At higher temperatures, this superstructure transforms into a (PbSnSe)(TiSe) alloyed structure. The rate of decay of superlattice reflections of the (SnSe)(TiSe) (PbSe)(TiSe) superstructure was used as the indicator of the progress of the reaction. We show that increasing the number of TiSe layers does not decrease the rate at which the SnSe and PbSe layers alloy, suggesting that at these temperatures it is reduction of the SnSe 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 layers.
由于固态反应中界面处反应时间长、反应温度高且反应体积小，原子尺度上的固态反应动力学尚未得到深入研究。所有这些条件在追踪反应路径时都带来了重大的分析挑战。在此，我们使用原位和非原位X射线衍射、原位X射线反射率、高角度环形暗场扫描透射电子显微镜以及能量色散X射线光谱来研究层状(PbSnSe)(TiSe)异质结构形成的机理途径，其中m是重复结构中TiSe层的可变数量。薄膜前驱体通过气相沉积以元素调制层的形式沉积到具有独立层的Pb和Sn的人工超晶格中，形成一个大小为最终结构两倍的重复单元。在低温下，前驱体仅经历结晶过程以形成中间(SnSe)(TiSe)(PbSe)(TiSe)超结构。在较高温度下，该超结构转变为(PbSnSe)(TiSe)合金结构。(SnSe)(TiSe)(PbSe)(TiSe)超结构的超晶格反射的衰减速率被用作反应进程的指标。我们表明，增加TiSe层的数量并不会降低SnSe和PbSe层合金化的速率，这表明在这些温度下，SnSe还原为SnSe和Se是合金形成的速率限制因素，而不是Sn和Pb通过TiSe层的相关扩散。
