Diffusion and phase transformations at 1D interfaces studied in 2D materials

Curious about interfaces and phase transformations in 2D materials?

Pawan Kumar, Deep Jariwala, and their colleagues at the University of Pennsylvania published a study using their Hummingbird Scientific in-situ MEMS Heating + Biasing TEM sample holder to investigate high-temperature phase transitions in 2D PtSe2 crystals. The 2-dimensional (2D) nature of the few-layer crystals resulted in unique 1D interfaces during the conversion to selenium-poor phases like PtSe and PtSe1-x.

4D STEM strain mapping further demonstrated large strain variations consistent with a high selenium vacancy concentration. Upon heating to 550 °C and above, the enhanced selenium vacancy diffusion and mobility to the crystal edges causes a Kirkendall effect in the 2D material. The extracted activation energy of the phase transformation indicates the process is mediated by selenium vacancies. This could allow tuning of 1D edge chemistry for improvement of near-infrared optoelectronics, wherein the various phases formed across the 1D boundary may enable new electronic states along the interface.

Figures showing a)Schematic of temperature induced changes to vacancy concentration and diffusion inducing phase transformations in PtSe2, b) Example strain mapping and higher strain in Se-poor areas, indicating higher vacancy concentration, c) In-situ TEM image sequence during heating at 550 °C, showing growth of transformed phase, d) measured activation energy for PtSe2 to PtSe1-x.

While high temperature was maintained, in-situ energy dispersive X-ray spectroscopy (EDS) confirmed the phase and interface chemistry, while in-situ strain mapping allowed isolation of the mechanisms which drive diffusion and phase transformation. This work highlights the use of in-situ electron microscopy techniques such as ultrafast heating to gain crucial mechanistic insights into structural transformations in nanoscale and 2D materials.

Reference:  Pawan Kumar, Andrew C. Meng, Kiyoung Jo, Eric A. Stach, and Deep Jariwala, Nano Letters 2022 22 (12), 4733-4740 DOI: 10.1021/acs.nanolett.2c00874, Full paper Copyright © 2022 American Chemical Society

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