Understanding filamentary growth and rupture by Ag ion migration through single-crystalline 2D layered CrPS4

• Published in: NPG Asia materials Vol. 12; no. 1
• Main Authors: Lee, Mi Jung, Kim, Sung-Hoon, Lee, Sangik, Yoon, Chansoo, Min, Kyung-Ah, Choi, Hyunsoo, Hong, Suklyun, Lee, Sungmin, Park, Je-Geun, Ahn, Jae-Pyoung, Park, Bae Ho
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 18-12-2020; Nature Publishing Group
• Subjects: 639/301/1005/1007; 639/925/357/1018; Biomaterials; Chemistry and Materials Science; Chromium; Configurations; Crystal defects; Crystal structure; Crystallinity; Density functional theory; Electric potential; Electrolytes; Electrolytic cells; Energy Systems; Filaments; Gold; Insulators; Ion migration; Lattice vacancies; Layered materials; Material properties; Materials Science; Metallizing; Microscopy; Molten salt electrolytes; Optical and Electronic Materials; Silver; Single crystals; Solid electrolytes; Structural Materials; Surface and Interface Science; Synapses; Thin Films; Two dimensional materials; Voltage
• ISSN: 1884-4049; 1884-4057
• DOI: 10.1038/s41427-020-00272-x

Memristive electrochemical metallization (ECM) devices based on cation migration and electrochemical metallization in solid electrolytes are considered promising for neuromorphic computing systems. Two-dimensional (2D) layered materials are emerging as potential candidates for electrolytes in reliable ECM devices due to their two-dimensionally confined material properties. However, electrochemical metallization within a single-crystalline 2D layered material has not yet been verified. Here, we use transmission electron microscopy and energy-dispersive X-ray spectroscopy to investigate the resistive switching mechanism of an ECM device containing a single-crystalline 2D layered CrPS
4
electrolyte. We observe the various conductive filament (CF) configurations induced by an applied voltage in an Ag/CrPS
4
/Au device in the initial/low-resistance/high-resistance/breakdown states. These observations provide concrete experimental evidence that CFs consisting of Ag metal can be formed inside single-crystalline 2D layered CrPS
4
and that their configuration can be changed by an applied voltage. Density functional theory calculations confirm that the sulfur vacancies in single-crystalline CrPS
4
can facilitate Ag ion migration from the active electrode layer. The electrically induced changes in Ag CFs inside single-crystalline 2D layered CrPS
4
raise the possibility of a reliable ECM device that exploits the properties of two-dimensionally confined materials.
2D materials: How neuromorphic chips get wired up
A 2D layered material has provided critical insights toward improving the stability of devices that can emulate biological synapses. Researchers in Seoul, South Korea, led by Jae-Pyoung Ahn at the Korea Institute of Science and Technology and Bae Ho Park from Konkuk University have used high-resolution microscopy to investigate electrochemical metallization cells, a new type of non-volatile computer memory. These cells, made by infusing insulators with tiny metallic filaments, can mimic the low and high conductivity states of synapses. When the team looked at the structure of a chromium thiophosphate insulator composed of numerous 2D crystal layers, they found that the pathways the filaments follow are guided by specific crystal defects. These findings may help manufacturers produce more reliable electrochemical metallization cells by reducing the likelihood of random, non-uniform filament formation.
We report on the formation and rupture of CFs through Ag ion migration inside a single-crystalline 2D van der Waals (vdW) solid electrolyte material within an ECM device structure. This study provides clear experimental evidence that CFs consisting of Ag can be formed inside single-crystalline 2D layered chromium thiophosphate (CrPS
4
) and their configuration can be changed by an applied voltage. Density functional theory calculations confirm that the Ag ion migration is an energetically favorable process. The electrically induced changes in Ag CFs inside single-crystalline CrPS
4
raise the possibility of a reliable ECM device that exploits the properties of two-dimensionally confined materials.