Ambient-Stable 2D Transition Metal Halides via Organic-Inorganic Encapsulation

Two-dimensional transitional metal halides have recently attracted significant attention due to their thickness-dependent and electrostatically tunable magnetic properties [1]. However, this class of materials is highly reactive chemically, which leads to irreversible degradation and catastrophic dissolution within seconds in ambient conditions, severely limiting subsequent characterization, processing, and applications. Here, we explore scalable strategies for imparting ambient stability to the prototypical transition metal halide CrI₃ by exploring the surface chemical implications of atomic layer deposition (ALD) encapsulation layers [2]. While direct ALD of alumina seemingly imparts ambient stability to multi-layer CrI₃, surface analysis with X-ray photoelectron spectroscopy reveals that reactive ALD alumina precursors chemically convert the top surface of CrI₃, which compromises magnetic properties in atomically thin samples. In contrast, by first assembling a noncovalent organic buffer layer of perylenetetracarboxylic dianhydride (PTCDA), subsequent ALD alumina results in minimal surface chemical perturbation, thereby preserving the magnetic properties of CrI₃ down to the monolayer limit as confirmed by magneto-optical Kerr effect characterization. Furthermore, this organic-inorganic encapsulation scheme imparts long-term ambient stability, which enables device fabrication and characterization in ambient conditions including field-effect transistors and photodetectors. Since this approach arrests photocatalytic decomposition pathways, encapsulated CrI₃ samples can also be irradiated with intense light sources in ambient conditions, thereby facilitating advanced optical spectroscopic methods such as optothermal measurements of CrI₃ thermal conductivity. Overall, by elucidating the surface chemistry of CrI₃, this work has resulted in a robust organic-inorganic encapsulation scheme that can be generalized to other transition metal halides and related van der Waals layered magnetic materials, thus accelerating future fundamental studies and efforts to realize ambient-stable spintronic [3], neuromorphic [4], and quantum information technologies [5].<br/> <br/>[1] H. Bergeron, <i>et al.</i>, <i>Chemical Reviews</i>, <b>121</b>, 2713 (2021).<br/>[2] J. T. Gish, <i>et al.</i>, <i>ACS Nano</i>, <b>15</b>, 10659 (2021).<br/>[3] M. E. Beck and M. C. Hersam, <i>ACS Nano</i>, <b>14</b>, 6498 (2020).<br/>[4] V. K. Sangwan and M. C. Hersam, <i>Nature Nanotechnology</i>, <b>15</b>, 517 (2020).<br/>[5] X. Liu and M. C. Hersam, <i>Nature Reviews Materials</i>, <b>4</b>, 669 (2019).