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High pressure structural and lattice dynamics study of bulk and few-layer α-In2Se3

high pressure,2D Materials

In the context of searching for ultrathin and robust ferroelectric two-dimensional (2D) materials, α-In₂Se₃ has drawn considerable attention owing to the existence of intercoupled in-plane (IP) and out-of-plane (OOP) ferroelectricity [1]. This makes α-In₂Se₃ a potential candidate for emerging artificial intelligence, information processing, and memory applications. It is expected that pressure will significantly affect the physicochemical properties of α-In₂Se₃ and recent research results on other layered compounds, such as graphene and Transition Metal Dichalcogenide, further support this hypothesis. However, to our knowledge, the overall structural behavior and the optical properties of the α-In₂Se₃ phase under high pressure (HP) have not been systematically investigated.

In this study, the overall structural evolution of both bulk and few-layer α-In₂Se₃ under pressure was explored by performing a concomitant in-situ synchrotron angle dispersive powder x-ray diffraction (XRD) and Raman spectroscopy study in a diamond anvil cell (DAC) up to 60 GPa (at room temperature) for both techniques. Helium and Neon were used as pressure-transmitting media (PTM), for the bulk and few-layer α-In₂Se₃, respectively. Helium remains fairly hydrostatic up to the highest pressure in this study, while Neon is slightly non-hydrostatic when pressure above 40 GPa [2]. The pressure inside the sample chamber was determined by ruby luminescence method [3] and/ or the gold Equation of state (EOS) [4].

In the case of bulk α-In₂Se₃, the results from both experimental methods reveal a pressure-induced structural phase transition from α-In₂Se₃ to a monoclinic β′-In₂Se₃ structure at ≈ 1 GPa, in agreement with previous studies [5-7]. Based on our detailed measurements using both experimental techniques and F−f formalism, the β′-In₂Se₃ structure remains stable up to 45 GPa, without a clear indication of a phase transition towards the previously reported β-In₂Se₃ phase [6-7]. With further pressure increase, another phase transition towards phase IV was observed, which can be indexed with an orthorhombic solid solution structure. Its low volume is not only more stable in terms of enthalpy but also points towards a crystal structure with three atoms per primitive unit cell. Based on the Raman results, the expected stoichiometry is still In₂Se₃. Therefore, the only plausible explanation is the formation of a solid solution, with 0.4 occupancy by In and 0.6 occupancy by Se at the relevant crystallographic positions.

A few-layer α-In₂Se₃ has also been investigated under pressure. However, to our knowledge, in contrast to the case of other 2D materials (such as graphite) there is no previous reported methodology in determining the number of layers based on Raman measurements. Therefore, a novel straightforward approach was developed to estimate the thickness of the specimens using Raman spectroscopy. Moreover, a backside micromachining procedure for loading the few-layer α-In₂Se₃ together with a custom made SiO₂/Si substrate inside a typical diamond anvil cell was developed. Finally, high-pressure Raman measurements were performed. Bulk and few-layer α-In₂Se₃, appear to follow identical structural evolution under pressure. However, the last observed phase transition ( b’ phase to IV phase) at 45 GPa appears to be abrupt in the case of few-layer α-In₂Se_3, while it involves a coexistence of the two phases, for a significant pressure range, for bulk α-In₂Se_3. This is attributed to the difference in the morphology between specimens, i.e. single-crystal and powder in the case of few-layer and bulk α-In₂Se_3, respectively.



Reference:

[1]    W. Ding, J. Zhu, Z. Wang, Y. Gao, D. Xiao, Y. Gu, Z. Zhang, and W. Zhu. Prediction of intrinsic two-dimensional ferroelectrics in In₂Se₃ and other III₂-VI₃ Van Der Waals materials. Nature communications, 8(1):14956, 2017.

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[4]    O. L. Anderson, D. G. Isaak, and S. Yamamoto. Anharmonicity and the equation of state for gold. Journal of Applied Physics, 65(4):1534–1543, Feb. 1989.

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[6]    R. Vilaplana, S. G. Parra, A. Jorge-Montero, P. Rodríguez-Hernández, A. Munoz, D. Errandonea, A. Segura, and F. J. Manjón. Experimental and theoretical studies on α-In₂Se₃ at high pressure. Inorganic chemistry, 57(14):8241–8252, 2018.

[7]    J. Zhao and L. Yang. Structure evolutions and metallic transitions in In₂Se₃ under high pressure. The Journal of Physical Chemistry C, 118(10):5445–5452, 2014.