[Paper Review] High entropy van der Waals materials (Review article)

Tianping Ying, Tongxu Yu+3 more|arXiv (Cornell University)|Aug 28, 2022

MXene and MAX Phase Materials59 references47 citations

TL;DR

This review introduces high entropy van der Waals materials (HEX), a new class of 2D materials formed by integrating high entropy concepts—random distribution of multiple cations—into van der Waals layered structures. By combining configurational entropy from multi-element solid solutions with the tunable degrees of freedom in 2D van der Waals systems (stacking, twisting, intercalation), HEX enables exploration of emergent quantum phenomena and enhanced catalytic performance, with successful demonstrations in high entropy dichalcogenides, phosphorus tri-chalcogenides, halides, and MXenes.

ABSTRACT

By breaking the restrictions on traditional alloying strategy, the high entropy concept has promoted the exploration of the central area of phase space, thus broadening the horizon of alloy exploitation. This review highlights the marriage of the high entropy concept and van der Waals systems to form a new family of materials category, namely the high entropy van der Waals materials (HEX, HE = high entropy, X= anion clusters) and describe the current issues and next challenges. The design strategy for HEX has integrated the local feature (e.g., composition, spin, and valence states) of structural units in high entropy materials and the holistic degrees of freedom (e.g., stacking, twisting, and intercalating species) in van der Waals materials, and has been successfully employed for the discovery of high entropy dichalcogenides, phosphorus tri-chalcogenides, halogens, and MXene. The rich combination and random distribution of the multiple metallic constituents on the nearly-regular 2D lattice give rise to a flexible platform to study the correlation features behind a range of selected physical properties, e.g., superconductivity, magnetism, and metal-insulator transition. The deliberate design of structural units and their stacking configuration can also create novel catalysts to enhance their performance in a bunch of chemical reactions.

Motivation & Objective

• To establish a new materials category, high entropy van der Waals materials (HEX), by merging high entropy alloy principles with 2D van der Waals systems.
• To address the limitation of bulk high entropy materials in catalysis and surface applications by enabling 2D forms with high surface-to-volume ratios.
• To explore the interplay between local disorder (multi-element substitution) and global structural degrees of freedom (stacking, twisting, intercalation) in 2D systems.
• To demonstrate the design and discovery of HEX materials, including high entropy dichalcogenides, phosphorus tri-chalcogenides, halides, and MXenes.
• To highlight the potential of HEX for emergent quantum phenomena and advanced catalytic applications through controlled structural engineering.

Proposed method

• Adopting a 'Chinese checkers' design strategy, where multiple metallic cations are randomly distributed across structural units (e.g., octahedra, prisms) in 2D lattices.
• Using characteristic structural units (e.g., AB3, AB4, AB6) as building blocks to describe and engineer the short-range order in high entropy 2D systems.
• Integrating configurational entropy from multi-element solid solutions with the intrinsic degrees of freedom of van der Waals materials: stacking order, twist angles, and intercalation species.
• Leveraging the 2D nature of van der Waals materials to enable exfoliation and surface-sensitive applications, while preserving high configurational entropy.
• Employing advanced characterization techniques (e.g., electron microscopy, X-ray diffraction, spectroscopy) to confirm random cation distribution and structural integrity.
• Applying theoretical frameworks to model entropy contributions in 2D vs. 3D systems, defining S2D = 2kB ln Ω to quantify entropy deficit and compensation mechanisms.

Experimental results

Research questions

• RQ1 How can the high entropy concept be successfully extended from 3D bulk alloys to 2D van der Waals materials to form a new class of materials?
• RQ2 What are the dominant degrees of freedom in HEX that enable tuning of electronic, magnetic, and catalytic properties?
• RQ3 How does the 2D confinement affect the configurational entropy and stability of high entropy systems compared to 3D bulk counterparts?
• RQ4 Can the random distribution of multiple cations in 2D lattices lead to emergent quantum phenomena such as superconductivity or metal-insulator transitions?
• RQ5 To what extent can structural engineering (stacking, twisting, intercalation) enhance catalytic performance in high entropy 2D materials?

Key findings

• High entropy van der Waals materials (HEX) have been successfully synthesized in multiple families, including high entropy dichalcogenides, phosphorus tri-chalcogenides, halides, and MXenes.
• The random distribution of multiple metallic cations on a nearly regular 2D lattice enables a flexible platform to study correlations in superconductivity, magnetism, and metal-insulator transitions.
• The 2D nature of HEX introduces a 33% entropy deficit compared to 3D high entropy systems, which can be compensated by stacking disorder, intercalation, and structural fluctuations.
• The integration of local configurational entropy with global structural degrees of freedom (stacking, twisting, intercalation) enables unprecedented tunability of physical and chemical properties.
• HEX materials exhibit enhanced catalytic performance due to the high density of active sites and tunable electronic structures from multi-element solid solutions.
• The structural unit approach (e.g., octahedral, prismatic, triangular) provides a robust framework to describe and design new HEX materials, analogous to Lego-like assembly of functional 2D building blocks.