High Entropy van der Waals Materials

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 describes 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 is successfully used 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.

Keywords: 2D materials; high entropy materials; superconductors; van der Waals materials.

Figure 1

The “Chinese checkers” design strategy of high‐entropy vdW materials. The color balls placed at each corner of the hexagram represent the optional multi‐elements that can be used to construct the structural units. Several typical structural units are exhibited in the voids of the hexagon circumscribing the hexagram. Their corresponding point symmetry groups are marked at the corner of the hexagon. Inside the checkerboard, a HES₆ octahedron is sketched for illustration.

Figure 2

Crystal growth and element distribution of HEX materials.^{[ 17} , ^{19 ]} a) Schematic diagram of the chemical vapor transport method suitable for the growth of HES₂, HESe₂, HECl₂, HEBr₂, HEI₂, and HEPS₃. b) Optical images of single crystals of (TiVCrNbTa)Se₂ and (MnFeCoNi)PS₃ with subcentimeter sizes. c) Quantitative element mapping of (TiVCrNbTa)Se₂ by EPMA. d) Atomic‐resolution scanning transmission electron microscopy high‐angle annular dark‐field (STEM‐HAADF) image of a flake of (MoWVNbTa)S₂, where the highlighted region (red box) shows the [001] projection. e) STEM‐HAADF image showing local variations in the intensity of atomic columns due to varying composition of cations. f) The intensity profile of the green box region is illustrated in (e), where the legends correspond to the predominant elements in each atomic column. b,c) Adapted with permission.^[17] Copyright 2021, American Chemical Society. d,f)   Wiley‐VCH GmbH.

Figure 3

The exfoliation of several typical HEX single crystals. a) Optical image of the exfoliated (MnFeCoNi)PS₃ on a SiO₂/Si wafer. b) Atomic force microscope (AFM) topographic image of the squared area shown in (a). The cross‐sectional profile along the black line is superimposed. c) Optical image of as‐cleaved monolayer and few layers (TiVCrNbTa)S₂ by using Al₂O₃‐assisted exfoliation method. Layer numbers have been indicated. d) A typical device fabricated on monolayer (TiVCrNbTa)S₂. Adapted with permission.^[17] Copyright 2021, American Chemical Society.

Figure 4

The intercalation of HEX. a) X‐ray diffraction patterns of polycrystalline (black), single‐crystalline (orange), and potassium‐intercalated (blue) (TiVCrNbTa)S₂. b–d) XPS peaks of S 2p, Ta 4f, and V 2p states for raw, monovalent (K), and divalent (Ba) intercalated (TiVCrNbTa)S₂. A continuous redshift by increasing the doping content can be seen in S 2p and Ta 4f. However, the distinction between the monovalent and divalent doping effect cannot be distinguished in V 2p (as well as in the rest transition metals), indicating the uneven and element‐selective charge distribution within the high‐entropy system. Adapted with permission.^[17] Copyright 2021, American Chemical Society.

Figure 5

Structure information from Raman spectroscopy. a) Temperature dependence of Raman spectra of bulk FePS₃. Reproduced with permission.^{[ 32 ]} Copyright 2016, American Chemical Society. b) Temperature dependence of Raman spectra of (MnFeCoNi)PS₃ with an antiferromagnetic transition at 70 K. c) Plots of full width at half maximum (FWHM) of Raman peak of P₁ (88 cm⁻¹, 92.7 cm⁻¹) and P₈ (383 cm⁻¹) versus temperature in FePS₃ and HEPS₃. The broken lines indicate the Neel transition temperatures for FePS₃ and HEPS₃ at 118 and 70 K, respectively.

Figure 6

Corrosion resistance of HE‐dichalcogenides. a–c) Corrosion resistance measurements of (TiVCrNbTa)Se₂ and its individual components against acid (HNO₃), base (NaOH), and organic (butylamine mixed in tetrahydrofuran) reagents. Adapted with permission.^[17] Copyright 2021, American Chemical Society.

Figure 7

The dependence of phase behavior on the characteristic elemental and structural features in different HEX systems. a) Conventional Hume–Rothery rule using electronegativity mismatch (Δχ) versus atomic size mismatch (δ) plot. The single phase and the phase‐separated ones intermingled and the size mismatch failed to be used as a valid criterion. b) Replot of the phase diagram using the mismatch of the M‐X bond (β) as the abscissa with its definition specified in the text. In practice, these bond lengths are extracted from the ICSD database, and their average value can be automatically calculated using a crystalline 3D visualization program such as VESTA.^{[ 41 ]} c) The exception of the HEPS₃ system, lying outside of other HE‐systems. Filled and open notations denote single and multiphase formation, respectively.

Figure 8

Electrical transport properties of HE‐dichalcogenides. a) Composition‐controlled metal‐insulator transition of (TiVCrNbTa)S₂ with the variation of its entropy content. b) 2D variable‐range hopping of the resistivity in the (TiVCrNbTa)S₂ system. The change of the linear dependence of ln ρ versus T from T ^−1/3 to T ^−1/2 indicates the opening of a Coulomb gap (Efros–Shkovskii gap) at low temperature. c) Thickness‐dependent resistivity of (TiVCrNbTa)S₂ from 12 to 300 K. An interesting observation is the anomalous increasing conductivity with decreasing the sample thickness, contrasting to the behavior of the majority of 2D materials. d) Superconductivity in (CoAu)_0.2(RhIrPdPt)_0.8Te₂. The transition temperatures can be gradually suppressed by increasing external magnetic fields. Adapted with permission.^{[ 17 ]} Copyright 2021, American Chemical Society.

Figure 9

a,b) Illustration of the concept of isolated atoms in the catalysis by controlling the concentration of expensive or toxic elements to fit the molecular geometry of the desired reaction. c) The current density of (MoWVNbTa)S₂ and Ag nanoparticles plotted against voltage versus RHE using linear sweep voltammetry test mode. (c) Adapted with permission.^{[ 19 ]} Copyright 2021, Wiley‐VCH GmbH.

Figure 10

Electrochemical characterizations of Co _x (VMnNiZn)_{1− x} PS₃ for HER. a) Linear sweep voltammetry curves with the variation of entropy. b) HER free‐energy diagram of corresponding edge sites, inset illustrates the adsorption of H on the edge S site (yellow balls). Negative ΔG (HE‐S and S) indicates that the adsorption of hydrogen on S is more favored than the desorption process, and vice versa. c) Basal‐plane models of P sites (P1–P3) and S sites (S1–S9) in Co_0.6(VMnNiZn)_0.4PS₃, where the surface P shows the lowest HER free energy. d) Calculated reaction energy of water dissociation for Co_0.6(VMnNiZn)_0.4PS₃ and CoPS₃, including Co, V, Mn, Ni, and Zn sites. Adapted with permission.^{[ 23 ]} Copyright 2021, American Chemical Society.

Figure 11

Long‐range magnetic ordering in a highly disordered system. a) Magnetic configurations of individual MPS₃ (M = Mn, Fe, Co, Ni). The crystal structure of HEPS₃ is identical to MPS₃ with the transition metal sites occupied by multiple elements. b) Magnetization of (MnFeCoNi)PS₃ with the external magnetic field H = 500 Oe perpendicular to or within the ab‐plane. The antiferromagnetic transition (T_N) and spin glass transition temperature (T _g) are marked. c,d) Two alternative models of magnetic ordering in the stoichiometric (MnFeCoNi)PS₃. e) Heat capacity of (MnFeCoNi)PS₃ from 2 to 280 K. The magnetic contribution (red) is extracted by subtracting the lattice‐specific heat (black) by fitting the Debye formula above 160 K. b,e) Adapted with permission.^[17] Copyright 2021, American Chemical Society.

Figure 12

High entropy induced electron donation and superconductivity in the HEPSe₃ system.^{[ 18 ]} a) XPS of P 2p in (MnFeCdIn)PSe₃ compared with that of FePSe₃. b) Metal–superconductor transition of (MnFeCdIn)PSe₃ with the external pressure. c) P—P distance (circles) and Neel temperature (squares) in MPSe₃ as a function of entropy. Insets illustrate the breaking of the P—P dimer into two Ps with lone‐pair electrons caused by the entropy‐driven electron donation. d) Semiconductor–metal transition and the emergence of superconductivity with the applied external pressure for (MnFeCdIn)PSe₃ phase. R _300K: resistivity at 300 K. The lone‐electron pair acts as a charge reservoir that releases electrons under high pressure and is responsible for the out‐of‐plane collapse.

Figure 13

Out‐of‐plane collapse in pressurized HEPSe₃. a,b) Color contour of the (113) and (300) diffraction peaks for Fe_0.8Mn_0.1Cd_0.05In_0.05PSe₃ and (FeMnCd)_0.25In_0.17PSe₃ under external physical pressure. c–e) Pressure‐dependent lattice parameters and volume of the unit cell for both samples. The pressure‐dependent V of both samples by using the third‐order Birch–Murnaghan equation of state. Reproduced with permission.^{[ 18 ]} Copyright 2022, The Authors.

Figure 14

Characterization of high‐entropy MXene morphology and structure. a) A schematic illustration of the soft‐chemical etching method for obtaining HE‐MXene from HE‐MAX. b) SEM micrographs of HE‐MAX, HE‐MXenes, and a single flake image of TiVNbMoC₃T_x on an alumina substrate. c) Schematic illustration of the nucleation and growth behavior of lithium guided by strains on HE‐MXene atomic layers. a,c) Reproduced with permission.^{[ 20 ]} Copyright 2021, Wiley‐VCH GmbH. b) Reproduced with permission.^{[ 21 ]} Copyright 2021, American Chemical Society.

Figure 15

The full set of degrees of freedom in high entropy vdW materials. These freedoms can be used separately or combined to realize bountiful physical properties or chemical performance. Adapted with permission.^{[ 17 ]} Copyright 2021, American Chemical Society.