Two-dimensional (2D) materials, such as transition metal dichalcogenides (TMDs), black phosphorus (BP), MXene and borophene, have aroused extensive attention since the discovery of graphene in 2004. They have wide range of applications in many research fields, such as optoelectronic devices, energy storage, catalysis, owing to their striking physical and chemical properties. Among them, anisotropic 2D material is one kind of 2D materials that possess different properties along different directions caused by the intrinsic anisotropic atoms’ arrangement of the 2D materials, mainly including BP, borophene, low-symmetry TMDs (ReSe2 and ReS2) and group IV monochalcogenides (SnS, SnSe, GeS, and GeSe). Recently, a series of new devices has been fabricated based on these anisotropic 2D materials. In this review, we start from a brief introduction of the classifications, crystal structures, preparation techniques, stability, as well as the strategy to discriminate the anisotropic characteristics of 2D materials. Then, the recent advanced applications including electronic devices, optoelectronic devices, thermoelectric devices and nanomechanical devices based on the anisotropic 2D materials both in experiment and theory have been summarized. Finally, the current challenges and prospects in device designs, integration, mechanical analysis, and micro-/nano-fabrication techniques related to anisotropic 2D materials have been discussed. This review is aimed to give a generalized knowledge of anisotropic 2D materials and their current devices applications, and thus inspiring the exploration and development of other kinds of new anisotropic 2D materials and various novel device applications.

Recent advances in anisotropic two-dimensional materials and device applications

Monolayer KAgX are a class of novel two-dimensional (2D) layered materials with efficient optical absorption and superior carrier mobility, signifying their potential application prospect in photovoltaic (PV) and thermoelectric (TE) fields. Motivated by the recent theoretical studies on the KAgX monolayer, we carried out systematic investigations on the TE performance of KAgS and KAgSe monolayers, employing density functional theory (DFT) and semiclassical Boltzmann transport equation (BTE). For both KAgSe and KAgS monolayers, large Gruneisen parameters, low group velocities, and short phonon scattering time greatly hinder their heat transport and result in an ultralow thermal conductivity, 0.26 and 0.33 W m-1 K-1 at 300 K, respectively. A twofold degeneracy appearing at the Γ point and the abrupt slope of the density of states (DOS) near the Fermi level give rise to high Seebeck coefficients of KAgX monolayers. Due to the ultralow thermal conductivity and excellent electronic transport performance, the ZT values as high as 4.65 (3.11) and 4.05 (2.63) at 500 (300) K in the n-type doping for KAgSe and KAgS monolayers are obtained. The exceptional performance of KAgX monolayers sheds light on their immense potential applications in the medium-temperature (around 300-500 K) thermoelectric devices and greatly stimulates further experimental synthesis and validation.

KAgX (X = S, Se): High-Performance Layered Thermoelectric Materials for Medium-Temperature Applications.

The high-throughput (HT) computational method is a useful tool to screen high-performance functional materials. In this work, using the deformation potential method under the single band model, we evaluate the carrier relaxation time and establish an electrical descriptor (χ) characterized by the carrier effective masses based on the simple rigid band approximation. The descriptor (χ) can be used to reasonably represent the maximum power factor without solving the electron Boltzmann transport equation. Additionally, the Gruneisen parameter (γ), a descriptor of the lattice anharmonicity and lattice thermal conductivity, is efficiently evaluated using the elastic properties, omitting the costly phonon calculations. Applying two descriptors (χ and γ) to binary chalcogenides, we HT compute 243 semiconductors and screen 50 promising thermoelectric materials. For these theoretically determined compounds, we successfully predict some previously experimentally and theoretically investigated promising thermoelectric materials. Additionally, 9 p-type and 14 n-type previously unreported binary chalcogenides are also predicted as promising thermoelectric materials. Our work provides not only new thermoelectric candidates with perfect crystalline structure for the future investigations but also reliable descriptors to HT screen high-performance thermoelectric materials.

Screening Promising Thermoelectric Materials in Binary Chalcogenides through High-Throughput Computations.

Monolayer KCuTe is a new-type of two-dimensional (2D) semiconductor material with high carrier mobility and large power energy conversion efficiencies, suggesting its potential application in thermoelectric (TE) and photoelectric fields. Based on the density functional theory (DFT) and semiclassical Boltzmann transport equation, the electronic and phonon transport properties of monolayer KCuTe are systematically studied. Our results show that it possesses an ultralow lattice thermal conductivity value of nearly ∼0.13 W m−1 K−1 at 300 K, mainly attributed to its small phonon group velocity, large Gruneisen parameters, and strong phonon–phonon scattering. Furthermore, the intralayer opposite phonon vibrations greatly restrict the heat transport. Monolayer KCuTe shows an ideal direct band gap of ∼1.21 eV, and a high twofold degeneracy appearing at the Γ point gives a high Seebeck coefficient of ∼2070 μV K−1, leading to high TE performance. Using the transport coefficients together with constant electron relaxation time, the figure of merit (ZT) can reach 2.71 at 700 K for the p-type doping, which is comparable to the well-known TE material SnSe (2.6 ± 0.3 at 935 K). Our theoretical studies may provide perspectives to TE applications of monolayer KCuTe and stimulate further experimental synthesis.

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Ultralow lattice thermal conductivity and high thermoelectric performance of monolayer KCuTe: a first principles study

Rhenium Diselenide Anchored on Reduced Graphene Oxide as Anode with Cyclic Stability for Potassium‐Ion Battery

The present study is the first attempt towards establishing computational insights into the structural, electronic, mechanical, dynamical and thermal properties of the tetragonal phases of potassium chalcoargentates (KAgX). We find that the lattice thermal conductivity of KAgX is anisotropic, with values of 0.553 (0.279), 0.509 (0.369) and 0.221 (0.125) Wm−1K−1 at room temperature (300 K) along the a-axis (c-axis) for KAgS, KAgSe and KAgTe, respectively. The calculated values of the lattice thermal conductivity are very small, especially along the c-axis. This highlights the potential of using KAgX in designing thermoelectric materials, since low lattice thermal conductivity is a requisite for maximizing the dimensionless figure of merit which defines the efficiency of a system in converting thermal to electrical energy and vice versa.

First-principles study of structural stability, electronic properties and lattice thermal conductivity of KAgX (X = S, Se, Te)

First principles study of the structural, stability properties and lattice thermal conductivity of bulk ReSe2

This study is the first attempt towards establishing computational insight into the structural, electronic, mechanical, dynamical and thermoelectric properties of the monoclinic phases of NaSbS2 and NaSbSe2. The mechanical properties are predicted using the Hill approximation. Dynamical stability was investigated by computing the phonon frequency to check for the absence of imaginary modes. Lattice thermal conductivity was calculated by using a single-mode relaxation-time approximation in the linearized phonon Boltzmann equation from first-principles an-harmonic lattice dynamics calculations. We found that the lattice thermal conductivity of NaSbS2 and NaSbSe2 are anisotropic, with values ranging between 0.753 and 1.173 Wm−1 K−1 at room temperature (300 K). The calculated values of the lattice thermal conductivity are small, especially along the x-axis. The charge transport properties are predicted using Boltzmann transport equations. The highest values attained for the figure of merit are high as 4.22 and 2.88 when the electron concentration is 1018 cm−3 at 600 K for NaSbS2 and NaSbSe2, respectively. This highlights the potential of using NaSbS2 and NaSbSe2 in designing thermoelectric materials since low lattice thermal conductivity and high figure of merit are a requisite for maximizing the efficiency of thermoelectric materials.

Mahmoud M. A. Mahmoud

Papers

First-principles study of structural stability, electronic properties and lattice thermal conductivity of KAgX (X = S, Se, Te)

First principles study of the structural, stability properties and lattice thermal conductivity of bulk ReSe2

Structural, stability and thermoelectric properties for the monoclinic phase of NaSbS2 and NaSbSe2: A theoretical investigation