Ultrahigh electron mobility induced by strain engineering in direct semiconductor monolayer Bi2TeSe2

Two-dimensional BiSbTeX2 (X = S, Se, Te) and their Janus monolayers as efficient thermoelectric materials

Today, there is a huge need for highly efficient and sustainable energy resources to tackle environmental degradation and energy crisis. We have analyzed the electronic, mechanical and thermoelectric (TE) characteristics of two-dimensional (2D) BiSbTeX2 (X = S, Se and Te) and Janus BiSbTeXY (X/Y = S, Se and Te) monolayers by implementing first principles simulations. These monolayers' dynamic stability and thermal stability have been demonstrated through phonon dispersion spectra and ab initio molecular dynamics (AIMD) simulations, respectively. The band structure of these monolayers can be tuned by applying uniaxial and biaxial strains. The investigated lattice thermal conductivity (κl) for these monolayers lies between 0.23 and 0.37 W m-1 K-1 at 300 K. For a more precise calculation of the scattering rate, we implemented electron-phonon coupling (EPC) and spin-orbit coupling effects to calculate the transport properties. For p(n)-type carriers, the power factor of these monolayers is predicted to be as high as 2.08 × 10-3 W m-1 K-2 and (0.47 × 10-3 W m-1 K-2) at 300 K. The higher thermoelectric figure of merit (ZT) of p-type carriers at 300 K is obtained because of their very low value of κl and high power factor. Our theoretical investigation predicts that these monolayers can be potential candidates for fabricating highly efficient thermoelectric power generators.

Exploring Bonding Configurations in MnBi2Te4-Type Materials

We perform a systematic investigation of several crystal structures, based on monolayer MnBi2Te4, of the form MnBiBiiXi2Xii2 using first-principles calculations. Our analysis shows that the most energetically favorable bonding configuration of the constituent elements in monolayer MnBiBiiXi2Xii2 is determined by the bond length between the Mn atom and its nearest X-site atoms. Tuning the bonding configuration of the material alters the magnetic, electronic, and topological properties. We also calculate the magnetic exchange parameters and magnetic anisotropy energy of the predicted structures. The calculations show that the elements at the X sites mainly determine the magnetic properties. Finally, we propose a stable phase of monolayer MnBi2S2Te2 (i.e., γ-MnBi2S2Te2) that exhibits the quantum anomalous Hall effect (QAHE). This study demonstrates that the bonding configuration of MnBi2Te4-type materials provides avenues for tuning the magnetic, electronic, and topological properties of van der Waals (vdW) materials.

Design of an Oxide Monolayer with High ZT by a Strong Anharmonicity Unit

In this paper, a new strategy to obtain a transition-metal oxide (TMO) thermoelectric monolayer is demonstrated. We show that the TMO thermoelectric monolayer can be achieved by the replacement of a transition-metal atom with a cluster, which is composed of heavy transition atoms with abundant valence electrons. Specifically, the transition-metal atom in the XO2 (X = Ti, Zr, Hf) monolayer is replaced by the [Ag6]4+ cluster and a stable structure Ag6O2 is achieved. Due to the abundant valence electrons in the [Ag6]4+ cluster unit, n-type Ag6O2 has high electrical conductivity, which leads to a satisfactory power factor. More importantly, Ag6O2 has an extremely low phonon thermal conductivity of 0.16 W·m-1·K-1, which is one of the lowest values in thermoelectric materials. An in-depth study reveals that the extremely low value originates from the strong phonon anharmonicity and weak metal bond of the [Ag6]4+ cluster unit. Due to the satisfactory power factor and ultralow phonon thermal conductivity, Ag6O2 has high ZT at 300-700 K, and the maximum ZT is 3.77, corresponding to an energy conversion efficiency of 22.24%. Our results demonstrate that replacement of the transition-metal atom by an appropriate cluster is a good way to obtain a TMO thermoelectric monolayer.

Unusual Janus Bi2TeSe2 Topological Insulators Displaying Second-Harmonic Generation, Linear-in-Temperature Resistivity, and Weak Antilocalization

Well-established knowledge about inversion-symmetric Bi2TexSe3-x topological insulators characterizes the promising new-generation quantum device. Noticeably, the inversion asymmetric phase containing different surface electronic structures may create an extra topological phenomenon pointing to a new device paradigm. Herein, Janus Bi2TeSe2 single-crystal nanosheets with an unconventional stacking sequence of Se-Bi-Se-Bi-Te are realized via chemical vapor deposition growth, which is clarified by atomically resolved AC-STEM and elemental mapping. An obvious polarization-dependent second-harmonic generation with a representative 6-fold rotational symmetry is detected due to the broken out-of-plane mirror symmetry in this system. Low-temperature transport measurements display a strange metal-like linear-in-temperature resistivity. Anomalous conductance peaks under low magnetic fields induced by the weak antilocalization effect of topological surface states and the two-dimensional transport-dominated anisotropic magnetoresistance are revealed. These findings correlate the Janus Bi2TeSe2 phase with emerging physics topics, which would inspire fresh thoughts in well-developed Bi3TexSe3-x topological insulators and open up opportunities for exploring hybrid nonlinear optoelectronic topological devices.

Strain Tunable Thermoelectric Material: Janus ZrSSe Monolayer

Thermoelectric (TE) performance of the Janus ZrSSe monolayer under biaxial strain is systematically explored by the first-principles approach and Boltzmann transport theory. Our results show that the Janus ZrSSe monolayer has excellent chemical, dynamical, thermal, and mechanical stabilities, which provide a reliable platform for strain tuning. The electronic structure and TE transport parameters of the Janus ZrSSe monolayer can be obviously tuned by biaxial strain. Under 2% tensile strain, the optimal power factor PF of the n-type-doped Janus ZrSSe monolayer reaches 46.36 m W m^-1 K^-2 at 300 K. This value is higher than that of the most classical TE materials. Under 6% tensile strain, the maximum ZT values for the p-type- and n-type-doped Janus ZrSSe monolayers are 4.41 and 4.88, respectively, which are about 3.83 and 1.49 times the results of no strain, respectively. Such high TE performance can be attributed to high band degeneracy and short phonon relaxation time under strain, causing simultaneous increase of the Seebeck coefficient and suppression of the phonon thermal transport. Present work demonstrates that the Janus ZrSSe monolayer is a promising candidate as a strain-tunable TE material and stimulates further experimental synthesis.

Strain engineering of hyperbolic plasmons in monolayer carbon phosphide: a first-principles study

Natural and tunable in-plane hyperbolic plasmons have so far been elusive, and hence few two-dimensional hyperbolic materials have been theoretically and experimentally discovered. Here, comprehensive first-principles calculations were conducted to study the electronic and plasmonic properties of biaxially strained monolayer carbon phosphide (β-CP). We found that (i) a compressed β-CP hosts strong anisotropic Dirac-shaped fermions with robust modulated Fermi velocity, (ii) for biaxial strain of -3% an unprecedented ultra-wide hyperbolic window is extended continuously from terahertz (9 THz) to mid-visible (blue light, 693 THz), (iii) the tunable optical Van Hove singularity as the origin of hyperbolic plasmons in deformed β-CP is disclosed, (iv) an elliptic to hyperbolic transition in the σ-near-zero regime is demonstrated in terahertz frequencies (9 THz), (v) the propagation angle of the concave wavefront can be actively tuned using biaxial strains, and (vi) hyperbolic dispersion reorientation from one principal axis to another orthogonal one under compressive strains larger than 8% is observed. This study sheds new light on the unique properties of hyperbolic two-dimensional (2D) materials having exotic optoelectronic characteristics which are promising candidates for anisotropic light control with ultimate dexterity in the flat optics.

Anisotropic thermal and electrical transport properties induced high thermoelectric performance in an Ir2Cl2O2 monolayer

In recent years, the energy crisis and global warming have been urgent problems that need to be solved. As is known, thermoelectric (TE) materials can transfer heat energy to electrical energy without air pollution. High-throughput calculations as a novel approach are adopted by screening promising TE materials. In this paper, we use first-principles calculations combined with the semiclassical Boltzmann transport theory to estimate the TE performance of monolayer Ir₂Cl₂O₂ according to the prediction that Ir₂Cl₂O₂ has potential as a good TE material via high-throughput calculations. The low thermal conductivities of 1.73 and 4.68 W mK^-1 of Ir₂Cl₂O₂ along the x- and y-axes are calculated, respectively, which exhibits the strong anisotropy caused by the difference in group velocities of low-frequency phonon modes. Then, the electronic transport properties are explored, and the figure of merit ZT is eventually obtained. The maximum ZT value reaches 2.85 (0.40) along the x-axis (y-axis) at 700 K, revealing that the TE properties of the Ir₂Cl₂O₂ monolayer are highly anisotropic. This work reveals that the anisotropic layer Ir₂Cl₂O₂ exhibits high TE performance, which confirms that it is feasible to screen excellent TE materials via high-throughput calculations.

2D XBiSe3(X = Ga, In, Tl) monolayers with high carrier mobility and enhanced visible-light absorption

The geometrical configurations of the XBiSe₃ (X = Ga, In, Tl) monolayers are identified by employing the first-principles density functional theory calculations, and the stabilities are confirmed by phonon dispersion, formation energy, and ab initio molecular dynamics simulation, respectively. The bandgap and band edges, the density of states, the optical absorption, mobility, and effect of strain engineering are evaluated to understand the photoelectronic properties of the monolayers. The results show that the XBiSe₃ monolayers have the indirect bandgaps of 1.14-1.69 (1.20-1.84) eV by HSE06(GW), leading to the enhanced optical absorption from the visible to near-ultraviolet region. The large mobility of the electron and hole are also observed, which is helpful for the separation and transfer of the photogenerated carrier pair. The band edges and bandgaps, as well as the optical absorptions, can effectively be tuned by strain engineering. It should be noted that the band edges of the InBiSe₃ monolayer could satisfy the condition of redox potential for the hydrogen evolution reaction under the compressive strain heavier than -3%, implicating this monolayer can also be used for photocatalytic water splitting to produce hydrogen. Therefore, these monolayers have potential applications in photocatalytic materials or photoelectronic devices such as energy harvesters and visible-light sensors.

First-Principle Investigations on the Electronic and Transport Properties of PbBi2Te2X2 (X = S/Se/Te) Monolayers

This paper reports first-principles calculations on PbBi2Te2S2, PbBi2Te2Se2 and PbBi2Te4 monolayers. The strain effects on their electronic and thermoelectric properties as well as on their stability have been investigated. Without strain, the PbBi2Te4 monolayer exhibits highest Seebeck coefficient with a maximum value of 671 μV/K. Under tensile strain the highest power factor are 12.38×1011 Wm−1K−2s−1, 10.74×1011 Wm−1K−2s−1 and 6.51×1011 Wm−1K−2s−1 for PbBi2Te2S2, PbBi2Te2Se2 and PbBi2Te4 at 3%, 2% and 1% tensile strains, respectively. These values are 85.9%, 55.0% and 3.3% larger than those of the unstrained structures.

The effect of mechanical strain on the Dirac surface states in the (0001) surface and the cohesive energy of the topological insulator Bi2Se3

The band gap (E g) engineering and Dirac point tuning of the (0001) surface of 8 QLs (quintuple layers) thick Bi2Se3 slab are explored using the first-principles density functional theory calculations by varying the strain. The strain on the Bi2Se3 slab primarily varies the bandwidth, modifies the p z - orbital population of Bi and moves the Dirac point of the (0001) surface of Bi2Se3. The Dirac cone feature of the (0001) surface of Bi2Se3 is preserved for the entire range of the biaxial strain. However, around 5% tensile uniaxial strain and even lower value of volume conservation strain annihilate the Dirac cone, which causes the loss of topological (0001) surface states of Bi2Se3. The biaxial strain provides ease in achieving the Dirac cone at the Fermi energy (E F) than the uniaxial and volume conservation strains. Interestingly, the transition from direct E g to indirect E g state of the (0001) surface of Bi2Se3 is observed in the volume conservation strain-dependent E g. The strain on Bi2Se3, significantly modifies the conduction band of Se2 atoms near E F compared to Bi and Se1, and plays a vital role in the conduction of the (0001) surface of Bi2Se3. The atomic cohesive energy of the Bi2Se3 slab is very close to that of (0001) oriented nanocrystals extracted from the Raman spectra. The strain-dependent cohesive energy indicates that at a higher value of strain, the uniaxial and volume conservation strain provides better stability than that of the biaxial strain (0001) oriented growth of the Bi2Se3 nanocrystals. Our study establishes the relationship between the strained lattice and electronic structures of Bi2Se3, and more generally demonstrates the tuning of the Dirac point with the mechanical strain.

Mechanical strength and flexibility in [Formula: see text]-4H borophene

Very recently, a novel phase of hydrogenated borophene, namely [Formula: see text]-4H, has been synthesized in a free-standing form. Unlike pure borophenes, this phase shows very good stability in the air environment and possesses semiconducting characteristics. Because of the interesting stiffness and flexibility of borophenes, herein, we systematically studied the mechanical properties of this novel hydrogenated phase. Our results show that the monolayer is stiffer (Y[Formula: see text] = [Formula: see text]195 N/m) than group IV and V 2D materials and even than MoS[Formula: see text], while it is softer than graphene. Moreover, similar to other phases of borophene, the inherent anisotropy of the pure monolayer increases with hydrogenation. The monolayer can bear biaxial, armchair, and zigzag strains up to 16, 10, and 14% with ideal strengths of approximately 14, 9, and 12 N/m, respectively. More interestingly, it can remain semiconductor under this range of tension. These outstanding results suggest that the [Formula: see text]-4H is a promising candidate for flexible nanoelectronics.

Tunable valley polarization, magnetic anisotropy and Dzyaloshinskii-Moriya interaction in two-dimensional intrinsic ferromagnetic Janus 2H-VSeX (X = S, Te) monolayers

Two-dimensional (2D) Janus materials are a novel kind of 2D materials, which have potential applications in nanoelectronics, optoelectronics and spintronics. However, a 2D Janus material combined with intrinsic ferromagnetism, electric dipole moment, valley polarization and Dzyaloshinskii-Moriya interaction (DMI) remains rarely reported. Here, the electronic structure and magnetic properties of 2D intrinsic ferromagnetic Janus 2H-VSeX (X = S, Te) monolayers are investigated systematically using the density-functional theory. Janus 2H-VSeX (X = S, Te) monolayers are intrinsic ferromagnetic semiconductors with in-plane magnetic anisotropy (IMA). The valley splitting of Janus 2H-VSeX (X = S, Te) monolayers appears by considering the spin-orbit coupling (SOC) effect and out of plane magnetization. Additionally, spontaneous vertical electric dipole moment and a large DMI are also found in Janus 2H-VSeX (X = S, Te) monolayers due to the broken inversion symmetry. Moreover, the valley splitting and DMI can be significantly increased by applying in-plane biaxial strain. These results provide an interesting 2D intrinsic ferromagnetic Janus material, which has potential applications in spintronic and valleytronic devices.

Monolayer Bi2Se3-xTex: novel two-dimensional semiconductors with excellent stability and high electron mobility

Two-dimensional materials play a vital role in next-generation microelectronics, optoelectronics and flexible electronics due to their novel physical properties caused by quantum-confinement effects. In this work, we investigate the stability and the possibility of exfoliation of monolayer Bi₂Se_3-xTe_x (x = 0, 1, 2) using first-principles calculations. Our calculations show that these materials are indirect bandgap semiconductors, and the elastic modulus is smaller than other conventional materials, which indicates better flexibility. We find that the electron mobility of monolayer Bi₂SeTe₂ along the armchair direction is higher than that of black phosphorene, reaching 2708 cm² V^-1 s^-1, and the electron mobility of monolayer Bi₂Se₃ along the zigzag direction is about 24 times larger than the hole mobility. The remarkable electron mobilities and highly anisotropic properties of these new monolayers pave the way for future applications in high-speed (opto)electronic devices.