Theoretical design of porous dodecagonal germanium carbide (d-GeC) monolayer.

Porous nanosheet materials have recently emerged as attractive candidates to serve as nanofiltration membranes. Through first-principles calculations based on density functional theory (DFT) calculations, we propose a new porous dodecagonal GeC (d-GeC) monolayer. We show that the d-GeC monolayer exhibits excellent energetic, mechanical, dynamic, and thermal stabilities. The d-GeC monolayer shows semiconducting properties with an indirect band gap of 1.73 eV (2.53 eV) PBE(HSE06). We also show that the d-GeC monolayer can serve as a good membrane for molecular and atomic permeation due to its low value of estimated diffusion energy barriers. Our results demonstrate the potential of the d-GeC monolayer for the design of nanofiltration membrane technology.

Exploring room-temperature ferromagnetism in WXBC (X = W, Mn, Fe) monolayers.

Two-dimensional (2D) transition metal boron-carbide is a novel material that has unique properties suitable for advanced spintronics and storage applications. Through first-principles calculations based on density functional theory (DFT) calculations, we report a new class of stable 2D ceramic WXBC (X = W, Mn, Fe) monolayers. We find that all WXBC monolayers prefer a ferromagnetic ground state with metallic electronic property. DFT calculations proved that WXBC monolayers exhibit good energetic, mechanical, and dynamic stabilities. More importantly, these monolayers exhibit large magnetic anisotropy energy (MAE) of 1213 µeV, 247 µeV and 20 µeV per magnetic atom for W2BC, WMnBC, and WFeBC, respectively. An out-of-plane easy axis (EA) magnetization direction is found for W2BC whereas the EA for WMnBC and WFeBC are in-plane. By performing Monte Carlo (MC) simulations based on the 2D Heisenberg model, we predict Curie temperatures (T C) of 155 K for the W2BC monolayer. The Berezinskii-Kosterlitz-Thouless transition (BKT) temperature values of WMnBC and WFeBC are as high as 374.69 K and 417.39 K. For further investigations, the adsorption properties of Li, Na, and K atoms on WXBC (atm-WXBC) systems are examined. It is revealed that the initial ferromagnetic metallic properties of bare WXBC monolayers are maintained for all atm-WXBC systems. The obtained strong chemisorption energies are high enough to make adsorbed Li, Na, and K immobile on WXBC surfaces. All these findings demonstrate the unique potential of WXBC monolayers as multifunctional candidates for advanced magnetic device and storage applications.

Comprehensive Study of Lithium Adsorption and Diffusion on Janus Mo/WXY (X, Y = S, Se, Te) Using First-Principles and Machine Learning Approaches.

The structural asymmetry of two-dimensional (2D) Janus transition-metal dichalcogenides (TMDs) produces internal dipole moments that result in interesting electronic properties. These properties differ from the regular (symmetric) TMD structures that the Janus structures are derived from. In this study, we, first, examine adsorption and diffusion of a single Li atom on regular MX2 and Janus MXY (M = Mo, W; XY = S, Se, Te) TMD structures at various concentrations using first-principles calculations within density functional theory. Lithium adsorption energy and mobility differ on the top and bottom sides of each Janus material. The correlation between Li adsorption energy, charge transfer, and bond lengths at different coverage densities is carefully examined. To gain more physical insight and prepare for future investigations into regular TMD and Janus materials, we applied a supervised machine learning (ML) model that uses clusterwise linear regression to predict the adsorption energies of Li on top of 2D TMDs. We developed a universal representation with a few descriptors that take into account the intrinsic dipole moment and the electronic structure of regular and Janus 2D layers, the side where the adsorption takes place, and the concentration dependence of adatom doping. This representation can easily be generalized to be used for other impurities and 2D layer combinations, including alloys as well. At last, we focus on analyzing these structures as possible anodes in battery applications. We conducted Li diffusion, open-circuit voltage, and storage capacity simulations. We report that lithium atoms are found to easily migrate between transition-metal (Mo, W) top sites for each considered case, and in these respects, many of the examined Janus materials are comparable or superior to graphene and regular TMDs. In addition, we report that the side with higher electronegative chalcogen atoms is suitable for Li adsorption and only MoSSe and MoSeTe can be suitable for full coverage of Li atoms on the surface. Bilayer Li adsorption was hindered due to negative open-circuit voltage. Bilayer Janus structures are better suited for battery applications due to less volumetric expansion/contraction during the discharge/charge process and having higher storage capacity. Janus monolayers undergo a transition from semiconducting to metallic upon adsorption of a single Li ion, which would improve anode conductivity. The results imply that the examined Janus structures should perform well as electrodes in Li-ion batteries.

Aluminum and lithium sulfur batteries: a review of recent progress and future directions.

Advanced materials with various micro-/nanostructures have attracted plenty of attention for decades in energy storage devices such as rechargeable batteries (ion- or sulfur based batteries) and supercapacitors. To improve the electrochemical performance of batteries, it is uttermost important to develop advanced electrode materials. Moreover, the cathode material is also important that it restricts the efficiency and practical application of aluminum-ion batteries. Among the potential cathode materials, sulfur has become an important candidate material for aluminum-ion batteries cause of its considerable specific capacity. Two-dimensional materials are currently potential candidates as electrodes from lab-scale experiments to possible pragmatic theoretical studies. In this review, the fundamental principles, historical progress, latest developments, and major problems in Li-S and Al-S batteries are reviewed. Finally, future directions in terms of the experimental and theoretical applications have prospected.

Ferromagnetic TM2BC (TM = Cr, Mn) monolayers for spintronic devices with high Curie temperature.

Transition metal boro-carbide (TM2BC) structures crystallize in the layered orthorhombic structure in their bulk phases. In this study, however, we find that TM2BC (TM = Cr, Mn) prefer a tetragonal (t) crystal structure in their monolayer phases due to the occurrence of strong sp2 bonds between the metal and B/C atoms instead of sp3 + sp2 bonds which exist in the orthorhombic phase. The calculated energy difference between the orthorhombic and the tetragonal structures based on density functional theory (DFT) is more than 1 eV per unit cell. In addition, t-Cr2BC and t-Mn2BC monolayers are dynamically and thermally stable with their magnetic metal electronic structures. For further investigations, we combine our DFT calculations with the Monte Carlo simulations and find that both t-TM2BC monolayers show ferromagnetic properties. The calculated Curie temperatures are 846 K and 128 K for t-Cr2BC and t-MnBC sheets, respectively. In addition, we examine the magnetic anisotropy energies (MAE) of t-TM2BC monolayers and find that both structures prefer out-of-plane as the easy axis magnetization direction and the applied electric field can easily modulate the MAE of the monolayers. Our theoretical calculations reveal that t-TM2BC (TM = Cr, Mn) sheets have great potential for the future design of controllable spintronic devices with their tunable MAE properties.

Engineering the Electronic, Thermoelectric, and Excitonic Properties of Two-Dimensional Group-III Nitrides through Alloying for Optoelectronic Devices (B1-xAlxN, Al1-xGaxN, and Ga1-xInxN).

Recently, two-dimensional (2D) group-III nitride semiconductors such as h-BN, h-AlN, h-GaN, and h-InN have attracted attention because of their exceptional electronic, optical, and thermoelectric properties. It has also been demonstrated, theoretically and experimentally, that properties of 2D materials can be controlled by alloying. In this study, we performed density functional theory (DFT) calculations to investigate 2D B1-xAlxN, Al1-xGaxN, and Ga1-xInxN alloyed structures. We also calculated the thermoelectric properties of these structures using Boltzmann transport theory based on DFT and the optical properties using the GW method and the Bethe-Salpeter equation. We find that by changing the alloying concentration, the band gap and exciton binding energies of each structure can be tuned accordingly, and for certain concentrations, a high thermoelectric performance is reported with strong dependence on the effective mass of the given alloyed monolayer. In addition, the contribution of each e-h pair is explained by investigating the e-h coupling strength projected on the electronic band structure, and we find that the exciton binding energy decreases with increase in sequential alloying concentration. With the ability to control such properties by alloying 2D group-III nitrides, we believe that this work will play a crucial role for experimentalists and manufacturers focusing on next-generation electronic, optoelectronic, and thermoelectric devices.

Electronic properties of bare and functionalized two-dimensional (2D) tellurene structures.

Recently, 2D tellurene (Te) structures have been experimentally synthesized. These structures possess high carrier mobility and stability which make them ideal candidates for applications in electronics, optoelectronics and energy devices. We performed density functional theory (DFT) and molecular dynamics (MD) simulations to investigate the stability and electronic structure of 2D α- and ß-Te sheets, and hydrogen, oxygen, and fluorine functionalized counterparts, including spin-orbit coupling effects. Our calculations show that bare α and ß-Te sheets are stable with band gaps of 0.44 eV and 1.02 eV respectively. When functionalized, α and ß monolayers exhibit metallic properties, except for hydrogenated ß-Te, which exhibits semiconducting properties with a band gap of 1.37 eV. We see that H, O and F destabilize the structure of α-Te. We also find that F and H cause ß-Te layers to separate into functionalized atomic chains and O causes ß-Te to transform into a Te3O2-like structure. We also studied single atom and molecule binding on the Te surface, the effects of adatom coverage, and the effects of functionalized Te on a GaSe substrate. Our results indicate that tellurene monolayers and functionalized counterparts are not only suitable for future optoelectronic devices, but can be used as metallic contacts in nanoscale junctions.

Unusual Pressure-Driven Phase Transformation and Band Renormalization in 2D vdW Hybrid Lead Halide Perovskites.

The application of high pressure allows control over the unit cell and interatomic spacing of materials without any need for new growth methods or processing while accessing their materials properties in situ. Under these extreme pressures, materials may assume new structural phases and reveal novel properties. Here, unusual phase transition and band renormalization effects in 2D van der Waals Ruddlesden-Popper hybrid lead halide perovskites, which have shown extraordinary optical properties and immense potential in light emission and conversion technologies, are reported. The results show that (CH3 (CH2 )3 NH3 )2 (CH3 NH3 )Pb2 Br7 (n = 2) layers undergo two distinct phase transitions related to PbBr6 octahedra, butylammonium (BA), and methylammonium (MA) molecule tilting motion that leads to rather unique/anomalous bandgap variation with pressure. In contrast, (CH3 (CH2 )3 NH3 )PbBr4 (n = 1) lacks MA molecules and possesses only one pressure-induced phase transition related to PbBr6 octahedra and BA tilting. In this range, the bandgap reduces monotonically, much similar to other inorganic semiconductors and display surprisingly large redshift from 3 to 2.4 eV. Together with theoretical calculations, this study offers unique insights into these pressure-induced changes and extends the understanding of these highly anisotropic layered soft organic perovskite materials under extreme conditions.

Monitoring the electronic, thermal and optical properties of two-dimensional MoO2 under strain via vibrational spectroscopies: a first-principles investigation.

This study presents the electronic, mechanical, thermal, vibrational and optical properties of the MoO2 monolayer under the effect of biaxial and uniaxial compressive/tensile strain, using first-principles calculations based on density functional theory. It has been found that the mechanical strength of MoO2 is higher than other MoX2 (X = S, Se, Te) monolayers. Dynamical stability analysis shows that MoO2 is stable up to 6% compressive and at least 8% tensile strain. Strain dependent Raman modes are investigated along the biaxial directions. It was obtained that phonon softening and hardening occurred under tensile and compressive strain owing to the increase and decrease of the bond lengths of the MoO2 structure. Our results also imply that the electronic band structure of the MoO2 monolayer can be tuned with strain and the energy bandgap decreases with increasing biaxial tensile strain up to 4%. For larger values of strain, a semiconductor to semimetal transition is observed; however, this kind of transition is not observed for uniaxial tensile strain. Besides, we report that MoO2 has a negative thermal expansion coefficient (TEC) in the range of extremely low-temperatures (0 K to 33 K) similar to other 2D MoX2 monolayers. For temperatures above 600 K, it possesses a positive TEC with an approximate maximum value of 12 × 10-6 K-1. We carried out optical property calculations by solving the Bethe-Salpeter equation and found that the MoO2 monolayer has two strongly bound excitons below the quasiparticle absorption edge. Overall, our results shed light on experimental studies and suggest that the MoO2 monolayer should be an excellent candidate for new design layered semiconductors, electronics, and optoelectronic devices.

Effects of the number of layers on the vibrational, electronic and optical properties of alpha lead oxide.

We have investigated the effects of the number of layers on the structure, vibrational, electronic and optical properties of α-PbO using first principles calculations. Our theoretical calculations have shown that four Raman active modes of α-PbO tend to red-shift from bulk to monolayer due to a decrease of the force constants and an increase of the bond lengths. It has been shown that while bulk and multilayer α-PbO have an indirect band gap, the monolayer form has a direct band gap value of 2.59 eV. Although lead atoms have 5d states, spin-orbit coupling does not significantly affect the band structure of α-PbO. The computed cleavage energy value (0.67 J m-2) confirms that monolayer PbO can be easily obtained from its bulk counterpart by exfoliation. In addition to the band structure, we also calculated the optical properties and absorbed photon flux Jabs of α-PbO structures to investigate the possibility of solar absorption. Our calculations reveal that while monolayer and bilayer PbO have relatively large band gaps and lower absorption coefficients, their Jabs values are not ideal for solar absorption devices. In contrast, the multilayer and bulk phases of the α-PbOs show good overlap with the solar spectrum and yield high electrical current values. Our calculations have indicated that ultrathin films of α-PbO (such as 3 nm thickness) could be excellent candidates for solar cells. We believe that our work can be utilized to improve electronic and optical devices based on lead oxide structures.

Chemical and substitutional doping, and anti-site and vacancy formation in monolayer AlN and GaN.

We investigated the effects of chemical/substitutional doping, hydrogenation, and anti-site and vacancy defects on the atomic, optoelectronic and magnetic properties of AlN and GaN monolayers. Upon doping of selected atoms, AlN and GaN monolayers can acquire magnetic properties, and their fundamental band gaps are modified by the localized gap states. Spin-polarized gap states broaden into bands at patterned coverage of adatoms, whereby half-metallic or magnetic semiconducting properties can be attained. Specific adatoms adsorbed to Ga atoms break the nearest vertical Ga-N bonds in the GaN bilayer in the heackelite structure and result in changes in the electronic and atomic structure. While adjacent and distant pairs of anion + cation vacancies induce spin polarization with filled and empty gap states, anti-site defects remain nonmagnetic; but both defects induce dramatic changes in the band gap. Fully hydrogenated monolayers are stable only for specific buckled geometries, where one geometry can also lead to an indirect to direct band gap transition. Also, optical activity shifts to the ultra-violet region upon hydrogenation of the monolayers. While H2 and O2 molecules are readily physisorbed on the surfaces of the monolayers with weak van der Waals attraction, they can be dissociated into constituent atoms at the vacancy site of the cation. Our study performed within density functional theory shows that the electronic, magnetic and optical properties of AlN and GaN monolayers can be tuned by doping and point defect formation in order to acquire diverse functionalities.

Diffusion quantum Monte Carlo and density functional calculations of the structural stability of bilayer arsenene.

We have studied the structural stability of monolayer and bilayer arsenene (As) in the buckled (b) and washboard (w) phases with diffusion quantum Monte Carlo (DMC) and density functional theory (DFT) calculations. DMC yields cohesive energies of 2.826(2) eV/atom for monolayer b-As and 2.792(3) eV/atom for w-As. In the case of bilayer As, DMC and DFT predict that AA-stacking is the more stable form of b-As, while AB is the most stable form of w-As. The DMC layer-layer binding energies for b-As-AA and w-As-AB are 30(1) and 53(1) meV/atom, respectively. The interlayer separations were estimated with DMC at 3.521(1) Å for b-As-AA and 3.145(1) Å for w-As-AB. A comparison of DMC and DFT results shows that the van der Waals density functional method yields energetic properties of arsenene close to DMC, while the DFT + D3 method closely reproduced the geometric properties from DMC. The electronic properties of monolayer and bilayer arsenene were explored with various DFT methods. The bandgap values vary significantly with the DFT method, but the results are generally qualitatively consistent. We expect the present work to be useful for future experiments attempting to prepare multilayer arsenene and for further development of DFT methods for weakly bonded systems.