Lattice Instability and Ultralow Lattice Thermal Conductivity of Layered PbIF.

Understanding the interplay between various design strategies (for instance, bonding heterogeneity and lone pair induced anharmonicity) to achieve ultralow lattice thermal conductivity (κl) is indispensable for discovering novel functional materials for thermal energy applications. In the present study, we investigate layered PbXF (X = Cl, Br, I), which offers bonding heterogeneity through the layered crystal structure, anharmonicity through the Pb2+ 6s2 lone pair, and phonon softening through the mass difference between F and Pb/X. The weak interlayer van der Waals bonding and the strong intralayer ionic bonding with partial covalent bonding result in a significant bonding heterogeneity and a poor phonon transport in the out-of-plane direction. Large average Grüneisen parameters (≥2.5) demonstrate strong anharmonicity. The computed phonon dispersions show flat bands, which suggest short phonon lifetimes, especially for PbIF. Enhanced Born effective charges are due to cross-band-gap hybridization. PbIF shows lattice instability at a small volume expansion of 0.1%. The κl values obtained by the two channel transport model are 20-50% higher than those obtained by solving the Boltzmann transport equation. Overall, ultralow κl values are found at 300 K, especially for PbIF. We propose that the interplay of bonding heterogeneity, lone pair induced anharmonicity, and constituent elements with high mass difference aids the design of low κl materials for thermal energy applications.

Monolayer, Bilayer, and Bulk BSi as Potential Anode Materials of Li-Ion Batteries.

Monolayer, bilayer, and bulk BSi are studied to explore their application potential as anode materials of Li-ion batteries. Structural stability and metallicity are obtained in each case. The Li storage capacities of monolayer and bilayer BSi are 1378 and 689 mAh g-1 , respectively, with average open circuit voltages of 1.30 and 0.47 V as well as Li diffusion barriers of 0.48 and 0.27 eV. Bulk BSi realizes a layered structure in the presence of a small amount of Li and its Li diffusion barrier of 0.48 eV is identical to that of graphite and lower than that of bulk Si (0.58 eV). The Li storage capacity of bulk BSi is found to be 689 mAh g-1 , i. e., much higher than that of graphite (372 mAh g-1 ). The volume expansion turns out to be 33 % and the chemical bonds remain intact at full lithiation, outperforming the 72 % volume expansion of bulk Si at the same capacity and thus pointing to excellent cyclability.

Enhanced out-of-plane electromechanical response of Janus ZrSeO.

Two-dimensional piezoelectric materials have attracted great attention as they could play a vital role in nano-electromagnetic systems. Herein, we investigate the compelling piezoelectric properties of Janus ZrSeO in monolayer and bulk structures using density functional theory calculations with a van der Waals correction. One of the two independent out-of-plane piezoelectric coefficients (e31) of the bulk ZrSeO is as high as 287.60 pC m-1, which is over five times larger than that of monolayer ZrSeO due to charge changes in the internal structure within each Zr, Se, and O layer. Interestingly, another large negative out-of-plane piezoelectric stress coefficient (e33) of bulk ZrSeO (-467.40 pC m-1) results from the displacement difference between the electronic and ionic center positions, which is at least three times larger than those previously reported for Janus Mo/W/Hf-based transition metal dichalcogenides. The charge transformation between atoms under strain induces negative piezoelectric stress, a process that is clarified using maximally localized Wannier functions (MLWF) and Bader charge analysis. This research also reveals the dependence of piezoelectricity in Janus MXY on the metal (M = Zr, Hf, W, Mo) and chalcogenide (X,Y = S, Se, O) components, which are directly proportional to the electronegativity and the atomic size difference.

Semiconducting Chalcogenide Alloys Based on the (Ge, Sn, Pb) (S, Se, Te) Formula with Outstanding Properties: A First-Principles Calculation Study.

Very recently, a new class of the multicationic and -anionic entropy-stabilized chalcogenide alloys based on the (Ge, Sn, Pb) (S, Se, Te) formula has been successfully fabricated and characterized experimentally [Zihao Deng et al., Chem. Mater. 32, 6070 (2020)]. Motivated by the recent experiment, herein, we perform density functional theory-based first-principles calculations in order to investigate the structural, mechanical, electronic, optical, and thermoelectric properties. The calculations of the cohesive energy and elasticity parameters indicate that the alloy is stable. Also, the mechanical study shows that the alloy has a brittle nature. The GeSnPbSSeTe alloy is a semiconductor with a direct band gap of 0.4 eV (0.3 eV using spin-orbit coupling effect). The optical analysis illustrates that the first peak of Im(ε) for the GeSnPbSSeTe alloy along all polarization directions is located in the visible range of the spectrum which renders it a promising material for applications in optical and electronic devices. Interestingly, we find an optically anisotropic character of this system which is highly desirable for the design of polarization-sensitive photodetectors. We have accurately predicted the thermoelectric coefficients and have calculated a large power factor value of 3.7 × 1011 W m-1 K-2 s-1 for p-type. The high p-type power factor is originated from the multiple valleys near the valence band maxima. The anisotropic results of the optical and transport properties are related to the specific tetragonal alloy unit cell.

The effect of non-analytical corrections on the phononic thermal transport in InX (X = S, Se, Te) monolayers.

We investigate the effect of non-analytical corrections on the phonon thermal transport properties in two-dimensional indium chalcogenide compounds. The longitudinal optical (LO) and transverse optical (TO) branches in the phonon dispersion are split near the Γ-point. The lattice thermal conductivity of monolayer InS is increased by 30.2% under non-analytical corrections because of the large LO-TO splitting at Γ-point. The predicted lattice thermal conductivities with non-analytical corrections at room temperature are 57.1 W/mK, 44.4 W/mK and 33.1 W/mK for the monolayer InS, InSe and InTe, respectively. The lattice thermal conductivity can be effectively reduced by nanostructures because the representative mean free paths are found very large in these monolayers. By quantifying the relative contribution of the phonon modes to the lattice thermal conductivity, we predict that the longitudinal acoustic branch is the main contributor to the lattice thermal conductivity. Due to the low lattice thermalconductivities of these monolayers, they can be useful in the nanoscale thermoelectric devices.

Ultrahigh and anisotropic thermal transport in the hybridized monolayer (BC2N) of boron nitride and graphene: a first-principles study.

Heat removal has become a significant challenge in the miniaturization of electronic devices, especially in power electronics, so semiconducting materials with suitable band gaps and high lattice thermal conductivity are highly desired. Here, through first-principles calculations, we theoretically predict an ultra-high and anisotropic lattice thermal conductivity in monolayer BC2N. The predicted values of lattice thermal conductivity at room-temperature are 893.90 W m-1 K-1 and 1275.79 W m-1 K-1 along the armchair and zigzag directions, respectively. These values are probably the highest that have ever been reported for two-dimensional semiconducting materials. Such high lattice thermal conductivities are attributed to the high vibrational frequencies, large phonon group velocities, long phonon lifetime, low phonon anharmonicity, and strong bonding in monolayer BC2N. We also calculate the electrical and electronic thermal conductivities, which are also very high. Based on these theoretical findings, we expect monolayer BC2N to be an adequate candidate for thermal management in nanoelectronic devices.

Strain engineering of phonon thermal transport properties in monolayer 2H-MoTe2.

The effect of strain on the phonon properties such as phonon group velocity, phonon anharmonicity, phonon lifetime, and lattice thermal conductivity of monolayer 2H-MoTe2 is studied by solving the Boltzmann transport equation based on first principles calculations. The phonon thermal transport properties of the unstrained monolayer 2H-MoTe2 are compared to those of the strained case under different biaxial tensile strains. One of the common features of two-dimensional materials is the quadratic nature near the Γ point of the out-of-plane phonon flexural mode that disappears by applying tensile strain. We find that the lattice thermal conductivity of the monolayer 2H-MoTe2 is very sensitive to strain, and the lattice thermal conductivity is reduced by approximately 2.5 times by applying 8% biaxial tensile strain due to the reduction in phonon group velocities and phonon lifetime. We also analyze how the contribution of each mode to lattice thermal conductivity changes with tensile strain. These results highlight that tensile strain is a key parameter in engineering phonon thermal transport properties in monolayer 2H-MoTe2.

Ultra low lattice thermal conductivity and high carrier mobility of monolayer SnS2 and SnSe2: a first principles study.

Using density functional theory, we systematically investigate the lattice thermal conductivity and carrier mobility of monolayer SnX2 (X = S, Se). The room-temperature ultra low lattice thermal conductivities found in monolayer SnS2 (6.41 W m-1 K-1) and SnSe2 (3.82 W m-1 K-1) are attributed to the low phonon velocity, low Debye temperature, weak bonding interactions, and strong anharmonicity in monolayer SnX2. The predicted values of lattice thermal conductivity are lower than those of other two-dimensional materials such as stanene, phosphorene, monolayer MoS2, and bulk SnX2. High phonon-limited carrier mobilities are obtained for the monolayer SnX2. For example, the electron mobility of monolayer SnS2 is 756.60 cm2 V-1 s-1 and the hole mobility is 187.44 cm2 V-1 s-1. The electron mobility of these monolayers is higher than their hole mobility due to the low effective mass of electrons and low deformation constants, which makes them n-type materials. Due to their ultra low lattice thermal conductivities coupled with high carrier mobilities, monolayer SnX2 materials may be promising materials for thermoelectric applications.

Thermoelectric and phonon transport properties of two-dimensional IV-VI compounds.

We explore the thermoelectric and phonon transport properties of two-dimensional monochalcogenides (SnSe, SnS, GeSe, and GeS) using density functional theory combined with Boltzmann transport theory. We studied the electronic structures, Seebeck coefficients, electrical conductivities, lattice thermal conductivities, and figures of merit of these two-dimensional materials, which showed that the thermoelectric performance of monolayer of these compounds is improved in comparison compared to their bulk phases. High figures of merit (ZT) are predicted for SnSe (ZT = 2.63, 2.46), SnS (ZT = 1.75, 1.88), GeSe (ZT = 1.99, 1.73), and GeS (ZT = 1.85, 1.29) at 700 K along armchair and zigzag directions, respectively. Phonon dispersion calculations confirm the dynamical stability of these compounds. The calculated lattice thermal conductivities are low while the electrical conductivities and Seebeck coefficients are high. Thus, the properties of the monolayers show high potential toward thermoelectric applications.

Adsorption and diffusion of mono, di, and trivalent ions on two-dimensional TiS2.

A comparative study of the monovalent (Li, Na, and K) and multivalent (Be, Mg, Ca, and Al) metal ion adsorption and diffusion on an electronically semi-metallic two-dimensional nanosheet of 1T structured TiS2 is presented here to contribute to the search for abundant, cheap, and nontoxic ingredients for efficient rechargeable metal ion batteries. The total formation energy of the metal ion adsorption and the Bader charge analysis show that the divalent Mg and Ca ions can have a charge storage density double that of the monovalent Li, Na, and K ions, while the Be and Al ions form metallic clusters even at a low adsorption density because of their high bulk energies. The adsorption of Mg ions shows the lowest averaged open circuit voltage (0.13 V). The activation energy barriers for the diffusion of metal ions on the surface of the monolayer successively decrease from Li to K and Be to Ca. Mg and Ca, being divalent, are capable of storing a higher power density than Li while K and Na have a higher rate capability than the Li ions. Therefore, rechargeable Li ion batteries can be totally or partially replaceable by Mg ion batteries, where high power density and high cell voltage are required, while the abundant, cheap, and fast Na ions can be used for green grid applications.