Exploration of AB3Si3 (A = Na/K/Rb/Cs) compounds under moderate pressure.

We discovered the composition of ternary AB3Si3 (A = Na/K/Rb/Cs) compounds in the moderate pressure range of 0-100 GPa using first-principles structural prediction and systematically analyzed their structures, stability, electronic and optical properties within the framework of density functional theory. The AB3Si3 compounds exhibit a diverse phase diagram, including nine structures that are selected based on formation energies, along with a known clathrate RbB3Si3 structure with Pm3̄n symmetry. All predicted phases are thermodynamically and dynamically stable within the studied pressure range. In particular, the KB3Si3 compound with a direct band gap of 1.0 eV is identified as a promising candidate for photovoltaic materials beyond silicon-based materials, among which boron atoms form a unique regular octahedral structure; in contrast, NaB3Si3 and RbB3Si3 compounds are shown to have metallicity. Our findings enrich crystal structures of alkali-metal borosilicides and provide valuable insights into their potential applications.

Theoretical design of two-dimensional carbon nitrides.

The study of two-dimensional (2D) materials has attracted considerable attention owing to their unique but fascinating properties. Here we systematically explored 2D carbon nitride monolayer sheets via the particle swarm optimization algorithm in combination with density functional theory. As a result of structural searches, four carbon nitride monolayers are predicted with stable stoichiometries of C5N2, C2N, C3N2and CN. These predicted structures are semiconductors with an optimal band gap for solar cell application as indicated in our electronic simulations. Our current results also reveal the high tensile strengths of the predicted structures compared to known porous carbon nitride monolayer sheets. This work may provide a route for the design of 2D candidates in the application of photovoltaic materials.

Predicted CsSi compound: a promising material for photovoltaic applications.

Exploration of photovoltaic materials has received enormous interest for a wide range of both fundamental and applied research. Therefore, in this work, we identify a CsSi compound with a Zintl phase as a promising candidate for photovoltaic material by using a global structure prediction method. Electronic structure calculations indicate that this phase possesses a quasi-direct band gap of 1.45 eV, suggesting that its optical properties could be superior to those of diamond-Si for capturing sunlight from the visible to the ultraviolet range. In addition, a novel silicon allotrope is obtained by removing Cs atoms from this CsSi compound. The superconducting critical temperature (Tc) of this phase was estimated to be of 9 K in terms of a substantial density of states at the Fermi level. Our findings represent a new promising CsSi material for photovoltaic applications, as well as a potential precursor of a superconducting silicon allotrope.

Promoted Electronic Coupling of Acoustic Phonon Modes in Doped Semimetallic MoTe2.

As a prototype of the Weyl superconductor, layered molybdenum ditelluride (MoTe2) encompasses two semimetallic phases (1T' and Td) which differentiate from each other via a slight tilting of the out-of-plane lattice. Both phases are subjected to serious phase mixing, which complicates the analysis of its origin of superconductivity. Herein, we explore the electron-phonon coupling (EPC) of the monolayer semimetallic MoTe2, without phase ambiguity under this thickness limit. Apart from the hardening or softening of the phonon modes, the strength of the EPC can be strongly modulated by doping. Specifically, longitudinal and out-of-plane acoustic modes are significantly activated for electron doped MoTe2. This is ascribed to the presence of rich valley states and equispaced nesting bands, which are dynamically populated under charge doping. Through comparing the monolayer and bilayer MoTe2, the strength of EPC is found to be less likely to depend on thickness for neutral samples but clearly promoted for thinner samples with electron doping, while for hole doping, the strength alters more significantly with the thickness than doping. Our work explains the issue of the doping sensitivity of the superconductivity in semimetallic MoTe2 and establishes the critical role of activating acoustic phonons in such low-dimensional materials.

Additive Molecules Adsorbed on Monolayer PbI2: Atomic Mechanism of Solvent Engineering for Perovskite Solar Cells.

Solvent engineering is highly essential for the upscaling synthesis of high-quality metal halide perovskite materials for solar cells. The complexity in the colloidal containing various residual species poses great difficulty in the design of the formula of the solvent. Knowledge of the energetics of the solvent-lead iodide (PbI2) adduct allows a quantitative evaluation of the coordination ability of the solvent. Herein, first-principles calculations are performed to explore the interaction of various organic solvents (Fa, AC, DMSO, DMF, GBL, THTO, NMP, and DPSO) with PbI2. Our study establishes the energetics hierarchy with an order of interaction as DPSO > THTO > NMP > DMSO > DMF > GBL. Different from the common notion of forming intimate solvent-Pb bonds, our calculations reveal that DMF and GBL cannot form direct solvent-Pb2+ bonding. Other solvent bases, such as DMSO, THTO, NMP, and DPSO, form direct solvent-Pb bonds, which penetrate through the top iodine plane and possess much stronger adsorption than DMF and GBL. A strong solvent-PbI2 adhesion (i.e., DPSO, NMP, and DMSO), associated with a high coordinating ability, explains low volatility, retarded precipitation of the perovskite solute, and tendency of a large grain size in the experiment. In contrast, weakly coupled solvent-PbI2 adducts (i.e., DMF) induces a fast evaporation of the solvent, accordingly a high nucleation density and small grains of perovskites are observed. For the first time, we reveal the promoted absorption above the iodine vacancy, which implies the need for pre-treatment of PbI2 like vacuum annealing to stabilize solvent-PbI2 adducts. Our work establishes a quantitative evaluation of the strength of the solvent-PbI2 adducts from the atomic scale perspective, which allows the selective engineering of the solvent for high-quality perovskite films.

Surface asymmetry induced turn-overed lifetime of acoustic phonons in monolayer MoSSe.

Recent successful growth of asymmetric transition metal dichalcogenides via accurate manipulation of different chalcogen atoms in top and bottom surfaces demonstrates exotic electronic and chemical properties in such Janus systems. Within the framework of density functional perturbation theory, anharmonic phonon properties of monolayer Janus MoSSe sheet are explored. By considering three-phonons scattering, out-of-plane flexural acoustic (ZA) mode tends to undergo a stronger phonon scattering than transverse acoustic (TA) mode and the longitudinal acoustic (LA) mode with phonon lifetime of ZA (1.0 ps) < LA (23.8 ps) < TA (25.8 ps). This is sharply different from the symmetric MoS2 where flexural ZA mode has the weakest anharmonicity and is least scattered. Moreover, utilizing non-equilibrium Green function method, ballistic thermal conductance at room temperature is found to be around 0.11 nWK-1nm-2, lower than that of MoS2. Our work highlights intriguing phononic properties of such MoSSe Janus layers associated with asymmetric surfaces.

Fast Intercalation of Lithium in Semi-Metallic γ-GeSe Nanosheet: A New Group-IV Monochalcogenide for Lithium-Ion Battery Application.

Existence of van der Waals gaps renders two-dimensional (2D) materials ideal passages of lithium for being used as anode materials. However, the requirement of good conductivity significantly limits the choice of 2D candidates. So far, only graphite is satisfying due to its relatively high conductivity. Recently, a new polymorph of layered germanium selenide (γ-GeSe) was proven to be semimetal in its bulk phase with a higher conductivity than graphite while its monolayer behaves semiconducting. In this work, by using first-principles calculations, the possibility was investigated of using this new group-IV monochalcogenide, γ-GeSe, as anode in Li-ion batteries (LIBs). The studies revealed that the Li atom would form an ionic adsorption with adjacent selenium atoms at the hollow site and exist in cationic state (lost 0.89 e to γ-GeSe). Results of climbing image-nudged elastic band showed the diffusion barrier of Li was 0.21 eV in the monolayer limit, which could activate a relatively fast diffusion even at room temperature on the γ-GeSe surface. The calculated theoretical average voltages ranged from 0.071 to 0.015 V at different stoichiometry of Lix GeSe with minor volume variation, suggesting its potential application as anode of LIBs. The predicted moderate binding energy, a low open-circuit voltage (comparable to graphite), and a fast motion of Li suggested that γ-GeSe nanosheet could be chemically exfoliated via Li intercalation and is a promising candidate as the anode material for LIBs.