Decoding Van der Waals Impact on Chirality Transfer in Perovskite Structures: Density Functional Theory Insights.

Chiral organic-inorganic perovskites exhibit unique physicochemical properties driven by the symmetry of monovalent organic cations. However, an atomistic understanding of how chiral cations transfer their chirality to the inorganic framework and the role played by van der Waals (vdW) interactions in this process is still incomplete. In this work, we report a theoretical investigation, based on density functional theory calculations within the Perdew-Burke-Ernzerhof (PBE) formulation for the exchange-correlation functional, into the role of the vdW interactions in the chirality transfer process. For that, we selected several vdW corrections, namely, Grimme (D2, D3, D3(BJ)), Tkatchenko-Scheffler (TS, TS+SCS, TS+HSI), density-dependent energy correction (dDsC), and many-body scattering (MBD) energy method correction. For the chiral perovskite systems, we selected a set of chiral organic-inorganic perovskites with several dimensions, namely, from zero-dimensional to three-dimensional, each having enantiomers with R and S configurations. Based on a statistical treatment of the relative errors of all lattice parameters with respect to experimental data, we found that D3, D3(BJ), TS, TS+SCS, TS+HSI, and MBD vdW are the most accurate corrections to describe the equilibrium structural properties of chiral perovskites using the PBE method. We identify chirality-induced sequential asymmetries of distorted octahedrons and propose angular descriptors to quantify them, where the orientations of these distortions depend on the R or S nature of the chiral cations. Furthermore, we demonstrate the importance of accurate vdW interactions in precisely describing these asymmetric distortions. By means of binding energies and charge-transfer analysis, we show that the impact of vdW corrections on the charge distribution leads to a subtle strengthening of hydrogen bonds between chiral cations and inorganic octahedra, resulting in an increase in the binding energy. Finally, we identified that the Rashba-Dresselhaus effect in two-dimensionality is refined by vdW interactions.

Prediction of Effective Photoelectron and Hole Separation in Type-I MoS2/PtSe2 van der Waals Junction.

Understanding the dynamics of charge transfer at vertical heterostructures of transition metal dichalcogenide monolayers is fundamentally important for future technological applications, given the unique feature of van der Waals interactions at the interface. Here, we employ time-dependent density functional theory formalism combined with molecular dynamics to investigate photoexcited electrons and holes in the type-I MoS2/PtSe2 van der Waals heterobilayer. While type-I junctions have been traditionally viewed as being ineffective in photocarrier separation, we show that here a different mechanism from type-II is at play, which effectively separates photoelectrons from photoholes. The key is the phonon bottleneck, arising from the characteristically different dynamic band alignments in the valence and conduction bands, respectively, which only affects the transfer of holes but not electrons. The disparity between electron and hole transfer rates offers a new direction for effective control of charge separation at interfaces.

Ab initioscreening of two-dimensional CuQxand AgQxchalcogenides.

Two-dimensional (2D) chalcogenides have attracted great interest from the scientific community due to their intrinsic physical-chemical properties, which are suitable for several technological applications. However, most of the reported studies focused on particular compounds and composition, e.g., MoS2, MoSe2, WS2, and WSe2. Thus, there is an increased interest to extend our knowledge on 2D chalcogenides. Here, we report a density functional theory (DFT) screening of 2D coinage-metal chalcogenides (MQx), whereM= Cu, Ag,Q= S, Se, Te,x= 0.5, 1.0, 1.5, 2.0, with the aim to improve our atomistic understanding of the physical-chemical properties as a function of cation (M), anion (Q), and composition (x). Based on 258 DFT calculations, we selected a set of 22 stableMQxmonolayers based on phonons analyses, where we identified 9 semiconductors (7 AgQxand 2 CuQx), with band gaps from 0.07 eV up to 1.67 eV, while the remaining systems have a metallic character. Using all 258 systems, we found a logarithmic correlation between the average weighted bond lengths and effective coordination number of cations and anions. As expected, the monolayer cohesive energies increase with the radius of theQspecies (i.e., from S to Te). Furthermore, an increase in the anion size diminishes the work function for nearly allMQxmonolayers, which can be explained by the nature of the electronic states at the valence band maximum.

The role of the A-cations in the polymorphic stability and optoelectronic properties of lead-free ASnI3 perovskites.

Tin-based ASnI3 perovskites have been considered excellent candidates for lead-free perovskite solar cell applications; however, our atomistic understanding of the role of the A-cations, namely, CH3NH3 (methylammonium, MA), CH3PH3 (methylphosphonium, MP) and CH(NH2)2 (formamidinium, FA), in the physical chemistry properties is far from satisfactory. For the first time, we report a density functional theory investigation of the MPSnI3 perovskite and non-perovskite phases as well as their comparison with the MASnI3 and FASnI3 phases, where we considered the role of the A-cation orientations in the structural stability of the ASnI3 phases. The orthorhombic structure is the most stable studied phase, which agrees with experimentally reported phase-transition trends. In contrast with the cation size and the weak hydrogen bonding interactions, which contribute to structural cohesion between the inorganic framework and A-cation, the dipole-dipole interactions play an important role to drive the structures to the lowest energy configurations. From our analysis, the inorganic framework dominates the optical properties, band structure, and density of states around the band edges. Broader absorption and smaller band gap energies occur for the perovskite structures compared to the low-dimensional hexagonal/pseudo-hexagonal non-perovskites.

Ab initio investigation of topological phase transitions induced by pressure in trilayer van der Waals structures: the example of h-BN/SnTe/h-BN.

The combination of two-dimensional crystals through the formation of van der Waals bilayers, trilayers, and heterostructures has been considered a promising route to design new materials due to the possibility of tuning their properties through the control of the number of layers, alloying pressure, strain, and other tuning mechanisms. Here, we report a density functional theory study on the interlayer phonon coupling and electronic structure of the trilayer h-BN/SnTe/h-BN, and the effects of pressure on the encapsulation of this trilayer system. Our findings demonstrated the establishment of a type I junction in the system, with a trivial bandgap of 0.55 eV, which is 10 % lower than the free-standing SnTe one. The almost inert h-BN capping layers allow a topological phase transition at a pressure of 13.5 GPa, in which the system evolves from a trivial insulator to a topological insulator. In addition, with further increase of the pressure up to 35 GPa, the non-trivial energy bandgap increases up to 0.30 eV. This behavior is especially relevant to allow experimental access to topological properties of materials, since large non-trivial energy bandgaps are required.

Edge, size, and shape effects on WS2, WSe2, and WTe2 nanoflake stability: design principles from an ab initio investigation.

An improved atomistic understanding of the W-based two-dimensional transition-metal dichalcogenides (2D TMDs) is crucial for technological applications of 2D materials, since the presence of tungsten endows these materials with distinctive properties. However, our atomistic knowledge on the evolution of the structural, electronic, and energetic properties and on the nanoflake stability of such materials is not properly addressed hitherto. Thus, we present a density functional theory (DFT) study of stoichiometric (WQ2)n nanoflakes, with Q = S, Se, Te, and n = 1,…,16, 36, 66, and 105. We obtained the configurations with n = 1,…,16 through the tree growth algorithm whereas the nanoflakes with n = 36, 66, and 105 were generated from fragments of 2D TMDs with an abundant diversity of shapes and edge configurations. We found that all the most stable nanoflakes present the same Q-terminated edge configuration. Furthermore, in isomers with n = 1,…,16 sizes, nanoflakes with triangular shapes and their derivatives, such as the rhombus geometry, define magic numbers, whereas for n > 16, triangular shapes were also found for the most stable structures, because they preserve the edge configuration. A strong modulation of the Hirshfeld charges, depending on chalcogen species and core or edge position, is also observed. The modulation of the Hirshfeld charge due to the nature of the W metal atoms makes the energetic 1D → 1T' transition of (WQ2)n differ in nanoflake size in relation to (MoQ2)n nanoflakes. Our analysis shows the interplay between edge configuration, coordination environment, and shape that determines the stability of nanoflakes, and allows us to describe design principles for stable 1T' stoichiometric nanoflakes of various sizes.

The role of the alkali and chalcogen atoms on the stability of the layered chalcogenide [Formula: see text] (A = alkali-metal; M = metal-cations; Q = chalcogen) compounds: a density functional theory investigation within van der Waals corrections.

There is a great interest to design two-dimensional (2D) chalcogenide materials, however, our atomistic understanding of the major physical parameters that drive the formation of 2D or three-dimensional (3D) chalcogenides is far from satisfactory, in particular, for complex quaternary systems. To address this problem, we selected a set of quaternary 2D and 3D chalcogenide compounds, namely, [Formula: see text] (A = Li, K, Cs; Q = S, Se, Te), which were investigated by density functional theory calculations within van der Waals (vdW) corrections. Employing experimental crystal structures and well designed crystal modifications, we found that the average atomic radius of the alkali-metal, A, and chalcogen, Q, species play a crucial role in the stability of the 2D structures. For example, the 2D structures are energetically favored for smaller [Formula: see text] and larger [Formula: see text] average atomic radius, while 3D structures are favored at intermediate average atomic radius. Those results are explained in terms of strain minimization and Coulomb repulsion of the anionic species in the structure framework. Furthermore, the equilibrium lattice parameters are in excellent agreement with experimental results. Thus, the present insights can help in the design of stable quartenary 2D chalcogenide compounds.