Molecular Origin of Properties of Organic–Inorganic Hybrid Perovskites: The Big Picture from Small Clusters

We show that the electronic properties, including the band gap, the gap deformation potential, and the exciton binding energy as well as the chemical stability of organic–inorganic hybrid perovskites can be traced back to their corresponding molecular motifs. This understanding allows one to quickly estimate the properties of the bulk semiconductors from their corresponding molecular building blocks. New hybrid perovskite admixtures are proposed by replacing halogens with superhalogens having compatible ionic radii. The mechanism of the boron-hydride based hybrid perovskite reacting with water is investigated by using a cluster model

Zn in the +III Oxidation State

The possibility that the group 12 elements Zn, Cd, and Hg can exist in an oxidation state of +III or higher has fascinated chemists for decades. It took nearly 20 years before experiments could confirm the theoretical prediction that Hg indeed can exist in the +IV oxidation state. While this unusual property of Hg is attributed to relativistic effects, Zn, which is much less massive than Hg, has not been expected to have an oxidation state higher than +II. Using density functional theory, we have shown that an oxidation state of +III for Zn can be realized by choosing specific ligands with large electron affinities. We demonstrate this by a systematic study of the interaction of Zn with the ligands F, BO₂, and AuF₆, whose electron affinities are progressively higher (3.4, 4.5, and 8.4 eV, respectively). The discovery of higher oxidation states of elements can help in the formulation of new reactions and hence in the development of new chemistry

Giant Valley Splitting and Valley Polarized Plasmonics in Group V Transition-Metal Dichalcogenide Monolayers

Two-dimensional group VI transition-metal dichalcogenides (TMDs) provide a promising platform to encode and manipulate quantum information in the valleytronics. However, the two valleys are energetically degenerate, protected by time-reversal symmetry (TRS). To lift this degeneracy, one needs to break the TRS by either applying an external magnetic field or using a magnetic rare-earth oxide substrate. Here, we predict a different strategy to achieve this goal. We propose that the ferromagnetic group V TMD monolayer, in which the TRS is intrinsically broken, can produce a larger valley and spin splitting. A polarized ZnS(0001) surface is also used as a substrate, which shifts the valleys to the low-energy regime (near the Fermi level). Moreover, by calculating its collective electronic excitation behaviors, we show that such a system hosts a giant valley polarized terahertz plasmonics. Our results demonstrate a new way to design and use valleytronic devices, which are both fundamentally and technologically significant

Atomic-Level Design of Water-Resistant Hybrid Perovskites for Solar Cells by Using Cluster Ions

Organic–inorganic hybrid perovskites have emerged as the most promising material in the development of next-generation solar cells. However, the stability of these materials exemplified by CH₃NH₃PbI₃ is the most pressing challenge; it readily decomposes when exposed to moisture. Here, we show how one can use a particular type of cluster ions, known as pseudohalides, to enhance the water resistance of the hybrid perovskite, while maintaining its favorable electronic properties. Starting with a simple physical model, we propose a new class of water-resistant hybrid perovskites as solar-cell absorbers based on the cluster ions by using DFT calculations and <i>ab initio</i> molecular dynamics. Limitations of applying the currently known pseudohalides for our purpose are also discussed

Cluster-Inspired Design of High-Capacity Anode for Li-Ion Batteries

One of the greatest challenges in next-generation Li-ion batteries (LIB) is to develop high charge capacity anodes with long cycle life. Inspired by the experimental observation of a magic Ti₁₃C₂₂ cluster and its layer-by-layer growth, we have constructed one-dimensional nanowires using Ti₉C₁₃ clusters as well as those based on planar-tetracoordinate carbon-containing (ptC) Ti₈C₁₂ metcar and TiC clusters with bulk cubic crystal structure. Using density functional theory and molecular dynamics we studied their energetic and thermal stability as well as their potential for LIB anode. The Ti₉C₁₃ nanowire is found to be stable up to 2500 K and has a charge capacity five times larger than that of the graphite anode being used today. Furthermore, unlike silicon nanotube anode materials, the Ti₉C₁₃ nanowire does not suffer from volume expansion/contraction during lithiation/delithiation processes

Nitrate Superhalogens as Building Blocks of Hypersalts

Using density functional theory (DFT) with a generalized gradient approximation for the exchange and correlation potential, we have studied the geometrical structure and electronic properties of NO_<i>x</i> (<i>x</i> = 1–3), Li­(NO₃)_<i>x</i> (<i>x</i> = 1,2), Mg­(NO₃)_<i>x</i> (<i>x</i> = 1–3), and Al­(NO₃)_<i>x</i> (<i>x</i> = 1–4) clusters. To validate the accuracy of the DFT-based method, calculations were also performed on small clusters using coupled cluster method with singles and doubles and noniterative inclusion of triples (CCSD­(T)). With an electron affinity of 4.03 eV, NO₃ behaves as a superhalogen molecule and forms the building block of hyperhalogens when interacting with metal atoms such as Li, Mg, and Al. This is confirmed by calculating the adiabatic detachment energies (ADEs) of Li­(NO₃)₂, Mg­(NO₃)₃, and Al­(NO₃)₄, which are 5.69, 6.64, and 6.42 eV, respectively. We also demonstrate that these hyperhalogens can form salts when counter balanced by a cation such as K

Assembling π‑Conjugated Molecules with Negative Gaussian Curvature for Efficient Carbon-Based Metal-Free Thermoelectric Material

The development of efficient, lightweight, cost-effective, and environmentally friendly thermoelectric materials is critical for energy conversion devices. However, none of the existing thermoelectric materials satisfy these requirements. Herein, we predict a novel carbon-based metal-free thermoelectric material denoted as bct-C₈₀S₁₆ that is composed of a π-conjugated saddle-shaped molecular unit with a negative Gaussian curvature, leading to a low lattice thermal conductivity while maintaining a high charge mobility. The resulting peak figure of merit (<i>ZT</i>) of 2.41 at 1000 K is much larger than those of conventional Bi- and Pb-based thermoelectric materials. Additionally, bct-C₈₀S₁₆ is highly porous and light, with a mass density of 1.11 g/cm³. Such a high thermoelectric performance and low mass density would make this metal-free semiconducting material promising for practical applications in space-based technologies

ψ‑Graphene: A New Metallic Allotrope of Planar Carbon with Potential Applications as Anode Materials for Lithium-Ion Batteries

Using state-of-the-art first-principles calculations, we propose a new two-dimensional (2D) carbon allotrope constructed by polymerizing the carbon skeletons of <i>s</i>-indacenes, named PSI (ψ)-graphene. We show that ψ-graphene has the lowest energy among all hitherto reported 2D allotropes of carbon composed of 5–6–7 carbon rings and is dynamically and thermally stable. This structure is metallic with robust metallicity against external strain. In addition, we find that the adsorption of Li atoms on ψ-graphene is exothermic, and the diffusion energy barrier is low and comparable to that of graphene. Furthermore, ψ-graphene can achieve a maximum Li storage capacity equivalent to that of LiC₆, suggesting its potential as an anode material for Li-ion batteries (LIBs). In addition, we show that increasing the number of hexagons in this structure can enhance the thermodynamic stability of the sheet while maintaining its metallicity. This study provides new insights into the design of new metallic carbon for nanostructured anode materials in the next generation of LIBs

Electronic Structure and Stability of Mono- and Bimetallic Borohydrides and Their Underlying Hydrogen-Storage Properties: A Cluster Study

Using gradient corrected density functional theory and a cluster-based model we have studied the stability and hydrogen-storage properties of monometallic borohydrides, M­(BH₄)₃, and bimetallic borohydrides, KM­(BH₄)₄ (M = Al, Ga, Sc). A systematic study of BH_<i>x</i> (<i>x</i> = 1–4), M­(BH₄)_<i>n</i> (M = Al, Ga, Sc; <i>n</i> = 1–4), and KM­(BH₄)₄ reveals many interesting results. (i) The vertical detachment energy of BH₄^– is larger than that of any halogen atom; hence, BH₄ can be classified as a superhalogen. (ii) When a metal atom, M, is surrounded with BH₄ moieties whose number exceed the valence of the metal atom by one, a new class of highly electronegative molecules referred to as hyperhalogens can be formed. (iii) Both BH₄^– and M­(BH₄)₄^– can serve as the building blocks of super- and hyper-salts, respectively, when counterbalanced with a metal cation such as K⁺. (iv) The energy required to remove a hydrogen atom from a bimetallic borohydride such as KAl­(BH₄)₄ is found to be less than that from the corresponding monometallic borohydride, namely Al­(BH₄)₃, thus making bimetallic borohydrides potential candidates for hydrogen-storage materials. We hope these results will stimulate experimental investigation into new super- and hyperhalogens and their corresponding salts as potential hydrogen-storage materials