Can CF₃‑Functionalized La@C₆₀ Be Isolated Experimentally and Become Superconducting?

Superconducting behavior even under harsh ambient conditions is expected to occur in La@C₆₀ if it could be isolated from the primary metallofullerene soot when functionalized by CF₃ radicals. We use <i>ab initio</i> density functional theory calculations to compare the stability and electronic structure of C₆₀ and the La@C₆₀ endohedral metallofullerene to their counterparts functionalized by CF₃. We found that CF₃ radicals favor binding to C₆₀ and La@C₆₀ and have identified the most stable isomers. Structures with an even number <i>m</i> of radicals are energetically preferred for C₆₀ and structures with odd <i>m</i> for La@C₆₀ due to the extra charge on the fullerene. This is consistent with a wide HOMO–LUMO gap in La@C₆₀(CF₃)_<i>m</i> with odd <i>m</i>, causing extra stabilization in the closed-shell electronic configuration. CF₃ radicals are both stabilizing agents and molecular separators in a metallic crystal, which could increase the critical temperature for superconductivity

Can CF₃‑Functionalized La@C₆₀ Be Isolated Experimentally and Become Superconducting?

Superconducting behavior even under harsh ambient conditions is expected to occur in La@C₆₀ if it could be isolated from the primary metallofullerene soot when functionalized by CF₃ radicals. We use <i>ab initio</i> density functional theory calculations to compare the stability and electronic structure of C₆₀ and the La@C₆₀ endohedral metallofullerene to their counterparts functionalized by CF₃. We found that CF₃ radicals favor binding to C₆₀ and La@C₆₀ and have identified the most stable isomers. Structures with an even number <i>m</i> of radicals are energetically preferred for C₆₀ and structures with odd <i>m</i> for La@C₆₀ due to the extra charge on the fullerene. This is consistent with a wide HOMO–LUMO gap in La@C₆₀(CF₃)_<i>m</i> with odd <i>m</i>, causing extra stabilization in the closed-shell electronic configuration. CF₃ radicals are both stabilizing agents and molecular separators in a metallic crystal, which could increase the critical temperature for superconductivity

Microscopic Mechanism of the Helix-to-Layer Transformation in Elemental Group VI Solids

We study the conversion of bulk Se and Te, consisting of intertwined <i>a</i> helices, to structurally very dissimilar, atomically thin two-dimensional (2D) layers of these elements. Our <i>ab initio</i> calculations reveal that previously unknown and unusually stable δ and η 2D allotropes may form in an intriguing multistep process that involves a concerted motion of many atoms at dislocation defects. We identify such a complex reaction path involving zipper-like motion of such dislocations that initiate structural changes. With low activation barriers ≲0.3 eV along the optimum path, the conversion process may occur at moderate temperatures. We find all one-dimensional (1D) and 2D chalcogen structures to be semiconducting

Tiling Phosphorene

We present a scheme to categorize the structure of different layered phosphorene allotropes by mapping their nonplanar atomic structure onto a two-color 2D triangular tiling pattern. In the buckled structure of a phosphorene monolayer, we assign atoms in “top” positions to dark tiles and atoms in “bottom” positions to light tiles. Optimum sp³ bonding is maintained throughout the structure when each triangular tile is surrounded by the same number <i>N</i> of like-colored tiles, with 0 ≤ <i>N</i> ≤ 2. Our <i>ab initio</i> density functional calculations indicate that both the relative stability and electronic properties depend primarily on the structural index <i>N</i>. The proposed mapping approach may also be applied to phosphorene structures with nonhexagonal rings and 2D quasicrystals with no translational symmetry, which we predict to be nearly as stable as the hexagonal network

Microscopic Mechanism of the Helix-to-Layer Transformation in Elemental Group VI Solids

We study the conversion of bulk Se and Te, consisting of intertwined <i>a</i> helices, to structurally very dissimilar, atomically thin two-dimensional (2D) layers of these elements. Our <i>ab initio</i> calculations reveal that previously unknown and unusually stable δ and η 2D allotropes may form in an intriguing multistep process that involves a concerted motion of many atoms at dislocation defects. We identify such a complex reaction path involving zipper-like motion of such dislocations that initiate structural changes. With low activation barriers ≲0.3 eV along the optimum path, the conversion process may occur at moderate temperatures. We find all one-dimensional (1D) and 2D chalcogen structures to be semiconducting

Structural Transition in Layered As_1–<i>x</i>P_<i>x</i> Compounds: A Computational Study

As a way to further improve the electronic properties of group V layered semiconductors, we propose to form in-layer 2D heterostructures of black phosphorus and gray arsenic. We use ab initio density functional theory to optimize the geometry, determine the electronic structure, and identify the most stable allotropes as a function of composition. Because pure black phosphorus and pure gray arsenic monolayers differ in their equilibrium structure, we predict a structural transition and a change in frontier states, including a change from a direct-gap to an indirect-gap semiconductor, with changing composition

Microscopic Mechanism of the Helix-to-Layer Transformation in Elemental Group VI Solids

We study the conversion of bulk Se and Te, consisting of intertwined <i>a</i> helices, to structurally very dissimilar, atomically thin two-dimensional (2D) layers of these elements. Our <i>ab initio</i> calculations reveal that previously unknown and unusually stable δ and η 2D allotropes may form in an intriguing multistep process that involves a concerted motion of many atoms at dislocation defects. We identify such a complex reaction path involving zipper-like motion of such dislocations that initiate structural changes. With low activation barriers ≲0.3 eV along the optimum path, the conversion process may occur at moderate temperatures. We find all one-dimensional (1D) and 2D chalcogen structures to be semiconducting

Graphitic Phase of NaCl. Bulk Properties and Nanoscale Stability

We applied the ab initio approach to evaluate the stability and physical properties of the nanometer-thickness NaCl layered films and found that the rock salt films with a (111) surface become unstable with thickness below 1 nm and spontaneously split to graphitic-like films for reducing the electrostatic energy penalty. The observed sodium chloride graphitic phase displays an uncommon atomic arrangement and exists only as nanometer-thin quasi-two-dimensional films. The graphitic bulk counterpart is unstable and transforms to another hexagonal wurtzite NaCl phase that locates in the negative-pressure region of the phase diagram. It was found that the layers in the graphitic NaCl film are weakly bounded with each other with a binding energy order of 0.1 eV per stoichiometry unit. The electronic band gap of the graphitic NaCl displays an unusual nonmonotonic quantum confinement response

Two-Dimensional Phosphorus Carbide: Competition between sp² and sp³ Bonding

We propose previously unknown allotropes of phosphorus carbide (PC) in the stable shape of an atomically thin layer. Different stable geometries, which result from the competition between sp² bonding found in graphitic C and sp³ bonding found in black P, may be mapped onto 2D tiling patterns that simplify categorizing of the structures. Depending on the category, we identify 2D-PC structures that can be metallic, semimetallic with an anisotropic Dirac cone, or direct-gap semiconductors with their gap tunable by in-layer strain

Two-Dimensional Phosphorus Carbide: Competition between sp² and sp³ Bonding

We propose previously unknown allotropes of phosphorus carbide (PC) in the stable shape of an atomically thin layer. Different stable geometries, which result from the competition between sp² bonding found in graphitic C and sp³ bonding found in black P, may be mapped onto 2D tiling patterns that simplify categorizing of the structures. Depending on the category, we identify 2D-PC structures that can be metallic, semimetallic with an anisotropic Dirac cone, or direct-gap semiconductors with their gap tunable by in-layer strain