Off-Plane Dielectric Screening of Few-Layer Graphdiyne and Its Family

We performed first-principles calculations on few-layer graphdiyne (GDY) and its family, sp–sp² hybrid carbon atomic layers, for an off-plane, static dielectric screening. The vertical dielectric constants of semiconducting GDY structures are finite and independent of the thickness. However, unlike the widely accepted wisdom that the static metallic screening is infinite, those of metallic GDY structures are finite and dependent on their thickness. Furthermore, the vertical dielectric screening can be tuned by varying the interlayer distance. We also studied the dielectric properties of heterostructures of GDY/its family; the vertical dielectric constant has an equivalent value from the two distinct values of the two distinct monostructures. The dielectric screening behaviors are well described by the uniform dielectric slab model. In addition, the band gaps can be widely tuned from 0 to 0.8 eV, by varying the thickness and electric field. Our results provide a method for engineering the dielectric constant and band gap of GDY and its family for applications of supercapacitors and nanodevices

Carbyne from First Principles: Chain of C Atoms, a Nanorod or a Nanorope

We report an extensive study of the properties of carbyne using first-principles calculations. We investigate carbyne’s mechanical response to tension, bending, and torsion deformations. Under tension, carbyne is about twice as stiff as the stiffest known materials and has an unrivaled specific strength of up to 7.5 × 10⁷ N·m/kg, requiring a force of ∼10 nN to break a single atomic chain. Carbyne has a fairly large room-temperature persistence length of about 14 nm. Surprisingly, the torsional stiffness of carbyne can be zero but can be “switched on” by appropriate functional groups at the ends. Further, under appropriate termination, carbyne can be switched into a magnetic semiconductor state by mechanical twisting. We reconstruct the equivalent continuum elasticity representation, providing the full set of elastic moduli for carbyne, showing its extreme mechanical performance (<i>e.g.</i>, a nominal Young’s modulus of 32.7 TPa with an effective mechanical thickness of 0.772 Å). We also find an interesting coupling between strain and band gap of carbyne, which is strongly increased under tension, from 2.6 to 4.7 eV under a 10% strain. Finally, we study the performance of carbyne as a nanoscale electrical cable and estimate its chemical stability against self-aggregation, finding an activation barrier of 0.6 eV for the carbyne–carbyne cross-linking reaction and an equilibrium cross-link density for two parallel carbyne chains of 1 cross-link per 17 C atoms (2.2 nm)

Engineering Two-Dimensional Nodal Semimetals in Functionalized Biphenylene by Fluorine Adatoms

We propose a band engineering scheme on the biphenylene network, a newly synthesized carbon allotrope. We illustrate that the electronic structure of the biphenylene network can be significantly altered by controlling conditions affecting the symmetry and destructive interference of wave functions through periodic fluorination. First, we investigate the mechanism for the appearance of a type-II Dirac fermion in a pristine biphenylene network. We show that the essential ingredients are mirror symmetries and stabilization of the compact localized eigenstates via destructive interference. While the former is used for the band-crossing point along high symmetry lines, the latter induces highly inclined Dirac dispersions. Subsequently, we demonstrate the transformation of the biphenylene network’s type-II Dirac semimetal phase into various Dirac phases such as type-I Dirac, gapped type-II Dirac, and nodal line semimetals through the deliberate disruption of mirror symmetry or modulation of destructive interference by varying the concentration of fluorine atoms

Improvement of Gas-Sensing Performance of Large-Area Tungsten Disulfide Nanosheets by Surface Functionalization

Semiconducting two-dimensional (2D) transition metal dichalcogenides (TMDCs) are promising gas-sensing materials due to their large surface-to-volume ratio. However, their poor gas-sensing performance resulting from the low response, incomplete recovery, and insufficient selectivity hinders the realization of high-performance 2D TMDC gas sensors. Here, we demonstrate the improvement of gas-sensing performance of large-area tungsten disulfide (WS₂) nanosheets through surface functionalization using Ag nanowires (NWs). Large-area WS₂ nanosheets were synthesized through atomic layer deposition of WO₃ followed by sulfurization. The pristine WS₂ gas sensors exhibited a significant response to acetone and NO₂ but an incomplete recovery in the case of NO₂ sensing. After AgNW functionalization, the WS₂ gas sensor showed dramatically improved response (667%) and recovery upon NO₂ exposure. Our results establish that the proposed method is a promising strategy to improve 2D TMDC gas sensors

Enhanced Hydrogen-Storage Capacity and Structural Stability of an Organic Clathrate Structure with Fullerene (C₆₀) Guests and Lithium Doping

An effective combination of host and guest molecules in a framework type of architecture can enhance the structural stability and physical properties of clathrate compounds. We report here that an organic clathrate compound consisting of a fullerene (C₆₀) guest and a hydroquinone (HQ) host framework shows enhanced hydrogen-storage capacity and good structural stability under pressures and temperatures up to 10 GPa and 438 K, respectively. This combined structure is formed in the extended β-type HQ clathrate and admits 16 hydrogen molecules per cage, leading to a volumetric hydrogen uptake of 49.5 g L^–1 at 77 K and 8 MPa, a value enhanced by 130% compared to that associated with the β-type HQ clathrate. A close examination according to density functional theory calculations and grand canonical Monte Carlo simulations confirms the synergistic combination effect of the guest–host molecules tailored for enhanced hydrogen storage. Moreover, the model simulations demonstrate that the lithium-doped HQ clathrates with C₆₀ guests reveal exceptionally high hydrogen-storage capacities. These results provide a new playground for additional fundamental studies of the structure–property relationships and migration characteristics of small molecules in nanostructured materials