Research

We explore the applicability of graphynes, twodimensional carbon sheets with sp- and sp2-bonds, as sodium (Na)-ion battery anodes using first-principles density functional theory. We found that voltages attainable from the charging−discharging of Na into multilayer graphyne are proper for use as anodes. The composite is ∼C6Na2 at the maximum Na concentration, corresponding to gravimetric and volumetric capacities of ∼837 mAh g−1 and ∼1056 mAh cm−3, respectively. These are significantly greater than the corresponding values (372 mAh g−1 and 818 mAh cm−3) of graphite for lithium. We ascribe the enhancement of the capacities to their nanoporous structures with sp- and sp2-bonded carbon atoms, which effectively bind multiple Na atoms. We propose that sp−sp2 carbon sheets can be promising candidates for high-capacity Na-ion battery anodes.

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.

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.

2D transition metal dichalcogenides (TMDs) have attracted much attention for their gas sensing applications due to their superior responsivity at typical room temperature. However, low power consumption and reliable selectivity are the two main requirements for gas sensors to be applicable in future electronic devices. Herein, a p-type (WSe2/WS2) and n-type (MoS2/WSe2) photovoltaic self-powered gas sensor is demonstrated using 2D TMD heterostructures for the first time. The gas sensors are operated by the photovoltaic effect of 2D TMD heterostructures, which are uniformly synthesized by the vacuum-based synthesis. The gas sensing properties of the WSe2/WS2 and MoS2/WSe2 heterostructure gas sensors are investigated for NO2 and NH3 with changing gas concentration, and each sensor exhibits selectivity to NO2 and NH3. From the results, it is confirmed that the 2D TMD heterostructures can be a viable platform for highly sensitive, selective gas sensor applications without external bias due to their photovoltaic features. Further, this study contributes toward revealing the gas sensor mechanism in 2D TMD heterostructure.

The atomic or molecular assembly on 2D materials through the relatively weak van der Waals interaction is quite different from the conventional heteroepitaxy and may result in unique growth behaviors. Here, it is shown that straight 1D cyanide chains display universal epitaxy on hexagonal 2D materials. A universal oriented assembly of cyanide crystals (AgCN, AuCN, and Cu0.5Au0.5CN) is observed, where the chains are aligned along the three zigzag lattice directions of various 2D hexagonal crystals (graphene, h‐BN, WS2, MoS2, WSe2, MoSe2, and MoTe2). The potential energy landscape of the hexagonal lattice induces this preferred alignment of 1D chains along the zigzag lattice directions, regardless of the lattice parameter and surface elements as demonstrated by first‐principles calculations and parameterized surface potential calculations. Furthermore, the oriented microwires can serve as crystal orientation markers, and stacking‐angle‐controlled vertical 2D heterostructures are successfully fabricated by using them as markers. The oriented van der Waals epitaxy can be generalized to any hexagonal 2D crystals and will serve as a unique growth process to form crystals with orientations along the zigzag directions by epitaxy.

Precise three-dimensional (3D) atomic structure determination of individual nanocrystals is a prerequisite for understanding and predicting their physical properties. Nanocrystals from the same synthesis batch display what are often presumed to be small but possibly important differences in size, lattice distortions, and defects, which can only be understood by structural characterization with high spatial 3D resolution. We solved the structures of individual colloidal platinum nanocrystals by developing atomic-resolution 3D liquid-cell electron microscopy to reveal critical intrinsic heterogeneity of ligand-protected platinum nanocrystals in solution, including structural degeneracies, lattice parameter deviations, internal defects, and strain. These differences in structure lead to substantial contributions to free energies, consequential enough that they must be considered in any discussion of fundamental nanocrystal properties or applications.