Kaiming Deng

Half-metallicity in organic single porous sheets.

The unprecedented applications of two-dimensional (2D) atomic sheets in spintronics are formidably hindered by the lack of ordered spin structures. Here we present first-principles calculations demonstrating that the recently synthesized dimethylmethylene-bridged triphenylamine (DTPA) porous sheet is a ferromagnetic half-metal and that the size of the band gap in the semiconducting channel is roughly 1 eV, which makes the DTPA sheet an ideal candidate for a spin-selective conductor. In addition, the robust half-metallicity of the 2D DTPA sheet under external strain increases the possibility of applications in nanoelectric devices. In view of the most recent experimental progress on controlled synthesis, organic porous sheets pave a practical way to achieve new spintronics.

Atomically Thin Transition-Metal Dinitrides: High-Temperature Ferromagnetism and Half-Metallicity

High-temperature ferromagnetic two-dimensional (2D) materials with flat surfaces have been a long-sought goal due to their potential in spintronics applications. Through comprehensive first-principles calculations, we show that the recently synthesized MoN2 monolayer is such a material; it is ferromagnetic with a Curie temperature of nearly 420 K, which is higher than that of any flat 2D magnetic materials studied to date. This novel property, made possible by the electron-deficient nitrogen ions, render transition-metal dinitrides monolayers with unique electronic properties which can be switched from the ferromagnetic metals in MoN2, ZrN2, and TcN2 to half-metallic ones in YN2. Transition-metal dinitrides monolayers may, therefore, serve as good candidates for spintronics devices.

Palladium nanoparticles supported on graphitic carbon nitride-modified reduced graphene oxide as highly efficient catalysts for formic acid and methanol electrooxidation

A covalently coupled hybrid of graphitic carbon nitride (g-C3N4) and reduced graphene oxide (rGO) is fabricated via an in situ chemical synthesis approach and used to load Pd nanoparticles. It is found that the Pd nanoparticles with an average diameter of 3.83 nm are evenly deposited on the g-C3N4–rGO surface. Compared with the Pd–rGO and commercial Pd–AC (Pd–activated carbon) catalysts, the ternary Pd–g-C3N4–rGO nanocomposite exhibits excellent electrocatalytic properties toward both formic acid and methanol electrooxidation, such as extremely large electrochemically active surface area (ECSA) values, significantly high forward peak current densities and reliable long-term stability. The enhanced electrochemical activity can be attributed to the specific characteristics of the unique nanostructure of Pd–g-C3N4–rGO and the concerted effects of the individual components, including the superior conductivity of rGO, the high specific surface area of the mesoporous structure, the good structural stability due to the covalent interactions between g-C3N4 and rGO, and the highly dispersed Pd nanoparticles as a result of the effect of planar groups of g-C3N4.

Efficient band structure tuning, charge separation, and visible-light response in ZrS2-based van der Waals heterostructures

As a fast emerging topic, van der Waals heterostructures can modify two-dimensional (2D) layered materials with desired properties, thus greatly extending the applications of these materials. Via state-of-the-art first-principles calculations, we systematically study four types of van der Waals heterostructures formed by monolayer graphene, h-BN, g-C3N4, and polyphenylene on ZrS2 nanosheets. A direct band gap can be obtained in the graphene/ZrS2 heterostructure, endowing graphene with the real ability to be applied in nanoelectronics, whereas the van der Waals interactions of graphene significantly broadens the optical absorption of ZrS2. The conduction band and valence band of the four heterostructures are contributed by the ZrS2 layer and the other layer, respectively, meaning good charge separation is achieved. We proposed that the strained h-BN/ZrS2 and g-C3N4/ZrS2 heterostructures satisfy fundamental aspects for photocatalytic water splitting, with the reduction and oxidation levels well inside their band gaps. By forming heterostructures with ZrS2, the optical properties of h-BN, g-C3N4 and polyphenylene show a remarkable improvement in the visible-light region. The findings in this study will be of broad interest in van der Waals heterostructure research and in the photocatalysis field.

Lithium-doped MOF impregnated with lithium-coated fullerenes: A hydrogen storage route for high gravimetric and volumetric uptakes at ambient temperatures

We theoretically demonstrated that by the impregnation of Li-decorated IRMOF-10 with Li-coated C(60), the hydrogen storage capacity is improved to be 6.3 wt% and 42 g L(-1) at 100 bar and 243 K. Both the gravimetric and volumetric hydrogen uptakes reach the 2015 DOE target at near ambient conditions.

Excited states of the 3d transition metal monoxides

The electron affinities and low-lying excited states of all the 3d transition metal monoxide molecules are studied using the density functional theory (DFT) and time-dependent (TD) DFT method. The calculated results are compared with the available theoretical ones and used to assign the features of these monoxides in photoelectron spectroscopies. It shows that TDDFT, by and large, can be used to get good results for the excited states of the open-shell transition metal oxides. The effect of basis sets on the calculated results is also discussed.

Two-Dimensional Hexagonal Transition-Metal Oxide for Spintronics

Two-dimensional materials have been the hot subject of studies due to their great potential in applications. However, their applications in spintronics have been blocked by the difficulty in producing ordered spin structures in 2D structures. Here we demonstrated that the ultrathin films of recently experimentally realized wurtzite MnO can automatically transform into a stable graphitic structure with ordered spin arrangement via density functional calculation, and the stability of graphitic structure can be enhanced by external strain. Moreover, the antiferromagnetic ordering of graphitic MnO single layer can be switched into half-metallic ferromagnetism by small hole-doping, and the estimated Curie temperature is higher than 300 K. Thus, our results highlight a promising way toward 2D magnetic materials.

Geometric and electronic structures of metal-substitutional fullerene C59Sm and metal-exohedral fullerenes C60Sm

The geometric and electronic structures of metal-substituted fullerene C59Sm and exohedral fullerenes C60Sm are studied using the density-functional theory. The geometric optimization shows that the replacement of a C atom with a Sm in C60 yields a stable substitutionally doped fullerene C59Sm, and among the five possible optimized geometries for C60Sm, the most favorable exohedral sites are above the center of a hexagon and a pentagon ring. The calculations for electronic structures show that the magnetic moment of Sm is preserved for all the stable structures as tiny hybridization takes place between the orbitals of the Sm atom and those of their neighboring carbons. Because of the small energy gaps and the half occupation of the highest occupied molecular orbitals, all the stable C60Sm isomers are inferred to be conductors.

Stabilization of the Metastable Lead Iodide Perovskite Phase via Surface Functionalization

Metastable structural polymorphs can have superior properties and applications to their thermodynamically stable phases, but the rational synthesis of metastable phases is a challenge. Here, a new strategy for stabilizing metastable phases using surface functionalization is demonstrated using the example of formamidinium lead iodide (FAPbI3) perovskite, which is metastable at room temperature (RT) but holds great promises in solar and light-emitting applications. We show that, through surface ligand functionalization during direct solution growth at RT, pure FAPbI3 in the cubic perovskite phase can be stabilized in nanostructures and thin films at RT without cation or anion alloying. Surface characterizations reveal that long-chain alkyl or aromatic ammonium (LA) cations bind to the surface of perovskite structure. Calculations show that such functionalization reduces the surface energy and plays a dominant role in stabilizing the metastable perovskite phase. Excellent photophysics and optically pumped lasing from the stabilized single-crystal FAPbI3 nanoplates with low thresholds were demonstrated. High-performance solar cells can be fabricated with such directly synthesized stabilized phase-pure FAPbI3 with a lower bandgap. Our results offer new insights on the surface chemistry of perovskite materials and provide a new strategy for stabilizing metastable perovskites and metastable polymorphs of solid materials in general.

Superhalogens as building blocks of two-dimensional organic–inorganic hybrid perovskites for optoelectronics applications

Organic-inorganic hybrid perovskites, well known for their potential as the next generation solar cells, have found another niche application in optoelectronics. This was demonstrated in a recent experiment (L. Dou, et al., Science, 2015, 349, 1518) on atomically thin (C4H9NH3)2PbBr4, where, due to quantum confinement, the bandgap and the exciton binding energy are enhanced over their corresponding values in the three-dimensional bulk phase. Using density functional theory we show that when halogen atoms (e.g. I) are sequentially replaced with superhalogen molecules (e.g. BH4) the bandgap and exciton binding energy increase monotonically with the superhalogen content with the exciton binding energy of (C4H9NH3)2Pb(BH4)4 approaching the value in monolayer black phosphorus. Lead-free admixtures (C4H9NH3)2MI4-x(BH4)x (M = Sn and Ge; x = 0-4) also show a similar trend. Thus, a combination of quantum confinement and compositional change can be used as an effective strategy to tailor the bandgap and the exciton binding energy of two-dimensional hybrid perovskites, making them promising candidates for optoelectronic applications.