Ultrahigh hydrogen storage capacity of holey graphyne.

Holey graphyne (HGY), a novel two-dimensional (2D) single-crystalline carbon allotrope, was recently synthesized by Castro-Stephens coupling reaction. The naturally existing uniform periodic holes in the 2D carbon-carbon network demonstrate its promising potential in energy storage. Herein, we conduct density functional theory (DFT) calculation and ab initio molecular dynamics simulations (AIMD) to predict the H storage properties of a single-layer HGY sheet modified by Li metal atoms. The DFT calculations demonstrate that Li atoms can bind strongly to the HGY sheet without forming clusters, and each Li atom can anchor four H2 molecules with an average adsorption energy of about -0.22 eV/H2. The largest H storage capacity of the doped HGY sheet can reach as high as 12.8 wt%, showing that the Li/HGY complex is an ideal H storage material at ambient conditions. In addition, we investigate the polarization mechanism of the storage media and find that the polarization originates from the electric field induced by both the ionic Li atoms and the weak polarized H2 molecules. Finally, the desorption mechanism of the adsorbed H2 molecules is thoroughly investigated using a kinetic AIMD method.

Ultra-high hydrogen storage capacity of holey graphyne.

Holey graphyne (HGY), a novel two-dementional 2D single-crystalline carbon allotrope, was recently synthesized by Castro-Stephens coupling reaction. The naturally existing uniform periodic holes in the 2D carbon-carbon network demonstrate its promising potential in the energy storage. Herein, we conducted density functional theory (DFT) calculation to predict the hydrogen storage capacity of HGY sheet. It is found the Li-decorated single-layer HGY can serve as a promising candidate for hydrogen storage. Our DFT calculations demonstrate that Li atoms can bind strongly to the HGY sheet without the formation of Li clusters, and each Li atom can anchor four H2 molecules with the average adsorption energy about 0.22 eV/H2. The largest hydrogen storage capacity of the doped HGY sheet can reach as high as 12.8 wt%, largely surpassing the target of the U. S. DOE (9 wt%), showing the Li/HGY complex is an ideal hydrogen storage material at ambient conditions. In addition, we investigate the polarization mechanism of the storage media and find that the polarization is originated from both the electric field induced by the ionic Li decorated on the HGY and the weak polarized hydrogen molecules dominated the H2 adsorption process.

Ambient Degradation-Induced Spin Paramagnetism in Phosphorene.

The sizeable direct bandgap, high mobility, and long spin lifetimes at room temperature offer black phosphorus (BP) potential applications in spin-based semiconductor devices. Toward these applications, a critical step is creating a magnetic response in BP, which is arousing much interest. It is reported here that ambient degradation of BP, which is immediate and inevitable and greatly changes the semiconducting properties, creates magnetic moments, and any degree of degradation leads to notable paramagnetism. Its Landau factor g measured is ≈1.995, revealing that the magnetization mainly results from spin rather than orbital moments. Such magnetism most likely results from the unsaturated phosphorus in the vacancies which are stabilized by O adatoms. It can be tuned by changing any one of the ambient factors of ambient temperature, humidity, and light intensity, and can be stabilized by exposing BP in argon. The findings highlight the importance of evaluating the effect of ambient degradation-induced magnetism on BP's spin-based devices. The work seems an essential milestone toward the forthcoming research upsurge on BP's magnetism.

Half-metallicity in a honeycomb-kagome-lattice Mg3C2 monolayer with carrier doping.

To obtain high-performance spintronic devices with high integration density, two-dimensional (2D) half-metallic materials are highly desired. Herein, we proposed a stable 2D material, i.e., the Mg3C2 monolayer, with a honeycomb-kagome lattice based on the particle-swarm optimization algorithm and first-principles calculations. This monolayer is an anti-ferromagnetic (AFM) semiconductor in its ground state. We have also demonstrated that a transition from an AFM semiconductor to a ferromagnetic half-metal in this 2D material can be induced by carrier (electron or hole) doping. The half-metallicity arises from the 2pz orbitals of the carbon (C) atoms for the electron-doped system and from the C 2px and 2py orbitals in the case of hole doping. Our findings highlight a new promising material with controllable magnetic and electronic properties towards 2D spintronic applications.

Metal-free spin and spin-gapless semiconducting heterobilayers: monolayer boron carbonitrides on hexagonal boron nitride.

The interfaces between monolayer boron carbonitrides and hexagonal boron nitride (h-BN) play an important role in their practical applications. Herein, we respectively investigate the structural and electronic properties of two metal-free heterobilayers constructed by vertically stacking two-dimensional (2D) spintronic materials (B4CN3 and B3CN4) on a h-BN monolayer from the viewpoints of lattice match and lattice mismatch models using density functional calculations. It is found that both B4CN3 and B3CN4 monolayers can be stably adsorbed on the h-BN monolayer due to the van der Waals interactions. Intriguingly, we demonstrate that the bipolar magnetic semiconductor (BMS) behavior of the B4CN3 layer and the spin gapless semiconductor (SGS) property of the B3CN4 layer can be well preserved in the B4CN3/BN and B3CN4/BN heterobilayers, respectively. The magnetic moments and spintronic properties of the two systems originate mainly from the 2pz electrons of the carbon atoms in the B4CN3 and B3CN4 layers. Furthermore, the BMS behavior of the B4CN3/BN bilayer is very robust while the electronic property of the B3CN4/BN bilayer is sensitive to interlayer couplings. These theoretical results are helpful both in understanding the interlayer coupling between B4CN3 or B3CN4 and h-BN monolayers and in providing a possibility of fabricating 2D composite B4CN3/BN and B3CN4/BN metal-free spintronic materials theoretically.

First-principles prediction of a new planar hydrocarbon material: half-hydrogenated 14,14,14-graphyne.

Graphynes, novel allotropic forms of carbon, have become a rising star in two-dimensional materials science due to the diverse geometric structures and excellent electronic properties. In this paper, first-principles calculations were performed to investigate a favorable path for successive hydrogenation of 14,14,14-graphyne and electronic properties of the resulting novel planar structure. Pairs of hydrogen atoms prefer to arrange themselves on opposite sides of acetylenic bonds within the basal plane due to the collective stabilization mediated by cooperative buckling of the original linear acetylenic chains. Progressive hydrogenation favors proceeding along linear directions in a row-by-row manner. A new strictly planar sp-sp(2)-bonded hydrocarbon is formed when half of the sp-hybridized carbon atoms in the chains are hydrogenated. In contrast to the zero-band-gap feature of pristine 14,14,14-graphyne, this hydrocarbon possesses a moderate direct band gap. A possible experimental realization of the proposed two-dimensional hydrocarbon was also discussed. This novel planar hydrocarbon material can not only broaden the application field of graphyne family in electronic and optoelectronic devices but also enrich the databases of carbon-based two-dimensional materials.

Identifying the effects of oxygen on the magnetism of WS2 nanosheets.

In this paper, the microstructure and magnetic properties of the exfoliated and sulfurized WS2 nanosheets were researched to identify the effects of oxygen on magnetism. The exfoliated WS2 nanosheets were prepared by a liquid exfoliation method, and then the sulfurized WS2 nanosheets were obtained after sulfurization of the exfoliated WS2 nanosheets. The exfoliated WS2 nanosheets show paramagnetism, and contain 1T and 2H phases, sulfur vacancies and some oxygen. The sulfurized WS2 nanosheets with an intrinsic structure exhibit an ordered magnetic signature. A combination of detailed experimental research and first-principles calculation demonstrates that oxygen in the structure of WS2 nanosheets would not induce magnetic moments, which can even suppress the spin-polarized edge states. These results identified the effects of oxygen on the magnetism of WS2 nanosheets and would promote its application in spintronics.

Constructing van der Waals Heterogeneous Photocatalysts Based on Atomically Thin Carbon Nitride Sheets and Graphdiyne for Highly Efficient Photocatalytic Conversion of CO2 into CO.

Atomically thin two-dimensional (2D) carbon nitride sheets (CNs) are attracting attention in the field of photocatalytic CO2 reduction. Because of the rapid recombination of photogenerated electron-hole pairs and limited more active sites, the photocatalytic efficiency of CNs cannot meet the actual requirements. Here, atomically thin 2D/2D van der Waals heterostructures of metal-free graphdiyne (GDY)/CNs are fabricated through a simple electrostatic self-assembly method. Experimental characterizations along with first-principles calculations show that the introduction of GDY in CNs promoted the transport of photogenerated carriers in the melon chain, thus suppressing the recombination of photogenerated electron-hole pairs. Both in situ FTIR measurements and DFT calculation confirm that the introduced GDY served as the CO2 adsorption site and enhanced the CO2 adsorption capacity of the CNs/GDY heterostructure. Thanks to the 2D/2D van der Waals heterojunction, the optimized CNs/GDY enhances significantly the CO generation rate up to 95.8 µmol g-1 that is 19.2-fold higher than that of CNs. This work provides a viable approach for the design of metal-free van der Waals heterostructure-based photocatalysts with high catalytic activity.

Realization of Strong Room-Temperature Ferromagnetism in Atomically Thin 2D Carbon Nitride Sheets by Thermal Annealing.

The presence of the intrinsic band gap of 3.06 eV makes atomically thin carbon nitride sheets (CNs) a promising spin-based semiconductor material. However, the absence of localized spins makes the pristine CNs intrinsically nonmagnetic. Here we report the realization of strong room-temperature (RT) ferromagnetism with a high Curie temperature of ca. 524.2 K in atomically thin 2D CNs by annealing pristine CNs at 700 °C. In particular, the RT saturated magnetization reaches as high as 0.71 emu/g, which is the highest value reported so far in carbon-based materials. The structural characterization combined with density functional theory calculations reveals that (i) the seven C-C bonds per unit cell were formed after annealing and (ii) the C-C bonds can introduce high-density localized spins and realize the long-range ferromagnetic couplings among these spins.

Electronic properties of α-graphyne on hexagonal boron nitride and α-BNyne substrates.

The upsurge in the research of α-graphyne (α-GY) has occurred due to the existence of a Dirac cone, whereas the absence of band gap impedes its semiconductor applications. Here, the electronic properties of α-GY on hexagonal boron nitride (h-BN) and α-BNyne (α-BNy) monolayers are investigated using first-principles calculations. Through engineering heterostructures, the band gap opening can be achieved and has different responses to the substrate and stacking sequence. Intriguingly, the band gap of α-GY/α-BNy with Ab1 stacking mode is up to 77.5 meV in the HSE06 functional, which is distinctly greater than K B T at room temperature. The characteristic Dirac band of α-GY is preserved on the α-BNy substrate, while it changes into a parabolic band on the h-BN substrate. Additionally, we also find that changing the interlayer distance is an alternative strategy to realize the tunable band gap. Our results show that by selecting a reasonable substrate, the linear band structure and thus the high carrier mobility as well as the distinct band gap opening could coexist in α-GY. These prominent properties are the key quantity for application of α-GY in nanoelectronic devices.

Increasing Solar Absorption of Atomically Thin 2D Carbon Nitride Sheets for Enhanced Visible-Light Photocatalysis.

Atomically thin 2D carbon nitride sheets (CNS) are promising materials for photocatalytic applications due to their large surface area and very short charge-carrier diffusion distance from the bulk to the surface. However, compared to their bulk counterpart, CNS' applications always suffer from an enlarged bandgap and thus narrowed solar absorption range. Here, an approach to significantly increase solar absorption of the atomically thin CNS via fluorination followed by thermal defluorination is reported. This approach can greatly increase the visible-light absorption of CNS by extending the absorption edge up to 578 nm. The modulated CNS loaded with Pt cocatalyst as a photocatalyst shows a superior photocatalytic hydrogen production activity under visible-light irradiation to Pt-CNS. Combining experimental characterization with theoretical calculations shows that this approach can introduce cyano groups into the framework of CNS as well as the accompanied nitrogen vacancies at the edges, which leads to both narrowing the bandgap and changing the charge distribution. This study will provide an effective strategy to increase solar absorption of carbon-nitride-based photocatalysts for solar energy conversion applications.

Ultrasmall and Monolayered Tungsten Dichalcogenide Quantum Dots with Giant Spin-Valley Coupling and Purple Luminescence.

Monolayered tungsten dichalcogenide quantum dots (WS2 QDs) have various potential applications due to their large spin-valley coupling and excellent photoluminescence (PL) properties. What is expected is that with the decrease in lateral size of QDs, the stronger quantum confinement effect will dramatically strengthen the spin-valley coupling and widen the band gap. However, ultrasmall monolayered WS2 QDs prepared by ion intercalation unavoidably undergo the problem of structural defects, which will create defect levels and significantly change their properties. In this study, we report that by annealing defective monolayered WS2 QDs in sulfur vapor, pristine monolayered WS2 QDs with an ultrasmall lateral size of ca. 1.8-3.8 nm can be obtained. The results show that the ultrasmall monolayered WS2 QDs exhibit a giant spin-valley coupling of ca. 821 meV. Moreover, the pristine ultrasmall monolayered WS2 QDs show purple PL centered at 416 nm, and the defect PL peaks in defective WS2 QDs can be effectively removed by annealing. All of these results afford the ultrasmall monolayered QDs various applications such as in optoelectronics, spintronics, valleytronics, and so on.