Yuzhen Liu

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.

The geometric, optical, and magnetic properties of the endohedral stannaspherenes M@Sn12 (M=Ti, V, Cr, Mn, Fe, Co, Ni)

The geometric, optical, and magnetic properties of the M@Sn(12) clusters (M=Ti, V, Cr, Mn, Fe, Co, Ni) are studied using the relativistic density-functional method. The geometric optimization shows that the ground states of these clusters are probably very close to the I(h) structure. Our calculations demonstrate that the optical gaps of the M@Sn(12) can be tuned from infrared to green, and the magnetic moments of them vary from 2 mu(B) to 5 mu(B) by doping d transition metal atoms into Sn(12) cage, suggesting that M@Sn(12) could be a new class of potential nanomaterials with tunable magnetic and optical properties.

N-substituted defective graphene sheets: promising electrode materials for Na-ion batteries

Using density functional theory calculations, we have investigated the adsorption of Na on pristine and N-substituted defective graphene sheets (graphitic, pyridinic, and pyrrolic structures) and explored their application in Na-ion batteries. The adsorption energy and the charge transfer of Na on the various types of sheet were calculated. The effects of N-substitution were also studied by electronic structure analysis, including the total electronic density of states, partial electron density of states, and charge density differences. The results show that electron-rich structures have a negative influence on Na binding, while electron-deficient structures are beneficial. The Na storage capacities of different sheets were evaluated by optimizing multiple Na atom adsorbed structures. We found that more Na atoms can be stored on electron-deficient sheets, making them promising for practical application as electrode materials in Na-ion batteries.

Lithium decoration of three dimensional boron-doped graphene frameworks for high-capacity hydrogen storage

Based on density functional theory and the first principles molecular dynamics simulations, a three-dimensional B-doped graphene-interconnected framework has been constructed that shows good thermal stability even after metal loading. The average binding energy of adsorbed Li atoms on the proposed material (2.64 eV) is considerably larger than the cohesive energy per atom of bulk Li metal (1.60 eV). This value is ideal for atomically dispersed Li doping in experiments. From grand canonical Monte Carlo simulations, high hydrogen storage capacities of 5.9 wt% and 52.6 g/L in the Li-decorated material are attained at 298 K and 100 bars.

Graphdiyne as a High-Efficiency Membrane for Separating Oxygen from Harmful Gases: A First-Principles Study

We theoretically explored the adsorption and diffusion properties of oxygen and several harmful gases penetrating the graphdiyne monolayer. According to our first-principles calculations, the oxidation of the acetylenic bond in graphdiyne needs to surmount an energy barrier of ca. 1.97 eV, implying that graphdiyne remains unaffected under oxygen-rich conditions. In a broad temperature range, graphdiyne with well-defined nanosized pores exhibits a perfect performance for oxygen separation from typical noxious gases, which should be of great potential in medical treatment and industry.

Influences of lithium doping and fullerene impregnation on hydrogen storage in metal organic frameworks

Using the grand canonical ensemble Monte Carlo method, two similar metal organic frameworks (isoreticular MOFs [IRMOF]-12 and -14) and their modified structures by doping lithium (Li) atoms above the organic units and/or impregnating with fullerenes in their cavities have been employed to investigate the capacities of H2 storage. Our simulations show that the H2 uptakes of Li-C60@Li-IRMOF-12 and Li-C60@Li-IRMOF-14 achieve the U.S. Department of Energy targets before 2017 both in gravimetric density and in volumetric density at 243 K and 100 bar. Combining the results of IRMOF-10-based structures, we further study the relationships between the H2 uptakes and the physical properties of the materials to identify the influence factors on the H2 storage at room temperature.

Nanoporous MoS2 monolayer as a promising membrane for purifying hydrogen and enriching methane

We present a theoretical prediction of a highly efficient membrane for hydrogen purification and natural gas upgrading, i.e. laminar MoS2 material with triangular sulfur-edged nanopores. We calculated from first principles the diffusion barriers of H2 and CO2 across monolayer MoS2 to be, respectively, 0.07 eV and 0.17 eV, which are low enough to warrant their great permeability. The permeance values for H2 and CO2 far exceed the industrially accepted standard. Meanwhile, such a porous MoS2 membrane shows excellent selectivity in terms of H2/CO, H2/N2, H2/CH4, and CO2/CH4 separation (>103, >  103, >  106, and  >  104, respectively) at room temperature. We expect that the findings in this work will expedite theoretical or experimental exploration on gas separation membranes based on transition metal dichalcogenides.

Will a graphitic-like ZnO single-layer be an ideal substrate for graphene?

The interaction between graphene and substrates may destroy the intrinsic properties of graphene, and reduce the potential applications of graphene in electronic devices. Here, we use first-principles calculations to explore the possibility of a graphitic ZnO layer as an ideal substrate for graphene. Taking graphitic ZnO with and without oxygen vacancies, we found that the intrinsic linear dispersion of graphene is well retained. Additionally, the resultant bilayer structure of graphene and the graphitic ZnO layer shows much better optical properties compared with separate graphene and graphitic ZnO. Moreover, we also found that both the band dispersion and Fermi velocity of the bilayer structured graphene are robust towards an external electric field. Therefore, our results indicate that a graphitic ZnO layer may be a suitable substrate for graphene in real applications.

Biaxial strain effect on the electronic and magnetic phase transitions in double perovskite La2FeMnO6: A first-principles study

The influence of biaxial strain on both the electronic and magnetic properties in double perovskite La2FeMnO6 is investigated by using density-functional calculations. The results show that La2FeMnO6 exhibits ferromagnetic semiconductor at ambient condition and turns into ferromagnetic half-metal under the whole tensile strain applied in this work, while the compound transfers into ferromagnetic metal under the compressive strain within −8% and into ferrimagnetic semiconductor with the compressive strain beyond −9%. For both ferromagnetic half-metallic and metallic La2FeMnO6, they exhibit very slight change in the electronic states and magnetic moments of the ions, comparing with those of the compound at ambient condition. The electronic configurations of Fe and Mn originating from the high-spin state Fe3+ and intermediate-spin state Mn3+ in ferromagnetic La2FeMnO6 emerge in the low-spin state and high-spin state in ferrimagnetic La2FeMnO6, respectively, when the compressive strain is beyond −9%.

Tailoring the perpendicular exchange bias in [Pt/Co/CoO]n multilayer by tensile stress on curved substrate

The effects of the tensile stress on exchange bias of [Pt/Co/CoO]n multilayer are investigated by depositing the magnetic film onto the ordered curved substrate composed of a polystyrene nanosphere monolayer film. The square ratio of the loop decreases first and increases then with the reduction of tensile stress when the nanosphere size increases. The square ratio and exchange bias field are enhanced significantly when 60 nm polystyrene nanosphere arrays are chosen as the substrate, which is ascribed to the increased interfacial uncompensated antiferromagnetic spins created by the periodical knots between neighbor spheres and the formation of the entire domain wall. In addition, the easy axis of [Pt/Co/CoO]n multilayer is tailored when the CoO sublayer thickness changes, which is due to the gradual development of a tilted anisotropy. When CoO is 1.9 and 2.6 nm, the exchange coupling with tilted magnetic easy axis is obtained in the 45° direction.