Pei Liang

Atomic Mechanism of Electrocatalytically Active Co-N Complexes in Graphene Basal Plane for Oxygen Reduction Reaction.

Superior catalytic activity and high chemical stability of inexpensive electrocatalysts for the oxygen reduction reaction (ORR) are crucial to the large-scale practical application of fuel cells. The nonprecious metal/N modified graphene electrocatalysts are regarded as one of potential candidates, and the further enhancement of their catalytic activity depends on improving active reaction sites at not only graphene edges but also its basal plane. Herein, the ORR mechanism and reaction pathways of Co-N co-doping onto the graphene basal plane have been studied by using first-principles calculations and ab initio molecular dynamics simulations. Compared to singly N-doped and Co-doped graphenes, the Co-N co-doped graphene surface exhibits superior ORR activity and the selectivity toward a four-electron reduction pathway. The result originates from catalytic sites of the graphene surface being modified by the hybridization between Co 3d states and N 2p states, resulting in the catalyst with a moderate binding ability to oxygenated intermediates. Hence, introducing the Co-N4 complex onto the graphene basal plane facilitates the activation of O2 dissociation and the desorption of H2O during the ORR, which is responsible for the electrocatalyst with a smaller ORR overpotential (∼1.0 eV) that is lower than that of Co-doped graphene by 0.93 eV. Our results suggest that the Co-N co-doped graphene is able to compete against platinum-based electrocatalysts, and the greater efficient electrocatalysts can be realized by carefully optimizing the coupling between transition metal and nonmetallic dopants in the graphene basal plane.

Layer-Dependent Dopant Stability and Magnetic Exchange Coupling of Iron-Doped MoS2 Nanosheets

Using density-functional theory calculations including a Hubbard U term we explore structural stability, electronic and magnetic properties of Fe-doped MoS2 nanosheets. Unlike previous reports, the geometry and the stability of Fe dopant atoms in MoS2 nanosheets strongly depend on the chemical potential and the layer number of sheets. The substitution Fe dopant atoms at the Mo sites are energetically favorable in monolayer MoS2 and the formation of intercalated and substitutional Fe complexes are preferred in bilayer and multilayer ones under the S-rich regime that is a popular condition for the synthesis of MoS2 nanosheets. We find that the Fe dopants prefer to the ferromagnetic coupling in monolayer MoS2 and the antiferromagnetic coupling in bilayer and multilayer ones, suggesting the layer dependence of magnetic exchange coupling (MEC). The transition of MEC in Fe-doped MoS2 sheets induced by the change of layer number arises from the competition mechanism between the double-exchange and superexchange couplings. The findings provide a route to facilitate the design of MoS2-based diluted magnetic semiconductors and spintronic devices.

Unveiling the atomic structure and electronic properties of atomically thin boron sheets on an Ag(111) surface

Two-dimensional (2D) boron sheets (i.e., borophene) have a huge potential as a basic building block in nanoelectronics and optoelectronics; such a situation is greatly promoted by recent experiments on fabrication of borophene on silver substrates. However, the fundamental atomic structure of borophene on the Ag substrate is still under debate, which greatly impedes further exploration of its properties. Herein, the atomic structure and electronic properties of borophene on an Ag(111) surface have been studied using first-principles calculations and ab initio molecular dynamics simulations. Our results reveal that there exist three energetically favorable borophene structures (β5, χ1, and χ2) on the Ag(111) surface and their simulated STM images are in good agreement with experimental results, suggesting the coexistence of boron phases during the growth. All these stable borophene structures have a planar structure with slight surface buckling (∼0.15 Å) and relatively high hexagonal vacancy density (1/6 and 1/5) and exhibit typical metallic conductivity. These findings not only can be applied to solve the experimental controversies about the atomic structure of borophene on the Ag substrate but also provide a theoretical basis for exploring the fundamental properties and applications of 2D boron sheets.

Structural, electronic, and optical properties of hydrogenated few-layer silicene: Size and stacking effects

The size and stacking effects on the structural, electronic, and optical properties of hydrogenated few-layer silicenes (HFLSs) are investigated systematically by the first-principle calculations within density functional theory. It is found that both the formation energies and band gaps of HFLSs increases with the reduction of layer thickness. The high formation energies imply the relatively lower structural stability in the thinner HFLSs due to their high surface/volume ratio. With the reduction of layer thickness, the increasing band gaps lead to an obvious blue shift of optical absorption edge in the HFLSs. Among three different stacking HFLSs with the same thickness, the ABC-stacking one has the lowest formation energy and the largest band gap due to the strong interactions of Si layers. Moreover, the structural transition of HFLSs from the ABC-stacking sequence to the AA-stacking one will cause a relative red shift of optical absorption peaks. The results indicate that the electronic and optical pro...

Tailoring electronic properties of InAs nanowires by surface functionalization

The effect of surface functionalization on the electronic properties of InAs nanowires is investigated by the first-principle calculations. Several surface adsorption species (H, F, Cl, Br, and I) with different coverages are considered. It is found that the electronic structures of InAs nanowires are sensitive to the coverage and adsorption sites of the passivating atoms. The band-gap magnitude of InAs nanowires depends on the suppression of surface states as determined by the charge-compensation ability of passivating atoms to surface atoms. For the halogen passivation, the weak charge-compensation ability induces the band-gap reduction when compared to the hydrogen passivation. The results provide us a feasible way to engineer the bandgap of nanowires by the modification of surface species.

Crystal facet effect on structural stability and electronic properties of wurtzite InP nanowires

The crystal-facet effect on the structural stability and electronic properties of wurtzite InP nanowires (NWs) with different side-facets are investigated by using first-principles calculation within density-function theory. The surface-energy calculation suggests that side-facet structures of InP NWs are unreconstructed due to the fact that the low-index {11¯00} and {112¯0} facets with paired In-P dimers satisfy the electron counting rule. The calculated formation energies indicate that the structural stability of InP NWs strongly depends on their side-facets. Among considered InP NWs with different side-facets, the {11¯00} faceted NWs present the highest stability due to the relative low surface atom ratio, which is in good agreement with experimental observations where wurtzite InP NWs prefer to be surrounded by {11¯00} facets. The size dependence of NW band gap indicates that the band gap (Eg) of uniform-sized InP NWs with different side-facets follows the trend, Eg-{112¯0} > Eg-{11¯00}-{112¯0} > Eg-{...

Interface effect on electronic and optical properties of antimonene/GaAs van der Waals heterostructures

The integration of two-dimensional (2D) materials with III–V semiconductor surfaces leads to the formation of 2D/3D van der Waals (vdW) heterostructures without the constraint of lattice matching, which offers new opportunities to improve electronic and optoelectronic properties. Here we explore the structural, electronic, and optical properties of various potential Sb/GaAs heterostructures consisting of a Sb monolayer (i.e., antimonene) on GaAs(111) substrates by using first-principles calculations within the density-functional theory. Our results demonstrate that the vdW interaction is crucial for the stability of Sb/GaAs heterointerfaces, but the interfacial coupling strength and band-structure characteristics of the heterostructures are strongly affected by the interface structures. We find that all stable Sb/GaAs heterostructures exhibit a type-II band alignment and have relatively small band gaps (0.71–1.39 eV) as compared to those of the independent Sb monolayer and GaAs substrates. Moreover, the formation of Sb/GaAs vdW heterostructures can lead to the separation of carriers and a high optical absorption coefficient in the visible-light range, which makes Sb/GaAs heterostructures a potential candidate for optoelectronic devices, such as solar cells.