Dan Cao

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.

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...

Strain driven enhancement of ferroelectricity and magnetoelectric effect in multiferroic tunnel junction

The strain effect on the ferroelectric and magnetoelectric coupling in multiferroic tunnel junction (MFTJ) Co/BaTiO3/Co has been investigated systematically by using first-principles calculations within density functional theory. It is found that both in-plane compressive strain and uniaxial tensile strain lead to the enhancement of ferroelectric polarization stability and intensity of magnetoelectric coupling in the MFTJ. There is a transition from the paraelectric phase to the ferroelectric phase for the BaTiO3 layer in MFTJ when the loaded in-plane compressive strain increases up to -2.8% and the corresponding average ferroelectric polarization is about 0.13 C m(-2). Meanwhile, the calculated surface magnetoelectric coefficients increase with increasing in-plane compressive strain. Similar phenomena have been also observed in the case of uniaxial tensile strain implemented in MFTJ. The results suggest that the ferroelectric polarization and magnetoelectric coupling in multiferroic tunnel junctions can be controlled by strain and we expect that this study can provide a theoretical basis for the design of spintronic devices.

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}u2009>u2009Eg-{11¯00}-{112¯0}u2009>u2009Eg-{...

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.

Defect Engineering in MoSe2 for the Hydrogen Evolution Reaction: From Point Defects to Edges

Superior catalytic activity and high chemical stability of inexpensive electrocatalysts for the hydrogen evolution reaction (HER) are crucial to the large-scale production of hydrogen from water. The nonprecious two-dimensional MoSe2 materials emerge as a potential candidate, and the improvement of their catalytic activity depends on the optimization of active reaction sites at both the edges and the basal plane. Herein, the structural stability, electrocatalytic activity, and HER mechanisms on a series of MoSe2 catalytic structures including of point defects, holes, and edges have been explored by using first-principles calculations. Our calculated results demonstrate that thermodynamically stable defects (e.g., VSe, VSe2, SeMo, and VMo3Se2) and edges (e.g., Mo-R and Se-R) in MoSe2 are very similar to the case of MoS2, but their HER activity is higher than that of the corresponding structures in MoS2, which is in good agreement with experimental observations. Furthermore, a Fermi-abundance model is proposed to explain the fundamental correlation between the HER activity of various MoSe2 catalysts and their intrinsic electronic structures, and this model is also applicable for assessing the HER activity of other types of catalysts, such as MoS2 and Pt. Moreover, two different HER mechanisms have been revealed in the MoSe2 catalytic structures: the Volmer-Tafel mechanism is preferred for the VSe and VSe2 structures, whereas the Volmer-Heyrovsky mechanism is more favorable for other MoSe2 catalytic structures. The present work suggests that MoSe2 with appropriate defects and edges is able to compete against the Pt-based catalysts and also opens a route to design highly active electrocatalysts for the HER.

Tuning competition between metallic and insulating phases with strain, light and electric field in thin films of La0.39Pr0.24Ca0.37MnO3

The electric field and photo illumination effects in thin films of La0.39Pr0.24Ca0.37MnO3(LPCMO)/SrTiO3 with different thicknesses (15, 40 and 120 nm) have been systematically investigated. X-ray diffraction measurements demonstrate that these grown films are epitaxial and c axis oriented. Due to strain relaxation, these films are in different strain states. It is found that the strain state can greatly change not only the electrical properties but also the responses to external perturbations. The thinner film experiences a larger tensile in-plane strain and has a more robust insulating state. The low-temperature insulating state in the 120 nm sample is readily destroyed by a small electric field or light illumination without a bias voltage. Both the critical light density and the critical electric field used to collapse the insulating state in the 40 nm LPCMO film are much larger than those in the 120 nm sample. It is revealed that the bias electric field and light illumination play complementary roles. Light illumination can reduce the critical bias electric field. The 15 nm sample exhibits a robust insulating state, which cannot be destroyed by a combination of a bias voltage of 200 V (E = 4 × 105 V m−1) and light illumination (λ = 532 nm, P = 2 mW cm−2). Large transient photoconductivity has been observed in the 40 nm sample under low bias voltages and in the 15 nm sample under all bias voltages. The observed transient photoconductivity and persistent photoconductivity are believed to have different origins.

Characterization of barrier heights in Nd0.7Sr0.3MnO3/Nb : SrTiO3 junctions with current–voltage characteristics

Barrier heights in heterojunctions composed of Nd0.7Sr0.3MnO3 and Nb doped SrTiO3 have been characterized with current–voltage characteristics by using various methods. It is revealed that an inappropriate assumption of the transport mechanism would usually lead to the underestimation of the barrier height. Provided that the transport mechanism is correctly identified the barrier height deduced with current–voltage characteristics could be in accordance with Andersons model and typical internal photoemission results. It is suggested that for manganite–titanate heterojunctions introducing the ideality factor in the thermionic emission saturation current density equation might largely counteract the underestimation of the barrier height.

Interfacial structure, ferroelectric stability, and magnetoelectric effect of magnetoelectric junction FeCo/BaTiO3/FeCo with alloy electrode

The interfacial structures, ferroelectric instability, and magnetoelectric coupling effect of FeCo/BaTiO3/FeCo junction with alloy electrode have been studied systematically using the first-principles calculations within density functional theory. Owing to different interfacial coupling between the electrode and BaTiO3 barrier layer, there are four possible interfacial structures, including TiO2/Fe, BaO/Fe, TiO2/Co, and BaO/Co interfaces. Among the four interfacial structures, the TiO2/Fe interface is the most stable one. With the decreasing thickness of BaTiO3 barrier layer in the junction, there exists a critical size of ferroelectricity with a thickness of ~4.5 formula unit cells. However, the ferroelectric polarization of magnetoelectric junction with the alloy FeCo electrode is larger than that of the junction with the Co electrode using a same size due to the stronger screening ability of alloy FeCo electrode. Meanwhile, we find that the induced magnetic moments of Ti atoms are nearly from the contribution of the first TiO2 layer adjacent to the interfaces irrespective of the thickness of ferroelectric barrier. In other words, the induced magnetic moments of Ti atoms only maintain an atom-layer thickness. Moreover, the induced magnetic moments of Ti atoms and local magnetic moments of Fe atoms at the two interfaces of tunneling junction depend on the polarization orientation, leading to a sizable ME coupling effect.

Series resistance effects in La0.5Ca0.5MnO3/SrTiO3:Nb(0 0 1) heterojunctions

The effects of series resistance in heterojunctions composed of La0.5Ca0.5MnO3 (LCMO) and Nb:SrTiO3 with 0.05 wt.% and 0.7 wt.% of Nb doping (0.05NbSTO and 0.7NbSTO) have been investigated in detail using current–voltage curves with and without light illumination. Two linear plots, namely, dV/dlnJ versus J and H(J)[≡V-(nk B T/q)ln(J/A**T 2)] versus J, have been used to extract the series resistance. These two plots give very close values. The extracted series resistance exhibits a monotonous increase with decreasing temperatures, which could explain the observed anomalous temperature dependence of short circuit current in LCMO/0.7NbSTO.