Strain drived band aligment transition of the ferromagnetic VS_2/C_3N van der Waals heterostructure

Exploring two-dimensional(2 D) magnetic heterostructures is essential for future spintronic and optoelectronic devices.Herein,using first-principle calculations,stable ferromagnetic ordering and colorful electronic properties are established by constructing the VS_2/C_3 N van der Waals(vdW) heterostructure.Unlike the semiconductive properties with indirect band gaps in both the VS_2 and C_3 N monolayers,our results indicate that a direct band gap with type-Ⅱ band alignment and p-doping characters are realized in the spin-up channel of the VS_2/C_3 N heterostructure,and a typical type-Ⅲband alignment with a broken-gap in the spin-down channel.Furthermore,the band alignments in the two spin channels can be effectively tuned by applying tensile strain.An interchangement between the type-Ⅱ and type-Ⅲ band alignments occurs in the two spin channels,as the tensile strain increases to 4%.The attractive magnetic properties and the unique band alignments could be useful for prospective applications in the next-generation tunneling devices and spintronic devices.     

Structural,electronic,and magnetic behaviors of 5d transition metal atom substituted divacancy graphene:A first-principles study

Structural, electronic, and magnetic behaviors of 5d transition metal(TM) atom substituted divacancy(DV) graphene are investigated using first-principles calculations. Different 5d TM atoms(Hf, Ta, W, Re, Os, Ir, and Pt) are embedded in graphene, these impurity atoms replace 2 carbon atoms in the graphene sheet. It is revealed that the charge transfer occurs from 5d TM atoms to the graphene layer. Hf, Ta, and W substituted graphene structures exhibit a finite band gap at high symmetric K-point in their spin up and spin down channels with 0.783 μB, 1.65 μB, and 1.78 μB magnetic moments,respectively. Ir and Pt substituted graphene structures display indirect band gap semiconductor behavior. Interestingly, Os substituted graphene shows direct band gap semiconductor behavior having a band gap of approximately 0.4 e V in their spin up channel with 1.5 μB magnetic moment. Through density of states(DOS) analysis, we can predict that d orbitals of 5d TM atoms could be responsible for introducing ferromagnetism in the graphene layer. We believe that our obtained results provide a new route for potential applications of dilute magnetic semiconductors and half-metals in spintronic devices by employing 5d transition metal atom-doped graphene complexes.     

Electronic and optical properties of GaN–MoS_2 heterostructure from first-principles calculations

Heterostructures(HSs) have attracted significant attention because of their interlayer van der Waals interactions. The electronic structures and optical properties of stacked GaN–MoS_2 HSs under strain have been explored in this work using density functional theory. The results indicate that the direct band gap(1.95 e V) of the Ga N–MoS_2 HS is lower than the individual band gaps of both the GaN layer(3.48 e V) and the MoS_2 layer(2.03 eV) based on HSE06 hybrid functional calculations. Specifically, the GaN–MoS_2 HS is a typical type-II band HS semiconductor that provides an effective approach to enhance the charge separation efficiency for improved photocatalytic degradation activity and water splitting efficiency.Under tensile or compressive strain, the direct band gap of the GaN–MoS_2 HS undergoes redshifts. Additionally, the GaN–MoS_2 HS maintains its direct band gap semiconductor behavior even when the tensile or compressive strain reaches 5% or-5%. Therefore, the results reported above can be used to expand the application of Ga N–MoS_2 HSs to photovoltaic cells and photocatalysts.     

Crystal structures and sign reversal Hall resistivities in iron-based superconductors Li_x(C_3H_(10)N_2)_(0.32)FeSe(0.15 <x< 0.4)

Heavy electron-doped FeSe-derived materials have attracted attention due to their uncommon electronic structures with only ‘electron pockets', and they are different from other iron-based superconductors. Here, we report the crystal structures, superconductivities and normal state properties of two new Li-doped FeSe-based materials, i.e.,Li0.15(C_3H_(10)N_2)_(0.32) FeSe(P-4) and Li_x(C_3H_(10)N_2)_(0.32) FeSe(P4/nmm, 0.25 x 0.4) with superconducting transition temperatures ranging from 40 K to 46 K. The determined crystal structures reveal a coupling between Li concentration and the orientation of 1,3-diaminopropane molecules within the largely expanded FeSe layers. Superconducting fluctuations appear in the resistivity of the two superconductors and are fitted in terms of the quasi two-dimensional(2 D) Lawrence–Doniach model. The existence of a crossing point and scaling behavior in the T-dependence of diamagnetic response also suggests that the two superconductors belong to the quasi-2 D system. Interestingly, with the increase of temperature, a sign of Hall coefficient(R_H) reversing from negative to positive is observed at ～185 K in both phases, suggesting that‘hole pockets' emerge in these electron-doped FeSe materials. First principle calculations indicate that the increase in FeSe layer distance will lift up a ‘hole band' associated with d_(x~2-y~2) character and increase the hole carriers. Our findings suggest that the increase in two dimensionalities may lead to the sign-reversal Hall resistivity in Li_x(C_3H_(10)N_2)_(0.32) FeSe at high temperature.     

Electronic Structures of the Filled Tetrahedral Semiconductor Li3AlN2

The first-principles total energy calculations with the local density approximation （LDA） and the plane wave pseudopotential method are employed to investigate the structural properties and electronic structures of Li3AlN2. The calculated lattice constants and internal coordination of atoms agree well with the experimental results. Detailed studies of the electronic structure and the charge-density redistribution reveal the features of the strong ionicity bonding of Al-N and Al-Li, and strong hybridizations between Li and N in Li3AlN2. Our band structure calculation verifies Li3AlN2 is a direct gap semiconductor with the LDA gap value of about 2.97eV and transition at Г.     

HfN_2 monolayer: A new direct-gap semiconductor with high and anisotropic carrier mobility

Searching for two-dimensional(2 D) stable materials with direct band gap and high carrier mobility has attracted great attention for their electronic device applications.Using the first principles calculations and particle swarm optimization(PSO) method,we predict a new 2 D stable material(HfNZ monolayer) with the global minimum of 2 D space.The HfNZ monolayer possesses direct band gap(～1.46 eV) and it is predicted to have high carrier mobilities(～10~3 cm~2·V~(-1)·s~(-1))from deformation potential theory.The direct band gap can be well maintained and flexibly modulated by applying an easily external strain under the strain conditions.In addition,the newly predicted HfN_2 monolayer possesses good thermal,dynamical,and mechanical stabilities,which are verified by ab initio molecular dynamics simulations,phonon dispersion and elastic constants.These results demonstrate that HfN_2 monolayer is a promising candidate in future microelectronic devices.     

Position Dependent Spontaneous Emission Spectra of a Λ-Type Atomic System Embedded in a Defective Photonic Crystal

We investigate the position dependent spontaneous emission spectra of a Λ-type three-level atom with one transition coupled to the free vacuum reservoir and the other one coupled to a double-band photonic band gap reservoir with a defect mode in the band gap.It is shown that,for the atom at the defect location,we have a two-peak spectrum with a wide dark line due to the strong coupling between the atom and the defect mode.While,when the atom is far from the defect location(or in the absence of the defect mode),the spectrum has three peaks with two dark lines due to the coupling between the atom and the photonic band gap reservoir with the largest density of states near the band edges.On the other hand,we have a four-peak spectrum for the atom at the space in between.Moreover,the average spontaneous emission spectra of the atoms uniformly embedded in high dielectric or low dielectric regions are described.It is shown that the atoms embedded in high(low) dielectric regions far from the defect location,effectively couple to the modes of the lower(upper) photonic band.However,the atoms embedded in high dielectric or low dielectric regions at the defect location,are coupled mainly to the defect modes.While,the atoms uniformly embedded in high(low) dielectric regions with a normal distance from the defect location,are coupled to both of defect and lower(upper) photonic band modes.     

Structural Transition from Ordered to Disordered of BeZnO_2 Alloy

Employing Monte Carlo simulations based on the cluster expansion,the special quasi-random structures and first-principles calculations,we systematically investigate the structure transition of BeZnO₂ alloys from the ordered to the disordered phase driven by the increased synthesis temperature,together with the solid-state phase diagram.It is found that by controlling the ordering parameter at the mixed sublattice,the band structure can vary continuously from a wide direct band gap of ...     

Enhancing visible-light photocatalytic activity of α-Bi2O3 via non-metal N and S doping

In recent years, some important research indicated that the visible-light activity of photocatalysts could be enhanced via incorporating p-block non-metal elements into the lattice. In this paper, we investigated the electronic structures of pure and different non-metal （C, N, S, F, Cl, and Br） doped α-Bi2O3 using first-principles calculations based on the density functional theory. The band structures, the electronic densities of states, and the effective masses of electrons and holes for doped α-Bi2O3 were obtained and analyzed. The N and S dopings narrowed the band gap and reduced the effective mass of the carriers, which are beneficial for the photocatalytic performance. The theoretical predication was further confirmed by the experimental results.     

Predicted novel insulating electride compound between alkali metals lithium and sodium under high pressure

The application of high pressure can fundamentally modify the crystalline and electronic structures of elements as well as their chemical reactivity, which could lead to the formation of novel materials. Here, we explore the reactivity of lithium with sodium under high pressure, using a swarm structure searching techniques combined with first-principles calculations, which identify a thermodynamically stable Li–Na compound adopting an orthorhombic oP8 phase at pressure above 355 GPa. The formation of Li–Na may be a consequence of strong concentration of electrons transfering from the lithium and the sodium atoms into the interstitial sites, which also leads to open a relatively wide band gap for Li NaoP8. This is substantially different from atoms sharing or exchanging electrons in common compounds and alloys. In addition, lattice-dynamic calculations indicate that Li Na-oP8 remains dynamically stable when pressure decompresses down to 70 GPa.     

Degeneracy and Split of Defect States in Photonic Crystals

One-dimensional photonic crystals with two or more structural defects are studied. We observed an interesting characteristic of transmission band structure of photonic crystals with defects using the transmission-matrix-method simulation. The transmission states in the wide photonlc band gap caused by defects reveal degeneracy and split in certain conditions. Every split state is contributed by coupling of all defects in a photonic crystal.Using the tight-binding method, we obtain an approximate analytic expression for the split frequency of photonic crystals with two structural defects.     

Polymerization of Silicon-Doped Heterofullerenes： an Ab Initio Study

We perform the calculations on geometric and electronic structures of Si-doped heterofullerene C5oSi10 and its derivatives, a C40Si20-C40Si20 dimer and a C40Si20-based nanowire by using density-functional theory, The optimized configuration of the C40Si20-based nanowire exhibits a regular dumbbell-shaped chain nanostructure. The electronic structure calculations indicate that the HOMO-LUMO gaps of the heterofullerene-based materials can be greatly modified by substitutionally doping with Si atoms and show a decreasing trend with increase cluster size. Unlike the band structures of the conventional wide band gap silicon carbide nanomaterials, the C40Si20- based nanowire has a very narrow direct band gap of 0.087eV.     

Mechanical,elastic, anisotropy,and electronic properties of monoclinic phase of m-Si_xGe_(3-x)N_4

The structural,mechanical,elastic anisotropic,and electronic properties of the monoclinic phase of m-Si_3N_4,mSi_2GeN_4,m-SiGe_2N_4,and m-Ge_3N_4are systematically investigated in this work.The calculated results of lattice parameters,elastic constants and elastic moduli of m-Si_3N_4and m-Ge_3N_4are in good agreement with previous theoretical results.Using the Voigt–Reuss–Hill method,elastic properties such as bulk modulus B and shear modulus G are investigated.The calculated ratio of B/G and Poisson’s ratio v show that only m-SiGe_2N_4should belong to a ductile material in nature.In addition,m-SiGe_2N_4possesses the largest anisotropic shear modulus,Young’s modulus,Poisson’s ratio,and percentage of elastic anisotropies for bulk modulus ABand shear modulus AG,and universal anisotropic index AUamong m-Si_xGe_(3-x)N_4(x=0,1,2,3.)The results of electronic band gap reveal that m-Si_3N_4,m-Si_2GeN_4,m-SiGe_2N_4,and m-Ge_3N_4 are all direct and wide band gap semiconducting materials.     

Emerging properties of two-dimensional twisted bilayer materials

Recent studies in van der Waals coupled two-dimensional(2D) bilayer materials have demonstrated a new freedom for material engineering by the formation of moiré pattern. By tuning the twist angle between two layers, one can modulate their electronic band structures and therefore the associated electrical transport and optical properties, which are distinct from the original ones of each individual layer. These new properties excite great passion in the exploration of new quantum states and possible applications of 2D bilayers. In this article, we will mainly review the prevailing fabrication methods and emerging physical properties of twisted bilayer materials and lastly give out a perspective of this topic.     

Investigation of optoelectronic properties of pure and Co substituted α-Al_2O_3 by Hubbard and modified Becke–Johnson exchange potentials

Advanced GGA + U(Hubbard) and modified Becke–Johnson(mBJ) techniques are used for the calculation of the structural, electronic, and optical parameters of α-Al2-x CoxO3(x = 0.0, 0.167) compounds. The direct band gaps calculated by GGA and m BJ for pure alumina are 6.3 eV and 8.5 eV, respectively. The m BJ approximation provides results very close to the experimental one(8.7 eV). The substitution of Al with Co reduces the band gap of alumina. The wide and direct band gap of the doped alumina predicts that it can efficiently be used in optoelectronic devices. The optical properties of the compounds like dielectric functions and energy loss function are also calculated. The rhombohedral structure of theα-Al2-x CoxO3(x = 0.0, 0.167) compounds reveal the birefringence properties.     

Electronic properties of size-dependent MoTe_2/WTe_2 heterostructure

Lateral two-dimensional(2D) heterostructures have opened up unprecedented opportunities in modern electronic device and material science. In this work, electronic properties of size-dependent MoTe_2/WTe_2 lateral heterostructures(LHSs)are investigated through the first-principles density functional calculations. The constructed periodic multi-interfaces patterns can also be defined as superlattice structures. Consequently, the direct band gap character remains in all considered LHSs without any external modulation, while the gap size changes within little difference range with the building blocks increasing due to the perfect lattice matching. The location of the conduction band minimum(CBM) and the valence band maximum(VBM) will change from P-point to Γ-point when m plus n is a multiple of 3 for A-mn LHSs as a result of Brillouin zone folding. The bandgap located at high symmetry Γ-point is favourable to electron transition, which might be useful to optoelectronic device and could be achieved by band engineering. Type-II band alignment occurs in the MoTe_2/WTe_2 LHSs, for electrons and holes are separated on the opposite domains, which would reduce the recombination rate of the charge carriers and facilitate the quantum efficiency. Moreover, external biaxial strain leads to efficient bandgap engineering. MoTe_2/WTe_2 LHSs could serve as potential candidate materials for next-generation electronic devices.     

Analysis of tensile strain enhancement in Ge nano-belts on an insulator surrounded by dielectrics

Ge nano-belts with large tensile strain are considered as one of the promising materials for high carrier mobility metal- oxide-semiconductor transistors and efficient photonic devices. In this paper, we design the Ge nano-belts on an insulator surrounded by Si3N4 or SiO？ for improving their tensile strain and simulate the strain profiles by using the finite difference time domain （FDTD） method. The width and thickness parameters of Ge nano-belts on an insulator, which have great effects on the strain profile, are optimized. A large uniaxial tensile strain of 1.16% in 50-nm width and 12-nm thickness Ge nano-belts with the sidewalls protected by Si3N4 is achieved after thermal treatments, which would significantly tailor the band gap structures of Ge-nanobelts to realize the high performance devices.     

A theoretical investigation of the band alignment of type-I direct band gap dilute nitride phosphide alloy of GaN_xAs_yP_(1-x-y)/GaP quantum wells on GaP substrates

The GaP-based dilute nitride direct band gap material Ga（NAsP） is gaining importance due to the monolithic integra- tion of laser diodes on Si microprocessors. The major advantage of this newly proposed laser material system is the small lattice mismatch between GaP and Si. However, the large threshold current density of these promising laser diodes on Si substrates shows that the carrier leakage plays an important role in Ga（NAsP）/GaP QW lasers. Therefore, it is necessary to investigate the band alignment in this laser material system. In this paper, we present a theoretical investigation to optimize the band alignment of type-I direct band gap GaNxAsyP1-x-y/GaP QWs on GaP substrates. We examine the effect of nitrogen （N） concentration on the band offset ratios and band offset energies. We also provide a comparison of the band alignment of type-I direct band gap GaNxAsyP1-x-y/GaP QWs with that of the GaNxAsyP1-x-y/Al2Ga1-2P QWs on GaP substrates. Our theoretical calculations indicate that the incorporations of N into the well and AI into the barrier improve the band alignment compared to that of the GaAsP/GaP QW laser heterostructures.     

Tailoring edge and interface states in topological metastructures exhibiting the acoustic valley Hall effect

In this study, we investigate the acoustic topological insulator or topological metastructure, where an acoustic wave can exist only in an edge or interface state instead of propagating in bulk. Breaking the structural symmetry enables the opening of the Dirac cone in the band structure and the generation of a new band gap, wherein a topological edge or interface state emerges.Further, we systematically analyze two types of topological states that stem from the acoustic valley Hall effect mechanism;one type is confined to the boundary, whereas the other type can be observed at the interface between two topologically different structures. Results denote that the selection of different boundaries along with appropriately designed interfaces provides the acoustic waves in the band gap range with abilities of one-way propagation, dual-channel propagation, immunity from backscattering at sharp corners, and/or transition between propagation at interfaces and boundaries. Furthermore, we show that the acoustic wave propagation paths can be tailored in diverse and arbitrary ways by combing the two aforementioned types of topological states.     

Tunable band gap and optical properties of surface functionalized Sc_2C monolayer

Using the density functional theory, we have investigated the electronic and optical properties of two-dimensional Sc_2C monolayer with OH, F, or O chemical groups. The electronic structures reveal that the functionalized Sc_2C monolayers are semiconductors with a band gap of 0.44–1.55 eV. The band gap dependent optical parameters, like dielectric function, absorption coefficients, reflectivity, loss function, and refraction index were also calculated for photon energy up to 20 eV. At the low-energy region, each optical parameter shifts to red, and the peak increases obviously with the increase of the energy gap. Consequently, Sc_2C monolayer with a tunable band gap by changing the type of surface chemical groups is a promising 2D material for optoelectronic devices.