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[en] Highlights: • The I impurity forms stronger bonding with zigzag graphene nanoribbons (ZGNR). • I-passivation results in AFM ground state. • Presence of I as passivating element further stabilized adsorbed I adatoms on ZGNR. • I adatom is expected to migrate on graphene surface with diffusion time of 1.47 ps • The magnetic stabilization is further enhanced in H2-ZGNR-I configuration. - Abstract: Graphene, being perfect 2-D structure, exhibits electronic properties which are sensitive to the presence of any impurtiy/adsorbed adatom. In the present work, interaction of I atoms has been investigated with graphene and zigzag graphene nanoribbon (ZGNR) by considering it, as a passivating element as well as an adsorbed adatom. Three different possible combinations of I passivation have been explored which include: one edge I and other edge H passivation (H-ZGNR-I), both edges passivation I (I-ZGNR-I) and one edge I passivation while other edge is passivated by H in sp3 manner (H2-ZGNR-I). Similarly, three adsorption sites namely top (T), bridge (B) and hole (H) have been considered at ZGNR as well as on planar graphene sheet. It is revealed that passivation of I on ZGNR is energetically favorable and settled in antiferromagnetic (AFM) ground state. Further, it is observed that H-ZGNR-I is the most stable configuration after pristine (H-ZGNR-H) followed by H2-ZGNR-I and I-ZGNR-I configurations. Our observations show that except I-ZGNR-I, all the structures exhibit magnetic stabilization well above the thermal excitations at room temperature which ensures their applicability for practical applications. Moreover, I adsorption always prefers T site on ZGNR/graphene sheet. Analysis of I migration on 2-D graphene indicates that diffusion barrier is always less than the thermal excitation energy (∼26 meV) and the diffusion time varies from ∼1.5 ps to ∼2.2 ps. Present findings suggest for stronger binding of I atoms with ZGNR whereas the same with graphene is comparatively weak and exhibits spontaneous migration.
[en] The Kitaev model is an exactly-soluble quantum spin model, whose ground state provides a canonical example of a quantum spin liquid. Spin excitations from the ground state are fractionalized into emergent matter fermions and fluxes. The flux excitation is pointlike in two dimensions, while it comprises a closed loop in three dimensions because of the local constraint for each closed volume. In addition, the fluxes obey global constraints involving (semi)macroscopic number of fluxes. We here investigate such global constraints in the Kitaev model on a three-dimensional lattice composed of nine-site elementary loops, dubbed the hypernonagon lattice, whose ground state is a chiral spin liquid. We consider two different anisotropic limits of the hypernonagon Kitaev model where the low-energy effective models are described solely by the fluxes. We show that there are two kinds of global constraints in the model defined on a three-dimensional torus, namely, surface and volume constraints: the surface constraint is imposed on the even-odd parity of the total number of fluxes threading a two-dimensional slice of the system, while the volume constraint is for the even-odd parity of the number of the fluxes through specific plaquettes whose total number is proportional to the system volume. In the two anisotropic limits, therefore, the elementary excitation of fluxes occurs in a pair of closed loops so as to satisfy both two global constraints as well as the local constraints.
[en] Highlights: • The thermoelectric quantities perform the sensitivity to the inter-dot coupling strength. • Double Fano resonances can be created to largely enhance the thermoelectric effect at low-temperature. • Thermoelectric figure of merit can be improved due to the coexistence of local bipolar effect and Fano resonance. • Thermoelectric figure of merit can be optimized by adjusting the dot-lead coupling strengths. - Abstract: The thermoelectric transport properties of a parallel-coupled double quantum dot (PCDQD) system with side-coupled quantum dots (QDs) is investigated by using the Keldysh non-equilibrium Green's function technique. The thermoelectric quantities, including the thermal conductance, thermopower, and thermoelectric figure of merit denoted by ZT, are sensitive to the inter-dot coupling strength. With the help of side-coupled QD, unusual double Fano resonances are created in the conductance spectra to largely enhance the thermoelectric effect at low-temperature. Benefited from the coexistence of local bipolar effect and Fano resonance, the ZT can be improved by one-fold higher than that of original PCDQD system. Moreover, when the asymmetry parameter α, which indicates the geometric arrangement of coupled QDs with a given lead, takes appropriate value, the optimization of ZT can be achieved at high temperature. Our work suggests that the side-coupled QDs scheme holds promise for the designing of high-efficiency thermoelectric conversion devices.
[en] Highlights: • Calculated band gap of Ba5Ta4O15 is improved to 4.05 eV using the TB-mBJ potential. • Mo–P co-doping can prevent the Fermi level at the acceptor states and the donor states. • Band gap of Mo–P co-doped Ba5Ta4O15 has been narrowed to 2.15 eV. • Band edges of the Mo–P co-doped Ba5Ta4O15 straddle the water redox potentials. - Abstract: Based on density functional calculations, Mo and P co-doped Ba5Ta4O15 compared with their mono-doping was studied for splitting water. The results showed that Mo–P co-doping significantly reduced the energy gap of Ba5Ta4O15 from 4.05 eV to 2.15 eV, being almost the optimum value for utilizing solar energy as much as possible. The top of valence band and the bottom of conduction band are both compatible with the oxidation-reduction potentials of water. More importantly, Mo–P co-doping prevents the filled spin-down states of Mo and the empty spin-down states of P from arising due to the charge compensation of Mo–P pairs. We propose that Mo–P co-doped Ba5Ta4O15 is one of the most promising photocatalyst candidates for solar water splitting.
[en] Highlights: • The different effects of interlayer distances on electronic structures of phosphorene/h-BN heterostructure are proposed. • The band structure types and gap values can be tuned effectively by the interlayer distances. • The direct-indirect band transition is mainly attributed to the opposite behaviors of the Ppz and Ppx,py bonding states on conduction band. - Abstract: By using first-principles calculations, we systemically investigate the electronic properties of phosphorene/h-BN heterostructure with different interlayer distances. Our results show that the electronic states in the vicinity of the Fermi level are completely dominated by phosphorene, and the system exhibits type-I band alignment consequently. Moreover, we also reveal the variation of the band structure of phosphorene/h-BN heterostructure with different interlayer distances. The band gap undergoes a direct to indirect transition as decreasing the interlayer distance. The mechanism of the band gap transition can be attributed to the different energy levels shifts, according to different electronic orbital characters on the band edge. In specific, the energy level of the Ppz bonding state shifts up while that of the Ppx,py bonding state falls down, along with the enhancement of the interactions between phosphorene and h-BN.
[en] Highlights: • Single step nanocrystalline growth of Cu2ZnSnSe4 nanoparticles by solvothermal method. • XRD and Raman studies reveal single phase growth of CZTSe. • Stochiometric growth of material confirmed by EDAX analysis. - Abstract: Cu2ZnSnSe4 (CZTSe) nanocrystals were prepared by a solvo-thermal method using elemental copper, zinc, tin and selenium powders as precursors and ethylenediamine as a stabilizer and chelating medium. The particles were synthesized at 180 °C after which they were annealed at 300 °C and 400 °C for 1 h. The influence of annealing the nanocrystals, on the crystal structure, particle size, morphology, composition and optical properties has been investigated. As-grown particles were found to contain secondary phases, which subsequently transformed into CZTSe nanocrystals upon annealing. XRD and Raman results confirmed that the samples recrystallized into kesterite CZTSe on annealing at 400 °C. SEM studies show that the as-grown material has a platelet like morphology, and after annealing at 400 °C, freshly formed nanosized particles (10–20 nm) appear. TEM studies reveal that the grain size of CZTSe nanoparticles is about 5–10 nm. Optical studies show that the band gap energy exhibits a blue shift in the range 1.3–1.6 eV, indicative of quantum confinement of electrons.
[en] We study the effects of quantum fluctuations on the dynamical generation of a gap and on the evolution of the spin-wave spectra of a frustrated magnet on a triangular lattice with bond-dependent Ising couplings, analog of the Kitaev honeycomb model. The quantum fluctuations lift the subextensive degeneracy of the classical ground-state manifold by a quantum order-by-disorder mechanism. Nearest-neighbor chains remain decoupled and the surviving discrete degeneracy of the ground state is protected by a hidden model symmetry. We show how the four-spin interaction, emergent from the fluctuations, generates a spin gap shifting the nodal lines of the linear spin-wave spectrum to finite energies.
[en] Distribution of a local field B(x) on the surface of Bi2Sr2CaCu2O8+d single crystal and its time dependence are measured using a micro Hall-probe array with 7 elements. From these data, the activation energy dependence on B(x) and on the current density J are estimated. The three-dimensional mapping of in B, J and U space gives us information on spatial behavior of in the sample.
[en] In this study, through DFT and TDDFT computational methods and by using benzimidazobenzophenanthroline (BBL) as an acceptor and tetraphenyldibenzoperiflanthene (DBP) as a donor, a Donor-Acceptor (D-A) system was devised on the purpose of designing and simulating organic solar cells. The optimization of this system was done in the basic state using the basis set and the method of B3LYP/6-311 + G*. The energy of HOMO and LUMO orbitals and the electron localization function (ELF) were also investigated. The LUMO orbital energy of the acceptor (A) was 0.51 eV less than that of the donor (D), which is close to the ideal value. The computation of the excited state was performed by using the CAM-B3LYP method and the same basis set. The hole-electron theory implemented in the excited state visually proved a charge transfer (CT) for the D-A system, which was based on the computations in the ground state. Typically, organic solar cells have a CT at one wavelength. The CT results show that there are two considerable CTs at 356- and 487-nm wavelengths, indicating the acceptable efficiency of this system.
[en] Transition metal trichalcogenides (TMTs), a family of van der Waals materials, have gained increasing interests from the discovery of magnetism in few-layer forms. Although TMTs with 3d transition metal elements have been studied extensively, much less is explored for the 4d and 5d cases, where the interesting interplay between electron correlations and the relativistic spin-orbit coupling is expected. Using ab initio calculations, we here investigate the electronic property of TMTs with 4d and 5d transition metal elements. We show that the band structures exhibit multiple node-like features near the Fermi level. These are the remnant of multiple Dirac cones that were recently discovered in the monolayer cases. Our results indicate that the peculiar two-dimensional multiple Dirac cones are concealed even in the layered bulk systems.