1,721,133 research outputs found
A monolayer transition-metal dichalcogenide as a topological excitonic insulator
Full text linkTopological insulators have been studied primarily with regard to the behaviour of electrons. A theoretical study now shows that a single layer of a metal dichalcogenide can become a topological insulator for excitons.Monolayer transition-metal dichalcogenides in the T ' phase could enable the realization of the quantum spin Hall effect(1) at room temperature, because they exhibit a prominent spin-orbit gap between inverted bands in the bulk(2,3). Here we show that the binding energy of electron-hole pairs excited through this gap is larger than the gap itself in the paradigmatic case of monolayer T ' MoS2, which we investigate from first principles using many-body perturbation theory(4). This paradoxical result hints at the instability of the T ' phase in the presence of spontaneous generation of excitons, and we predict that it will give rise to a reconstructed 'excitonic insulator' ground state(5-7). Importantly, we show that in this monolayer system, topological and excitonic order cooperatively enhance the bulk gap by breaking the crystal inversion symmetry, in contrast to the case of bilayers(8-16) where the frustration between the two orders is relieved by breaking time reversal symmetry(13,15,16). The excitonic topological insulator is distinct from the bare topological phase because it lifts the band spin degeneracy, which results in circular dichroism. A moderate biaxial strain applied to the system leads to two additional excitonic phases, different in their topological character but both ferroelectric(17,18) as an effect of electron-electron interaction
Optical properties of lead-free double perovskites by ab initio excited-state methods
Full text linkWe discuss the nature of the optical excitations of Cs2AgBiBr6, the archetypal compound of lead-free double perovskites. Such quaternary material shows an indirect electronic band gap with a broad optical absorption spectrum above 2 eV. By means of ab initio excited-state methods we show that the first absorption peak is due to a bound direct exciton (near the X point of the Brillouin zone), while the photoluminescence spectrum is explained in terms of phonon-assisted radiative recombination of indirect-bound excitons with transferred momenta along the L-X and Gamma-X directions. To address the role of metal and halide atoms on the electronic and optical properties of this materials class, we investigate two additional ternary double perovskites, i.e., Cs2In2X6 (X = F, Br). On the basis of the accurate determination of the absorption coefficients and minimum gaps, we estimate the spectroscopic limited maximum efficiency of solar cells based on such compounds, providing relevant information for their application in photovoltaics
Electronic and optical properties of GaN(110) surface
No full textElectronic and optical properties of GaN(110) surfac
Role of Quantum-confinement in Anatase nanosheets
No full textDespite most of the applications of anatase nanostructures rely on photoexcited charge processes, yet profound theoretical understanding of fundamental related properties is lacking. Here, by means of ab initio ground and excited-state calculations, we reveal, in an unambiguous way, the role of quantum confinement effect and of the surface orientation, on the electronic and optical properties of anatase nanosheets (NSs). The presence of bound excitons extremely localized along the (001) direction, whose existence has been recently proven also in anatase bulk, explains the different optical behavior found for the two orientations — (001) and (101) — when the NS thickness increases. We suggest also that the almost two-dimensional nature of these excitons can be related to the improved photoconversion efficiency observed when a high percentage of (001) facet is present in anatase nanocrystals
Multiple Linear Dichroism Inversions in SnO Monolayers for Polarization-Sensitive UV Photodetection: An Ab Initio Investigation
Full text linkTin monoxide (SnO) undergoes a phase transition from litharge-like tetragonal (space group P4/nmm) to orthorhombic geometry (layer group pmmn) in passing from multilayer to monolayer crystals. By means of ab initio ground and excited-state methods, we explore the impact of the reduced pmmn spatial symmetry on the electronic and optical properties of SnO monolayers. As a consequence of the in-plane anisotropy, the electronic states of the band edges show asymmetric projections onto the px and py atomic orbitals along orthogonal directions in the Brillouin zone. This results in optical absorption and exciton properties that are highly sensitive to the direction of in-plane polarized light. In contrast to typical linear dichroic materials, which generally favor the absorption of one polarization over the orthogonal one across a wide frequency range, we show that SnO monolayers display linear dichroism inversion. Here, the energy ordering of the exciton states causes the two orthogonal polarizations to be absorbed with different intensities depending on the light frequency. We observe multiple inversions of the linear dichroism across wavelengths from 200 to 400 nm. These properties make SnO monolayers promising candidates for further exploration of low-symmetry, two-dimensional materials for advanced applications in polarization-sensitive nanoscale devices. In addition, we propose utilizing optical dichroism measurements as a means to probe the recently predicted ferroelastic-to-paraelastic transition of SnO monolayers
Electronic and Optical Properties of SiGe alloys within first-principles schemes
No full textElectronic and Optical Properties of SiGe alloys within first-principles scheme
Ab-Initio Calculation of the optical Properties of BN(110) Surface
No full textWe study the optical properties of the nonpolar (110) surface of cubic Boron Nitride calculated within the first-pinciple DFT-LDA scheme. The reflectance anisotropy (RA) spectrum is analyzed in relation to the better known spectrum of the GaAs(110) surface. The influence of the ionicity on the surface relaxation, the surface states character and the surface optical spectra, is studied. Comparisons with existing data for the surface under study and results to the GaN(110) surface are also given
First-Principles Calculations of Exciton Radiative Lifetimes in Monolayer Graphitic Carbon Nitride Nanosheets: Implications for Photocatalysis
Full text linkIn this work, we report on the exciton radiative lifetimes of graphitic carbon nitride monolayers in the triazine-based (gC3N4-t) and heptazine-based (gC3N4-h) forms, as obtained by means of ground-state plus excited-state ab initio calculations. By analyzing the exciton fine structure, we highlight the presence of dark states and show that the photogenerated electron-hole (e-h) pairs in gC3N4-h are remarkably long-lived, with an effective radiative lifetime of 260 ns. This fosters the employment of gC3N4-h in photocatalysis and makes it attractive for the emerging field of exciton devices. Although very long intrinsic radiative lifetimes are an important prerequisite for several applications, pristine carbon nitride nanosheets show very low quantum photoconversion efficiency, mainly due to the lack of an efficient e-h separation mechanism. We then focus on a vertical heterostructure made of gC3N4-t and gC3N4-h layers, which shows a type-II band alignment and looks promising for achieving net charge separation
Boosted Solar Light Absorbance in PdS2/PtS2Vertical Heterostructures for Ultrathin Photovoltaic Devices
Full text linkTransition-metal dichalcogenides (TMDs) represent a class of materials whose archetypes, such as MoS2 and WS2, possess exceptional electronic and optical properties and have been massively exploited in optoelectronic applications. The layered structure allows for their exfoliation to two-dimensional samples with atomic thickness (≲ 1 nm), promising for ultrathin, ultralight devices. In this work, by means of state-of-the-art ab initio many-body perturbation theory techniques, we focus on single-layer PdS2 and PtS2 and propose a novel van der Waals heterostructure with outstanding light absorbance, reaching up to 50% in the visible spectrum and yielding a maximum short-circuit current of 7.2 mA/cm2 under solar irradiation. The computed excitonic landscape predicts a partial charge separation between the two layers and the momentum-forbidden lowest-energy state increases the carrier diffusion length. Our results show that the employment of vertical heterostructures with less conventional TMDs, such as PdS2/PtS2, can greatly boost light absorbance and favor the development of more efficient, atomic-thin photovoltaic devices
First-Principles Nonequilibrium Green's Function Approach to Ultrafast Charge Migration in Glycine
No full textWe investigate the photoinduced ultrafast charge migration phenomenon in the glycine molecule using a recently proposed nonequilibrium Green's functions (NEGF) approach. We first consider the dynamics resulting from the sudden removal of an electron in the valence shells, finding a satisfactory agreement with available data. Then we explicitly simulate the laser-induced photoionization process and study the evolution of the system after the pulse. We disentangle polarization and correlation effects in the electron dynamics and assign the main frequencies to specific elements of the reduced one-particle density matrix. We show that electronic correlations renormalize the bare frequencies, redistribute the spectral weights, and give rise to new spectral features
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