Quasiparticle energies, exciton level structures and optical absorption spectra of ultra-narrow ZSiCNRs

Abstract

The hydrogen-passivated N chain zigzag silicon carbide nanoribbons (N-ZSiCNRs) are indexed by their width N (the number of zigzag Si-C chains across the nanoribbon). Based on first-principles GW and Bethe-Salpeter equation (BSE) approaches, we investigated the quasiparticle band structures, exciton level structures and optical absorption spectra of the ultra-narrow N-ZSiCNRs with N ¼ 2-3. It is found that the GW band gap of 3-ZSiCNR is 0.804 eV, which is more than two times larger than the HSE06 band gap (0.401 eV). The GW band gap of 2-ZSiCNR is 2.911 eV, which is also almost more than two times larger than the HSE06 band gap (1.621 eV). These results indicate that for 1-dimensional structure materials, HSE06 approaches underestimate the band gap of the system. The GW + BSE calculations demonstrate that the optical absorption spectra of the N-ZSiCNRs are dominated by edge-state-derived excitons with large binding energy, composed of a characteristic series of exciton states. It is found that the edge-state excitons of N-ZSiCNR belong to charge-transfer excitons, where the excited electron is confined to a Si edge while the hole is located on a C edge. The exciton binding energy increases with decreasing width N, which indicates that the quantum confinement effect enhances with decreasing width N. The excitons in 2-ZSiCNR can have a binding energy up to 1.78 eV. In addition, the exciton level structure and wave function are classified. It is very interesting to find a relationship between the node of the exciton wave functions and the incoming polarization light exciton excitation. For example, in the longitudinal optical absorption spectra, if the exciton whose wave function possesses an odd number of nodes is optically active, then the exciton whose wave function possesses an even number of nodes is optically inactive. In contrast, in the transverse optical absorption spectra, the exciton whose wave function possesses an odd number of nodes is optically inactive, while the exciton whose wave function possesses an even number of nodes is optically active.

GeC/GaN vdW Heterojunctions: A Promising Photocatalyst for Overall Water Splitting and Solar Energy Conversion

“…Subsequent to obtaining the DFT-PBE-vdW eigenvalues and wave functions, the GW band structure was calculated non-self-consistently, i.e., within the G 0 W 0 approximation, using the BerkeleyGW package. , A 12 × 12 × 1 k -point grid, 16.0 Ry energy cutoff, and 320 unoccupied bands are undertaken to calculate the dielectric function and self-energy, where the Coulomb truncation scheme was adopted.…”

“…Excitonic effects (electron–hole interactions) are included by solving the Bethe–Salpeter equation (BSE) in terms of the two-particle Green’s function, using the BerkeleyGW code. , The optical absorption coefficient α­(ω) is given as , where ε′(ω) and ε″(ω) are, respectively, the real and imaginary parts of the dielectric function ε­(ω), and c is the speed of light in a vacuum . To obtain converged optical spectra, the kernel K eh (describing the interaction between excited electrons and holes) of the GaN monolayer (GeC monolayer and GeC/GaN vdW heterostructure) is computed on a sparse k -point grid of 24 × 24 × 1 and then interpolated onto a denser k -point grid of 48 × 48 × 1. , It is well known that summing over only some of the degenerate space will break symmetry and lead to incorrect oscillator strengths in absorption . Since we are interested only in the low-energy region of the optical spectra, the degeneracy-allowed six highest valence bands and four lowest conduction bands were incorporated during BSE calculations.…”

A new crystal family of GaNGeC quaternary compounds including direct band gap semiconductors and metals

“…In this study, most calculations pertaining to DFT (the standard density functional theory) and GW (the G 0 W 0 approximation) electronic band structures, 12,20,21 elastic constants and GW +BSE optical spectra (calculated using the combination of GW and Bethe Salpeter equation (BSE) method) 12,20,21 are performed using the Vienna Ab initio Simulation Package (VASP). 22,23 The projector augmented wave pseudo potential, 23 plane wave basis set, and Perdew Burke Ernzerhof (PBE) exchange correlation functional, 24 including vdW dispersive corrections 25 and spinorbit coupling (SOC), were applied.…”

“…As listed in Table 1, S-1, S-2, S-3,S-4, S-5 and S-6 crystals are semiconductors, whereas S-7 and S-8 are metals. With regard to the semiconductors, the GW band structures 12,20,21 are shown, and for metals, the PBE band structures are displayed (Fig. 4).…”

“…The light absorption capacity of S-1, S-2, S-3, S-4, S-5 and S-6 crystals has been evaluated using the function of light absorption coefficients changing with photon energy, which is calculated using the GW+BS. 12,20,21 Moreover, in order to reveal the anisotropy of the optical properties of the material, the optical absorption spectra corresponding to light polarized parallel to the x, y and z directions have been considered, as shown in Fig. from an inter-band transition between the VBM and CBM at the R point.…”