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Electronic structure calculations based on many-body perturbation theory (e.g. GW or the random-phase approximation (RPA)) require function evaluations in the complex time and frequency domain, for example inhomogeneous Fourier transforms or analytic continuation from the imaginary axis to the real axis. For inhomogeneous Fourier transforms, the time-frequency component of the GreenX library provides time-frequency grids that can be utilized in low-scaling RPA and GW implementations. In addition, the adoption of the compact frequency grids provided by our library also reduces the computational overhead in RPA implementations with conventional scaling. In this work, we present low-scaling GW and conventional RPA benchmark calculations using the GreenX grids with different codes (FHI-aims, CP2K and ABINIT) for molecules, two-dimensional materials and solids. Very small integration errors are observed when using 30 time-frequency points for our test cases, namely $<10^{-8}$ eV/electron for the RPA correlation energies, and 10 meV for the GW quasiparticle energies., Comment: 15 pages, 6 figures

Two dimensional (2D) semiconducting light absorbers, have recently considered as promising components to improve the efficiency in the photocatalytic hydrogen production via water splitting. In this work, by employing density functional theory computations, we introduced novel SiX2 (X = P, As) nanosheets in tetragonal (penta-) and orthorhombic (rec-) phases, as promising light absorber semiconductors for overall water splitting. The predicted nanomembranes exhibit good mechanical, dynamical and thermal stabilities. They also show small cleavage energies in the range of 0.31 J/m2 to 0.39 J/m2, comparable to that of the graphene and thus suggesting the feasibility of their experimental exfoliation. Notably, predicted monolayers are semiconductors with indirect band gaps of 2.65 eV for penta-SiP2, 2.35 eV for penta-SiAs2, 1.89 eV for rec-SiAs2, and a direct band gap of 2.21 eV for rec-SiP2. These nanomaterials however show relatively large interlayer quantum confinement effects, resulting in smaller band gap values for bilayer lattices. We observed a huge difference between the electron and hole mobilities for penta-SiP2 and rec-SiAs2 monolayers and highly directional dependent electron and hole mobilities in rec-SiP2, yielding an effective separation of photogenerated charge carriers. Remarkably, these novel nanomembranes show strong absorption in the visible region of light as well as suitable band edge positions for photocatalytic water splitting reaction, specifically under neutral conditions.

Carbon nitride two-dimensional (2D) materials are among the most attractive class of nanomaterials, with wide range of application prospects. As a continuous progress, most recently, two novel carbon nitride 2D lattices of C3N5 and C3N4 have been successfully experimentally realized. Motivated by these latest accomplishments and also by taking into account the well-known C3N4 triazine-based graphitic carbon nitride structures, we predicted two novel C3N6 and C3N4 counterparts. We then conducted extensive density functional theory simulations to explore the thermal stability, mechanical, electronic and optical properties of these novel nanoporous carbon-nitride nanosheets. According to our results all studied nanosheets were found to exhibit desirable thermal stability and mechanical properties. Non-equilibrium molecular dynamics simulations on the basis of machine learning interatomic potentials predict ultralow thermal conductivities for these novel nanosheets. Electronic structure analyses confirm direct band gap semiconducting electronic character and optical calculations reveal the ability of these novel 2D systems to adsorb visible range of light. Extensive first-principles based results by this study provide a comprehensive vision on the stability, mechanical, electronic and optical responses of C3N4, C3N5 and C3N6 as novel 2D semiconductors and suggest them as promising candidates for the design of advanced nanoelectronics and energy storage/conversion systems.

In this work, density functional theory calculations were carried out to explore the mechanical response, dynamical/thermal stability, electronic/optical properties and photocatalytic features of monoclinic As2X3 (X=S, Se and Te) nanosheets. Acquired phonon dispersions and ab-initio molecular dynamics results confirm the stability of studied nanomembranes. Observation of relatively weak interlayer interactions suggests that the exfoliation techniques can be potentially employed to fabricate nanomembranes from their bulk counterparts. The studied nanosheets were found to show highly anisotropic mechanical properties. Notably, new As2Te3 2D lattice predicted by this study is found to exhibit unique superstretchability, which outperforms other 2D materials. In addition, our results on the basis of HSE06 functional reveal the indirect semiconducting electronic nature for the monolayer to few-layer and bulk structures of As2X3, in which a moderate decreasing trend in the band-gap by increasing the thickness can be established. The studied nanomaterials were found to show remarkably high and anisotropic carrier mobilities. Moreover, optical results show that these nanosheets can absorb the visible light. In particular, the valence and conduction band edge positions, high carrier mobilities and optical responses of As2Se3 nanosheets were found to be highly desirable for the solar water splitting. The comprehensive vision provided by this study not only confirm the stability and highly attractive electronic and optical characteristics of As2S3, As2Se3 and As2Te3 nanosheets, but also offer new possibilities to design superstretchable nanodevices.

New two diemensional structures nanoribbon including phosphorus and germanium atoms are introduced for the nanoelectronic applications. Under various bias voltages, the electronic transport in the systems have been studied within the noneqilibrium Green's function formalism. The $I-V$ characteristics have been extracted. DOS and $T(E,V_{bias})$ have been investigated and show that the charge transport occurs when the bias voltage reaches about 1 \textit{V}. The calculated MPSH shows that the spatial distribution of orbital levels has been affected by the electrodes. The studied structures have a bandgap of about 0.7 \textit{eV} which absorbs light in the visible range and thus could be an interesting contender for solar cells applications., Comment: 8 pages, 11 figures

We derive the matrix elements of Coulomb interaction between states with different number of electrons and holes in a semiconductor nanocrystal within the 8-band $\mathbf{k}\cdot\mathbf{p}$ theory. These matrix elements are responsible for multiple exciton generation which may contribute to the enhancement of the efficiency of solar cells. Our calculations are performed within the multi band envelope function formalism based on the states resulting from diagonalization of the 8-band $\mathbf{k}\cdot\mathbf{p}$ Hamiltonian. We study in detail and compare two contributions to the inter-band Coulomb coupling: the mesoscopic one, which involves only the envelope functions and relies on band mixing, and the microscopic one, that relies on the Bloch parts of the wave functions and is non-zero even between single- band states. We show that these two contributions are of a similar order of magnitude. We study also the statistical distribution of the magnitudes of the inter-band Coulomb matrix elements and show that the overall coupling to remote states decays according to a power law favorable for the convergence of numerical computations., Comment: 8 pages, 5 figures; minor corrections with respect to v2

The population dynamics of single- and bi-exciton states in semiconductor nanocrystals is modeled numerically in the presence of Coulomb coupling between single- and two-exciton states and a dissipation channel in order to study the transient bi-exciton population that occurs in an optically excited semiconductor nanocrystal. The results show that the system evolution strongly changes if the dissipation is included. In a certain range of parameters, the growth of the exciton number (MEG process) is fast (on picosecond time scale) and the following decay (Auger process) is much slower (hundreds of picoseconds). In some cases, the maximum occupation of the bi-exciton state increases when dissipation is included. The dynamics of an ensemble of nanocrystals with a certain size dispersion is studied by averaging over the energy of the bi-exciton state which can be different for each single nanocrystal. The validity of Markov and secular approximation is also verified., Comment: 11 pages, 12 figures