Luigi Cigarini

Bio: Luigi Cigarini is an academic researcher from University of Ostrava. The author has contributed to research in topics: Pseudopotential & Density functional theory. The author has an hindex of 2, co-authored 4 publications receiving 5 citations.

Lattice dynamics in the conformational environment of multilayered hexagonal boron nitride (h-BN) conveys to peculiar infrared optical responses

Abstract: Stacking mismatches in hexagonal boron nitride (h-BN) nanostructures affect their photonic, mechanical, and thermal properties. To access information about the stacked configuration of layered ensembles, highly sophisticated techniques like X-ray photoemission spectroscopy or electron microscopy are necessary. Here, instead, by taking advantage of the geometrical and chemical nature of h-BN, we show how simple structural models, based on shortened interplanar distances, can produce effective charge densities. Accounting these in the non-analytical part of the lattice dynamical description makes it possible to access information about the composition of differently stacked variants in experimental samples characterized by infrared spectroscopy. The results are obtained by density functional theory and confirmed by various functionals and pseudopotential approximations. Even though the method is shown on h-BN, the conclusions are more general and show how effective dielectric models can be considered as valuable theoretical pathways for the vibrational structure of any layered material.

Lattice dynamics in the conformational environment of multilayered hexagonal boron nitride (h-BN) results in peculiar infrared optical responses.

Abstract: Stacking mismatches in hexagonal boron nitride (h-BN) nanostructures affect their photonic, mechanical, and thermal properties. To access information about the stacked configuration of layered ensembles, highly sophisticated techniques like X-ray photoemission spectroscopy or electron microscopy are necessary. Here, instead, by taking advantage of the geometrical and chemical nature of h-BN, we show how simple structural models, based on shortened interplanar distances, can produce effective charge densities. Accounting these in the non-analytical part of the lattice dynamical description makes it possible to access information about the composition of differently stacked variants in experimental samples characterized by infrared spectroscopy. The results are obtained by density functional theory and confirmed by various functionals and pseudopotential approximations. Even though the method is shown using h-BN, the conclusions are more general and show how effective dielectric models can be considered as valuable theoretical pathways for the vibrational structure of any layered material.

Lattice dynamics in the conformational environment of bulk hexagonal boron nitride (h-BN) conveys a peculiar optical response

Abstract: We report a detailed study of the stability and dynamical lattice properties of five possible differently stacked configurations of bulk hexagonal boron nitride (h-BN). The results are obtained upon comparison among different theoretical methods, namely Local Density Approximation (LDA) and Generalized Gradient Approximation (GGA) Density Functional Theory (DFT) approaches. We devote particular attention to the prominent role played by the pseudopotential approximation in the theoretical framework and differently from previous implementations, we obtain the best performances from GGA rather than LDA. The major complication, nevertheless, consists in the necessity of a complex approach for the non-analytical part of the dynamical matrix. Our conclusions are based on the agreement of the calculated data with a set of infrared spectroscopy measurements, taken from the experimental literature. Besides, we apply the theoretical model to derive, from geometrical considerations, an empirical method for the calculation of the non-analytical part of the dynamical matrix for potential use in discerning techniques.

Stacking and Registry Effects in Layered Materials: The Case of Hexagonal Boron Nitride

Abstract: The interlayer sliding energy landscape of hexagonal boron nitride (h-BN) is investigated via a van der Waals corrected density functional theory approach. It is found that the main role of the van der Waals forces is to anchor the layers at a fixed distance, whereas the electrostatic forces dictate the optimal stacking mode and the interlayer sliding energy. A nearly free-sliding path is identified, along which band gap modulations of ∼0.6 eV are obtained. We propose a simple geometric model that quantifies the registry matching between the layers and captures the essence of the corrugated h-BN interlayer energy landscape. The simplicity of this phenomenological model opens the way to the modeling of complex layered structures, such as carbon and boron nitride nanotubes.

Alternative Stacking Sequences in Hexagonal Boron Nitride.

Abstract: The relative orientation of successive sheets, i.e. the stacking sequence, in layered two-dimensional materials is central to the electronic, thermal, and mechanical properties of the material. Often different stacking sequences have comparable cohesive energy, leading to alternative stable crystal structures. Here we theoretically and experimentally explore different stacking sequences in the van der Waals bonded material hexagonal boron nitride (h-BN). We examine the total energy, electronic bandgap, and dielectric response tensor for five distinct high symmetry stacking sequences for both bulk and bilayer forms of h-BN. Two sequences, the generally assumed AA' sequence and the relatively unknown (for h-BN) AB (Bernal) sequence, are predicted to have comparably low energy. We present a scalable modified chemical vapor deposition method that produces large flakes of virtually pure AB stacked h-BN; this new material complements the generally available AA' stacked h-BN.

Optical coherence in atomic monolayer transition metal dichalcogenides limited by electron-phonon interactions

Abstract: We systematically investigate the excitonic dephasing of three representative transition-metal dichalcogenides, namely, ${\mathrm{MoS}}_{2}$, ${\mathrm{MoSe}}_{2}$, and ${\mathrm{WSe}}_{2}$ atomic monolayer thick and bulk crystals, in order to gain a proper understanding of the factors that determine the optical coherence in these materials. Coherent nonlinear optical spectroscopy and temperature dependent absorption, combined with theoretical calculations of the phonon spectra, indicate electron-phonon interactions, to be the limiting factor. Surprisingly, the excitonic dephasing, differs only slightly between atomic monolayers and high quality bulk crystals, which indicates that material imperfections are not the limiting factor in atomically thin monolayer samples. The temperature dependence of the electronic band gap and the excitonic linewidth combined with ``ab initio'' calculations of the phonon energies and the phonon density of states reveal a strong interaction with the $E$' and $E$'' phonon modes.

Type‐II van der Waals Heterostructures Based on AsP and Transition Metal Dichalcogenides: Great Promise for Applications in Solar Cell

Abstract: As an effective means to adjust the properties of 2D materials, type‐II van der Waals (vdW) heterostructures have been under extensive research due to their significantly reduced carrier recombination probability and extended carrier lifetime. Herein, stable vdW heterostructures based on arsenic phosphorus (AsP) and transition metal dichalcogenides are designed. The geometry, electronic, and optical properties for type‐II AsP/MX2 heterostructures (M = Mo, W; X = S, Se) by first‐principle calculations are systematically explored and their application in solar cell materials is predicted. AsP/MX2 heterostructures are indirect semiconductors with the quasiparticle bandgap ranging from 1.49 to 2.02 eV. They effectively widen the light absorption of AsP monolayers in visible and ultraviolet regions. It is worth noting that AsP/WSe2 heterostructure can form a built‐in electric field (0.832 eV Å−1) and have a minor exciton binding energy (0.22 eV), suggesting that it is a potential solar cell material. The power conversion efficiency is more than 15%. The results will provide a theoretical basis for sustainable energy applications of AsP‐based vdW heterostructures in the future.

Quasiparticle energies and significant exciton effects of monolayered blue arsenic phosphorus conformers.

Abstract: Low-dimensional systems have strong multi-body interactions and fewer geometric constraints due to the screening effect of the Coulomb interaction. We use the single-shot GW-Bethe Salpeter equation (G0W0-BSE) to calculate the electronic and optical properties of six-blue arsenic phosphorus (β-AsP) conformers. The results show significant anisotropic exciton effects of covering visible regions, which apparently changed the light absorption. The maximum exciton binding energy is up to 0.99 eV, which is more extensive than the black phosphorus monolayer (0.9 eV). We predict that the different orbital contributions to valence bands may cause the anisotropic exciton effect difference. Our results indicate that β-AsP monolayers with the large binding energies of exciton hold a great promise for applications in optoelectronic devices.