Electronic excitations of borophene: novel graphical tool for the electron density

Chasing and describing electronic excitations is of paramount importance both for the understanding of materials' properties as well as for designing functionalities in new frontiers applications.

In this thesis project, we propose a new approach for the visual description of electronic excitations, via the accurate determination and visualization of the electron density. When a system is submitted to an external perturbation, it reacts via a change in the electronic density. The rate of change is normally captured by the density-density response function (the polarizability - in linear response) or higher order susceptibilities. The change in the density (upon perturbation) can be accurately described viaTime-Dependent Density Functional Theory (TDDFT) [1,2] or via the solution of the Bethe-Salpeter Equation (BSE) [3,4] within the Green's functions formalism [5,6]. The visualization in real space and in real time of the density change constitutes a new tool for the description of electronic properties of materials (for it carries information about the band-structure, the collective plasmonic excitations, as well asthe excitonic features). However the ab initio determination of the full density response is never carried out for two important reasons: i) in standard situations one calculates only the macroscopic compontent, necessary, for instance, to evaluate optical absorption, reflectivity or electron energy loss spectra; ii) the full matrix is much more cumbersome to evaluate than just the macroscopic component (which is only one number). The full polarizability matrix, and so the full description of the density change upon perturbation, beside containing all spectra cited above (and many others), permits also the description of excitations channels, plasmon formation and migrations, and relates to two-particle correlation functions in a non-trivial way. All this in a visual, intuitive, and graphically attractive way: it is a movie in real space and real time. Preliminary results in bulk silicon [7], graphite [8] and LiF [9] show really encouraging. But the field in which this approach shows full promise is ab initio plasmonics in 2D materials [10,11].

Bidimensional materials have attracted an enormous interest in the last couple of decades, due to the impressive wide range of properties and potential applications in electronic devices, energy storage, plasmonics. This tumultuous activity does not concern only graphene [12,13], but also boron nitride [14], silicene [15], germanene [16], phosphorene [17], transition metal dichalcogenides [18], antimonene [19], arsenene [20]. Our group has strongly contributed in the particular field of electronic excitations and excitonic effects [21,22] on 2D materials.

Recently, borophene, a single layer of boron atoms (predicted in 1996 [23]), has been synthesized [24,25]. Up to four different allotropic forms have been studied, both experimentally and theoretically, that show promising electronic properties (conductivity - including superconducting phase - reduction properties for catalysis, hydrogen storage capacity, etc.). Due to the specific electronic and structural configuration, as well as its plasmon features in the visible range, borophene constitutes the ideal prototype for both experimental 2D plasmonics, and realistic numerical calculations.

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