Research

The study of excitons in 2D materials is one of my favourite topics. Excitons are bound states of an electron-hole pair interacting via the Coulomb potential. Excitons determine the optical properties of semiconducting materials and the functionality in optoelectronic applications like LEDs or solar cells. The physics of excitons in semiconducting 2D materials (boron nitride and single-layer MoS2) is particularly interesting and intriguing. 2D materials have a large exciton binding energy (one order of magnitude larger than in 3D materials) and the strong dependence on the number of layers. We apply the Bethe-Salpeter Equation and the GW method to obtain the exciton states of 2D materials. We have established that the exciton binding energy depends strongly on the number of layers due to the change in the dielectric screening. The image of the left shows the density of the exciton wave function of the ground state. It is constructed fixing the position of the hole on top of the molybdenum atom and writing the electron density. Nice contributionas are: the seminal paper in MoS2, the paper on interlayer excitons with long radiative lifetime in heterobilayers (E Torun, HPC Miranda, A Molina-Sánchez, L Wirtz,Physical Review B 97, 245427 (2018)), and the effect of the temperature on the electronic and optical properties of MoS2 (A. Molina-Sánchez, M. Palummo, A. Marini, and L. Wirtz, Physical Review B 93, 155435 (2016)).

The existence of magnetism in 2D materials open the opportunity of investigating fundamental questions of condensed matter physics. We are especially interested in the interplay of magnetic and optical properties (Kerr effect and magnetic dichroism), the effects of magnetization in the excitonic states, the nonlinear optical properties, and the properties of heterostructures that combine semiconductor and magnetic 2D materials. In 2019 I started to work on magnetic 2D materials, in collaboration with J. Fernández-Rossier (INL). In this paper, we have studied the magneto-optic Kerr effect (MOKE) using ab initio methods of chromium trihalides; CrI3, CrBr3 and CrCl3. We have found a dependence of the MOKE intensity with the ligand atom and the strength of the spin-orbit interaction.

In 2011 I started to investigate 2D Materials. My first work studied phonons in single-layer and bulk MoS2. Often, properties of 2D materials behave in a counter-intuitive way with respect to their bulk counterparts. That is also the case of phonons in monolayer MoS₂. In this article, we have explained the trend of the Raman active phonon modes of single-layer and multi-layer MoS₂ and WSe₂. We have used calculated the phonon modes with ab initio methods, using density functional perturbation theory. The Raman spectra behaviour is the consequence of a two-fold effect; lattice renormalization and the change in the dielectric screening with the increasing of the number of layers. Below I showed the E2g in-plane mode. The animation has been generated using the phonon website of Henrique Miranda.

In this work, we have applied ab initio methods, together with non-equilibrium Green’s function theory to perform simulations of ultrafast spectroscopy in transition metal dichalcogenides. We have simulated the carrier dynamics photo-generated by an intense and circularly polarized laser in WSe2. We reported the dynamics of the spin/valley polarization. Another work in the field of ultrafast carrier dynamics is the Intravalley Spin–Flip Relaxation Dynamics in Single-Layer WS2 (Z. Wang et al., Nano Letters 18, 6882–6891 (2018)).

Perovskites are promising active materials for solar cells. In collaboration with the experimental colleagues of UMDO at the ICMUV we study

I collaborate in this nice work of Henrique Miranda in ab initio calculations of Raman spectroscopy in 2D materials. The calculations include the excitonic effects in the Raman response. The figure shows a k-map the optical and Raman susceptibility for the main active modes at two different frequencies. This work was the core of Henrique thesis. The thesis contains detailed information of the methodology to calculate Raman scattering using first principles methods.

I had the luck of being part of the work of Fulvio Paleari in phonon-assisted absorption/emission spectra in Boron Nitride. Boron nitride has a strong luminescence in the UV range and an indirect bandgap. Spite of being a popular 2D crystal, its luminescence properties has only been partially understood. Our results explained recent luminescence experiments and reveal the exciton-phonon coupling channels responsible for the emission lines. This work was the main publication of Fulvio thesis. In the thesis you can find more details about this work.