2D Materials

2D materials

We investigate the atomic and electronic structure and other interesting properties of 2D materials. These materials include not only graphene and its derivatives but also other 2D materials like silicene, germanene, stanene, metal dichalcogenides, and others.

2D Transition Metal Dichalcogenides (TMDs)

The chemical formula of 2D TMDs is MX2, where M is a transition metal and X a chalcogen (S, Se, Te). In contrast to graphene, which does not have a band gap, 2D TMDs show versatile electronic properties that vary from metallic to insulating, depending on M. Our research has focused on the electronic and plasmonic properties of these materials. In particular, we have predicted the direct to indirect bandgap crossover in strained single layer MoSe2; and we have investigated the plasmon dispersions and revealed the important role played by the interband transitions and strong local fields in these 2D materials.

Ongoing research is dealing with photo-excited states in TMDs, in particular valley- and spin-selective photoexcitation and possible excitonic signatures in time-resolved photoemission spectroscopy. Furthermore, we are exploring the possibility of light-induced Floquet states of matter in TMDs.

Elemental 2D Materials

The experimental realization of graphene and the discovery of its unique properties started the exploration of other 2D materials with properties not less intriguing compared to graphene. Below we describe our research on various 2D materials beyond graphene.

  • Silicene

Silicene is a silicon counterpart of graphene in which Si atoms are arranged in a honeycomb lattice. Unlike carbon, silicon atoms exclusively prefer sp3 hybridization. Due to this and the fact that the bond distance between silicon atoms is rather large to maintain a strong pi-bonding, silicene can not stay planar and is stabilized via slight buckling. Silicene was experimentally synthesized on Ag(111) substrate and linear bands near the Fermi level was observed in this system. We have shown that, these linear bands should not be attributed to solely silicene or Ag but to the strong hybridization between the two. Recently, we have revealed the atomic structure of √3x√3 reconstructed silicene that is frequently observed in experiments. Finally, we have shown that, these √3x√3 structures may act as a building block for a layered allotrope of silicon.

  • Germanene

Germanene is another novel 2D material akin to graphene. Recently, it has been successfully synthesized on metal surfaces. In collaboration with experimental groups, we have revealed the atomic structure of germanene overlayer on Au(111) surface with ab initio calculations.

  • Stanene

We predict from first-principles calculations a novel structure of stanene with dumbbell units (DB), and show that it is a two-dimensional topological insulator with inverted band gap which can be tuned by compressive strain. Furthermore, we propose that the boron nitride sheet and reconstructed (2×2) InSb(111) surfaces are ideal substrates for the experimental realization of DB stanene, maintaining its non-trivial topology.

Excitons in TiO2

Although Titanium Dioxide (TiO2) is one of the most investigated materials for light-energy conversion applications, the nature of its fundamental charge excitations is still unknown. We provide here a complete characterization of the localization character and energetics of charge-transfer excitons in TiO2 in both rutile and anatase phases. Our surprising finding is that even though anatase TiO2 is a 3D crystal, the lowest energy excitonic state has 2D character.

We combine state of the art static and time-resolved optical spectroscopies with many-body perturbation theory (GW + Bethe-Salpeter) to study the electronic and optical properties of TiO2. The computed optical spectra and exciton binding energies are in full agreement with angle-resolved photoemission spectroscopy and spectroscopic ellipsometry (see panel a in the figure below). Furthermore, we demonstrate that for the case of anatase, the direct optical gap is dominated by a strongly bound exciton with a two-dimensional character (panel b) [1]. Additionally, ultrafast spectroscopy measurements revealed that the validity of our results extends to colloidal nanoparticles and defective samples, used in light-energy conversion applications. Next we plan to investigate the interplay between these excitonic quasiparticles and coherent acoustic phonons in TiO2 nanoparticles. Another promising outlook is the study of exciton q dispersion to get insight into how these quasiparticles propagate inside the crystal.

TiO2

[1] Baldini, L. Chiodo, A. Dominguez, M. Palummo, S. Moser, M. Yazdi-Rizi, G. Auböck, B. P. P. Mallett, H. Berger, A. Magrez, C. Bernhard, M. Grioni, A. Rubio, and M. Chergui: Strongly bound excitons in anatase TiO2 single crystals and nanoparticles. Nature Communications Nature Communications 8, Article number: 13 (2017)