Probing the properties of materials with ab initio quantum-mechanical calculations

The use of so-called first-principles (or ab initio) simulations of the formation and dynamics of materials has a prevalent role in modern research for various fields of physics and chemistry. The HellasGrid infrastructure has allowed members of the Computational Condensed Matter Physics and Materials Science Group (NTUA-CCMP) of the Physics Department at the National Technical University of Athens to perform systematic and extensive ab initio studies on a number of materials that show great potential for technological applications. The calculations have utilized the state of the art approach of density-functional theory (DFT) which enables the solution of the many-body electron problem in extended systems with high accuracy.

Graphene and graphene-like two-dimensional (2D) materials stand out as prominent members of the rapidly expanding family of nano-materials. Using DFT calculations the NTUA-CCMP group has probed [1] [2] the atomic-scale mechanisms that facilitate the formation of hydrogenated graphene, also known as graphane, a wide band gap semiconductor that, in principle, could be combined with graphene in all-carbon electronics. Similar recent DFT studies [3] have examined the formation and properties of complex 2D hydrogenated silicene and germanene (the silicon and germanium analogues of graphene) and showed that these materials could have interesting applications in nano-electronics and nano-mechanical systems. Moreover, DFT calculations analyzed the structural details of mono-layer films of silicon and germanium, either on metallic substrates, [4] or as free-standing ultra-thin films [5].

tsertseris

Figure 1: Spin densities in a TiB2/MnB2 superlattice with one MnB2 layer and 5 TiB2 layers in the unit cell. Magnetizations of successive MnB2 layers are parallel and antiparallel in the top and bottom panels, respectively.

In the same spirit, extensive ongoing ab initio investigations [6],[7] have targeted the physical characteristics of complex structures of metallic di-borides, a large class of quasi-2D materials. These investigations have already identified two key features of di-boride systems, the presence of the so-called inter-layer exchange coupling in magnetic superlattices (Fig. 1) and the stability of nano-columns in TiB2 with excess of boron. Both effects could lead to important applications in, for example, magnetic recording, spintronics, and low-dimensional electronics.

Finally, DFT simulations [8],[9] have been employed to study transformations of PCBM crystals and the dependence of their electronic properties on impurities. PCBM is a fullerene chemical derivative with a bucky-ball C60 core and a functional tail. Despite the fact that PCBM molecules are renowned for their high-efficiency as electron acceptors in organic photovoltaics (OPV), the type of crystals that these molecules form remained elusive. Systematic DFT investigations [8] of continuous PCBM crystal transformations addressed this open issue and identified several locally stable types of PCBM crystals. In addition, recent ab initio studies [9] described the particulars of oxygen and water insertion in PCBM crystals and the role of these impurities as degradation agents in PCBM–based OPV systems.

Contact Details:

  • L. Tsetseris, Assistant Professor, NTUA, leont (at) mail.ntua.gr

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