Magnetic Interactions, Polarity Effects and Phase Transitions in Low Dimensional Covalent Systems: transition metal dichalcogenides nanoribbons

When: Wednesday, 5th November (2014), 12:00h
Place: Departamento de Física Teórica de la Materia Condensada, Facultad Ciencias, Module 5, Seminar Room (5th Floor).
Speaker: Ana María Llois, Condensed Matter Department, GAIyAN-Comisión Nacional de Energía Atómica, Buenos Aires. Department of Physics JJ Giambiagi, FCEyN, Univ. of Buenos Aires.

In the year 2004, Novoselov et al successfully isolated not only graphene but also layers of some transition metal dichalcogenides (TMDs). TMDs are well known for their laminar structure. They consist, in the bulk, of three atoms thick monolayers (trilayers) held together through weak Van der Waals interactions. Each monolayer consists of a plane of transition metal atoms sandwiched by two planes of chalcogen ones. There is a strong molecular bonding between metal and chalcogen atoms within the monolayers, which present an ionic as well as covalent character. Many TMDs have been thoroughly studied in the last 30 years due to their many potential applications. In this talk I am going to address interesting effects switched on by low dimensionality and edge effects in both, semiconducting as well as metallic TMD’s nanoribbons. Starting with semiconducting TMD’s nanostructures we focus in particular on MoS2 and WS2, which have gained an increasing importance in a number of recent technological applications. Relying on first principles simulations, we predict a metal-to- semiconductor transition for zigzag ribbons of small witdh and monolayer thickness due to the structural and electronic flexibility associated to polarity effects. This finding opens the possibility for controlling the ribbon type during synthesis, in compounds of major technological importance. We finish this exposition focusing on NbS2 , a metallic dichalcogenide whose monolayers belong to the 1H polytype. Bulk NbS2 is non magnetic. It presents neither charge nor spin density waves, at variance with other related dichalcogenides, such as bulk NbSe2, which develops charge density waves below 40 K, due to 2D Fermi surface instability (nesting). We have shown that zigzag nanoribbons cut out from the 1H NbS2 monolayers present a magnetic ground state triggered by the zigzag edges and that this ground state is actually a spin density wave. In this presentation, the origin and features of this ground state are going to be traced back, analysed and compared with the cases of NbSe2, TaS2 and the semiconducting MoS2.