Electronic, Vibrational and Thermoelectric Properties of Two-Dimensional Materials
- Author(s): Wickramaratne, Darshana
- Advisor(s): Lake, Roger K
- et al.
The discovery of graphene's unique electronic and thermal properties has moti-
vated the search for new two-dimensional materials. Examples of these materials
include the layered two-dimensional transition metal dichalcogenides (TMDC) and
metal mono-chalcogenides. The properties of the TMDCs (eg. MoS2, WS2, TaS2,
TaSe2) and the metal mono-chalcogenides (eg. GaSe, InSe, SnS) are diverse - ranging from semiconducting, semi-metallic and metallic. Many of these materials exhibit strongly correlated phenomena and exotic collective states such as exciton condensates, charge density waves, Lifshitz transitions and superconductivity. These properties change as the film thickness is reduced down to a few monolayers.
We use first-principles simulations to discuss changes in the electronic and the
vibrational properties of these materials as the film thickness evolves from a single atomic monolayer to the bulk limit. In the semiconducting TMDCs (MoS2, MoSe2,WS2 and WSe2) and monochalcogenides (GaS, GaSe, InS and InSe) we show confining these materials to their monolayer limit introduces large band degeneracies or non-parabolic features in the electronic structure. These changes in the electronic structure results in increases in the density of states and the number of conducting modes. Our first-principles simulations combined with a Landauer approach show these changes can lead to large enhancements up to an order of magnitude in the thermoelectric performance of these materials when compared to their bulk structure. Few monolayers of the TMDCs can be misoriented with respect to each other due to the weak van-der-Waals (vdW) force at the interface of two monolayers. Misorientation of the bilayer semiconducting TMDCs increases the interlayer van-der-Waals gap distance, reduces the interlayer coupling and leads to an increase in the magnitude
of the indirect bandgap by up to 100 meV compared to the registered bilayer.
In the semi-metallic and metallic TMDC compounds (TiSe2, TaS2, TaSe2) a phase
transition to a charge density wave (CDW) ground state occurs at a temperature that is unique to each material. Confining these materials to a single monolayer or few-monolayers can increase or decrease their CDW transition temperature and change the magnitude of the CDW energy gap. We show the low energy Raman modes observed in 1T-TaSe2 and 1T-TaS2 in their CDW ground state can emerge from zone folded phonons due to the reconstruction of the lattice in the bulk and monolayer structures. In 1T-TiSe2 the driving mechanism of the CDW is excitonic condensation.We show the excitonic gap of the monolayer and bilayer structures can increase by up to a factor of 3 compared to the excitonic gap of the bulk structure.