Electrostatic control of electronic and structural orderings in two-dimensional materials

The band structure of a solid crystal is not only dependent on the potential of periodic atoms but also the electron-ion and electron-electron interactions. When dimensionality reduces to two, the latter part becomes prominent owning to greatly enhanced Coulomb interaction between quasiparticles. Meanwhile, the atomic thickness of two-dimensional materials allows full access to the interior of materials through electrostatic doping. Thus, electrostatic doping becomes one main approach to exploit the fundamental physics and practical application in the 2D limit. Among the family of two-dimensional systems, transition metal dichalcogenides (TMDs) are semiconductors with bandgaps, covering a broad spectrum. In addition, new degrees of freedom, such as topological edges and valley index, are found in TMDs and open the door to a new generation of energy-efficient optoelectronic devices carrying more information./nThis dissertation first presents optimal thermoelectric effect in TMDs, simultaneously optimizing Seebeck coefficient and electrical conductivity. Even though both of them are determined by the electronic structure of the material, large Seebeck coefficient requires a large asymmetry near Fermi level, opposite to that for electrical conductivity. We employed electrostatic doping to lift up Fermi level and balance this tradeoff. And a recorded-high power factor up to 8.5 mW m−1 K−2 at room temperature was reported here due to its large effective mass and unique density of state under strong quantum confinement. Additionally, Seebeck coefficient at various temperatures reveals the phonon-limited scattering mechanism at two-dimensional materials. If limiting the carrier density to a low level, the electrical conductivity gives a carrier hopping length around 3nm, which indicates the density of active defects inside. The demonstrated high, electronically modulated power factor in 2D TMDCs holds promise for efficient thermoelectric energy conversion. And the discoveries of carriers’ scattering and transport mechanisms are significant to understand quasiparticles’ interaction at low dimensionality./nWhen the density of quasiparticles goes beyond critical density, quasiparticles not only impact electronic structure but also have the possibility to reconstruct the configuration of lattice. For the first time, we demonstrated monolayer molybdenum ditelluride (MoTe2) reforms form hexagonal phase to monoclinic one through electrostatic doping. A hysteretic behavior has been observed, which is a feature of structural changes as well as the existence of kinetic barrier. Microscopically, the crystal orientation of the electrostatic induced monoclinic phase is found to be consistent with the original crystal orientation of hexagonal phase. It leads to the reversibility of the structural phase transition in terms of both phases and orientation. This discovery opens new possibilities for developing phase-change memory based on atomically thin membranes. The carrier density (metallicity) is also shown to greatly affect the photoluminescence efficiency by controlling the pathway of exciton recombination and hence improves the performance of optoelectronics.

Advisor: Zhang, Xiang