Thickness- and Symmetry-Induced Electronic Transport Properties of Semimetallic Transition Metal Dichalcogenides

Abstract

This thesis presents experimental study of how thickness and symmetry would impact the electron transport properties of MoTe₂ and WTe₂, two members of the transition metal dichalcogenide family.

In the first experiment, as presented in Chapter 3, we used Raman spectroscopy and electron transport measurement to study the phase transition in semimetallic MoTe₂. It is known that bulk MoTe₂ goes through a first-order phase transition around ∼250 K. The high temperature phase is monoclinic. At low temperatures, it crystallizes in the inversion symmetry breaking orthorhombic phase. Our Raman spectroscopy measurement suggests that for flakes below certain thickness, ∼12 nm, only the inversion symmetry breaking phase exist, even up to and beyond room temperature. The corresponding electron transport measurement also shows no evidence of phase transition around ∼250 K.

Since there is a distinction between the bulk and thin MoTe₂ in terms of how it crystallizes, the band structure of thin MoTe₂ is expected to be different as well. This could be evident in the electron transport measurement. In Chapter 4, we performed thickness dependent magnetotransport measurements on MoTe₂. Our findings show both the electron and hole carrier density decreased in thin MoTe₂. Also, the magnetoresistance, which is large in bulk MoTe₂, is suppressed systematically with reduced thickness.

The third experiment follows from a recent non-linear anomalous Hall effect work on few-layer WTe₂. This effect produces second harmonic voltage and zero frequency (DC) voltage in the transverse direction when an oscillating electric field is applied in the longitudinal direction. In Chapter 5, we reproduced this effect in WTe₂, and extended the frequency to the radio frequency range.