Quantum capacitance and noise measurements in van der Waals heterostructures

Abstract

The discovery of graphene has revolutionized the field of condensed matter physics and opened new prospects for a wide range of other two-dimensional materials. Further with the advancement of fabrication techniques, one can reassemble atomically thin layers in van der Waals heterostructures, which shows exotic physics like Hofstadter’s butterfly, Valley Hall effect, Coulomb drag, Mott insulator, even denominator fractional quantum Hall effect and superconductivity. The transport properties in these systems are governed by the electronic band dispersion; therefore, it is desirable to directly probe the electronic density of states, which is beyond the conventional transport measurement. In this thesis, we have carried out the quantum capacitance measurements to probe the thermodynamic density of states of van-der Waals heterostructures based on two-dimensional materials. In the first part of my thesis, we have investigated the band structure renormalization in monolayer graphene in presence of tunable one-dimensional super-lattice potential at both zero and finite magnetic fields and supported by our theoretical calculations. Furthermore, we have employed the magneto- capacitance spectroscopy to study the energetics of bilayer graphene in presence of electric and magnetic fields. Our results directly capture the phase transition between the different ground states of zeroth Landau level in bilayer graphene. At higher electric fields, we also observe the collapsing of the Landau levels, which was consistent with the existing theoretical predictions. As a part of the thesis, we study the anisotropic band dispersion of Black phosphorus, a two-dimensional material, using the above technique and could also probe the localized states near the band edge. In the second part of the thesis, we have investigated the transport and optoelectronic properties of transition metal chalcogenides (MoTe2) of different phases (2H and 1T’). The low-frequency 1/f noise measurements in our dual gated devices allow us to extract out the noise contribution originating from both the channel as well as contacts. We show that the origin of noise in the MoTe2 channel is due to carrier number fluctuation. The transport measurement of 1T’ MoTe2 shows two transitions depending on the thickness of the flakes, where the transition at low temperature is due to MIT (metal-insulator) transition and the transition at higher temperature is attributed to a structural phase transition. Finally, we measure the photo-responsivity in MoTe2-graphene van der Waals heterostructures and show that the photo-responsivity at the junction is largely enhanced as compared to bare MoTe2. By measuring photo-voltaic response of the junction using the ionic liquid gating, we also estimate the work function of MoTe2