Probing effect of disorder in the electrical and optical properties on van der Waals heterostructure

Abstract

In this thesis, we studied the transport properties of single-layer graphene and single-layer to few-layer MoS2. We also studied the optical properties of MoS2 based single-layer, twisted bilayer devices. The main observations of this thesis are summarized below: 1. We have observed that in graphene devices, the impurity number density increases with increasing temperature, irrespective of the quality of the sample. In high mobility devices, the mobility increases with decreasing temperature in contrast to low mobility devices where mobility decreased as we decrease the temperature. We found the temperature coefficient of resistance to depend strongly on the gate bias voltages. These observations have been explained using long-range and short-range scattering in the light of classical Boltzmann transport formalism. 2. We have probed the sulfur vacancy energy level (which lies about 400 meV below the conduction band minima) through conductance fluctuation spectroscopy in few-layer MoS2 devices encapsulated between hBN and HfO2. The resistance fluctuations in these devices were orders of magnitude lower than un-encapsulated devices fabricated directly on SiO2 substrates. The encapsulation also made the device extremely stable against environmental degradation. 3. We observed the defect-mediated peak in the PL emission of single-layer MoS2 samples in low temperatures, both on hBN substrate and SiO2 substrate. This defect state is present in all our samples, irrespective of the device quality indicating the intrinsic nature of the defect level. 4. We have observed the Raman mode stiffening as we decrease the temperature and below a certain temperature it saturates. On the other hand, the FWHM of the Raman modes decreases with temperature and saturates below a certain temperature. This temperature-dependent Raman shift and lifetime of the Raman modes in single-layer and natural bilayer MoS2 can be understood from three-phonon scattering process. We identified the presence of strain on hBN substrated MoS2 samples and nd that these samples have much lower impurity charge concentrations as compared to those on SiO2 substrates. 5. We have observed an enhancement in photoluminescence in our pre-annealed artificial MoS2 bilayer. When we perform thermal annealing at 300deg C in a high vacuum, the PL got quenched and the coupling between the layers improved. This evolution of inter-layer coupling resulted in the appearance of an `indirect' peak in PL emission, Raman shift in the out of plane Raman mode and also a new layer-breathing mode peak in low-frequency Raman measurement