Understanding And Engineering Electronic And Optoelectronic Properties Of 2d Materials And Their Interfaces PDF Download

In the pursuit of further miniaturization beyond Moore's law, tremendous effort has been dedicated to exploring the potential of two-dimensional (2D) materials for nanoscale electronic devices. 2D materials are a group of solid state materials that possess strong in-plane covalent bonds while individual atomic layers are held together by weak van der Waals (vdW) interactions. Hence, their bulk crystals can be exfoliated into few-layer or even atomically thin single-layers via micro-mechanical exfoliation techniques. These materials possess unique and exotic properties due to quantum confinement of importance to future electronics. However, many technical problems need to be solved to realize this goal. For example, as 2D material based devices become smaller down to the nanometer scale, the electrical contacts must also be reduced in scale which creates different characteristics from those of macroscopic counterparts. In addition, there are issues of reliability and stability with devices comprised of such materials. There is a need to understand the electronic and chemical properties of several interfaces that arise in such materials: metal-2D and 2D-2D junctions, for example. To this end, this thesis focuses on understanding nanoscale metal-2D semiconductor (SC) and 2D SC-2D SC junctions exploring: (1) electronic and optoelectronic behavior at the nanoscale junction of metal-MoS2 and dependence on the layer number (thickness), (2) realization of voltage selectable photodiodes based on a lateral in-plane MoS2-WSe2 heterojunctions, and (3) interfacial properties and (opto)electronic characteristics of a phosphorene-MoS2 vertical vdW p-n junction. The first part of this thesis explores the layer number dependent electrical characteristics of the MoS2-metal nanoscale junction using current imaging of MoS2 nanosheets consisting of regions of varying different thicknesses using conductive and photoconductive spectral atomic force microscopy (C- and PCS-AFM). The layer number dependence of the effective barrier was measured, by obtaining consecutive current images while changing bias voltages, showing it to be linear. At the same time, spatially resolved two-dimensional (2D) maps of local electrical properties are generated from simultaneously recorded local current-voltage (IV) data. Furthermore, the layer number dependent spectral photoresponse of MoS2 is investigated, which shows the highest response in single layer (1L) region. The photoresponse decreases for increasing layer number, but increases again between 4L and 1 OL due to increased light absorption. The photoresponse is also strongly dependent on the wavelength of the incident light, showing much higher currents for photon energies that are above the optical bandgap. The photoresponse in forward and reverse biases shows barrier symmetry for 1 L but asymmetry for 2, 3, and 4L, which further indicates a dominant role of the barrier on carrier transport at the junction. The second part of this thesis investigates the spatially resolved transverse electrical properties of the monolayer WSe2 -MoS2 lateral p-n heterostructures at their nanoscale junctions with metals both in the dark and under laser illumination. As in the first part of the thesis, C- and PCS-AFM, versatile tools to conveniently and efficiently interrogate layer-dependent electronic and optoelectronic characteristics in a MoS2 crystal containing regions of different thicknesses, which enables direct characterization and comparison of the different layer regions without the complexities associated with fabricating and testing of different individual field-effect transistor devices, are used for measurements. By performing current imaging using a PtIr-coated conductive tip on an ultrathin nanosheet that includes homogeneous crystals of WSe2 and MoS2 and a lateral junction region in between, many thousands of WSe2/MoS2/the junction-metal contact points form during imaging and directly compare their local properties at the same time under identical experimental conditions with the nanoscale spatial resolution. The third part of this thesis explores a new type of 2D vertical heterostructures that simultaneously possess desirable properties of constituent materials, paving the path for overcoming intrinsic shortcomings of each component material to be used as an active material in nanoelectronic devices. As a first example, a MoS 2-graphene vertical heterostructure is constructed and its charge transfer and photoluminescence (PL) at the interface are investigated. C-AFM and Raman spectroscopy show that there is a significant charge transfer between the two component materials. The PL intensity of monolayer MoS2 is noticeably quenched when in contact with a single layered graphene in comparison to that of a bare monolayer MoS2 crystal. Then, with the acquired understanding of the underlying physics at the 2D vdW heterointerfaces, the possibility of a black phosphorus (BP)-MoS2 vertical heterostructure as an ultrathin channel material for high-performance 2D (opto)electronic devices is studied. CVD-synthesized MoS2 and micromechanically exfoliated BP crystals are stacked together to form a vertical p-n heterostructure. Optical microscopy, AFM images, and Raman spectroscopy data show that the MoS2 thin films can be used as a passivation layer, protecting BP from deteriorating in ambient conditions for extended period of time or under an elevated temperature in an Ar environment. The IV characteristics of FET devices based on the vertical heterostuctures exhibit that the MoS2 layer has limited impact on superior carrier transport properties of the BP in the dark. Upon light illumination, photoconductivity of the BP-MoS2 heterostructure region increased compared to that of the bare BP region of the same flake, mainly due to the fact that a built-in electric field formed at the BP-MoS2 interface facilitates the dissociation of electron-hole pairs generated by light absorption.