Novel Transport Characterizations in Layered Two-Dimensional Materials and Bulk Chalcogenides

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The isolation of graphene by the simple Scotch-tape method led to the discovery of many novel two-dimensional (2D) materials. Since then, many new material growth and characterization techniques were proposed and many new phenomena were observed in layered 2D materials. Black phosphorous (BP) and topological insulator(TI) \ce{Bi2Se3} are examples of them. Besides the layered TIs and semiconducting transition metal dichalcogenides, many other bulk chalcogenides are of particular interest like the misfit layer chalcogenides (MLCs), phase change chalcogenides (PCCs), and many more. In this research, novel electrical transport techniques will be discussed to study the properties of 2D BP, \ce{Bi2Se3}, and two bulk chalcogenides: the MLC Bi-Cr-Se and the PCC \ce{K2Sb8Se13}. The central focus of this research is the semiconducting 2D black phosphorous which has moderate tunable band-gap and high mobility. It is also a 2D material with in-plane anisotropy caused by the puckered planar structure along the armchair direction. Previous study employed both optical techniques and a multiple-step etch fabrication process to quantify the anisotropic electrical tensor of such 2D materials. Here, the crystal orientation of an exfoliated BP flake is determined by purely electrical means. A sequence of three resistance measurements on an arbitrarily shaped flake with five contacts determines the three independent components of the anisotropic in-plane resistivity tensor, thereby revealing the crystal axes. The resistivity anisotropy ratio decreases linearly with increasing temperature $T$ and carrier-density reaching a maximum ratio of 3.0 at low temperatures and densities, while mobility indicates impurity scattering at low $T$ and acoustic phonon scattering at high $T$. Then the disorder-induced switching hysteresis common to 2D materials has been quantitatively addressed in this research using BP as an example. The widely observed hysteresis in the gate-dependent conductivity of many 2D materials was primarily attributed to the substrate and/or the surface adsorbates which has not been well quantified. By measuring the switching transient conductivity of BP field-effect transistor devices with step-like gate-bias change, the hysteresis-related heavy-tail transients in both pristine and highly disordered BP were modeled to within the dispersive diffusion frame. Along with the proposed continuous time random walk model, the characteristic energy of the trap tail which causes the slow relaxation and therefore the hysteresis, can be extracted from the temperature dependent behaviors. Due to the sensitivity of 2D materials to their physical and chemical environment, these materials function well as surface sensors and field effect transistor channels. Two of the most important parameters to characterize eventual device applications are the density and mobility in the presence of an interface disorder potential. A disorder-scaling method is introduced here to analyze gate-dependent conductivity as a function of changing disorder potential strength. It can extract relative changes in carrier density and mobility while the disorder potential is increased and decreased in strength revealing the underlying doping mechanism. This method was demonstrated with support from accurate Hall measurements on multilayer BP as well as its application on monolayer \ce{Bi2Se3} to study the surface adsorption/desorption process. Aside from 2D chalcogenides, our interests have extended to 3D chalcogenides of more complex structures. One is the semiconducting MLCs Bi-Cr-Se. The electrical properties of both bulk and thin flake devices were measured. A non-Gaussian resistance noise was observed in the exfoliated device indicating a possible way to characterize 2D magnetic dynamics with quasi-1D transport properties. A newly identified three-phase chalcogenide compound \ce{K2Sb8Se13} was also investigated. The electrical properties of all three phases and their respective transitions were measured which is essential for potential novel applications in electronics.

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  • 02/25/2019
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