Date of Award

5-2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Energy Science and Engineering

Major Professor

Kai Xiao

Committee Members

Gong Gu, David G. Mandrus, Philip D. Rack, David Geohegan

Abstract

The rise of two-dimensional (2D) materials has enabled the realization of ultra-thin electronic devices with a broad range of applications in transistors, memory devices, photodetectors, chemical sensors, and electronic displays. The optoelectronic functionality displayed by this unique material class is determined by the underlying phenomena relating to their crystal structure, quantum confinement, and heterogeneities such as defects, dopants, and atomic interface in their heterostructures.The first part of this thesis highlights the effects of heterogeneities in tuning the electronic functionalities of 2D materials. For example, the presence of sharp atomic interfaces could introduce p-n junction rectifying behavior which is the fundamental unit of electronic devices.The second part introduces a novel 2D anisotropic material, palladium diselenide (PdSe2), with a unique pentagonal, puckered structure unlike most other 2D materials with hexagonal building blocks. PdSe2 displays a strong layer-dependent optical and electronic properties. Density Functional Theory (DFT) calculations and absorption spectroscopy reveal that PdSe2 exhibit a wide-tunable indirect bandgap from ~0 eV in bulk to 1.3 eV in monolayer. Also, the anomalous layer-dependent Raman peak shifts around 5 – 9 cm-1 from bulk to monolayer for PdSe2 confirms the strong interlayer coupling in PdSe2.The third section discusses the field-effect transistor (FET) device performance of PdSe2, which shows a characteristic high carrier mobility as high as 158 cm2V-1s-1 and air stability for wide-tunable electronic applications. Also, PdSe2 devices show temperature-dependent conductivity with observed metal to insulator (MIT) transition.Finally, through plasma treatment, a new complementing metallic phase can be achieved from PdSe2 that forms a sharp atomic interface with negligible Schottky barrier heights. The phase transformation process is understood to be induced by the removal of selenium atoms. The entirely new material, Pd17Se15, with an electrically-conducting property, is used as a contact for PdSe2 devices which resulted in the reduction of the Schottky barrier present at the metal-semiconductor interface. This realization is an important step in the quest to eliminate contact resistance in 2D electronic devices.The ease of manipulating the structure of 2D materials, coupled with ample device engineering opportunities, makes 2D materials viable candidates for future nano-electronics.

Orcid ID

http://orcid.org/0000-0001-5596-5238

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