Date on Master's Thesis/Doctoral Dissertation

5-2023

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemical Engineering

Degree Program

Chemical Engineering, PhD

Committee Chair

Jasinski, Jacek

Committee Co-Chair (if applicable)

Gupta, Gautam

Committee Member

Gupta, Gautam

Committee Member

Sumanesekara, Gamini

Committee Member

Sathitsuksanoh, Noppadon

Committee Member

Fu, Xiao-An

Author's Keywords

Phosphorene; intercalation; nanoribbons; degradation; arsenic-phosphorous alloys

Abstract

Two-dimensional (2D) layered materials have sparked the interest of researchers because of their interesting properties drawn from quantum confinement effects, elevated density of states (DOS), increased surface area, increased active sites, etc. Phosphorene, an interesting addition to the 2D material family in 2014, has interesting properties such as in-plane anisotropy, elevated carrier mobility and thickness-dependent bandgaps that have made it a material of intense interest. Furthermore, there have been theoretical studies predicting that phosphorene could be made even more versatile by intercalating alkali atoms into its layers. Intercalation has proven to be a veritable tool for synthesis, modification, transformation, and phase transitions in 2D layered materials. This dissertation focuses on the experimental the mechanism behind cesium and sodium intercalation of phosphorene through spectroscopic studies and electrical transport measurements. Additionally, alloying phosphorene with grey arsenic was achieved and investigated as a solution to the stability problems of phosphorene that has prevented its large-scale deployment in devices and commercialization.Phosphorene degrades within hours when exposed to ambient conditions. A photo-assisted oxidation process occurs which yields phosphorous oxide species ( that etches away its surface; ultimately degrading it). The first project of this dissertation was focused on studying the effects of cesium (Cs) vapors on black phosphorene (BP) flakes at varied times and at a temperature gradient of 150oC. The x-ray diffraction (XRD) measurements of these samples suggest a reduction in the strength of the van der Waals interactions between BP layers leading to the loss of coherence of out-of-plane peaks. At the same time, the three main Raman modes of BP (, , and ) steadily redshifted as exposure times were increased, with modes and shifting faster than . After initial rapid downshifts of active BP phonon modes, this intercalation strategy showed its limits following prolonged exposure times. Saturation of BP flakes by Cs vapors ensued and the kinetics was fitted with an exponential decay function. Furthermore, the thermoelectric power (TEP) of cesiated BP exhibited an inversion in sign from positive to negative around 400 K, lending credence to the transformation of as-prepared BP which is a p-type semiconductor to an n-type equivalent due to Cs atom intercalation driven shifting of the Fermi level toward the conduction band of BP and the donation of electrons from Cs. The next project was on the electrochemical intercalation of sodium (Na) into BP flakes which eventually produced phosphorene nanoribbons (PNRs). PNRs have inspired strong research interests to explore their exciting properties that associate with the unique 2D structure of phosphorene as well as the additional quantum confinement of the nanoribbon morphology, providing new materials strategy for electronic and optoelectronic applications. Despite several attempts at their fabrication, the production of PNRs with narrow widths has been a great challenge. The study conducted in this project lead to the development of a facile and straightforward approach to synthesize PNRs via electrochemical process. The anisotropic Na+ diffusion barrier in black phosphorus (BP) along the [001] zigzag direction against the [100] armchair direction proved to be beneficial in achieving this feat. The produced PNRs displayed widths of good uniformity (10.3 3.8 nm) observed by high resolution transmission electronic microscopy (HR-TEM), and the suppressed B2g vibrational mode from Raman spectroscopy results. More interesting, when used in field-effect transistors (FETs), the synthesized bundles of PNRs exhibit the n-type behavior, which is dramatically different with from bulk BP flakes which are p-type. This work provides insights into a new synthesis approach of PNRs with confined width, paving the way towards to the development of phosphorene and other highly anisotropic nanoribbon materials for high quality electronic applications. Finally, the effects of oxygen and humidity on black phosphorous (BP) and black arsenic phosphorous () flakes using Raman spectroscopy and in-situ electric transport measurements (four-probe resistance and thermoelectric power, TEP) was investigated. The results show that the incorporation of arsenic into the lattice of BP renders it more stable, with the degradation times for BP, , and being 4, 5, and 11 days, respectively. The P-P Raman peak intensities were found to decrease with exposure to oxygen and moisture. The TEP measurements confirmed that both BP and are p-type semiconductors with the TEP of stabilizing more slowly than that of BP. In addition, the four-probe resistance of BP and stabilized significantly faster when exposed to air after being degassed in a vacuum. This was attributed to the charge transfer between the oxygen redox potential of air and the Fermi energy (EF) of the semiconductors.

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