Graduation Year

2020

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Physics

Major Professor

Humberto R Gutierrez, Ph.D.

Committee Member

Lilia Woods, Ph.D.

Committee Member

Matthias Batzill, Ph.D.

Committee Member

Sarath Witanachchi, Ph.D.

Committee Member

Venkat Bhethanabotla, Ph.D.

Keywords

Alloys, Group-III Monochalcogenides, Heterostructures, Photoluminescence, Raman Spectroscopy, Transition Metal Dichalcogenides

Abstract

Atomically thin two-dimensional (2D) materials have attracted a growing interest in the lastdecade from the fundamental point of view as well as their potential applications in functional devices. Due to their high surface-to-volume ratio, the physical properties of 2D materials are very sensitive to the environmental factor such as surrounding media and illumination conditions (e.g. light-mater interaction). In the first part of this dissertation I will present recent advances in developing laser-assisted methods to tune the physical properties of 2D transition metal dichalcogenides (TMDs). We demonstrate laser-assisted chemical modification ultrathin TMDs, locally replacing selenium by sulfur atoms. The photo-conversion process takes place in a controlled reactive gas environment and the heterogeneous reaction rates are monitored via in situ real-time Raman and photoluminescence spectroscopies. The spatially localized photo-conversion resulted in a heterogeneous TMD structure, with chemically distinct domains, where the initial high crystalline quality of the film is not affected during the process. This has been further confirmed via transmission electron microscopy as well as Raman and Photoluminescence spatial maps. Additionally, we also applied this method for after-growth local electronic doping, where small amounts of chalcogen atoms are replaced by nitrogen increasing the hole concentration and hence the p-type doping. Our study demonstrates the potential of laser-assisted chemical reaction for on-demand synthesis of heterogeneous two-dimensional materials as well as the on-demand production of p-n homojunctions, with applications in nanodevices.

Later in the second part, I will also present studies of stability of 2D group-III monochalcogenides and the combined effect of environment and illumination conditions. Group-III monochacogenides such as GaSe and GaS have attracted considerable interest as two-dimensional (2D) alternatives to the traditional transition metal dichalcogenides. The production of large area films as well as the long-term ambient stability remain a challenge for scalable integration of these materials into the next generation of 2D circuitry and optoelectronic devices. In this part of the dissertation, I will present a simple atmosphericpressure Chemical Vapor Deposition method to synthesize continuous monolayers of GaSe & GaS. Additionally, we study the time-dependent ambient stability of bare and encapsulated monolayer samples by Raman spectroscopy using a laser-scanning method that minimizes the cumulative laser damage and allows a reasonable signal-to-noise ratio. This is the first systematic stability study in bare monolayers of GaSe. Our results reveal that bare GaSe monolayers can stand up to six hours in air before complete degradation, while encapsulation with transparent polymeric films can help to delay this process

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