Abstract

Inch-Scale High Throughput Metrology of Graphene

and Patterned Graphene Oxide

by

Dennis Pleskot

Master of Science, Graduate Program in Materials Science

and Engineering

University of California, Riverside, June 2013

Dr. Cengiz Ozkan, Chairperson

In order to fully utilize the unique properties of graphene, large-area sheets of the material must be produced. As the demand for large, continuous sheets of graphene increases, the need to quickly and effectively characterize such a material correspondingly increases. In tandem, the use of patterned graphene oxide in practical applications has also expanded at a rapid pace, leading to a greater need to characterize this material as well. In this study, fluorescence quenching microscopy was examined as a means of analyzing 4 in^{2 sheets of graphene and patterned graphene oxide in a fast and efficient manner. It was determined that fluorescence microscopy offers a number of advantages in imaging these materials over conventional techniques such as Raman spectroscopy, SEM, and TEM. Fluorescence microscopy proved to be a highly scalable technique that is able to image a graphitic material in substantially less time than would be possible in these other methods while still maintaining adequate resolution. Unlike SEM and TEM, fluorescence microscopy is also a non-destructive characterization technique, allowing the samples that were actually characterized to be used in practical applications. Additionally, a clear contrast between graphene oxide and pristine graphene could be observed. Analysis of the fluorescence contrast histogram shows that graphene oxide displays a unique intensity peak and can thus not only be distinguished from pristine graphene but also defects in the sample, the presence of multiple layers, and changes in the uniformity of the graphene surface. These observations and results demonstrate the expanding usefulness of fluorescence microscopy as a high-throughput characterization technique.}

ABSTRACT OF THE DISSERTATION

Strongly Interacting Electrons and Holes in Ultrathin MoTe2 Heterostructures

by

Dennis Pleskot

Doctor of Philosophy, Graduate Program in Materials Science and Engineering

University of California, Riverside, March 2019

Dr. Nathaniel Gabor, Chairperson

Heterostructures composed of atomic layer materials (ALMs) bonded through van der Waals (vdW) interactions have demonstrated great potential for use in next generation optoelectronic devices. However, the role of strong charge carrier interactions in the interlayer photocurrent in such devices is not fully understood. One such ALM, molybdenum ditelluride (MoTe2), has a band gap energy near 1.0 eV, suggesting that MoTe2 is a promising material for ultrasensitive infrared optical photodetectors and next-generation solar photocells. Like other ALMs, the photoresponse in MoTe2 should give experimental access to strong many-body phenomena. Here, we report on the advanced fabrication and dynamic photoresponse of graphene-MoTe2-graphene vdW heterostructure photocells. We find that the power dependence of the interlayer photocurrent is well described by a single power law at low power, where the power law exponent parameterizes the nonlinearity of the photoresponse. We develop a detailed 2-particle Auger recombination model that accounts for careful time integration of the dynamics, resulting in an analytic solution that reproduces the non-linear power dependence. Additionally, we observed sharp suppression of conventional power law behavior above a critical power threshold, accompanied with a critical phase transition in the spatial expansion of the suppressed photocurrent region. We attribute this transition to the emergence of an electron-hole liquid phase that is remarkably stable at room temperature and moderate operating conditions. Additional measurements of the dynamic photoresponse support the existence of this phase, allowing us to fully explore the evolution of electron-hole pairs with increasing density in MoTe2.