PhD Defense by Neha Kondekar
THE SCHOOL OF MATERIALS SCIENCE AND ENGINEERING
GEORGIA INSTITUTE OF TECHNOLOGY
Under the provisions of the regulations for the degree
DOCTOR OF PHILOSOPHY
on Tuesday, July 14, 2020
BlueJeans Video Conferencing
will be held the
"Investigation of Phase Transformations in Transition Metal Dichalcogenides During Synthesis and Post Growth”
Prof. Matthew McDowell, Advisor, ME/MSE
Prof. Eric Vogel, MSE
Prof. Faisal Alamgir, MSE
Prof. Mark Losego, MSE
Prof. Satish Kumar, ME
Transition metal dichalcogenides (TMDCs) are promising candidates for use in beyond-Moore electronics, photovoltaics, sensing, and energy storage devices. The highly anisotropic crystal structure of TMDCs is characterized by chemically active edge sites and relatively inert basal plane layers, which are bonded to each other by weak van der Waals forces. To tailor TMDCs’ electronic, chemical and optical properties for different applications, careful control of their crystallographic orientation, composition, and layer thickness is required. While the commercialization of electronic devices requires uniform TMDC growth over large areas, enhancement of catalytic activity necessitates that as many edge sites as possible are exposed to the surface. To ensure optimal operation of TMDC-based transistors, engineering the interface with different contact metals (including at elevated temperatures and in the presence of surface oxides) is important to reduce the resulting contact resistance across the interface. Furthermore, the presence of other metals can influence the final morphology of TMDC crystallites during their synthesis and alter their resulting properties. In all these cases, to obtain optimal optoelectronic and catalytic properties, understanding the role of metal/TMDC interfaces in controlling the properties and performance of these materials is crucial.
The first part of this dissertation discusses experiments in which MoS2 crystals with different crystallographic surface orientations are interfaced with other materials (Li, Ge or Ag). It then proceeds to explore – using data obtained from in situ X-ray photoelectron spectroscopy (XPS) – the interfacial phase transformations that occur during interface formation. Reactions across horizontally aligned MoS2 layers are compared with vertically aligned MoS2 layers (i.e., predominantly edge sites exposed). An important finding is that the edge sites feature a native surface oxide that prevents the reaction between Ge and MoS2, but the same oxide is not sufficient to prevent the reactive Li species from reducing MoS2 to Mo metal. Ag shows very little reaction with either of the samples. The material with exposed basal planes features surface defects (vacancies, steps, ledges etc.) that act as sites for the onset of interfacial transformations, which occur readily with Ge and Li. Using the Ni-MoS2 system, further investigations were performed on the influence of elevated temperatures on interfacial reactions. Overall, the results show that MoS2 orientation affects the extent of interfacial reaction in the case of some contact materials, but not in others; furthermore, the varied chemistry of different MoS2 surfaces likely contributes to these differences.
Next, the influence of Ni on the growth of crystalline MoS2 is investigated using in situ transmission electron microscopy (TEM). An amorphous MoS2 precursor is annealed on different substrates – including on Ni films of varying thicknesses – to nucleate and grow polycrystalline MoS2. The results indicate that Ni assists lateral growth of MoS2 crystals, as they end up being approximately 10 times larger than those without Ni. Other transition metals (Fe, Cu and Co) were also examined, but only Ni caused a significant change in the MoS2 crystallite size. Based on in situ TEM observations and other ex situ results, it is postulated that this result is due to Ni atoms or NiSx clusters increasing the mobility of Mo and S atoms at the MoS2 edge planes, which enables faster edge growth rates.
In the third section, knowledge gained from the fundamental in situ studies of the Ni-MoS2 system is used to selectively pattern MoS2 growth during chemical vapor deposition (CVD) onto Si substrates featuring 300 nm SiO2 on their surface. Growth of MoS2 on thin (~2 nm) patterned Ni films but not on the surrounding SiO2/Si substrate was observed. Au thin films were also used to seed growth of MoS2 crystals, but once nucleated, MoS2 crystals preferred to grow over the SiO2/Si substrate. Utilizing the different interactions of MoS2 with these different materials during growth, crystalline MoS2 was selectively grown over Ni regions which in turn were contacted by Au regions. This configuration was utilized to demonstrate fabrication of transistor structures. Further studies are needed to explore the effects of Ni dopants on the electronic properties of patterned devices.
Overall, this dissertation elucidates the dynamic structural and phase evolution of MoS2 crystals both during synthesis, as well as upon interfacing the as-grown crystallites with different materials. The combination of in situ and ex situ measurements provide important observations of the dynamic phase transformations during and after growth. These results help to fill a knowledge gap that exists regarding the engineering of TMDC materials: prior work has provided understanding of the atomic-scale structure of active TMDCs for catalysis and optoelectronic applications, but the best pathways to synthesize and engineer these materials are not always clear. These findings provide a basis for future engineering of TMDC crystal size and morphology via addition of different transition metals as well as stabilization of phases across TMDC heterointerfaces. This phase engineering could be important for a range of applications, including catalysis and next-generation electronic and optical devices.