Physical Vapor Deposition of Zinc Phthalocyanine Nanostructures on Graphene Templated Substrates: A Study of Reactor Design, Growth Kinetics, and Surface Morphology
Open Access
- Author:
- Mirabito, Timothy
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- March 04, 2020
- Committee Members:
- David W Snyder, Thesis Advisor/Co-Advisor
Joan Marie Redwing, Committee Member
Suzanne E Mohney, Committee Member
John C Mauro, Program Head/Chair - Keywords:
- Physical Vapor Deposition
Graphene
Zinc Phthalocyanine
ZnPc
Nanostructures - Abstract:
- Zinc phthalocyanine (ZnPc) is an important member of the organic semiconducting phthalocyanine family, well known for its electrical properties and chemical stability. With the recent push for new solar technology and energy efficient electronics, the interest in small molecule organic semiconductors has accelerated. Like many organic materials, high electrical and optical performance is contingent upon the crystalline quality, control over the morphology, control over the orientation, and a selective growth area. Of these challenges, one is dependent on the growth conditions and three are affected by both the substrate interaction and growth conditions. Therefore, a growth process is required that produces high quality ZnPc structures with immense control over the orientation and morphology in a specific deposition area. This thesis studies the effect of physical vapor deposition (PVD) reactor design, process conditions, and graphene-based substrates on the quality, surface morphology and interfacial properties of single crystal ZnPc. A vertical PVD reactor was designed and built to take advantage of the growth kinetics of ZnPc. By scaling down the reactor geometry from traditional thermal evaporators, control over the source material temperature, thermal gradient, and substrate temperature, for a wide range of growth conditions, could be attained without the use of a turbo pump. As part of the reactor study, a new pressure-based deposition method was developed, allowing for consistent results and stable growth conditions. This combination of reactor design and process method produced flexible morphologies from thin films to nanostructures verified using scanning electron microscopy (SEM). The effects of graphene layers and substrate material on the surface morphology were also studied. Here, the number of graphene layers is shown to produce a change in structure from nanowires to island clusters. Substrate material underlying single layer graphene is also shown to produce a change in morphology between high aspect ratio nanowires on single crystal substrates, to island clusters on amorphous silicon dioxide. Raman spectroscopy and kelvin probe atomic force microscopy are implemented to characterize a novel AB stacked isolated graphene domain and the work function of each distinct graphene layer, showing the 100 meV variation between layers that may be controlling the growth morphology of ZnPc. The flexibility of the reactor and process is also demonstrated by growing both common polymorphs, α-ZnPc and β-ZnPc. The structure and controlled orientation of each polymorph is carried out using SEM, and X-ray diffraction. In each case, the introduction of graphene and a defined substrate temperature of 200 ⁰C were shown to change the orientation between a horizontal and vertical configuration while maintaining the desired polytype. Post-growth annealing of α-ZnPc is demonstrated to transition to β-ZnPc at a temperature of 300 ⁰C. Finally, an initial study of the influence of graphene defects is reported. Using a series plasma treated graphene-based substrates, the growth morphology is characterized using both SEM and Raman spectroscopy. The results show no correlation between graphene defect density and growth morphology. Additionally, Raman characterization of ZnPc growth upon single and multi-layered graphene areas are shown to cause no defects in the graphene as part of the nucleation or growth process.