Electronic Coupling and Morphology in Organic - Two Dimensional Heterostructures

Amsterdam, Samuel

doi:https://doi.org/10.21985/n2-66kt-ad12

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Electronic Coupling and Morphology in Organic - Two Dimensional Heterostructures

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Mixed-dimensional heterojunctions between two-dimensional (2D) materials and organic semiconductors is a rapidly growing field. This is motivated by the promise of leveraging the extraordinary properties of 2D materials with the synthetic tunability and reconfigurability of organic electronics, allowing the realization of new physics or devices that are not possible in either constituent material alone. Early approaches to understanding these interfaces relied on simple band alignment considerations, approximating the organic molecule as two orbital energies in a vacuum. While this approach is useful, fully realizing the possibilities of this material combination require understanding how organic molecules couple to 2D materials, and in turn how 2D materials affect the structure of the organic thin films. This thesis presents several examples of organic / 2D heterojunctions whose unique properties cannot be explained solely by considering the constituent materials in isolation. The electronic coupling between metallophthalocyanines (MPc) and molybdenum disulfide (MoS2) shows emergent states due to mixing of the molecular orbitals with band states, and Raman enhancement in this system is found to depend on the energy of non-frontier orbitals. MPc are also found to interact with defects in the MoS2, quenching their low temperature photoluminescence (PL). Density functional theory calculations reveal this quenching to be a result of the MPc stabilizing dark, negatively charged defect states over bright, neutral ones. Finally, optical – morphological relationships are characterized in the heterojunction between pentacene and hexagonal boron nitride (hBN). hBN is found to template a new thin-film phase of pentacene which has lower energy free excitons, lower interfacial strain, and long-lived PL decay compared to the typical thin-film phase observed on SiO2. Taken together, these results show the importance of detailed characterization of organic / 2D heterostructures beyond their band alignment. While this increases complexity in the design of these systems, it also provides many opportunities. Controlling electronic coupling, defect trapping, and thin film phase provide a wide range of tools for exploring new physics and fine-tuning the properties of organic / 2D heterojunctions. Understanding and controlling these interactions are necessary to fully leverage the unique opportunities provided by such mixed-dimensional interfaces.

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