Chemical Functionalization of Two-Dimensional Nanomaterials

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The limits of miniaturization of electronic device components and the steady need for faster computation power have motivated the discovery and cultivation of low-dimensional materials. Among these, two-dimensional (2D) materials have exhibited a wide range of superlative optoelectronic, thermal and mechanical properties. The interest in 2D materials took-off with the isolation of Graphene in 2004. However, despite graphene’s initial promise for use in electronic devices, it is a zero band gap semimetal, and therefore its utility in semiconductor applications is limited. Other 2D materials such as transition metal dichalcogenides (TMDC) offer promising alternatives to graphene due to their inherent band gap. Furthermore, by stacking distinct 2D materials alone or in combination with other low dimensional materials to form heterostructures, new electronics with unique functionalities can be realized. This dissertation presents multiple examples of heterostructures based on 2D materials. Van der Waals (vdW) interactions, which dominate the interlayer bonding of 2D layered materials, are exploited for the non-covalent functionalization of graphene and TMDC. First, vdW heterostructures were grown and characterized. In particular, molybdenum disulfide (MoS2) was grown on epitaxial graphene (EG), resulting in rotationally commensurate MoS2 atop EG. The nature of the vdW interactions in this system mitigates the large lattice mismatch of the two materials, resulting in a nearly strain-free MoS2 with reduced defect density, sharp interfaces, and consistent material quality. While EG can effectively template MoS2 growth, the electronic interlayer coupling between the two hinders the intrinsic properties of MoS2 (e.g., photoluminescence/electrical transport). Thus, in the next stage, a computationally driven transfer method was developed to selectively transfer isolated MoS2 crystals from EG. The transferred MoS2 exhibits good electrical performance as measured in a field-effect transistor and enhanced photoluminescence intensity. Building off of these results, we extend the vdW heterostructure concept beyond 2D layered materials and demonstrate mixed-dimensional heterostructures comprised of organic and 2D materials. A monolayer of organic molecules was self-assembled on graphene. The molecules assemble into domains consisting of one-dimensional molecular stripes. These domains are further arranged into a 6-fold symmetric network consistent with the symmetry of the underlying graphene lattice. The interaction between the organic molecules and the graphene creates a unique hierarchical self-assembly that can be used for templating metallic nanostripes pattern. This work highlights the importance of vdW bonding not only in 2D materials but also in materials of other dimensions. The vdW interactions that lead to the heterostructures presented here will inform future efforts for large-scale growth, transfer, and templating of layered materials which in turn will allow for their incorporation in electronic and optoelectronic device applications

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  • 01/30/2019
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