Colloidal, quasi two-dimensional nanomaterials termed nanoplatelets (NPLs) exhibit remarkable optical properties that arise from their anisotropic structure that features quantum confinement along only one dimension. Though leveraging these properties for improved devices requires oriented assemblies, very few studies have investigated routes to controlled orientation. The ability to precisely manipulate NPL orientation would also allow for meaningful studies of optical, magnetic, and electronic properties of NPLs to be implemented, the results of which would otherwise be very difficult to interpret.<br/><br/>Here, we achieve orientation of CdSe NPLs through two methods that are evaluated via transmission electron microscopy and small angle x-ray scattering. First, CdSe NPLs are embedded into polymer films and stretched biaxially to induce “face-down” orientation of NPLs in the plane of the film. Alternatively, NPLs can be oriented into either a “face-down” or “edge-up” configuration via a Langmuir-Blodgett type approach, in which a dispersion of NPLs in alkanes is dropcast onto an antisolvent of controlled polarity.<br/><br/>We apply our methodology to the preparation of oriented films of CdSe NPLs for magneto-optical characterizations focused on exciton spatial extent. Given the lack of lateral quantum confinement in a NPL, it is unclear whether the exciton is laterally delocalized, or if the strong quantum confinement induced by its thin dimension leads to a localized exciton. The spatial extent of the exciton is crucial to arriving at an understanding of NPL behavior, such as how rates of electronic processes scale with dimension. By tracking the shift in exciton energy as a function of magnetic field up to 60 T, the in-plane exciton radius can be deduced. We find that despite the common assumption that the exciton is spatially delocalized in the plane of the NPL, the exciton appears largely spheroidal. This work follows similar studies of exciton spatial extent in monolayer transition metal dichalcogenides and provides clear insight into the spatial extent of excitons in these promising, highly processible and scalable, two-dimensional materials.

Orientation of CdSe nanoplatelets for advanced magneto-optical characterization

2019 Spring National Meeting

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