Photophysics of Colloidal Two-Dimensional Semiconductor Nanoparticles

Brumberg, Alexandra

doi:https://doi.org/10.21985/n2-zbnt-0a82

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Photophysics of Colloidal Two-Dimensional Semiconductor Nanoparticles

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Recent progress in the field of nanomaterials has enabled significant advances in optoelectronic devices such as solar cells, light-emitting diodes, photocatalysts, and sensors. Nanoparticles feature superior optical and electronic properties that arise from quantum confinement and therefore cannot be attained used bulk materials. However, further developments in the field of nanotechnology require increased knowledge regarding how to further improve the properties of these novel materials. Because nanoparticles are quantum confined, methods of altering nanoparticles properties extend beyond just changes of material composition to also those of nanoparticle size and shape. The modification of shape is especially useful when it involves changing the number of nanoscale dimensions, as quantum confinement in one, two, or three dimensions results in drastically different optical and electronic properties. However, because the electronic behavior of one- and two-dimensional nanomaterials is not captured by either that of nanoparticles confined in all three dimensions (i.e. zero-dimensional nanomaterials), nor by that of bulk materials, experimental investigations into their photophysical behaviors are needed to gain fundamental understanding regarding how nanoparticle dimensionality affects optoelectronic behavior. This dissertation investigates the effect of nanoparticle dimensionality on the photophysical behavior of semiconductor nanoplatelets (NPLs)—colloidal, two-dimensional nanoparticles that are quantum confined in only one dimension. In each study, time-resolved spectroscopy is employed to characterize the optical or structural properties of CdSe NPLs following photoexcitation, in order to gain insight into the behavior of NPLs in conditions relevant to optoelectronic applications. In the first study, a magneto-optical experiment is used to determine the lateral spatial extent of the excited electron-hole pair (“exciton”), which dictates electronic interactions between NPLs and other materials. Although the exciton in two-dimensional materials is often assumed to be spatially extended throughout the plane of the material, we show that the exciton in CdSe NPLs is relatively spherical in shape. An example of a type of electronic interaction that is affected by exciton spatial extent is demonstrated in the study of electron transfer between pairs of nanoparticles with varied dimensionalities presented in the following chapter, which reveals that systems containing NPLs undergo charge transfer more rapidly than those containing zero-dimensional particles (“quantum dots”). Another type of electronic interaction is that of Auger recombination, an undesirable process through which excess electronic energy is transferred to a third carrier (either an electron or hole) instead of light. Here, we investigate the scaling of rates of Auger recombination with respect to NPL size, revealing that the universal volume scaling relationship present for zero- and one-dimensional nanoparticles does not carry over to two-dimensional NPLs. Furthermore, despite the common assumption that multiexciton recombination is dominated by nonradiative (Auger) recombination in quantum-confined systems, measurements of Auger lifetimes as a function of temperature reveal that multiexcitons become radiative at low temperatures in NPLs. Finally, the effect of intense photoexcitation on NPL crystal structure (and therefore optoelectronic properties) is probed using time-resolved X-ray diffraction, revealing that, following photoexcitation, the NPL lattice loses crystallinity more significantly in the out-of-plane direction than in the in-plane direction. Although this is cause for concern as the out-of-plane direction is the one that dictates optoelectronic properties, the excitation intensity required to induce disordering in NPLs is significantly higher than that observed for quantum dots, suggesting NPLs are still the more promising material for optoelectronics. Together, the studies in this dissertation delve into many of the photophysical processes that bear relevance to optoelectronic devices, showing that nanoparticle dimensionality is an important factor to consider in the design of nanotechnology.

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