Master of Science
Light harvesting can be referred to the use of an ensemble of different nanoparticles, or quantum dots, or other absorbers to optimize the ability to capture a given spectrum of electromagnetic radiation (for example the solar spectrum under specific atmospheric conditions) in a light-absorbing system. To this end, different nanoparticles play complementary functions within the system and absorb or scatter light at different wavelength intervals. Light harvesting finds applications in fields as diverse as solar cells, photosynthesis and photocatalysis. Graphene supporting a set of different semiconducting nanoparticles has often been proposed as light harvesters. To further this concept, my thesis has explored the effect of few-layer graphene nanosheets on the optical properties of colloidal molybdenum disulfide quantum dots (MoS2-QDs) and investigated the synthesis of tin/tin oxide nanoparticles (SnOx-NPs) with a process compatible with their integration on graphene. Using synchrotron X-ray absorption near-edge spectroscopy, we find that the density of unoccupied electronic-states in MoS2-QDs is profoundly affected by their Van der Waals interaction with few-layer graphene nanosheets (principally affecting the S K-edge) as well as quantum confinement (mainly shifting the Mo L3 edge). As far as tin-based nanoparticles are concerned, our revisitation, via X-ray photoelectron spectroscopy (XPS), of a popular synthesis approach based on stannous acetate reduction, shows that this method is normally not yielding to metallic Sn-NPs, but tin monoxide, even in oxygen-free atmospheres. Only the subsequent disproportionation of SnO may perhaps lead to SnO2 and metallic tin because of heat released from the reduction process. As SnO-NPs are an excellent optical absorber in the visible-near infrared photon energy range, they will be an excellent complement to MoS2-QDs on few-layer graphene nanosheets, in a novel graphene-based light-harvesting system.
Summary for Lay Audience
Light-harvesting is one of the most important aspects of technological development. It finds applications in the fields of solar cells, photosynthesis, photocatalysis, and more. The application of nanoparticles and quantum dots for light harvesting has attracted a lot of interest due to their unique physical properties. Different nanoparticles can absorb or scatter light at different wavelengths, therefore using a set of various types of nanoparticles with would allow capturing a required light spectrum for the light-harvesting. Graphene is a 2-dimensional form of graphite but has different optical and electrical properties such as high conductivity. This material with supporting semiconductor nanoparticles on its surface has often been proposed as a promising light harvester. To further this concept, this thesis explored the effect of graphene with several layers on the optical properties of colloidal molybdenum disulfide nanoparticles (MoS2-QDs) and studied the way of synthesis of tin/tin oxide (SnOx) nanoparticles, which would allow their simple integration onto the graphene surface. By studying chemical composition and electronic states with the help of high-energy X-rays we have observed that weak interaction forces, which are called van der Waals interactions, are affecting sulfur species in MoS2-QDs, while functionalization of quantum dots has mostly affected Mo species. As for the tin-based nanoparticles, after synthesis, they have been extensively studied with the help of x-ray and the photoelectric effect, which they produce, when interacting with the material. Based on this analysis it has been concluded that the popular method of reducing tin containing material to achieve metallic tin nanoparticles resulted in the creation of tin monoxide even in the oxygen-free atmosphere. As a result, we can say that only transformation of SnO into the SnO2 and Sn nanoparticles is happening due to the heat released during the reduction reaction of initial tin containing material. Overall, the combination of SnO nanoparticles with MoS2-QDs on few-layer graphene nanosheets could lead to a novel graphene-based harvesting system, as these nanoparticles would complement each other and absorb different parts of the spectrum.
Vidish, Denys, "Functional nanoparticles: tin monoxide and molybdenum disulfide quantum dots on graphene nanosheets" (2021). Electronic Thesis and Dissertation Repository. 8171.
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