Semiconductor nanocrystals (NCs) have emerged as promising fluorophores in a plethora of applications including lighting and display technologies. Cd/Pb-chalcogenide-based NCs are by far the most studied classes of semiconductor NCs due to their exemplary luminescence properties. However, their toxicity poses a limit to their widespread application and use in biological systems, nanomedicine, as biomarkers, etc. Therefore, the search for alternatives that can replace Cd/Pb-chalcogenide-based NCs as fluorophores in various applications is a topic of rigorous research. This PhD thesis delves into the development of synthetic strategies for one such class of materials that can potentially replace Cd/Pb-chalcogenide-based NCs in various applications. I-III-VI semiconductor NCs, containing earth abundant metals which are comparatively less toxic than Cd and Pb have emerged as a suitable alternative. In this group, Cu-In-S/Se (CIS/Se) based NCs have gained significant interest due to their nontoxic nature and interesting optical properties. The principal aim of this thesis is to develop synthetic strategies to obtain morphologically vivid CIS/Se NCs and study their optical properties.
Due to the multiple reactive species present in ternary /quaternary NCs, direct method of synthesis wherein all precursors are reacted at the same time exhibit problems of inhomogeneous size, shape, and compositions, along with binary byproducts formed in addition to the desired ternary/quaternary NCs. In view of this limitation of direct method of synthesis, a cation exchange (CE) pathway of synthesis has been developed. In this approach, a binary NC is first synthesized using a conventional direct method, which then serves as a host lattice for the incoming third or fourth cation thus leading to the synthesis of ternary or quaternary multicomponent NCs. Employing this route enables the preservation of the morphology and crystal structure of the host NC after the exchange process, leading to better control over size, shape, and composition of the desired NCs. In this thesis, 0D spherical Cu-Zn-In-Se (CZISe) NCs were synthesized using a CE approach starting with binary Cu2-xSe NCs and thereafter the composition dependence of their optical properties was studied. The synthesized quaternary CZISe NCs exhibited intensive tuneable photoluminescence (PL) in the near infrared (NIR) range and narrow PL band widths in comparison to the band widths generally observed in this class of materials. Long-chain organic ligands on the surface of colloidal NCs limit carrier mobility, and hence surface modification of the NCs becomes necessary for applications where carrier mobility is an important aspect, e.g., in solar cell fabrication. Thus, surface modification of the synthesized CZISe NCs was also explored to make the NCs compatible for prospective applications of solar energy harvesting.
In addition to 0D NCs, two-dimensional (2D) NCs have gained significant interest due to their unique anisotropic optical properties. For example, extremely narrow PL band widths were exhibited for CdSe nanoplatelets (NPLs) due to the strong confinement of the NPLs in the thickness direction. These 2D NCs have also been utilized in a wide array of applications, particularly in thin film photovoltaics and optoelectronics, and therefore investigation of 2D morphologies of I-III-VI based NCs is also of utmost interest. In this thesis, 2D Cu-Zn-In-S (CZIS) NPLs were synthesized which exhibited rectangular morphology and were unstacked due to the synthetic strategy employed. CIS NPLs were synthesized using a seed-mediated approach and a subsequent CE with Zn enabled the synthesis of CZIS NPLs. Subsequently, a ZnS shell growth leading to the formation of CZIS/ZnS NPLs resulted in the enhancement of PL intensity. As compared to 2D CIS NCs the Se counterpart is less studied and very few reports of 2D CISe-based NCs are present in literature and the reported 2D CISe based NCs have not exhibited any PL. Due to the narrower band gap of CISe than CIS, it is possible to push the PL into the NIR range which unlocks new applications and therefore developing synthetic strategies for 2D CISe based NCs which exhibit PL in the NIR range was also explored in this synthesis. CISe NPLs were synthesized using a similar seed-mediated approach used for CIS NPLs, but the difference in reactivities of S and Se required significant optimization of the synthesis parameters. A subsequent CE with Zn resulted in the synthesis of CZISe NPLs with inherent PL in the NIR range with very narrow PL band widths.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:86939 |
| Date | 29 August 2023 |
| Creators | Bora, Ankita |
| Contributors | Eychmüller, Alexander, Kaskel, Stefan, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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