Multinary Chalcogenide Nanocrystals: Synthesis, Characterisation and Electronic Structure Modelling
Embargoed until 2024-06-30
Author
Date
2023Type
- Doctoral Thesis
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Abstract
Semiconductor chalcogenide nanocrystals have been in the limelight of material research since the 1990s. Diverse fields such as thermoelectrics, photovoltaics, photodetection or biomedical imaging benefit from the extensive library of nanomaterials and their particular physical and chemical properties. Containing a few 100s to 10'000s of atoms, these particles exhibit unique size-dependent properties due to quantum confinement. Initially focused on binary materials such as cadmium selenide and lead sulfide, synthesis protocols rapidly expanded to diverse types of nanocrystals containing multiple elements with composition, shape and surface control. Earth-abundant and low-toxic elements replace cadmium, lead and mercury for increased sustainability and heterogeneous structures such as inorganic shells further diversify achievable properties.
Chalcogenide nanocrystals typically display a periodic arrangement of various positively charged metal cations and negatively charged anions (sulphur, selenium and tellurium), forming a crystal lattice. The lattice type depends on various factors such as composition and temperature. Within a solid solution, the relative atomic concentration changes while lattice symmetry is preserved. The crystal may contain an array of disorder and defects, which may significantly impact electronic, optical and thermal properties. While the structure is non-trivial to investigate due to the small crystallite size, advanced measurement and modelling techniques such as electron microscopy and X-ray spectroscopy reveal structure-property relations. Combining complementary methods enables detailed insight into the structure of homogeneous and heterogeneous particles as well as dynamic processes.
The aim of this thesis is to extend the scope of nanocrystal synthesis, investigate composition dependent properties, as well as understand the nature of atomic ordering in the nanocrystal lattice and implications for measurable optical properties. Resulting precisely designed, uniform nanocrystals will enable the bottom-up fabrication of highly functional devices for a variety of applications.
The synthesis development of multinary nanocrystals requires rigorous balancing of precursor reactivity. Relevant synthesis parameters were evaluated to produce stoichiometric silver-antimony-telluride (AgSbTe2) nanocrystals with small sizes and narrow size distributions. Composition control was achieved through adjusting the cation precursor ratio. A significantly larger solid solution range than known in bulk Ag-Sb-Te was enabled due to nanoscaling. This increased composition tunability may further enhance thermoelectric properties previously measured in AgSbTe2.
The detailed study of synthesis parameters and resulting nanocrystal properties reveals elaborate details on synthesis dynamics. The elemental composition of copper-antimony-selenide nanocrystals depends on time, temperature and precursor concentrations. Studying these trends illuminates the growth mechanism and permits predictable synthesis of highly uniform, stoichiometric Cu3SbSe4 and off-stoichiometric nanocrystals. The defect tolerance is increased compared to bulk, leading to a larger composition range without phase separation. With tight-binding computational modelling the resulting trend is correlated with an increased presence of copper vacancies. This combined study of experiment and electronic structure modelling provides the basis for future development of Cu3SbSe4 for mid-infrared absorption and thermoelectric devices.
Ordered Vacancy Compounds within the silver-indium-selenide solid solution (such as Ag3In5Se9) are promising materials for biomedical imaging and energy harvesting and exhibit a periodic arrangement of vacancies and different cations. Tight-binding statistics reveal superior optical properties for a homogeneous distribution of cations in the crystal lattice. Inorganic zinc selenide shell growth synthesis significantly increases photoluminescence. Maximum values are measured up to two days of room temperature storage after shell growth. This phenomenon is linked to slow cation rearrangement with simulations and time-dependent photoluminescence measurements.
These results promise widespread possibilities for developing novel chalcogenide nanocrystals. The understanding and control of lattice ordering is crucial for device performance and should be diligently studied in relevant nanomaterials. An interdisciplinary approach integrating experimental and computational methods will fuel the development of tailored nanomaterials, enabling desperately sought for technological advances for a more sustainable society. Show more
Permanent link
https://doi.org/10.3929/ethz-b-000619265Publication status
publishedExternal links
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Publisher
ETH ZurichSubject
Nanocrystals; chalcogenide nanocrystal; Synthesis; Tight-binding calculationsOrganisational unit
03895 - Wood, Vanessa / Wood, Vanessa
Funding
175889 - Multi-length Scale Engineering of Thermal Properties of Nanocrystals and their Composite Films: Fundamentals and Applications (SNF)
185902 - QSIT - Quantum Science and Technology (SNF)
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