Characterisation of Structure-Property Relationships in Novel Chalcogenides Systems
Citation:
Simonian, Tigran, Characterisation of Structure-Property Relationships in Novel Chalcogenides Systems, Trinity College Dublin, School of Chemistry, Chemistry, 2024Download Item:
Abstract:
As devices become smaller and head towards the atomic limits, minute changes in their structure, in the form of defects, can have an oversized effect on the device properties. This is especially true for novel classes of chalcogenide-based materials, which typically possess weaker bond strengths than their oxide counterparts. As such, careful structure-property relationship establishment is required to fully exploit the material's properties and ensure adequate device's lifetimes and efficiencies.
In this work, several novel chalcogenide systems are investigated through the combination of transmission electron microscopy (TEM) and computational methods such as density functional theory (DFT) calculations. Firstly, the thermoelectric properties of TlGaSe2 were investigated. Stacking faults were seen throughout the samples using scanning TEM (STEM), which were correlated with diffraction measurements and simulations of both. Stacking fault energies were calculated with DFT to be negative, implying that the faults are intrinsic to the system. Due to this, first-principles calculations were more practical to use to qualitatively study the effects of these stacking faults on the thermoelectric properties than physical experimentation. Preliminary results from electron transport studies indicate negligible change with the inclusion of the stacking fault, as to be expected for a thermoelectric material. However, initial phonon band structure calculations indicate a change in phonon frequencies when the stacking faults are included, which may imply a lowering of thermal conductivity.
The lack of control of this stacking order inspired the exploration of chalcogenide van der Waals heterostructures, such as GaS/GaSe heterostructures. DFT simulations predict a Rashba band splitting at the valence band maximum of the structures when spin-orbit coupling effects are included. The size of this splitting, along with the band gap of the structures, is highly dependent on the twist angle between the two monolayers of GaS and GaSe. These multiple degrees of freedom are highly sought after for optoelectronic and spintronic applications and thus warranted further investigation. Several heterostructures were constructed and imaged. Despite the further sample optimisation required to get to the monolayer limit, the imaging showed successful construction of the heterostructures and plans to probe the band gaps and Rashba splitting using EELS are discussed.
While the systems above are interesting in their own context, they are not yet commercially viable within the short- to medium-term timelines. Hence, the field of chalcogenide perovskites for photovoltaic applications is explored with the BaZrS(3-y)Se(y) alloy system. Photoconductivity spectroscopy measurements show a decreasing band gap with increasing Se content, in line with previous and current theoretical studies. Cross-sectional STEM imaging highlights the vast number of in-plane and out-of-plane anti-phase boundaries (APBs) throughout the films. Mapping of the A-site cation distances reveals a tripartite of out-of-plane lattice spacings, which are in line with reflection pairs seen in HRXRD spectra. The lack of strain from the surface of the films to the substrate is indicative of fully relaxed films, where the strain relaxation mechanism is the APBs and the buffered interface with the substrate.
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Science Foundation Ireland (SFI)
Centre for Doctoral Training in Advanced Characterisation of Materials
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https://tcdlocalportal.tcd.ie/pls/EnterApex/f?p=800:71:0::::P71_USERNAME:SIMONIATDescription:
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Author: Simonian, Tigran
Sponsor:
Science Foundation Ireland (SFI)Centre for Doctoral Training in Advanced Characterisation of Materials
Advisor:
Nicolosi, ValeriaPublisher:
Trinity College Dublin. School of Chemistry. Discipline of ChemistryType of material:
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