Characterisation and Defect Analysis of 2D Layered Ternary Chalcogenides

TlGaX₂ [X = Se, S] is a family of layered 2-D ternary chalcogenides. These p-type semiconductors have band gaps within the green to ultraviolet range, which makes them ideal candidates for optoelectronic applications.<br/>Current examples of such applications, such as phototransistors and detectors, make use of multistep processes, such as mechanical exfoliation with a PMMA transfer or thin film synthesis via thermal evaporation, which makes potential future scalability of these devices cumbersome. Liquid phase exfoliation (LPE) is a far more facile process that has been shown to work for a large number of layered van der Waals materials. Herein, we demonstrate that these TlGaX₂ materials can be exfoliated with a facile one step sonication in IPA, leaving behind little to no residue.<br/>Despite structure-property relationships in materials being highly important for their future device applications, this has to date not been fully addressed for this family of semiconductors. For example, while it was reported that Se vacancies in TlGaSe₂ can lead to a change in the nature and size of the bandgap, a quantitative relationship was not determined.<br/>In this work, we use high-resolution scanning transmission electron microscopy (HRSTEM), in combination with EDX/EELS, to experimentally address this aspect of defects in TlGaSe₂. Our experiments clearly indicate the presence of stacking faults (as were seen previously in XRD studies with bulk TlGaSe₂, and with isostructural KInS₂) and surface relaxation in LPE-TlGaSe2. Using complimentary density functional theory (DFT) simulations, we explore the effect of defects on the electronic structure. Furthermore, density functional perturbation theory (DFPT) is used to quantify the effect of the stacking faults on phonon suppression, and how this relates to the thermoelectric properties of the materials.