Characterisation and Defect Analysis of 2D Layered Ternary Chalcogenides

Session

Poster Session 1

Authors

Mr. Tigran Simonian (1, 3, 4), Dr. Ahin Roy (1, 3, 4), Prof. Zdeněk Sofer (2), Prof. Valeria Nicolosi (1, 3, 4)

Affiliations

1. Advanced Microscopy Lab, Trinity College Dublin
2. University of Chemistry and Technology Prague
3. CRANN/AMBER, Trinity College Dublin
4. School of Chemistry, Trinity College Dublin

Keywords

TlGaSe2, HRTEM, HRSTEM, EDX, EELS, DFT, Characterisation

Abstract text

TlGaX₂ [X = Se, S, Te] is a family of layered 2-D ternary chalcogenides. These p-type semiconductors have band gaps within the green to ultraviolet range [1] [2] [3], which makes them ideal candidates for optoelectronic applications.

Current examples of such applications, such as phototransistors [4] and detectors [5] [6], make use of multistep processes, such as mechanical exfoliation with a PMMA transfer [4], or thin film synthesis via thermal evaporation [6], 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 [7]. Herein, we demonstrate that these TlGaX₂ materials can be exfoliated with a facile one step sonication in IPA, leaving behind little to no residue.

While there have been a number of studies into the electronic structure of these materials, an exact description of their band structures has yet to be established [6]. Interestingly, despite defect quantification being an important structure-property relationship in semiconductors, none of its aspects, e.g. stoichiometry, charge states, structural defects, etc., have yet been fully addressed. 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 [2].

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 [8], and with isostructural KInS₂ [9]) and surface relaxation in LPE-TlGaSe2. Furthermore, we show a transient formation of a 1-D structure along the edges of TlGaSe₂ flakes, catalysed by intense electron beam exposure in the microscope. Using complimentary density functional theory (DFT) simulations, we explore the effect of defects on the electronic structure, and rationalise the transient formation of the 1-D structure.

References

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