Probing Antisite Defects and Their Mobility in Layered PtSe₂ with Low-Voltage Aberration-Corrected STEM

The successful synthesis/exfoliation of Transition Metal Dichalcogenides (TMDs) covered the need for 2D materials with an energy bandgap, which is essential for transistor applications. PtSe₂ belongs to Noble-Metal Dichalcogenides, a subcategory of TMDs, consisting of metals from group 10 of the periodic table. It exhibits layer-dependent electronic properties, allowing it to be employed either as a semiconductor or semimetal. PtSe₂ has a broad range of applications in sensing, optoelectronics, and photonics [1], while properties can be tuned through defect engineering. The occurrence of magnetism in PtSe₂ has been attributed to Pt vacancies [2] while stacking sequences different than the 1T, can also enhance its performance as a piezoresistive sensor [3]. However, till now the research focused on isolated vacancies, and remains unknown the effect of antisite and complex point defects in the electronic structure and corresponding physical properties of PtSe₂. In this work, samples were exfoliated using either mechanical or liquid-phase exfoliation methods [4]. The structural characterization of exfoliated samples was performed with low-voltage aberration-corrected scanning-transmission electron microscopy (STEM) imaging using the Nion UltraSTEM operated at 60 kV. Multi-frame averaging was utilized to reduce beam damage, while custom scan patterns minimized beam-induced movements. Electronic properties of defected PtSe₂ were calculated using FHI-aims all-electron code, while geometries were optimized using FHI-vibes. Multislice STEM imaging simulations were accomplished with abTEM code [5]. Different point defects, including vacancies, antisites, and as well complex cases were detected in ultrathin flakes of PtSe₂. The presence of point defects was validated with Multislice STEM imaging simulations using the same experimental conditions under which images were acquired. The converged beam in STEM imaging induced beam irradiation effects. Fast frame image series were utilized to study the ‘in-situ’ creation of Pt antisite defects and their mobility across hopping into different atomic positions. The energetic pathways of antisite defects was studied using the Nudged Elastic Band (NEB) method. Finally, the structure-property correlation, regarding the effect of realistic defect cases in the thermoelectrical properties of PtSe₂, was also investigated using the Boltzmann Transport Equation.<br/><br/><b>References</b><br/>[1] Wang, G. et al., Layered PtSe₂ for Sensing, Photonic, and (Opto-)Electronic Applications. Advanced Materials <b>33</b>, (2021).<br/>[2] Avsar, A. et al. Defect induced, layer-modulated magnetism in ultrathin metallic PtSe₂. Nature Nanotechnology <b>14</b>, (2019).<br/>[3] Kempt, R. et al. Stacking Polymorphism in PtSe₂ Drastically Affects Its Electromechanical Properties. Advanced Science <b>9</b>, (2022).<br/>[4] Nicolosi, V. et al. Liquid Exfoliation of Layered Materials. Science <b>340 </b>(2013).<br/>[5] Madsen, J. & Susi, T. The abTEM code: transmission electron microscopy from first principles. Open Research Europe <b>1</b>, (2021).