Atom-Resolved Engineering of Defect Complexes in a Monolayer Semiconductor Imaged with 4D-STEM

Defect complexes have the potential to introduce desirable functionalities in host semiconductors [1,2], but directing their formation at the atomic level is a challenging task. Here, we demonstrate the atomic-scale engineering of various defect complexes, primarily adopting linear configurations, in monolayer MoS₂. By applying electron ptychography to our four-dimensional-scanning transmission electron microscopy (4D-STEM) datasets [3,4], we achieve a deep sub-Ångstrom resolution of 0.35 Å, enabling the measurement of subtle lattice displacements with an accuracy of up to 2 pm. Moreover, we can track atomic dynamics during defect complex nucleation and reconstruction with high elemental sensitivity. Through systematic control of the electron dose and Re-doping concentration, we uncover the formation thresholds for these complexes. Notably, sulfur vacancy lines manifest when vacancy density reaches ~5 x 10¹³ cm^-2 (~2.5 at. %) and undergo a significant out-of-plane lattice distortion as the density reaches 8 x 10¹³ cm^-2 (~4 at. %). Re-dopant lines emerge at a dopant concentration of 4 x 10¹³ cm^-2 (~4 at. %). Metastable sulfur interstitial lines exhibit delicate formation conditions, requiring both a high Re-doping level of 6 x 10¹³ cm^-2 (~6 at. %) and a cumulative electron dose of 3 x 10⁵ e/Ų. Interestingly, these formation thresholds reveal smaller values than those typically measured through mesoscopic techniques such as X-ray photoelectron spectroscopy (XPS), albeit on the same order of magnitude. This highlights the scalability of our atom-resolved fabrication conditions to mesoscopic property engineering in materials. We show how diverse driving forces, such as defect-configuration energetics, structural polymorphism, and defect-defect coupling, can be effectively harnessed to assemble single-defects in a highly ordered manner.<br/><br/>Funding from the Ministry of Education (MOE) Singapore, under AcRF Tier 3 (MOE2018-T3-1-005) and AcRF Tier 1 (R-284-000-179-133) are kindly acknowledged.<br/><br/>[1] Loh, L., et al. (2021). Substitutional doping in 2D transition metal dichalcogenides. <i>Nano Research</i>, 14, 1668-1681.<br/>[2] Loh, L., et al. (2021). Impurity-Induced Emission in Re-Doped WS₂ Monolayers. <i>Nano Letters</i>, 21(12), 5293-5300.<br/>[3] Ning, S., et al. (2022). Accurate and Robust Calibration of the Uniform Affine Transformation Between Scan-Camera Coordinates for Atom-Resolved In-Focus 4D-STEM Datasets. <i>Microscopy and Microanalysis</i>, 28(3), 622-632.<br/>[4] Ning, S., et al. (2023). An integrated constrained gradient descent (iCGD) protocol to correct scan-positional errors for electron ptychography with high accuracy and precision. <i>Ultramicroscopy</i>, 248, 113716.