Detection of Negative Charge Induced by Single Vanadium Dopant Atoms in 2D WSe₂ by 4D-STEM

Structural anomalies in 2D materials have been known as the key to locally modify the electrical, optical and magnetic properties. In order to tailor the material properties and to explore their functionalities, the ability to survey the local electric properties together with their structural configuration at the atomic scale is essential. Recently, a new imaging technique called Center of Mass (CoM), sensitive to the local electrostatic field, has been demonstrated in a Scanning Transmission Electron Microscope (STEM)[1-2]. However, the lack of quantitative understanding and interpretation of CoM images is the main reason why this imaging mode is not yet routinely used for the study of 2D materials.<br/>In this work, we explore the use of the CoM technique for atomic scale mapping of the local electrostatic field and potential around single atom V dopants in WSe₂/graphene heterostructure grown by molecular beam epitaxy (MBE). The quantitative analysis is achieved by comparing the experimentally obtained E-field and potential maps to the Density Functional Theory based multislice STEM image simulations taking into account the influence of key microscope parameters such as: convergence angle, defocus and lens aberrations. The residual three-fold astigmatisms were measured using a ptychographic probe reconstruction for each experimental 4D-dataset in order to generate the reliable and directly comparable simulated potential maps. A negative charge around V dopants is then detected as a drop in the electrostatic potential maps.<br/>Finally, the technique is applied for imaging the electrostatic potential landscape in complex structural configurations in the presence of growth related defects such as Se vacancies and inversion domain boundaries. The separation of background signal from the projected total potential map allowed to extract both the quantitative charge field and the local electrostatic potential directly relating to the individual atom components. The latter was used to determine the precise atomic position of dopants and defects. The results showed the formation of potential wells of different forms arising around vanadium substitutes. A strong background variation is observed and which cannot be explained by the independent atom model simulations. Therefore, we suppose that the potential drop is seen as a consequence of charge related phenomena, where the shape and the depth of the potential drops might be determined by a complex interaction between defects, as predicted by the DFT calculations [3].<br/>Our results demonstrate the capability of the CoM method to map the electrostatic potential including charge effect, opening the perspective for atomic scale analysis of charge effects and interactions between charged defects in synthesized 2D materials.<br/>[1] N. Shibata et al., Nature Physics, 8 (2012) 611<br/>[2] K. Müller et al., Nature Communications, 5 (2014) 5653<br/>[3] D. Dosenovic et al, submitted to ACS Nano (under review)