Notably, monolayer MoS 2 with a direct bandgap of 1.8 eV overcomes the weakness of zero-bandgap graphene, [5,6] which makes it a promising candidate for transistors. To continue transistor scaling and achieve high performance, MoS 2 fieldeffect transistors (FETs) have been the subject of intensive research. For example, different structures of MoS 2 FETs were studied by Lee et al., who found that the device performance can be improved using trilayer MoS 2 with dielectric and gate electrodes of hexagonal boron nitride (hBN) and graphene for higher mobility. [7] Recently, Sebastian et al. reported a comprehensive study of key FET parameters by statistical measurements to benchmark device-to-device variation in 2D FETs. [8] Meanwhile, versatile MoS 2 -based devices, such as photodetectors, [9][10][11] light-emitting diodes, [12,13] memory devices, [14][15][16] sensors, [17,18] and memtransistors, [19][20][21] have also been explored to broaden the range of device applications.To assess the technological viability of 2D-based devices for practical applications, it is necessary to better investigate the limits of materials in electronic and optoelectronic devices. In this regard, the electrical breakdown associated with material damages, which generally stems from avalanche multiplication, is a critical issue in device failures. The breakdown behavior of 2D-based devices has been widely discussed through measurements of current density. [22,23] The thermal effect that occurs by Joule heating on electrical breakdown is analyzed with simulations, [24] and multilayer MoS 2 FETs are also studied to understand the heat dissipation with respect to the device layer number. [25] To date, the mechanism of breakdown is mostly deduced by measured and simulated data, whereas a direct observation of the structural variation of materials with bias remains limited. Since Joule heating is expected to occur under electric fields, there will be a high probability of material damage. Previous researches have revealed the thermal evolution in 2D materials by in situ heating, showing the possible damage to the material such as nanoislands, scrolls, and folded layers. [26] Therefore, we will focus on the occurrence of structural transformation in this work. Meanwhile, an advanced technique for monitoring the dynamic behavior and acquiring atomic information 2D materials have great potential for not only device scaling but also various applications. To prompt the development of 2D electronics and optoelectronics, a better understanding of the limitation of materials is essential. Material failure caused by bias can lead to variations in device behavior and even electrical breakdown. In this study, the structural evolution of monolayer MoS 2 with high bias is revealed via in situ transmission electron microscopy at the atomic scale. The biasing process is recorded and studied with the aid of aberrationcorrected scanning transmission electron microscopy. The effects of electron beam irradiation and biasing are also discussed through the combi...
