Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 113, Issue 31, Pages 8583-8588Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1605982113
Keywords
MoS2; microwave impedance microscopy; edge states; electrical inhomogeneity; metal-insulator transition
Categories
Funding
- US Department of Energy (DOE), Office of Science, Basic Energy Sciences [DE-SC0010308]
- Welch Foundation [F-1814, F-1255]
- Office of Naval Research [N00014-1110190]
- Nanomanufacturing Systems for Mobile Computing and Energy Technologies (NASCENT) Engineering Research Center [EEC-1160494]
- China 973 Program [2013CB921900, 2012CB921300]
- DOE [DE-FG03-02ER45958]
- U.S. Department of Energy (DOE) [DE-SC0010308] Funding Source: U.S. Department of Energy (DOE)
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The understanding of various types of disorders in atomically thin transition metal dichalcogenides (TMDs), including dangling bonds at the edges, chalcogen deficiencies in the bulk, and charges in the substrate, is of fundamental importance for TMD applications in electronics and photonics. Because of the imperfections, electrons moving on these 2D crystals experience a spatially nonuniform Coulomb environment, whose effect on the charge transport has not been microscopically studied. Here, we report the mesoscopic conductance mapping in monolayer and few-layer MoS2 field-effect transistors by microwave impedance microscopy (MIM). The spatial evolution of the insulator-to-metal transition is clearly resolved. Interestingly, as the transistors are gradually turned on, electrical conduction emerges initially at the edges before appearing in the bulk of MoS2 flakes, which can be explained by our first-principles calculations. The results unambiguously confirm that the contribution of edge states to the channel conductance is significant under the threshold voltage but negligible once the bulk of the TMD device becomes conductive. Strong conductance inhomogeneity, which is associated with the fluctuations of disorder potential in the 2D sheets, is also observed in the MIM images, providing a guideline for future improvement of the device performance.
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