Direct Observation of 2D Electrostatics and Ohmic Contacts in Template-Grown Graphene/WS Heterostructures.

Large-area two-dimensional (2D) heterojunctions are promising building blocks of 2D circuits. Understanding their intriguing electrostatics is pivotal but largely hindered by the lack of direct observations. Here graphene-WS heterojunctions are prepared over large areas using a seedless ambient-pressure chemical vapor deposition technique. Kelvin probe force microscopy, photoluminescence spectroscopy, and scanning tunneling microscopy characterize the doping in graphene-WS heterojunctions as-grown on sapphire and transferred to SiO with and without thermal annealing. Both p-n and n-n junctions are observed, and a flat-band condition (zero Schottky barrier height) is found for lightly n-doped WS, promising low-resistance ohmic contacts. This indicates a more favorable band alignment for graphene-WS than has been predicted, likely explaining the low barriers observed in transport experiments on similar heterojunctions. Electrostatic modeling demonstrates that the large depletion width of the graphene-WS junction reflects the electrostatics of the one-dimensional junction between two-dimensional materials.