  Two-dimensional (2D) materials and heterostructures have recently gained wide
attention due to potential applications in optoelectronic devices. However, the
optical properties of the heterojunction have not been properly characterized
due to the limited spatial resolution, requiring nano-optical characterization
beyond the diffraction limit. Here, we investigate the lateral monolayer
MoS2-WS2 heterostructure using tip-enhanced photoluminescence (TEPL)
spectroscopy on a non-metallic substrate with picoscale tip-sample distance
control. By placing a plasmonic Au-coated Ag tip at the heterojunction, we
observed more than three orders of magnitude photoluminescence (PL) enhancement
due to the classical near-field mechanism and charge transfer across the
junction. The picoscale precision of the distance-dependent TEPL measurements
allowed for investigating the classical and quantum tunneling regimes above and
below the ~320 pm tip-sample distance, respectively. Quantum plasmonic effects
usually limit the maximum signal enhancement due to the near-field depletion at
the tip. We demonstrate a more complex behavior at the 2D lateral
heterojunction, where hot electron tunneling leads to the quenching of the PL
of MoS2, while simultaneously increasing the PL of WS2. Our simulations show
agreement with the experiments, revealing the range of parameters and
enhancement factors corresponding to various regimes. The controllable
photoresponse of the lateral junction can be used in novel nanodevices.
