Imaging In-plane Charge-transfer Excitons in a twisted-WS_2­ Moiré Superlattice

The transition-metal dichalcogenides (TMD) moire superlattice provides a versatile platform to study novel quantum phases, such as strongly correlated behavior and nontrivial topological phases. Exotic elementary excitations such as moire excitons emerge in such TMDC moire superlattices, which has been detected by various optical spectroscopic measurements. The moire potential can significantly modulate how the excited electron and hole bond with each other, while its microscopic nature remains unclear. Previous works mostly attributed it to the Umklapp scattering of the Wannier exciton7-10, while recent studies pointed out that moire potentials could even modify the internal structure of the excitons and yield a new type of in-plane charge transfer (ICT) excitons, where the bound electrons and holes are spatially separated. In this work, we performed a microscopic study of the ICT exciton in a twisted WS2 (t-WS2) moire superlattice combining nanoscale photocurrent measurement and ab initio calculations. Combining laser excitation and scanning tunneling microscopy (STM), photocurrent spectroscopy and mapping with sub-moire spatial resolution were performed. Because of the charge separation in the ICT excitons, the STM tip can selectively tunnel the electron or hole within the exciton depending on its spatial position. Our photocurrent map shows a spatial oscillating photocurrent polarity, providing strong evidence for the existence of ICT exciton in this t-WS2 moire superlattice. This result agrees perfectly with ab initio calculated ICT exciton wavefunction. By investigating the response of the oscillating photocurrent under an in-plane electric field, we further confirm the bound nature of the ICT excitons and rule out the potential electron-hole plasma.