Nano-imaging photoresponse in a moir\'e unit cell

TL;DR

This study uses nanoscale imaging to explore the complex photoresponse within a single moiré unit cell of twisted bilayer graphene, revealing spatially dependent optoelectronic behavior driven by the lattice structure.

Contribution

It introduces sub-diffraction photocurrent spectroscopy as a novel method to investigate microscopic optoelectronic mechanisms in moiré superlattices.

Findings

Photoresponse varies spatially within the moiré unit cell.

Sign and magnitude of photocurrent depend on lattice orientation.

Photo-thermoelectric effect explains the observed photocurrent.

Abstract

Graphene-based moir\'e superlattices have recently emerged as a unique class of tuneable solid-state systems that exhibit significant optoelectronic activity. Local probing at length scales of the superlattice should provide deeper insight into the microscopic mechanisms of photoresponse and the exact role of the moir\'e lattice. Here, we employ a nanoscale probe to study photoresponse within a single moir\'e unit cell of minimally twisted bilayer graphene. Our measurements reveal a spatially rich photoresponse, whose sign and magnitude are governed by the fine structure of the moir\'e lattice and its orientation with respect to measurement contacts. This results in a strong directional effect and a striking spatial dependence of the gate-voltage response within the moir\'e domains. The spatial profile and carrier-density dependence of the measured photocurrent point towards a photo-thermoelectric induced response that is further corroborated by good agreement with numerical simulations. Our work shows sub-diffraction photocurrent spectroscopy is an exceptional tool for uncovering the optoelectronic properties of moir\'e superlattices.