  The design of stacks of layered materials in which adjacent layers interact
by van der Waals forces[1] has enabled the combination of various
two-dimensional crystals with different electrical, optical and mechanical
properties, and the emergence of novel physical phenomena and device
functionality[2-8]. Here we report photo-induced doping in van der Waals
heterostructures (VDHs) consisting of graphene and boron nitride layers. It
enables flexible and repeatable writing and erasing of charge doping in
graphene with visible light. We demonstrate that this photo-induced doping
maintains the high carrier mobility of the graphene-boron nitride (G/BN)
heterostructure, which resembles the modulation doping technique used in
semiconductor heterojunctions, and can be used to generate spatially-varying
doping profiles such as p-n junctions. We show that this photo-induced doping
arises from microscopically coupled optical and electrical responses of G/BN
heterostructures, which includes optical excitation of defect transitions in
boron nitride, electrical transport in graphene, and charge transfer between
boron nitride and graphene.
