Photoresponse of Natural van der Waals Heterostructures

Van der Waals heterostructures (vdWH) consisting of 2D materials offer a platform to obtain materials by design and are very attractive owing to unique electronic states. Research on 2D vdWHs has so far been focused on fabricating individually stacked atomically thin unary or binary crystals including graphene, h-BN, and 2D semiconductors. Here we present our study of the optoelectronic properties of a naturally occurring vdWH, known as franckeite, which is a layered crystal composed of lead, tin, antimony, iron, and sulfur. We found that franckeite (60 nm < d < 100 nm) behaves as a narrow band gap semiconductor demonstrating a wide-band photoresponse. We have observed the band-edge transition at ∼1500 nm (∼830 meV). Laser-power and temperature-resolved photocurrent measurements reveal that the photocarrier generation and recombination are dominated by continuously distributed trap states within the band gap. We calculated the optical absorption properties via density functional theory. Moreover, the device has a fast photoresponse with a rise time as fast as ∼1 ms. Our study provides a fundamental understanding of the optoelectronic behavior in a complex naturally occurring vdWH, and may pave an avenue toward developing nanoscale optoelectronic devices with tailored properties.