  Van der Waals (vdW) heterostructures consisting of two dimensional materials
offer a platform to obtain material by design and are very attractive owing to
novel electronic states. Research on 2D van der Waals heterostructures (vdWH)
has so far been focused on fabricating individually stacked atomically thin
unary or binary crystals. Such systems include graphene (Gr), hexagonal boron
nitride (h-BN) and member of the transition metal dichalcogenides family. Here
we present our experimental study of the opto-electronic properties of a
naturally occurring vdWH, known as Franckeite, which is a complex layered
crystal composed of lead, tin, antimony, iron and sulfur. We present here that
thin film franckeite (60 nm < d < 100 nm) behave as narrow band gap
semiconductor demonstrating a wide band photoresponse. We have observed the
band-edge transition at ~ 1500 nm (~830 meV) and high external quantum
efficiency (EQE~3%) at room temperature. Laser power resolved and temperature
resolved photocurrent measurements reveal that the photo-carrier generation and
recombination are dominated by continuously distributed trap states within the
band gap. To understand wavelength resolved photocurrent, we also calculated
the optical absorption properties via density functional theory. Finally, we
have shown that the device has fast photoresponse with rise time as fast as ~ 1
ms. Our study provides a fundamental understanding of the optoelectronic
behavior in a complex naturally occurring vdWH and can open up the
possibilities of producing new type of nanoscale optoelectronic devices with
tailored properties.
