  Van der Waals heterostructures comprised of two-dimensional (2D) materials
offer a platform to obtain materials by design with unique electronic
properties. Franckeite (Fr) is a naturally occurring van der Waals
heterostructure comprised of two distinct alternately stacked semiconducting
layers; (i) SnS$_2$ layer and (ii) Pb$_3$SbS$_4$. Though both layers in the
heterostructure are semiconductors, the photoluminescence from Franckeite
remains elusive. Here, we report the observation of photoluminescence (PL) from
Franckeite for the first time. We observed two PL peaks at ~ 1.93 eV and ~ 2.11
eV. By varying the temperature from 1.5 K to 80 K, we found that the PL peak
position redshifts and the integrated intensity decreases slowly as we increase
the temperature. We observed linear dependence of photoluminescence integrated
intensity on excitation laser power indicating that the photoluminescence is
originating from free excitons in the SnS$_2$ layer of Fr. By comparing the PL
from Fr with the PL from a monolayer MoS$_2$, we determined that the PL quantum
efficiency from Fr is an order of magnitude lower than that of a monolayer
MoS$_2$. Our study provides a fundamental understanding of the optical behavior
in a complex naturally occurring van der Waals heterostructure, and may pave an
avenue toward developing nanoscale optical and optoelectronic devices with
tailored properties.
