  As a 2D ferromagnetic semiconductor with magnetic ordering, atomically thin
chromium triiodide is the latest addition to the family of two-dimensional (2D)
materials. However, realistic exploration of CrI3-based devices and
heterostructures is challenging, due to its extreme instability under ambient
conditions. Here we present Raman characterization of CrI3, and demonstrate
that the main degradation pathway of CrI3 is the photocatalytic substitution of
iodine by water. While simple encapsulation by Al2O3, PMMA and hexagonal BN
(hBN) only leads to modest reduction in degradation rate, minimizing exposure
of light markedly improves stability, and CrI3 sheets sandwiched between hBN
layers are air-stable for >10 days. By monitoring the transfer characteristics
of CrI3/graphene heterostructure over the course of degradation, we show that
the aquachromium solution hole-dopes graphene.
