  Single- and few-layer chromium triiodide (CrI3), which has been intensively
investigated as a promising platform for two-dimensional magnetism, was usually
prepared by mechanical exfoliation. Here, we report on the growth of
single-layer CrI3 by molecular beam epitaxy under ultrahigh vacuum. The atomic
structures and local density of states have been revealed by scanning tunneling
microscopy (STM). Iodine trimers, each of which consists of three I atoms
surrounding a three-fold Cr honeycomb center, have been identified as the basic
units of the topmost I layer. Different superstructures of single-layer CrI3
with characteristic periodicity around 2-4 nm were obtained on Au(111), but
only pristine structure was observed on graphite. At elevated temperatures (423
K), CrI3 was partially decomposed, resulting in the formation of single-layer
chromium diiodide. Our bias-dependent STM images suggest that the unoccupied
and occupied states are distributed spatial-separately, which is consistent
with our density functional theory calculations. The effect of charge
distribution on the superexchange interaction in single-layer CrI3 was
discussed.

Materials Science