  Monolayer group-V transition metal dichalcogenides in their 1T phase have
recently emerged as a platform to investigate rich phases of matter, such as
spin liquid and ferromagnetism, resulting from strong electron correlations.
Although 1T phase NbSe2 does not occur naturally in bulk form, it has been
discovered that the 1T and 1H phases can coexist when monolayer NbSe2 is grown
via molecular beam epitaxy (MBE). This discovery has inspired theoretical
investigations predicting collective phenomena such as ferromagnetism in two
dimensions. Here, by controlling the MBE growth parameters, we demonstrate the
successful growth of single-phase 1T-NbSe2. By combining scanning tunneling
microscopy/spectroscopy and ab initio calculations, we show that this system is
a charge-transfer insulator, with the upper Hubbard band located above the
valence band maximum. Furthermore, by creating a vertical 1T/2H NbSe2
heterostructure, we find evidence of exchange interactions between the
localized magnetic moments in 1T phase and the metallic/superconducting phase,
as manifested by Kondo resonances and Yu-Shiba-Rusinov bound states.
