  Two-dimensional (2D) transition metal nitrides (TMNs) are new members in the
2D materials family with a wide range of applications. Particularly, highly
crystalline and large area thin films of TMNs are potentially promising for
applications in electronic and optoelectronic devices; however, the synthesis
of such TMNs has not yet been achieved. Here, we report the synthesis of
few-nanometer thin Mo5N6 crystals with large area and high quality via in situ
chemical conversion of layered MoS2 crystals. The structure and quality of the
ultrathin Mo5N6 crystal are confirmed using transmission electron microscopy,
Raman spectroscopy and X-ray photoelectron spectroscopy. The large lateral
dimensions of Mo5N6 crystals are inherited from the MoS2 crystals that are used
for the conversion. Atomic force microscopy characterization reveals the
thickness of Mo5N6 crystals is reduced to about 1/3 of the MoS2 crystal.
Electrical measurements show the obtained Mo5N6 samples are metallic with high
electrical conductivity (~ 100 {\Omega} sq-1), which is comparable to graphene.
The versatility of this general approach is demonstrated by expanding the
method to synthesize W5N6 and TiN. Our strategy offers a new direction for
preparing 2D TMNs with desirable characteristics, opening a door for studying
fundamental physics and facilitating the development of next generation
electronics.
