  Two-dimensional molybdenum disulfide (MoS2) is an excellent channel material
for ultra-thin field effect transistors. However, high contact resistance
across the metal-MoS2 interface continues to limit its widespread realization.
Here, using atomic-resolution analytical scanning transmission electron
microscopy (STEM) together with first principle calculations, we show that this
contact problem is a fundamental limitation from the bonding and interactions
at the metal-MoS2 interface that cannot be solved by improved deposition
engineering. STEM analysis in conjunction with theory shows that when MoS2 is
in contact with Ti, a metal with a high affinity to form strong bonds with
sulfur, there is a release of S from Mo along with the formation of small
Ti/TixSy clusters. A destruction of the MoS2 layers and penetration of metal
can also be expected. The design of true high-mobility metal-MoS2 contacts will
require the optimal selection of the metal or alloy based on their bonding
interactions with the MoS2 surface. This can be advanced by evaluation of
binding energies with increasing the number of atoms within metal clusters.
