I agree with Alexander. Such as ZnO is n-type intrinsically, due to native oxygen vacancy (which is a defect).

Hi  Bharat,

I do not have the expertise in MoS2. Yet the attached paper might help you.

I I do not know exactly, but I think that the reason for this in the character of defects. The dominant type of defect is excess metal atoms in the space between the layers. It is clear that this donor doping.

I agree with Alexander. Such as ZnO is n-type intrinsically, due to native oxygen vacancy (which is a defect).

I think majority of electrons come from Sulphur atoms rather than metal atoms.

May be. I think please do XPS

oxide prone material usually shows n-type character due to surface trapping and defects

MoS2 is an intrinsic semiconductor, but since field effect transistors made from MoS2 are Schottky-barrier FETs it is the work function of the metal used for the contacts that determines the polarity of the device.  See the linked paper where high work function contacts were used to make a p-type device.  Fermi-level pinning and other effects can muddy the waters a bit, but I think the most simple picture is just the work function of the contacts determining how the bands meet at the interface which determines the transistor polarity.

http://pubs.acs.org/doi/abs/10.1021/nl4043505

The conduction type depends on the the presence of either crystallographic geometrical and chemical defects. Only point defects in substitution sites can contribute to the material carrier type and concentration. The n-type means that one extra valence electron atoms or defects are substituting either the Molybdenum or the sulfur.

Best wishes

I am more curious about phosphorene which has all P atoms bonded with other three P atoms and no free carriers (either electrons or holes) after bonding. Why it is p-type semiconductor?

Most of the metal oxides show either n-type of p-type semiconducting characteristics. It may be attributed to the point defects present in the metal oxide. If there is oxigen related defect/vacancy then it will show n-type behaviour other wise p-type.

I don't know exactly but same kind of phenomenon may be valid for MoS2 as well, where sulphar related defects would cause n-type bahaviour.

I agree with Satyendra Mourya