Chemisorption-induced n-doping of MoS2 by oxygen

Both chemisorption and physisorption affect the electronic properties of two-dimensional materials, such as MoS2, but it remains a challenge to probe their respective roles experimentally. Through repeated in-situ electrical measurements of few-layer MoS2 field-effect transistors in an ultrahigh vacuum system with well-controlled oxygen partial pressure (6 x 10(-8) mbar-3 x 10(-7) mbar), we were able to study the effect of chemisorption on surface defects separately from physically adsorbed oxygen molecules. It is found that chemisorption of oxygen results in n-doping in the channel but negligible effect on mobility and on/off ratio of the MoS2 transistors. These results are in disagreement with the previous reports on p-doping and degradation of the device's performance when both chemisorption and physisorption are present. Through the analysis of adsorptiondesorption kinetics and the first-principles calculations of electronic properties, we show that the experimentally observed n-doping effect originates from dissociative adsorption of oxygen at the surface defects of MoS2, which lowers the conduction band edge locally and makes the MoS2 channel more n-type-like as compared to the as-fabricated devices. (C) 2016 AIP Publishing LLC.