National University of Singapore (NUS) physicists found that contacts made of molybdenum disulfide (MoS2) and tungsten diselenide (WSe2) on gold metal are both p-type, while the same contacts with chalcogen vacancy defects become n-type. Non-local exchange and correlation effects are critical in determining the energy level alignment and the contact polarity. The results of the study, published in npj 2D Materials and Applications, show that the different contact polarities observed experimentally for MoS2/gold and WSe2/gold interfaces stem from the distinct nature of the defects in these two materials.