Understanding the Intrinsic Water Wettability of Molybdenum Disulfide (MoS2)

2D semiconductors allow for unique and ultrasensitive devices to be fabricated for applications ranging from clinical diagnosis instruments to low-energy light-emitting diodes (LEDs). Graphene has championed research in this field since it was first fabricated; however, its zero bandgap creates many challenges. Transition metal dichalcogenides (TMDCs), e.g., MoS2, have a direct bandgap which alleviates the challenge of creating a bandgap in graphene-based devices. Water wettability of MoS2 is critical to device fabrication/performance and MoS2 has been believed to be hydrophobic. Herein, we report that water contact angle (WCA) of freshly exfoliated MoS2 shows temporal evolution with an intrinsic WCA of 69.0 +/- 3.8 degrees that increases to 89.0 +/- 3.1 degrees after 1 day exposure to ambient air. ATR-FTIR and ellipsometry show that the fresh, intrinsically mildly hydrophilic MoS2 surface adsorbs hydrocarbons from ambient air and thus becomes hydrophobic.