Yu Ren Zhou - University of Pennsylvania
Co-Author(s): Carl H. Naylor and A. T. Charlie Johnson, University of Pennsylvania, Philadelphia, PA
Monolayer Molybdenum Disulfide (MoS2) is a two-dimensional semiconductor with many interesting properties such as a tunable band gap and elastic modulus comparable to that of graphene. Due to the monolayer nature of MoS2, every atom on its surface is exposed to the environment maximizing its sensing capabilities. Chemical vapor sensing has many important applications, including trace detection of alcohol (breathalyzer tests) and explosives (security and law enforcement). An exciting emerging application of vapor sensing is cancer diagnosis, since it has been shown that dogs could identify cancer patients by odor detection. Monolayer MoS2 was grown by chemical vapor deposition (CVD). The reaction of sulfur vapor with ammonium heptamolydate on a silicon/silicon oxide substrate at approximately 800°C forms monolayer MoS2 flakes. MoS2 was transferred from the growth substrate onto pre-fabricated devices on a silicon/silicon oxide substrate with gold source/drain electrodes, thus making arrays of MoS2 field effect transistors (FETs). The bias and gate voltages were kept constant while exposing the FETs to various concentrations and types of chemical vapors. The changes in current levels were recorded for various exposures to determine the sensitivity of the MoS2 FETs to vapors. A reliable method of mass-producing MoS2 FETs was developed, allowing the simultaneous fabrication of thousands of FETs with a yield exceeding 95%. Up to 30% decrease in current was measured upon exposure of the transistors to 33 vol% saturated propionic acid vapor, confirming sensitivity of MoS2 to external vapors. Up to 20% increase in current was measured upon exposure to 33 vol% ammonia; these opposite changes imply protonation of ammonia and deprotonation of propionic acid at the MoS2 surface. Introduction of dimethyl methyl phosphonate (DMMP) was observed to have little effect on the channel resistance, but caused up to 40% decrease in the contact resistance at 33 vol% saturated vapor. These results imply that DMMP interacts almost exclusively with the gold-MoS2 contact, and that charge transfer from DMMP molecules to the contact may be decreasing the Schottky barrier. There are many directions of further work related to this project. Firstly, the vapor sensing properties of related materials such as Tungsten Disulfide (WS2) and Tungsten Ditelluride (WTe2) may be examined. Secondly, MoS2 could be functionalized with biomolecules such as DNA or proteins to increase the interaction between organic vapors and the MoS2 surface. Finally, the effects of vapors on other properties of MoS2, such as its optical absorption spectrum, could be investigated.
References: McCulloch, M., Jezierski, T., Broffman, M., Hubbard, A., Turner, K., & Janecji, T. Diagnostic accuracy of canine scent detection in early- and late-stage lung and breast cancers. Integrative Cancer Therapies 2006, 5(1), 30-39.
Jiang, J. W. Graphene vs. MoS2: A short review. Frontiers of Physics 2014, 10(106801), 1-16.
Funder Acknowledgement(s): We acknowledge support of the NSF EFRI Research Experience and Mentoring (REM) program and the support of the University of Pennsylvania's VIPER program.
Faculty Advisor: A. T. Charlie Johnson, firstname.lastname@example.org
Role: I was responsible for all stages of this project. I synthesized monolayer MoS2 using a pre-developed CVD process, patterned gold electrodes on silicon wafers using electron-beam lithography, and transferred MoS2 onto those electrodes to create FETs. I ran vapor sensing trials on the MoS2 FETs using a pre-existing apparatus. Along with Carl H. Naylor, I analyzed data collected from the vapor sensing trials and discussed conclusions of the collected data sets.