2-D materials as well as layered heterostructures have motivated several research works owing to their novel attributes and potential application in next-generation electronic as well as photonic gadgets. Monolayer transition metal dichalcogenides including tungsten disulfide and molybdenum disulfide are semiconductors with high carrier mobility, 1.6-2.0eV bandgaps, and strong absorption at visible wavelengths. Transition metal dichalcogenides heterostructures develop type II heterojunctions, which allows for quick interlayer relaxation of the produced excitons. This is important for several applications such as photodetectors, light emitting diodes, and solar cells.
The uniform and large-area production for 2D materials will enable a practical implementation of the materials.
Unfortunately, degradation of transition metal dichalcogenides by oxidation appears as a potential threat for practical applications. Oxidation of transition metal dichalcogenides of silicon oxide stored under ambient condition initiates in a few weeks after growth. This can be accelerated in harsh environments such as high temperature, high humidity, oxygen plasma treatment, UV radiation, and current flow across defect sites.
This temporal degradation of TMDs via in-air oxidation presents an obstacle, not only in many scientific studies, but also in any practical device applications. It should be noted that the issue of in-air oxidation has been neglected by majority of researchers, since they can test (for publication) as-fabricated devices before the materials degrade.
Irrespective that hBN or polymer encapsulation can minimize oxidation, several applications such as DNA sensors, gas sensors and other bio-applications demand contact between the transition metal dichalcogenides and the environment. The pursuit for increased air-stability of TMDs is therefore essential for the eventual practical use of TMDs and the coordinated improvement in their applications.
Understanding the mechanism of degradation through in-air oxidation of TMDs and identifying conditions for oxidation free TMDs will be a groundbreaking development of 2-D heterostructures for practical applications.
Researchers led by Professor EH Yang at Stevens Institute of Technology in New Jersey work on elucidating the role of hBN or other 2D substrates, which will lead to realizing anti-oxidation of 2D heterostructures. The EH Yang group already demonstrated the epitaxial growth of tungsten disulfide on graphene through chemical vapor deposition, and found that graphene significantly increased the air-stability of tungsten disulfide. When they kept tungsten disulfide-graphene on the silicon oxide growth substrate, the tungsten disulfide begun to oxidize after several months, and while freely suspended, the tungsten disulfide was oxidation-free after ten months of air exposure. Their research work was highlighted as a cover issue of Advanced Materials.
“No systematic studies exist on how the oxidation can be eliminated via synthetic techniques. Also, there is no long-term solution for practicality of these materials unless the issues concerning in-air oxidation are fully addressed“, said Professor EH Yang. The first author, Dr. Kyungnam Kang added, “our group works on directly tackling the issue of in-air oxidation. Anti-oxidant 2D semiconductors has been demonstrated. The initial results provide a solid foundation for practical device applications“.
The research team performed tungsten disulfide growth through a low-pressure chemical vapor deposition process on chemical vapor deposition-grown graphene. The graphene layer was priori transferred on a silicon dioxide substrate. The process generated millimeter sized polycrystalline tungsten disulfide monolayers and single crystalline monolayers that spun tens of micrometers.
The authors elucidated the effects of the substrate-dependent oxidation of tungsten disulfide linked to surface electric fields and explored antioxidation of tungsten disulfide on graphene suspended in air. The authors observed tungsten disulfide on silicon oxide substrate was oxidized within a few weeks, and was significantly oxidized in 4 months. The oxidation process occurred at localized areas with defects.
Polycrystalline tungsten disulfide monolayers on graphene-silicon oxide substrate exhibited a considerably suppressed in-air oxidation owing to the screening of surface electric field by graphene, reducing potential initiation sites for oxidation. However, suspended tungsten disulfide-graphene did not display oxidation for a period of ten months in ambient conditions. This was attributed to lack of defects as well as local electric fields. The authors also initiated interior oxidation on a tungsten disulfide single crystal through conductive atomic force microscope that oxidized tungsten disulfide artificially. This suggested the purpose of localized electric filed effect in the course of tungsten oxidation.
Finding conditions for growing oxidation-free TMDs will mark a milestone in realizing TMD device applications far beyond the current research level. Synthesis and fabrication of 2D materials with a long shelf-life (i.e., with oxidation-resistant TMDs) can be one of the ground-breaking steps towards practical optoelectronics applications, including valleytronics, photodetectors, optical modulators, mode-locked lasers, ultrafast saturation, solar cells and bio-imaging devices.
Kyungnam Kang, Kyle Godin, Young Duck Kim, Shichen Fu, Wujoon Cha, James Hone, and Eui-Hyeok Yang. Graphene-Assisted Antioxidation of Tungsten Disulfide Monolayers: Substrate and Electric-Field Effect. Advanced Materials 2017, 29, 1603898.Go To Advanced Materials