Recent progress in large-area synthesis of monolayer molybdenum disulphide, a new two-dimensional direct-bandgap semiconductor, is paving the way for applications in atomically thin electronics. Little is known, however, about the microstructure of this material. Here we have refined chemical vapour deposition synthesis to grow highly crystalline islands of monolayer molybdenum disulphide up to 120 μm in size with optical and electrical properties comparable or superior to exfoliated samples. Using transmission electron microscopy, we correlate lattice orientation, edge morphology and crystallinity with island shape to demonstrate that triangular islands are single crystals. The crystals merge to form faceted tilt and mirror twin boundaries that are stitched together by lines of 8- and 4-membered rings. Density functional theory reveals localized mid-gap states arising from these 8-4 defects. We find that mirror twin boundaries cause strong photoluminescence quenching whereas tilt boundaries cause strong enhancement. Meanwhile, mirror twin boundaries slightly increase the measured in-plane electrical conductivity, whereas tilt boundaries slightly decrease the conductivity.
Bibliographical noteFunding Information:
Overall project coordination, sample growth, and electrical and optical characterization were supported as part of the Center for Re-Defining Photovoltaic Efficiency Through Molecular-Scale Control, an Energy Frontier Research Center funded by the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences under Award DE-SC0001085. A.M.v.d.Z was supported by the EFRC as a research fellow. Electron microscopy was performed at and supported by the Cornell Center for Materials Research, an NSF MRSEC (NSF DMR-1120296). P.Y.H. was supported under the National Science Foundation Graduate Research Fellowship Grant No. DGE-0707428. D.A.C. was supported by a Columbia University Presidential fellowship and a GEM PhD Fellowship sponsored by the Center for Functional Nanomaterials at Brookhaven National Lab. T.C.B. was supported under the Department of Energy Office of Science Graduate Fellowship Program (DOE SCGF), administered by ORISE-ORAU under Contract No. DE-AC05-06OR23100. The authors thank S. Gondarenko, R. Hovden, I. Meric, J. Ravichandran, L. Wang, J. Richmond-Decker and P. Kim for helpful discussions.
All Science Journal Classification (ASJC) codes
- Materials Science(all)
- Condensed Matter Physics
- Mechanics of Materials
- Mechanical Engineering