| AVS 66th International Symposium & Exhibition | |
| 2D Materials | Monday Sessions |
| Session 2D+EM+MI+NS-MoM |
| Session: | Properties of 2D Materials including Electronic, Magnetic, Mechanical, Optical, and Thermal Properties I |
| Presenter: | Peiyu Chen, University of Oxford, UK |
| Authors: | P. Chen, University of Oxford, UK W. Xu, University of Oxford, UK Y. Gao, University of Oxford, UK P. Holdway, University of Oxford, UK J.H. Warner, University of Oxford, UK M.R. Castell, University of Oxford, UK |
| Correspondent: | Click to Email |
Monolayer MoS2 crystals grown on amorphous substrates such as SiO2 are randomly oriented. However, when MoS2 is grown on crystalline substrates, the crystal shapes and orientations are also influenced by their epitaxial interaction with the substrate. In the first part of this talk, we present the results from chemical vapor deposition growth of MoS2 on three different terminations of single crystal strontium titanate (SrTiO3) substrates: (111), (110), and (001). On all three terminations of SrTiO3, the monolayer MoS2 crystals try to align their <2 -1 -1 0>-type directions (i.e., the sulfur-terminated edge directions) with the <1 -1 0>-type directions on SrTiO3. This arrangement allows near-perfect coincidence epitaxy between seven MoS2 unit cells and four SrTiO3 unit cells. On SrTiO3(110), this even distorts the crystal shapes and introduces an additional strain detectable by photoluminescence (PL). Our observations can be explained if the interfacial van der Waals (vdW) bonding between MoS2 monolayers and SrTiO3 is greatest when maximum commensuration between the lattices is achieved. Therefore, a key finding of this study is that the vdW interaction between MoS2 and SrTiO3 substrates determines the supported crystal shapes and orientations by epitaxial relations.
Monolayer MoS2 is also a wide-bandgap semiconductor suitable for use in high-temperature electronics. It is therefore important to understand its thermal stability. In the second part, we uncover the thermal degradation behavior of monolayer MoS2 supported on SrTiO3 in ultrahigh vacuum (UHV) because of sulfur loss. MoS2 was found to degrade on the (111), (110), and (001) terminations of SrTiO3 substrates in a similar way. The sulfur loss begins at 700 °C, at which point triangular etch trenches appear along the sulfur-terminated edge directions of the MoS2 crystals (in scanning tunneling microscopy). The sulfur vacancies can be filled byannealing the crystals in a hot sulfur atmosphere, and the optical properties (by Raman spectroscopy and PL) of monolayer MoS2 can nearly be fully recovered. At higher UHV annealing temperatures, the remaining Mo is oxidized by the SrTiO3 substrates into MoO2 and MoO3. The initial sulfur loss and the formation of MoOx are confirmed by X-ray photoelectron spectroscopy. The sulfur annealing no longer takes effect when all the Mo has been oxidized, which happens at a temperature between 800 °C and 900 °C in UHV. The MoS2 crystal shapes are stable upon annealing until the residual MoO3 particles evaporate at above 1000 °C. This infers that any triangular crystals that look intact under low-magnification optical microscopy and SEM may not mean pristine MoS2.