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. 2017 Dec 5;7:16938. doi: 10.1038/s41598-017-16970-6

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© The Author(s) 2017

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Photoluminescence enhancement and non-radiative rate suppression by oxygen bonding in monolayer MoS₂ grown on r-sapphire. (a) Comparison of the PL spectra of monolayer MoS₂ grown on c-sapphire (MoS₂/c-sapphire; red curve) to that of monolayer MoS₂ grown on r-sapphire (MoS₂/r-sapphire; blue curve) reveals that the PL of MoS₂/r-sapphire is substantially brighter and shifted to higher energies. (b) Likewise, the excited state lifetime of MoS₂/r-sapphire (red curve) is clearly enhanced compared that of MoS₂/c-sapphire. (c) As a direct result of the longer excited state lifetimes, the scaling of the emission intensity of the PL with excitation density of MoS₂/r-sapphire (blue curve) shows an earlier onset of exciton-exciton annihilation (i.e., sublinear behavior) than that of MoS₂/c-sapphire which is linear over the same range of excitation densities and ~100× dimmer. (d–e) The XPS spectra in the Mo range of MoS₂ on r-sapphire (d) and c-sapphire (e). The Mo-O bonding is also clearly identified in Mo 3d range when the growth is on r-sapphire (d) but negligible on c-sapphire. (e) The reduced oxygen concentration agrees well with the quenched PL intensity in Fig. 3h, indicating the oxygen doping is the key factor of the PL enhancement on r-sapphire. (f–g) XPS spectra in the S range of MoS₂ on r-sapphire (f) and c-sapphire (g). In addition to the expected S 2p peak, S-O bonding is observed in both as-grown samples. After the sulfurization, the S-O bonding can be reduced on c-sapphire, indicating the S-O in this case is due to the unsulfurized MoO_x. (h) PL spectra of MoS₂/r-sapphire before and after sulfurization. The PL quenches ~51% after the sulfurization. (i) PL spectra of MoS₂/c-sapphire before and after sulfurization. The PL is ~10 meV blue shifted after the sulfurization, which may be due to the reduction of S vacancies after the sulfurization.