Oxygen Passivation Mediated Tunability of Trion and Excitons in ${\mathrm{MoS}}_{2}$

Oxygen Passivation Mediated Tunability of Trion and Excitons in ${MoS}_{2}$

Pranjal Kumar Gogoi, Zhenliang Hu, Qixing Wang, Alexandra Carvalho, Daniel Schmidt, Xinmao Yin, Yung-Huang Chang, Lain-Jong Li, Chorng Haur Sow, A. H. Castro Neto, Mark B. H. Breese, Andrivo Rusydi, and Andrew T. S. Wee

Phys. Rev. Lett. 119, 077402 – Published 16 August 2017

Abstract

Using wide spectral range in situ spectroscopic ellipsometry with systematic ultrahigh vacuum annealing and in situ exposure to oxygen, we report the complex dielectric function of ${MoS}_{2}$ isolating the environmental effects and revealing the crucial role of unpassivated and passivated sulphur vacancies. The spectral weights of the $A$ (1.92 eV) and $B$ (2.02 eV) exciton peaks in the dielectric function reduce significantly upon annealing, accompanied by spectral weight transfer in a broad energy range. Interestingly, the original spectral weights are recovered upon controlled oxygen exposure. This tunability of the excitonic effects is likely due to passivation and reemergence of the gap states in the band structure during oxygen adsorption and desorption, respectively, as indicated by ab initio density functional theory calculation results. This Letter unravels and emphasizes the important role of adsorbed oxygen in the optical spectra and many-body interactions of ${MoS}_{2}$ .

Received 6 February 2017

DOI:https://doi.org/10.1103/PhysRevLett.119.077402

Physics Subject Headings (PhySH)

Excitons Optical conductivity Trions

Dichalcogenides Quantum wells

Density functional theory Ellipsometry

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Pranjal Kumar Gogoi^1,2,*, Zhenliang Hu¹, Qixing Wang¹, Alexandra Carvalho³, Daniel Schmidt², Xinmao Yin¹, Yung-Huang Chang⁴, Lain-Jong Li⁵, Chorng Haur Sow^1,3, A. H. Castro Neto^1,3, Mark B. H. Breese^1,2, Andrivo Rusydi^1,2,6,†, and Andrew T. S. Wee^1,3,‡

¹Department of Physics, Faculty of Science, National University of Singapore, Singapore 117542, Singapore
²Singapore Synchrotron Light Source, National University of Singapore, 5 Research Link, Singapore 117603, Singapore
³Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, Singapore 117542, Singapore
⁴Department of Electrophysics, National Chiao Tung University, Hsinchu 30010, Taiwan
⁵Physical Science and Engineering Division, King Abdullah University of Science and Technology (KAUST), Thuwal 23955, Saudi Arabia
⁶NUSNNI-NanoCore, National University of Singapore, Singapore 117576, Singapore

^*phypkg@nus.edu.sg
^†phyandri@nus.edu.sg
^‡phyweets@nus.edu.sg

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Vol. 119, Iss. 7 — 18 August 2017

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Figure 1
Optical spectra of monolayer ${MoS}_{2}$ measured at 300 K. The comparison of $ϵ_{1}$ ( $ϵ_{2}$ ) measured before and after annealing in UHV at 700 K is shown in (a),(b). Evolution of the excitonic peaks $A$ and $B$ are shown in (c) and (d) when exposed to nitrogen and oxygen after annealing at 700 K. The direction of evolution of the peak structures (with time) is indicated by the arrows. Note that results shown in (a) and (b) are from UHV measurements, while (c) and (d) are from measurements where the sample chamber is flushed with the respective gases.
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Figure 2
Real part of the optical conductivity and spectral weight for different annealing temperatures and measured at 300 K after cooling down. Real part of the optical conductivity for different annealing temperatures are shown in (a). The spectral weight for the full range (0.6–5.7 eV) and region II (1.7–2.2 eV) are plotted in (b) and (c), respectively. The spectral weight for regions I (0.6–1.7 eV), III (2.2–3.3 eV), and IV (3.3–5.7 eV) are shown in (d). In (b), (c), and (d), the dashed lines are guides to the eye.
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Figure 3
Contributions of excitons and trion to the optical spectra of ${MoS}_{2}$ . Data and fit of $ϵ_{2}$ using Lorentzian-Gaussian line shapes for $X_{Bound}$ , $X_{T}$ , $X_{A}$ , and $X_{B}$ are shown in (a), (b), and (c) for annealing temperatures of 300, 500, and 700 K, respectively. Spectral weights of $X_{Bound}$ , $X_{T}$ , and $X_{A}$ are depicted in (d). The ratio of the spectral weight of $X_{T}$ to $X_{T}$ , $X_{A}$ , and $X_{Bound}$ together against annealing temperature are shown in (e). Difference in the energy positions for $X_{A}$ and $X_{T}$ with respect to annealing temperature is plotted in (f). In (d), (e), and (f), the dashed lines are guides to the eye. The shaded portions in (e) and (f) indicate regions where desorption of chemisorbed oxygen affects the respective quantities conspicuously.
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Figure 4
Various configurations of oxygen adsorption with corresponding energies. (a) Sulphur, molybdenum, and oxygen are represented by yellow, blue, and red spheres, respectively. The sulphur vacancy site is represented by the cross. (b) Nudged elastic band calculation of the energy barrier for migration of an oxygen molecule towards a sulphur vacancy and respective trapping. The energy barrier, measured from the starting point, is 56 meV. The calculation was performed in the spin-averaged state. (c) Calculated ionization levels for relevant defects (all energies are in eV). VBM and CBM refer to the valence band maximum and conduction band minima, respectively. (d) Representation of the charge density of the trapped electrons at sulfur vacancies.
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Physical Review Letters

Oxygen Passivation Mediated Tunability of Trion and Excitons in MoS2

Pranjal Kumar Gogoi, Zhenliang Hu, Qixing Wang, Alexandra Carvalho, Daniel Schmidt, Xinmao Yin, Yung-Huang Chang, Lain-Jong Li, Chorng Haur Sow, A. H. Castro Neto, Mark B. H. Breese, Andrivo Rusydi, and Andrew T. S. Wee

Phys. Rev. Lett. 119, 077402 – Published 16 August 2017

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Oxygen Passivation Mediated Tunability of Trion and Excitons in ${MoS}_{2}$