Exciton and trion dynamics in atomically thin ${\mathrm{MoSe}}_{2}$ and ${\mathrm{WSe}}_{2}$: Effect of localization

Exciton and trion dynamics in atomically thin ${MoSe}_{2}$ and ${WSe}_{2}$ : Effect of localization

T. Godde, D. Schmidt, J. Schmutzler, M. Aßmann, J. Debus, F. Withers, E. M. Alexeev, O. Del Pozo-Zamudio, O. V. Skrypka, K. S. Novoselov, M. Bayer, and A. I. Tartakovskii

Phys. Rev. B 94, 165301 – Published 5 October 2016

Abstract

We present a detailed investigation of the exciton and trion dynamics in naturally doped ${MoSe}_{2}$ and ${WSe}_{2}$ single atomic layers as a function of temperature in the range 10–300 K under above band-gap laser excitation. By combining time-integrated and time-resolved photoluminescence (PL) spectroscopy, we show the importance of exciton and trion localization in both materials at low temperatures. We also reveal the transition to delocalized exciton complexes at higher temperatures where the exciton and trion thermal energy exceeds the typical localization energy. This is accompanied by strong changes in PL including suppression of the trion PL and decrease of the trion PL lifetime, as well as significant changes for neutral excitons in the temperature dependence of the PL intensity and the appearance of a pronounced slow PL decay component. In ${MoSe}_{2}$ and ${WSe}_{2}$ studied here, the temperatures where such strong changes occur are observed at around 100 and 200 K, respectively, in agreement with their inhomogeneous PL linewidth of 8 and 20 meV at $T \approx 10 K$ . The observed behavior is a result of a complex interplay between influences of the specific energy ordering of bright and dark excitons in ${MoSe}_{2}$ and ${WSe}_{2}$ , sample doping, trion, and exciton localization and various temperature-dependent nonradiative processes.

Received 19 August 2016

DOI:https://doi.org/10.1103/PhysRevB.94.165301

Physics Subject Headings (PhySH)

Electronic structure Excitons Semiconductors Trions

2-dimensional systems Transition metal dichalcogenides

Time-resolved photoluminescence

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

T. Godde¹, D. Schmidt², J. Schmutzler², M. Aßmann², J. Debus², F. Withers³, E. M. Alexeev¹, O. Del Pozo-Zamudio¹, O. V. Skrypka¹, K. S. Novoselov³, M. Bayer², and A. I. Tartakovskii¹

¹Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, United Kingdom
²Experimentelle Physik 2, Technische Universität Dortmund, D-44221 Dortmund, Germany
³School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, United Kingdom

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Vol. 94, Iss. 16 — 15 October 2016

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Figure 1
Time-integrated (TI) and time-resolved (TR) photoluminescence (PL) for single-monolayer ${MoSe}_{2}$ samples. (a) TIPL spectra in the range 12–110 K. Exciton ( $X$ ) and trion ( $X^{*}$ ) peaks are labeled. Spectra are multiplied by factors shown on the graph. (b) TRPL PL traces for $T = 12$ K for $X$ (squares) and $X^{*}$ (circles) (sample 1). Lines show the fitting functions. The inset shows the $X$ PL decay time and $X^{*}$ rise time as a function of gate voltage in sample 5. The gate voltage is used to vary the carrier concentration in the film with a positive voltage corresponding to a higher concentration. (c) and (d) TRPL traces for $X^{*}$ (sample 1) in (c) and $X$ (sample 2) in (d) for a range of temperatures. Normalized PL intensities are shown.
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Figure 2
Temperature dependence of time-integrated (TI) and time-resolved (TR) photoluminescence (PL) for single-monolayer ${MoSe}_{2}$ samples. (a) Spectrally integrated TIPL intensity. Dependencies for $X$ (squares) and $X^{*}$ (circles) are shown for samples 1 and 2. (b) $X$ (squares) and $X^{*}$ (circles) PL decay times in samples 1 (solid symbols) and 2 (open symbols). The horizontal lines mark the setup resolution for the two experiments: the dashed line is for the 12 ps resolution corresponding to the data shown with the open symbols; the solid line is for 4 ps resolution corresponding to the data shown with the solid symbols. (c) The ratio of the time integral of the function used to fit the slow PL decay component, $I_{slow}$ , and the total integral under the PL decay curve, $I_{tot}, r = I_{slow} / I_{tot}$ .
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Figure 3
Time-integrated (TI) and time-resolved (TR) photoluminescence (PL) for single-monolayer ${WSe}_{2}$ samples. (a) A TIPL spectrum measured at $T = 9$ K in sample 4. Neutral exciton ( $X$ ) and trion ( $X^{*}$ ) peaks as well as localized exciton peaks $P_{1}, P_{2}$ , and $P_{3}$ are labeled. (b) TIPL in a temperature range 9–293 K. Individual spectra at each temperature are normalized. Features as in graph (a) are labeled. (c) and (d) TRPL traces for $X$ (c) and $X^{*}$ (d) in sample 3 for a range of temperatures. Normalized PL intensities are shown.
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Figure 4
Temperature dependence of time-integrated (TI) and time-resolved (TR) photoluminescence (PL) for single-monolayer ${WSe}_{2}$ samples. (a) Spectrally integrated TIPL intensity. Dependencies for $X$ (squares) and all PL in the range shown in Fig. 3 (triangles) are shown. (b),(c) $X$ (squares) and $X^{*}$ (circles) PL decay times. The data for the slow and fast decay components are shown in (b) and (c), respectively. Open and solid symbols show $X$ PL decay data for two different samples. (d) The ratio of the time integral of the function used to fit the slow PL decay component, $I_{slow}$ , and the total integral under the PL decay curve, $I_{tot}, r = I_{slow} / I_{tot}$ . Data for both $X$ (squares) and $X^{*}$ (circles) are shown.
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Physical Review B

covering condensed matter and materials physics

Exciton and trion dynamics in atomically thin ${MoSe}_{2}$ and ${WSe}_{2}$ : Effect of localization

T. Godde, D. Schmidt, J. Schmutzler, M. Aßmann, J. Debus, F. Withers, E. M. Alexeev, O. Del Pozo-Zamudio, O. V. Skrypka, K. S. Novoselov, M. Bayer, and A. I. Tartakovskii

Phys. Rev. B 94, 165301 – Published 5 October 2016

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Physical Review B

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Exciton and trion dynamics in atomically thin MoSe2 and WSe2: Effect of localization

T. Godde, D. Schmidt, J. Schmutzler, M. Aßmann, J. Debus, F. Withers, E. M. Alexeev, O. Del Pozo-Zamudio, O. V. Skrypka, K. S. Novoselov, M. Bayer, and A. I. Tartakovskii

Phys. Rev. B 94, 165301 – Published 5 October 2016

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Exciton and trion dynamics in atomically thin ${MoSe}_{2}$ and ${WSe}_{2}$ : Effect of localization