Observation of two distinct negative trions in tungsten disulfide monolayers

Abdelaziz Boulesbaa, Bing Huang, Kai Wang, Ming-Wei Lin, Masoud Mahjouri-Samani, Christopher Rouleau, Kai Xiao, Mina Yoon, Bobby Sumpter, Alexander Puretzky, and David Geohegan

Phys. Rev. B 92, 115443 – Published 25 September 2015

Abstract

Ultrafast pump-probe spectroscopy of two-dimensional tungsten disulfide monolayers $(2 D W S_{2})$ grown on sapphire substrates revealed two transient absorption spectral peaks that are attributed to distinct negative trions at $\sim 2.02 eV (T_{1})$ and $\sim 1.98 eV (T_{2})$ . The dynamics measurements indicate that trion formation by the probe is enabled by photodoped 2D ${WS}_{2}$ crystals with electrons remaining after trapping of holes from excitons or free electron-hole pairs at defect sites in the crystal or on the substrate. Dynamics of the characteristic absorption bands of excitons $X_{A}$ and $X_{B}$ at $\sim 2.03$ and $\sim 2.40 eV$ , respectively, were separately monitored and compared to the photoinduced absorption features. Selective excitation of the lowest exciton level $X_{A}$ using $λ_{pump} < 2.4 eV$ forms only trion $T_{1}$ , implying that the electron remaining from dissociation of exciton $X_{A}$ is involved in the creation of this trion with a binding energy $\sim 10 meV$ with respect to $X_{A}$ . The absorption peak corresponding to trion $T_{2}$ appears when $λ_{pump} < 2.4 eV$ , which is just sufficient to excite exciton $X_{B}$ . The dynamics of trion $T_{2}$ formation are found to correlate with the disappearance of the bleach of the $X_{B}$ exciton, indicating the involvement of holes participating in the bleach dynamics of exciton $X_{B}$ . Static electrical-doping photoabsorption measurements confirm the presence of an induced absorption peak similar to that of $T_{2}$ . Since the proposed trion formation process here involves exciton dissociation through hole trapping by defects in the 2D crystal or substrate, this discovery highlights the strong role of defects in defining optical and electrical properties of 2D metal chalcogenides, which is relevant to a broad spectrum of basic science and technological applications.

Received 29 April 2015
Revised 28 August 2015

DOI:https://doi.org/10.1103/PhysRevB.92.115443

Physics Subject Headings (PhySH)

Excitons Trions

2-dimensional systems Dichalcogenides Monolayer films

Ultrafast pump-probe spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Abdelaziz Boulesbaa^*, Bing Huang, Kai Wang, Ming-Wei Lin, Masoud Mahjouri-Samani, Christopher Rouleau, Kai Xiao, Mina Yoon, Bobby Sumpter, Alexander Puretzky, and David Geohegan

Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, 1 Bethel Valley Road, Oak Ridge, Tennessee, 37831, USA

^*Author to whom correspondence should be addressed: boulesbaaa@ornl.gov

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Vol. 92, Iss. 11 — 15 September 2015

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Figure 1
Band-edge optical transitions in 2D- $W S_{2}$ . (a), Schematic representation of the lowest excitonic transitions $X_{A}$ and $X_{B}$ at the $K$ -point and $K^{'} = - K$ point. Spin states are indicated by small “up” and “down” arrows. (b), Transient absorption 3-D plot upon excitation at $2.1 eV$ . Because $2.1 eV$ excitation is insufficient for $X_{B} (2.4 eV)$ generation, the observed depletion of the $X_{B}$ transition is due to a state-filling effect rather than generation of $X_{B}$ (see Fig. S5 and corresponding supplementary text). (c), $X_{A}$ bleach dynamics upon excitation at different energies (symbols) fitted to a bi-exponential function (solid lines). One fast component describes the hot carriers relaxation, and the second component describes electron-hole recombination. A third component to describe the positive portion of the signal was added ( $+ 0.6$ and $+ 0.2 mOD$ in case of $2.1$ and $3.1 eV$ excitations, respectively). The fitting parameters are listed in Table S1. (d), TAS recorded 500 ps after excitation as indicated. In the case of excitation at $2.1 eV$ , the spectrum (open circles) was fit to one Gaussian describing $T_{1}$ (olive solid line). During the fit of the measured spectrum (filled squares) in the case of excitation at $3.1 eV$ with the sum of two Gaussians (solid dark line), the center and the width of $T_{1}$ were fixed, but the center and the width of $T_{2}$ were returned by the converged fit (shaded areas). The fitting parameters are listed in Table S6.
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Figure 2
Origins of $T_{2}$ and XX. (a), TAS collected after excitation at $3.1 eV$ at time-delays as indicated. (b), TAS collected after excitation at $2.1 eV$ at time-delays as indicated. (c), Dynamics of XX plotted at $1.94 eV$ upon excitation at $3.1$ and $2.1 eV$ , fit to a bi-exponential function (green solid line). The fitting parameters are listed in Table S3. For clarity, the signal amplitude in the case of excitation at $3.1 eV$ was divided by 2 which does not affect its dynamics. (d), Measured dynamics of $T_{2}$ upon excitation at $3.1 eV$ plotted at $1.98 eV$ (filled blue circle) fit to one exponential rise ( $3.7 ps$ time constant) and two exponential decays, see Table S3 (solid pink plot). The fitting parameters are listed in Table S3. For comparison, bleach recoveries of $X_{A}$ and $X_{B}$ upon excitation at $3.1 eV$ are plotted with their amplitudes normalized and shifted to match the amplitude of $T_{2}$ .
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Figure 3
Scheme representative for the experimental setup used for electro-optical static transient absorption measurements (left panel), and absorbance change spectra at different n-doping levels as indicated (right panel).
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Figure 4
Mechanism of $T_{1}$ and $T_{2}$ generation at the $K$ and $K^{'}$ points in a pump-probe experiment. (Left panel) Exciton $X_{A}$ (blue circles) is generated upon quasi-resonant excitation with respect to $X_{A}$ by the pump pulse. Following dissociation of $X_{A}$ (at both $K$ and $K^{'}$ ) through hole trapping at the substrate or a defect site (ST), the remaining electron at the $K (K^{'})$ valley joins (dashed arrows) the newly generated electron-hole pairs (red circles) by the probe at $K^{'} (K)$ to form $T_{1}$ . (Right panel) Upon generation of exciton $X_{B}$ (blue circles) using sufficient pump photon energy, and following its dissociation through hole trapping at ST, the remaining electron at $C_{1}$ binds to the newly generated electron-hole pairs (red circles) through the $V_{1} \to C_{1}$ trion transition upon absorption of probe photons. To account for $T_{1}$ and $T_{2}$ binding energies, they are depicted lower than $C_{2}$ and $C_{1}$ levels, respectively.
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