Elsevier

Applied Surface Science

Volume 463, 1 January 2019, Pages 52-57
Applied Surface Science

Full Length Article
Unraveling biexciton and excitonic excited states from defect bound states in monolayer MoS2

https://doi.org/10.1016/j.apsusc.2018.08.205Get rights and content

Highlights

  • Systematic observation of biexciton from 4 K to room temperature by photoluminescence study of monolayer MoS2.

  • Distinction of biexciton from A- trion and ground state of A exciton in MoS2.

  • Exciton bound to sulfur vacancies have been distinguished from biexciton having binding energy ∼ 60 meV.

Abstract

Being direct band gap semiconductor, two-dimensional monolayer (ML) MoS2 has remarkable optical transitions arising from excitons, trions, biexciton and defects mediated bound states. However, experimental realization of biexciton in ML MoS2 has been challenging due to broad spectral feature of exciton. Here, we report on systematic observation of biexciton (AXX ∼ 1.90 eV) along with A- trion ∼ 1.92 eV and ground state exciton A1s ∼ 1.96 eV in ML MoS2 at 4 K, by laser-power and temperature-dependent non-resonant photoluminescence (PL) spectroscopy. At low temperatures the excited state of A exciton has also been observed, A2s ∼ 2.13 eV, which consequently merges in thermal broadening of B excitons, with rise in temperature. With excitation energy and power dependent PL, emission arising from exciton bound to sulfur vacancy (∼1.82 eV) have been distinguished from biexciton. Thus understanding of such excitonic states and biexcitons is useful for future quantum information processing, optoelectronic, photonics and THz applications.

Graphical abstract

Systematic observation of biexciton AXX and A2s excited state of A exciton along with ground state (A1s) in monolayer MoS2 at 4 K, by temperature dependent and power dependent non-resonant photoluminescence spectroscopy.

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Introduction

The rigorous scientific exploration of two dimensional (2D) transition metal dichalcogenides (TMDCs) is motivated by confinement mediated versatile properties [1], [2], [3]. Among these materials, MoS2 [4], [5], WS2 [6], [7], WSe2 [8], [9], are stable semiconducting compounds with thickness tunable optical band gap in visible region and strong light matter interaction [1], [2], [4]. Owing to the confinement effect, the ML semiconducting TMDCs with direct band gap has been explored for efficient light matter interactions [10], [11], photo detectors [10], photovoltaic [11], tera-hertz devices [12] and valley selective polarization [13] or valleytronics [14], [15], [16]. Optoelectronic properties are implicit with dynamic behavior of photogenerated quasiparticle, known as A and B excitons [17], [18], trions [19] and biexcitons [20], [21], [22], appearing due to strong Coulomb interactions driven by reduced dimensionality and weak dielectric screening. Biexcitons in monolayer TMDCs have large binding energy in the range of 50–70 meV [21], [23], comparable with binding energies of trions and excitons, therefore, expected to be stable even up to room temperature. Interestingly, excitons and trions have been studied extensively but the research on biexciton of MoS2 is still scarce. This strong binding energy necessitates the full understanding about biexcitons for fundamental studies of many-body interactions as well as for several applications such as biexciton lasing devices [24], quantum logic gates [25].

Current research on excitons of TMDCs suggests that although exciton has been studied rigorously yet it has not been explored completely. Several theoretical and experimental reports have shown that excitons exhibit a series of excited states (1s, 2s, 3s, 2p and 3p), called Rydberg states in analogy to the hydrogen series [17], [18], [26], [27]. The ability to spectrally observe Rydberg state is critically dependent on material quality, synthesis route, surrounding dielectric environment, surface passivation. Pristine ML of MoS2, WS2, WSe2, obtained through mechanical exfoliation provide best quality flakes to observe less intense excited states of excitons. Access to series of Rydberg states can be used to estimate free particle band gap as well as binding energy of excitonic ground state, which are otherwise difficult to measure in ML TMDCs. In this context, binding energies of A exciton of WS2 (∼ 0.32 eV) and B exciton of MoS2 (∼ 0.44 eV) along with quasiparticle band gaps have been estimated by Rydberg states [17], [18]. Rydberg states in ML MoS2 [17], [26], WS2 [18] and WSe2 [27], [28] have gained enormous interests for many-body interactions and applications in future quantum as well as optoelectronic technologies [26].

Optical absorption and emission from direct band gap thin layers carries information about the electronic band structure and nature of excitons, trions and biexcitons [4]. Owing to the broad excitonic feature of A exciton, the energy of A exciton lies in a wide range starting from 1.92 to 1.96 eV (Supporting information, Table S1) and in some cases A exciton overlaps with A- trion energy. This ambiguity in assignment of trion and exciton energy results in uncertainty in determination of binding energy. Moreover, unlike to other TMDCs such as WS2 [29], MoSe2 [22] where excitonic features are sharp and biexcitons can be easily identified, determination of biexciton in ML MoS2 is challenging. Further, observation of Rydberg state of A exciton has also been reported frequently for ML WS2 and WSe2 even at room temperature because of larger spin-orbit splitting between A and B exciton for WS2 (∼ 0.4 eV) [17], [18] and WSe2 (∼ 0.45 eV) [27]. On the other hand, owing to relatively smaller spin-orbit splitting for ML MoS2 (∼ 0.15 eV), though Rydberg series for B exciton have been observed [17], [26], yet excited states of A exciton remains challenging. To access excitonic states and their properties, various experiments such as linear one-photon absorption [18], [27], ultra-fast mid-infrared spectroscopy [26], two-photon photoluminescence excitation [17], [27], [30] and nonlinear wave-mixing spectroscopy [31] have been adopted.

We report on observation of biexciton, AXX ∼ 1.90 eV with binding energy ∼ 60 meV and A2s (∼ 2.13 eV) Rydberg state for ML MoS2, using non-resonant and power dependent photoluminescence at cryogenic temperatures. Observed broad asymmetric emission from exciton bound to sulfur vacancy has unusually different temperature dependent PL response, thus distinguished from excitonic states. With systematic analysis of laser power and temperature dependent study and due to high binding energy of AXX, we found that AXX exists even at 300 K, though indistinguishable due to thermal broadening of A and B exciton.

Section snippets

Materials and methods

High quality 2H-MoS2 natural crystal was used for mechanical exfoliation by scotch tape [32], followed by transferring on to Si (0 0 1) substrate with 300 nm SiO2, (SiO2/Si). Prior to flakes transfer, substrate was sequentially sonicated in acetone and isopropanol for 15 min each. Subsequently, substrate was dipped in a freshly prepared piranha solution (H2SO4:H2O2, 3:1) for 30 min and thoroughly rinsed with de-ionized water followed by blow drying with nitrogen gas and keeping on hot plate at

Result and discussion

Optical image of MoS2 flake on SiO2/Si substrate is shown as inset of Fig. 1(a). Grey scale contrast value is estimated to be ∼0.1 for optical image of ML MoS2, along the arrow in inset. Shown in Fig. 1(b), absence of interlayer modes (< 60 cm−1) [33] and difference ∼ 19 cm−1 between two characteristic Raman modes of E2g1 (∼ 386 cm−1) and A1g(∼ 405 cm−1) confirms the presence of high quality ML [34], [35]. Observed PL has been understood by three known transitions appearing from A- trion, A and

Conclusion

In summary, we report on observation of biexciton AXX and A2s excited state of A exciton and distinction of defect bound excitons from excitonic states in ML MoS2. Observation of such states in ML MoS2 is challenging due to large spectral broadening of A exciton and small spin orbit gap between A1s and B1s excitonic states. After careful analysis of low temperature dependent non-resonant and power dependent PL spectrum, we have identified the optical transitions from biexciton (AXX ∼ 1.90 eV),

Supporting information

Details of various features in PL spectra, Fitting of PL spectrum, Exciton-phonon coupling parameters, Defects in 2D MoS2.

Acknowledgements

AS would like to acknowledge IIT Mandi for temperature dependent Raman and Photoluminescence Facility.

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