Full Length ArticleUnraveling biexciton and excitonic excited states from defect bound states in monolayer MoS2
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.
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 (∼ 386 cm−1) and (∼ 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.
References (45)
- et al.
Emerging photoluminescence in monolayer MoS2
Nano Lett.
(2010) - et al.
Colloquium: excitons in atomically thin transition metal dichalcogenides
Rev. Mod. Phys.
(2018) - et al.
Raman spectroscopy of few-quintuple layer topological insulator Bi2Se3 nanoplatelets
Nano Lett.
(2011) - et al.
Atomically thin MoS2: a new direct-gap semiconductor
Phys. Rev. Lett.
(2010) - et al.
Single-layer MoS2 transistors
Nat Nano
(2011) - et al.
Evolution of electronic structure in atomically thin sheets of WS2 and WSe2
ACS Nano
(2013) Large excitonic effects in monolayers of molybdenum and tungsten dichalcogenides
Phys. Rev. B
(2012)- et al.
Photovoltaic and photothermoelectric effect in a double-gated WSe2 device
Nano Lett.
(2014) - et al.
High-performance single layered WSe2 p-FETs with chemically doped contacts
Nano Lett.
(2012) - et al.
Single-layer MoS2 phototransistors
ACS Nano
(2011)
Two-dimensional crystals: managing light for optoelectronics
ACS Nano
Surface optical rectification from layered MoS2 crystal by THz time-domain surface emission spectroscopy
ACS Appl. Mater. Interfaces
Valley polarization of trions and magnetoresistance in heterostructures of MoS2 and yttrium iron garnet
ACS Nano
Control of valley polarization in monolayer MoS2 by optical helicity
Nat. Nanotechnol.
Valley polarization in MoS2 monolayers by optical pumping
Nat. Nanotechnol.
Valley-selective circular dichroism of monolayer molybdenum disulphide
Nat. Commun.
Observation of excitonic rydberg states in monolayer MoS2 and WS2 by photoluminescence excitation spectroscopy
Nano Lett.
Exciton binding energy and nonhydrogenic rydberg series in monolayer WS2
Phys. Rev. Lett.
Tightly bound trions in monolayer MoS2
Nat. Mater.
Excited biexcitons in transition metal dichalcogenides
Nano Lett.
Observation of biexcitons in monolayer WSe2
Nat. Phys.
Excited state biexcitons in atomically thin MoSe2
ACS Nano
Cited by (55)
Rapid synthesis of MoS<inf>2</inf>–Ag nanocomposites via photoreduction for optical tuning and surface-enhanced Raman scattering applications
2023, Journal of MateriomicsCitation Excerpt :The PL peaks were analyzed using Lorentzian fitting method, which is commonly used method for analysis of PL spectra, since excitonic optical emission could be approximated using Lorentzian function [48]. Therefore, PL peaks were split into three peaks, A− trions (green curves), A excitons (blue curves), and B excitons (magenta curves), and the conversion of A− trions to A excitons after the photoreduction was investigated [49,50]. For as-prepared MoS2, it exhibited that A− trion peak at 1.844 eV (672 nm), A exciton peak at 1.869 eV (663 nm), and B exciton peak at 1.985 eV (625 nm), respectively.
Giant photoresponsivity of transfer free grown fluorographene – MoS<inf>2</inf> heterostructured ultra-stable transistors
2021, Materials TodayCitation Excerpt :Though atomic layers such as graphene or metallic TMDs with zero band gap limit their applications in field effect transistors (FETs) and other optoelectronic circuitries, semiconducting TMDs having direct electronic transition in monolayer form have shown potential application in photodetectors, biosensors, light-emitting diodes, and solar cells [1,2,4–7]. Further, these TMDs are found to be exhibiting the presence of quasiparticles like excitons, trions, and biexcitons in their photoluminescence (PL) studies conducted at ambient or sub-ambient temperatures [8,9]. The quantum confinement and limited dielectric screening effects of these TMDs make the excitons extremely stable even at room temperature with high binding energies of ~500–600 meV [9], though this high binding energy can limit their applications in photodetectors and solar cells [7,10].
Sulfur Vacancy Related Optical Transitions in Graded Alloys of Mo<inf>x</inf>W<inf>1-x</inf>S<inf>2</inf> Monolayers
2024, Advanced Optical MaterialsSpectroscopic Study of the Excitonic Structure in Monolayer MoS<inf>2</inf> under Multivariate Physical and Chemical Stimuli
2024, Physica Status Solidi (A) Applications and Materials Science