Excitons In Monolayer MoS2 Research Articles

Biexcitons-plasmon coupling of Ag@Au hollow nanocube/MoS2 heterostructures based on scattering spectra.

The strong interaction between light and matter is one of the current research hotspots in the field of nanophotonics, and provides a suitable platform for fundamental physics research such as on nanolasers, high-precision sensing in biology, quantum communication and quantum computing. In this study, double Rabi splitting was achieved in a composite structure monolayer MoS2 and a single Ag@Au hollow nanocube (HNC) in room temperature mainly due to the two excitons in monolayer MoS2. Moreover, the tuning of the plasmon resonance peak was realized in the scattering spectrum by adjusting the thickness of the shell to ensure it matches the energy of the two excitons. Two distinct anticrossings are observed at both excitons resonances, and large double Rabi splittings (90 meV and 120 meV) are obtained successfully. The finite-difference time domain (FDTD) method was also used to simulate the scattering spectra of the nanostructures, and the simulation results were in good agreement with the experimental results. Additionally, the local electromagnetic field ability of the Ag@Au hollow HNC was proved to be stronger by calculating and comparing the mode volume of different nanoparticles. Our findings provides a good platform for the realization of strong multi-mode coupling and open up a new way to construct nanoscale photonic devices.

Observation of ~100% valley-coherent excitons in monolayer MoS2 through giant enhancement of valley coherence time

In monolayer transition metal dichalcogenide semiconductors, valley coherence degrades rapidly due to a combination of fast scattering and inter-valley exchange interaction. This leads to a sub-picosecond valley coherence time, making coherent manipulation of exciton a highly challenging task. Using monolayer MoS2 sandwiched between top and bottom graphene, here we demonstrate fully valley-coherent excitons by observing ~100% degree of linear polarization in steady state photoluminescence. This is achieved in this unique design through a combined effect of (a) suppression in exchange interaction due to enhanced dielectric screening, (b) reduction in exciton lifetime due to a fast inter-layer transfer to graphene, and (c) operating in the motional narrowing regime. We disentangle the role of the key parameters affecting valley coherence by using a combination of calculation (solutions of Bethe-Salpeter and Maialle-Silva-Sham equations) and a careful choice of design of experiments using four different stacks with systematic variation of screening and exciton lifetime. To the best of our knowledge, this is the first report in which the excitons are found to be valley coherent in the entire lifetime in monolayer semiconductors, allowing optical readout of valley coherence possible.

Direct Observation of Room‐Temperature Intravalley Coherent Coupling Processes in Monolayer MoS2

AbstractComprehensive understanding of the interactions between excited states with spin‐valley polarizations is crucial for the applications of monolayer 2D transition metal dichalcogenides (TMDs) in spin‐valleytronics. Here, by broadband femtosecond transient absorption spectroscopy with circularly polarized pump/probe lights, a systematic investigation on the many‐body interactions of band‐edge valley excitons in monolayer MoS2 is performed, where intravalley coherent coupling processes are directly observed at room temperature. Namely, when the intravalley mixing states in the superposition region of A‐exciton and B‐exciton states are excited, there is an intravalley coherent coupling between A‐exciton and B‐exciton in K valley (or A′‐exciton and B′‐exciton in K′ valley) with an oscillation frequency of ≈7–8 THz, rather than the intervalley excitonic coupling in general. The findings elucidate intravalley mixing and coherent oscillations in valley excitons of monolayer TMDs at room temperature, which can give insight into the many‐body coupling mechanisms of excited valley excitons in 2D TMDs.

Ultrafast Exciton Dynamics in Scalable Monolayer MoS2 Synthesized by Metal Sulfurization.

Excitons in monolayer transition metal dichalcogenides (TMDs) have exceptionally large binding energies and dominate the optical properties of materials. Exploring the relaxation behavior of excitons is crucial for understanding the fundamental physics as well as the performance of TMD-based optoelectronic devices. However, ultrafast carrier dynamics is sensitive to the structural defects and surface conditions of TMDs, depending on the growth or transfer process. Here, we utilized pump-probe transient absorption (TA) spectroscopy with a white-light probe to investigate the dynamics of excitons in monolayer MoS2 synthesized by the metal sulfurization method. The sulfurization method was used for the fabrication of large-scale, continuous, and uniform thin films with a controllable number of layers. The excitation dynamics of the wafer-size monolayer MoS2 is found to be comparable to that of monolayer MoS2 flakes grown by chemical vapor deposition (CVD). The dominant processes of carrier relaxation in the monolayer MoS2 are exciton–exciton annihilation (hundreds of femtoseconds), the trapping of the excitons by surface states (a few picoseconds), and interband carrier-phonon scattering (tens of picoseconds). Moreover, the induced absorption due to mid-gap defects, which is often observed for samples fabricated by growth methods, such as CVD, is not observed for our continuous and uniform monolayer films. Understanding the charge carrier dynamics of the exciton in the scalable and uniform monolayer MoS2 can provide physical insights that are valuable in the design and development of complex 2D devices.

Slow Cooling of High‐Energy C Excitons Is Limited by Intervalley‐Transfer in Monolayer MoS2

AbstractHigh‐energy C exciton in 2D transition metal dichalcogenides with strong photon absorption can be utilized prospectively in light‐harvesting and opto‐electric devices. Here, a detailed study on C exciton dynamics in monolayer MoS2 is presented by femtosecond transient absorption (TA) spectroscopy. In the experiment, the C exciton with unique parallel band structure exhibits a slow process within tens of picoseconds (≈10–50 ps) as well as a fast process within several picoseconds (≈1–6 ps) instead of previous reported ultrafast cooling (<500 fs) process. From TA spectroscopy, the experimental results confirmed that the relatively slow cooling of C exciton is mainly limited by the rates of intervalley transfer rather than Pauli blocking effect from band‐edge excitons. The timescale of intervalley transfer is longer than that of intraband relaxation of C exciton, therefore, leading to the slow cooling of C exciton. Moreover, intervalley transfer time are estimated with two processes of 5.1 ± 0.6 ps and 69.5 ± 8 ps when the pump fluence is ≈127 µJ cm−2. This work provides further understanding of the ultrafast dynamics of C exciton in monolayer MoS2 and opens new opportunities for opto‐electric related applications.

Tuning carrier concentration in a superacid treated MoS2 monolayer

The effect of bis(trifluoromethane) sulfonimide (TFSI, superacid) treatment on the optical properties of MoS2 monolayers is investigated by means of photoluminescence, reflectance contrast and Raman scattering spectroscopy employed in a broad temperature range. It is shown that when applied multiple times, the treatment results in progressive quenching of the trion emission/absorption and in the redshift of the neutral exciton emission/absorption associated with both the A and B excitonic resonances. Based on this evolution, a trion complex related to the B exciton in monolayer MoS2 is unambiguously identified. A defect-related emission observed at low temperatures also disappears from the spectrum as a result of the treatment. Our observations are attributed to effective passivation of defects on the MoS2 monolayer surface. The passivation reduces the carrier density, which in turn affects the out-of-plane electric field in the sample. The observed tuning of the carrier concentration strongly influences also the Raman scattering in the MoS2 monolayer. An enhancement of Raman scattering at resonant excitation in the vicinity of the A neutral exciton is clearly seen for both the out-of-plane A′1 and in-plane E′ modes. On the contrary, when the excitation is in resonance with a corresponding trion, the Raman scattering features become hardly visible. These results confirm the role of the excitonic charge state plays in the resonance effect of the excitation energy on the Raman scattering in transition metal dichalcogenides.

Effects of contact material on complex excitonic behaviour of monolayer MoS2

We investigated the optical behaviour of excitons in monolayer MoS2 prepared on a 60 nm-thick metal (Au) ring structure patterned on SiO2. CVD-grown monolayer MoS2 was transferred to the metal structure and placed in contact with three different environments simultaneously: SiO2, Au metal and air. The optical properties of the transferred MoS2 on the substrates were characterized through micro-photoluminescence (μ-PL) measurements. PL peaks were observed near ∼1.82 eV for defect-related emission, ∼1.92 eV for an A exciton, which includes a neutral exciton and a trion, and ∼2.1 eV for a B exciton. The optical band gap and exciton-phonon interaction of the samples on the three substrates were investigated by measuring the temperature-dependent photoluminescence.

Physical mechanism on exciton-plasmon coupling revealed by femtosecond pump-probe transient absorption spectroscopy

The unclear mechanism of exciton-plasmon coupling interaction in monolayer MoS2-Ag nanoparticles hybrid, as a longstanding target in molecular nanotechnology and catalysis, is systemically investigated with transmission spectra and femtosecond pump-probe transient absorption spectroscopy in this paper. The properties of exciton in monolayer MoS2 are strongly enhanced due to the local surface plasmon resonance (LSPR) induced by Ag nanoparticles, and manifested by obvious changes in transmission spectra. Furthermore, we discuss the dynamic processes of exciton-plasmon coupling interaction with the femtosecond transient absorption spectroscopy, which indicates that there are three lifetimes, Auger scattering, electron-electron interaction and electron-phonon interaction, and illustrate the reason of the enlarged lifetime in hybrid system. Meanwhile, the intensity of A excitonic state in femtosecond transient absorption spectroscopy is significantly enhanced by LSPR, instead of excitonic state B. In conclusion, our study can promote the deeper understanding and illustrate the unique merits of the exciton-plasmon coupling interaction in the monolayer MoS2-Ag nanoparticles hybrid system.

Self-energy effect and Coulomb potential modulation of the exciton in monolayer MoS2 on polar substrate

We theoretically investigate the correction of exciton binding energy in monolayer MoS2 resulting from the exciton couples with surface optical (SO) phonons induced by polar substrate. The total correction of binding energy can be divided into the self-energy effect and modification of Coulomb potential using the unitary transformation method. We find that both the self-energy and Coulomb potential vary from tens of meV to several hundreds of meV depending on the cut-off wave vector of SO phonon modes, polarizability of substrate materials and internal distance between the monolayer MoS2 and polar substrate. An effective Coulomb potential is obtained by combining the modified term into the Coulomb potential. This potentially could be widely used in various two-dimensional materials. Our theoretical results not only propose the ways to externally control the exciton binding energy in experiment, but also enrich the understanding of the exciton properties in the dielectric environment.

Exciton Hall effect in monolayer MoS2.

The spontaneous Hall effect driven by the quantum Berry phase (which serves as an internal magnetic flux in momentum space) manifests the topological nature of quasiparticles and can be used to control the information flow, such as spin and valley. We report a Hall effect of excitons (fundamental composite particles of electrons and holes that dominate optical responses in semiconductors). By polarization-resolved photoluminescence mapping, we directly observed the Hall effect of excitons in monolayer MoS2 and valley-selective spatial transport of excitons on a micrometre scale. The Hall angle of excitons is found to be much larger than that of single electrons in monolayer MoS2 (ref.), implying that the quantum transport of the composite particles is significantly affected by their internal structures. The present result not only poses a fundamental problem of the Hall effect in composite particles, but also offers a route to explore exciton-based valleytronics in two-dimensional materials.

Valley-Coherent Hot Carriers and Thermal Relaxation in Monolayer Transition Metal Dichalcogenides.

We show room-temperature valley coherence in MoS2, MoSe2, WS2, and WSe2 monolayers using linear polarization-resolved hot photoluminescence (PL) at energies close to the excitation, demonstrating preservation of valley coherence before sufficient scattering events. The features of the copolarized hot luminescence allow us to extract the lower bound of the binding energy of the A exciton in monolayer MoS2 as 0.42 (±0.02) eV. The broadening of the PL peak is found to be dominated by a Boltzmann-type hot luminescence tail, and using the slope of the exponential decay, the carrier temperature is extracted in situ at different stages of energy relaxation. The temperature of the emitted optical phonons during the relaxation process is probed by exploiting the corresponding broadening of the Raman peaks due to temperature-induced anharmonic effects. The findings provide a physical picture of photogeneration of valley-coherent hot carriers and their subsequent energy relaxation pathways.

Exciton binding energy in bulk MoS2: A reassessment

Transmittance and photocurrent (PC) spectroscopy has been used to study absorption in bulk 2H-MoS2 at energies close to its direct bandgap at the K-point of the Brillouin zone. Spectral lineshape analysis using the hydrogenic exciton model, together with temperature dependence of absorption and PC spectrum, and also bias dependence of PC, suggests that the feature previously identified as the n = 2 excited state transition of the A exciton, and used to estimate its binding energy Eb, has a different origin. The feature is reproduced in simulations only after including the recently identified H-point exciton transition. A consistent picture, which explains Eb in terms of other experimentally and theoretically determined parameters, emerges when excitons in bulk MoS2 are considered as quasi 2-dimensional with Eb∼84 meV for the A exciton at the K-point. This value when scaled appropriately matches fairly with a measured Eb of the A exciton in monolayer MoS2.

Plasma functionalization for cyclic transition between neutral and charged excitons in monolayer MoS2.

Monolayer MoS2 (1L-MoS2) has photoluminescence (PL) properties that can greatly vary via transition between neutral and charged exciton PLs depending on carrier density. Here, for the first time, we present a chemical doping method for reversible transition between neutral and charged excitons of 1L-MoS2 using chlorine-hydrogen-based plasma functionalization. The PL of 1L-MoS2 is drastically increased by p-type chlorine plasma doping in which its intensity is easily tuned by controlling the plasma treatment duration. We find that despite their strong adhesion, a post hydrogen plasma treatment can very effectively dedope chlorine adatoms in a controllable way while maintaining robust structural integrity, which enables well-defined reversible PL control of 1L-MoS2. After exhaustive chlorine dedoping, the hydrogen plasma process induces n-type doping of 1L-MoS2, degrading the PL further, which can also be recovered by subsequent chlorine plasma treatment, extending the range of tunable PL into a bidirectional regime. This cyclically-tunable carrier doping method can be usefully employed in fabricating highly-tunable n- and p-type domains in monolayer transition-metal dichalcogenides suitable for two-dimensional electro-optic modulators, on-chip lasers, and spin- and valley-polarized light-emitting diodes.