Direct measurement of biexcitons in monolayer WS2

Tunable exciton–photon couplings are of great importance for cavity quantum electrodynamics (QED), polariton chemistry, optical computing, and bosonic lasing. In this regard, two-dimensional (2D) excitons in transition metal dichalcogenides (TMDs) have attracted tremendous interest─the huge oscillator strengths endow sensitive responses to optical modes, and their excitonic properties can be actively tuned by external stimuli. However, tunable coupling with spin-forbidden dark excitons in monolayer TMDs has rarely been demonstrated, and how the bright/dark-exciton–photon couplings coexist in one system is still a mystery. Herein, we utilize waveguide and antenna modes in a gold nanosphere-on-mirror cavity to match the in-plane and out-of-plane dipole moments of bright and dark excitons in monolayer WS2, respectively, and simultaneously. Strong bright-exciton–photon couplings with a coupling strength of up to 60 meV are observed at room temperature. We show that the waveguide mode can be tuned in wavelengths by controlling the spacer thickness and demonstrate the anti-crossing feature of polaritons. Next, by increasing the excitation angle, we dynamically enlarge the relative contribution from the antenna mode coupled with dark excitons and suppress the waveguide mode coupled with bright states. Moreover, a tunable bright exciton linewidth is achieved as a result of suppressed homogeneous broadening caused by the dominant dark exciton decays under strong dark-exciton–photon couplings. This is not only proof of tunable couplings with dark excitons but also an insightful finding that reveals the novel interactions between bright- and dark-exciton–photon hybrids in a single optical cavity, opening new possibilities in tunable QED.

Being atomically thin, flexible, and exhibiting considerable light emission and ultrafast non-equilibrium dynamics, semiconducting transition metal dichalcogenides (TMDs) have been considered as promising candidates for next-generation optoelectronic devices. The optical and electronic properties of TMDs are governed by a rich landscape of tightly bound excitons, including regular bright excitons, as well as optically inaccessible dark exciton states. Recently, strain engineering of monolayer TMDs has been introduced to tune their optical properties, such as the exciton transition energy, exciton-phonon coupling, or the Stokes shift [1-3].Transport of charge carriers is crucial for nanoelectronics. In conventional materials, electronic transport can be conveniently controlled by external electric fields. However, the tightly bound excitons, being neutral particles, are only weakly affected by electrical fields. We demonstrate that mechanical strain can also be used to manipulate the transport of excitons in TMDs. To this end, we apply homogeneous tensile strain to a WS2 monolayer by bending the substrate [1], which causes a redshift of the X0 exciton photoluminescence. By measuring the spatiotemporal photoluminescence after near-resonant excitation with femtosecond laser pulses, we map the spread of excitons and extract the strain-dependent diffusion coefficient [4].Furthermore, we demonstrate the propagation of excitons in an inhomogeneous strain landscape. We create inhomogeneous tensile strain in TMD monolayers by transferring them onto patterned substrates with nanopillars or by a nanoimprint technique [5]. Due to the redshift of the exciton resonances with applied strain, excitons are expected to move towards high-strain regions in an inhomogeneous strain field - the so-called "funneling" effect. We verify this behavior for the "bright" TMD material monolayer MoSe2. In the case of the "dark" monolayer WS2, we observe exactly the opposite effect. Here, the excitons are expelled from the high-strain regions ("anti-funneling") [6]. By comparing our experimental results with a microscopic theory, we explain this observation by the drift of momentum-dark KΛ excitons, which, in contrast to bright excitons, shift to higher energies with strain.Our joint experiment-theory study highlights the dominant role of momentum-dark excitons for the dynamics in monolayer TMDs and provides crucial design guidelines for TMD devices based on exciton transport.

First-principles study on the electronic and optical properties of WS2 and MoS2 monolayers

Density functional theory calculations of hydrogen molecule adsorption on monolayer molybdenum and tungsten disulfide

Many potential applications of monolayer transition metal dichalcogenides (TMDs) require both high photoluminescence (PL) yield and high electrical mobilities. However, the PL yield of as prepared TMD monolayers is low and believed to be limited by defect sites and uncontrolled doping. This has led to a large effort to develop chemical passivation methods to improve PL and mobilities. The most successful of these treatments is based on the nonoxidizing organic “superacid” bis(trifluoromethane)sulfonimide (TFSI) which has been shown to yield bright monolayers of molybdenum disulfide (MoS2) and tungsten disulfide (WS2) but with trap-limited PL dynamics and no significant improvements in field effect mobilities. Here, using steady-state and time-resolved PL microscopy we demonstrate that treatment of WS2 monolayers with oleic acid (OA) can greatly enhance the PL yield, resulting in bright neutral exciton emission comparable to TFSI treated monolayers. At high excitation densities, the OA treatment allows for bright trion emission, which has not been demonstrated with previous chemical treatments. We show that unlike the TFSI treatment, the OA yields PL dynamics that are largely trap free. In addition, field effect transistors show an increase in mobilities with the OA treatment. These results suggest that OA serves to passivate defect sites in the WS2 monolayers in a manner akin to the passivation of colloidal quantum dots with OA ligands. Our results open up a new pathway to passivate and tune defects in monolayer TMDs using simple “wet” chemistry techniques, allowing for trap-free electronic properties and bright neutral exciton and trion emission.

The type-II PtSe2/WS2 van der Waals heterostructure: A high efficiency water-splitting photocatalyst

Electronic properties of pure and doped NbS2 and WS2 monolayers

We developed a comprehensive theoretical framework focusing on many-body effects of exciton species in monolayer WS2, including bright and dark excitons, and intra- and inter-valley biexcitons, to investigate valley dynamics in monolayer WS2 subjected to a tilted magnetic field . In particular, the evolution of the exciton population densities and the many-body particle scatterings are considered to calculate the valley polarization () as a function of the magnetic field. We found valley splittings for the dark exciton and biexciton energy levels that are larger than those of bright excitons, of −0.23 meV T−1. For example, −0.46 meV T−1 for dark excitons and −0.69 meV T−1 for bright-dark intra-valley biexcitons. Furthermore, inter-valley bright-dark excitons have an opposite valley energy splittings of +0.23 meV T−1. For the samples pumped by linearly polarized light, exhibits distinct magnetic field dependence for different types of many-body particle states. Among them, the of the intra-valley bright-dark biexcitons increases with increasing magnetic field and reaches nearly 50% at B = 65 T. The brightened dark exciton, on the other hand, exhibits vanishing , indicating long valley relaxation time. Remarkably, the inter-valley bright-dark biexciton shows unconventional behavior with an inverted . The opposite for intra- and inter-valley bright-dark biexcitons, coupled with their large valley splitting and long valley lifetime may facilitate their coherent manipulation for quantum computing.

Two-dimensional materials have attracted the attention of many researchers. Especially transition metal dichalcogenides (TMDs) like MoS2, WS2, etc., grants a wide scale of the band gap. TMDCs, MoS2 and WS2 monolayers have similar electronic and structural properties. WS2 has a great surface to volume ratio, a wide band gap range, high thermal and oxidative stability. It also has the peak carrier mobility and least effective mass than other TMDCs. So, it has been used in many applications like solar cells, LED, rechargeable batteries and sensors. In this work, we have analysed the stability and the electronic properties of monolayer and doped WS2 with Cobalt (Co), Iron (Fe) and Nickel (Ni) using density functional theory (DFT). The stability of the system has been studied by the formation energy. The electronic properties are analysed by band structure, the density of states, charge transfer, chemical potential, and total energy of the systems. These results show that the formation energy of the doped system is increasing with a negative magnitude which proves that the doped structures are more stable. We have observed reasonable changes in the band structure and density of states for transition metal doped WS2 while comparing with WS2 monolayer. We concluded that the doped WS2 shows better results than monolayer WS2 in the stability and improved electronic properties. These results may provide a prospective insight for making gas sensing devices.

Fundamental understandings on the dynamics of charge carriers and excitonic quasiparticles in semiconductors are of central importance for both many-body physics and promising optoelectronic and photonic applications. Here, we investigated the carrier dynamics and many-body interactions in two-dimensional (2D) transition metal dichalcogenides (TMDs), using monolayer WS2 as an example, by employing femtosecond broadband pump-probe spectroscopy. Three time regimes for the exciton energy renormalization are unambiguously revealed with a distinct red-blue-red shift upon above-bandgap optical excitations. We attribute the dominant physical process in the three typical regimes to free carrier screening effect, Coulombic exciton–exciton interactions and Auger photocarrier generation, respectively, which show distinct dependence on the optical excitation wavelength, pump fluences and/or lattice temperature. An intrinsic exciton radiative lifetime of about 1.2 picoseconds (ps) in monolayer WS2 is unraveled at low temperature, and surprisingly the efficient Auger nonradiative decay of both bright and dark excitons puts the system in a nonequilibrium state at the nanosecond timescale. In addition, the dynamics of trions at low temperature is observed to be significantly different from that of excitons, e.g., a long radiative lifetime of ~ 108.7 ps at low excitation densities and the evolution of trion energy as a function of delay times. Our findings elucidate the dynamics of excitonic quasiparticles and the intricate many-body physics in 2D semiconductors, underpinning the future development of photonics, valleytronics and optoelectronics based on 2D semiconductors.

Alternative channel materials for future ultra-scaled electronic devices have been intensively pursued nowadays since the feature size of silicon-based transistors has been scaled down to their physical limit. Atomically-thin semiconducting transitional metal dichalcogenides (TMDCs) including WS2, MoS2, WSe2, MoSe2, e. have shown a lot of unique properties compared to their bulk crystals, such as indirect-to-direct bandgap transitions, strong spin-orbit coupling and valley polarization. In particular, monolayer WS2 has shown the highest theoretical room temperature electron mobility among other semiconducting TMDCs as a result of its low effective mass. Combined with the large exciton/trion binding energy with high photoluminescence quantum yield, monolayer WS2 is a strong candidate as a potential channel material for high-efficiency optoelectronic applications.However, it is still challenging to grow wafer-sized, highly uniform and strictly monolayer TMDCs continuous film through the conventional chemical vapor deposition (CVD) due to the uncontrollable growth kinetics. The evaporation rates and amounts of the heated precursors are uncontrollable because the saturation vapor pressure of the precursor is exponentially dependent on the temperature inside the furnace. The sulfurized film is thus consisted of a mixture of monolayer, bilayer and multiple layers of TMDCs. The film with such unreproducible quality is not applicable for real industrial applications.In this work, we provide a self-limiting growth strategy based on modified CVD process to prepare the wafer-sized monolayer TMDCs. Theoretical simulations were performed to understand the fundamental thermodynamically mechanism of the strictly monolayer growth. The property-variation in TMDCs due to difference in electronic structure between different layers of TMDCs can be significantly reduced based on this new approach. The following figure indicates that the PL spectra detected from different spots (spot 1 to 5) of the WS2 film. All spectra measured from the 4'' wafer-sized sample show characteristics unique to monolayer WS2. This poses a reliable route for the growth of large-area monolayer TMDCs, which is essential for their reliable and robust applications in nanoelectronic devices. Figure 1

2D materials exhibit unique electronic states due to quantum confinement. Among the Group-VI chalcogenides, direct mono-layer WS2 is the most prominent where screening is non-localized, having strongly bound excitons with large binding energies and a pronounced deviation of the excitonic states from the hydrogenic series. State-of-the-art experimental and theoretical methods to determine excitonic Rydberg series employ optical spectroscopy and Bethe-Salpeter (BSE) equation, respectively, but incur high costs, paving the way to develop analytical approaches. We present a generalized hydrogenic model by employing a fractional version of the Coulomb-like potential to capture the excitonic Rydberg series of the fundamental optical transition in mono-layer WS2, based on the fractional scaling of the electron-hole pair interactions through the tuning of the fractional-space parameter β, benchmarked with experimental data and that of with numerical computation of the hydrogenic solution involving the Rytova-Keldysh (R-K) potential model. The enhanced electron-hole interactions lead to a strong dielectric contrast between the mono-layer WS2 and its surrounding environment and causes the deviation of the low-lying excitonic states from the hydrogenic series. The fractional Coulomb potential (FCP) model captures the first two non-hydrogenic states at β < 3, to fit a Coulomb-like to logarithmic change with respect to the excitonic radius and the higher hydrogenic states to have Coulombic interactions at β ≈ 3 in mono-layer WS2. A comparison of the proposed model with an existing model based on Wannier theory reveals a reduction in the relative mean square error of up to 30% for the excitonic series, with only the ground state captured as non-hydrogenic by the latter.

Transition metal dichalcogenides MX2 (M = Mo/W; X = S/Se) exhibit excellent optical and electrical properties. However, the atomically thin thickness induced low absorption cross sections impede their further applications as efficient absorbers and emitters. Herein, we report the emission enhancement and exciton species modulation in monolayer (ML) WS2 via the decoration of CdTe quantum dots (QDs). The ML WS2 was synthesized by thermal evaporation and showed evident neutral (Ao) and charged (A−) exciton emissions with variable A−-to-Ao ratios under different excitation powers and temperatures. The A− emission played an important role at low temperature (80 K) and high excitation power (5 mW). After the decoration of the CdTe QDs, the photoluminescence (PL) intensity of the ML WS2 enhanced greatly. Moreover, the Ao emission was dominant in WS2 + CdTe even under high excitation power and low temperature. The transfer of numerous holes from CdTe to WS2 induced the nonradiative recombination probability reduction, and p-type doping was critical to the observed PL enhancement and exciton species modulation in WS2 + CdTe. Our results provide a flexible strategy to improve the PL properties of atomically thin WS2 and further deepen the understanding of exciton-physics in ML MX2 for various applications.

Transition metal dichalcogenides and related layered materials in their monolayer and a few layers thicknesses regime provide a promising optoelectronic platform for exploring the excitonic- and many-body physics. Here, we have investigated the effects of nanoparticle-induced local strain on the optical properties of exciton, X0, and trion, X−, in monolayer WS2. Biaxial tensile strain up to 2.0% was quantified and verified by monitoring the changes in three prominent Raman modes of WS2: E2g1(Γ), A1g, and 2LA(M). We obtained an increase of 34 meV in X− binding energy with an average tuning rate of 17.5 ± 2.5 meV/% strain across all the samples irrespective of the surrounding dielectric environment of monolayer WS2 and the sample preparation conditions. Strain-induced linewidth broadening and deformation potentials of both X0 and X− emission elucidate that X− binding energy increases due to strain-enhanced electron–phonon coupling. This work holds relevance for future X−-based nano-opto-electro-mechanical systems and devices.

Recently, transition metal dichalcogenide (TMDCs) monolayers have garnered significant interest due to their unique physical properties. Hence, this study investigates the thermal and electrical properties of T′-WS2 monolayers doping with transition metal atoms (Mo and Cr) through first-principles calculations. The research results show that the lattice thermal conductivities of the four doped monolayers (Mo1-WS2, Mo2-WS2, Cr1-WS2, and Cr2-WS2) at 300 K are significantly reduced by 55.49%–58.67% compared to the pristine T′-WS2 monolayer. Metal atom doping will affect the thermal conductivity through a synergistic effect in phonon heat capacity, phonon lifetime, and phonon group velocity. Moreover, doping Cr and Mo atoms can transform the pristine T′-WS2 monolayer into a direct bandgap semiconductor, and Cr atom doping exhibits a stronger electronic modulation capability than Mo atom doping. The total density of states near the Fermi level in the Cr doped system is significantly enhanced, increasing by orders of magnitude compared to the T′-WS2 monolayer. Our findings display the impact of Mo and Cr doping on the phonon heat transport and electronic properties of the T′-WS2 monolayer, which would provide key data support for TMDCs based electronic devices.