Engineering Relaxation-Paths of C-Exciton for Constructing Band Nesting Bypass in WS2 Monolayer

Unveiling the Radiative Electron-Hole Recombination of MoS2 Nanostructures at Extreme pH Conditions

Nanostructures of MoS2 are in wide research for optoelectronic, energy and biological applications. Opto-electronic and biological applications requires the tuning of photoluminescence properties of MoS2 nanostructures. In this article, nanosized MoS2 is hydrothermally synthesized, and photoluminescence at extreme pH conditions (pH 1 and 13) is examined. As the photoluminescence gives a key to probe the radiative electron-hole recombination, here, photoluminescence emissions are used as an indicator to suggest the pattern of electron-hole recombination in the material at extreme pH conditions. Raman spectroscopy, dynamic light scattering, Scanning electron microscopic image and energy dispersive x-ray analysis are done for material confirmation. At pH 1 and 13 as-synthesized nanostructured MoS2 exhibited both upconversion and downconversion photoluminescence. The intensity of photoluminescence is varied with respect to pH. Excitation-dependent photoluminescence mechanisms and preliminary understanding on the ratio of quantum yields and life span of excited state of as-synthesized nanostructured MoS2 are unveiled here.

Ultrafast broadband spectroscopy of widely spread excitonic features in WSe2 nanosheets

The performance of an optoelectronic device is largely dependent on the light harvesting properties of the active material as well as the dynamic behaviour of the photoexcited charge carriers upon absorption of light. Recently, atomically thin two-dimensional transition metal dichalcogenides (2D TMDCs) have garnered attention as highly prospective materials for advanced ultrathin solar cells and other optoelectronic applications, owing to their strong interaction with electromagnetic radiation, substantial optical conductivity, and impressive charge carrier mobility. WSe2 is one such extremely promising solar energy material. It has absorption throughout the UV-Vis-NIR region with the existence of four excitonic features, just like MoS2 and WS2. However, stability issues and the absence of any robust synthetic route limit their practical applications. Herein, we have successfully synthesized atomically thin stable WSe2 nanosheets using a very effective colloidal hot injection method and further studied the optical properties of this material using femtosecond transient absorption spectroscopy. We probed all four excitonic features of WSe2 spread throughout the visible region. The dynamics of the high-energy excitons were found to be distinctively slower when compared to their band edge counterparts, adding an additional advantage in optoelectronic applications. We delved further into the factors governing exciton dynamics within WSe2, uncovering the strong influence of the electronic band structure. Importantly, our study highlights the importance of all four excitonic features in a 2D TMDC material, which emerge in the system irrespective of the excitation wavelength and influence each other.

Defect-Mediated Exciton Localization and Relaxation in Monolayer MoS2

Defects in chemical vapor deposition (CVD)-grown monolayer MoS2 are unavoidable and provide a powerful approach to creating single-photon emitters and quantum information systems through localizing excitons. However, insight into the A- trion and B/C exciton localization in monolayer MoS2 remains elusive. Here, we investigate defect-mediated A- trion and B/C exciton localization and relaxation in CVD-grown monolayer MoS2 samples via transient absorption spectroscopy. The localization rate of A- trions is five times faster than B excitons, which is attributed to the distinctions in the Bohr radius, diffusion rate, and multiphonon emission. Furthermore, we obtain unambiguous experimental evidence for the direct excitation of localized C excitons. Varying gap energy at the band-nesting region revealed by first-principles calculations explains the anomalous dependence of localized C exciton energy on delay time. We also find that the rapid dissociation of localized C excitons features a short characteristic time of ∼0.14 ps, while the measured relaxation time is much longer. Our results provide a comprehensive picture of the defect-mediated excitonic relaxation and localization dynamics in monolayer MoS2.

Low-Dimensional-Materials-Based Photodetectors for Next-Generation Polarized Detection and Imaging

The vector characteristics of light and the vectorial transformations during its transmission lay a foundation for polarized photodetection of objects, which broadens the applications of related detectors in complex environments. With the breakthrough of low-dimensional materials (LDMs) in optics and electronics over the past few years, the combination of these novel LDMs and traditional working modes is expected to bring new development opportunities in this field. Here, the state-of-the-art progress of LDMs, as polarization-sensitive components in polarized photodetection and even the imaging, is the main focus, with emphasis on the relationship between traditional working principle of polarized photodetectors (PPs) and photoresponse mechanisms of LDMs. Particularly, from the view of constitutive equations, the existing works are reorganized, reclassified, and reviewed. Perspectives on the opportunities and challenges are also discussed. It is hoped that this work can provide a more general overview in the use of LDMs in this field, sorting out the way of related devices for "more than Moore" or even the "beyond Moore" research.

Self-powered and broadband opto-sensor with bionic visual adaptation function based on multilayer γ-InSe flakes

Visual adaptation that can autonomously adjust the response to light stimuli is a basic function of artificial visual systems for intelligent bionic robots. To improve efficiency and reduce complexity, artificial visual systems with integrated visual adaptation functions based on a single device should be developed to replace traditional approaches that require complex circuitry and algorithms. Here, we have developed a single two-terminal opto-sensor based on multilayer γ-InSe flakes, which successfully emulated the visual adaptation behaviors with a new working mechanism combining the photo-pyroelectric and photo-thermoelectric effect. The device can operate in self-powered mode and exhibit good human-eye-like adaptation behaviors, which include broadband light-sensing image adaptation (from ultraviolet to near-infrared), near-complete photosensitivity recovery (99.6%), and synergetic visual adaptation, encouraging the advancement of intelligent opto-sensors and machine vision systems.

Probing Excitonic Rydberg States by Plasmon Enhanced Nonlinear Optical Spectroscopy in Monolayer WS2 at Room Temperature

The optoelectronic properties of two-dimensional (2D) transition metal dichalcogenide (TMDC) monolayers such as WS₂ are largely dominated by excitons due to strong Coulomb interactions in these 2D confined monolayers, which lead to formation of Rydberg-like excitonic states below the free quasiparticle band gap. The precise knowledge of high order Rydberg excitonic states is of great importance for both fundamental understanding such as many-electron effects and device applications such as optical switching and quantum process information. Bright excitonic states could be probed by linear optical spectroscopy, while probing dark excitonic states generally requires nonlinear optical (NLO) spectroscopy. Conventional optical methods for probing high-order Rydberg excitonic states were generally performed at cryogenic temperatures to ensure enough signal-to-noise ratio (SNR) and narrow line width. Here we have designed a hybrid nanostructure of monolayer WS₂ integrated with a plasmonic cavity and investigated their NLO properties at the single particle level. Giant enhancement in NLO responses, stronger excitonic resonance effects, and narrowed line widths of NLO excitation spectra were observed when monolayer WS₂ was placed in our carefully designed plasmonic cavity. Optimum enhancement of 1000-, 3000-, and 3800-fold were achieved for two-photon photoluminescence (2PPL), second harmonic generation (SHG), and third-harmonic generation (THG), respectively, in the optimized cavity structure. The line width of SHG excitation spectra was reduced from 43 down to 15 meV. Plasmon enhanced NLO responses brought improved SNR and spectral resolution, which allowed us to distinguish discrete excitonic states with small energy differences at room temperature. By using three complementary NLO techniques in combination with linear optical spectroscopy, energies of Rydberg excitonic states of A (1s, 2s, 2p, 3s, 3p, 4s), B (1s), and C and D excitons of monolayer WS₂ have been accurately determined, which allow us to determine exciton binding energy and quasiparticle bandgap. It was interesting to find that the 2p lies 30 meV below 2s, which lends strong support to the theoretical prediction of nonlocal dielectric screening effects based on a non-hydrogenic model. Our results show that plasmon enhanced NLO spectroscopy could serve as a general method for probing high order Rydberg excitonic states of 2D materials.