Ultrafast quantum beats of anisotropic excitons in atomically thin ReS2

Quantum Beats between Spin-Singlet and Spin-Triplet Interlayer Exciton Transitions in WSe2-MoSe2 Heterobilayers

The long-lived interlayer excitons (IXs) of semiconducting transition metal dichalcogenide heterobilayers are prime candidates for developing various optoelectronic and valleytronic devices. Their photophysical properties, including fine structure, have been the focus of recent studies, and the presence of two spin states, namely, spin-singlet and spin-triplet, has been experimentally confirmed. However, the existence of the interaction between these states and their nature remains unknown to date. Here, we demonstrate the presence of coherent coupling between the spin-singlet and spin-triplet IXs of a WSe2-MoSe2 heterobilayer utilizing quantum beat spectroscopy via a home-built Michelson interferometer. As a clear signature of coherent coupling, the quantum beat signal has been observed for the first time between closely spaced transitions of IXs. The observed strong damping of the quantum beat signals with fast dephasing times of 270-400 fs indicates that fluctuations giving rise to inhomogeneous broadening in the photoluminescence emission of these states are uncorrelated.

Advances in Two-Dimensional Materials for Optoelectronics Applications

The past one and a half decades have witnessed the tremendous progress of two-dimensional (2D) crystals, including graphene, transition-metal dichalcogenides, black phosphorus, MXenes, hexagonal boron nitride, etc., in a variety of fields. The key to their success is their unique structural, electrical, mechanical and optical properties. Herein, this paper gives a comprehensive summary on the recent advances in 2D materials for optoelectronic approaches with the emphasis on the morphology and structure, optical properties, synthesis methods, as well as detailed optoelectronic applications. Additionally, the challenges and perspectives in the current development of 2D materials are also summarized and indicated. Therefore, this review can provide a reference for further explorations and innovations of 2D material-based optoelectronics devices.

The exchange between anions and cations induced by coupled plasma and thermal annealing treatment for room-temperature ferromagnetism

Two-dimensional (2D) materials, with outstanding magnetic properties at room temperature, are highly desirable for the future spintronic and nanoscale electronic industry. However, most of the 2D systems are not of magnetic nature due to thermal fluctuations. Herein, we propose a novel strategy to induce robust room-temperature ferromagnetism in the originally nonmagnetic 2D ReS₂ by the exchange between anions and cations. The vacancies are created by argon plasma treatment, which lowers the formation energy of point defects. The subsequent annealing facilitates the movement of the cations into the anion sites, giving rise to antisite defects, which leads to a significant increase in the magnetization. First-principles calculations demonstrate that the point defect with respect to the antisite substitution from Re to S is responsible for the extraordinary room-temperature ferromagnetism. This work opens a new door to the design of spin electronic structures by controllable antisite defects.

Bright and Dark Exciton Coherent Coupling and Hybridization Enabled by External Magnetic Fields

Magnetic field- and polarization-dependent measurements on bright and dark excitons in monolayer WSe₂ combined with time-dependent density functional theory calculations reveal intriguing phenomena. Magnetic fields up to 25 T parallel to the WSe₂ plane lead to a partial brightening of the energetically lower lying exciton, leading to an increase of the dephasing time. Using a broadband femtosecond pulse excitation, the bright and partially allowed excitonic state can be excited simultaneously, resulting in coherent quantum beating between these states. The magnetic fields perpendicular to the WSe₂ plane energetically shift the bright and dark excitons relative to each other, resulting in the hybridization of the states at the K and K' valleys. Our experimental results are well captured by time-dependent density functional theory calculations. These observations show that magnetic fields can be used to control the coherent dephasing and coupling of the optical excitations in atomically thin semiconductors.

Near-Infrared Self-Powered Linearly Polarized Photodetection and Digital Incoherent Holography Using WSe2/ReSe2 van der Waals Heterostructure

Polarization-sensitive photodetection has attracted considerable attention as an emerging technology for future optoelectronic applications such as three-dimensional (3D) imaging, quantum optics, and encryption. However, traditional photodetectors based on Si or III-V InGaAs semiconductors cannot directly detect polarized light without additional optical components. Herein, we demonstrate a self-powered linear-polarization-sensitive near-infrared (NIR) photodetector using a two-dimensional WSe₂/ReSe₂ van der Waals heterostructure. The WSe₂/ReSe₂ heterojunction photodiode with semivertical geometry exhibits excellent performance: an ideality factor of 1.67, a broad spectral photoresponse of 405-980 nm with a significant photovoltaic effect, outstanding linearity with a linear dynamic range wider than 100 dB, and rapid photoswitching behavior with a cutoff frequency up to 100 kHz. Strongly polarized excitonic transitions around the band edge in ReSe₂ lead to significant 980 nm NIR linear-polarization-dependent photocurrent. This linear polarization sensitivity remains stable even after exposure to air for longer than five months. Furthermore, by leveraging the NIR (980 nm)-selective linear polarization detection of this photodiode under photovoltaic operation, we demonstrate digital incoherent holographic 3D imaging.

Exciton-Dominated Ultrafast Optical Response in Atomically Thin PtSe2

Strongly bound excitons are a characteristic hallmark of 2D semiconductors, enabling unique light-matter interactions and novel optical applications. Platinum diselenide (PtSe₂ ) is an emerging 2D material with outstanding optical and electrical properties and excellent air stability. Bulk PtSe₂ is a semimetal, but its atomically thin form shows a semiconducting phase with the appearance of a band-gap, making one expect strongly bound 2D excitons. However, the excitons in PtSe₂ have been barely studied, either experimentally or theoretically. Here, the authors directly observe and theoretically confirm excitons and their ultrafast dynamics in mono-, bi-, and tri-layer PtSe₂ single crystals. Steady-state optical microscopy reveals exciton absorption resonances and their thickness dependence, confirmed by first-principles calculations. Ultrafast transient absorption microscopy finds that the exciton dominates the transient broadband response, resulting from strong exciton bleaching and renormalized band-gap-induced exciton shifting. The overall transient spectrum redshifts with increasing thickness as the shrinking band-gap redshifts the exciton resonance. This study provides novel insights into exciton photophysics in platinum dichalcogenides.

Doping-Mediated Lattice Engineering of Monolayer ReS2 for Modulating In-Plane Anisotropy of Optical and Transport Properties

ReS₂ exhibits strong anisotropic optical and electrical responses originating from the asymmetric lattice. Here, we show that the anisotropy of monolayer (1L) ReS₂ in optical scattering and electrical transport can be practically erased by lattice engineering via lithium (Li) treatment. Scanning transmission electron microscopy revealed that significant strain is induced in the lattice of Li-treated 1L-ReS₂, due to high-density electron doping and the resultant formation of continuous tiling of nanodomains with randomly rotating orientations of 60°, which produced a nearly isotropic response of polarized Raman scattering and absorption of Li-treated 1L-ReS₂. With Li treatment, the in-plane conductance of 1L-ReS₂ increased by an order of magnitude, and its angle dependence became negligible. Our result that the asymmetric phase was converted into the isotropic phase by electron injection could significantly expand the optoelectronic applications of polymorphic two-dimensional transition metal dichalcogenides.

Femtosecond Transfer and Manipulation of Persistent Hot-Trion Coherence in a Single CdSe/ZnSe Quantum Dot

Ultrafast transmission changes around the fundamental trion resonance are studied after exciting a p-shell exciton in a negatively charged II-VI quantum dot. The biexcitonic induced absorption reveals quantum beats between hot-trion states at 133 GHz. While interband dephasing is dominated by relaxation of the P-shell hole within 390 fs, trionic coherence remains stored in the spin system for 85 ps due to Pauli blocking of the triplet electron. The complex spectrotemporal evolution of transmission is explained analytically by solving the Maxwell-Liouville equations. Pump and probe polarizations provide full control over amplitude and phase of the quantum beats.

2D Re-Based Transition Metal Chalcogenides: Progress, Challenges, and Opportunities

The rise of 2D transition-metal dichalcogenides (TMDs) materials has enormous implications for the scientific community and beyond. Among TMDs, ReX2 (X = S, Se) has attracted significant interest regarding its unusual 1T' structure and extraordinary properties in various fields during the past 7 years. For instance, ReX2 possesses large bandgaps (ReSe2: 1.3 eV, ReS2: 1.6 eV), distinctive interlayer decoupling, and strong anisotropic properties, which endow more degree of freedom for constructing novel optoelectronic, logic circuit, and sensor devices. Moreover, facile ion intercalation, abundant active sites, together with stable 1T' structure enable them great perspective to fabricate high-performance catalysts and advanced energy storage devices. In this review, the structural features, fundamental physicochemical properties, as well as all existing applications of Re-based TMDs materials are comprehensively introduced. Especially, the emerging synthesis strategies are critically analyzed and pay particular attention is paid to its growth mechanism with probing the assembly process of domain architectures. Finally, current challenges and future opportunities regarding the controlled preparation methods, property, and application exploration of Re-based TMDs are discussed.

Polarized Light-Emitting Diodes Based on Anisotropic Excitons in Few-Layer ReS2

An on-chip polarized light source is desirable in signal processing, optical communication, and display applications. Layered semiconductors with reduced in-plane symmetry have inherent anisotropic excitons that are attractive candidates as polarized dipole emitters. Herein, the demonstration of polarized light-emitting diode based on anisotropic excitons in few-layer ReS₂ , a 2D semiconductor with excitonic transition energy of 1.5-1.6 eV, is reported. The light-emitting device is based on minority carrier (hole) injection into n-type ReS₂ through a hexagonal boron nitride (hBN) tunnel barrier in a metal-insulator-semiconductor (MIS) van der Waals heterostack. Two distinct emission peaks from excitons are observed at near-infrared wavelength regime from few-layer ReS₂ . The emissions exhibit a degree of polarization of 80% reflecting the nearly 1D nature of excitons in ReS₂ .

Light Polarization-Controlled Conversion of Ultrafast Coherent-Incoherent Exciton Dynamics in Few-Layer ReS2

Coherent light-matter interaction can transiently modulate the quantum states of matter under nonresonant laser excitation. This phenomenon, called the optical Stark effect, is one of the promising candidates for realizing ultrafast optical switches. However, the ultrafast modulations induced by the coherent light-matter interactions usually involve unwanted incoherent responses, significantly reducing the overall operation speed. Here, by using ultrafast pump-probe spectroscopy, we suppress the incoherent response and modulate the coherent-to-incoherent ratio in the two-dimensional semiconductor ReS₂. We selectively convert the coherent and incoherent responses of an anisotropic exciton state by solely using photon polarizations, improving the control ratio by 3 orders of magnitude. The efficient modulation was enabled by transient superpositions of differential spectra from two nondegenerate exciton states due to the light polarization dependencies. This work provides a valuable contribution toward realizing ideal ultrafast optical switches.

Strong One-Dimensional Characteristics of Hole-Carriers in ReS2 and ReSe2

Each plane of layered ReS₂ and ReSe₂ materials has 1D chain structure, from which intriguing properties such as 1D character of the exciton states and linearly polarized photoluminescence originate. However, systematic studies on the 1D character of charge carriers have not been done yet. Here, we report on systematic and comparative studies on the energy-momentum dispersion relationships of layered transition metal dichalcogenides ReS₂ and ReSe₂ by angle resolved photoemission. We found that the valence band maximum or the minimum energy for holes is located at the high symmetric Z-point for both materials. However, the out-of-plane (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${k}_{z}$$\end{document}kz) dispersion for ReSe₂ (20 meV) is found to be much smaller than that of ReS₂ (150 meV). We observe that the effective mass of the hole carriers along the direction perpendicular to the chain is about 4 times larger than that along the chain direction for both ReS₂ and ReSe₂. Remarkably, the experimentally measured hole effective mass is about twice heavier than that from first principles calculation for ReS₂ although the in-plane anisotropy values from the experiment and calculations are comparable. These observation indicate that bulk ReS₂ and ReSe₂ are unique semiconducting transition metal dichalcogenides having strong one-dimensional characters.