Dark Exciton States Research Articles

Theoretical investigation of non-Förster exciton transfer mechanisms in perylene diimide donor, phenylene bridge, and terrylene diimide acceptor systems.

The rates of exciton transfer within dyads of perylene diimide and terrylene diimide connected by oligophenylene bridge units have been shown to deviate significantly from those of Förster's resonance energy transfer theory, according to single molecule spectroscopy experiments. The present work provides a detailed computational and theoretical study investigating the source of such a discrepancy. Electronic spectroscopy data are calculated by time-dependent density functional theory and then compared with experimental results. Electronic couplings between the exciton donor and the acceptor are estimated based on both the transition density cube method and transition dipole approximation. These results confirm that the delocalization of the exciton to the bridge parts contributes to significant enhancement of donor-acceptor electronic coupling. Mechanistic details of exciton transfer are examined by estimating the contributions of the bridge electronic states, vibrational modes of the dyads commonly coupled to both donor and acceptor, inelastic resonance energy transfer mechanism, and dark exciton states. These analyses suggest that the contribution of common vibrational modes serves as the main source of deviation from Förster's spectral overlap expression.

Intense Dark Exciton Emission from Strongly Quantum-Confined CsPbBr3 Nanocrystals

Dark exciton as the lowest-energy (ground) exciton state in metal halide perovskite nanocrystals is a subject of much interest. This is because the superior performance of perovskites as the photon source combined with long lifetime of dark exciton can be attractive for many applications of exciton. However, the direct observation of the intense and long-lived dark exciton emission, indicating facile access to dark ground exciton state, has remained elusive. Here, we report the intense photoluminescence from dark exciton with microsecond lifetime in strongly confined CsPbBr3 nanocrystals and reveal the crucial role of confinement in accessing the dark ground exciton state. This study establishes the potential of strongly quantum-confined perovskite nanostructures as the excellent platform to harvest the benefits of extremely long-lived dark exciton.

Measurement of the spin-forbidden dark excitons in MoS2 and MoSe2 monolayers

Excitons with binding energies of a few hundreds of meV control the optical properties of transition metal dichalcogenide monolayers. Knowledge of the fine structure of these excitons is therefore essential to understand the optoelectronic properties of these 2D materials. Here we measure the exciton fine structure of MoS2 and MoSe2 monolayers encapsulated in boron nitride by magneto-photoluminescence spectroscopy in magnetic fields up to 30 T. The experiments performed in transverse magnetic field reveal a brightening of the spin-forbidden dark excitons in MoS2 monolayer: we find that the dark excitons appear at 14 meV below the bright ones. Measurements performed in tilted magnetic field provide a conceivable description of the neutral exciton fine structure. The experimental results are in agreement with a model taking into account the effect of the exchange interaction on both the bright and dark exciton states as well as the interaction with the magnetic field.

Observation of intravalley phonon scattering of 2s excitons in MoSe2 and WSe2 monolayers

We present a high-resolution resonance Raman study of hBN encapsulated MoSe2 and WSe2 monolayers at 4 K using excitation energies from 1.6 eV to 2.25 eV. We report resonances with the WSe2 A2s and MoSe2 A2s and B2s excited Rydberg states despite their low oscillator strength. When resonant with the 2s states we identify new Raman peaks which are associated with intravalley scattering between different Rydberg states via optical phonons. By calibrating the Raman scattering efficiency and separately constraining the electric dipole matrix elements, we reveal that the scattering rates for k = 0 optical phonons are comparable for both 1s and 2s states despite differences in the envelope functions. We also observe multiple new dispersive Raman peaks including a peak at the WSe2 A2s resonance that demonstrates non-linear dispersion and peak-splitting behavior that suggests the dispersion relations for dark excitonic states at energies near the 2s state are extremely complex.

Robust room temperature emissions of trion in darkish WSe2 monolayers: effects of dark neutral and charged excitonic states

Owing to nonzero charge and spin degrees of freedom, trions offer unprecedented tunability and open new paths for applications in devices based on 2D semiconductors. However, in monolayer WSe2, the trion photoluminescence is commonly detected only at low temperatures and vanishes at room temperature, which undermines practical applications. To unveil how to overcome this obstacle, we have developed a comprehensive theory to probe the impact of different excitonic channels on the trion emission in WSe2 monolayers, which combines ab initio tight-binding formalism, Bethe–Salpeter equation and a set of coupled rate equations to describe valley dynamics of excitonic particles. Through a systematic study in which new scattering channels are progressively included, we found that, besides the low electron density, strong many-body correlations between bright and dark excitonic states quenches the trion emission in WSe2. Therefore, the reduction of scatterings from bright to dark states is required to achieve trion emission at room temperature for experimentally accessible carrier concentrations.

Synthesis of Two‐Dimensional Perovskite by Inverse Temperature Crystallization and Studies of Exciton States by Two‐Photon Excitation Spectroscopy

AbstractTwo‐dimensional (2D) organic–inorganic hybrid perovskites (OIHPs), a natural multiple‐quantum‐well structure with quasi‐2D electronic properties, have recently emerged as a promising class of semiconducting materials for photovoltaic and optoelectronic applications. However, facile synthesis of high‐quality 2D OIHPs single crystals is still lacking. The layer dependence of the exciton binding energy of (C4H9NH3)2PbI4 (C4PI), a widely studied 2D OIHP, is still debated. Herein, a novel synthesis technique based on inverse temperature crystallization in a binary‐solvent system is used to prepare 2D OIHPs and a systematic study of excitonic states of the synthesized 2D OIHPs by two‐photon excitation (TPE) spectroscopy is conducted. The obtained TPE spectra indicate that the exciton binding energies are similar for C4PI nanosheets and bulk crystals with different number of layers, most likely due to the intrinsically weak interlayer coupling. Further, the dark excitonic 2p states of (C6H5(CH2)2NH3)2PbI4 (PEPI) and C4PI are also observed by TPE spectroscopy. The results provide a novel synthesis protocol and insight into exciton properties of 2D OIHPs.

Fine structures of valley-polarized excitonic states in monolayer transitional metal dichalcogenides

Abstract Monolayer transitional metal dichalcogenides (TMDCs), a new class of atomically thin semiconductor, respond to optical excitation strongly with robust excitons, which stem from the reduced screening in two dimensions. These excitons also possess a new quantum degree of freedom known as valley spin, which has inspired the field of valleytronics. The strongly enhanced Coulomb interaction allows the exciton to bind with other particles to form new excitonic states. However, despite the discovery of trions, most of the excitonic states in monolayer TMDCs remain elusive until recently, when new light was shed into the fascinating excitonic fine structures with drastically improved sample quality through boron nitride encapsulation. Here, we review the latest research progress on fine structures of excitonic states in monolayer TMDCs, with a focus on tungsten-based TMDCs and related alloy. Many of the new excitonic complexes inherit the valley degree of freedom, and the valley-polarized dark excitonic states are of particular interest because of their long lifetime and possible long valley coherence time. The capability of resolving the excitonic fine structures also enables the investigation of exciton–phonon interactions. The knowledge of the interlayer between excitons and other particles not only advances our understanding of many-body effects in the monolayer TMDCs but also provides guidance on future applications based on TMDCs.

Linear and nonlinear optical probing of various excitons in 2D inorganic-organic hybrid structures

Nonlinear optical properties, such as two-(or multi-) photon absorption (2PA), are of special interest for technologically important applications in fast optical switching, in vivo imaging and so on. Highly intense infrared ultrashort pulses probe deep into samples and reveal several underlying structural perturbations (inter-layer distortions, intra-layer crumpling) and also provide information about new excited states and their relaxation. Naturally self-assembled inorganic-organic multiple quantum wells (IO-MQWs) show utility from room-temperature exciton emission features (binding energies ~200–250 meV). These Mott type excitons are highly sensitive to the self-assembly process, inorganic network distortions, thickness and inter-layer distortions of these soft two-dimensional (2D) and weak van der Waal layered hybrids. We demonstrate strong room-temperature nonlinear excitation intensity dependent two-photon absorption induced exciton photoluminescence (2PA-PL) from these IO-MQWs, excited by infrared femtosecond laser pulses. Strongly confined excitons show distinctly different one- and two-photon excited photoluminescence energies: from free-excitons (2.41 eV) coupled to the perfectly aligned MQWs and from energy down-shifted excitons (2.33 eV) that originate from the locally crumpled layered architecture. High intensity femtosecond induced PL from one-photon absorption (1PA-PL) suggests saturation of absorption and exciton-exciton annihilation, with typical reduction in PL radiative relaxation times from 270 ps to 190 ps upon increasing excitation intensities. From a wide range of IR excitation tuning, the origin of 2PA-PL excitation is suggested to arise from exciton dark states which extend below the bandgap. Observed two-photon absorption coefficients (β ~75 cm/GW) and two-photon excitation cross-sections (η2σ2 ~ 110GM), further support the evidence for 2PA excitation origin. Both 1PA- and 2PA-PL spatial mappings over large areas of single crystal platelets demonstrate the co-existence of both free and deep-level crumpled excitons with some traces of defect-induced trap state emission. We conclude that the two-photon absorption induced PL is highly sensitive to the self-assembly process of few to many mono layers, the crystal packing and deep level defects. This study paves a way to tailor the nonlinear properties of many 2D material classes. Our results thus open new avenues for exploring fundamental phenomena and novel optoelectronic applications using layered inorganic-organic and other metal organic frameworks.

Strong coupling of two quantum dots with a microcavity in the presence of an external and tilted magnetic field

Including an external magnetic field B in a Tavis-Cummings model enable us to consider four exciton states in each quantum dot, two brights and two darks. With a continuous and incoherent pump to a cavity, we explore the conditions of occupation and emission as a function of B. We have found that although the dark excitons are optically inactive, these show a trace of emission as a result of both the strong coupling between light and matter and the interaction between bright and dark exciton states mediated by the magnetic field. Further, tuning one of the quantum dots at resonance with the cavity, we report the set of parameters that allows us to change the resonance condition between exciton states from different quantum dots, namely, we use the magnetic field as a control parameter to select which exciton state and which quantum dot will be mostly coupled to the cavity.

Band‐Edge Exciton Fine Structure and Exciton Recombination Dynamics in Single Crystals of Layered Hybrid Perovskites

Abstract2D perovskite materials have recently reattracted intense research interest for applications in photovoltaics and optoelectronics. As a consequence of the dielectric and quantum confinement effect, they show strongly bound and stable excitons at room temperature. Here, the band‐edge exciton fine structure and in particular its exciton and biexciton dynamics in high quality crystals of (PEA)2PbI4 are investigated. A comparison of bulk and surface exciton lifetimes yields a room temperature surface recombination velocity of 2 × 103 cm s−1 and an intrinsic lifetime of 185 ns. Biexciton emission is evidenced at room temperature, with a binding energy of ≈45 meV and a lifetime of 80 ps. At low temperature, exciton state splitting is observed, which is caused by the electron–hole exchange interaction. Transient photoluminescence resolves the low‐lying dark exciton state, with a bright/dark splitting energy estimated to be 10 meV. This work contributes to the understanding of the complex scenario of the elementary photoexcitations in 2D perovskites.

Observation of Intralayer and Interlayer Excitons in Monolayered WSe2/WS2 Heterostructure

The atomically thin two-dimensional semiconductors of the material family of transition-metal dichalcogenides exhibit astonishing many-particle physics dominated by the formation of tightly bound planar-confined excitons due to strong in-plane Coulomb interaction. In addition to bright excitonic features, the presence of various dark excitonic states also has been experimentally observed very recently in this material family. In addition to that, a different type of exciton emerges when a van-der-Waals heterostructure is assembled by deterministically stacking two different monolayer TMDCs. Here, we demonstrate a WSe2/WS2 type-II heterostructure where the electrons transfer to the WS2 layer whereas the holes transfer to the WSe2 layer, thereby giving rise to interlayer excitons. The interlayer interaction depends on the coupling strength of the heterostructure which determines the luminescence spectrum. Such spectrum features contributions from individual layers as well as the heterobilayer configuration. These results and findings open new approaches for bandgap engineering using strongly hybridized bandstructures.

Large and controllable spin-valley splitting in two-dimensional WS2/h−VN heterostructure

Inspired by the profound physical connotations and potential application prospects of the valleytronics, we design a two-dimensional (2D) WS2/h-VN magnetic van der Waals (vdW) heterostructure and study the control of valley degree of freedom through the first-principles calculations. A considerable spin splitting of 627 meV is obtained at the K valley, accompanied with a strong suppression of that at the K' valley. An intrinsic large valley splitting of 376 meV is generated in the valence band, which corresponds to an effective Zeeman magnetic field of 2703 T. Besides of the valence band, the conduction band of WS2 possesses a remarkable spin splitting also, and valley labelled dark exciton states are present at the K' valley. The strengths of spin and valley splitting relied on the interfacial orbital hybridization are further tuned continually by the in-plane strain and interlayer spacing. Maximum spin and valley splitting of 654 and 412 meV are finally achieved, respectively, and the effective Zeeman magnetic field can be enhanced to 2989T with a -3 % strain. Time-reversal symmetry breaking and the sizable Berry curvature in the heterostructure lead to a prominent anomalous Hall conductivity at the K and K' valleys. Based on these finding, a prototype filter device for both the valley and spin is proposed.

Magnetic field mixing and splitting of bright and dark excitons in monolayer MoSe2

Monolayers of semiconducting transition metal dichalcogenides (TMDCs) with unique spin-valley contrasting properties and remarkably strong excitonic effects continue to be a subject of intense research interests. These model 2D semiconductors feature two fundamental intravalley excitons species–optically accessible ‘bright’ excitons with anti-parallel spins and optically inactive ‘dark’ excitons with parallel spins. For applications exploiting radiative recombination of bright excitons or long lifetime dark excitons, it is essential to understand the radiative character of the exciton ground state and establish the energy separation between the lowest energy bright and dark excitons. Here, we report a direct spectroscopic measure of dark excitons in monolayer MoSe2 encapsulated in hexagonal boron nitride. By applying strong in-plane magnetic field, we induce mixing and splitting of bright and dark exciton branches, which enables an accurate spectroscopic determination of their energies. We confirm the bright character of the exciton ground state separated by a 1.5 meV gap from the higher energy dark exciton state, much smaller compared to the previous theoretical expectations. These findings provide critical information for further improvement of the accurate theoretical description of TMDCs electronic structure.

Theory of exciton dynamics in time-resolved ARPES: Intra- and intervalley scattering in two-dimensional semiconductors

Time- and angle-resolved photoemission spectroscopy (trARPES) is a powerful spectroscopic method to measure the ultrafast electron dynamics directly in momentum-space. However, band gap materials with exceptional strong Coulomb interaction such as monolayer transition metal dichlacogenides (TMDC) exhibit tightly bound excitons, which dominate their optical properties. This rises the question whether excitons, in particular their formation and relaxation dynamics, can be detected in photoemission. Here, we develope a fully microscopic theory of the temporal dynamics of excitonic time- and angle resolved photoemission with particular focus on the phonon-mediated thermalization of optically excited excitons to momentum-forbidden dark exciton states. We find that trARPES is able to probe the ultrafast exciton formation and relaxation throughout the Brillouin zone.

Ultrafast dynamics in monolayer transition metal dichalcogenides: Interplay of dark excitons, phonons, and intervalley exchange

Understanding the ultrafast coupling and relaxation mechanisms between valleys in transition metal dichalcogenide semiconductors is of crucial interest for future valleytronic devices. Recent ultrafast pump-probe experiments showed an unintuitive significant bleaching at the excitonic $B$ transition after optical excitation of the energetically lower excitonic $A$ transition. Here, we present a possible microscopic explanation for this surprising effect. It is based on the joint action of exchange coupling and phonon-mediated thermalization into dark exciton states and does not involve a population of the B exciton. Our work demonstrates how intra- and intervalley coupling on a femtosecond timescale governs the optical valley response of 2D semiconductors.

Magnetic brightening, large valley Zeeman splitting, and dynamics of long-lived A and B dark excitonic states in monolayer WS2

We theoretically investigate the magnetic brightening, valley Zeeman splitting, and valley dynamics of long-lived A and B dark excitons (A-DEs and B-DEs) in monolayer ${\mathrm{WS}}_{2}$, subjected to tilted magnetic fields. Biexciton states and indirect intervalley excitons, with the latter composed of a hole in the $K$ valley and an electron in the $\mathrm{\ensuremath{\Lambda}}$ valley, are also taken into account. We reveal that both in-plane and out-of-plane field components greatly affect the magnetic brightening process due to correlations among different excitonic quasiparticles but in distinct roles. Specifically, the in-plane component primarily enables the dark-state brightening, while the perpendicular one not only modifies the bright-dark splitting inside a given valley, but also lifts the valley degeneracy of either bright or dark excitonic states. Moreover, in the presence of a tilted magnetic field with a nonzero in-plane component, we observe that both A-DEs and B-DEs can be brightened, with the latter requiring a much stronger field for a certain intensity of light emission. Remarkably, we find that the valley splitting of A-DEs is around twice as large as that of A bright excitons but vanishes for B-DEs, as a result of the combined effect of the spin and orbital magnetic moments on the energy shifts of the two valleys. In addition, as the tilt angle of the field with respect to the normal direction of ${\mathrm{WS}}_{2}$ sheet increases from ${0}^{\ensuremath{\circ}}$ to ${180}^{\ensuremath{\circ}}$, the bright-dark splittings for A and B excitons exhibit an opposite behavior, but both feature an ``X''-like pattern when the field is oriented in-plane. Finally, it is observed that the temperature greatly affects the light emission of bright and brightened dark states, while the corresponding temperature dependence is distinct for A and B excitons.

Hyperfine Interactions and Slow Spin Dynamics in Quasi-isotropic InP-based Core/Shell Colloidal Nanocrystals.

Colloidal InP core nanocrystals are taking over CdSe-based nanocrystals, notably in optoelectronic applications. Despite their use in commercial devices, such as display screens, the optical properties of InP nanocrystals and especially their relation to the exciton fine structures remain poorly understood. In this work, we show that the ensemble magneto-optical properties of InP-based core/shell nanocrystals investigated in strong magnetic fields up to 30 T are strikingly different from other colloidal nanostructures. Notably, the mixing of the lowest spin-forbidden dark exciton state with the nearest spin-allowed bright state does not occur up to the highest magnetic fields applied. This lack of mixing in an ensemble of nanocrystals suggests an anisotropy tolerance of InP nanocrystals. This striking property allowed us to unveil the slow spin dynamics between Zeeman sublevels (up to 400 ns at 15 T). Furthermore, we show that the unexpected magnetic-field-induced lengthening of the dark exciton lifetime results from the hyperfine interaction between the spin of the electron in the dark exciton with the nuclear magnetic moments. Our results demonstrate the richness of the spin physics in InP quantum dots and stress the large potential of InP nanostructures for spin-based applications.

Valley dynamics of different trion species in monolayer WSe2

We develop a model, taking into account optically inactive excitonic dark states, of both spin-forbidden and momentum-forbidden types, to explore valley dynamics of different trion species in monolayer WSe2. We observe two distinct regimes, determined by two typical bright-dark scattering processes, i.e., the spin-flip and spin-conserving ones. When the spin-flip scattering is comparable to the spin-conserving one, we find that the valley polarizations (VPs) of all excitonic states including excitons and singlet and triplet trions are copolarized with respect to the excitation laser, consistent with available experimental measurements. Interestingly, when the spin-conserving scattering dominates over the spin-flip one, we observe the reversal of VP of the singlet trion. This fascinating valley control opens the door to an envisaged valley field-effect transistor (FET) (an analog of spin FET). Furthermore, we reveal that the reversed VP strengthens until it becomes stabilized, offering a promising platform for coherent control of valley pseudospin. Our findings should stimulate experiments probing the VP reversal in samples with strong spin-conserving scattering.

Dark exciton based strain sensing in tungsten-based transition metal dichalcogenides

The trend towards ever smaller high-performance devices in modern technology requires novel materials with new functionalities. The recent emergence of atomically thin two-dimensional (2D) materials has opened up possibilities for the design of ultra-thin and flexible nanoelectronic devices. As truly 2D materials, they exhibit an optimal surface-to-volume ratio, which results in an extremely high sensitivity to external changes. This makes these materials optimal candidates for sensing applications. Here, we exploit the remarkably diverse exciton landscape in monolayer transition metal dichalcogenides to propose a novel dark-exciton-based concept for ultra sensitive strain sensors. We demonstrate that the dark-bright-exciton separation can be controlled by strain, which has a crucial impact on the activation of dark excitonic states. This results in a pronounced intensity change of dark excitons in photoluminescence spectra, when only 0.05 $\%$ strain is applied. The predicted extremely high optical gauge factors of up to 8000 are promising for the design of optical strain sensors.

Temperature dependence of photoluminescence properties of water-soluble ZnSe quantum dots

We have investigated the optical properties of ZnSe quantum dots (QDs) prepared by a hydrothermal method. The photoluminescence (PL)-decay profiles become slower with an increase in temperature up to 160 K, contrary to an ordinary behaviour due to thermal quenching. The temperature dependence of the PL-decay profile is explained by a three-state model consisting of a ground state and two excited states of the lower-lying bound-exciton and higher-lying dark-exciton states. The analysis of the temperature dependence of the decay time indicates that the dark-exciton state contributes to the PL-decay process in the ZnSe QDs.