Strong Excitonic Effects Research Articles

Crossover of Frenkel and Wannier-Mott Excitons Through Halide Composition Tuning in Mixed Halide Perovskites.

Using first-principles G0W0 (G0 is one-electron Green's function and W0 is the dynamical screening Coloumb potential) coupled Bethe-Salpeter equation (BSE)calculations with spin-orbit coupling, exceptionally strong excitonic effects are identified in several bismuth-based vacancy-ordered mixed halide double perovskites. These perovskites are thermodynamically stable with negative formation energy. For Cs3Bi2X9 (X = Cl,Br,I) double perovskites, both the bandgap and excitonic binding energy decrease as the size of the halogen atom increases. The excitonic effects can be tuned in mixed halide perovskites such as Cs3Bi2I6Cl3, Cs3Bi2I6Br3, Cs3Bi2Br6I3, Cs3Bi2Cl6Br3, Cs3Bi2Br6Cl3, and Cs3Bi2Cl6I3. This study reports the exciton radiative lifetimes of the vacancy-ordered perovskites, revealing that these excitons exhibit long radiative lifetimes, particularly for Cs3Bi2Br6I3 with 11141 at 300 K and 24 at 5 K. The long radiative lifetimes are linked to the delocalization of the exciton (Wannier-Mott type) in real space, whereas the more localized exciton (Frenkel type) in Cs3Bi2Cl6Br3 results in shorter radiative lifetimes of 155 at 300 K and 334 ns at 5 K. Due to their long exciton lifetime, these materials present interesting opportunities for photovoltaicapplications.

Polarization-Dependent Plasmon-Induced Doping and Strain Effects in MoS2 Monolayers on Gold Nanostructures.

Monolayers of transition-metal dichalcogenides, such as MoS2, have attracted significant attention for their exceptional electronic and optical properties, positioning them as ideal candidates for advanced optoelectronic applications. Despite their strong excitonic effects, the atomic-scale thickness of these materials limits their light absorption efficiency, necessitating innovative strategies to enhance light-matter interactions. Plasmonic nanostructures offer a promising solution to overcome those challenges by amplifying the electromagnetic field and also introducing other mechanisms, such as hot electron injection. In this study, we investigate the vibrational and optical properties of MoS2 monolayer deposited on gold substrates and gratings, emphasizing the role of strain and plasmonic effects using conventional spectroscopic techniques. Our results reveal significant biaxial strain in the supported regions and a uniaxial strain gradient in the suspended ones, showing a strain-induced exciton and carrier funneling effect toward the center of the nanogaps. Moreover, we observed an additional polarization-dependent doping mechanism in the suspended regions. This effect was attributed to localized surface plasmons generated within the slits, as confirmed by numerical simulations, which may decay nonradiatively into hot electrons and be injected into the monolayer. Photoluminescence measurements further demonstrated a polarization-dependent trion-to-A exciton intensity ratio, supporting the hypothesis of additional plasmon-induced doping. These findings provide a comprehensive understanding of the strain-mediated funneling effects and plasmonic interactions in hybrid MoS2/Au nanostructures, offering valuable insights for developing high-efficiency photonic devices and quantum technologies, including polarization-sensitive detectors and excitonic circuits.

One-Dimensional Excitonic Insulator of M6Te6 (M = Mo, W) Atomic Wires.

Coulomb attraction with weak screening can trigger spontaneous exciton formation and condensation, resulting in a strongly correlated many-body ground state, namely, the excitonic insulator. One-dimensional (1D) materials natively have ineffective dielectric screening. For the first time, we demonstrate the excitonic instability of single atomic wires of transition metal telluride M6Te6 (M = Mo, W), a family of 1D van der Waals (vdW) materials accessible in the laboratory. The many-body GW and Bethe-Salpeter equation scheme shows giant exciton binding energies up to 1.6 eV for these narrow-gap semiconductors, much exceeding their single-particle band gaps and implying a robust thermal-equilibrium exciton Bose-Einstein condensation with high critical temperatures. The excitonic instability of these single atomic wires is attributed to their small dielectric constant, same parity and ultraflat dispersion for the band edge states. Our work shed light in exploiting the strong excitonic effects in 1D vdW materials to realize macroscopic quantum phenomena.

Boosting Exciton Dissociation and Charge Transfer in Fluorine-Substituted Covalent Organic Frameworks with Biomimetic Electron Pumps for Remarkable Photocatalytic Extraction of Uranium.

Visible-light-driven photocatalytic uranium extraction based covalent organic frameworks (COFs) are green and sustainable, but their performance is severely restricted by a strong exciton effect. Herein, inspired by the physiology of cardiac pacing, a novel fluorine-based COF (PyF-DaS-COF) with a biomimetic electronic pump has been fabricated and used for the photocatalytic extraction of uranium. Both experimental and theoretical calculations confirm that strongly electronegative fluorine plays a crucial role in exciton dissociation and charge transfer. The enhanced electron push-pull effect increases the intrinsic separation driving force of charge separation. Furthermore, fluorine substitution thermodynamically favors the generation of the crucial *OOH intermediate in the uranium reduction reaction. As a result, the PyF-DaS-COF achieves a record k value (T = 293 K) of 0.082 min-1 with an extraction capacity of 991.5 mg g-1. Importantly, PyF-DaS-COF achieves a removal rate of over 99% in real uranium-containing wastewater. The current work provides unique insights into designing novel and effective COFs for controlling radioactive contamination.

Directional Chiral Exciton Emission via Topological Polarization Singularities in all Van der Waals Metasurfaces

AbstractMonolayer transition metal dichalcogenides (TMDs) with strong exciton effects have enabled diverse light emitting devices, however, their Ångstrom thickness makes it challenging to efficiently manipulate exciton emission by themselves. Although their nanostructured multi‐layer counterparts can effectively manipulate optical field at deep subwavelength thickness scale, these indirect band gap multi‐layer TMDs are lack of strong luminescence, hindering their applications in light emitting devices. Here, the integration of monolayer TMDs is presented with nanostructured multi‐layer TMDs, combining both strong exciton emission and optical manipulation in a single ultra‐thin platform. Leveraging the topological polarization singularities in the all van der Waals metasurfaces, chiral exciton emission is experimentally demonstrated whose directionality can be controlled by tailoring metasurface's symmetry. These results provide an approach that can realize exciton emitting devices fully based on 2D materials with controllable polarization and directionality, and may unfold a new avenue for multi‐functional 2D semiconductor light sources.

Metal‐Ion‐Coordinated Microenvironments in Covalent Organic Frameworks for Enhanced Photocatalytic CO2 Reduction

AbstractMetal‐coordinated covalent organic frameworks (M‐COFs) have garnered significant attention for their potential in carbon dioxide (CO2) photoreduction. However, the efficient conversion of CO2 to (carbon monoxide) CO remains challenging because of the strong exciton effects from the Coulombic interactions of electron‐hole pairs within the metal‐associated coordination environment. Herein, three benzoxazole (BBO)‐based COF photocatalysts coordinated with cobalt (Co) ions featuring distinct coordination geometries: Co‒N‒O2, Co‒N‒O3, and Co‒N2‒O2 are strategically designed. The exciton dissociation is precisely modulated and the photocatalytic reduction of CO2 is enhanced. Among them, BBO‐COFBPY‐Co demonstrates a significant increase in CO production rate from 5, 024.87 to 10, 552.15 µmol g−1 h−1, and an improvement in the selectivity from 80% to 91%, coupled with excellent stability. Spectral characterizations and density functional theory (DFT) theoretical calculations elucidate that the judicious tuning of the Co atom coordination environment optimized the local electronic structure of the BBO‐COFs which significantly boosts the exciton dissociation, facilitates charge carrier migration within the framework, mitigates the recombination of photogenerated electron‐hole pairs, and reduce the energy barrier of the rate‐determining step. This study provides insight into the pivotal role of metal coordination microenvironments in enhancing the photocatalytic CO2 reduction process and paving the way for new strategies in exciton regulation within COFs.

Regulation of Exciton Effects in Functionalized Conjugated Polymers by B-N Lewis Pairs for Visible-Light Photocatalysis.

Strong excitonic effects are common in organic conjugated polymer semiconductors, severely hindering the generation of free charge carriers for conducting photocatalysis. Therefore, exploring new channels to modulate exciton dissociation in polymers is far-reaching in facilitating photocatalysis. A series of B-N Lewis pair functionalized conjugated polymers have been developed to minimize exciton effects by modulating charge transfer pathways. Theoretical studies have shown that introducing B-N Lewis pairs can dramatically increase the distance of charge transfer (D index) and the amount of electron transfer and reduce the Coulomb attraction energy (EC), which contributes to breaking the equilibrium of the coexistence of excitons and charge carriers. Further experimental results show that the singlet excitons are efficiently dissociated into more free-charge carriers under photoexcitation to participate in surface reactions. The optimized polymer PyPBM shows an exponential increase in photocatalytic hydrogen and hydrogen peroxide production performance by visible light illumination.

Regulation of Exciton Effects in Functionalized Conjugated Polymers by B‐N Lewis Pairs for Visible‐Light Photocatalysis

Strong excitonic effects are common in organic conjugated polymer semiconductors, severely hindering the generation of free charge carriers for conducting photocatalysis. Therefore, exploring new channels to modulate exciton dissociation in polymers is far‐reaching in facilitating photocatalysis. A series of B‐N Lewis pair functionalized conjugated polymers have been developed to minimize exciton effects by modulating charge transfer pathways. Theoretical studies have shown that introducing B‐N Lewis pairs can dramatically increase the distance of charge transfer (D index) and the amount of electron transfer and reduce the Coulomb attraction energy (EC), which contributes to breaking the equilibrium of the coexistence of excitons and charge carriers. Further experimental results show that the singlet excitons are efficiently dissociated into more free‐charge carriers under photoexcitation to participate in surface reactions. The optimized polymer PyPBM shows an exponential increase in photocatalytic hydrogen and hydrogen peroxide production performance by visible light illumination.

Organic-Inorganic Interfacial Dipole Induced by Energy Level Alignment for Efficient Photocatalytic Sterilization.

Photocatalytic molecules are considered to be one of the most promising substitutions of antibiotics against multidrug-resistant bacterial infections. However, the strong excitonic effect greatly restricts their efficiency in antibacterial performance. Inspired by the interfacial dipole effect, a Ti3C2 MXene modified photocatalytic molecule (MTTTPyB) is designed and synthesized to enhance the yield of photogenerated carriers under light irradiation. The alignment of the energy level between Ti3C2 and MTTTPyB results in the formation of an interfacial dipole, which can provide an impetus for the separation of carriers. Under the role of a dipole electric field, these photogenerated electrons can rapidly migrate to the side of Ti3C2 for improving the separation efficiency of photogenerated electrons and holes. Thus, more electrons can be utilized to produce reactive oxygen species (ROS) under light irradiation. As a result, over 97.04% killing efficiency can be reached for Staphylococcus aureus (S. aureus) when the concentration of MTTTPyB/Ti3C2 was 50 ppm under 660 nm irradiation for 15 min. A microneedle (MN) patch made from MTTTPyB/Ti3C2 was used to treat the subcutaneous bacterial infection. This design of an organic-inorganic interface provides an effective method to minimize the excitonic effect of molecules, further expanding the platform of inorganic/organic hybrid materials for efficient phototherapy.

Spin-Locked WS2 Vortex Emission via Photonic Crystal Bound States in the Continuum.

Owing to their strong exciton effects and valley polarization properties, monolayer transition-metal dichalcogenides (1L TMDs) have unfolded the prospects of spin-polarized light-emitting devices. However, the wavefront control of exciton emission, which is critical to generate structured optical fields, remains elusive. In this work, the experimental demonstration of spin-locked vortex emission from monolayer Tungsten Disulfide (1L WS2) integrated with Silicon Nitride (SiNx) PhC slabs is presented. The symmetry-protected bound states in the continuum (BIC) in the SiNx PhC slabs engender azimuthal polarization field distribution in the momentum space with a topological singularity in the center of the Brillouin zone, which imposes the resonantly enhanced WS2 exciton emission with a spin-correlated spiral phase front by taking advantage of the winding topologies of resonances with the assistance of geometric phase scheme. As a result, exciton emission from 1L WS2 exhibits helical wavefront and doughnut-shaped intensity beam profile in the momentum space with topological charges locked to the spins of light. This strategy on spin-dependent excitonic vortex emission may offer the unparalleled capability of valley-polarized structured light generation for 1L TMDs.

Insights into nitrogen-doped BiOBr with oxygen vacancy and carbon quantum dots photocatalysts for the degradation of sulfonamide antibiotics: Actions to promote exciton dissociation and carrier migration

The strong excitonic effect significantly influences the efficiency of electron-hole pair generation. Therefore, promoting the dissociation of excitons into free charge carriers has drawn much attention. In this study, nitrogen (N)-doped BiOBr photocatalyst (BOBNC) modified with carbon quantum dots (CQDs) was synthesized via a solvothermal method. We demonstrated through photoluminescence and photoelectrochemical techniques the synergistic effect of oxygen vacancies and CQDs, facilitating exciton dissociation and charge carrier migration, consequently significantly enhancing the electron density in the material. Compared to pure BiOBr, degradation experiments revealed that the optimized doping ratio of 0.5BOBNC with CQDs increased the rate constant for sulfamethoxazole (SIZ) by 19.89 times. Furthermore, based on quenching experiments, electron spin resonance (ESR) tests, and DFT calculations, h+, O2•− and 1O2 were identified as the primary reactive species for SIZ degradation, and a photocatalytic mechanism was proposed. Additionally, the influence of various environmental factors on the photocatalytic system was investigated. In conclusion, this work not only contributes to a profound understanding of BiOBr exciton dissociation but also presents a promising photocatalytic technique for the remediation of diverse water environments.

A Systematic Study of the Temperature Dependence of the Dielectric Function of GaSe Uniaxial Crystals from 27 to 300 K.

We report the temperature dependences of the dielectric function ε = ε1 + iε2 and critical point (CP) energies of the uniaxial crystal GaSe in the spectral energy region from 0.74 to 6.42 eV and at temperatures from 27 to 300 K using spectroscopic ellipsometry. The fundamental bandgap and strong exciton effect near 2.1 eV are detected only in the c-direction, which is perpendicular to the cleavage plane of the crystal. The temperature dependences of the CP energies were determined by fitting the data to the phenomenological expression that incorporates the Bose-Einstein statistical factor and the temperature coefficient to describe the electron-phonon interaction. To determine the origin of this anisotropy, we perform first-principles calculations using the mBJ method for bandgap correction. The results clearly demonstrate that the anisotropic dielectric characteristics can be directly attributed to the inherent anisotropy of p orbitals. More specifically, this prominent excitonic feature and fundamental bandgap are derived from the band-to-band transition between s and pz orbitals at the Γ-point.

Improving ZnO ultraviolet Schottky photodetectors by inserting a MoS2 layer

ZnO Schottky junction photodetectors (PDs) has become a research focus due to their wide application, high-performance and low-cost. However, their development has been limited by the low photo-response. Two-dimensional MoS2 exhibit numerous advantages, including a tunable band gap, a strong exciton effect, and broadband absorption. In this work, we incorporate MoS2 into ZnO thin films using a simple spin-coating method to create Schottky ultraviolet (UV) PDs that show a remarkably increased responsivity (by 885%) and a substantially improved photo-current (by one order of magnitude). Furthermore, they exhibit a relatively low noise equivalent power (5.11×10−13 W) and a high normalized detectivity 2.96×1011 Jones. This outstanding performance can be attributed to the enhanced absorption and efficient charge transfer of ZnO film after introduction of MoS2 layer. In this study, an easy method is presented for creating high-performance ZnO Schottky PDs.

Photocatalytic benzylamine coupling dominated by modulation of linkers in donor-acceptor covalent organic frameworks

The strong excitonic effect is widely prevalent in covalent organic frameworks (COFs)-based catalysts, while the high exciton binding energies (Eb) of COFs seriously limit their photocatalytic performance. Herein, theoretical calculations are conducted to evaluate the Eb values of potential COFs derived from benzo[1,2-b:3,4-b'-b'']trithiophene-2,5,8-tricarbaldehyde (BTT) and 2,4,6-trihydroxybenzene-1,3,5-tricarbaldehyde (Bp) as aldehyde monomers. Diverse aromatic amine linkers including dipyridine diamine (BPy), biphenyl diamine (BPh), and p-phenylenediamine (MPh) with different molecular lengths and N-sites are used as linker models to regulate the excitonic effect. The obtained BTT-BPy-COF with a long BPy linker and the lowest calculated Eb value possesses the best photocatalytic performance for benzylamine coupling, with conversion and selectivity up to 99% accompanied by a high oxidation rate, outperforming most of the reported works. Further mechanistic investigation reveals that the bipyridine-N sites in BTT-BPy-COF exhibit strong electron transfer as well as enhanced O2 adsorption and activation that is beneficial to photocatalytic benzylamine coupling.

Thermoplasmonic TiN boosts photocatalysis in covalent-organic frameworks

Covalent-organic frameworks (COFs) hold promise for photocatalysis, but their efficiency is hindered by strong excitonic effects that limit charge separation and transfer. Here, we demonstrate a novel strategy to overcome this challenge by integrating thermoplasmonic TiN nanoparticles with a β-ketoenamine-linked COFs (TFP-BD). The enhanced local electromagnetic field generated by TiN promotes the separation of electron-hole pairs in TFP-BD. This enhancement is evidenced by a doubled carrier lifetime (∼0.99 ns) and photocurrent density (1.67 μA cm⁻²) in the 3% TiN@TFP-BD composite compared to pristine TFP-BD. Additionally, thermoplasmonic heating in 3% TiN@TFP-BD (69.2 °C) surpasses that of TFP-BD (62.15 °C), accelerating mass transport during the photocatalytic process. This synergistic effect leads to a remarkable hydrogen evolution rate (HER) of 37.59 mmol g⁻¹ h⁻¹, an 8.5-fold improvement over the pristine COFs. Our work highlights the potential of thermoplasmonic enhancement for solar fuel production and establishes a new approach to mitigate excitonic limitations in COF-based photocatalysts.

Strong Linearly Polarized Light Emission by Coupling Out-of-Plane Exciton to Anisotropic Gap Plasmon Nanocavity.

With exceptional quantum confinement, 2D monolayer semiconductors support a strong excitonic effect, making them an ideal platform for exploring light-matter interactions and as building blocks for novel optoelectronic devices. Different from the well-known in-plane excitons in transition metal dichalcogenides (TMD), the out-of-plane excitons in indium selenide (InSe) usually show weak emission, which limits their applications as light sources. Here, by embedding InSe in an anisotropic gap plasmon nanocavity, we have realized plasmon-enhanced linearly polarized photoluminescence with an anisotropic ratio up to ∼140, corresponding to degree of polarization (DoP) of ∼98.6%. Such polarization selectivity, originating from the polarization-dependent plasmonic enhancement supported by the "nanowire-on-mirror" nanocavity, can be well tuned by the InSe thickness. Moreover, we have also realized an InSe-based light-emitting diode with polarized electroluminescence. Our research highlights the role of excitonic dipole orientation in designing nanophotonic devices and paves the way for developing InSe-based optoelectronic devices with polarization control.

Sustained robust exciton emission in suspended monolayer WSe2 within the low carrier density regime for quantum emitter applications

The development of semiconductor optoelectronic devices is moving toward low power consumption and miniaturization, especially for high-efficiency quantum emitters. However, most of these quantum sources work at low carrier density regions, where the Shockley–Read–Hall (SRH) recombination may be dominant and seriously reduce the emission efficiency. In order to reduce the effect of carrier trapping and sustain a strong photoluminescence (PL) emission under low power pumping conditions, we investigated the influence of “suspending” a monolayer of tungsten diselenide (WSe2), a novel two-dimensional quantum material. Not only the PL intensity but also the fundamental photoluminescence quantum yield (PLQY) has exhibited a huge, order-scale enhancement through suspending; even surprisingly, we found the PLQY improvement to be far significant under small pumping powers and observed an exponential increase in tendency toward an even lower carrier density region. With its strong excitonic effect, suspended WSe2 offers a solution to reduce carrier trapping and participate in non-radiative processes. Moreover, in the low-power range, where SRH recombination dominates, suspended WSe2 exhibited a remarkably higher percentage of excitonic radiation compared to contacted WSe2. Herein, we quantitatively demonstrate the significance of the suspended WSe2 monolayer in a low carrier density region, highlighting its potential for developing compact, low-power quantum emitters in the future.

Prediction of BiS2-type pnictogen dichalcogenide monolayers for optoelectronics

In this work, we introduce a 2D materials family with chemical formula MX2 (M={As, Sb, Bi} and X={S, Se, Te}) having a rectangular 2D lattice. This materials family has been predicted by systematic ab-initio structure search calculations in two dimensions. Using density-functional theory and many-body perturbation theory, we study the structural, vibrational, electronic, optical, and excitonic properties of the predicted MX2 family. Our calculations reveal that the predicted SbX2 and BiX2 monolayers are stable while the AsX2 layers exhibit an in-plane ferroelectric instability. All materials display strong excitonic effects and good optical absorption within the infrared-to-visible range. Hence, these monolayers can harvest solar energy and serve in optoelectronics applications. Furthermore, our results indicate that exfoliation of the predicted MX2 monolayers from their bulk counterparts is experimentally viable.

Temperature-driven journey of dark excitons to efficient photocatalytic water splitting in β-AsP.

Limited availability of photogenerated charge carriers in two-dimensional (2D) materials, due to high exciton binding energies, is a major bottleneck in achieving efficient photocatalytic water splitting (PWS). Strong excitonic effects in 2D materials demand precise attention to electron-electron correlation, electron-hole interaction and electron-phonon coupling simultaneously. In this work, we explore the temperature-dependent electronic and optical responses of an efficient photocatalyst, blue-AsP (β-AsP), by integrating electron-phonon coupling into state-of-the-art GW + BSE calculations. Interestingly, strong electron-lattice interaction at high temperature promotes photocatalytic water splitting with an increasing supply of long-lived dark excitons. This work presents an atypical observation contrary to the general assumption that only bright excitons enhance the PWS due to prominent absorption. Dark excitons, due to the low recombination rate, exhibit long-lived photogenerated electron-hole pairs with high exciton lifetime increasing with temperature up to ∼0.25 μs.

A first principles study on the stability and electronic and optical properties of 2D SbXY (X = Se/Te and Y = I/Br) Janus layers.

Motivated by the exceptional optoelectronic properties of 2D Janus layers (JLs), we explore the properties of group Va antimony-based JLs SbXY (X = Se/Te and Y = I/Br). Using Bader charges, the electric dipole moment in the out-of-plane direction of all the JLs is studied and the largest dipole moment is found to be in the SbSeI JL. Our results on the formation energy, phonon spectra, elastic constants, and ab initio molecular dynamics (AIMD) simulation provide insights into the energetic, vibrational, mechanical, and thermal stability of JLs. After confirming the stability, the three-dimensional phase diagram is investigated to propose the experimental conditions required to fabricate the predicted JLs. Then, the electronic band structure is calculated using different levels of theory, namely, the generalized gradient approximation (GGA), GGA + spin-orbit coupling (GGA + SOC), hybrid Heyd-Scuseria-Ernzerhof (HSE) functional, and many-body perturbation theory-based Green's function method (GW). According to the HSE results, JLs show band gaps between 1.653 and 1.852 eV. The GGA + SOC calculations reveal Rashba spin splitting in these JLs. The calculated carrier mobility using deformation potential theory shows that the electrons have exceptionally high mobility compared to holes, which assists the spatial separation of both charge carriers. The optical spectra are determined using GGA, HSE, and GW methods. With respect to GGA results, HSE and GW optical spectra show a blue shift. More accurate calculations using the GW-Bethe Salpeter equation (BSE) yield optical absorption spectra that are dominated by strong excitonic effects with the excitonic binding energy (BEex) in the range of 550-800 meV. Compared to the GW-BSE method, the Mott-Wannier (MW) model predicts a lower BEex. A strong e-h coupling is observed for dispersions along K-M in the Brillouin zone from the fat band analysis. Our study suggests that the SbSeI JL is a potential candidate for photocatalytic and photovoltaic applications due to its largest dipole moment and low excitonic binding energy.