Excitonic Effects Research Articles

Two-Dimensional Hybrid Organic-Inorganic Perovskite Nanosheets for Gb/s Visible-Light Communication

With the rapid development of solid-state lighting and the congestion of radio-frequency communication data traffic, visible-light communication (VLC) has emerged as a versatile technology for simultaneous illumination and communication. However, the conventional color-converting phosphors integrated with light-emitting diodes (LEDs), or laser diodes (LDs) usually have limited optical modulation bandwidth due to the long carrier recombination lifetime, which is not suitable for high-speed multiple-wavelength data transfer based on phosphor-conversion VLC systems. Herein, we demonstrate a hybrid organic-inorganic perovskite nanosheets (NSs), i.e., (C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sub> NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> PbI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> (PEPI), passivated by polymer, as a fast-acting color-converting phosphor for VLC. Compared to the PEPI micro-plates ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula> Ps), the NSs exhibit a stronger excitonic effect with a shorter fluorescence lifetime of 877 ± 4.7 ps, leading to a broad −3-dB bandwidth of 192.8 MHz. Given the large bandwidth, a net data rate of 0.93 Gb/s was achieved based on an orthogonal frequency-division multiplexing (OFDM) modulation scheme. These investigations verified the feasibility of using two-dimensional hybrid organic-inorganic perovskite materials as a promising phosphor for future multi-Gb/s color-pure wavelength-division multiplexing systems.

Bi vacancy simultaneous manipulation of bulk adsorption and carrier utilization to replenish the mechanism of Cr(VI) photoreduction at universal pH

Constructing atomic-level cation vacancies is an effective strategy to modulate the electronic properties of host materials, thus promoting charge transfer and redox reaction kinetics. Herein, ultrathin Bi24O31Br10 nanosheets with cationic vacancy was synthesized via an inventive precipitation strategy by adding appropriate arginine. The difunctional arginine can changes the pH of the synthetic system and selectively deprive Bi atoms on the surface to form Bi vacancies through strong affinity between functional groups and cations. The abundant Bi vacancy decrease the Gibbs free energy change and provide channels to promote bulk adsorption of Cr(VI), thus effectively reduces the carrier migration distance during in situ bulk photocatalytic reduction of Cr(VI) and induces the formation of new impurity states to improve the utilization of visible light. Bi vacancy mitigates strong excitonic effects confined by significantly reducing exciton binding energies, thus effectively promote the separation of photo-generated charges and suppress reorganization of photon-generated carrier. Benefiting from these merits, these exclusive VBi-Bi24O31Br10 ultra-thin nanosheets exhibit superior reduction performance for Cr(VI), and the removal rate was more than 90% at universal pH (2–10). This work provides a new idea for the construction of cation vacancies and a feasible method for the removal of Cr(VI) under alkaline conditions.

Chalcogenide Perovskites (ABS3; A = Ba, Ca, Sr; B = Hf, Sn): An Emerging Class of Semiconductors for Optoelectronics.

Chalcogenide perovskites have received considerable interest in the photovoltaic research community because of their stability, nontoxicity, and lead-free composition. However, because of the huge computational cost, theoretical study focusing on excitonic and polaronic properties is not explored rigorously. Herein, we capture the excitonic and polaronic effects in a series of chalcogenide perovskites ABS3, where A = Ba, Ca, Sr and B = Hf, Sn, by employing state-of-the-art hybrid density functional theory and many-body perturbative approaches, viz., GW and BSE. We find that they possess an exciton binding energy larger than that of 3D inorganic-organic hybrid perovskites. We examine the interplay of electronic and ionic contributions to the dielectric screening and conclude that the electronic contribution is dominant over the ionic contribution. Using the Feynman polaron model, polaron parameters are computed, and charge-separated polaronic states are less stable than bound excitons. Finally, the theoretically calculated spectroscopic limited maximum efficiency suggests that among all chalcogenide perovskites, CaSnS3 could serve as the best choice for photovoltaic applications.

Electron energy-loss spectra of functionalized titanium and zirconium MXene monolayers, including excitonic effects

Using the full-potential linearized augmented plane wave (FP-LAPW)-based energy-loss near-edge structure (ELNES), the present study deals with the electronic properties of titanium and zirconium MXene monolayers. Specifically, the carbon and nitrogen K-edges of M2CT2 and M2NT2 (M = Ti, Zr; T = F, O, OH) MXene monolayers have been calculated and compared with those of the existing experimental spectra. On going from the bulk systems to MXene monolayers, the main spectral features occur at higher energies as a result of the shorter M-X bond lengths in the monolayers. As fingerprints of the terminal groups, in the C K- and N K-edges of Ti- and Zr-based terminated MXenes, the oscillator strengths and overall spectral shapes are changed, and in comparison to bare MXenes, the main spectral features are red-shifted. Moreover, on going from O termination to OH and then to F, one can observe a slight red-shift in the K-edge spectra of Ti2C(N) and Zr2C(N). While the peaks originating from the px + py states are modified through the appending of terminal elements, showing an obvious red-shift, the terminal substituent has no significant effect on features engendered by pz character. In addition, including excitonic effects on the K-edge absorption spectra considerably changes the response of MXene semiconductors, resulting in red-shifts of the ELNES spectral features.

One-dimensional excitons in long phosphorene atomic chains

The electronic structure of phosphorene atomic chains (PACs) embedded in various dielectric environments is studied theoretically by using a configuration interaction approach beyond the conventional double-excitation scheme. While the nominal single-particle gap of the PACs is shown to roughly obey an expected scaling law of ${L}^{\ensuremath{-}2}$, where $L$ measures the length of PACs, the quasiparticle shift is found to gradually converge to a value that is insensitive to the dielectric environment as $L$ increases. In the meantime, exciton binding energies are revealed to obey a simple scaling law of $0.96/(\ensuremath{\kappa}L)\ensuremath{-}0.02$, with $\ensuremath{\kappa}$ being the effective dielectric constant. As $L$ goes to infinity, the quasiparticle and excitonic effect are both found to be greatly suppressed as if the long-range electron-electron interactions are quenched in long PACs.

Suppressing the Excitonic Effect in Covalent Organic Frameworks for Metal-Free Hydrogen Generation.

Photocatalytic hydrogen generation is a promising solution for renewable energy production and plays a role in achieving carbon neutrality. Covalent organic frameworks (COFs) with highly designable backbones and inherent pores have emerged as novel photocatalysts, yet the strong excitonic effect in COFs can impede the promotion of energy conversion efficiency. Here, we propose a facile approach to suppress the excitonic effect in COFs, which is by narrowing the band gap and increasing the dielectric screening via a rational backbone design and chemical modifications. Based on the GW-BSE method, we uncover a linear relationship between the electronic dielectric constant and the inverse square of the optical band gap of COFs of the Lieb lattice. We further demonstrate that both reduced exciton binding energy and enhanced sunlight absorption can be simultaneously realized in COFs with a narrow band gap. Specifically, we show that one of our designed COFs whose exciton binding energy is nearly half that of g-C3N4 is capable of metal-free hydrogen production under near-infrared light irradiation. Our results showcase an effective method to suppress the excitonic effect in COFs and also pave the way for their applications in photocatalytic, photovoltaic, and other related solar energy conversions.

Long-range transport and ultrafast interfacial charge transfer in perovskite/monolayer semiconductor heterostructure for enhanced light absorption and photocarrier lifetime.

Atomically thin two-dimensional transition metal dichalcogenides (TMDs) have shown great potential for optoelectronic applications, including photodetectors, phototransistors, and spintronic devices. However, the applications of TMD-based optoelectronic devices are severely restricted by their weak light absorption and short exciton lifetime due to their atomically thin nature and strong excitonic effect. To simultaneously enhance the light absorption and photocarrier lifetime of monolayer semiconductors, here, we report 3D/2D perovskite/TMD type II heterostructures by coupling solution processed highly smooth and ligand free CsPbBr3 film with MoS2 and WS2 monolayers. By time-resolved spectroscopy, we show interfacial hole transfer from MoS2 (WS2) to the perovskite layer occurs in an ultrafast time scale (100 and 350fs) and interfacial electron transfer from ultrathin CsPbBr3 to MoS2 (WS2) in ∼3 (9) ps, forming a long-lived charge separation with a lifetime of >20ns. With increasing CsPbBr3 thickness, the electron transfer rate from CsPbBr3 to TMD is slower, but the efficiency remains to be near-unity due to coupled long-range diffusion and ultrafast interfacial electron transfer. This study indicates that coupling solution processed lead halide perovskites with strong light absorption and long carrier diffusion length to monolayer semiconductors to form a type II heterostructure is a promising strategy to simultaneously enhance the light harvesting capability and photocarrier lifetime of monolayer semiconductors.

Unconventional excitonic states with phonon sidebands in layered silicon diphosphide

Complex correlated states emerging from many-body interactions between quasiparticles (electrons, excitons and phonons) are at the core of condensed matter physics and material science. In low-dimensional materials, quantum confinement affects the electronic, and subsequently, optical properties for these correlated states. Here, by combining photoluminescence, optical reflection measurements and ab initio theoretical calculations, we demonstrate an unconventional excitonic state and its bound phonon sideband in layered silicon diphosphide (SiP2), where the bound electron–hole pair is composed of electrons confined within one-dimensional phosphorus–phosphorus chains and holes extended in two-dimensional SiP2 layers. The excitonic state and emergent phonon sideband show linear dichroism and large energy redshifts with increasing temperature. Our ab initio many-body calculations confirm that the observed phonon sideband results from the correlated interaction between excitons and optical phonons. With these results, we propose layered SiP2 as a platform for the study of excitonic physics and many-particle effects.

Excitonic effects on high-harmonic generation in Mott insulators

To study excitonic effects on high-harmonic generation (HHG) in Mott insulators, we investigate pumped nonequilibrium dynamics in the one-dimensional extended Hubbard model. By employing time-dependent calculations based on the exact diagonalization and infinite time-evolving block decimation methods, we find the strong enhancement of the HHG intensity around the exciton energy. The subcycle analysis in the sub-Mott-gap regime shows that the intensity region of the time-resolved spectrum around the exciton energy splits into two levels and oscillates following the driving electric field. This excitonic dynamics is qualitatively different from the dynamics of free doublon and holon but favorably contributes to HHG in the Mott insulator.

Azobenzene-Substituted Triptycenes: Understanding the Exciton Coupling of Molecular Switches in Close Proximity.

Herein, we report a series of azobenzene‐substituted triptycenes. In their design, these switching units were placed in close proximity, but electronically separated by a sp3 center. The azobenzene switches were prepared by Baeyer–Mills coupling as key step. The isomerization behavior was investigated by 1H NMR spectroscopy, UV/Vis spectroscopy, and HPLC. It was shown that all azobenzene moieties are efficiently switchable. Despite the geometric decoupling of the chromophores, computational studies revealed excitonic coupling effects between the individual azobenzene units depending on the connectivity pattern due to the different transition dipole moments of the π→π* excitations. Transition probabilities for those excitations are slightly altered, which is also revealed in their absorption spectra. These insights provide new design parameters for combining multiple photoswitches in one molecule, which have high potential as energy or information storage systems, or, among others, in molecular machines and supramolecular chemistry.

Saddle-point exciton signature on high-order harmonic generation in two-dimensional hexagonal nanostructures

The disclosure of basic nonlinear optical properties of graphene-like nanostructures with correlated electron-hole nonlinear dynamics over a wide range of frequencies and pump field intensities is of great importance for both graphene fundamental physics and for the expected novel applications of two-dimensional (2D) hexagonal nanostructures in extreme nonlinear optics. In the current paper, the nonlinear interaction of 2D hexagonal nanostructures with the bichromatic infrared driving field taking into account the many-body Coulomb interaction is investigated. Numerical investigation in the scope of the Bloch equations within the Houston basis that take into account $e\text{--}e$ and $e\text{--}h$ interactions in the Hartree-Fock approximation reveals significant excitonic effects in the high-harmonic generation process in 2D hexagonal nanostructures such as graphene and silicene. It is shown that, due to the correlated electron-hole nonlinear dynamics around the van Hove singularity, spectral caustics in the high-harmonic generation spectrum are induced near the saddle-point excitonic resonances.

Exciton dynamics and photoresponse behavior of the in situ annealed CsSnBr3 perovskite films synthesized by thermal evaporation

The CsSnBr3 photodetectors are fabricated by thermal evaporation and 75 °C in situ annealing, and the effect of in situ annealing on the morphology, structure, exciton dynamics and photoresponse of thermally evaporated CsSnBr3 films are investigated. Especially, temperature dependent steady-state photoluminescence (PL) and transient PL decaying have been analyzed in details for understanding the exciton dynamics. Meanwhile, effect of annealing on the activation energy for trap sites (E a), exciton binding energy (E b), activation energy for interfacial trapped carriers (ΔE), trap densities and carriers mobilities are studied and the annealed (A-CsSnBr3) reveals obviously lower E b and trap density together with notably higher carrier mobility than those of the unannealed (UA-CsSnBr3). Temperature dependence of the integrated PL intensity can be ascribed to the combining effect of the exciton dissociation, exciton quenching through trap sites and thermal activation of trapped carriers. The temperature dependent transient PL decaying analysis indicates that the PL decaying mechanism at low and high temperature is totally different from that in intermediate temperature range, in which combing effect of free exciton and localized state exciton decaying prevail. The beneficial effects of the in situ annealing on the photoresponse performance of the CsSnBr3 films can be demonstrated by the remarkable enhancement of the optimal responsivity (R) after in situ annealing which increases from less than 1 A W−1 to 1350 A W−1 as well as dramatically improved noise equivalent power, specific detectivity D* and Gain (G).

Multistable excitonic Stark effect

The optical Stark effect is a tell-tale signature of coherent light--matter interaction in excitonic systems, wherein an irradiating light beam tunes exciton transition frequencies. Here we show that, when excitons are placed in a nanophotonic cavity, the excitonic Stark effect can become highly nonlinear, exhibiting multivalued and hysteretic Stark shifts that depend on the history of the irradiating light. This multistable Stark effect (MSE) arises from feedback between the cavity mode occupation and excitonic population mediated by the Stark-induced mutual tuning of the cavity and excitonic resonances. Strikingly, the MSE manifests even for very dilute exciton concentrations and can yield discontinuous Stark shift jumps of order meV. We expect that the MSE can be realized in readily available transition metal dichalcogenide excitonic systems placed in planar photonic cavities at modest pump intensities. This phenomenon can provide new means to engineer coupled states of light and matter that can persist even in the single exciton limit.

First-principle study on the stability, mechanical, electronic, and optical properties of two-dimensional scandium oxyhalides

Based on the first-principle calculation, we report a new kind of two-dimensional scandium oxyhalide monolayers. The formation energy, exfoliation energy, elastic constants, phonon spectra, molecular dynamic simulations, electronic structures, and optical properties were calculated. Analytical results confirmed that all the monolayers are energetically (with the exception of ScOF and ScOI), mechanically, dynamically, and thermally stable, and these monolayers are free-standing. The monolayers are widely indirect band gap semiconductors with band gaps obtained by the HSE06 scheme from 2.3 eV to 6.5 eV. The G0W0 plus the Bethe-Salpeter equation method reveals that ScOF, ScOCl, and ScOBr have large excitonic binding energies of 1.750, 1.123, and 0.622 eV, respectively. The high optical absorptions are observed in the UV light regions. Given their suitable band edges, large excitonic effects, and high optical absorptions in the UV light regions, ScOCl and ScOBr demonstrate great potential applications as photocatalysts for water splitting and solar-blind ultraviolet photodetectors.

Boosting exciton dissociation by regulating dielectric constant in covalent organic framework for photocatalysis

Polymer semiconductor photocatalysts usually suffer from the high exciton binding energy because of the low dielectric constant. Herein, two crystalline polymer photocatalyst prototypes, neutral covalent organic framework (COF) and ionic covalent organic framework ( i COF), were employed to investigate the exciton effect. After polarization by the ionic sites with high dipole moment, the i COF endows a greatly increased dielectric constant to reach the low exciton binding energy of 23 meV, below the thermal energy under room temperature ( k B T, 26 meV). Thus, enhanced generation, separation, and transport of photogenerated charge carriers over the i COF catalyst resulted in a hydrogen peroxide production rate of 10.01 mmol g -1 h -1 in the photocatalytic oxygen reduction coupling with water oxidation reaction, which is almost 7 times higher than the neutral COF under identical conditions. This work demonstrates a promising way to tune the dielectric properties of polymer photocatalysts in order to regulate the exciton effect and related photocatalytic behavior. • Highly crystalline i COF was obtained in a microwave-assisted solvothermal route • The highly polar ionic sites enhanced the dielectric constant of the COF matrix • Low exciton binding energy was reached to allow spontaneous exciton dissociation • Rapid H 2 O 2 production was observed under visible-light irradiation in pure water Photocatalytic production of high-energy chemicals such as H 2 O 2 plays an essential role in seeking sustainable energy. Organic polymeric semiconductors that can be prepared from the earth’s abundant elements are preferential photocatalysts with advantages such as tunability. Nonetheless, the inherent low dielectric properties of organic polymers result in high exciton binding energies that inhibit the spontaneous exciton dissociation and thus restrict photocatalytic activity. Herein, we demonstrated that polarizing the covalent organic framework by integration of ionic moieties, fabricating i COF, can greatly increase the dielectric constant and thus reduce the exciton binding energy down to k B T (26 meV). The spontaneous exciton dissociation was obtained, accelerating the separation and transport of photogenerated carriers and thus a high H 2 O 2 production rate. This work opens a new avenue to boost the efficiency of organic semiconductors for solar-fuel production. The high polarization of the ionic sites greatly increases the dielectric constant of the COF matrix and thus reduces the exciton binding energy, effectively accelerating the H 2 O 2 production rate under visible-light irradiation.

Recommendation of Orbitals for G0W0 Calculations on Molecules and Crystals

The many-body GW approximation, especially the G0W0 method, has been widely used for condensed matter and molecules to calculate quasiparticle energies for ionization, electron attachment, and band gaps. Because G0W0 calculations are well-known to have a strong dependence on the orbitals, the goal of the present work is to provide guidance on the choice of density functional used to generate orbitals and to recommend a choice that gives the most broadly accurate results. We have systematically investigated the dependence of G0W0 calculations on the orbitals for 100 molecules and 8 crystals by considering orbitals obtained with a diverse set of Kohn-Sham (KS) and generalized KS (GKS) functionals (63 functionals plus Hartree-Fock). The percentage of Hartree-Fock exchange employed in density functionals has been found to have strong influence on the predicted molecular ionization energy and crystal fundamental band gaps (with optimum values between 40 and 56%), but to have less effect on predicting molecular electron affinities. The low cost of the Gaussian implementation, even with hybrid functionals in periodic calculations, the better performance of global hybrids as compared to range-separated hybrids of either than screened exchange or long-range-corrected type, and the relatively low cost of global-hybrid-functional periodic calculations using Gaussians means that one can employ global-hybrid functionals at a very reasonable cost and obtain more accurate band gaps of semiconductors than are obtained by the methods currently widely employed, namely local gradient approximations. We single out three global-hybrid functionals that give especially good results for both molecules (100 in the test set) and crystals (8 in the test set, for all of which our benchmark data are the proper band gap rather than an optical band gap uncorrected for exciton effects).

Observation of in-plane exciton-polaritons in monolayer WSe2 driven by plasmonic nanofingers.

The transition metal dichalcogenides (TMDs) have drawn great research attention, motivated by the derived remarkable optoelectronic properties and the potentials for high-efficient excitonic devices. The plasmonic nanocavity, integrating deep-sub wavelength confinement of optical mode with dramatic localized field enhancement, provides a practical platform to manipulate light-matter interaction. In order to obtain strong exciton-plasmon coupling effects, it's crucial to match the vibration direction ofexciton to the available strong localized in-plane electricfield. Herein, we demonstrate the coupling effect of in-plane exciton in monolayer tungsten diselenide (WSe2) to deterministic gap-plasmon field which is produced by nanometrically gapped collapsed nanofingers. The gap-plasmon field which is completely parallel to the in-plane excitons in WSe2 will drive a strong exciton-plasmon coupling at room temperature. More interestingly, it is experimentally observed that the luminescence of exciton-polariton cannot be influenced by the temperature in the range from 77K to 300K due to the presence of nanofingers. According to the theoretical analysis results, we attribute this finding to the dielectric screening effect arising from the extremely strong localized electric field of plasmonic nanofingers. This work proposes a feasible way to harness and manipulate the exciton of low-dimensional semiconductor, which might be potential for quantum optoelectronics.

Carbon vacancy-mediated exciton dissociation in Ti3C2Tx/g-C3N4 Schottky junctions for efficient photoreduction of CO2

Earth-abundant g-C3N4 is a promising photocatalyst for CO2 reduction, but its practical application is severely limited by the excitonic effect of g-C3N4 derived from strong binding energy and lack of electron-enriched active sites. Herein, we design a novel 2D/2D Schottky junction photocatalysts comprising of Ti3C2Tx-modified defective g-C3N4 nanosheets with carbon vacancy (denoted as Ti3C2Tx/Vc-CN) by a self-assembly method. The carbon vacancies in g-C3N4 promote exciton dissociation into free charge, while the formed Schottky junctions between Ti3C2Tx and Vc-CN further enables a directional charge transfer, thus providing an electron-rich catalytic surface for the CO2 reduction. Thanks to the synergy of promoted exciton dissociation and directional electron transfer, the optimal 20% Ti3C2Tx/Vc-CN display a high CO evolution rate of 20.54 µmol·g−1·h−1 under visible light irradiation, which is 7.4 times higher than that of bare CN. This work highlights the synergy of the promoted exciton dissociation and directional electron transfer in the activity enhancement of photocatalytic CO2 reduction.

Many-body effects in the X-ray absorption spectra of liquid water

SignificanceIn X-ray absorption spectroscopy, an electron-hole excitation probes the local atomic environment. The interpretation of the spectra requires challenging theoretical calculations, particularly in a system like liquid water, where quantum many-body effects and molecular disorder play an important role. Recent advances in theory and simulation make possible new calculations that are in good agreement with experiment, without recourse to commonly adopted approximations. Based on these calculations, the three features observed in the experimental spectra are unambiguously attributed to excitonic effects with different characteristic correlation lengths, which are distinctively affected by perturbations of the underlying H-bond structure induced by temperature changes and/or by isotopic substitution. The emerging picture of the water structure is fully consistent with the conventional tetrahedral model.

Single-particle exciton-initiated photo-catalytic reaction mapping on defective cuprous oxide

Although the exciton-triggered photocatalysis has recently been confirmed, so far it is limited to the low-dimensional semiconductors, and the characterization approaches are impossible to image and characterize the photo-catalytic reaction on single semiconductor photocatalysts. Herein by combining dark-field microscopy with 3, 3′, 5, 5′-tetramethylbenzidine as the chromogenic probe, we visually map the exciton-initiated photo-catalytic reaction on defective Cu2O microcrystals in real time at the single-particle level. Single-particle imaging results reveal the heterogeneity in reactivity among different individuals and at different regions within the same microcrystal. Moreover, the reaction rate constant of this photocatalytic reaction is determined as well. The theoretical simulations suggest that the introduction of Mn2+ can modulate the electronic structure and enhance the oxygen affinity of Cu2O, facilitating the production of 1O2. This high spatio-temporal-resolution imaging approach is general, which is also appropriate for investigating the exciton effects on Fe2+-doped Cu2O microcrystals.