Excitonic Effects Research Articles

Origin for dark catalytic performance of sulfide-based photocatalysis: Vital role of sulfur vacancy in charge storage and release

Effective storage and release of photogenerated charge carriers is a feasible strategy to resolve the light-dependence of photocatalytic technology. Herein, a sulfur vacancy-rich (Sv) 1T@2H-MoS2/Cd0.5Zn0.5S junction is developed. The strong ability of sulfur vacancies for storage and release of photogenerated charge carriers enables the 1T@2H-MoS2/Cd0.5Zn0.5S junction to work efficiently under intermittent light. In case of stopping illumination, Sv-1T@2H-MoS2/Cd0.5Zn0.5S junction still shows catalytic activity, leading to the continuous degradation of ofloxacin in the following dark period. Under intermittent lighting, the ofloxacin removal efficiency is 91.7 %, and the energy consumption was reduced by 62 % in comparison with that under continuous visible-light irradiation. The origin for the post-illumination catalytic activity in the dark was explored, and the excitonic effects were also taken into account in 1O2 generation.

Reticular Materials for Photocatalysis.

Photocatalysis leverages solar energy to overcome the thermodynamic barrier, enabling efficient chemical reactions under mild conditions. It can greatly reduce reliance on traditional energy sources and has attracted significant research interest. Reticular materials, including metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), represent a class of crystalline materials constructed from molecular building blocks linked by coordination and covalent bonds, respectively. Reticular materials function as heterogeneous catalysts, combining well-defined structures and high tailorability akin to homogeneous catalysts. In this review, the regulation of light absorption, charge separation, and surface reactions in the photocatalytic process through precise molecular-level design based on the features of reticular materials is elaborated. Notably, for MOFsmicroenvironment modulation around catalytic sites affects photocatalytic performance is delved, with emphasis on their unique dynamic and flexible microenvironments. For COFs, the inherent excitonic effects due to their fully organic nature is discussed and highlight the strategies to regulate excitonic effects for charge- and/or energy-transfer-mediated photocatalysis. Finally, the current challenges and future directions in this field, aiming to provide a comprehensive understanding of how reticular materials can be optimized for enhanced photocatalysis is discussed.

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.

Exciton Enhanced Nonlinear Optical Responses in Monolayer h-BN and MoS2: Insight from First-Principles Exciton-State Coupling Formalism and Calculations.

Excitons are vital in the photophysics of materials, especially in low-dimensional systems. The conceptual and quantitative understanding of excitonic effects in nonlinear optical (NLO) processes is more challenging compared to linear ones. Here, we present an ab initio approach to second-order NLO responses, incorporating excitonic effects, that employs an exciton-state coupling formalism and allows for a detailed analysis of the role of individual excitonic states. Taking monolayer h-BN and MoS2 as two prototype 2D materials, we calculate their second harmonic generation (SHG) susceptibility and shift current conductivity tensor. We find strong excitonic enhancement in the NLO responses requires that the resonant excitons are not only optically bright themselves but also able to couple strongly to other bright excitons. Our results explain the occurrence of two strong peaks in the SHG of monolayer h-BN and why the A and B excitons of MoS2 unexpectedly exhibit minimal excitonic enhancement in both SHG and shift current generation.

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.

Multiple conformal-contact transfer of large-area crack-free transition metal dichalcogenide stacks

Abstract Atomically-thin two-dimensional (2D) transition metal dichalcogenides (TMDs) have emerged as an ideal platform for both physics investigation and device applications. By stacking different layers into homo- or hetero-structures, an extra degree of freedom is involved in further tuning their properties, thereby boosting scenarios in twistronics, moiré photonics and optoelectronics. However, interfacial imperfections such as contaminations and cracks, frequently occur during the layer stacking sequence and accumulate layer by layer, greatly degenerating the interface quality. In this study, we developed a multiple conformal-contact transfer method to construct TMD stacks with crack-free intrinsic interfaces. The design of a deformable buffer layer is crucial to guarantee the conformal contact and intact transfer of each layer, contributing to the successful construction of centimetre-scale TMD stacks up to 8 layers. Precise control over spatial location and interlayer twist angle is also feasibly achieved, evidenced by the stacking-dependent interlayer exciton (IE) effects in WS2-WSe2 heterostructures. This work provides a facile and precise approach for architecting 2D stacks with perfect interfaces, which will further accelerate the customized design for their device functionalization.

Excitonic Effects on the Ultrafast Nonlinear Optical Response of MoS2 and Fluorinated Graphene/MoS2 Heterostructure Films for Photonic Applications.

In the present work, the ultrafast nonlinear optical (NLO) response of some molybdenum disulfide (MoS2), fluorinated graphene (FG), and FG/MoS2 heterostructure thin films was studied using the Z-scan and optical Kerr effect techniques employing femtosecond laser pulses at different excitation wavelengths (i.e., 400, 570, 610, 660, 800, and 1200 nm). The experiments have shown that the NLO response of the MoS2 and MoS2/FG films was significantly enhanced when the films were excited with 400, 610, and 660 nm laser pulses due to resonance effects with the close-lying excitons in these nanostructures. For a better evaluation of the resonant enhancement of the NLO response, measurements were also carried out at off-resonant wavelengths, i.e., at 570, 800, and 1200 nm. The presence of excitons in the MoS2 and MoS2/FG films resulted in strong saturable absorption and self-defocusing, with exceptionally large values of third-order susceptibilities χ(3) ranging from 10-12 to 10-13 esu. In addition, the NLO response of the MoS2/FG heterostructure was found to be stronger than that of the individual MoS2 and FG films, most probably attributed to interlayer carrier transfer. The determined NLO parameters of the studied nanostructures were found to be comparable to, and in some cases exceeded, those of other reported 2D materials known to exhibit a strong NLO response as well. These findings not only advance the fundamental understanding of the contributions of excitons on the NLO response/properties of transition metal dichalcogenide-based ultrathin films but also highlight the importance of excitons for tailoring their NLO response in view of various applications in advanced optoelectronics and photonic devices.

A DFT study on the role of excitons and electric field-induced symmetry breaking and topological properties of ZrBr

The control of band gap and topological properties is a crucial objective in the field of materials science, particularly for the application of 2D materials such as topological insulators. Zirconium monobromide (ZrBr), a topological insulator with band inversion, holds significant potential for various applications. In this study, we used Density Functional Theory (DFT) with different approximations namely, Generalized Gradient Approximation (GGA) with and without spin orbit coupling (SOC) as well as hybrid functional to investigate the electronic structure and the band gap of this compound. In addition, we introduced the excitonic effect by considering GW plus Bethe–Salpeter equation (GW-BSE) to compute the optical band gap and quasi-particle energy. We found that ZrBr present a low static dielectric function and weak exciton binding energy, generally lower than those of conventional semiconductors. This behavior is due to the reduced Coulomb interaction in the material. To further explore the material’s behavior, we apply an electric field in three directions to observe symmetry breaking and its impact on band gap tuning. This comprehensive analysis aims to explore the understanding and the potential applications of ZrBr as a topological insulator.

Giant excitonic magneto-Stark effect in wide GaAs/AlGaAs quantum wells

We have studied the magneto-Stark effect of exciton states with large wave vectors, significantly exceeding the wave vector of light. This magneto-Stark effect can be called “giant” in comparison with a similar effect observed in bulk materials in comparable magnetic fields. In this work, we propose a microscopic model of the “giant” magneto-Stark effect. The model does not contain any free parameters. The numerical results obtained in the framework of this model quantitatively describe the experimental results published earlier in Ref. S. Y. Bodnar et al., (2017) for a heterostructure with a wide GaAs/AlGaAs quantum well in a magnetic field.

Two-dimensional Sc2O3 monolayer with tunable ultrawide bandgap for solar-blind ultraviolet photodetector

A novel 2D scandia (Sc2O3) monolayer was identified using an evolutionary algorithm-based crystal structure prediction method. The Sc2O3 monolayer exhibits a P3‾m1 symmetry and excellent energetic, thermal, dynamical, and mechanical stability, as well as good mechanical flexibility. The monolayer possesses an ultrawide indirect band gap of 6.246 eV. The monolayer is transparent in the visible light zone, while the large exciton effect leads to significant absorption in the solar-blind and vacuum ultraviolet regions. The band gap of this monolayer can be reduced monotonically by external biaxial tensile strain, resulting in the absorption spectrum covering the entire solar blind spectral region when the load reaches 5.0 %. Additionally, the monolayer has an ultra-high in-plane dielectric constant of approximately 50. The superior stability, flexibility, and strain-tunable electronic and optical properties, as well as the ultra-high dielectric constant, suggest its potential application in a solar-blind photodetector in harsh environments.

Beyond the random phase approximation (RPA): First principles calculation of the valence EELS spectrum for KBr including local field, quasiparticle, excitonic and spin orbit coupling effects

The low energy region (< 50 eV) of the electron energy loss spectrum (EELS) can contain a great deal of spectral detail associated with excitations of the valence electrons. Calculation of the spectra from first principles can assist with interpretation and the most widely used method is the random phase approximation (RPA), usually neglecting local field effects (LFE). For KBr this approach is insufficient due to the importance of quasiparticle and excitonic effects. Calculations including these multi-electron effects are shown to give much improved agreement with the experimental spectra, and the inclusion of spin-orbit coupling (SOC) reproduces the excitonic doublet just above band-edge onset. A review of the complex theory behind these methods is given along with practical guidance on performing these calculations.

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.

Oxygen vacancy and boron dual sites enable efficient oxygen activation for emerging contaminant degradation: Boosted exciton dissociation via built-in electric field modulation

Two-dimensional (2D) layered materials are promising photocatalysts for environmental remediation through O2 activation. However, the inherent robust excitonic effects from the confined-layer structure hinder the generation of highly oxidative free radicals from the preferred charge transfer pathway. Taking 2D BiOBr as a prototype, we implemented a superficial surface boronizing method to incorporate oxygen vacancy (OV)-boron (B) dual sites onto the surface. The OV-B dual sites disorder the surface atomic arrangement of BiOBr and induce strong built-in electric field (BEF). This effectively weakens exciton binding energy, boosts exciton dissociation into free charges, and facilities the transfer and separation of free charges, which collectively enables charge transfer to surpass excitonic effects as the dominant O2 activation pathway. Consequently, BiOBr with OV-B dual sites outperforms pristine and mono-OV modified BiOBr in both O2 activation and emerging contaminant degradation. This work offers insightful strategies for modulating 2D photocatalysts to optimize O2 activation towards efficient contaminant removal.

Tuning the optical absorption and exciton bound states of germanene by chemical functionalization

We present a comprehensive study of buckled honeycomb germanene functionalized with alternately bonded side groups hydroxyl (–H), methyl (–CH3) and trifluoro methyl (–CF3). By means of most modern theoretical and computational methods we determine the atomic geometries versus the functionalizing groups. The quasiparticle excitation effects on the electronic structure are taken into account by means of exchange-correlation treatment within the GW framework. The Bethe–Salpeter equation is solved ab initio to derive optical spectra including excitonic and quasiparticle effects. Band edge excitons are investigated in detail. The binding properties are compared with those resulting from model studies. The functionalization leads to significantly modified band structures compared with pristine germanene. The Dirac bands near the K point are destroyed and direct gaps appear at the \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Gamma$$\\end{document} point. Together with the many-body effects, quasiparticle gaps of 2.3, 1.8 or 1.0 eV result for –H, –CH3 and –CF3 functionalization. Totally different absorption spectra are found for in-plane and out-of-plane light polarization. Strongly bound excitons are visible below the quasiparticle band edge with binding energies of about 0.5, 0.4 or 0.3 eV. The nature of these band-gap excitons is investigated via their wave function, the contribution of various interband combinations and the dipole selection rules.

Vertical stacking approach for tailoring electronic and optical properties of ultrathin two-dimensional vdW heterostructures of P-MAB (M=Ti, Zr, Hf; A=S, B=Se)

The vertical stacking in the form of heterostructures, is an effective way to enhance the performance of corresponding isolated monolayers. Strong interactions between Janus monolayers (MAB) and Phosphorus (P) monolayers result in significant improvements in both physical and chemical properties compared to single layer configurations. Using first-principle (hybrid) calculations, we explore electronic structure and optical properties of P–MAB (M = Ti, Zr, Hf; A = S, B=Se) van der Waals heterostructures, for potential applications in electronic and optoelectronic devices. Ab initio molecular dynamic (AIMD) simulations confirm the thermal stability, while phonon spectra confirm the dynamical stabilities of these systems. Our analysis reveals that all configurations maintain semiconducting nature with an indirect bandgap. The type-II band alignment in the P–MAB heterostructures is demonstrated through calculations of the partial density of states (PDOS). Bader charge analysis and planar-averaged charge density differences indicate an electron transfer from the P monolayers to the MAB monolayer. Furthermore, our investigation of the dielectric function ϵ2(ω) shows that the lowest energy shift are primarily influenced by excitonic effects.

Metallic-Substrate-Induced Renormalization of Quasiparticle Band Gaps and Excitonic Effects in Biphenylene Nanoribbons

Metallic-Substrate-Induced Renormalization of Quasiparticle Band Gaps and Excitonic Effects in Biphenylene Nanoribbons

N vacancies and OH/C=O co-modified g-C3N4 photoactivated peroxymonosulfate for the degradation of tetracycline: Synergistic promotion effect between excitons and carriers

Activation of peroxymonosulfate (PMS) to generate reactive oxygen species (ROS) for pollutant degradation via metal-free photocatalysts is an important green wastewater treatment and remediation method, but the pathway for generating ROS via exciton effects in catalysts is often overlooked. In this study, OH/C=O co-modified N-defective rod-shaped carbon nitride (Nv-CN-O) was designed and synthesized by soft template and alkali hydrothermal method. In the Nv-CN-O/PMS/Light system, there was a synergistic degradation of tetracycline (TC) by carriers and excitons, with a degradation rate 2.15 times higher than that of CN/PMS/Light. N vacancies and OH/C=O enhanced the adsorption to PMS, accelerated the charge transfer process, and activated PMS to generate ·O2− dominated reactive species. The functional group OH/C=O promoted the production of triplet excitons that through the energy transfer pathway to promote the production of 1O2. Abundant ROS were involved in the TC degradation process, and three possible TC degradation pathways were proposed. Nv-CN-O has strong anti-interference ability in actual water bodies, shows a promoting effect on the degradation of TC, which has practical application potential.

Photoinduced Nonadiabatic Dynamics of a Single-Walled Carbon Nanotube-Porphyrin Complex.

Single-walled carbon nanotubes (SWCNTs) have gained a lot of attention in the past few decades due to their promising optoelectronic properties. In addition, SWCNTs can form complexes that have good chemical stability and transport properties with other optical functional materials through noncovalent interactions. Elucidating the detailed mechanism of these complexes is of great significance for improving their optoelectronic properties. Nevertheless, simulating the photoinduced dynamics of these complexes accurately is rather challenging since they usually contain hundreds of atoms. To save computational efforts, most of the previous works have ignored the excitonic effects by employing nonadiabatic carrier (electron and hole) dynamics simulations. To properly consider the influence of excitonic effects on the photoinduced ultrafast processes of the SWCNT-tetraphenyl porphyrin (H2TPP) complex and to further improve the computational efficiency, we developed the nonadiabatic molecular dynamics (NAMD) method based on the extended tight binding-based simplified Tamm-Dancoff approximation (sTDA-xTB), which is applied to study the ultrafast photoinduced dynamics of the noncovalent SWCNT-porphyrin complex. In combination with statically electronic structure calculations, the present work successfully reveals the detailed microscopic mechanism of the ultrafast excitation energy transfer process of the complex. Upon local excitation on the H2TPP molecule, an ultrafast energy transfer process occurs from H2TPP (SWCNT-H2TPP*) to SWCNT (SWCNT*-H2TPP) within 10 fs. Then, two slower processes corresponding to the energy transfer from H2TPP to SWCNT and hole transfer from H2TPP to SWCNT take place in the 1 ps time scale. The sTDA-xTB-based electronic structure calculation and NAMD simulation results not only match the previous experimental observations from static and transient spectra but also provide more insights into the detailed information on the complex's photoinduced dynamics. Therefore, the sTDA-xTB-based NAMD method is a powerful theoretical tool for studying the ultrafast photoinduced dynamics in large extended systems with a large number of electronically excited states, which could be helpful for the subsequent design of SWCNT-based functional materials.

Exploring the potential of α-Ge(1 1 1) monolayer in photocatalytic water splitting for hydrogen production

In this study, the structural, electronic, and optical properties of 2D α-Ge(1 1 1) are investigated using Density Functional Theory (DFT) calculations, complemented by many-body perturbation theory calculations based on the GW/BSE approach. The thermodynamic stability of this material is assessed through ab initio molecular dynamics simulations (AIMD), and their dynamic stability is confirmed via phonon dispersion calculations. The analysis of the optical properties reveals significant absorption peaks in both visible and ultraviolet regions, with an absorption edge at 47 eV (1.87 eV without excitonic effects). The band edges are well-aligned with water redox potentials at neutral pH, making them suitable for water-splitting applications. For other pH levels, we find the process may be feasible through the participation of different excited states populated by light absorption. Remarkably, the α-Ge(1 1 1) monolayer demonstrates a predicted solar-to-hydrogen conversion efficiency of 34.80 %, outperforming many other two-dimensional materials. These findings position the α-Ge(1 1 1) monolayer as a promising candidate for developing efficient photocatalytic materials for hydrogen generation via overall water splitting.

Boron nitride quantum dots supported hollow NH2-MIL-125 drive photo-Fenton-PMS system for photocatalytic tetracycline degradation: Contribution of tannic acid etching

NH2-MIL-125(Ti) materials had great potential for photocatalytic applications but had low activity due to exciton effect and narrow absorption range of visible light. The surface oxygen-containing negative functional groups of boron nitride quantum dots (BNQDs) could overcome these defects, but due to the low load capacity, a higher specific surface area of the substrate was usually required. In this paper, a hollow Ti-MOF material was developed by etching technology. The hollow structure formed by tannic acid etching broadened the absorption range of visable light and provided more alternative surfaces for loading BNQDs. The 85.2% of high tetracycline (TC) removal efficiency for the best sample (BNQDs-5@20-Ti-MOF + PMS) was obtained, which was about 56.8 and 1.9 times of the 20-Ti-MOF and BNQDs-5@20-Ti-MOF, respectively. BNQDs-5@20-Ti-MOF + PMS system showed a great TC degradation efficiency in a wide pH range (pH = 5–9). In addition, reaction temperature and the inorganic ions did not show significant inhibition effect for TC removal. Both free radical and non-free radical pathways were involved in the TC degration by BNQDs-5@20-Ti-MOF + PMS system, among which O2•− and 1O2 played the key roles. Interestingly, multiple 1O2 production paths contributed to the high efficiency and stability of BNQDs-5@20-Ti-MOF + PMS system. This study revealed a reasonable combination of Ti-MOF and BNQDs, which provided a new efficient photocatalyst for environmental remediation.