Excitonic Effects Research Articles

An investigation of dynamical stability, electronic, and optical properties of sodium metaborate (NaBO2): A many-body G0W0+BSE approach

This study investigated the electronic, optical, and phonon properties of bulk sodium metaborate (NaBO2) using density functional theory (DFT), and the many-body G0W0 plus Bethe-Salpeter equation (BSE) approach. The investigation of the phonon dispersion curve indicates the dynamic stability of the NaBO2 compound in the rhombohedral phase, which is in good agreement with experimental results. Also, cohesive energy calculation shows the thermodynamic stability of this material. The band structure results indicated that the direct band gap of NaBO2 at the Γ point is 3.81 eV/4.16 eV within LDA/PBE approximation, which increased to 6.78 eV/6.57 eV by considering the quasi-particle effects in G0W0@LDA/PBE calculations. Considering the electron-hole effects in the G0W0+BSE approach, the optical gap showed more accurate results, and the value of 5.83 eV/5.71 eV was obtained for the LDA/PBE starting point. Therefore, excitonic effects within the G0W0+BSE approximation, in comparison to G0W0+RPA, leads to the redshift of the optical absorption spectrum. The optical results estimate the exciton binding energy of 0.95 eV/0.86 eV within LDA/PBE approximation. The investigation of the optical absorption spectrum introduces sodium metaborate as a transparent material with the prospect of use in optical devices.

Non-adiabatic Exciton Dynamics in van der Waals Heterostructures.

In this Perspective, we introduce a first-principles method that combines time-dependent density functional theory with non-adiabatic molecular dynamics (NAMD) to explore exciton dynamics in two-dimensional (2D) van der Waals (vdW) heterostructures. The theoretical foundation and computational efficiency of the method are discussed and compared with those of related methods (e.g., GW-BSE). Using three 2D vdW heterostructures as examples, we demonstrate that the proposed method can provide a reliable description of many-body electron-hole interactions crucial to exciton dynamics. With much lower computational costs than the GW-BSE method, the proposed method represents a particularly promising theoretical tool to probe exciton dynamics in solids. Moreover, we find that the NAMD simulations widely used in the literature cannot capture the excitonic effect in 2D materials and often yield incorrect results because they are formulated in a single-particle picture. The instances where the single-particle picture fails are pointed out and contrasted with the many-body simulation results.

Electronic and optical properties of the buckled and puckered phases of phosphorene and arsenene

Using first-principles calculations, we have investigated the structural, electronic, and optical properties of phosphorene and arsenene, group V two-dimensional materials. They have attracted the scientific community’s interest due to their possible applications in electronics and optoelectronics. Since phosphorene and arsenene are not planar monolayers, two types of structures were considered for each system: puckered and buckled arrangements. Computations of band gap were performed within the GW approach to overcome the underestimation given by standard DFT and predict trustable band gap values in good agreement with experimental measurements. Our calculated electronic band gaps lie in the range from near-infrared to visible light, suggesting potential applications in optoelectronics devices. The computed electronic band gaps are 2.95 eV and 1.83 eV for blue and black phosphorene systems. On the other hand, the values for buckled and puckered arsenene are 2.56 eV and 1.51 eV, respectively. Moreover, the study of the optical properties has been dealt by computing the dielectric function imaginary part, which was obtained using the Bethe–Salpeter approach. The use of this technique allows the consideration of excitonic effects. Results indicate strong exciton binding energies of 830 meV for blue phosphorene, 540 meV for black phosphorene, 690 meV for buckled arsenene, and 484 meV for puckered arsenene. The results of our study suggest the possibility of using these materials in electronic and optoelectronic devices.

High anisotropy in titanium trisulfide monolayer: Ultrahigh carrier mobilities and large excitonic absorption

Two-dimensional titanium trisulfide (TiS3) attracts much attention due to its high carrier mobilities and anisotropic physical properties. In this study, based on the ab-initio method, the vibrational, carrier transport and excitonic properties for TiS3 monolayer are investigated in detail. The results show that highly anisotropic structure of monolayer TiS3 leads to its anisotropic physical properties. The hole mobilities are larger than electron mobilities, which can be further enhanced by strain engineering. A strong absorption peak at 0.59 eV for light polarization along b direction is observed within the quasi-particle bandgap, which is 15 times larger than that at 0.98 eV for light polarization along a direction. The absorption peaks are contributed dominantly by bright excitons whose binding energies are 0.43 eV and 0.82 eV along a and b directions, both of which are delocalized Wannier-Mott types. The ultrahigh and flexibly strain-engineering transport properties and the significant excitonic effects of anisotropic TiS3 monolayer make possible applications in flexible electronic and optoelectronic devices.

Excitonic Effects in the Optical Properties of Scandium and Hafnium MXene Semiconductors from First‐Principles Calculations

The present study uses a full‐potential linearized augmented plane wave method and Bethe–Salpeter equation to calculate and interpret the optical properties of Sc2CT2 (T = O, F, and OH) and Hf2CO2 MXene semiconductors, including real and imaginary parts of the dielectric function along with energy loss function. Excitonic structures are observed below calculated bandgaps in these compounds. Sc2CO2 and Sc2CF2 have three excitonic features, whereas Sc2C(OH)2 and Hf2CO2 show two. As compared to other compounds, Sc2CO2 has a larger exciton binding energy. The energy loss functions display spectral features near the bandgap regions due to the excitonic structures in the imaginary part of the dielectric function.

Enhanced Exciton Effect and Singlet Oxygen Generation Triggered by Tunable Oxygen Vacancies on Bi2MoO6 for Efficient Photocatalytic Degradation of Sodium Pentachlorophenol.

Construction of the tunable oxygen vacancies (OVs) is widely utilized to accelerate molecular oxygen activation for boosting photocatalytic performance. Herein, the in-situ introduction of OVs on Bi2MoO6 was accomplished using a calcination treatment in an H2/Ar atmosphere. The introduced OVs can not only facilitate carrier separation, but also strengthen the exciton effect, which accelerates singlet oxygen generation through the energy transfer process. Superior carrier separation and abundant singlet oxygen played a crucial role in favoring photocatalytic NaPCP degradation. The optimal BMO-001-300 sample exhibited the fastest NaPCP degradation rate of 0.033 min-1, about 3.8 times higher than that of the pristine Bi2MoO6. NaPCP was effectively degraded and mineralized mainly through dechlorination, dehydroxylation and benzene ring opening. The present work will shed light on the construction and roles of OVs in semiconductor-based photocatalysis and provide a novel insight into ROS-mediated photocatalytic degradation.

Quantitative analysis of the blue-green single-photon emission from a quantum dot in a thick tapered nanowire

Quantum dots (QDs) acting as single-photon emitters in the blue-green range are fabricated and characterized at cryogenic temperature. They consist in CdSe dots inserted in (Zn,Mg)Se nanowires with a thick shell. Photoluminescence spectra, decay curves, and autocorrelation functions were measured under nonresonant continuous-wave and pulsed excitation. An analytical approach is applied simultaneously to the decay curves and correlation functions. It allows a quantitative description of how these two quantities are affected by the exciton rise due to biexciton feeding, the bright exciton decay, the effect of the dark exciton, and the reexcitation between two laser pulses. Linewidths at our limit of resolution (200 $\ensuremath{\mu}\mathrm{eV}$) are recorded. The reported correlation counts vary from a full control by reexcitation from traps, to a small contribution of reexcitation by mobile carriers or other QDs, as low as 5%.

Hot exciton effect in photoluminescence of monolayer transition metal dichalcogenide

AbstractHot excitons are usually neglected in optical spectroscopy in two‐dimensional semiconductors for the sake of momentum conservation, as the majority of hot excitons are out of light cones. In this letter, we elaborate on the contribution of hot excitons to optical properties of monolayer Molybdenum diselenide (MoSe2) with photoluminescence (PL) and PL excitation (PLE) spectroscopy. With the excitation‐intensity‐dependent PL, temperature‐dependent PL and PLE experiments combined with the simulations, we experimentally distinguish the influences of the exciton temperature and the lattice temperature in the PL spectrum. It is concluded that the acoustic phonon‐assisted PL accounts for the non‐Lorentzian high energy tail in the PL spectrum, and the hot exciton effect is significant to linear optical properties of transition metal dichalcogenides. Besides, the effective exciton temperature is found to be several tens of Kelvin higher than the lattice temperature at non‐resonant optical excitation. It indicates that the exciton temperature needs to be carefully taken into account when considering the exciton‐related quantum phase phenomena such as exciton condensation. It is experimentally demonstrated that the effective exciton temperature can be tuned by excitation energy.Key points The acoustic phonon‐assisted photoluminescence (PL) accounts for the non‐Lorentzian high‐energy tail in the PL spectrum. “Hot” excitons play a significant role in optical properties of two‐dimensional transition metal dichalcogenides. The effective exciton temperature could be tuned by excitation energy.

Frenkel Excitons in Vacancy-Ordered Titanium Halide Perovskites (Cs2TiX6)

Low-cost, nontoxic, and earth-abundant photovoltaic materials are long-sought targets in the solar cell research community. Perovskite-inspired materials have emerged as promising candidates for this goal, with researchers employing materials design strategies including structural, dimensional, and compositional transformations to avoid the use of rare and toxic elemental constituents, while attempting to maintain high optoelectronic performance. These strategies have recently been invoked to propose Ti-based vacancy-ordered halide perovskites (A2TiX6; A = CH3NH3, Cs, Rb, or K; X = I, Br, or Cl) for photovoltaic operation, following the initial promise of Cs2SnX6 compounds. Theoretical investigations of these materials, however, consistently overestimate their band gaps, a fundamental property for photovoltaic applications. Here, we reveal strong excitonic effects as the origin of this discrepancy between theory and experiment, a consequence of both low structural dimensionality and band localization. These findings have vital implications for the optoelectronic application of these compounds while also highlighting the importance of frontier-orbital character for chemical substitution in materials design strategies.

Energy transfer to Gd3+ from the self-trapped exciton in ScPO4:Gd3+ — In the case of persistent luminescence

We report a persistent luminescence phenomenon of ScPO4:Gd3+ phosphor, which exhibits long-lasting ultraviolet emission peaking at 313 nm after the end of X-ray irradiation. Unlike the steady-state luminescence of the phosphor excited by ionizing irradiation, no quantum-cutting emission is involved in the persistent luminescence. We ascribe the presence of persistent luminescence, as well as the absence of quantum cutting, to the effect of self-trapped exciton on the electron and energy transfers. This work provides some insights to the understanding of Gd3+-activated ultraviolet persistent luminescence.

Realizing efficient exciton dissociation in an all-organic heterojunction photocatalyst for highly improved photocatalytic H2 evolution

Although graphitic carbon nitride is a promising photocatalyst in the field of energy conversion and environmental purification, the intrinsic properties like excitonic effects and sluggish charge transfer restrict further photocatalytic applications. To circumvent these limitations, the novel all-organic heterojunction photocatalysts were constructed by anchoring organic carbon dots (O-dots) on porous graphitic carbon nitride nanosheets (O-dots/CNS). Results demonstrated that excitons can be effectively dissociated into electrons and holes at the interface of O-dots/CNS heterojunction, followed by holes injected to O-dots and electrons accumulated in CNS to realize efficient charge separation. Consequently, the O-dots/CNS with the optimized hydrogen (H2) evolution performance could be reached 1564.5 μmol h−1g−1 under the visible light irradiation. This work not only presents new ideas for rational design photocatalytic reaction system from exciton and charge carrier, but also broaden the applications of this new kind of organic dots in the field of energy conversion.

Electronic and Optical Properties of Rocksalt Mg1-xZnxO and Wurtzite Zn1-xMgxO with Varied Concentrations of Magnesium and Zinc.

The structural, electronic and optical properties of rocksalt Mg1-xZnxO and wurtzite Zn1-xMgxO with the concentration of Zn and Mg varying from 0.125 to 0.875 were investigated using density functional theory (DFT), DFT+U, linear response theory and the Bethe-Salpeter equation. According to the experimental band gap for varied concentrations of magnesium and zinc, modeling the supercell was utilized for the varied concentrations of Mg/Zn/O compounds in order to not only avoid constructing the complicated interface systems that are observed in the experiments but also take into account the excitonic effects that usually require huge computational resources. From the calculated density of states, the Zn states are highly related to the edge of the conduction band minimum and responsible for the width of bandgap. In addition, the contribution of Zn-d states is below expectations as they are located away from the VBM. As for the optical response, an increase in Zn concentration would cause a red-shifted spectrum, on the whole. In contrast, the higher concentration of Mg also triggers the blue-shift of the optical spectrum. In addition, anisotropic properties could be found in the spectrum with consideration of the excitonic effects, whereas there is no apparent difference in optical response based on linear response theory. In addition, the optical features of this work reflect the characteristic peaks of the literature around the absorption onset.

First-principles study of metallic impurities induced 355 nm UV laser absorption in fused silica

To date, it is controversial which metallic impurities play crucial role in the laser induced degradation and damage of fused silica at 355 nm. In this work, a First -principles study for the influences of impurities in fused silica are presented, which are usually contaminated during manufacture and post-processing processes. Our results show that Ca, K, Fe and Ce can lead to obvious optical absorption near 355 nm. Moreover, we find that, for K and Ca, the absorption intensity near 355 nm is highly sensitive to the concentration. Our results indicate that K and Ca can easily induce degradation and damage of fused silica at 355 nm. These two impurities should be concerned during manufacture and post-processing processes. The detailed theoretical study reveal that Ca and K can induce obvious impurity states in band gap and the optical absorption near 355 nm is originated from the transition between impurity states to conduction band. Our results also reveal that, for fused silica with Ca impurity, the exciton effect plays an important role for the optical absorptions near 355 nm, while it is not significant for K. Our work helps to explore laser damage mechanism and improve manufacture and post-processing techniques of fused silica.

Interaction of excitons with magnetic topological defects in 2D magnetic monolayers: localization and anomalous Hall effect

Novel 2D material CrI3 reveals unique combination of 2D ferromagnetism and robust excitonic response. We demonstrate that the possibility of the formation of magnetic topological defects, such as Néel skyrmions, together with large excitonic Zeeman splitting, leads to giant scattering asymmetry, which is the necessary prerequisite for the excitonic anomalous Hall effect. In addition, the diamagnetic effect breaks the inversion symmetry, and in certain cases can result in exciton localization on the skyrmion. This enables the formation of magnetoexcitonic quantum dots with tunable parameters.

The suppressing of excitonic effects in Cu-chalcogenides for solar cell applications

Excitonic effect can usually significantly enhance the optical absorption coefficient in wide-gap insulators. Cu-chalcogenides are widely used as thin-film solar cell absorbers due to their high light utilization efficiency. Theoretical investigations on the excitonic absorption in Cu-chalcogenides are rare because of the complicated two-particle nature of an exciton. Here, we theoretically evaluate the continuum-excitonic effect in CuGaS2 and CuInS2 by solving the Bethe-Salpeter equation. Despite the large bandgap of 2.4 eV for CuGaS2, the excitonic effect induces moderate absorption enhancement when compared with the single-particle scenario. Moreover, the experimental results are better reproduced using the latter approach, indicating that the intrinsic excitonic effect is nearly suppressed in the practical experiments. The moderate intrinsic excitonic effect and its further suppressing in the experiments are explained by strong electronic screening effects in these materials. Justification of the simple single-particle approach would be convenient for future theoretical studies, which otherwise require more complex methods with more expensive computational efforts.

Hologram imaging quality improvement by ionization controlling based on the self-trapped excitons with double-pulse femtosecond laser

Abstract Holograms hidden inside transparent materials are important for information encryption storage because of their advantages of secrecy, and could completely avoid information loss caused by surface wear. Inside the transparent material, the modified filaments array was need for hologram fabrication to change the optical phase or amplitude of incident laser, which is sensitive to the change of refractive index. Then the uniformity of modified filaments inside transparent materials is highly required. In this study, by tuning the interval time of the double-pulse processing, holograms with improved imaging quality were fabricated by double-pulse femtosecond laser and the effect and mechanism of self-trapped excitons (STEs) on the ablation have been systematically studied. The imaging quality of the hologram fabricated with double-pulse laser was superior to that of the one fabricated with the single-pulse laser and 350 fs was verified to be the best time interval for double-pulse processing. The evolution of the electrons dynamics was investigated by using the pump-probe technology. With the double-pulse time interval increasing, the residual electrons, excitons, STEs, and defects caused by the first sub-pulse would become dominated sequentially. The results demonstrated the controllability of STEs and quality improvement of final structures by double-pulse femtosecond laser processing.

Two-dimensional heterostructures formed by graphenelike ZnO and MgO monolayers for optoelectronic applications

Two-dimensional heterostructures are an emerging class of materials for novel applications because of extensive engineering potential by tailoring intriguing properties of different layers as well as the ones arising from their interface. A systematic investigation of mechanical, electronic, and optical properties of possible heterostructures formed by bilayer structures graphenelike ZnO and MgO monolayers is presented. Different functionality of each layer makes these heterostructures very appealing for device applications. ZnO layer is convenient for electron transport in these structures, while MgO layer improves electron collection. At the outset, all of the four possible stacking configurations across the heterostructure are mechanically stable. In addition, stability analysis using phonon dispersion reveals that the AB stacking formed by placing the Mg atom on top of the O atom of the ZnO layer is also dynamically stable at zero temperature. Henceforth, we have investigated the optical properties of these stable heterostructures by applying many-body perturbation theory within the framework of GW approximation and solving the Bethe-Salpeter equation. It is demonstrated that strong excitonic effects reduce the optical band gap to the visible light spectrum range. These results show that this new two-dimensional form of ZnO/MgO heterostructures open an avenue for novel optoelectronic device applications.

Zero-Threshold Optical Gain in Electrochemically Doped Nanoplatelets and the Physics Behind It.

Colloidal nanoplatelets (NPLs) are promising materials for lasing applications. The properties are usually discussed in the framework of 2D materials, where strong excitonic effects dominate the optical properties near the band edge. At the same time, NPLs have finite lateral dimensions such that NPLs are not true extended 2D structures. Here we study the photophysics and gain properties of CdSe/CdS/ZnS core-shell-shell NPLs upon electrochemical n doping and optical excitation. Steady-state absorption and PL spectroscopy show that excitonic effects are weaker in core-shell-shell nanoplatelets due to the decreased exciton binding energy. Transient absorption studies reveal a gain threshold of only one excitation per nanoplatelet. Using electrochemical n doping, we observe the complete bleaching of the band edge exciton transitions. Combining electrochemical doping with transient absorption spectroscopy, we demonstrate that the gain threshold is fully removed over a broad spectral range and gain coefficients of several thousand cm-1 are obtained. These doped NPLs are the best performing colloidal nanomaterial gain medium reported to date, with the lowest gain threshold and broadest gain spectrum and gain coefficients that are 4 times higher than in n-doped colloidal quantum dots. The low exciton binding energy due to the CdS and ZnS shells, in combination with the relatively small lateral size of the NPLs, results in excited states that are effectively delocalized over the entire platelet. Core-shell NPLs are thus on the border between strong confinement in QDs and dominant Coulombic effects in 2D materials. We demonstrate that this limit is in effect ideal for optical gain and that it results in an optimal lateral size of the platelets where the gain threshold per nm2 is minimal.

Excitonic Effects in X-ray Absorption Spectra of Fluoride Salts and Their Surfaces

Given their natural abundance and thermodynamic stability, fluoride salts may appear as evolving components of electrochemical interfaces in Li-ion batteries and emergent multivalent ion cells. This is due to the practice of employing electrolytes with fluorine-containing species (salt, solvent, or additives) that electrochemically decompose and deposit on the electrodes. Operando X-ray absorption spectroscopy (XAS) can probe the electrode–electrolyte interface with a single-digit nanometer depth resolution and offers a wealth of insights into the evolution and Coulombic efficiency or degradation of prototype cells, provided that the spectra can be reliably interpreted in terms of local oxidation state, atomic coordination, and electronic structure about the excited atoms. To this end, we explore fluorine K-edge XAS of mono- (Li, Na, and K) and di-valent (Mg, Ca, and Zn) fluoride salts from a theoretical standpoint and discover a surprising level of detailed electronic structure information about these materials despite the relatively predictable oxidation state and ionicity of the fluoride anion and the metal cation. Utilizing a recently developed many-body approach based on the ΔSCF method, we calculate the XAS using density functional theory and experimental spectral profiles are well reproduced despite some experimental discrepancies in energy alignment within the literature, which we can correct for in our simulations. We outline a general methodology to explain shifts in the main XAS peak energies in terms of a simple exciton model and explain line-shape differences resulting from the mixing of core-excited states with metal d character (for K and Ca specifically). Given ultimate applications to evolving interfaces, some understanding of the role of surfaces and their terminations in defining new spectral features is provided to indicate the sensitivity of such measurements to changes in interfacial chemistry.

Edge carboxylation-induced charge separation dynamics of graphene quantum dot/cellulose nanocomposites

Graphene quantum dot (GQD)@cellulose nanocomposites possess optoelectronic properties of interest for photovoltaic applications. However, the optoelectronic properties related to the shapes and edge types of GQDs have not been fully explored. In the present work, we investigate the effects of carboxylation on the energy alignment and charge separation dynamics at the interface of GQD@cellulose nanocomposites using density functional theory calculations. Our results show that the GQD@cellulose nanocomposites composed of hexagonal GQDs with armchair edges exhibit better photoelectric performance than those composed of other types of GQDs. Carboxylation stabilizes the energy level of the highest occupied molecular orbital (HOMO) of the triangular GQDs with armchair edges but destabilizes the HOMO energy level of cellulose, resulting in hole transfer from the GQDs to cellulose upon photoexcitation. However, the calculated hole transfer rate is lower than the nonradiative recombination rate because excitonic effects dominate the dynamics of charge separation in GQD@cellulose nanocomposites.