Excitonic Effects Research Articles

Band-structure tunability via the modulation of excitons in semiconductor nanostructures: manifestation in photocatalytic fuel generation.

Understanding the energetics of electron transfer at the semiconductor interface is crucial for the development of solar harvesting technologies, including photovoltaics, photocatalysis, and solar fuel systems. However, modern artificial photosynthetic materials are not efficient and limited by their fast charge recombination with high binding energy of excitons. Hence, reducing the exciton binding energy can increase the generation of charge carriers, which improve the photocatalytic activities. Extensive research has been dedicated to improving the exciton dissociation efficiency through rational semiconductor design via heteroatom doping, vacancy engineering, the construction of heterostructures, and donor-π-acceptor (D-π-A) interfaces to extend the charge carrier migration, promoting the dissociation of excitons. Consequently, functionalized photocatalysts have demonstrated remarkable photocatalytic performances for solar fuel production under visible light irradiation. This review provides the fundamental aspects of excitons in semiconductor nanostructures, having a high binding energy and ultrafast exciton formation together with promising photo-redox properties for solar to fuel conversion application. In particular, this review highlights the significant role of the excitonic effect in the photocatalytic activity of newly developed functional materials and the underlying mechanistic insight for tuning the performance of nanostructured semiconductor photocatalysts for water splitting, CO2 reduction, and N2 fixation reactions.

Regulating excitonic effects in non-oxide based XPSe3 (X = Cd, Zn) monolayers towards enhanced photocatalysis for overall water splitting.

The non-oxide 2D materials have garnered considerable interest due to their potential utilization as photocatalysts, which offer a superior substitute to metal-oxide-based photocatalysts. This study investigates the impact of the dielectric environment on the size and binding energy of excitons in atomically thin, experimentally synthesized semiconducting monolayers [XPSe3, X = (Cd, Zn)] to address the critical problem of electron-hole recombination, which significantly hinders the efficiency of most photocatalysts. We employ a precise non-hydrogenic model surpassing the hydrogenic-based Mott-Wannier model. Our findings are among the first few demonstrations of an increase in exciton size (and decrease in exciton binding energy) as environmental screening increases. These findings have implications for photocatalytic water splitting and are not limited to metal phosphorus trichalcogenides, but can be applied to other classes of 2D materials as well. This work also compares metal-oxide photocatalysts, which have been the focus of much research over the past five decades, to non-oxide-based metal phosphorus trichalcogenide photocatalysts, which offer a superior alternative due to their ability to address issues such as light-harvesting inability in the visible spectrum and unwanted charge recombination centres. Furthermore, the implications of this study extend beyond photocatalysts and are significant for the design and development of next-generation optoelectronic devices that incorporate excitonic processes, such as solar cells, photodetectors, LEDs, etc.

Thickness-modulated optical nonlinearity of colloidal CdSe-CdS core-shell nanoplatelets: large two-photon absorption and self-focusing effects.

As a one-dimensional quantum confined material, colloidal semiconductor nanoplatelets have been widely studied as potential nonlinear materials due to their strong exciton effect and large two-photon absorption cross-section similar to that of two-dimensional materials. In this work, CdSe-CdS core-shell nanoplatelets were synthesized and third-order nonlinear optical properties related to shell thickness were measured using the Z-scan method. Measurement revealed a monotonic increase in the imaginary part of the third-order nonlinear susceptibility (Imχ(3)) of CdSe-CdS nanoplatelets, ranging from 0.62 × 10-13 esu to 2.43 × 10-13 esu, with the growth of shell thickness. The real part of the third-order nonlinear susceptibility (Reχ(3)) shows a non-monotonic change between 4.28 × 10-13 esu and 1.99 × 10-13 esu. The trends were further elucidated by analyzing the optical properties of the nanoplatelets, such as absorption, photoluminescence, and quantum yield, and understanding the variations in defect distribution, exciton binding energy, and quantum confinement effects. The results indicated that the appropriate passivation of the CdS shell effectively enhanced the luminescent performance and third-order nonlinearity of the nanoplatelets, while the induced defects and weakened quantum confinement effects due to the continued shell growth resulted in the opposite effect.

Trends in energy and charge transfer in 2D and integrated perovskite heterostructures.

Two-dimensional (2D) van der Waals (vdW) heterostructured transition metal dichalcogenides (TMDs) open up new possibilities for a wide range of optoelectronic applications. Interlayer couplings are responsible for several fascinating physics phenomena, which are in addition to the multifunctionalities that have been discovered in the field of optoelectronics. These couplings can influence the overall charge, or the energy transfer processes via stacking, separation, and dielectric angles. This focused review article summarizes the most recent and promising strategies for interlayer exciton emission in 2D or integrated perovskites and TMD heterostructures. These types of devices require a thorough comprehension and effective control of interlayer couplings in order to realize their functionalities and improve performance, which is demonstrated in this article with the energy or charge transfer mechanisms in the individual devices. An ideal platform for examining the interlayer coupling and the related physical processes is provided by a summary of the recent research findings in 2D perovskites and TMDs. Furthermore, it would encourage more investigation into the comprehension and regulation of excitonic effects and the related optoelectronic applications in vdW heterostructures over a broad spectral response range. Finally, the current challenges and prospects are summarized in this paper.

Optical properties of orthorhombic germanium sulfide: unveiling the anisotropic nature of Wannier excitons.

To fully explore exciton-based applications and improve their performance, it is essential to understand the exciton behavior in anisotropic materials. Here, we investigate the optical properties of anisotropic excitons in GeS encapsulated by h-BN using different approaches that combine polarization- and temperature-dependent photoluminescence (PL) measurements, ab initio calculations, and effective mass approximation (EMA). Using the Bethe-Salpeter Equation (BSE) method, we found that the optical absorption spectra in GeS are significantly affected by the Coulomb interaction included in the BSE method, which shows the importance of excitonic effects besides it exhibits a significant dependence on the direction of polarization, revealing the anisotropic nature of bulk GeS. By combining ab initio calculations and EMA methods, we investigated the quasi-hydrogenic exciton states and oscillator strength (OS) of GeS along the zigzag and armchair axes. We found that the anisotropy induces lifting of the degeneracy and mixing of the excitonic states in GeS, which results in highly non-hydrogenic features. A very good agreement with the experiment is observed.

First-principles study of the bandgap renormalization and optical property of β-LiGaO2

The β-LiGaO2 with an orthorhombic wurtzite-derived structure is a candidate ultrawide direct-bandgap semiconductor. In this work, using the non-adiabatic Allen–Heine–Cardona approach, we investigate the bandgap renormalization arising from electron–phonon coupling. We find a sizable zero-point motion correction of –0.362 eV to the gap at Γ, which is dominated by the contributions of long-wavelength longitudinal optical phonons. The bandgap of β-LiGaO2 decreases monotonically with increasing temperature. We investigate the optical spectra by comparing the model Bethe–Salpether equation method with the independent-particle approximation. The calculated optical spectra including electron–hole interactions exhibit strong excitonic effects, in qualitative agreement with the experiment. The contributing interband transitions and the binding energy for the excitonic states are analyzed.

Excitonic Enhancement and Excited Excitonic States in CsPbBr3 Halide Perovskite Single Crystals

Halide perovskites are novel photonics materials promising numerous applications in fields such as photovoltaics, LED light sources, microlasers, and radiation detectors. Many halide perovskites are direct-gap semiconductors, and Wannier-Mott excitons play a significant role in their optical properties near the fundamental absorption edge. The high oscillator strength of these states favors applications where efficient interaction with light is required. In this work, to study excitonic states in CsPbBr3 halide perovskite single crystals, the reflection spectroscopy at temperatures from 4 K was used. A reflection coefficient up to 70% was observed for the n=1 exciton state, followed by weak excited states of excitons with n=2 and n=3. It should be noted that the Sommerfeld enhancement factor should be considered for a correct description of the behavior of the dielectric constant, taking into account excitonic effects.

Oxygen Vacancy‐Mediated Exciton Effect in Hierarchical BiOBr Enables Dichotomy of Energy Transfer and Electron Transfer in Photocatalysis

AbstractUsing solar energy through green and simple artificial photosynthesis systems are considered as a promising way to solve the energy and environmental crisis. However, one of the important primary steps of photosynthesis, i.e., energy transfer, is long being ignored especially in inorganic semiconducting systems due to the small exciton binding energies. Herein, the simultaneous interrogation of the charge transfer and energy transfer steps in a photoexcitation process is proposed by utilizing few‐layered nanosheet‐assembled hierarchical BiOBr nanotubes with rich oxygen vacancies (OVs) as efficient multifunctional photocatalysts. Benefiting from the integrated 1D/2D structure and abundant OV defects, the excitonic effect strikes a delicate balance in the optimized BiOBr photocatalyst, showing not only improved charge carrier separation and transfer but also enhanced exciton generation. As a result, the hierarchical BiOBr nanotubes exhibit high efficiency toward photocatalytic CO2 reduction with an impressive CO evolution rate of 135.6 µmol g−1 h−1 without cocatalyst or photosensitizer. The dominant reactive oxygen species of singlet oxygen (1O2) are discriminated for the first time, which originated from an energy transfer process, with electrophilic character, whereas the minor effect of superoxide anion radical (•O2−) with a nucleophilic rate‐determining step in the photocatalytic aerobic oxidation of sulfides.

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.