Excitonic Effects Research Articles

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.

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 350fs 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.

Influence of external fields on the exciton binding energy and interband absorption in a double inverse parabolic quantum well

Ground-state binding energy and optical absorption coefficient of exciton in a symmetric double inverse parabolic quantum well system in the presence of external electric and magnetic fields are investigated theoretically. Numerical calculations have been carried out with the variational technique considering the single-band effective mass approximations. Results reveal that the excitonic binding energy and interband optical absorption depend on significantly on the geometrical and structural parameters defining carriers’ double inverse parabolic confinement potential, the aluminum concentration at the well center, the electric field, and the magnetic field. The exciton binding energy is a decreasing (increasing) function of the electric field (magnetic field). The increase in aluminum concentration creates a blueshift at the resonance peak positions. As a result, the tunability of the excitonic effects is expected to be of importance in developing stable and high-efficiency nanoscale excitonic optoelectronic devices . • Analyses of double inverse parabolic quantum well. • Exciton binding energy. • Interband transitions. • Electric and magnetic field effects.

Surface Boronizing Can Weaken the Excitonic Effects of BiOBr Nanosheets for Efficient O2 Activation and Selective NO Oxidation under Visible Light Irradiation.

The photocatalytic O2 activation for pollutant removal highly depends on the controlled generation of desired reactive oxygen species (ROS). Herein, we demonstrate that the robust excitonic effect of BiOBr nanosheets, which is prototypical for singlet oxygen (1O2) production to partially oxidize NO into a more toxic intermediate NO2, can be weakened by surface boronizing via inducing a staggered band alignment from the surface to the bulk and simultaneously generating more surface oxygen vacancy (VO). The staggered band alignment destabilizes excitons and facilitates their dissociation into charge carriers, while surface VO traps electrons and efficiently activates O2 into a superoxide radical (•O2-) via a one-electron-transfer pathway. Different from 1O2, •O2- enables the complete oxidation of NO into nitrate with high selectivity that is more desirable for safe indoor NO remediation under visible light irradiation. This study provides a facile excitonic effect manipulating method for layered two-dimensional photocatalysts and sheds light on the importance of managing ROS production for efficient pollutant removal.

Probing Excitonic Rydberg States by Plasmon Enhanced Nonlinear Optical Spectroscopy in Monolayer WS2 at Room Temperature.

The optoelectronic properties of two-dimensional (2D) transition metal dichalcogenide (TMDC) monolayers such as WS2 are largely dominated by excitons due to strong Coulomb interactions in these 2D confined monolayers, which lead to formation of Rydberg-like excitonic states below the free quasiparticle band gap. The precise knowledge of high order Rydberg excitonic states is of great importance for both fundamental understanding such as many-electron effects and device applications such as optical switching and quantum process information. Bright excitonic states could be probed by linear optical spectroscopy, while probing dark excitonic states generally requires nonlinear optical (NLO) spectroscopy. Conventional optical methods for probing high-order Rydberg excitonic states were generally performed at cryogenic temperatures to ensure enough signal-to-noise ratio (SNR) and narrow line width. Here we have designed a hybrid nanostructure of monolayer WS2 integrated with a plasmonic cavity and investigated their NLO properties at the single particle level. Giant enhancement in NLO responses, stronger excitonic resonance effects, and narrowed line widths of NLO excitation spectra were observed when monolayer WS2 was placed in our carefully designed plasmonic cavity. Optimum enhancement of 1000-, 3000-, and 3800-fold were achieved for two-photon photoluminescence (2PPL), second harmonic generation (SHG), and third-harmonic generation (THG), respectively, in the optimized cavity structure. The line width of SHG excitation spectra was reduced from 43 down to 15 meV. Plasmon enhanced NLO responses brought improved SNR and spectral resolution, which allowed us to distinguish discrete excitonic states with small energy differences at room temperature. By using three complementary NLO techniques in combination with linear optical spectroscopy, energies of Rydberg excitonic states of A (1s, 2s, 2p, 3s, 3p, 4s), B (1s), and C and D excitons of monolayer WS2 have been accurately determined, which allow us to determine exciton binding energy and quasiparticle bandgap. It was interesting to find that the 2p lies 30 meV below 2s, which lends strong support to the theoretical prediction of nonlocal dielectric screening effects based on a non-hydrogenic model. Our results show that plasmon enhanced NLO spectroscopy could serve as a general method for probing high order Rydberg excitonic states of 2D materials.

Temperature-Dependent Carrier Extraction and the Effects of Excitons on Emission and Photovoltaic Performance in Cs0.05FA0.79MA0.16Pb(I0.83Br0.17)3 Solar Cells.

The photovoltaic parameters of triple cation perovskite [Cs0.05FA0.79MA0.16Pb(I0.83Br0.17)3] solar cells are investigated focusing on the electro-optical properties and differences in performance at low and high temperatures. The signature of a parasitic barrier to carrier extraction is observed at low temperatures, which results in a loss of performance at T < 200 K. Intensity-dependent measurements indicate extraction across this parasitic interface is limited by a combination of the exciton binding energy and thermionic emission. However, the photovoltaic performance of the device is recovered at low intensity─where the photocarrier generation rate threshold is lower than the thermionic extraction rate. Loss of solar cell performance is also observed to be strongly correlated to an increase in photoluminescence intensity, indicating inhibited carrier extraction results in strong radiative recombination and that these systems do not appear to be limited by significant thermally activated non-radiative processes. Evidence of limited carrier extraction due to excitonic effects is also observed with a strong anti-correlation in photoluminescence and carrier extraction observed at lower temperatures.

Excitonic Bloch–Siegert shift in CsPbI3 perovskite quantum dots

Coherent interaction between matter and light field induces both optical Stark effect and Bloch–Siegert shift. Observing the latter has been historically challenging, because it is weak and is often accompanied by a much stronger Stark shift. Herein, by controlling the light helicity, we can largely restrict these two effects to different spin-transitions in CsPbI3 perovskite quantum dots, achieving room-temperature Bloch–Siegert shift as strong as 4 meV with near-infrared pulses. The ratio between the Bloch–Siegert and optical Stark shifts is however systematically higher than the prediction by the non-interacting, quasi-particle model. With a model that explicitly accounts for excitonic effects, we quantitatively reproduce the experimental observations. This model depicts a unified physical picture of the optical Stark effect, biexcitonic optical Stark effect and Bloch–Siegert shift in low-dimensional materials displaying strong many-body interactions, forming the basis for the implementation of these effects to information processing, optical modulation and Floquet engineering.

Piezoelectricity and optical properties of janus MXY (M = Sb, As; X = Te, Se; Y = Br, I) monolayers

Two-dimensional (2D) piezoelectric semiconductor nanomaterials with strong optical absorbance in the visible range are promising applications in piezotronics, phototronics, and piezo-phototronics. We determine and analyze the stability, electronic structures, piezoelectricity, and optical properties of MXY (M = Sb, As; X = Te, Se; Y = Br, I) monolayers. Analyses of stabilities disclose these monolayers are energetically, mechanically, thermodynamically, and dynamically stable. Both HSE06-SOC and G0W0 approaches reveal that all the monolayers have indirect gaps falling in the visible optical spectrum. The intrinsic mirror asymmetry not only induces appreciable in-plane and vertical piezoelectricity, which is comparable with the Janus transition metal dichalcogenide monolayers, but also exhibit appreciable intrinsic build-in electronic field. The robust excitonic effects shift the optical absorption spectra down to the visible light region and extend to the ultraviolet region for these monolayers. Finally, the monolayers show large in-plane and out-of-plane static dielectric constants which are larger than most reported monolayers. All the excellent properties suggest that the monolayers are potential candidates for piezotronics, piezo-phototronics, and optoelectronic devices for harvesting solar energy.

Size-dependent third-order optical nonlinearity in colloidal CdSe/CdS core/crown nanoplatelets with quasi-type Ⅱ band-structure

Colloidal semiconductor nanoplatelets, a kind of one-dimensional quantum confined materials, have been widely investigated due to their remarkable excitonic effect, in which excellent two-photon absorption property has been proved. In this work, 4.5 monolayers thick CdSe nanoplatelets laterally extended with CdS were synthesized and their crown size-dependent third-order nonlinear optical properties were measured through a picosecond Z-scan method. The detailed investigation combining the time-related photoluminescence spectroscopy reveals the effective passivation of the CdSe boundary with same area size of the core could improve the quantum yield up to 85% and third-order nonlinearity. In addition, the two-photon absorption cross-section can reach up to 6.8 × 106 GM due to the quasi-type Ⅱ band structure of these core/crown nanoplatelets.

Understanding inclusive quantum dots hollow CN@CIZS heterojunction for enhanced photocatalytic CO2 reduction

Photocatalytic conversion of carbon dioxide (CO2) to fuel or useful chemicals is of great significance for energy and environmental issues. Herein, we successfully prepare a hollow g-C3N4@Cu-In-Zn-S quantum dots (CN@CIZS QDs) photocatalytic heterojunction employing the method of electrostatic self-assembly. The hollow structure, multiple exciton effect of quantum dots and the discrepancy of band structure generate excellent photocatalytic performance. The yield of CO2 photocatalytic reduction to CO and CH4 respectively is 23.04 μmol g−1 and 13.92 μmol g−1 after simulated visible light irradiation for 6 h for hollow CN@CIZS QDs 20 wt% sample. Moreover, the results of in-situ characterization and other photochemical tests strongly demonstrate the significant optimization of the photocatalytic performance. The research offers a feasible green thought for the resource utilization of CO2.

Strong Optical Excitation and High Thermoelectric Performance in 2D Holey‐Phosphorene Monolayer

Through density functional theory (DFT)‐based computations, a systematic exploration of the newly predicted 2D phosphorene allotrope, namely holey‐phosphorene (HP), is carried out. It is revealed that HP shows a semiconducting nature with an indirect bandgap of 0.83 eV upon Perdew‐Burke‐Ernzerhof (PBE) functional. Then, to survey the optical features, a (G0W0)‐based approach is employed to solve the Bethe–Salpeter equation to derive the intra‐layer excitonic effects. It is derived via the absorption spectrum, that HP presents an excitonic binding strength of 1.47/1.96 eV along the x/y‐direction with the first peak of the absorption at 0.92/0.43 eV for the x/y‐direction. The thermoelectric properties are also explored in detail and reveal a very high thermal power value along with an enhanced figure of merit (ZT) of about 3.6. The 2D HP monolayer for thermoelectric performance has high thermoelectric conversion efficiency (TCE) and is estimated to be about 22. All these outstanding findings may be attributed to the quantum confinement effect of the porous geometry of the 2D HP nanosheet, thereby confirming its relevance as a prospect for high‐performance optoelectronic and thermoelectric engineering systems.

Emerging exotic properties of two-dimensional ternary tetrahexagonal BCN: Tunable anisotropic transport properties with huge excitonic effects for nanoelectronics and optoelectronics

A two-dimensional ternary tetrahexagonal BCN monolayer (th-BCN) is designed by applying Stone-Wales transformation to its pentagonal counterpart (p-BCN). Using first-principles calculations, we examine the dynamical, mechanical, thermal, and environment stability of th-BCN. The monolayer exhibits superior anisotropic electronic and optical properties compared to its counterparts (the domains of C and BN in the tetrahexagonal network and p-BCN). Direct band gap of 3.05 eV and excellent band edge alignment with the oxidation and reduction reactions of water heralds the use of th-BCN in photocatalytic water splitting. The band gap energy and band edge positions can be further adjusted by strain. The anisotropic high carrier mobility of th-BCN improves fast carrier separation and low recombination rate. The electronic transport direction can be easily controlled by strain engineering. For instance, one can rotate the preferred transport direction of hole by 90° with a very small uniaxial strain. Many-body effects beyond DFT reveal that the monolayer has strong light-harvesting capability (105cm−1) in the visible-ultraviolet region and exhibits strong excitonic effects with large excitonic binding energy of 0.98 eV. All these appealing properties make th-BCN a promising and tunable anisotropic 2D material for nanoelectronic and optoelectronic applications.

Effect of self-trapped excitons in the optical properties of manganese-alloyed hexagonal-phased metal halide perovskite

Emission induced by self-trapped excitons (STEs) has recently been studied in several metal halide perovskites (MHPs), particularly in low-dimensional MHPs. STEs generally occur in low-dimensional MHPs owing to their soft lattices, strong electron–phonon coupling, and lattice distortion. However, the formation of STEs in all-inorganic high-dimensional MHPs, specifically in hexagonal phases, is challenging. From this perspective, we studied the formation of STEs in all-inorganic hexagonal-phase CsCdCl3 MHPs and further investigated the role of free excitons and STEs in the optical properties of CsCd(1-x)MnxCl3 MHPs. Mn2+ ion-activated CsCd(1-x)MnxCl3 MHPs were synthesized, and their crystal structures were refined. The CsCd(1-x)MnxCl3 MHPs exhibited orange emission upon 295 nm excitation with a high photoluminescence quantum yield of 74 %. Moreover, CsCd(1-x)MnxCl3 MHPs exhibited good thermal and chemical stability. We also investigated the effect of Mn2+ doping on CsCd(1-x)MnxCl3 single crystals, polycrystals, and nanocrystals. This work offers deep insights into the photophysical properties of MHPs as well as provides a promising approach to achieving high-quality, thermally, and chemically stable white-light emission.

Two-Dimensional Ternary Pentagonal BCN: A Promising Photocatalyst Semiconductor for Water Splitting with Strong Excitonic Effects

Developing sustainable and stable visible-light-driven photocatalysts to convert solar energy by splitting water into H${}_{2}$ and O${}_{2}$ is a great challenge in applied research. The authors' calculations show that two-dimensional BCN is a very promising semiconductor for optoelectronic applications. Its excellent stability, sizable band gap, high carrier mobility, and distinct excitonic peaks are promising for photovoltaics, with its valence and conduction bands ideally straddling the oxidation and reduction potentials of water. Its optical absorption coefficient in the visible and near-ultraviolet regions is comparable to that of the perovskites currently employed in solar cells.