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Semiconducting MXenes are an intriguing two-dimensional (2D) material class with promising electronic and optoelectronic properties. Here, we focused on recently prepared Hf-based MXenes, namely, Hf3C2O2 and Hf2CO2. Using the first-principles calculation and excited state corrections, we proved their dynamical stability, reconciled their semiconducting behavior, and obtained fundamental gaps by using the many-body GW method (indirect 1.1 and 2.2 eV; direct 1.4 and 3.5 eV). Using the Bethe–Salpeter equation, we subsequently provided optical gaps (0.9 and 2.7 eV, respectively), exciton binding energies, absorption spectra, and other properties of excitons in both Hf-based MXenes. The indirect character of both 2D materials further allowed for a significant decrease of excitation energies by considering indirect excitons with exciton momentum along the Γ-M path in the Brillouin zone. The first bright excitons are strongly delocalized in real space while contributed by only a limited number of electron–hole pairs around the M point in the k-space from the valence and conduction band. A diverse range of excitonic states in Hf3C2O2 MXene lead to a 4% and 13% absorptance for the first and second peaks in the infrared region of absorption spectra, respectively. In contrast, a prominent 28% absorptance peak in the visible region appears in Hf2CO2 MXene. Results from radiative lifetime calculations indicate the promising potential of these materials in optoelectric devices requiring sustained and efficient exciton behavior. [ABSTRACT FROM AUTHOR]

We present a detailed theoretical analysis of polarized absorption spectra and linear dichroism of cyanine dye aggregates whose unit cells contain two molecules. The studied threadlike ordered system with a molecular exciton delocalized along its axis can be treated as two chains of conventional molecular aggregates, rotated relative to each other at a certain angle around the aggregate axis. Our approach is based on the general formulas for the effective cross section of light absorption by a molecular aggregate and key points of the molecular exciton theory. We have developed a self-consistent theory for describing the orientational effects in the absorption and dichroic spectra of such supramolecular structures with nonplanar unit cell. It is shown that the spectral behavior of such systems exhibits considerable distinctions from that of conventional cyanine dye aggregates. They consist in the strong dependence of the relative intensities of the J- and H-type spectral bands of the aggregate with a nonplanar unit cell on the angles determining the mutual orientations of the transition dipole moments of constituting molecules and the aggregate axis as well as on the polarization direction of incident light. The derived formulas are reduced to the well-known analytical expressions in the particular case of aggregates with one molecule in the unit cell. The calculations performed within the framework of our excitonic theory combined with available vibronic theory allow us to quite reasonably explain the experimental data for the pseudoisocyanine bromide dye aggregate. [ABSTRACT FROM AUTHOR]

Transient absorption (TA) spectroscopy of semiconductor nanocrystals (NCs) is often used for excited state population analysis, but recent results suggest that TA bleach signals associated with multiexcitons in NCs do not scale linearly with exciton multiplicity. In this manuscript, we probe the factors that determine the intensities and spectral positions of exciton and biexciton components in the TA spectra of CdSe quantum dots (QDs) of five diameters. We find that, in all cases, the peak intensity of the biexciton TA spectrum is less than 1.5 times that of the single exciton TA spectrum, in stark contrast to a commonly made assumption that this ratio is 2. The relative intensities of the biexciton and exciton TA signals at each wavelength are determined by at least two factors: the TA spectral intensity and the spectral offset between the two signals. We do not observe correlations between either of these factors and the particle diameter, but we find that both are strongly impacted by replacing the native organic surface-capping ligands with a hole-trapping ligand. These results suggest that surface trapping plays an important role in determining the absolute intensities of TA features for CdSe QDs and not just their decay kinetics. Our work highlights the role of spectral offsets and the importance of surface trapping in governing absolute TA intensities. It also conclusively demonstrates that the biexciton TA spectra of CdSe QDs at the band gap energy are less than twice as intense as those of the exciton. [ABSTRACT FROM AUTHOR]

The development of lasing materials with weak pile‐up effect from triplet excitons, low lasing threshold, and long operational lifetime is much significant for truly continuous wave operation and electrically pumping organic lasers. However, most of organic gain media has been verified to have extreme quenching effect from triplet excitons due to its wide‐band distribution of triplet absorption spectrum. Here, it is found that the phenanthroimidazole derivatives as gain media have congenital advantages on handling triplet's issues. A very low amplified spontaneous emission (ASE) threshold of 0.56 µJ cm−2 and a long operational lifetime of 5.77 × 105 pump pulses in air atmosphere are well obtained. When increasing the pump repetition frequency from 20 Hz to 20 kHz, the operational lifetimes of the thin films have small reduction. Under a high pump repetition frequency of 200 kHz, which is quasi‐continuous wave operational regime, the PI2PhDPO thin film exhibits very low ASE threshold of 0.78 µJ cm−2 due to the tiny overlap between triplet absorption and ASE spectra. [ABSTRACT FROM AUTHOR]

In the era of intelligence, the output colors of foundational phosphors are expected to be controlled by programs, while current activators‐determined afterglow candidates with fixed spectral channel have limitations in creating customized colors. Here, a long‐lived warm‐white emission is successfully demonstrated originating from self‐trapped excitons (STEs) in non‐toxic Cs2NaInCl6:Ag,Bi, ranging from 400 to 850 nm, of which the afterglow color can be easily customized using filters. This investigation indicates that 3% of Ag alloying breaks the dark STEs and introduces traps for efficient afterglow, while 3% Bi doping further improves the quantum yield to ≈100% and greatly enhances afterglow by ≈100‐fold compared with the initial intensity, allowing for an impressive afterglow persistence of over 20,000 s. Intriguingly, the self‐trapped defect bands from Jahn‐Teller distortions are prolonged from hundreds of femtoseconds to several hours, and they are first detected in the steady‐state absorption spectra after the cessation of excitation sources, contributing to the concluded dynamic afterglow model for STEs. And its intelligent application is corroborated by designed multi‐channel information system. These findings offer a novel scheme for understanding dynamic luminescence of STEs and supply an exemplification of designing white afterglow phosphors. [ABSTRACT FROM AUTHOR]

We report the absorption spectra and photophysical properties of homo and hetero‐aggregate assemblies of a strongly emissive N‐annulated perylene dye (P) and of a dyad made of P and a methyl viologen derivative (P‐MV), in ethanol‐water solutions. In homo‐aggregate assemblies of P, the π‐π* fluorescence of the isolated chromophore is replaced by excimer emission at lower energy, with a lifetime of 900 ps, due to excimer formation from the initially prepared excitons. In homo‐aggregate assemblies of P‐MV, photoinduced charge separation, with formation of P+‐MV− species, occurs in 3 ps with a charge recombination of 20 ps. In hetero‐aggregate P/P‐MV systems, the light energy absorbed by the P components delocalizes over various P subunits, and when a P‐MV unit is reached, charge separation occurs; however, excimer emission is present for P/P‐MV ratio larger than 3 : 1, indicating that delocalized excitons within the hetero‐aggregate systems extend over a limited number of P chromophores. [ABSTRACT FROM AUTHOR]

Electroabsorption (EA) measurements can be used to identify the type of excitons contributing to the absorption spectra of semiconductors. However, the shape of the EA spectrum may vary depending on the mode of measurement due to the optical interference effects. Analysis without considering these effects may lead to erroneous conclusions. In this work, we present EA measurements and analysis for reflection mode measurements considering optical interference effects. We compared the inferences with transmission mode measurements and discuss the limitations. We identified the nature of excitons associated with each transition in the absorption spectrum of poly[(2,5-bis(2-hexyldecyloxy)-phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c]-[1,2,5]-thiadiazole)] thin film from EA measurements. The bands at 1.89, 2.05, and 2.27 eV had a mixed nature consisting of charge transfer and Frenkel characteristics. Of these, the band at 2.05 eV showed the strongest charge transfer characteristic. From thickness dependent measurements, we showed that the interference effects increase with the thickness of the semiconductor layer. The nature of excitons, however, could still be deduced qualitatively from reflection mode EA measurements. [ABSTRACT FROM AUTHOR]

The optoelectronic properties of quantum confined semiconductor nanocrystals depend critically on the band edge electron and hole levels and their exciton fine structures. Transient absorption (TA) spectroscopy has been widely used to probe the dynamics of photogenerated electrons, holes, and excitons in these materials through their state filling induced bleach of the band edge exciton transition. Such effects, in principle, reflect the band edge fine structures and are well understood for the conduction band electrons. However, the valence band hole state filling signals remain poorly understood due to the complexity of the valence band level structure and the presence of fast hole trapping in many materials. Herein, we report a study of the valence band hole state filling effect by comparing the TA spectra of CdSe quantum dots (QDs) with different degrees of hole trapping and by selective removal of the conduction band electrons to adsorbed methyl viologen molecules. We observe that in CdSe/CdS core/shell QDs with a high photoluminescence quantum yield of 81%, the valence band hole contributes to 22% ± 1% of the exciton bleach, while a negligible hole state filling signal is observed in CdSe core only QDs with a photoluminescence quantum yield of 17%. This hole state filling effect can be explained by a simplified valence band edge hole model that contains two sets of twofold degenerate hole levels that are responsible for the higher energy bright exciton and lower energy dark exciton states, respectively. Our result clarifies the TA spectral features of the valence band holes and provides insights into the nature of single hole states in CdSe-based QDs. [ABSTRACT FROM AUTHOR]

We theoretically propose a hybrid nanomechanical resonator (NR) system, where a doubly clamped suspended NR with an embedded quantum dot driven by two-tone fields is coupled to another NR via the Coulomb interaction, and we investigate the absorption spectra of the probe field under different exciton-pump field detuning. In the condition of pump on-resonance, the absorption spectrum gives a method to determine the coupling strength of the two NRs. In the pump off resonance, the absorption spectra exhibit double-Fano resonance, which is determined by the interaction of the two NRs, the frequencies of the two NRs, as well as the pump detuning. Furthermore, the double-Fano resonances are accompanied by the rapid normal phase dispersion, which indicates the slow- and fast-light effect. The results show that the group velocity index is tunable by the interaction of the two NRs, the detuning, and the different resonator frequencies, which can achieve the conversion from fast light to slow light. [ABSTRACT FROM AUTHOR]

Due to the exceptional photophysical properties and great potential applications in solid state lighting, zero-dimensional halide perovskites with broadband emitting have recently attracted much attention. In this work, to reveal the origin of the broadband absorption and emission spectra of 0D (C 4 N 2 H 14 Br) 4 SnBr x I 6 − x (x = 3), the ground state and exciton dynamics properties have been studied by hybrid PBE0 functional and the GW+BSE method. Different self-trapped excitons (STEs) are obtained, and their emission energies by PBE0 are consistent well with experimental data. By considering the vibrational broadening of emission, our calculated full width at half maximum of emission accords well with the experiment. The absorption of free exciton and STEs is also calculated by considering the quasiparticle and excitonic effect. Our results show that these light-induced trapped states can be observed by experiments, as confirmed in other similar systems by the femtosecond transient absorption spectra. [ABSTRACT FROM AUTHOR]

The absorption spectra of thin films (CsCl)1–x(CuCl)x were studied at T = 90 K in the spectral range 2–6 eV. From the analysis of the spectra, the formation of CsCu2Cl3 and Cs2CuCl3 compounds in the CsCl–CuCl system was created. The Cs2CuCl3 compound is stable only in a vacuum; in air, it apparently decomposes into CsCu2Cl3 with a disordered crystal lattice and CsCl. The exciton spectrum of both compounds is interpreted on the basis of transitions in the Cu+ ion. The one-dimensional character of excitons in CsCu2Cl3 and the two-dimensional character in Cs2CuCl3 were established. [ABSTRACT FROM AUTHOR]

We present here a theory of Majorana excitons, photo-excited conduction electron-valence band hole pairs, interacting with Majorana Fermions in a Kitaev chain of semiconductor quantum dots embedded in a nanowire. Using analytical tools and exact diagonalization methods, we identify the presence of Majorana zero modes in the nanowire absorption spectra. [ABSTRACT FROM AUTHOR]

Recently, interlayer and intralayer excitons in transition metal dichalcogenide heterobilayers have been studied both experimentally and theoretically. In spite of a growing interest, these layer-resolved excitons in the presence of external stimuli, such as strain, remain not fully understood. Here, using density-functional theory calculations with many-body effects, we explore the excitonic properties of vertically stacked MoSe2/WSe2 heterobilayer in the presence of in-plane biaxial strain of up to 5%. We calculate the strain dependence of exciton absorption spectrum, oscillator strength, wave function, and binding energy by solving the Bethe–Salpeter equation on top of the standard GW approach. We identify the interlayer and intralayer excitons by analyzing their electron-hole weights and spatial wave functions. We show that with the increase in strain magnitude, the absorption spectrum of the interlayer and intralayer excitons is red-shifted and re-ordered, and the binding energies of these layer-resolved excitons decrease monotonically and almost linearly. We derive the sensitivity of exciton binding energy to the applied strain and find that the intralayer excitons are more sensitive to strain than the interlayer excitons. For instance, a sensitivity of −7.9 meV/% is derived for the intra-MoSe2-layer excitons, which is followed by −7.4 meV/% for the intra-WSe2-layer excitons, and by −4.2 meV/% for the interlayer excitons. Our results indicate that interlayer and intralayer excitons in vertically stacked MoSe2/WSe2 heterobilayer are efficiently tunable by in-plane biaxial strain. [ABSTRACT FROM AUTHOR]

Experimental temperature dependences (77–293 K) of absorption and photoluminescence spectra of CdSe/ZnS semiconductor quantum dots (QDs) were analyzed based on modern models of exciton–phonon coupling. It was proven that the first excitonic transition absorption band formed with participation primarily of optical phonons of the CdSe core while the photoluminescence properties reflected the interaction of the ZnS layer with optical phonons. Photoluminescence quenching for CdSe/ZnS QDs of various diameters upon formation of nanoassemblies with porphyrin molecules was analyzed in the framework of a quantum-mechanical model. It was shown that the quenching rate constant decreased with increasing QD diameter and that the quenching process under quantum confinement conditions was due to the electron of an excited electron–hole pair tunneling onto the QD surface with subsequent localization on surface traps. [ABSTRACT FROM AUTHOR]

Vibrational-electronic (vibronic) resonance and its possible role in energy and charge transfer have been experimentally and theoretically investigated in several photosynthetic proteins. Using a dimer modeled on a typical photosynthetic protein, we contrast the description of such excitons provided by an exact basis set description, as opposed to a basis set with reduced vibrational dimensionality. Using a reduced analytical description of the full Hamiltonian, we show that in the presence of vibrational excitation both on electronically excited as well as unexcited sites, constructive interference between such basis states causes vibronic coupling between excitons to become progressively stronger with increasing quanta of vibrational excitation. This effect leads to three distinguishing features of excitons coupled through a vibronic resonance, which are not captured in basis sets that restrict ground state vibrations: (1) the vibronic resonance criterion itself, (2) vibronically assisted perfect delocalization between sites even though purely electronic mixing between the sites is imperfect due to energetic disorder, and (3) the nuclear distortion accompanying vibronic excitons becoming increasingly larger for resonant vibronic coupling involving higher vibrational quanta. In terms of spectroscopically observable limitations of reduced basis set descriptions of vibronic resonance, several differences are seen in absorption and emission spectra but may be obscured on account of overwhelming line broadening. However, we show that several features such as vibronic exciton delocalization and vibrational distortions associated with electronic excitations, which ultimately dictate the excited state wavepacket motions and relaxation processes, are fundamentally not described by basis sets that restrict ground state vibrations. [ABSTRACT FROM AUTHOR]

Photo-induced relaxation processes leading to excimer formations or other traps are in the focus of many investigations of optoelectronic materials because they severely affect the efficiencies of corresponding devices. Such relaxation effects comprise inter-monomer distortions in which the orientations of the monomer change with respect to each other, whereas intra-monomer distortions are variations in the geometry of single monomers. Such distortions are generally neglected in quantum chemical investigations of organic dye aggregates due to the accompanied high computational costs. In the present study, we investigate their relevance using perylene-bisimide dimers and diindenoperylene tetramers as model systems. Our calculations underline the importance of intra-monomer distortions on the shape of the potential energy surfaces as a function of the coupling between the monomers. The latter is shown to depend strongly on the electronic state under consideration. In particular, it differs between the first and second excited state of the aggregate. Additionally, the magnitude of the geometrical relaxation decreases if the exciton is delocalized over an increasing number of monomers. For the interpretation of the vibronic coupling model, pseudo-Jahn–Teller or Marcus theory can be employed. In the first part of this paper, we establish the accuracy of density functional theory-based approaches for the prediction of vibrationally resolved absorption spectra of organic semiconductors. These investigations underline the accuracy of those approaches although shortcomings become obvious as well. These calculations also indicate the strength of intra-monomer relaxation effects. [ABSTRACT FROM AUTHOR]

The vibrationally resolved absorption spectra of zinc phthalocyanine (ZnPc) aggregates (up to 70 monomers) are explored using the non-Markovian stochastic Schrödinger equation. Various types of local excitations, charge-transfer (CT) excitations, and exciton–phonon couplings are explicitly included in a comprehensive model Hamiltonian, which is parameterized by first-principles calculations. The absorption spectral simulations clarify that the two absorption bands in the Q-band region observed in experiments can be assigned to the contribution from the CT-mediated interactions, rather than the mixtures of different-type aggregates, as prevailingly assumed. Furthermore, the relative intensities of the two bands are found to be closely related to the intermolecular distance and molecular number in a ZnPc aggregate. From the investigation of the decoherence process after optical excitation, it is found that CT states can induce coherence regeneration as the time scale of charge separation is much faster than that of the vibration-induced decoherence. However, they would instead boost the decoherence process as the two time scales become comparable. The two different effects of CT states may suggest a novel way to regulate the decoherence process in excitation energy relaxation. [ABSTRACT FROM AUTHOR]

We study the exciton localization and resulting optical response for disordered tubular aggregates of optically active molecules. It has previously been shown that such tubular structures allow for excitons delocalized over more than a thousand molecules, owing to the combined effects of long-range dipole–dipole interactions and the higher-dimensional (not truly one-dimensional) nature of the aggregate. Such large delocalization sizes prompt the question to what extent in experimental systems the delocalization may still be determined by the aggregate size (diameter and length) and how this affects the aggregate's optical response and dynamics. We perform a systematic study of the size effects on the localization properties using numerical simulations of the exciton states in a cylindrical model structure inspired by the previously derived geometry of a cylindrical aggregate of cyanine dye molecules (C8S3). To characterize the exciton localization, we calculate the participation ratio and the autocorrelation function of the exciton wave function. We also calculate the density of states and absorption spectrum. We find strong effects of the tube's radius on the localization and optical properties in the range of parameters relevant to the experiment. In addition, surprisingly, we find that even for tubes as long as 750 nm, the localization size is limited by the tube's length for disorder values that are relevant to experimental circumstances, while observable effects of the tube's length in the absorption spectrum still occur for tube lengths up to about 150 nm. The latter may explain the changes in the optical spectra observed during the aging process of bromine-substituted C8S3 aggregates. For weak disorder, the exciton wave functions exhibit a scattered, fractal-like nature, similar to the quasi-particles in two-dimensional disordered systems. [ABSTRACT FROM AUTHOR]

The complex interplay between molecules and plasmonic metal nanoparticles (MNPs) presents a set of particular characteristics in absorption/scattering spectra such as excitonic splitting, asymmetric line shapes, plasmon-induced absorption enhancement and transparencies, etc. Although the MNP-molecule systems have been intensively investigated experimentally and theoretically, the construction of a theoretical framework which can produce all the disparate experimental observations and account for the electron-phonon (e-p) coupling is still in progress. Here, we present a theoretical approach which can account for both the plasmon-exciton coupling and the e-p interaction and produce all the spectral line shapes ranging from Fano antiresonance to Rabi splitting by simply tuning the coupling strength or plasmon damping rate. Additionally, we demonstrate the evolution of vibronic spectra and exciton dynamics with the coupling strength, plasmon damping rate, and detuning energy. It is found that the vibronic structures appearing in Rabi-like spectra are worse resolved, wider, and more largely shifted than those appearing in the Fano regime, attributed to the more significant deformation of the molecular vibrational wavepacket in the Rabi-like regime than that in the Fano regime as the molecular e-p interaction increases. The positive/negative value of detuning energy can induce different degrees of the vibrational wavepacket deformation and subsequently a different effect on the spectra in different coupling regimes. [ABSTRACT FROM AUTHOR]

A methodology is described to extract the absorption coefficient spectrum and exciton oscillator strength of GaN layers and GaN/AlGaN quantum wells by analyzing microtransmittance experiments in high-quality, free-standing membranes with thicknesses in the 160–230 nm range. The absorbance of a subwavelength GaN membrane is found to be an oscillating function of its thickness, in keeping with the standing wave effect. We analyze our results using two alternative models including interference effects and extract identical absorption coefficient values. The room-temperature absorption coefficient of bulk GaN membranes at the main exciton peak is found to be 9 × 104 cm−1. In the case of GaN/AlGaN quantum wells, the enhancement and blue shift of the excitonic absorption are observed, as a result of quantum confinement. [ABSTRACT FROM AUTHOR]

The "hot exciton" mechanism based on high-lying reverse intersystem crossing process has great advantages in achieving high-performance deep-blue organic light-emitting diodes (OLEDs). Nevertheless, how to solve the loss of high-lying excitons to improve device performance further is a pressing and challenging issue. To break through this shackle, a novel deep-blue emitter based on "exciton recovery" strategy is successfully design, namely CAT. By combining the transient absorption spectrum, theoretical calculation, magneto-electroluminescence, and transient-electroluminescence measurements, the multi-channeled pathways of excitons utilization via hot exciton and triplet-triplet annihilation processes is comprehensively demonstrated, and the proportion of singlet excitons by each channel. Finally, a high exciton utilization efficiency is successfully achieved, and the non-doped OLED based on CAT exhibit an excellent external quantum efficiency of 10.39% with the CIE coordinates of (0.15, 0.087). Furthermore, the sensitized blue fluorescent OLED by CAT as host also achieves excellent performance. More importantly, the operational lifetime of the "multi-channel" sensitized device is evaluated for the first time, performing the remarkable LT50 (lifetime to 50% of the initial luminance) of 320 h at 540 cd m-2. These results fully reveal the significance of the "exciton recovery" strategy and give new inspiration for exploring highperformance blue OLEDs. [ABSTRACT FROM AUTHOR]

The theoretical investigation of excited state for large photoactive systems plays the fundamental role in understanding various optical processes in material and biological system. Frenkel exciton (FE) model describing the excitation of the whole system as a collective effect of quasi‐particles of excitons, that is, bound electron–hole pairs, is well‐known as a simple but powerful theoretical scheme to present a clear and insightful physical picture for complicated excited state problems. In this mini‐review, we summarize our recent developments of quantum chemical methods based on exciton models and their related applications for large photoactive systems. It is shown that our developed ab initio renormalized exciton model (REM) and block interaction product state (BIPS) schemes provide new efficient and automatic low‐scaling excited state methods for both localized and delocalized excited states in large systems. Illustrative examples including simulations of both absorption and emission spectrum in large sized molecular aggregates, indicate the exciton model based methods provide promising computational tools for unravel the mechanism of photophysical and photochemical processes in large photoactive systems. [ABSTRACT FROM AUTHOR]

[Display omitted] • The lattice modification of Cs 2 NaInCl 6 was conducted by Ag and Zr dopants. • The emissive color temperature was tailored from warm- to cool-white with Zr content. • Double self-trapped excitons (STEs)-based pure-white afterglow was realized, which could last over 5 h. • 40 % of Zr dopants enhanced the initial afterglow intensity for ∼ 100-fold, compared with Cs 2 Na 0.9 Ag 0.1 InCl 6. • The bands of STEs were observed in steady-state absorption spectra thanks to energy storage. The incommensurable differences between the feeding and actual doping ratios of luminescent dopants and excessive reliance on rare-earth ions become two key problems in halide perovskite-based afterglow materials, restricting the production cost and further application. Here, we prepare a series of novel lead- and rare-earth-free halide afterglow phosphors Cs 2 (Na 0.9 Ag 0.1 In) 1- x Zr x Cl 6 under an ambient condition, in which the Zr dopants modify the lattice thus tailoring the emission spectrum and trap distribution, simultaneously. The focused Cs 2 (Na 0.9 Ag 0.1 In) 0.6 Zr 0.4 Cl 6 with mainly size distribution of ∼ 0.6–1.8 μm exhibits ∼ 350–850 nm ultrabroad white luminescence under ultra-violet excitation, and the introduced traps endow the white emission with a long-persistence of over 5 h after pre-irradiation. The absorption bands of self-trapped excitons (STEs) are observed in the steady-state absorption spectra during the afterglow process, suggesting the general accepted picosecond-scaled lifetimes of STEs have been successfully prolonged to hour-scale, which provide a potential opportunity for investigating the specific behavior of STEs. [ABSTRACT FROM AUTHOR]

Mixed-cation lead mixed-halide perovskites simultaneously possess structural stability and high power conversion efficiency. A thorough study of both carrier and exciton dynamics is needed to understand the photophysical properties that underpin its superior photovoltaic performance. By utilizing a broadband transient absorption spectroscopy, we observe the carrier and exciton dynamics in a FA0.85Cs0.15Pb(I0.97Br0.03)3 (FCPIB) perovskite by simultaneously resolving the carrier and exciton contribution to the transient change of the absorption spectra, from which the carrier density and exciton oscillator strength can be determined. Our data reveal a quick and significant conversion of the photogenerated carriers to excitons, on top of the usual carrier recombination process. Moreover, the decay of carrier density shows a change of kinetics from a second-order recombination at high pump fluence to a third-order recombination at low pump fluence. Our analysis utilizes band anharmonicity, presents an independent determination of electronic temperature and quasi-Fermi energy, and reveals an interesting interplay among the processes of carrier cooling, exciton formation/decay and carrier recombination, all as a function of time after photoexcitation. Our work demonstrates the use of pump fluence as a knob to tune the relative populations of carriers and excitons in halide perovskite materials. Lead halide perovskites have many suitable features as materials for solar cells and a better understanding of the underlying physics will help engineer improved performance values. Here, the authors use broadband transient absorption spectroscopy to probe the excited state dynamics of mixed-halide perovskites and the role of excitons and carriers. [ABSTRACT FROM AUTHOR]

Fluorescence properties of quantum dots (QDs) are critically affected by their redox states, which is important for practical applications. In this study, we investigated the optical properties of MoSe2-metallic phase quantum-dots (MoSe2-mQDs) depending on the pH variation, in which the MoSe2-mQDs were dispersed in water with two sizes (Φ~3 nm and 12 nm). The larger MoSe2-mQDs exhibited a large red-shift and broadening of photoluminescence (PL) peak with a constant UV absorption spectra as varying the pH, while the smaller ones showed a small red-shift and peak broadening, but discrete absorption bands in the acidic solution. The excitation wavelength-dependent photoluminescence shows that the PL properties of smaller MoSe2-mQDs are more sensitive to the pH change compared to those of larger ones. From the time-resolved PL spectroscopy, the excitons dominantly decaying with an energy of ~3 eV in pH 2 clearly show the shift of PL peak to the lower energy (~2.6 eV) as the pH increases to 7 and 11 in the smaller MoSe2-mQDs. On the other hand, in the larger MoSe2-mQDs, the exciton decay is less sensitive to the redox states compared to those of the smaller ones. This result shows that the pH variation is more critical to the change of photophysical properties than the size effect in MoSe2-mQDs. [ABSTRACT FROM AUTHOR]

Stacking atomically thin films enables artificial construction of van der Waals heterostructures with exotic functionalities such as superconductivity, the quantum Hall effect, and engineered light-matter interactions. In particular, heterobilayers composed of monolayer transition metal dichalcogenides have attracted significant interest due to their controllable interlayer coupling and trapped valley excitons in moiré superlattices. However, the identification of twist-angle-modulated optical transitions in heterobilayers is sometimes controversial since both momentum-direct (K–K) and -indirect excitons reside on the low energy side of the bright exciton in the monolayer constituents. Here, we attribute the optical transition at ∼1.35 eV in the WS2/WSe2 heterobilayer to an indirect Γ–K transition based on a systematic analysis and comparison of experimental photoluminescence spectra with theoretical calculations. The exciton wavefunction obtained by the state-of-the-art GW-Bethe-Salpeter equation approach indicates that both the electron and hole of the excitons are contributed by the WS2 layer. Polarization-resolved k -space imaging further confirms that the transition dipole moment of this optical transition is dominantly in-plane and is independent of the twist angle. The calculated absorption spectrum predicts that the so-called interlayer exciton peak coming from the K–K transition is located at 1.06 eV, but with a much weaker amplitude. Our work provides new insight into the steady-state and dynamic properties of twist-angle-dependent excitons in van der Waals heterostructures. [ABSTRACT FROM AUTHOR]

The photo-Hall-effect was applied to the study of electron dynamics in semi-insulating Fe-doped GaN. High-powered light-emitting diodes of wavelengths k = 940, 536, 449, 402, and 365 nm were used to excite steady-state free-electron volume concentrations Dn = 105 - 108cm-3, depending on k and intensity I0. Electron lifetime s was determined from the energy E dependence of the excited sheet electron concentration Dns through the relationship Dns = I0sA(E), where the absorbance A(E) is a known function of sample thickness d and absorption coefficient a, and the energy dependence of a is taken from a theory of deep-center photoionization. The major sample impurities were Fe, Si, and C, with [Fe] ⪢ [Si] and [C]. Fitted lifetimes s ranged from 15 to 170 ps, depending on [Fe]. It was found that Dns ∞ I0 for [Si]>[C] and ∞ I01/2 for [Si] < [C]; the latter dependence arises possibly from self-compensation of neutral C impurities by N-vacancy donors. For [Si] > [C], some of the neutral Fe3+ is converted to Fe2+ with ground state Fe2+(5E) and excited state Fe2+(5T2); a fit of n vs. temperature T over the range of 290-325K in the dark establishes E5E with respect to the conduction band: ECB - E5E = 0.564 eV-b5ET, where b5E = 3.6 × 10-4 eV/K. At room temperature, 294K, ECB - E5E = 0.46 eV and ECB - E5T2 = 0.07 eV. [ABSTRACT FROM AUTHOR]

In the present work, we accurately calculate the absorption spectrum of liquid ammonia up to 13 eV using many-body perturbation approach. The electronic bandgap of liquid NH3 is perfectly described as the combination of density functional theory, Coulomb-hole screened exchange, and G0W0 approximation to the electronic self-energy, yielding a direct gap (Γ→ Γ) of 7.71 eV, fully consistent with the experimentally measured gap from photo-emission spectroscopy. With respect to the NH3 optical properties, the entire spectrum in particular the low lying first absorption band is extremely affected by electron-hole interactions, leading to a fundamental redistribution of spectral weights of the independent-particle spectrum. Three well separated but broad main peaks are identified at 7.0, 9.8, and 11.8 eV with steadily increasing intensities in excellent agreement with the experimental data. Furthermore, we observe a giant net blue-shift of the first absorption peak of about 1.4 eV from gaseous to liquid phase as the direct consequence of many-body effects, allowing the associated liquid ammonia absorption band exciton to delocalize and feel more effectively the repulsion effects imposed by the surrounding solvent shells. Further, the spectrum is insensitive to the coupling of resonant and anti-resonant contributions. Concerning electronic response structure of liquid NH3, it is most sensitive to excitations at energies lower than its electronic gap. [ABSTRACT FROM AUTHOR]

In the present paper, accurate optical absorption spectrum of liquid H2O is calculated in the energy range of 5-20 eV to probe the nature of water's excited states by means of many body perturbation approach. Main features of recent inelastic X-ray measurements are well reproduced, such as a bound excitonic peak at 7.9 eV with a shoulder at 9.4 eV as well as the absorption maximum at 13.9 eV, followed by a broad shoulder at 18.4 eV. The spectrum is dominated by excitonic effects impacting the structures of the spectrum at low and higher energy regimes mixed by single particle effects at high energies. The exciton distribution of the low-energy states, in particular of S1, is highly anisotropic and localized mostly on one water molecule. The S1 state is essentially a HOCO-LUCO (highest occupied crystal orbital - lowest unoccupied crystal orbital) transition and of intra-molecular type, showing a localized valence character. Once the excitation energy is increased, a significant change in the character of the electronically excited states occurs, characterized through emergence of multiple quasi-particle peaks at 7.9 eV in the quasi-particle (QP) transition profile and in the occurring delocalized exciton density distribution, spread over many more water molecules. The exciton delocalization following a change of the character of excited states at around 7.9 eV causes the blue shift of the first absorption band with respect to water monomer S1. However, due to reduction of the electronic band gap from gas to liquid phase, following enhanced screening upon condensation, the localized S1 state of liquid water is red-shifted with respect to S1 state of water monomer. For higher excitations, near vertical ionization energy (11 eV), quasi-free electrons emerge, in agreement with the conduction band electron picture. Furthermore, the occurring red and blue shift of the excited states are independent of the coupling of resonant and anti-resonant contributions to the spectrum. [ABSTRACT FROM AUTHOR]

Excitonic effects in 1D semiconductors can be qualitatively different from those in higher dimensions. In particular, the Sommerfeld factor, the ratio of the above-band-edge excitonic continuum absorption to free electron-hole pair generation, has been shown to be less than 1 (suppressed) in 1D systems while it is larger than 1 (enhanced) in 2D and 3D systems. Strong continuum suppression indeed exists in semiconducting single-wall carbon nanotubes, a prototypical 1D semiconductor. However, absorption spectra for carbon nanotubes are typically fit with a combination of Lorentzians and a polynomial background baseline with little physical meaning. Here, we performed absorption measurements in aligned single-chirality (6,5) carbon nanotube films. The obtained spectra were fit with our theoretical model obtained by solving the Boltzmann scattering equation involving fifty-nine different types of transitions among three different types of quasiparticles. Specifically, we took into account microscopic interactions between photons, phonons, and excitons, including their dispersions, which unambiguously demonstrated that the background absorption is due to phonon-assisted transitions from the semiconductor vacuum to finite-momentum continuum states of excitons. The excellent agreement we obtained between experiment and theory suggests that our numerical technique can be seamlessly extended to compute strongly out-of-equilibrium many-body dynamics and time-resolved spectra in low-dimensional materials. [Display omitted] [ABSTRACT FROM AUTHOR]

The temperature-driven plasmon-exciton coupling in thermoresponsive dextran-graft-PNIPAM/Au nanoparticle/CdTe quantum dot (D-g-PNIPAM/Au NPs/CdTe QDs) hybrid nanosystem was studied. A significant (0.84 eV) splitting of the absorption peak was observed in the absorption spectrum of the nanosystem, which reflects the fact of formation of plexcitons, occurring due to strong plasmon-exciton coupling. An increasing with time plasmonic enhancement of the photoluminescence of CdTe QDs was revealed, as a result of the penetration of quantum dots into the volume of the D-g-PNIPAM/Au NP hybrid nanosystem and bonding to it. The heating–cooling cycle of the aqueous solution of the studied nanosystem leads to a reversible quenching-recovery alteration of the QD photoluminescence. The quenching was rationalized as a result of an increased probability of nonradiative resonance energy transfer (RET) from CdTe QDs to Au NPs, which occurs due to shortening of the NP-QD distance, caused by shrinking of the macromolecule due to cooling-induced lower critical solution temperature phase transition. Increasing the NP-QD distance in the heating stage recovers the QD PL intensity. The observed effect opens up opportunities for the controlled reversible temperature-driven tuning of the photoluminescence intensity of D-g-PNIPAM/Au NP/CdTe QD nanosystem, which is highly important for its potential use in photonics and biomedical applications. [ABSTRACT FROM AUTHOR]

Low‐dimensional metal halide perovskites have been widely regarded due to their unique crystal structure and photophysical properties. Herein, blue‐emission CsPbBr3 quantum dots (QDs) or nanocrystals (NCs) with extremely small crystal size (1–2 nm) but high photoluminescence quantum yield (PLQY, 72.4%) are synthesized with a new surface ligand, single (6‐amino‐6‐deoxy) beta cyclodextrin (6A‐βCD), which also acts as the confined‐growth template for such small QDs. Detailed photoluminescence spectrum and femtosecond transient absorption spectrum study proves that the blue emission with multiple peaks originates from both band‐edge radiation and exciton recombination. Furthermore, as a result of the small size, exciton localization in the form of self‐trapped excitons (STEs) and significant quantum size confinement effect are both important reasons for the efficient and wide blue emissions of these QDs. This work provides a new strategy not only to synthesize ultrasmall perovskite QDs but also to develop blue perovskite‐QD light‐emitting devices. [ABSTRACT FROM AUTHOR]

A protecting group strategy was employed to synthesise a series of indolenine squaraine dye oligomers up to the nonamer. The longer oligomers show a distinct solvent dependence of the absorption spectra, that is, either a strong blue shift or a strong red shift of the lowest energy bands in the near infrared spectral region. This behaviour is explained by exciton coupling theory as being due to H‐ or J‐type coupling of transition moments. The H‐type coupling is a consequence of a helix folding in solvents with a small Hansen dispersity index. DOSY NMR, small angle neutron scattering (SANS), quantum chemical and force field calculations agree upon a helix structure with an unusually large pitch and open voids that are filled with solvent molecules, thereby forming a kind of clathrate. The thermodynamic parameters of the folding process were determined by temperature dependent optical absorption spectra. [ABSTRACT FROM AUTHOR]

The absorption spectra of RbCu2Cl3 and Rb2Cu3Cl5 thin films were studied in the 2−6 eV spectral and 90−450 K temperature ranges. The localization of excitons in CuCl 4 3 − structural elements of crystal lattice of compounds, three-dimensional nature of excitons in RbCu2Cl3 and two-dimensional nature of excitons in Rb2Cu3Cl5 were established. The exciton spectra of RbCu2Cl3 and Rb2Cu3Cl5 thin films were interpreted based on the transitions in the Cu+ ion. [ABSTRACT FROM AUTHOR]

We have studied light matter interaction in quantum dot and exciton polaritonic coupler hybrid systems. The coupler is made by embedding two slabs of an excitonic material (CdS) into a host excitonic material (ZnO). An ensemble of non-interacting quantum dots is doped in the coupler. The bound exciton polariton states are calculated in the coupler using the transfer matrix method in the presence of the coupling between the external light (photons) and excitons. These bound exciton-polaritons interact with the excitons present in the quantum dots and the coupler is acting as a reservoir. The Schrödinger equation method has been used to calculate the absorption coefficient in quantum dots. It is found that when the distance between two slabs (CdS) is greater than decay length of evanescent waves the absorption spectrum has two peaks and one minimum. The minimum corresponds to a transparent state in the system. However, when the distance between the slabs is smaller than the decay length of evanescent waves, the absorption spectra has three peaks and two transparent states. In other words, one transparent state can be switched to two transparent states when the distance between the two layers is modified. This could be achieved by applying stress and strain fields. It is alsofound that transparent states can be switched on and off by applying an external control laser field. [ABSTRACT FROM AUTHOR]

We conduct polarization-dependent ultrafast spectroscopy to study the dynamics of singlet fission (SF) in tetracene single crystals. The spectrotemporal species for singlet and triplet excitons in transient absorption spectra are found to be strongly dependent on probe polarization. By carefully analyzing the polarization dependence, the signals contributed by different transitions related to singlet excitons have been disentangled, which is further applied to construct the correlation between dynamics of singlet and triplet excitons. The anisotropy of exciton dynamics provides an alternative approach to tackle the long-standing challenge in understanding the mechanism of singlet fission in organic semiconductors. [ABSTRACT FROM AUTHOR]

We extend our previous work on singlet exciton fission in isolated dimers to the case of crystalline materials, focusing on pentacene as a canonical and concrete example. We discuss the proper interpretation of the character of low-lying excited states of relevance to singlet fission. In particular, we consider a variety of metrics for measuring charge-transfer character, conclusively demonstrating significant charge-transfer character in the low-lying excited states. The impact of this electronic structure on the subsequent singlet fission dynamics is assessed by performing real-time master-equation calculations involving hundreds of quantum states. We make direct comparisons with experimental absorption spectra and singlet fission rates, finding good quantitative agreement in both cases, and we discuss the mechanistic distinctions that exist between small isolated aggregates and bulk systems. [ABSTRACT FROM AUTHOR]

We examine the excitonic properties of layered configurations of low dimensional transition metal dichalcogenides (LTMDCs) using the fractional dimensional space approach. The binding energies of the exciton, trion, and biexciton in LTMDCs of varying layers are analyzed, and linked to the dimensionality parameter α, which provides insight into critical electro-optical properties (relative oscillator strength, absorption spectrum, exciton-exciton interaction) of the material systems. The usefulness of α is highlighted by its independence of the physical mechanisms underlying the confinement effects of geometrical structures. Our estimates of the binding energies of exciton complexes for the monolayer configuration of transition metal dichalcogenides suggest a non-collinear structure for the trion and a positronium-molecule-like square structure for the biexciton. [ABSTRACT FROM AUTHOR]

This study looks at the three-level optical Stark effect of excitons in GaAs cylindrical quantum wires, utilizing the renormalized wave function theory. By applying the three-level model consisting of the first two electron levels connected via a powerful pump laser and the first hole level, we observe the appearance of the excitonic optical Stark effect through the appearance of two separated peaks in the exciton absorption spectra. In addition, the strong impact of the pump laser detuning and the wire radius on the optical Stark effect are also put under thorough examination. Finally, a brief guidance for experimental verification is also suggested. [ABSTRACT FROM AUTHOR]

In this paper, we study the exciton absorption spectra in InGaAs prolate ellipsoidal quantum dots when a strong pump laser resonant with electron quantized levels is active. Our obtained results by renormalized wavefunction theory show that, under suitable conditions, the initial exciton absorption peak is split into two new peaks as the evidence of the existence of the three-level optical Stark effect of excitons. We have suggested an explanation of the origin of the effect as well as investigating the effect of pump field energy, size, and geometric shape of the quantum dots on effect characteristics. The comparison with the results obtained in the spherical quantum dots implies the important role of geometric shape of the quantum structures when we examine this effect. [ABSTRACT FROM AUTHOR]

Cesium lead halide (CsPbX3, X = Cl, Br, and I) perovskite nanocrystals (NCs) exhibit efficient anti‐Stokes photoluminescence (ASPL) upon sub‐gap single‐photon excitation, which has great potential for all‐solid‐state laser cooling. However, the mechanism responsible for the ASPL still remains elusive. Here, ASPL properties of CsPb(BrI)3 NCs and CsPbI3 NCs in inorganic solid matrix are investigated. Excitation wavelength‐ and power‐dependent ASPL demonstrates that it is a phonon‐assisted single‐photon process. Femtosecond transient absorption spectra show that ASPL is mediated by the Urbach tail states, where localized excitons are generated upon sub‐gap excitation. Through electron–phonon coupling, these localized excitons change into free exciton to generate the exciton bleaching, and then thermally dissociate into free carriers accompanied by band edge bleaching, leading to the efficient ASPL. Such a two‐step up‐conversion process is further confirmed by the carrier temperature extracted from the band edge bleaching band. This work deepens the understanding of ASPL in CsPbX3 NCs, and is valuable for the development of materials for solid‐state laser cooling. [ABSTRACT FROM AUTHOR]

This study looks at the three-level optical Stark effect of excitons in GaAs cylindrical quantum wires, utilizing the renormalized wave function theory. By applying the three-level model consisting of the first two electron levels connected via a powerful pump laser and the first hole level, we observe the appearance of the excitonic optical Stark effect through the appearance of two separated peaks in the exciton absorption spectra. In addition, the strong impact of the pump laser detuning and the wire radius on the optical Stark effect are also put under thorough examination. Finally, a brief guidance for experimental verification is also suggested. [ABSTRACT FROM AUTHOR]

The influence of two-photon processes on absorption in a system of excitons and biexcitons in the case of two-pulse interaction is studied. It is shown that taking into account two-photon processes leads to the possibility of observing the asymmetric Autler–Townes effect. At certain values of incident fields, the Autler–Townes effect can vanish, which is associated with the quantum interference of all interaction mechanisms. The possibility of controlling the absorption spectrum by varying the field intensities and resonance detunings of the incident radiation is predicted. [ABSTRACT FROM AUTHOR]

Molecular excitons play a central role in natural and artificial light harvesting, organic electronics, and nanoscale computing. The structure and dynamics of molecular excitons, critical to each application, are sensitively governed by molecular packing. Deoxyribonucleic acid (DNA) templating is a powerful approach that enables controlled aggregation via sub-nanometer positioning of molecular dyes. However, finer sub-Angstrom control of dye packing is needed to tailor excitonic properties for specific applications. Here, we show that adding rotaxane rings to squaraine dyes templated with DNA promotes an elusive oblique packing arrangement with highly desirable optical properties. Specifically, dimers of these squaraine:rotaxanes exhibit an absorption spectrum with near-equal intensity excitonically split absorption bands. Theoretical analysis indicates that the transitions are mostly electronic in nature and only have similar intensities over a narrow range of packing angles. Compared with squaraine dimers, squaraine:rotaxane dimers also exhibit extended excited-state lifetimes and less structural heterogeneity. The approach proposed here may be generally useful for optimizing excitonic materials for a variety of applications ranging from solar energy conversion to quantum information science. DNA templating is a useful strategy to control the positioning and aggregation of molecular dyes on a sub-nanometer scale, but sub-angstrom control is desirable for the precise tailoring of excitonic properties. Here, the authors show that templating squaraine dyes functionalized with rotaxane rings promotes an elusive oblique packing arrangement and extended excited-state lifetimes. [ABSTRACT FROM AUTHOR]

Ce+3 doped CsLaSiS 4 single crystals were synthesized using high-temperature flux synthesis. XRD analysis showed that the products are isostructural to CsLaSiS 4 and crystallize in orthorhombic space group Pnma. Absorption spectra were studied at room temperature, and photoluminescence properties were examined in the temperature range of 5–310 K. The energy of interband transitions E g = 3.75 eV was determined at room temperature in the Tauc model. At room temperature, only a non-elementary d → f emission band of Ce+3 ions was observed in the 520 nm region. The luminescence kinetics upon excitation by a pulsed cathode beam or X-ray synchrotron radiation exhibited a dominant nanosecond component. Decay time, as well as build-up time, decreased with increasing concentration of Ce+3 ions. Concentration quenching of Ce+3 emission was not observed up to 11.9 mol% of Ce+3 ions. At a low temperature of 5 K, new wide emission bands at 422 and 688 nm appeared in the photoluminescence spectrum. It is shown that the 422 nm band in the photoluminescence spectrum corresponds to host emission, specifically the luminescence of self-trapped excitons (STE). The 688 nm emission band corresponds to defect-related luminescence. STE emission is quenched according to the Mott law at temperatures above 26 K with an activation energy of 20 meV. An efficient energy transfer channel from the STE to the Ce+3 ion via the radiative resonance mechanism is observed. The emission of Ce+3 ions and defect-related luminescence can be excited by the intracenter way, due to electron-hole recombination, or by the creation of defect bound excitons. Based on the obtained spectroscopic data, a band scheme illustrating the processes of relaxation of electronic excitations at T = 5 K in Ce+3 doped CsLaSiS 4 crystals is proposed. [Display omitted] • Ce+3 doped CsLaSiS 4 single crystals were obtained through high-temperature flux synthesis. • Inter configurational d-f transitions in Ce3+ ions were studied upon both UV- and X-ray excitation. • Luminescence kinetic studies were conducted using a pulsed cathode beam and X-ray synchrotron radiation. • Self-trapped excitons, defect-bound excitons, and impurity luminescence were observed at T = 5 K. [ABSTRACT FROM AUTHOR]

The dynamics of the transient absorption spectra of H*- and J-aggregates of an indotricarbocyanine dye was studied using femtosecond pump-probe spectroscopy. Relaxation of the induced transmission of the H*-aggregates occurs with time constants of ~3 and ~30 ps with radiation pumping at λmax = 400 nm and ~30 ps with pumping at λmax = 800 nm. For J aggregates biexponential relaxation with time constants of ~1 and ~20 ps is observed in both cases. The fast component of the kinetics of transmission of the band of the H* aggregates with pumping at λmax =400 nm is due to the possibility of direct transition from the upper edge of the exciton band of the aggregate to the ground state, while the slow component is present as a result of concurrent fast transition from the upper edge of the exciton band to the lower edge and subsequent relaxation from this to the ground state. The fast component of the kinetics of transmission of the band of the J aggregates is determined by the possibility of transition from the unthermalized exciton state to the ground state, while the slow component is due to transition to the ground state after relaxation within the limits of the exciton band. The greater length of delocalization of the excitons in the J aggregates (nine molecules) compared with the H* aggregates (four molecules) appears not only as decrease of the half-width of the stationary absorption band but also as more rapid quenching of the excited states. It was shown that the H* aggregates have a narrow stationary absorption band at 514 nm and weak absorption with a maximum at 756 nm. With pumping at λmax = 800 nm there is a bathochromic shift of the maxima of the transmission band of the system as a result of the appearance of nonuniform broadening. [ABSTRACT FROM AUTHOR]

The optical properties of an isolated firefly luciferin anion are investigated by using first-principles calculations, employing the many-body perturbation theory to take into account the excitonic effect. The calculated photoabsorption spectra are compared with the results obtained using the timedependent density functional theory (TDDFT) employing the localized atomic orbital (AO) basis sets and a recent experiment in vacuum. The present method well reproduces the line shape at the photon energy corresponding to the Rydberg and resonance excitations but overestimates the peak positions by about 0.5 eV. However, the TDDFT-calculated positions of some peaks are closer to those of the experiment. We also investigate the basis set dependency in describing the free electron states above vacuum level and the excitons involving the transitions to the free electron states and conclude that AO-only basis sets are inaccurate for free electron states and the use of a plane wave basis set is required [ABSTRACT FROM AUTHOR]

We explore the interaction between a quantum emitter and a metal nanoring by numerical solution of coupled Maxwell-Liouville equations. When the quantum emitter and nanoring are subjected to an incident plane wave, coupling between the quantum emitter and a dark plasmon supported by the nanoring gives rise to a similar lineshape to the familiar Fano type. It results from the excitation of a dark plasmon via intermediary participation of the quantum emitter. The dark plasmon is characterized through the width and shift parameters of the emitter peak in the absorption spectrum of the nanoparticle. Our results are obtained with the help of finite-difference time-domain method and a recently proposed symmetry-adapted averaging approach. [ABSTRACT FROM AUTHOR]

This paper reports on the defect correlated self-quenching and spectroscopic investigation of calcium titanate (CaTiO3) phosphors. A series of CaTiO3 phosphors doped with trivalent europium (Eu3+) and codoped with potassium (K+) ions were prepared by the solid state reaction method. The X-ray diffraction results revealed that the obtained powder phosphors consisted out of a single-phase orthorhombic structure and it also indicated that the incorporation of the dopants/co-dopants did not affect the crystal structure. The scanning electron microscopy images revealed the irregular morphology of the prepared phosphors consisting out of lm sized diameter particles. The Eu3+ doped phosphors illuminated with ultraviolet light showed the characteristic red luminescence corresponding to the 5D0→7Fj transitions of Eu3+. As a charge compensator, K+ ions were incorporated into the CaTiO3:Eu3+ phosphors, which enhanced the photoluminescence (PL) intensities depending on the doping concentration of K+. The concentration quenching of Eu3+ in this host is discussed in the light of ion-ion interaction, electron phonon coupling, and defect to ion energy transfer. The spectral characteristics and the Eu-O ligand behaviour were determined using the Judd-Ofelt theory from the PL spectra instead of the absorption spectra. The CIE (International Commission on Illumination) parameters were calculated using spectral energy distribution functions and McCamy's empirical formula. Photometric characterization indicated the suitability of K+ compensated the CaTiO3:Eu3+ phosphor for pure red emission in light-emitting diode applications. [ABSTRACT FROM AUTHOR]

We report the detailed balance analysis of the photovoltaic conversion efficiency of nanocrystal solar cells with discrete excitonic absorption profiles. We calculated the conversion efficiency limits of single junction, multiexciton, and tandem solar cells as a function of exciton peak energy and width. We delineate the width of the absorption spectrum in terms of the energetic width of an isolated nanocrystal and the physical width due to the nanocrystal diameter distribution in the ensemble. Our results suggest that the conversion efficiency is less strongly impacted by the ensemble distribution than by the energetic peak width. [ABSTRACT FROM AUTHOR]