Biexciton Binding Energy Research Articles

Exciton dynamics in CsPbBr3 single crystal: LT splitting energy, exciton-polariton dispersion, and biexciton binding energy.

Metal halide perovskite materials (MHPs) are promising for several applications due to their exceptional properties. Understanding excitonic properties is essential for exploiting these materials. For this purpose, we focus on CsPbBr3 single crystals, which have higher crystal quality, are more stable, and have no Rashba effect at low temperatures compared to other 3D MHPs. We have estimated exciton energy positions, longitudinal-transverse splitting energy, and damping energy using low-temperature reflection spectra. Under high excitation intensity, two biexciton emissions (M-emission) and exciton-exciton scattering emission (P-emission) were observed. We assign the two M-emissions to the emission to the states of longitudinal and transverse excitons, i.e., ML and MT emissions. From the energy position of the MT emission, the biexciton binding energy has been estimated to be ∼2meV. By analyzing P-emission obtained from the back side of the sample, we have estimated the exciton binding energy to be 17.8-23.7meV. This estimation minimizes the influence of the wavenumber distribution in the scattering process. In addition, time-resolved transmittance measurements using pulsed white light have revealed the group velocity dispersion. Comparing experimental results with theoretical calculations using the Lorentz model clarifies that exciton dynamics in CsPbBr3 can be described with a simple Lorentz model. These insights enhance the understanding of exciton behavior and support the development of exciton-based devices using MHPs.

Anomalous Behavior in Dark-Bright Splitting Impacts the Biexciton Binding Energy in (BA)2(MA)n-1PbnBr3n+1 (n = 1-3).

Two-dimensional Ruddlesden-Popper series are an excellent system for tuning physical properties of the perovskite by controlling the layer number (n). For instance, bandgap and exciton binding energies of the series gradually increase upon reducing n via enhanced quantum and dielectric confinements. Here, we present findings that challenge the anticipated trend in electron-hole exchange interaction within (BA)2MAn-1PbnBr3n+1 (n = 1-3), which causes spin-dependent exciton level splitting into bright and dark states, where the latter is partially visible near the surface of the Br-based two-dimensional Ruddlesden-Popper series. Contrary to expectations, the smallest gap between bright and dark exciton levels is observed from n = 2 at 10 K. This anomaly results in the strongest biexciton binding between two dark excitons occurring at n = 2, rather than at n = 1 as initially hypothesized. The observed anomaly arises from a phase transition induced by octahedral tilting occurring only for n = 2 near 100 K as confirmed by temperature-dependent optical and X-ray diffraction measurements. Our results show that Coulomb interaction need not vary gradually with n, which can impact the optoelectronic properties of the Ruddlesden-Popper series.

Modulation of High-Intensity Optical Properties in CdS/CdSe/CdS Spherical Quantum Wells by CdSe Layer Thickness.

Spherical quantum wells (SQWs) have proven to be excellent materials for suppressing Auger recombination due to their expanded confinement volume. However, research on the factors and mechanisms of their high-intensity optical properties, such as multiexciton properties and third-order optical nonlinearities, remains incomplete, limiting further optimization of these properties. Here, a series of CdS/CdSe (xML)/CdS SQWs with varying CdSe layer thicknesses were prepared. The modulation effects of CdSe shell variations on the PL properties, defect distribution, biexciton binding energy, and third-order optical nonlinearities of the SQWs were investigated, and their impact on the material's multiexciton properties was further analyzed. Results showed that the typical CdS/CdSe(3ML)/CdS sample exhibited a large volume-normalized two-photon absorption cross-section (18.17 × 102 GM/nm3) and favorable biexciton characteristics. Optical amplification was observed at 12.4 μJ/cm2 and 1.02 mJ/cm2 under one-photon (400 nm) and two-photon (800 nm) excitation, respectively. Furthermore, different amplified spontaneous emission spectra were observed for the first time under one/two-photon excitation. This phenomenon was attributed to thermal effects overcoming the biexciton binding energy. This study provides valuable insights for further optimizing multiexciton gain characteristics in SQWs and developing optical gain applications.

Biexciton in Prolate Ellipsoidal Quantum Dot: Optical-Magnetic Properties

In this study, we present a theoretical framework for assessing the ground state energy of a biexciton within an ellipsoidal quantum dot subjected to an external magnetic field. Our method involves employing the variational method and the geometrical adiabatic approximation. We calculate the biexciton energy and binding energy, considering their dependence on the magnetic field strength and the small geometrical parameter of the ellipsoidal quantum dot. Our findings reveal an increase in magnetobiexciton energy with the strength of the magnetic field. Additionally, we analyze the biexciton’s magnetization concerning the external magnetic field and the small semiaxis of the ellipsoidal quantum dot. Moreover, we provide an estimation of the magnetobiexciton recombination radiative lifetime in the ellipsoidal quantum dot, which falls within the nanosecond range.

Electronic Structure of Biexcitons in Metal Halide Perovskite Nanoplatelets.

A theoretical description of biexcitons in metal halide perovskite nanoplatelets is presented. The description is based on a variational effective mass model, including polaronic effects by means of a Haken potential. The strong quantum and dielectric confinements are shown to squeeze the biexciton under the polaronic radius, which greatly enhances Coulomb attractions and (to a lesser extent) repulsions. This explains the need for effective dielectric constants approaching the high-frequency limit in previous simulations, and the binding energies exceeding 40 meV observed in single-monolayer nanoplatelets. Biexcitons are formed by a pair of weakly interacting excitons, with a roughly rectangular geometry. This translates into a constant ratio between biexciton and exciton binding energies (2D Haynes rule) well below the ideal value of ΔBX/ΔX = 0.228 proposed for squared biexcitons. The ratio is independent of the number of monolayers in the platelet, but it does depend on the lateral and dielectric confinement.

Biexcitons and quadrons in self-assembled quantum dots

We theoretically study biexcitons and quadrons in quantum dots with parabolic confinement and give a complete comparison between the two excitations. The calculation of quadron and biexciton binding energies as functions of electron-to-hole confinement potentials and mass ratios, using the unrestricted Hartree–Fock method, shows the essential differences between biexcitons and quadrons. The crossover between the negative and positive binding energies is indicated. The effect of an external magnetic field on the quadron and biexciton binding energies has also been investigated. In addition, the crossover between anti-binding and binding of both excited quadron and biexciton states in a certain range of the electron-to-hole oscillator length ratios has been found.

Electrochemical control of photoluminescence in pure wurtzite CdSe/CdS core/shell nanocrystals

Spectroelectrochemical methods are employed to systematically investigate the impacts of external charge filling in three pure wurtzite CdSe/CdS core/shell nanocrystals (NCs) with different shell thicknesses. We observed that electrons can penetrate the thick CdS shell and inject into the conduction band with an abnormal brightening, while holes are blocked. The formed negative trions result in a different spectral shift according to the size of the NCs, with a redshift of 3.7 nm for 12.5 nm NCs and almost no movement for 23.4 nm NCs. Meanwhile, due to the formation of biexcitons, a variational spectral blueshift with the NCs' size can be determined in time-resolved photoluminescence spectroscopy. This indicates that the binding energy of both biexcitons and negatively charged excitons can be regulated by controlling the shell thickness. Extra charges can also be used to activate/passivate trap states, thereby confirming that the 12.5 nm sample has better surface/interface quality.

Direct Visualization of Confinement and Many‐Body Correlation Effects in 2D Spectroscopy of Quantum Dots

AbstractThe size tunable color of colloidal semiconductor quantum dots (QDs) is probably the most elegant illustration of the quantum confinement effect. As explained by the simple “particle‐in‐a‐box” model, the transition energies between the levels increase when the “box” becomes smaller. To investigate quantum confinement effects, typically a well‐defined narrow size distribution of the nanoparticles is needed. In this contribution, how coherent electronic two‐dimensional spectroscopy (2DES) can directly visualize the quantum size effect in a sample with broad size distribution of QDs is demonstrated. The method is based on two features of the 2DES – the ability to resolve inhomogeneous broadening and the capability to reveal correlations between the states. In QD samples, inhomogeneous spectral broadening is mainly caused by the size distribution and leads to elongated diagonal peaks of the spectra. Since the cross peaks correlate the energies of two states, they allow drawing conclusions about the size dependence of the corresponding states. It is also found that the biexciton binding energy changes between 3 and 8 meV with the QD size. Remarkably, the size dependence is non‐monotonic with a clear minimum.

Size Dependence of Trion and Biexciton Binding Energies in Lead Halide Perovskite Nanocrystals.

Lead halide perovskite nanocrystals (NCs) have attracted much attention as light-source materials for light-emitting diodes, lasers, and quantum light emitters. The luminescence properties of perovskite NCs and the performance of NC-based light-source devices depend on trion and biexciton dynamics. Here, we examined the size dependence of trion and biexciton binding energies by conducting low-temperature single-dot spectroscopy on three different perovskite NCs: CsPbBr3, CsPbI3, and FAPbBr3. While the photoluminescence spectral widths of the all-inorganic CsPbBr3 and CsPbI3 NCs were narrow, compared with those of the organic-inorganic hybrid FAPbBr3 NCs, the binding energies of trions and biexcitons of all three samples showed similar size dependences, independent of the A-site cation and halogen. The effective-mass approximation calculations implied the importance of dynamical dielectric screening on the formation of trions and biexcitons.

Spatial Heterogeneity of Biexcitons in Two-Dimensional Ruddlesden-Popper Lead Iodide Perovskites.

Quantum confinement in two-dimensional (2D) Ruddlesden-Popper (RP) perovskites leads to the formation of stable quasi-particles, including excitons and biexcitons, the latter of which may enable lasing in these materials. Due to their hybrid organic-inorganic structures and the solution phase synthesis, microcrystals of 2D RP perovskites can be quite heterogeneous, with variations in excitonic and biexcitonic properties between crystals from the same synthesis and even within individual crystals. Here, we employ one- and two-quantum two-dimensional white-light microscopy to systematically study the spatial variations of excitons and biexcitons in microcrystals of a series of 2D RP perovskites BA2MAn-1PbnI3n+1 (n = 2-4, BA= butylammonium, MA = methylammonium). We find that the average biexciton binding energy of around 60 meV is essentially independent of the perovskite layer thickness (n). We also resolve spatial variations of the exciton and biexciton energies on micron length scales within individual crystals. By comparing the one-quantum and two-quantum spectra at each pixel, we conclude that biexcitons are more sensitive to their environments than excitons. These results shed new light on the ways disorder can modify the energetic landscape of excitons and biexcitons in RP perovskites and how biexcitons can be used as a sensitive probe of the microscopic environment of a semiconductor.

Probing the Multiexcitonic Dynamics in CsPbI3 Nanocrystals across the Temperature‐Induced Reversible Phase Transitions

AbstractCsPbI3 nanocrystals (CPI NCs) have become a trending research topic due to their impressive potential in functional optoelectronic devices and optical gain applications. Their optical responses are governed by carrier dynamics, which is greatly influenced by temperature and corresponding phase structure due to the effects of inherently electron‐phonon coupling. Notably, CPI NCs have been identified to adopt an unexpectedly stable cubic phase from room temperature to liquid helium temperature. Here, using in situ cryogenic electron diffraction measurements, it is unambiguously demonstrated that CPI NCs undergo consecutive cubic‐tetragonal‐orthorhombic phase transitions from 298 to 100 K. The corresponding temperature‐dependent multiexcitonic dynamics are investigated in each phase by combining time‐resolved photoluminescence and transient absorption spectroscopy. In addition to the temperature dependency, the lifetime of both excitons and biexcitons evidently depends on the phase structures of the CPI NCs, highlighting the crucial effect of crystal structure on the carrier dynamics. Moreover, the biexciton binding energy increases with higher crystal symmetry due to the decrease of the dielectric constant. The findings shed light on the structural phase transition and its relationship to the carrier dynamics in all‐inorganic perovskite NCs, which provides critical insight into the structure‐performance relationship in CPI NCs for promising applications in optoelectronic devices.

Biexciton binding energy in CH3NH3PbBr3 as determined by multiphoton photoluminescence excitation spectroscopy

Biexciton binding energy in CH3NH3PbBr3 as determined by multiphoton photoluminescence excitation spectroscopy

Spectral Fingerprint of Quantum Confinement in Single CsPbBr3 Nanocrystals.

Lead halide perovskite nanocrystals are promising materials for classical and quantum light emission. To understand these outstanding properties, a thorough analysis of the band-edge exciton emission is needed, which is not reachable in ensemble and room-temperature studies because of broadening effects. Here, we report on a cryogenic-temperature study of the photoluminescence of single CsPbBr3 nanocrystals in the intermediate quantum confinement regime. We reveal the size-dependence of the spectral features observed: the bright triplet exciton energy splittings, the trion and biexciton binding energies, and the optical phonon replica spectrum. In addition, we show that bright triplet energy splittings are consistent with a pure exchange model and that the variety of polarization properties and spectra recorded can be rationalized simply by considering the orientation of the emitting dipoles and the populations of the emitting states.

Localized Bound Multiexcitons in Engineered Quasi-2D Perovskites Grains at Room Temperature for Efficient Lasers.

Reducing the excitation threshold to minimize the Joule heating is critical for the realization of perovskite laser diodes. Although bound excitons are promising for low threshold laser, how to generate them at room temperature for laser applications is still unclear in quasi-2D perovskite-based devices. In this work, via engineering quasi-2D perovskite PEA2 (CH3 NH3 )n -1 Pbn Br3 n +1 microscopic grains by the anti-solvent method, room-temperature multiexciton radiative recombination is successfully demonstrated at a remarkably low pump density of 0.97 µJ cm-2 , which is only one-fourth of that required in 2D CdSe nanosheets. In addition, the well-defined translational momentum in quasi-2D perovskite grains can restrict the Auger recombination which is detrimental to radiative emission. Furthermore, the quasi-2D perovskite grains are favorable for increasing binding energies of excitons and biexcitons and so as the related radiative recombination. Consequently, the prepared <n= 8> phase quasi-2D perovskite film renders a threshold of room-temperature stimulated emission as low as 13.7 µJ cm-2 , reduced by 58.6% relative to the amorphous counterpart with larger grains. The findings in this work are expected to facilitate the development of solution-processable perovskite multiexcitonic laser diodes.

Lateral quantum confinement regulates charge carrier transfer and biexciton interaction in CdSe/CdSeS core/crown nanoplatelets

Charge carrier dynamics essentially determine the performance of various optoelectronic applications of colloidal semiconductor nanocrystals. Among them, two-dimensional nanoplatelets provide new adjustment freedom for their unique core/crown heterostructure. Herein, we demonstrate that by fine-tuning the core size and the lateral quantum confinement, the charge carrier transfer rate from the crown to the core can be varied by one order of magnitude in CdSe/CdSeS core/alloy-crown nanoplatelets. In addition, the transfer can be affected by a carrier blocking mechanism, i.e., the filled carriers hinder further possible transfer. Furthermore, we found that the biexciton interaction is oppositely affected by quantum confinement and electron delocalization, resulting in a non-monotonic variation of the biexciton binding energy with the emission wavelength. This work provides new observations and insights into the charge carrier transfer dynamics and exciton interactions in colloidal nanoplatelets and will promote their further applications in lasing, display, sensing, etc.

Excitonic Spin-Coherence Lifetimes in CdSe Nanoplatelets Increase Significantly with Core/Shell Morphology.

We report spin-polarized transient absorption for colloidal CdSe nanoplatelets as functions of thickness (2-6 monolayer thickness) and core/shell motif. Using electro-optical modulation of co- and cross-polarization pump-probe combinations, we sensitively observe spin-polarized transitions. Core-only nanoplatelets exhibit few-picosecond spin lifetimes that weakly increase with layer thickness. The spectral content of differenced spin-polarized signals indicate biexciton binding energies that decrease with increasing thickness and smaller values than previously reported. Shell growth of CdS with controlled thicknesses, which partially delocalize the electron from the hole, significantly increases the spin lifetime to ∼49 ps at room temperature. Implementation of ZnS shells, which do not alter delocalization but do alter surface termination, increased spin lifetimes up to ∼100 ps, bolstering the interpretation that surface termination heavily influences spin coherence, likely due to passivation of dangling bonds. Spin precession in magnetic fields both confirms long coherence lifetime at room temperature and yields the excitonic g factor.

Many-Body Correlations and Exciton Complexes in CsPbBr3 Quantum Dots.

All-inorganic lead-halide perovskite (LHP) (CsPbX3 , X=Cl, Br, I) quantum dots (QDs) have emerged as a competitive platform for classical light-emitting devices (in the weak light-matter interaction regime, e.g., LEDs and laser), as well as for devices exploiting strong light-matter interaction at room temperature. Many-body interactions and quantum correlations among photogenerated exciton complexes play an essential role, for example, by determining the laser threshold, the overall brightness of LEDs, and the single-photon purity in quantum light sources. Here, by combining cryogenic single-QD photoluminescence spectroscopy with configuration-interaction (CI) calculations, the size-dependent trion and biexciton binding energies are addressed. Trion binding energies increase from 7to 17meV for QD sizes decreasing from 30to 9nm, while the biexciton binding energies increase from 15to 30meV, respectively. CI calculations quantitatively corroborate the experimental results and suggest that the effective dielectric constant for biexcitons slightly deviates from the one of the single excitons, potentially as a result of coupling to the lattice in the multiexciton regime. The findings here provide a deep insight into the multiexciton properties in all-inorganic LHP QDs, essential for classical and quantum optoelectronic devices.

Universal scaling laws for charge-carrier interactions with quantum confinement in lead-halide perovskites

Lead halide perovskites open great prospects for optoelectronics and a wealth of potential applications in quantum optical and spin-based technologies. Precise knowledge of the fundamental optical and spin properties of charge-carrier complexes at the origin of their luminescence is crucial in view of the development of these applications. On nearly bulk Cesium-Lead-Bromide single perovskite nanocrystals, which are the test bench materials for next-generation devices as well as theoretical modeling, we perform low temperature magneto-optical spectroscopy to reveal their entire band-edge exciton fine structure and charge-complex binding energies. We demonstrate that the ground exciton state is dark and lays several millielectronvolts below the lowest bright exciton sublevels, which settles the debate on the bright-dark exciton level ordering in these materials. More importantly, combining these results with spectroscopic measurements on various perovskite nanocrystal compounds, we show evidence for universal scaling laws relating the exciton fine structure splitting, the trion and biexciton binding energies to the band-edge exciton energy in lead-halide perovskite nanostructures, regardless of their chemical composition. These scaling laws solely based on quantum confinement effects and dimensionless energies offer a general predictive picture for the interaction energies within charge-carrier complexes photo-generated in these emerging semiconductor nanostructures.

In English

A theory of exciton quasimolecules (formed from spatially separated electrons and holes) in a nanosystem consisting of double quantum dots (QDs) of germanium synthesized in a silicon matrix is presented. It is shown that the binding energy of the singlet ground state of the quasimolecule of an exciton is considerably larger than the binding energy of biexciton in a silicon single crystal by almost two orders of magnitude. It is shown that the exciton quasimolecule formation is of the threshold character and possible in a nanosystem, where D is the distance between the surfaces of QD that satisfies the following condition: (where and are some critical distances). Using the variational method, we obtain the binding energy of the exciton quasimolecule singlet ground state of the system as a function of the distance between the surfaces of QD D, and the QD radius a. It is shown that the convergence of two QDs up to a certain critical value of the distance between the surfaces of QD DC leads to the effective overlapping of the electron wave functions and the appearance of exchange interactions. As a result, the exciton quasimolecules can be formed from the QDs. It is shown that the existence of such a critical distance DC arises from the quantum size effects. Dimensional quantization of electrons and holes motion leads to the following fact: as the distance between the surfaces of the QD DC decreases, the decrease in the energies of interaction of the electrons and holes entering into the Hamiltonian of the exciton quasimolecule cannot compensate for the increase in the kinetic energy of the electrons and holes. At larger values of the distance between the surfaces of the QD D, , the exciton quasimolecule breaks down into two excitons (consisting of spatially separated electrons and holes), localized over the QD surfaces. The fact that the energy of the ground state of singlet excitonic quasimolecule is in the infrared range of the spectrum, presumably, allows us to use a quasimolecule for the development of new elements of silicon infrared nanooptoelectronics.

Confined Excitons in Spherical-Like Halide Perovskite Quantum Dots.

Quantum dots (QDs) offer unique physical properties and novel application possibilities like single-photon emitters for quantum technologies. While strongly confined III-V and II-VI QDs have been studied extensively, their complex valence band structure often limits clear observations of individual transitions. In recently emerged lead-halide perovskites, band degeneracies are absent around the bandgap reducing the complexity of optical spectra. We show that for spherical-like CsPbBr3 QDs with diameters >6 nm, excitons confine with respect to their center-of-mass motion leading to well-pronounced resonances in their absorption spectra. Optical pumping of the lowest-confined exciton with femtosecond laser pulses not only bleaches all excitons but also reveals a series of distinct induced absorption resonances which we attribute to exciton-to-biexciton transitions and are red-shifted by the biexciton binding energy (∼40 meV). The temporal dynamics of the bleached excitons further support our exciton confinement model. Our study provides the first insight into confined excitons in CsPbBr3 QDs and gives a detailed understanding of their linear and nonlinear optical spectra.