Exciton Splitting Research Articles

Squaraine Dyes for Single‐Component Shortwave Infrared‐Sensitive Photodiodes and Upconversion Photodetectors

AbstractSensitive detection of shortwave infrared (SWIR) light using organic dyes will be a significant advance toward many applications in industry and research. Furthermore, from a fabrication and optimization view, photogeneration of charges in diodes consisting of a single dye layer will be highly attractive. However, SWIR dyes are scarce and organic photodiodes usually utilize a donor–acceptor materials combination to split excitons into charges. Here, it is demonstrated that single‐component layers of several SWIR squaraine dyes operate as efficient photodetectors, with peak external quantum efficiency > 40% beyond 1000 nm and sensitivity out to 1300 nm. Photocurrents show a superlinear dependence on reverse bias. It is shown that this results from a field‐assisted exciton dissociation mechanism, and not from field‐dependent charge injection or extraction. SWIR photodiodes are combined with organic light‐emitting diodes to fabricate upconversion photodetectors – devices that convert SWIR photons directly into visible light. Upconverters are characterized by a low turn‐on voltage (1.5 V) and a high luminance contrast (on‐off ratio 16 000) and SWIR‐to‐visible (λ = 575 nm) photon conversion efficiency (1.85%). Upconversion photodetectors emerge as a promising alternative to the current inorganic‐based imaging technology.

Band splitting under strain: A route to broad-spectrum absorption

A strategy to split bright exciton bands is proposed to achieve broad-spectrum absorption in two-dimensional semiconductors. Based on ab initio many-body perturbation theory and Bethe-Salpeter equation calculations, we predict that post-transition-metal chalcogenides monolayers can realize broadband absorption under biaxial tensile strains. We find that top valence bands responsible for the strongly absorbing excitons exhibit disparate bonding characteristics and symmetries, thus responding to the strain differently. A moderate strain on the monolayers could yield large energy splitting of the exciton bands and a redshift of the band gap, broadening the absorption across the solar spectrum. The bright excitons have mixed inter- and intra-layer features, rendering them with relatively long lifetimes and strong absorption amplitudes. This work provides a new perspective to engineer two-dimensional materials with superior optoelectronic properties.

Exciton-polaron interactions in metal halide perovskite nanocrystals revealed via two-dimensional electronic spectroscopy.

Metal halide perovskite nanocrystals have been under intense investigation for their promise in optoelectronic devices due to their remarkable physics, such as liquid/solid duality. This liquid/solid duality may give rise to their defect tolerance and other such useful properties. This duality means that the electronic states are fluctuating in time, on a distribution of timescales from femtoseconds to picoseconds. Hence, these lattice induced energy fluctuations that are connected to polaron formation are also connected to exciton formation and dynamics. We observe these correlations and dynamics in metal halide perovskite nanocrystals of CsPbI3 and CsPbBr3 using two-dimensional electronic (2DE) spectroscopy, with its unique ability to resolve dynamics in heterogeneously broadened systems. The 2DE spectra immediately reveal a previously unobserved excitonic splitting in these 15nm NCs that may have a coarse excitonic structure. 2D lineshape dynamics reveal a glassy response on the 300fs timescale due to polaron formation. The lighter Br system shows larger amplitude and faster timescale fluctuations that give rise to dynamic line broadening. The 2DE signals enable 1D transient absorption analysis of exciton cooling dynamics. Exciton cooling within this doublet is shown to take place on a slower timescale than within the excitonic continuum. The energy dissipation rates are the same for the I and Br systems for incoherent exciton cooling but are very different for the coherent dynamics that give rise to line broadening. Exciton cooling is shown to take place on the same timescale as polaron formation, revealing both as coupled many-body excitation.

Influence of Organic Spacer Cation on Dark Excitons in 2D Perovskites

AbstractThe organic spacer cation plays a crucial role in determining the exciton fine structure in 2D perovskites. Here, magneto‐optical spectroscopy is used to gain insight into the influence of the organic spacer on dark excitons in Ruddlesden–Popper (RP) perovskites. Using modest magnetic field strengths (<1.5 T), the optically forbidden dark exciton state can be identified and its emission properties significantly modulated via application of in‐plane magnetic fields, up to temperatures of 15 K. At low temperatures, an increase in collected photoluminescence efficiency of >30% is demonstrated, signifying the critical role of the dark exciton state for light‐emitting applications of 2D perovskites. The exciton fine structure and the degree of magnetic‐field‐induced mixing are significantly impacted by the choice of organic spacer cation, with 4–methoxyphenylethylammonium (MeO‐PEA) showing the largest effect due to larger bright–dark exciton splitting. This study distinguishes between interior (bulk) and surface dark‐exciton emission, showing that bright–dark exciton splitting differs between the interior and surface. The results emphasize the significance of the organic spacer cation in controlling the exciton fine structure in 2D perovskites and have important implications for the development of optoelectronic technology based on 2D perovskites.

Nanographene‐Based Metal‐Organic Framework Thin Films: Optimized Packing and Efficient Electron‐Hole Separation Yielding Efficient Photodetector

AbstractOrganic semiconductors, specifically polycyclic aromatic chromophores like pentacene, coronene, and nanographenes (hexa‐peri‐hexabenzocoronene, HBC), are often utilized in optoelectronic applications due to their intriguing single‐molecule properties. However, the integration of these aromatic compounds into optoelectronic devices frequently encounters significant obstacles, primarily due to the unpredictable and challenging control over the structure and morphology of their condensed phase. Rather than resorting to chemical modifications or advanced thin‐film manufacturing methods such as chemical vapor deposition, the so‐called metal‐organic framework (MOF) approach is employed to create a highly photoresponsive, nanographene‐based thin film employing a layer‐by‐layer, liquid‐phase epitaxy method. It is demonstrated that the novel Cu‐HBC MOF thin film exhibits excellent photoresponsive behavior under ultraviolet illumination, including a fast response time (˂ 0.02 s) and high current switching ratio (≈104), which significantly outperforms an isostructural MOF Zn‐HBC. Furthermore, it is demonstrated that the metal nodes inside the MOF can be used to enhance the photoresponse by efficient exciton splitting.

Strain‐Free GaSb Quantum Dots as Single‐Photon Sources in the Telecom S‐Band

AbstractGenerating single photons in the telecommunication wavelength range from semiconductor quantum dots (QDs) and interfacing them with spins of electrons or holes is of high interest in recent years, with research mainly focusing on indium‐based QDs. However, there is not much data on the optical and spin properties of gallium antimonide (GaSb) QDs, despite it being a physically rich system with an indirect to direct bandgap crossover in the telecom wavelength range. This work investigates the (quantum‐) optical properties of GaSb QDs, which are fabricated by filling droplet‐etched nanoholes in an aluminum gallium antimonide (AlGaSb) matrix. Photoluminescence (PL) features from isolated and highly symmetric QDs are observed that exhibit narrow linewidth in the telecom S‐band and show an excitonic fine structure splitting of up to µeV. Moreover, time‐resolved measurements of the decay characteristics of an exciton are performed and the second‐order photon autocorrelation function of the charge complex is measured to , revealing clear antibunching and thus proving the capability of this material platform to generate non‐classical light.

A comparison increasing the photodegradation power of a Ag/g–C3N4 /CoNi–LDH nanocomposite: Photocatalytic activity toward water treatment

For environmental applications, it is crucial to rationally design and synthesize photocatalysts with positive exciton splitting and interfacial charge transfer. Here, a novel Ag-bridged dual Z-scheme Ag/g-C3N4/CoNi-LDH plasmonic heterojunction was successfully synthesized using a simple method, with the goal of overcoming the common drawbacks of traditional photocatalysts such as weak photoresponsivity, rapid combination of photo-generated carriers, and unstable structure. These materials were characterized by XRD, FT–IR, SEM, TEM UV–Vis/DRS, and XPS to verify the structure and stability of the heterostructure. The pristine LDH, g-C3N4, and Ag/g-C3N4/CoNi-LDH composite were investigated as photocatalysts for water remediation, an environmentally motivated process. Specifically, the photocatalytic degradation of tetracycline was studied as a model reaction. The performance of the supports and composite catalyst were determined by evaluating both the degradation and adsorption phenomenon. The influence of several experimental parameters such as catalyst loading, pH, and tetracycline concentration were evaluated. The current study provides important data for water treatment and similar environmental protection applications.

Excitonic Coupling in Fluorene-Based Bichromophoric Systems: Vibrational Quenching and the Transition from Weak to Intermediate Coupling.

Excimeric systems (i.e., excited dimers) have well served as model compounds for the study of the delocalization of electronic energy over weakly interacting chromophores. However, there remain relatively few isolated systems in which such interactions can be studied experimentally at a level to afford detailed comparisons with theory. In this Article, we examine a series of covalently and noncovalently linked dimers of fluorene, as a model aromatic chromophore, where the formation of excimers requires a π-stacked, cofacial orientation at van der Waals contact. Building upon a series of seminal prior studies that examined vibronic quenching of the excitation interaction in van der Waals dimers, the key question that we sought to address here is whether a single quenching factor could reproduce experimental excitonic splittings across a series of covalently and noncovalently linked bichromophoric systems built from the same chromophore. In comparing experimentally measured excitonic splittings with calculated static splittings using time-dependent density functional methods, we find that all systems save one fall on a line with a slope of 0.080(8), reflecting a vibrational quenching of roughly 1 order of magnitude. The outlier, which shows a significantly reduced quenching factor, represents a cyclophane-linked system where the fluorene moieties are constrained in a cofacial arrangement. We argue that this system evidences the transition from the weak to intermediate coupling regime.

The Stokes Shift and Exciton Fine Structure in Strongly Confined CsPbBr3 Perovskite Nanoplatelets.

The Stokes shift is negligible in bulk perovskites but large in two-dimensional (2D) CsPbBr3 perovskites. The issue has attracted a lot of discussion, but it remains controversial. Here, we report the temperature-dependent absorption and photoluminescence (PL) spectra of CsPbBr3 perovskite nanoplatelets (NPLs). We observe a temperature-dependent Stokes shift changing from 26 to 41 meV. This phenomenon was attributed to the exciton fine structure according to the great difference in peak width. The triple bright exciton levels all participate in the absorption process and result in a wide absorption peak, while only the lowest exciton level contributes to photon emission and exhibits a relatively narrow PL peak. The PL decay curves also present the characterization of bright and dark exciton couplings at low temperatures. The splitting of triple bright excitons is induced by the morphology anisotropy of the 2D structure, so the large Stokes shift is proposed to be an intrinsic property of 2D perovskites.

Size Control of the Mechanism of Exciton Polarization in Metal Oxide Nanocrystals through Fermi Level Pinning.

Light-matter interaction in certain aliovalently doped metal oxide nanocrystals (NCs) results in the generation of localized surface plasmon resonance (LSPR) in the near- to mid-infrared, allowing for their implementation in various technologies, including photovoltaics, sensing, and electrochromics. These materials could also facilitate coupling between plasmonic and semiconducting properties, making them highly interesting for electronic and quantum information technologies. In the absence of dopants, free charge carriers can arise from native defects such as oxygen vacancies. Here we show using magnetic circular dichroism spectroscopy that the exciton splitting in In2O3 NCs is induced by both localized and delocalized electrons and that contributions from the two mechanisms are strongly dependent on the NC size, owing to Fermi level pinning and the formation of a surface depletion layer. In large NCs, the angular momentum transfer from delocalized cyclotron electrons to the excitonic states is the dominant mechanism of exciton polarization. This process diminishes with decreasing NC size, owing to the rapidly reduced volume of the plasmonic core. On the other hand, exciton polarization in small NCs is dominated by localized electron-spin-induced splitting of the excitonic states. This mechanism is independent of NC size, suggesting that wave functions of localized spin states on NC surfaces do not overlap with the excitonic states. The results of this work demonstrate that the effects of individual and collective electronic properties on excitonic states can be simultaneously controlled by NC size, making metal oxide NCs a promising class of materials for quantum, spintronic, and photonic technologies.

Fermi polaron fine structure in strained van der Waals heterostructures

The fine structure of attractive Fermi polarons in van der Waals heterostructures based on monolayer transition metal dichalcogenides in the presence of elastic strain is studied theoretically. The charged excitons (trions), three particle bound states of two electrons and a hole or two holes and an electron, do not show any strain-induced fine structure splitting compared to neutral excitons whose radiative doublet is split by the strain into linearly polarized components. The correlation of the trions with Fermi sea holes gives rise to attractive Fermi polarons. We show that this results in a fine structure splitting of the polaron into states polarized along the main axes of the strain tensor. This effect is related to the bosonic statistics of Fermi polarons. We develop a microscopic theory of the effect and calculate the strain-induced splitting of Fermi polarons for both tungsten- and molybdenum-based monolayers, identifying the role of inter- and intravalley exciton–electron interactions. The fine structure splitting of the attractive Fermi polaron is proportional to both the excitonic splitting and the Fermi energy. The Fermi polaron fine structure in bilayers is briefly analyzed, and the role of electron and trion localization in moiré potentials is discussed.

Two-photon excitation with finite pulses unlocks pure dephasing-induced degradation of entangled photons emitted by quantum dots

Semiconductor quantum dots have emerged as an especially promising platform for the generation of polarization-entangled photon pairs. However, it was demonstrated recently that the two-photon excitation scheme employed in state-of-the-art experiments limits the achievable degree of entanglement by introducing which-path information. In this work, the combined impact of two-photon excitation and longitudinal acoustic phonons on photon pairs emitted by strongly-confining quantum dots is investigated. It is found that phonons further reduce the achievable degree of entanglement even in the limit of vanishing temperature due to phonon-induced pure dephasing and phonon-assisted one-photon processes, which increase the reexcitation probability. In addition, the degree of entanglement, as measured by the concurrence, decreases with rising temperature and/or pulse duration, even if the excitonic fine-structure splitting is absent and when higher electronic states are out of reach. Furthermore, in the case of finite fine-structure splittings, phonons enlarge the discrepancy in concurrence for different laser polarizations.

New coercivities and Curie temperatures emerged in van der Waals homostructures of Fe3GeTe2

Stacking of two-dimensional van der Waals (vdW) materials has emerged as a vital approach for investigating the complex interplay between coexisting materials, leading to novel electronic, photonic, and spintronic phenomena such as strong electronic correlation, enhanced Zeeman splitting of excitons, and the magnetic proximity effect. While most of the previous focus was on the electronic and photonic properties of vdW hetero-/homostructures, the study of magnetism in hetero-/homostructures remains lacking, though phenomena such as layer-dependent magnetism already highlighted the critical role of interlayer coupling in magnetism of layered systems. Here, we report an interesting two-step magnetic hysteresis loop in the homostructure of the layered vdW magnet ${\mathrm{Fe}}_{3}{\mathrm{GeTe}}_{2}$ (FGT). The two coercivities in the homostructure differ from that of each constituent FGT layer, indicating that the two constituents interact mutually but do not merge into a thicker uniform material. The Curie temperature (${T}_{\mathrm{C}}$) of the homostructure is in between that of the thinner and thicker constituents, further implying that the two constituent layers interact yet without forming a naturally occurring thicker layer. Our results highlight the unexplored opportunities that the rich interplay within vdW magnet homostructures can be employed to create emerging magnetic properties and develop novel multistate devices.

Promoter G-Quadruplex Binding Styryl Benzothiazolium Derivative for Applications in Ligand Affinity Ranking and as Ethidium Bromide Substitute in Gel Staining.

Fluorescent compounds that can preferentially interact with certain nucleic acids are of great importance in new drug discovery in a multitude of functions including fluorescence-based displacement assays and gel staining. Here, we report the discovery of an orange emissive styryl-benzothiazolium derivative (compound 4) which interacts preferentially with Pu22 G-quadruplex DNA among a pool of nucleic acid structures containing G-quadruplex, duplex, and single-stranded DNA structures as well as RNA structures. Fluorescence-based binding analysis revealed that compound 4 interacts with Pu22 G-quadruplex DNA in a 1:1 DNA to ligand binding stoichiometry. The association constant (Ka) for this interaction was found to be 1.12 (±0.15) × 106 M-1. Circular dichroism studies showed that the binding of the probe does not cause changes in the overall parallel G-quadruplex conformation; however, signs of higher-order complex formation were seen in the form of exciton splitting in the chromophore absorption region. UV-visible spectroscopy studies confirmed the stacking nature of the interaction of the fluorescent probe with the G-quadruplex which was further complemented by heat capacity measurement studies. Finally, we have shown that this fluorescent probe can be used toward G-quadruplex-based fluorescence displacement assays for ligand affinity ranking and as a substitute for ethidium bromide in gel staining.

Zero-field quantum beats and spin decoherence mechanisms in CsPbBr3 perovskite nanocrystals

Coherent optical manipulation of exciton states provides a fascinating approach for quantum gating and ultrafast switching. However, their coherence time for incumbent semiconductors is highly susceptible to thermal decoherence and inhomogeneous broadening effects. Here, we uncover zero-field exciton quantum beating and anomalous temperature dependence of the exciton spin lifetimes in CsPbBr3 perovskite nanocrystals (NCs) ensembles. The quantum beating between two exciton fine-structure splitting (FSS) levels enables coherent ultrafast optical control of the excitonic degree of freedom. From the anomalous temperature dependence, we identify and fully parametrize all the regimes of exciton spin depolarization, finding that approaching room temperature, it is dominated by a motional narrowing process governed by the exciton multilevel coherence. Importantly, our results present an unambiguous full physical picture of the complex interplay of the underlying spin decoherence mechanisms. These intrinsic exciton FSS states in perovskite NCs present fresh opportunities for spin-based photonic quantum technologies.

Effects of Opposite Atoms on Electronic Structure and Optical Absorption of Two‐Dimensional Hexagonal Boron Nitride

AbstractThe first‐principles many‐body GW and Bethe–Salpeter equation (BSE) calculations on the 2D hexagonal boron nitride (2D‐hBN) to explore the effects of opposite atoms on the electronic structure and linear one‐photon absorption (OPA) are performed. Five AA‐ and AB‐stacked bilayer and eight AAB‐stacked trilayer structures are considered. The AAB‐stacked trilayer hBN (TL‐BN) structures are constructed by mixing the AA‐ and AB‐stacked bilayer hBN (BL‐BN). It is shown that the GW approximation gives rise to different types (i.e., indirect or direct) of fundamental bandgaps from the independent particle approximation for all structures except those dominated by the B–B opposite. The stacking modes dominated by the B–B opposite have a direct fundamental bandgap in both approximations. The OPA spectra are calculated by solving the BSE combined with the GW quasi‐particle correction. The binding energy and Davydov splitting of excitons of TL‐BN strongly depend on the opposite atoms and are related to the role of the stacking BL‐BN substructure. Finally, taking the six‐layer and below AB‐stacked structures as examples, it is shown that the B–B opposite unit is helpful in constructing the turbostratic‐phase‐like stacking structures with a direct fundamental bandgap which are more suitable for optoelectronic applications.

Structure of the Charge-Transfer State in PM6/Y6 and PM6/Y6:YT Composites Studied by Electron Spin Echo Technique

Nowadays, Y-shaped non-fullerene acceptors become increasingly important in organic photovoltaics (OPV). Their use in binary and ternary bulk heterojunction composites continuously pushes up the efficiency of OPV devices. However, the mechanism of OPV performance enhancement by the third component of a ternary composite is rarely understood. In the present work, pulse EPR technique was used to reveal the similarities and the differences of photoinduced charge separation process in binary PM6/Y6 and ternary PM6/Y6:Y-T composites, where PM6 is polymer donor, Y6 and Y-T are different non-fullerene acceptors. Out-of-phase electron spin echo signal was detected for both composites, which is the signature of the charge-transfer state (CT state) formed at the donor/acceptor interface upon exciton splitting. Nearly identical distribution of the distances between the electron and the hole constituting the CT state was obtained for these composites from the analysis of this signal. In both cases the average electron-hole distance was 3.5 nm. It implies that OPV efficiency increase with Y-T addition is not caused by the increased probability of CT state dissociation followed by free charge generation for PM6/Y6:Y-T composite.

Gradient Type‐II CdSe/CdSeTe/CdTe Core/Crown/Crown Heteronanoplatelets with Asymmetric Shape and Disproportional Excitonic Properties (Small 11/2023)

Multi-Crown Type-II Nanoplatelets In article number 2205729, Hilmi Volkan Demir and co-workers discuss the recombination dynamics of CdSe/CdSeTe/CdTe core/crown/crown nanoplatelets (NPLs) with a step-wise gradient electronic structure. The NPLs possess disproportional electron and hole wavefunction distributions, in which the activation/deactivation of direct and indirect channels is possible by adjusting the domains' geometry. These NPLs demonstrate exceptionally high two-photon absorption cross-section and low dark-bright exciton splitting energy.

A Hybrid Magneto‐Optic Capacitive Memory with Picosecond Writing Time

AbstractThe long‐term future of information storage requires the use of sustainable nanomaterials in architectures operating at high frequencies. Interfaces can play a key role in this pursuit via emergent functionalities that break out from conventional operation methods. Here, spin‐filtering effects and photocurrents are combined at metal‐molecular‐oxide junctions in a hybrid magneto‐capacitive memory. Light exposure of metal‐fullerene‐metal oxide devices results in spin‐polarized charge trapping and the formation of a magnetic interface. Because the magnetism is generated by a photocurrent, the writing time is determined by exciton formation and splitting, electron hopping, and spin‐dependent trapping. Transient absorption spectroscopy measurements show changes in the electronic states as a function of the magnetic history of the device within picoseconds of the optical pumping. The stored information is read using time‐resolved scanning magneto optic Kerr effect measurements during microwave irradiation. The emergence of a magnetic interface in the picosecond timescale opens new paths of research to design hybrid magneto‐optic structures operating at high frequencies for sensing, computing, and information storage.

Gradient Type-II CdSe/CdSeTe/CdTe Core/Crown/Crown Heteronanoplatelets with Asymmetric Shape and Disproportional Excitonic Properties.

Characterized by their strong 1D confinement and long-lifetime red-shifted emission spectra, colloidal nanoplatelets (NPLs) with type-II electronic structure provide an exciting ground to design complex heterostructures with remarkable properties. This work demonstrates the synthesis and optical characterization of CdSe/CdSeTe/CdTe core/crown/crown NPLs having a step-wise gradient electronic structure and disproportional wavefunction distribution, in which the excitonic properties of the electron and hole can be finely tuned through adjusting the geometry of the intermediate crown. The first crown with staggered configuration gives rise to a series of direct and indirect transition channels that activation/deactivation of each channel is possible through wavefunction engineering. Moreover, these NPLs allow for switching between active channels with temperature, where lattice contraction directly affects the electron-hole (e-h) overlap. Dominated by the indirect transition channels over direct transitions, the lifetime of the NPLs starts to increase at 9 K, indicative of low dark-bright exciton splitting energy. The charge transfer states from the two type-II interfaces promote a large number of indirect transitions, which effectively increase the absorption of low-energy photons critical for nonlinear properties. As a result, these NPLs demonstrate exceptionally high two-photon absorption cross-sections with the highest value of 12.9 × 106 GM and superlinear behavior.