Behavior Of Excitons Research Articles

Probing Nanoscale Exciton Funneling at Wrinkles of Twisted Bilayer MoS2 Using Tip-Enhanced Photoluminescence Microscopy.

In twisted bilayer (t2L) two-dimensional (2D) transition metal dichalcogenides, local strain at wrinkles strongly modulates the local exciton density and PL energy resulting in an exciton funneling effect. Probing such exciton behaviors especially at nanometer length scales is beyond the limit of conventional analytical tools due to the limited spatial resolution and low sensitivity. To address this challenge, herein we applied high-resolution tip-enhanced photoluminescence (TEPL) microscopy to investigate exciton funneling at a wrinkle in a t2L MoS2 sample with a small twist angle of 0.5°. Owing to a spatial resolution of <10 nm, excitonic behavior at nanoscale sized wrinkles could be visualized using TEPL imaging. Detailed investigation of nanoscale exciton funneling at the wrinkles revealed a deformation potential of -54 meV/%. The obtained results provide novel insights into the inhomogeneities of excitonic behaviors at nanoscale and would be helpful in facilitating the rational design of 2D material-based twistronic devices.

Shape-Tuned Multiphoton-Emitting InP Nanotetrapods.

As the properties of a semiconductor material depend on the fate of the excitons, manipulating exciton behavior is the primary objective of nanomaterials. Although nanocrystals exhibit unusual excitonic characteristics owing to strong spatial confinement, studying the interactions between excitons in a single nanoparticle remains challenging due to the rapidly vanishing multiexciton species. Here, a platform for exciton tailoring using a straightforward strategy of shape-tuning of single-crystalline nanocrystals is presented. Spectroscopic and theoretical studies reveal a systematic transition of exciton confinement orientation from 3D to 2D, which is solely tuned by the geometric shape of material. Such a precise shape-effect triggers a multiphoton emission in single nanotetrapods with arms longer than the exciton Bohr radius of material. In consequence, the unique interplay between the multiple quantum states allows a geometric modulation of the quantum-confined Stark effect and nanocrystal memory effect in single nanotetrapods. These results provide a useful metric in designing nanomaterials for future photonic applications.

New insight into carrier transport in 2D layered perovskites

2D layered halide perovskites exhibit anisotropic carrier transport. Using scanning photocurrent microscopy, in this issue of <i>Chem</i>, Shrestha et al. report novel findings in 2D layered Ruddlesden-Popper perovskites, including a long carrier in-plane diffusion length and exciton behavior and dissociation, unveiling new insight not previously obtained through spectroscopic techniques.

Elevated temperature spectroscopic ellipsometry analysis of the dielectric function, exciton, band-to-band transition, and high-frequency dielectric constant properties for single-crystal ZnGa2O4

We report the elevated temperature (22 °C ≤ T ≤ 600 °C) dielectric function properties of melt grown single crystal ZnGa2O4 using a spectroscopic ellipsometry approach. A temperature dependent Cauchy dispersion analysis was applied across the transparent spectrum to determine the high-frequency index of refraction yielding a temperature dependent slope of 3.885(2) × 10−5 K−1. A model dielectric function critical point analysis was applied to examine the dielectric function and critical point transitions for each temperature. The lowest energy M0-type critical point associated with the direct bandgap transition in ZnGa2O4 is shown to red-shift linearly as the temperature is increased with a subsequent slope of −0.72(4) meV K−1. Furthermore, increasing the temperature results in a reduction of the excitonic amplitude and increase in the exciton broadening akin to exciton evaporation and lifetime shortening. This matches current theoretical understanding of excitonic behavior and critically provides justification for an anharmonic broadened Lorentz oscillator to be applied for model analysis of excitonic contributions.

Dynamics of Excitons in Conjugated Molecules and Organic Semiconductor Systems.

The exciton, an excited electron-hole pair bound by Coulomb attraction, plays a key role in photophysics of organic molecules and drives practically important phenomena such as photoinduced mechanical motions of a molecule, photochemical conversions, energy transfer, generation of free charge carriers, etc. Its behavior in extended π-conjugated molecules and disordered organic films is very different and very rich compared with exciton behavior in inorganic semiconductor crystals. Due to the high degree of variability of organic systems themselves, the exciton not only exerts changes on molecules that carry it but undergoes its own changes during all phases of its lifetime, that is, birth, conversion and transport, and decay. The goal of this review is to give a systematic and comprehensive view on exciton behavior in π-conjugated molecules and molecular assemblies at all phases of exciton evolution with emphasis on rates typical for this dynamic picture and various consequences of the above dynamics. To uncover the rich variety of exciton behavior, details of exciton formation, exciton transport, exciton energy conversion, direct and reverse intersystem crossing, and radiative and nonradiative decay are considered in different systems, where these processes lead to or are influenced by static and dynamic disorder, charge distribution symmetry breaking, photoinduced reactions, electron and proton transfer, structural rearrangements, exciton coupling with vibrations and intermediate particles, and exciton dissociation and annihilation as well.

Anomalous emission behavior of excitons at low temperature in monolayer WS2

We report on the anomalous emission behavior of excitons (X) in monolayer WS2 using temperature dependent photoluminescence spectroscopy. In general, PL emission from excitons enhances with decreasing temperature due to suppression of phonon mediated non-radiative transitions. Here, we observe that excitonic PL is temperature independent, although with decreasing temperature, the emission intensity for trion (X −) and biexciton or defect-bound excitons (XX/L 1) increases up to 123 K and then decreases. Analysis of experimental data with a model derived from the Boltzmann distribution and Saha equation reveal conversion of excitons into trions, biexcitons or defect-bound excitons and an increase of spin forbidden dark state with reduction in temperature. These findings could provide better strategies for designing future quantum devices.

Relaxation and darkening of excitonic complexes in electrostatically doped monolayer WSe2 : Roles of exciton-electron and trion-electron interactions

We present photoluminescence measurements in monolayer WSe$_2$, which point to the importance of the interaction between charged particles and excitonic complexes. The theoretical analysis highlights the key role played by exchange scattering, referring to cases wherein the particle composition of the complex changes after the interaction. For example, exchange scattering renders bright excitonic complexes dark in monolayer WSe$_2$ on accounts of the unique valley-spin configuration in this material. In addition to the ultrafast energy relaxation of hot excitonic complexes following their interaction with electrons or holes, our analysis sheds light on several key features that are commonly seen in the photoluminescence of this monolayer semiconductor. In particular, we can understand why the photoluminescence intensity of the neutral bright exciton is strongest when the monolayer is hole-doped rather than charge neutral or electron-doped. Or the reason for the dramatic increase of the photoluminescence intensity of negatively-charged excitons (trions) as soon as electrons are added to the monolayer. To self-consistently explain the findings, we further study the photoluminescence spectra at different excitation energies and analyze the behavior of the elusive indirect exciton.

Boosting the Optoelectronic Performance by Regulating Exciton Behaviors in a Porous Semiconductive Metal–Organic Framework

Exciton behaviors including exciton formation and dissociation dynamics play an essential role in the optoelectronic performance of semiconductive materials but remain unexplored in semiconductive metal-organic frameworks (MOFs). Herein, we reveal that the exciton behaviors in semiconductive MOFs can be regulated by framework-guest interactions, a feature often not achievable in traditional inorganic or organic semiconductors. Incorporation of the electron-deficient molecule within the pores of a terbium-based semiconductive MOF (Tb2L2·4H2O·6DMF, L = TATAB3-, 4,4',4″-s-triazine-1,3,5-triyltri-p-aminobenzoate, DMF = N,N-dimethylformamide) results in efficient energy transfer from the MOF skeleton to molecular acceptors, with a yield of up to 77.4%. This interaction facilitates distinctive exciton type conversion, giving rise to modified conductivity and photoelectric performance. We further fabricated a MOF-based X-ray detection device to demonstrate how the new architecture bolsters the optoelectronic efficiency, which outperforms the properties of parent semiconductive MOFs, with more than 60 times and 40 times enhancement of the photocurrent on-off ratio and detection sensitivity, respectively. With judiciously optimized exciton behaviors, the detection device exhibits a high sensitivity of 51.9 μC Gyair-1 cm-2 and records a charge carrier mobility-lifetime product of 1.12 × 10-3 cm2 V-1 among MOF-based X-ray detectors, which are competitive with values for commercially available detectors. These findings demonstrate a rational synthetic approach to designing exciton arrangements to improve the optoelectronic efficiency of semiconductive MOFs.

Self‐assembly into Polymorphic 2D Nanosheets with Crystallization‐Induced Emission Enhancement

Comprehensive SummaryOrganic micro/nanocrystals with polymorphic tailorability and perfect platform to modulate decode the complex relationship among molecular structures, supramolecular/noncovalent interactions, aggregates and exciton behaviors. Herein, we precisely tune the polymorphic two‐dimensional organic nanosheets (2DONs) of cyano‐spirocyclic aromatic hydrocarbon (2'‐CN‐SFX), where the crystal growth processes are dominated by precursor concentration. The low concentration (ca. 2 mmol/L) affords the rhombus‐like nanosheets with monoclinic P21/c space group, in which molecules adopt antiparallel and slipped stacking mode. In contrast, the high concentration (ca. 4 mmol/L) favors herringbone‐like molecular packing pattern and then generates shuttle‐like nanosheets with monoclinic C2/c space group. Both above polymorphic nanosheets exhibit unprecedented crystallization‐induced emission enhancement (CIEE) feature that increases the photoluminescence quantum yields (PLQYs) from 16% in solution, 21% in amorphous film to 37%—39% in crystalline states. This result will offer promising opportunities for nanophotonics and organic intelligent semiconductors.

Intrinsically Viscoelastic Supramolecular Conjugated Polymer toward Suppressing Coffee-Ring Effect

Intrinsically Viscoelastic Supramolecular Conjugated Polymer toward Suppressing Coffee-Ring Effect

Investigating the dielectric properties and exciton diffusion in C70 derivatives.

In recent years, the dielectric constant (εr) of organic semiconductors (OSCs) has been of interest in the organic photovoltaic (OPV) community due to its potential influence on the exciton binding energy. Despite progress in the design of high εr OSCs and the accurate measurement of the εr, the effects of the synthetic strategies on specific (opto)electronic properties of the OSCs remain uncertain. In this contribution, the effects of εr on the optical properties of five new C70 derivatives and [70]PCBM are investigated. Together with [70]PCBM, the derivatives have a range of εr values that depend on the polarity and length of the side chains. The properties of the singlet excitons are investigated in detail with steady-state and time-resolved spectroscopy and the exciton diffusion length is measured. All six derivatives show similar photophysical properties in the neat films. However, large differences in the crystallinity of the fullerene films influence the exciton dynamics in blend films. This work shows that design principles for OSCs with a higher εr can have a very different influence on the performance of traditional BHJ devices and in neat films and it is important to consider the neat film properties when investigating the optoelectronic properties of new materials for OPV.

Cross-Scale Synthesis of Organic High-k Semiconductors Based on Spiro-Gridized Nanopolymers.

High dielectric constants in organic semiconductors have been identified as a central challenge for the improvement in not only piezoelectric, pyroelectric, and ferroelectric effects but also photoelectric conversion efficiency in OPVs, carrier mobility in OFETs, and charge density in charge-trapping memories. Herein, we report an ultralong persistence length (lp ≈ 41 nm) effect of spiro-fused organic nanopolymers on dielectric properties, together with excitonic and charge carrier behaviors. The state-of-the-art nanopolymers, namely, nanopolyspirogrids (NPSGs), are synthesized via the simple cross-scale Friedel-Crafts polygridization of A2B2-type nanomonomers. The high dielectric constant (k = 8.43) of NPSG is firstly achieved by locking spiro-polygridization effect that results in the enhancement of dipole polarization. When doping into a polystyrene-based dielectric layer, such a high-k feature of NPSG increases the field-effect carrier mobility from 0.20 to 0.90 cm2 V−1 s−1 in pentacene OFET devices. Meanwhile, amorphous NPSG film exhibits an ultralow energy disorder (<50 meV) for an excellent zero-field hole mobility of 3.94 × 10−3 cm2 V−1 s−1, surpassing most of the amorphous π-conjugated polymers. Organic nanopolymers with high dielectric constants open a new way to break through the bottleneck of efficiency and multifunctionality in the blueprint of the fourth-generation semiconductors.

Out-of-plane dipole-modulated photogenerated carrier separation and recombination at Janus-MoSSe/MoS2 van der Waals heterostructure interfaces: an ab initio time-domain study.

Out-of-plane mirror symmetry-breaking provides a powerful tool for engineering the electronic properties and the exciton behavior of two-dimensional materials. Here, by combining time-domain density functional theory with nonadiabatic dynamics, we investigate the underlying mechanism of how the vertical dipole moment modulates the photoexcited carrier transport and the electron-hole recombination dynamics in polar Janus MoSSe/MoS2 stacked heterostructures. It is shown that the stronger nonadiabatic coupling, interlayer-state delocalization and the built-in electric field caused by charge redistribution facilitate a more rapid photocarrier separation across the interface in the S/S stacked bilayer compared with the S/Se bilayer, explaining the experimentally observed stronger photoluminescence quenching effect in the S/S heterostructure. We also found that the photocarrier recombination of the heterostructure with the S/Se interface has a timescale up to nanoseconds, which is ∼4 times longer than that of the S/S bilayer. Such a prolonged recombination time originates from the dipole-weakened nonadiabatic coupling between occupied and unoccupied states instead of quantum coherence and the band gap effect. Overall, Janus MoSSe/MoS2 heterostructures exhibit superior photocatalytic activity, reflecting the ultrafast photocarrier separation triggered by the built-in electric field, suppressed carrier recombination, high solar-to-hydrogen conversion efficiency and the strong absorption coefficient expanding from visible-light to near-infrared-light. The above atomistic and time-domain findings reveal the intrinsic dipole as an effective freedom to regulate the nonadiabatic photocarrier dynamics in Janus-based 2D heterostructures for efficient energy harvesting and optoelectronic applications.

Strong coupling between monolayer quantum emitter WS2 and degenerate/non-degenerate surface lattice resonances.

Strong light-matter coupling manifested by Rabi splitting has drawn considerable interest owing to its fundamental significance for impressive interaction enhancement in the fields of ultrafast active plasmonic devices and quantum information. In this paper, we investigate the coherent optical properties of a plasmonic system consisting of periodic metal nanoparticle arrays covered by a WS2 thin film of atomic layer thickness. The coupling factor, energy splitting, and temporal dynamics of this coherent coupling phenomenon are quantitatively revealed by finite-difference time-domain (FDTD) simulation and a full quantum mechanical model proves that the exciton behavior of the fermionic quantum emitter WS2 is carefully modulated by bosonic surface lattice resonances. This work may pave the way for coherent modulation of polariton and plasmon devices and can potentially open up diverse exciting possibilities like nanoscale light sources, single-photon emitters, and all-optical transistors.

Multidimensional Tungsten Oxides for Efficient Solar Energy Conversion

Dimensionalities of materials have more profound connotations than barely the external morphologies. The mechanical, optical, electrical, and magnetic properties of a certain material can be significantly influenced by the spatial degrees of freedom of interactions/excitons. Herein, how multidimensionalities affect their physicochemical properties from the perspective of photoactive tungsten oxide (WO3) materials, which further influence their performances in solar energy conversion applications are looked at. Benefiting from the fruitful research efforts on WO3‐based photocatalysts and photoelectrodes, the key procedures in photoenergy conversion including light absorption, charge separation, and surface reaction, are discussed to elucidate the unique features, advantages, and challenges of the single‐dimensional WO3 heterostructures. Then the optoelectronic properties and performance of mixed‐dimensional WO3 heterostructures are investigated to show the synergistic effect between the specifically combined dimensionalities. The dynamic behaviors of excitons or charge carriers within the mixed‐dimensional structures and at their interfaces are inspected, with several photophysical or chemical phenomena noted for designing other photoactive metal oxide semiconductors. A bumper harvest of WO3 photovoltaics, photocatalysts, and photoelectrodes beyond dimensionality is on the way, hence, it is an opportune chance to evaluate the progress and foresee future directions in this fast‐moving research field.

Multiwavelength magnetic coding of helical luminescence in ferromagnetic 2D layered CrI3

SummaryTwo-dimensional (2D) van der Waals (vdW) ferromagnets have opened new avenues for manipulating spin at the limits of single or few atomic layers, and for creating unique magneto-exciton devices through the coupling of ferromagnetic (FM) orders and excitons. However, 2D vdW ferromagnets explored so far have rarely possessed exciton behaviors; to date, FM CrI3 have been revealed to show ligand-field photoluminescence correlated with FM ordering, but typically with a broad emission peak. Here, we report a straightforward approach to realize strong coupling of narrow helical emission and FM orders in CrI3 through microsphere cavity. The resonant whispering-gallery modes (WGM) of SiO2 microspheres cause strong oscillation helical emissions with a full width at half-maximum (FWHM) of ∼5 nm under continuous wave excitation. Reversible magnetic coding of helical luminescence is realized in the range of 950–1100 nm. This work enables numerous opportunities for creating magnetic encoding lasing for photonic integrated chips.

Evolution of the Nature of Excitons and Electronic Couplings in Hybrid 2D Perovskites as a Function of Organic Cation π‐Conjugation

Abstract2D perovskites have attracted much attention, due to their organic–inorganic hybrid nature and layered configuration. Multi‐quantum‐well structures then generally form since the inorganic frameworks and organic spacers have typically well‐separated frontier energy levels. Here, it is focused on the opposite picture where wavefunction hybridization occurs between the frontier orbitals of the two components. Such a hybridization emerges upon tuning the strength of electronic coupling between the inorganic and organic layers. A series of model diammonium organic spacers is theoretically designed with varying extent of π‐conjugation along their backbones. Wavefunction hybridization is realized in a Dion‐Jacobson 2D perovskite combining PbI4 inorganic layers with anthracene‐bis(ethan‐1‐ammonium) organic spacers. An analysis of the electronic band structures points to electronic couplings as high as 30–40 meV between the organic and inorganic components. Such couplings can lead to the formation of interfacial hybrid excitons or to the appearance of Dexter‐type energy transfer conducive to phosphorescence in the organic layers. Overall, the results highlight that a variety of excitonic behaviors could be observed by tuning the degree of conjugation of the organic cations and the structural proximity and electronic couplings between the organic and inorganic constituents.

Quantitative Correlation of Triplet Exciton Management in Host with the Device Lifetime of Blue Phosphorescent Organic Light‐Emitting Diodes

AbstractThe improvement of the device stability of blue phosphorescent organic light‐emitting diodes (PhOLEDs) has proven to be a challenging issue in terms of enhancing the efficiency of blue organic light‐emitting diodes in practical applications. This work comprehensively investigates the exciton dynamics of electroplex hosts and quantitatively correlates the steady‐state triplet excitons in the host with the device lifetime of state‐of‐the‐art blue PhOLEDs. The kinetic processes of electrically generated singlet excitons, triplet excitons, and polarons explored via transient electroluminescence and numerical modeling reveal that the triplet exciton density in the host is governed by reverse intersystem crossing and the triplet–triplet annihilation rates. A degradation modeling that takes into account the simultaneous material degradation due to the triplet excitons in the host and the dopant is newly established. The results indicate that the suppressed host degradation due to the reduction (1.5×) in host triplet excitons leads to enhanced operational stability. The characterization method and the numerical modeling in this work facilitate the determination of the exciton and polaron behavior of the host and allow for predicting the host‐dependent device lifetime of PhOLEDs for specific host materials.

Magnetic Effect of Dopants on Bright and Dark Excitons in Strongly Confined Mn-Doped CsPbI3 Quantum Dots

We investigated the magnetic effect of Mn2+ ions on an exciton of Mn-doped CsPbI3 quantum dots (QDs), where we looked for the signatures of an exciton magnetic polaron known to produce a large effective magnetic field in Mn-doped CdSe QDs. In contrast to Mn-doped CdSe QDs that can produce ∼100 T of magnetic field upon photoexcitation, manifested as a large change in the energy and relaxation dynamics of a bright exciton, Mn-doped CsPbI3 QDs exhibited little influence of a magnetic dopant on the behavior of a bright exciton. However, a μs-lived dark exciton in CsPbI3 QDs showed 40% faster decay in the presence of Mn2+, equivalent to the effect of ∼3 T of an external magnetic field. While further study is necessary to fully understand the origin of the large difference in the magneto-optic property of an exciton in two systems, we consider that the difference in antiferromagnetic coupling of the dopants is an important contributing factor.

Intrinsically Stretchable and Stable Ultra‐Deep‐Blue Fluorene‐Based Polymer with a High Emission Efficiency of ≈90% for Polymer Light‐Emitting Devices with a CIEy = 0.06

AbstractThe manufacture of robust ultra‐deep‐blue polymer with an intrinsic stretchability is an extreme challenge toward fabricating flexible light‐emitting optoelectronic devices. As a key precondition for the realization of the intrinsically stretchable behavior, easily interchain slippages also cause the formation of intermolecular excited states, which is negative to the color purity and the emission efficiency of blue‐emitting film and devices. Herein, a series of π‐interrupted‐conjugated alternating polymers with a robust ultra‐deep‐blue emission and intrinsically stretchable properties are demonstrated. Interestingly, the model polymers (P4, P6, and P8) present stable ultra‐deep‐blue emission with an extremely high efficiency of ≈90% and a Commission internationale de l'éclairage (CIE) coordinates of (0.15, 0.06) in solid‐state, attributed to the excellent singlet excitonic behavior without any obvious polaron formation. More importantly, significantly different to rigid π‐conjugated polymers, the model P8 film displays an efficient deep blue emission even under a tensile rate of ≈15%, associated with the densely interchain entanglement and interpenetration. Finally, preliminary P8‐based polymer light‐emitting diodes also show a stable ultra‐deep‐blue emission with a CIE of (0.15, 0.06). All results above confirm the effectiveness of the π‐interrupted‐conjugated strategy to construct a robust deep‐blue polymer with an excellent intrinsic stretchability, which is a great potential for applying in flexible light‐emitting devices.