There is a widespread conviction,fuelled by simple effective mass theoretical modelling,that the enhanced optical properties of quantum shells(i.e.,CdS/CdSe/CdS core/shell in a shell structures)derive from a combination of relaxed carrier confinement,decreased overlap between electron and hole wave functions,reduced Coulomb interaction between photoinduced charge carriers and exciton-exciton repulsion,achieved in these nanoscopic spherical quantum wells(SQWs)by shape engineering.Confirming the origins of such properties by means of a more sophisticated theoretical framework is however important to ensure that future efforts at further improvement do not pursue the wrong strategy.Using the atomistic semiempirical pseudopotential method,we show that most of the assumptions behind such effects are incorrect,and that the origins of the peculiar optical properties of quantum shells are to be found elsewhere.
Colloidal semiconductor quantum dots(QDs)exhibit broadband light absorption,continuously tunable narrowband emission,and high photoluminescence quantum yields.As such,they represent promising materials for use in light-emitting diodes,solar cells,detectors,and lasers.Single-QD spectroscopy can remove the ensemble averaging to reveal the diverse optical properties and exciton dynamics of QD materials at the single-particle level.The results of relevant research can serve as guidelines for materials science community in tailoring the synthesis of QDs to develop novel applications.This paper reviews recent progress in exciton dynamics revealed by single-QD spectroscopy,focusing on the exciton and multi-exciton dynamics of single colloidal CdSe-based QDs and perovskite QDs.Finally,potential future directions for single-QD spectroscopy and exciton dynamics are briefly considered.
Perovskite nanocrystal(PNC)solids are promising materials for optoelectronic applications.Recent studies have shown that exciton diffusion in PNC solids occurs via alternate exciton hopping(EH)and photon recycling(PR).The energy disorder induced by the size distribution is a common factor in PNC solids,and the impact of this energy disorder on the exciton diffusion remains unclear.Here,we investigated the exciton diffusion in CsPbBr3 NC solids with a Gaussian size distribution of 11.2±6.8 nm via steady and time-resolved photoluminescence(PL)spectroscopy with multiple detection bands in transmission mode.Our results indicated that exciton diffusion was controlled by a downhill transfer among the different energy sites through the disordered energy landscape,as confirmed by the accompanying low-temperature PL analysis.A detailed examination revealed that the acceptor distribution in tandem with the reabsorption coefficient determined the contribution of EH and PR to exciton transfer between different energy sites.Consequently,the exciton diffusion mechanism varied in PNC solids of different thicknesses:in a thin solid with a thickness of several hundred nanometers,the exciton transfer was dominated by efficient EH and PR from the high-energy sites to the lower-energy sites;in a few-micrometer-thick solid,transfer from the medium-energy sites toward the lower-energy sites also became prominent and occurred mainly through PR.These findings enhance the understanding of the vital role that the acceptor distribution plays in the exciton diffusion process in PNC solids,providing important insights for optoelectronic applications based on PNC solids.Our work also exploits the use of commonly available tools for in-depth exciton diffusion studies,which reveals the interior diffusion information that is usually hidden in surface sensitive PL imaging methods.
In our study of super quantum discord between two excitonic qubits inside a coupled semiconductor quantum dots system,our primary focus is to uncover the impact of weak measurement on its quantum characteristics.To achieve this,we analyze how varying the measurement strength x,affects this super quantum correlation in the presence of thermal effects.Additionally,we assess the effect of this variation on the system's evolution against its associated quantum parameters;external electric fields,exciton-exciton dipole interaction energy and F?rster interaction.Our findings indicate that adjusting x to smaller values effectively enhances the super quantum correlation,making weak measurements act as a catalyst.This adjustment ensures its robustness against thermal effects while preserving the non-classical attributes of the system.Furthermore,our study unveils that the effect of weak measurements on this latter surpasses the quantum effects associated with the system.Indeed,manipulating the parameter x allows the weak measurement to function as a versatile tool for modulating quantum characteristics and controlling exciton-exciton interactions within the coupled semiconductor quantum dots system.
Over the past few decades,significant progress has been made in thin-film optoelectronic devices based on transition metal dichalcogenides.However,the exciton states'sensitivity to the environment presents challenges for device applications.This study reports the evolution of photoinduced exciton states in monolayer tungsten disulfide in a low-pressure environment to help elucidate the physical mechanism of the transition between neutral and charged excitons.At 222 mTorr,the transition rate between excitons comprises two components:0.09 s–1 and 1.68 s–1.Based on this phenomenon,we developed a pressure-tuning method that allows for a tuning range of approximately 40%of exciton weight.Our study demonstrates that the intensity of neutral exciton emission from monolayer tungsten disulfide follows a power-law distribution in relation to pressure,indicating a highly sensitive pressure dependence.We provide a nondestructive and highly sensitive method for exciton conversion through in situ optical manipulation.This highlights the potential development of monolayer tungsten disulfide for pressure sensors and explains the impact of environmental factors on the product quality in photovoltaic devices.In addition,it demonstrates the promising future of monolayer transition metal dichalcogenides in applications such as photovoltaic devices and miniature biochemical sensors.
Stacking single layers of atoms on top of each other provides a fundamental way to achieve novel material systems and engineer their physical properties,which offers opportunities for exploring fundamental physics and realizing next-generation optoelectronic devices.Among the two-dimensional(2D)-stacked systems,transition metal dichalcogenide(TMDC)heterostructures are particularly attractive because they host tightly-bonded interlayer excitons which possess various novel and appealing properties.These interlayer excitons have drawn significant research attention and hold high potential for the application in unique optoelectronic devices,such as polarization-and wavelength-tunable single photon emitters,valley Hall transistors,and possible high-temperature superconductors.The development of these devices requires a comprehensive understanding of the fundamental properties of these interlayer excitons and the impact of electric fields on their behaviors.In this review,we summarize the recent advances on the understanding of interlayer exciton dynamics under electric fields in TMDC heterostructures.We put emphasis on the electrical modulation of interlayer excitons’emission,the valley Hall transport of charge carriers after the separation of interlayer excitons by an electric field,and the correlation physics of interlayer excitons and charges under electrical doping and tuning.Challenges and perspectives are finally discussed for the application of TMDC heterostructures in future optoelectronics.
By using one-dimensional tight-binding model modified to include electron-electric field interaction and electron-electron interaction,we theoretically explore the polarization process of exciton and biexciton in cis-polyacetylene.The dynamical simulation is performed by adopting the non-adiabatic evolution approach.The results show that under the effect of moderate electric field,when the strength of electron-electron interaction is weak,the singlet exciton is stable but its polarization presents obvious oscillation.With the enhancement of interaction,it is dissociated into polaron pairs,the spin-flip of which can be observed through modulating the interaction strength.For the triplet exciton,the strong electron-electron interaction restrains its normal polarization,but it is still stable.In the case of biexciton,the strong electron-electron interaction not only dissociate it,but also flip its charge distribution.The yield of the possible states formed after the dissociation of exciton and biexciton is also calculated.
Charge generation,a critical process in the operation of organic solar cell(OSC),requires thorough investigation in an ultrafast perspective.This work demonstrates that the utilization of alloy model for the non-fullerene acceptor(NFA)component can regulate the crystallization properties of active layer films,which in turn affects exciton diffusion and hole transfer(HT),ultimately influencing the charge generation process.By incorporating BTP-eC7 as a third component,without expanding absorption range or changing molecular energy levels but regulating the ultrafast exciton diffusion and HT processes,the power conversion efficiency(PCE)of the optimized PM6:BTP-eC9:BTP-eC7 based ternary OSC is improved from 17.30%to 17.83%,primarily due to the enhancement of short-circuit current density(JSC).Additionally,the introduction of BTP-eC7 also reduces the trap state density in the photoactive layer which helps to reduce the loss of JSC.This study introduces a novel approach for employing ternary alloy models by incorporating dual acceptors with similar structures,and elucidates the underlying mechanism of charge generation and JSC in ternary OSCs.
Carbon nitride,a typical low-dimensional conjugated polymer photocatalyst,features a high exciton binding energy due to the weak dielectric screening and the strong Coulombic attraction of photogenerated electrons and holes.The reduction of the exciton binding energy of carbon nitride to promote the conversion from excitons into free carriers is the first priority for the improvement of charge-transfer-dependent photocatalytic reaction activity.In this paper,by introducing a variety of polar metal cations to carbon nitride,it is demonstrated that the charge distribution of the heptazine ring can be improved by ion polarization,which effectively promotes the dissociation of excitons into electrons and holes.The sodium ion shows the best modification effect,which enhances the rate of both photocatalytic hydrogen and hydrogen peroxide production by about 50%.Characterization shows that the introduction of strongly polar metal cations contributes to the reduction of the exciton dissociation energy of carbon nitride.This study provides a new perspective and a convenient method for the exciton modulation engineering of low-dimensional photocatalysts.
