Multiexciton Recombination Research Articles

Ultrafast and Radiation-Hard Lead Halide Perovskite Nanocomposite Scintillators.

The use of scintillators for the detection of ionizing radiation is a critical aspect in many fields, including medicine, nuclear monitoring, and homeland security. Recently, lead halide perovskite nanocrystals (LHP-NCs) have emerged as promising scintillator materials. However, the difficulty of affordably upscaling synthesis to the multigram level and embedding NCs in optical-grade nanocomposites without compromising their optical properties still limits their widespread use. In addition, fundamental aspects of the scintillation mechanisms are not fully understood, leaving the scientific community without suitable fabrication protocols and rational guidelines for the full exploitation of their potential. In this work, we realize large polyacrylate nanocomposite scintillators based on CsPbBr3 NCs, which are synthesized via a novel room temperature, low waste turbo-emulsification approach, followed by their in situ transformation during the mass polymerization process. The interaction between NCs and polymer chains strengthens the scintillator structure, homogenizes the particle size distribution and passivates NC defects, resulting in nanocomposite prototypes with luminescence efficiency >90%, exceptional radiation hardness, 4800 ph/MeV scintillation yield even at low NC loading, and ultrafast response time, with over 30% of scintillation occurring in the first 80 ps, promising for fast-time applications in precision medicine and high-energy physics. Ultrafast radioluminescence and optical spectroscopy experiments using pulsed synchrotron light further disambiguate the origin of the scintillation kinetics as the result of charged-exciton and multiexciton recombination formed under ionizing excitation. This highlights the role of nonradiative Auger decay, whose potential impact on fast timing applications we anticipate via a kinetic model.

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.

Tuning energy transfer efficiency in quantum dots mixture by controling donor/acceptor ratio* *Project supported by the National Natural Science Foundation of China (Grant Nos. 52025023, 51991342, 52021006, 11888101, and 61922028), the Key R&D Program of GuangdongProvince, China (Grant Nos. 2020B010189001, 2019B010931001, and 2018B030327001), the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB33000000), Beijing Natural Science Foundation,

Improving the emission performance of colloidal quantum dots (QDs) is of paramount importance for their applications on light-emitting diodes (LEDs), displays and lasers. A highly promising approach is to tune the carrier recombination channels and lifetime by exploiting the energy transfer process. However, to achieve this precise emission optimization, quantitative modulation on energy transfer efficiency is highly desirable but still challenging. Here, we demonstrate a convenient approach to realize tunable energy transfer efficiency by forming QDs mixture with controllable donor/acceptor (D/A) ratio. With the mixing ratio ranging from 16/1 to 1/16, the energy transfer efficiency could be effectively tuned from near zero to ∼ 70%. For the high mixing ratio of 16/1, acceptors obtain adequate energy supplied by closely surrounding donors, leading to ∼ 2.4-fold PL enhancement. While for the low mixing ratio, the ultrafast and efficient energy extraction process directly suppresses the multi-exciton and Auger recombination in the donor, bringing about a higher threshold. The facile modulation of emission performance by controllably designed mixing ratio and quantitatively tunable energy transfer efficiency will facilitate QD-based optoelectronic and photovoltaic applications.

Temporal shaping of single photons by engineering exciton dynamics in a single quantum dot

The majority of photonic quantum information technologies rely on single photons that have high purity and indistinguishability. Although solid-state quantum emitters can serve such single photons on demand, their asymmetric temporal and spatial mode profiles limit the optimal efficiency and fidelity of quantum interaction. Here, we demonstrate single-photon pulses at a telecom wavelength with a Gaussian-like temporal mode profile from a cavity-coupled single quantum dot. Engineering the exciton dynamics via multi-exciton cascade recombination and cavity detuning enables us to modify the rise and decay dynamics of single excitons. Furthermore, the cascade recombination process temporally retards the single-exciton emission from the background emission, leading to possible purification of single photons at high excitation power. In addition, coupling quantum dots into a low Q cavity mode leads to a Gaussian-like spatial mode profile, which brings a high collection efficiency. This approach paves the way for producing single photons with an optimized temporal and spatial waveform.

Enhanced Multiexciton Emission Property in Gradient Alloy Core/Shell CdZnSeS/ZnS Quantum Dots: Balance between Surface Passivation and Strain-Induced Lattice Defect

The effect of surface/interface defects on multiexciton recombination is studied with CdZnSeS/ZnS alloy core/shell quantum dots (QDs) with different shell thicknesses. Through pump-power-related ti...

Auger Recombination and Multiple Exciton Generation in Colloidal Two-Dimensional Perovskite Nanoplatelets: Implications for Light-Emitting Devices

Improving the understanding of multiple exciton interactions and dynamics in semiconductor nanostructures is mandatory for their successful use as photoactive materials in light convertors such as electroluminescent diodes, lasers, or single-photon sources. Here high-fluence and high-energy excitation effects are investigated in strongly confined two-dimensional (2D) lead iodide perovskite nanoplatelets (NPLs) using time-resolved photoluminescence and femtosecond transient absorption spectroscopy. Nonradiative Auger recombination (AR) is the dominant pathway for multiexciton recombination. Its dynamics are found to be subquadratic with the exciton density. Indeed, because of the limited exciton wave-function delocalization length, AR is limited by exciton diffusion in the 2D plane at moderate excitation fluence and takes place in several hundreds of picoseconds, with typical recombination rates on the order of 10–2 cm2/s. At high excitation fluence leading to an average interexciton distance comparable with the exciton delocalization length, the measured “intrinsic” AR time is faster than 10 ps and independent of the NPL composition. The strong dependence of the AR rate on the interexciton distance allows us to identify the recombination resulting from multiple exciton generation, involving the reaction of “geminate biexcitons”, upon excitation at low fluence with high-energy photons.

Ultrafast multiexciton Auger recombination of CdSeS

Multiexciton generation is a process where multiple excitons are generated by absorbing single photons. Efficient multiexciton generation in quantum dots may be a revolutionary discovery, because it provides a new method to improve the solar-to-electric power conversion efficiency in quantum dots-based solar cells and to design novel quantum dots-based multielectron or hole photocatalysts. However, the mechanism of ultrafast multiexciton generation and recombination remain unclear. In this paper, alloy-structured quantum dots, CdSeS, are prepared by the hot injection method. The generation and recombination mechanism of charge carriers in quantum dots samples are discussed in detail. The bivalent band structure of alloy-structured quantum dots is determined by ultraviolet-visible absorption spectra. It is found that the 1S&lt;sub&gt;3/2&lt;/sub&gt;(h)-1S(e) (or 1S), 2S&lt;sub&gt;3/2&lt;/sub&gt;(h)-1S(e) (or 2S) and 1P&lt;sub&gt;3/2&lt;/sub&gt;(h)-1P(e) (or 1P) exciton absorption bands of these quantum dots are at 510 nm, 468 nm and 430 nm, respectively. Femtosecond transient absorption spectroscopy and nanosecond time-resolved photoluminescence spectroscopy are used to investigate the ultrafast exciton generation and recombination dynamics in the alloy-structured quantum dots. By fitting the transient kinetics of 1S exciton bleach, an average biexciton decay time is obtained to be about 80 ps, which is almost twice the decay time of traditional quantum dots (less than 50 ps). Combined with the recently developed ultrafast interface charge separation technology that can extract multiple excitons before their annihilation, it will have a promising application prospect. Moreover, there is a hole relaxation on a the time scale of 5-6 ps via a phonon coupling pathway to lower-energy hole states in addition to the above-described ultrafast exciton-exciton annihilation process in 2S and 1P excitons. Furthermore, by nanosecond time-resolved photoluminescence spectroscopy, it can be concluded that the charge separated state is long-lived (200 ns). Our findings provide a valuable insight into the understanding of ultrafast multiexciton generation and recombination in quantum dots. These results are helpful to understand the intrinsic photo-physics of multiexciton generation in quantum dots, to implement the photovoltaic and optoelectronic applications, and to ascertain the exciton relaxation dynamics of quantum dots.

Resonant Energy Transfer can Trigger Multiexciton Recombination in Dense Quantum Dot Ensembles.

Core/shell quantum dots/quantum rods are nanocrystals with typical application scenarios as ensembles. Resonance energy transfer is a possible process between adjacent nanocrystals. Highly excited nanocrystals can also relax energy by multiexciton recombination, competing against the energy transfer. The two processes have different dependencies and can be convolved, resulting in collective properties different from the superposition of the individual nanocrystals. A platform to study the interplay of energy transfer and multiexciton recombination is presented. CdSe/CdS quantum dot/quantum rods encapsulated in amphiphilic micelles with an interparticle distance control by spacer ligands are used for time-resolved photoluminescence and transient absorption experiments. At exciton populations around one, the ensemble starts to be in a state where energy transfer can trigger multiexciton Auger recombination, altering the collective dynamics.

Continuous-wave infrared optical gain and amplified spontaneous emission at ultralow threshold by colloidal HgTe quantum dots.

Colloidal quantum dots (QDs) raise more and more interest as solution-processable and tunable optical gain materials. However, especially for infrared active QDs, optical gain remains inefficient. Since stimulated emission involves multifold degenerate band-edge states, population inversion can be attained only at high pump power and must compete with efficient multi-exciton recombination. Here, we show that mercury telluride (HgTe) QDs exhibit size-tunable stimulated emission throughout the near-infrared telecom window at thresholds unmatched by any QD studied before. We attribute this unique behaviour to surface-localized states in the bandgap that turn HgTe QDs into 4-level systems. The resulting long-lived population inversion induces amplified spontaneous emission under continuous-wave optical pumping at power levels compatible with solar irradiation and direct current electrical pumping. These results introduce an alternative approach for low-threshold QD-based gain media based on intentional trap states that paves the way for solution-processed infrared QD lasers and amplifiers.

Spatiotemporal dynamics of excitons in monolayer and bulk WS2.

Spatiotemporal dynamics of excitons in monolayer and bulk WS2 at room temperature is studied by transient absorption microscopy in the reflection geometry. Excitons are formed from photocarriers injected by a tightly focused 390 nm pump pulse, and monitored by detecting different reflection of a time-delayed and spatially scanned 620 nm probe pulse. We obtain exciton lifetimes of 22 ± 1 and 110 ± 10 ps in monolayer and bulk WS2, respectively. Both lifetimes are independent of the exciton density, showing the absence of multi-exciton recombination processes. Exciton diffusion coefficients of 60 ± 20 and 3.5 ± 0.5 cm(2) s(-1) are obtained in monolayer and bulk samples, respectively. These results provide a foundation for understanding excitons in this new material and its optoelectronic applications.

Sample-averaged biexciton quantum yield measured by solution-phase photon correlation.

The brightness of nanoscale optical materials such as semiconductor nanocrystals is currently limited in high excitation flux applications by inefficient multiexciton fluorescence. We have devised a solution-phase photon correlation measurement that can conveniently and reliably measure the average biexciton-to-exciton quantum yield ratio of an entire sample without user selection bias. This technique can be used to investigate the multiexciton recombination dynamics of a broad scope of synthetically underdeveloped materials, including those with low exciton quantum yields and poor fluorescence stability. Here, we have applied this method to measure weak biexciton fluorescence in samples of visible-emitting InP/ZnS and InAs/ZnS core/shell nanocrystals, and to demonstrate that a rapid CdS shell growth procedure can markedly increase the biexciton fluorescence of CdSe nanocrystals.

Small Bright Charged Colloidal Quantum Dots

Using electrochemical charge injection, the fluorescence lifetimes of negatively charged core/shell CdTe/CdSe QDs are measured as a function of core size and shell thickness. It is found that the ensemble negative trion lifetimes reach a maximum (∼4.5 ns) for an intermediate shell thickness. This leads to the smallest particles (∼4.5 nm) with the brightest trion to date. Single dot measurements show that the negative charge suppresses blinking and that the trion can be as bright as the exciton at room temperature. In contrast, the biexciton lifetimes remain short and exhibit only a monotonous increase with shell thickness, showing no correlation with the negative trion decays. The suppression of the Auger process in small negatively charged CdTe/CdSe quantum dots is unprecedented and a significant departure from prior results with ultrathick CdSe/CdS core/shell or dot-in-rod structures. The proposed reason for the optimum shell thickness is that the electron-hole overlap is restricted to the CdTe core while the electron is tuned to have zero kinetic energy in the core for that optimum shell thickness. The different trend of the biexciton lifetime is not explained but tentatively attributed to shorter-lived positive trions at smaller sizes. These results improve our understanding of multiexciton recombination in colloidal quantum dots and may lead to the design of bright charged QDs for more efficient light-emitting devices.

Observation of Biexcitons in Nanocrystal Solids in the Presence of Photocharging

In nanocrystal quantum dots (NQDs), generating multiexcitons offers an enabling tool for enhancing NQD-based devices. However, the photocharging effect makes understanding multiexciton kinetics in NQD solids fundamentally challenging, which is critically important for solid-state devices. To date, this lack of understanding and the spectral-temporal aspects of the multiexciton recombination still remain unresolved in solid NQD ensembles, which is mainly due to the confusion with recombination of carriers in charged NQDs. In this work, we reveal the spectral-temporal behavior of biexcitons (BXs) in the presence of photocharging using near-unity quantum yield CdSe/CdS NQDs exhibiting substantial suppression of Auger recombination. Here, recombinations of biexcitons and single excitons (Xs) are successfully resolved in the presence of trions in the ensemble measurements of time-correlated single-photon counting at variable excitation intensities and varying emission wavelengths. The spectral behaviors of BXs and Xs are obtained for three NQD samples with different core sizes, revealing the strength tunability of the X-X interaction energy in these NQDs. The extraction of spectrally resolved X, BX, and trion kinetics, which are otherwise spectrally unresolved, is enabled by our approach introducing integrated time-resolved fluorescence. The results are further experimentally verified by cross-checking excitation intensity and exposure time dependencies as well as the temporal evolutions of the photoluminescence spectra, all of which prove to be consistent. The BX and X energies are also confirmed by theoretical calculations. These findings fill an important gap in understanding the spectral dynamics of multiexcitons in such NQD solids under the influence of photocharging effects, paving the way to engineering of multiexciton kinetics in nanocrystal optoelectronics, including NQD-based lasing, photovoltaics, and photodetection.

Size-Dependent Trap-Assisted Auger Recombination in Semiconductor Nanocrystals

The acceleration of Auger-type multicarrier recombination in semiconductor nanocrystals impedes the development of many quantum-dot photonics, solar-cell, lighting, and lasing technologies. To date, only multiexciton and charged-exciton Auger recombination channels are known to show strong size dependence in nanocrystals. Here, we report the first observation of strongly accelerated "trap-assisted" Auger recombination rates in semiconductor nanocrystals. Trap-assisted Auger recombination in ZnO nanocrystals, involving the recombination of conduction-band electrons with deeply trapped holes via nonradiative energy transfer to extra conduction-band electrons, has been probed using time-resolved photoluminescence and transient absorption spectroscopies. We demonstrate that this trap-assisted Auger recombination accelerates dramatically with decreasing nanocrystal size, having recombination times of >1 ns in the largest nanocrystals but only ~80 ps in the smallest. These trap-assisted Auger recombination rates are shown to scale with inverse nanocrystal radius squared (1/τ(Auger) ~ R(-2)). Because surface carrier traps are ubiquitous in colloidal semiconductor nanocrystals, such fast trap-assisted Auger recombination is likely more prevalent in semiconductor nanocrystal photophysics than previously recognized.

Hyper-Branched CdTe Nanostructures Based on the Self-Assembling of Quantum Dots and Their Optical Properties

As the priority of interconnects and active components in nanoscale optical and electronic devices, three-dimensional hyper-branched nanostructures came into focus of research. Recently, a novel crystallization route, named as "nonclassical crystallization," has been reported for three-dimensional nanostructuring. In this process, Quantum dots are used as building blocks for the construction of the whole hyper-branched structures instead of ions or single-molecules in conventional crystallization. The specialty of these nanostructures is the inheritability of pristine quantum dots' physical integrity because of their polycrystalline structures, such as quantum confinement effect and thus the luminescence. Moreover, since a longer diffusion length could exist in polycrystalline nanostructures due to the dramatically decreased distance between pristine quantum dots, the exciton-exciton interaction would be different with well dispersed quantum dots and single crystal nanostructures. This may be a benefit for electron transport in solar cell application. Therefore, it is very necessary to investigate the exciton-exciton interaction in such kind of polycrystalline nanostructures and their optical properites for solar cell application. In this research, we report a novel CdTe hyper-branched nanostructures based on self-assembly of CdTe quantum dots. Each branch shows polycrystalline with pristine quantum dots as the building units. Both steady state and time-resolved spectroscopy were performed to investigate the properties of carrier transport. Steady state optical properties of pristine quantum dots are well inherited by formed structures. While a suppressed multi-exciton recombination rate was observed. This result supports the percolation of carriers through the branches' network.

Multiexcitons in Semiconductor Nanocrystals: A Platform for Optoelectronics at High Carrier Concentration

It is well-known that the shape, size, and composition of semiconductor nanocrystals give rise to a quantized manifold of electronic states, that is, excitons. In addition, these nanocrystals can support multiple excitations per particle under relatively modest conditions. Beyond a laboratory curiosity, these multiexcitons dictate a wide variety of optoelectronic properties of semiconductor nanocrystals including those from lasers, light-emitting diodes, photon sources, and possibly photovoltaics. Whereas their existence has been known for some time, observation of the structure and dynamics of multiexcitons has remained elusive due to their ultrafast lifetimes. In this Perspective, we discuss the first glimpses of the structural dynamics of multiexcitons in CdSe semiconductor nanocrystals as revealed by excitonic state-resolved femtosecond pump/probe spectroscopy. These measurements of multiexciton formation, cooling, and recombination are related to the optical gain performance of nanocrystals. In parti...

Auger process in CdS/ZnS core-shell quantum dots

Auger processes are investigated for CdS/ZnS core-shell quantum dots. Auger recombination (AR) lifetime and electron relaxation inside the core are computed. Using the effective-mass theory and by solving a three-dimension Schrödinger equation we predict the dependence of Auger relaxation on size of core-shell nanocrystals. We considered in this work different AR processes: the excited electron (EE), excited hole (EH), multiexciton AR type. Likewise, Auger multiexciton recombination rates are predicted for biexciton. Our results show that biexciton AR type is more efficient than the other AR process (excited electron (EE) and excited hole (EH)). We also found that electron Auger relaxation P → S is very efficient in core-shell nanostructures.

(Multi)exciton Dynamics and Exciton Polarizability in Colloidal InAs Quantum Dots

We report steady state and dynamic properties of (multi)excitons in InAs quantum dots using femtosecond time-resolved transient absorption and time-resolved terahertz spectroscopy. The polarizability of confined excitons in the ground state is found to be of the order of 104 Å3, increasing sharply with quantum dot radius as R3. The size-dependence of the exciton polarizability can be quantitatively reproduced by using multiband tight binding calculations. Following the generation of a single exciton with excess electronic energy, electron intraband relaxation from the 1Pe → 1Se level occurs on a time scale of 0.8 ps for particles of 2.2 nm radius, whereas hot hole cooling occurs faster than 150 fs. The intraband relaxation dynamics are consistent with an Auger process involving electron−hole energy transfer, indicating an electron−hole coupling time of 0.8 ps. Finally, when multiple excitons are generated through multiphoton absorption, multiexciton recombination dynamics occurs on 1−100 ps time scales. We illustrate the challenges associated with reliably extracting the time constants of multiexciton recombination from our data, which are helpful for elucidating the multiexciton recombination mechanism. Our results demonstrate that the exciton dynamics in InAs quantum dots are governed by the relatively low dielectric constant of InAs (resulting in strong carrier−carrier interactions) and the low energy spacing between valence levels (resulting from the relatively high value of the hole effective mass, and allowing for rapid phonon-mediated hole relaxation).

Femtosecond excitation correlation spectroscopy of single-walled carbon nanotubes: Analysis based on nonradiative multiexciton recombination processes

We studied the nonlinear time-resolved luminescence signals due to multiexciton recombination processes in single-walled carbon nanotubes (SWNTs) using femtosecond excitation correlation (FEC) spectroscopy. From theoretical analysis of the FEC signals, we found that the FEC signals in the long time range are dominated by the single exciton decay in SWNTs, where the exciton-exciton annihilation process is efficient. Our results provide a simple method to clarify the single exciton decay dynamics in low-dimensional materials.

Photoluminescence dynamics and reduced Auger recombination inSi1−xGex/Sisuperlattices under high-density photoexcitation

Optical gain and stimulated emission processes in Si nanostructures are controlled by the dynamics of high-density carriers. Here, we report photoluminescence (PL) dynamics and multiexciton recombination in ${\text{Si}}_{1\ensuremath{-}x}{\text{Ge}}_{x}/\text{Si}$ superlattices (SLs) under high-density excitation. Saturation of the PL intensity and rapid PL decay are observed as the excitation laser intensity is increased. These phenomena occur due to nonradiative Auger recombination of the electron-hole pairs. We find that the Auger process in ${\text{Si}}_{1\ensuremath{-}x}{\text{Ge}}_{x}/\text{Si}$ SLs is less pronounced than that in the ${\text{Si}}_{1\ensuremath{-}x}{\text{Ge}}_{x}/\text{Si}$ single quantum wells. Our results show that coupled nanostructures have an advantage in efficient light emission and the control of many-body carrier dynamics.