Band-edge exciton in quantum dots of semiconductors with a degenerate valence band: Dark and bright exciton states

Charge Transfer from Quantum-Confined 0D, 1D, and 2D Nanocrystals

The properties of colloidal quantum-confined semiconductor nanocrystals (NCs), including zero-dimensional (0D) quantum dots, 1D nanorods, 2D nanoplatelets, and their heterostructures, can be tuned through their size, dimensionality, and material composition. In their photovoltaic and photocatalytic applications, a key step is to generate spatially separated and long-lived electrons and holes by interfacial charge transfer. These charge transfer properties have been extensively studied recently, which is the subject of this Review. The Review starts with a summary of the electronic structure and optical properties of 0D-2D nanocrystals, followed by the advances in wave function engineering, a novel way to control the spatial distribution of electrons and holes, through their size, dimension, and composition. It discusses the dependence of NC charge transfer on various parameters and the development of the Auger-assisted charge transfer model. Recent advances in understanding multiple exciton generation, decay, and dissociation are also discussed, with an emphasis on multiple carrier transfer. Finally, the applications of nanocrystal-based systems for photocatalysis are reviewed, focusing on the photodriven charge separation and recombination processes that dictate the function and performance of these materials. The Review ends with a summary and outlook of key remaining challenges and promising future directions in the field.

Reversible Ligand Detachment from CdSe Quantum Dots Following Photoexcitation

The nanocrystal-ligand boundaries of colloidal quantum dots (QDs) mediate charge and energy transfer processes that underpin photochemical and photocatalytic transformations at their surfaces. We used time-resolved infrared spectroscopy combined with transient electronic spectroscopy to probe vibrational modes of the carboxylate anchoring groups of stearate ligands attached to cadmium selenide (CdSe) QDs that were optically excited in solid nanocrystal films. The vibrational frequencies of surface-bonded carboxylate groups revealed their interactions with surface-localized holes in the excited states of the QDs. We also observed transient and reversible photoinduced ligand detachment from CdSe nanocrystals within their excited state lifetime. By probing both surface charge distributions and ligand dynamics on QDs in their excited states, we open a pathway to explore how the nanocrystal-ligand boundary can be understood and controlled for the design of QD architectures that most effectively drive charge transfer processes in solar energy harvesting and photoredox catalysis applications.

Observation of Dark States in Two-Dimensional Electronic Spectra of Chlorosomes

Observations of low-lying dark states in several photosynthetic complexes challenge our understanding of the mechanisms behind their efficient energy transfer processes. Computational models are necessary for providing novel insights into the nature and function of dark states, especially since these are not directly accessible in spectroscopy experiments. Here, we will focus on signatures of dark-type states in chlorosomes, a light-harvesting complex from green sulfur bacteria well-known for uniting a broad absorption band with very efficient energy transfer. In agreement with experiments, our simulations of two-dimensional electronic spectra capture the ultrafast exciton transfer occurring in 100s of femtoseconds within a single chlorosome cylinder. The sub-100 fs process corresponds to relaxation within the single-excitation manifold in a single chlorosome tube, where all initially created populations in the bright exciton states are quickly transferred to dark-type exciton states. Structural inhomogeneities on the local scale cause a redistribution of the oscillator strength, leading to the emergence of these dark-type exciton states, which dominate ultrafast energy transfer. The presence of the dark-type exciton states suppresses energy loss from an isolated chlorosome via fluorescence quenching, as observed experimentally. Our results further question whether relaxation to dark-exciton states is a leading process or merely competes with transfer to the baseplate within the photosynthetic apparatus of green sulfur bacteria.

Positive Trions in InP/ZnSe/ZnS Colloidal Nanocrystals

InP-based colloidal nanocrystals are being developed as an alternative to cadmium-based materials. However, their optical properties have not been widely studied. In this paper, the fundamental magneto-optical properties of InP/ZnSe/ZnS nanocrystals are investigated at cryogenic temperatures. Ensemble measurements using two-photon excitation spectroscopy revealed the band-edge hole state to have 1Sh symmetry, resolving some controversy on this issue. Single nanocrystal microphotoluminescence measurements provided increased spectral resolution that facilitated direct detection of the lowest energy confined acoustic phonon mode at 0.9 meV, which is several times smaller than the previously reported values for similar nanocrystals. Zeeman splitting of narrow spectral lines in a magnetic field indicated a bright trion emission. A simple trion model was used to identify a positive trion charge. Furthermore, the Zeeman split spectra allowed the direct measurement of both the electron and hole g-factors, which match existing theoretical predictions.

Electrically Tunable Single Polaritonic Quantum Dot at Room Temperature

Exciton-polaritons confined in plasmonic cavities are hybridized light-matter quasiparticles, with distinct optical characteristics compared to plasmons and excitons alone. Here, we demonstrate the electric tunability of a single polaritonic quantum dot operating at room temperature in electric-field tip-enhanced strong coupling spectroscopy. For a single quantum dot in the nanoplasmonic tip cavity with variable dc local electric field, we dynamically control the Rabi frequency with the corresponding polariton emission, crossing weak to strong coupling. We model the observed behaviors based on the quantum confined Stark effect in the strong coupling regime.

A tight-binding model for illustrating exciton confinement in semiconductor nanocrystals

The Brus equation describes the relation between the lowest energy of an electron-hole pair and the size of a semiconductor crystallite. However, taking the strong confinement regime as a starting point, the equation does not cover the transition from weak to strong confinement, the accompanying phenomenon of charge-carrier delocalization, or the change in the transition dipole moment of the electron-hole pair state. Here, we use a one-dimensional, two-particle Hubbard model for interacting electron-hole pairs that extends the well-known tight-binding approach through a point-like electron-hole interaction. On infinite chains, the resulting exciton states exhibit the known relation between the Bohr radius, the exciton binding energy, and the effective mass of the charge carriers. Moreover, by introducing infinite-well boundary conditions, the model enables the transition of the exciton states from weak to strong confinement to be tracked, while straightforward adaptations provide insights into the relation between defects, exciton localization, and confinement. In addition, by introducing the dipole operator, the variation of the transition dipole moment can be mapped when shifting from electron-hole pairs in strong confinement to delocalized and localized excitons in weak confinement. The proposed model system can be readily implemented and extended to different multi-carrier states, thus providing researchers a tool for exploring, understanding, and teaching confinement effects in semiconductor nanocrystals under different conditions.

Two-Dimensional Electronic Spectroscopy Reveals Dynamics within the Bright Fine Structure of CdSe Quantum Dots

Semiconductor quantum dots are characterized by a discrete excitonic structure featuring coarse as well as fine structure. The lowest fine structure states have splittings into bright-dark states which are now well confirmed by single dot spectroscopy. In contrast, the splitting of the lowest coarse exciton into bright-bright fine structure states has not been observed nor the dynamics between these states. Here, we use the unique combination of time and energy resolution of two-dimensional electronic spectroscopy to directly observe the fine structure splittings into a bright-bright doublet. These splittings are strongly size dependent, with population relaxation on the <100 fs time scale.

Tuning Energy Transfer Pathways in Halide Perovskite-Dye Hybrids through Bandgap Engineering

Lead halide perovskite nanocrystals, which offer rich photochemistry, have the potential to capture photons over a wide range of the visible and infrared spectrum for photocatalytic, optoelectronic, and photon conversion applications. Energy transfer from the perovskite nanocrystal to an acceptor dye in the form of a triplet or singlet state offers additional opportunities to tune the properties of the semiconductor-dye hybrid and extend excited-state lifetimes. We have now successfully established the key factors that dictate triplet energy transfer between excited CsPbI3 and surface-bound rhodamine dyes using absorption and emission spectroscopies. The pendant groups on the acceptor dyes influence surface binding to the nanocrystals, which in turn dictate the energy transfer kinetics, as well as the efficiency of energy transfer. Of the three rhodamine dyes investigated (rhodamine B, rhodamine B isothiocyanate, and rose Bengal), the CsPbI3-rose Bengal hybrid with the strongest binding showed the highest triplet energy transfer efficiency (96%) with a rate constant of 1 × 109 s-1. This triplet energy transfer rate constant is nearly 2 orders of magnitude slower than the singlet energy transfer observed for the pure-bromide CsPbBr3-rose Bengal hybrid (1.1 × 1011 s-1). Intriguingly, although the single-halide CsPbBr3 and CsPbI3 nanocrystals selectively populate singlet and triplet excited states of rose Bengal, respectively, the mixed halide perovskites were able to generate a mixture of both singlet and triplet excited states. By tuning the bromide/iodide ratio and thus bandgap energy in CsPb(Br1-xIx)3 compositions, the percentage of singlets vs triplets delivered to the acceptor dye was systematically tuned from 0 to 100%. The excited-state properties of halide perovskite-molecular hybrids discussed here provide new ways to modulate singlet and triplet energy transfer in semiconductor-molecular dye hybrids through acceptor functionalization and donor bandgap engineering.

Enhanced Spin Polarization from Biaxially Strained Colloidal Quantum Dots

Electron and hole spin polarization is crucial for quantum dots to be used in spin lasers and quantum information processing. However, the degree of spin polarization in II-VI and III-V semiconductor quantum dots is low because of the degenerated valence band. Here, we increase the light and heavy hole degeneracy by introducing biaxial strain into CdSe-based quantum dots, enabling the degree of spin polarization to be increased from 20% to 50% under photoexcitation. The optical gain threshold measurement further reveals that the increase in polarization helps to reduce the gain threshold.

Hot Excitons Cool in Metal Halide Perovskite Nanocrystals as Fast as CdSe Nanocrystals

The idea of phonon bottlenecks has long been pursued in nanoscale materials for their application in hot exciton devices, such as photovoltaics. Decades ago, it was shown that there is no quantum phonon bottleneck in strongly confined quantum dots due to their physics of quantum confinement. More recently, it was proposed that there are hot phonon bottlenecks in metal halide perovskites due to their physics. Recent work has called into question these bottlenecks in metal halide perovskites. Here, we compare hot exciton cooling in a range of sizes of CsPbBr3 nanocrystals from weakly to strongly confined. These results are compared to strongly confined CdSe quantum dots of two sizes and degrees of quantum confinement. CdSe is a model system as a ruler for measuring hot exciton cooling being fast, by virtue of its efficient Auger-assisted processes. By virtue of 3 ps time resolution, the hot exciton photoluminescence can now be directly observed, which is the most direct measure of the presence of hot excitons and their lifetimes. The hot exciton photoluminescence decays on nearly the same 2 ps time scale on both the weakly confined perovskite and the larger CdSe quantum dots, much faster than the 10 ps cooling predicted by transient absorption experiments. The smaller CdSe quantum dot has still faster cooling, as expected from quantum size effects. The quantum dots of perovskites show extremely fast hot exciton cooling, decaying faster than detection limits of <1 ps, even faster than the CdSe system, suggesting the efficiency of Auger processes in these metal halide perovskite nanocrystals and especially in their quantum dot form. These results across a range of sizes of nanocrystals reveal extremely fast hot exciton cooling at high exciton density, independent of composition, but dependent upon size. Hence these metal halide perovskite nanocrystals seem to cool heavily following quantum dot physics.

Exciton and biexciton transient absorption spectra of CdSe quantum dots with varying diameters

Transient absorption (TA) spectroscopy of semiconductor nanocrystals (NCs) is often used for excited state population analysis, but recent results suggest that TA bleach signals associated with multiexcitons in NCs do not scale linearly with exciton multiplicity. In this manuscript, we probe the factors that determine the intensities and spectral positions of exciton and biexciton components in the TA spectra of CdSe quantum dots (QDs) of five diameters. We find that, in all cases, the peak intensity of the biexciton TA spectrum is less than 1.5 times that of the single exciton TA spectrum, in stark contrast to a commonly made assumption that this ratio is 2. The relative intensities of the biexciton and exciton TA signals at each wavelength are determined by at least two factors: the TA spectral intensity and the spectral offset between the two signals. We do not observe correlations between either of these factors and the particle diameter, but we find that both are strongly impacted by replacing the native organic surface-capping ligands with a hole-trapping ligand. These results suggest that surface trapping plays an important role in determining the absolute intensities of TA features for CdSe QDs and not just their decay kinetics. Our work highlights the role of spectral offsets and the importance of surface trapping in governing absolute TA intensities. It also conclusively demonstrates that the biexciton TA spectra of CdSe QDs at the band gap energy are less than twice as intense as those of the exciton.

Band-Edge Energy Levels of Dynamic Excitons in Cube-Shaped CdSe/CdS Core/Shell Nanocrystals

An electron-hole pair in a cube-shaped CdSe/CdS core/shell nanocrystal exists in the form of dynamic excitons across the strongly and weakly confined regimes under ambient temperatures. Photochemical doping is applied to distinguish the band-edge electron and hole levels, confirming an effective mass model with universal constants. Reduction of the optical bandgap upon epitaxy of the CdS shells is caused by lowering the band-edge electron level and barely affecting the band-edge hole level. Similar shifts of the electron levels, yet retaining the hole levels, can switch the order in energy of the three lowest-energy transitions. Thermal distribution of 1-4 electrons among the two thermally accessible electron levels follows number-counting statistics, instead of Fermi-Dirac distribution.

Electric-Field Fluctuations as the Cause of Spectral Instabilities in Colloidal Quantum Dots

Spectral diffusion (SD) represents a substantial obstacle toward implementation of solid-state quantum emitters as a source of indistinguishable photons. By performing high-resolution emission spectroscopy for individual colloidal quantum dots at cryogenic temperatures, we prove the causal link between the quantum-confined Stark effect and SD. Statistically analyzing the wavelength of emitted photons, we show that increasing the sensitivity of the transition energy to an applied electric field results in amplified spectral fluctuations. This relation is quantitatively fit to a straightforward model, indicating the presence of a stochastic electric field on a microscopic scale, whose standard deviation is 9 kV/cm, on average. The current method will enable the study of SD in multiple types of quantum emitters such as solid-state defects or organic lead halide perovskite quantum dots, for which spectral instability is a critical barrier for applications in quantum sensing.

Spectral widths and Stokes shifts in InP-based quantum dots

InP-based quantum dots (QDs) have Stokes shifts and photoluminescence (PL) line widths that are larger than in II-VI semiconductor QDs with comparable exciton energies. The mechanisms responsible for these spectral characteristics are investigated in this paper. Upon comparing different semiconductors, we find the Stokes shift decreases in the following order: InP > CdTe > CdSe. We also find that the Stokes shift decreases with core size and decreases upon deposition of a ZnSe shell. We suggest that the Stokes shift is largely due to different absorption and luminescent states in the angular momentum fine structure. The energy difference between the fine structure levels, and hence the Stokes shifts, are controlled by the electron-hole exchange interaction. Luminescence polarization results are reported and are consistent with this assignment. Spectral widths are controlled by the extent of homogeneous and inhomogeneous broadening. We report PL and PL excitation (PLE) spectra that facilitate assessing the roles of homogeneous and different inhomogeneous broadening mechanisms in the spectra of zinc-treated InP and InP/ZnSe/ZnS particles. There are two distinct types of inhomogeneous broadening: size inhomogeneity and core-shell interface inhomogeneity. The latter results in a distribution of core-shell band offsets and is caused by interfacial dipoles associated with In-Se or P-Zn bonding. Quantitative modeling of the spectra shows that the offset inhomogeneity is comparable to but somewhat smaller than the size inhomogeneity. The combination of these two types of inhomogeneity also explains several aspects of reversible hole trapping dynamics involving localized In3+/VZn2- impurity states in the ZnSe shells.

Minimizing heat generation in quantum dot light-emitting diodes by increasing quasi-Fermi-level splitting

Minimizing heat accumulation is essential to prolonging the operational lifetime of quantum dot light-emitting diodes (QD-LEDs). Reducing heat generation at the source is the ideal solution, which requires high brightness and quantum efficiency at low driving voltages. Here we propose to enhance the brightness of QD-LEDs at low driving voltages by using a monolayer of large QDs to reduce the packing number in the emitting layer. This strategy allows us to achieve a higher charge population per QD for a given number of charges without charge leakage, enabling enhanced quasi-Fermi-level splitting and brightness at low driving voltage. Due to the minimized heat generation, these LEDs show a high power conversion efficiency of 23% and a T95 operation lifetime (the time for the luminance to decrease to 95% of the initial value) of more than 48,000 h at 1,000 cd m-2.

Highly stable and pure single-photon emission with 250 ps optical coherence times in InP colloidal quantum dots

Quantum photonic technologies such as quantum communication, sensing or computation require efficient, stable and pure single-photon sources. Epitaxial quantum dots (QDs) have been made capable of on-demand photon generation with high purity, indistinguishability and brightness, although they require precise fabrication and face challenges in scalability. By contrast, colloidal QDs are batch synthesized in solution but typically have broader linewidths, low single-photon purities and unstable emission. Here we demonstrate spectrally stable, pure and narrow-linewidth single-photon emission from InP/ZnSe/ZnS colloidal QDs. Using photon correlation Fourier spectroscopy, we observe single-dot linewidths as narrow as ~5 µeV at 4 K, giving a lower-bounded optical coherence time, T2, of ~250 ps. These dots exhibit minimal spectral diffusion on timescales of microseconds to minutes, and narrow linewidths are maintained on timescales up to 50 ms, orders of magnitude longer than other colloidal systems. Moreover, these InP/ZnSe/ZnS dots have single-photon purities g(2)(τ = 0) of 0.077-0.086 in the absence of spectral filtering. This work demonstrates the potential of heavy-metal-free InP-based QDs as spectrally stable sources of single photons.

Theory of Photoluminescence Spectral Line Shapes of Semiconductor Nanocrystals

Single-molecule photoluminescence (PL) spectroscopy of semiconductor nanocrystals (NCs) reveals the nature of exciton-phonon interactions in NCs. Understanding the homogeneous spectral line shapes and their temperature dependence remains an open problem. Here, we develop an atomistic model to describe the PL spectrum of NCs, accounting for excitonic effects, phonon dispersion relations, and exciton-phonon couplings. We validate our model using single-NC measurements on CdSe/CdS NCs from T = 4 to 290 K, and we find that the slightly asymmetric main peak at low temperatures is comprised of a narrow zero-phonon line (ZPL) and acoustic phonon sidebands. Furthermore, we identify the specific phonon modes that give rise to the optical phonon sidebands. At temperatures above 200 K, the spectral line width shows a stronger dependence upon the temperature, which we demonstrate to be correlated with higher order exciton-phonon couplings. We also identify the line width dependence upon reorganization energy, NC core sizes, and shell thicknesses.

Time-Frequency Signatures of Electronic Coherence of Colloidal CdSe Quantum Dot Dimer Assemblies Probed at Room Temperature by Two-Dimensional Electronic Spectroscopy

Electronic coherence signatures can be directly identified in the time-frequency maps measured in two-dimensional electronic spectroscopy (2DES). Here, we demonstrate the theory and discuss the advantages of this approach via the detailed application to the fast-femtosecond beatings of a wide variety of electronic coherences in ensemble dimers of quantum dots (QDs), assembled from QDs of 3 nm in diameter, with 8% size dispersion in diameter. The observed and computed results can be consistently characterized directly in the time-frequency domain by probing the polarization in the 2DES setup. The experimental and computed time-frequency maps are found in very good agreement, and several electronic coherences are characterized at room temperature in solution, before the extensive dephasing due to the size dispersion begins. As compared to the frequency-frequency maps that are commonly used in 2DES, the time-frequency maps allow exploiting electronic coherences without additional post-processing and with fewer 2DES measurements. Towards quantum technology applications, we also report on the modeling of the time-frequency photocurrent response of these electronic coherences, which paves the way to integrating QD devices with classical architectures, thereby enhancing the quantum advantage of such technologies for parallel information processing at room temperature.

Colloidal Semiconductor Nanocrystal Lasers and Laser Diodes

Lasers and optical amplifiers based on solution-processable materials have been long-desired devices for their compatibility with virtually any substrate, scalability, and ease of integration with on-chip photonics and electronics. These devices have been pursued across a wide range of materials including polymers, small molecules, perovskites, and chemically prepared colloidal semiconductor nanocrystals, also commonly referred to as colloidal quantum dots. The latter materials are especially attractive for implementing optical-gain media as in addition to being compatible with inexpensive and easily scalable chemical techniques, they offer multiple advantages derived from a zero-dimensional character of their electronic states. These include a size-tunable emission wavelength, low optical gain thresholds, and weak sensitivity of lasing characteristics to variations in temperature. Here we review the status of colloidal nanocrystal lasing devices, most recent advances in this field, outstanding challenges, and the ongoing progress toward technological viable devices including colloidal quantum dot laser diodes.

Exciton Dephasing by Phonon-Induced Scattering between Bright Exciton States in InP/ZnSe Colloidal Quantum Dots

Decoherence or dephasing of the exciton is a central characteristic of a quantum dot (QD) that determines the minimum width of the exciton emission line and the purity of indistinguishable photon emission during exciton recombination. Here, we analyze exciton dephasing in colloidal InP/ZnSe QDs using transient four-wave mixing spectroscopy. We obtain a dephasing time of 23 ps at a temperature of 5 K, which agrees with the smallest line width of 50 μeV we measure for the exciton emission of single InP/ZnSe QDs at 5 K. By determining the dephasing time as a function of temperature, we find that exciton decoherence can be described as a phonon-induced, thermally activated process. The deduced activation energy of 0.32 meV corresponds to the small splitting within the nearly isotropic bright exciton triplet of InP/ZnSe QDs, suggesting that the dephasing is dominated by phonon-induced scattering within the bright exciton triplet.

Resolving the Emission Transition Dipole Moments of Single Doubly Excited Seeded Nanorods via Heralded Defocused Imaging

Semiconductor nanocrystal emission polarization is a crucial probe of nanocrystal physics and an essential factor for nanocrystal-based technologies. While the transition dipole moment for the lowest excited state to ground state transition is well characterized, the dipole moment of higher multiexcitonic transitions is inaccessible via most spectroscopy techniques. Here, we realize direct characterization of the doubly excited-state relaxation transition dipole by heralded defocused imaging. Defocused imaging maps the dipole emission pattern onto a fast single-photon avalanche diode detector array, allowing the postselection of photon pairs emitted from the biexciton-exciton emission cascade and resolving the differences in transition dipole moments. Type-I¹/₂ seeded nanorods exhibit higher anisotropy of the biexciton-to-exciton transition compared to the exciton-to-ground state transition. In contrast, type-II seeded nanorods display a reduction of biexciton emission anisotropy. These findings are rationalized in terms of an interplay between the transient dynamics of the refractive index and the excitonic fine structure.

Electronic Structure and Excited State Dynamics of Cadmium Chalcogenide Nanorods

The cylindrical quasi-one-dimensional shape of colloidal semiconductor nanorods (NRs) gives them unique electronic structure and optical properties. In addition to the band gap tunability common to nanocrystals, NRs have polarized light absorption and emission and high molar absorptivities. NR-shaped heterostructures feature control of electron and hole locations as well as light emission energy and efficiency. We comprehensively review the electronic structure and optical properties of Cd-chalcogenide NRs and NR heterostructures (e.g., CdSe/CdS dot-in-rods, CdSe/ZnS rod-in-rods), which have been widely investigated over the last two decades due in part to promising optoelectronic applications. We start by describing methods for synthesizing these colloidal NRs. We then detail the electronic structure of single-component and heterostructure NRs and follow with a discussion of light absorption and emission in these materials. Next, we describe the excited state dynamics of these NRs, including carrier cooling, carrier and exciton migration, radiative and nonradiative recombination, multiexciton generation and dynamics, and processes that involve trapped carriers. Finally, we describe charge transfer from photoexcited NRs and connect the dynamics of these processes with light-driven chemistry. We end with an outlook that highlights some of the outstanding questions about the excited state properties of Cd-chalcogenide NRs.

Optically Excited Lasing in a Cavity-Based, High-Current-Density Quantum Dot Electroluminescent Device

Laser diodes based on solution-processable materials can benefit numerous technologies including integrated electronics and photonics, telecommunications, and medical diagnostics. An attractive system for implementing these devices is colloidal semiconductor quantum dots (QDs). The progress towards a QD laser diode has been hampered by rapid nonradiative Auger decay of optical-gain-active multicarrier states, fast device degradation at high current densities required for laser action, and unfavorable competition between optical gain and optical losses in a multicomponent device stack. Here we resolve some of these challenges and demonstrate optically excited lasing from fully functional high-current density electroluminescent (EL) devices with an integrated optical resonator. This advance has become possible due to excellent optical gain properties of continuously graded QDs and a refined device architecture, which allows for highly efficient light amplification in a thin, EL-active QD layer.

Measuring the Ultrafast Spectral Diffusion and Vibronic Coupling Dynamics in CdSe Colloidal Quantum Wells using Two-Dimensional Electronic Spectroscopy

We measure the ultrafast spectral diffusion, vibronic dynamics, and energy relaxation of a CdSe colloidal quantum wells (CQWs) system at room temperature using two-dimensional electronic spectroscopy (2DES). The energy relaxation of light-hole (LH) excitons and hot carriers to heavy-hole (HH) excitons is resolved with a time scale of ∼210 fs. We observe the equilibration dynamics between the spectroscopically accessible HH excitonic state and a dark state with a time scale of ∼160 fs. We use the center line slope analysis to quantify the spectral diffusion dynamics in HH excitons, which contains an apparent sub-200 fs decay together with oscillatory features resolved at 4 and 25 meV. These observations can be explained by the coupling to various lattice phonon modes. We further perform quantum calculations that can replicate and explain the observed dynamics. The 4 meV mode is observed to be in the near-critically damped regime and may be mediating the transition between the bright and dark HH excitons. These findings show that 2DES can provide a comprehensive and detailed characterization of the ultrafast spectral properties in CQWs and similar nanomaterials.

It Is a Trap!: The Effect of Self-Healing of Surface Defects on the Excited States of CdSe Nanocrystals

Colloidal semiconductor nanocrystals have attracted much interest due to their unique optical properties, with applications ranging from displays to biomedical imaging. Nanocrystal optical properties depend on the structure of the surface, where defects can lead to traps. CdSe nanocrystals undergo surface reorganization, or self-healing, to eliminate defects, removing midgap traps from the band structure. However, the effect of this process on the optical spectrum is not well studied. Here, we show that self-healing not only eliminates midgap traps from the band structure but also brightens the spectrum and causes the excitonic states to emerge as the dominant features, in agreement with experimental annealing studies. We find that self-healing can lead to new traps like bonded Se-Se or Cd-Cd dimers, and their behavior is different from that of undercoordinated atom traps. These results suggest that eliminating traps requires a balance of allowing enough surface reorganization to eliminate undercoordinated atoms, but not so much that dimeric traps form.

Big Data in a Nano World: A Review on Computational, Data-Driven Design of Nanomaterials Structures, Properties, and Synthesis

The recent rise of computational, data-driven research has significant potential to accelerate materials discovery. Automated workflows and materials databases are being rapidly developed, contributing to high-throughput data of bulk materials that are growing in quantity and complexity, allowing for correlation between structural-chemical features and functional properties. In contrast, computational data-driven approaches are still relatively rare for nanomaterials discovery due to the rapid scaling of computational cost for finite systems. However, the distinct behaviors at the nanoscale as compared to the parent bulk materials and the vast tunability space with respect to dimensionality and morphology motivate the development of data sets for nanometric materials. In this review, we discuss the recent progress in data-driven research in two aspects: functional materials design and guided synthesis, including commonly used metrics and approaches for designing materials properties and predicting synthesis routes. More importantly, we discuss the distinct behaviors of materials as a result of nanosizing and the implications for data-driven research. Finally, we share our perspectives on future directions for extending the current data-driven research into the nano realm.

A Comparative Study of the Band-Edge Exciton Fine Structure in Zinc Blende and Wurtzite CdSe Nanocrystals

In this paper, we studied the role of the crystal structure in spheroidal CdSe nanocrystals on the band-edge exciton fine structure. Ensembles of zinc blende and wurtzite CdSe nanocrystals are investigated experimentally by two optical techniques: fluorescence line narrowing (FLN) and time-resolved photoluminescence. We argue that the zero-phonon line evaluated by the FLN technique gives the ensemble-averaged energy splitting between the lowest bright and dark exciton states, while the activation energy from the temperature-dependent photoluminescence decay is smaller and corresponds to the energy of an acoustic phonon. The energy splittings between the bright and dark exciton states determined using the FLN technique are found to be the same for zinc blende and wurtzite CdSe nanocrystals. Within the effective mass approximation, we develop a theoretical model considering the following factors: (i) influence of the nanocrystal shape on the bright-dark exciton splitting and the oscillator strength of the bright exciton, and (ii) shape dispersion in the ensemble of the nanocrystals. We show that these two factors result in similar calculated zero-phonon lines in zinc blende and wurtzite CdSe nanocrystals. The account of the nanocrystals shape dispersion allows us to evaluate the linewidth of the zero-phonon line.

Inverse size-dependent Stokes shift in strongly quantum confined CsPbBr3 perovskite nanoplates

Colloidal semiconductor nanocrystals (NCs) are used as bright chromatic fluorophores for energy-efficient displays. We focus here on the size-dependent Stokes shift for CsPbBr₃ nanocrystals. The Stokes shift, i.e., the difference between the wavelengths of absorption and emission maxima, is crucial for display application, as it controls the degree to which light is reabsorbed by the emitting material reducing the energetic efficiency. One major impediment to the industrial adoption of NCs is that slight deviations in manufacturing conditions may result in a wide dispersion of the product's properties. A data-driven analysis of over 2000 reactions comparing two data sets, one produced via standard colloidal synthesis and the other via high-throughput automated synthesis is discussed. We show that differences in the reaction conditions of colloidal CsPbBr₃ nanocrystals yield nanocrystals with opposite Stokes shift size-dependent trends. These match the morphologies of two-dimensional nanoplatelets (NPLs) and nanocrystal cubes. The Stokes shift size dependence trend of NPLs and nanocubes is non-monotonic indicating different physics is at play for the two nanocrystal morphologies. For nanocrystals with cubic shape, with the increase of edge length, there is a significant decrease in Stokes shift values. However, for NPLs with the increase of thickness (1-4 ML), Stokes shift values will increase. The study emphasizes the transition from a spectroscopic point of view and relates the two Stokes shift trends to 2D and 0D exciton dimensionalities for the two morphologies. Our findings highlight the importance of CsPbBr₃ nanocrystal morphology for Stokes shift prediction.

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.

Optical and Scintillation Properties of Record-Efficiency CdTe Nanoplatelets toward Radiation Detection Applications

Colloidal CdTe nanoplatelets featuring a large absorption coefficient and ultrafast tunable luminescence coupled with heavy-metal-based composition present themselves as highly desirable candidates for radiation detection technologies. Historically, however, these nanoplatelets have suffered from poor emission efficiency, hindering progress in exploring their technological potential. Here, we report the synthesis of CdTe nanoplatelets possessing a record emission efficiency of 9%. This enables us to investigate their fundamental photophysics using ultrafast transient absorption, temperature-controlled photoluminescence, and radioluminescence measurements, elucidating the origins of exciton- and defect-related phenomena under both optical and ionizing excitation. For the first time in CdTe nanoplatelets, we report the cumulative effects of a giant oscillator strength transition and exciton fine structure. Simultaneously, thermally stimulated luminescence measurements reveal the presence of both shallow and deep trap states and allow us to disclose the trapping and detrapping dynamics and their influence on the scintillation properties.

Electron Spin Coherence in CdSe Nanocrystals in a Glass Matrix

The coherent spin dynamics of electrons in CdSe nanocrystals embedded in a glass matrix with diameters from 3.3 up to 6.1 nm are investigated by time-resolved Faraday ellipticity at room and cryogenic temperatures. Only one Larmor precession frequency is detected, which corresponds to the larger of the two precession frequencies and thus g-factor values found in the typical signal from solution-grown colloidal CdSe nanocrystals. We identify this frequency accordingly as associated with the spin precession of resident electrons localized in the nanocrystals in the vicinity of the surface. We provide a detailed theoretical analysis of the exciton level spin structure in the magnetic field and model the spin dynamics in CdSe nanocrystals of different symmetries. This allows us to exclude the exciton as the origin of the experimentally observed oscillating signal. At a cryogenic temperature of 6 K, an additional nonoscillating component emerges in the spin dynamics. We consider several possible origins of this signal and conclude that it is related to the hole spin polarization.

Recent progress and future prospects on halide perovskite nanocrystals for optoelectronics and beyond

As an emerging new class of semiconductor nanomaterials, halide perovskite (ABX₃, X = Cl, Br, or I) nanocrystals (NCs) are attracting increasing attention owing to their great potential in optoelectronics and beyond. This field has experienced rapid breakthroughs over the past few years. In this comprehensive review, halide perovskite NCs that are either freestanding or embedded in a matrix (e.g., perovskites, metal-organic frameworks, glass) will be discussed. We will summarize recent progress on the synthesis and post-synthesis methods of halide perovskite NCs. Characterizations of halide perovskite NCs by using a variety of techniques will be present. Tremendous efforts to tailor the optical and electronic properties of halide perovskite NCs in terms of manipulating their size, surface, and component will be highlighted. Physical insights gained on the unique optical and charge-carrier transport properties will be provided. Importantly, the growing potential of halide perovskite NCs for advancing optoelectronic applications and beyond including light-emitting devices (LEDs), solar cells, scintillators and X-ray imaging, lasers, thin-film transistors (TFTs), artificial synapses, and light communication will be extensively discussed, along with prospecting their development in the future.

Biexciton dynamics in halide perovskite nanocrystals

Lead halide perovskite nanocrystals are attracting considerable interest as next-generation optoelectronic materials. Optical responses of nanocrystals are determined by excitons and exciton complexes such as trions and biexcitons. Understanding of their dynamics is indispensable for the optimal design of optoelectronic devices and the development of new functional properties. Here, we summarize the recent advances on the exciton and biexciton photophysics in lead halide perovskite nanocrystals revealed by femtosecond time-resolved spectroscopy and single-dot spectroscopy. We discuss the impact of the biexciton dynamics on controlling and improving the optical gain.

Uniaxial Strain Engineering via Core Position Control in CdSe/CdS Core/Shell Nanorods and Their Optical Response

Anisotropic strain engineering has emerged as a powerful strategy for enhancing the optoelectronic performance of semiconductor nanocrystals. Here, we show that CdSe/CdS dot-in-rod structures offer a platform for fine-tuning the optical response of CdSe quantum dots through anisotropic strain. By controlling the spatial position of the CdSe core within a growing CdS nanorod shell, varying degrees of uniaxial strain can be introduced. Placing CdSe cores at the end of the CdS nanorod induces strong asymmetric compression along the c-axis of the wurtzite CdSe core, dramatically altering its absorption and emission characteristics, whereas CdSe cores located near the middle of the nanorod experience a comparatively weak uniaxial strain field. The change in absorption and emission spectra and dynamics for highly strained end-position CdSe/CdS nanorods is explained by (1) relative shifting of the valence band light hole and heavy hole levels and (2) introduction of a strong piezoelectric potential, which spatially separates the electron and hole wave functions. The ability to tune the degree of uniaxial strain through core position control in a nanorod structure creates opportunities for precisely modulating the electronic properties of CdSe nanocrystals while simultaneously taking advantage of dielectric and optical anisotropies intrinsic to 1D nanostructures.

Optically Switchable NIR Photoluminescence of PbS Semiconducting Nanocrystals using Diarylethene Photoswitches

Precisely modulated photoluminescence (PL) with external control is highly demanded in material and biological sciences. However, it is challenging to switch the PL on and off in the NIR region with a high modulation contrast. Here, we demonstrate that reversible on and off switching of the PL in the NIR region can be achieved in a bicomponent system comprised of PbS semiconducting nanocrystals (NCs) and diarylethene (DAE) photoswitches. Photoisomerization of DAE to the ring-closed form upon UV light irradiation causes substantial quenching of the NIR PL of PbS NCs due to efficient triplet energy transfer. The NIR PL fully recovers to an on state upon reversing the photoisomerization of DAE to the ring-open form with green light irradiation. Importantly, fully reversible switching occurs without obvious fatigue, and the high PL on/off ratio (>100) outperforms all previously reported assemblies of NCs and photoswitches.

Thickness-Dependent Dark-Bright Exciton Splitting and Phonon Bottleneck in CsPbBr3-Based Nanoplatelets Revealed via Magneto-Optical Spectroscopy

The optimized exploitation of perovskite nanocrystals and nanoplatelets as highly efficient light sources requires a detailed understanding of the energy spacing within the exciton manifold. Dark exciton states are particularly relevant because they represent a channel that reduces radiative efficiency. Here, we apply large in-plane magnetic fields to brighten optically inactive states of CsPbBr₃-based nanoplatelets for the first time. This approach allows us to access the dark states and directly determine the dark-bright splitting, which reaches 22 meV for the thinnest nanoplatelets. The splitting is significantly less for thicker nanoplatelets due to reduced exciton confinement. Additionally, the form of the magneto-PL spectrum suggests that dark and bright state populations are nonthermalized, which is indicative of a phonon bottleneck in the exciton relaxation process.

A Sensitizer of Purpose: Generating Triplet Excitons with Semiconductor Nanocrystals

The process of photon upconversion promises importance for many optoelectronic applications, as it can result in higher efficiencies and more effective photon management. Upconversion via triplet-triplet annihilation (TTA) occurs at low incident powers and at high efficiencies, requirements for integration into existing optoelectronic devices. Semiconductor nanocrystals are a diverse class of triplet sensitizers with advantages over traditional molecular sensitizers such as energetic tunability and minimal energy loss during the triplet sensitization process. In this Perspective, we review current progress in semiconductor nanocrystal triplet sensitization, specifically focusing on the nanocrystal, the ligand shell which surrounds the nanocrystal, and progress in solid-state sensitization. Finally, we discuss potential areas of improvement which could result in more efficient upconversion systems sensitized by semiconductor nanocrystals. Specifically, we focus on the development of solid-state TTA upconversion systems, elucidation of the energy transfer mechanisms from nanocrystal to transmitter ligand which underpin the upconversion process and propose novel configurations of nanocrystal-sensitized systems.

Simulations of nonradiative processes in semiconductor nanocrystals

The description of carrier dynamics in spatially confined semiconductor nanocrystals (NCs), which have enhanced electron-hole and exciton-phonon interactions, is a great challenge for modern computational science. These NCs typically contain thousands of atoms and tens of thousands of valence electrons with discrete spectra at low excitation energies, similar to atoms and molecules, that converge to the continuum bulk limit at higher energies. Computational methods developed for molecules are limited to very small nanoclusters, and methods for bulk systems with periodic boundary conditions are not suitable due to the lack of translational symmetry in NCs. This perspective focuses on our recent efforts in developing a unified atomistic model based on the semiempirical pseudopotential approach, which is parameterized by first-principle calculations and validated against experimental measurements, to describe two of the main nonradiative relaxation processes of quantum confined excitons: exciton cooling and Auger recombination. We focus on the description of both electron-hole and exciton-phonon interactions in our approach and discuss the role of size, shape, and interfacing on the electronic properties and dynamics for II-VI and III-V semiconductor NCs.

Energy Versus Electron Transfer: Managing Excited-State Interactions in Perovskite Nanocrystal-Molecular Hybrids

Energy and electron transfer processes in light harvesting assemblies dictate the outcome of the overall light energy conversion process. Halide perovskite nanocrystals such as CsPbBr₃ with relatively high emission yield and strong light absorption can transfer singlet and triplet energy to surface-bound acceptor molecules. They can also induce photocatalytic reduction and oxidation by selectively transferring electrons and holes across the nanocrystal interface. This perspective discusses key factors dictating these excited-state pathways in perovskite nanocrystals and the fundamental differences between energy and electron transfer processes. Spectroscopic methods to decipher between these complex photoinduced pathways are presented. A basic understanding of the fundamental differences between the two excited deactivation processes (charge and energy transfer) and ways to modulate them should enable design of more efficient light harvesting assemblies with semiconductor and molecular systems.

The Rise and Future of Discrete Organic-Inorganic Hybrid Nanomaterials

Hybrid nanomaterials (HNs), the combination of organic semiconductor ligands attached to nanocrystal semiconductor quantum dots, have applications that span a range of practical fields, including biology, chemistry, medical imaging, and optoelectronics. Specifically, HNs operate as discrete, tunable systems that can perform prompt fluorescence, energy transfer, singlet fission, upconversion, and/or thermally activated delayed fluorescence. Interest in HNs has naturally grown over the years due to their tunability and broad spectrum of applications. This Review presents a brief introduction to the components of HNs, before expanding on the characterization and applications of HNs. Finally, the future of HN applications is discussed.

Electrical control of biexciton Auger recombination in single CdSe/CdS nanocrystals

The Auger recombination effect is strongly enhanced in semiconductor nanocrystals due to the quantum confinement, and various strategies in chemical synthesis have been employed so far to suppress this nonradiative decay pathway of multiple excitons. Here we apply external electric fields on single CdSe/CdS giant nanocrystals at room temperature, showing that the biexciton Auger and single-exciton radiative rates can be averagely decreased by ∼40 and ∼10%, respectively. In addition to a reduced overlap of the electron-hole wavefunctions, the large decrease of biexciton Auger rate could be contributed by the enhanced exciton-exciton repulsion, while the electron-hole exchange interaction might be weakened to cause the relatively small decrease of the single-exciton radiative rate. The above findings have thus proved that the external electric field can serve as a post-synthetic knob to tune the exciton recombination dynamics in semiconductor nanocrystals towards their efficient applications in various optoelectronic devices.

Quantification of Exciton Fine Structure Splitting in a Two-Dimensional Perovskite Compound

Applications of two-dimensional (2D) perovskites have significantly outpaced the understanding of many fundamental aspects of their photophysics. The optical response of 2D lead halide perovskites is dominated by strongly bound excitonic states. However, a comprehensive experimental verification of the exciton fine structure splitting and associated transition symmetries remains elusive. Here we employ low temperature magneto-optical spectroscopy to reveal the exciton fine structure of (PEA)₂PbI₄ (here PEA is phenylethylammonium) single crystals. We observe two orthogonally polarized bright in-plane free exciton (FX) states, both accompanied by a manifold of phonon-dressed states that preserve the polarization of the corresponding FX state. Introducing a magnetic field perpendicular to the 2D plane, we resolve the lowest energy dark exciton state, which although theoretically predicted, has systematically escaped experimental observation (in Faraday configuration) until now. These results corroborate standard multiband, effective-mass theories for the exciton fine structure in 2D perovskites and provide valuable quantification of the fine structure splitting in (PEA)₂PbI₄.

Bright State Sensitized Triplet Energy Transfer from Quantum Dot to Molecular Acceptor Revealed by Temperature Dependent Energy Transfer Dynamics

Quantum dot (QD) sensitized molecular triplet excited state generation has been a promising alternative for traditional triplet state harvesting schemes. However, the correlation between QD bright/dark states and QD sensitized triplet energy transfer (TET) has been unclear. Herein, we studied the bright/dark states contribution to TET with CdSe/CdS core/shell QD-oligothiophene as the model system. Equilibrium between QD bright and dark states was tuned by changing temperature, and TET dynamics were monitored with transient absorption spectroscopy. Analysis of acceptor triplet excited state growth kinetics yields rates of TET from bright and dark states as 0.492 ± 0.011 ns^-1 and 0.0271 ± 0.0014 ns^-1 at 5 K, suggesting significant contribution of bright states to TET. The result was rationalized by bright state wave function components with the same electron/hole spin projections leading to nonzero TET probability. The study provides new insights into QD sensitized TET mechanisms and inspiration for future TET efficiency optimization through QD exciton engineering.

Nanophotonic Approach to Study Excited-State Dynamics in Semiconductor Nanocrystals

In semiconductor nanocrystals, excited electrons relax through multiple radiative and nonradiative pathways. This complexity complicates characterization of their decay processes with standard time- and temperature-dependent photoluminescence studies. Here, we exploit a simple nanophotonic approach to augment such measurements and to address open questions related to nanocrystal emission. We place nanocrystals at different distances from a gold reflector to affect radiative rates through variations in the local density of optical states. We apply this approach to spherical CdSe-based nanocrystals to probe the radiative efficiency and polarization properties of the lowest dark and bright excitons by analyzing temperature-dependent emission dynamics. For CdSe-based nanoplatelets, we identify the charge-carrier trapping mechanism responsible for strongly delayed emission. Our method, when combined with careful modeling of the influence of the nanophotonic environment on the relaxation dynamics, offers a versatile strategy to disentangle the complex excited-state decay pathways present in fluorescent nanocrystals as well as other emitters.

Thermally Activated Bright-State Delayed Blue Photoluminescence from InP Quantum Dots

Thermally activated delayed photoluminescence (TADPL) generated from organic chromophore-functionalized quantum dots (QDs) is potentially beneficial for persistent light generation, QD-based PL sensors, and photochemical synthesis. While previous research demonstrated that naphthoic acid-functionalized InP QDs can be employed as low-toxicity, blue-emissive TADPL materials, the electron trap states inherent in these nanocrystals inhibited the observation of TADPL emerging from the higher-lying bright states. Here, we address this challenge by employing the heterocyclic aromatic compound 8-quinolinecarboxylic acid (QCA), whose triplet energy is strategically positioned to bypass the electron trap states in InP QDs. Transient absorption and photoluminescence spectroscopies revealed the generation of bright-state TADPL from QCA-functionalized InP QDs resulting from a nearly quantitative Dexter-like triplet-triplet energy transfer (TTET) from photoexcited InP QDs to surface-anchored QCA chromophores followed by reverse TTET from these bound molecules to the InP QDs. This modification resulted in a 119-fold increase in the average PL intensity decay time with respect to the as-synthesized InP QDs.

Anomalous Emission Shift of CdSe/CdS/ZnS Quantum Dots at Cryogenic Temperatures

The band-gap energy of most bulk semiconductors tends to increase as the temperature decreases. However, non-monotonic temperature dependence of the emission energy has been observed in semiconductor quantum dots (QDs) at cryogenic temperatures. Here, using stable and highly efficient CdSe/CdS/ZnS QDs as the model system, we quantitatively reveal the origins of the anomalous emission red-shift (∼8 meV) below 40 K by correlating ensemble and single QD spectroscopy measurements. About one-quarter of the anomalous red-shift (∼2.2 meV) is caused by the temperature-dependent population of the band-edge exciton fine levels. The enhancement of electron-optical phonon coupling caused by the increasing population of dark excitons with temperature decreases contributes an ∼3.4 meV red-shift. The remaining ∼2.4 meV red-shift is attributed to temperature-dependent electron-acoustic phonon coupling.

Assessing the Environmental Effects Related to Quantum Dot Structure, Function, Synthesis and Exposure

Quantum dots (QDs) are engineered semiconductor nanocrystals with unique fluorescent, quantum confinement, and quantum yield properties, making them valuable in a range of commercial and consumer imaging, display, and lighting technologies. Production and usage of QDs are increasing, which increases the probability of these nanoparticles entering the environment at various phases of their life cycle. This review discusses the major types and applications of QDs, their potential environmental exposures, fates, and adverse effects on organisms. For most applications, release to the environment is mainly expected to occur during QD synthesis and end-product manufacturing since encapsulation of QDs in these devices prevents release during normal use or landfilling. In natural waters, the fate of QDs is controlled by water chemistry, light intensity, and the physicochemical properties of QDs. Research on the adverse effects of QDs primarily focuses on sublethal endpoints rather than acute toxicity, and the differences in toxicity between pristine and weathered nanoparticles are highlighted. A proposed oxidative stress adverse outcome pathway framework demonstrates the similarities among metallic and carbon-based QDs that induce reactive oxygen species formation leading to DNA damage, reduced growth, and impaired reproduction in several organisms. To accurately evaluate environmental risk, this review identifies critical data gaps in QD exposure and ecological effects, and provides recommendations for future research. Future QD regulation should emphasize exposure and sublethal effects of metal ions released as the nanoparticles weather under environmental conditions. To date, human exposure to QDs from the environment and resulting adverse effects has not been reported.

Enhanced emission directivity from asymmetrically strained colloidal quantum dots

Current state-of-the-art quantum dot light-emitting diodes have reached close to unity internal quantum efficiency. Further improvement in external quantum efficiency requires more efficient photon out-coupling. Improving the directivity of the photon emission is considered to be the most feasible approach. Here, we report improved emission directivity from colloidal quantum dot films. By growing an asymmetric compressive shell, we are able to lift their band-edge state degeneracy, which leads to an overwhelming population of exciton with in-plane dipole moment, as desired for high-efficiency photon out-coupling. The in-plane dipole proportion determined by back-focal plane imaging method is 88%, remarkably higher than 70% obtained from conventional hydrostatically strained colloidal quantum dots. Enhanced emission directivity obtained here opens a path to increasing the external quantum efficiencies notably.

Engineering Long-Lived Blue Photoluminescence from InP Quantum Dots Using Isomers of Naphthoic Acid

Leveraging triplet excitons in semiconductor quantum dots (QDs) in concert with surface-anchored molecules to produce long-lifetime thermally activated delayed photoluminescence (TADPL) continues to emerge as a promising technology in diverse areas including photochemical catalysis and light generation. All QDs presently used to generate TADPL in QD/molecule constructs contain toxic metals including Cd(II) and Pb(II), ultimately limiting potential real-world applications. Here, we report newly conceived blue-emitting TADPL-producing nanomaterials featuring InP QDs interfaced with 1- and 2-naphthoic acid (1-NA and 2-NA) ligands. These constitutional isomers feature similar triplet energies but disparate triplet lifetimes, translating into InP-based TADPL processes displaying two distinct average lifetime ranges upon cooling from 293 to 193 K. The time constants fall between 4.4 and 59.2 μs in the 2-NA-decorated InP QDs while further expanding between 84.2 and 733.2 μs in the corresponding 1-NA-ligated InP materials, representing a 167-fold time window. The resulting long-lived excited states enabled facile bimolecular triplet sensitization of ¹O₂ phosphorescence in the near-IR and promoted sensitized triplet-triplet annihilation photochemistry in 2,5-diphenyloxazole. We speculate that the discovery of new nanomaterials exhibiting TADPL lies on the horizon as myriad QDs can be readily derivatized using isomers of numerous classes of surface-anchoring chromophores yielding precisely regulated photophysical properties.

Dark and Bright Excitons in Halide Perovskite Nanoplatelets

Semiconductor nanoplatelets (NPLs), with their large exciton binding energy, narrow photoluminescence (PL), and absence of dielectric screening for photons emitted normal to the NPL surface, could be expected to become the fastest luminophores amongst all colloidal nanostructures. However, super-fast emission is suppressed by a dark (optically passive) exciton ground state, substantially split from a higher-lying bright (optically active) state. Here, the exciton fine structure in 2-8 monolayer (ML) thick Cs_{n - 1} Pb_n Br_{3n + 1} NPLs is revealed by merging temperature-resolved PL spectra and time-resolved PL decay with an effective mass model taking quantum confinement and dielectric confinement anisotropy into account. This approach exposes a thickness-dependent bright-dark exciton splitting reaching 32.3 meV for the 2 ML NPLs. The model also reveals a 5-16 meV splitting of the bright exciton states with transition dipoles polarized parallel and perpendicular to the NPL surfaces, the order of which is reversed for the thinnest NPLs, as confirmed by TR-PL measurements. Accordingly, the individual bright states must be taken into account, while the dark exciton state strongly affects the optical properties of the thinnest NPLs even at room temperature. Significantly, the derived model can be generalized for any isotropically or anisotropically confined nanostructure.