Search

A correlation between band alignment at an electron transport layer/perovskite interface and the locus of hysteresis in perovskite solar cells is proposed.

Overview of the electronic structure and relevant processes in light absorption and emission for understanding how absorbing and emitting states are connected.

Perovskite nanocrystals (PNCs) and their strongly confined versions have traditionally been synthesized via hot injection methods. Here we explore a microwave synthesis as a viable and more versatile alternative.

Here, we report mode-specific resonance Raman enhancements of ligands covalently bound to the surface of colloidal CdSe nanocrystals (NCs). By the systematic comparison of a set of structural derivatives, the extent of resonance Raman enhancement is shown to be directly related to the molecular symmetry of the bound ligands. The enhancement dependence on molecular symmetry is further discussed in terms of Franck-Condon and Herzberg-Teller contributions and their associated selection rules. We further show that resonance Raman may be used to distinguish between possible surface binding motifs of bidentate ligands under continuous wave excitation. More generally, this work demonstrates the usefulness of resonance Raman as a characterization tool when characterizing adsorbed molecular species on semiconductor NC surfaces.

Semiconductor nanoparticles (NP) or nanocrystals (NC) have been investigated for many decades, with particular acceleration in interest upon the discovery of quantum confinement effects thereby yielding quantum dots (QD) from certain well-grown NC. The term NP is commonly used in the case of metal and wide gap semiconductor nanocrystals. The term NC is commonly used in semiconductor nanocrystals, whether covalent II-VI or ionic perovskites, that are colloidally grown. The term QD applies to select semiconductor nanocrystals for whom their size is on the order of the excitonic Bohr radius. In the case of colloidal particles on the nanometer length scale, these terms are often used carelessly and interchangeably. The words have specific meaning in relationship to specific physical effects which give rise to specific key processes that can be measured. Here, we provide a Perspective on the ways in which size confers function across different families of NP. In this way, we aim to find ways to identify their similarities and differences by providing the correct semantics for discussion of the salient processes.

We monitor the time-resolved photoluminescence (t-PL) from CsPbBr3 perovskite nanocrystals with a time resolution of 3 ps, which is fast enough to resolve emission from potential multiexcitonic states. Being 15 nm in length and twice the Bohr length, these nanocrystals are either weakly confined or bulk-like. In contrast to this expectation of weak confinement, emission from multiexcitons is observed with binding energies consistent with strongly confined quantum dots. In addition to emission from biexcitons, emission from triexcitons is observed. The triexciton emission includes both S and P recombination channels. Excitation with different amounts of excess energy yields the same PL spectral dynamics, indicating that there are no hot carrier effects, and the electronic structure of the absorbing states is the same. The kinetics of the multiexciton populations are presented in two ways. The kinetics are first shown in a spectrally integrated form, showing faster t-PL at higher fluences independent of excitation excess energy. Both excess energies show the same saturation response. In the second way of presenting the kinetics, the multiexciton populations are decomposed and presented as transients and saturation curves. These decomposed spectra into exciton, biexciton, and triexciton populations enable further insight into their kinetics and fluence dependence.

Metal halide perovskites have emerged as promising materials for light-emitting and photovoltaic applications over recent years. However, unlike quantum dots (QDs) where emission occurs from a quantum-confined exciton state, the nature of emitting states within these bulk nanocrystals remains ambiguous. Herein, we present time-resolved photoluminescence (t-PL) measurements on bulk CsPbBr₃ nanocrystals (NCs). These data reveal that these bulk NCs exhibit a spectrum to the average photoluminescence lifetime, as is seen in CdSe. Notably, there is an absence of physical confinement within these perovskites due to their large size giving them bulk character. The presence of a lifetime spectrum suggests the existence of an excitonic structure. The presence of an excitonic structure suggests the possibility of quantum confinement. Unlike conventional QDs that have quantum confinement due to physical boundaries, perovskite nanocrystals (PNCs) may possess polaronic confinement.

Two-dimensional electronic spectroscopy of CdSe quantum dots shows no sign of fast hole trapping and excited state absorption dynamics reveal fine structure relaxation that could account for the lack of hole bleach.

We present the characterization of multiexciton interactions in semiconductor CdSe quantum dots through two-dimensional electronic spectroscopy (2DES) and quantum dynamics simulations. Our simulations reproduce the overall shape of the experimental 2DES allowing us to characterize the different species formed.

Two-Dimensional Electronic (2DE) spectroscopy is performed on bulk nanocrystals of semiconductor perovskites of two different classes. The anti-diagonal linewidths reveal liquid like structural dynamics due to ultrafast polaron formation on the 300 fs timescale.

Two-dimensional electronic spectroscopy (2DES) has recently been gaining popularity as an alternative to the more common transient absorption spectroscopy due to the combination of high frequency and time resolution of 2DES. In order to advance the reliable analysis of population dynamics and to optimize the time resolution of the method, one has to understand the numerous field matter interactions that take place at an early and negative time. These interactions have historically been discussed in one-dimensional spectroscopy as coherent artifacts and have been assigned to both resonant and non-resonant system responses during or before the pulse overlap. These coherent artifacts have also been described in 2DES but remain less well-understood due to the complexity of 2DES and the relative novelty of the method. Here, we present 2DES results in two model nanocrystal samples, CdSe and CsPbI3. We demonstrate non-resonant signals due to solvent response during the pulse overlap and resonant signals, which we assign to perturbed free induction decay (PFID), both before and during the pulse overlap. The simulations of the 2DES response functions at early and negative time delays reinforce the assignment of the negative time delay signals to PFID. Modeling reveals that the PFID signals will severely distort the initial picture of the resonant population dynamics. By including these effects in models of 2DES spectra, one is able to push forward the extraction of early time dynamics in 2DES.

Despite the impressive abilities of coherent multi-dimensional spectroscopy (CMDS), its’ implementation is limited due to the complexity of continuum generation and required phase stability between the pump pulse pair. In light of this, we have implemented a system producing sub-10 fs pulses with tunable central wavelength. Using a commercial OPA to drive a hollow-core fiber, the system is extremely simple. Output pulse energies lie in the 40-80 μJ range, more than sufficient for transmission through the pulse shaping optics and beam splitters necessary for CMDS. Power fluctuations are minimal, mode quality is excellent, and spectral phase is well behaved at the output. To demonstrate the strength of this source, we measure the two-dimensional spectrum of CdSe quantum dots over a range of population times and find clean signals and clear phonon vibrations. This combination of OPA and hollow-core fiber provides a substantial extension to the capabilities of CMDS.

Lead-halide perovskites have attracted tremendous attention, initially for their performance in thin film photovoltaics, and more recently for a variety of remarkable optical properties. Defect tolerance through polaron formation within the ionic lattice is a key aspect of these materials. Polaron formation arises from the dynamical coupling of atomic fluctuations to electronic states. Measuring the properties of these fluctuations is therefore essential in light of potential optoelectronic applications. Here we apply two-dimensional electronic spectroscopy (2DES) to probe the timescale and amplitude of the electronic gap correlations in CsPbI3 perovskite nanocrystals via homogeneous lineshape dynamics. The 2DES data reveal irreversible, diffusive dynamics that are qualitatively inconsistent with the coherent dynamics in covalent solids such as CdSe quantum dots. In contrast, these dynamics are consistent with liquid-like structural dynamics on the 100 femtosecond timescale. These dynamics are assigned to the optical signature of polaron formation, the conceptual solid-state analogue of solvation. Lead-halide perovskites have promising optoelectronic properties, determined by interplay of electronic and structural properties. Here the authors probe CsPbI3 nanocrystals by two-dimensional electronic spectroscopy, showing liquid-like structural dynamics signature of polaron formation.

Semiconductor nanocrystals have seen much use for their narrow emission line widths for display and lighting applications. Recent progress on semiconductor nanocrystals has suggested the possibilit...

Here, we report resonance Raman spectra of CdSe colloidal nanocrystals (NCs) passivated with organic ligands. In addition to the well-known longitudinal optical phonons, we observe ligand vibrations. The ligand vibrations are shown to be resonantly enhanced through electronic mixing with the states of the NC. These measurements were enabled by substituting the native ligands with thiophenol. Thiophenol serves as an ideal probe for exciton–ligand coupling as it is a widely employed Raman molecular tag and quenches background luminescence in CdSe. The ligand vibrations are shown to be resonantly enhanced through exciting NC transitions. We show that vibronic coupling is observable in CdSe with diameters from 2 to 6 nm and for both phosphonic acid and amine native ligands. The coupling is evidenced by both asymmetric and symmetric mode enhancement through Herzberg–Teller or Franck–Condon and Herzberg–Teller mechanisms, repectively. The ligand exchange quenching strategy may be generally applicable to study ...

Semiconductor nanocrystals are known to have properties of bulk semiconductors as well as molecules. Two rules that govern molecules are that there is no dual emission (Kasha) and there is no spectrum to the emission quantum yield (Vavilov). We show that the latter rule of molecular spectroscopy is generally violated in semiconductor nanocrystals. Through experiments and theory on CdSe and perovskite nanocrystals, these violations are shown to arise via hot carrier effects. Experiments and simple phenomenology reveal that quantum yield spectra arise because of enhanced hot carrier trapping rates. A semiclassical electron-transfer theory rationalizes a microscopic picture of the carrier kinetics. These effects are especially significant when quantifying syntheses of bright emitters such as perovskite nanocrystals. These effects are also a general approach to simple steady-state measurements of the action of hot carrier kinetics.

Semiconductor nanocrystals are being developed with increasingly complex shapes and geometries, often featuring complex shell structures. One aims to characterize these structures by different probes, beyond electronic spectroscopies. Vibrational spectroscopy is a useful tool to probe the phononic structure, but the commonly used frequency-domain methods can be plagued by artifacts due to charge-trapping dynamics. To circumvent these issues, coherent phonons may be measured in the time domain via excitonic state-resolved pump/probe spectroscopy. These measurements reveal several new observations on phononic processes, focusing on model systems of radially graded alloys of core/shell nanocrystals: CdSeCdXZn1–XS. The main new observation is frequency changes to the longitudinal optical phonon at high energy due to electronic mixing. This new, softened phonon mode appears via previously unobserved biexcitonic signals. The state-resolved measurements reveal insights into how the shelling process controls exci...

These authors show that perovskites support a new state of quantum confined excitonic polarons.

Two-quantum variants of two-dimensional electronic spectroscopy (2DES) have previously been used to characterize multi-exciton interactions in molecules and semiconductor nanostructures though many implementations are limited by phasing procedures or non-resonant signals. We implement 2DES using phase-cycling to simultaneously measure one-quantum and two-quantum spectra in colloidal CdSe quantum dots. In the pump–probe geometry, fully absorptive spectra are automatically acquired by measuring the sum of the rephasing and nonrephasing signals. Fifth-order two-quantum spectroscopy allows for direct access to multi-exciton states that may be obscured in excited state absorption signals due to population relaxation or third-order two-quantum spectra due to the non-resonant response.

The microscopic origin and timescale of the fluctuations of the energies of electronic states has a significant impact on the properties of interest of electronic materials, with implication in fields ranging from photovoltaic devices to quantum information processing. Spectroscopic investigations of coherent dynamics provide a direct measurement of electronic fluctuations. Modern multidimensional spectroscopy techniques allow the mapping of coherent processes along multiple time or frequency axes and thus allow unprecedented discrimination between different sources of electronic dephasing. Exploiting modern abilities in coherence mapping in both amplitude and phase, we unravel dissipative processes of electronic coherences in the model system of CdSe quantum dots (QDs). The method allows the assignment of the nature of the observed coherence as vibrational or electronic. The expected coherence maps are obtained for the coherent longitudinal optical (LO) phonon, which serves as an internal standard and confirms the sensitivity of the technique. Fast dephasing is observed between the first two exciton states, despite their shared electron state and common environment. This result is contrary to predictions of the standard effective mass model for these materials, in which the exciton levels are strongly correlated through a common size dependence. In contrast, the experiment is in agreement with ab initio molecular dynamics of a single QD. Electronic dephasing in these materials is thus dominated by the realistic electronic structure arising from fluctuations at the atomic level rather than static size distribution. The analysis of electronic dephasing thereby uniquely enables the study of electronic fluctuations in complex materials.

Strong confinement in semiconductor quantum dots enables them to host multiple electron–hole pairs or excitons. The excitons in these materials are forced to interact, resulting in quantum-confined multiexcitons (MXs). The MXs are integral to the physics of the electronic properties of these materials and impact their key properties for applications such as gain and light emission. Despite their importance, the electronic structure of MX has yet to be fully characterized. MXs have a complex electronic structure arising from quantum many-body effects, which is challenging for both experiments and theory. Here, we report on the investigation of the electronic structure of MX in colloidal CdSe QDs using time-resolved photoluminescence, state-resolved pump–probe, and two-dimensional spectroscopies. The use of varying excitation energy and intensities enables the observation of many signals from biexcitons and triexcitons. The experiments enable the study of MX structures and dynamics on time scales spanning 6 orders of magnitude and directly reveal dynamics in the biexciton manifold. These results outline the limits of the simple concept of binding energy. The methods of investigations should be applicable to reveal complex many-body physics in other nanomaterials and low-dimensional materials of interest.

Excitonic-state-resolved pump/probe spectroscopy is performed on semiconductor dot-in-rod nanocrystals. Using excitonic-state-resolved pumping we are able to resolve effects of the rod upon exciton...

The electronic structure of multiexcitons significantly impacts the performance of nanostructures in lasing and light-emitting applications. However, these multiexcitons remain poorly understood due to their complexity arising from many-body physics. Standard transient-absorption and photoluminescence spectroscopies are unable to unambiguously distinguish effects of sample inhomogeneity from exciton-biexciton interactions. Here, we exploit the energy and time resolution of two-dimensional electronic spectroscopy to access the electronic structure of the band-edge biexciton in colloidal CdSe quantum dots. By removing effects of inhomogeneities, we show that the band-edge biexciton structure must consist of a discrete manifold of electronic states. Furthermore, the biexciton states within the manifold feature distinctive binding energies. Our findings have direct implications for optical gain thresholds and efficiency droop in light-emitting devices and provide experimental measures of many-body physics in nanostructures.

The emission line widths of semiconductor nanocrystals yield insight into the factors that give rise to their electronic structure, thereby providing a path for utilizing nanocrystals in light emissive applications. Experiment and theory in conjunction reveal the contributions to line broadening to the core and surface emission bands. As nanocrystals become small, broad emission from the surface becomes prominent. In the case of the core emission, we reveal previously unobserved vibronic contributions in addition to the already well-known electronic structure of the band-edge exciton. As the temperature decreases, broad emission from the surface becomes prominent. This surface emission also exhibits vibronic contributions albeit more strongly. Analysis of the surface emission reveals the existence of a previously unobserved electronic structure of the surface in complete parallel to that of the core. The surface is characterized by a bright and dark state as well as a spectrum of bright states.

Semiconductor nanocrystals emit light from excitons confined to their core, as well as from their surfaces. Time-resolving the emission from the core yields information on the band edge exciton, which is now well understood. In contrast, the emission from the surface is ill-characterized and remains poorly understood, especially on long time scales. In order to understand the kinetics of charge trapping to the surface and electronic relaxation within the surface, we perform time-resolved emission spectroscopy on CdSe nanocrystals with strong surface emission. The time-resolved spectra reveal a time scale of electron transfer from core to surface much slower than previously thought. These spectra also unveil electron dynamics in the surface band, which gives rise to an average lifetime spectrum. These dynamics are explained by invoking two surface states. This simple model further rationalizes the role of ligands in tuning the surface emission of nanocrystals. These experimental results provide a critical ...

The size-dependent optical and electronic properties of semiconductor nanocrystal (NC) have been exploited over decades for various applications. This size dependence involves a transition from the regime of bulk colloids of ∼100 nm radius to quantum dots (QDs) of ∼10 nm radius, the details of which are material specific. To understand the transition from the QD regime (∼10 nm) to the molecular cluster regime (∼1 nm) of nanocrystals, we have carefully synthesized a set of CdSe nanocrystals with sizes ranging from 0.89 to 1.66 nm in radius. As the nanocrystals become small, the surface emission strongly increases in amplitude, and the core emission broadens and red-shifts. These effects are rationalized in terms of coupling to ligands via electron transfer theory. The core emission spectra arise from increased vibrational coupling of ligands for very small NC. The surface emission amplitudes arise from a size-dependent surface free energy. The transition from the QD to the molecular cluster regime is found...

Semiconductor nanocrystals have been extensively studied for optoelectronic applications including light emission, the focus of this review. Historically, the core of the nanocrystal was the main aspect of the system as it gives rise to the confined excitons and multiexcitons which absorb and emit light. In addition to the core, the surface or interface of these nanocrystals is also important by virtue of their small size. Yet, our understanding of the surface is in its early stages in terms of both chemical and electronic structure. Here, we review the ways in which the interface can control the excitonics, which gives rise to optoelectronic function. We focus our discussion on the ways in which the interface can control optical gain for nanocrystal lasers and white light generation for nanocrystal based light emitting diodes. These processes are connected to different interfacial structures. Finally, we discuss two new applications based on surface electronic structure control: optical switching and opt...

Coherent Multi-dimensional Spectroscopy is ideally suited to investigate many-body effects in semiconductor nanostructures. Here we employ 2D optical spectroscopy on the model system of CdSe quantum dots to reveal the structure of the bandedge biexciton.

The interface of semiconductor nanocrystals is a critical factor for determining their performance in light emissive applications. Traditional nanocrystals have an abrupt termination of the core/shell interface. Recent synthetic work has focused upon developing graded core/shell interfaces via alloying. Here, we employ femtosecond state-resolved pump/probe spectroscopy, temperature-dependent photoluminescence spectroscopy, and a microscopic theory of interfacial charge trapping to reveal the manner in which a graded interface controls the main optical gain metrics: threshold, bandwidth, and lifetime in the CdSe/Cd,Zn,S core/shell system. Photoluminescence spectroscopy in conjunction with semiclassical electron transfer theory reveals the absence of an interfacial electronic state. This absence of a surface/interfacial state is unique to these nanocrystals with a graded shell structure, enabling trap free performance. Excitonic state-resolved pump/probe spectroscopy reveals that the higher excitons do not ...

Excitonic state-resolved pump/probe spectroscopy and time correlate single photon counting were used to study exciton dynamics from the femtosecond to nanosecond time scales in CdSe/Cd,Zn,S nanocrystals. These measurements reveal the role of the core/shell interface as well as surface on non-radiative excitonic processes over three time regimes. Time resolved photoluminescence reports on how the interface controls slow non-radiative processes that dictate emission at the single excitonic level. Heterogeneity in decay is minimized by interfacial structure. Pump/probe measurements explore the non-radiative multiexcitonic recombination processes on the picosecond timescale. These Auger based non-radiative processes dictate lifetimes of multiexcitonic states. Finally state-resolved pump/probe measurements on the femtosecond timescale reveal the influence of the interface on electron and hole relaxation dynamics. We find that the interface has a profound influence on all three types of non-radiative processes which ultimately control light emission from nanocrystals.

We report on the performance of Ar-filled hollow-core fibers (HCFs) capillary driven by long pulses ( > 100 fs) of moderate energy ( < 600 μ J) for coherent multidimensional spectroscopy in the visible range. The source is characterized by high overall transmission and excellent spatial mode. Broadening is achieved by self-phase modulation. Notably, the uncompressed visible output is shorter than the input pulse. This peculiar observation is explained by the structure of self-phase modulated pulses, coupled with spectral filtering. By virtue of its simplicity, low requirements, spectral stability, and the excellent properties of its spatial output, HCFs can provide an interesting alternative to achieve broadband visible pulses for broadband optical excitation and multidimensional spectroscopy applications.

Semiconductor nanostructures have shown promise for light emission across various intensity regimes. Desired performance objectives of photoluminescence efficiency, low gain threshold, large gain lifetime and bandwidth have not been met by any one nanocrystal. A physical understanding of the design principles governing these objectives is also lacking. We show that a carefully engineered CdSe/Cd,Zn,S core/shell nanocrystal uniquely meets all criteria. The key factor allowing for these improvements is the gradual core/shell boundary, which decouples the surface electronic states.

Photoluminescence quenching studies have been used as a means to study different surface attributes of semiconductor nanocrystals. Here we compare the quenching characteristics of iodine and iodobenzene on different sizes of nanocrystals. Iodine quenches statically via the external heavy atom effect and charge transfer, whereas iodobenzene quenches dynamically and only by way of the external heavy atom effect. This qualitative study shows that in both cases quenching efficiency increases with decreasing size and that in the case of charge transfer quenching the core excitonic state is more efficiently quenched than the surface emission.

The surface of semiconductor nanocrystals is one of their defining features by virtue of their nanometer size. Yet the surface is presently among the most poorly understood aspects of nanocrystal science. This perspective provides an overview of spectroscopic work that has revealed the first insights into the nature of the surface, focusing upon CdSe nanocrystals. We focus on two aspects of surface processes in nanocrystals: the kinetics of surface trapping and the thermodynamics of core/surface equilibria. We describe femtosecond pump/probe spectroscopic experiments which reveal the signatures of carrier trapping at the surface. We also describe temperature dependent steady-state photoluminescence experiments which reveal new aspects of the surface. This work suggest that the surface emission is largely driven by homogeneous broadening via phonon progressions. The implications are that the surface electronic state bears similarity to the quantized excitonic core of the nanocrystal.

We present a detailed analysis of a setup capable of arbitrary amplitude, phase, and polarization shaping of broadband visible femtosecond pulses at 1 kHz via a pair of actively phase stabilized acousto-optic programmable dispersive filters arranged in a Mach-Zehnder interferometer geometry. The setup features phase stability values around λ/225 at 580 nm as well as degrees of polarization of at least 0.9 for any polarization state. Both numbers are important metrics to evaluate a setup's potential for applications based on polarization-shaped femtosecond pulses, such as fully coherent multi-dimensional electronic spectroscopy.

The intricate chemistry occurring at the surface of semiconductor nanocrystals is crucial to tailoring their optical properties to a myriad of applications. This perspective aims to re-evaluate long held ideas in semiconductor nanocrystal surface science in the light of a body of new and rich research. We start by reviewing recent developments in ligand chemistry, followed by a discussion of spectroscopic and computational approaches used for advancing the poorly-understood electronic structure of the surface. With the insights gained, we show how the surface impacts emissive behaviour and we summarize strategies to increase fluorescent quantum yield. This discussion is followed by a review of experimental approaches for quantitative analysis of the surface chemistry at concentrations relevant to spectroscopic measurements. We end by highlighting some new directions in ligand chemistry, namely all-inorganically passivated semiconductor nanocrystals and new applications of surface emission.

Semiconductor based light generation is of enormous contemporary interest, given that a large fraction of global energy is used for lighting. White-light semiconductor colloidal quantum dots may find application in future solid state lighting technologies. These dots possess two inherent emission bands, a narrow emissive band attributed to a quantum confined exciton, and a broad emission associated with surface trapping. White light CdSe colloidal semiconductor nanocrystals passivated with phosphonic acids were synthesized by a hot-injection method. Aliquots of this sample are then ligand exchanged with amine and thiol ligands. These samples are embedded in polystyrene films, and a series of temperature dependent photoluminescence measurements are performed. The spectral width as a function of temperature is plotted for all samples. These data are then analyzed in terms of three models. The results suggest that surface line shape broadness may be tied to strong electron-phonon coupling and is largely ligand dependent. The amine and phosphonic acid passivated samples showed large temperature dependence over the range studied, whereas the thiol passivated sample had a lower dependence. This is tentatively explained in terms of hole delocalization in the case of thiol passivation.

The realistic electronic structure of semiconductor nanocrystals is characterized by excitonic fine structure and atomistic symmetry breakings that are challenging to resolve experimentally. Exciton-phonon coupling is one of the most sensitive measures of the excitonic wave functions of the nanocrystals. Here, we exploit this sensitivity via chirped pulse and polarization resolved femtosecond pump/probe spectroscopy of colloidal CdSe nanocrystals. Pulse chirp measurements and simulations are used to explore the contributions of excited- and ground-state vibrational wavepackets to the observed coherent phonons in the pump/probe signals. Polarization resolved pump/probe spectroscopy is used to explore electronic and vibrational polarization anisotropies. We find no electronic polarization anisotropy, whereas vibrational anisotropy is preserved.

Optical pumping of semiconductor nanocrystals with femtosecond pulse sequences was performed in order to modulate multiexciton populations. We show for the first time that control of multiexciton populations produces high speed modulation of stimulated emission. Upon the basis of the speed of multiexcitonic processes in nanocrystals, we show modulation rates approaching 1 THz by virtue of strong quantum confinement effects. Employing femtosecond optical pulse sequences, we demonstrate all-optical logic using these nanocrystals in two forms: an AND gate, and an inverter, a key step toward all optical signal processing.

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...