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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.

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 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.

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...

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.

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.

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.

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.

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...

Semiconductor quantum dots are of interest as optical gain media for lasing applications. Here we report on efficient, broad bandwidth optical gain in the CdSe/ZnS/CdSe quantum dot/barrier/quantum shell nanocrystal. These nanocrystals are known to support spontaneous emission from both CdSe phases, offering promise for lasing applications via wave function engineering. The CdSe/ZnS/CdSe nanocrystals were found to have enhanced optical gain characteristics relative to CdSe quantum dots, as shown using femtosecond transient absorption spectroscopy. The enhancement of gain metrics such as bandwidth and efficiency arises from stimulated emission from quantum shell-enabled excitations. These shell-enabled excitations increase gain bandwidth via emission from new transitions and increase efficiencies via tailored biexciton interactions. This unique two-color character in both spontaneous emission and optical gain is rationalized by slow exciton cooling from the core/shell states into the core localized quantum ...

The ability to confine electrons and holes in semiconductor quantum dots (QDs) in the form of excitons creates an electronic structure which is both novel and potentially useful for a variety of applications. Upon optical excitation of the dot, the initial excitonic state may be electronically hot. The relaxation dynamics of this hot exciton is the primary event which controls key processes such as optical gain, hot carrier extraction, and multiple exciton generation. Here, we describe femtosecond state-resolved pump/probe experiments on colloidal CdSe quantum dots that provide the first quantitative measure of excitonic state-to-state transition rates. The measurements and modeling here reveal that there are multiple paths by which hot electrons and hot holes relax. The immediate result is that there is no phonon bottleneck for electrons or holes for excitons in quantum dots. This absence of phonon-based relaxation is confirmed by independent measurements of weak exciton–phonon coupling between the vario...

By direct observation of coherent acoustic phonons, we demonstrate a novel extrinsic piezoelectric response in colloidal CdSe semiconductor quantum dots. This response is driven by the migration of charges to the surface of the quantum dot on a vibrationally impulsive time scale. Surface- and fluence-dependent studies reveal that the observed carrier capture based piezo response is controllable and is at least an order of magnitude larger than the intrinsic piezo response of wurtzite CdSe.

Single dot spectroscopy is performed on two-color CdSe/ZnS/CdSe core/barrier/shell nanostructures. Unlike quantum dots cores, these systems have two phases with which to emit and ultimately examine for blinking analysis. These particles are brighter than conventional quantum dots and also show the photoluminescence (PL) intensity and energy fluctuations characteristic of quantum dots. Single dot spectral diffusion analysis yields no measureable energy shift correlation between the core and the shell on the 200 ms time scale. In contrast, the single dot PL from the CdSe shell has narrower linewidths than the CdSe core, indicating differences in its spectral diffusion on shorter timescales.

The first observation of state-resolved exciton−phonon couplings are reported for the lowest four eigenstates in strongly confined colloidal quantum dots for both optical and acoustic phonons. State-resolved pump/probe experiments show that the excitonic eigenstate has a strong influence upon the Frohlich coupling to optical phonons and a moderate influence on the deformation potential coupling to acoustic phonons. These experiments provide a general approach to measuring exciton−phonon couplings in quantum dots with eigenstate-specificity.

Semiconductor nanocrystals have been synthesized that support intrinsic dual emission from the excitonic core as well as the surface. By virtue of chemical control of the thermodynamics of the core/surface equilibria, these nanocrystals support ratiometric temperature sensing over a broad temperature scale. This surface-chemistry-based approach for creating intrinsic dual emission enables a completely new strategy for application of these nanocrystals in optical nanothermometry.

Excitation transfer pathways in colloidal core/barrier/shell nanomaterials were investigated in the CdSe/ZnS/CdSe system. Absorption of light in the outer CdSe shell results in emission from the band edge of the CdSe core. The CdSe quantum shell acts as a light harvester that indirectly increases the brightness of the CdSe quantum-dot core. Spectroscopic evidence suggesting that the CdSe core and shell are coupled by tunneling of excitons through the ZnS barrier is provided. Competition kinetic analysis shows that charge transport competes effectively with hole capture by pyridine at the outer CdSe shell and exciton relaxation within the outer CdSe shell.

Small CdSe semiconductor nanocrystals with diameters below 2 nm are thought to emit white light due to random surface defects which result in a broad distribution of midgap emitting states, thereby preventing rational design of small nanocrystal white light emitters. We perform temperature dependent photoluminescence experiments before and after ligand exchange and electron transfer simulations to reveal a very simple microscopic picture of the origin of the white light. These experiments and simulations reveal that these small nanocrystals can be physically modeled in precisely the same way as normal-sized semiconductor nanocrystals; differences in their emission spectra arise from their surface thermodynamics. The white light emission is thus a consequence of the thermodynamic relationship between a core excitonic state and an optically bright surface state with good quantum yield. By virtue of this understanding of the surface and the manner in which it is coupled to the core excitonic states of these nanocrystals, we show both chemical and thermodynamic control of the photoluminescence spectra. We find that using both temperature and appropriate choice in ligands, one can rationally control the spectra so as to engineer the surface to target color rendering coordinates for displays and white light emitters.

Several different compositions of semiconductor nanocrystals are subjected to numerous spectroscopic techniques to elucidate the nature of surface trapping in these systems. We find a consistent temperature-dependent relationship between core and surface photoluminescence intensity and marked differences in electron-phonon coupling for core and surface states based on ultrafast measurements and Resonance Raman studies, respectively. These results support a minimal model of surface charge trapping applicable to a range of nanocrystal systems involving a single surface state in which the trapped charge polarization leads to strong phonon couplings, with transitions between the surface and band edge excitonic states being governed by semiclassical electron-transfer theory.

Temperature-dependent photoluminescence spectra of a range of semiconductor nanocrystals are presented which reveal an unanticipated thermodynamic relationship between the core excitonic states and the surface of the nanocrystal. These results challenge the widely held view of the surface emission arising from an energetically broad distribution of midgap defect states at the surface. We show that a simpler model based on semiclassical electron transfer and invoking only a single surface state uniquely describes all observables.

Aging of semiconductor nanocrystals (NCs) is well-known to attenuate the spontaneous photoluminescence from the band edge excitonic state by introduction of nonradiative trap states formed at the NC surface. In order to explore charge carrier dynamics dictated by the surface of the NC, femtosecond pump/probe spectroscopic experiments are performed on freshly synthesized and aged CdTe NCs. These experiments reveal fast electron trapping for aged CdTe NCs from the single excitonic state (X). Pump fluence dependence with excitonic state-resolved optical pumping enables directly populating the biexcitonic state (XX), which produces further accelerated electron trapping rates. This increase in electron trapping rate triggers coherent acoustic phonons by virtue of the ultrafast impulsive time scale of the surface trapping process. The observed trapping rates are discussed in terms of electron transfer theory.

The elementary excitation in semiconductor quantum dots is the exciton, an excited electron-hole pair. The size and geometry of the dot confines the exciton thereby yielding quantum confinement effects. The simplest examples of size quantization effects include the spectrum of single exciton states which dominate the linear absorption spectrum and the Stokes shift for the spontaneous photoluminescence spectrum. Here, we report on our recent results exploring the structure and dynamics of both single excitons and multi-excitons in colloidal CdSe quantum dots using femtosecond state-resolved pump/probe spectroscopy. These experiments have revealed tremendous insight into the relaxation pathways of hot excitons, new aspects of exciton-phonon interactions, and in the first observation of the electronic structure of multi-excitons.

Employing real time observation of coherent acoustic phonons, we demonstrate a novel extrinsic piezoelectric response of quantum dots, that is quantized, tunable and an order of magnitude larger than their intrinsic piezo response.

We report on the electronic structure of the band-edge biexciton in colloidal CdSe quantum dots using femtosecond spectroscopy and atomistic many-body pseudopotential calculations. Time-resolved spectroscopy shows that optical transitions between excitonic and biexcitonic states are distinct for absorptive and emissive transitions, leading to a larger Stokes shift for the biexciton than for the single exciton. The calculations explain the experimental results by showing that there is a previously unobserved electronic substructure to the band-edge biexciton which yields two distinct families of transitions.

Using state-selective excitation to tailor multi-exciton interactions, we have demonstrated that optical gain in strongly confined semiconductor quantum dots is a completely universal, size independent, and intrinsic property of these materials.

Coherent optical and acoustic phonons are simultaneously observed in CdSe quantum dots with excitonic state-specificity for the first time. These experiments yield the size and eigenstate dependent coupling strengths for both modes.

Femtosecond carrier relaxation dynamics in colloidal CdSe quantum dots are measured which focus on the early time dynamics for different initial excitonic states. Selecting the initial excitonic states produces a clear influence upon the early time dynamics. The difference between the dynamics when initially populating different excitonic states provides a direct measurement of excitonic relaxation times in strongly confined colloidal semiconductor quantum dots with specificity to electrons and holes. These results furthermore show state specific biexciton interactions.

Femtosecond relaxation dynamics of colloidal CdSe quantum dots are measured for different initial excitonic states. These experiments show dramatic, previously unobserved dynamics at all probe wavelengths based upon preparation of the initial state.

We analyze epitaxially grown InGaN/GaN dot-in-a-wire heterostructures to relate growth and design properties to trends seen in photoluminescence (PL) and resonance Raman spectra. Temperature-dependent PL measurement of these dot-in-a-wire heterostructures illustrate an expected decrease in integrated PL emission and an unusual narrowing of peak width with increasing temperature. Information extracted from Resonance Raman spectra was utilized in a time-dependent model to analyze and to simulate PL spectra. These spectra were found to be in good agreement with the experimental PL data and provided insight into the broadening mechanisms affecting the samples. PL measurements were taken as a function of position on the sample and radial variation of peak energies was observed. This variation was attributed to the radial temperature gradient present during nanowire growth. These additional characteristics of the nanowire heterostructures will allow for increased understanding of these systems potentials for applications in white light emitting diodes.

The overall symmetry of the envelope functions for the four lowest energy excitonic states in colloidal CdSe quantum dots are assigned using excitonic state-resolved pump/probe spectroscopy. These experiments yield a direct test of the two prevailing approaches to quantum dot electronic structure: the effective mass and the atomistic approaches. The experimental results are in complete agreement with the atomistic approach, whereas the effective mass approach fails to assign the correct electron symmetries for higher excitonic states.