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Metal halide perovskite nanocrystals have been under intense investigation for their promise in optoelectronic devices due to their remarkable physics, such as liquid/solid duality. This liquid/solid duality may give rise to their defect tolerance and other such useful properties. This duality means that the electronic states are fluctuating in time, on a distribution of timescales from femtoseconds to picoseconds. Hence, these lattice induced energy fluctuations that are connected to polaron formation are also connected to exciton formation and dynamics. We observe these correlations and dynamics in metal halide perovskite nanocrystals of CsPbI3 and CsPbBr3 using two-dimensional electronic (2DE) spectroscopy, with its unique ability to resolve dynamics in heterogeneously broadened systems. The 2DE spectra immediately reveal a previously unobserved excitonic splitting in these 15 nm NCs that may have a coarse excitonic structure. 2D lineshape dynamics reveal a glassy response on the 300 fs timescale due to polaron formation. The lighter Br system shows larger amplitude and faster timescale fluctuations that give rise to dynamic line broadening. The 2DE signals enable 1D transient absorption analysis of exciton cooling dynamics. Exciton cooling within this doublet is shown to take place on a slower timescale than within the excitonic continuum. The energy dissipation rates are the same for the I and Br systems for incoherent exciton cooling but are very different for the coherent dynamics that give rise to line broadening. Exciton cooling is shown to take place on the same timescale as polaron formation, revealing both as coupled many-body excitation. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The decay of excited states via radiative and nonradiative paths is well understood in molecules and bulk semiconductors but less so in nanocrystals. Here, we perform time-resolved photoluminescence (t-PL) experiments on CsPbBr 3 metal-halide perovskite nanocrystals, with a time resolution of 3 ps, sufficient to observe the decay of both excitons and biexcitons as a function of temperature. The striking result is that the radiative rate constant of the single exciton increases at low temperatures with an exponential functional form, suggesting quantum coherent effects with dephasing at high temperatures. The opposing directions of the radiative and nonradiative decay rate constants enable enhanced brightening of PL from excitons to biexcitons due to quantum effects, promoting a faster approach to the quantum theoretical limits of light emission. Ab initio quantum dynamics simulations reproduce the experimental observations of radiation controlled by quantum spatial coherence enhanced at low temperatures.

Metal halide perovskite nanocrystals are under intense investigation for their outstanding optical and electronic properties. The presence of higher fine structure states, let alone nonequilibrium processes within the fine structure, and multiexcitonic fine structure remains poorly understood due to a lack of experimental probes. Here, we use time-resolved photoluminescence (t-PL) spectroscopy with an improvement from 100 to 3 ps resolution which reveals previously unobserved spectral dynamics from excitons to multiexcitons in 15 nm CsPbBr 3 nanocrystals. The simple and immediate observation from temperature dependence is a previously unobserved fine structure to the multiexcitons. Further insight is gleaned from t-PL spectral bandwidth trajectories at extrema in temperature and exciton density. The bandwidth trajectories reveal the presence of a previously unobserved fine structure in excitons as well as multiexcitons. The bandwidth trajectories reveal a complex history, from multiexciton recombination to exciton thermalization to Auger heating to lattice thermalization. Whereas the amplitude of these spectral effects is large, ∼60 meV, modeling suggests that the spectral effects are mostly phonon based illustrating the importance of the lattice on light emission from metal halide perovskite nanocrystals.