Exciton–polaron interactions in metal halide perovskite nanocrystals revealed via two-dimensional electronic spectroscopy

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

The case of semiconductor nanocrystals

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

Probing biexciton structure in CdSe nanocrystals using 2D optical spectroscopy

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

Probing biexciton structure in CdSe nanocrystals using 2D optical spectroscopy

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

Extending Semiconductor Nanocrystals from the Quantum Dot Regime to the Molecular Cluster Regime

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 to be at 1.2 nm radius, in contrast to the transition from the bulk to QD transition at the Bohr radius of 5.4 nm in CdSe. These size-dependent surface electronic phenomena may be used for light emission applications

Temperature Dependence of Emission Line Widths from Semiconductor Nanocrystals Reveals Vibronic Contributions to Line Broadening Processes

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

Toward Ratiometric Nanothermometry via Intrinsic Dual Emission from Semiconductor Nanocrystals

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

Ultrafast hole relaxation dynamics in quantum dots revealed by two-dimensional electronic spectroscopy

Elucidating the population dynamics of correlated electron-hole pairs (bound excitons) in semiconducting quantum dots (QDs) is key for developing our fundamental understanding of nanoscale photophysics as well as for the optimal design of devices, such as lasers. For decades, it was assumed that holes did not contribute to band edge bleach signals in QDs. Here, we employ two-dimensional electronic spectroscopy to monitor electron and hole dynamics in both CdSe and CdSe/CdS/ZnS QDs to probe electron and hole dynamics. Based on a combination of time and frequency resolution, we observe a previously unresolved bleaching signal in CdSe QDs on timescales faster than 30 fs due to hole cooling. Atomistic semiempirical pseudopotential calculations are used to rationalize the order of magnitude difference in the observed hole dynamics in CdSe and CdSe/CdS/ZnS QDs. This picture advances our understanding of QD excitonics past the prevailing continuum effective mass theories generally used to describe QD electronic structure and dynamics.</p