Two-Dimensional Fluctuation Correlation Spectroscopy (2D-FlucCS): A Method to Determine the Origin of Relaxation Rate Dispersion.

Relaxation rate dispersion, i.e., nonexponential or multicomponent kinetics, is observed in complex systems when measuring relaxation kinetics. Often, the origin of rate dispersion is associated with the heterogeneity in the system. However, both homogeneous (where all molecules experience the same rate but inherently nonexponential) and heterogeneous (where all molecules experience different rates) systems can exhibit rate dispersion. A multidimensional correlation analysis method has been demonstrated to detect and quantify rate dispersion observed in molecular rotation, diffusion, solvation, and reaction kinetics. One-dimensional (1D) autocorrelation function detects rate dispersion and measures its extent. Two-dimensional (2D) autocorrelation function measures the origin of rate dispersion and distinguishes homogeneous from heterogeneous. In a heterogeneous system, implicitly there exist subensembles of molecules experiencing different rates. A three-dimensional (3D) autocorrelation function measures subensemble exchange if present and reveals if the system possesses static or dynamic heterogeneity. This perspective discusses the principles, applications, and potential and also presents a future outlook of two-dimensional fluctuation correlation spectroscopy (2D-FlucCS). The method is applicable to any experiment or simulation where a time series of fluctuation in an observable (emission, scattering, current, etc.) around a mean value can be obtained in steady state (equilibrium or nonequilibrium), provided the system is ergodic.

Extracting quantitative dielectric properties from pump-probe spectroscopy.

Optical pump-probe spectroscopy is a powerful tool for the study of non-equilibrium electronic dynamics and finds wide applications across a range of fields, from physics and chemistry to material science and biology. However, a shortcoming of conventional pump-probe spectroscopy is that photoinduced changes in transmission, reflection and scattering can simultaneously contribute to the measured differential spectra, leading to ambiguities in assigning the origin of spectral signatures and ruling out quantitative interpretation of the spectra. Ideally, these methods would measure the underlying dielectric function (or the complex refractive index) which would then directly provide quantitative information on the transient excited state dynamics free of these ambiguities. Here we present and test a model independent route to transform differential transmission or reflection spectra, measured via conventional optical pump-probe spectroscopy, to changes in the quantitative transient dielectric function. We benchmark this method against changes in the real refractive index measured using time-resolved Frequency Domain Interferometry in prototypical inorganic and organic semiconductor films. Our methodology can be applied to existing and future pump-probe data sets, allowing for an unambiguous and quantitative characterisation of the transient photoexcited spectra of materials. This in turn will accelerate the adoption of pump-probe spectroscopy as a facile and robust materials characterisation and screening tool.

Nonequilibrium Carrier Transport in Quantum Dot Heterostructures.

Understanding carrier dynamics and transport in quantum dot based heterostructures is crucial for unlocking their full potential for optoelectronic applications. Here we report the direct visualization of carrier propagation in PbS CQD solids and quantum-dot-in-perovskite heterostructures using femtosecond transient absorption microscopy. We reveal three distinct transport regimes: an initial superdiffusive transport persisting over hundreds of femtoseconds, an Auger-assisted subdiffusive transport before thermal equilibrium is achieved, and a final hopping regime. We demonstrate that the superdiffusive transport lengths correlate strongly with the degree of energetic disorder and carrier delocalization. By tailoring the perovskite content in heterostructures, we obtained a superdiffusive transport length exceeding 90 nm at room temperature and an equivalent diffusivity of up to 106 cm2 s-1, which is 4 orders of magnitude higher than the steady-state values. These findings introduce promising strategies to harness nonequilibrium transport phenomena for more efficient optoelectronic devices.

Tetrafluoroborate-Induced Reduction in Defect Density in Hybrid Perovskites through Halide Management.

Hybrid-perovskite-based optoelectronic devices are demonstrating unprecedented growth in performance, and defect passivation approaches are highly promising routes to further improve properties. Here, the effect of the molecular ion BF4 - , introduced via methylammonium tetrafluoroborate (MABF4 ) in a surface treatment for MAPbI3 perovskite, is reported. Optical spectroscopy characterization shows that the introduction of tetrafluoroborate leads to reduced non-radiative charge-carrier recombination with a reduction in first-order recombination rate from 6.5 × 106 to 2.5 × 105 s-1 in BF4 - -treated samples, and a consequent increase in photoluminescence quantum yield by an order of magnitude (from 0.5 to 10.4%). 19 F, 11 B, and 14 N solid-state NMR is used to elucidate the atomic-level mechanism of the BF4 - additive-induced improvements, revealing that the BF4 - acts as a scavenger of excess MAI by forming MAI-MABF4 cocrystals. This shifts the equilibrium of iodide concentration in the perovskite phase, thereby reducing the concentration of interstitial iodide defects that act as deep traps and non-radiative recombination centers. These collective results allow us to elucidate the microscopic mechanism of action of BF4 - .

Ultrafast Tracking of Exciton and Charge Carrier Transport in Optoelectronic Materials on the Nanometer Scale.

We present a novel optical transient absorption and reflection microscope based on a diffraction-limited pump pulse in combination with a wide-field probe pulse, for the spatiotemporal investigation of ultrafast population transport in thin films. The microscope achieves a temporal resolution down to 12 fs and simultaneously provides sub-10 nm spatial accuracy. We demonstrate the capabilities of the microscope by revealing an ultrafast excited-state exciton population transport of up to 32 nm in a thin film of pentacene and by tracking the carrier motion in p-doped silicon. The use of few-cycle optical excitation pulses enables impulsive stimulated Raman microspectroscopy, which is used for in situ verification of the chemical identity in the 100-2000 cm-1 spectral window. Our methodology bridges the gap between optical microscopy and spectroscopy, allowing for the study of ultrafast transport properties down to the nanometer length scale.

Effect of T·T Mismatch on DNA Dynamics Probed by Minor Groove Binders: Comparison of Dynamic Stokes Shifts of Hoechst and DAPI.

Recognition of DNA base mismatches and their subsequent repair by enzymes is vital for genomic stability. However, it is difficult to comprehend such a process in which enzymes sense and repair different types of mismatches with different ability. It has been suggested that the differential structural changes of mismatched bases act as cues to the repair enzymes, although the effect of such DNA structural changes on surrounding water and ion dynamics is inevitable due to strong electrostatic coupling among them. Thus, collective dynamics of DNA, water, and ions near the mismatch site is believed to be important for mismatch recognition and repair mechanism. Here we show that introduction of a T·T mismatch in the minor groove of DNA induces dispersed (collective) power-law solvation dynamics (of exponent ∼0.24), measured by monitoring the time-resolved fluorescence Stokes shifts (TRFSS) of two popular minor groove binders (Hoechst 33258 and DAPI) over five decades of time from 100 fs to 10 ns. The same ligands however sense different dynamics (power-law of exponent ∼0.15 or power-law multiplied with biexponential relaxation) in the minor groove of normal-DNA. The similar fluorescence anisotropy decays of ligands measured in normal- and T·T-DNA suggest that Stokes shift dynamics and their changes in T·T-DNA purely originate from the solvation process, and not from any internal rotational motion of probe-ligands. The dispersed power-law solvation dynamics seen in T·T-DNA indicate that the ligands do not sense any particular (exponential) relaxation specific to T·T wobbling and/or other conformational changes. This could be the reason why T·T mismatch is recognized by enzymes with lower efficiency compared to purine-pyrimidine and purine-purine mismatches.

Rate and Amplitude Heterogeneity in the Solvation Response of an Ionic Liquid.

In contrast with conventional liquids, ionic liquids have solvation dynamics with more rate dispersion and with average times that do not agree with dielectric measurements. A kinetic analog of multidimensional spectroscopy is introduced and used to look for heterogeneity in simulations of coumarin 153 in [Im12][BF4]. Strong heterogeneity is found in the diffusive solvation rate. An unanticipated heterogeneity in the amplitude of the inertial solvation is also seen. Both heterogeneities exchange at the same rate. This rate is similar to the mean diffusive solvation time, putting it in the intermediate-exchange region. Overall, there are multiple violations of the assumptions usually invoked in the theory of reaction dynamics.

Power-Law Solvation Dynamics in G-Quadruplex DNA: Role of Hydration Dynamics on Ligand Solvation inside DNA.

G-quadruplex DNA (GqDNA) structures act as promising anticancer targets for small-molecules (ligands). Solvation dynamics of a ligand (DAPI: 4',6-diamidino-2-phenylindole) inside antiparallel-GqDNA is studied through direct comparison of time-resolved experiments to molecular dynamics (MD) simulation. Dynamic Stokes shifts of DAPI in GqDNA prepared in H2O buffer and D2O are compared to find the effect of water on ligand solvation. Experimental dynamics (in H2O) is then directly compared with the dynamics computed from 65 ns simulation on the same DAPI-GqDNA complex. Ligand solvation follows power-law relaxation (summed with fast exponential relaxation) from ~100 fs to 10 ns. Simulation results show relaxation below ~5 ps is dominated by water motion, while both water and DNA contribute comparably to dictate long-time power-law dynamics. Ion contribution is, however, found to be negligible. Simulation results also suggest that anomalous solvation dynamics may have origin in subdiffusive motion of perturbed water near GqDNA.

When is a single molecule heterogeneous? A multidimensional answer and its application to dynamics near the glass transition.

Even for apparently simple condensed-phase processes, bulk measurements of relaxation often yield nonexponential decays; the rate appears to be dispersed over a range of values. Taking averages over individual molecules is an intuitive way to determine whether heterogeneity is responsible for such rate dispersion. However, this method is in fundamental conflict with ergodic behavior and often yields ambiguous results. This paper proposes a new definition of rate heterogeneity for ergodic systems based on multidimensional time correlation functions. Averages are taken over both time and molecules. Because the data set is not subdivided, the signal-to-noise ratio is improved. Moment-based quantities are introduced to quantify the concept of rate dispersion. As a result, quantitative statements about the fraction of the dispersion due to heterogeneity are possible, and the experimental noise is further averaged. The practicality of this approach is demonstrated on single-molecule, linear-dichroism trajectories for R6G in poly(cyclohexyl acrylate) near its glass transition. Single-molecule averaging of these data does not provide useful conclusions [C. Y. Lu and D. A. Vanden Bout, J. Chem. Phys. 125, 124701 (2006)]. However, full-ensemble, two- and three-dimensional averages of the same data give clear and quantitative results: the rate dispersion is 95% ± 5% due to heterogeneity, and the rate exchange is at least 11 times longer than the mean rotation time and possibly much longer. Based on these results, we suggest that the study of heterogeneous materials should not focus on "ensemble" versus "single-molecule" experiments, but on one-dimensional versus multidimensional measurements.