Using recently developed laser-assisted synthesis technique (LAST), we demonstrate, for the first time, transient absorption (TA) exciton dynamics of W- and Mo-families of TMDs that have strong excitation fluence dependence and reach extremely long, several nanosecond lifetimes at the highest fluences. To put this in the context, all previous observations have shown tens of picosecond lifetimes at high powers (due to Auger recombination). Using a variety of experimental approaches (excitation wavelength and temperature dependent measurements) and comparing the results with TMD samples in a freestanding form, we trace our observations to the amount of strain produced in the LAST samples by the specifics of high temperature growth and substrate cooling. Using kinetic modeling, we relate these observations to the strain-induced modifications of electronic bands and associated population of intervalley dark excitons that can now interplay with intravalley (bright) excitations.
Hot carrier cooling in halide perovskites is governed by the interplay of carrier-carrier and carrier-phonon interactions, and so the study of their dynamics can reveal important underlying photophysical processes operating in these materials. Here, ultrafast pump-push-probe spectroscopy is used to isolate hot carrier dynamics in lead halide perovskite nanomaterials spanning a range of sizes and shapes. A weak size-dependence is shown by cuboidal CsPbBr3 nanocrystals, while 2D CsPbBr3 nanoplatelets and Ruddlesden-Popper (PEA)2PbI4 display a hot phonon bottleneck that becomes increasingly suppressed with greater excitonic character. This is attributed to an enhanced influence of carrier-carrier scattering in low-dimensional perovskites.
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