No abstract

Selective, polarity-based filtration of a reaction mixture during growth was used to track surface ligation during nanocrystal growth.

The model of nucleation and growth proposed by LaMer has guided our understanding of nanocrystal (NC) formation for over 70 years. However, LaMer’s model does not account for the effects of surface–ligand interactions on the availability of precursor, the resulting burst nucleation, or the growth of the NC. These interactions play a critical role in determining the electronic properties and physical structure of perovskite NCs. The surfaces of perovskite NCs are dynamic, with multiple processes occurring simultaneously, such as the adsorption and desorption of ligands and growth via incorporation of precursor or oriented attachment of small crystallites. These unstable surfaces are difficult to characterize during growth. A model of NC growth that includes the behavior and impact of ligands at the NC surface would enhance rational syntheses of perovskite nanomaterials with targeted properties. Here we discuss techniques and strategies that have provided insight into the nature of NC surfaces, emphasizing the need for new, multimodal, in situ measurements to characterize the role of surface ligands during perovskite NC growth.

Selected pharmaceutical chemicals, steroids and xenoestrogens (PCSXs) consisting of 29 endocrine modulators, therapeutic drugs, pesticides, detergents, plastics, and active ingredients in household products were measured in water, riverbed sediments and fish collected in a tributary embayment of the Potomac River (Hunting Creek, Alexandria, VA, USA) in the vicinity of wastewater discharge. A total of 17 PCSXs were found in the Hunting Creek samples, with steroid hormones (e.g., progesterone and 17α-ethinylestradiol), triclosan, dextromethorphan and bisphenol A being the most prominent micropollutants detected.The geospatial distribution of the PCSXs in Hunting Creek indicated that the steroids correlated with wastewater treatment plant discharge in all matrices, but such an association is tentative in Hunting Creek given the complex nature of urban sources of PCSXs and hydrodynamics in an urban tidal river. The sediment PCSX concentrations correlated with sediment total organic carbon content at all sampling sites. For the most part, the PCSXs showed an enrichment in fish tissue relative to sediments when concentrations were normalized to lipids and sediment organic carbon contents, but the influence of endogenous steroids is also an important consideration for these chemicals.

Methylammonium lead triiodide (MAPbI 3) nanocrystals (NCs) are emerging materials for a range of optoelectronic applications. Photophysical characterization is typically limited to structurally stable NCs owing to the long timescales required for many spectroscopies, preventing the accurate measurement of NCs during growth. This is a particular challenge for non-linear spectroscopies such as transient absorption. Here we report on the use of a novel single-shot transient absorption (SSTA) spectrometer to study MAPbI 3 NCs as they grow. Comparing the transient spectra to derivatives of the linear absorbance reveals that photogenerated charge carriers become localized at surface trap states during NC growth, inducing a TA lineshape characteristic of the Stark effect. Observation of this Stark signal shows that the contribution of trapped carriers to the TA signal declines as growth continues, supporting a growth mechanism with increased surface ligation toward the end of NC growth. This work opens the door to the application of time-resolved spectroscopies to NCs in situ, during their synthesis, to provide greater insight into their growth mechanisms and the evolution of their photophysical properties.