No abstract

Lead halide perovskite materials have attracted significant attention in the context of photovoltaics and other optoelectronic applications, and recently, research efforts have been directed to nanostructured lead halide perovskites. Collodial nanocrystals (NCs) of cesium lead halides (CsPbX3, X = Cl, Br, I) exhibit bright photoluminescence, with emission tunable over the entire visible spectral region. However, previous studies on CsPbX3 NCs did not address key aspects of their chemistry and photophysics such as surface chemistry and quantitative light absorption. Here, we elaborate on the synthesis of CsPbBr3 NCs and their surface chemistry. In addition, the intrinsic absorption coefficient was determined experimentally by combining elemental analysis with accurate optical absorption measurements. (1)H solution nuclear magnetic resonance spectroscopy was used to characterize sample purity, elucidate the surface chemistry, and evaluate the influence of purification methods on the surface composition. We find that ligand binding to the NC surface is highly dynamic, and therefore, ligands are easily lost during the isolation and purification procedures. However, when a small amount of both oleic acid and oleylamine is added, the NCs can be purified, maintaining optical, colloidal, and material integrity. In addition, we find that a high amine content in the ligand shell increases the quantum yield due to the improved binding of the carboxylic acid.

We quantitatively investigate the size-dependent optical properties of colloidal PbS nanocrystals or quantum dots (Qdots) by combining-the Qdot absorbance spectra with detailed elemental analysis of the Qdot suspensions. At high energies, the molar extinction coefficient epsilon increases With the Not volume d(3) and agrees with theoretical calculations using the Maxwell-Garnett effective medium theory and bulk values for the Qdot dielectric function. This demonstrates that quantum confinement has no influence on E in this spectral range, and it provides an accurate method to calculate the Qdot concentration. Around the band gap, epsilon only increases with d(1.3), and values are comparable to the epsilon of PbSe Qdots. The data are related to the oscillator strength f(if) of the band gap transition and results agree well with theoretical tight-binding calculations, predicting a linear dependence of f(if) on d. For both PbS and PbSe Qdots, the exciton lifetime tau is calculated from f(if). We find values ranging between 1 and 3 mu s, in agreement with experimental literature data from time-resolved luminescence spectroscopy. Our results provide a thorough general framework to calculate and understand the optical properties of suspended colloidal quantum dots. Most importantly, it highlights the significance of the local field factor in these systems

Inductively coupled plasma mass spectrometry (ICP-MS) was combined with UV–vis−NIR spectrophotometry and transmission electron microscopy to determine the nanocrystal composition and molar extinction coefficient ϵ of colloidal PbSe quantum dot (Q-PbSe) suspensions. The ICP-MS results show a nonstoichiometric Pb/Se ratio, with a systematic excess of lead for all samples studied. The observed ratio is consistent with a faceted spherical Q-PbSe model, composed of a quasi stoichiometric Q-PbSe core terminated by a Pb surface shell. At high photon energies, we find that ϵ scales with the nanocrystal volume, irrespective of the Q-PbSe size. From ϵ, we calculated a size-independent absorption coefficient. Its value is in good agreement with the theoretical value for bulk PbSe. At the band gap, ϵ is size-dependent. The resulting absorption coefficient increases quadratically with decreasing Q-PbSe size. Calculations of the oscillator strength of the first optical transition are in good agreement with theoretical tight binding calculations, showing that the oscillator strength increases linearly with Q-PbSe size.

The possibilities offered by 1 H solution NMR for the study of colloidal nanocrystal ligands are reviewed. Using CdSe and PbSe nanocrystals with tightly bound oleate ligands as examples, the solution NMR toolbox for ligand analysis is introduced, highlighting 1D 1 H, diffusion ordered (DOSY) and nuclear Overhauser effect (NOESY) spectroscopy as NMR techniques that enable bound ligands to be distinguished from free ligands. For each of the toolbox techniques, it is outlined how dynamic stabilization in the fast exchange regime affects the spectra obtained. Next, it is shown how the perturbation of a purified dispersion by dilution or the addition of excess ligands can be used to analyze the binding of ligands to a nanocrystal. Finally, saturation transfer difference (STD) spectroscopy is presented as an NMR technique that may complement the established toolbox.

Solution nuclear magnetic resonance spectroscopy (NMR) is used to identify and quantify the organic capping of colloidal PbSe nanocrystals (Q-PbSe). We find that the capping consists primarily of tightly bound oleic acid ligands. Only a minor part of the ligand shell (0-5% with respect to the number of oleic acid ligands) is composed of tri- n-octylphosphine. As a result, tuning of the Q-PbSe size during synthesis is achieved by varying the oleic acid concentration. By combining the NMR results with inductively coupled plasma mass spectrometry, a complete Q-PbSe structural model of semiconductor core and organic ligands is constructed. The nanocrystals are nonstoichiometric, with a surface that is composed of lead atoms. The absence of surface selenium atoms is in accordance with an oleic acid ligand shell. NMR results on a Q-PbSe suspension, stored under ambient conditions, suggest that oxidation leads to the loss of oleic acid ligands and surface Pb atoms, forming dissolved lead oleate.

Urban air pollution and smoke from fires have been modeled to reduce cloud formation by absorbing sunlight, thereby cooling the surface and heating the atmosphere. Satellite data over the Amazon region during the biomass burning season showed that scattered cumulus cloud cover was reduced from 38%in clean conditions to 0%for heavy smoke (optical depth of 1.3). This response to the smoke radiative effect reverses the regional smoke instantaneous forcing of climate from -28 watts per square meter in cloud-free conditions to +8 watts per square meter once the reduction of cloud cover is accounted for.

We use solution NMR techniques to analyze the organic/inorganic interface of CdSe quantum dots (Q-CdSe) synthesized using oleic acid as a surfactant. It is shown that the resulting Q-CdSe are stabilized by tightly bound oleic acid species that only exchange upon addition of free oleic acid. The NMR analysis points toward a two-step exchange mechanism where free ligands are initially physisorbed within the ligand shell to end up as bound, chemisorbed ligands in a second step. Importantly, we find that every ligand is involved in this exchange process. By addition of oleic acid with a deuterated carboxyl headgroup, we demonstrate that the bound ligands are oleate ions and not oleic acid molecules. This explains why a dynamic adsorption/desorption equilibrium only occurs in the presence of excess free oleic acid, which donates the required proton. Comparing the number of oleate ligands to the excess cadmium per CdSe quantum dot, we find a ratio of 2:1. This completes the picture of Q-CdSe as organic/inorganic entities where the surface excess of Cd(2+) is balanced by a double amount of oleate ligands, yielding overall neutral nanoparticles.