Real-Time X-ray Scattering Discovers Rich Phase Behavior in PbS Nanocrystal Superlattices during In Situ Assembly

Abstract

During the self-organization of colloidal semiconductor nanoparticles by solvent evaporation, nanoparticle interactions are substantially determined by the organic ligands covering the inorganic core. However, the influence of the ligand grafting density on the assembly pathway is often not considered in experiments. Here, we carry out an in situ synchrotron small-angle X-ray scattering and X-ray cross-correlation analysis study of the real-time assembly of oleic acid-capped PbS nanocrystals at a low ligand coverage of 2.7 molecules/nm2. With high temporal and spatial resolution, we monitor the transitions from the colloidal suspension through the solvated superlattice states into the final dried superstructure. In a single in situ experiment, we observe a two-dimensional hexagonal, hexagonal close-packed, body-centered cubic, body-centered tetragonal (with different degrees of tetragonal distortion), and face-centered cubic superlattice phases. Our results are compared to the self-organization of PbS nanocrystals with a higher ligand coverage up to 4.5 molecules/nm2, revealing different assembly pathways. This highlights the importance of determining the ligand coverage in assembly experiments to approach a complete understanding of the assembly mechanism as well as to be able to predict and produce the targeted superstructures.

Quantifying Structural Heterogeneity in Individual CsPbBr3 Quantum Dot Superlattices

“…These SL structural trends are congruent with previous studies on SL growth dynamics during evaporative self-assembly using other nanocrystal cores. , Many factors are known to influence nanocrystal assembly dynamics in solution, including size, shape and faceting, ligand density, concentration, and solubility. − Among these factors, the starting nanocrystal size likely plays an important role in our CsPbBr 3 SL assemblies because the other factors remain constant in a single growth experiment. Several studies utilizing in situ X-ray scattering and theoretical modeling have elucidated how nanocrystal size impacts attractive (van der Waal’s) and repulsive (ligand sterics and osmotic pressure) interactions between individual nanocrystals in SL growth. − These studies show that larger nanocrystals begin to assemble into ordered SLs at earlier time points and lower solution concentration than smaller nanocrystals due to their stronger attractive interactions.…”

“…Stronger associative interactions between larger QDs in the growth solution may cause these SLs to assemble into more strongly coupled local domains even when significant global disorder remains. 34 In summary, we have shown that STEM imaging coupled to FFT analysis is a facile method to characterize the structural properties of individual 3D SLs comprising cubic CsPbBr 3 QDs. The radial and angular FFT profiles provide information on the average QD CtC distance and size dispersity in the SL as well as the number of crystalline domains.…”

“…The size dependence may also arise in part from the van der Waal’s and osmotic interactions that govern the self-assembly process in solution. Stronger associative interactions between larger QDs in the growth solution may cause these SLs to assemble into more strongly coupled local domains even when significant global disorder remains …”

Assembly, Properties, and Application of Ordered Group II–VI and IV–VI Colloidal Semiconductor Nanoparticle Films

“…In situ SAXS, WAXS, GISAXS, and GIWAXS provide the possibility of studying the self-assembly kinetics and of understanding intermediate steps of superlattice formation. [26,73,74] For this a drop of colloidal nanocrystal dispersion is exposed to X-ray radiation and during the process of solvent evaporation scattered X-rays are probed in different angles depending upon the method. Once solvent evaporation starts, nanocrystals compress into ordered structures and X-ray scattering techniques with subsecond temporal and micrometer spatial resolution help to probe the stages of superlattice formation.…”

Time‐Resolved SAXS Study of Polarity‐ and Surfactant‐Controlled Superlattice Transformations of Oleate‐Capped Nanocubes During Solvent Removal

“…[24][25][26] Exposure of superlattices of oleic acid (OA) coated, cuboctahedral PbS and PbSe NPs to a nonpolar solvent (e.g., octane) was found to induce structural transformation between face-centered cubic (fcc), bodycentered-tetragonal (bct), body-centered cubic (bcc) or hexagonal closed-packed (hcp) structures. [11,24,27,28] Uptake of toluene was found to increase the interparticle distance and periodicity in superlattices of tetraoctylammonium bromide coated Au NP while exposure to ethanol resulted in an amorphous transformation. [20] However, while SAXS and WAXS can yield structural information of the superlattice, the observed superlattice structures and phase transitions have to the best of our knowledge not been related to quantitative information of the solvent amount and composition.In this study, time-resolved SAXS was used to study the structural transformation and lattice expansions during solvent removal of OA-coated iron oxide nanocube superlattices dispersed in a mixture of good and poor solvent with and without added OA.…”