Monitoring Nanocrystal Self‐Assembly in Real Time Using In Situ Small‐Angle X‐Ray Scattering

Abstract

involve a variety of interactions and driving forces between inorganic hard core, organic soft surface-coating ligands, and surrounding solvent molecules. [2,[7][8][9] As a consequence, a thorough understanding of a solvent-mediated assembly process is required to produce NC solids with programmable properties.Here, we have chosen lead sulfide (PbS) nanocrystals as a model system for the investigation of the in situ self-assembly due to the possibility to produce monodisperse particles in a colloidal solution with a precisely controlled size. Additionally, the unique electronic properties of semiconductor PbS NCs with tunable bandgap make them attractive for many potential technological applications, including solar cells, light-emitting diodes, transistors, photodetectors, etc. [10][11][12][13] The structure and degree of order in NC superlattices depend on several factors, such as NC size and shape, ligand length, grafting density, ligand-solvent interactions, NC concentration, solvent evaporation rate (in case of evaporative assembly), and cell geometry or substrate where the assembly takes place. Thus, recent studies on lead chalcogenide NC assemblies revealed the formation of face-centered cubic (fcc) or body-centered cubic (bcc) superstructures as well as lattice distorted facecentered tetragonal (fct) and body-centered tetragonal (bct) superlattices. [14][15][16][17][18] However, their assembling mechanism remains largely unresolved mainly due to the challenge in measuring intermediate transitions between the initial colloidal state and the final superlattice state, especially in real time.Recently, in situ scattering methods have emerged as a powerful characterization tool to study the nucleation and growth of nanoparticle superlattices. [19][20][21][22] Small-angle X-ray scattering (SAXS) allows for monitoring the self-assembly dynamics with subsecond temporal resolution over large sample volumes under controlled conditions (e.g., controlled solvent vapor environment and temperature) and provides realtime information on long range order in the assembled structures. Often, the grazing-incidence small-angle X-ray scattering (GISAXS) technique is applied for in situ measurements, where a drop of nanocrystal suspension is placed on a substrate and evaporated in a controlled manner. [16,20,23,24] However, at small incident angles of the X-ray beam GISAXS is able to probe only up to several monolayers and is thus limited to the surface regime. For bulk measurements transmission SAXS is Self-assembled nanocrystal superlattices have attracted large scientific attention due to their potential technological applications. However, the nucleation and growth mechanisms of superlattice assemblies remain largely unresolved due to experimental difficulties to monitor intermediate states. Here, the self-assembly of colloidal PbS nanocrystals is studied in real time by a combination of controlled solvent evaporation from the bulk solution and in situ small-angle X-ray scattering (SAXS) in transmission geometry. For the fi...

Structure–Transport Correlation Reveals Anisotropic Charge Transport in Coupled PbS Nanocrystal Superlattices

“…The occurrence of the two SL types (rhcp and bcc) found here may be related to the previously observed hcp–bcc transition for OA‐capped PbS NC SLs upon tailored solvent evaporation. [ 29 ] Similarly, our polycrystalline bcc SLs are assembled from PbS NCs dispersed in hexane, whereas hexane‐octane mixtures resulted in monocrystalline rhcp SLs. This invokes different solvent evaporation rates, which may lead to distinct SL unit cells.…”

Synchrotron Scattering Methods for Nanomaterials and Soft Matter Research

“…Computer simulations showed that crystallization at such a low volume fraction is only possible if attractive interactions between colloidal nanoparticles are significant [96]. The evaporation induced self-assembly of lead sulfide (PbS) nanoparticles was studied by in situ SAXS [97]. The initial colloidal liquid ordered to a swollen hexagonal closed-packed (hcp) superstructure along the cell walls before transforming to bcc structure upon drying.…”

Assembly by solvent evaporation: equilibrium structures and relaxation times

“…Still, despite the enormous success of SE, many fundamental questions are still unresolved, such as how much solvent (if any) remains in the final equilibrium structure, how the parameters controlling the evaporation process affect the quality of the final SLs, or the sequence of events and potential barriers that lead to crystallization. This includes identification of intermediate structures, such as an fcc to bcc (Bain) 14,15 or hcp to bcc 16 transition. Furthermore, it is well known 1 that SE and ST do not generally result in the same structures for the same conditions, raising issues related to metastability.…”

“…and Yang et al,11 all previous BNSLs have been obtained through heterogeneous nucleation, where crystallization occurs on a substrate. Recent experiments have began to characterize the dynamics of assembly14,16 in single component SLs, and establish that the final bcc SL proceeds through an intermediate fcc or hcp phase. In fact, the experiments in ref 16.…”

“…Recent experiments have began to characterize the dynamics of assembly14,16 in single component SLs, and establish that the final bcc SL proceeds through an intermediate fcc or hcp phase. In fact, the experiments in ref 16. report a hcp to bcc transition at around η HS ≈ 0.6, in agreement with the qualitative arguments provided above, even though there is evidence for the hcp phase at η HS ≈ 0.4 < η f HS = 0.49, in slight disagreement with our predictions.…”