Site-Specific Ligand Interactions Favor the Tetragonal Distortion of PbS Nanocrystal Superlattices

Abstract

We analyze the structure and morphology of mesocrystalline, body-centered tetragonal (bct) superlattices of PbS nanocrystals functionalized with oleic acid. On the basis of combined scattering and real space imaging, we derive a three-dimensional (3D) model of the superlattice and show that the bct structure benefits from a balanced combination of {100}PbS-{100}PbS and {111}PbS-{111}PbS interactions between neighboring layers of nanocrystals, which uniquely stabilizes this structure. These interactions are enabled by the coaxial alignment of the atomic lattices of PbS with the superlattice. In addition, we find that this preferential orientation is already weakly present within isolated monolayers. By adding excess oleic acid to the nanocrystal solution, tetragonal distortion is suppressed, and we observe assembly into a bilayered hexagonal lattice reminiscent of a honeycomb with grain sizes of several micrometers.

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

“…While the thermodynamically favored superlattice structures are fcc and hcp due to the highest packing of the spheres, the bcc structure is often observed in lead chalcogenide NC superlattices . It is believed that the bcc structure is formed due to anisotropic NC interactions caused by preferential ligand coverage on certain facets or because the bcc structure allows for more uniform distribution of ligands around the NC core . In our system, the core radius is R = 1.96 nm and the fully extended length of oleic acid (OA) is L OA = 2.29 nm (for calculations of ligand length see ref.…”

“…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 face‐centered tetragonal (fct) and body‐centered tetragonal (bct) superlattices . 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.…”

Templated self-assembly of one-dimensional CsPbX3 perovskite nanocrystal superlattices

“…[6][7][8] Self-assembled NP arrays including one, two, and three-dimensional superlattices, have attracted considerable attention in various research fields, [9][10][11][12] and created a wide range of applications for self-assembled nanostructures in functional nano-devices, including thermoelectrics, photovoltaics, catalysts, and sensors. [13][14][15][16] Generally, the ascribed mechanism of solution phase selfassembly of NPs relies on the strong interactions between longchain ligands on NP surfaces, 10,[16][17][18][19][20][21] or directional interactions between functional groups (polymers, DNA, inert nucleation materials), 9,14,19,22 yielding equilibrium structures with a balance between inter-ligand and inter-particle interactions. 19,23 However, the controllable assembly of inorganic nanocrystals into anisotropic structures still represents a significant challenge.…”

Quantifying Angular Correlations between the Atomic Lattice and the Superlattice of Nanocrystals Assembled with Directional Linking

“…24,63 We anticipate that the progressive exploration of synthetic MCs with increasing complexity, e.g, binary NC superlattices, nanorod assemblies, honeycomb lattices, etc. will benefit strongly from the present study.…”

“…The fact that the shorter cubic directions of the SL ("a") exhibit collinearity with the AL, and the longer cubic direction ("c") does not, supports the view that ligand-ligand interactions from specific facets of neighboring NCs are responsible for the often observed tetragonal distortion in PbS MCs. 24,63 We anticipate that the progressive exploration of synthetic MCs with increasing complexity, e.g, binary NC superlattices, nanorod assemblies, honeycomb lattices, etc. will benefit strongly from the present study.…”