Structural Features of PbS Nanocube Monolayers upon Treatment with Mono- and Dicarboxylic Acids and Thiols at a Liquid–Air Interface

This paper describes
the ordering of PbS nanocubes (NCs) within
free-standing monolayers (suspended on acetonitrile), upon exchanging
the native oleate ligands for a series of thiolate and carboxylate
ligands at the liquid–air interface. Treatment with either
carboxylic acids or thiols effectively decreases the inter-NC separation
of nearest-neighbor particles without etching the NC surface. Dicarboxylic
acids and dithiols bridge neighboring NCs with an interparticle separation
that is consistent with fully extended, bridging ligands. Monocarboxylic
acids and monothiols separate NCs by an amount governed by their length,
with long-chain ligands showing significant intercalation. 1H NMR spectroscopy shows carboxylic acids are more effective at replacing
the native oleate than are thiols, which we ascribe to the lower pKa values of carboxylic acids. The fast exchange
that occurs upon treatment with monocarboxylic acids kinetically traps
the clusters of particles in nonclosed packed geometries, so monolayers
treated with monocarboxylic acids are, on average, less ordered than
those treated with monothiols. Ex situ electron microscopy
and grazing incidence small-angle X-ray scattering (GISAXS) analyses
of deposited films on Si/SiO2 substrates show that NCs
exchanged with nonbridging ligands pack more efficiently at long length
scales than do NCs exchanged with bridging ligands, due primarily
to the creation of defects within the NC lattice in response to the
rigidity of the bridging ligand.