Interfacial Self-Assembly and Oriented Attachment in the Family of PbX (X = S, Se, Te) Nanocrystals

Abstract

The realization of materials with new optoelectronic properties draws much scientific attention toward the field of nanocrystal superstructures. Low-dimensional superstructures created by interfacial assembly and oriented attachment of PbSe nanocrystals are a striking example because theory showed that PbSe sheets with a honeycomb geometry possess non-trivial flat bands and Dirac cones in the valence and conduction bands. Here, we report on the formation of one-dimensional linear and zigzag structures and two-dimensional (2D) square and honeycomb structures for the entire lead chalcogenide family: PbX (X = S, Se, Te). We observe that PbTe, with a lower bulk melting temperature and enthalpy of formation than those of PbSe, shows a higher nanocrystal surface reactivity, such that the surface must be passivated and the reaction conditions moderated to obtain reasonably ordered superstructures. The present findings constitute a step forward in the realization of a larger family of atomically coherent 2D superstructures with variable IV–VI and II–VI compositions and with electronic properties dictated by the nanogeometry.

Understanding the Formation of PbSe Honeycomb Superstructures by Dynamics Simulations

“…The formation process of the PbSe silicene-honeycomb superlattice typically occurs at the solvent-air interface. During the process, ligands desorb from the NC f100g facets, allowing oriented attachment between PbSe NCs by opposite f100g facets [73]. In similar experiments [73,85,87], self-assembly of NCs into square superlattices was observed, with all NCs oriented with a f100g facet parallel to the superlattice plane, and with oriented attachment between PbSe NCs by opposite f100g facets also performed.…”

“…During the process, ligands desorb from the NC f100g facets, allowing oriented attachment between PbSe NCs by opposite f100g facets [73]. In similar experiments [73,85,87], self-assembly of NCs into square superlattices was observed, with all NCs oriented with a f100g facet parallel to the superlattice plane, and with oriented attachment between PbSe NCs by opposite f100g facets also performed. Although the relevant forces involved in the formation of these superlattices are known [93], the precise mechanisms are far from understood.…”

“…In this case, a close-packed NC superstructure is obtained, whose geometry is dictated mainly by entropic factors, i.e., the shape and size of the NCs [67][68][69][70][71]. NC-NC attractive interactions can be activated (e.g., by ligand-exchange [72] or facet-specific ligand-detachment [73] reactions), inducing NC self-assembly in different types of superstructures. A special class of NC superstructures, referred to as NC superlattices, is formed when NC self-assembly is followed by crystal-structure alignment and oriented attachment [74][75][76][77][78][79][80][81][82][83] of close-by NCs, resulting in atomically coherent nanogeometric materials.…”

“…The formation process of NC superlattices can be confined in 2D when the NCs are adsorbed at a fluid-fluid interface [84]. The NCs selfassemble in ordered monolayers at the fluid-fluid interface, and subsequently perform oriented attachment [73,[85][86][87][88], resulting in 2D atomically coherent nanogeometric materials. In this last class of processes, in addition to NC entropy and NC-NC interactions, also the different interactions of the NCs with the different molecules of the two fluids forming the interface (i.e., interface-adsorption effects) are involved in the superlattice formation mechanism.…”

“…First experimentally observed a few years ago [73,85,86,88], the PbSe silicene-honeycomb 2D superlattice is expected to exhibit a Dirac-type band structure, with the semiconductor band gap preserved [89][90][91]. Hence, such a material would combine the properties of semiconductors with those of graphene, making it very interesting for optoelectronic applications.…”