Influence of the ligand stripping on the transport properties of nanoparticle-based PbSe nanomaterials

Nanomaterials produced from the bottom-up assembly of nanocrystals may incorporate ∼1020–1021 cm–3 not fully coordinated surface atoms, i.e., ∼1020–1021 cm–3 potential donor or acceptor states that can strongly affect transport properties. Therefore, to exploit the full potential of nanocrystal building blocks to produce functional nanomaterials and thin films, a proper control of their surface chemistry is required. Here, we analyze how the ligand stripping procedure influences the charge and heat transport properties of sintered PbSe nanomaterials produced from the bottom-up assembly of colloidal PbSe nanocrystals. First, we show that the removal of the native organic ligands by thermal decomposition in an inert atmosphere leaves relatively large amounts of carbon at the crystal interfaces. This carbon blocks crystal growth during consolidation and at the same time hampers charge and heat transport through the final nanomaterial. Second, we demonstrate that, by stripping ligands from the nanocrystal surface before consolidation, nanomaterials with larger crystal domains, lower porosity, and higher charge carrier concentrations are obtained, thus resulting in nanomaterials with higher electrical and thermal conductivities. In addition, the ligand displacement leaves the nanocrystal surface unprotected, facilitating oxidation and chalcogen evaporation. The influence of the ligand displacement on the nanomaterial charge transport properties is rationalized here using a two-band model based on the standard Boltzmann transport equation with the relaxation time approximation. Finally, we present an application of the produced functional nanomaterials by modeling, fabricating, and testing a simple PbSe-based thermoelectric device with a ring geometry.