Morphology and Band Structure of Orthorhombic PbS Nanoplatelets: An Indirect Band Gap Material

Abstract

PbS quantum dots and nanoplatelets (NPLs) are of enormous interest in the development of optoelectronic devices. However, some important aspects of their nature remain unclear. Recent studies have revealed that colloidal PbS NPLs may depart from the rock-salt crystal structure of bulk and form an orthorhombic (Pnma) modification instead. To gain insight into the implications of such a change over the optoelectronic properties, we have synthesized orthorhombic PbS NPLs and determined the lattice parameters by means of selected area electron diffraction measurements. We have then calculated the associated band structure using density functional theory with Perdew–Burke–Ernzerhof functional for solids and with the GW approximation, including spin–orbit interactions. An indirect band gap is found, which may explain the weak luminescence reported in experiments. We derive effective masses for conduction and valence bands and deduce that quantum confinement along the a crystallographic axis (short axis of the NPL) reinforces the indirect band gap but that along b and c axes favors a direct gap instead. Calculations for colloidal nanoplatelets of 1.8 nm thickness, carried out with k·p theory, show that excitonic effects are strong, with binding energies of about 150 meV.