Electrospray Deposition of PbSe Quantum Dot Superlattices with Improved Uniformity and Structural Perfection

Abstract

Colloidal PbX (X = Se, S) quantum dots (QDs) can be self-assembled into highly ordered three-dimensional superlattice (SL) films and subsequently fused by ligand exchange to produce epitaxially interconnected SLs (epi-SLs) with strong inter-QD electronic coupling and the potential for fully delocalized electronic states. However, the conventional method of fabricating these films by drop casting a QD dispersion onto the surface of liquid ethylene glycol (EG) suffers from uncontrolled lateral liquid flows during spreading and drying that result in nonuniform, irreproducible SL films with substantial flow-induced QD positional disorder (riverlike flow patterns in the QD chains). Here, we use electrospray deposition (ESD) to make PbSe QD SL films on EG that are uniform over large areas (∼20 cm2) and have much less QD positional disorder compared to drop-cast SL films. The suppression of flow-induced structural disorder in ESD films is attributed to the unique ability of ESD to deposit a microscopically thin layer of QD dispersion that slowly dries in place without macroscopic liquid flow. SL film thickness is readily tuned by changing the sprayed volume. Moreover, epi-SLs made from ESD films have better positional order than epi-SLs made from drop-cast films. ESD enables the reproducible fabrication of 3D PbSe QD epi-SLs of excellent uniformity and structural quality for studying miniband transport and other collective electronic phenomena. Fabrication of films of CdSe QDs and ZnFe2O4 nanocrystals on EG shows that ESD is a general method for making high-quality, large-area nanocrystal films of uniform and controlled thickness for a variety of applications.