Bottom-up approach to fabricate nanostructured thin films from colloidal nanocrystal precursors

Abstract

<p>Control over microstructures at the nanoscale (<100nm) still seems challenging due to, among other things, the stochastic nature of nucleation in the bulk phase. The densification of assemblies of ligand-capped nanocrystals (colloidal nanocrystal assemblies, CNAs) could bypass this challenge that limits our control over the nanostructure and, therefore, the properties of materials. However, the removal of the ligands and the cracking that follows it are the two critical hurdles that have been stymieing this approach.</p> <p>We show that low-pressure plasma processing can effectively remove ligands from CNAs (down to 0.6 at.% of carbon which can be accounted for adventitious carbon) without harming the properties of the inorganic cores of the nanoparticles and the structure of CNAs. The cracking of CNAs is correlated with the structure of the CNAs, which can be controlled and easily predicted by Hansen solubility parameters of solvent in which the nanoparticles are dispersed. While a fully solvated ligand shell leads to the formation of close-packed ordered CNAs – which cracked after self-assembly or ligand removal – a partially solvated one results in interdigitation of the ligand shell yielding disordered CNAs, which remained crack-free after ligand removal and sintering up to a critical cracking thickness of 440nm. The process is demonstrated with particles of different compositions, ligands, sizes, shapes, as well as with binary systems. These findings allowed for the fabrication of cm2 crack-free, phase-pure polycrystalline films with tunable, near-monodisperse grain sizes using CNAs as precursors. We observed electrical conductivities of PbS films produced by this approach over 1 cm to be 1.370 S/cm which is comparable to those of bulk crystal. This conductivity value is remarkable considering the fact that the typical porosity in fully processed CNAs is around 40%.</p> <p>We simultaneously answered the fundamental question that how microstructure of CNAs evolves during ligand removal and studied its effect on microstructure related physical properties, e.g., mechanical properties. We further demonstrated that our bottom-up approach can control the grain boundary composition in the final materials by controlling the chemical structure and composition of the ligands and the characteristics of the plasma. We show that with our unprecedented control on grain boundary composition, we can selectively modify grain growth mechanisms, control phase transitions, and affect mechanical properties. By understanding the interaction of plasma species with CNAs and the mass transport in the system, we were able to accelerate the plasma etching rate by more than an order of magnitude.</p> <p>Finally, we demonstrate the applicability of our approach in developing an optics-free lithography in which CNAs are used as resist and as an active material. By selectively masking the interaction of plasma with CNAs with a hard mask we could change the solubility of the exposed regions. This patterning technology can pattern materials which are hard to pattern by traditional inorganic etching based pattern transfer (example, copper, gold).</p>