Chemically Driven Sintering of Colloidal Nanocrystals for Building Functional Devices from the Bottom Up

Additive and on-demand manufacturing is the key enabler for customizable, low-cost, and scalable functional devices that serve emerging sensing and computing applications. Such bottom-up fabrication requires ink-type materials with tailorable physical properties and assembly mechanisms versatile for multi-material and multi-layer deposition. Here, we present a general platform using highly tunable colloidal nanocrystals (NCs) as building blocks and chemical-treatment-assisted patterning and sintering to form structures ranging from tens of nanometers to hundreds of microns.
First, we investigate the structural and property evolution of NCs during the chemically driven sintering process. Treatment of organic-ligand capped Cu NC films with solutions of shorter, environmentally benign, and noncorrosive inorganic reagents, namely, SCN^– and Cl^–, effectively removes the organic ligands, drives NC grain growth, and limits film oxidation. We investigate the mechanism by systemically varying the Cu NC size, ligand reagent, and ligand treatment time and follow the change of their structures and electrical and optical properties. Cl^–-treated, 4.5 nm diameter Cu NC films yield the lowest DC resistivity, only 3.2 times that of bulk Cu, and metal-like dielectric functions at optical frequencies. We exploit the high conductivity of these chemically sintered Cu NC films and, in combination with photo- and nanoimprint-lithography, pattern multiscale structures to achieve high-Q radio frequency (RF) capacitive sensors and near-infrared (NIR) resonant optical metasurfaces.
Then, we integrate the NC materials and chemical treatment with a high-resolution inkjet printing technique. This general approach enables layer-by-layer printing with wide selections of NC inks, ligand reagents, substrates, and device architectures. Chemical-treatment-induced contraction and densification allow printed Ag NC structures to achieve the finest linewidth of 70 nm and filling ratio of up to 75%, achieving bulk-Ag-like conductivity for wide-gamut structural color gratings. By exploiting Ag, Au, and PbS NCs and compact ligands, we demonstrate all-printed multi-layer infrared photodiodes with 10-µm pixel sizes.