Stimuli-Responsive Surface Ligands for Direct Lithography of Functional Inorganic Nanomaterials

Conspectus Colloidal nanocrystals (NCs)have emerged as a diverse class ofmaterials with tunable composition, size, shape, and surface chemistry.From their facile syntheses to unique optoelectronic properties, thesesolution-processed nanomaterials are a promising alternative to materialsgrown as bulk crystals or by vapor-phase methods. However, the integrationof colloidal nanomaterials in real-world devices is held back by challenges in making patterned NC films with the resolution, throughput, andcost demanded by device components and applications. Therefore, suitableapproaches to pattern NCs need to be established to aid the transitionfrom individual proof-of-concept NC devices to integrated and multiplexedtechnological systems. In this Account, we discuss the developmentof stimuli-sensitivesurface ligands that enable NCs to be patterned directly with goodpattern fidelity while retaining desirable properties. We focus onrationally selected ligands that enable changes in the NC dispersibilityby responding to light, electron beam, and/or heat. First, we summarizethe fundamental forces between colloidal NCs and discuss the principlesbehind NC stabilization/destabilization. These principles are appliedto understanding the mechanisms of the NC dispersibility change uponstimuli-induced ligand modifications. Six ligand-based patterningmechanisms are introduced: ligand cross-linking, ligand decomposition,ligand desorption, in situ ligand exchange, ion/ligandbinding, and ligand-aided increase of ionic strength. We discuss examplesof stimuli-sensitive ligands that fall under each mechanism, includingtheir chemical transformations, and address how these ligands areused to pattern either sterically or electrostatically stabilizedcolloidal NCs. Following that, we explain the rationale behind theexploration of different types of stimuli, as well as the advantagesand disadvantages of each stimulus. We then discuss relevantfigures-of-merit that should be consideredwhen choosing a particular ligand chemistry or stimulus for patterningNCs. These figures-of-merit pertain to either the pattern quality(e.g., resolution, edge and surface roughness, layer thickness), orto the NC material quality (e.g., photo/electro-luminescence, electricalconductivity, inorganic fraction). We outline the importance of theseproperties and provide insights on optimizing them. Both the patternquality and NC quality impact the performance of patterned NC devicessuch as field-effect transistors, light-emitting diodes, color-conversionpixels, photodetectors, and diffractive optical elements. We alsogive examples of proof-of-concept patterned NC devices and evaluatetheir performance. Finally, we provide an outlook on further expandingthe chemistry of stimuli-sensitive ligands, improving the NC patternquality, progress toward 3D printing, and other potential researchdirections. Ultimately, we hope that the development of a patterningtoolbox for NCs will expedite their implementation in a broad rangeof applications.

Automated summary

Colloidal nanocrystals are a diverse class of materials with tunable properties and are a promising alternative to traditionally grown bulk crystals or vapor-phase methods. However, challenges in patterned film formation hinder the integration of these materials into real-world devices. This Account discusses the development of stimuli-sensitive surface ligands that enable the direct patterning of nanocrystals while preserving their desirable properties. The mechanisms and examples of ligand-based patterning are explained, along with the advantages and disadvantages of different stimuli.