Additive Manufacturing of Nanostructures Enabled by Ligand-Salt Stabilized Polar Nanocrystal Inks

<br/>Solution-processable semiconductor nanocrystal (NC) materials can enable the sustainable manufacturing of NC-enabled devices such as QD displays, nanophotonics, and optical quantum devices using techniques such as inkjet printing and large-area coating. A current limitation for deposition and nanoscale resolution solution-based additive manufacturing of NCs is a common requirement for nonpolar solvent systems to sufficiently stabilize NC suspensions for effective use as NC inks. In conventional printing, such as piezoelectric or liquid vaporization-based inkjet printing, non-polar inks are ejected from micromachined inkjet nozzles. In these techniques, stable printing of minimum drop volumes on the picoliter scale and printed feature dimensions down to ~10 microns are enabled in part by the low surface energy of the apolar inks. Due to the small zeta potential that the NCs have in solution; the practicality of these inks, however, is often hindered by the concentration limits of NCs in the solvent. To move towards emerging nanoscale additive nanomanufacturing with functional materials such as single NC quantum dot (QD) heterointegration or high resolution QD microdisplay fabrication. QD inks will require finite ink polarizability to enable true nanoscale deterministic positioning and attoliter scale deposition volumes. Here, we have utilized a nitrosonium tetrafluoroborate salt to ligand exchange shelled CdSe/CdS NCs to a polar solvent, N’N Dimethylformamide (DMF), that allows for high printability at low concentrations (M-10) to additively manufacture nanoscale structures. <br/>CdSe NCs in polar DMF were utilized in electrohydrodynamic ink jet (EHDIJ) printing to additively manufacture nanostructures with &lt;100 nm dimensions with high reproducibility. The nanoparticles were stabilized in the polar solvent through tetrafluoroborate ions via ligand exchange with nitrosonium tetrafluoroborate salt. Although the photoluminescent properties of the CdSe NCs were significantly degraded after ligand exchange, shelled CdSe/CdS NCs retained their high PL quantum yields and resulted in exceptionally stable NC/DMF suspension inks for EHDIJ at attoliter and sub-100nm feature sizes. These polar NC inks also allow for high printability at lower NC concentrations of NCs than typical non-polar inks such as octane/hexadecane mixtures. This work supports polar NC inks as a pathway towards additive manufacturing of integrated single NC quantum devices leveraging electrohydrodynamics to extract droplets 104 times smaller in volume than in conventional inkjet printing using simpler, zero moving part inkjet heads where droplet size is decoupled from the mechanical orifice size.. <br/>Electrohydrodynamic inkjet printing is a promising additive manufacturing method for fabricating nanostructures for electromagnetic enhancements and optical devices. Currently, subtractive electron beam lithography is a widely used method for generating nanoscale features. However, the requirement for conductive substrates, high cost, and high fabrication time limits the adoption of this method for research environments. We demonstrate here that sub 100nm additive nanomanufacturing of functional materials and single quantum dot printing of dilute polar NC inks is now possible.