Force- and time-dependent feature size and shape control in molecular printing via polymer-pen lithography

Preparation and characterization of dot arrays: Polymer-pen arrays with 8000 pens and 80-μm separation between tips were prepared and inked with MHA as previously described.^[1] The polymer-pen arrays were mounted on an NScriptor (NanoInk, Inc., USA) nanolithography platform that was used for all patterning experiments. Gold nanostructures were prepared by patterning the MHA on 25-nm-thick Au films with a 5-nm Ti adhesion layer thermally evaporated on Si〈100〉SiO_x wafers (Nova Electronic Materials, USA). The tip position, z-piezo extension dwell time between the inked tip arrays and the Au surfaces, and humidity were controlled by InkCAD (NanoInk, Inc., USA) software. Upon immersion of the patterned surface in an etching solution (20mM thiourea, 30mM FeNO₂, 20mM HCl, and 20mM octanol (aq)) for approximately 5 min, the Au was dissolved selectively from areas that are not capped by the MHA. The resulting Au features were analyzed by SEM (Hitachi S-4800, Japan) and a Zeiss Axiovert 200M (Carl Zeiss, Inc., Germany) optical microscope. Variable force patterning: A scale (PS121, Ohaus, USA) was placed beneath the substrate. The MHA-inked tip arrays were brought into contact with the surface so that deformation of the pyramids could be observed optically, but no force was detected by the scale, and this point was set at z-piezo extension=0 μm. Patterns were generated by forming dots with increasing z-piezo extensions (1-7 μm) and forming more dots while the force was decreased for each new dot (7-1 μm). Forces were determined for each different z-piezo extension and the edge lengths of 50 different features produced at each different value of z-piezo extension were measured to obtain an average feature size.