FluidFM as a lithography tool in liquid: spatially controlled deposition of fluorescent nanoparticles.

The atomic force microscope (AFM) is a powerful instrument for nanolithography, which is well characterized in air where the deposition process is steered by capillary action. In contrast, AFM patterning has been seldom achieved in liquid, mostly via electrochemical deposition. This study investigates the pressure-controlled local deposition of nanoparticles in a liquid environment using a FluidFM. Fluorescent 25 nm polystyrene nanospheres were chosen as nanoobjects to be dispensed because they enable both the in situ monitoring of the process by optical microscopy and the ex situ high-resolution characterization of the pattern by e.g. scanning electron microscopy. The FluidFM microchannel was filled with an aqueous solution of negatively charged nanoparticles to be delivered onto a glass surface coated with a polycation. An overpressure in the internal fluidic circuit leads to the deposition of nanoparticle dots and lines under the tip, while the force control regulates the contact between the probe and the surface. The nanoparticle adsorption process depends both on applied pressure and contact time (respectively tip velocity) and can be described using the Langmuir approximation for the random sequential adsorption model. Moreover, we observed that the force setpoint, which does not influence the capillary-driven mechanism in air, indeed affects the hydrodynamic resistance at the tip aperture and therefore the volumetric flow. The described method demonstrates the potential of FluidFM in depositing nano-sized objects in liquid with nanometre precision.