Functional scanning probes for sensing, actuation and deposition

Abstract

The imaging of objects by standard bright field microscopy is limited by the wavelength of light. A technique that allows for a higher resolution is atomic force microscopy (AFM). In this technique the topography of a surface is mapped by measuring the force acting on an extremely sharp tip when it is scanned over a surface of interest. In this thesis the functionalization of AFM probes is explored, which gives exciting additional possibilities next to imaging. Regarding actuation, the integration of an electrostatic microactuator in the probe was investigated, motivated by the drive towards high-speed (video-rate) AFM. To make the integration more straightforward, a silicon-on-insulator (SOI) compatible fabrication process for the fabrication of in-plane tips was developed and tested. With this process, sharp silicon nitride tips (best measured radius 8 nm) can be batch fabricated on monocrystalline silicon cantilevers with arbitrary shapes. Electrochemical sensing functionality in AFM was obtained by filling a fountain pen probe with mercury, resulting in an in-situ renewable mercury microelectrode. Both dropping mercury electrode and hanging mercury droplet configurations were obtained, depending on the pressure applied on the mercury. Chronoamperometric measurements and cyclic and square-wave voltammograms were obtained with the potential of microscale spatial resolution. Liquid deposition in AFM was studied by using nanofountain pen probes with various tip shapes. Arrays of spots were deposited in contact mode by spotting and writing lines which subsequently break up into droplets. Contactless deposition was achieved by using electrohydrodynamic deposition. On a dedicated setup, the electrospray onset voltage was studied as a function of gap height, liquid properties and applied external pressure. In a commercial AFM setup, liquid deposition was obtained at voltages down to 60 V at an initial gap of 640 nm. Dried deposits of dissolved sodium sulfate with sizes down to 50 nm were achieved. The nanoscale contactless deposition technique presented in this thesis opens up new possibilities in the field of nanolithography and 3D nanofabrication.