  We demonstrate the fabrication of sharp nanopillars of high aspect ratio onto
specialized atomic force microscopy (AFM) microcantilevers and their use for
high-speed AFM of DNA and nucleoproteins in liquid. The fabrication technique
uses localized charged-particle-induced deposition with either a focused beam
of helium ions or electrons in a helium ion microscope (HIM) or scanning
electron microscope (SEM). This approach enables customized growth onto
delicate substrates with nanometer-scale placement precision and in-situ
imaging of the final tip structures using the HIM or SEM. Tip radii of <10 nm
are obtained and the underlying microcantilever remains intact. Instead of the
more commonly used organic precursors employed for bio-AFM applications, we use
an organometallic precursor (tungsten hexacarbonyl) resulting in
tungsten-containing tips. Transmission electron microscopy reveals a thin layer
of carbon on the tips. Consequently, the interaction of the new tips with
biological specimens is likely very similar to that of standard carbonaceous
tips, with the added benefit of robustness. A further advantage of the
organometallic tips is that compared to carbonaceous tips they better withstand
UV-ozone cleaning treatments to remove residual organic contaminants between
experiments. Such contaminants are unavoidable during the scanning of soft
biomolecules in liquid. Our tips can also be grown onto the blunted tips of
previously-used cantilevers, thus providing a means to recycle specialized
cantilevers and restore their performance to the original manufacturer
specifications. Finally, a focused helium ion beam milling technique to reduce
the tip radii and thus further improve lateral spatial resolution in the AFM
scans is demonstrated.
