Transport-limited electrochemical formation of long nanosharp probes from tungsten

Abstract

We show that electrochemical formation of long probes with nanosharp tips can be controlled by choosing an appropriate thermodynamic pathway of metal to metal oxide and hydroxide transformation. Currently, convection-limited electropolishing (CLE) is extensively used. Nanosharp probes are produced by electrochemically etching a wire until it breaks into two pieces. This process is difficult to control because of the complexity of the associated hydrodynamic flows. We introduce transport-limited electropolishing (TLE), where the electrochemical reaction results in the formation of metal oxides and hydroxides which form a porous surface layer hindering the flow of electrolyte. The developed TLE method enables one to make long tapered needles. The taper can spread over more than 6 mm while the radius of tip curvature can be decreased down to 30 nm. These needles are strong and were successfully applied for piercing single smooth vascular muscle cells.

In situ X-ray nanotomography of metal surfaces during electropolishing

“…This design enabled stabilization of the meniscus and eliminated the influence of the hydrogen generated at the cathode. These two sets of experiments provided new insights onto the effects associated with the free surface of meniscus 14 30 and separate them from those occurring in the bulk of electrolyte.…”

“…Among different applications dealing with electrochemical polishing of metals, nanotechnology applications are, probably, the most demanding, attractive, and challenging 2 . Electropolishing is used in fabrication of sharp tips for scanning tunneling microscopy 3 , atomic force microscopy 4 , field ion microscopy 5 , Raman spectroscopy 6 , nanolithography 7 8 , micro/nano welding 9 10 , microcircuitry and sensorics 11 12 and many biomedical applications 13 14 15 . This method is able to provide the wire tips of different metals 16 17 18 19 20 21 22 with the radius of curvature of the apex varying from a few nanometers to submicrons.…”

“…In this work, we undertake a tour de force around a technique for the nanoscale in situ visualization of electrochemical processes accompanying electropolishing of tungsten wires. Tungsten was chosen due it’s high Young modulus and melting point making it an attractive candidate for a variety of applications in biomedicine 13 14 , scanning probe microscopy 4 6 12 16 , nanolithography 8 and others. To satisfy the requirements of in situ X-ray imaging and to accommodate a tungsten wire, electrolyte, and circuitry, we designed and built a special electrochemical cell.…”

“…This regime leads to the wire necking, a kind of field induced morphological instability which is difficult to control 30 . We used the regime of low voltage electropolishing 14 applying only 2 ± 0.5 V. The low voltage electropolishing offers rich opportunities for making sharp probes with different tip morphologies 14 .…”

“…Applying a rigorous image processing, we discovered that the tungstate oxide films growing next to the metal phase can be shielded from electrolyte by another layer of complex compounds. This shielding effect promotes the uniform and stable polishing of the entire submersed portion of the wire leading to the formation of the centimeter long needle 14 . This is the main difference between the low voltage electropolishing and conventional high voltage electropolishing generating only hundred micron long taper 30 .…”