Seeing atoms under viscous conditions
With today’s atomic force microscopy (AFM) instrumentation, atomic resolution imaging can routinely be achieved in aqueous environments. But what about imaging in liquids other than water? Reporting in Nanotechnology, an international research team at the Nanoscale Function Group at University College Dublin (UCD), and the Max Planck Institute for Polymer Research (MPIP), have shown that high-resolution imaging with dynamic AFM is possible even in highly viscous environments.
A wide variety of important processes, for example, in energy-storage materials, happen at solid-liquid interfaces. Here, the liquids are mostly non-aqueous solutions such as organic solvents and/or ionic liquids. These liquids are often much more viscous than water. The increased fluid viscosity leads to higher noise contributions from the force sensor (cantilever) used in AFM and thus degrades the detection limits and the resolution attainable using dynamic AFM imaging modes.
Seeing the atomic structure
The researchers performed their experiments on the Nanoscale Function Group’s home-built low-noise AFM. Using an atomically sharp diamond tip, they managed to “see” the atomic structure of graphite and mica surfaces in a glycerol-water mixture that was more than 30 times more viscous than pure water. The resolution they achieved during the experiments was unprecedented for such a high-viscosity liquid.
Fluid damping impacts
To explain these unexpected results, the authors conducted a thorough analysis on the impact that the fluid damping has on the performance limits of dynamic AFM imaging modes. One aspect that led to the high signal-to-noise ratio (SNR) was that the high damping of the liquid makes experiments at very low oscillation amplitudes possible – as low as 60 pm. This amplitude is much smaller than the diameter of a glycerol or a water molecule. Thus, the gap between the apex of the tip and the surface underneath is always free from liquid molecules. Effectively, this generates a quasi-vacuum environment, where forces between the tip and surface dominate the interactions. A second aspect that led to high SNR was that the presence of liquid molecules with reduced mobility at the solid-liquid interface generated strong force fields for the tip-sample interaction.
By studying a broader range of materials and liquids with high-resolution AFM, it is possible to shed light on the complex physics behind the interaction of liquid molecules with solid surfaces and their impact on the imaging mechanism in high-resolution AFM.
More information can be found in the journal Nanotechnology 25 175701.
Further reading
Interview with Hans-Jürgen Butt
High-speed AFM revealed in slow motion (July 2012)
Algorithm investigates different stable states of cantilever oscillation in AFM (July 2012)
About the author
Stefan A L Weber worked as a postdoc supported by the Alexander von Humboldt foundation and the Science Foundation Ireland in the groups of B J Rodriguez and S P Jarvis. Here, he learned all the secrets of low-noise AFM in liquids. In 2012 he became a project leader at the Max-Planck Institute for Polymer Research in Mainz, Germany, in the “Physics of Interfaces” group of H-J Butt.
