Scanning Probe Microscope Looks at Liquids' Surfaces

Kevin Robinson

Researchers at the Lawrence Berkeley National Laboratory have developed a scanning probe microscope that can show the surfaces of liquids with horizontal resolutions of about 100 Å and vertical resolutions of about 1 Å.

Dubbed scanning polarization force microscopy, the technique is similar to atomic force microscopy (AFM) in that it measures small forces between a cantilever tip and a sample to create an image.

Scanning polarization force microscopy reveals tiny droplets of KOH in water on a graphite surface. The image is about 8 3 8 µm, and the gray scale represents heights of up to 200 Å.

Devised by Lawrence Berkeley's Miquel Salmeron, the microscope applies electric voltage to the cantilever probe, polarizing it. The probe then scans the sample at a distance of about 100 Å. The electric polarization of the tip, where the charge is particularly strong, creates a charge of opposite polarity in the sample, making the two attract. By controlling and measuring this attractive force, scientists can map the topography of the sample.

Heretofore, the highest resolution for liquid surface imaging (a few microns) has been achieved with diffraction-limited optical images. AFM has not been successful because the probe usually touches the sample. With liquids, contact allows the tip to sink into the liquid, or capillary action pulls the liquid up the tip. Salmeron said the new device scans close enough to the surface to achieve the maximum resolution.

Conductive cantilever

The cantilever must be conductive for the system to work. Of the two types of tips sold currently, the doped silicon tips are conductive. Silicon nitride tips require a thin coating of metal before they are conductive.

"It turns out that anyone can make one of these," Salmeron said.