AFM and STM in Liquid Electrolytes

Depending on the composition of the liquid and the physical and chemical properties of the SPM probe and sample, operation of an SPM in a liquid can vary from slightly different to much different than in air.

AFM Applications

In contact AFM in air, a thin layer of liquid adsorbed on the sample surface causes attractive forces between the AFM tip and sample. These forces are greatly reduced or eliminated when the tip and sample are immersed in liquid. For this reason, AFM tip-sample force curves in fluid are generally, but not always, different from those in air. They show much less, and sometimes no, adhesion between the tip and sample surface during the “retract” segment of a force curve cycle. The fidelity—and sometimes resolution—of contact AFM images in liquid may depend less on tip/sample adhesive forces and more on the selection of the cantilever, as described below.

Viscous Damping

The mechanical response time of AFM cantilevers in fluid is increased as a result of viscous damping. Consequently, to preserve image fidelity, scanning speeds during contact AFM in liquids may have to be kept lower than when scanning in air. The increase in response time occurs because viscous damping in liquids is much greater than that in air, and the cantilever drags some liquid along with it when moving through the liquid (mass loading). For example, the viscous damping of an AFM cantilever in water is found to be 1–2 orders of magnitude greater than in air.

The amount of viscous damping in liquids can vary considerably for different kinds of commercially available cantilevers, depending on the cantilever geometry. Thus, the choice of AFM cantilevers may affect the image quality in a liquid more dramatically than in air. Generally, among the commonly used V-shaped nitride cantilevers, shorter and thicker cantilevers with larger spring constants are better suited for AFM imaging in liquids. However, these cantilevers can be less gentle on fragile and soft surfaces, as well as on loosely adsorbed species. Contact force must be controlled in order to avoid relocation of electrochemically deposited material by the AFM cantilever. You may need to experiment with various cantilevers to ascertain which ones better suit the imaging requirements of each run.

Optical Properties of the Electrolyte

Because the AFM relies on optical detection of a laser beam, the optical properties of the electrolyte can also affect the operation of the AFM. Opaque electrolytes diffuse the laser beam and in some cases can block the beam. The presence of large colloidal particles in the electrolyte can interfere with the operation of the AFM by sticking to the cantilever or by obscuring the path of the laser beam as it enters and exits the fluid cell. Such particles can sometimes be by-products of electrochemical reactions in the electrolyte. Air bubbles can create the same types of problems as colloidal particles. The workaround for some of the common problems associated with air bubbles and particles are described in Clearing Contaminants and Bubbles.

Electrochemical Reaction Dynamics

The volume of the electrochemical cell in an SPM is usually small compared to standard laboratory electrochemical cells, and the SPM probe—either STM tip or AFM cantilever-tip assembly—is held close to, and often in contact with, the sample, which is the working electrode.

Bruker personnel have sometimes observed noticeable differences in the surface area of the working electrode beneath the AFM cantilever as compared with areas away from the cantilever. Diffusion of reactant species from the bulk of the electrolyte to areas beneath the cantilever may be affected by the proximity of the cantilever (and the scanning tip) to the surface of the working electrode. This is also known as the tip-shielding effect.

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