Imaging Electrochemical Controlled Chemical Gradients Using Pulsed Force Mode Atomic Force Microscopy

The electrochemical formation of gradients in self assembled monolayers has been demonstrated recently ¹. The capacity to image these gradients provides useful information on the physical chemistry of electrochemical striping.

Imaging chemical gradients requires the ability to sense the chemical moiety on the top of the self-assembled monolayer. This has been accomplished by derivatizing an atomic force microscope (AFM) tip with molecules selected to have specific interactions with the sample in a technique known as chemical force microscopy ². Typical tapping mode AFM is then used to image the sample; the tip is oscillated vertically above the sample and the tip-sample interaction modulates the amplitude of the tip. The sample adhesion, sample stiffness, and sample topography all influence the oscillation amplitude of the tip.

Pulsed Force Mode (PFM) ³ is an extension for atomic force microscopes. The PFM electronics introduces a sinusoidal modulation to the z-piezo of the AFM with an amplitude between 10 to 500 nm at a user selectable frequency between 100 Hz and 2 kHz. With this repetition rate, a pseudo force distance curve can be measured. PFM provides several benefits as compared to other AFM modes. First, it is possible to image the topography of soft samples without introducing large lateral forces, which may alter or destroy the sample. The baseline correction adjusts the instrument at every modulation cycle, suppressing changes of the laser adjustment during scanning and eliminating long range forces, like Coulomb interaction, which can occur because of electric charges on tip or sample. Secondly, the quantitative information about the adhesive forces, acting between tip and sample can be extracted. This provides the means to image material contrast by using chemically modified tips with higher affinity to a specific structural element of the sample. The local stiffness can help users to identify different materials and allow quantitative analysis of the mechanical properties on a nanometer scale.

We have derivatized silicon nitride force modulation tips with octadecyltrichlorosilane (OTS) and patterned a gold surface by microcontact printing hexadecalthiol (HDT) stripes. The sample was then treated with an 3 mercacaptopropionic acid (MPA) solution to backfill the bare gold regions. By imaging the sample using chemical force PFM AFM we are able to separate topography, adhesion, and stiffness changes across our sample. Because our films are relatively flat, with 1 nm RMS roughness over 5 (m, the topography signal (Figure 1a) shows only long-range variations, (which are due to the glass substrate). In the adhesion channel (Figure 1b) we observe a strong adhesion force between the OTS coated tip and the HDT and a small interaction between the OTS and MPA. Finally the stiffness channel (Figure 1c) reveals that these values are similar for OTS and HDT.

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