Imaging stability in force-feedback high-speed atomic force microscopy

Abstract

We studied the stability of force-feedback high-speed atomic force microscopy (HSAFM) by imaging soft, hard, and biological sample surfaces at various applied forces. The HSAFM images showed sudden topographic variations of streaky fringes with a negative applied force when collected on a soft hydrocarbon film grown on a grating sample, whereas they showed stable topographic features with positive applied forces. The instability of HSAFM images with the negative applied force was explained by the transition between contact and noncontact regimes in the force-distance curve. When the grating surface was cleaned, and thus hydrophilic by removing the hydrocarbon film, enhanced imaging stability was observed at both positive and negative applied forces. The higher adhesive interaction between the tip and the surface explains the improved imaging stability. The effects of imaging rate on the imaging stability were tested on an even softer adhesive Escherichia coli biofilm deposited onto the grating structure. The biofilm and planktonic cell structures in HSAFM images were reproducible within the force deviation less than ~0.5 nN at the imaging rate up to 0.2 s per frame, suggesting that the force-feedback HSAFM was stable for various imaging speeds in imaging softer adhesive biological samples.

Coil-to-Bridge Transitions of Self-Assembled Water Chains Observed in a Nanoscopic Meniscus

“…The process becomes a kinetically activated one when the speed is higher than the threshold speed of ∼1 nm/s. In this speed regime, the number of oscillations and the transition positions are different between the approaching and retraction curves, as seen in the data reported earlier …”

“…In this speed regime, the number of oscillations and the transition positions are different between the approaching and retraction curves, as seen in the data reported earlier. 40 The observed shorter onset distance of ∼4 nm is the averaging effect of the larger probe on the force. The absence of the oscillatory feature in normal channels suggests that the nanoscale surface roughness of the larger probe results in an interference effect among FJC chain forces through the d variation in eq 2.…”

“…In this speed regime, the number of oscillations and the transition positions are different between the approaching and retraction curves, as seen in the data reported earlier. 40 …”

Direct observation of self-assembled chain-like water structures in a nanoscopic water meniscus

“…23,44,[50][51][52]66 These oscillatory forces appear to be unique characteristics of water in air because they have not been observed with the menisci formed by soft materials such as hydrocarbon films and biofilms. [57][58][59] The unique periodicities (~1 nm) in this study suggest that when the vapor, meniscus, and solid co-exist, the structure of confined water is different from the structure at the interface between bulk water and a solid surface. The gradual force increase and decrease during the periodicities suggest that the observed force-distance curves were generated not by a single evaporation or condensation process, but by an equilibrium process between evaporation (from meniscus to vapor) and condensation (from vapor to meniscus).…”

“…47,[53][54][55][56][57][58][59] The sample preparation, the experimental setup, and the determination of force conversion factor were detailed in our previous report. 54 It is worth noting that the Page 6 of 25 determination of the conversion factor requires the spring constant of the cantilever.…”