Deron A. Walters

Tapping mode atomic force microscopy in liquids

Tapping mode atomic force microscopy in liquids gives a substantial improvement in imaging quality and stability over standard contact mode. In tapping mode the probe‐sample separation is modulated as the probe scans over the sample. This modulation causes the probe to tap on the surface only at the extreme of each modulation cycle and therefore minimizes frictional forces that are present when the probe is constantly in contact with the surface. This imaging mode increases resolution and reduces sample damage on soft samples. For our initial experiments we used a tapping frequency of 17 kHz to image deoxyribonucleic acid plasmids on mica in water. When we imaged the same sample region with the same cantilever, the plasmids appeared 18 nm wide in contact mode and 5 nm in tapping mode.

Short cantilevers for atomic force microscopy

We have designed and tested a family of silicon nitride cantilevers ranging in length from 23 to 203 μm. For each, we measured the frequency spectrum of thermal motion in air and water. Spring constants derived from thermal motion data agreed fairly well with the added mass method; these and the resonant frequencies showed the expected increase with decreasing cantilever length. The effective cantilever density (calculated from the resonant frequencies) was 5.0 g/cm3, substantially affected by the mass of the reflective gold coating. In water, resonant frequencies were 2 to 5 times lower and damping was 9 to 24 times higher than in air. Thermal motion at the resonant frequency, a measure of noise in tapping mode atomic force microscopy, decreased about two orders of magnitude from the longest to the shortest cantilever. The advantages of the high resonant frequency and low noise of a short (30 μm) cantilever were demonstrated in tapping mode imaging of a protein sample in buffer. Low‐noise images were tak...

Studies of vibrating atomic force microscope cantilevers in liquid

An atomic force microscope (AFM) design providing a focused spot of order 7 μm in diameter was used to analyze the motion of vibrating cantilevers in liquid. Picking an operating frequency for tapping mode AFM operation in liquid is complex because there is typically a large number of sharp peaks in the response spectrum of cantilever slope amplitude versus drive frequency. The response spectrum was found to be a product of the cantilever’s broad thermal noise spectrum and an underlying fluid drive spectrum containing the sharp peaks. The geometrical shape of transverse cantilever motion was qualitatively independent of the fluid drive spectrum and could be approximately reproduced by a simple theoretical model. The measurements performed give new insights into the behavior of cantilevers during tapping mode AFM operation in liquid.

Modification of calcite crystal growth by abalone shell proteins: an atomic force microscope study.

A family of soluble proteins from the shell of Haliotis rufescens was introduced over a growing calcite crystal being scanned in situ by an atomic force microscope (AFM). Atomic step edges on the crystal surface were altered in shape and speed of growth by the proteins. Proteins attached nonuniformly to the surface, indicating different interactions with crystallographically different step edges. The observed changes were consistent with the habit modification induced by this family of proteins, as previously observed by optical microscopy. To facilitate further studies in this area, AFM techniques and certain AFM imaging artifacts are discussed in detail.

Bias-dependent molecular-level structure of electrical double layer in ionic liquid on graphite.

Here we report the bias-evolution of the electrical double layer structure of an ionic liquid on highly ordered pyrolytic graphite measured by atomic force microscopy. We observe reconfiguration under applied bias and the orientational transitions in the Stern layer. The synergy between molecular dynamics simulation and experiment provides a comprehensive picture of structural phenomena and long and short-range interactions, which improves our understanding of the mechanism of charge storage on a molecular level.

Rapid imaging of calcite crystal growth using atomic force microscopy with small cantilevers

Using a 26 μm cantilever with a resonant frequency of 100 kHz in water, we were able to obtain sequential images of calcite crystal steps growing from a screw dislocation. The small cantilever permitted acquisition of 250 nm images at scan rates of 104 lines/s (1.2 s/image). From this sequence we directly measured critical step lengths (the length of the shortest step that can advance) of 6–21 nm. These values provided a rough estimate of (0.25±0.13 J/m2) for the step energy per unit length per unit step height on the (104) face of calcite.

Atomic Force Microscopy of the Nacreous Layer in Mollusc Shells

We present atomic force microscopy (AFM) observations of the aragonite tablets of mature nacre in two types of mollusc, a bivalve (Atrina sp.) and a gastropod (Haliotis rufescens). By imaging in liquids it was possible to dissolve away the nacre layer by layer to reveal both the structure of a single tablet and its relation to vertically adjacent tablets. Atrina tablets (inner face) had a concave appearance; the central depression was surrounded by elongate rings that mimicked the orientation and aspect ratio of the unit cell viewed along the c axis. The tablet surfaces had a rough texture, and flat (001) planes of aragonite were rarely observed. Unit cell orientations were generally aligned both vertically and laterally between tablets of Atrina. Etching tablets with HCI initially removed the elongate rings and produced etch rows parallel to the a axis. Further etching of bleached Atrina nacre lifted off individual tablets to reveal underlying nacreous layers, showing no morphological registry between vertically adjacent tablets. The nacreous structure of Haliotis differed from Atrina in three ways: (i) the tablets were flatter and showed no elongate rings; (ii) the positions of the central depressions approximately repeated between nacreous layers, showing that the (presumed) nucleation sites line up along a given stack; and (iii) the unit cell orientations were not preserved between laterally adjacent tablets but were approximately aligned between vertically adjacent tablets.

Atomic force microscope for small cantilevers

We have designed and built an atomic force microscope (AFM) with optical beam deflection detection providing a focused spot size of 1.6 micrometers in diameter. This small spot size was implemented with a variable focus adjustment that allows us to re-focus on each cantilever. This design opens up the usage of a new range of small cantilevers with low-noise characteristics. We have microfabricated novel aluminum cantilevers with dimensions as small as 9 micrometers in length and 2.5 micrometers in width and have characterized them with this new AFM. The resonance frequency of the smallest cantilever was 2.5 MHz in air and 0.94 MHz in water. We demonstrated the imaging capabilities of the AFM head by imaging abalone nacre with a 10 micrometers long cantilever using the tapping mode in liquid at a drive frequency of 442 KHz.

Atomic force microscopy using small cantilevers

We have applied a new generation of short cantilevers with high resonant frequencies to tapping mode atomic force microscopy of a process in situ. Crystal growth in the presence of protein has been imaged stably at 79 lines/s (1.6 s/image), using a 26 micrometers long cantilever with a spring constant of 0.66 N/m at a tapping frequency of 90.9 kHz. This high scan speed nearly eliminated distortion in the step edge motion and allowed imaging of finer features along the step edges. Atomic force microscopy with short cantilevers therefore allows higher resolution imaging of crystal growth in space as well as time.

Generalized Hertz model for bimodal nanomechanical mapping

Summary Bimodal atomic force microscopy uses a cantilever that is simultaneously driven at two of its eigenmodes (resonant modes). Parameters associated with both resonances can be measured and used to extract quantitative nanomechanical information about the sample surface. Driving the first eigenmode at a large amplitude and a higher eigenmode at a small amplitude simultaneously provides four independent observables that are sensitive to the tip–sample nanomechanical interaction parameters. To demonstrate this, a generalized theoretical framework for extracting nanomechanical sample properties from bimodal experiments is presented based on Hertzian contact mechanics. Three modes of operation for measuring cantilever parameters are considered: amplitude, phase, and frequency modulation. The experimental equivalence of all three modes is demonstrated on measurements of the second eigenmode parameters. The contact mechanics theory is then extended to power-law tip shape geometries, which is applied to analyze the experimental data and extract a shape and size of the tip interacting with a polystyrene surface.