BackgroundAccurate mechanical characterization by the atomic force microscope at the highest spatial resolution requires that topography is deconvoluted from indentation. The measured height of nanoscale features in the atomic force microscope (AFM) is almost always smaller than the true value, which is often explained away as sample deformation, the formation of salt deposits and/or dehydration. We show that the real height of nano-objects cannot be obtained directly: a result arising as a consequence of the local probe-sample geometry.Methods and FindingsWe have modeled the tip-surface-sample interaction as the sum of the interaction between the tip and the surface and the tip and the sample. We find that the dynamics of the AFM cannot differentiate between differences in force resulting from 1) the chemical and/or mechanical characteristics of the surface or 2) a step in topography due to the size of the sample; once the size of a feature becomes smaller than the effective area of interaction between the AFM tip and sample, the measured height is compromised. This general result is a major contributor to loss of height and can amount to up to ∼90% for nanoscale features. In particular, these very large values in height loss may occur even when there is no sample deformation, and, more generally, height loss does not correlate with sample deformation. DNA and IgG antibodies have been used as model samples where experimental height measurements are shown to closely match the predicted phenomena.ConclusionsBeing able to measure the true height of single nanoscale features is paramount in many nanotechnology applications since phenomena and properties in the nanoscale critically depend on dimensions. Our approach allows accurate predictions for the true height of nanoscale objects and will lead to reliable mechanical characterization at the highest spatial resolution.
Standard models accounting for capillary interactions typically involve expressions that display a significant decay in force with separation. These forces are commonly investigated in the nanoscale with the atomic force microscope.Here we show that experimental observations are not predicted by these common expressions in dynamic interactions. Since in dynamic atomic force microscopy methods the cantilever is vibrated over the surface, the nanoscopic tip is submitted to nonlinear interactions with the sample in a periodic fashion. That is, the force dependencies involved in dynamic interactions in the nanoscale can be probed. We describe two extreme experimental scenarios in these dynamic interactions and interpret them as single and multiple asperity cases. In both extremes there is a predominantly attractive component of the net force that is relatively independent of distance and that ranges several nanometers above the surface. The distance dependence approximates that of a square well. Experimental data have been acquired for cantilevers of different stiffness and fundamental resonant frequency indicating that the distance dependencies provided here are valid for a relatively large range of frequencies. The reproducibility of our experiments and the accurate prediction of the experimental data that we present imply that future investigations should take the phenomena that we report into account to describe and interpret dynamic capillary interactions.
We describe fundamental energy dissipation in dynamic nanoscale processes in terms of the localization of the interactions. In this respect, the areal density of the energy dissipated and the effective area of interaction in which each process occurs are calculated for four elementary dissipative processes. It is the ratio between these two, which we term M that provides information about how localized the interactions are.We show that neither the phase lag, nor the magnitude of the energy dissipated alone provide information about energy localization but M has to be considered instead.Energy dissipation, nanoscale processes, viscosity, hysteresis, atomic force microscopy
A way to operate fundamental mode amplitude modulation atomic force microscopy is introduced which optimizes stability and resolution for a given tip size and shows negligible tip wear over extended time periods (∼24 h). In small amplitude small set-point (SASS) imaging, the cantilever oscillates with sub-nanometer amplitudes in the proximity of the sample, without the requirement of using large drive forces, as the dynamics smoothly lead the tip to the surface through the water layer. SASS is demonstrated on single molecules of double-stranded DNA in ambient conditions where sharp silicon tips (R ∼ 2–5 nm) can resolve the right-handed double helix.
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