We provide a method to characterize the tip radius of an atomic force microscopy in situ by monitoring the dynamics of the cantilever in ambient conditions. The key concept is that the value of free amplitude for which transitions from the attractive to repulsive force regimes are observed, strongly depends on the curvature of the tip. In practice, the smaller the value of free amplitude required to observe a transition, the sharper the tip. This general behavior is remarkably independent of the properties of the sample and cantilever characteristics and shows the strong dependence of the transitions on the tip radius. The main advantage of this method is rapid in situ characterization. Rapid in situ characterization enables one to continuously monitor the tip size during experiments. Further, we show how to reproducibly shape the tip from a given initial size to any chosen larger size. This approach combined with the in situ tip size monitoring enables quantitative comparison of materials measurements between samples. These methods are set to allow quantitative data acquisition and make direct data comparison readily available in the community.

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.

Measuring the level of hydrophilicity of heterogeneous surfaces and the true height of water layers that form on them in hydrated conditions has a myriad of applications in a wide range of scientific and technological fields. Here, we describe a true non-contact mode of operation of atomic force microscopy in ambient conditions and a method to establish the source of apparent height. A dependency of the measured water height on operational parameters is identified with water perturbations due to uncontrolled modes of imaging where intermittent contact with the water layer, or even the surface, might occur. In this paper we show how to (1) determine when the water is being perturbed and (2) distinguish between four different interaction regimes. Each of the four types of interaction produces measurements ranging from fractions of the true height in one extreme to values which are as large as four times the real height in the other. We show the dependence of apparent height on the interaction regime both theoretically and empirically. The agreement between theory and experiment on a BaF2(111) sample displaying wet and un-wet regions validates our results.

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.

Dielectrophoretic assembly and atomic force microscopy modification of reduced graphene oxide J. Appl. Phys. 110, 114515 (2011) Quasi in situ scanning force microscope with an automatic operated reaction chamber Rev. Sci. Instrum. 82, 113709 (2011) Mapping of conservative and dissipative interactions in bimodal atomic force microscopy using open-loop and phase-locked-loop control of the higher eigenmode Appl. Phys. Lett. 99, 074103 (2011) Wideband phase-locked loop circuit with real-time phase correction for frequency modulation atomic force microscopy Rev. Sci. Instrum. 82, 073707 (2011) Additional information on J. Appl. Phys. In ambient conditions, nanometric water layers form on hydrophilic surfaces covering them and significantly changing their properties and characteristics. Here we report the excitation of subharmonics in amplitude modulation atomic force microscopy induced by intermittent water contacts. Our simulations show that there are several regimes of operation depending on whether there is perturbation of water layers. Single period orbitals, where subharmonics are never induced, follow only when the tip is either in permanent contact with the water layers or in pure noncontact where the water layers are never perturbed. When the water layers are perturbed subharmonic excitation increases with decreasing oscillation amplitude. We derive an analytical expression which establishes whether water perturbations compromise harmonic motion and show that the predictions are in agreement with numerical simulations. Empirical validation of our interpretation is provided by the observation of a range of values for apparent height of water layers when subharmonic excitation is predicted.