Quantification of van der Waals forces in bimodal and trimodal AFM.

The multifrequency formalism is generalized and exploited to quantify attractive forces, i.e., van der Waals interactions, with small amplitudes or gentle forces in bimodal and trimodal atomic force microscopy (AFM). The multifrequency force spectroscopy formalism with higher modes, including trimodal AFM, can outperform bimodal AFM for material property quantification. Bimodal AFM with the second mode is valid when the drive amplitude of the first mode is approximately an order of magnitude larger than that of the second mode. The error increases in the second mode but decreases in the third mode with a decreasing drive amplitude ratio. Externally driving with higher modes provides a means to extract information from higher force derivatives while enhancing the range of parameter space where the multifrequency formalism holds. Thus, the present approach is compatible with robustly quantifying weak long range forces while extending the number of channels available for high resolution.

Hydration Dynamics and the Future of Small-Amplitude AFM Imaging in Air.

Here, we discuss the effects that the dynamics of the hydration layer and other variables, such as the tip radius, have on the availability of imaging regimes in dynamic AFM-including multifrequency AFM. Since small amplitudes are required for high-resolution imaging, we focus on these cases. It is possible to fully immerse a sharp tip under the hydration layer and image with amplitudes similar to or smaller than the height of the hydration layer, i.e., ~1 nm. When mica or HOPG surfaces are only cleaved, molecules adhere to their surfaces, and reaching a thermodynamically stable state for imaging might take hours. During these first hours, different possibilities for imaging emerge and change, implying that these conditions must be considered and reported when imaging.

Rapid discrimination of chemically distinctive surface terminations in 2D material based heterostructures by direct van der Waals identification.

We demonstrate that surfaces presenting heterogeneous and atomically flat domains can be directly and rapidly discriminated via robust intensive quantifiables by exploiting one-pass noninvasive methods in standard atomic force microscopy (AFM), single ∼2 min passes, or direct force reconstruction, i.e., ∼103 force profiles (∼10 min collection time), allowing data collection, interpretation, and presentation in under 20 min, including experimental AFM preparation and excluding only sample fabrication, in situ and without extra experimental or time load. We employ a misfit SnTiS3 compound as a model system. Such heterostructures can be exploited as multifunctional surface systems and provide multiple support sites with distinguishable chemical, mechanical, or opto-electronic distinct properties. In short, they provide an ideal model system to exemplify how current AFM methods can significantly support material discovery across fields.

Explaining doping in material research (Hf substitution in ZnO films) by directly quantifying the van der Waals force.

Non-monotonic behavior has been observed in the optoelectronic properties of ZnO thin films as doped with Hf (HZO). Here we propose that two competing mechanisms are responsible for such behaviour. Specifically, we propose that provided two crystal orientations dominate film growth, only one of them might be responsible for direct Hf substitution. Nonmonotonic behaviour is expected at once by considering that preferential growth of the crystal that allows for direct Hf substitution is inhibited by Hf concentration in the manufacturing process. This inhibition would also be a thermodynamic consequence of Hf substitution. Maxima in Hf substitution is thus reached at a point where enough crystals exhibit the preferential orientation, and where enough Hf is present on the surface for substitution. Outside this optimum scenario, Hf substitution can only decrease. We interpret the surface phenomena by discussing surface energy and the van der Waals forces as measured experimentally by means of atomic force microscopy.

Insights into graphene wettability transparency by locally probing its surface free energy.

In this work, we study the surface energy of monolayer, bilayer and multilayer graphene coatings, produced through exfoliation of natural graphite flakes and chemical vapor deposition. We employ bimodal atomic force microscopy and micro-Raman spectroscopy for high spatial resolution and large area scanning of force of adhesion on the regions of the graphene/SiO2 surface with different graphene layers. Our measurements show that the interface conditions between graphene and SiO2 dominate the experimentally observed graphene surface energy. This finding sheds new light on the controversy surrounding graphene transparency studies. By separating the surface energy into polar and non-polar interactions, our findings suggest that monolayer graphene is nearly van der Waals opaque but partially transparent (near 60%) to polar interactions, which is further supported by characterizing graphene on the copper surface and two levels of density functional theory simulation. In addition to providing quantitative insight into the surface interactions of complicated graphene coatings, this work demonstrates a new route to nondestructively monitor the interface between graphene and coated substrates.

Direct Measurement of the Magnitude of the van der Waals Interaction of Single and Multilayer Graphene.

Vertical stacking of monolayers via van der Waals (vdW) assembly is an emerging field that opens promising routes toward engineering physical properties of two-dimensional materials. Industrial exploitation of these engineering heterostructures as robust functional materials still requires bounding their measured properties so as to enhance theoretical tractability and assist in experimental designs. Specifically, the short-range attractive vdW forces are responsible for the adhesion of chemically inert components and are recognized to play a dominant role in the functionality of these structures. Here, we reliably quantify the strength of ambient vdW forces in terms of an effective Hamaker coefficient for chemical vapor deposition-grown graphene and show how it scales by a factor of two or three from single to multiple layers on standard supporting surfaces such as copper or silicon oxide. Furthermore, direct measurements on freestanding graphene provide the means to discern the interplay between the vdW potential of graphene and its supporting substrate. Our results demonstrated that the underlying substrates could be controllably exploited to enhance or reduce the vdW force of graphene surfaces. We interpret the physical phenomena in terms of a Lifshitz theory-based analytical model.

Surface alteration of calcite: interpreting macroscopic observations by means of AFM.

Wettability has been recognized to play a fundamental role in the efficacy of water flooding processes of carbonate oil and gas reservoirs. However, the theoretical mechanism governing this process is still not entirely understood. This can be partly attributed to the absence of ad hoc tools and standardized sample-preparation methodologies for comprehensive transient characterization of the mineral surface. Here, we use atomic force microscopy (AFM) to investigate the effect of different calcite sample-preparation methodologies in estimating the macroscopic water static contact angle (SCA). Single crystal calcite surfaces are aged in deionized (DI) water baths, for different exposure times, and dried by different techniques, to reveal SCA discrepancies. Trends and observations are explained with the use of time-dependent adhesion maps of the surface obtained by bimodal AFM. In this context, the AFM interpretation of macroscopic observations provides a means to single out the different factors influencing wettability, thus allowing for a more standardized description of the processes responsible for the modification of the affinity between the mineral rock and injected water.

Multifrequency AFM: from origins to convergence.

Since the inception of the atomic force microscope (AFM) in 1986, influential papers have been presented by the community and tremendous advances have been reported. Being able to routinely image conductive and non-conductive surfaces in air, liquid and vacuum environments with nanoscale, and sometimes atomic, resolution, the AFM has long been perceived by many as the instrument to unlock the nanoscale. From exploiting a basic form of Hooke's law to interpret AFM data to interpreting a seeming zoo of maps in the more advanced multifrequency methods however, an inflection point has been reached. Here, we discuss this evolution, from the fundamental dilemmas that arose in the beginning, to the exploitation of computer sciences, from machine learning to big data, hoping to guide the newcomer and inspire the experimenter.

Divergent surface properties of multidimensional sp (2) carbon allotropes: the effect of aging phenomena.

Despite the current interest in the scientific community in exploiting divergent surface properties of graphitic carbon allotropes, conclusive differentiation remains elusive even when dealing with parameters as fundamental as adhesion. Here, we set out to provide conclusive experimental evidence on the time evolution of the surface properties of highly oriented pyrolytic graphite (HOPG), graphene monolayer (GML) and multiwalled carbon nanotubes (MWCNTs) as we expose these materials to airborne contaminants, by providing (1) statistically significant results based on large datasets consisting of thousands of force measurements, and (2) errors sufficiently self-consistent to treat the comparison between datasets in atomic force microscopy (AFM) measurements. We first consider HOPG as a model system and then employ our results to draw conclusions from the GML and MWCNT samples. We find that the surface properties of aged HOPG are indistinguishable from those of aged GML and MWCNT, while being distinct from those of cleaved HOPG. Herein, we provide a sufficient body of evidence to disregard any divergence in surface properties for multidimensional sp (2) carbon allotropes that undergo similar aging processes.

The power laws of nanoscale forces under ambient conditions.

We report a power law derived from experimental atomic force microscopy (AFM) data suggesting a nano to mesoscale transition in force-distance dependencies. Our results are in relative agreement with the Hamaker and Lifshitz theories for van der Waals forces for the larger tip radii only.