Preparation of probe tips with well-defined spherical apexes for quantitative scanning force spectroscopy

Schwarz, Udo D.; Zwörner, O.; Köster, Peter; Wiesendanger, R.

doi:10.1116/1.589488

Cited by 13 publications

(4 citation statements)

References 18 publications

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“…Single-asperity friction forces obtained with an AFM are proportional to the true contact area if the friction is dominated by interfacial friction, the shear strength of the interface is constant, and the contact pairs are isotropic linearly elastic materials. 41,43,45,50,51,88,89 In these cases, the friction varies with load in a nonlinear way. In contrast, the friction of alkanephosphonic acid SAMs varied linearly with load.…”

Section: Discussionmentioning

confidence: 99%

Atomic contributions to friction and load for tip–self-assembled monolayers interactions

et al. 2008

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Scanning force microscopies ͑SFM͒ are being routinely used to examine the mechanical and tribological properties of materials with the goal of obtaining information, such as Young's Moduli and shear strengths from the experimental data ͓Unertl, J. Vac. Sci. Technol. A 17, 1779 ͑1999͔͒. Analysis of data obtained from an SFM experiment typically requires the use of continuum mechanics models to extract materials properties. When applying these models care must be taken to ensure that the experimental conditions meet the requirements of the model being applied. For example, despite many successful applications of the Johnson-Kendall-Roberts ͑JKR͒ model to SFM data, it does not take into account the presence of a compliant layer on the sample surface. Recent AFM experiments that examined the friction of self-assembled monolayers ͑SAMs͒ have confirmed that friction versus load data cannot be fit by the JKR model. The authors suggest that the penetration of the SAM by the tip gives rise to an additional contribution to friction due to "plowing" ͓Flater et al., Langmuir 23, 9242 ͑2007͔͒. Herein, molecular-dynamics simulations are used to study atomic contact forces between a spherical tip in sliding contact with a SAM. These simulations show that different regions around the tip contribute in unanticipated ways to the total friction between the tip and the monolayer and allow for the number and location of monolayer atoms contributing friction to be determined. The use of atomic contact forces within the monolayer, instead of forces on the rigid tip layers, allows for the contributions to friction force ͑and load͒ to be deconvoluted into forces that resist ͑repel͒ and assist ͑attract͒ tip motion. The findings presented here yield insight into the AFM experiments of SAMs and may have important consequences for the adaptation of continuum contact models for the contact between a sphere and surface where penetration into the sample is possible.

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Section: Discussionmentioning

confidence: 99%

Atomic contributions to friction and load for tip–self-assembled monolayers interactions

et al. 2008

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“…Quantitative nanotribology measurements were obtained with two different probe tips with curved apexes with radii of 45 ± 3 and 150 ± 10 nm. The tips were formed by coating the tips of tungsten carbide-coated silicon cantilever probes (MikroMasch, Wilsonville, OR) with a smooth hydrogenated amorphous carbon film using electron beam induced decomposition (EBID) in a transmission electron microscope (TEM). , This process forms a dense hydrocarbon coating on the tip, but the exact composition and hybridizations of the carbon atoms are not known. Both tips were imaged at high resolution by TEM before and after the AFM experiments to measure tip shape and radius and to determine the extent to which the tip may have changed during the experiment.…”

Section: Methodsmentioning

confidence: 99%

Atomic-Scale Friction on Diamond: A Comparison of Different Sliding Directions on (001) and (111) Surfaces Using MD and AFM

et al. 2007

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Atomic force microscopy (AFM) experiments and molecular dynamics (MD) simulations were conducted to examine single-asperity friction as a function of load, surface orientation, and sliding direction on individual crystalline grains of diamond in the wearless regime. Experimental and simulation conditions were designed to correspond as closely as state-of-the-art techniques allow. Both hydrogen-terminated diamond (111)(1 x 1)-H and the dimer row-reconstructed diamond (001)(2 x 1)-H surfaces were examined. The MD simulations used H-terminated diamond tips with both flat- and curved-end geometries, and the AFM experiments used two spherical, hydrogenated amorphous carbon tips. The AFM measurements showed higher adhesion and friction forces for (001) vs (111) surfaces. However, the increased friction forces can be entirely attributed to increased contact area induced by higher adhesion. Thus, no difference in the intrinsic resistance to friction (i.e., in the interfacial shear strength) is observed. Similarly, the MD results show no significant difference in friction between the two diamond surfaces, except for the specific case of sliding at high pressures along the dimer row direction on the (001) surface. The origin of this effect is discussed. The experimentally observed dependence of friction on load fits closely with the continuum Maugis-Dugdale model for contact area, consistent with the occurrence of single-asperity interfacial friction (friction proportional to contact area with a constant shear strength). In contrast, the simulations showed a nearly linear dependence of the friction on load. This difference may arise from the limits of applicability of continuum mechanics at small scales, because the contact areas in the MD simulations are significantly smaller than the AFM experiments. Regardless of scale, both the AFM and MD results show that nanoscale tribological behavior deviates dramatically from the established macroscopic behavior of diamond, which is highly dependent on orientation.

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“…Optical microscopy was used to measure the in-plane dimensions of the lever, namely, length and width, and transmission electron microscopy (TEM) was used to measure the small critical dimensions of the cantilevers, such as tip radius, tip height, and lever thickness. The tip shape was determined using TEM. − TEM images were taken using either a JEOL 100CX or a JEOL 200CX (Tokyo, Japan), with resolution for high-magnification tip images of approximately 3 nm. The radius of the tip was determined by a fitting a circle to the end of the tip.…”

Section: Methodsmentioning

confidence: 99%

Nanotribology of Octadecyltrichlorosilane Monolayers and Silicon: Self-Mated versus Unmated Interfaces and Local Packing Density Effects

2007

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We use atomic force microscopy (AFM) to determine the frictional properties of nanoscale single-asperity contacts involving octadecyltrichlorosilane (OTS) monolayers and silicon. Quantitative AFM measurements in the wearless regime are performed using both uncoated and OTS-coated silicon AFM tips in contact with both uncoated and OTS-coated silicon surfaces, providing four pairs of either self-mated or unmated interfaces. Striking differences in the frictional responses of the four pairs of interfaces are found. First, lower friction occurs with OTS present on either the tip or substrate, and friction is yet lower when OTS is present on both. Second, the shape of the friction versus load plot strongly depends on whether the substrate is coated with OTS, regardless of whether the tip is coated. Uncoated substrates exhibit the common sublinear dependence, consistent with friction being directly proportional to the area of contact. However, coated substrates exhibit an unusual superlinear dependence. These results can be explained qualitatively by invoking molecular plowing as a significant contribution to the frictional behavior of OTS. Direct in situ comparison of two intrinsic OTS structural phases on the substrate is also performed. We observe frictional contrast for different local molecular packing densities of the otherwise identical molecules. The phase with lower packing density exhibits higher friction, in agreement with related previous work, but decisively observed here in single, continuous images involving the same molecules. Lateral stiffness measurements show no distinction between the two OTS structural phases, demonstrating that the difference in friction is not due to divergent stiffnesses of the two phases. Therefore, the packing density directly affects the interface's intrinsic resistance to friction, that is, the interfacial shear strength.

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Preparation of probe tips with well-defined spherical apexes for quantitative scanning force spectroscopy

Cited by 13 publications

References 18 publications

Atomic contributions to friction and load for tip–self-assembled monolayers interactions

Atomic contributions to friction and load for tip–self-assembled monolayers interactions

Atomic-Scale Friction on Diamond: A Comparison of Different Sliding Directions on (001) and (111) Surfaces Using MD and AFM

Nanotribology of Octadecyltrichlorosilane Monolayers and Silicon: Self-Mated versus Unmated Interfaces and Local Packing Density Effects

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