Dynamic atomic force microscopy methods

Friction, wear and lubrication between materials in contact are of fundamental importance in many pure and applied sciences. Owing to the development of experimental and computer-simulation techniques for studying these phenomena at the atomic scale, an understanding is beginning to emerge of the molecular mechanisms of tribology in thin films and at surfaces.

Nanotribology: friction, wear and lubrication at the atomic scale

A few years after the invention of the scanning tunneling microscope (STM), the atomic force microscope (AFM) was developed Instead of measuring tunneling current, a new physical quantity could be investigated with atomic-scale resolution: the force between a small tip and a chosen sample surface This paper reviews progress and recent results obtained with AFM and other closely related techniques in the field of nanotribology, and attempts to point out many of the unresolved questions that remain The goal of this paper is to demonstrate that AFM is capable of producing atomic-scale knowledge As such, the authors will focus upon some of the contributions of the AFM to nanotribology They will almost exclusively discuss results that shed light on the actual atomic and molecular processes taking place, as opposed to the more applied investigations of microscale properties which are also carried out with AFM They will accompany this discussion by mentioning related theoretical efforts and simulations, although their main emphasis will be upon experimental results and the techniques used to obtain them, as well as suggested future directions In many ways, AFM techniques for quantitative, fundamental nanotribology are only in a nascent stage; certain key issues such as force calibration,more » tip characterization, and the effects of the experimental environment, are not fully resolved or standardized The authors thus begin with a critical discussion of the relevant technical aspects with using AFM for nanotribology 289 refs« less

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Scratching the Surface: Fundamental Investigations of Tribology with Atomic Force Microscopy.

Mapping the spatial arrangement of chemical functional groups and their interactions is of significant importance to problems ranging from lubrication and adhesion to recognition in biological systems. A force microscope has been used to measure the adhesive and friction forces between molecularly modified probe tips and organic monolayers terminating in a lithographically defined pattern of distinct functional groups. The adhesive interactions between simple CH(3)/CH(3), CH(3)/COOH, and COOH/COOH functional groups correlate directly with friction images of sample surfaces patterned with these groups. Thus, by monitoring the friction between a specifically functionalized tip and sample, one can produce friction images that display predictable contrast and correspond to the spatial distribution of functional groups on the sample surface. Applications of this chemically sensitive imaging technique are discussed.

https://science.sciencemag.org/content/sci/265/5181/2071.full.pdf

Functional Group Imaging by Chemical Force Microscopy

The controlled transfer of organized monolayers of amphiphilic molecules from the airwater interface to a solid substrate was the first molecular-scale technology for the creation of new materials. However, the potential benefits of the technology envisioned by Langmuir and Blodgett in the 1930s have yet to be fully realized. Problems of reproducibility and defects and the lack of basic understanding of the packing of complex molecules in thin films have continued to thwart practical applications of Langmuir-Blodgett films and devices made from such films. However, modern high-resolution x-ray diffraction and scanning probe microscopy have proven to be ideal tools to resolve many of the basic questions involving thin organic films. Here, studies are presented of molecular order and organization in thin films of fatty acid salts, the prototypical system of Katharine Blodgett. Even these relatively simple systems present liquid, hexatic, and crystalline order; van der Waals and strained layer epitaxy on various substrates; wide variations in crystal symmetry and interfacial area with counterions; modulated superstructures; and coexisting lattice structures. The wide variety of possible structures presents both a challenge and an opportunity for future molecular design of organic thin-film devices.

https://science.sciencemag.org/content/sci/263/5154/1726.full.pdf

Langmuir-Blodgett films

THE study of chemical phase separation in multicomponent thin organic films typically involves the addition of a dye which is selectively more soluble in one of the phases, thereby making it possible to probe the domain structures by fluorescence microscopy1–4. The resolution of this approach is generally limited to tens of micrometres. The atomic force microscope, on the other hand, has recently proved useful for imaging organic thin films down to the atomic scale5–9, but this technique provides details of the overall film topography, rather than the chemical composition. Here we show that the recently developed friction force microscope10–13, which simultaneously measures both the normal and lateral forces on the scanning tip, can be used to image and identify compositional domains with a resolution of ∼5 A. Although the topography of the individual domains can be imaged with a standard atomic force microscope, it is the additional information provided by the friction measurement that allows them to be chemically differentiated.

Friction measurements on phase-separated thin films with a modified atomic force microscope

A correlation between friction and elasticity is drawn from measurements on organic thin films with a modified scanning force microscope (SFM). Local elastic compliance has been measured simultaneously with both topography and friction on the submicrometer scale on thin films of phase-separated mixtures of fluorocarbons and hydrocarbons. On the fluorocarbon domains, higher friction and lower Young`s modulus than on the hydrocarbon domains have been found. Variations in pH during sample preparation lead to differences in film formation, detected with the SFM as changes in topography, elasticity, and friction. On increasing pH, both the Young`s modulus and friction force are found to decrease. This unexpected result is discussed in terms of film formation and cohesive energy mechanisms.

Y. Gotoh

Papers

Friction measurements on phase-separated thin films with a modified atomic force microscope

Force Microscopy Study of Friction and Elastic Compliance of Phase-Separated Organic Thin Films

Friction force microscopy of mixed Langmuir-Blodgett films