Imaging in AFM

AFM, especially when used in noncontact modes, is an ideal tool to image delicate morphological details of surfaces of polymer films [8a]. When these films are coated with gold for scanning electron microscopy (SEM), features with typical dimensions of less than —50 run become buried and may disappear from SEM images. In AFM imaging this is not a problem as no conductive coating is required. 

In order to obtain a true image in AFM (as well as in any other microscopy or spectroscopy experiment), the observed result and the instrumental line-shape must be deconvoluted. This is often possible in spectroscopy, but often impossible in AFM, especially for artefacts at length scales that are comparable with the tip dimensions. This tip-imaging artefact cannot be eliminated. If the sample is an insulator it may also be locally charged. Image artefacts are also introduced due to surface deformation. 

Figure Bl.19.19. Examples of inaccessible features in AFM imaging. L corresponds to the AFM tip. The dotted curves show the image that is recorded in the case of (a) depressions on the underside of an object and (b) mounds on the top surface of an object. M- L andMoi correspond to convolutions of the surface features with the tip shape. (Taken from [ ], figure 2.)...

In AFM, the relative approach of sample and tip is nonnally stopped after contact is reached. Flowever, the instrument may also be used as a nanoindenter, measuring the penetration deptli of the tip as it is pressed into the surface of the material under test. Infomiation such as the elastic modulus at a given point on the surface may be obtained in tliis way [114], altliough producing enough points to synthesize an elastic modulus image is very time consuming. 

Allen M J, Hud N V, Balooch M, Tench R J, Siekhaus W J and Balhorn R 1992 Tip-radius-induced artifacts in AFM images of protamine-complexed DMA fibers Ultramicroscopy 42-A4 1095... 

One of the key issues of mechanical behavior of multicomponent materials such as TPV is the stmcture and properties of the interface regions. The phase image in Figure 20.1 Id shows a part of TPV sample with few mbber domains surrounded by iPP matrix. An extended rectangle area outlined with a white dotted box includes several interfaces between mbber domains and the plastic matrix. Examination of the interfaces is a challenging task and one possible approach is AFM-based... 

Atomic force microscope (AFM) is a powerful nanotechnology tool for molecular imaging and manipulations. One major factor limiting resolution in AFM to observe individual biomolecules such as DNA is the low sharpness of the AFM tip that scans the sample. Nanoscale 1,3,5,7-tetrasubstituted adamantane is found to serve as the molecular tip for AFM and may also find application in chemically well-defined objects for calibration of commercial AFM tips [113]. 

Sample preparation for AFM analysis is relatively simple. Generally, a desired amount of sample is absorbed onto a smooth and clean substrate surface, for example, a freshly cleaved mica surface. For example, to prepare a food macromolecule sample for AFM imaging in air, the diluted macromolecule solution is disrupted by vortexing. Then, a small aliquot (tens of microliters) of vortexed solution is deposited onto a surface of freshly cleaved mica sheet by pipette. The mica surface is air dried before the AFM scan. A clean surrounding is required to avoid the interference of dust in the air. Molecular combing or fluid fixation may be applied to manipulate the molecule to get more information. 

Whenever possible, we perform experiments in the same part of the surface before and after some operation. The operation may involve exposure of the surface to a reactant such as water or 02, for example or it may be some switch in the functionality of the microscope, for instance, imaging first in AFM mode and then in STM mode. For reactions, we simply stop the scan and retract the tip by up to 1000 nm, which ensures it does not crash when we admit gases or vapors to the chamber via the high-precision leak valves. 

Precise thickness measurements by TEM require sections transverse to the basal lamellar surface. Conversely, only lamellae that can be identified as untilted "edge-on" or "flat-on" in AFM images are suitable for thickness analysis. The average thickness obtained by these techniques is based on sampling microscopic areas and will only be correct if the morphology is uniform in the sample. Micrographs taken from different areas of the specimen are usually studied, and statistical analysis of histograms used for quantitative analysis [255,256]. 

The concept of resolution in AFM is different from radiation-based microscopies because AFM imaging is a three-dimensional imaging technique. There is an important distinction between images resolved by wave optics and those resolved by scanning probe techniques. The former is limited by diffraction, whereas the latter is limited primarily by apical probe geometry and sample geometry. Usually the width of a DNA molecule is loosely used as a measure of resolution, because it has a known diameter of 2.0 nm in its B form. 

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