Atomic force microscopy imaging principle

A new alternative to solve this problem is atomic force microscopy (AFM) which is an emerging surface characterization tool in a wide variety of materials science fields. The method is relatively easy and offers a subnanometer or atomic resolution with little sample preparation required. The basic principle involved is to utilize a cantilever with a spring constant weaker than the equivalent spring between atoms. This way the sharp tip of the cantilever, which is microfabricated from silicon, silicon oxide or silicon nitride using photolithography, mechanically scans over a sample surface to image its topography. Typical lateral dimensions of the cantilever are on the order of 100 pm and the thickness on the order of 1 pm. Cantilever deflections on the order of 0.01 nm can be measured in modem atomic force microscopes. 

On the other hand, optical microscopy, confocal microscopy, ellipsometry, scanning electron microscopy (SEM), scanning tunneling microscopy (STM), atomic force microscopy (AFM) and total internal reflection fluorescence (TIRF) are the main microscopic methods for imaging the surface structure. There are many good books and reviews on spectroscopic and chemical surface analysis methods and microscopy of surfaces description of the principles and application details of these advanced instrumental methods is beyond the scope of this book. 

AFM (atomic force microscopy) was developed about five years after STM (see Atomic force microscopy). It relies upon the measurement of the force of interaction between a sharp tip and a sample. Being based upon the measurement of force rather than current, it is applicable in principle to any material. The technique has great versatility, and as in the case of STM, imaging may be carried out under ambient or fluid conditions. The tip is attached to a flexible cantilever, which is rastered across a sample surface. As the interaction force between the tip and the sample changes, the deflection of the cantilever varies. The cantilever deflection is readily measured (by optical deflection in most commercial systems) and is proportional to the interaction force leading to quantification, provided the spring constant of the lever is known. Either the cantilever or the sample is mounted on a piezoelectric crystal in order to exact fine control over the relative movements of the tip. 

Within the numerous proximal probe techniques developed in the years following the breakthrough inventions of the STM (1) and AFM (5), scanning force microscopy (8) represents a family of scanning probe techniques that rely in their contrast mechanism on various forces between probe tip and sample (9-12) (see Atomic Force Microscopy). In order to provide a basis for an understanding and appreciation of the SFM work on polymers (13-18), as presented in this review, the basic principles of SFM, as well as selected imaging modes, are briefly discussed. 

Since the invention of the atomic force microscope (AFM) by Binning et al. (48), which can generate atomic-scale images of materials, this new tool has often been used in combination with electron microscopy to examine the surface and porosity of pillared clays. The principle of this technique is based on scanning the surface of a sample with a very sharp tip, brought within close proximity of the sample, to map the contours of the surface. Hartman et al. (49) demonstrated the ability of the AFM to image molecular-scale features of montmorillonite and illite. Occelli et al. (50,51) conducted a profound characterization of Al-pillared... 

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