AFM operation

In this section, we will describe some experiments which we have performed using the above-mentioned nano-Raman microscope. Figure 2.6a shows the Raman spectmm of an adenine nanocrystal of height 7 nm and width 30 nm [19]. Several Raman bands are observed as the probe tip is near enough to the sample (AFM operation is made in contact mode). These bands, except the one appearing at 924 cm, are assigned as the vibrational modes, inherent to the adenine molecule, according to the molecular orbital calculation. For examples, two major bands, one at... 

FIGURE 6.2 Diagrams of different AFM operating modes. (A) Contact mode and (B) dynamic mode for topographic imaging. (C) Force spectroscopy mode for interaction probing. Reprinted with permission from Liu and Wang (2010). 

Figure 3.3 The interaction distances of the different modes of AFM operation.

The versatility of AFM is exemplified by the number of different operation modes, which have been employed with various degrees of success for the analysis of DNA molecules on surfaces. As mentioned before, AFM operates by measuring the attractive or repulsive forces between a tip and the specimen using a feedback system, with the cantilever deflection yielding the actual topography of the specimen. Different setups of the feedback and cantilever deflection result in different AFM operation modes, as summarised in Table 1. 

In the early years of AFM operation, the cantilevers were cut from a metal foil, and the tips were made from crushed diamond particles, picked up by a piece of eyebrow hair, and painstakingly glued manually on the cantilevers. This situation has changed completely since the methods for mass production of cantilevers with integrated tips were developed. A review of various methods for making cantilevers using standard microfabrication techniques was published by Albrecht et al. (1990), and an improved method is described by Akamine et al. (1990). Those AFM cantilevers with integrated tips are now available commercially. 

The AFM operates in an analogous fashion. A sharp tip mounted on a weak cantilever is rastered across the sample. Light interference or reflection is used to measure the deflection of the cantilever. The light signal measured as a function of sample location is used to generate a map of the surface topography. Because mechanical forces are responsible for the deflection of the probe arm, conductors as well as insulators can be studied. STM and AFM provide local real space images of the surface and are thus... 

In the longer term, picoindentation instruments are likely to be widely used to extend the technique to a still smaller scale, with the help of techniques developed for atomic force microscopy. Already, plastic deformation at depths of a few atomic layers, as well as the effect of surface forces, have been quantified by means of depth-load measurements, using a point force microscope, i.e. an AFM operated in static (non-scanning) mode (Burnham Colton, 1989). 

AFM operation requires a minimum of vibrations. These vibrations refer to not only building vibrations, but also vibrations caused by airflow, persons walking in the lab, and equipment, such as personal computers, the AFM controller, etc. To damp out vibrations, AFM scan units are typically placed on passive and/or active vibration damping systems, such as ... 

The imaging of biologically relevant polymers must be performed in most cases under liquids in order to reduce the forces between the scanned probe tip and the sample surface or, more importantly, to ensure that the polymers of interest retain their integrity, shape, and biological function when studied by AFM approaches. This Chapter will provide an introduction to AFM operation under liquid and will elucidate the peculiarities of force microscopy operation in conjunction with a liquid cell. Finally, ex situ and in situ studies of biopolymeric specimens will be highlighted. 

As mentioned, in AFM studies of biopolymers the use of a suitable liquid cell is indispensable in many cases. On the one hand, biopolymers or even living cells may be studied in vitro under natural conditions (pH, temperature, salt, etc.) and variations of these conditions is often possible during the experiment [71-74], on the other hand excessive normal and lateral forces can be reduced to a minimum, which still allows one to image and study the biopolymeric samples non-invasively [79-81], Hence we will first provide an introduction to the use of the mentioned liquid cells and then treat contact mode AFM and intermittent contact mode AFM operation under liquid. The procedures and operation principles discussed can of course be readily extended to the study of non-biological polymers (see e.g. Chap. 4). 

Fig. 3.37 Schematic of AFM operation under liquid without rubber ring. A water drop, which is spanned between liquid cell and sample, encloses the imaged area incl. tip and cantilever completely...

For TM-AFM operation under liquid, the system is set up similar to the contact mode in liquid. By contrast, the cantilevers used are typically much softer than for operation in air. Often conventional contact mode levers (spring constant of 0. 3-few N/m) are used single beam cantilevers work in some cases better than triangular levers. The tuning of the lever is performed after the crude approach has... 

---