Atomic force microscope principles

Figure Bl.19.18. Schematic of an atomic force microscope showing the optical lever principle.

DNA arrays have been also generated by Dip-Pen Nanolithography (DPN) [80]. DPN involves the transfer of NAs directly from a coated Atomic Force Microscope (AFM) tip to the substrate of interest by virtue of direct molecular diffusion. Using this technique, thiol-modified ONDs have been patterned onto gold substrates and acrylamide-modified ONDs onto glass sHdes that were previously modified with mercaptopropyltrimethoxysilane. Feature sizes ranging from many micrometers to less than 100 nanometers could be obtained. The deposition of two different OND sequences onto the same substrate has also been reported [80], but the appHcation of this principle to the fabrication of high-density arrays remains to be addressed. 

The Atomic Force Microscope. The principle setup of an AFM is comparable to that of an STM, except that the tunneling tip is replaced by a force sensor (cantilever). A schematic view of an AFM is shown in Fig. 4. A sharp, not necessarily conductive tip, is mounted on the end of a spring. 

Fig. 4. Principle of a atomic force microscope. A sharp tip is brought close to the sample. The forces acting between tip and sample lead to a deflection of the spring.

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. 

Non-contact atomic force microscope (AFM) and N2 absorption measurements on beta zeolites reveal the extreme irregularity of the external crystal surface which can make up a considerable proportion of the total surface area. A catalytic test, the acylation of 2-methoxynaphthalene, shows that active sites on the outer surface play an important role in the catalytic activity of the zeolite. Attempts to influence the external surface area and its catalytic activity through synthesis or post-synthesis modification such as dealumination show that the principle influence on the external surface comes from the synthesis procedure. 

Like the STM, the atomic force microscope (AFM) uses a very sharp tip to probe and map sample topography. The AFM detects near-field forces between the tip and sample, instead of detecting the tunneling current. Knowledge of the near-field forces acting between tip and sample is necessary for us to understand the AFM working principles. There are several types of the near-field forces which are briefly described as follows. 

The introduction and development of Micro-Thermal Analysis are described and discussed by Duncan Price in Chapter 3. The atomic force microscope (AFM) forms the basis of both scanning thermal microscopy (SThM) and instruments for performing localised thermal analysis. The principles and operation of these techniques, which exploit the abilities of a thermal probe to act both as a very small heater and as a thermometer, in the surface characterisation of materials are described in detail. The... 

Biosensors using atomic force microscopes (AFMs) are devices which employ an atomic force microscope for biological recognition events. The principle of biosensors using atomic force microscopy is mainly based on the mass-sensitive detection of binding events that change the deflection of a cantilever whose surface is modified with immobilized bioreceptors. 

Biosensors Using Atomic Force Microscopes, Fig. 1 (a) Basic components and working principle of AFM. A sharp tip fixed at the end of a fiexible cantilever is raster scanned over the surface of a sample. As the tip interacts with the surface, the cantilever deflects, and its deflections are monitored by a laser and a photodiode and then used to reconstruct the topography of the sample, (b) A schematic diagram of AFM as a biosensor in detecting... 

Hong S, Woo J, Shin H, Jeem J, Pak Y, Colla E, Setter N, Kim E, No K (2006) Principle of ferroelectric domain imaging using atomic force microscope. J Appl Phys 89(2) 1377-1386... 

The most common and possibly most versatile SPM is the atomic force microscope (AFM). The AFM relies on the principle of the atomic forces exerted between two objects as they are brought close to each other (Fig. 2). At a distance of tens to hundreds of angstroms, the interatomic forces are attractive (predominantly due to long-range van der Waals forces). On approaching the surface to a distance of a few angstroms, the force becomes repulsive. To observe this phenomenon. 

The principle of microcalorimetry is iUustraled with Fig. A.10.7 (pTA of TA Instruments, Inc.). The tip of an atomic force microscope, AFM, is replaced by a Pt-wire that can be heated and modulated, as is illustrated in detail with Fig. 3.96. A typical resolution is about 1.0 pm with heating rates up to 1,000 K min. A temperature precision of +3 K and a modulation frequency up to 100 kHz has been reached. The figure shows the control circuit for localized thermal analysis. In this case the probe contacts the surface at a fixed location with a programmed force, controlled by the piezoelectric feedback of the AFM. A reference probe is attached next to the sample probe with its tip not contacting the sample, allowing for... 

The principle is shown in Fig. 6.30, illustrating how the invention arose from the perennial problem of capillary condensation in the fine gap between tip and surface. An atomic force microscope (AFM) tip is positioned close to a flat gold substrate. If the air is damp, then moisture appears in the very fine capillary gap... 

Principles of scanning tunneling and atomic force microscopes... 

The atomic force microscope (AFM) was designed for high-resolution surface topography analysis. The basic measuring principle is sketched in Fig. 2.34. A sample is scanned by a sharp tip attached to a sensitive cantilever spring via a... 

Figure 3.38 Working principle of an atomic force microscope (AFM).

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... 

One application of these principles is in the fabrication of microtips of the type used in atomic force microscopes (AFM). A fiber can be dipped in an acid bath to etch the material away. The radius of the fiber diminishes as time progresses, which causes the height of the meniscus to drop in accordance with equation (2.28). The process goes on until all that is left is a sharp tip resting on the surface of the acid bath (Figure 2.15). 

Biosensors using atomic force microscopes (AFMs) are devices which employ an atomic force microscope for biological recognition events. The principle of biosensors... 

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