AFM microscopes

CNTs have been used as AFM tips and there appears to be every likelihood that extremely narrow structures can be probed [179]. WS2 could be mounted on the ultrasharp Si tip following a similar methodology. These tips were tested in an AFM microscope by imaging a replica of high aspect ratio, and it was observed that these WS2 nanotube tips provide a considerable improvement in the image quality compared to the conventional ultrasharp Si tips [258]. 

The other landmarks which happened in the middle of the 8O s were a demonstration of the possibility of appheation of the STM apparatus for lithography (Fig. b), and for controlled atom handling. The invention of AFM microscope opened the route for the construction of several different force microscopies (SPM techniques). This way the use of STM/SPM techniques as a Feynman Machine finally had been realized, the STM apprenticeship time came to an end and the time of travel begau... 

In recent decades, the areas of application of SPMs has increased rapidly. Because this subject is becoming very large only a few examples will be delineated here. The STM and AFM microscopes allow ns to manipnlate the sensor (STM = tip, AFM = cantilever) at a distance of the order of nanometers (A) from the snbstrate nnder ambient conditions. This means that we can develop so-called nanotechnology prodncts if we conld control the movement of the sensor within this range of separation. In the following some major developments are mentioned however, as the possibilities are many, the list is not complete dne to space considerations and the rapid rate at which advances are being made. 

The same procedure of the powder preparation was repeated for the microscope study to determine the particle size directly with an optical microscope and an atomic force microscope (AFM). Microscope images were made for samples extracted from the suspension settled in a wide glass for 2, 2.5 and 3 h. 

A tunable CO2 laser has been combined with an atomic-force-microscopy (AFM) microscope to form an apertureless near-held-imaging system [17]. This technique can produce spahal resoluhon of up to A/lOO with high throughput however, the tunable range of the CO2 laser is hmited to a region of the IR spectrum that is not parhcularly informahve for most IR chromophores (2300 cm ). 

Investigations on the worn surfaces were performed using a scanning electron and an atomic force (AFM) microscope. It is believed that SCF are... 

These conditions are often difficult to achieve, especially on rough surfaces where z, is necessarily large in order to prevent the tip from crashing into the surface. In such instances, the flux of molecules from two adjacent pores cannot be completely resolved unless using instruments with nanometer-resolution imaging capability, for example, a combined SECM-AFM microscope. 

Fig. VIII-1. Schematic illustration of the scanning tunneling microscope (STM) and atomic force microscope (AFM). (From Ref. 9.)...

Fig. Vni-3. (a) Atomic force microscope (AFM) and (b) transmission electron microscope (TEM) images of lead selenide particles grown under arachidic acid monolayers. (Pi Ref. 57.)...

Detection of cantilever displacement is another important issue in force microscope design. The first AFM instrument used an STM to monitor the movement of the cantilever—an extremely sensitive method. STM detection suffers from the disadvantage, however, that tip or cantilever contamination can affect the instrument s sensitivity, and that the topography of the cantilever may be incorporated into the data. The most coimnon methods in use today are optical, and are based either on the deflection of a laser beam [80], which has been bounced off the rear of the cantilever onto a position-sensitive detector (figme B 1.19.18), or on an interferometric principle [81]. 

It is remarkable that tire roots of the SFA go back to the early 1960s [1], Tabor and Winterton [2] and Israelachvili and Tabor [3] developed it to the current state of the art some 15 years before the invention of the more widely used atomic force microscope (AFM) (see chapter B1.19). 

The atomic force microscope (ATM) provides one approach to the measurement of friction in well defined systems. The ATM allows measurement of friction between a surface and a tip with a radius of the order of 5-10 nm figure C2.9.3 a)). It is the tme realization of a single asperity contact with a flat surface which, in its ultimate fonn, would measure friction between a single atom and a surface. The ATM allows friction measurements on surfaces that are well defined in tenns of both composition and stmcture. It is limited by the fact that the characteristics of the tip itself are often poorly understood. It is very difficult to detennine the radius, stmcture and composition of the tip however, these limitations are being resolved. The AFM has already allowed the spatial resolution of friction forces that exlribit atomic periodicity and chemical specificity [3, K), 13]. 

The avidin-biotin complex, known for its extremely high affinity (Green, 1975), has been studied experimentally more extensively than most other protein-ligand systems. The adhesion forces between avidin and biotin have been measured directly by AFM experiments (Florin et al., 1994 Moy et al., 1994b Moy et al., 1994a). SMD simulations were performed on the entire tetramer of avidin with four biotins bound to investigate the microscopic detail of nnbinding of biotin from avidin (Izrailev et al., 1997). 

That simulation study [49] aimed at a microscopic interpretation of single molecule atomic force microscope (AFM) experiments [50], in which unbinding forces between individual protein-ligand complexes have been m( asured... 

Newer techniques that are responding to the need for atomic level imaging and chemical analysis include scanning tunneling microscopes (STMs), atomic force microscopes (AFMs) (52), and focused ion beams (FIBs). These are expected to quickly pass from laboratory-scale use to in-line monitoring apphcations for 200-mm wafers (32). 

The molecular dipstick microscope is related to the AFM. It measures lubricant film thickness. The probe is lowered into the oil film on a surface (like the automobile engine crankcase dipstick). The tip is attracted to the surface by the surface tension of the film but repelled by van der Waal s forces from the hard substrate. By noting the height of the probe from the two surfaces as it makes contact, the film thickness can be measured with a precision of about 0.5 nm. 

Apart from the application of XPS in catalysis, the study of corrosion mechanisms and corrosion products is a major area of application. Special attention must be devoted to artifacts arising from X-ray irradiation. For example, reduction of metal oxides (e. g. CuO -> CU2O) can occur, loosely bound water or hydrates can be desorbed in the spectrometer vacuum, and hydroxides can decompose. Thorough investigations are supported by other surface-analytical and/or microscopic techniques, e.g. AFM, which is becoming increasingly important. 

It may be that in years to come, interatomic potentials can be estimated experimentally by the use of the atomic force microscope (Section 6.2.3). A first step in this direction has been taken by Jarvis et al. (1996), who used a force feedback loop in an AFM to prevent sudden springback when the probing silicon tip approaches the silicon specimen. The authors claim that their method means that force-distance spectroscopy of specific sites is possible - mechanical characterisation of the potentials of specific chemical bonds . 

It is not easy to determine detailed properties of the tube terminations using STM or AFM. These microscopes cannot image undercut surfaces and the tip shape is convoluted with the cap shape of the nanotube. How ever, the tips may have very sharp edges... 

The development of a host of scanning probe devices such as the atomic force microscope (AFM) [13-17] and the surface forces apparatus (SFA) [18-22], on the other hand, enables experimentalists to study almost routinely the behavior of soft condensed matter confined by such substrates to spaces of molecular dimensions. However, under conditions of severe confinement a direct study of the relation between material properties and the microscopic structure of confined phases still remains an experimental challenge. 

A very similar technique is atomic force microscope (AFM) [38] where the force between the tip and the surface is measured. The interaction is usually much less localized and the lateral resolution with polymers is mostly of the order of 0.5 nm or worse. In some cases of polymer crystals atomic resolution is reported [39], The big advantage for polymers is, however, that non-conducting surfaces can be investigated. Chemical recognition by the use of specific tips is possible and by dynamic techniques a distinction between forces of different types (van der Waals, electrostatic, magnetic etc.) can be made. The resolution of AFM does not, at this moment, reach the atomic resolution of STM and, in particular, defects and localized structures on the atomic scale are difficult to see by AFM. The technique, however, will be developed further and one can expect a large potential for polymer applications. 

Binnig et al. [48] invented the atomic force microscope in 1985. Their original model of the AFM consisted of a diamond shard attached to a strip of gold foil. The diamond tip contacted the surface directly, with the inter-atomic van der Waals forces providing the interaction mechanism. Detection of the cantilever s vertical movement was done with a second tip—an STM placed above the cantilever. Today, most AFMs use a laser beam deflection system, introduced by Meyer and Amer [49], where a laser is reflected from the back of the reflective AFM lever and onto a position-sensitive detector. 

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