Atomic force microscopy operating modes

The force microscope, in general, has several modes of operation. In the repulsive-force or contact mode, the force is of the order of 1-10 eV/A, or 10 -10 newton, and individual atoms can be imaged. In the attractive-force or noncontact mode, the van der Waals force, the exchange force, the electrostatic force, or magnetic force is detected. The latter does not provide atomic resolution, but important information about the surface is obtained. Those modes comprise different fields in force microscopy, such as electric force microscopy and magnetic force microscopy (Sarid, 1991). Owing to the limited space, we will concentrate on atomic force microscopy, which is STM s next of kin. 

Although the resolution of atomic force microscopy (AFM) is basically inferior to that of STM, the technique has the advantage that insulating materials can also be used as substrates. In AFM the forces acting between the tip and the sample surface are detected. The probe tip mounted on a flexible cantilever scans over the sample. AFM can be operated in contact mode, exploiting repulsive forces, as well as in non-contact mode, exploiting attractive forces. In the contact mode the probe tip is in direct contact with the sample surface (Fig. 7.8). Either the tip is passed over the sample surface at constant height (CHM,... 

Ueyama H, Sugawara Y, Morita S (1998) Stable operation mode for dynamic nonconctact atomic force microscopy. Appl Phys A 66 S295... 

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). 

Figure 5.13 The dynamic non-contact mode. The frequency shift of the cantilever oscillation is used to operate the feedback loop. (Reproduced with permission from P.C. Braga and D. Ricci (eds), Atomic Force Microscopy, Humana Press. 2004 Humana Press.)...

Atomic force microscopy (AFM) allows the topography of a sample to be scanned by using a very small tip made from silicon nitride. The tip is attached to a cantilever that is characterised by its spring constant, resonance frequency, and a quality factor. The sample rests on a piezoceramic tube which can be moved horizontally x,y motion) and vertically (z motion). Displacement of the cantilever is measured by the position of a laser beam reflected from the mirrored surface on the top side of the cantilever, whereby the reflected laser beam is detected by a photodetector. AFM can be operated in either contact or a noncontact mode. In contact mode the tip travels in close contact with the surface, whereas in noncontact mode the tip hovers 5-10 nm above the surface. 

Electron microscopy D E PACKHAM TEM and STEM applied to adhesion studies Scanning electron microscopy B C COPE Mode of operation, examples of use Atomic force microscopy G J LEGGETT Basis, scope limitations Scanning probe microscopy G J LEGGETT Basis and use... 

Atomic Force Microscopy (AFM). AFM measurements were performed with a commercial scanning probe microscope (Nanoscope III, Digital Instruments, Santa Barbara, CA). Measurements of surface topography and lateral force were made simultaneously by operating the instrument in contact mode while scanning the cantilever laterally. All measurements were performed in ambient air on freshly plasma cleaned samples. 

Atomic Force Microscopy (ATM). A Nanoscope IIIA Multimode (Digital Instruments, Santa Barbara, CA) was used for atomic force microscopy measurements. The AFM was operated in force mode, with a scan rate of 1 Hz, and a 2 -piezo total displacement of 500 nm, without calibrating the scanner before each measurement (routine calibration only). Both ap-proach/extension and retraction force curves of the cantilever were recorded. 

Different modes of operation of AFM may be applied to the characterisation of biopolymers. As biopolymers differ in their rigidity (Young s modulus), their responses to the normal and sheer forces apphed by the AFM tip will also differ. As different modes of AFM are available, the user can forego resolution in order to minimise sample distortion (as in the case of tapping mode atomic force microscopy), or improve the resolution using noncontact or cryogenic AFM. 

Soft biopolymers such as DNA and RNA are prone to tip-induced damage, often resulting from the relatively large capillary forces (when imaging in air) or friction forces experienced in contact mode, hi this latter case, tapping mode atomic force microscopy (TM-AFM) may be used [40-42]. We will briefly discuss the most commonly employed modes of operation of AFM that are routinely used to obtain sample topography. The most common modes are contact mode AFM (CM-AFM) [2,43] and TM-AFM [44,45]. 

The structural properties of the film were investigated by atomic force microscopy (AFM) performed on a XE-100 Park system equipped with 910 ACTA cantilever, operating in the tapping mode. The results were analysed in XEl software, fiom which the root mean square roughness (Rrms) value and average grain size were estimated. 

In 1991, Albrecht et al. invented frequency modulation atomic force microscopy (FM-AFM) for operating d3mamic-mode AFM in vacuum environments. Before this invention, it was common to operate d3mamic-mode AFM with the amplitude detection method, which is referred to as amplitude modulation AFM (AM-AFM). In AM-AFM, the tip-sample distance is regulated such that the oscillation amplitude of the cantilever (A) is kept constant. 

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