Non-contact mode

Laser beam deflection offers a convenient and sensitive method of measuring cantilever deflection. In the non-contact mode, the AFM derives topographic images from measurements of attractive forces the tip does not touch the sample. On the other hand, in the contact mode, repulsion forces between the tip and the sample produce topographic images. 

The AFM can be operated in many ways. The main classes of interaction are contact mode, tapping mode, and non-contact mode. Table 3.2 shows the modes of operation for AFM and the types of forces of interaction working in the individual modes of operation. 

Non-contact mode (NC-AFM) Intermittent contact mode (TM-AFM) Lateral force mode Magnetic force Thermal scanning 

Strong (repulsive)—constant force or constant height Weak (attractive)—vibrating probe Strong (repulsive)—vibrating probe 

Frictional forces that exert a torque on the scanning cantilever Magnetic field of the surface Distribution of thermal conductivity 

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The second mode of operation is the non-contact mode, in which the distance between tip and sample is much larger, between 2 and 30 nm. In this case one describes the forces in terms of the macroscopic interaction between bodies. Magnetic force microscopy, in which the magnetic domain structure of a solid can be imaged, is an example of the non-contact mode operation. 

A third mode, which has recently become the standard for work on surfaces that are easily damaged, is in essence a hybrid between contact and non-contact modes, and is sometimes called the tapping mode. In this case the cantilever is brought into oscillation such that the tip just touches the surface at the maximum deflection towards the sample. When the oscillating cantilever approaches the maximum deflection, it starts to feel the surface and the oscillation becomes damped, which is... 

A third mode, which has recently become the standard for work on surfaces that are easily damaged, is in essence a hybrid between contact and non-contact mode,... 

Figure 7.13 Left interaction potential and force between an atom at the apex of the tip and an atom in the surface. Tip-surface interactions can be described by a summation of these potentials over all combinations of atoms from the tip and the surface. Right interaction potential between the tip, approximated as a sphere, and a plane surface, valid in the non-contact mode of force microscopy. To stress the long-range character of the non-contact potential, the Lennard-Jones interaction potential between two atoms has been included as well (dotted line).

The details of the development of the EBRD process have been described by Pietsch et al.[187] There are two alternative operation modes in addition to the above continuous non-contact mode. The first one may be referred to as continuous contact atomization. In this mode, liquid metal contacts the bottom surface of the container instead of melt dripping, and then flows continuously from the center to the rim of the container. The second one may be termed discontinuous non-contact atomization. In this mode, the container is first filled by dripping melt while it is rotating at a very low speed of about 3 x 10-3 radians/s. The rotating speed of the container is then enhanced to about 14 radians/s while the metal or alloy is remelted and atomized. More than one focused electron beam may be used to provide energy for melting metal. 

The technique of atomic force microscopy (AFM) has permitted the direct observation of single polysilane molecules. Poly[//-decyl-(high molecular weight (4/w = 5,330,000 and Mn = 4,110,000), PSS, helicity, and rigid rod-like structure due to the aliphatic chiral side chains, was deposited from a very dilute (10-10 Si-unit) dm-3] toluene solution onto a (hydrophobic) atomically flat (atomic layer steps only present) sapphire (1012) surface. After drying the surface for a few minutes in a vacuum, AFM images were taken at room temperature in air in the non-contact mode.204,253 An example is shown in Figure 22, in which the polymer chain is evident as a yellow trace. 

In the non-contact mode (Fig. 6b), AFM acquires the topographic images from measurements of attractive forces in close proximity of the surface, as the tip does not touch the sample and the cantilever oscillates close to the sample surface [12]. This mode is difficult to work with in ambient conditions due to the interference of the capillary forces. Very stiff cantilevers are needed so that the attraction does not overcome the spring constant of the cantilever. However, the lack of contact with the sample means that this mode should be the least invasive and hence cause the least disruption. The disadvantage of this method is that the tip may jump into contact with the surface due to attractive forces. 

Example 12.3. Cylindrical micelles formed from CTAB in aqueous medium and adsorbed to gold(lll) are shown on the cover of this book. The AFM image were taken in a special non-contact mode to avoid destmction of the micelles. The cylinders orient parallel to steps in the gold. 

Contact SFM can be performed at much lower forces in vacuum than in air. Due to elimination of the thin water layer, also operation in non-contact mode became much more stable at distances near to the surface. UHV-SFM in contact mode allows easily nanometer resolution and even atomic-scale features could be resolved by non-contact SFM using a special feedback scheme [98,172]. Thus, UHV conditions strongly advance non-destructive imaging of soft surfaces and make the measurements more reproducible and quantitative. 

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,... 

Both AM and FM modes were initially meant to be non-contact modes, i.e., the cantilever was far away from the surface and the net force between the front atom of the tip and the sample was clearly attractive. The AM mode was later used very successfully in ambient conditions at a... 

Newtons) between a sharp tip and atoms in the surface [40]. The tip is mounted on a flexible arm, called the cantilever, and is positioned at subnanometer distance from the surface. If the sample is scanned under the tip in the x-y plane, it feels the attractive or repulsive force from the surface atoms and hence it is deflected in the z-direction. Various methods exist to measure these deflections [37, 40-42]. Before we describe the equipment and applications to catalysts, we will briefly examine the theoretical aspects of AFM, which can be applied in either the contact mode or the non-contact mode. 

Another method used for starch is the non-contact mode, where the starch granules are simply spread onto an adhesive tape fixed on the AFM sample holder. Using this technique, both nodules (20-50 nm in diameter) and smooth areas without any visible features were observed at the surface of potato starch granules.114 Multiple freeze-thaw cycles have revealed an internal, lamellar structure of chain clusters bundled into blocklets 50-300 nm in diameter.115... 

The topology of the microstructure was investigated by atomic force microscopy (DualScope/DME, Herlev, Denmark) in non-contacting mode. The scan speed of the cantilever was set to 50 xm/s at a constant force of0.16nN. 

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