Sharp tip

NSOM Near-Held scanning optical microscopy [103a] Light from a sharp tip scatters off sample Surface structure to 3 nm... 

The field ion microscope (FIM) has been used to monitor surface self-diflfiision in real time. In the FIM, a sharp, crystalline tip is placed in a large electric field in a chamber filled with Fie gas [14]. At the tip. Fie ions are fonned, and then accelerated away from the tip. The angular distribution of the Fie ions provides a picture of the atoms at the tip with atomic resolution. In these images, it has been possible to monitor the diflfiision of a single adatom on a surface in real time [15]. The limitations of FIM, however, include its applicability only to metals, and the fact that the surfaces are limited to those that exist on a sharp tip, i.e. difhision along a large... 

Advances have been made in directly measuring the forces between two surfaces using freshly cleaved mica surfaces mounted on supports (15), and silica spheres in place of the sharp tip of an atomic force microscopy probe (16). These measurements can be directly related to theoretical models of surface forces. 

Scanning force microscopes use a sharp tip mounted on a flexible cantilever. When the tip comes within a few A of the sample s surface, repulsive van der Waals forces... 

These problems have been improved in recent years by the microfabrication of sharp tips with radii less than 10 nm, the observation in an SEM or STEM of the exact radius before and after the experiment, the use of robust carbon-nanotube probes, and general improvements in control electronics. However, another method used initially was the attachment of a small colloid particle in place of the AFM tip. These particles were considered a reasonably good approximation to a single-asperity contact their radii were accurately known and remained the same for the duration of the experiment. Such probes have also been used to investigate colloids where surface roughness is an important aspect of the colloid interaction. 

The STM method was developed by Binnig and Rohrer, who received the Nobel Prize for their invention. STM is the most mature of the scanning probe methods. A sharp tip (curvature of the order 100 A) is brought close to a surface and a low potential difference (the bias voltage) is applied between sample and tip. 

Atomic force microscopy (AFM) or, as it is also called, scanning force microscopy (SFM) is based on the minute but detectable forces - of the order of nano Newtons -between a sharp tip and atoms on the surface. The tip is mounted on a flexible arm, called a cantilever, and is positioned at a subnanometre 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. The deflection can be measured with a laser and photo detectors as indicated schematically in Fig. 4.29. Atomic force microscopy can be applied in two ways. 

The combination of atomic force microscopy (AFM) and Raman spectroscopy is another approach to attain high spatial resolution. AFM also employs a sharp tip close to a sample surface. When the tip is made of metal and light is irradiated onto the tip and surface, Raman scattering is largely enhanced. In this way, a spatial resolution of 15 nm is achieved [2]. 

Picardi et al. introduced a method to fabricate a sharp Au tip for STM by electrochemical etching [31]. The efficiency of TERS for a thin BCB dye layer using the etched sharp tip was then compared with that using an Au-coated AFM tip. 

The basic idea of AFM is to use a sharp tip scanning over the surface of a sample while sensing the interaction between the tip and the sample (Dufrene, 2008b). The tip with a flexible cantilever (in some AFM models the sample) is mounted on a piezoelectric scanner which can move... 

Tapered or v-shaped blades give good penetration with lightweight coat application. If this type of blade is chamfered on both sides of the V (which in fact effectively creates a rounding off), there is produced, in effect, a small diameter roller. This shape results in a high hydraulic force which enables the rubber compound to be forced further into the cloth structure than would be the case with the sharp tip of the conventional V blade. 

Field emission microscopy was the first technique capable of imaging surfaces at resolution close to atomic dimensions. The pioneer in this area was E.W. Muller, who published the field emission microscope in 1936 and later the field ion microscope in 1951 [23]. Both techniques are limited to sharp tips of high melting metals (tungsten, rhenium, rhodium, iridium, and platinum), but have been extremely useful in exploring and understanding the properties of metal surfaces. We mention the structure of clean metal surfaces, defects, order/disorder phenomena,... 

STM is based on the tunneling of electrons between the surface and a very sharp tip [36,49]. As explained in the Appendix, the cloud of electrons at the surface is not entirely confined to the surface atoms but extends into the vacuum (this effect causes the electric dipole layer at the surface that contributes to the work function). When an extremely fine tip (see Fig. 7.18) approaches the surface to within a few angstroms, the electron clouds of the two start to overlap. A small positive potential... 

Figure 7.18 Scanning tunneling microscopy is based on the tunneling of electrons between the surface and an atomically sharp tip positioned at a few angstroms above the surface. The tunneling current depends sensitively on the distance s between the tip and the surface. An image of the surface is obtained by scanning the tip horizontally over the surface. A control system keeps the tunneling current and therefore the distance between tip and surface constant the scans are a plot of the vertical position of the piezoelectrically driven tip versus its horizontal position.

---