Cantilever tip

Most NC-AFMs use a frequency modulation (FM) teclmique where the cantilever is mounted on a piezo and serves as the resonant element in an oscillator circuit [101. 102]. The frequency of the oscillator output is instantaneously modulated by variations in the force gradient acting between the cantilever tip and the sample. This teclmique typically employs oscillation amplitudes in excess of 20 mn peak to peak. Associated with this teclmique, two different imaging methods are currently in use namely, fixed excitation and fixed amplitude. 

Figure 4.29. Experimental set-up for atomic force microscopy. The sample is mounted on a piezo electric scanner and can be positioned with a precision better than 0.01 nm in thex, y, and z directions. The tip is mounted on a flexible arm, the cantilever. When the tip is attracted or repelled by the sample, the deflection of the cantilever/tip assembly is...

Figure 2.4 A scanning electron microscopy image of an AFM cantilever tip covered with a thin silver film.

K., Sekkat, Z. and Kawata, S. (1999) Near-field scanning optical microscope using a metallized cantilever tip for nanospectroscopy. Proc. SPIE, 3791,40-48. 

Figure 7.14 Experimental set-up for atomic force microscopy. The sample is mounted on a piezoelectric scanner and can be positioned with a precision better than 0.01 nm in the x, y, and z direction. The tip is mounted on a flexible arm the cantilever. When the tip is attracted or repelled by the sample, the deflection of the cantilever/tip assembly is measured as follows. A laser beam is focussed at the end of the cantilever and reflected to two photodiodes, numbered 1 and 2. If the tip bends towards the surface, photodiode 2 receives more light than 1, and the difference in intensity between 1 and 2 is a measure of the deflection of the cantilever and thus of the force between the sample and the tip. With four photodiodes, one can also measure the sideways deflection of the tip, for example at an edge on the sample surface.

Fig.13 Schematic view of a typical setup of single molecule pulling experiments and an illustration of the generic free energy potential G(z) [87]. The x-position denominates the cantilever and the z-position denominates the cantilever tip to which one end of DNA molecule is attached. Reprinted with permission...

Fig. 14 Two topview AFM images of A-DNA in AFM tapping mode imder ambient conditions a before and b after mechanically cutting a dsDNA strand in the center of the image (circles) with the AFM cantilever tip [106]. Reprinted with permission...

If vibration is applied both through the cantilever tip (at frequency heterodyne detection. The AFM tip detects the oscillating force at the difference frequency cot — cos, very much like a heterodyne radio receiver. This technique is known as heterodyne force microscopy (HFM Cuberes et al. 2000). Once again, the tip-surface force non-linearity plays a critical role. The low-frequency beating oscillation carries information on the phase of the original high-frequency oscillations. 

Zt deflection of the cantilever (tip) from the equilibrium position Zc position of the cantilever base relative to the ample surface A contact area... 

Quantitative evaluation of a force-distance curve in the non-contact range represents a serious experimental problem, since most of the SFM systems give deflection of the cantilever versus the displacement of the sample, while the experimentalists wants to obtain the surface stress (force per unit contact area) versus tip-sample separation. A few prerequisites have to be met in order to convert deflection into stress and displacement into tip-sample separation. First, the point of primary tip-sample contact has to be determined to derive the separation from the measured deflection of the cantilever tip and the displacement of the cantilever base [382]. Second, the deflection can be converted into the force under assumption that the cantilever is a harmonic oscillator with a certain spring constant. Several methods have been developed for calibration of the spring constant [383,384]. Third, the shape of the probe apex as well as its chemical structure has to be characterised. Spherical colloidal particles of known radius (ca. 10 pm) and composition can be used as force probes because they provide more reliable and reproducible data compared to poorly defined SFM tips [385]. 

The ability to fabricate small cantilevers, tips, and wires160 (and to cross the wires, in some circumstances), opens the possibility of making nanoscale sensors161. A number of these systems have been demonstrated in laboratory experiments in most cases, SAMs have provided the functionality that gives the systems their selectivity for particular analytes162,163. 

Figure 17.10 SEM picture of Pt coated AFM cantilever tip after measurement...

If we look more closely at these changes, which was done by fem simulations by Prume in [10], we can find a correction factor, which helps to derive the electrical properties precisely for measurements down to 1 /rm2. Figure 17.12 shows the influence of the cantilever tip and the body part of the cantilever in fem simulation. 

The SPM technique allows measurements of the local sample surface potential. The NanoScope recorded two passes. In the first pass, the sample surface topography was obtained by the standard tapping mode. The surface potential was measured during the second pass carried out in the lift mode (lift height was 100 nm). Here the cantilever s vibration is turned off and an oscillating voltage U c cos cot is applied directly to the cantilever tip. This creates an oscillating electrostatic force F at the frequency co on the cantilever ... 

Until fairly recently, the theories described in Secs. II and III for particle-surface interactions could not be verified by direct measurement, although plate-plate interactions could be studied by using the surface forces apparatus (SFA) [61,62]. However, in the past decade two techniques have been developed that specifically allow one to examine particles near surfaces, those being total internal reflection microscopy (TIRM) and an adapted version of atomic force microscopy (AFM). These two methods are, in a sense, complementary. In TIRM, one measures the position of a force-and torque-free, colloidal particle approximately 7-15 fim in dimension as it interacts with a nearby surface. In the AFM method, a small (3.5-10 jam) sphere is attached to the cantilever tip of an atomic force microscope, and when the tip is placed near a surface, the force measured is exactly the particle-surface interaction force. Hence, in TIRM one measures the position of a force-free particle, while in AFM one measures the force on a particle held at a fixed position. 

The advent of the atomic force microscope has allowed surface properties at nearly molecular length scales to be measured directly for the first time. Recently, a method has been proposed whereby a small ( 3.5 /nn) particle is attached to the cantilever tip of the commercially available, Nanoscope II AFM [67,68]. The particles are attached with an epoxy resin. When the cantilever tip is placed close to a planar surface, the AFM measures directly the interaction force between the particle and the surface. A primary difference between this technique and the surface forces apparatus (SFA) is the size of the substrates, since the SFA generally requires smooth surfaces approximately 2 cm in diameter. Other differences are discussed by Ducker et al. [68]. For our purposes, it suffices to note that the AFM method explicitly incorporates the particle-wall geometry that is the focus of this chapter. 

A major advance for colloid scientists was to attach colloidal particles to the AFM tips so that surface forces relevant to colloidal dispersions could be measured [29-31. For this purpose a colloidal particle (radius 2-25 gm) is glued to the cantilever tip, which enables interaction forces to be measured between this particle and a flat surface or even another particle. For ease of quantitative analysis, spherical particles are preferred. 

The employed technique for this purpose was the so-called colloidal-probe AFM (Atomic Force Microscopy). A carbon microparticle with high degree of carbonization was attached to the top of the cantilever tip, forming the colloidal probe, and its interaction force with cleaved graphite was measured within a liquid cell filled with organic liquid, controlled at a desired temperature above the bulk freezing point of the liquid. The two surfaces will form a slit-shaped nanospace because the radius of the particle is far larger than the separation distance concerned here. 

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