Laser-beam deflection technique

After these initial and promising lateral-force results, the friction force microscopy (FFM) was introduced. The FFM is a modified SFM with a four-quadrant photodiode, based on the laser beam deflection technique (Meyer Amer 1988) (Fig. 2.17). The beam is emitted by a low-voltage laser diode and reflected from the rear side of the cantilever to the four-quadrant photodiode. With this detection scheme, normal and torsional forces can be measured simultaneously. The torsional forces correspond to the lateral forces measured with the instrument of Mate et al. (1987). In 1993, Ovemey introduced the threefold measurement of topography, friction and elasticity on a polymer sample using an ITM. With this latest achievement, a wide spectram of tribological information was opened up, limited only by the lattice parameters of the sample. 

We have developed a laser beam deflection technique which, although similar to the method employed by Amer s group, differs in several important respects. U4-15) In particular, our method employs highly focused heating and probe laser beams, both incident normal to the sample surface, and the experiments are performed at high modulation frequencies of up to 10 MHz. 

Figure 1. Schematic depiction of laser beam deflection technique used for the thin-film thickness measurement experiments.

The laser-beam deflection technique was first employed to determine the vibrational relaxation times of hydrogen [9] and deuterium [10]. The... 

Fig. 14. Rate coefficients derived from laser-beam deflection technique [40].

The most precise work to date is that obtained by the laser-beam deflection technique over the temperature range 3500—8000° K. The precision of the data is displayed in Fig. 14. Rate coefficients for hydrogen dissociation for several colliding partners including the hydrogen atom were calculated as... 

Shock tube studies of the decomposition of Oj have revealed the presence of an incubation period which precedes the observation of a steady rate of dissociation [11, 13, 58]. The length of the incubation period at a particular temperature was related to the vibrational relaxation time for oxygen at that temperature using previously measured relaxation data [59, 60]. The decomposition has been observed in a variety of inert diluents (He, Ar, Kr and Xe) in a wide range of oxygen concentrations (1—50%) over an extensive temperature range (2850—8500°K) by several different analytical methods including ARAS [13] and the laser-beam deflection technique [11]. 

The complexity introduced by including reactions (15)—(19) precludes the analysis of the entire density gradient oscillogram as was done for homonuclear dissociations. The determination of atomic efficiencies for heteronuclear decompositions is not possible using the laser-beam deflection technique. 

Using the laser beam deflection technique, A. Buguin was able to measure the film s profile during the dewetting process (Figure 7.16) in the inertial regime Re 1), both in the presence and in the absence of shock waves. 

FIGURE 7.16. (a) Laser beam deflection technique used to measure the profile z x) of the film during wetting (b) profile in the regime Fr < I, when shock waves and ripples are absent (c) profile for four different thicknesses 8 cm away from the nucleation point in the regime FV > 1 the ripples are the signature of shock waves (courtesy A. Buguin). 

The cantilever bending-technique requires a sensitive displacement detection such as a capacitance probe (Klokholm 1976, 1977), optical interferometry (Sontag and Tam 1986), a tunnelling tip (Wandass et al. 1988) or angular detection (e.g. laser beam deflection, Son-tag and Tam 1986 Trippel 1977 Tam and Schroeder 1988 Betz 1997 Sander et al. 1998). 

The shortcomings of EQCM methods in investigating ion exchange processes were somewhat compensated by the probe beam deflection technique [248]. This technique is based on the measurement of the extent of deflection of a parallel laser beam passing over a polymer coated electrode surface at a very small distance. When there is an ion flux near the electrode... 

SFM cantilevers and tips are often made of silicon or silicon nitride SFM tips possess radii of curvature at the apex between few and several tens to hundreds of nanometers (Fig. 1). A piezoelectric transducer is used in order to position the sample accurately. Depending on the scanner type (piezo tube length and design) the maximum scan sizes vary between ca 1 /rm and several himdred micrometers, with an accuracy of positioning in the best cases of O.Ol nm. The cantilever deflection is typically monitored by an optical beam deflection technique (Fig. la). Other possibilities to measure the deflection include STM, piezoresistive, capacitance, or interferrometric detection schemes (22). In the optical beam deflection setup, laser light is reflected off the end of the cantilever and is collected by a position-sensitive photodiode. For instance, a 4-quadrant photodiode can simultaneously measure deflections in vertical (surface normal) and horizontal (lateral) direction (Fig. la). 

Bulk analytical methods, including laser beam deflection and X-ray or neutron diffraction techniques, have been used to measure lattice strains and stresses. These methods, however, are limited to measuring lattice volume expansion due to li-ion intercalation on the molecular scale. They cannot detect extended defects, cracks, or microfractures, and thus cannot be directly used for mechanics analysis of particles or porous particle networks. Even with this limitation, diffraction or deflection techniques can stiU provide valuable information on lattice structural... 

Chung, K.Y. and Kim, K.-B. (2002) Investigation of structural fatigue in spinel electrodes using in situ laser probe beam deflection technique. J. Electrochem. Soc., 149 (1), A79. 

Figure 3.5.14 Techniques of force measurement in scanning force microscopy, (a) Scanning eiectron microscopy micrographs of a cantiiever and tip etched from siiicon in bottom view (upper) and side view (lower), (b) Laser beam deflection and (c) interferometer schemes for the measurement of the cantiiever bending.

For SFM, maintaining a constant separation between the tip and the sample means that the deflection of the cantilever must be measured accurately. The first SFM used an STM tip to tunnel to the back of the cantilever to measure its vertical deflection. However, this technique was sensitive to contaminants on the cantilever." Optical methods proved more reliable. The most common method for monitoring the defection is with an optical-lever or beam-bounce detection system. In this scheme, light from a laser diode is reflected from the back of the cantilever into a position-sensitive photodiode. A given cantilever deflection will then correspond to a specific position of the laser beam on the position-sensitive photodiode. Because the position-sensitive photodiode is very sensitive (about 0.1 A), the vertical resolution of SFM is sub-A. 

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