Position-sensitive photodiodes

Fig. 5. Block diagram of contact atomic force microscope system in which cantilever deflection monitored optically with position-sensitive photodiode...

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. 

One of the simplest and most efficient methods for amplifying the tiny deflections of the cantilever during imaging relies on a laser-beam being reflected from the back of the cantilever to a position-sensitive photodiode (Figure 3.2). Any change in the bend of the cantilever will move the position of the reflected laser-spot centred on the photodiode. The change in the bend of the cantilever corresponds to the displacement... 

Figure 3.2 A schematic diagram of a typical AFM instrumental set-up. A laser is aligned to reflect off the back of the cantilever onto a position-sensitive photodiode (PSPD), which allows the accurate tracking of the small cantilever deflections which result as the probe is scanned over the sample surface (or, as in the case of this instrument, the sample is scanned beneath a stationary probe)...

As the tip is scanned over the sample, or the sample is scanned under the tip, forces between the tip and the sample surface cause spatial deflections and oscillations of the cantilever. The key information gathered in AFM comes with measuring those deflections, quantified by means of an optical lever system, coupled with a position-sensitive photodiode (Marinello et al. 2009). 

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). 

Time-resolved SAXS experiments were conducted on two different beamlines at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratories. Initial experiments were conducted on Beamline X3A2 at a wavelength of 0.154 nm (2). The scattered intensity was collected by a linear position sensitive photodiode detector coupled to an optical multichannel analyzer. In these early experiments, real-time SAXS scattering during crystallization at 45 C was analyzed for one composition of each of the model blends, as well as the scattering from neat PEO crystallized at two temperatures. 

The latter relation is known as Kelvin s equation. Methods for creating propagating capillary ripples typically involve either a mechanical or electrocapillary disturbance of the fluid interface [189-191]. The laser is more appropriate because it does not necessitate physical contact with the fluid surface [497]. The wave characteristics, which are necessary for the evaluation of the interfacial properties through the dispersion relation, are often determined by the reflection of a laser beam from the fluid surface to a position-sensitive photodiode. 

The true friction forces are obtained by mass force compensation in which the mass forces related to zero normal force, determined at a specific frequency, are subtracted from the total force readings. An example of the adopted mass force compensation is given in Fig. 6. The synchronising of the two force recordings is performed by using the simultaneously recorded readings of an optical position sensor, which is located close to the friction measurement cell. The sensor is based on an IR-LED and a position sensitive photodiode detector. 

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