Piezo excitation

This piezo excitation method works fine in air and vacuum, while it causes problems in liquid. In the method, an alternating current (AC) voltage signal is applied across the piezo actuator to induce its mechanical vibration. This vibration, in turn, gives rise to an acoustic wave, which propagates to the cantilever and excites its vibration. 

In a typical design, the acoustic wave propagates to any components mechanically coupled with the piezo actuator. This uncontrolled propagation excites vibrations of spurious resonances in the propagation paths. In air and vacuum, the influence from the spurious resonances is often negligible since their Q factors are lower than that of the cantilever resonance. In liquid, the Q factor of the cantilever drops to less than 10, which is often lower than that for a spurious resonance. Hence, the amplitude and phase versus frequency curves obtained in liquid are heavily distorted because of the influence of the spurious resonances. 

Clean amplitude and phase curves are desirable in d3mamic-mode AFM, especially with the FM detection method. In FM-AFM, a cantilever is always oscillated at its resonance frequency using the self-oscillation circuit. This self-oscillation loop works as a feedback circuit to keep f at —90°. This feedback control operates on the linear slope of the phase versus frequency curve around the resonance frequency, as shown in Fig. 18.1b. Thus, large distortions in the phase curve result in instability of the cantilever excitation and deteriorate the accuracy of the force measurements by FM-AFM. 

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Campbell, G. A., Mutharasan, R. Monitoring of the self-assembled monolayer of 1-hexa-decanethiol on a gold surface at nanomolar concentration using a piezo-excited millimeter-sized cantilever sensor. Langmuir 2005, 21 (25), 11568-11573... 

In spite of the clean amplitude and phase curves obtained by the magnetic and photothermal excitation methods, the piezo excitation method has been the most widely used, even in liquid. This is mainly because of its simple setup and wide applicability. The method does not require any modifications on the cantilever, which is very attractive for commercial instruments. Therefore, considerable efforts have been made for improving the performance of the piezo excitation method. 

The use of air-bome ultrasound for the excitation and reception of surface or bulk waves introduces a number of problems. The acoustic impedance mismatch which exists at the transducer/air and the air/sample interfaces is the dominant factor to be overcome in this system. Typical values for these three media are about 35 MRayls for a piezo-ceramic (PZT) element and 45 MRayls for steel, compared with just 0.0004 MRayls for air. The transmission coefficient T for energy from a medium 1 into a medium 2 is given by... 

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. 

In the fonner, the excitation amplitude to the lever (via the piezo) is kept constant, thus, if the lever experiences a damping close to the surface the actual oscillation amplitude falls. The latter involves compensatmg the excitation amplitude to keep the oscillation amplitude of the lever constant. This mode also readily provides a measure of the dissipation during the measurement [100]. 

Figure 6 shows an outline of a PAS instrument designed for fast time-resolved measurements. The excitation light is a laser pulse of some 20 ns duration, at a wavelength which falls within the absorption spectrum of the sample (e.g. 337 nm with a nitrogen laser). Total absorption of this pulse then deposits an energy E in the sample and this will decay in the course of time into heat which will give rise to the pressure sensed by the detector usual microphones have slow response times, so that piezo-electric devices are used to improve the instrument s time resolution [43]. 

If the time consumption is acceptable and the image drift is negligible, a scan line can be scanned twice to separate topography and electrical properties. In this case, a first scan in contact or better in a dynamic mode without an electrical excitation is performed. The tip is lifted and for the following second scan the z-piezo is controlled in a way that the tip follows the same topography as for the first scan (constant tip-sample distance or interleave scan). During this second line scan, one of the above-mentioned measurements of electrical properties can be performed [396]. 

Fig. 35 Simultaneously measured a,b topography c,d repulsive force e,f a.c. current amplitude on graphite before (a, c, e) and after (b, d, f) Fourier space filtering. The Fourier transform parameters for the inverse transformation of b are taken from the Fourier transformation of e. Scan width 5.5 nm, 500x500 pixel, scan speed 50 nm/s, repulsive force 100 nN, a.c. excitation 3.9 mV at 102 kHz. In order to minimise the piezo amplifier noise, a weak feedback was used and the topography contrast is smaller than 100 pm. Force contrast 1 nN, current contrast from 5.5 to 7 nA...

ZnO is a wide band gap semiconductor, which is used for various applications. Based on textured ZnO films one can build highly effective piezo field emitters. On the other hand ZnO is a very effective electron-excited phosphor. ZnO films easily withstand electron fluence more than 1 W/cm. ZnO films doped with Al, Ga, or In have a low resistivity of about 10 " Qcm and a high transparency of about 90%. This is sufficient for applications as a front contact in solar cells, liquid crystal displays etc. Dielectric ZnO films have a high electromechanical coupling factor that allow using ZnO in various surface acoustic wave (SAW) devices such as delay lines, delay-line filters, resonators, transducers and SAW convolvers. 

Cantilevers in AFM function as force transducers converting unknown force to measurable deflection. The value of the unknown force can then be expressed by Hookean mechanics following spring constant calibrations. In addition to static point loads, cantilevers can also be vibrated, e.g., by an oscillation piezo to which the fixed end of the beam is attached (or by other approaches). Excitation frequency, oscillation amplitude, and phase relationships are variables that govern dynamic tapping (intermittent contact) imaging. This problem will be discussed in the next section. 

The prime differences among the different AFM modes, such as CM (discussed above) and intermittent CM, as elucidated in the following section, are the feedback parameters and the choice of the cantilever. For intermittent contact (tapping) mode AFM, a stiff cantilever (k typically 10—50 N/m) with a resonance frequency of 100—400 kHz is chosen. The cantilever, which is inserted in an identical manner as for CM into the cantilever holder, is excited to vibrate by an integrated piezo actuator. Instead of deflection (contact force), the amplitude of the forced oscillating lever is detected, analyzed, and utilized in the feedback loop (Fig. 2.20). 

The pump-probe pulses are obtained by splitting a femtosecond pulse into two equal pulses for one-color experiments, or by frequency converting a part of the output to the ultraviolet region for bichromatic measurements. The relative time delay of the two pulses is adjusted by a computer-controlled stepping motor. Petek and coworkers have developed interferometric time-resolved 2PPE spectroscopy in which the delay time of the pulses is controlled by a piezo stage with a resolution of 50 attoseconds [14]. This set-up made it possible to probe decoherence times of electronic excitations at solid surfaces. 

The components of a TS system are shown in Figure 11.39 (130). A resonance stethoscope is used to transmit crystalline vibrations in the sample to the audio frequency range where they are converted to electrical signals by use of a piezo-electric crystal. The stethoscope is constructed of quartz, which, because of its high Q value, operates mechanically both as a tuned pick-up sensor and as a self-exciting resonator. The unit incorporates a sample-holder head shaped as an acoustic transformer and fitted with a transmitter rod that mechanically matches the piezo-electric cell fixed on a heavy recoil... 

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