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Ferroelectrics Characterization and Modeling Part 2

Tham khảo tài liệu 'ferroelectrics characterization and modeling part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Near-Field Scanning Optical Microscopy Applied to the Study of Ferroelectric Materials 25 2. Experimental details of the AFM This section consists of a brief introduction to the AFM technique followed by the description of the commercial electronics used by experimental set-up in this work. As a peculiarity we can mention that the SPM techniques were proposed many years ago but they could not be developed until the 80s because of such techniques required positioning systems of great precision. Nowadays thanks to existence of piezoelectric positioners and scanners the tip-sample distance can be controlled with a precision in the order of the Angstrom. As a result the AFM resolution is limited by other effects different from relative tip-sample motion precision. 2.1 The AFM The basis of the AFM is the control of the local interaction between the microscope probe and the material surface. The probe usually a silicon nano-tip is located at the end of a micro-cantilever. To obtain images of the sample topography the distance between the tip and the sample is kept constant by an electric feedback loop. The AFM working principle varies depending on the operation mode. In the case of ferroelectric surfaces the most used method is the non-contact mode due to the fact that such mode allows the simultaneous measurement of electrostatic interactions Eng et al. 1998 1999 . Working in non-contact mode an external oscillation is induced to the cantilever by means of a mechanical actuator. In our commercial AFM Nanotec Electronica S.L. a Schaffer-Kirchoff laser is mounted in the tip holder for monitoring the cantilever motion. The laser beam 3mW at 659 nm wavelength is aligned in order to be focused in the cantilever see Fig. 2a impinging the reflected light in a four-quadrant photodetector Fig. 2b . In this way the cantilever oscillation can be determined by comparison between the signals measured in the four diodes of the detector. If the frequency of the external excitation is