Growth and physical properties of crystalline rubrene - BOA Bicocca ...

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34 Experimental techniques Figure 3.6: Scheme of the raster scanning mode of an AFM. The collected images are consituted by the assembly of the points collected during the scanning in the fast-scan direction. right parts it is possible to obtain the amount of deflection in the vertical or horizontal direction, respectively. The sample is mounted on a piezoelectric scanner, whose motion in the three dimensions can be controlled with nanometric precision by varying the applied bias. The AFM performs a raster scanning of the sample surface, as illustrated in figure 3.6. The angle between the fast-scan direction and the long axis of the cantilever can be changed at will, in order to achieve the desired experimental conditions. Finally, a feedback circuit is present, making it possible to readjust the scanner vertical extension in function of the cantilever deflection, as will be explained with more details in the following paragraphs. Base working modes The two main AFM working modes are the contact and semi-contact (also called tapping) modes. Choosing one of these two working modes it is possible to obtain information about the topography, the chemical composi- tion, various physical properties and the orientation of the single crystalline domains constituting a crystalline thin film. It is also possible to obtain molecular resolution images of a crystalline surface. In general the interactions between the AFM tip and the sample surface are due to contributions from a wide range of different forces, among which the main ones are:
3.2 Sample characterization 35 Figure 3.7: Plot of the tip-sample interaction potential as a function of the tipsample distance. For the different ranges of tip-sample distance the corresponding AFM working modes and the kinds of the preponderant tip-sample interactions are indicated. • Van der Waals forces • Short-range repulsive forces • Capillary forces • Electrostatic and magnetic forces The cantilever deflection is due to the sum of all these forces, which can then be measured knowing the elastic constant of the cantilever. As shown in figure 3.7, depending on the AFM working mode in use and more generally on the tip-sample distance, different kind of forces can dominate the tip-sample interaction from time to time. The calculation of the exact amount of the combination of all those interactions is in general quite complex, but for the general use of an AFM this is not necessary, and thus it won’t be described in more detail. The maximum resolution of an AFM is determined by various factors. First of all, it is necessary to distinguish between lateral and vertical resolution. Indeed, the vertical resolution is only limited by the precision of the vertical movements of the scanner. The best attainable resolution with
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Page 10 and 11: x Introduction molecular oxygen inc
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84 Oxidation dynamics of rubrene ep
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118 Conclusions and Perspectives On
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120 Conclusions and Perspectives
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122 Bibliography [13] D. A. Bernard
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124 Bibliography [41] V. Coropceanu
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126 Bibliography [71] A. J. Maliaka
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128 Bibliography [101] C. Kittel, I
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130 Bibliography [130] D. Beljonne,
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132 Bibliography
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134 PhD activity Attended courses
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