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

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36 Experimental techniques the scanner used in this work is 0.1 nm. The maximum lateral resolution is instead determined by the tip size and curvature. An AFM tip has generally a rounded apex, whose curvature radius is, for the most commonly used tips, around 50 Å. This leads to a limit resolution of around 20 Å at best, since only a part of the tip interacts with the sample. In contact mode the tip is brought to a distance from the sample surface for which the tip-sample interaction is of the repulsive kind. When, during scanning, the tip encounters a protrusion or a valley on the surface, the tip-sample distance and thus the cantilever deflection vary. In response to the change of the cantilever deflection, the feedback circuit vary the vertical position of the scanner, in order to restore the original cantilever deflection. Standard AFM topographic images are constructed by mapping the variations in the vertical extension of the scanner as a function of the tip position over the sample surface. In figure 3.8a an AFM height image collected in contact mode is reported, in which the grays scale is used to represent the relative height of each point. In figure 3.8b the grays scale is instead used to represent the cantilever deflection in each point. The color of this image is uniform with the exception of the steps present on the surface, since the feedback circuit reacts to the change in the cantilever deflection in correspon- dence of a step, restoring it to the original value. It can also be observed that the second image corresponds to the first derivative of the first one. With an AFM working in contact mode it is possible, in parallel to the aquisition of height images, to get images conveying further information such as: the presence of regions with different chemical or physical properties (Lateral Force Microscopy, LFM), the orientation of the surface crystalline domains (Trans- verse Shear Microscopy, TSM) or molecular resolution images[84, 86–89]. The intensity of the tip-sample interaction during scanning in contact mode is large enough to damage the surface of the softer samples, such as most organic materials. Even if the interaction with the tip can be signifi- cantly reduced, for example by working in dry nitrogen atmosphere in order to remove the contribution from capillary forces, it is better, if working in contact mode is not absolutely necessary (for example in order to obtain molecular resolution images), to make use of a complementary technique, called semi-contact mode. In tapping mode the cantilever is forced to oscil- late at a frequency ν close to its resonant frequency, thanks to a piezoelectric element placed in the tip holder. The tip-sample distance is then adjusted to
3.2 Sample characterization 37 Figure 3.8: Contact mode AFM image of the surface of a (3 × 3) µm 2 region of a rubrene thin film grown on top of a rubrene single crystal. (a) Height image in which lighter grays correspond to higher regions of the surface. (b) Deflection image collected on the same region, in which the grays scale is used to represent the cantilever deflection in each point. a value such that, during each oscillation, the tip passes from a non-contact regime to a contact regime, and then back to a non-contact regime. Variations in the average tip-sample distance lead to variations of the root mean square (RMS) oscillation amplitude of the cantilever. Thus, if during the scanning the tip encounters a valley or a protrusion on the sample surface there is a variation in the RMS oscillation amplitude of the cantilever. In analogy with what happens for contact-mode measurements, the feedback circuit restores the initial oscillation amplitude by varying the vertical extension of the scanner. A morphological image of the surface can then be reconstructed by assigning to each image pixel the corresponding value of the scanner vertical extension. Height and amplitude images collected in tapping mode would look exactly like the height and deflection images reported in figure 3.8 and collected in contact mode. Since in tapping mode the AFM tip is in contact with sample surface only in the lowest point of the cantilever oscillation, the duration of the tip- sample interaction is minimized with respect to contact mode. This leads to the main advantage of tapping mode over contact mode measurements, i.e. minimum sample damage due to tip-sample interactions. During the scanning of a surface in tapping mode it is also possible to collect phase-contrast images, in addition to height images, constructed
Page 1: Università degli Studi di Milano-B
Page 4 and 5: iv Contents 4 Rubrene thin films 47
Page 6 and 7: vi Contents
Page 8 and 9: viii Introduction ties of these mat
Page 10 and 11: x Introduction molecular oxygen inc
Page 12 and 13: 2 Organic semiconductors Figure 1.1
Page 14 and 15: 4 Organic semiconductors 1.3.1 Opti
Page 16 and 17: 6 Organic semiconductors Figure 1.2
Page 18 and 19: 8 Organic semiconductors 1.3.2 Elec
Page 20 and 21: 10 Organic semiconductors
Page 22 and 23: 12 Rubrene: physical and chemical p
Page 38 and 39: 28 Experimental techniques Figure 3
Page 42 and 43: 32 Experimental techniques 3.2 Samp
Page 48 and 49: 38 Experimental techniques by repre
Page 50 and 51: 40 Experimental techniques Fω =
Page 52 and 53: 42 Experimental techniques In order
Page 56 and 57: 46 Experimental techniques whole di
Page 58 and 59: 48 Rubrene thin films for a long ti
Page 60 and 61: 50 Rubrene thin films Parameter Val
Page 62 and 63: 52 Rubrene thin films Figure 4.3: M
Page 64 and 65: 54 Rubrene thin films series of dif
Page 66 and 67: 56 Rubrene thin films present in th
Page 68 and 69: 58 Rubrene thin films Figure 4.6: T
Page 70 and 71: 60 Rubrene thin films Figure 4.7: M
Page 72 and 73: 62 Rubrene thin films for a rubrene
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Page 82 and 83: 72 Rubrene thin films The presence
Page 84 and 85: 74 Rubrene thin films present. Sinc
Page 86 and 87: 76 Rubrene thin films ≈ −33.5 k
Page 88 and 89: 78 Rubrene thin films
Page 90 and 91: 80 Oxidation dynamics of rubrene ep
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86 Oxidation dynamics of rubrene ep
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100 Oxidation dynamics of rubrene e
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102 Influence of oxygen on electric
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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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