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

More documents

Recommendations

Info

20 Rubrene: physical and chemical properties Figure 2.8: Contour plot showing the excitation intensity distribution (top) and the PL intensity distribution (bottom) on the (b,c) surface of a rubrene single crystal. From [66] direction for which charge carrier mobility reaches its maximum values[66]. 2.6 Oxidation of crystalline rubrene As already noted in section 2.2 rubrene molecules are strongly affected by oxidation. The understanding of the dynamics and effects of this process on the properties of crystalline rubrene is thus mandatory. The main source of contaminants and defects in crystalline rubrene is the presence of impurities in the source material which are incorporated during the crystal growth; rubrene peroxide have been shown to be the dominant impurity in commercial rubrene powder, and the presence of small amounts of oxygen in the inert gas flow used for crystal growth has been shown to increase the amount of such impurities in the as-grown crystals[67, 68]. Purification of the source material through several sublimation cycles and a carefully controlled growth environment are thus mandatory in order to get high quality samples in a reproducible manner. A study of the post-growth oxidation of crystalline rubrene exposed to ambient conditions for several days has been carried out by mass spectrometry[19]. In figure 2.9 the concentration of rubrene peroxide molecules in rubrene thin films of different crystalline quality as a function of depth is reported. While in amorphous or dendritic films a large amount of rubrene peroxide is found even in the deeper layers, in rubrene crystallites it is present only in the first layers. The most likely reason for this behavior is that in
2.6 Oxidation of crystalline rubrene 21 Figure 2.9: Depth profiles of rubrene peroxide concentration in rubrene thin films with different degrees of crystallinity as determined by mass spectrometry. From [19]. crystalline rubrene there is less room for the molecular conformational change upon rubrene peroxide formation. Nonetheless this result demonstrate the presence of a non negligible amount of rubrene peroxide even in the first few layers of rubrene crystals exposed to air. This has also been confirmed by Mitrofanov et al., comparing the PL spectra of pristine rubrene single crystals with those of rubrene single crystals oxidized on purpose by exposing them to a pure oxygen atmosphere for several hours[69]. The PL spectra collected at various temperatures, reported in figure 2.10a and referring to pristine (left) and oxidized (right) samples, show a clear change of the PL bands upon oxidation, in particular with the appearance of a band around 650 nm, attributed to the electronic transitions from a bandgap state induced by rubrene peroxide. By two-photon spectroscopy the authors have also been able to measure the PL spectra of the oxidized samples as a function of depth in the crystal. The results, reported in figure 2.10b, show that the rubrene peroxide PL band is present only in the spectra collected from the sample surface, while the spectra collected from the bulk resemble those of pristine rubrene crystals, in agreement with the mass spectrometry data previously discussed. The actual origin of the 650 nm PL band, which is often related to oxygen induced bandgap states, is however still unclear. Recently Chen et al. have carried out a systematic study on this particular band and their find- ings suggest that it actually originates from amorphous inclusions in rubrene
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 46 and 47: 36 Experimental techniques the scan
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
Page 74 and 75: 64 Rubrene thin films Figure 4.9: (
Page 76 and 77: 66 Rubrene thin films Figure 4.11:
Page 78 and 79: 68 Rubrene thin films Figure 4.12:
Page 80 and 81:
70 Rubrene thin films Figure 4.13:
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
Page 92 and 93:
Page 94 and 95:
Page 96 and 97:
Page 98 and 99:
Page 100 and 101:
Page 102 and 103:
Page 104 and 105:
Page 106 and 107:
Page 108 and 109:
Page 110 and 111:
100 Oxidation dynamics of rubrene e
Page 112 and 113:
102 Influence of oxygen on electric
Page 114 and 115:
Page 116 and 117:
Page 118 and 119:
Page 120 and 121:
Page 122 and 123:
Page 124 and 125:
Page 126 and 127:
Page 128 and 129:
118 Conclusions and Perspectives On
Page 130 and 131:
120 Conclusions and Perspectives
Page 132 and 133:
122 Bibliography [13] D. A. Bernard
Page 134 and 135:
124 Bibliography [41] V. Coropceanu
Page 136 and 137:
126 Bibliography [71] A. J. Maliaka
Page 138 and 139:
128 Bibliography [101] C. Kittel, I
Page 140 and 141:
130 Bibliography [130] D. Beljonne,
Page 142 and 143:
132 Bibliography
Page 144:
134 PhD activity Attended courses
show all

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

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?