1. b a The crystal lattice of rubrene is orthorhombic:  =  =  = 90 0 and a = 14.44 Å b = 7.18 Å c = 26.97 Å Density = 1.26 g/cm 3 Z = 4 The space group Aba2 In most of the rubrene crystals the largest natural facet is (a,b), at which we probe charge carrier transport by depositing a FET structure. The crystals are usually elongated along the b axis, and the two natural angles can be identified if the corners are not broken off:  = 127 0, and  = 116.5 0 In FET measurements, the mobility is at maximum along the b axis (see next slide). The c axis is perpendicular to the drawing.   Normally, the crystals are elongated in this direction Crystal structure of Rubrene (1,1,0) (1,0,0) c (0,0,1) 1-in rubrene crystal (RU 2009) Henn, Williams, Gibbons, J. Appl. Cryst. 4, 256 (1971) (see actual text of the paper on the last slide)

2. Molecular Packing in Rubrene Strong  -  overlap along the b axis   b ~ 20 cm 2 /Vs, a record for mobility in organic semiconductors b a rubrene HOMO herringbone packing the a-b plane, flat crystal facet c a b rubrene V. Sundar et al., Science 303, 1644-1646 (2004) V. Podzorov et al., Phys. Rev. Lett. 93, 086602 (2004)

3. a (7 Å) b (14 Å) c (27 Å) d (~15 Å) Molecular steps in AFM image on the left correspond to one top layer of rubrene molecules. The terraces are separated by d = c/2 = ~15 Å. 3D crystal structure of Rubrene

4. Optical absorption and photoluminescence of Rubrene H. Najafov et al, PRL 96 056604 (2006)