J. Vrijmoeth et al., SINGLE CRYSTALLITES IN PLANAR POLYCRYSTALLINE OLIGOTHIOPHENE FILMS - DETERMINATION OF ORIENTATION AND THICKNESS BY POLARIZATION MICROSCOPY, Journal of applied physics, 83(7), 1998, pp. 3816-3824
Thin films of evaporated oligothiophenes (alpha-nT, n = 4-8) show a ''
planar polycrystalline'' structure: each of the individual crystallite
s has a random azimuthal orientation, the (a, b) face of its unit cell
is aligned with the surface plane, We introduce a technique to determ
ine the orientation and thickness of such aligned thiophene crystals b
y optical polarization microscopy. Due to the optical birefringence of
the crystal, It appears with different colors in the microscope depen
dent on its orientation and thickness. To support the method proposed,
we solve Maxwell's equations and obtain quantitative agreement with t
he observed colors. The organic crystal shows biaxial anisotropy. For
unsubstituted quaterthiophene, alpha-4T, we find effective refractive
indices n(b) = 1.84 +/- 0.1 and n(a) = 1.61 +/- 0.1 for waves under no
rmal incidence. Our conclusions are fully confirmed by atomic force mi
croscopy with molecular resolution. Our analyses result in a simple re
cipe to obtain the directions of the rr and b crystal axes from the op
tical experiment. (C) 1998 American Institute of Physics. [S0021-8979(
98)02007-6].