Molecular orientation and electronic properties of oriented organic thin films by means of low-energy electron transmission.

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We investigated the correlation between the molecular orientation and the electron- transport properties for thin films of long alkyl molecules, Langmuir-Blodgett (LB) films of cadmium arachidate, LB films of polydiacetylene, evaporated films of tetratetracontane and thin films of related molecules, with low-energy electron transmission (LEET) technique. LEET spectra were measured as a function of film thickness, crystallinity, temperature and time after the film deposition.We found that (i) degree of normal molecular orientation depends on the film thickness and the orientation becomes better for the thickness larger than 4 molecular layers, (ii) a poor orientation for thinner films, 2 molecular layers, is a results of poor crystallinity, (iii) the bottom of the conduction band is located at 0.4-0.5 ev above the vacuum level and the energy position shifts to low energy side by heating due to thermal excitation of molecular vibrations, (iii) the effective efficiency of the hot- electron injection into the film is larger at low temperature, and (iv) the electron transmission through the band gap can not explained by the tunnel theory.Further, angle-resolved photoemission experiments were performed with synchrotron radiation for oriented thin films of 18-pentatriacontanone (stearone). The normal molecular orientation and energy band dispersion were observed simultaneously with this method.