Structural and orientational transitions in dibenzopentacene thin films on Bi(111)

The structural and orientational transitions in the thin films of dibenzopentacene grown on Bi(111) have been investigated with low temperature scanning tunneling microscopy. At the initial stage, dibenzopentacene molecules form two-dimensional (2D) gas over the substrate surface. With the coverage increasing, three condensed phases have been observed in the monolayer regime. (i) The low-density phase is composed of parallel rows of flat-lying molecules, which exhibit single-molecule chirality in the homochiral domains. (ii) Some molecules are intercalated into the low-density phase with edge-on orientation and form single-molecule chains. (iii) The high-density phase is composed of parallel trimer rows, and each trimer contains one flat-lying molecule and two edge-on molecules. (iv) In the multilayer regime, 3D molecular islands form on the first layer, indicating Stranski-Krastanov growth mode. These results provide insights into the correlation between molecular orientation and structures of thin films.