Growth of organic ultrathin films studied by Penning ionization electron and ultraviolet photoelectron spectroscopies: Pentacene
Authors: Ozaki Hiroyuki 
Published: July 2002

Penning ionization electron spectroscopy was applied to ultrathin pentacene films [monolayer (0.3 nm thick) to dozens of layers] prepared by vapor deposition under different conditions. Remarkable differences were found among the Penning ionization electron spectra (PIES). The local electron distribution of each molecular orbital (MO) protruding from the film surface was probed and the relation between the MO shape and the molecular orientation was investigated. Deposition onto a metal substrate without a crystallographical surface yields a crystalline film at room temperature. The molecules are oriented with the long axes almost perpendicular to the substrate and make the σ bands of the PIES by far stronger than the π bands. In the pure π region, the π9 and π7 MOs having large distribution at the long-axis end provide more intense bands than other π MOs. On the metal substrate held at 213 K, molecules form an amorphous film with the long axes inclined a little on average. The π and σ bands exhibit comparable intensities and no specific band is enhanced. When 1 monolayer equivalence (MLE) of pentacene is deposited onto a graphite substrate at 123 K, a monolayer of flat-lying molecules is obtained. The π MOs provide more enhanced bands than the σ MOs but the π9 and π7 MOs with little distribution around the C–H bonds are harder to detect than other MOs in the pure π region. Furthermore, the growth of each film was investigated using Penning spectroscopy and ultraviolet photoelectron spectroscopy in combination. Spectral dependence upon amount of deposition revealed three modes of film growth, which correspond to the three molecular aggregations. The crystalline “film” cannot cover the substrate to ca. 30 MLE because molecules landed on the substrate move around and gather to form crystallites which grow three-dimensionally. But, the crystallite formation is inhibited on the cooled metal substrate owing to the low mobility of molecules. The rough surface is completely covered at 3–5 MLE and the molecules are accumulated randomly but uniformly in thickness with further deposition. On the graphite substrate, every new monolayer of flat-lying molecules is formed at 123 K and piled up in succession to form a layered film. With increasing number of layers, however, the surface molecules become inclined little by little. Finally, at 60 MLE they are tilted to the same extent as in an amorphous film. The structures and growth modes were found consistent with the stability or sublimation properties of these and related films as well as with the relaxation shifts reflected in the positions of the first PIES bands. It was also indicated that the aggregation of the outermost molecules is considerably different between the amorphous and layered film of 60 MLE in spite of similar, somewhat-tilted orientation. That is, the molecules mutually overlay and sterically prevent the neighbors from desorbing in the former, whereas the molecules lack upper-side neighbors and are very liable to desorb in the latter.