Structure and bonding of monolayer and multilayer films of acenes adsorbed to silver(111).

摘要

Linear acenes are very promising for use as the semiconductor material in organic thin film transistors (OTFT). The organic-metal interface in these devices has been identified as crucial to device performance. In this dissertation, a series of linear acenes adsorbed to Ag(111) was studied using thermally programmed desorption (TPD). In all cases, repulsive interactions were found within the first monolayer due to mutual dipole-dipole repulsions. The dipoles are a result of charge transfer in the formation of the adsorbate-substrate bond. Standard charge transfer models were used to model the repulsive interactions for benzene and naphthalene. For tetracene and pentacene, models were constructed that account for the short-range interactions by considering charge and dipole delocalization. It was found that the desorption energy and dipole moment increases linearly with the number of double bonds of the acene. In addition, it was found that the desorption energy and dipole moment are also linearly related.; Multilayer films of linear acenes adsorbed on Ag(111) at 90 K all show the formation of metastable phases with the exception of benzene. For naphthalene, increasing the coverage could activate the conversion of the metastable phase to a more stable phase. For tetracene and pentacene, the conversion of the metastable phase to a more stable phase could be thermally activated. In all cases, the desorption energy of the most stable multilayer phase was found to agree well with the bulk sublimation energy.; In addition, the monolayer structure of the aromatic molecule aniline adsorbed to Ag(111) was studied using TPD and high resolution electron energy loss spectroscopy (HREELS). TPD showed that aniline is weakly chemisorbed to Ag(111) with dipole-dipole repulsive interactions. An intensity analysis of the HREELS spectrum revealed that the aniline has a slight tilt with respect to the surface, with the amino group closer to the surface than the phenyl group. A deconvolution of the HREELS spectrum showed that the vibrational mode frequencies are not perturbed significantly from the gas phase values. It was determined that the interaction between aniline and Ag is primarily through the amino group.