Covalent Attachment and Characterization of Perylene Monolayers on Si(111) and TiO2 for Electron-Selective Carrier Transport.

Abstract

We functionalized chemically oxidized Si(111) and TiO2 surfaces with covalently attached rylene molecules and demonstrated further chemical conversion of the attached species. Base-catalyzed activation of perylene tetracarboxylic dianhydride (PTCDA) preceded reaction with phenylaminosilane-terminated surfaces, yielding surface-bound perylene via an imide linkage. Transflection infrared (IR) spectroscopy of the carbonyl vibrational region elucidated the presence of anhydride, imide, and ester species following each reaction stage. The presence of both anhydride and imide IR features following reaction with PTCDA validates successful perylene attachment. Subsequent functionalization of the surface-attached perylenes yielded IR spectra with little or no detectable anhydride features that indicate successful conversion to ester or imide species based on respective reactions with alkyl bromides or aryl amines. X-ray photoelectron spectroscopy quantified fractional coverages of surface-attached perylene species following a post-deposition derivatization with fluorine-containing alkyl bromides and with anilines. Overlayer model interpretation of the photoelectron results determined a perylene surface coverage of ∼15% relative to the surface density of Si(111) atop sites and a ∼10% surface coverage of imide-terminated perylene species. The interpreted coverage data yield an approximate conversion efficiency for the anhydride-to-imide derivatization at surface-attached perylenes of ∼66%. We discuss the present results in terms of possible coverage and packing on oxide-free silicon surfaces and the utilization of covalently attached rylene species as electron-transporting and hole-blocking connecting layers in molecular electronics and tandem-junction photovoltaic designs.