Towards modelling the vibrational signatures of functionalized surfaces: carboxylic acids on H-Si(111) surfaces

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

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.

Effects of the basis set and of the exchange-correlation functional on the Inelastic Electron Tunneling signatures of 1,4-benzenedithiol

The effects of the atomic basis set and of the exchange-correlation (XC) functional on the Inelastic Electron Tunneling (IET) spectra have been investigated by considering the prototypical 1,4-benzenedithiol molecule. These studies have been completed by tackling the reliability of the same methods for predicting the IR absorption spectrum of the same molecule. The main conclusions are (i) the B3LYP XC functional is suitable to predict the relative vibrational frequencies, (ii) provided a scaling factor is used, the root mean square error on the vibrational frequencies goes down to 18 cm(-1), (iii) triple-ζ basis sets and in particular the cc-pVTZ basis set is a good compromise between accuracy and computational needs, (iv) basis set effects on the IET intensities are larger than those of the XC functional, and (v) the cc-pVTZ, cc-pVQZ, and aug-cc-pVDZ basis sets provide consistent IET intensities.

Analyzing the vibrational signatures of thiophenol adsorbed on small gold clusters by DFT calculations

Using density functional theory, we calculate the IR and Raman signatures of the thiophenol (TP) molecule adsorbed on gold clusters by mimicking the different types of adsorption sites, and we analyze these signatures by using advanced tools implemented into the pyvib2 program. First, we follow the evolution of the vibrational normal modes from the isolated TP molecule to those of TP adsorbed on different clusters to highlight the influence of the site of adsorption on the vibrational motions. The use of the overlap matrix between the modes enables mode permutations, mode mixings, and mode splittings to be highlighted, all of which depend not only on the adsorption but also on the type of cluster and its symmetry. Second, the IR and Raman signatures were analyzed by using group coupling matrices and atomic contribution patterns based on the Hug decomposition scheme. Key results include 1) the fact that Raman spectroscopy is more sensitive than IR spectroscopy with respect to the nature of the coordination site, 2) an IR criterion that distinguishes between on-top coordination (onefold coordinated) with respect to the bridge (twofold coordinated) and hexagonal close-packed hollow site coordination (threefold coordinated), and 3) the best agreement to the experimental Raman spectrum with regard to signatures in the 500 to 1200 cm(-1) region is obtained for bridged, twofold coordination.