Thin-Film Organic Heteroepitaxy.

Incorporating crystalline organic semiconductors into electronic devices requires understanding of heteroepitaxy given the ubiquity of heterojunctions in these devices. However, while rules for commensurate epitaxy of covalent or ionic inorganic material systems are known to be dictated by lattice matching constraints, rules for heteroepitaxy of molecular systems are still being written. Here, it is found that lattice matching alone is insufficient to achieve heteroepitaxy in molecular systems, owing to weak intermolecular forces that describe molecular crystals. It is found that, in addition, the lattice matched plane also must be the lowest energy surface of the adcrystal to achieve one-to-one commensurate molecular heteroepitaxy over a large area. Ultraviolet photoelectron spectroscopy demonstrates the lattice matched interface to be of higher electronic quality than a disordered interface of the same materials.

Electronic Properties of Cyano Ionic Liquids: a Valence Band Photoemission Study.

The valence band spectra of three cyano-ionic liquids based on 1-ethyl-3-methylimidazolium (Im2,1+) paired with thiocyanate (SCN-), dicyanamide (N(CN)2-), and tricyanomethanide (C(CN)3-) have been measured using ultraviolet and X-ray photoemission spectroscopy. Experimental spectra are compared to their corresponding density of states, weighted by photoemission cross sections, calculated for clusters of ions pairs of increasing size. Thus, this study bridges single ion approaches to 3D periodical DFT studies and enables the exploration of the different aspects of electronic structure establishment in ILs. Even for a relatively small cluster size, the relative energy of cation and anions states shifts by an amount that corresponds closely to that expected from the Madelung energy of a bulk IL, and the photoemission cross section-weighted DOS spectra are in good agreement with the measured valence bands. Trends in the relative energy and ionic character of the frontier orbitals across this series of cyano-ILs are discussed.

Helical Peptides Design for Molecular Dipoles Functionalization of Wide Band Gap Oxides.

The use of helical hexapeptides to establish a surface dipole layer on a TiO2 substrate, with the goal of influencing the energy levels of a coadsorbed chromophore, is explored. Two helical hexapeptides, synthesized from 2-amino isobutyric acid (Aib) residues, were protected at the N-terminus with a carboxybenzyl group (Z) and at the C-terminus carried either a carboxylic acid or an isophthalic acid (Ipa) anchor group to form Z-(Aib)6-COOH or Z-(Aib)6-Ipa, respectively. Using a combination of vibrational and photoemission spectroscopies, bonding of the two peptides to TiO2 surfaces (either nanostructured or single-crystal TiO2(110)) was found to be highly dependent on the anchor group, with Ipa establishing a monolayer much more efficiently than COOH. Furthermore, a monolayer of Z-(Aib)6-Ipa on TiO2(110) was exposed for different binding times to a solution of a zinc tetraphenylporphyrin (ZnTPP) derivative terminated with an Ipa anchor group (ZnTPP-P-Ipa). Photoemission spectroscopy revealed that ZnTPP-P-Ipa partly displaced Z-(Aib)6-Ipa, forming a coadsorbed monolayer on the oxide surface. The presence of the peptide molecular dipole shifted the HOMO levels of the ZnTPP group to lower energy by ∼300 meV, in accordance with a simple parallel plate capacitor model. These results suggest that a mixed-layer approach, involving coadsorption of a strong molecular dipole compound with a chromophore, is a versatile method to shift the energy levels of such chromophores with respect to the band edges of the substrate.