Improving Orientation, Packing Density, and Molecular Arrangement in Self-Assembled Monolayers of Bianchoring Ferrocene-Triazole Derivatives by "Click" Chemistry.

This work describes the self-assembled monolayers (SAMs) of two ferrocene derivatives with two anchoring groups (at the bottom and at the top of the SAM) deposited on ultraflat template-stripped gold substrates by cyclic voltammetry and analyzed by complementary surface characterization techniques. The SAM of each molecule is deposited by three different protocols: direct deposition (one step), click reaction on the surface (two steps), and reverse click reaction on the surface (two steps). The SAM structure is well studied to determine the SAM orientation, SAM arrangement, and ferrocene position within the SAM. Electron transfer kinetics have also been studied, which agree with the quality of each SAM. With the help of two anchoring groups and click-chemistry active functional groups, we have shown that the two molecules can be deposited by controlling the position of ferrocene at either end. We further investigated the involvement of the triazole five-membered ring in the electron transfer mechanism. We have found that a carbon spacer between ferrocene and triazole improves the SAM packing. This study enhances the understanding of tethering thiol and thiol acetate anchoring groups on gold by a controlled orientation, which may help in the development of functional molecular devices requiring two anchoring groups.

Van der Waals Heteroepitaxy of Air-Stable Quasi-Free-Standing Silicene Layers on CVD Epitaxial Graphene/6H-SiC.

Graphene, consisting of an inert, thermally stable material with an atomically flat, dangling-bond-free surface, is by essence an ideal template layer for van der Waals heteroepitaxy of two-dimensional materials such as silicene. However, depending on the synthesis method and growth parameters, graphene (Gr) substrates could exhibit, on a single sample, various surface structures, thicknesses, defects, and step heights. These structures noticeably affect the growth mode of epitaxial layers, e.g., turning the layer-by-layer growth into the Volmer-Weber growth promoted by defect-assisted nucleation. In this work, the growth of silicon on chemical vapor deposited epitaxial Gr (1 ML Gr/1 ML Gr buffer) on a 6H-SiC(0001) substrate is investigated by a combination of atomic force microscopy (AFM), scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and Raman spectroscopy measurements. It is shown that the perfect control of full-scale almost defect-free 1 ML Gr with a single surface structure and the ultraclean conditions for molecular beam epitaxy deposition of silicon represent key prerequisites for ensuring the growth of extended silicene sheets on epitaxial graphene. At low coverages, the deposition of Si produces large silicene sheets (some hundreds of nanometers large) attested by both AFM and SEM observations and the onset of a Raman peak at 560 cm-1, very close to the theoretical value of 570 cm-1 calculated for free-standing silicene. This vibrational mode at 560 cm-1 represents the highest ever experimentally measured value and is representative of quasi-free-standing silicene with almost no interaction with inert nonmetal substrates. From a coverage rate of 1 ML, the silicene sheets disappear at the expense of 3D Si dendritic islands whose density, size, and thickness increase with the deposited thickness. From this coverage, the Raman mode assigned to quasi-free-standing silicene totally vanishes, and the 2D flakes of silicene are no longer observed by AFM. The experimental results are in very good agreement with the results of kinetic Monte Carlo simulations that rationalize the initial flake growth in solid-state dewetting conditions, followed by the growth of ridges surrounding and eventually covering the 2D flakes. A full description of the growth mechanism is given. This study, which covers a wide range of growth parameters, challenges recent results stating the impossibility to grow silicene on a carbon inert surface and is very promising for large-scale silicene growth. It shows that silicene growth can be achieved using perfectly controlled and ultraclean deposition conditions and an almost defect-free Gr substrate.

Influence of chlorine substitution on the self-assembly of zinc phthalocyanine.

The adsorption and ordering of zinc phthalocyanine (ZnPc) and octachloro zinc phthalocyanine (ZnPcCl(8)) on an Ag(111) surface is studied in situ by scanning tunneling microscopy under ultrahigh vacuum. Two-dimensional self-assembled supramolecular domains are observed for these two molecules. We show how substituting chlorine atoms for half of the peripheral hydrogen atoms on ZnPc influences the self-assembly mechanisms. While intermolecular interactions are dominated by van der Waals forces in ZnPc molecular networks, ZnPcCl(8) molecular packing undergoes a sequential phase evolution driven by the creation of C-Cl...H-C hydrogen bonds between adjacent molecules. At the end of this evolution, the final molecular assembly involves all possible hydrogen bonds. Our study also reveals the influence of molecule-substrate interactions through the presence of fault lines generating a stripe structure in the molecular film.