Reversible Anion‐Driven Switching of an Organic 2D Crystal at a Solid–Liquid Interface

Ionic self‐assembly of charged molecular building blocks relies on the interplay between long‐range electrostatic forces and short‐range, often cooperative, supramolecular interactions, yet has been seldom studied in two dimensions at the solid–liquid interface. Here, we demonstrate anion‐driven switching of two‐dimensional (2D) crystal structure at the Au(111)/octanoic acid interface. Using scanning tunneling microscopy (STM), three organic salts with identical polyaromatic cation (PQPC₆⁺) but different anions (perchlorate, anthraquinonedisulfonate, benzenesulfonate) are shown to form distinct, highly ordered self‐assembled structures. Reversible switching of the supramolecular arrangement is demonstrated by in situ exchange of the anion on the pre‐formed adlayer, by changing the concentration ratio between the incoming and outgoing anion. Density functional theory (DFT) calculations reveal that perchlorate is highly mobile in the adlayer, and corroborate why this anion is only resolved transiently in STM. Surprisingly, the templating effect of the anion persists even where it does not become part of the adlayer 2D fabric, which we ascribe to differences in stabilization of cation conformations by the anion. Our results provide important insight into the structuring of mixed anion–cation adlayers. This is essential in the design of tectons for ionic self‐assembled superstructures and biomimetic adaptive materials and valuable also to understand adsorbate–adsorbate interactions in heterogeneous catalysis.     

Carbon monoxide oxidation and nitrous oxide reduction on Rh/Pt(1 1 1) electrodes

Rhodium adlayers on Pt(1 1 1) substrates have been prepared by electrodeposition from dilute Rh³⁺ acidic solutions. Resulting deposition rates are lower than 0.03 ML min⁻¹. Pseudomorphic growth of the first monolayer has been confirmed by scanning tunneling microscopy (STM) as well as the formation of small compact islands in the submonolayer range. Carbon monoxide oxidation and nitrous oxide reduction have been studied on Rh/Pt(1 1 1) electrodes. The oxidation of carbon monoxide is catalyzed by the presence of very low coverages of rhodium as demonstrated by the negative shift of the CO oxidation profile. Results are compatible with a bifunctional mechanism for catalysis including CO diffusion in the Pt domains toward the edges of the islands (splitting of the voltammetric oxidation profile). The reduction of nitrous oxide occurs at different potential and with different rates on Pt domains, at the center of the Rh islands and at their edges, being the latter sites especially active. In any case, the adsorptive and catalytic activity of the adlayers differ from those of the bulk Pt(1 1 1) and Rh(1 1 1) electrodes. The existence of strain in the film together with a diminution in the coordination number for adatoms at the edges of the islands are considered to be at the origin of the observed behavior.     

Adlayer‐Free Large‐Area Single Crystal Graphene Grown on a Cu(111) Foil

To date, thousands of publications have reported chemical vapor deposition growth of “single layer” graphene, but none of them has described truly single layer graphene over large area because a fraction of the area has adlayers. It is found that the amount of subsurface carbon (leading to additional nuclei) in Cu foils directly correlates with the extent of adlayer growth. Annealing in hydrogen gas atmosphere depletes the subsurface carbon in the Cu foil. Adlayer‐free single crystal and polycrystalline single layer graphene films are grown on Cu(111) and polycrystalline Cu foils containing no subsurface carbon, respectively. This single crystal graphene contains parallel, centimeter‐long ≈100 nm wide “folds,” separated by 20 to 50 µm, while folds (and wrinkles) are distributed quasi‐randomly in the polycrystalline graphene film. High‐performance field‐effect transistors are readily fabricated in the large regions between adjacent parallel folds in the adlayer‐free single crystal graphene film.     

云母-石墨烯界面纳米尺度受限水层的湿润-去湿润研究

研究受限于纳米尺度空间的水或者其他分子液体的性质在生物学、材料科学、摩擦学、微流控等领域有重要的意义。但直接通过显微镜观察界面上的纳米尺度液体的润湿动态过程仍然是个挑战。近年来受益于石墨烯等二维晶体材料的发现与发展,由石墨烯作为覆盖材料形成的受限体系为这一问题提供了很好的解决方式。通过原子力显微镜原位成像发现,由石墨烯与云母组成的受限体系中包含水层,一层水层的高度约为0.37 nm,而通过改变周围空气的湿度可以调控受限体系中水层的湿润-去湿润可逆性变化。同时实验中观察到了水层随湿度变化时呈现出的不同的生长方式。平整的石墨烯覆盖层提供了一种新的尺寸可调整的纳米通道,可对纳米尺度的受限水层实现高分辨成像,为纳米流体器件的发展提供了可能性。     

Adlayer structure of octa-alkoxy-substituted copper(II) phthalocyanine on Au(111) by electrochemical scanning tunneling microscopy

Electrochemical scanning tunneling microscopy (ECSTM) has been used to examine the adlayer of octa-alkoxy-substituted copper(II) phthalocyanines (CuPc(OC(8)H(17))(8)) on Au(111) in 0.1 M HClO(4), where the molecular adlayer was prepared by spontaneous adsorption from a benzene solution containing this molecule. Topography STM scans revealed long-range ordered, interweaved arrays of CuPc(OC(8)H(17))(8) with coexistent rectangular and hexagonal symmetries. High-quality STM molecular resolution yielded the internal molecular structure and the orientation of CuPc(OC(8)H(17))(8) admolecules. These STM results could shed insight into the method of generating ordered molecular assemblies of phthalocyanine molecules with long-chained substitutes on metal surface.     

Fluorescence and electrochemistry studies of pyrene-functionalized surface adlayers to probe the microenvironment formed by cholesterol

We have synthesized a series of pyrene/cholesterol co-functionalized adlayers on quartz, oxidized silicon, indium-doped tin oxide and gold substrates. The pyrene derivative is N-1-pyrenesulfonyl-ethylenediamine (PSEDA) and the cholesterol derivative is cholesterol-ethylenediamine (Chol-NH₂), which was bound covalently to substrates through epoxide functionalities. X-ray photoelectron spectroscopy shows covalent attachment of both moieties. Optical ellipsometry shows an increase of ca. 5 Å with pyrene/cholesterol co-attachment on oxidized silicon wafers, and an increase of ca. 12 Å when only pyrene was added. Steady-state fluorescence measurements indicate the presence of cholesterol reduces the efficiency of pyrene excimer formation and provides a less polar environment as sensed by the PSEDA I₁/I₃ band ratio. The amount of pyrene excimer formed depends on the reaction time for the adlayer co-deposition reaction. Cyclic voltammetry shows that covalently bound PSEDA is oxidized at ca. 540 mV and physisorbed PSEDA is oxidized at ca. 780 mV. AC voltammetry shows that Chol-NH₂ in the adlayer reduces the electron transfer rate for the PSEDA redox reaction.