Andri Schütz

Synthesis of a covalent monolayer sheet by photochemical anthracene dimerization at the air/water interface and its mechanical characterization by AFM indentation

Covalent monolayer sheets in 2 hours: spreading of threefold anthracene-equipped shape-persistent and amphiphilic monomers at the air/water interface followed by a short photochemical treatment provides access to infinitely sized, strictly monolayered, covalent sheets with in-plane elastic modulus in the range of 19 N/m.

Synthesis of two-dimensional analogues of copolymers by site-to-site transmetalation of organometallic monolayer sheets

Monolayer sheets have gained attention due to the unique properties derived from their two-dimensional structure. One of the key challenges in sheet modification/synthesis is to exchange integral parts while keeping them intact. We describe site-to-site transmetalation of Zn(2+) in the netpoints of cm(2)-sized, metal-organic sheets by Fe(2+), Co(2+), and Pb(2+). This novel transformation was done both randomly and at predetermined patterns defined by photolithography to create monolayer sheets composed of different netpoints. All transmetalated sheets are mechanically strong enough to be spanned over 20 × 20 μm(2) sized holes. Density functional theory calculations provide both a model for the molecular structure of an Fe(2+)-based sheet and first insights into how transmetalation proceeds. Such transmetalated sheets with random and patterned netpoints can be considered as two-dimensional analogues of linear copolymers. Their nanoscale synthesis presents an advance in monolayer/polymer chemistry with applications in fields such as surface coating, molecular electronics, device fabrication, imaging, and sensing.

Square-micrometer-sized, free-standing organometallic sheets and their square-centimeter-sized multilayers on solid substrates.

Oligofunctional terpyridine-based monomers are spread at an air/water interface, where they are connected with transition metal salts such as Fe(II) into mechanically coherent monolayer sheets of macroscopic dimension. The conversions of these processes are determined by XPS for several monomer/metal ion combinations. The sheets are transferred onto TEM grids, the 20 × 20 square micrometer sized holes of which can be spanned. AFM indentation experiments provide in-plane elastic moduli which are compared with naturally occurring sheets such as graphene. The new organometallic sheets are also used to create multilayer assemblies on square centimeter length scales on solid substrates. Finally some directions are provided where this research can lead to in future and where its application potential lies.

Approaching two-dimensional copolymers: photoirradiation of anthracene- and diaza-anthracene-bearing monomers in Langmuir monolayers.

By using structurally similar amphiphilic monomers, it is shown that compressed monolayers of varying amounts of such monomers at the air/water interface can be converted by photo-irradiation into the corresponding covalently connected monolayer sheets. Since one of the monomers carries three anthracene units and the other three 1,8-diaza-anthracene units, the growth reaction is proposed to take place through photochemically achieved [4+4]-cycloaddition between pairs of these units that are co-facially (face-to-face) arranged, to furnish the corresponding covalent dimers. While evidence for both homodimers is amply available, the existence of the heterodimer needs to be established with the help of a model reaction to support the conceptual aspect of this work, copolymerization in two dimensions. The sheet copolymers exhibit substantial robustness in that they can be spanned over 20 × 20 μm(2)-sized holes without rupturing under their own weight. X-ray photoelectron spectroscopy (XPS) studies reveal that the monomers are incorporated into the sheet copolymers according to feed. These results establish existence of the first covalent sheet copolymer, which is considered a step ahead towards novel 2D materials.

Minimally invasive characterization of covalent monolayer sheets using tip-enhanced Raman spectroscopy.

Synthetic covalent monolayer sheets and their subclass, two-dimensional polymers are of particular interest in materials science because of their special dimensionality which renders them very different from any bulk matter. However, structural analysis of such entities is rather challenging, and there is a clear need for additional analytical methods. The present study shows how tip-enhanced Raman spectroscopy (TERS) can be performed on monomer monolayers and the covalent sheets prepared from them by [4 + 4]-cycloaddition to explore rather complex structural and mechanistic issues. TERS is a surface analytical method that combines the high lateral resolution of scanning probe microscopy (SPM) with a greatly enhanced Raman scattering intensity. The high spatial resolution (<60 nm) and the significantly improved sensitivity (contrast factor of >4000) compared to confocal Raman microscopy provides new insights into the formation of this new and exciting material, namely significant consumption of the reactive units (anthracenes) and exclusion of the alternative [4 + 2]-cycloaddition. Moreover, due to the high lateral resolution, it was possible to find a first spectroscopic hint for step growth as the dominant mechanism in the formation of these novel monolayer sheets. In addition, TERS was used to get first insights into the phase behavior of a comonomer mixture.