Rational Design of 2D Supramolecular Networks Switchable by External Electric Fields.

The reversible formation of hydrogen bonds is a ubiquitous mechanism for controlling molecular assembly in biological systems. However, achieving predictable reversibility in artificial two-dimensional (2D) materials remains a significant challenge. Here, we use an external electric field (EEF) at the solid/liquid interface to trigger the switching of H-bond-linked 2D networks using a scanning tunneling microscope. Assisted by density functional theory and molecular dynamics simulations, we systematically vary the molecule-to-molecule interactions, i.e., the hydrogen-bonding strength, as well as the molecule-to-substrate interactions to analyze the EEF switching effect. By tuning the building block's hydrogen-bonding ability (carboxylic acids vs aldehydes) and substrate nature and charge (graphite, graphene/Cu, graphene/SiO2), we induce or freeze the switching properties and control the final polymorphic output in the 2D network. Our results indicate that the switching ability is not inherent to any particular building block but instead relies on a synergistic combination of the relative adsorbate/adsorbate and absorbate/substrate energetic contributions under surface polarization. Furthermore, we describe the dynamics of the switching mechanism based on the rotation of carboxylic groups and proton exchange, which generate the polarizable species that are influenced by the EEF. This work provides insights into the design and control of reversible molecular assembly in 2D materials, with potential applications in a wide range of fields, including sensors and electronics.

Oxidative etching of S-vacancy defective MoS2 monolayer upon reaction with O2.

The reactions of O2 with S vacancy sites within a MoS2 monolayer were investigated using density functional theory calculations. We considered the following defects: single S vacancy, double S vacancy, two adjacent S vacancies and two S vacancies separated by a sulphur atom. We found that the surface distribution of S vacancy sites plays a key role in determining the surface reactivity towards O2. We observed the desorption of SO2 only for the last vacancy distribution. For the other cases, the surface becomes passivated with very stable structures having O atoms on the original vacancy sites and in some cases an SO group in an adjacent position. The ab initio molecular dynamics simulations showed that the impingement of the O2 molecule on an S vacancy site produces a stable chemisorbed O2 molecule with an upright configuration. The surface reactions initiate after the O2 molecule switches to the lying-down configuration which favours the breakage of the O-O bond and the concurrent formation of S-O bonds. In the most reactive vacancy site configuration, the dissociation of the first O2 molecule produces an SO intermediate which finally leads to desorption of SO2 after oxygen abstraction from the other adjacent O2 molecule. The formation of a MoO3 moiety within the monolayer was also observed in the molecular dynamic simulations at higher oxidation levels.

The Role of Tris(2-carboxyethyl)phosphine Reducing Agent in the Controlled Formation of α,ω-Alkanedithiols Monolayers on Au(111) with Monocoordinated and Bicoordinated Configurations.

The addition of the reducing agent tris(2-carboxyethyl) phosphine (TCEP) during the formation of α,ω-alkanedithiols monolayers on Au(111) using the immersion method produces the assembly of monolayers with bicoordinated molecules (both S-terminal groups bound to the surface) that have a reductive desorption potential that is more positive than for monolayers with monocoordinated molecules in a standing up configuration. We show that the use of TCEP either during formation of the monolayer or as a post treatment procedure allows the controlled formation of monolayers with bicoordinated or monocoordinated configurations. Density functional theory (DFT) calculations were performed to elucidate the role of TCEP in the formation of the bicoordinated configuration. We investigated the TCEP-dithiol interaction in ethanol solvent as well as the coadsorption of trimethylphosphine with 1,2-ethanedithiol on Au(111). The Brønsted base character of the phosphine facilitates the H exchange from the -SH groups of the dithiol to the phosphorus atom of TCEP with very low activation energy barriers, thus allowing the thiolate groups to bind to the Au(111) surface, thus yielding the bicoordinated configuration. Dithiol lifting mechanisms such as H exchange between S atoms and the formation of intra/inter layer disulfide bonds have much higher energy barriers.

A pH-Sensitive Supramolecular Switch Based on Mixed Carboxylic Acid Terminated Self-Assembled Monolayers on Au(111).

We show that homogeneously mixed self-assembled monolayers (SAMs) of mercaptoalkanoic acids of different chain lengths can be used to build up a pH-sensitive supramolecular switch. The acids with short and long alkyl chains interact via the strong hydrogen bond between carboxylic acid groups. The pH acts as a trigger by breaking or restoring the hydrogen bond interaction in basic or acidic solutions, respectively. The corresponding changes in the monolayer structure were determined by ellipsometry, surface-enhanced Raman spectroscopy, and contact angle measurements. Density functional theory (DFT) calculations were performed to elucidate the structures of interacting molecules compatible with the surface coverage obtained from electrochemical reductive desorption experiments. The simplicity of the preparation procedure assures a high reproducibility whereas the stability of the homogeneous mixed SAM guarantees the reversibility of the switching process.

The role of the organic layer functionalization in the formation of silicon/organic layer/metal junctions with coinage metals.

The design of silicon/alkyl layer/metal junctions for the formation of optimal top metal contacts requires knowledge of the mechanistic and energetic aspects of the interactions of metal atoms with the modified surface. This involves (a) the interaction of the metal with the terminal groups of the organic layer, (b) the diffusion of metal atoms through the organic layer and (c) the reactions of metal atoms with the silicon surface atoms. The diffusion through the monolayer and the metal catalyzed breakage of Si-C bonds must be avoided to obtain high quality junctions. In this work, we performed a comprehensive density functional theory investigation to identify the reaction pathways of all these processes. In the absence of a reactive terminal group, gold atoms may penetrate through a compact alkyl monolayer on Si(111) with no energy barrier. However, the presence of thiol terminal groups introduces a high energy barrier which blocks the diffusion of metals into the monolayer. The diffusion barriers increase in the order Ag < Au < Cu and correlate with the stability of metal-thiolate complexes whereas the barriers for the formation of metal silicides increase in the order Cu < Au < Ag in correlation with the increasing metallic radii. The reactivity of gold clusters with functionalized Si(111) surfaces was also investigated. Metal silicide formation can only be avoided by a compact monolayer terminated by a reactive functional group. The mechanistic and energetic picture obtained in this work contributes to understanding of the factors that influence the quality of top metal contacts during the formation of silicon/organic layer/metal junctions.

On the mechanism of silicon activation by halogen atoms.

Despite the widespread use of chlorinated silicon as the starting point for further functionalization reactions, the high reactivity of this surface toward a simple polar molecule such as ammonia still remains unclear. We therefore undertook a comprehensive investigation of the factors that govern the reactivity of halogenated silicon surfaces. The reaction of NH3 was investigated comparatively on the Cl-Si(100)-2 × 1, Br-Si(100)-2 × 1, H-Si(100)-2 × 1, and Si(100)-2 × 1 surfaces using density functional theory. The halogenated surfaces show considerable activation with respect to the hydrogenated surface. The reaction on the halogenated surfaces proceeds via the formation of a stable datively bonded complex in which a silicon atom is pentacoordinated. The activation of the halogenated Si(100)-2 × 1 surfaces toward ammonia arises from the large redistribution of charge in the transition state that precedes the breakage of the Si-X bond and the formation of the Si-NH2 bond. This transition state has an ionic nature of the form Si-NH3(+)X(-). Steric effects also play an important role in surface reactivity, making brominated surfaces less reactive than chlorinated surfaces. The overall activation-energy barriers on the Cl-Si(100)-2 × 1 and Br-Si(100)-2 × 1 surfaces are 12.3 and 19.9 kcal/mol, respectively, whereas on the hydrogenated Si(100)-2 × 1 surface the energy barrier is 38.3 kcal/mol. The reaction of ammonia on the chlorinated surface is even more activated than on the bare Si(100)-2 × 1 surface, for which the activation barrier is 21.3 kcal/mol. Coadsorption effects in partially aminated surfaces and in the presence of reaction products increase activation-energy barriers and have a blocking effect for further reactions of NH3.