Solvent-Mediated Modulation of the Au–S Bond in Dithiol Molecular Junctions

Gold–dithiol molecular junctions have been studied both experimentally and theoretically. However, the nature of the gold–thiolate bond as it relates to the solvent has seldom been investigated. It is known that solvents can impact the electronic structure of single-molecule junctions, but the correlation between the solvent and dithiol-linked single-molecule junction conductance is not well understood. We study molecular junctions formed with thiol-terminated phenylenes from both 1-chloronaphthalene and 1-bromonaphthalene solutions. We find that the most probable conductance and the distribution of conductances are both affected by the solvent. First-principles calculations show that junction conductance depends on the binding configurations (adatom, atop, and bridge) of the thiolate on the Au surface, as has been shown previously. More importantly, we find that brominated solvents can restrict the binding of thiols to specific Au sites. This mechanism offers new insight into the effects of the solvent environment on covalent bonding in molecular junctions

Impact of Surface Adsorption on Metal–Ligand Binding of Phenanthrolines

Phenanthrolines are a class of ligands known to bind with many different metal cations to form complexes. The aromatic backbone of phenanthroline also allows for preferential adsorption on few-layer graphene (FLG) films via π–π stacking. Here we investigate the effects of adsorption and the resulting steric restrictions on the binding ability of four different phenanthroline derivatives: phenanthroline, neocuproine, bathophen­anthroline, and bathocuproine. In solution, a wide range of metal cations tested formed complexes with these ligands, but only Cu2+ and Ag+ showed evidence of binding to ligands adsorbed onto FLG, as measured by the chemiresistive response of the films. The substituents present on each ligand affected the magnitude of the response in different ways. Raman and X-ray photoelectron spectroscopy (XPS) were used to study two different systems in more detail: iron with phenanthroline, which shows a response in solution (ferroin) but not on the FLG surface (purported monoligand complex), and copper with neocuproine, which responds both in solution (bis­[neocuproine]­copper­(I)) and on the FLG surface (monoligand complex). Raman and XPS data indicate complexing of copper by surface-bound ligands. Theoretical calculations show that the copper–neocuproine monocomplex has a higher adsorption energy and binding energy to a graphene surface than the iron–phenanthroline monocomplex. The reduction of copper­(II) to copper­(I) by the surface-bound neocuproine further results in a stronger charge transfer response from the sensor. The results of this study provide insights into the mechanisms of solid-state sensing of metal cations for water quality detection based on steric and electrochemical restrictions induced by surface adsorption

Tuning the Chemical and Mechanical Properties of Conductive MoS₂ Thin Films by Surface Modification with Aryl Diazonium Salts

Molybdenum disulfide (MoS2) is a promising material for applications in sensors, energy storage, energy conversion devices, solar cells, and fuel cells. Because many of those applications require conductive materials, we recently developed a method for preparing a conductive form of MoS2 (c-MoS2) using dilute aqueous hydrogen peroxide in a simple and safe way. Here, we investigate modulating the chemical and mechanical surface properties of c-MoS2 thin films using diazonium chemistry. In addition to a direct passivation strategy of c-MoS2 with diazonium salts for electron-withdrawing groups, we also propose a novel in situ synthetic pathway for modification with electron-donating groups. The obtained results are examined by Raman spectroscopy and X-ray photoelectron spectroscopy. The degree of surface passivation of pristine and functionalized c-MoS2 films was tested by exposing them to aqueous solutions of different metal cations (Fe2+, Zn2+, Cu2+, and Co2+) and detecting the chemiresistive response. While pristine films were found to interact with several of the cations, modified films did not. We propose that a surface charge transfer mechanism is responsible for the chemiresistive response of the pristine films, while both modification routes succeeded at complete surface passivation. Functionalization was also found to lower the coefficient of friction for semiconducting 2H-MoS2, while all conductive materials (modified or not) also had lower coefficients of friction. This opens up a pathway to a palette of dry lubricant materials with improved chemical stability and tunable conductivity. Thus, both in situ and direct diazonium chemistries are powerful tools for tuning chemical and mechanical properties of conductive MoS2 for new devices and lubricants based on conductive MoS2

Defect Engineering of Graphene to Modulate pH Response of Graphene Devices

Graphene-based pH sensors are a robust, durable, sensitive, and scalable approach for the sensitive detection of pH in various environments. However, the mechanisms through which graphene responds to pH variations are not well-understood yet. This study provides a new look into the surface science of graphene-based pH sensors to address the existing gaps and inconsistencies among the literature concerning sensing response, the role of defects, and surface/solution interactions. Herein, we demonstrate the dependence of the sensing response on the defect density level of graphene, measured by Raman spectroscopy. At the crossover point (ID/IG = 0.35), two countervailing mechanisms balance each other out, separating two regions where either a surface defect induced (negative slope) or a double layer induced (positive slope) response dominates. For ratios above 0.35, the pH-dependent induction of charges at surface functional groups (both pH-sensitive and nonsensitive groups) dominates the device response. Below a ratio of 0.35, the response is dominated by the modulation of charge carriers in the graphene due to the electric double layer formed from the interaction between the graphene surface and the electrolyte solution. Selective functionalization of the surface was utilized to uncover the dominant acid–base interactions of carboxyl and amine groups at low pH while hydroxyl groups control the high pH range sensitivity. The overall pH-sensing characteristics of the graphene will be determined by the balance of these two mechanisms