Heterojunctions of semiconductors and metals are the fundamental building blocks of modern electronics. Coherent heterostructures between dissimilar materials can be achieved by composition, doping, or heteroepitaxy of chemically different elements. Here, we report the formation of coherent single-layer 1H−1T MoS 2 heterostructures by mechanical exfoliation on Au(111), which are chemically homogeneous with matched lattices but show electronically distinct semiconducting (1H phase) and metallic (1T phase) character, with the formation of these heterojunctions attributed to a combination of lattice strain and charge transfer. The exfoliation approach employed is free of tape residues usually found in many exfoliation methods and yields single-layer MoS 2 with millimeter (mm) size on the Au surface. Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), scanning tunneling microscopy (STM), and scanning tunneling spectroscopy (STS) have collectively been employed to elucidate the structural and electronic properties of MoS 2 monolayers on Au substrates. Bubbles in the MoS 2 formed by the trapping of ambient adsorbates beneath the single layer during deposition, have also been observed and characterized. Our work here provides a basis to produce two-dimensional heterostructures which represent potential candidates for future electronic devices.

The performance of electrochemical devices using ionic liquids (ILs) as electrolytes can be impaired by water uptake. This work investigates the influence of water on the behavior of hydrophilic and hydrophobic ILswith ethylsulfate and tris(perfluoroalkyl)trifluorophosphate or bis(trifluoromethyl sulfonyl)imide (TFSI) anions, respectivelyon electrified graphene, a promising electrode material. The results show that water uptake slightly reduces the IL electrochemical stability and significantly influences graphene’s potential of zero charge, which is justified by the extent of anion depletion from the surface. Experiments confirm the dominant contribution of graphene’s quantum capacitance (C Q ) to the total interfacial capacitance (C int ) near the PZC, as expected from theory. Combining theory and experiments reveals that the hydrophilic IL efficiently screens surface charge and exhibits the largest double layer capacitance (C IL ∼ 80 μF cm–2), so that C Q governs the charge stored. The hydrophobic ILs are less efficient in charge screening and thus exhibit a smaller capacitance (C IL ∼ 6–9 μF cm–2), which governs C int already at small potentials. An increase in the total interfacial capacitance is observed at positive voltages for humid TFSI-ILs relative to dry ones, consistent with the presence of a satellite peak. Short-range surface forces reveal the change of the interfacial layering with potential and water uptake owing to reorientation of counterions, counterion binding, co-ion repulsion, and water enrichment. These results are consistent with the charge being mainly stored in a ∼2 nm-thick double layer, which implies that ILs behave as highly concentrated electrolytes. This knowledge will advance the design of IL-graphene-based electrochemical devices.

In mechanochemistry, the application of controlled forces is key to altering reaction rates and pathways to direct product yields and selectivity. However, a fundamental knowledge gap exists between what is occurring on the atomic scale in mechanically driven reactions and the resulting macroscale outcomes. Two-dimensional (2D) materials, such as graphene, proffer a model system to study the impact of mechanical forces, such as strain, on chemical reactivity, as force distributions may be applied across a well-organized atomic-scale structure comprising a single layer of C atoms. Here, using Raman micro-spectroscopy and first-principles calculations, we have investigated the reaction of graphene, under varying degrees of strain, with 4-nitrobenzenediazonium tetrafluoroborate (4-NBD). We find that only with increased out-of-plane distortion (shifting the C atoms of graphene from sp 2 toward sp 3 electronic states) would the reactivity be increased, with larger out-of-plane distortions yielding greater reactivity. Density functional theory (DFT) calculations reveal that increasing the curvature of graphene decreases the activation barrier of 4-NBD functionalization and enhances the thermodynamic favorability of the reaction. Furthermore, we find that curvature affects the orientation of the graphene 2p z orbitals, and we then relate the thermodynamic feasibility of 4-NBD functionalization with the orbital orientation. These studies point to how the precise application of forces can be used to direct the functionalization of graphene for C−C bond forming reactions, which has significant implications for controlling its corresponding electronic structure in a well-defined fashion.

Graphene has unique mechanical, electronic, and optical properties that make it of interest for an array of applications. These properties can be modulated by controlling the architecture of graphene and its interactions with surfaces. Self-assembled monolayers (SAMs) can tailor graphene–surface interactions; however, spatially controlling these interactions remains a challenge. Here, we blend colloidal lithography with varying SAM chemistries to create patterned architectures that modify the properties of graphene based on its chemical interactions with the substrate and to study how these interactions are spatially arrayed. The patterned systems and their resulting structural, nanomechanical, and optical properties have been characterized using atomic force microscopy, Raman and infrared spectroscopies, scattering-type scanning near-field optical microscopy, and X-ray photoelectron spectroscopy.

There is significant interest in using single- and few-layer molybdenum disulfide (MoS2) in nanoscale devices, stemming from the ability to tune its optoelectronic properties. One method investigated to tune the mechanical and electronic properties of MoS2 is through aryl radical addition reactions with compounds such as 4-nitrobenzenediazonium tetrafluoroborate (4-NBD). Here we investigated this for single-layer MoS2 (SLM) and multilayer MoS2 (MLM) on Au(111) substrates. The optical, chemical, and tribological properties of SLM and MLM on Au(111) were investigated before and after functionalization with an aqueous solution of 4-NBD. Atomic force microscopy (AFM) was used to characterize the pristine and functionalized MoS2 on Au(111) and illustrated the formation of a film on the MoS2 surfaces from the diazonium compound. This film density was greater on SLM than on MLM, likely due to the increased electronic coupling between SLM and the Au(111) substrate. Interestingly, we find that the films formed appear to be weakly bound on the surface of SLM and MLM and were easily worn away through AFM when contact forces exceeded 5 and 3 nN, respectively, suggesting limited covalent binding to the surfaces. While some of the diazonium reacts with the surfaces, the remaining diazonium film on the surface of SLM and MLM is stabilized through electron transfer from the underlying Au(111) substrate yielding a self-terminating dendritic growth on the surface with a film thickness of about 1.5 nm, regardless of the thickness of the MoS2 or the initial concentration of the diazonium solution.