Molybdenum disulfide (MoS2) is a semiconducting transition metal dichalcogenide that is known to be a catalyst for both the hydrogen evolution reaction (HER) as well as for hydro-desulfurization (HDS) of sulfur-rich hydrocarbon fuels. Specifically, the edges of MoS2 nanostructures are known to be far more catalytically active as compared to unmodified basal planes. However, in the absence of the precise details of the geometric and electronic structure of the active catalytic sites, a rational means of modulating edge reactivity remain to be developed. Here we demonstrate using first-principles calculations, X-ray absorption spectroscopy, as well as scanning transmission X-ray microscopy (STXM) imaging that edge corrugations yield distinctive spectroscopic signatures corresponding to increased localization of hybrid Mo 4d states. Independent spectroscopic signatures of such edge states are identified at both the S L2,3 and S K-edges with distinctive spatial localization of such states observed in S L2,3-edge STXM imaging. The presence of such low-energy hybrid states at the edge of the conduction band is seen to correlate with substantially enhanced electrocatalytic activity in terms of a lower Tafel slope and higher exchange current density. These results elucidate the nature of the edge electronic structure and provide a clear framework for its rational manipulation to enhance catalytic activity.

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 functional implementation of graphene as a solid boundary lubricant requires the ability to control its frictional response across a variety of interfaces. This is challenging, as being a single atomic layer thick, the nanotribological properties of graphene depend highly on the competing interaction strengths with the converse sides of the top and bottom contacts of the interfaces it is placed in between. One method to modulate these interactions is to tune the surface chemistry (of one or both counter-faces) with self-assembled monolayers (SAMs). To fully understand the effects on the graphene/SAM (G-SAM) composite interfaces formed, however, first necessitates a basic understanding of graphene–SAM interactions. To explore graphene–SAM adhesive and frictional interactions over a range of chemical functionalities, SAMs were used to functionalize atomic force microscopy (AFM) tips with varying terminal end-groups (−NH2, −CH3, and –phenyl, compared to unfunctionalized −OH terminated reference tips). AFM pull-off force measurements and thermal gravimetric analysis (TGA) were used to evaluate the work of adhesion (mJ/m2) and interaction energy (kcal/mol) of the functionalized tips with graphene. Friction force microscopy (FFM) measurements were performed with the same functionalized AFM tips to examine the graphene-molecule frictional response. Tip–graphene interaction strength was increased for hydrophobic and aromatic functional groups. The frictional response was found to depend on a balance of graphene-molecule adhesion and shear strain.

No abstract

Polyacetylene materials, particularly two-dimensional polyacetylene thin films, have widespread applications in modern electronics and energy harvesting devices. One way to fabricate polyacetylene thin films is to attach functional groups that can covalently bond to underlying surfaces. Thiol-terminated polydiacetylenes, with their highly conjugated backbones, provide abundant possibilities to fabricate robust monolayer thin films when attached to Au surfaces. Similar to diacetylene solids or Langmuir−Blodgett films, the polymerization of diacetylene thiolates (DATs) on Au(111) surfaces can be initiated by UV radiation. The highly conjugated structures, combined with the covalent bonds to the Au surfaces, generate superior rigidity as well as distinct electronic properties as compared to typical alkanethiolate monolayers on Au substrates. Here, we explore the diacetylene thiol, HS(CH 2 ) 3 CC−CC(CH 2 ) 4 CH 3 (C3-DAT), assembled on Au(111) surfaces before and after polymerization. The monolayers were found to exhibit higher friction following polymerization due to structural changes in the films. Electronically, although I−V (dI/dV) measurements show similar spectra for both C3-DAT and C3-poly-DAT monolayer films, size-dependent differences in the electronic properties were found to arise for C3-poly-DAT islands of ca. 10−20 nm when embedded into a dodecanethiolate matrix, which can be attributed to the Coulomb blockade.