Surface Curvature Enhances the Electrotunability of Ionic Liquid Lubrication.

Ionic liquids (ILs) are a promising class of lubricants that allow dynamic friction control at electrified interfaces. In the real world, surfaces inevitably exhibit some degree of roughness, which can influence lubrication. In this work, we deposited single-layer graphene onto 20 nm silica nanoparticle films to investigate the effect of surface curvature and electrostatic potential on both the lubricious behavior and interfacial layering structure of 1-ethyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide on graphene. Normal force and friction force measurements were conducted by atomic force microscopy using a sharp silicon tip. Our results reveal that the friction coefficient at the lubricated tip-graphene contacts significantly depends on surface curvature. Two friction coefficients are measured on graphene peaks and valleys with a higher coefficient measured at lower loads (pressures), whereas only one friction coefficient is measured on smooth graphene. Moreover, the electrotunability of the friction coefficient at low loads is observed to be significantly enhanced in peaks and valleys compared with smooth graphene. This is associated with the promoted overscreening of surface charge on convex interfaces and the steric hindrance at concave interfaces, which leads to more layers of ions (electrostatically) bound to the surface, i.e., thicker boundary films (electrical double layers). This work opens new avenues to control IL lubrication on the nanoscale by combining topographic features and an electric field.

Two-Legged Molecular Walker and Curvature: Mechanochemical Ring Migration on Graphene.

Attaining controllable molecular motion at the nanoscale can be beneficial for multiple reasons, spanning from optoelectronics to catalysis. Here we study the movement of a two-legged molecular walker by modeling the migration of a phenyl aziridine ring on curved graphene. We find that directional ring migration can be attained on graphene in the cases of both 1D (wrinkled/rippled) and 2D (bubble-shaped) curvature. Using a descriptor approach based on graphene's frontier orbital orientation, we can understand the changes in binding energy of the ring as it translates across different sites with variable curvature and the kinetic barriers associated with ring migration. Additionally, we show that the extent of covalent bonding between graphene and the molecule at different sites directly controls the binding energy gradient, propelling molecular migration. Importantly, one can envision such walkers as carriers of charge and disruptors of local bonding. This study enables a new way to tune the electronic structure of two-dimensional materials for a range of applications.

Water in the Electrical Double Layer of Ionic Liquids on Graphene.

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 (CQ) to the total interfacial capacitance (Cint) 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 (CIL ∼ 80 µF cm-2), so that CQ governs the charge stored. The hydrophobic ILs are less efficient in charge screening and thus exhibit a smaller capacitance (CIL ∼ 6-9 µF cm-2), which governs Cint 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.

Using Patterned Self-Assembled Monolayers to Tune Graphene-Substrate Interactions.

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.

Formation of Coherent 1H-1T Heterostructures in Single-Layer MoS2 on Au(111).

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 MoS2 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 MoS2 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 MoS2 monolayers on Au substrates. Bubbles in the MoS2 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.