Thin Friction The rubbing motion between two surfaces is always hindered by friction, which is caused by continuous contacting and attraction between the surfaces. These interactions may only occur over a distance of a few nanometers, but what happens when the interacting materials are only that thick? Lee et al. (p. 76; see the Perspective by Müser and Shakhvorostov) explored the frictional properties of a silicon tip in contact with four atomically thin quasi–two dimensional materials with different electrical properties. For all the materials, the friction was seen to increase as the thickness of the film decreased, both for flakes supported by substrates and for regions placed above holes that formed freely suspended membranes. Placing graphene on mica, to which it strongly adheres, suppressed this trend. For these thin, weakly adhered films, out-of-plane buckling is likely to dominate the frictional response, which leads to this universal behavior. A universal trend is observed for the friction properties of thin films on weakly adhering substrates. Using friction force microscopy, we compared the nanoscale frictional characteristics of atomically thin sheets of graphene, molybdenum disulfide (MoS2), niobium diselenide, and hexagonal boron nitride exfoliated onto a weakly adherent substrate (silicon oxide) to those of their bulk counterparts. Measurements down to single atomic sheets revealed that friction monotonically increased as the number of layers decreased for all four materials. Suspended graphene membranes showed the same trend, but binding the graphene strongly to a mica surface suppressed the trend. Tip-sample adhesion forces were indistinguishable for all thicknesses and substrate arrangements. Both graphene and MoS2 exhibited atomic lattice stick-slip friction, with the thinnest sheets possessing a sliding-length–dependent increase in static friction. These observations, coupled with finite element modeling, suggest that the trend arises from the thinner sheets’ increased susceptibility to out-of-plane elastic deformation. The generality of the results indicates that this may be a universal characteristic of nanoscale friction for atomically thin materials weakly bound to substrates.

Frictional Characteristics of Atomically-Thin Sheets

Melamine-derived carbon sponges for oil-water separation

To meet the increasing demands for effective heat management of electronic devices, a graphene-based polymeric composite is considered to be one of the candidate materials owing to the ultrahigh thermal conductivity (TC) of graphene. However, poor graphene dispersion, low quality of exfoliated graphene, and strong phonon scattering at the graphene/matrix interface restrict the heat dissipation ability of graphene-filled composites. Here, a facile and versatile approach to bond graphene foam (GF) with polydimethylsiloxane (PDMS) is proposed, and the corresponding composite with considerable improvement in TC and insulativity is fabricated. First, three-dimensional GF was coated with polydopamine (PDA) via π–π stack and functional groups from PDA reacted with 3-aminopropyltriethoxysilane (APTS). Then, the modified GF was compressed (c-GF) to enhance density and infiltrated with PDMS to get the c-GF/PDA/APTS/PDMS composite. As a result, these processes endow the composite with high TC of in-plane 28.77 W m–1...

Three-Dimensional Graphene Foam-Filled Elastomer Composites with High Thermal and Mechanical Properties

2D semiconductors allow for unique and ultrasensitive devices to be fabricated for applications ranging from clinical diagnosis instruments to low-energy light-emitting diodes (LEDs). Graphene has championed research in this field since it was first fabricated; however, its zero bandgap creates many challenges. Transition metal dichalcogenides (TMDCs), e.g., MoS2, have a direct bandgap which alleviates the challenge of creating a bandgap in graphene-based devices. Water wettability of MoS2 is critical to device fabrication/performance and MoS2 has been believed to be hydrophobic. Herein, we report that water contact angle (WCA) of freshly exfoliated MoS2 shows temporal evolution with an intrinsic WCA of 69.0 ± 3.8° that increases to 89.0 ± 3.1° after 1 day exposure to ambient air. ATR-FTIR and ellipsometry show that the fresh, intrinsically mildly hydrophilic MoS2 surface adsorbs hydrocarbons from ambient air and thus becomes hydrophobic.

Understanding the Intrinsic Water Wettability of Molybdenum Disulfide (MoS2)

ConspectusGraphitic carbons are important solid materials with myriad applications including electrodes, adsorbents, catalyst support, and solid lubricants. Understanding the interaction between water and graphitic carbons is critically important for both fundamental material characterization and practical device fabrication because the water–graphitic interface is essential to many applications. Research interests in graphene and carbon nanotubes over the past decades have brought renewed interest to elucidate wettability of graphitic carbons and understand their interaction with the surrounding environment.Research on this topic can be traced back to the 1940s, and the prevailing notion has been that graphitic carbons are hydrophobic. Though there have been different voices, this conclusion is supported by many previous water contact angle tests and well accepted by the community since sp2 carbon is nonpolar in nature. However, recent results from our groups showed that graphitic surfaces are intrinsica...

Are Graphitic Surfaces Hydrophobic

Because of the atomic thinness of graphene, its integration into a device will always involve its interaction with at least one supporting substrate, making the surface energy of graphene critical to its real-life applications. In the current paper, the contact angle of graphene synthesized by chemical vapor deposition (CVD) was monitored temporally after synthesis using water, diiodomethane, ethylene glycol, and glycerol. The surface energy was then calculated based on the contact angle data by the Fowkes, Owens–Wendt (extended Fowkes), and Neumann models. The surface energy of fresh CVD graphene grown on a copper substrate (G/Cu) immediately after synthesis was determined to be 62.2 ± 3.1 mJ/m2 (Fowkes), 53.0 ± 4.3 mJ/m2 (Owens–Wendt) and 63.8 ± 2.0 mJ/m2 (Neumann), which decreased to 45.6 ± 3.9, 37.5 ± 2.3, and 57.4 ± 2.1 mJ/m2, respectively, after 24 h of air exposure. The ellipsometry characterization indicates that the surface energy of G/Cu is affected by airborne hydrocarbon contamination. G/Cu ex...

http://pubs.acs.org/doi/pdf/10.1021/la5018328

Caitlyn Conaway

Papers

Study on the surface energy of graphene by contact angle measurements.