Diana Berman

Macroscale superlubricity enabled by graphene nanoscroll formation

Slip sliding away Many applications would benefit from ultralow friction conditions to minimize wear on the moving parts such as in hard drives or engines. On the very small scale, ultralow friction has been observed with graphite as a lubricant. Berman et al. achieved superlubricity using graphene in combination with crystalline diamond nanoparticles and diamondlike carbon (see the Perspective by Hone and Carpick). Simulations showed that sliding of the graphene patches around the tiny nanodiamond particles led to nanoscrolls with reduced contact area that slide easily against the amorphous diamondlike carbon surface. Science, this issue p. 1118; see also p. 1087 Nanodiamonds wrapped with graphene sheets lead to ultralow friction against a diamondlike carbon surface. [Also see Perspective by Hone and Carpick] Friction and wear remain as the primary modes of mechanical energy dissipation in moving mechanical assemblies; thus, it is desirable to minimize friction in a number of applications. We demonstrate that superlubricity can be realized at engineering scale when graphene is used in combination with nanodiamond particles and diamondlike carbon (DLC). Macroscopic superlubricity originates because graphene patches at a sliding interface wrap around nanodiamonds to form nanoscrolls with reduced contact area that slide against the DLC surface, achieving an incommensurate contact and substantially reduced coefficient of friction (~0.004). Atomistic simulations elucidate the overall mechanism and mesoscopic link bridging the nanoscale mechanics and macroscopic experimental observations.

Graphene as a protective coating and superior lubricant for electrical contacts

Potential for graphene to be used as a lubricant for sliding electrical contacts has been evaluated. Graphene, being deposited as a sporadic flakes on the gold substrate sliding against titanium nitride ball shows not only significant improvement in tribological behavior by reducing both friction (by factor of 2–3) and wear (by 2 orders) but also, even more importantly, demonstrates stable and low electrical resistance at the sliding contacts undergoing thousands of sliding passes regardless of the test environment (i.e., both in humid and dry conditions).

Sequential Infiltration Synthesis for the Design of Low Refractive Index Surface Coatings with Controllable Thickness

Control over refractive index and thickness of surface coatings is central to the design of low refraction films used in applications ranging from optical computing to antireflective coatings. Here, we introduce gas-phase sequential infiltration synthesis (SIS) as a robust, powerful, and efficient approach to deposit conformal coatings with very low refractive indices. We demonstrate that the refractive indices of inorganic coatings can be efficiently tuned by the number of cycles used in the SIS process, composition, and selective swelling of the of the polymer template. We show that the refractive index of Al2O3 can be lowered from 1.76 down to 1.1 using this method. The thickness of the Al2O3 coating can be efficiently controlled by the swelling of the block copolymer template in ethanol at elevated temperature, thereby enabling deposition of both single-layer and graded-index broadband antireflective coatings. Using this technique, Fresnel reflections of glass can be reduced to as low as 0.1% under normal illumination over a broad spectral range.

Metal-induced rapid transformation of diamond into single and multilayer graphene on wafer scale

The degradation of intrinsic properties of graphene during the transfer process constitutes a major challenge in graphene device fabrication, stimulating the need for direct growth of graphene on dielectric substrates. Previous attempts of metal-induced transformation of diamond and silicon carbide into graphene suffers from metal contamination and inability to scale graphene growth over large area. Here, we introduce a direct approach to transform polycrystalline diamond into high-quality graphene layers on wafer scale (4 inch in diameter) using a rapid thermal annealing process facilitated by a nickel, Ni thin film catalyst on top. We show that the process can be tuned to grow single or multilayer graphene with good electronic properties. Molecular dynamics simulations elucidate the mechanism of graphene growth on polycrystalline diamond. In addition, we demonstrate the lateral growth of free-standing graphene over micron-sized pre-fabricated holes, opening exciting opportunities for future graphene/diamond-based electronics.

Contact voltage-induced softening of RF microelectromechanical system gold-on-gold contacts at cryogenic temperatures

A series of experiments were performed in vacuum environments to investigate the impact of rf micromechanical system switch contact voltage versus resistance for gold-on-gold contacts at cryogenic temperatures. The purpose of this work was twofold as follows: (1) to examine whether asperity heating models already validated for high temperature contacts were also applicable at cryogenic temperatures and (2) to explore the implications and validity of prior suggestions that contact temperatures between 338 and 373 K are high enough to dissociate adsorbed film and/or push them aside but low enough to prevent asperities from becoming soft and adherent. Measurements on two distinct switch types, fabricated at independent laboratories, were performed in the temperature range 79–293 K and for contact voltages ranging from 0.01 to 0.13 V. Contact resistance values at all temperatures were observed to be lower for higher contact voltages, consistent with the aforementioned asperity heating models, whereby increase...

Impact of adsorbed organic monolayers on vacuum electron tunneling contributions to electrical resistance at an asperity contact

Electrical contact resistance measurements are reported for RF micro-electromechanical switches situated within an ultrahigh vacuum system equipped with in situ oxygen plasma cleaning capabilities. Measurements were performed on fused (permanently adhered) switches with Au/Au contacts and functioning switches with Au/RuO2 contacts in both the presence and absence of adsorbed monolayers of pentane and dodecane. For switches adhered in the closed position, adsorption occurs only in regions external to direct contact. For functioning switches, however, it can occur either within or exterior to the contact. The data are analyzed within the framework of two distinct geometries, to explore how the presence of adsorbed molecules in regions close to the contact may impact vacuum tunneling contributions to the experimentally measured resistance: (1) The resistance associated with direct contact in parallel with a vacuum tunneling path, which upon uptake of the monolayer is replaced by the molecular resistance and ...

Approaches for Achieving Superlubricity in Two-Dimensional Materials

Controlling friction and reducing wear of moving mechanical systems is important in many applications, from nanoscale electromechanical systems to large-scale car engines and wind turbines. Accordingly, multiple efforts are dedicated to design materials and surfaces for efficient friction and wear manipulation. Recent advances in two-dimensional (2D) materials, such as graphene, hexagonal boron nitride, molybdenum disulfide, and other 2D materials opened an era for conformal, atomically thin solid lubricants. However, the process of effectively incorporating 2D films requires a fundamental understanding of the atomistic origins of friction. In this review, we outline basic mechanisms for frictional energy dissipation during sliding of two surfaces against each other, and the procedures for manipulating friction and wear by introducing 2D materials at the tribological interface. Finally, we highlight recent progress in implementing 2D materials for friction reduction to near-zero values-superlubricity-across scales from nano- up to macroscale contacts.

Operando tribochemical formation of onion-like-carbon leads to macroscale superlubricity

Stress-induced reactions at the sliding interface during relative movement are known to cause structural or chemical modifications in contacting materials. The nature of these modifications at the atomic level and formation of byproducts in an oil-free environment, however, remain poorly understood and pose uncertainties in predicting the tribological performance of the complete tribosystem. Here, we demonstrate that tribochemical reactions occur even in dry conditions when hydrogenated diamond-like carbon (H-DLC) surface is slid against two-dimensional (2D) molybdenum disulfide along with nanodiamonds in dry nitrogen atmosphere. Detailed experimental studies coupled with reactive molecular dynamics simulations reveal that at high contact pressures, diffusion of sulfur from the dissociated molybdenum disulfide led to amorphization of nanodiamond and subsequent transformation to onion-like carbon structures (OLCs). The in situ formation of OLCs at the sliding interface provide reduced contact area as well as incommensurate contact with respect to the H-DLC surface, thus enabling successful demonstration of superlubricityStress-induced tribochemical reactions that reduce friction at sliding interfaces typically require liquid lubricants. Here, the authors discover the nanoscale tribocatalytic formation of onion-like carbon from 2D MoS2 and nanodiamond under dry and oil-free conditions, providing superlubricity at the macroscale.

Discontinuous fatty acid elongation yields hydroxylated seed oil with improved function

The biosynthesis of ‘unusual’ fatty acids with structures that deviate from the common C16 and C18 fatty acids has evolved numerous times in the plant kingdom. Characterization of unusual fatty acid biosynthesis has enabled increased understanding of enzyme substrate properties, metabolic plasticity and oil functionality. Here, we report the identification of a novel pathway for hydroxy fatty acid biosynthesis based on the serendipitous discovery of two C24 fatty acids containing hydroxyl groups at the 7 and 18 carbon atoms as major components of the seed oil of Orychophragmus violaceus, a China-native Brassicaceae. Biochemical and genetic evidence are presented for premature or ‘discontinuous’ elongation of a 3-OH intermediate by a divergent 3-ketoacyl-CoA (coenzyme A) synthase during a chain extension cycle as the origin of the 7-OH group of the dihydroxy fatty acids. Tribology studies revealed superior high-temperature lubricant properties for O. violaceus seed oil compared to castor oil, a high-performance vegetable oil lubricant. These findings provide a direct pathway for designing a new class of environmentally friendly lubricants and unveil the potential of O. violaceus as a new industrial oilseed crop.Vegetable oils are used in cooking, manufacturing and supplying biofuel. Now, researchers have identified two new fatty acids in the seed oil of a Brassicaceae species whose biosynthesis involves an unprecedented fatty acid hydroxylation pathway.

Effect of Substrate Support on Dynamic Graphene/Metal Electrical Contacts

Recent advances in graphene and other two-dimensional (2D) material synthesis and characterization have led to their use in emerging technologies, including flexible electronics. However, a major challenge is electrical contact stability, especially under mechanical straining or dynamic loading, which can be important for 2D material use in microelectromechanical systems. In this letter, we investigate the stability of dynamic electrical contacts at a graphene/metal interface using atomic force microscopy (AFM), under static conditions with variable normal loads and under sliding conditions with variable speeds. Our results demonstrate that contact resistance depends on the nature of the graphene support, specifically whether the graphene is free-standing or supported by a substrate, as well as on the contact load and sliding velocity. The results of the dynamic AFM experiments are corroborated by simulations, which show that the presence of a stiff substrate, increased load, and reduced sliding velocity lead to a more stable low-resistance contact.