Achieving superlubricity (or near zero friction) at macroscales has been of prime importance to tribologists mainly because of tremendous advantages it may offer in terms of energy savings that is otherwise wasted to friction and wear. Earlier reports of structural superlubricity in layered materials at nanoscale were based on structural incommensurability between sliding lattice planes [1] and then recently, few interesting approaches have been proposed and demonstrated to extend structural superlubricity from micro to centimeter scale [2-3]. However, sustaining superlubricity at macroscale (which can be reproducibly translated into true engineering scale) has been very challenging with crystalline solids so far, since even small defect or disorder on the surface could hamper this effect along with other dissipative forces at play during dynamic sliding. In this work, we experimentally demonstrated that superlubricity can be realized at macroscales with sliding a diamond-like carbon (DLC) surface against graphene mixed with nanodiamonds [4]. We showed that during sliding, graphene patches wrap around nanodiamonds reducing the contact area and DLC provides a perfect incommensurate surface to achieve superlubric state in dry atmosphere for extended time periods. We performed detailed large-scale molecular dynamics simulations which closely elucidated the mesoscopic link that bridges the nanoscale mechanics and macroscopic experimental observations, thus introducing a new mechanism to explain our experimental results. Our discovery offers a direct pathway for designing smart frictionless tribological systems for practical applications of industrial interest.

References:

1. Superlubricity of graphite

Dienwiebel et al. Physical Review Letters, 92(12), 126101 (2004)

2. Observation of Microscale Superlubricity in Graphite

Liu et al. Physical Review Letters, 108, 205503 (2012)

3. Superlubricity in centimetres-long double-walled carbon nanotubes under ambient conditions, Zhang et al. Nature

Nanotechnology, 8, 912 (2013)

4. Macroscale superlubricity enabled by graphene nanoscroll formation

Berman et al. Science, 348, 6239, 1118 (2015)

This work was performed at the Center for Nanoscale Materials, a U.S. Department of Energy Office of Science User Facility under Contract No. DE-AC02-06CH11357.

Assemblies of graphene oxide (GO) sheets into macroscopic membranes are of great interest because GO membranes have been demonstrated to have excellent mechanical properties and great potential for applications in solution. Extraordinary stability of GO films in water has been noted, which is a prerequisite for their membrane applications in solution. However, this is counterintuitive because GO sheets become negatively charged on hydration and the electrostatic repulsion between the sheets should make the membrane disintegrate. In this study, we have discovered a long-overlooked reason behind this apparent contradiction. Our findings show that while neat GO membranes do, indeed, readily disintegrate in water, the films become stable if they are crosslinked by multivalent cationic metal contaminants. Such metal contaminants can be introduced unintentionally during the synthesis and processing of GO, most notably on filtration with anodized aluminium oxide filter discs that corrode to release significant amounts of aluminium ions. This new insight improves the understanding of interlayer interaction in GO membranes and their durability, which is crucial for proper use of GO thin films. The finding also has wide implications in interpreting the processing–structure–property relationships of GO and other lamellar membranes. Strategies to avoid and mitigate metal contamination, as well as those taking advantage of this effect for synthesizing new layered materials are discussed.

¹Nature Materials 12, 118 (2013).

²Appl. Phys. Lett. 105,081601 (2014).

³Appl. Mater. Interf. (2016) in press; J. Vac. Sci. Technol. A 33, 020605 (2015).

In this talk I will dicuss various approaches for deposition of 2D semi-conducting transition metal dichalcogenide films including MoS2, WS2, ReS2 and Phosphorene. These approaches range from mechanical exfoliation to state-of-art chemical vapor deposition methods. I will describe the unique optoelectronic properties of these materials in the monolayer limit. I will discuss various methods to charaterize the state of defectiveness of these films. We find that while the Raman spectra of these films are insensitive to defects, their photoluminescence reveals a distinct defect-related spectral feature located about 0.1 eV below the neutral free A-exciton peak. This peak originates from defect-bound neutral excitons and intensifies as the sheet is made more defective. This spectral feature is observable in air under ambient conditions (room temperature and atmospheric pressure), which allows for a relatively simple way to determine the defectiveness of 2D semiconducting nanosheets. I will also discuss doping of MoS2 and WS2 sheets in which Mo and W atoms are substituted by transition metal impurity atoms. I will show that both p and n type doping is possible using this approach and will discuss the effect of such doping on the electronic and optoelectronic properties of the material. Finally I will consider aging of 2D MoS2 and WS2 sheets. The aging will include both effect of airborne contaminants as well as oxidation and structural degradation of such films.