Ze Liu

Observation of microscale superlubricity in graphite.

Through experimental study, we reveal superlubricity as the mechanism of self-retracting motion of micrometer sized graphite flakes on graphite platforms by correlating respectively the lock-up or self-retraction states with the commensurate or incommensurate contacts. We show that the scale-dependent loss of self-retractability is caused by generation of contact interfacial defects. A HOPG structure is also proposed to understand our experimental observations, particularly in term of the polycrystal structure. The realisation of the superlubricity in micrometer scale in our experiments will have impact in the design and fabrication of micro/nanoelectromechanical systems based on graphitic materials.

Interlayer binding energy of graphite: A mesoscopic determination from deformation

Despite interlayer binding energy is one of the most important material properties for graphite, there is still lacking report on its direct experimental determination. In this paper, we present a novel experimental method to directly measure the interlayer binding energy of highly oriented pyrolytic graphite (HOPG). The obtained values of the binding energy are 0.27(±0.02)J/m , which can serve as a benchmark for other theoretical and experimental works.

A graphite nanoeraser

We present here a method for cleaning intermediate-size (up to 50 nm) contamination from highly oriented pyrolytic graphite and graphene. Electron-beam-induced deposition of carbonaceous material on graphene and graphite surfaces inside a scanning electron microscope, which is difficult to remove by conventional techniques, can be removed by direct mechanical wiping using a graphite nanoeraser, thus drastically reducing the amount of contamination. We discuss potential applications of this cleaning procedure.

Stripe/kink microstructures formed in mechanical peeling of highly orientated pyrolytic graphite

Mechanical exfoliation is nowadays the primary method to produce isolated graphenes. A stripe/kink microstructure is observed in our graphite flakes produced by mechanical exfoliation of highly oriented pyrolytic graphite (HOPG). It composes a series of parallel stripes with width of about 100 microns separated by kinking microstructures (∼2 microns) in the graphite flake plane. The formation of such structure is attributed to the sliding between adjacent layers of the HOPG under the mechanical peeling. A theoretical model is presented to understand the persistence of such kinking structures in terms of the interlayer shear force locking effect.

Binding and interlayer force in the near-contact region of two graphite slabs: Experiment and theory

Via a novel experiment, Liu et al. [Phys. Rev. B 85, 205418 (2012)] estimated the graphite binding energy, specifically the cleavage energy, an important physical property of bulk graphite. We re-examine the data analysis and note that within the standard Lennard-Jones model employed, there are difficulties in achieving internal consistency in the reproduction of the graphite elastic properties. By employing similar models which guarantee consistency with the elastic constant, we find a wide range of model dependent binding energy values from the same experimental data. We attribute some of the difficulties in the determination of the binding energy to: (i) limited theoretical understanding of the van der Waals dispersion of graphite cleavage, (ii) the mis-match between the strong bending stiffness of the graphite-SiO2 cantilever and the weak asymptotic inter-layer forces that are integrated over to produce the binding energy. We find, however, that the data do support determination of a maximum inter-layer force that is relatively model independent. We conclude that the peak force per unit area is 1.1 ± 0.15 GPa for cleavage, and occurs at an inter-layer spacing of 0.377 ± 0.013 nm.

Mechanics and Multidisciplinary Study for Creating Graphene-Based van der Waals Nano/Microscale Devices

Elastic resonators are the core elements for various types of nano/micro scale instruments and devices (e.g. gyroscopes, mass and acceleration sensors, AFM, SNOM). However due to the inevitable thermal dissipation in the elastic deforming modes their quality factor dramatically reduces as size shrinks, which is the bottleneck challenge for the application in nano devices. Van der Waals (vdW) oscillators recently invented (Zheng QS, Jiang Q, Phys Rev Lett, 88:045503, 2002) have two orders of magnitude higher in both motion speed and quality factor, that are the two major factors determining the performance of various nano/microscale devices, for example nano/micromechanical gyroscopes. Based on the vdW oscillators a completely new class of nano/micro devices is proposed. Furthermore the recently discovered self-retraction motion between two large scale sheared graphite flakes (Zheng QS, et al, Phys Rev Lett, 100:067205, 2008) has greatly promoted the graphene based vdW devices. By combining with the mature microfabrication technology for mass production, the graphene-based vdW sliding devices offer a great candidate for a new type of nano/micro devices, as well as high-density/high-speed hard diskettes. In this paper we report new experimental and theoretical advances in these fields, including self-retraction motion and dissipation mechanisms, challenges in surface physics and chemistry, novel stripe/kink structures arising from instabilities, transferring, self-assembling, and ultrahigh-speed record technology.