Mechanical exfoliation is a widely used method to isolate high quality graphene layers from bulk graphite. In our recent experiments, some ordered microstructures, consisting of a periodic alternation of kinks and stripes, were observed in thin graph
ite flakes that were mechanically peeled from highly oriented pyrolytic graphite (HOPG). A theoretical model is presented in this paper to understand the formation of such ordered microstructures, based on elastic buckling of a graphite flake being subjected to a bending moment. The width of the stripes predicted from this model agrees reasonably well with our experimental measurements.
Hexagonal layered crystalline materials, such as graphene, boron nitride, tungsten sulfate, and so on, have attracted enormous attentions, due to their unique combination of atomistic structures and superior thermal, mechanical, and physical properti
es. Making use of mechanical buckling is a promising route to control their structural morphology and thus tune their physical properties, giving rise to many novel applications. In this paper, we employ finite element analysis (FEA), molecular dynamic (MD) simulations and continuum modeling to study the mechanical buckling of a column made of layered crystalline materials with the crystal layers parallel to the longitudinal axis. It is found that the mechanical buckling exhibits a gradual transition from a bending mode to a shear mode of instability with the reduction of slenderness ratio. As the slenderness ratio approaches to zero, the critical buckling strain {epsilon}cr converges to a finite value that is much smaller than the materials mechanical strength, indicating that it is realizable under appropriate experimental conditions. Such a mechanical buckling mode is anomalous and counter-intuitive. The critical buckling strain {epsilon}cr predicted by our continuum mechanics model agrees very well with the results from the FEA and MD simulations for a group of typical hexagonal layered crystalline materials. MD simulations on graphite indicate the continuum mechanics model is applicable down to a scale of 20 nm. This theoretical model also reveals that a high degree of elastic anisotropy is the origin for the anomalous mechanical buckling of a column made of layered crystalline materials in the absence of structural slenderness. This study provides avenues for engineering layered crystalline materials in various nano-materials and nano-devices via mechanical buckling.
Some highly ordered compounds of graphene oxide (GO), e.g., the so-called clamped and unzipped GO, are shown to have piezoelectric responses via first-principles density functional calculations. By applying an electric field perpendicular to the GO b
asal plane, the largest value of in-plane strain and strain piezoelectric coefficient, d31 are found to be 0.12% and 0.24 pm/V, respectively, which are comparable with those of some advanced piezoelectric materials. An in-depth molecular structural analysis reveals that deformation of the oxygen doping regions in the clamped GO dominates its overall strain output, whereas deformation of the regions without oxygen dopant in the unzipped GO determines its overall piezoelectric strain. This understanding explains the observed dependence of d31 on oxygen doping rate, i.e., higher oxygen concentration giving rise to a larger d31 in the clamped GO whereas leading to a reduced d31 in the unzipped GO. As the thinnest two-dimensional piezoelectric materials, GO has a great potential for a wide range of MEMS/NEMS actuators and sensors.
Reported values (0.2 MPa ~ 7.0 GPa) of the interlayer shear strength (ISS) of graphite are very dispersed. The main challenge to obtain a reliable value of ISS is the lack of precise experimental methods. Here we present a novel experimental approach
to measure the ISS, and obtain the value as 0.14 GPa. Our result can serve as an important basis for understanding mechanical behavior of graphite or graphene-based materials.
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 incom
mensurate 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.
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 int
erlayer binding energy of highly oriented pyrolytic graphite (HOPG). The obtained values of the binding energy are 0.27($pm $0.02)J/m$^{2}$, which can serve as a benchmark for other theoretical and experimental works.
