STM observation of a box-shaped graphene nanostructure appeared after mechanical cleavage of pyrolytic graphite
Graphical abstract
Introduction
From the moment of graphene discovery and until the present time, several methods of its preparation have been suggested [1], [2], [3], [4], [5]. Among the suggested methods, the method of mechanical exfoliation of graphene planes from highly oriented pyrolytic graphite (HOPG) [1], [5] deserves a special mention since mechanical exfoliation of graphene planes apparently underlies mechanism of formation of the spatial box-shaped graphene (BSG) nanostructure described in the present work.
A surface of HOPG having unusual appearance is presented in Fig. 1 [6]. The surface has been either formed or uncovered after mechanical cleavage. As a rule, plane atomically smooth areas with sizes from several hundreds of nanometers to several microns are produced after cleaving this sort of graphite [7]. In the case considered, the graphite surface represents a multilayer system of parallel hollow channels which plane facets/walls are apparently graphene sheets.
A periodical microstructure that appeared after mechanical cleavage of HOPG is described in work [8]. The microstructure is a system of parallel folds periodically repeating through approximately 100 μm. The width of a fold area makes about 2 μm. The microstructure consists of several graphite layers and reaches 12 μm in depth. The microstructure and the detected nanostructure have some similarities: both structures extend in one dimension, periodically repeat, their folds are formed across the cleaving front, they both have layer shifts and channels with quadrangular cross-section. The observed similarities may imply similarity of the processes of formation of those surface structures, i.e., scalability of the phenomenon when passing from micrometer to nanometer fold sizes.
The main objectives of the presented work are
- (1)
Demonstration of the existence of BSG nanostructure.
- (2)
Analysis of the sizes and morphology of elements of BSG nanostructure.
- (3)
Development of a possible mechanism (a qualitative model) of the BSG nanostructure formation.
- (4)
A brief estimation of the prospects of possible applications of BSG nanostructure (to prove the need of further research).
Theoretical analysis and computer modeling of the discovered nanostructure as well as attempts of its reproduction are planned to be implemented at the next stages of the research. Based on the study of the BSG nanostructure, possible areas of its application were defined: detectors, catalytic cells, nanochannels of fluidic devices, nanomechanical resonators, multiplication channels of electrons, hydrogen storage and some others.
The notions of a channel wall and a channel facet used below are close to each other. Wall, as a rule, refers to a flat surface common to two adjacent channels. Facets usually refer to outer flat surfaces of the upper channel layer.
Section snippets
Specimen and measurement method
HOPG (Research Institute of Graphite, Russia) with mosaic spread angle 0.8° (density 2.24 g/cm3, purity 99.999%) was used as a specimen. The specimen was as thin as 0.3 mm strip of 2 mmÿ4 mm. Electrical insulation adhesive tape (KLL, Taiwan) of polyvinylchloride 0.13 mm in thickness was used for cleavage. The images of the BSG nanostructure of 512ÿ512 points were obtained with the scanning tunneling microscope (STM) Solver P4 (NT-MDT Co., Russia) in the air at room temperature, in the constant
Experimental observations
The following experimental facts point out the small thickness of the walls/facets of the detected nanostructure. First, the direct measurement of the wall thickness (see the white arrows in Fig. 1(a)) of an open channel gives a size of order of 1 nm (an open channel is the one that has no top facets). Second, the direct measurement of the facet thickness (see the black arrows in the inset) also gives a size of order of 1 nm.
Third, during the raster scanning, the STM tip seems to cause plastic
Analysis of the observations
Since the nanostructure under consideration looks periodical, we may expect well noticeable maxima, corresponding to the observed periodicity, to be present at the two-dimensional Fourier spectrum of the nanostructure. Fig. 1(a) gives a good idea of the directions along which some possible periodicities could propagate. These directions are defined by α + 90° = 152.7° and β 90° = 53.8° angles. Along the first of the above directions, the channels themselves repeat periodically; along the second one
Formation mechanism
Below is a qualitative description of the probable formation mechanism of the detected BSG nanostructure. It is assumed that the box-shaped nanostructure arises as a result of a mechanical cleaving performed by an adhesive tape. Fig. 8 shows the HOPG cleaving method that possibly enables the formation of the searched for box-shaped nanostructure. At first glance, the method might seem just insignificantly different from the existing one. Nevertheless, there are several specific peculiarities,
Discussion
The detected nanostructure has been formed as a result of a number of inelastic deformations. At the moment of the nanostructure formation, the ultimate relative elongation apparently approached the maximum permissible level for graphene (13% for the armchair orientation; 20% for the zigzag orientation) [11], [29], [30] or even exceeded it at some locations (see the structure defects in Fig. 1 in the form of ruptured upper facets). Immediately as the box-shaped nanostructure is being
Conclusions
The key points of the research can be summarized as follows:
- (1)
A previously unknown 3D box-shaped graphene nanostructure has been detected on highly oriented pyrolytic graphite after mechanical cleavage.
- (2)
The discovered nanostructure is a multilayer system of parallel hollow nanochannels having quadrangular cross-section with typical width of a nanochannel facet 25 nm, typical wall/facet thickness 1 nm and length 390 nm and more.
- (3)
An original mechanism has been proposed that qualitatively explains the
Acknowledgments
This work was supported by the Russian Foundation for Basic Research (project 16-08-00036) and by the Ministry of Education and Science of the Russian Federation (contracts 14.429.11.0002, 14.578.21.0009). I thank O.E. Lyapin for critical reading of the manuscript and discussions; Dr. O.V. Sinitsyna for discussion on dislocation networks; Dr. A.L. Gudkov, Prof. E.A. Ilyichev and Assoc. Prof. E.A. Fetisov for their support and encouragement.
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