Elsevier

Applied Surface Science

Volume 360, Part B, 1 January 2016, Pages 451-460
Applied Surface Science

STM observation of a box-shaped graphene nanostructure appeared after mechanical cleavage of pyrolytic graphite

https://doi.org/10.1016/j.apsusc.2015.09.222Get rights and content

Highlights

  • A previously unknown 3D box-shaped graphene (BSG) nanostructure has been detected.

  • The nanostructure is a multilayer system of parallel nanochannels having quadrangular cross-section.

  • Typical width of a nanochannel facet makes 25 nm, typical wall/facet thickness is 1 nm.

  • A mechanism qualitatively explaining the nanostructure formation has been proposed.

  • Possible applications of the BSG nanostructure are briefly discussed.

Abstract

A description is given of a three-dimensional box-shaped graphene (BSG) nanostructure formed/uncovered by mechanical cleavage of highly oriented pyrolytic graphite (HOPG). The discovered nanostructure is a multilayer system of parallel hollow channels located along the surface and having quadrangular cross-section. The thickness of the channel walls/facets is approximately equal to 1 nm. The typical width of channel facets makes about 25 nm, the channel length is 390 nm and more. The investigation of the found nanostructure by means of a scanning tunneling microscope (STM) allows us to draw a conclusion that it is possible to make spatial constructions of graphene similar to the discovered one by mechanical compression, bending, splitting, and shifting graphite surface layers. The distinctive features of such constructions are the following: simplicity of the preparation method, small contact area between graphene planes and a substrate, large surface area, nanometer cross-sectional sizes of the channels, large aspect ratio. Potential fields of application include: ultra-sensitive detectors, high-performance catalytic cells, nanochannels for DNA manipulation, nanomechanical resonators, electron multiplication channels, high-capacity sorbents for hydrogen storage.

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 1⿿2 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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