Research Papers

Formation of Carbon Nanoscrolls During Wedge-Based Mechanical Exfoliation of HOPG

[+] Author and Article Information
B. Jayasena

Nanyang Technological University,
School of Mechanical
and Aerospace Engineering,
Singapore 639798, Singapore
e-mail: Jaya0033@e.ntu.edu.sg

S. Subbiah

Nanyang Technological University,
School of Mechanical
and Aerospace Engineering,
Singapore 639798, Singapore
e-mail: sathyans@ntu.edu.sg

C. D. Reddy

Institute of High Performance Computing,
A*STAR, Singapore 138632, Singapore
e-mail: reddy@ihpc.a-star.edu.sg

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MICRO- AND NANO-MANUFACTURING. Manuscript received August 18, 2013; final manuscript received December 18, 2013; published online January 24, 2014. Assoc. Editor: Ashutosh Sharma.

J. Micro Nano-Manuf 2(1), 011003 (Jan 24, 2014) (7 pages) Paper No: JMNM-13-1063; doi: 10.1115/1.4026325 History: Received August 18, 2013; Revised December 18, 2013

Carbon nanoscrolls (CNS) of various forms are observed when highly ordered pyrolytic graphite (HOPG) is mechanically exfoliated using a wedge. We present two hypothesis of how such scrolls form. The first hypothesis is based on microscopy evidence of pre-existing folds in layer edges of the HOPG. The second hypothesis is based on the literature evidence that graphene sheets when subject to deformation can result in defects on the torn edges. The sample preparation process can induce such defects in the HOPG layers. We show using molecular simulations that the interaction of the moving wedge with certain fold geometries can trigger scroll formation, confirming the first hypothesis. To test the second hypothesis, we show using molecular simulations, that layers with edge defects, upon interacting with the moving wedge, can also form scrolls. In reality, both these factors could simultaneously cause scrolls to form. Opportunities exist in fine-tuning this wedge-based mechanical exfoliation process to synthesize CNS for use in potential applications.

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Grahic Jump Location
Fig. 1

Experimental set-up of wedge based mechanical exfoliation. (a) Schematic diagram showing the wedge positioned with respect to the HOPG specimen to induce cleaving. (b) Experimental set up where the HOPG and wedge are placed on high precision slides for positioning and movement to cause cleaving

Grahic Jump Location
Fig. 2

Experimentally observed nanoscroll formation in layers synthesized using wedge-based mechanical exfoliation. Different forms of scrolls are evident: sheets with scrolls on two edges (a); fully rolled up sheets (b), (f); stack of sheets rolled together with axial sliding of the roll creating a conical shape (c), (d), and (e).

Grahic Jump Location
Fig. 3

(a) Pre-existing folds hypothesis; view of the scotched tape peeled side-surface of the HOPG sample indicate presence of folds/curls or distorted sections before exfoliation; wedge interacts with such folds may wind newly exfoliated layers around the pre-existing folds, (b) dDefective edge hypothesis; the HOPG trimming process defective edges on a planar graphene sheet. This can trigger scrolls to form when energy is supplied by the moving wedge which also releasing layers.

Grahic Jump Location
Fig. 4

Molecular simulation results with various pre-existing initial folds. (a) Model has a semi-circular fold with no overlap. This results in no scroll formation. (b) Model has an almost complete circular fold but with no overlap. This results in only a folded layer with no scroll formation. (c) Model has an involute shape fold with strong overlap. This results in scroll formation. The scrolls are also seen to slide out axially forming a conical shape similar to that seen in experimental results.

Grahic Jump Location
Fig. 5

(a) Scrolling at a larger cleaving depth (8.8 Å) generates multi-layer scrolls and (b) sStarting with a larger initial fold (involute) radius results in equilibrating to a different folded shape with additional cleaved layers.

Grahic Jump Location
Fig. 6

Effect of wedge position on scroll formation; (a) Illustrating the wedge position at two different cleaving depths, (b) Scroll formation at a cleaving depth of 2 Å—wedge is seen to cleave and scroll up additional layers, and (c) Scroll formation at a cleaving depth of 2 Å above the HOPG top layer; the wedge cleaves only the initial folded layer and no additional layers are cleaved.

Grahic Jump Location
Fig. 7

Variation of potential energy and development of scroll formation after inducing 5—6 defects on the armchair edge of a free standing graphene sheet; a planar sheet rolls into an “S” shape after 75 ps, starts forming (overlapping) CNS after 100 ps and the fully wrapped CNS forms after 135 ps.

Grahic Jump Location
Fig. 8

Effect of 5–6 type of defects during the exfoliation process; (a) and (b) are simulation results with the same cleaving depth (2 Å) but different lateral starting positions of the wedge. (c) Shows simulations results at a higher cleaving depth of 8 Å.

Grahic Jump Location
Fig. 9

Comparison of cleaving force at cleaving depths of insertions at 2 Å in pre-existing fold with and without any defect structures; larger initial peak and lower steady state value indicate the effect of initial wedge engagement and the self-induced scrolling nature of the process. Defect induced structures display a less force at the wedge - material interface.

Grahic Jump Location
Fig. 10

Comparison of without (pre-defined) and with defect scrolling; (a) and (b) indicate the variation of potential energy against the displacement, initial section showed the stick slip type of variation and fitted linear curve in enlarge potion indicate a increasing and decreasing pattern.



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