Planer nano-graphenes from camphor by CVD

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Abstract

Next to conducting polymers and carbon nanotubes (curved, closed graphenes), the material that can bring revolution in electronics is planer graphene (PG)/planer few layer graphenes (PFLG). Synthesis of PG and PFLG by simple, economical and re-producible method was a challenge. We synthesized PFLG films from camphor pyrolysis on nickel substrates by simple, cost effective thermal CVD method and studied using HR-TEM, visible Raman spectroscopy, XRD and FE-SEM. This opens the possibility that the controlled and large area synthesis of PGs and PFLGs is possible by CVD based methods, for possible large area electronic applications.

Graphical abstract

Synthesis of planer nano-graphenes have been achieved from the pyrolysis of camphor by simple, re-producible thermal CVD method. This should open-up many more possibilities for planer graphene based electronic devices.

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Introduction

Graphene is a single layer of carbon atoms densely packed into a benzene ring structure. Graphene can be made by extracting individual plane (s) of carbon atoms from graphite crystals and are generally used to describe properties of many carbon nanomaterials, including graphite, large fullerenes and nanotubes. Carbon nanotubes can be thought of as graphene sheet (s) rolled up into nanometer sized cylinders. Many of the applications that are thought for carbon nanotubes, can be considered for graphenes. Planer graphene has been presumed not to exist in free-state, being unstable with respect to formation of curved structures such as soot, fullerenes and nanotubes. The first attempt to produce individual graphene sheet by exfoliation dates to the work done by Brodie [1] in 1859. Since then, different attempts were made for the synthesis of planer single layer graphenes and PFLGs [2], [3], [4], [5], with little success. To our knowledge, Novoselov et al. reported recently the most successful method for the synthesis of PFLG [6], [7]. They have made planer few layer graphene (PFLG) films and are observed to be metallic in nature. Ballistic charge transport, linear current–voltage (IV) characteristic and huge sustainable currents (>108 A/cm2) makes them an interesting candidate for applications in electronic devices. Graphene transistors show modest on-off resistance ratio (less than ∼30 at 300 K) which is sufficient for logic circuits. It is also possible to increase this ratio by using p–n junctions, local gates or point contact geometry. Further, the transistor fabricated using PFLG is the ‘first metallic transistor’ made in which the active material was a metallic graphene, i.e. the channel was made from a metallic material [6], [7]. The transistor application of graphene is very exciting, in analogy to carbon nanotubes. Planer graphenes have many unique properties [8], [9], [10], [11], [12], [13]. The application of single sheet graphenes in nanocomposites as a filler is demonstrated by Schniepp et al. [14]. Application of nano-graphenes for the storage of molecular hydrogen is also suggested [15], [16]. Single graphene sheet has been detected by Horiuchi et al. in a carbon nanofilm [17]. Jang et al. obtained US patent for a complex process for the fabrication of nano-scaled graphene plates [18]. Controlled, easy and low cost synthesis of graphene/PFLG is still a challenge and not much efforts have been made in this direction. The chemical approach developed by Prof. Mullen for synthesis of ‘graphene molecules’ is very interesting and important [19]. However, in the last 10 years, chemical vapor deposition (CVD) based methods are developed to synthesize curved closed graphenes (i.e. carbon nanotubes) with a good success, worldwide. It will be interesting and important if these methods can ultimately produce planer graphene/PFLGs – that can be used for fabricating electronic devices. Considering these facts, we are looking for the possibility of synthesis of planer graphenes/PFLGs by CVD methods. Via this communication, we disclose our important observation that, indeed, planer graphenes/PFLGs film can be synthesized by simple thermal CVD method. We obtained PFLG film on Ni substrates from pyrolysis of camphor by thermal CVD. Large amount of camphor yielded pyrolytic graphite (PG) films. Our experiments indicate that PFLGs can be directly synthesized by CVD method and that its controlled synthesis, patterning and manipulation should be possible in the near future. Further, the method has good prospects to be scaled up.

Section snippets

Experimental

A one-meter long quartz tube (diameter 50 mm) serves as a CVD reactor kept horizontally inside two horizontal furnaces. Camphor (0.1–0.5 gm) is evaporated in the first furnace (180 °C) and pyrolyzed in the second furnace (700–850 °C) with argon as carrier gas. In each experiment, 3–4 samples of Ni sheets (2 × 2 cm2, NILACO Corporation, Japan) are kept on the alumina boat in the centre of the second furnace. The substrates are used as received and were cleaned ultrasonically with acetone and methanol.

Results and discussions

Fig. 1 shows TEM image of PFLG film. In the top left corner of the image, a PFLG sheet is visible which is folded a bit at the places marked by arrows. In the region marked by two arrows, graphitic structure with interlayer spacing of about 0.34 nm is clearly visible, corresponding to graphite 0 0 0 2 spacing. Centre of the image shows yet another PFLG film edge. Fig. 2 shows the high resolution image of this central PFLG film edge. Inset shows the intensity pattern along the line marked in Fig. 2.

Conclusions

In conclusion, for the first time, we experimentally demonstrated that planer few layer nano-graphenes (PFLGs) can be synthesized by simple thermal CVD method. Method is simple, cost effective and has capability to scale-up. Further, such synthesis has been carried out from a natural, environmental friendly, low cost precursor – Camphor. Efforts are now directed towards understanding the growth mechanism of the PFLG film and to synthesize defect free films over a silicon wafer size area.

Acknowledgements

Financial support from JSPS is appreciated. We thank Referees of this manuscript for giving useful suggestions.

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