Review.2 – Properties of graphene produced by the high pressure-high temperature growth process

Micro & Nano Letters, 3, 1 (2008) 29-34

Authors: 
F. Parvizi, D. Teweldebrhan, S. Ghosh, I. Calizo, A. A. Balandin, H. Zhu, R. Abbaschian

After the successful preparation of graphene for the first time in 2004 at the University of Manchester, research on preparing and developing applications for graphene has taken off. This is due to the amazing properties that graphene exhibits, such as high thermal and electrical conductivity, incredible strength and lightness, and flexibility. Since 2004, various methods have been devised to prepare graphene and the journal article reviewed here represents just one of those methods.

Article Summary:  
A novel method for the synthesis of single-layer graphene is reported by the researchers, known as the high pressure-high temperature (HPHT) growth process. The graphene layers were grown in a split sphere growth apparatus, where the temperature and pressure were closely regulated in order to generate the desired single-layered graphene flakes. A small diamond seed was planted inside the vessel to initiate graphene growth, along with bulk graphite as reactant. A Fe-Ni molten catalyst was used, on a stabilized zirconia substrate infiltrated with CsCl. Experiments were performed with values in the range of  5 – 6 GPa and 1300 – 1700 °C. The prepared graphene was then characterized by micro-Raman spectroscopy, scanning electron microscopy (SEM) and the transient plane source (TPS) technique. The growth process produced graphitic layers and the single graphene layer was taken from the top of the graphitic layers. The Raman spectrum of bulk graphite was compared to the spectra for the produced single-layer graphene and graphitic layers. Through this comparison it was determined that the graphene had higher crystallinity and lower defect concentration than the graphitic layers. SEM was used to confirm that the graphene layers had uniform composition. Finally, the TPS technique was used to measure the thermal conductivity of the prepared graphitic layers. The cross-plane thermal conductivity at room temperature was reported to be 1.3 W/m•K, which is lower than previously reported, but may be due to the presence of leftover catalyst and other impurities.

Some interesting points to be taken from this journal article:

• The split sphere growth apparatus has been previously used for synthetic diamond growth, but has found another useful application here for the production of single-layer graphene
• Using different pressures and temperatures during the reactions may lead to the ability to tune the HPHT process to produce high-quality graphene layers
• Moving forward, the ability to prepare high-quality graphene in good yields will become the objective of many scientists and engineers working on graphene-related research. The applications for graphene will become virtually limitless if its synthesis becomes economically viable.

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