Controlled synthesis of nanographites and characterization of their electronic and magnetic properties

  • Aradhana Dissanayake

Western Sydney University thesis: Master's thesis

Abstract

Nanographites with conjugated open edges are a class of sp2 bonded nanocarbon system which is situated between aromatic molecules and bulk graphite. Their properties strongly depend on the topology of the sp2 carbon network. In particular, the non-bonding edge localized states near the Fermi level crucially affect the electronic states which govern the electronic and structural properties of the material. One fascinating feature is the edge magnetism that originates from the spin alignment of edge localized spins in contrast to the diamagnetic nature of bulk graphite; thus it is not surprising that unusual properties of nanographites strongly depend on the preparation conditions that alter the surface-to-bulk-volume ratio and the topology of the carbon lattice. High energy ball milling (HEBM) technique is an effective method of producing nanographites by structural disintegration of bulk graphite. This process not only reduces the crystallite sizes into nanoscale, but also crucially modifies the peripheral regions changing the sub-surface electronic structure en route to nanosize. Since the HEBM produces a large amount of nanographites with a wide range of properties, this technique can be easily scaled up to industrial production of nanographites by tuning the milling variables. This work details the development of a series of nanographites with controlled crystallite sizes and controlled defect concentrations by HEBM in order to improve the understanding of the impact of milling on the topology of sp2 carbon network. Nanographites were prepared by milling of macrocrystalline graphite in a stainless steel grinding container with zirconium oxide interior and zirconium oxide grinding balls in order to minimize the likelihood of ferromagnetic contaminations that could affect the properties of the nanographites. Milling was done in an argon atmosphere to avoid the oxidation and related modifications at the active edges/defects of the carbon lattice. Milling conditions such as; the milling duration (1, 3, 10 and 30 hrs), the grinding ball diameter (10 and 20 mm) and, the milling speed (200 and 400 rpm) were varied to produce, by design, various nanographite materials with progressively decreasing out-of-plane crystallite sizes, Lc, and in-plane crystallite sizes, La. The average out-of-plane crystallite sizes reduce from 51 to a minimum of 4 nm and in-plane crystallite size reduces from 60 to a minimum of 7 nm upon milling. The subsequent changes in structural, electronic and magnetic properties of the graphite en route to nanosize were studied. After the preliminary studies, the properties of ball milled graphites prepared at 400 rpm with 10 mm grinding balls were further characterized as they represent a series of nanographites with a wide range of morphologies, defect concentrations and crystallite sizes.
Date of Award2012
Original languageEnglish

Keywords

  • graphene
  • carbon
  • nanoscience
  • magnetic properties
  • nanotechnology
  • nanostructures
  • carbon composites

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