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The intriguing properties and nanotechnology applications of graphene and graphene analogs

(Nanowerk Spotlight) Although graphene properties and applications have already been well-discussed in the literature, it also is important to understand how 2D chemistry of graphene and graphene analogs is related to various applications.
Graphene functionalization – including metal decoration, metal substrates, intercalation, doping, and hybridization – modifies the unique 2D features of graphene. In this way, the electronic and physical properties of graphene can be controlled toward the given purpose such as highly effective novel electronic device applications.
Already, graphene functionalization such as adsorption, intercalation, and doping toward device applications has attracted great attention. A new review article in ACS Applied Materials & Interfaces ("Graphene and Graphene Analogs toward Optical, Electronic, Spintronic, Green-Chemical, Energy-Material, Sensing, and Medical Applications") addresses the following topics:
  • Graphene: Structural Properties of Defects, Edges, Nanoribbons, Metal Substrates, and Patterned Circuits;
  • Graphene-Based Applications: Photoluminescence, Electronics, Spintronics, Magnetics, Superconductors, and Protecting Materials;
  • Graphene-Based Green Chemistry and Energy Materials;
  • Graphene-Based Sensing and Medical Applications.
  • Various types of graphene defects including grain boundaries
    Various types of graphene defects including grain boundaries (GB) (typical width and height of each defect are described). Each GB can be differentiated by different populations of adatoms because their adsorption energies depend on the GB type. For example, as compared with the absolute binding energy 0.4 eV of a gold adatom on pristine graphene, gold adatoms on stitched GBs are 0.4-0.7 eV higher, those on the upper/lower edges of overlapped GBs are 0.3-0.6 eV/0.1 eV higher, and those on wrinkles are ∼0.1 eV higher because of the enhanced C sp3 character. (© American Chemical Society) (click on image to enlarge)
    Since the advent of graphene, two-dimensional (2D) chemistry has emerged with noncovalent adsorption on graphene, intercalation inside graphene bilayer to few-layers, and functionalization of graphene.
    The techniques developed for synthesizing graphene can be grouped into several methods including mechanical cleavage, epitaxial growth, chemical vapor deposition (CVD), and organic synthesis methods.
    As compared with the 3D bulk system, the boundaries of the bulk systems or the interface between two different types of bulk systems are often related to 2D systems.
    Two-dimensional materials show the characteristic density of states (DOS) which is nearly constant to energy variation due to the confinement effect, in contrast to those of 3D materials which are nearly proportional to the 3/2th power of the energy variation.
    Since these characteristic DOSs can lead to new types of materials with exotic electronic properties, the physical and chemical behaviors are quite different from the bulk properties.
    In this regard, the isolation of 2D materials has been an intriguing topic. However, it has been known that the conventional long-range positional order is absent in 2D systems (Physical Review Letters, "Absence of Ferromagnetism or Antiferromagnetism in One- or Two-Dimensional Isotropic Heisenberg Models"). Nevertheless, it was suggested that long-range orientation order may exist.
    Thus, 2D solids have been characterized by quasi-long-range positional order and true long-range orientation order, so as to show almost all the properties of crystals.
    The Kosterlitz-Thouless transition, which might be responsible for order-disorder transitions in 2D systems, must be viewed as an upper bound for 2D quasilong-range positional order, and so the grain boundary formation and vacancy condensation mechanisms have appeared. In this regard, GB and dislocation have been critical issues in the preparation of practical 2D materials.
    Nowadays, thanks to the usefulness of graphene with unusual material features, graphene functionalization such as adsorption, intercalation, and doping toward device applications has attracted great attention.
    raman graphene 2D band showing a stark contrast between two regions of graphene (lighter areas)/h-BN (darker areas) on a Si/SiO2 substrate
    (a) Raman graphene 2D band showing a stark contrast between two regions of graphene (lighter areas)/h-BN (darker areas) on a Si/SiO2 substrate. (b) Illustration of growth mechanism for graphene/amorphous-carbon (a-C) heterostructures. (c) Raman maps of graphene/a-C line pattern based on the Raman frequency of ωG. (© Nature Publishing Group; © American Chemical Society) (click on image to enlarge)
    Since graphene properties and applications have already been well-discussed in the literature, wthe authors of this review rather focus on how conceptual understanding of 2D chemistry of graphene and graphene analogs is related to various applications.
    In the final section of the review, the authors discuss outlook and future challenges for the field.
    "All the exotic characteristics of graphene, patterned graphene, and graphene analogs will be further utilized in diverse future high-tech research fields including information-technology/biotechnology fused science and technology as well as industrial applications and medical applications including brain information communications," they conclude.
    By Michael is author of two books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology and Nanotechnology: The Future is Tiny. Copyright © Nanowerk
     

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