Novel Phases of Elemental Boron

Research on the chemistry of elemental boron is developing rapidly. The existence of quasi planar and tubular boron clusters was predicted by theory in the 1990ties and confirmed by experiment recently.  Up to now, however, very little is known about the properties of these novel boron phases.
To deepen the understanding of these materials Jens Kunstmann (in cooperation with A. Quandt, University of Greifswald) studied boron sheets and boron nanotubes using ab initio methods [1,2].

Boron Sheetsa broad boron sheet

A broad boron sheet (see figure to the right) is the analogue of a single graphite sheet and the precursor of boron nanotubes. The sheet has linear chains of sp hybridized sigma-bonds (yellow contours) lying only along its so called armchair direction, a high stiffness, and anisotropic bonds properties. The puckering of the sheet was explained as a mechanism to stabilize the sp sigma-bonds. The anisotropic bond properties of the boron sheet lead to a two-dimensional reference lattice structure, which is rectangular rather than triangular. As a consequence the chiral angles of related boron nanotubes range from 0° to 90° (in contrast to 0° to 30° for carbon nanotubes) [1].

Boron Nanotubesstrain energyboron nanotube

The morphologies of the boron sheet and boron nanotubes are strongly related, as shown in the figure to the right (which on the left shows an armchair boron nanotube). Therefore one can consider the sheet, described above, as the precursor of boron nanotubes. Given the electronic properties of the boron sheet, we could demonstrate that all of the related boron nanotubes are metallic, irrespective of their radius and chiral angle, and we also postulated the existence of helical currents in ideal chiral nanotubes. Furthermore, we showed that the strain energy of boron nanotubes will depend on their radii, as well as on their chiral angles (see right part of the figure). This is a rather unique property among nanotubular systems, and it could be the basis of a different type of structure control within nanotechnology. Zigzag boron nanotubes seem to have little or even no strain energy, which could imply that these types of boron nanotubes do not exist at all [1]. This interpretation also explains the morphology of so called constricted boron nanotubes, that were reported by us earlier [2].

  1. J. Kunstmann and A. Quandt
    Broad boron sheets and boron nanotubes: An ab initio study of structural, electronic, and mechanical properties

    Physical Review B 74, 035413 (2006), (arXiv)
  2. J. Kunstmann and A. Quandt
    Constricted boron nanotubes
    Chemical Physics Letters 402, 21 (2005) , (arXiv)