Size, Shape and Phase Stability of Diamond Nanostructures, and the Effect of Nitrogen Impurities
Dr. Amanda Barnard (University of Oxford, UK)

The transformation of nanodiamonds into carbon-onions (and vice versa) has been observed experimentally and has been modelled computationally at various levels of sophistication. Presented here are results of first principles calculations investigating the structural and phase stability of quasi-zero dimensional (0-D) and quasi-one dimensional (1-D) diamond nanostructures as a function of size and shape. In the case of 0-D structures, depending on the size and shape of the particles, the formation of intermediary structure are predicted. Neither pure carbon-onion nor a nanodiamond, these structures (known as “bucky-diamonds”) exhibit diamond-like cores encased in an onion-like shells, in agreement with experimental observations. Similarly, in the case of 1-D nanostructures, the formation of “bucky-wires” are predicted (depending upon the diamond nanowire morphology) with a diamond nanowire encased in a partial nanotubular shell.

However, when synthesized, most diamond nanostructures contain unintentional nitrogen impurities. Since the reliability of nanoscale devices using these materials will be largely dependent upon the position and stability of dopants and impurities within the particles, it is highly desirable to know whether nitrogen will be preferentially located within the core or at the surface; or possibly at edges or corners. The issue is further complicated by bucky-diamonds, which offer alternative substitution sites with naturally reduced coordination in the outer shells. To illustrate this, a systematic series of density functional tight binding (DFTB) simulations have been performed examining the configuration and potential energy surface (PES) of substitutional N in isolated nanodiamond and bucky-diamond, the shape of which offer combinations of {100}, {110} and {111} surfaces, {100}/{110}, {100}/{111}, {110}/{110} and {111}/{111} edges, and {100}/{110}/{110} and {100}/{111}/{111} corners. By comparing the PES for N substitution along “substitution paths” from the nanoparticle centre to each of these termini we can predict that N is unlikely to be positioned in the core these nanoparticles; although the particular (energetically preferred) position will be sensitive to the specific size, shape and degree of surface passivation.