Photoemission study of onionlike carbons produced by annealing nanodiamonds

Abstract

Photoelectron spectroscopy has been used to study the products resulting from high temperature phase transformation of nanodiamonds (ND). Depending on the temperature of annealing various particles with a diamond core covered by nanometer sized fullerene-like shells, and onionlike carbon (OLC) were formed. Analysis of the $\mathrm{C}1s$ photoemission lines of the intermediates of ND transformation, prepared at temperatures of 1420 and 1600 K and then exposed to atmosphere, reveals the presence of oxygen-containing groups and both ${\mathit{sp}}^{\mathit{2}}$ and ${\mathit{sp}}^{\mathit{3}}$ carbon. The ${\mathit{sp}}^{\mathit{2}}$ component for these samples has binding energies of $284.70\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ (for the sample prepared at 1420 K) and $284.50\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ (for the sample prepared at 1600 K). A difference of $1.3\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ in the binding energy of the ${\mathit{sp}}^{\mathit{3}}$ and ${\mathit{sp}}^{\mathit{2}}$ components was observed. The ${\mathit{sp}}^{\mathit{2}}$ component for OLC prepared at 1800, 1900, and 2140 K has a binding energy of $284.45\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The shift towards higher binding energies of the ${\mathit{sp}}^{\mathit{2}}$ component of the samples prepared at lower temperatures is explained by significant curvature of graphite layers formed in the initial stages of graphitization. The observed increase in density of states at the Fermi level for the samples prepared at 1600, 1800, and 1900 K is associated with an accumulation of different types of defects in the curved graphite layers during graphitization of diamond. The Lorentzian widths of $\mathrm{C}1s$ photoemission lines from OLC are large compared with those of HOPG. The possible reasons for this broadening are discussed.