Formation of Amorphous Carbon and Graphite in CVD Diamond upon Annealing: A HREM, EELS, Raman and Optical Study

Abstract

Polycrystalline diamond thick films were subjected to annealing in vacuum at temperatures of 1350-1450 C. The films were examined by optical absorption, Raman spectroscopy, transmission electron microscopy and electron energy loss spectroscopy. The formation of amorphous carbon and/or of well-crystallized graphite layers up to 20 nm thick was evidenced along grain boundaries. Intra-granular nanometer-sized graphite islands were also observed, sometimes as transformed micro-twin bands. The diamond-to-graphite transition occurs in such a way that three (111) diamond planes transform into two (0002) graphitic sheets. The internal graphitization causes a severe degradation of the optical quality of the diamond films.

Optical spectroscopy of the surface of nanoporous diamond films

“…Preparation of porous samples requires selective removal of inclusions of graphite-like carbon, concentrated on the grain boundaries, by oxidation of the films. Etching in an H 2 SO 4 + K 2 Cr 2 O 7 solution led to selective but only partial removal of the graphite-like material both from the surface of the diamond/graphite-like nanorods [23] in the UNCD-25 sample and from the regions of non-diamond carbon, transformed as a result of high-temperature annealing [27], within the interior volume of UNCD-5. Annealing in air at a temperature of ~450 o C-500 o C and above also led to further selective etching of the non-diamond carbon in the UNC diamond films, a decrease in absorption in the range from the UV to the far IR, a decrease in electrical conductivity by several orders of magnitude [24], and changes in the Raman spectra.…”

“…The UNCD-5 sample initially was a continuous film, and our first step in forming an extended surface was to graphitize the grain boundaries by successive annealing under vacuum in a graphite furnace (pressure 10 -3 Pa, annealing time 1 h) at temperatures from 800 o C to 1510 o C, and our second step was to etch out the grain boundaries. Vacuum annealing of UNCD-5 increased the conductivity by eight orders of magnitude while simultaneously reducing the bandgap width from 0.6 eV to 0.2 eV [19], which suggests partial transformation of sp 3 phase to sp 2 phase at the intercrystallite boundaries within the interior volume of the film [27]. In order to remove the thin surface layer of graphite appearing on the diamond surface (both the film and the substrate), after high-temperature vacuum annealing the samples were etched in an H 2 SO 4 + K 2 Cr 2 O 7 solution at a temperature of ~200 o C for ~100 seconds, followed by boiling twice in deionized water to remove acid residues.…”

Microstructural features and properties of the nano-crystalline SiC/BN(C) composite ceramic prepared from the mechanically alloyed SiBCN powder

“…r⁄ at 289 eV. These are the features of carbon atoms bonded with other carbon atoms in the well sp 2 hybridized graphite rings [28]. It is generally accepted that in sp 2 hybridized graphite, every carbon atom holds r bond with three other ones in the ring, and possesses p bond with the counterpart in the adjacent atom layer.…”

Synthetic diamond electrodes: Photoelectrochemical behavior of vacuum-annealed undoped polycrystalline diamond films

“…3b) allows suggesting that the annealing of diamond films gives birth to two effects that act in opposite directions on the photoelectrochemical behavior of the samples. 2 Using the absorption coefficient for our films given in [18], we estimated the light penetration depth for a sample annealed at 1600°C as approximately 100 µm, which is less than the film thickness (~400 µm) but exceeds the average crystallite size (≤80 µm). The other one is the "graphitization" of the intercrystalline boundaries.…”