Size dependence of structural stability in nanocrystalline diamond

Abstract

We describe experiments which demonstrate that carbon atoms introduced into a fused-silica substrate by means of MeV ion implantation can, after suitable annealing, form nanocrystalline diamond. Unlike other methods of creating diamond, the coalescence of the carbon into diamond nanocrystals occurs when the samples are heated in a conventional furnace and does not require the application of high external pressures, or any pre-existing diamond template. Following a dose of 5ϫ10 16 C/cm 2 into fused quartz and after annealing in forming gas ͑4% hydrogen in argon͒, perfect cubic diamond crystallites of 5-7 nm diameter are formed. For higher doses, the same annealing treatments produce larger crystallites which are comprised of other varieties of solid carbon phases. We conclude that diamond is the stable form of carbon provided that the crystallite size is sufficiently small ͑less than 7 nm͒ and that the nanocrystallites are appropriately surface passivated.

Structural properties of highly conductive ultra-nanocrystalline diamond films grown by hot-filament CVD

“…Notably, while the core-loss EELS spectrum shows different feature for diamond (σ * -band near 289.5 eV) and the sp 2 -bonded carbon (π * -band near 284.5 eV), it is the plasmon-loss EELS spectrum, which can unambiguously differentiate the a-C (the noncrystalline sp 2 -bonded carbon) and the graphite phases (the crystalline sp 2 -bonded carbon) in sp 2 -bonded carbon. [19][20][21][22] In plasmon-loss EELS spectrum, a-C phase exhibits a peak near 22 eV (ω a -band), whereas the graphite phase shows a peak near 27 eV (ω g -band). [19][20][21][22] In contrast, the diamond phase exhibits a peak at 33 eV (ω d2 -band) corresponding to bulk plasmon with a shoulder at 23 eV (ω d1 -band) corresponding to surface plasmon and the ω d1 /ω d2 peak ratio is ∼1/ √ 2.…”

“…[19][20][21][22] In plasmon-loss EELS spectrum, a-C phase exhibits a peak near 22 eV (ω a -band), whereas the graphite phase shows a peak near 27 eV (ω g -band). [19][20][21][22] In contrast, the diamond phase exhibits a peak at 33 eV (ω d2 -band) corresponding to bulk plasmon with a shoulder at 23 eV (ω d1 -band) corresponding to surface plasmon and the ω d1 /ω d2 peak ratio is ∼1/ √ 2. [19][20][21][22] The core-loss EELS spectrum of highly conducting nanocrystalline diamond films ( figure 6(a)) exhibits an abrupt rise near ∼289.5 eV (σ * -band) and a deep valley near 302 eV with a smaller peak near ∼284.5 eV (π * -band).…”

“…[19][20][21][22] In contrast, the diamond phase exhibits a peak at 33 eV (ω d2 -band) corresponding to bulk plasmon with a shoulder at 23 eV (ω d1 -band) corresponding to surface plasmon and the ω d1 /ω d2 peak ratio is ∼1/ √ 2. [19][20][21][22] The core-loss EELS spectrum of highly conducting nanocrystalline diamond films ( figure 6(a)) exhibits an abrupt rise near ∼289.5 eV (σ * -band) and a deep valley near 302 eV with a smaller peak near ∼284.5 eV (π * -band). These bands indicate the presence of predominantly sp 3 -bonded carbon, the diamond, along with some proportion of sp 2 -bonded carbon.…”

Structural Stability of Clean, Passivated, and Partially Dehydrogenated Cuboid and Octahedral Nanodiamonds Up to 2 Nanometers in Size

“…(2) The size range from 1.5-2 nm up to 5-10 nm is the nanodiamond regime in which the diamond phase is thermodynamically the most stable form for pure carbon nanoparticles. Gamarnik 24 25 used a thermodynamic model to calculate the size-dependent threshold preference for stable pure carbon nanodiamond over nanographite as 10.2 nm at 25 C, 6.1 nm at 545 C, 4.8 nm at 800 C, and 4.3 nm at 1100 C. Jiang et al 26 modeled the phase transition thermodynamics between nanodiamond and nanographite including the effects of surface stress on the internal pressure of the nanoparticle, and found the transition size to nanodiamond decreases from ∼11 nm at 0 K to ∼4 nm at 1500 K; they note the experimental observation that 5 nm nanodiamonds are transformed into nanographite at 1073 K. (Carbon implantation experiments 27 also suggest that diamond is the stable form of carbon for crystallites <7 nm that are appropriately surface passivated.) Winter and Ree 28 29 used primarily empirical (PM3 and AM1 Hartree-Fock) models to predict that small nanodiamond clusters formed in the detonation of high explosives are more stable than graphite below approximately 33,000-70,000 atoms (depending upon the computational method used), corresponding to a particle size of 6-8 nm; in another study using other methods, Barnard et al 22 estimated 24,398 atoms (∼5.2 nm) as the upper limit.…”

Plasma-enhanced chemical vapor deposition of nanocrystalline diamond

“…Bursill and co-workers [73][74][75] carefully performed structural analysis on nanocrystalline diamond powder and studied the surface and bulk plasmon response derived from the low-loss spectra using highresolution TEM (HR-TEM)/PEELS and STEM/PEELS. Prawer and co-workers [56,76] also reported the EEL spectra of nanocrystalline diamond synthesized by detonation and ion implantation.…”