Diamond formation by thermal activation of graphite

Comparison of diamond nanoparticles captured on the floating and grounded membranes in the hot filament chemical vapor deposition process

Negatively charged diamond nanoparticles are known to be generated in the gas phase of the hot filament chemical vapor deposition (HFCVD) process. However, the structures of these nanoparticles remain unknown. Also, the effect of charging on the stability of nanodiamond structures has not been studied experimentally. Here, by installing a capturing apparatus in an HFCVD reactor, we succeeded in capturing nanoparticles on the floating and grounded SiO, carbon, and graphene membranes of a copper transmission electron microscope grid during HFCVD. We examined the effect of charge on the crystal structure of nanodiamonds captured for 10 s under various conditions and identified four carbon allotropes, which are i-carbon, hexagonal diamond, n-diamond, and cubic diamond, by analyzing 150 d-spacings of ∼100 nanoparticles for each membrane. Nanoparticles captured on the floating membrane consisted mainly of cubic diamond and n-diamond, whereas those captured on the grounded membrane consisted mainly of i-carbon. Diamond particles deposited for 8 h on the floating silicon (Si) substrate exhibited an octahedron shape with well-developed facets, and a high-intensity 1332 cm-1 Raman peak, whereas diamond particles deposited on the grounded Si substrate showed a spherical shape partially covered with crystalline facets with a broad G-band Raman peak. These results indicate that charging stabilizes the diamond structure.

Stimulated Raman scattering of light in suspension of diamond microparticles in ethanol and in water

The spectra of stimulated Raman scattering of light in ethanol and in water suspensions containing diamond microparticles with sizes 0.2-0.3 μm were investigated. An excitation radiation source was a pulsed ruby laser with a generation wavelength λ₀ = 694.3 nm, a pulse duration τ_p ≈ 20 ns, a maximum beam energy of E_max = 0.6 J, a spectral width Δν = 0.015 cm^-1, and a beam divergence 3.5·10^-4 rad. For the first time, the observation of stimulated Raman scattering of light at a boson peak in suspension of diamonds microcrystals with close sizes (0.2-0.3 μm) in a liquid is reported. The corresponding spectra were recorded using a Fabry-Perot interferometer. In this case, the frequency shift of the stimulated Stokes Raman scattering depended on the size of the diamond microparticles introduced into the liquid and amounted to ~1 cm^-1. In addition, stimulated Raman scattering by a fundamental optical mode with a frequency shift ν = 1331 cm^-1 was observed. In this case, the Raman spectra were recorded using a small-sized spectrometer with a multi-element receiver, detecting radiation in the range of 200-1000 nm. At a sufficiently high intensity of the exciting radiation, the Stokes and anti-Stokes satellites were simultaneously present in the spectrum of stimulated Raman scattering. The obtained results on stimulated scattering of diamond microparticles in liquids are of interest for estimating the sizes of microcrystals from scattering spectra at a boson peak, as well as for creating a frequency comb of emitters based on stimulated Raman scattering with a large frequency shift.

Direct laser writing of nanodiamond films from graphite under ambient conditions

Synthesis of diamond, a multi-functional material, has been a challenge due to very high activation energy for transforming graphite to diamond, and therefore, has been hindering it from being potentially exploited for novel applications. In this study, we explore a new approach, namely confined pulse laser deposition (CPLD), in which nanosecond laser ablation of graphite within a confinement layer simultaneously activates plasma and effectively confine it to create a favorable condition for nanodiamond formation from graphite. It is noteworthy that due to the local high dense confined plasma created by transparent confinement layer, nanodiamond has been formed at laser intensity as low as 3.7 GW/cm(2), which corresponds to pressure of 4.4 GPa, much lower than the pressure needed to transform graphite to diamond traditionally. By manipulating the laser conditions, semi-transparent carbon films with good conductivity (several kΩ/Sq) were also obtained by this method. This technique provides a new channel, from confined plasma to solid, to deposit materials that normally need high temperature and high pressure. This technique has several important advantages to allow scalable processing, such as high speed, direct writing without catalyst, selective and flexible processing, low cost without expensive pico/femtosecond laser systems, high temperature/vacuum chambers.

Thermodynamics of Diamond Nucleation on the Nanoscale

To have a clear insight into the diamond nucleation upon the hydrothermal synthesis and the reduction of carbide (HSRC), we performed the thermodynamic approach on the nanoscale to elucidate the diamond nucleation taking place in HSRC supercritical-fluid systems taking into account the capillary effect of the nanosized curvature of the diamond critical nuclei, based on the carbon thermodynamic equilibrium phase diagram. These theoretical analyses showed that the nanosize-induced interior pressure of diamond nuclei could drive the metastable phase region of the diamond nucleation in HSRC into the new stable phase region of diamond in the carbon phase diagram. Accordingly, the diamond nucleation is preferable to the graphite phase formation in the competing growth between diamond and graphite upon HSRC. Meanwhile, we predicted that 400 MPa should be the threshold pressure for the diamond synthesis by HSRC in the metastable phase region of diamond, based on the proposed thermodynamic nucleation on the nanoscale.

Diamond formation by reduction of carbon dioxide at low temperatures

This Communication reports a low-temperature diamond synthesis technique, in which diamonds (10-250 mum) can form at a temperature as low as 440 degrees C by reduction of dense CO2 with metallic Na. The X-ray diffraction pattern of a powder sample shows three reflection peaks, indexed with 111, 220, and 311, corresponding unambiguously to cubic diamond. The Raman spectrum of the product exhibits an intense first-order peak at 1332 cm-1, which is the characteristic signature of the cubic diamond, indicating the formation of well-crystallized diamond. Carbon dioxide is a nontoxic low-energy molecule, abundant on earth. This novel reduction method could allow studies of large-size diamond growth using CO2 as the carbon source.

Novel pathway to the growth of diamond on cubic beta-SiC(001)

By carrying out first-principles calculations on diamond-forming processes, we predict a method for the heteroepitaxial growth of diamond on cubic beta-SiC(001). In the method, we used two processes: (i) the preformation of an sp(3)-like surface configuration of beta-SiC(001) by the adsorption of group-V surfactants; (ii) the successive growth of diamond by the segregation of the surfactants onto a surface and the desorption of surface hydrogen. Analyzing the segregation energies, we found that the atomic size effect plays a crucial role in the surfactant-mediated growth of diamond on beta-SiC(001).