Microwave plasma enchanced synthesis of nanocrystalline diamond films

CVD diamond films are usually grown using gas mixtures of 1% hydrocarbon (eg CH4) and 99% H2. Diamond films using C{sub 60}/Ar and CH4/Ar discharges are nanocrystalline. Fragmentation of C{sub 60} in Ar discharge occurs via the C{sub 2} pathway. Quantum chemical calculations support the idea that the chemically highly energetic C{sub 2} molecule with adsorption energy=8 eV can grow diamond without intervention of atomic hydrogen, while hydrocarbon precursors with adsorption energies={similar_to}1 eV require the intervention of atomic hydrogen. The very high nucleation rates are responsible for the nanostructure (crystallite size 15 {mu}m) grown from Ar discharges. Diamond nucleation occurs in preference to graphite nucleation as a result of the thermodynamic stability of nanocrystalline diamond

Spatially resolved electron energy loss spectroscopy on n-type ultrananocrystalline diamond films

The addition of nitrogen to the synthesis gas during synthesis of ultrananocrystalline-diamond (UNCD) films results in films uniquely exhibiting very high n-type electrical conductivity even at ambient temperatures. This result is due to the formation of nanowires having elongated diamond core nanostructures and a sp2-bonded C sheath surrounding the core. The work presented here provides detailed confirmation of this important result through spatially resolved-electron energy loss spectroscopy. The direct observation of nitrogen incorporated in the sheath has been enabled. The incorporation of this nitrogen provides strong support to a plausible mechanism for the n-type conduction characteristic of the UNCD films

Multiwavelength Raman spectroscopy of diamond nanowires present in n-type ultrananocrystalline films

Multiwavelength Raman spectroscopy is employed to investigate ultrananocrystalline diamond films deposited by the plasma enhanced chemical vapor deposition technique. Recently, we have shown that the addition of nitrogen in the gas source during synthesis induce the formation of diamond n-type films, exhibiting the highest electrical conductivity at ambient temperature. This point is related with the formation of elongated diamond nanostructures and the presence of sp2-bonded carbon in these films. The Raman results presented here confirm these aspects and provide a better and deeper understanding of the nature of these films and their related optical and electronic properties

Growth of (110) Diamond using pure Dicarbon

We use a density-functional based tight-binding method to study diamond growth steps by depositing dicarbon species onto a hydrogen-free diamond (110) surface. Subsequent C_2 molecules are deposited on an initially clean surface, in the vicinity of a growing adsorbate cluster, and finally, near vacancies just before completion of a full new monolayer. The preferred growth stages arise from C_2n clusters in near ideal lattice positions forming zigzag chains running along the [-110] direction parallel to the surface. The adsorption energies are consistently exothermic by 8--10 eV per C_2, depending on the size of the cluster. The deposition barriers for these processes are in the range of 0.0--0.6 eV. For deposition sites above C_2n clusters the adsorption energies are smaller by 3 eV, but diffusion to more stable positions is feasible. We also perform simulations of the diffusion of C_2 molecules on the surface in the vicinity of existing adsorbate clusters using an augmented Lagrangian penalty method. We find migration barriers in excess of 3 eV on the clean surface, and 0.6--1.0 eV on top of graphene-like adsorbates. The barrier heights and pathways indicate that the growth from gaseous dicarbons proceeds either by direct adsorption onto clean sites or after migration on top of the existing C_2n chains.Comment: 8 Pages, 7 figure

Theoretical evaluation of matrix effects on trapped atomic levels

We suggest a theoretical model for calculating the matrix perturbation on the spectra of atoms trapped in rare gas systems. The model requires the ''potential curves'' of the diatomic system consisting of the trapped atom interacting with one from the matrix and relies on the approximation that the total matrix perturbation is a scalar sum of the pairwise interactions with each of the lattice sites. Calculations are presented for the prototype systems Na in Ar. Attempts are made to obtain ab initio estimates of the Jahn-Teller effects for excited states. Comparison is made with our recent Matrix-Isolation Spectroscopic (MIS) data. 10 refs., 3 tabs

Thermoelectric power factors of nanocarbon ensembles as a function of temperature

Thermoelectric power factors of nanocarbon ensembles have been determined as a function of temperature from 400 to 1200 K. The ensembles, composed of mixtures of nanographite or disperse ultrananocrystalline diamond with B 4 C B4C , are formed into mechanically rigid compacts by reaction at 1200 K with methane gas and subsequently annealed in an argon atmosphere at temperatures up to 2500 K. The ensembles were characterized using scanning electron microscopy, Raman, x-ray diffraction, and high resolution transmission electron microscopy techniques and found to undergo profound nanostructural changes as a function of temperature while largely preserving their nanometer sizes. The power factors increase strongly both as a function of annealing temperature and of the temperature at which the measurements are carried out reaching 1 µW/K 2 ¿cm 1 µW/K2¿cm at 1200 K without showing evidence of a plateau. Density functional “molecular analog” calculations on systems based on stacked graphene sheets show that boron substitutional doping results in a lowering of the Fermi level and the creation of a large number of hole states within thermal energies of the Fermi level [P. C. Redfern, D. M. Greun, and L. A. Curtiss, Chem. Phys. Lett. 471, 264 (2009)]. We propose that enhancement of electronic configurational entropy due to the large number of boron configurations in the graphite lattice contributes to the observed thermoelectric properties of the ensembles

Diamond nanowires and the insulator-metal transition in ultrananocrystalline diamond films

Further progress in the development of the remarkable electrochemical, electron field emission, high-temperature diode, and optical properties of n-type ultrananocrystalline diamond films requires a better understanding of electron transport in this material. Of particular interest is the origin of the transition to the metallic regime observed when about 10% by volume of nitrogen has been added to the synthesis gas. Here, we present data showing that the transition to the metallic state is due to the formation of partially oriented diamond nanowires surrounded by an sp2-bonded carbon sheath. These have been characterized by scanning electron microscopy, transmission electron microscopy techniques (high-resolution mode, selected area electron diffraction, and electron-energy-loss spectroscopy), Raman spectroscopy, and small-angle neutron scattering. The nanowires are 80–100nm in length and consist of ~5nm wide and 6–10nm long segments of diamond crystallites exhibiting atomically sharp interfaces. Each nanowire is enveloped in a sheath of sp2-bonded carbon that provides the conductive path for electrons. Raman spectroscopy on the films coupled with a consideration of plasma chemical and physical processes reveals that the sheath is likely composed of a nanocarbon material resembling in some respects a polymer-like mixture of polyacetylene and polynitrile. The complex interactions governing the simultaneous growth of the diamond core and the sp2 sheath responsible for electrical conductivity are discussed as are attempts at a better theoretical understanding of the transport mechanism

Growing carbon nanotubes by microwave plasma-enhanced chemical vapor deposition

A processing route has been developed to grow bundles of carbon nanotubes on substrates from methane and hydrogen mixtures by microwave plasma-enhanced chemical vapor deposition, catalyzed by iron particles reduced from ferric nitrate. Growth takes place at about 900°C leading to nanotubes with lengths of more than 20 μm and diameters on the nanometer scale

First-principles study of the effect of charge on the stability of a diamond nanocluster surface

Effects of net charge on the stability of the diamond nanocluster are investigated using the first-principles pseudopotential method with the local density approximation. We find that the charged nanocluster favors the diamond phase over the reconstruction into a fullerene-like structure. Occupying the dangling bond orbitals in the outermost surface, the excess charge can stabilize the bare diamond surface and destabilize the C-H bond on the hydrogenated surface. In combination with recent experimental results, our calculations suggest that negative charging could promote the nucleation and further growth of low-pressure diamond.open8

Control of diamond film microstructure by Ar additions to CH4/H2 microwave plasmas

The transition from microcrystalline to nanocrystalline diamond films grown from Ar/H2/CH4 microwave plasmas has been investigated. Both the cross-section and plan-view micrographs of scanning electron microscopy reveal that the surface morphology, the grain size, and the growth mechanism of the diamond films depend strongly on the ratio of Ar to H2 in the reactant gases. Microcrystalline grain size and columnar growth have been observed from films produced from Ar/H2/CH4 microwave discharges with low concentrations of Ar in the reactant gases. By contrast, the films grown from Ar/H2/CH4 microwave plasmas with a high concentration of Ar in the reactant gases consist of phase pure nanocrystalline diamond, which has been characterized by transmission electron microscopy, selected area electron diffraction, and electron energy loss spectroscopy. X-ray diffraction and Raman spectroscopy reveal that the width of the diffraction peaks and the Raman bands of the as-grown films depends on the ratio of Ar to H2 in the plasmas and are attributed to the transition from micron to nanometer size crystallites. It has been demonstrated that the microstructure of diamond films deposited from Ar/H2/CH4 plasmas can be controlled by varying the ratio of Ar to H2 in the reactant gas. The transition becomes pronounced at an Ar/H2 volume ratio of 4, and the microcrystalline diamond films are totally transformed to nanocrystalline diamond at an Ar/H2 volume ratio of 9. The transition in microstructure is presumably due to a change in growth mechanism from CH3· in high hydrogen content to C2 as a growth species in low hydrogen content plasmas