Role of acetylenic bonds in the mechanical,electronic and optical properties of yne-diamonds

Yne-diamonds are novel carbon allotropes designed by inserting acetylenic bonds into the framework of diamond. Varying the ratio of acetylenic bonds yields a new family of carbon allotropes consisting of sp- and sp³-hybridized atoms. The study of these novel carbon frameworks is becoming a topic of increasing interest. Here, we report our systematic studies on the stability, mechanical, electronic and optical properties of yne-diamonds. Our calculations indicate that yne-diamonds are mechanically stable, although they are energetically less favorable than diamond due to sp-hybridized carbon atoms in the frameworks. In contrast to early estimation, yne-diamonds are unlikely superhard materials, but exhibit good ductility. The band gaps of yne-diamonds vary from 0.165 eV to 4.850 eV, as the ratio of acetylenic bonds increases. The energetically most favorable yne-diamond has a direct band gap of 2.916 eV. The optical properties of these carbon allotropes are also discussed.     

Carbon isotope fractionation associated with HPHT crystallization of diamond

We have studied the isotope properties of carbon phases including carbon source, diamond, carbon dissolved in metal melt and the gas phase formed at diamond crystallization experiment in the closed system Fe–Ni–C at 1450 °С and 5.5 GPa during 17.5 h. The δ¹³C value of the grown diamond decreases in direction of growth from − 26.5 to − 27.1‰, when the δ¹³C value of initial carbon is − 27.1‰. Carbon isotope composition of the melt is by 3.2‰ lower than that of coexisting diamond. The data obtained agree well with the model calculations of isotope fractionation on crystallization from solution in a closed system.     

Layer-by-Layer assembly and redox properties of undoped HPHT diamond particles

The surface properties of undoped diamond particles are investigated by a combination of zeta potential measurements in solution and electrochemical studies in thin layer assemblies. High-Pressure High-Temperature (HPHT) 500 nm diamond particles exhibit positive and negative zeta potentials depending on pH. The estimated point of zero zeta potential (pzzp) was 6.6, while mobility measurements provided an average charge per particle of ?(843 ± 31)e at high pH. The charge indicates that approximately 50 ppm of surface atoms involves ionisable impurities. The positive charge measured at low pH is of similar magnitude and could be related to nitrogen impurities. The surface charge in basic solutions allows the electrostatic adsorption of diamond particles on poly(diallyldimethylammonium chloride) (PDADMAC) modified In-doped SnO₂ electrodes (ITO). The particle number density shows a strong dependence on pH, with a maximum value of (1.7 ± 0.3) × 10⁸ cm^?2. Electrochemical studies carried out in the absence of redox species in solution revealed signals associated with sp² type surface states. Analysis of electrochemical responses concluded that 1 × 10⁴ redox centres per particle are involved in a single electron transfer process. We demonstrate that this simple yet versatile approach is rather sensitive to the extent of sp² hybridisation at the surface of diamond powders.     

Formation of bismuth-core-carbon-shell nanoparticles by bismuth immersion during plasma CVD synthesis of thin diamond films

This paper describes an attempt to synthesize bismuth-doped diamond by plasma CVD. Solid bismuth was inserted into the reaction plasma of CH₄ and H₂. Examination by TEM showed that most of the bismuth was included as Bi nanoparticles in the carbon nanospheres, which segregated at the grain boundary of the diamond polycrystals. Diffraction peaks corresponding to the carbon allotrope Chaoite were observed at the grain boundaries. The Raman spectra showed very complex features between 100–1600 cm^− 1, suggesting the existance of molecule-like species.     

Thermal stability and surface modifications of detonation diamond nanoparticles studied with X-ray photoelectron spectroscopy

Detonation nanodiamond (ND) particles were dispersed on silicon nitride (SiN_x) coated sc-Si substrates by spin-coating technique. Their surface density was in the 10¹⁰–10¹¹ cm^?2 range. Thermal stability and surface modifications of ND particles were studied by combined use of X-ray Photoelectron Spectroscopy (XPS) and Field Emission Gun Scanning Electron Microscopy (FEG SEM). Different oxygen-containing functional groups could be identified by XPS and their evolution versus UHV annealing temperature (400–1085 °C) could be monitored in situ. The increase of annealing temperature led to a decrease of oxygen bound to carbon. In particular, functional groups where carbon was bound to oxygen via one σ bond (C–OH, C–O–C) started decomposing first. At 970 °C carbon–oxygen components decreased further. However, the sp²/sp³ carbon ratio did not increase, thus confirming that the graphitization of ND requires higher temperatures. XPS analyses also revealed that no interaction of ND particles with the silicon nitride substrate occurred at temperatures up to about 1000 °C. However, at 1050 °C silicon nitride coated substrates started showing patch-like damaged areas attributable to interaction of silicon nitride with the underlying substrate. Nevertheless ND particles were preserved in undamaged areas, with surface densities exceeding 10¹⁰ cm^?2. These nanoparticles acted as sp³-carbon seeds in a subsequent 15 min Chemical Vapour Deposition run that allowed growing a 60–80 nm diamond film. Our previous study on Si(100) showed that detonation ND particles reacted with silicon between 800 and 900 °C and, as a consequence, no diamond film could be grown after Chemical Vapour Deposition (CVD). These findings demonstrated that the use of a thin silicon nitride buffer layer is preferable insofar as the growth of thin diamond films on silicon devices via nanoseeding is concerned.     

Annealing-induced structural changes of carbon onions: High-resolution transmission electron microscopy and Raman studies

The first- and second-order Raman spectra of carbon nano-onions (CNOs), produced via annealing of detonation nanodiamonds with a mean grain size of ∼5 nm in the argon ambience at the maximal temperature of annealing process (T_MAX) varying from 1500 to 2150 °C, are analyzed together with the high-resolution transmission electron microscopy (HRTEM) images. The combined analysis provides a deep insight into the annealing-induced atomic-scale structural modifications of the CNO nanoparticles. The Raman and HRTEM data unambiguously demonstrate the reduction in the number of defects in the CNO structure, as well as indicate the conversion from the diamond sp³-bonded carbon phase to the sp²-bonded carbon phase with increasing T_MAX and its almost full completion for T_MAX = 1600 °C.     

Exciton and piezoelectricity in insulating 2-dimensional boron carbide

Using first-principles density functional theory, we predict a hexagonal structure of boron carbide with two shells, which consists of the sp² hybridized boron and carbon in (001) plane and the pz–pz (σ) bonding carbon along [001] direction. The calculated results show that the structure is thermodynamically stable and possesses lower formation energy than other candidates. In addition, the quasiparticle calculations within the GW approximation reveal that the boron carbide, which is a two dimensional insulator, exhibits the indirect band gap of 2.4 eV and large exciton bonding energy of 1.35 eV. In optical absorption spectra, a bright Frenkel class bound exciton has been discovered at about 2.98 eV, which is desirable for light emitting applications. Besides, the piezoelectric coefficient (e₂₂) of −2.38×10⁻¹⁰ Cm⁻¹ is predicted for monolayer boron carbide, which indicates that the monolayer boron carbide is a potential candidate for piezoelectric applications in the nanoelectromechanical systems.     

Optical properties of impact diamonds from the Popigai astrobleme

Impact diamonds from Popigai astrobleme were found to consist of different carbon phases: cubic and hexagonal diamond with sp³ bonding according to X-ray structural analysis as well as amorphous, crystalline and disordered graphite with sp²-bonding (Raman scattering). The sizes of graphite domains vary from 10 to 100 nm. Fundamental absorption edge for Popigai impact diamonds is shifted ~ 0.5 eV to lower energies in comparison with kimberlite diamonds (5.47 eV) as a result of the lonsdaleite input, in good agreement with ab initio calculations (E_g = 5.34 and 4.55 eV for 3C cubic and 2H hexagonal diamonds, respectively). Yellowish color of impact diamonds is due to Rayleigh light scattering on structural defects whereas graphite is responsible for gray to black coloring. In the mid-IR region there is a multi-phonon absorption of 3C diamond in the 1800 to 2800 cm^− 1 range and some new bands at 969, 1102, 1225, and 1330 cm^− 1 in the one-phonon region. Micro-Raman study shows inclusions of side noncarbon minerals (quartz, magnetite, and hematite) some of which contain Cr^3 + impurity. The vibration modes of cubic diamond and lonsdaleite exhibited in the Raman spectra were elucidated by the first-principles studies. Popigai impact diamonds demonstrate a broad-band luminescence in 2.1, 2.38, and 2.84 eV components similar to that for nanocrystal polycrystalline 3C diamond. All emissions are excited at band-to-band transitions whereas the last two are observed also at excitation into 2.4 and 3.0 bands supposedly as a result of intracenter processes within the H3(NVN) and NV⁰ centers.     

MD simulation of phase transformations in liquid carbon

The supercooled liquid of carbon is investigated by means of molecular-dynamics simulation. The dynamics of a glass and a supercooled liquid is compared and the glass transition temperature is determined by two methods: analyzing (i) the temperature dependence of thermodynamic coefficients and (ii) relaxation time of liquid. The pressure dependences of the glass transition temperature and the diamond melting temperature are found. The percolation properties of structures of sp³ atoms formed in liquid carbon with different numbers of embedded diamond crystallites are investigated. It is shown that the percolation cluster of 4-fold coordinated atoms forms when their total concentration in structure reaches a value close to 0.38 irrespective of the number of embedded crystallites. It turns out that the stability of diamond crystallites embedded into supercooled carbon liquid correlates with the presence of the percolation cluster of 4-fold coordinated atoms. The correspondence of diamond crystallite stability with percolation disappears at a temperature more than 5000 K. The topological criterion for the definition of tetrahedral amorphous carbon is proposed: amorphous carbon is tetrahedral if a percolation cluster exists in it and the embedded diamond crystallites are stable.     

Sp3/sp2 character of the carbon and hydrogen configuration in micro- and nanocrystalline diamond

Hot filament and microwave plasma CVD micro- nanocrystalline diamond films are analysed by visible and ultra-violet excitation source Raman spectroscopy. The sample grain size varies from 20 nm to 2 μm. The hydrogen concentration in samples is measured by SIMS and compared to the grain size, and to the ratio of sp² carbon bonds determined by Raman spectroscopy from the 1332 cm^− 1 diamond peak and the sp² 1550 cm^− 1 G band. Hydrogen concentration appears to be proportional to the sp² bonds ratio. The 3000 cm^− 1 CH_x stretching mode band intensity observed on the Raman spectra is decreasing with the G band intensity. Thermal annealing modifies the sp² phase structure and concentration, as hydrogen outdiffuses.     

Hydrogen incorporation,bonding and stability in nanocrystalline diamond films

The hydrogen concentration in hot filament and microwave plasma CVD nanocrystalline diamond films is analysed by secondary ion mass spectrometry and compared to the film grain size. The surface and bulk film carbon bonds are analysed respectively by X-ray photoelectron spectroscopy (XPS) and ultra-violet Raman spectroscopy. XPS results show the presence of the hydrogenated p-type surface conductive layer. The respective intensities of the 1332 cm^− 1 diamond peak, of the G and D bands related to sp² phases, and of the 3000 cm^− 1 CH_x stretching mode band, are compared on Raman spectra. The samples are submitted to thermal annealing under ultra-high vacuum in order to get hydrogen out-diffusion. XPS analysis shows the surface desorption of hydrogen. Thermal annealing modifies the sp² phase structure as hydrogen out diffuses.     

X-ray photoemission spectroscopy of nitrogen-doped UNCD/a-C:H films prepared by pulsed laser deposition

Nitrogen-doped ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) films were deposited by pulsed laser deposition (PLD). Nitrogen contents in the films were controlled by varying a ratio in the inflow amount between nitrogen and hydrogen gases. The film doped with a nitrogen content of 7.9 at.% possessed n-type conduction with an electrical conductivity of 18 Ω^? 1 cm^? 1 at 300 K. X-ray photoemission spectra, which were measured using synchrotron radiation, were decomposed into four component spectra due to sp², sp³ hybridized carbons, C=N and C–N. A full-width at half-maximum of the sp³ peak was 0.91 eV. This small value is specific to UNCD/a-C:H films. The sp²/(sp³ + sp²) value was enhanced from 32 to 40% with an increase in the nitrogen content from 0 to 7.9 at.%. This increment probably originates from the nitrogen incorporation into an a-C:H matrix and grain boundaries of UNCD crystallites. Since an electrical conductivity of a-C:H does not dramatically enhance for this doping amount according to previous reports, we believe that the electrical conductivity enhancement is predominantly due to the nitrogen incorporation into grain boundaries.     

Piezoresistive effect of individual electrospun carbon nanofibers for strain sensing

Piezoresistive behavior of individual electrospun carbon nanofibers (CNF) was studied for the first time via a microelectromechanical systems platform. The gage factor of CNFs was found to vary from 1.96 to 2.55, not correlating with nanofiber diameter. The measured strain sensitivity of electrical resistance of individual CNFs could not be solely explained based on strain induced dimensional changes of CNFs, pointing to piezoresistivity in nanofibers. The microstructure of CNFs was studied via TEM imaging and Raman spectroscopy, suggesting the presence of sp² and sp³ hybridized carbon atoms in CNFs. The piezoresistivity of CNFs was explained in light of their hybrid structure. A one-dimensional model was adopted to relate CNFs piezoresistivity to their microstructure and electron tunneling between sp² hybridized regions through sp³ hybridized regions. The calibrated model revealed tunneling distances of 0.15–0.3 nm between sp² hybridized atoms. Moreover, our study pointed to the degree of graphitization and elastic mismatch between differently hybridized carbon atom regions in CNFs as critical parameters controlling CNFs’ piezoresistivity. This study sets the stage for the utilization of CNFs, not just as load bearing elements, but also as multifunctional nanoscale components with strain sensing capabilities, for instance in Nanoelectro-mechanical systems applications.     

Iron-mediated growth of epitaxial graphene on SiC and diamond

Ordered graphene films have been fabricated on Fe-treated SiC and diamond surfaces using the catalytic conversion of sp³ to sp² carbon. In comparison with the bare SiC (0 0 0 1) surface, the graphitization temperature is reduced from over 1000 °C to 600 °C and for diamond (1 1 1), this new approach enables epitaxial graphene to be grown on this surface for the first time. For both substrates, a key development is the in situ monitoring of the entire fabrication process using real-time electron spectroscopy that provides the necessary precision for the production of films of controlled thickness. The quality of the graphene/graphite layers has been verified using angle-resolved photoelectron spectroscopy, scanning tunneling microscopy and low energy electron diffraction. Graphene is only formed on treated regions of the surface and so this offers a method for fabricating and patterning graphene structures on SiC and diamond in the solid-state at industrially realistic temperatures.     

Effects of plasma treatments on the nitrogen incorporated nanocrystalline diamond films

The nitrogen incorporated nanocrystalline diamond (NCD) films were grown on n-silicon (100) substrates by microwave plasma enhanced chemical vapor deposition (MPECVD) using CH₄/Ar/N₂ gas chemistry. The effect of surface passivation on the properties of NCD films was investigated by hydrogen and nitrogen-plasma treatments. The crystallinity of the NCD films reduced due to the damage induced by the plasma treatments. From the crystallographic data, it was observed that the intensity of (111) peak of the diamond lattice reduced after the films were exposed to the nitrogen plasma. From Raman spectra, it was observed that the relative intensity of the features associated with the transpolyacetylene (TPA) states decreased after hydrogen-plasma treatment, while such change was not observed after nitrogen-plasma treatment. The hydrogen-plasma treatment has reduced the sp²/sp³ ratio due to preferential etching of the graphitic carbon, while this ratio remained same in both as-grown and nitrogen-plasma treated films. The electrical contacts of the as-grown films changed from ohmic to near Schottky after the plasma treatment. The electrical conductivity reduced from ~ 84 ohm^– 1 cm^− 1 (as-grown) to ~ 10 ohm^– 1 cm^− 1 after hydrogen-plasma treatment, while the change in the conductivity was insignificant after nitrogen-plasma treatment.     

Photoluminescence quenching of graphene oxide by metal ions in aqueous media

The photoluminescence (PL) quenching of water-soluble graphene oxide (GO) solution was systematically investigated in the presence of transition metal ions. Their PL spectra were analyzed by the Stern–Volmer equation, and the trend of the quenching efficiency was Fe²⁺ > Co²⁺ > Ni²⁺ > Cd²⁺ > Hg²⁺. The results of the steady-state and time-resolved PL spectra of the GO solution suggested that the PL quenching was related to the new non-radiative optical transitions from the bridging states due to the hybridization of the sp³ orbitals of GO and the 3d orbitals of metal ions, proven by density functional theory calculations. The overall results indicated that the bridging states from the hybridization of GO sp³ and unfilled 3d orbitals (Fe²⁺) in comparison with filled 3d orbitals (Hg²⁺) were highly localized, and their energy levels were more suitable for being non-radiative transition states.     

The control of BN and BC bonds in BCN films synthesized using pulsed laser deposition

A novel chemical bond transformation from BN to BC was observed in BCN films synthesized using pulsed laser deposition (PLD). BCN film prepared by using green laser (λ=532 nm) induced two IR absorption at 1370 and 800 cm⁻¹. This film was dominated by amorphous carbon phase and sp² hybridized BN bonds. BCN film deposited using an ultraviolet (UV) laser (λ=266 nm ) induced an addition infrared (IR) absorption at 1250 cm⁻¹. As we deposit BCN film by deep-UV laser (λ=213 nm), the absorption at 1370 and 800 cm⁻¹ disappeared while the absorption at 1250 cm⁻¹ remained. According to X-ray photoelectron spectroscopy (XPS), BC bonds with a carbon rich composition were formed. The formation of BC bonds in BCN films was also sensitive to deposition gas pressure and substrate temperature. Reactive carbon and boron species were needed to enable BC bonds that hybridized the carbon and the BN phases. A low substrate temperature was required to avoid competition with sp² hybridized pure BN and pure carbon bonds.     

Room temperature dc electrical conductivity studies of electron-beam irradiated carbon nanotubes

The influence of electron-beam (E-beam) irradiation on the electrical (electronic) properties of single- (SW) and multi-walled (MW) carbon nanotube grown by microwave chemical vapor deposition is investigated. These films were subjected to a constant energy of 50 keV (50 A/cm²) from a scanning electron microscope gun for 2.5, 5.5, 8.0, and 15 h continuously — such conditions resemble increased temperature and/or pressure regime, enabling a degree of structural fluidity. To assess the structural modifications and electrical properties, the films were analyzed before and after irradiation. The experiments show that with increased exposure to ≥ 8–9 h, occasionally found individual bundles of single-wall nanotubes tend to collapse or pinch, graphitize/amorphize, and oxidize within the area of the electron-beam focus. Dramatic improvement in the I–V properties for single-walled (from semiconducting to quasi-metallic) and relatively small but systematic behavior for multi-walled with increasing exposure is discussed in terms of the critical role of controlled introduction of defects. The contact resistance decreases by orders of magnitude when exposed to electron beam and for all of the measurements the values ranged between 80 Ω and 10 kΩ at room temperature. These results also indicated that multi-walled nanotubes tend to reach a state of saturation degradation assessed by four-probe conductivity measurements. It is suggestive that there may be local gradual re-organization, i.e. sp^2 + δ, sp³ C ⇔ sp² C. More importantly, they provided a contrasting comparison between metallic/semiconducting (single/double-wall) and invariably metallic (multi-wall) carbon nanotubes.     

Hydrogen plasma etching of diamond films deposited on graphite

Poly- and nanocrystalline diamond films have been deposited using microwave plasma enhanced CVD with gas mixtures of x%CH₄/15%H₂/Ar (x = 0.5, 1, 3, and 5). After deposition the resulting films were exposed to a hydrogen plasma etching for 30 min. The hydrogen plasma produced preferential etching of non-diamond carbon on the surface of the samples and the development of steps and pits. Raman spectroscopy and X-ray photoelectron spectroscopy analyses on the etched films showed increased sp³/sp² ratio and decreased surface oxygen. The etch mechanism proposed is regression of pre-existing steps and step flow.