Partial hydrogenation of the open surface of graphene, epitaxially grown by chemical vapor deposition (CVD) on a Cu(111) substrate, leads to the formation of a crystalline sp3 hybridized carbon monolayer stabilized by interface C-Cu covalent bonding. This transition is reversible, with heating yielding almost complete restoration of the original graphene-copper structure. The graphene-Cu system is characterized by weak van der Waals interactions and this is the first transformation to yield C-Cu bonding. Through extensive spectroscopic characterization (Raman, X-ray photoelectron, X-ray absorption fine structure and valence-band photo-emission spectroscopies) and theoretical analysis based on density functional theory (DFT), we find transformation from weak van der Waals binding in the graphene-Cu system to covalent bonding between partially (topside) hydrogenated graphene and the Cu(111) surface, with the potential to revert back to its initial physisorbed state via dehydrogenation through heating. This reversible control over the graphene-Cu interaction opens new avenues for the design and manipulation of graphenebased devices. Furthermore, this sp3 hybridized carbon monolayer, with its C-metal substrate bonds, could potentially serve as a seed layer for the growth of large-area diamond films.

Natural diamonds were (and are) formed (thousands of million years ago) in the upper mantle of Earth in metallic melts at temperatures of 900-1,400 degrees C and at pressures of 5-6GPa (refs.(1,2)). Diamond is thermodynamically stable under high-pressure and high-temperature conditions as per the phase diagram of carbon(3). Scientists at General Electric invented and used a high-pressure and high-temperature apparatus in 1955 to synthesize diamonds by using molten iron sulfide at about 7GPa and 1,600 degrees C (refs.(4-6)). There is an existing model that diamond can be grown using liquid metals only at both high pressure and high temperature(7). Here we describe the growth of diamond crystals and polycrystalline diamond films with no seed particles using liquid metal but at 1atm pressure and at 1,025 degrees C, breaking this pattern. Diamond grew in the subsurface of liquid metal composed of gallium, iron, nickel and silicon, by catalytic activation of methane and diffusion of carbon atoms into and within the subsurface regions. We found that the supersaturation of carbon in the liquid metal subsurface leads to the nucleation and growth of diamonds, with Si playing an important part in stabilizing tetravalently bonded carbon clusters that play a part in nucleation. Growth of (metastable) diamond in liquid metal at moderate temperature and 1atm pressure opens many possibilities for further basic science studies and for the scaling of this type of growth.

Chemical vapor deposition of carbon precursors on Cu-based substrates at temperatures exceeding 1000 °C is currently a typical route for the scalable synthesis of large-area high-quality single-layer graphene (SLG) films. Using molecules with higher activities than CH 4 may afford lower growth temperatures that might yield fold- and wrinkle-free graphene. The kinetics of growth of graphene using hydrocarbons other than CH 4 are of interest to the scientific and industrial communities. We measured the growth rates of graphene islands on Cu(111) foils by using C 2 H 2 , C 2 H 4 , C 2 H 6 and CH 4 , respectively (each mixed with H 2 ). From such kinetics data we obtain the activation enthalpy (Δ H ≠ ) of graphene growth as shown in parentheses (C 2 H 2 (0.93±0.09 eV); C 2 H 4 (2.05±0.19 eV); C 2 H 6 (2.50±0.11 eV); CH 4 (4.59±0.26 eV)); C 2 H y (y=2, 4, 6) show similar growth behavior but CH 4 is different. Computational fluid dynamics and density functional theory simulations suggest that C 2 H y differs from CH 4 due to different values of adsorption energy and the lifetime of relevant carbon precursors on the Cu(111) surface. Combining experimental and simulation results, we find that the rate determining step (RDS) is the dissociation of the first C−H bond of CH 4 molecules in the gas phase, while the RDS using C 2 H y is the first dehydrogenation of adsorbed C 2 H y that happens with assistance of H atoms adsorbed on the Cu(111) surface. By using C 2 H 2 as the carbon precursor, high-quality single-crystal adlayer-free SLG films are achieved on Cu(111) foils at 900 °C.

We report the growth of amorphous carbon nanowalls with molten salt electrolytes and a carbonate carbon source at 600 degrees C on home-made Cu(111) foil as the growth substrate (and cathode). The nanometer thick nanowalls grow preferentially along symmetric slip lines on the Cu(111) surface and their ordered arrangement appears to also be dictated by the electrosynthesis parameters. Computational chemistry suggests that nucleation of carbon growth is favored at the slip lines (atomic steps) of the Cu(111) surface. The electrodeposited carbon structures can be varied by tuning the potential on the electrodes and temperature of the molten salt. The macro, micro, and nanoscale structure of the nanowalls was studied and is reported.

The authors report the growth of micrometer-long single-crystal graphene ribbons (GRs) (tapered when grown above 900 °C, but uniform width when grown in the range 850 °C to 900 °C) using silica particle seeds on single crystal Cu(111) foil. Tapered graphene ribbons grow strictly along the Cu<101> direction on Cu(111) and polycrystalline copper (Cu) foils. Silica particles on both Cu foils form (semi-)molten Cu-Si-O droplets at growth temperatures, then catalyze nucleation and drive the longitudinal growth of graphene ribbons. Longitudinal growth is likely by a vapor-liquid-solid (VLS) mechanism but edge growth (above 900 °C) is due to catalytic activation of ethylene (C2 H4 ) and attachment of C atoms or species ("vapor solid" or VS growth) at the edges. It is found, based on the taper angle of the graphene ribbon, that the taper angle is determined by the growth temperature and the growth rates are independent of the particle size. The activation enthalpy (1.73 ± 0.03 eV) for longitudinal ribbon growth on Cu(111) from ethylene is lower than that for VS growth at the edges of the GRs (2.78 ± 0.15 eV) and for graphene island growth (2.85 ± 0.07 eV) that occurs concurrently.

We report a study of the kinetics of dissolution of (100) and (110) single-crystal diamond plates ("D(100)" and "D(110)") in thin films of nickel (Ni) and cobalt (Co). This dissolution occurs at the metal-D(100) or metal-D(110) interface and was studied in the presence and also in the absence of water vapor at temperatures near 1000 degrees C. The single-crystal D(100) dissolves in Ni, and also in Co, in the temperature range 900-1050 degrees C. The dissolution is too slow to measure below 900 degrees C. In an argon (Ar) atmosphere (under an Ar(g) flow at 1000 sccm and 1 atm pressure, with no water vapor present in the reaction chamber) and at any temperature in the range 900-1050 degrees C, the metal film is rapidly saturated with dissolved carbon (C), thin graphite films form on the free metal surface and at the metal-D interface during heating at or above 650 degrees C, and the dissolution of the diamond then stops. For addition of water vapor, its partial pressure was controlled by using a water bubbler immersed in a constant temperature bath and Ar(g) was used as the carrier gas. We discovered two different regimes (I and II) for the kinetics of dissolution of D(100) and D(110), in which the rate-determining step was the removal of carbon atoms on the open metal surface (regime I, lower partial pressure of water vapor) or dissolution of diamond at the metal-diamond interface (regime II, higher partial pressure of water vapor) that yielded different Arrhenius parameters. Time-of-flight-secondary ion mass spectrometry depth profiles show the concentration gradient of C from a certain depth into the metal film surface down to the M-D(100) interface, and residual gas analyzer measurements show that the gas products formed in the presence of water vapor on the metal surface are CO and H2. It was found that the rate of dissolution of diamond in Co was higher than that in Ni for both D(100) and D(110) and for both regimes I and II, and possible reasons are suggested. We also found that D(111) could not be dissolved at the Ni/D(111) and Co/D(111) interface in the presence of water vapor (over the same range of sample temperatures). The reaction paths for dissolution of C at the M-D(100) or M-D(110) interface and for removal of C from the free surfaces of Ni and Co were assessed through density functional theory modeling at 1273 K.

The electrical and optoelectronic properties of nanometer-sized ZnO structures are highly influenced by its native point defects. Understanding and controlling these defects are essential for the development of high-performance ZnO-based devices. Here, an electrical device consisting of a polycrystalline ZnO-coated silica nanospring was fabricated and used to characterize the electrical and photoconductive properties of the ZnO layer using near-UV (405 nm) and sub-bandgap (532 and 633 nm) excitation sources. We observe a photocurrent response with all three wavelengths and notably with 532 nm green illumination, which is the energy associated with deep oxygen vacancies. The polycrystalline ZnO-coated silica nanospring exhibits a high responsivity of 1740 A W-1 with the 405 nm excitation source. Physical models are presented to describe the photocurrent rise and decay behavior of each excitation source where we suggest that the rise and decay characteristics are highly dependent on the energy of the excitation source and the trapping of electrons and holes in intermediate defect levels in the bandgap. The energy levels of the trap depths were determined from the photoconductive decay data and are matched to the reported energy levels of singly and doubly ionized oxygen vacancies. A phenomenological model to describe the dependence of the saturation photocurrent on excitation intensity is presented in order to understand the characteristics of the observed breaks in the slopes of the saturation photocurrent versus excitation intensity profile.

We report a new approach to making highly dense, oriented, and crystalline graphite films from heat-treated and pressed graphene oxide (G-O). By introducing small-diameter reduced graphene oxide (rG-O) flakes into the graphene oxide starting material, we found that after heat treatment at 3,000 degrees C, the sample density and atomic order substantially improved over a film composed, at the outset, only of pure G-O flakes. A subsequent mechanical press increased the density but reduced the atomic order. A second 3,000 degrees C heat treatment restored the graphitic structure with graphitization metrics exceeding even those of the first heat treatment. The optimized graphitic film with an original concentration of 15 wt% reduced G-O in G-O gave well-oriented graphitic films with a density of 2.1 g cm(-3), cross-plane thermal conductivity of 5.65 W m(-1) K-1, and in-plane thermal conductivity of 2,025 +/- 25 W m(-1) K-1.

High-quality AB-stacked bilayer or multilayer graphene larger than a centimetre has not been reported. Here, we report the fabrication and use of single-crystal Cu/Ni(111) alloy foils with controllable concentrations of Ni for the growth of large-area, high-quality AB-stacked bilayer and ABA-stacked trilayer graphene films by chemical vapour deposition. The stacking order, coverage and uniformity of the graphene films were evaluated by Raman spectroscopy and transmission electron microscopy including selected area electron diffraction and atomic resolution imaging. Electrical transport (carrier mobility and band-gap tunability) and thermal conductivity (the bilayer graphene has a thermal conductivity value of about 2,300 W m −1 K −1 ) measurements indicated the superior quality of the films. The tensile loading response of centimetre-scale bilayer graphene films supported by a 260-nm thick polycarbonate film was measured and the average values of the Young’s modulus (478 GPa) and fracture strength (3.31 GPa) were obtained.

Notwithstanding the numerous density functional studies on the chemically induced transformation of multilayer graphene into a diamond-like film carried out to date, a comprehensive convincing experimental proof of such a conversion is still lacking. We show that the fluorination of graphene sheets in Bernal (AB)-stacked bilayer graphene grown by chemical vapour deposition on a single-crystal CuNi(111) surface triggers the formation of interlayer carbon–carbon bonds, resulting in a fluorinated diamond monolayer (‘F-diamane’). Induced by fluorine chemisorption, the phase transition from (AB)-stacked bilayer graphene to single-layer diamond was studied and verified by X-ray photoelectron, UV photoelectron, Raman, UV-Vis and electron energy loss spectroscopies, transmission electron microscopy and density functional theory calculations. The fluorination of graphene sheets in bilayer graphene grown by chemical vapour deposition on a single-crystal CuNi(111) surface results in a fluorinated diamond monolayer.

Notwithstanding numerous density functional studies on the chemically induced transformation of multilayer graphene into a diamond-like film, a comprehensive convincing experimental proof of such a conversion is still lacking. We show that the fluorination of graphene sheets in Bernal (AB)-stacked bilayer graphene (AB-BLG) grown by chemical vapor deposition on a single crystal CuNi(111) surface triggers the formation of interlayer carbon-carbon bonds, resulting in a fluorinated diamond monolayer (F-diamane). Induced by fluorine chemisorption, the phase transition from AB-BLG to single layer diamond was studied and verified by X-ray photoelectron, ultraviolet photoelectron, Raman, UV-Vis, electron energy loss spectroscopies, transmission electron microscopy, and DFT calculations.

Single-crystal metals have distinctive properties owing to the absence of grain boundaries and strong anisotropy. Commercial single-crystal metals are usually synthesized by bulk crystal growth or by deposition of thin films onto substrates, and they are expensive and small. We prepared extremely large single-crystal metal foils by "contact-free annealing" from commercial polycrystalline foils. The colossal grain growth (up to 32 square centimeters) is achieved by minimizing contact stresses, resulting in a preferred in-plane and out-of-plane crystal orientation, and is driven by surface energy minimization during the rotation of the crystal lattice followed by "consumption" of neighboring grains. Industrial-scale production of single-crystal metal foils is possible as a result of this discovery.

The initial phases of amorphous silica nanospring formation via a vapor-liquid-solid mechanism are reported. The low temperature eutectic of Au-Si results in the formation of an asymmetrical shaped catalyst at the early stages of nanospring formation. As solid silica is formed below the Au-Si catalyst the system lowers its surface free energy and forms multiple amorphous silica nanowires beneath a common catalyst, as opposed to a single nanowire. The diameter of one of the nanowires forming the nanospring ranges between 10-20 nm. The difference in growth rates of the individual nanowires creates an asymmetry in the interfacial surface tension on the boundaries of the Au-Si catalyst/nanowires interface. Using Stokes' theorem it is shown that there is a variable work of adhesion on the outer boundary of the Au-Si catalyst/nanowire interface of a nanospring, which is defined as an effective contact angle anisotropy. The anisotropic growth on the catalyst/nanowire boundary results in the nanowires coherently coiling into to a single, larger, helical structure with an overall diameter of 70-500 nm.

A significant improvement of the response characteristics of a redox chemical gas sensor (chemiresistor) constructed with a single ZnO coated silica nanospring has been achieved with the technique of lock-in signal amplification. The comparison of DC and analog lock-in amplifier (LIA) AC measurements of the electrical sensor response to toluene vapor, at the ppm level, has been conducted. When operated in the DC detection mode, the sensor exhibits a relatively high sensitivity to the analyte vapor, as well as a low detection limit at the 10 ppm level. However, at 10 ppm the signal-to-noise ratio is 5 dB, which is less than desirable. When operated in the analog LIA mode, the signal-to-noise ratio at 10 ppm increases by 30 dB and extends the detection limit to the ppb range.

A redox chemical sensor (chemiresistor) was constructed with a single ZnO coated silica nanospring. The chemiresistor response to toluene vapor as a function of the sensor temperature (T(NS)) and vapor temperature (T(V)) was measured and analyzed. The maximum sensitivity of the single ZnO coated nanospring device occurred at the sensor temperature (T(NS)) of 310 °C and at the vapor temperature (T(V)) of 250 °C. The characteristics of the electrical response of a single ZnO coated nanospring device were compared to those of a ZnO thin film. The single ZnO nanospring sensor was less responsive to small changes in toluene concentration relative to the ZnO thin film, but has a lower ultimate detection level. A computational model to simulate an electrical response of the single nanospring sensor and the thin film indicated that the differences between their response characteristics is due to the geometry of the nanospring and corresponding periodic boundary conditions imposed by the nanospring geometry, which is absent in a thin film.

A chemical sensor (chemiresistor) was constructed from a xenon light bulb by coating it with a 3-D zinc oxide coated silica nanospring mat, where the xenon light bulb serves as the sensor heater. The sensor response to toluene as a function of xenon light bulb sensor temperature (TLB) and vapor temperature (TV) was observed and analyzed. The optimum operational parameters in terms of TLB and TV were determined to be 435 °C and 250 °C, respectively. The activation energy of toluene oxidation (Ed) on the ZnO surface was determined to be 87 kJ·mol−1, while the activation energy of oxidation (Ea) of the depleted ZnO surface was determined to be 83 kJ·mol−1. This study serves as proof of principle for integrating nanomaterials into an inexpensive sensor platform, which can also be used to characterize gas-solid, or vapor-solid, redox processes.

The photocurrent of individual gallium nitride (GaN) nanowires decorated with Au nanoparticles as function of the wavelength of light (405 nm (blue), 532 nm (green), and 632.8 nm (red)) and nanowire diameter (80 to 400 nm) is reported. The photocurrent scales with photon energy but oscillates with nanowire diameter. The oscillations are described in terms of the scattering of surface plasmon polaritons into allowed transverse magnetic electromagnetic modes of the nanowire that have maximum intensities in the undepleted region of the nanowire. These oscillations do not occur below a nanowire diameter of ~200 nm due to the depletion layer formed at the Au-GaN interface, which completely depletes the nanowire, that is, there is an insufficient density of carriers that can be excited into the conduction band. On the basis of estimations of the depletion depth and solutions of the Helmholtz equation, the maxima in the photocurrent for d > 200 nm are assigned to the two lowest azimuthally symmetric transverse magnetic eigenmodes: (m = 0, n = 1) and (m = 0, n = 2), which have maximum electric field intensities within the undepleted region of the GaN nanowire. The outcome of this work could have far reaching implications on the development of nanophotonics.

The potential silica nanospring (NS) supported cobalt catalyst (Co/SiO2 -NS) for Fischer-Tropsch synthesis (FTS) was investigated, and the results were compared with those of a conventional silica gel supported cobalt catalyst (Co/SiO2-gel). Co/SiO2-gel and Co/SiO2-NS catalysts were prepared using the incipient wetness impregnation method and a thermal assisted reduction process, respectively, and characterized by scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS), transmission electron microscopy (TEM), N2 physisorption, X-ray powder diffraction (XRD), and H2-temperature programmed reduction (H2-TPR). The catalysts were evaluated for their conversion of syngas to products in a quartz fix-bed micro-reactor (230 °C, atm pressure). The FTS products were trapped and characterized by GC-MS to determine conversion efficiency. The products (alkanes) for both catalysts ranged from C1 to C21 and would be a suitable substrate for diesel. The results show that the NS approach for a FTS catalyst support shows promise for generating fuels from syngas. Future work will focus on optimizing Co/SiO2-NS catalyst for improved conversion efficiencies.

Chemiresistors (conductometric sensor) were fabricated on the basis of novel nanomaterials--silica nanosprings ALD coated with ZnO. The effects of high temperature and UV illumination on the electronic and gas sensing properties of chemiresistors are reported. For the thermally activated chemiresistors, a discrimination mechanism was developed and an integrated sensor-array for simultaneous real-time resistance scans was built. The integrated sensor response was tested using linear discriminant analysis (LDA). The distinguished electronic signatures of various chemical vapors were obtained at ppm level. It was found that the recovery rate at high temperature drastically increases upon UV illumination. The feasibility study of the activation method by UV illumination at room temperature was conducted.

Chemiresistors were constructed using 3-D silica nanospring mats coated with a contiguous film of ZnO nanocrystals. Chemiresistors with an average ZnO nanocrystal radius <3 nm, or >20 nm, were found to exhibit a relative change in conductance of a factor of 50 upon exposure to a gas flow of 20% O2 and 80% N2 with ∼500 ppm of toluene and an operational temperature of 400 °C. Samples with an average ZnO nanocrystal radius of 15 nm were found to be the most responsive with a relative conductance change of a factor of 1000. The addition of metal nanoparticles (average radius equal to 2.4 nm) onto the surface of the ZnO nanocrystals (average radius equal to 15 nm) produced a relative change in conductance of a factor of 1500. For the optimum conditions (T = 400 °C, grain size ∼15 nm) well-defined spikes in conductance to explosive vapors (TNT, TATP) were obtained for 0.1 ms exposure time at ppb levels.